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首页 >> 中国造纸杂志社 >> 国际造纸 >> 摘要 >> 《Paper and Biomaterials》2019年第1期摘要
Wet Pressing Models to Reduce Energy Consumption in Papermaking
J. David McDonald1,*, Richard J. Kerekes2, Joe R. Zhao3
1. JDMcD Consulting Inc., Vaudreuil-Dorion, Quebec, J7V 0G1, Canada
2. Pulp and Paper Centre, the University of British Columbia, Vancouver, BC V6T 1Z4, Canada
3. Tri-Y Environmental Research Institute, Vancouver, BC V5M 3H9, Canada
Abstract: Improved wet pressing reduces the need for drying and consequently energy needed in papermaking. Accordingly, it is desirable to optimize wet pressing, but the process is very complex with many interacting variables. It is therefore desirable to employ a mathematical model that accounts for the major variables to estimate the effects of changes in equipment and operating variables. This paper descibes such a model called the Decreasing Permeability Model (DPM). Mill and pilot plant applications over a wide range of basis weights for paper and paper board are described.
 
Keywords: wet pressing; Decreasing Permeability Model (DPM); papermaking; water removal; rewet; mathemeitical model
 
 
 
*Corresponding author:
J. David McDonald, PhD;
research interests: papermaking, mechanical pulping, and bio-materials;
E-mail: mcdonald.jdavid@gmail.com
 
 
 
1    Introduction
Improved wet pressing is a major means of energy conservation by reducing the need for drying in paper and paper board production. In non-integrated mills, steam for drying must be generated by combustion of gas, oil, coal or biomass. Kraft and TMP mills often have excess steam, but even low-pressure steam can now be used in condensing turbines to produce electricity rather than to dry paper. Thus, removing more water mechanically in the pressing process reduces energy consumption and may liberate steam that could be converted into high-value energy. There are other potential benefits as well, such as improved runnability and higher machine speeds which result in greater production rates. Given these benefits, there is a strong desire to optimize press sections. The objective of this study is to describe a model of wet pressing that enables this.
2    Overview
Pressing is a very complex process affected by many variables:
·Press nip load
·Peak pressure
·Machine speed
·Properties of the furnish
·Felt construction and double felting
·Paper temperature
·In-going web solids
·Rewet
·Grammage
Given the complexity of wet pressing, it is not easy to optimize the process. A change in one variable often affects the influence of another variable. For good results, all the variables must be considered simultaneously. This is best done by a mathematical model. Although a number of models of pressing have been developed over the years[1], only one meets the full extent of the needs described above. This is the Decreasing Permeability Model (DPM) developed by McDonald and Kerekes[2-7] described below.
3    Decreasing Permeability Model (DPM)
The DPM describes the moisture after pressing, m, as the sum of 3 terms: a flow term, mf, an equilibrium term, me, and a rewet term, mr. These are expressed as moisture ratios, i.e., mass water/mass fibre, as shown in Equation (1):
    m=mf +me+mr                                                           (1)
3.1    Flow term, mf
This term represents the moisture remaining in the web after pressing for a given time under a given pressure. The product of pressure and time is often expressed as the press impulse, I. Flow must overcome resistance to flow through the web which is represented by compressibility factor, n, specific permeability, A, and basis weight, W. The flow term is given by:
 
 
Where:
mo—moisture ratio before press;
me—equilibrium moisture ratio attained after pressing for infinite time at the peak nip pressure;
A—specific permeability, g/m;
n—compressibility factor;
I—press impulse (pressure×time = line load/speed), kPa·s;
W—basis weight, kg/m2;
v—kinematic viscosity, m2/s.
3.2    Equilibrium term, me
This term represents moisture after pressing for an infinite time. This equilibrium moisture depends on furnish coefficients , d and peak pressure, Pp:
      me=Ppd                                                                 (3)
The furnish coefficients are a function of pulp type, yield, drying history, refining and chemical treatment.
3.3    Rewet term, mr
Rewet is water that has been pressed from the web but returns to the web after the press nip. As with the other remaining water, this must be removed in drying. Because rewet is a surface phenomenon that occurs at the web/felt interface, it is commonly expressed as mass of moisture per area of paper, R. Consequently, R is divided by basis weight W to be expressed as a moisture ratio:
 
 
Rewet has two components: (a) flow rewet, mf, which is water that flows back into the web as a result of capillary forces and (b) separation rewet, Rs which is the water layer between the felt and paper that stays with the paper when they separate. The total rewet has the following form[8]:
 
 
Where:
R—rewet, g/m2;
t—the contact time between the felt and paper starting in the expanding nip, s;
D—the diameter of the batt fibres, b—a geometrical factor related to the felt structure, mg/m3;
mf —the minimum moisture ratio achieved inside the press nip (Equation (2)).
 
 
Where:
p—surface tension of the water in the paper, N/m;
rp,0—a constant representative of the paper, g2/m4;
—viscosity of water, N·s/m2.
4    Full DPM equation
Combining the above terms, we obtain the full DPM[6-7]:
 
 
This equation describes the full range of wet pressing, for example over the full range of basis weights. As shown in Fig.1[1], at high basis weights, the flow term mf dominates pressing, giving a “flow-controlled regime”. In contrast, at lower basis weights, equilibrium moisture and rewet dominate pressing, giving a “pressure controlled” regime.
 
 
 
 
 
 
 
 
 
 
Fig.1    The first term on the right-hand side of the DPM dominates the flow-controlled regime whereas the equilibrium moisture, me, and rewet terms are responsible for the behaviour in the pressure-controlled regime[1]
5    Approximations
The full DPM in Equation (7) contains variables which may be unknown or difficult to determine. In these cases, some approximations are possible under certain circumstances.
In the flow-controlled regime, when the pressed moisture is much greater than equilibrium moisture, a simplified form of Equation (7) shown below may give a good approximation of the full DPM equation[4-5]:
 
 
Equation (8) may also give a good representation of pressing when equilibrium moisture is important and the extent of rewet is unknown. This is possible by accounting for the effects of these variables through smaller values of A and larger values of n as shown in Table 1[6]. Physically, the changed coefficients represent a smaller effective permeability and higher power dependence on moisture ratio. An example of the usefulness of this approximation for R=0 is the excellent fit of Equation (8) to data from a Canadian newsprint machine survey[3]. The fit had a coefficient of determination (R2) of 0.95.
Table 1    Approximate equations for newsprint[6]
Equations         n       A/(g·m1)  R/(g·m2)
(8)    3.74 5.12×109 0
(2 & 3)      2.97 6.84×108 0
(7)    2.65 1.03×107 5
(7)    2.32 1.52×107 10
 
 
On the other hand, when basis weight is small, the flow term mf becomes small and therefore neither rewet nor equilibrium moisture can be neglected. This region depends on many factors such as furnish, press impulse, and incoming moisture. Generally, for basis weights less than 50 g/m2, pressed moisture can be approximated as[8-9]:
 
 
Finally, we note here that caution must be exercised in use of these approximate equations to ensure that they are employed within their limits of applicability.
6    Multiple press nips and double-felted presses
Most paper machines have more than one press nip. The moisture after the last press nip can be determined by using the calculated moisture after the first press as the ingoing moisture to the second press in Equation (8) and repeating this process until the final press. The same result can be obtained more simply by using the sum of the press impulses from each nip in Equation (8), which has been shown to be mathematically equivalent[5].
Double-felting allows water to flow through both sides of the pressed paper such that it behaves like a sheet with basis weight W/2. Because of the W2 factor in the DPM, this is equivalent to multiplying the specific permeability (A) by 4[10]. In addition, because there are two felt surfaces in contact with the paper, rewet (R) will be doubled[11].
7    Applications
7.1    Light basis weight grades
Pressing a 50 g/m2, 100% TMP furnish was evaluated on a pilot machine having a three-roll, inclined rolling nip press followed by a shoe press[5]. Moisture samples were collected after the shoe press for a variety of loadings, machine speeds, and web temperatures. The furnish dependent coefficients (A and n) were determined independently by pressing handsheets on a pilot press. Moisture values after pressing were measured and compared to predicted values from Equation (8) with and without rewet, as shown in Fig.2. The difference between these of 9 g/m2 was interpreted as rewet.
 
 
 
 
 
 
 
 
Fig.2    Measured versus predicted solid content after a shoe press on a pilot paper machine. The difference in results was attributed to rewet[5]
7.2    Medium basis weight grades
A second pilot machine trial using the above newsprint furnish examined a range of basis weights from 25 to 100 g/m2[4-5]. Moisture ratio was measured after the second rolling nip at a speed of 610 m/min with nip loads 60 and 80 kN/m. Fitting Equation (8) to this data gave the furnish dependent coefficients (A and n) and the rewet (R=23 g/m2). The fitted curves are shown in Fig.3.
It is apparent that this range of basis weights spans the pressure controlled and flow-controlled regimes for this furnish. Below 50 g/m2, it would be possible to model dewatering by Equation (9), that is, by a linear equation in 1/W with equilibrium moisture at intercept (1/W=0). This is described in more detail in a later example.
7.3    High basis weight grades
7.3.1    Corrugating medium
This study employed the DPM to improve pressing on a commercial paper machine for corrugating medium[10]. In the first pressing stage, two straight-through presses were replaced with a three-roll inclined press followed by a third press. In the second stage, the third press was rebuilt to increase the nip load by almost a factor of 2. Both rebuilds were accompanied by higher machine speeds to increase production. The DPM was employed to predict moistures for the contemplated changes. The estimates for the final press for the three configurations were in good agreement with the mill results, as shown in Table 2[10].
 
7.3.2    Linerboard
This application considered pressing a 205 g/m2 linerboard composed of a 160 g/m2 filler layer with either a white or brown 45 g/m2 top-liner[10]. The board was pressed in two top-felted press nips followed by two bottom-felted nips. The furnish dependent coefficients for each furnish (A and n) were determined by pressing handsheets in a pilot press. The filler layer was the most difficult component to dewater, followed by the brown and white liner. The pressed moisture was approximated by weighting the calculated pressed moisture ratios of each component by their relative fibre masses[10]:
 
 
Estimates from the DPM gave solid contents of 38.8% and 39.4% for the brown and white liner respectively. This agreed well with the measured solid content of 39% after the final press.  Additional estimates were employed to justify the installation of a shoe press to increase the machine speed and production.
8    Theoretical estimate of upper limit of pressing
This case addressed a question often asked by papermakers: what is the upper limit of dryness from wet pressing?  Equally important, what are the barriers to reaching this limit? These issues were addressed recently by application of the DPM for both low basis weight and high basis weight[11].
In the pressure-controlled regime, as expected, equilibrium moisture content and rewet are critical. Thus, lowering either of these will permit higher dryness. For grades that are not bulk sensitive, increasing the peak pressure in the nip will lower the equilibrium moisture although high pressure has a diminishing effect as shown by Equation (3).
The influence of rewet is shown in Fig.4 as the slope of the line 1/W. Three levels of rewet are shown with the smallest approaching R=0. At this level, equilibrium moisture would be the only water left in the paper.
In recent work[8], we explored the factors which affect rewet. In the pressure-controlled regime, the limit is primarily a function of felt design and contact time between felt and paper after the press nip. Appropriate choice of felt and immediate separation of the paper from the felt at the nip exit will lower rewet[7-8,12]. In the flow-controlled regime, rewet is not independent of basis weight. As we have shown[8,12], for heavy-weight papers, rewet increases with pressed moisture ratio. Combining Equations (1), (4) and (5), we obtain:
 
Lastly, an example of an upper limit of pressing is shown in Fig.5 at differing levels of rewet for a TMP furnish at 50 g/m2. In the absence of any rewet, it is apparent that the upper dryness limit would approach 65%[11].
9    Summary and conclusions
This paper has described applications of the DPM to improve wet pressing in order to reduce energy consumption in drying. Questions remain, however, on the upper limit of dryness attainable, in particular factors affecting rewet and equilibrium moisture content. A better understanding of these variables may lead to higher dryness contents that are currently attained.
References
[1] McDonald J D, Kerekes R J. Pragmatic Mathematical Models of Wet Pressing in Papermaking[J]. Bioresources, 2017, 12(4): 9520-9537.
[2] Kerekes R J, McDonald J D. A decreasing permeability model of wet pressing: theory[J]. TAPPI J., 1991, 74(12): 150-156.
[3] McDonald J D, Kerekes R J. A Decreasing Permeability Model of Wet Pressing: Applications[J]. TAPPI J., 1991, 74(12): 142-149.
[4] McDonald J D, Kerekes R J. A Decreasing Permeability Model of Wet Pressing with Rewetting[J]. TAPPI J., 1995, 78(11): 107-111.
[5] McDonald J D, Hamel J, Kerekes R J. Design Equation for Paper Machine Press Sections[J]. J. Pulp Pap. Sci., 2000, 26(11): 401-406.
[6] Kerekes R J, McDonald E M, McDonald J D. Decreasing Permeability Model of Wet Pressing: Extension to Equilibrium Conditions[J]. J-FOR, 2014, 3(2): 46-51.
[7] McDonald J D, McDonald E M, Kerekes R J. The Impact of Felt Design on Paper Machine Press Dewatering[J]. J-FOR, 2014, 3(2): 53-57.
[8] McDonald J D, Kerekes  R J. Rewet in Wet Pressing of Paper[J]. TAPPI J., 2018, 17(9): 479-487.
[9] Sweet J S. A Basic Study of Water Removal at the Press[J]. Pulp Paper Mag. Can., 1961, 62(8): T367-T371.
[10] McDonald J D, Amini J. Predicting the press dewatering of heavyweight grades[J]. TAPPI J., 2000, 83(2):79-82.
[11] McDonald J D, Kerekes R J. Estimating limits of wet pressing on paper machines[J]. TAPPI J., 2017, 16(2): 81-87.
[12] McDonald J D, Pikulik I I. Postnip rewet of newsprint[J]. Nordic Pulp and Paper Research Journal, 1988, 3(3): 115-119. 
 
 
 
 
Comparative Study of Isolated Polysaccharides from Triploid Poplar Using Different Solvents and Chemicals
YingYing Chai1, Ning Zhao1, YunShan Ju1, QingTao Fan2, Kun Wang1,*
 
1. Beijing Key Laboratory of Lignocellulosic Chemistry, MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China
2. Beijing Institute of Science and Technology Information, Beijing 100044, China
 
Abstract: The conversion of lignocellulose to value-added products is normally focused on fuel production; however, large-scale biorefineries require a cost-effective pretreatment process that can effectively fractionate the three main constituents of lignocellulose for the production of chemicals, fuels, and materials. In this study, a hemicellulosic biopolymer from poplar was fractionated by a mild organosolv process and the effects of various chemicals (sodium hydroxide, triethylamine, and formic acid) and alcohols on the fractionation efficiency and structural variation of hemicellulose were examined. Comparative studies indicated that an acidic catalyst decreased the purity of hemicelluloses by partial degradation of cellulose, and the core of the hemicellulosic biomacromolecule could be released and dissolved under alkaline conditions with 5.8%~19.0% yields. In addition, the use of alcohol with longer alkyl chains facilitated the release of the hemicellulosic biomacromolecule by partially cleaving the ether bonds in the lignin-carbohydrate complex (LCC); this is probably due to steric hindrance. The thermal degradation behavior showed that complete pyrolysis was easily achieved for the hemicellulosic polymer with minimal branches irrespective of its molecular weight.
 
