Ind. Eng. Chem. Res. 2003, 42, 5445-5449
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Carboxyl Groups in Wood Fibers. 2. The Fate of Carboxyl Groups during Alkaline Delignification and Its Application for Fiber Yield Prediction in Alkaline Pulping X.-S. Chai, Q. X. Hou, and J. Y. Zhu* Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street, N.W., Atlanta, Georgia 30318
Detailed characterizations of the content of carboxyl groups in wood fibers derived from alkaline delignification of loblolly pine using various processes were conducted in this study (part 2) using the method presented in the preceding paper, part 1. It was demonstrated that the content of hexenuronic acid (HexA) groups can be used to account for the carboxyl groups associated with hemicelluloses in wood fibers. The difference between the measured contents of carboxyl and HexA groups can be used to account for the carboxyl groups associated with wood lignin. As a result, the calculated content of carboxyl groups associated with lignin correlated very well to the pulp number. An empirical correlation for predicting fiber yield with the measured contents of carboxyl and HexA groups was developed on the basis of mass conservation. Good agreement between measured and predicted fiber yield for the two wood species studied was obtained. Introduction Wood fibers derived from chemical pulping contain a significant amount of carboxylic acid groups. These carboxyl groups play an important role in wood fiber modifications for processing high-quality products. For example, carboxyl groups were reported to be responsible for the retention of various wet-end additives in fiber suspension.1-2 They also improved interfiber bonding during pressing and drying and consequently the mechanical properties of dried paper sheets.3-5 In part 1 of this study,6 we presented a rapid, accurate, and automated analytical method for the determination of carboxylic acid groups in wood fibers using headspace gas chromatography. The robustness of the method enabled us to conduct a systematic and detailed study of the effect of wood pulping and bleaching process conditions on the fate of carboxyl groups in wood fibers. The objective of this part of the study was to understand the fate of carboxyl groups during wood pulping and fiber bleaching processes, which could help optimize wood fiber production process conditions to achieve maximum preservation of carboxyl groups in wood fibers for producing high-quality materials. Specifically, this study was focused on the two most frequently practiced industrial processes: alkaline pulping and oxygen delignification. Experimental Section Alkaline Wood Pulping. All wood pulping experiments were carried out in 500 mL bomb digesters. Eight bomb digesters were mounted on a rotating drum and heated in a glycol oil bath. Oven dried (o.d.) loblolly pine wood chips (50 g) were used in each digester and subjected to digestion by the cooking liquor. The sulfidity (S, defined as the ratio of the concentrations of * To whom correspondence should be addressed. Current address: USDA Forest Products Laboratory, One Gifford Pinchot Dr., Madison, WI 53726. E-mail:
[email protected].
Na2S and (Na2S + NaOH) where the concentrations are expressed as of Na2O) of the cooking liquor was varied from 0% to 30%. The active alkali (AA, defined as NaOH + Na2S as of Na2O) charge on wood of the cooking liquor varied between 15% for low-sulfidity cooking (S ) 15%) and 18% for soda (S ) 0) and high-sulfidity (S ) 30%) cooking. The cooking liquor/wood ratio was 4:1. The temperature of the cooking liquor was ramped from 23 to 170 °C in 70 min (2.1 °C/min) and then maintained at 170 °C. Pulping time was varied for a set of digestions in different bombs conducted under the same pulping condition to obtain time-dependent information on carboxyl groups and other species (i.e., hexenuronic acid groups, HexA) in wood fibers. At the end of each pulping experiment, the fibers were completely disintegrated in a laboratory blender and thoroughly washed with tap water in a basket with 200-mesh screen. Fiber pads were then prepared in a handsheet machine for the measurement of fiber yields, kappa numbers, HexA, and carboxyl groups. A gravimetric method was used to determine the fiber yield based on the o.d. weight of the fiber obtained from pulping and the initial o.d. wood chips used in the digester. The chemical strength of the pulping liquor, such as active alkali charge and sulfidity, was determined using the ABC titration method.7 The wood pulp kappa number, a measure of the reactivity of the residual lignin in wood fibers, was determined by TAPPI standard method, T236 cm-85.8 Conventional UV spectroscopy was used to determine the dissolved lignin concentration in the pulping liquor. Alkaline Oxygen Delignification. A 1 L pressurized reactor equipped with a stirrer was used to conduct oxygen delignification of loblolly pine kraft fibers with kappa number of 20.9 and moisture content of 68.6%. The reacting solution was made of 5 mL of MgSO4 of concentration 20 g/L, 12.8 mL of NaOH of concentration 78.3 g/L, and 294.7 mL of distilled water. A 40 g sample of o.d. fibers was mixed with the reacting solution in the reactor with a fiber consistency of 10% and MgSO4
10.1021/ie0209733 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003
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Figure 1. Time-dependent profiles of content of carboxyl groups in wood fibers during conventional alkaline pulping of loblolly pine with different process conditions.
