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Ind. Eng. Chem. Res. 2008, 47, 8430–8435
Energy Saving in Papermaking through Filler Addition Chunxu Dong,†,‡ Delong Song,†,‡ Timothy Patterson,‡,§ Art Ragauskas,‡,| and Yulin Deng*,†,‡ School of Chemical and Biomolecular Engineering, Institute of Paper Science and Technology, School of Mechanical Engineering, and School of Chemistry and Biochemistry, Georgia Institute of Technology, 500 10th Street N.W., Atlanta, Georgia 30332
The main purposes of filler addition in papermaking are to improve paper properties and reduce cost. The cost reduction is attributed to the low cost of the filler itself and to the energy savings that occur during the papermaking process. In this work, the effects of clay filler addition on the energy savings during the papermaking process, including water drainage, pressing, and drying, were systematically investigated. Experimental results indicated that the addition of filler could dramatically increase the drainage rate and the water removal rates during pressing and drying. With 20% filler addition, the drainage time decreased by 20% compared to the unfilled paper. At a filler content of 23%, the solids content of the handsheets after pressing increased at least 5 absolute points, and the drying rates increased by 20%. All these improvements can offer considerable benefits for paper mills, including enhanced machine speeds and large energy savings. Introduction Papermaking is a water-based manufacturing process in which dewatering processes, including drainage, pressing, and evaporating (drying), are critical steps.1-5 In the formation/wire section of paper machines, most of water in the furnish is drained by gravity or vacuum. The solids content of the wet web leaving the wire is around 20-25%. In the press section, more water is removed via mechanical pressing, and the solids content of web can be increased to 40-50%. The wet web after press section enters the dryer section and is further dried to a solids content of 90-95%. Water removal is a costly process, in term of energy, but the relative amounts of energy required for drainage, pressing, and drying are not the same. Although only a very small portion of total water (about 1% or less) is removed in the drying section,4,5 the energy consumption in this part is intensive. It is reported that about 60% of energy consumption, or about 80% of steam consumption in paper mills, occurs at the drying section.2,6 Normally, the ratio of energy cost in the drying section to that in press section is more than 15:1.7 It has been suggested that, for 1% solids content increase in the press section, 4% energy can be saved in the drying section.7 Therefore, any improvement in water removal during the pressing would directly result in considerable cost savings. Increasing the solids content of the web entering the drying section and improving the drying rate are two effective means to reduce the energy cost of drying. Water removal during papermaking is a complicated process. It is well accepted that there are basically two types of water in a wet web, i.e. free water (also called unbound water) and bound water.4 Free water is mainly located in the macropores of web (between fibers or in fiber wall and lumens). Bound water is distributed in the micropores in the fiber walls. The bound water can be further divided into freezable bound water and nonfreezable bound water.5 The former can freeze at a lower temperature under the normal freezing point. However, the nonfreezable bound water does not freeze at all because of its chemical interactions with the cellulose. Park et al. provided a * Corresponding author. E-mail:
[email protected]. † School of Chemical and Biomolecular Engineering. ‡ Institute of Paper Science and Technology. § School of Mechanical Engineering. | School of Chemistry and Biochemistry.
review of the water types in cellulose fibers.5 They concluded that free water during drying is removed first followed by freezing bound water, and nonfreezing bound water was hardest to be evaporated. Addition of fillers to the papermaking furnish has a long history in the paper industry.8 In modern papermaking, filler is actually the second most used raw material in some paper grades. With filler addition, some paper properties, such as opacity, smoothness, and printing quality, can be improved. However, reducing the papermaking cost is also a reason to use fillers. As mineral fillers are less expensive than wood fibers, the cost of raw materials of papermaking can be reduced with the addition of filler. Furthermore, the addition of filler can also significantly improve the efficiency and economics of papermaking process. It is believed that the addition of filler in papermaking can improve drainage and water remove rates in the pressing and drying process.9-13 Liimatainen et al. found that the filtration resistance of beaten pulps was reduced by adding precipitated calcium carbonate (PCC) fillers, which results in improved drainage.9 Gerteiser and Laufman also found that the use of rhombohedral calcium carbonate could facilitate drainage.10 Kenage and co-workers concluded that the addition of ground limestone could increase the solids content of web entering the dryer section, and the drying rates of filler loaded papers were also higher than those of paper without fillers.11 Zhao et al. found that the dewatering in the pressing section could be enhanced with the addition of clay fillers based on a pilot trial.12 In addition, Allan et al. suggested that the presence of calcium carbonate filler within the fibers could significantly reduce water-holding capacity of fibers, thereby leading to faster drainage and higher drying rates.13 Although the improvement of water removal caused by filler addition was reported by many researchers, these results are either mainly based on mill observations or limited measurement in laboratory, and a systematic study on the effect of filler addition on drainage, pressing, and drying has not been conducted. In this study, the effects of clay filler on the water removal behaviors were investigated. The related mechanism of water removal was discussed. The energy saving by filler addition was also calculated. A package board grade sheet with a basis weight of 200 g/m2 was selected as the target products
10.1021/ie8011159 CCC: $40.75 2008 American Chemical Society Published on Web 10/03/2008
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Figure 3. Effect of clay filler on drainage time of linerboard pulp. Figure 1. The schematic drawing of the dynamic drainage analyzer (DDA).