Keywords: organosolv fractionation; biorefinery; hemicelluloses, thermal property
 
 
YingYing Chai, master candidate;
E-mail: bjfu140534126@163.com
 
*Corresponding author:
Kun Wang, professor, PhD tutor; research interest: integrated utilization of biomass and bioenergy;
E-mail: wangkun@bjfu.edu.cn
 
 
1    Introduction
The increasing demand for energy and materials has attracted increasing concerns over greenhouse-gas-induced climate change and future fossil petroleum shortage. To mitigate these threats, new technologies are being developed to balance economic growth and the environmental impact by utilizing biomass components to replace petroleum-derived energy carriers and chemicals[1-2]. As the concept of “BIOREFINERY” is extensively prevalent recently, the utilization of all the components of biomass is becoming an increasingly important topic for governments and researchers, in particular regarding the applications of hemicelluloses[3-5]. Hemicelluloses, also termed heteropolysaccharides or polyoses, are non-cellulosic polysaccharides consisting of D-xylose, D-mannose, L-arabinose, D-glucose,
D-galactose, and 4-O-methyl-D-glucuronic acid residues with different fractions and different substituents. The main hemicellulose of hardwoods is O-acetylated 4-O-methyl-glucuronic acid xylan or glucuronoxylan. According to Willfor’s report, xylan is the major hemicellulose constituent in poplar species (15.9% to 22.4%) followed by mannan (0.9% to 3.4%), 4-O-methyl-glucuronic acid (2.2% to 2.8%), galacturonic acid (2.3% to 2.8%), and glucuronic acid (0.1% to 0.3%)[6]. Xylan, in most cases, is extensively connected with cellulose by hydrogen bonds and van der Waals forces, which determine its vital role in the recalcitrance of cell walls. Thus, xylan is of critical importance in cellulose enzymatic hydrolysis.
The consideration of co-products in value engineering and the target costing of lignocellulosic biorefining are important aspects for an overall economic evaluation and subsequent implementation of new technology[7]. An effective pretreatment strategy is to develop a platform where cellulose is recovered for bioethanol, and other components in biomass are also converted to valuable coproducts. Lignin depolymerization into bio-based aromatics via ‘lignin-first’ processes has been reported[8]; however, the utilization of high hemicellulose retention presents several key challenges, economically and technologically. Rapid progress has been made for the fractionation of hemicellulosic biomacromolecules using various pretreatment strategies, including hydrothermal[9-10], dilute-acid[11-12], alkali[13-14], steam explosion[15], ultrasound-assisted extraction[16], and ultrafiltration[17-18] processes. The organosolv pretreatment has been proposed to provide both chemical and physical modification of the substrate, including the disruption of hydrogen bonds in crystalline cellulose and the removal of lignin and hemicelluloses that embed within the cellulose fibers. In addition, the pretreatment results in an increase in the fiber porosity and surface area of cellulose and a reduction in the degree of polymerization (DP) for subsequent enzymatic hydrolysis[19-21]. Historically, ethanol and methanol have primarily been used in organosolv pretreatment because of their low cost and volatility, which are beneficial for solvent recovery. A variety of organic solvents, temperature and pressure conditions, and catalysts have been extensively investigated. After the pretreatment, the solid and liquid forms are separated and high-purity solid lignin is precipitated and isolated by decreasing the concentration of the organic solvent in the liquid; the solvent is then recycled and reused. Industrially, a feasible pretreatment processes should have good pulp yield, maintain high quality of the other two main constituents, and be cost-effective.
Triploid Populus tomentosa Carr., a kind of fast-growing poplar widely planted in China to prevent wind erosion and control desertification, has considerable economic and ecological importance[22]. In recent years, triploid poplars have been rapidly used in the production of paper, boards, and bioethanol. In previous studies, the variation of the crystal configuration in cellulose extracted from triploid poplars was comparatively investigated after dissolution-regeneration treatment with different solvent systems. The bioconversion efficiency of the regenerated cellulose changed depending on the solvent[23]. In addition, the structural variations of hemicellulose and lignin, isolated from triploid poplars after an acidic biorefinery process, was studied[24]. The flexibility of the biorefinery process is an important novel consideration, reflecting the developing scenario regarding the variety of feedstock and product choices. As attention shifts from biofuels to multi-product biorefineries, this flexibility will be required to support decision-making for the future biorefining industry. Following these premises, the aims of this study were to apply the organosolv pretreatment under different conditions (acidic or basic, different alcohols) to separate hemicelluloses from poplar. The fractionation efficiency was evaluated. The fractionated hemicellulosic samples were characterized by Fourier Transform Infrared Spectroscopy (FT-IR), Gel Permeation Chromatography (GPC), Thermogravimetry Analysis (TGA), and High-Pressure Liquid Chromatography (HPLC).
2    Experimental
2.1    Materials
A four-year-old triploid poplar (Populus tomentosa Carr.) was cut from Shandong Province, China. After being debarked and air-dried, the tree trunks were chipped using a custom-designed chipper. The chips were ground and the fraction passing a 40-mesh was collected for chemical analysis. The main chemical composition is: wax (4.2%), glucose (44.5%), xylose (19.8%), arabinose (3.6%), Klason lignin (21.4%), and ash (5.1%). The standard deviation was less than 5%. All the other reagents and chemicals are analytical grade unless otherwise stated.
2.2    Organosolv fractionation
Prior to organosolv fractionation, the poplar powder was first dewaxed with a toluene-ethanol mixture (2∶1, V/V) in a Soxhlet extractor for 6 h. The extractive-free sample (10.0 g) was then treated with a 70% aqueous alcohol solution with or without catalyst, and the hemicellulosic fractionations were precipitated in ethanol after being neutralized with HCl/NaOH, as described in Scheme 1. In detail, four kinds of aqueous alcohol solvents i.e., methanol, ethanol, n-propanol, and n-butanol, all at 70% (V/V), and three catalysts i.e., sodium hydroxide, triethylamine, and formic acid, all with a fixed concentration of 1% (W/V), were employed in the current study. The fractionation process was carried out in a round-bottom flask with an external heater and a condenser and a wood to liquid ratio of 1∶20 (g/mL)[25-26]. After reaching the designated temperature (80℃), the mixtures were incubated (3 h), cooled to room temperature, and filtrated under vacuum. The liquid phase was neutralized to pH value 5~6 with concentrated HCl or 2 mol/L NaOH and further concentrated under reduced pressure. The hemicellulosic components were fractionated by adding three equivalent volumes of 95% ethanol and centrifuging at 1000 g for 30 min. All the hemicellulosic samples were freeze-dried and kept in a desiccator at room temperature for further analysis. To reduce errors and confirm the results, each experiment was repeated twice under the same conditions. The yields are given as the average of the replicates, on a dry-weight basis.
 
 
 
 
 
 
 
 
 
 
 
 
Scheme 1    Organosolv fractionation of the hemicellulosic fractions from poplar
2.3    Wet-chemical analysis
The monosaccharide components in the hemicellulosic fractions were obtained by hydrolyzing 5 mg samples with 6% H2SO4 for 2.5 h at 105℃[27-28]. After the hydrolysis, the samples were diluted 50-fold, filtered, and injected into the HPAEC system (Dionex ICS3000, USA) with a pulsed amperometric detector and an ion-exchange Carbopac PA-1 column (4×250 mm). The neutral sugars were separated in 18 mmol/L NaOH (carbonate-free and purged with nitrogen) at a rate of 0.5 mL/min. The run time was 45 min followed by a 10-min elution with 0.2mol/L NaOH to wash the column, and then a 15-min elution with 18 mmol/L NaOH to reequilibrate the column. Calibration was performed with a standard solution of L-rhamnose, L-arabinose, L-glucose, L-galactose, D-mannose, and D-xylose.
The composition of residual lignin associated with hemicellulose was measured via a degradation process with nitrobenzene oxidation under alkaline conditions[29]. The separation of phenolics was achieved with a HPLC system (1200 series, Agilent Technologies, USA) on a ZORBAX Eclipse XDB-C18 column (4.6×250 mm). The identification of phenolic acids and aldehydes was carried out by comparing the retention times and UV spectra (DAD, diode array detector) of the eluting peaks of the authentic standard compounds (p-hydroxybenzoic acid, vanillic acid, syringic acid, ferulic acid, p-coumaric acid, p-hydroxybenzaldehyde, vanillin, syringaldehyde, acetovanillone, and acetosyringone), which were purchased from Sigma (Sigma-Aldrich Corp., St. Louis, MO, USA). All analyses were run at least twice, and the errors for the sugar and aromatic-compounds analysis were less than 1% and 8%, respectively.
The measurement of the molecular weights of the hemicellulosic fractions by a GPC system on a PL Aquagel-OH mixed column (300×7.5 mm, Agilent) was described in previous papers and was conducted using a HPLC system equipped with a refractive index detector (RID)[30]. The eluent was 0.02 mol/L NaCl in 5mmol/L sodium phosphate buffer (pH value=7.5) with a flow rate of 0.1 mL/min. To calibrate the column and calculate the molecular weight of hemicelluloses, monodisperse polysaccharide of known molecular weight was used as the standard.
2.4    FT-IR analysis
FT-IR spectra of the hemicellulosic preparations were recorded using an FT-IR spectrophotometer (Tensor 27, Bruker, Germany) using a KBr disc containing 1% finely ground samples. The FT-IR absorption was measured within the range of 400~4000cm1 with an accumulation of 32 scans and a resolution of 2 cm1.
2.5    Thermal analysis
A Shimadzu (Japan) DTG-60 simultaneous thermogravimeric/differential thermal analysis (TGA-DTA) apparatus was employed for thermal stability tests. This apparatus detects mass loss with a resolution of 0.1 mg. To study the difference of heat and mass transfer, the sample weight was maintained at ~5 mg. Samples were heated up to 600℃ at a constant heating rate of 10℃/min, and purified nitrogen (99.9995%) was used as the carrier gas to provide an inert atmosphere for pyrolysis and to remove the gaseous and condensable products, minimizing any secondary vapor-phase interactions. The thermal decomposition temperature was taken at the onset of significant weight loss (0.5%) after the initial moisture loss.
3    Results and discussion
3.1    Fractional yield and characteristics
The effectiveness of organosolv process is based on its ability to progressively break down and modify the lignin macromolecule until the resulting molecular fragments become small enough to be dissolved in the aqueous liquor. Based on the concept of biorefinery, optimal pretreatment conditions are defined to ensure efficient hydrolysis of the water-soluble cellulose stream with an acceptable recovery of the total carbohydrates, mainly hemicelluloses. As can be seen, the ethanol organosolv process without any catalysts yielded only 1.7% hemicellulose, containing 70.9% xylose and 22.5% glucose as the main sugars (Table 1). Under acidic conditions, although the yield of hemicellulose slightly increased to 3.7%, the content of glucose significantly increased to 37.3% and that of xylose correspondingly decreased to 51.2%. Clearly, the partial degradation of amorphous cellulose occurred under acidic conditions and the dissolved glucose oligosaccharide was precipitated and fractionated. The alkaline condition with the organic base TEA did not efficiently improve the separation of hemicelluloses as only a 1.1% yield was obtained. The relative content of xylose was notably low, almost equal to that of the PE+Acid sample. This can probably be ascribed to the fact that the hemicellulosic fraction obtained from TEA extraction had a multi-branched structural system on the backbone chain. In comparison, the use of an inorganic basic catalyst was more efficient to fractionate hemicelluloses. As shown, 5.8%~19.0% hemicelluloses were isolated under such conditions; the fraction gradually increased with the length of the alkyl chain in different alcohols. The fractionation of hemicelluloses involves the partial degradation of the hemicellulose biomacromolecule and the dissolution of the degraded fragments; hence, the type of alcohol has a critical impact on the fractionation efficiency. This is probably due to steric hindrance i.e., an alkaline alcohol solution with a longer alkyl chain is more beneficial for the process of releasing the hemicellulosic biomacromolecule from the lignocellulosic matrix by partially cleaving the ether bonds in the LCC.
Once the influence of pretreatment conditions on the recovery and sugar composition of the carbohydrate polymers was tested, the molecular weight distribution of the hemicellulosic samples was investigated. Fig.1 presents the GPC curves of the hemicellulosic samples, and the data given in Table 1 show the weight-average, number-average molecular weight (Mw and Mn, respectively), and polydispersity (Mw/Mn) values as a function of the organosolv conditions. The polydispersity of obtained hemicellulosic fractions was obviously affected by the acid-base property of the solvents (Fig.1(a)). The pure ethanol aqueous solution only extracted the hydrophilic part of hemicelluloses with low molecular weight. The Mw and Mn values of the PE sample were determined to be 24300 and 9000, respectively. The core biomacromolecule of hemicelluloses could be easily hydrolyzed under acidic conditions and then fractionated, resulting in the enhanced yield and Mw value. When TEA was used as a catalyst, the degradation of hemicelluloses was limited and the Mw value correspondingly increased, although the fractionation efficiency was still low. In comparison, the inorganic base catalyst is suitable for hemicellulose isolation with its original biomacromolecular structure. It is surprising to notice that the Mw value of the PE+TEA sample (256300) was much higher than that of the PE+NaOH sample (110400), but the main peak of the distribution curve was in the relatively low molecular-weight region. The explanation is that the Mw value of PE+TEA was probably overestimated because of the minimum presence of the “tail” of the curve for values higher than 1×106 and the resulting high polydispersity (9.2). This data also further confirmed the result from the chemical analysis that the PE+TEA product was a highly-branched hemicellulosic polymer. In terms of different alcohols, the trend of the Mw variation was found to be similar to that of the yields i.e., it increases with the alkyl chain length in alcohols. As discussed above, an alcohol with longer alkyl chains could probably contribute to cleaving the ether bonds in the LCC, leading to an increased efficiency for hemicellulose fractionation. The core of the hemicellulosic biomacromolecule was released and then dissolved in the alkaline solvent, and the values of the Mw accordingly increased; in addition, there is a clear shift of the molecular weight distribution to higher values.
The presence of associated lignin with the fractionated hemicelluloses is likely due to the LCC, which is linked via ether and ester bonds. Alkaline nitrobenzene oxidation provides an estimate of the total amount of lignin and an indication of the composition of the phenolic units[30]. As the data show, two major products were identified i.e., syringic acid and vanillin, which both represented 62.9%~87.5% of the total phenolic monomers (Table 2). Small amounts of p-hydroxybezonic acid, vanillic acid, and syringaldehyde as well as traces of the oxidation products, acetovanillone and acetosyringone, were also found to be present in the obtained mixtures. Overall, the addition of catalysts, whether acidic or basic, could break the LCC links and decrease the content of associated lignin as well as improve the purity of the hemicellulosic fraction. Meanwhile, the S/G ratio was maintained in the range of 1.48 to 2.11, indicating that the sub-structure of the associated lignin was not significantly influenced under such mild conditions.
3.2    FT-IR analysis
FT-IR spectroscopy has been used to identify polysaccharides, check their purity, determine their structure, and investigate complexing and inter-molecular interactions[31]. All spectra shown in Fig.2 are typical results for hemicellulosic carbohydrates. No significant differences in the main absorption intensities could be observed among all spectra. The O—H stretching vibrations are observed at 3000~3200 cm1 and the peak at 2921 cm1 is due to the methyl and methylene C—H stretching vibrations. The band at 1458 cm1 is complex, having a contribution from —CH2 bending in xylan as well as —CH3 deformation (asymmetric) in lignin, suggesting that small amounts of lignin are associated with the hemicellulosic fractions. The low intensities of the bands at 1372, 1325, 1269, and 1232 cm1 indicate the methyl C—H wagging and —OH, —CH2, and C—H bending, respectively. The similarities in the polysaccharide fingerprint region (900~1200 cm1) of all samples indicated similar sugar types and biomolecular structures in all hemicellulosic fractions. Under detailed examination, the main difference was the disappearance of the ester band at 1740 cm1 in all catalytically extracted hemicellulosic fractions, undoubtedly resulting from the saponification reaction of acetyl groups and methyl esters under acidic and alkaline conditions.
3.3    Thermal stability
Pyrolysis, which is basically a polymeric structure-cracking process, converts the lignocellulosic material into a volatile fraction and char. The knowledge of thermal degradation is crucial to understand the polymeric thermal stability[32]. Although no clear correlation has been established to the specific application of this study, the thermal stability is closely related to the structural characteristics and aggregation status of polymers, and thereby, was measured to provide further information. Hemicellulose consists of various saccharides with a random and amorphous structure, the branches are very easy to be removed from the main stem and then degraded to volatile constituents at low temperatures[33]. During the pyrolysis, acetic acid is formed during the elimination of acetyl groups originally linked to the xylose unit, furfural is formed by the dehydration of the xylose unit, formic acid is formed from the carboxylic groups of uronic acid, and methanol is formed from the methoxyl groups of uronic acid[34]. Clearly, the TGA data displayed in Fig.3 indicated that the selected hemicellulosic samples started to decompose at about 180℃, 205℃, and 240℃ for PE, PE+Acid and PE+NaOH, respectively, reflecting the quantitative determination of the degradation behavior. However, it is surprising to note that the weight of the pyrolysis residue for PE+NaOH is much lower than that of the residue for the other two samples. A probable explanation could be that a complete pyrolysis was more easily achieved for the hemicellulosic polymer with minimal branches which has the highest Mw value among the three samples despite the high onset temperature.
The magnitude and location of peaks found in the differential thermal analysis (DTA) curve also provide information on the thermal properties of the polymer according to the temperature. According to the literature[33], the pyrolysis of hemicelluloses is exothermic, which is due to the charring process. The obvious exothermic peaks were distinctly observed in the DTA curves for all samples, and a slight shift towards the high-temperature region was also observed for PE+Acid and PE+NaOH (Fig.3), indicating increased thermal stability. The peak at about 280℃ is attributed to the cracking and abscission of C—C and C—O bonds connected with the main branch of hemicelluloses, which is due to its high reactivity. Thereby, it is reasonable to observe that for the hemicellulosic fraction with the highest molecular weight (PE+NaOH), this peak appeared at the highest temperature.
4    Conclusions
The hemicellulosic fractions were isolated via organosolv pretreatment based on a biorefinery process. The hemicelluloses fractions were rich in xylose, principally resulting from 4-O-methyl-D-xylans, and the acidic catalyst decreased the purity of hemicelluloses by partial degradation of cellulose. Although the core of hemicellulosic biomacromolecule could be maintained and isolated under alkaline conditions, the use of alcohol with longer alkyl chains was beneficial for the fractionation efficiency because of the partial cleavage of the ether bonds in the lignin-carbohydrate complex (LCC). The main difference observed in FT-IR spectra  was the disappearance of the ester group, which was ascribed to the saponification reaction of acetyl groups and methyl esters under all catalytic conditions. Mild organosolv pretreatment with alkaline catalysts is an effective strategy for the valorization of poplar, and the basic understanding of the fractionated hemicellulose hopefully provides new opportunities for future applications.
Acknowledgments
This work was financially supported by the Fundamental Research Funds for the Central Universities (2017TP13), the National Key R&D Program of China (2016YFD0600803), 2018 National Student Research Training Program (201710022033) and the Innovation Program of College of Materials Science and Technology. This paper was also supported by the 2017 the international Clean Energy Talent program (No.201702660054). The authors also thank the colleagues for their valuable suggestions during the course of this work.
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Manufacturing of High-efficiency Air Filter Paper Using Ultra-fine Fibers
Yong-Chil Ro, Yong-Il Ri*, Guang-Jin Jong
Institute of Paper Engineering, State Academy of Sciences, Pyongyang, D P R Korea
 
Abstract: In this paper, the manufacturing of high-efficiency air filter paper is reported. The air filter paper was produced using ultra-fine fibers and wateroat fibers mercerized by alkali, using an electrospinning apparatus with multiple rings. The high efficiency air filter paper has an antibacterial effect after adding a chitosan-copper complex which is harmless to humans. As a result of the measurement, the filtering efficiency of the air filter paper is approximately 99.998% and its antibacterial efficiency is approximately 99.5%.
 