and NaOH charge on the fibers of 0.25% and 2.51%, respectively. The reactor was closed and set into the heating vessel. Oxygen was then injected into the reactor to pressurize the reactor to 7.8 atm (100 psig). The reactor was heated to 90 °C and maintained during the reaction process. The retention time was varied from 10 to 124 min for different batches of delignification to obtain time-dependent information. At the end of each experiment, the reactor was opened quickly, and the solution was squeezed out of the wood fibers into a sample bottle for carbonate analysis. The fiber slurry was filtered through a Buchner funnel and washed completely for kappa number determination. Determination of Chemical Compounds in Wood Fibers. The analysis of carboxyl groups in wood fibers was accomplished using phase conversion reaction (PRC) headspace gas chromatography (HSGC), which was presented in part 1 of this study.6 The spectroscopic technique for the measurements of HexA content in wood fibers that we developed9 was employed in the present study. In this method, 22 mmol/L (0.6%) mercuric chloride and 0.7% sodium acetate were used to make a hydrolysis solution. A known amount of wood fibers was added into a vial that contained the hydrolysis solution. Then, it was placed in a water bath with a temperature range 60-70 °C for 30 min. After the solution was cooled to room temperature, spectroscopic measurements of the absorption of the solution at 260 and 290 nm were conducted. The measured absorbances were used for HexA content calculations. Detailed descriptions of the methods can be found in our previous work.9 Results and Discussion Time-Dependent Carboxyl Group Content in Wood Fibers during Pulping. Figure 1 shows the time-dependent data of measured carboxyl group content in wood fibers derived from various chemical pulping processes of loblolly pine. It was found that carboxyl groups in wood fibers decrease continuously as alkaline pulping process proceeds after the pulping temperature reaches 170 °C. The measured carboxyl group content data were not accurate at an early stage of the pulping process because that wood chips were not completely disintegrated into fibers, causing erroneous measurements. Therefore, only the results obtained when the pulping temperature reached 170 °C were shown. Because carboxyl groups are mainly associated with lignin and hemicelluloses in wood, it is expected that their content in wood fibers decreases due to both
Figure 2. Time-dependent profiles of contents of HexA and carboxyl groups in wood fibers during conventional kraft pulping of loblolly pine.
lignin removal and hemicellulose dissolution as delignification proceeds. Moreover, both sulfidity and alkali charge affect carboxyl group content as they do delignification (Figure 1). High sulfidity and alkali charge and the addition of catalyst anthraquinone (AQ) facilitate delignification and thus increase the removal of carboxyl groups on fibers. The carboxyl group data reported in Figure 1 are the total carboxylic acid group contents in wood fibers that include those associated with both lignin and hemicelluloses. It will be interesting to identify the carboxyl groups that are associated with lignin in wood fibers, which can yield information on degree of delignification and fiber yield, as will be discussed later in this paper. 4-O-Methyl glucuronic acid is the only compound in hemicelluloses that is associated with carboxylic acid groups. It is well-known that 4-O-methyl glucuronic acid can be completely catalyzed by alkali to form HexA and methanol in alkaline solutions;10-11 therefore, we can use the amount of HexA on wood fibers to account for the amount of carboxyl groups associated with hemicelluloses in the fibers once the alkali catalyzed reaction of 4-O-methyl glucuronic acid is completed (typically after the pulping temperature reaches the maximum pulping temperature in batch cooking, i.e., about 60 min into the pulping process in this study). Figure 2 shows the time-dependent profiles of HexA, the content of the measured total carboxyl groups, along with the content of carboxyl groups associated with lignin in the wood fibers calculated by subtracting the HexA content from the total carboxyl group content measured from the same fiber sample. The wood fibers were obtained from four kraft pulping experiments that were terminated at different pulping times (all terminated after pulping temperature reached 170 °C to ensure complete catalyzation of 4-O-methyl glucuronic acid into HexA and methanol). The content of carboxyl groups associated with lignin in wood fibers decreases as delignification proceeds. The content of HexA in wood fibers also decreases slightly as pulping proceeds due to the dissolution of hemicelluloses. The amount of carboxyl groups associated with lignin is about the same as that associated with hemicelluloses for wood fibers obtained from the cooking that lasted for 160 min. The maximum amount of HexA was found to be about 70 µmol/g fiber, indicating that the carboxyl groups associated with hemicelluloses in wood are about 42 µmol/g wood based on the estimated fiber yield of 0.60. Relationship between Carboxyl Groups and Dissolved Lignin/Kappa Number of Pulp. The time-
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Figure 3. Relationship between content of carboxyl groups associated with lignin in wood fibers and dissolved lignin in the pulping liquor.