Figure 2. A typical curve of vacuum vs drainage time.
because the energy required for production of such high basis weight grades is high. Experimental Section Materials. Unbleached kraft pulp (60% hardwood and 40% soft wood) was refined in a Valley beater to a freeness of 400 mL Canadian Standard Freeness (CSF). Georgia’s Kaolin clay filler with a surface area of 11 m2/g and aspect ratio of 80 was provided by Imerys (Atlanta, GA). Percol-175 (high molecular weight, low charge density, cationic polyacrylamide retention aid) was obtained from Ciba Specialty Chemicals (Suffolk, VA). Drainage Test. The drainage test was performed by using a dynamic drainage analyzer (DDA), and its drainage unit was illustrated in Figure 1. With a shear force in stock and vacuum applied, the DDA can simulate the drainage on paper machines. The linerboard pulp with various amounts of clay was diluted to a total solid consistency of 0.5 wt% for all DDA tests. A 5 ppm retention aid of Percol-175 (0.1 wt% based on total solid weight) was added at a stirrer speed of 800 rpm. As the drainage process starts, water begins to go through the forming wire and flow to the vacuum vessel which has a vacuum of 0.2 bar. When air starts being sucked through the sheet, there will be a sudden drop of vacuum, as shown in Figure 2. The duration from the starting point to the point with the vacuum sudden drop is reported as drainage time. Handsheet Preparation. Handsheets were made by using a British handsheet mold. The linerboard pulp was diluted to 1 wt%, and various amounts of clay fillers were added. Retention aid Percol 175 was used to retain fine matter (wood fines and filler particles) at 0.1 wt% based on total solid weight for both filled and unfilled paper sheets. After the addition of filler and retention aid, the slurry was stirred for 20 s at 1000 rpm. Handsheets with a target basis weight of 200 g/m2 were produced. After a cursory wet pressing (50 psi, 5 min), all
handsheets were cut to a standard size for the water removal tests. The filler content was determined by ashing the paper in a muffler oven according to the standard TAPPI method T211. Water Removal Tests (pressing and drying). The wet pressing test was performed by using a MTS testing apparatus, which consists of three main parts: test frame, MTS 458.20 Micro Console, and PC-based mate. The sample with a felt (about 15% moisture content) was pressed at a certain pressure (610 or 1150 psi) for several cycles. The contact time for each press cycle is about 0.03 s. The mass of sheet samples before and after press was measured immediately. After the wet pressing test, the sample was dried at 105 °C by a Mettler Toledo LJ16 moisture analyzer. The weight of the sheet was recorded every 30 s, and the rate of water removal was calculated. Thermal Analysis. The thermal behaviors of fiber-water system and filler-water system were investigated by using the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. The TGA traces were obtained by using a TG/DTA320 instrument (Seiko Instruments Inc., CA) in a heating mode from 35 to 280 °C with a heating rate of 10 °C/min in a nitrogen atmosphere. The samples were all about 6 mg, and the initial water content was about 48 wt%. The DSC tests were performed using a DSC 2500 instrument (Seiko Instruments Inc., CA), and all the samples were cooled from 20 to -40 °C with a rate of 10 °C/min. Contact Angle Measurement. Dynamic contact angles of water were measured at 25 °C by a FTÅ200 dynamic contact angle analyzer (First Ten Ångstroms, Portsmouth, VA). The fully dried sheet samples were put on the glass slides, and a deionized water drop was put on the sample using an automatic syringe pump. After the water droplets contacted the surface of the sheet for 10 s, the images of water drops were captured, and the contact angles were calculated automatically using those images. Results and Discussion Effects of Fillers on Drainage Rates. The drainage behavior, as an indication of a specific filtration resistance, is a critical issue during sheet formation. For a given type of fiber, its refining level, filler addition amount, additive types, and additive dosages are important variables which can affect the drainage rates. We kept all the other conditions fixed in this study. From Figure 3 we can see that the drainage time decreased dramatically with the addition of filler (by replacing fibers with filler), agreeing well with previous work.9,10 In this case, the drainage time decreased by 20% from 9.4 s to 7.5 s by adding 20% filler. We believe that the improvement of drainage is attributed to the hydrophobicity and high porosity of fillers. It is reported
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Figure 4. Solid content of handsheets after pressing. Figure 6. Effects of fillers on water removal rate (directly after the wet pressing section).