Keywords: ultra-fine fiber; wateroat (Zizania latifolia) fiber; chitosan-copper complex; air filter paper
 
Yong-Chil Ro, PhD candidate; E-mail: arirangip@star-co.net.kp
 
*Corresponding author:
Yong-Il Ri, PhD, professor; research interests: innovative papermaking and product development of specialty paper, pulping, and papermaking;
E-mail: arirangip@star-co.net.kp
1    Introduction
With the development of science and technology, the standard of the requirements for the environment is rising. The rapid development of modern science and industry imposes the need for a higher level of air filtering. The industries including electronic, medicine, chemical, biological industries, and food processing, require miniaturization, high precision, purification, quality, and an indoor environment of higher reliability. These requirements impose raises of need for high efficiency air filter papers. Therefore, manufacturing high efficiency filters that can fully satisfy the requirements of consumers is very important[1-2].
The technology of the electrospinning-made ultra-fine fibers is developing rapidly. It is possible to manufacture high efficiency air filter papers using ultra-fine fibers[3-6]. The chitosan extracted from crab shell, and its derivatives provide antibacterial properties to the paper without harming human body[7-9].
In order to manufacture air filter papers with a 99.998% filtering efficiency and the antibacterial function, we conducted research on making the air filter paper by mixing the electrospinning-made ultra-fine fibers as the main filtering material and wateroat fibers mercerized by alkali as frame fibers and then adding a chitosan-copper complex, which is a natural sterilizer and harmless to the human body.
2    Experimental
2.1    Raw materials and equipments
2.1.1    Raw materials
Wateroat (Zizania latifolia), acetate cellulose (substitution degree of 2.7), acetone (AR), chitosan (degree of deacetylation 95%, self-prepared), copper sulphate (CuSO4, AR), polyvinyl alcohol (PVA, average degree of polymerization of 1500), sodium hydroxide (NaOH, AR), and acetic acid (CH3COOH, AR).
 
2.1.2    Experimental equipment
Hollander beater, table paper machine (200) and air permeability tester (made by experimental equipment factory of Academy of Science, D P R Korea), scanning electron microscope (SEM, QUANTA 200), tensile strength tester (ZL-300A), pore tester (HAD-KJ-10), and a laser particle counter (PMS400-LASASP 100).
2.2    Preparation of experimental materials
2.2.1    Preparation of wateroat fiber mercerized by alkali
Wateroat fibers mercerized by alkali are thinner and longer than wateroat fibers; therefore, they are suitable for manufacturing high efficiency filter paper.
Mercerized wateroat fibers were manufactured in the following steps: wateroat→cutting→cooking→washing→screening→bleaching→drying→mercerization→fraction→mercerized wateroat fibers.
The wateroat was cut into a length of 3~5 cm for cooking. Cooking condition was as follows: NaOH 15%, temperature 125℃, time 2.5 h, and the ratio of solid to liquid was 1∶3.5.
The wateroat fiber pulp was bleached in two stages and NaClO was used as a bleaching agent. The specifics of the first stage are as follows: pulp consistency 5%, temperature 55℃, time 3 h, and available chlorine 1.5%. The specifics of the second stage are as follows: pulp consistency 5%, temperature 55℃, time 2 h, and available chlorine 1%.
The bleached pulp was dried for mercerization. The conditions for mercerization are as follows: NaOH consistency 20%, time 10 min, and temperature 20℃. The parenchyma cell was separated from the pulp using an inclined screen.
 
2.2.2    Preparation of ultra-fine fibers
Using acetate cellulose as a main raw material for ultra-fine fibers, the acetate cellulose was dissolved in 8 wt%~10 wt%  acetone and it was used as a spinning solution. The ultra-fine fibers were fabricated via a needleless electrospinning apparatus with multiple rings that we developed. Copper wire of 1 mm diameter was used for the multiple rings to make the diameter of 8 cm, and the distance between rings was 4 cm.
Electrospinning was performed at a voltage of 35 kV and the spinning distance was 17 cm.
 
2.2.3    Preparation of chitosan-copper complex
Refined chitosan with 95% deacetylation degree was made from crab shells[11]. It was solved in 0.5% acetate solution. Then, 0.35 mol CuSO4 was added in 1 mol chitosan units and it underwent a reaction for 2 h with stirring. The pH value of the reaction medium was adjusted to 6.5. The product of chitosan-copper complex powder was separated from the solution after the reaction, multiple washes with distilled water, and drying.
The chitosan-copper complex was then solved in an acetic acid solution with a pH value of 2 at 90℃ and used as an antibacterial agent.
 
2.2.4    Manufacture of high-efficiency air filter paper
The air filter paper manufactured using only ultra-fine fibers has very poor strength. If natural fibers are mixed with ultra-fine fibers, the strength of the air filter paper can be increased.
Ultra-fine fiber was beaten by the free-beating method in the Hollander beater. After beating, its average length was between 1.5 mm and 2.5 mm.
Mercerized wateroat fibers were defibered and mixed with ultra-fine fibers. By changing the mixing ratio of ultra-fine fibers and mercerized wateroat fibers, sheets were manufactured for testing using a table paper machine. Sheets were dehydrated using a vacuum dehydrator and dried using an experimental cylinder dryer. Then, PVA solution was sprayed on the sheets to increase the strength of the air filter paper. Finally, the solution of chitosan-copper complex was sprayed to add antibacterial properties to the air filter paper.
2.3    Analysis and measurement
Mercerized wateroat fibers and ultra-fine fibers were examined in terms of SEM, pore diameter, and permeability of the filter paper. These were tested according to the ISO 1924.2, ISO5636/1, and EN868-3 standards, respectively.
3    Results and discussion
3.1    Characteristics of mercerized wateroat fibers
The morphology of mercerized wateroat fibers before and after the fraction process was investigated using a digital microscope. The results are shown in Fig.1. As shown in Fig.1, there only existed mercerized wateroat fibers in the raw material after the fraction process. It is shown that the mercerized wateroat fibers manufactured in the above-mentioned process can be used as raw material for high-efficiency filter paper.
The diameters of mercerized wateroat fibers were investigated using SEM images. As shown in Fig.2, its diameter is 2~4 m and length is 0.8~1.6 mm.
3.2    Characteristics of ultra-fine fibers
The diameter of ultra-fine fibers was investigated using SEM images. As shown in Fig.3, the diameter of the ultra-fine fiber is 0.3~0.7 m and its length is 5~10 mm.
3.3    Analysis of the chitosan-copper complex
The high-efficiency air filters used in medicine should be such that they do not permit microorganisms in the air to infiltrate.
It is possible that the high-efficiency air filter paper may be infected by germs in air during use. To prevent this phenomena, the high-efficiency air filter paper should have antibacterial properties. Chitosan-copper complex can be used as the antibacterial agent[11]. The Fourier-transform infrared FT-IR spectra of the synthesized chitosan-copper complex and chitosan are shown in Fig.4. The peak at environs of 450 cm1 reveals that the compound has a coordination bond of copper and nitrogen. The peaks at 400~500 cm1 reveal that the chitosan-copper complex is successfully synthesized.
3.4 Characteristics of the high-efficiency air filter paper
3.4.1    Air permeability of air filter paper with different ultra-fine fiber content
The air permeability of air filter paper with different ultra-fine fiber content is shown in Table 1. An increase in the content of ultra-fine fibers led to a decrease in the air permeability of the air filter paper. This is because the increase in the content of ultra-fine fibers leads to a decrease in the porosity of the air filter paper.
Table 1    Air permeability of air filter paper with different ultra-fine fiber content
No.   Ultra-fine fiber content /%       Air permeability (100 Pa)
/(L·m2·s1)
1       55     28.1
2       65     27.2
3       75     26.4
 
 
3.4.2    Pore diameter of air filter paper with different ultra-fine fiber content
The pore diameter of air filter paper with different ultra-fine fiber content is shown in Table 2.
Table 2    Pore diameters of air filter paper with different ultra-fine fiber content
No.   Ultra-fine fiber content/%        Pore diameter/m
                   Average   Max
1       55     10     13
2       65     9       11
3       75     8       10
 
As shown in Table 2, the pore diameter of the air filter paper decreases with an increase in the ultra-fine fiber content. It reveals that ultra-fine fibers play an important role in the formation of small pores in air filter paper.
 
3.4.3    The strength of air filter paper with different PVA dosage
To increase the strength of air filter paper, a series of comparative experiments were performed using the PVA solution, and the results are shown in Table 3.
Table 3    Strength of air filter paper with different PVA dosage
No.   PVA dosage/% Breaking length/m
1       0       170
2       0.3   290
3       0.6   350
4       0.9   370
 
 
As shown in Table 3, when PVA dosage increases, the strength of the air filter paper also increases.
As PVA dosage increases, the air permeability of the air filter paper decreases and the flowing resistance increases. When PVA dosage is 0.6%, its mechanical strength increases to two-fold that of the original air filter paper; simultaneously, changes to the diameter and air permeability are slight. Thus the PVA dosage was determined to be 0.6%.
 
3.4.4    Filtering efficiency of air filter paper
The filtering efficiency of the air filter paper was measured using the DOP method with dioctyl phthalate[12-13]. The filtering efficiency of air filter paper was measured by fixing the base weight of the air filter paper to be 90 g/m2 and change the content of ultra-fine fibers in the air filter paper. When the content of ultra-fine fibers increases, the filtering efficiency also increases, but the change is not as significant as expected. When the content of ultra-fine fibers is 75%, the filtering efficiency of the air filter paper is approximately 99.998%.
 
3.4.5    Antibacterial effect of the chitosan-copper complex
Chitosan and chitosan-copper complexes have high antibacterial function. These materials have antibacterial capacities because their molecules contain —NH2. Generally, materials that comprise a cell nucleus have negative charge. However, —NH2 has positive charge and can combine with the materials of a cell nucleus and palsies their function. Therefore, the cells stop growing[11,14].
Different amount of chitosan-copper complex solution was sprayed on the high-efficiency air filter paper. After drying, the barley malt culture fluid was added on the papers. Then, the paper was placed in a 30℃ atmosphere and inoculated naturally for 5 days. Finally, the research on the antibacterial efficiency of high-efficiency air filter paper was carried out by measuring the areas contaminated and not contaminated by germs. Non-antibacterial paper is used as a control sample. The results are shown in Table 4.
Table 4    Antibacterial efficiency of the air filter paper
No.   Amount of chitosan-copper complex/%  Antibacterial effect
/%
1       0       —
2       0.005        75.5
3       0.010        88.4
4       0.015        97.2
5       0.020        99.1
6       0.025        99.5
7       0.030        99.5
 
 
As shown in Table 4, the antibacterial effect of air filter paper increases with the increasing amount of the chitosan-copper complex sprayed on the air filter paper. When the amount of chitosan-copper complex sprayed on the air filter paper is 0.025% by weight, the antibacterial effect reaches its maximum potential and antibacterial efficiency is observed to be approximately 99.5%. When the amount of the chitosan-copper complex is increased to more than 0.025% by weight, the antibacterial effect does not increase further.
As shown Fig.5, when the amount of chitosan-copper complex was 0.025%, no strain growth was observed (Fig.5(a)), whereas well-developed strains were observed in the absence of chitosan-copper complex (Fig.5(b)), indicating the excellent antibacterial efficiency of the chitosan-copper complex.
 
3.4.6    Technical characteristics of the high-efficiency air filter paper
The technical characteristics of the high-efficiency air filter paper with ultra-fine fiber of 75 wt% is shown in Table 5.
As shown in Table 5, the technical characteristics of the high-efficiency air filter paper are observed to be similar to that of the HEPA filter.
4    Conclusions
The high-efficiency filter paper can be manufactured with the wateroat fibers mercerized by alkali and ultra-fine fibers, which are produced through needleless electrospinning using multiple rings. Especially, the results have proved that mercerized wateroat fibers can be used to manufacturing high-efficiency air filter paper. In addition, the technology that prevents contamination of high-efficiency filter paper from germs spread in air has been established by sprafing chitosan-copper complex on the air filter paper.
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Preparation of Electrospun PVDF Nanofiber Composite Filter Medium and Its Application in Air Filtration
 
Qi Du1,2, Na Wang1,2, Wen Liu1,2, XueFeng Chen1,2, Yue Xu1,2,
Ying Wan1,2
1. China National Pulp and Paper Research Institute Co., Ltd., Beijing, 100102, China;
2. National Engineering Laboratory of Pulp and Paper, Beijing, 100102, China
Abstract: Nanofibrous media with both high particle interception efficiency and robust air permeability has broad technological applications in areas including individual protection, industrial security, and environmental governance. However, producing such filtration media has proven to be extremely challenging. Here we reported an approach to preparing and fabricating a polyvinylidene fluoride (PVDF) nanofiber composite filter medium composed of 2D PVDF nanofiber nets and a stable substrate via one-step electrospinning for effective air filtration. PVDF nanofibers are obtained by adjusting the electrospinning process. With the combined properties of ultrasmall diameter, high porosity, and a bonded scaffold, the resulting PVDF nanofiber composite filter medium exhibits a robust high filtration efficiency of 99.901% (equivalent to an F9 rating) for 0.4 m particles and a long service life (a large dust holding capacity of 36 g/m2) for ultrafine airborne particles based on the sieving principle and surface filtration behavior. The successful synthesis of PVDF nanofibers medium would not only make it a promising candidate for air filtration, but also provide new insights into the design and development of composite nanofiber structures for various applications.
 
Keywords: electrospinning; filtration efficiency; nanofiber; dust holding capacity; air filtration
 
Qi Du, engineer; research interests: specialty paper;
E-mall: duqi_0204@163.com
 
1    Instroduction
Fiber Filters, which are widely used in air filtration, are attractive for particle filtration owing to their cost-effectiveness and ease of scalable synthesis from various materials[1-2]. When fibrous media is used for air filtration, two factors generally need to be considered. One is the fiber diameter, which determines the filtration efficiency of the filter medium[3-4]; the other is the filter medium’s structure, which affects the air permeability of the medium and reduces the energy cost[2, 5].
Nanofiber-based filtration media is currently attracting increasing attention because it can enhance the filtration performance (especially the filtration efficiency)[6-7]. There are many techniques for making nanofibers, such as template synthesis[8], sea-island spinning[9], phase-separation[10], plasma treatment[11], and electrospinning[12]. Compared with other technologies, electrospinning has become the most effective method for the preparation of ultrafine fibers[13-14] owing to its simple equipment, easy operation, and sustainable production. In recent years, with the development of electrospinning technology, hundreds of polymers have been prepared as nanofiber materials by electrospinning[15-16], e.g., polyacrylonitrile (PAN)[17], PAN/poly(acrylic acid)[18], polyetherimide[19], poly(lactic acid)[20], and polyamide-66 (PA-66)[21]. However, these materials still have some disadvantages: inadequate filtration performance, weak mechanical properties, and short service life.
Polyvinylidene fluoride (PVDF) is attractive for electrospinning owing to its combined properties of flexibility, low weight, low thermal conductivity, high chemical corrosion resistance, and heat resistance. In this study, PVDF was used as a spun polymer to prepare nanofibers, which were incorporated into a substrate. To improve the filtration performance (especially the filtration efficiency and dust-holding capacity (DHC)) of the filter medium, the effect of the electrospinning parameters on its performance and the effect of the electrospun PVDF nanofibers on the filtration performance of air filters were investigated.
2    Experimental
2.1    Raw materials
PVDF powder (Mw=300000) was purchased from Solvay (Shanghai) Co., Ltd., N,N-dimethylformamide (DMF) and acetone were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). Bilayer composite air filter paper (120 g/m2) was supplied by our laboratory as the substrate to receive the electrospun PVDF nanofibers. All chemicals were of analytical grade and were used as received without further purification.
2.2    Preparation of PVDF solutions
A preliminary study found that a spinning solution consisting of PVDF powder dissolved in a mixed solvent system of DMF and acetone is most suitable for electrospinning. It was found that when the volume ratio of DMF and acetone was 2∶3 and the mass fraction of PVDF in the spinning solution was 12 wt%, PVDF nanofibers with better quality could be obtained.
The PVDF powder was first dissolved in DMF at a certain concentration and stirred at 40℃ for 4 h until a transparent homogeneous solution was obtained. Then, the PVDF/DMF solution was cooled to room temperature, and a calculated amount of acetone was added. The volume ratio of DMF and acetone in the resulting PVDF/DMF/acetone solution was 2∶3. The blended PVDF/DMF/acetone solution was stirred for another 2 h to ensure complete dispersion of PVDF in the solvent.
2.3    Fabrication of PVDF nanofibers
PVDF nanofibers for deposition on the bilayer composite air filter substrate were fabricated using FM-1205 spinning equipment (BeijingFutureMaterialSci-techCo.,Ltd., China). A schematic diagram of the electrostatic spinning process and nanofiber composite air filter medium is shown Fig.1.
To electrospin PVDF fibers, the PVDF/DMF/acetone solution was placed in a 10 mL plastic syringe equipped with a stainless steel needle as the spinneret. A digitally controlled syringe pump was used to feed the polymer solution into the needle tip at a constant feeding rate of 0.005 mm/s. A metal roller wrapped in bilayer composite filter paper was electrically connected to the negative high-voltage DC power supply and used as the fiber collector. A stainless steel needle with an inside diameter of 0.4 mm was electrically connected to the positive high-voltage DC power supply. During the electrospinning process, the PVDF solution was exposed to a high DC electrical field, which was generated by applying a positive voltage of 27 kV and a negative voltage of 3 kV to a 15 cm gap between the spinneret and the fiber collector. The ambient temperature was 35℃, and the relative humidity was kept at 35%. After electrospinning, a nonwoven nanofibrous PVDF membrane was formed on the bilayer composite air filter paper. The obtained nanofiber composite air filter medium was then dried in vacuum at 60℃ for 4 h to remove the residual solvent before any further characterization.
2.3    Characterization
2.3.1 Pore size of nanofiber composite air filter medium and diameter of PVDF nanofibers
The morphology of the nanofiber composite air filter medium was examined by scanning electron microscopy (SEM) (S-3400N, Hitachi Ltd., Japan) after the medium was coated with gold. The fiber diameter of each PVDF nanofiber layer was measured by an image analyzer (Image-Pro Plus 6.0), and the pore size of the PVDF nanofiber layers was obtained by analyzing a series of SEM images using the grid method.
 