Figure 4. Relationship between content of carboxyl groups associated with lignin in wood fibers and wood pulp kappa number in various alkaline pulping processes.
dependent content of carboxyl groups associated with lignin in wood fibers should be correlated to the degree of delignification, as discussed previously. Figure 3 shows the relationship between carboxyl groups associated with lignin and the dissolved lignin measured in the pulping liquor. It was found that the carboxyl groups associated with lignin are inversely proportional to the dissolved lignin in the pulping liquor according to the following universal linear equation, independent of the pulping process parameters:
y ) 261.8 - 2.678x
(1)
where y is the carboxyl group content in wood fibers (in µmol/g fiber) and x is dissolved lignin concentration (in g/L) in the pulping liquor. The slope of eq 1 varies with wood species and the ratio of wood to cooking liquor. The intercept on the vertical coordinate of 261.8 µmol/g of the linear correlation equation can be considered as the total carboxyl groups in the initial wood chips. The inverse linear relationship indicates that using the measured HexA content in the wood fibers can account very well for the carboxyl groups associated with hemicelluloses in the fibers. Furthermore, the inverse linear relationship also suggests that there is no change in the amount of carboxyl groups associated with lignin throughout the pulping process. Subtracting the carboxyl groups associated with hemicelluloses from the total measured carboxyl groups results in an excellent approximation to the carboxyl groups associated with lignin. Pulp kappa number is a measure of the reactivity of residual lignin on wood fibers and is often used in industrial practice to measure the degree of delignification. It is well-known that kappa number is linearly proportional to the content of residual lignin (Klason lignin) on fibers.12 Recent studies13-15 indicate that HexA groups on wood fibers can also react with oxidants (such as potassium permanganate or bleaching chemicals) to contribute to kappa number, especially for fibers derived from hardwood species. However, the relative contribution from HexA to kappa number for unbleached softwood fibers is not as significant as that for unbleached hardwood fibers due to the high lignin content in unbleached softwood fibers. The data obtained from loblolly pine (a softwood) fibers in the present study indicate that the linear relationship between Klason lignin on wood fibers and kappa number is not distorted significantly by the contribution of HexA to kappa number. A universal linear correlation
Figure 5. Time-dependent profiles of the contents of carboxyl and HexA groups in wood fibers during a process oxygen delignification of wood fibers.
between the calculated carboxyl groups associated with lignin and pulp kappa number was obtained, independent of pulping process conditions (Figure 4). Equation 2 shows the least-squares fit of the data in Figure 4 and can be used to estimate the carboxyl groups associated with lignin on wood fibers on the basis of the pulp kappa number:
y ) 28.3 - 0.938x
(2)
Figures 3 and 4 indicate that a large amount of carboxyl groups that are associated with wood lignin will be removed through the delignification process. It is not possible to preserve these carboxyl groups through changing pulping process conditions. Our previous study11 indicated that it was possible to reduce HexA dissolution and thereby preserve carboxyl groups associated with hemicelluloses by changing pulping process conditions. Reduction of Carboxyl Groups in Fibers during the Oxygen Delignification Process. Figure 5 shows the time-dependent profiles of the contents of carboxyl groups and HexA in wood fibers during an oxygen delignification process. The data demonstrate that the content of carboxyl groups in wood fibers decreases continuously as oxygen delignification proceeds; however, the dissolution of HexA is negligible. Therefore, the measured carboxyl groups in wood fibers should be directly related to delignification. The linear relationships between the carboxyl group content and dissolved lignin in the oxygen bleaching stream and pulp kappa number are displayed in Figures 6 and 7, respectively. Fiber Yield in Alkaline Pulping. Wood fiber yield (defined as the amount of o.d. wood fibers resulted from
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Figure 6. Relationship between content of carboxyl groups associated with lignin in wood fibers and dissolved lignin in the bleaching stream of oxygen delignification.
Figure 8. Comparison of measured fiber yields with those predicted by the empirical correlation of eq 7 for various alkaline pulping processes.
mine carbohydrates in the spent pulping liquor. Instead of relying on the composition of the spent pulping liquor to obtain fiber yield, we focused on the wood fiber itself for fiber yield estimation. From eq 1, we can estimate lignin removal through pulping, i.e.