Figure 5. The effect of press cycles on solids content of handsheets (1150 psi).
that wood fibers adsorb more water than mineral fillers.4 For the handsheets with the same basis weight, increasing filler content means a decrease in the amount of fibers and a change in the internal pore structure of the handsheets, resulting in a lowered the total water absorbability and increased drainage rates. On the other hand, a more open structure of fiber web can be formed after the addition of filler particles, which facilitates water drainage.9,10 Effects of Fillers on Water Removal in the Wet Pressing Section. In the press section, water is removed by mechanical means as much as practically possible. Typically the solids content of web leaving the press section is about 45%. In this study, the pressing behavior of handsheets with different filler contents was determined. Figure 4 shows the effects of filler content on solids content of handsheets after wet pressing. The ingoing solids content of the sheets was about 25%. For a given pressure, the solids content increased almost linearly with the increase of filler addition. In order to simulate multinip press in paper mills, press with multicycles was also conducted. Results can be seen from Figures 5 that, for a given filler loading level, the solids content of handsheets increased with the increase of press cycles, and the slopes for the handsheets with different filler addition levels are almost the same. The second press could provide around a 4 point increase of solids content. In the following three press cycles (i.e., third, fourth, fifth press cycle), the increase of solids content is only 1-2 points. That means it is getting harder and harder to remove water when the solids content is at a high level, in this case approaching 40%. In addition, the more filler the handsheets contain, the higher solids content could be obtained for each press cycle. In this experiment, only the handsheets containing the highest filler levels (i.e., 23%) reached solids content higher than 45%. The increase of water removal efficiency under mechanical pressure by adding filler is mainly due to the different absorb-
Figure 7. Effects of filler content on water removal rate (the same initial water content).
abilities of fiber wall and filler particles. In general, cellulose is hydrophilic, the fibers have a lumen inside, and the wetted cellulose walls contain many micropores which contain bound water. In contrast, Kaolin clay is an inorganic and dense material. It is impossible for water molecules to penetrate into the filler particles, and water mainly exists as free water around the particles. That means, for a fixed solids mass and total water content, the higher the filler content, the greater the amount of free water. Because the pressing is mainly to remove the free water, in the press section, more water can be removed from filler loaded papers, and a higher solids content can be achieved. Effects of Fillers on Water Removal in the Drying Section. Although a significant portion of water in the sheet was removed in the press section, the paper sheet still contains a large amount of free water and bound water, which is either located within fibers or bonded on fiber surface via hydrogen
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Figure 8. The heat flow of handsheets at different water contents (left: without filler; right: with 23% filler addition).
Figure 9. The energy used to release frozen water.
Figure 10. TGA curves of the fibers with different filler content (the same initial water content).
Figure 11. Schematic diagram of water within a pore of paper. It is shown that a large contact angle (less hydrophilic) results in a high meniscus radius r.