2.3.2 DHC, filtration resistance and filtration efficiency
The filtration performances of all the air filter materials with developed pore structure were measured using a filtration test bench[15]. The measurement protocol was based on the ISO 5011 international standard for evaluating the basic performance characteristics of inlet air cleaning equipment for internal combustion engines and compressors. The results were compared and analyzed. An in-house-developed bench was used for the filtration tests, and the test conditions are shown in Table 1. Dust was fed through the filtration chambers using a compressed air particle disperser. The particle size distribution of fine dust is plotted in Fig.2.
The test bench (Fig.3) is composed of two filtration chambers:
(1) The tested filter is placed inside the first chamber (upstream chamber), which contains flowing air charged with dust before filtration.
(2) A high-efficiency particulate air filter is placed inside the second chamber (downstream chamber) to collect the fine dust particles that cross the tested filter and reach the chamber.
The measured filtration properties are the pressure drop, fractional filtration efficiency, and DHC. The DHC is the quantity of captured particles inside the tested filter; the difference between the initial and final weights of the tested element gives the DHC in g/m2. Thus, the DHC is obtained using Equation(1).
      DHC=(M2M1)/A                                                   (1)
Where, M1 is the initial weight of the test element, M2 is the final weight of the test element, and A is the test area of the tested filter.
3    Results and discussion
3.1    Characterization of PVDF nanofibers
In this study, PVDF nanofibers of nearly uniform diameter were successfully produced by controlling the PVDF/DMF/acetone electrospinning solution, DC voltage (30 kV), and receiving distance (15 cm). The organic solvent acetone was added to decrease the viscosity and surface tension of the electrospinning solution owing to its low viscosity and low density. Representative SEM micrographs of PVDF nanofibers obtained using various electrospinning times are shown in Fig.4(a)~Fig.4(c). The images reveal randomly oriented nonwoven scaffold structures that support ultrathin 2D nanonets.
The diameter distribution of the PVDF nanofibers is shown in Fig.4(d). It can be seen that under the experimentally determined process parameters, the diameter of the electrospun PVDF nanofibers is distributed between 60 nm and 350 nm, and falls mainly between 60 nm and 150 nm. Fibers in this range account for nearly 80% of the total, whereas fibers with diameter of 150~350 nm account for only about 20%. This shows that PVDF nanofibers with a smooth morphology and small diameter can be obtained under the spinning conditions determined in this study.
The number of nanofibers in the PVDF layer could be controlled by varying the electrospinning time. This method seems to be suitable for mass production. As shown in Fig.4(a)~Fig.4(c), the density of nanofibers in the PVDF layer gradually varied from sparse to dense as the electrospinning time increased. For a short time of 5 min, as shown in Fig.4(a), only a small quantity of PVDF nanofibers was deposited onto the substrate. However, when the electrospinning time was extended to 10 min, as shown in Fig.4(b), the number of PVDF nanofibers on the substrate increased significantly, and they formed a simple network structure. When the electrospinning time was further increased to 15 min, as shown in Fig.4(c), a dense nanofibrous PVDF layer with a complex network structure, indicating good filtration performance for fine dust particles, was formed on the substrate.
3.2    Pore structure of PVDF nanofiber layer
The pore structure was examined by measuring the pore size and pore size distribution using the grid method to investigate the effect of the electrospinning time on the structure of the nanofibrous PVDF layer, and thus to reveal the unique superiority of the nanonets and cavity structures for airborne particle filtration. Fig.5 shows the pore diameter and pore size distributions of the nanofibrous PVDF layers formed using various electrospinning times. The results show that the pore size distribution of the nanofibrous PVDF layers gradually changes from wide to narrow as the electrospinning time increases from 5 min to 15 min.
Fig.5(a)~Fig.5(c) show that as the electrospinning time increases, the pore size distribution of the electrospun PVDF fiber layer is first dispersed and then concentrated. As shown in Fig.5(a), when the electrospinning time is 5 min, the pore size distribution of the electrospun PVDF fiber layer is between 0.43 m and 5 m, and the distribution is relatively broad; when the electrospinning time  increases to 10 min, as shown in Fig.5(b), the pore size distribution becomes narrower, and the pore size is continuously distributed in the low-diameter region. When the electrospinning time is 15 min, as shown in Fig.5(c), the pore size distribution is concentrated between 0 and 1.5 m. Fig.5(d) shows that as the electrospinning time increases, the maximum, minimum, and average pore diameters of the PVDF nanofiber layers decrease. When the electrospinning time increases from 5 min to 15 min, the maximum diameter decreases from 4.9 m to 1.5 m, the minimum diameter decreases from 0.4 m to 0.2 m, and the average diameter decreases from 1.0 m to 0.7 m. The reason is that as the electrospinning time increases, the number of fibers in the nanofiber layer increases gradually, and the fibers are deposited more densely. The small pore size structure, in which the particulates form a filter cake on the surface of the filter material and achieve surface filtration, enables the filter medium to remove particulates from polluted air. 
3.3 Influence of electrospinning time on the filtration efficiency
The filtration efficiency of the PVDF nanofiber composite air filter media fabricated using various electrospinning times under a face velocity of 60 L/min is demonstrated in Fig.6. The filtration efficiency of the nanofiber composite air filter materials versus the electrospinning time (5, 10, and 15 min), which controlled the number of PVDF nanofibers, improved to varying degrees when the electrospinning time increased to 15 min. Obviously, the filtration efficiency exhibited a sharp rise for particle sizes of ≤5 m but improved slightly for particle sizes of ≥5 m.
The filtration efficiency of the substrate for 0.4 m particles was only 76.275% when the PVDF nanofibers were not reassembled on the substrate. However, when PVDF nanofibers were incorporated into the substrates by electrospinning, the filtration efficiency of the nanofiber composite air filter materials with electrospinning times of 5, 10, and 15 min were 90.144%, 99.901%, and 99.978%, respectively. Obviously, the filtration efficiency was greatly improved and reached a relatively steady value when the electrospinning time was 10 min, and it could easily meet the standard for high-efficiency particulate air filters (>99.97%) when the electrospinning time was 15 min; traditional filters cannot meet this standard, therefore this result further reveals the key role of the nanofibers and multilayer composite structures in air filtration.
3.4    Influence of electrospinning time on DHC
To judge the quality of filter materials, it is generally necessary to consider their overall filtration performance.The filtration efficiency is not the only evaluation parameter for judging the quality of filter materials. In fact, when fibrous media are used for air filtration, two factors need to be considered. One is the filtration efficiency, which determines the quality classification of filter materials; the other is the DHC, which can indirectly reflect the service life of filter materials. Therefore, in the following, the influence of electrospinning time on the filter medium’s DHC is investigated. Consequently, the relationship between the electrospinning time and the DHC of the fibrous medium is examined, and the qualitative and quantitative effects of the DHC are clearly demonstrated.
The influence of the electrospinning time on the DHC is shown in Fig.7. The highest DHC is obtained for the substrate without PVDF nanofibers owing to its internal structure with many large pores, which allows particles to be deposited in the pores of the filter medium without rapidly clogging them. Although the substrate has a high DHC, its filtration efficiency is very low.Therefore, it cannot satisfy the requirements for high-efficiency filtering.
The DHC of the PVDF nanofiber composite filter medium decreased with electrospinning time. For electrospinning times of 5, 10, and 15 min, the DHC of the PVDF nanofiber composite filter medium decreased to 41, 36, and 34 g/m2, respectively. In fact, each fiber type affects the medium filtration ability as well as contributing to the mechanical behavior of the filtration structure. The fine fibers make an essential contribution to the particle capture improvement. The decrease in the fiber diameter results in a higher number of fibers for an equivalent filter medium packing density, which increases the statistical probability that dust particles encounter fibers. This increase potentially leads to an improvement in the filtration behavior, as shown in Fig.6. However, as the electrospinning time increases, the number of fibers in the nanofiber layer gradually increases, and the fibrous arrangement becomes increasingly compact. This fibrous arrangement causes clogging at the interface between the PVDF nanofibers and the substrate layer, which quickly increases the pressure drop and thus reduces the DHC of the medium significantly despite its good filtration efficiency.
To improve the filtration performance of fibrous media, the DHC and the filtration efficiency must be balanced. When the electrospinning time was 10 min, the nanofiber composite filter material had high filtration efficiency and high DHC, which can meet the requirements for high-efficiency filtration.
4    Conclusion
In summary, we designed and fabricated PVDF nanofiber composite filter media composed of 2D PVDF nanofibers and a stable substrate for effective air filtration via one-step electrospinning. PVDF nanofiber formation is promoted by adjusting the electrospinning process. When the volume ratio of DMF and acetone is 2∶3, the mass fraction of PVDF in the spinning solution is 12%, the acceptable distance is 15 cm, the electrospinning time is 10 min, and the spinning voltage is 30 kV, the PVDF nanofibers with smooth morphology and stable size can be obtained. With combined properties of ultrasmall diameter, high porosity, and a bonded scaffold, the resulting PVDF nanofiber composite filter medium exhibits a robust high filtration efficiency of 99.901% (equivalent to an F9 rating) for 0.4 m particles and a long service life (large dust holding capacity of 36 g/m2) for ultrafine airborne particles based on the sieving principle and surface filtration behavior.
Acknowledgments
This work was supported by the Science and Technology Project of Chaoyang District, Beijing, China (CYGX1709) and the National Key R & D Program of China (2017YFE0101500).
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Evaluation and Optimization of Old Magzines Deinking Process of a Modified Lignin Sulfonates Deinking Agent
ShuBin Wu*, SiJia Chen, YuanYuan Zhao
State Key Laboratory of Pulp and Paper Engineering, School of Light Industry and Engineering,
South China University of Technology, Guangzhou, Guangdong Province, 510640, China
 
Abstract: Deinking is an important process in waste paper recycling process, and this study mainly focuses on the properties of the deinking agent and process optimization. DIA-30 prepared in our laboratory, and FS and LD, were the three representative deinking agents studied herein. Brightness, effective residual ink concentration (ERIC), dirt, macro-stickies, fiber loss during repulping, flotation, and washing processes were measured to evaluate the performance of the deinking agents and optimize process conditions. Each of the deinking agents exhibited different advantages: DIA-30 was optimal for deinking efficiency, brightness enhancement, as well as dirt and macro-stickies removal, whereas LD was optimal for ink removal during flotation process but performed poorly in terms of brightness enhancement. FS showed a relatively balanced performance compared to those of DIA-30 and LD in the deinking process, but DIA-30 is the best option for deinking due to its outstanding comprehensive properties. The optimal repulping conditions for the deinking of 100% old magzines (OMG) using DIA-30 are as follows: repulping temperature, 60℃; pulp consistency, 13 wt%; repulping time, 20 min; H2O2 dosage, 0.5 wt% (oven-dried pulp); NaOH dosage, 1.2 wt% (oven-dried pulp); Na2SiO3 dosage, 0.3 wt% (oven-dried pulp); and optimum DIA-30 dosage, 0.1 wt%.
 
Keywords: deinking agent; optimization; secondary fiber; waste paper; old magzines
 
*Corresponding author:
ShuBin Wu, professor, PhD tutor; research interests: fiber recycling and biomass conversion and value added utilization;
E-mail: shubinwu@scut.edu.cn
 
1    Introduction
Increasing environmental concerns and rising demand for paper have rendered continuous waste paper recycling an essential process in paper industry. Used papers such as newsprint, photocopied papers, and laser-printed papers can be recycled by removing the ink to obtain a brighter pulp prior to the papermaking process. Over the past decade, the recovery and utilization of waste paper as a raw material for papermaking have increased drastically[1-2].
Deinking aims to detach and remove printing ink from the fibers to be recycled to improve the resulting optical characteristics of pulp and paper. This is the most important process for using waste paper as a raw material to manufacture different paper products[3-4]. The removal of ink particles is achieved by a combination of chemical and mechanical forces in the deinking process followed by repulping, flotation, and washing processes. Repulping process separates, swells and flexes fibers, and detaches fibers from the ink. Chemical deinking requires large amounts of sodium silicate, hydrogen peroxide, and surfactants to promote ink detachment. The removal of the detached ink from the pulp slurry is achieved by washing and 鹢tation[5-6]. During flotation process, the foaming agent is used to generate a stable foam for ink removal. A collector may be used to agglomerate small ink particles for flotation removal, and a dispersant is often used to prevent ink particles from being redeposited onto the fiber. The technological requirements of the factory must balance the amount of foam, as it is needed to increase the ink removal rate, but excess foam results in fiber loss[7]. Moreover, different separation processes function optimally at different particle size ranges, but washing is only successful when the ink particles are very small, whereas flotation process requires larger particles[8].
Secondary fiber recycling factories should be adapted to practical situations, as pulps have varying recycling potentials. The most prominent factors affecting recycling potentials are the fiber composition, initial beating degree of the original fibers, extent of wet pressing, drying conditions, calendaring effects, chemical additives, and deinking effects[9].
As deinking agent types and deinking conditions exert enormous influence on the deinking results, many studies have focused on optimizing the deinking process and deinking agent formula. Li et al[10] studied the effect of flotation parameters on the deinking efficiency of domestic OMG. The authors reported the optimized flotation conditions as: pulp consistency, 12 wt%; temperature, 50℃; flotation time, 6 min; air to pulp ratio, 40%; pulp flow velocity, 5.0 m/s; and pH value 11.0. Under these c, on, ditions, the brightness of the flotation deinking pulp reached 77.0% ISO. Zhang et al[11] optimized the OMG deinking process with high pulp consistency and showed that a brightness of 60% could be achieved under the following conditions: pulp consistency, 15 wt%; temperature, 60~80℃; time, 30 min; stirring velocity, 300 r/min; NaOH dosage, 3 wt%; Na2SiO3 dosage, 1 wt%; deinking agent dosage, 0.4 wt%; and H2O2 dosage, 1 wt%. Wu group developed some methods of preparing deinking agents and surfactants. The deinking agents are used for secondary fibers deinking such as waste books and periodicals paper and mixed office waste paper. The effect is very obvious in terms of whiteness increase and ink removal, and the deinking agents are simple and easy to prepare[12-15].
In this study, the pulp brightness, ERIC value, dirt, macro-stickies, fiber loss ratio through repulping, flotation, and washing processes were measured to assess the performance of deinking agents. The properties of a laboratory-made deinking agent with modified lignin sulfonates added (DIA-30) were determined and process optimization was performed.
2    Experimental
2.1    Evaluation of different deinking agents
As a raw material, 100% OMG was used in this section. OMG was first torn into 20 mm×20 mm pieces and placed in sealable bags for 24 h to balance the water retention. The repulping process was performed in a FORMAXN-100 high consistency pulper under the following conditions: pulp consistency, 13 wt%; repulping temperature, 70℃; repulping time, 20 min; NaOH dosage, 0.78 wt% (oven-dried pulp); Na2SiO3 dosage, 0.43 wt% (oven-dried pulp); and H2O2 dosage, 1.8 wt% ((oven-dried pulp). A certain amount of hot water was added to the pulper, following which the chemicals were added successively, NaOH, Na2SiO3, 230 g oven-dried OMG pieces, deinking agent, H2O2, and finally hot water until the pulp consistency reached 13 wt% at a rotating velocity of 300 r/min.
DIA-30 made in our laboratory consists of a non-ionic surfactant (alkylphenol ethoxylate) and modified lignin sulfonates. FS consists of an anionic surfactant mixed with a fatty acid salt and LD consists of polyoxyethylene ether non-ionic surfactant. The deinking agent dosage was 0.2 wt% (oven-dried pulp) for all agents.
The control group without deinking agent added was subjected to the same treatment for comparison. Deinking efficiency was calculated from Equation (1):
Deinking efficiency (%)=(ERIC-ERICdeinked)/ERIC                                                                                
(1)
The flotation process was performed in a VOITH delta25 deinking and flotation cell at 50℃ for 5 min. A total of 10 g of oven-dried pulp was diluted with water until the pulp consistency reached 1 wt%.
The washing process was performed in a 200 mesh bag using tap water.
Three handsheet pieces of 200 g/m2 were produced in a Buchner funnel (12.5 cm diameter) according to GB/T 8940.2—2002 for the brightness test. The brightness was tested using a Technibrite micro TB-1c instrument according to TAPPI T128 sp-02.
Three handsheet pieces of 60 g/m2 were produced in a ZQJ1B-1 standard sheet former for the ERIC value test. The ERIC value was tested using a Color Touch ERIC-950 residual ink tester according to TAPPI T567 om-04.
Three 20-g oven-dried pulp samples were prepared to test macro-stickies using a Pulmac MasterScreenTM device. Macro-stickies were scanned and analyzed using a Hewlett-Packard J192A scanner and Spec&Scan analysis software, respectively, according to TAPPI T277 om-07. The 20-g oven-dried pulp sample was diluted to a consistency of 1 wt% and separated in fluffer. The pulp was then poured into a Pulmac MasterScreenTM device. The residue was collected on specialized black wet strength filter paper (≥20 cm diameter). A specialized white coated paper was covered on the black wet strength filler paper and the papers were placed into a vacuum dryer (pressure 0.8 MPa, temperature 95℃, time 8 min). The white coated paper was stripped away and the impurities were washed off without using water and subsequently covered with an organosilicon-coated paper (≥20 cm diameter). Vacuum drying was performed for 5 min under the same condition as listed above. The unwashed impurities were then marked using a marker pen to reduce the disturbance in the scan.
Six handsheet pieces of 200 g/m2 were produced in a ZQJ1B-1 standard sheet former for the dirt test. The dirt was tested and analyzed using a Hewlett-Packard J192A scanner and Spec&Scan analysis software in reference to GB/T 8940.2—2002 and TAPPI T563 om-08.
Flotation residues were washed using a 200 mesh bag and weighed after drying in the oven. The fiber loss ratio was calculated from Equation (2):
Fiber loss ratio (%)=residue pulp mass (oven-dried pulp)/pulp mass after flotation (oven-dried pulp)        (2)
The test methods and devices of the follow-up experiments were the same as listed above.
2.2    Deinking condition optimization
2.2.1 Optimization of deinking conditions in the repulping process
Based on Section 2.1, DIA-30 was chosen to perform the orthogonal optimization test to determine the optimal deinking conditions. The DIA-30 dosage was fixed at 0.2 wt%. The optimal repulping temperature, repulping time, and dosages of NaOH, Na2SO3, H2O2 were determined, and the experimental scheme is shown in Table 1.
The raw material used in this study was 100% OMG. The repulping process was performed at a pulp consistency of 13 wt%, and flotation process was performed at 50℃ for 5 min at a pulp consistency of 1 wt%. Washing process was performed in a 200 mesh bag using water.
 