∆Mlignin ) k′1[A1 - (carboxyl - HexA)]
Figure 7. Relationship between content of carboxyl groups associated with lignin in wood fibers and wood pulp kappa number in an oxygen delignification process.
pulping as a percentage of o.d. wood chip used at the beginning of cooking) is an important parameter in wood fiber production. Unfortunately, it is very difficult to obtain an accurate fiber yield in industrial wood pulping practice. A very primitive method16 that uses a basket containing wood chips suspended in a digester was often used for yield determination in industrial practice. One of the objectives of this study is to develop an empirical correlation for fiber yield determination from the measured content of acid groups in wood fibers. Wood consists mainly of celluloses, hemicelluloses, and lignin, though the composition varies with wood species. Therefore, the mass of wood can be expressed as
Mwood ) Mcelluloses + Mhemicelluloses + Mlignin + Mother (3) During wood pulping, lignin removal is also accompanied by the loss of carbohydrates, mainly hemicelluloses. Therefore, fiber yield can be estimated by measuring the amounts of these compounds lost in the pulping process. Lignin removal can be easily accounted for by the measurements of dissolved lignin in the spent pulping liquor. However, no simple method can deter-
(4)
where A1 ) 261.8 µmol/g fiber is the maximum attainable carboxyl groups associated with lignin based on the data presented in Figure 3. The loss of carbohydrate can be mainly attributed to the dissolution of hemicelluloses, which can be accounted for by the reduction of HexA from its maximum value through the pulping process, i.e.
∆Mhemicellulose ) k′2(A2 - HexA)
(5)
where A2 ) 70 µmol/g fiber is the maximum attainable HexA content based on the data presented in Figure 2. A1 and A2 are constants for a given wood species. Assuming that there is no loss of cellulose and the loss of other compounds (e.g., extractives) is a constant for a given species, then fiber yield can be expressed as
yield (%) ) 100 - k1[A1 - (carboxyl - HexA)] k2(A2 - HexA) - k3 (6) where k1 and k2 are calibration (empirical) constants and k3 is the yield loss of other materials including extractives and other unaccounted losses such as reject loss in the pulping process. For simplicity, eq 6 can be expressed as
yield (%) ) y0 + y1 × carboxyl + y2 × HexA (7) Multiple linear regression analysis was conducted for the 11 experiments of various pulping process conditions presented in Figure 1 to obtain the correlation coefficients y0 ) 35.84, y1 ) 0.1482, and y2 ) -0.1353 (unit of carboxyl groups and HexA is in µmol/g fiber). It
Table 1. Comparisons between Values Predicted by Equation 7 with y0 ) 46.16, y1 ) 0.1352, and y2 ) -0.2036 and Measured Fiber Yields of Laboratory Alkaline Pulping of Maple sulfidity (%)
alkali charge AA (%)
carboxyl groups (µmol/g fiber)
HexA (µmol/g fiber)
measured yield (%)
predicted yield (%)
relative difference (%)
31
16
96.91 127.78 123.66 110.65 146.76
60.50 74.79 58.70 61.64 32.39
46.78 48.87 48.32 50.14 59.98
46.95 48.21 50.93 48.57 59.41
0.4 -1.4 5.4 -3.1 -1.0
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should be pointed out that these empirical coefficients are functions only of wood species, independent of pulping conditions, which makes the correlation very useful. The r2 of the fit was 0.916. The estimated standard deviation of the fitted fiber yield was only 1.5%, indicating the validity of the approach taken to derive fiber yield in this study. Figure 8 compared the experimentally measured fiber yield with those predicted by eq 7. A near-unity linear correlation between the measured and predicted yields was obtained with proportional coefficient of 0.999 and r2 ) 0.916. To further demonstrate the validity and applicability of eq 7 for fiber yield prediction in alkaline pulping, we applied eq 7 to another set of laboratory alkaline pulping experiments of maple13 (a hardwood). The fiber yield of the five experiments ranged from 46% to 60%. The contents of carboxyl acid and HexA groups were measured using the method and procedures that we developed earlier.6,9 Table 1 compares the measured and predicted fiber yields. The maximum relative difference is about 5%. Considering that the standard deviation of the measured yield of around 1% is equivalent to a relative difference of 2% (assuming 50% fiber yield), the fiber yields predicted by eq 7 are excellent. Conclusions Detailed characterizations of the content of carboxyl groups in wood fibers of loblolly pine derived from various alkaline delignification processes and conditions were conducted in this study. The results indicated that the content of HexA groups on wood fibers can be used to account for the carboxyl groups associated with hemicelluloses in wood fibers. The difference between the measured contents of carboxyl and HexA groups in wood fibers linearly correlates very well with the dissolved lignin in the pulping liquor or in the bleaching streams, indicating that this difference can be used to account for the amount of carboxyl groups associated with residual lignin in wood fibers. The calculated carboxyl groups associated with lignin linearly correlated with pulp kappa number well, as expected. One of the most important contributions of this study is the development of an empirical correlation for the prediction of fiber yield in alkaline pulping using the measured contents of HexA and carboxyl groups based on mass conservation. The predicted fiber yields of the 16 alkaline pulping experiments of loblolly pine and maple showed good agreement with those measured. Acknowledgment This work was supported by the state of Georgia (Grants PP99-MP-9 and PP02-MP-03) and the U.S. Department of Energy (DE-FC07-96ID13438).