bonding.1 To remove this water, thermal drying is needed. In this study, the handsheets after wet pressing were dried using a moisture balance, which allowed the change of solids content
with time to be measured. We choose the solids content of 93% (a common solid content at the reel in a real papermaking process) as a full dry point. In practice, however, the moisture content of filler loaded paper sheet probably may be lower than that in the case of a corresponding unfilled sheet since the filler density is different from fiber density. This issue needs to be considered by future researchers. It clearly shows in Figure 6 that the higher the filler content in paper, the faster the paper can reach the targeted dryness. For unfilled paper, 9 min are needed to fully dry the sheet. However, for papers with 23% fillers, the required fully dry time is only 6.5 min, suggesting the drying time can be reduced about 28% if 23% filler is loaded in paper. Because the handsheets carried out different amounts of water after wet pressing, the initial solids contents of handsheets for the above drying tests varied. Therefore, the difference in the drying time discussed above does not solely reflect the drying performance but takes into account both the solids content of web entering the drying section and the rate of water removal via evaporation. In order to evaluate the pure effect of drying section, we carried out a second set of tests in which the initial solids contents of all sheets were adjusted to the same level. Figure 7 illustrates the water drying time for three different sheets with the same initial solid content of 37%. Although the curves are almost identical during the initial drying process, they split when the solids content reaches 80%. This difference becomes larger when the target dryness (93% paper solid content) is achieved. Figure 7b indicates that 7.5 min are needed to reach 93% dryness for unfilled paper, and only 6.5 min are required for 23% filler containing paper to achieve the 93% dryness, suggesting about 13.3% drying time reduction in the drying section only. Therefore, it can be concluded that not only can the solid content of the sheet be significantly increased but the water evaporation rate on a dryer can be also improved by adding fillers, although the effects of the latter are less significant than those the former. Mechanism of Water Removal. Thermal Analysis of Drying Process. During the drying process of wet web, free water is evaporated first, followed by freezing bound water, and then nonfreezing bound water, which is the most difficult to be removed.5 That is to say, the total drying time of wet web is highly dependent on the amount of different types of water, and DSC measurements were conducted to obtain some information about it. Figure 8 illustrates the heat flow at different temperatures for unfilled paper and filler loaded paper. In order to quantitatively compare the frozen water contents in different samples, the adsorbed heats, which are the signal peak areas in
8434 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Figure 12. Contact angles of water on the fiber sheet (a) and clay sheet (b) after 10 s.
the figures, were determined by integral approach, and the data are summarized in Figure 9. It is clearly shown that the total energy of phase change (corresponding to free and freezingbound water in sheets) for paper with clay fillers was larger than that for paper without clay filler, which means there was more free water and freezing-bound water in the filler loaded sheets. As the total water contents (i.e., initial water contents) are the same, the amount of nonfreezable bound water in filler loaded paper is less than that in unfilled paper. Therefore, the filler loaded paper can be dried faster than unfilled paper. The TGA tests were also conducted to evaluate the water removal rate. All samples used for tests had the same initial weight and water content. From Figure 10, we can see that the water in pure clay fillers is evaporated faster than that in wood fibers. The results suggest that the addition of filler in paper can improve the drying rates. Thermodynamics of Water Evaporation. The driving force to evaporate water is vapor pressure. The higher the vapor pressure, the faster the rate of water evaporation. The bound water in paper mainly locates in micropores, which is shown in Figure 11. The vapor pressure can be determined by the Kelvin equation: pr 2σM )(1) p Fr where R is the universal gas constant, T is temperature, pr is the actual vapor pressure above the meniscus surface, p is the saturated vapor pressure at a flat surface, σ is the surface tension, M is the molar weight, F is the density, and r is the radius of the bubble. It can be seen that the vapor pressure above a curved surface is an indication of the evaporation rate of liquid, and it increases with the increase of the radius of bubble. Figure 11 indicates that the larger the contact angle, the larger the radius of the bubble, corresponding to a larger vapor pressure and fast drying rate. Figure 12 illustrates the water drops on two different samples (a pure fiber sheet and a clay plate) after 10 s. It is clear that the contact angle of the water drop on the clay sheet was much higher than that on the fiber sheet. This means the vapor pressure of water in a sheet will increase with the addition of clay, and a higher water evaporation rate can be achieved. This result would suggest that the addition of the clay filler might increase the drying rate of the paper sheets. Energy Savings. Since it is more economical to remove water by pressing than evaporation, eliminating more water in the wet pressing section, i.e. achieving higher solids contents, can substantially save thermal energy in the drying section. For example, we track 1 kg of oven-dried fiber. If the solids content of wet web is 41%, the mass of water contained in the web would be: RT ln