2.2.2    Optimization of the deinking agent dosage
Based on previous experiments, the following conditions: repulping temperature, 60℃; repulping time, 20 min; H2O2 dosage, 0.5 wt%; Na2SiO3 dosage, 0.3 wt%; NaOH dosage, 1.2 wt%; and pulp consistency, 13 wt% were determined to be the optimal repulping conditions when 100% OMG was used as a raw material. DIA-30 was used as the deinking agent to perform deinking agent dosage optimization experiments.
The processing conditions of flotation and washing are the same as mentiond in section 2.2.1.
3    Results and discussion
3.1    Evaluation of different deinking agents
As shown in Fig.1, it is clear that the final brightness with the addition of the deinking agent improved compared to that of the control group, indicating that the deinking agents contribute to the increased brightness. DIA-30, FS, and LD all exhibited good performance in terms of improving brightness, where the pulp brightness reached as high as ≥85.0% ISO. DIA-30 was the best deinking agent in terms of brightness improvement, showing a 13.9% pulp brightness increase after washing process compared with the pulp brightness after repulping process. The different performances between deinking agents indicate that the deinking agents function differently during the repulping, flotation, and washing processes.
As shown in Fig.1, it is clear that the total deinking efficiency of the processes with deinking agents was higher than that of the control group. This indicates that adding deinking agents promotes the peeling of the ink. LD was the most efficient deinking agent for flotation, achieving a deinking efficiency of 48.6%. DIA-30 exerted a striking effect on the deinking process, producing the lowest final ERIC value of the three deinking agents and showing the best total deinking efficiency of 76.4%. As listed in Table 2, DIA-30 showed the best performance in terms of dirt removal and macro-stickies removal.
Adding deinking agents generates foam, resulting in increased fiber removal. However, the differences in the fiber loss ratios of different deinking agents was negligible.
DIA-30 is a well-performing deinking agent for the deinking process, providing optimal pulp brightness increase, deinking efficiency, and dirt/macro-stickies removal. LD was proficient in terms of ink removal during flotation process but achieved poor brightness ratings and mediocre dirt and macro-stickies removal. FS exhibited a relatively balanced performance between those of DIA-30 and LD with respect to deinking efficiency, brightness, and dirt/macro-stickies removal in the deinking process.
Table 2    Effect of the different deinking agents on dirt and macro-stickies removal and fiber loss
Deinking agent         Dirt removal efficiency/%         Macro-stickies area removal efficiency/%       Fiber loss ratio/%
         Area          Counts              
Control group  72.0 75.1 60.3 1.9
DIA-30      77.2 85.4 80.9 2.3
FS     74.2 84.7 73.0 2.6
LD    73.4 82.7 63.3 2.7
 
 
3.2    Optimization of the deinking conditions
3.2.1 Optimization of deinking conditions in the repulping process
Repulping is the first stage of the deinking process, significantly influencing the result of the following stages. Therefore, repulping temperature, time, NaOH dosage, Na2SiO3 dosage, and H2O2 dosage were optimized in this section. DIA-30 was chosen as the deinking agent to conduct this optimization experiment.
From Table 3, it is clear that the brightness differed greatly depending on the repulping conditions. A final brightness of 85.0% ISO was obtained under condition No.1, which was the highest obtain, while condition No.9 results in the lowest brightness.
Table 4 shows that ERIC drops rapidly after a series of processes, and the final ERIC value for the different conditions all were approximately 50×106. The total deinking efficiency differed depending on the treatment conditions, of which the (maxmin) value (No.7No.2) reached 12.6%.
From the data listed in Table 5, different conditions showed large differences in terms of dirt and macro-stickies removal. The maximum dirt area removal efficiency was 81.5% and dirt count removal efficiency was 80.6% for conditions No.4 and No.9, respectively. Condition No.1 showed minimal dirt removal, with a dirt area and count removal efficiencies of only 33.5% and 33.0%, respectively. Conditions No.5 and No.7 performed perfectly in terms of macro-stickies area removal, both achieving a high removal efficiency of 64.0% while condition No.15 showed the worst performance. In terms of the macro-stickies count removal, conditions No.8 and No.14 showed removal efficiencies of >58.0% and the fiber loss ratio under each condition differed slightly.
A summary of the data is listed in Table 6, as analyzed by the direct-viewing analysis method. The data show the degree of the influence of the factors on each index in the form of A, B, C, D, E, where A represents the greatest effect while E represents the least significant effect. The optimal conditions were selected according to the degree of the factors’ effect. The range was calculated from Equation (3). The data of the condition in each row indicate the optimal condition for the index in the leftmost column.
Range=max (average of the deinking results under identical conditions of repulping temperature/time/NaOH dosage/H2O2 dosage/Na2SiO3 dosage)min (average of the deinking results under identical conditions of repulping temperature/time/NaOH dosage/H2O2 dosage/Na2SiO3 dosage)                         (3)
Based on the comprehensive analysis above, to achieve higher brightness, deinking efficiency, dirt/macro-stickies removal efficiency, lower energy consumption, and chemical dosage, we concluded that a repulping temperature of 60℃, repulping time of 20 min, H2O2 dosage of 0.5 wt%, Na2SiO3 dosage of 0.3 wt%, and NaOH dosage of 1.2 wt% were optimal for deinking process.
3.2.2    Optimization of the deinking agent dosage
The DIA-30 deinking agent dosage was studied in this section as it is an important factor in the deinking process. Based on Table 7, the final brightness was similar for different dosage of DIA-30 at approximately 80% ISO compared mith the control group, DIA-30 (0.1 wt%) exhibited the best total brightness increase of 11.6% ISO. However, increased dosages of DIA-30 did not further increase the pulp brightness.
ERIC value showed a declining trend after repulping, flotation, and washing processes, as indicated in Table 8. The total deinking efficiency was 81.6% when treated with DIA-30 (0.1 wt%), which was the optimal ink removal.
Macro-stickies removal and dirt removal efficiencies show no explicit tendency in Table 9 with increasing DIA-30 dosage. However, DIA-30 (0.1 wt%) showed satisfactory performance in terms of macro-stickies removal. The macro-stickies area removal efficiency of the DIA-30 (0.1 wt%) was significantly higher than its macro-stickies count removal efficiency as DIA-30 (0.1 wt%) worked well on big size macro-stickies removal. The change of fiber loss ratio was negligible.
Considering the various factors, DIA-30 is advantageous for the deinking process combined with flotation, washing process, and macro-stickies removal with an optimum dosage of 0.1 wt% under the following conditions: repulping temperature, 60℃; repulping time, 20 min; H2O2 dosage, 0.5 wt%; Na2SiO3 dosage, 0.3 wt%; NaOH dosage, 1.2 wt%; and pulp consistency, 13 wt%.
4    Conclusions
Deinking agents and conditions significantly influenced the deinking results. The deinking agent properties and optimized repulping conditions were determined based on the above analysis.
(1) DIA-30 is optimal for improving brightness and macro-stickies removal and achieved better ink removal when matched with subsequent washing. The optimized dosage of DIA-30 was determined to be 0.1 wt%. LD showed good ink removal during the flotation process, but poor brightness. However, when flotation process was performed for 5 min, a significant amount of foam was produced, which will have adverse effects on flotation process. FS exhibited a relatively balanced performance in the deinking process between those of the DIA-30 and FS.
(2) The optimal repulping conditions for deinking of 100% OMG using DIA-30 is as follows: repulping temperature, 60℃; pulp consistency, 13 wt%; repulping time, 20 min; H2O2 dosage, 0.5 wt% (oven-dried pulp); NaOH dosage, 1.2 wt% (oven-dried pulp); and Na2SiO3 dosage, 0.3 wt% (oven-dried pulp).
Acknowledgments
The authors are grateful for the financial support from the science and technology project of Guangzhou city (Contract No. 201607020025).
References
[1] Lee K C, Tong W Y, Ibrahim D, et al. Evaluation of Enzymatic Deinking of Non-impact Ink Laser-printed Paper Using Crude Enzyme from Penicillium rolfsii c3-2(1) IBRL[J]. Appl Biochem Biotechnol, 2017, 181(1): 451-463.
[2] McKinney R W J. Waste paper preparation and contamination removal [J]. Technology of Paper Recycling, 1995: 47-129.
[3] Behin J, Vahed Sh. Effect of alkyl chain in alcohol deinking ofrecycled fibers by flotation process[J]. Colloids and Surfaces A: Physicochem. Eng. Aspects, 2007, 297(1-3): 131-141.
[4] Singh A, Yadav R D, Kaur A. An ecofriendly cost effective enzymatic methodology for deinking of school waste paper[J]. Bioresource Technology, 2012, 120(3): 322-327.
[5] Hong R Y, Su L Q, Chen S. Comparison of cutinases in enzymic deinking of old newsprint[J]. Cellulose, 2017, 24(11): 5089-5099.
[6] Rutland M, Pugh R J. Calcium soaps in flotation deinking; fundamental studies using surface force and coagulation techniques[J]. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1997, 125(1): 33-46.
[7] Luo Q, Deng Y L, Zhu J Y. Foam control using a foaming agent spray: a novel concept for flotation deinking of waste paper[J]. Ind. Eng. Chem. Res, 2003, 42(15): 3578-3583.
[8] Lassus A. Deinking chemistry, recycle fiber and deinking[J]. Papermaking Science and Technology, 2000: 241-265.
[9] Klofta J L, Miller M L. Effects of deinking on the recycle potential of papermaking fibers[J]. Pulp & Paper Canada, 1994, 95(8): 320-323.
[10] Li R G, Chen K F, Li B, et al. Optimization of Flotation Parameters in Old Book Paper Deinking[J]. Journal of Shaanxi University of Science and Technology, 2006(5): 12-17.
[11] Zhang S F, Dang R C. Optimization of old book paper deinking process with high consistency[J]. Journal of Shaanxi University of Science and Technology, 2005, 23(5): 19-23.
[12] Wu S B, Mao C P, Zhao Y Y, et al. The deinking agent of lignin modified product and its preparation method: China, 20110326939.3[P]. 2012-06-20.
[13] Wu S B, Zhao Y Y, Li B, et al. Deinking agent made from gutter oil as the main raw material and preparation method: China, 201110398772.1[P]. 2013-11-13.
[14] Wu S B, Mao C P, Zhao Y Y, et al. The method of preparing surfactant by using lignin thermal chemical degradation as raw material: China, 201110282376.2[P]. 2014-07-30.
[15] Wu S B, Li B, Zhao Y Y, et al. The simultaneous removal agent and the preparation method of the mixed impurities of waste paper ink and Macro-stickies: China, 20110348449.3[P]. 2013-11-27. 
 
 
Design Method of Curved-bar Refining Plates for Disc Refiner
Huan Liu1, JiXian Dong1,2,*, XiYa Guo1,3, RuiFan Yang1, Hui Jing1,
XiaoJun Jiang4
 
1. College of Mechanical and Electrical Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi Province, 710021, China
2. Shaanxi Research Institute of Agricultural Products Processing Technology, Xi’an, Shaanxi Province, 710021, China
3. College of Art and Design, Shaanxi University of Science and Technology, Xi’an, Shaanxi Province, 710021, China
4. Nantong Huayan Casting Co., Ltd., Nantong, Jiangsu Province, 226403, China
 
Abstract: Straight- and curved-bar refining plates are two important types of plates commonly used in disc refiners in the papermaking industry. Theoretically, the curved-bar refining plate has a relatively uniform bar interaction angle, which indicates uniform refining effects. The bar angle of the curved bar was proposed and two typical curved-bar plates, the three-stage radial curved-bar plate and isometric curved-bar plate, were designed in this paper. The arc equations of the curved-bar center line and curved-bar edges were established and finally, the specific edge load (SEL) of the curved-bar plate was derived. The determination of bar parameters was discussed, which provides a theoretical basis for the design of curved-bar plates.
 
Keywords: disc refiner; refining plates; curved bar; design method
 
Huan Liu, Ph.D candidate;
E-mail: liuhsust@126.com
 
*Corresponding author:
JiXian Dong, professor, PhD tutor; research interests: light chemical machinery;
E-mail:djx@sust.edu.cn
 
1    Introduction
The disc refiner is an important piece of equipment to improve the properties of fiber and pulp in the pulp and paper industry. During the refining process, the pulp is fed into a groove-type rotating refining area composed of a stator and a rotor. It is subjected to complex mechanical actions, such as shearing and compression of the bar as well as friction between fibers, so that the fiber morphology and the properties of pulp are changed, and finally, the desired properties of the target paper are achieved.
The plate is the core working part of the disc refiner, and its bar profile directly affects the refining quality, efficiency, and energy consumption. Plates are usually assembled by many segments with the same bar structure, which includes the bar width, groove width, bar height, bar angle, and dams[1]. Many plates with different bar parameters are used in mills, so it is important to understand how the bar profile affects the changes in the pulp and fiber.
According to the form of the bar distributed in the refining area, the plate type can be roughly divided into straight-bar and curved-bar plates. The conventional bar angle of a straight-bar plate is 15°~20°. It is the most widely used type of refining plate at present, but has a disadvantage in that the bar-interaction angle changes during the interaction between the rotor and stator. If the straight-bar plate has a bar angle of 10°, the bar-interaction angle will vary between 15° and 40°, with an average angle of 30°[2]. The pulp flow in the refining area is unstable, or the pulp layer would be blocked because of the change in the bar-interaction angle; as a result, the quality of the fibers refined at different positions is uneven, which affects the paper properties. Theoretically, there is a certain curvature on the curved bar, so that using curved-bar plate in refining process will solve the problem of excessive changes in the interaction angle to a certain extent, especially for the logarithmic spiral-bar plate, which creates a constant bar-interaction angle. The quality of the fiber or pulp will be approximately uniform based on theoretical analysis, but the mathematical analysis and experimental research of the curved-bar plate need to be further developed.
Many researchers hold different views on the refining characteristics of the curved-bar plate. The medium-density fiberboard (MDF) spiral plate and LemaxX series plate proposed by Andritz[3] adopt a spiral-bar design to ensure the stability of the bar-interaction angle and uniformity of the pulp quality. Some studies expressed that the cutting effect of the curved-bar plate is weaker than that of the straight bar[4-6], but the actual refining effect has not been tested and verified experimentally.
As a kind of ordinary bar plate, the curved-bar plate has many applications in the pulp and paper industry, which can be divided into radial type and isometric type according to the distribution of bars[7]. The arc of the radial curved bar is distributed along the radial direction and along the circumference of the center of the circle, as shown in Fig.1. Moreover, there are more bars in the inner part of the refining zone and fewer in the outer part, which will reduce the effective refining area when only one level of curved bars is placed in the refining zone. To solve this problem, the bars are usually arranged again at a larger groove width. The isometric curved-bar plate, which has the same groove width in the refining zone, can also solve the problem mentioned above, as shown in Fig.2. The isometric-bar plates are usually manufactured with several identical segments, and their refining effects are uniform compared to those of radial curved-bar plates, theoretically.
At present, the design of refining plate is based on specific edge load (SEL) theory[8], which has limitations in some degree because it gives less consideration to the bar parameters, such as bar angle and bar width. The design of the plates should consider the refining intensity, residence time of fibers in the refining area, and hydraulic performance, and the bar parameters, such as bar width, groove width, bar angle, and dams, should be selected reasonably to achieve specific requirements while ensuring production. However, there is no clear basis for the design of curved-bar plates, which have complex bar edges, unlike the straight bar. In this study, the design of multi-stage radial curved-bar plates and isometric curved-bar plates was conducted.
2    Definition of bar angle of curved bar
Leider et al designed a curved-bar plate for the pulp and paper industry[9], as shown in Fig.3, using the angle (+90°) between AB and OB and  between the tangent of the curved-bar end point C and the radius OC to indicate the curved bar, in which the definition of the angles at starting point B and end point C are different. Hackl et al[10] designed a non-paper refining plate in which the curved bar is represented by the angles  and  at starting point B and ending point C, as shown in Fig.4, and the drawing and measuring of the tangent are complicated.
The above two methods define only one arc angle of the curved bar, which cannot express the full arc of the curved bar. Through analysis of predecessors, a new definition of the curved bar is proposed in this paper. The arc that passes through point C, the intersection of the center line of the segment and the center circle of the refining area, is called the curved-bar center line and is denoted by two angles  (the starting angle of the curved bar) and  (the bar angle of the curved bar), as shown in Fig.5. Therefore, the curved-bar center line is determined by the location B and the bar angle that measures the curvature of the curved bar.
3    Design of curved-bar refining plates
3.1    Design of radial curved-bar plate
The groove width of radial-bar refining plates is gradually increased from the inner part to the outer part of the refining area. If only a one-stage grinding tooth is used; the effective refining area is reduced, which yields a lower utilization efficiency of the refining zone. Therefore, multi-stage refining plates have been developed. Fig.6 shows a three-stage radial curved-bar plate, which inserts more bars between the adjacent bars where the bar angle of the different-stage curved bar is different. In this part, a three-stage radial curved-bar plate is designed as follows.
 