Literature Cited (1) Wagberg, L.; Bjorklund, M. On the Mechanism behind Wet Strength Development in Papers Containing Wet Strength Resins. Nordic Pulp Pap. Res. J. 1993, 8, 53. (2) Isogai, A.; Kitaoka, C.; Onabe, F. Effects of Carboxyl Groups in Pulp on Retention of Alkylketene Dimer. J. Pulp Pap. Sci. 1997, 23 (5), 215. (3) Katz, S.; Liebergott, N.; Scallan, A. A Mechanism for the Alkali Strengthening of Mechanical Pulps. Tappi J. 1981, 64 (7), 97. (4) Scallan, A. M. The Effect of Acidic Groups on the Swelling of Pulps: A Review. Tappi J. 1983, 66 (11), 73. (5) Barzyk, D.; Page, D. H.; Ragauskas, A. Acidic Group Topchemistry and Fiber-to-Fiber Specific Bond Strength. J. Pulp Pap. Sci. 1997, 23, 59. (6) Chai, X.-S.; Hou, Q. X.; Zhu, J. Y.; Chen., S.-L.; Wang, S. F.; Lucia, L. Carboxyl Groups in Wood Fibers. 1. Determination of Carboxyl Groups by Headspace Gas Chromatography. Ind. Eng. Chem. Res. 5440-5444. (7) Alkaline Pulping. In Pulp and Paper Manufacture, Vol. 1: Pulping of the Wood, 2nd ed.; MacDonald, R. G., Ed.; McGrawHill: New York, 1969; p 563. (8) Kappa Number of Pulp. In TAPPI Test Methods; TAPPI Press: Atlanta, Georgia, 1996; T236 cm-85. (9) Chai, X.-S.; Zhu, J. Y.; Li, J. A Simple and Rapid Method to Determine Hexenuronic Acid Groups in Chemical Pulp. J. Pulp Pap. Sci. 2001, 27 (5), 165. (10) Clayton, D. W. The Alkaline Degradation of Some Hardwood 4-O-Methyl-D-Glucuronoxylans. Sven. Papperstidn 1963, 66 (4), 115. (11) Chai, X.-S.; Yoon, S.-H.; Zhu, J. Y.; Li, J. The Fate of Hexenuronic Acid Groups During Alkaline Pulping of Loblolly Pine. J. Pulp Pap. Sci. 2001, 27 (12), 407. (12) Tasman, J. E.; Berzins, V. The Permanganate Consumption of Pulp Materials: III The Relationship of the KAPPA Number to the Lignin Content of Pulp Materials. Tappi 1957, 40 (9), 699. (13) Chai, X.-S.; Luo, Q.; Yoon, S.-H.; Zhu, J. Y. The Fate of Hexenuronic Acid Groups During Kraft Pulping of Hardwoods. J. Pulp Pap. Sci. 2001, 27 (12), 403. (14) Gellerstedt, G.; Li, J. An HPLC method for the quantitative determination of hexenuronic acid groups in chemical pulp. Carbohydr. Res. 1996, 294, 41. (15) Ikeda, T.; Hosoya, S.; Tomimura, Y.; Magara, K.; Ishihara, M. Contribution of LCC Bond Cleavage to the Kappa Number Reduction of Kraft Pulp. Proceedings of the 9th International Symposium on Wood Pulping Chemistry; Techn. Sect.; CPPA: Montreal, 1997. (16) MacLeod, M.; Radiotis, T.; Uloth, V.; Munro, F.; Tench, L. Basket Cases IV: Higher Yield with Paprilox Polysulfide-AQ Pulping of Hardwoods. Proceedings of 2001 Pulping Conference; TAPPI Press: Atlanta, GA, 2001.
Received for review December 3, 2002 Revised manuscript received July 30, 2003 Accepted August 2, 2003 IE0209733