m1 )
1 kg ( 41% ) × (1 - 41%) ) 1.439 kg
(2)
If the solids content of wet web is 46%, the mass of water contained in the web would be: m2 )
1 kg ( 46% ) × (1 - 46%) ) 1.174kg
(3)
The mass difference of water in webs with the above different solids content would be: m ) m1 - m2 ) 1.439 -1.174 ) 0.265 kg
(4)
That means, if the solids content of web entering the dryers increases from 41% to 46%, the total water which needs to be evaporated would decrease 0.265 kg/kg fiber. The saved heat energy can be calculatged using the following equation: Q ) m × CP × (T2 - T1) + m × ∆VH
(5)
where m is the mass of water, CP is specific heat capacity of water (4.2 × 103 J kg-1 °C-1, assuming it is a constant during the drying process), ∆VH is specific evaporation enthalpy of water, 2260 kJ kg-1; T1 is room temperature, 25 °C; T2 is evaporation temperature, 100 °C. As calculated above, the saved heat energy in the pressing section is 682.4 kJ/kg dry paper. Conclusions The effects of filler addition on behavior of drainage, pressing, and drying were systematically investigated. The results indicated the following: • With the other conditions fixed, the drainage time decreased with the addition of filler. At 20% filler loading level, the drainage time decreased by 20%, compared to the drainage time of stock without fillers. • For a given pressing condition, the higher the filler addition level, the higher the solids content of web leaving the press section. With 23% filler loaded, the solids content increased 5 absolute points, which leads to an energy saving of 682.4 kJ per kg dry paper and a 15% decreased drying time. • The TGA test indicated that the higher the filler content in paper, the faster the sample could be dried. • The higher rate of water evaporation of web containing fillers is attributed to two factors, including lower nonfreezing bound water content, and higher vapor pressure. The results of DSC results indicated that the nonfreezing-bound water content in filler loaded sheets was lower than that in unfilled papers. From the Kevin equation and contact angle results, we found that the water in filler loaded papers had a higher vapor pressure than that in unfilled papers. In general, the addition of filler could significantly improve water removal, resulting in energy saving or enhanced paper machine speeds. Acknowledgment The authors thank the member companies of the Institute of Paper Science and Technology, Georgia Institute of Technology,
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and the Georgia’s Traditional Industries in Pulp and Paper Production (TIP3) Manufacturing Process for their support of this research.
(8) Griggs, W. H. Use of Fillers in Papermaking. Tappi 1988, 71, 77– 81.
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(10) Gerteiser, N.; Laufmann, M. Effects of Natural Ground Calcium Carbonate on Wooddfree Papermaking Process and Paper Quality. Appita 1989, 42 (4), 295–300.
(1) Chance, J. L. Overview of the Dryer Section. In TAPPI 1991 Practical Aspects of Pressing and Drying Short Course Notes; TAPPI Press: Atlanta, GA, 1991; pp159-174. (2) Britt, K. W.; Unbehend, J. E.; Shridharan, R. Observations on Water Removal in Papermaking. Tappi 1986, 69, 76–79. (3) Attwood, D. How the Fiber-Water Relationship Affects the Drying Process. Paper Trade J. 1971, 155 (6), 31–35. (4) Karlsson M. Papermaking Part 2: Drying; Tappi Press: Atlanta, GA, 2000. (5) Park, S.; Venditti, R. A.; Jameel, H.; Pawlak, J. J. Hard-to-remove Water in Cellulose Fibers Characterized by Thermal Analysis: A Model for the Drying of Wood-based Fibers. Tappi 2007, 6, 10–16. (6) Peel, J. D. Paper Science and Paper Manufacture; Angus Wilde Publications Inc.: Vancouver, B.C. Canada, 1999. (7) Bermond, C. Establishing the Scientific Base for Energy Efficiency in Emerging Pressing and Drying Technologies. Appl. Thermal Eng. 1997, 17, 901–910.
(9) Liimatainen, H.; Kokko, S.; Rousu, P.; Niinimaki, J. Effect of PCC Filler on Dewatering of Fiber Suspension. Tappi 2006, 5 (11), 11–17.
(11) Kenaga, D. L.; Moore, E. E.; Meath, K. R. Drying Rate Studies of Ultrahigh Filled Paper and Paperboard. Tappi 1982, 65, 57–61. (12) Zhao, Y.; Kim, D.; White, D.; Deng, Y.; Patterson, T.; Jones, P.; Turner, E.; Ragauskas, A. J. Developing a New Paradigm for Linerboard Fillers. Tappi 2008, 7, 3–7. (13) Allan, G. G.; Carroll, J. P.; Devakula, M. L. P.; Gaw, K.; Joseph, A. A.; Pichitlamken, J. The Effect of Filler Location on the Drainage, Pressing, and Drying of Pulp and Paper. Tappi 1997, 80, 175–179.
ReceiVed for reView July 18, 2008 ReVised manuscript receiVed August 18, 2008 Accepted August 21, 2008 IE8011159
