Ind. Eng. Chem. Res. 2002, 41, 1955-1959
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kraft Pulping of Eucalyptus globulus: Kinetics of Residual Delignification Miguel A. Gilarranz,*,† Aurora Santos,‡ Julia´ n Garcı´a,‡ Mercedes Oliet,‡ and Francisco Rodrı´guez‡ Area de Ingenierı´a Quı´mica, Facultad de Ciencias, Universidad Auto´ noma de Madrid, Cantoblanco, 28049 Madrid, Spain, and Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, Universidad Complutense de Madrid, 28040 Madrid, Spain
Kraft pulping of Eucalyptus globulus was studied in order to develop a kinetic model for residual delignification. Pulping runs were carried out in the temperature range of 150-180 °C and the hydroxide ion concentration range of 0.7-1.6 M. A mean value of 95.5% was found for lignin conversion at the transition between bulk and residual delignification. A dependence of the transition point on pulping conditions was observed. The influence of cooking temperature on the rate constant was expressed by an Arrhenius-type equation. A power-law function was employed to model hydroxide ion concentration influence. The activation energy obtained was 86 kJ/mol, whereas a value of 0.59 was calculated for the kinetic order of the hydroxide ion concentration. Introduction Recently, developments in pulping systems have enabled the extension of the cooking stage to yield kraft pulps with low levels of lignin and bleaching requirements. The goal is to achieve the lowest possible Kappa number so that lignin content in the brown stock and, hence, the bleaching load is minimized.1 In these conditions, the use of alternative bleaching agents, such as ClO2, O2, H2O2, and O3, is economically feasible. It has been shown that if the cooking stage is prolonged from a Kappa number of 30-24 in softwood pulping, a reduction of 25% in the chemical demand of oxygen (COD) and of 30% in halogenated adsorbable organic componunds (AOX) contained in the bleaching effluents is possible.2 Likewise, when the Kappa number is lowered to 14 by the combined use of extended delignification and oxygen delignification, the reduction obtained in COD and AOX is of 50% and 60%, respectively.2 To reduce pulp lignin content variability and maintain stable digester operation, a good digester control is crucial. With more pulp and paper companies using continuous digesters to meet the increasing competitiveness in the global market place and tighter environmental regulations, digesters control is also of paramount importance to maximize the produced pulp quality and to reduce overall operating costs. Therefore, the development of reliable control algorithms is much needed to improve digesters design and performance.3,4 The modeling of delignification is extremely complex because of the multitude of reactions involved and the * Corresponding author. E-mail:
[email protected]. Fax: 34 913974187. Phone: 34 913975523. † Universidad Auto ´ noma de Madrid. ‡ Universidad Complutense de Madrid.
heterogeneous nature of lignin and wood sources. However, some common patterns can be found.5 Three distinct phases of delignification can be observed in most systems: an initial phase that involves the rapid removal of about 20% of the lignin, a slower stage of bulk delignification, and finally, an even slower residual delignification.6,7 Although the reaction patterns are not fully understood, most kinetic models describe delignification as the dissolution of three types of lignin present in wood from the beginning: initial, bulk, and residual lignin.6 Thus, delignification can be considered as the consecutive or simultaneous dissolution of initial, bulk, and residual lignin. When the three types of lignin react simultaneously, the superposition of the exponential decay processes results in the existence of three apparent phases with different reaction rates for the dissolution of lignin. In most cases, the differences in the delignification rate make it possible to assume that the three types of lignin dissolve consecutively. It has been observed that delignification is of apparent first-order with respect to the lignin content of wood in the three phases.9 Thus, most models have the same general expression3