3.1.1    Design of the 1st-radial arc of curved bar
The main technical parameters of the refining plate are shown in Fig.7. If the 1st-stage curved-bar angle is , the center line of the curved bar is arc AC, which has a bar angle of  and radius of Ra.
The origin O1 of the center line of the 1st-stage curved bar was selected as the pole, and the function of the circle of arc AC is expressed by Eq.(1).
      =Ra                                                                       (1)
 
 
 
 
 
 
 
 
 
 
Fig.7    The main technical parameters and design center line of 1st-stage curved bar of three-stage radical curved bar
If the bar width of the 1st-stage curved bar is b, the functions of its inner and outer arc, as shown in Fig.8, can be expressed by Eq.(2).
      =Ra±b/2                                                              (2)
Where the inner arc of the 1st-stage curved bar is obtained by subtraction and outer one is obtained by addition.
 
 
 
 
 
 
 
 
 
 
 
 
Fig.8    Design of the inner and outer arcs of the
1st-stage curved bar
At last, the number of 1st-stage curved bars, n, should be determined by the size of the refining plate and the process requirements. After this, the circumferential array of curved bars should be completed.
 
3.1.2    Design of the 2nd- and 3rd-radial arc of curved bar
The design of the 2nd- and 3rd-stage curved bars of the three-stage radial curved-bar plate is slightly different from that of the 1st-stage curved bar. According to the size of the refining plate, process requirements, and refining intensity, the refining zone was divided reasonably into the breaking zone, coarse refining zone, and refining zone. The values of R1 and R2 should be selected correctly, as shown in Fig.9 and Fig.10. The starting circle with radius Ri intersects the two center arcs of the adjacent 1st-stage curved bars. The radius of the center circle of area through which the 2nd-stage curved bar is (R1+Ro)/2, and the starting point and intermediate position of the 2nd-stage curved bar can be determined by the equal points of arcs FG and HI. In this paper, the starting point and intermediate position of the center arc of the 2nd-stage curved bar are determined by their two equal points, as shown in Fig.9.
The design of the 3rd-stage curved bar is the same as that of the 2nd-stage curved bar. It should be noted that the starting point is the equal points of arc KJ, which is part of the starting circle of the 3rd-stage curved bar cut by the adjacent curved-bar center arc of the 2nd-stage curved bar. The two equal points of KJ is defined as the starting point in this paper, as shown in Fig.10.
If the bar widths of the 2nd- and 3rd-stage curved bars are a and c and the bar angles are 1 and 2, the function of the center arc of the 2nd- and 3rd-stage curved bar can be obtained by Eq.(3).
      =RX                                                                      (3)
Thus, the function of the 2nd- and 3rd-stage curved bar is
      =RX±Y/2                                                             (4)
For the 2nd-stage curved bar, X=b and Y=a, and for the 3rd-stage curved bar, X=c and Y=c.
The circumferential array of the 2nd- and 3rd-stage curved bar is arranged according to the number of 1st-stage curved bars. Thus, the number of 2nd-stage curved bars is n(m1) and that of 3rd-stage curved bars is nm(z1).
Multi-stage radial curved-bar plates have the same bar angle in the same-stage curved bars, which are evenly distributed in the whole circumference. Therefore, the division of the plate does not affect the orientation of the curved-bar distribution. The plates with large diameters were manufactured by segments because of the complex overall manufacturing. Therefore, the center angle of the segment should be designed reasonably to equally divide the plate.
3.2    Design of isometric curved-bar plate
Compared with the radial curved-bar plates, the distribution of the bar width and groove width is more uniform from the inner part to the outer part of the refining zone. Theoretically, the isometric curved-bar plates have a uniform bar-interaction angle, which is conducive to uniformity of refining. The design of the isometric curved-bar plates is briefly introduced by using the bar angle of the curved bar proposed above. This article takes the right-hand rotational curved bar as an example.
 
3.2.1    Design of the center arc of isometric curved bar
The design of the center arc of the isometric curved bar is the same as that mentioned above. However, the intersection point B of the curved-bar center line, the center circle of the refining zone, and the center line of the segment are defined based on the design of the bar angle, as shown in Fig.11. The center arc of the center curved bar is determined when the bar angle  and starting point A are selected, and its function is similar to Eq.(1) and Eq.(3). After this, the function of the inner and outer arc can be determined considering the bar width b and groove width g, which are consistent with Eq.(2) and Eq.(4).
3.2.2    Design of curved bar on both sides of the center bar
If the groove width of the isometric curved-bar plate is g and the bar width is b, the equation of the 1st- stage curved-bar arc, as shown in Fig.12, on the left side (the left-hand bar is on the right side) is
      =R1±b/2+g                                                         (5)
And the functions of the 2nth arc, when n≥1, on both sides of the center curved bar can be expressed as
      =R1±b/2±g(g+b)                                              (6)
Similarly, the equations of the (2n+1)th arc, when n≥1, can be determined by Eq.(7).
      =R1±b/2±g±n(g+b)                                        (7)
When determining the function of curved-bar edges   on the left side of the center bar with Eq.(6) and Eq.(7), the arc can be obtained by addition. That of the right side can be obtained by subtraction. However, the choice of symbol is reversed for the left-hand bar.
The curved bars are arranged by the above arc equation. When the arc is full for the entire refining segment, the design of the isometric arc in a segment can be completed by trimming the arc out of the refining zone.
4    Bar profile of curved-bar plates
4.1    SEL of curved-bar plates
The SEL is a common way of quantifying refining intensity[8], which denotes the net energy applied to each meter of the bar crossing (J/m) and is calculated by Eq. (8).
 
 
where SEL is the refining intensity (J/m), Pnet is the net refining power (kW), n is the rotation speed (r/min), and CEL is the cutting edge length (km/r).
The SEL is proposed for straight-bar plates, and the calculation of CEL[11] (Eq.(9)), the common contact length of the opposite bars per revolution, is also for straight-bar plates, and cannot be applied to curved-bar plates.
 
 
Where r1 is the inner radius of the plate (mm), r2 is the outer radius of the plate (mm), nr is the total bar number of the rotor, ns is the total bar number of the stator, and  is the bar angle of the plate (°).
The calculation principle of the curved bar’s CEL is the same as that of the straight bar, as shown in Fig.13; the bar segments are divided into several zones and the number and length of the bars in each zone are counted. CEL of the curved-bar plate is calculated by the following equation.
 
 
Where i is the center angle of the curved-bar center line at zone i and Ri is the circle radius of the curved-bar center line at zone i.
SEL is affected by Pnet, n, and CEL of the refining process, and it should be selected reasonably for the refining process of different pulps. When refining softwood pulp, SEL should be 1.5~4.5 J/m, for hardwood pulp, it should be 0.5~1.5 J/m[12].
The refining plates are the core component of the refining process. Under the condition of constant Pnet and n, CEL can be adjusted by reasonable design of the bar parameters, through which SEL can reach the range mentioned above for effective refining. Through the analysis of Eq.(10), CEL is directly related to the configuration of the bar width, groove width, bar angle, etc., and the corresponding SEL can be matched by rationally designing the bar profile.
4.2    Bar-parameter determination of curved-bar plates
As a direct acting component of the refining process, the bar parameters of the refining plates have a direct influence on the refining effects. This paper introduces the design methods of two curved bars, but the selection of bar parameters is not provided, and will be briefly introduced below.
The selection of the bar profile, such as the bar width, groove width, and bar height, can be referred to that of the straight-bar plates. According to the literature[12], for the refining of softwood pulp, with a consistency of 3.5%~4.5%, the bar width of the refining plates can be chosen from 3.0 mm to 5.5 mm, and the groove width is generally between 5 mm and 7 mm. For refining hardwood pulp, the bar width is generally from 2.0 mm to 3.5 mm, and the width of the groove is generally from 3 mm to 4 mm. Generally, the width of the groove is usually 2~3 times the average fiber length.
The bar angle is one of the key bar parameters of refining plates, and is usually 15°~20° for straight-bar plates[13]. The bar angle of curved-bar plates can be selected as (+/2), in which  (15°~20°) is the bar angle of the straight-bar plate and  is the center angle of the segment, based on the different definition of the bar angle compared to that of the straight bar. Further, there is another angle that should be noted for the multi-stage radial curved-bar plate. The starting angle of a curved bar is usually between 0° and 45°, preferably between 15° and 30°. Here, it will be advantageous if  is larger than 1 and 1 is larger than 2[10].
5    Conclusions
As a common type of refining plate in the pulp and paper industry, curved-bar plates are weaker than straight-bar plates in fiber cutting, and theoretically, the refining performance provided by curved-bar plates is relatively uniform. However, the design of curved-bar plates is limited by a lack of theoretical basis. The work done by this study is described below.
The representation method of the curved-bar refining plates at home and abroad was analyzed and a new method to represent the bar angle of the curved bar was proposed in this paper to clearly describe the curved bar.
The multi-stage radial curved-bar plate and isometric curved-bar plate were designed based on the bar angle of the curved bar as described in this paper. Subsequently, the mathematical expressions of the curved-bar edges and the center arc of the curved-bar plate were established, which will provide a theoretical basis for parametric modeling of curved-bar plates and enhance the efficiency of their design.
The SEL of curved-bar plates was deduced compared to that of straight-bar plates, which can provide a theoretical basis for design of curved-bar plates. Furthermore, the determination of the bar profile was discussed based on that of straight-bar plate.
Acknowledgments
The authors gratefully acknowledge the funding by the National Natural Science Foundation (Grant No. 50745048).
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[10] Hackl M, Feichtinger K, Wendelin G. Rotor disk, US: 20120294725A1[P]. 2012.
[11] Technical Association of the Pulp and Paper Industry. TAPPI standard TIP 0508-05:Refiner Plate Intensity[S]. USA: Technical Association of the Pulp and Paper Industry, 2001.
[12] Hannu Paulapuro. Papermaking Part 1 Stock Preparation and Wet End[M]. 1st edition. Finland: Finnish Paper Engineers’ Association/Paperi ja Puu Oy, 2001: 47-48.
[13] He Beihai. Papermaking Principle and Engineering [M]. Beijing: China Light Industry Press, 2013: 32-34. .
 
 
 
A Principle of Producing Multi-ply Paper Using a Wire in an Inclined Wire Machine
Song-Il Pak, Yong-Il Ri*, Chol-Ho Kim
Institute of Paper Engineering, State Academy of Sciences, Pyongyang, D P R Korea
 
 
Abstract: In this study, we improved the dispersibility of the stocks in the headbox of an inclined wire machine to produce a distinct paper, and analyzed some factors affecting paper formation in the production of multi-ply paper. We used FLUENT6.3 to analyze the flow of the stocks in the headbox and select the structure of the diffusion part required for improving the dispersibility of fibers. Moreover, based on a simulation experiment, the optimal rational angle of the diffusion part () was found to be approximately 8°~10°, and it improved the paper formation in the case of usage of two plates. Using the equation for the formation of paper layers in the headbox of an inclined wire machine, we obtained a paper with the given basic weight by controlling the inclined angle of the wire (), initial height of water (H), and concentration of the stocks. We considered the effect of  and H of the stocks in the headbox on the fiber distribution, and according to the results,  should be set as approximately 20°~30° and H should be maximally high. When producing multi-ply paper by a wire, the line pressure of the couch roll should be maintained at 1.8~2.0 kN/m to avoid the damage to the paper sheets. In addition, we found the optimal structure parameter of the dehydrated roll was as follows: hole ratio of approximately 30% of the dehydrated roll surface area, width of 1.5~2.0 mm, slot pitch of 5~6 mm, slot depth of 2~3 mm, and inclined angle of diffusion part () of 5°.
 
Keywords: inclined wire machine; headbox; fiber suspensions fluid; multi-ply paper; papermaking
 
Song-Il Pak, PhD candidate;
E-mail: arirangip@star-co.net.kp
 
*Corresponding author:
Yong-Il Ri, PhD, professor; research interests: innovative papermaking and product development of specialty paper, pulping, and papermaking;
E-mail: arirangip@star-co.net.kp
 
1    Introduction
A paper machine, as an equipment for producing paper, can be classified into two types, namely, cylinder mold and Fourdrinier type, and in newspaper production, the double wire type is widely used[1].
The inclined wire type is a transformative type of a Fourdrinier paper machine, and its drainage combines the principles of both the cylinder mold and Fourdrinier type.
A primitive inclined wire machine was developed in the 1930s to produce a distinct paper using long fibers, and the wire of this type is very similar to the wire of a Fourdrinier machine[2-3].
An inclined wire machine is not suitable for producing ordinary paper because it is an equipment for manufacturing a distinct paper without long fibers which have poor properties in the papermaking process at a very low pulp concentration (0.01%~0.05%). An inclined wire machine operates at a low pulp concentration, and therefore, it needs a large amount of water during the papermaking process.
We chose a rational structure for the headbox that was suitable for the properties of 50″- and 100″-paper machines to produce various types of distinct and functional papers, and we developed the principle for papermaking by using an inclined wire machine.
2 Analysis of fluid flow in headbox of inclined wire machine and choice of its rational structure
2.1 Mathematical modeling for flow of stocks in inclined wire headbox
There have been significant developments in the study of structure of the headbox to improve the dispersibility of fibers. For example, a control system for the dilution factor and basic weight of the paper in its section was realized[4], and the fiber distribution (formation) was improved by developing a new vibration system by the calculation and design of a high-speed machine[5].
We analyzed the flow property of the stocks by using FLUENT6.3 in the headbox of an inclined wire machine, which is used for producing a distinct paper.
Modeling the flow field of a fluid mathematically, we obtained the following equations[6].
Mass conservation:
 
 
Where  and U=(U1,U2,U3) are the density and velocity of the fluid in time t and at position x=(x1, x2, x3), respectively.
Momentum conservation:
 
 
Where p is the pressure and >0 is the viscosity coefficient.
Turbulence model: this yields the equation for determining turbulence viscosity coefficient t. Various models are used for its calculation, and here we use the standard k- model[7].
 
 
 
 
(4)
Where k is the turbulence kinetic energy,  is its rate of dissipation, k and  are the turbulence Prandtl numbers for k and , respectively, Gk is the generated turbulence kinetic energy due to the mean velocity gradient, and G1, G2 are the given constants.
Turbulence viscosity coefficient t is computed by combining k and  as follows:
 
 
Where C is a constant.
Based on the flow models (1)~(4) defined in Equations (1)~(4), we analyzed the property of the paper stock flow in the headbox of an inclined wire machine.
2.2 Flow property of stocks and selection of structu, re o, f diffusion part
We allow the flow of the headbox in the width direction to smooth and unify the suspension of the pulp fibers that enter the bath of the headbox using the diffusion part to ensure a flat flow of the paper stock with a simple structure. Fig.1 shows the structure of the diffusion part.
As shown in Fig.1, width B of the diffusion part has to be equal to width b of the bath of headbox, and it is selected based on the size of the machine.
Height H and inclined angle  of the diffusion part entering the bath of the headbox are determined by the condition that the entrance of the diffusion part is not larger than the cross-section of the entrance connected to the supplying pipe of raw materials. This implies that it must satisfy B·H≤b·h.
If the cross-section of the entrance of the diffusion part entering the bath of the headbox is larger than the one connected to the supplying pipe of the raw materials, then the air will be mixed in the stocks, air drops in the bath will rise, and bubbles will be formed, which is not good in the flow and deckle of the stocks. Moreover, the stocks entering from the pipe supplying raw materials are distributed along the width direction of the machine and the flow rate decreases; consequently, the fibers in the stocks become entangled.
To analyze the flow property of the stocks, we assume that the stocks enter the entrance of the diffusion part at a rate of 10 m/s along the width direction of the headbox.
Fig.2~Fig.4 display the results of the analysis of the flow property of the stocks.
According to the results of the simulation with FLUENT6.3, the rational diffusion angle () is approximately 8°~10°.
If we set  as 10°, then length L of the diffusion part from the supplying pipe of raw materials to the entrance of the bath of the headbox will increase. Therefore, the size of the machine is larger, and consequently, the setting area of the wire parts in the process for producing the paper is larger.
To overcome this issue, the entrance part of the supplying pipe of the raw materials may be made into a circular cone. Then, combining it with the entrance of the diffusion part, we can decrease L.
2.3    Flow property of stocks in bath of headbox
The stocks are distributed uniformly along the width direction of the machine by the diffusion part entering the bath of the headbox. Subsequently, the stocks keep an equilibrium on the flow through plate, and then are dehydrated through the wire, and a sheet is formed.
We analyze the flow property of the stocks based on the diffusion part in the bath of the inclined wire headbox by using FLUENT6.3. Then, we assume that the flow of the stocks through the diffusion part is uniform along the width direction at a rate of 5 m/s.
Fig.5~Fig.7 show the distribution of the flow velocity of the stocks in the bath of the inclined wire headbox.
As shown in Fig.5, without a plate in the bath of the headbox, the stocks enter through the entrance without a decrease in the flow rate. Consequently, the flow rate of the stocks is higher than the wire rate. Therefore, in the paper formation, damage occurs in the region of the paper sheets, the fibers of stocks in the bath are not distributed sufficiently in all the directions, and the difference of strength in the across-vertical direction of the paper increases. Moreover, after the stocks leave the entrance of the bath, a rotation occurs, which has a negative effect on the paper sheet formation.
The case of one plate in the bath of the headbox (Fig.6) provides a better condition for forming paper sheets than the case without a plate. Because here, the stocks enter through the entrance of the bath, there is a decrease in the flow rate. In comparison, after passing the plate, there is a small rotation, which might slightly affect the paper sheet formation.
In the case of two plates (Fig.7), for the stocks entering the entrance of the bath, the flow rate becomes sufficiently stable, which helps the stocks form uniformly. The fibers of the stocks are sufficiently distributed in all the directions, and the difference of strength of across-vertical direction of the produced paper decreases.
3 Equation for formation of paper layers in inclined wire machine
The process for the formation of paper layers depends on filtering pressure P. Therefore, the basic equation for the formation of paper layers can be expressed as follows:
 
 
Where:
q—basic weight (kg/m2),
q0—wire coefficient (constant depending on the property of the wire, kg/m2),
t—time for the formation of the paper layers (s),
P—difference between the filtering pressures (Pa),
k—filtering coefficient (m/s),
c—concentration coefficient (kg/m3),
g—gravitational acceleration (m/s2).
In addition, concentration coefficient c may be represented as follows:
 
 
Where:
c0—concentration of the paper stock,
cw—concentration of white water,
cz—concentration of the fiber layers on the wire,
T—ratio of the mass of goods on the raw material, where c0>>cw, cz>>c0, and T≈1.
Therefore, we know c≈c0. This implies that concentration coefficient c is almost equal to the concentration of the stocks.
In Equation (5), P is not a constant and it is a function of time t, i.e., P(t).
The height of the pulp in the bath of the inclined wire decreases directly to the length (L) of the interval in the formation of the paper layers, and the difference between the filtering pressures varies linearly with it.
Setting the velocity of the wire as V and the length of the interval in the formation of the paper layers as L, we can express the difference between the filtering pressures as follows:
 
 
Where P0 is the difference between the filtering pressures initially.
Substituting Equation (7) into Equation (5) yields:
 
 
Integrating the above by using the boundary condition (q is (0, q), t is (0, )), we have:
 
 
Where  is the time when the formation of the paper layers is completed.
Using  and substituting it to Equation (9), we obtain:
 
 
 
or
 
 
Equations (10) and (11) are the equations for the formation of the paper layers in the inclined wire machine.
Using the unit measures in the paper technology, Equations (10) and (11) are follows:
 
 
 
 
Where:
the unit of q and q0 is g/m2,
the unit of c is %,
the unit of k is 106 m/s,
the unit of H0 is m.
Using Equations (12) and (13), we can calculate the length of the interval in the formation of paper layers needed to obtain a paper of a given basic weight under the given conditions (the rate of the wire, concentration of the stocks, and initial filtering height of water).
In contrast, setting the angle of the inclined wire as  , we have:
 
 
Substituting Equation (14) into Equations (12) and (13) yields:
 
 
 
 
 
Using Equations (12) and (13), we can obtain a paper with a given basic weight by controlling the angle of the inclined wire, rate of the wire, initial height of water, and concentration of the stocks.
In addition, we can calculate the angle of the inclined wire suitable for producing the paper with operation of the machine and of a given basic weight.
In contrast, as we know from the above equations, we must increase the concentration of the stocks to reduce the waste of water and further increase the productivity of the paper machine.
However, there exists a limitation in increasing the concentration of the stocks to maintain the quality of the production. Therefore, we must determine the appropriate concentration of the stocks to improve the quality of the production and reduce the waste of water. Moreover, we must design the rational elements of the wire parts and decide the operational conditions.
4 Principle for producing multi-ply paper by one wire
The principle for producing multi-ply paper using one wire in an inclined wire machine is demonstrated here.
The stocks completed in the composite process are rectified by passing the diffusion part and plate through the headbox with a controlled concentration.
In a paper machine, we can set 1~3 headboxes and we can adjust the numbers of headboxes according to the requirement. We can place stocks having the same or different properties in each headbox. When we place stocks having the same property in several headboxes, we can manufacture a thick paper with a more basic weight. Conversely, when we place different stocks in the headboxes, we can obtain multi-ply paper having different properties.
The basic weight of the paper in each layer is 50~80 g/m2.
If we want to manufacture a two-layer paper using the machine, then we add the sheets formed in two headboxes, and similarly, a three-layer paper is manufactured by adding the sheets formed in three headboxes. This implies that to manufacture an N-layer paper, N headboxes must be set up. The size of a headbox is approximately 1600~1800 mm.
4.1 Effect of inclined angle of headbox on paper sheet formation
In our experimentation, we used non-bleaching sulphate pulp (NUKP) and mixed-waste paper pulp as the materials of the stocks. The experiment was performed under the following conditions: extracting condition 36″, beating degree 42°SR, extracting concentration 0.2%, maximal height of water 200 mm, and extracting rate of 15 m/min. Varying the inclined angle of the headbox as 10°, 20°, 30°, and 40°, we measure the directivity of the fibers and thickness uniformity of the stocks.
The directivity of the fibers in the extracting papers can be determined by the naked eye and difference of  breaking length (BL) between MD and CD of the paper is shown in Table 1.
The method for measuring the thickness is as follows: we first cut a piece of paper with length of 50 cm from the extracting paper and make pieces in size of  10 cm×10 cm. We then measure the thickness of each piece approximately 5~10 times, and denote its average as . The instrument for measuring the thickness is a caliper (ZHD-4) with a diameter of 16 mm.
The thickness uniformity of the paper is calculated by the following equation:
 
 
 
 
Where:
Df —thickness uniformity of the paper, %,
D—dispersion of the measured value, mm,
XDi—thickness measurement value of each piece, mm,
—average of the thickness measurement values, mm,
n—measurement number,
The experimental results are presented in Table 1.
Table 1    Effect on paper sheet formation based on inclined angle of the headbox
Inclined angle of the headbox/(°)    Difference of BL between MD and CD/m        Formation
/103 m
10     50     0.63
20     63     0.71
30     76     0.82
40     145  1.57
 
MD=machine direction; CD=cross direction; BL=breaking length.
Table 1 shows that the difference of BL between MD and CD of the paper gradually increases as the inclined angle of the headbox increases, and it rapidly increases when the inclined angle of the headbox is larger than 30°. The reason is that as the inclined angle increases, a stopping phenomenon occurs for the stocks in the headbox bath, and so, paper layers are formed owing to the fibers being dragged in the movement direction of the wire.
 Furthermore, the thickness uniformity in the extracting paper gradually deteriorates as the inclined angle increases. As it is larger than 30°, there is a rapid deteriorated trend. This implies that as the inclined angle of the headbox increases, the fibers of the stocks in the headbox bath are not distributed uniformly and are intertwined; hence, the formation of paper deteriorates.
In contrast, in the case that the inclined angle of the headbox is small, it is good for the formation of paper sheets. However, in this scenario, the water height of the stocks must be kept constant when extracting the paper in the headbox. This enlarges the installation area of the wire part because the length in the formation of the paper sheet is longer. Therefore, the angle of the inclined wire headbox in the production of multi-ply paper must be selected rationally to reduce the setting area of the wire parts in accordance with the qualitative index of the paper and properties of medium- and small-sized plants. Moreover, we believe that it may be rational to select the setting inclined angle of the headbox between 20° and 30°.
4.2 Effect of water height of stocks in headbox on paper formation
In our experiment, we used mixed-waste paper pulp with a beating degree of 42°SR as the material of the stocks. We analyzed the effect on the formation of paper sheets by varying the water height of the stocks in the headbox. This was done when extracting papers of 80 g/m2 under the condition of paper stock concentration of 0.3% and inclined angle of the headbox of 25°.
The experimental results are exhibited in Fig.8. Fig.8 shows that we can increase the maximal height of the water of the stocks by controlling the dewatering quantity in the interval in which the paper sheet is formed when extracting the papers from the inclined wire headbox. Consequently, the difference of BL between MD and CD of papers is reduced and paper formation is improved. Increasing the water height of the stocks in the headbox is advantageous to the formation of paper sheets because with increasing water height, the fibers in the stocks become smoothly distributed in all directions. Therefore, we must raise the water height of stocks as much as possible when extracting papers in the inclined wire headbox.
4.3 Determination of optimal line pressure in couch roll
A large increase in the line pressure in a couch roll corresponds to a dry paper sheet and significant quantity of water exhausting from it. Finally, the dehydration in the dehydrating roll becomes abnormal and the paper is destroyed. Oppositely, a small line pressure in a couch roll implies a small quantity of water exhausting from the stocks. Moreover, even though the dehydration in the dehydrating roll is normal, there occurs a phenomenon in which the paper sheets are separated from the blanket owing to the low drying level of the wetting stocks after passing the couch roll.
To determine the optimal line pressure of the couch roll, we used mixed-waste paper pulp as the material and performed the experiment under the following conditions: inclined angle of the headbox of 25°, concentration of the stocks of 0.3%, beating degree of 42°SR, extracting rate of 15 m/min, and basic weight of 80 g/m2.
We set up a vacuum absorption box in the wire part and measure the dryness of the paper sheets and thickness uniformity of the wetting paper sheets formed in the headbox before and after passing through the couch roll. The pressure in the vacuum absorption box is 12.5 kPa.
Table 2 shows that if the line pressure of the couch roll is higher than 1.8 kN/m, then the dewatering quantity from the wet sheet increases and water through the dewatering roll is not completely dewatered and it flows on the sheet. Consequently, the paper destruction phenomenon occurs when the line pressure is higher than 2.0 kN/m. Therefore, it is better to set the line pressure as 1.8~2.0 kN/m when producing multi-ply paper.
Table 2    Dewatering character of sheet according to line pressure of the couch roll
Line pressure/(kN·m1)         Consistency of sheet/%
        1.2
        1.4
        1.6
        1.8
        2.0
        2.2   8.7
9.5
10.0
11.0
11.7
12.6
 
 
4.4Structure of dewatering roll and determination of inclined angle () of wire
Fig.9 shows the structure of new dewatering roll. The distinct feature of the new dewatering roll is different from that of the previous one in that it is not covered with a rubber layer on the surface of the roll, which is  instead made of steel or cast iron. We bore a screw hole in it and placed a resinous net on it.
 
 
 
 
 
Fig.9    Structure of new dewatering roll
The screw hole is bored to spread out in both sides, starting from the center of the roll.
Based on the experiment, the most rational ratio of the holes is approximately 30% on the surface area of the roll, width of the screw hole is 1.5~2.0 mm, and its pitch is 5~6 mm. The depth is determined by the wall width of the main roll body, and in general, it is 2~3 mm.
As shown in Fig.10, the wetting paper sheets formed on the wire change the direction by the guidance roll and enter the space between the dehydrating roll and couch roll.
 
 
 
 
 
 
 
Fig.10    Position of dewatering roll
If the wire entered horizontally or with an upper incline in the space between the dehydrating roll and couch roll, then the exhausted water under the pressing force flows onto the paper sheets formed on the wire, thereby destroying the paper sheets. Therefore, the wire must enter with a low incline in the space between the dehydrating roll and couch roll.
In addition, a large  is also unreasonable in the installation of the headboxes and dewatering rolls; therefore, we must determine the rational . To this end, we measure the length (Lw) of the water overflowing the wire when the paper sheets are compressed and exhausted in the dewatering roll, changing .
We use NUKP and mixed-waste paper pulp with a beating degree of 42°SR. The experiment was conducted under the following conditions: inclined angle of the headbox of 25°, concentration of the stocks of 0.3%, extracting rate of 15 m/min, and basic weight of 80 g/m2.
We present the results in Table 3.
According to Table 3, water does not overflow from the couch roll to reach the wire and that the paper sheet formation is not destructed when the inclined angles of the headbox are 4° and 5° for the NUKP and mixed-waste paper pulp, respectively. Therefore, for the rational installation of the dehydrating roll to produce multi-ply paper, it is better to set the entering inclined angle as 5°.  
As mentioned above, we developed the principle for producing multi-ply paper by using a wire in an inclined wire machine and practiced it. It was found that the structure of the machine was simple.
5    Conclusion
In this study, we chose the structure of the headbox in which we could obtain a paper product with an ideal fiber distribution using an inclined wire machine. Moreover, we demonstrated the principle for producing multi-ply paper using a wire.
We analyzed the flow properties of the stocks in the headbox of an inclined wire machine by using FLUENT6.3. According to the simulation, the rational angle of the diffusion part () was approximately 8°~10°, and it was optimal to use two plates during the paper formation.
Using the equation for the formation of the paper layers in the headbox of an inclined wire machine, we could obtain a paper with a given basic weight by controlling the angle of the inclined wire, initial height of the water, and concentration of the stocks.
We explored the effect of the inclined angle of the wire part and the water height of the stocks in the headbox on the fiber distribution. It was concluded that the rational inclined angle of the wire () was approximately 20°~30° and water height (H) could be set maximally high.
For the production of multi-ply paper by a wire, the line pressure of the couch roll should be maintained within 1.8~2.0 kN/m, which would not cause damage during the paper formation in the sheet region.
We found the optimum structure parameters for the dehydrated roll were as follows: hole ratio in the dehydrated roll of approximately 30% of the roll surface area, width of 1.5~2.0 mm, slot pitch of 5~6 mm, slot depth of 2~3 mm, and inclined angle of diffusion part () of 5°.
References
[1] Ri Y I, et al. Versatility Paper Manufacturing Principle by Using the Inclined Wire Dewatering Method[J]. Bulletin of the Academy of Science, 2017(3): 24-25.
[2] Zhang J M. Light Industry Machine[M]. Beijing: China Light Industry Press, 2007.
[3] Duan M P. Light Industry Machine[M]. Beijing: China Light Industry Press, 1996.
[4] Chen H, Tang W, Liu W B, et al. Structure and Control of a Hydraulic Headbox with Dilution Water[J]. China Pulp & Paper, 2013, 32(12): 38-44.
[5] Zhang F, Gao Z F, Liu H, et al. Design of a New Shaking Device for High Speed Paper Machine[J]. China Pulp & Paper, 2013, 32(12): 52-55.
[6] Kundu P K, Cohen I M, Dowling D R, et al. Fluid mechanics[M]. Waltham: Academic Press, 2001.
[7] White F M. Viscous fluid flow[M]. New York: McGraw-Hill, 1974. 
 
 
 
Status, Prospects & Perspectives of Indian Paper Industry
 
Rakesh Kumar Jain,  Vikas Kumar
United Nations Industrial Development Organisations, UNIDO IC ISID, New Delhi, India
 
Abstract: Indian paper industry has been showing a positive demand growth unlike Europe and USA, despite couple of issues being confronted by the paper industry such as availability of good quality raw material, scale of operation, cost reduction and environment. In view of the increasing competition among the domestic and the global players, sustainability, green production gaining significance, there is a growing need to upgrade, develop and adopt appropriate advanced technologies tailer made to the requirement of Indian paper industry. In light of above, UNIDO implemented a project supported by DIPP, Govt. of India aimed towards capacity building of the Nodel Research Institute (CPPRI), Indian paper industry’s associations and Indian paper industry. The present article highlights key findings of various activities carried out under the project such as diagnostic assessment of the paper industry, demonstration of identified technologies, dissemination of the findings among the industries and twinning with the international institutions having relevant expertise and training.
 
Keywords: Indian paper industry; raw material; scale of operation; environment
 
Rakesh Kumar Jain, PhD, technical expert; research  interests: chemical recovery, energy and environmental management and bio technology in pulp and paper;
E-mail: dr.rkjain56@rediffmail.com
1    Introduction
Indian paper industry is one of the most flourishing and thriving industries. It is the 15th largest in the world and accounts for about 4.0% of the world’s paper production.
The paper industry plays a significant role in Indian economy with a turnover of over INR 56000 crores and a resultant contribution of around INR 5500 crores to the national exchequer.
There are 863 pulp and paper mills in India with an operational installed capacity of around 21 million tons per annum as against total installed capacity of 25 million tons per annum of paper and paperboard. The paper industry provides direct employment opportunities to about 0.5 million people and indirect employment to over 1.5 million people.
2    Structure of Indian paper industry
Indian paper industry uses diverse raw materials like wood, agro residues and recycled fibres. Of around 610 operational mills, 67% of the paper production is contributed by recycled fibre-based mills (519 mills), 22% by wood-based mills (22 mills) and the remaining 11% of the production is contributed by agro-based mills (69 mills).
Despite the fact that Indian paper industry holds its importance in the national economy, unfortunately the industry stands highly fragmented; predominately consisting of small/medium scale mills based on recycled fibres and agro residue fibres, and few of the large mills using wood. Structure of Indian paper industry is shown in Fig.1. The operational capacity of the mills varies from 10 to 1500 t/d. Out of around 600 units which are operational, more than 400 units having capacities of below 100 t/d. Out of which 267 mills have an installed capacity of below 50 t/d. Further, there are only 40 mills having capacities of more than 300 t/d, of which only 20 mills have the capacity of more than 500 t/d.
 