-
dL ) (k1[OH]a + k2[OH]b[SH]c)(L - L∞) dt
(1)
Equation 1 is applied for delignification with different parameters for each period. A review of the models proposed for initial and bulk delignification can be found in a previous work.9 The residual phase of delignification is by far the least investigated, and different conclusions can be found in the literature about the variables affecting residual delignification. It has been found that the amount of lignin to be removed in the
10.1021/ie0108907 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/23/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002
residual stage of delignification depends on the hydroxide ion concentration and, to some extent, on hydrogen sulfide ion concentration and ionic strength; however, the residual delignification rate only depends on alkali concentration and pulping temperature.6 The lack of dependence on hydrogen sulfide ion has also been reported.8,10-12 Thus, the models for residual delignification can be simplified to
-
dL ) k[OH]a(L - L∞) dt
Table 1. Experimental Conditions for the Cooking Runsa run
temp (°C)
[OH]0 (M)
run
temp (°C)
[OH]0 (M)
1 2 3 4 5 6
150 150 150 160 160 160
0.7 1.2 1.6 0.7 1.2 1.6
7 8 9 10 11 12
170 170 170 180 180 180
0.7 1.2 1.6 0.7 1.2 1.6
a
(2)
Lindgren and Lindstro¨m6,11 employed a modification of eq 2, where the exponent of hydroxide concentration was assumed to be the unity. The dependence of residual delignification on hydroxide ion concentration has been observed for many types of wood; however, in the case of wheat straw pulping,12 residual delignification was found to be independent of hydroxide ion concentration. Guidici and Park3 proposed an alternative model where the condensation and dissolution of residual lignin is considered. In addition to models where the rate-determining step is the chemical reaction, some models have been proposed to take into account the diffusion in wood.13 The majority of the literature works about kraft residual delignification have been carried out with softwoods (spruce, pine, Douglas fir, etc.) and hardwoods such as birch. However, little basic information is available about the kinetics of delignification of eucalypt woods during kraft pulping. This paper deals with the kinetic modeling of the residual delignification of Eucalyptus globulus in kraft pulping. In a previous work, the kinetics of initial and bulk delignification was studied.9 In conventional industrial kraft pulping, the cook is usually interrupted before the transition to residual delignification to avoid excessive degradation of cellulose fibers and the loss of pulp yield. However, this phase is of greater interest as a result of efforts to prolong the delignification in kraft pulping processes. Thus, the study of the residual stage of delignification is particularly interesting because residual delignification determines the final pulp lignin content. Experimental Section The pulping setup employed consists of a 4-L stainless steel batch digester equipped with a recirculation pump, an external heating system, and the measurement and control of both pressure and temperature. The liquor recirculation line is provided with a liquor autosampler and a heat exchanger. Further details about it can be found in a previous work.9 The runs were carried out using selected E. globulus wood chips with a thickness lower than 5 mm. Bark, oversized chips, knots, and other irregularities were removed by handsorting. The chips (545 g, oven dried basis) were introduced into the digester along with the pulping liquor at a liquor-to-wood ratio of 5.5 L of liquor/ kg of oven dried wood. The autoclave was purged with nitrogen, and an overpressure of 2 atm was applied and held throughout the cooking run. The overpressure helps to avoid pulping liquor boiling and to prevent the cavitation of the recirculation pump. The system was heated at a rate of 3 °C/min. The cooking time was measured from the moment that the programmed temperature was reached. When the cooking time was completed, the digester was cooled to room temperature
[SH]0 ) 0.18 M; liquor-to-wood ratio ) 5.5 L/kg.