 
 
 
 
 
 
 
 
Fig.1    Structure of Indian paper industry
Indian paper industry is categorized into different sectors based on raw materials used or by the variety of paper produced.
3 Indian paper industry segments based on raw material
Indian paper industry is typically divided into three major sectors based on the raw materials used. These are wood-based, agro-based and recycled fibre-based sectors. The consumption of different raw materials by the paper industry depends upon the variety of paper produced, availability of raw material and environmental factors to certain extent.
The distribution of Indian paper industry based on the type of raw material used for making paper viz. wood, agro residues and recycled fibre is given in Table 1.
Table 1    Structure of Indian Paper Industry
Raw material   No. of mills/units     Capacity*
/(Mt·a-1)         Production
 share/%
Wood-based (large integrated)        31     4.12 22
Agro-based (medium scale)     144  1.86 11
Recycle fibre-based (medium and small scale)        629  11.38        67
Total        863  17.34        100
* The capacity is based on production of operating mills.
The raw material consumption pattern has changed over the last few decades. In early seventies the share of wood-based raw material was 84% whereas the agro-based and recycled fibre-based raw material contributed only 7% and 9%, respectively. Presently, wood-based large integrated paper mills having installed capacities in range of 250~1500 t/d, have production share of around 22%. The medium sized agro-based paper mills have capacities of 30~550 t/d with a production share of 11% whereas the recycled fibre-based paper mills operate in the range of 10~1400 t/d contributing to 67% of total production (Fig.2). Till about a decade ago, the wood-based, agro-based and recycled fibre-based paper mills contributed 31%, 22% and 47% respectively to the total production. This spurt in mills shifting to use of recycled fibres over other raw materials is seen mainly for environmental compliance.
4 Indian paper industry segments based on products
Indian paper industry mainly produce writing & printing paper, packaging paper (industrial grade) and newspaper.
In terms of volume, highest contribution to Indian paper production comes from packaging paper followed by writing & printing paper and newspaper. Out of the total production of 17.4 million tons of paper and paperboard, writing & printing paper constitutes 35%, packaging paper 55% and newspaper around 10%. However, certain speciality papers such as security papers and check papers are imported in India.
The writing & printing paper comprises mainly of uncoated varieties viz. cream wove paper, maplitho paper; branded copier paper is mainly produced from wood-based raw materials with a little share from agro-based and recycled fibre-based raw materials, whereas the packaging paper, classified into kraft paper, boards, poster paper and others including duplex board and grey board  is mainly produced by the small and medium sized recycled fibre-based and agro-based mills. Newspaper is produced by mills utilizing mainly recycled fibres as raw material. Table 2 presents the production of various grade papers from different raw materials in Indian paper industry[1].
India produces many varieties of papers, namely, paper, coated paper and some specialty paper. Varieties under writingrs. There are approximately 610 operational paper mills in India, of which twelve are major players.
5    Demand & supply scenario
The paper industry in India looks extremely positive as the demand for upstream market of paper products, like, tissue paper, tea bag tissue, filter paper, light weight online coated paper, medical grade coated paper, etc., is growing up.
In spite of the continual focus on digitization, India’s requirement for paper is anticipated to rise 53% in the next six years, principally due to a sustained boost in the number of school-going children in rustic areas. Growing consumerism, modern retailing, rising literacy and the growing use of documentation will continue demand for writing & printing paper buoyant. The exponential enlargement of e-commerce in India has opened up the latest horizon and could donate significantly to the demand where the paper is being lengthily used for packaging. Though India’s per capita utilization is quite low compared to global peers, things are looking up and a requirement is set to rise from the present 13.2 kg to an estimated 20 kg by 2020. This indicates there is a lot of headroom for the development of paper industry in India. From a demand point of view, each one kilogram incremental per capita utilization results in supplementary demand of more than one million tons a year. Besides, strategy aspects also have a key position to play in the development of Indian paper industry. The government’s continued focus on literacy, amplified consumerism, an increase in organized retail are predictable to positively affect paper consumption and demand in India.
The following prime grades of paper imported from USA, Europe, Dubai and Singapore are label stock, wet strength papers, tea bag tissue, soft tissue, filter paper, insulation kraft, extensible kraft, decorative laminates, overlay tissue, thermal papers, digital papers, coated paper and board and some specialities. The volume of the import of paper and paperboard was around 1.34 million tons (per annum) in 2007~2008 which increased to around 1.48 million tons in 2015~2016, contributing around 8.55% of the total consumption of paper and paperboard. India exports 0.64 million tons of paper & paperboard per annum. Paper exports account for a meagre 3.70% of the total paper and paperboard production.
Nearly half of the newspaper demand in India is met by imports. 1.50 million tons of newspaper was imported in 2015~2016, which is higher than 1.33 million tons for the previous year.
India exports following grades of papers to Middle East, South Eastern countries, Eastern Europe and USA: A4 copiers paper, wood-free (mostly from bamboo and agro-waste by several small mills), MG varieties (from small agro-based mills), coated duplex (mostly from recycled fibre-based mills) and large quantity of converted products like stationery items, calendars, books, magazines, children’s play books and comics. The export of newspaper from India is negligible.
6    Growth and future projections
Driven by the need to meet the rising demand of paper in India, the paper industry witnessed more than two-folds increase in the paper and paperboard manufacturing capacity in the past ten years. Installed capacities increased from 7.32 million tons per annum in 2005~2006 to 21.30 million tons per annum in 2015~2016 with significant capacity expansions occurring during 2009~2010 and incremental capacity additions in subsequent years. The main driver in growth of Indian paper industry has been the positive growth in domestic paper demand. As per the data available, Indian paper industry has indicated a steady growth with an average rate of around 6%~7%, which has been indicated by the growth in the capacity which has increased from 12.7 million tons (2010~2011) to 21.30 million tons (2015~2016)[2]. 
Based on the average growth rate of around 7.8% and the average consumption growth of around 7.4% in last decade, the projected production of paper in India in 2025 is expected to be 25 million tons and consumption around 27 million tons (approx).
Indian paper industry can be more competitive by adding improvements of key ports, roads and railways and communication facilities, revision of forest policy is required for wood-based paper mills so that plantation can be raised by paper industry, cooperatives of farmers, and state government. Degraded forest land should be made available to the paper industry for raising plantations.
7    Major issues & challenges
Major issues confronting Indian pulp and paper industry are high cost of production caused by inadequate availability & non-availability of good-quality fibrous raw materials, uneconomical scale of operation, high cost of basic inputs including energy and water, technological obsolescence and environmental challenges. Despite a positive demand outlook, there are several barriers to the growth of the paper industry. Some of the major issues confronting the paper industry in India are summarized below.
7.1    Raw material availability
Scare availability and quality of raw material is one of the biggest challenges faced by Indian paper industry. Unlike the developed countries, India is a fibre deficient country. The paper industry depends on a mixed source of raw materials consisting of wood, agro residues and recycled fibres. However, the supply of each source is limited forcing the paper industry to depend on imported pulp, wood chips and waste paper. Various factors exert influence on consistent supply of raw materials.
Since the last several years, Indian paper industry has been plagued by inadequate supply of wood. The paper industry does not have access to the forest lands which are owned by the government. Only felling from the forests mainly bamboo, hard wood and eucalyptus are available to the paper industry. Government regulations on captive plantation by the paper industry also restrict the use of degraded forest lands for plantation of pulpable species of trees.
The availability of agro residues is affected by cycles in agricultural produce. Other factors limiting their availability for the paper industry include:
·Utilization of bagasse as an alternative fuel in sugar industry.
·Recovering of only 75% wheat straw due to prevailing harvesting mechanism.
·High cost of transportation of loose straw (40% higher).
Availability of indigenous waste paper is poor as most of the post-consume paper finds alternate use as in packaging. Waste paper recovery rate in India is as low as 35% compared to 55%~60% in developed nations. Further in the absence of an effective waste paper collection, sorting and grading system, the quality of waste paper is inferior. The paper industry relies on waste paper imports to meet its demand of waste paper.
7.2    Cost of basic input
Rising cost of various inputs to the paper industry like fibrous raw materials, chemicals and coal severely hampered the profitability and competitive edge of the industry. The price of coal in India has increased over the years despite decline in international coal prices. The prices of fibrous materials in India have followed a rising trend, due to unfavorable demand supply scenario in the country. However in the present times due to renewed thrust on agro-forestry and softening of pulp prices, the situation has eased substantially. A comparison of cost structure in Indian mill and European mill indicates higher raw material cost for Indian mills at 57% compared to only 40% in European mill.
7.3    Low scale of operation and obsolete technologies
Indian paper industry is highly fragmented predominately consisting of small units based on recycled fibres and agro residue fibres. While only 169 mills have capacities over 100 t/d and around 424 mills in India have installed capacities ranging from 10 to 100 t/d. Most of the large scale paper mills have made substantial investments on capacity enhancements and modernization to reap the benefits of economies of scale, the smaller scale mills still operate on obsolete technologies. As a result, the small scale mills continue to have poor productivity and low operating margins.
The major reasons responsible for technological obsolescence in pulp and paper sector are:
·Industry is generating below par returns on investments. This is due to high cost of basic inputs and complexity of scale of operation. Due to low returns on investment, paper industries are not able to mobilise funds for modernisation. 
·Interest component on investment is very high in India as compared to other countries. Due to low rate of interest the mills abroad are able to invest heavily on technology/modernization and therefore, are cost competitive.
·The capital to turnover ratio in case of Indian paper industry is very poor when compared to other industrial sectors in India (e.g. 3∶1 in paper industry compared to 1∶1 in cement industry).
7.4    Environmental concerns
The major environmental issues faced by the paper industry include: high effluent load, high color load, recycled fibres, black liquor treatment in small scale agro-based mills, solid waste treatment, concern over air pollution center with growing public awareness. For compliance to strict environment regulations, the paper industry needs to invest heavily in:
·Fibre recovery system.
·Tertiary treatment systems like membrane filtration (ultra-filtration/reverse osmosis), activated carbon filter, pressure sand filter.
·Up-gradation/modification of existing ETPs etc. to increase reuse/recycle of back liquid or treated mill effluent and zero discharge level.
7.5    Lack of skilled manpower
Indian paper industry faces a severe shortfall of skilled manpower. Presently there are only seven institutions offering B. Tech/M.Sc. Degree/Diploma courses in the area of pulp and paper. Availability of process stream technical manpower from existing seven institutions is quite low (less than 300 persons) against a requirement of 571 persons per year at the level of B. Tech. & Diploma. This shows that there exists a big gap between requirement & availability of technically trained manpower to meet the present and the growing demand of the paper industry.
7.6    Technological challenges
The major technological challenges being faced by Indian paper industry range from issues related to the raw materials handling, yield, process and energy efficiency, quality, and handling of internal process water and treatment of black liquid and solid waste including de-inked pulp (DIP) sludges and plastic waste[3]. Some of the challenges may be addressed by technology or through investments at the different mills, while others cannot be solved.
7.7    Raw material storage and handling
7.7.1    Wood-/agro-based paper mills
The availability and the high cost of raw materials have been the major issues with wood- and agro-based mills. One of the reasons for the high price in case of straws could be the higher handling charges for transportation of straws in loose form without compact bailing. This has a direct bearing on the cost of the raw material being delivered to the mills.
 
7.7.2    Recycled fibre-based mills
Sorting of recycled fibres is not optimal. This issue cannot be solved in the individual mills alone. Sorting systems to remove both waste components such as plastic and to improve the sorting of paper into different product categories (tailored for the product; the right fibre to the right product) would be highly beneficial. This would help ensure good product quality and minimum process variations.
7.8    Pulping of wood-/agro-based paper mills
In most of the large scale wood-based paper mills, pulping of wood-based raw material is carried out in continuous (stationary) digesters, using rapid displacement heat (RDH) pulping process followed by the oxygen delignification (ODL) process. In medium/large scale agro-based mills producing a bleached variety of writing & printing grade paper and paperboard, the most common practice of pulping is the use of continuous Pandia digesters with or without using an ODL step. Whereas, a few of the agro-based mills producing unbleached variety of paper, still use energy inefficient batch digesters. With regards to pulping of recycled fibres, various types of pulpers (ranging from low consistency to high consistency) are used including efficient drum pulpers (only in few mills).
7.9    Bleaching
A majority of the medium-scale agro-based paper mills have bleaching sequences with elemental chlorine stages causing a concern for pulp quality and environment. The majority of large-scale wood/agro-based paper mills use elemental chlorine free (ECF) bleaching sequences. Whereas few of the medium scale agro-based mills which still use elemental chlorine as bleaching agents.
Recycled fibres-based mills producing bleached variety use obsolete and environmentally unfriendly calcium hypochlorite as the bleaching chemicals which are added directly to the hydra-pulper and/or in the chest before the decker washer.
7.10    Product quality and pulp yield
Due to the fibre properties of the raw material available indigenously, this issue can be partly addressed in recycled fibre-based mills through proper fibre processing by optimizing the process conditions for pulping in hydra-pulper and in bleaching (use of proper bleaching equipment lay out and use of environmentally friendly bleaching chemicals).
7.11 Stickies/slime issues related to closing of the water systems
Closing up of the water system to reduce fresh water consumption and effluent discharge from the mills results in the build-up of slime and stickies. This issue is extremely important, and creates huge quality and chemical usage issues unless the internal water management in the mills is handled correctly.
7.12    Environmental management
7.12.1    Effluent treatment
All the mills have high focus on effluent treatment. Small scale paper mills follow more or less the same regulations as larger scale mills, thus costs related to effluent treatment plant investment and energy/chemicals in effluent treatment systems are considerable.
Closing up of the water system and reduced fresh water consumption has resulted in serious issues related to TDS and colour (in case of agro-/ wood-based mills).
 
7.12.2    Disposal/usage of solid waste (plastic waste/ DIP sludge)
This issue is especially important for mills using recycled fibres as their raw material. It cannot be solved in the mill alone; the collection system should apply better sorting systems to minimize the amount of plastic in the raw material delivered at each unit.
 
7.12.3    Wet end operation and chemical optimization
This is a typical challenge for mills where a majority of the costs are related to chemical usage, but also important for mills with low chemical usage. Chemical usage optimization will also affect product quality and emissions.
 
7.12.4    Lack of process automation
Essentially, there is a huge potential for automation and system integrators to work collaboratively with India’s pulp and paper companies and help them acquire the competitive edge. This means paper mills in India have tremendous opportunity to improve their profit margin by increasing their investments in automation systems and enterprise solutions, and integrating them to achieve collaborative production management.
    Enterprise solutions, such as enterprise resource planning systems, manufacturing execution systems or collaborative production management systems, and supply chain management systems have not received adequate attention from the paper industry management. Also there exists a lack of coordination between the automation department and IT department within the mill. As a result, the pulp and paper industry in India lags behind its Asian and global counterparts.
There are not many mills that have integrated wet-end systems in the overall control strategy. The paper mill, is the formative stage in a papermaking process and any forward control strategy results in impressive gains in terms of quality. Likewise, energy, being the significant portion of production cost, is getting less attention in terms of monitoring the overall consumption of power across various sections of the plant.
8 Indian paper industry revitalizing for con-solidation    
Indian paper industry could observe a round of consolidation and co-operation amid different players in the next few years to together leverage quick changing manufacturing technologies and smoothen diffident addition for raw materials. This could leave less than 10 big players in the domestic market as next to the present 28 major ones. The industry, that is extremely reliant on wood pulp for manufacturing of paper and paperbased products, is also attempting to broaden its raw material base to lower cost of invention. This also means that there is vast potential for the area, which can be met through the use of new technologies. Indian paper industry has invested about Rs 20000 crore on capacity enhancement, machinery upgrade, and acquisitions. While the sector is enthusiastic to enlarge capacity further, decisions in this regard will rely on how soon companies can develop their financials. The sector, which faced issue from rising input (wood) prices, is now enhanced due to a renewed thrust on agro-forestry and softening of flesh costs. Now with the beginning of some state-of-the-art pulp and paper machines. The paper industry will meet the need of lower operating costs and superior quality.
9    Conclusions
Indian paper industry is likely to see marginal improvement in demand from education and corporate sectors, aided by expected higher GDP g, rowth , of the country. Though India’s per capita utilization is quite low compared to global peers, things are looking up and a requirement is set to rise from the present 13 million tons to an estimated 20 million tons by 2020.
The government’s continued focus on literacy, amplified consumerism, an increase in organized retail are predictable to positively affect paper consumption and demand in india. This indicates there is a lot of headroom for development of paper industry in India. Paper mills’ continuous efforts on farm forestry as well as higher wood prices have led to increased availability of wood in nearby areas, thereby reducing average wood procurement costs for mills. 
Industry expects major sector companies to report a marginal improvement in revenue growth to 7%~8% driven primarily by volume growth. It is believed that high capacity utilization, strong demand outlook, moving into environmentally friendly & value-added products and capacity expansion are key signs of the attractiveness of the industry over the medium to long-term.
In spite of the continual focus on digitization, India’s requirement for paper is anticipated to rise 53% in the next six years, principally due to a sustained boost in the number of school-going children in rustic areas. Growing consumerism, modern retailing, rising literacy and the growing use of documentation will continue to demand for writing & printing paper buoyant. The exponential enlargement of e-commerce in the nation has opened up the latest horizon and could donate significantly to the demand where the paper is being lengthily used for packaging.
Acknowledgment
The author expresses heartfelt gratitude to Isaksson Anders and his team for useful discussions and their inputs in preparation of the manuscript.
References
[1] Thapliyal B P, Singh K, Tandon Rita. The Indian Paper Sector Status Report 2017 & Sustainabilty Prospets[C]//A research paper in Proceedings of the 13th International Technical Conference on Pulp, Paper and Allies Industries. New Delhi, India, 2017: 1-10.
[2] Jain R K. Compendium of Census Survey of Indian Paper Industry[M]. Saharanpur, India: Central Pulp & Paper Research Institute, 2016: 10-30.
[3] Jain R K, Eriksen Oyvind, Isaksson Ander, et al. Development and Adoption of Appropriate Technologies for Enchancining Sustainability and Competitiveness of the Indian Paper Industry[C]//A research paper in Proceedings of the 13th International Technical Conference on Pulp, Paper and Allies Industries. New Delhi, India, 2017: 41-52.
      
 
 
 
 
 
 
 
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