using a heat exchanger placed in the liquor recirculation pipe. Cooked wood was defiberized and washed with water until colorless washing water was obtained. The washed pulp was air-dried and stored. The ranges studied for hydroxide ion concentration in the white liquor and cooking temperature were 0.71.6 M and 150-180 °C, respectively. The sodium sulfide concentration was fixed at a value of 0.18 M. The experimental conditions for the runs are summarized in Table 1. A total of 10 black liquor samples were taken at regular intervals of time for each run. The lignin content in black liquors was determined by UV spectrophotometry.1,4,14 The aliquots of black liquor were diluted in 0.01 N NaOH and filtered with a 0.45 µm syringe filter, and their absorbance was measured at 280 nm. Lignin standards were prepared from the black liquors drained at the end of each experiment. A 100 mL sample of black liquor was precipitated with sulfuric acid at a pH of 2. The precipitate was filtered and redissolved in 0.1 N NaOH, precipitated again, and washed twice in the filter with distilled water.15 The lignin standards were prepared on lignin basis; therefore, lignin weight was corrected for ash and sulfur content. Ash content was determined at 500 °C, and sulfur was analyzed in a LECO CNS 2000 instrument. Lignin conversion was calculated from wood initial lignin content, lignin concentration in the black liquor content, and cooking yield. The results were checked by using the pulp Kappa number. A value of 0.16 was determined for the relationship between the pulp Kappa number and the pulp lignin content. After each run, an additional cook was carried for a pulping time, corresponding to the beginning of residual delignification, and the Kappa number of the resulting pulp was determined. A good agreement was found between the lignin conversion values obtained from liquor analysis and the pulp Kappa number. Hydroxide and hydrosulfide concentrations in the pulping liquor were determined as follows.9 Ten milliliters of black liquor was diluted with water to 100 mL. Twenty-five milliliters of a 25% barium chloride solution was added, and the solution was titrated with 0.5 N HCl until pH 9.3 was reached. In this step, the hydroxide ion was totally titrated. BaCl2 was employed to precipitate carbonate ion as barium carbonate. Five milliliters of a 25% formaldehyde solution was added to the sample and tritrated down again to pH 9.3. Formaldehyde complexes hydrosulfide ion, and simultaneously, hydroxide ion is released. The hydrosulfide ion concentration can be calculated from titrated hydroxide ion. Transition from Bulk to Residual Delignification. Figure 1 shows the logarithmic plot of lignin conversion data (1 - XL) versus the cooking time as scatter points. The two straight lines that can be observed from data trend in each run indicate a firstorder dependence of delignification with respect to lignin content. The first line represents the bulk delignifica-
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Figure 1. Experimental lignin conversion data and model predictions.
tion, which takes place up to a lignin conversion of about 95%. Beyond this value, delignification is much slower and takes place in the residual stage. Initial delignification is not observed in Figure 1 because of the total removal of initial lignin during the heating period. The transition from initial to bulk delignification was previously fixed at a value of 18%.9 The lignin conversion value at the transition from bulk to residual delignification (XLt) was obtained from the logarithmic plot of (1 - XL) versus time, where the transition was considered as the intercept of the slopes corresponding to bulk and residual phases. Data fitting of XLt, alkali charge, and temperature yielded the following equation:
XLt ) 0.0217[OH]0 + 6.9 × 10-4 T + 0.816
(3)
The values predicted by eq 3 range from 93.5% to 97.5%. These values are close to that of 97% reported in a previous work for the kraft pulping of E. globulus wood from another lot.9 The phase transition calculated is also in the same range as that observed for other wood species. Thus, in southern pine kraft lignin, a lignin conversion value of around 90% has been reported at the transition to the residual stage,4 and Chiang et al.8 found that, in the pulping of Douglas fir, the residual lignin amounts to 4% of the total lignin. Equation 3 predicts that the conversion at the phase transition increases slightly with the cooking temperature and hydroxide ion concentration, as is shown in Figure 1. Other works in the literature indicate that the transition from the initial to the bulk phase is independent of temperature and alkali and sulfide concentration, whereas the transition from the bulk to the residual phase is strongly influenced by cooking conditions.16 Lindgren and Lindstro¨m6 have also shown that, in the kraft pulping of spruce wood, the lignin conversion at the transition from bulk to residual stage can vary depending on the concentration of hydroxide ion, concentration of hydrogen sulfide ion, ionic strength, and temperature. Similar results were found for birch, wheat straw, and Gmelina arborea.11,12,17 It was also found that the transition point at which the change from bulk to residual delignification takes place is reduced to a lower lignin content if the temperature is increased.6,11 Likewise, the proportion of lignin in the final delignification phase is reduced by an increase in the alkali concentration.16 Therefore, the rise in bulk delignification extension with alkali concentration and cooking temperature observed in this work for kraft pulping E. globulus has also been reported for other species. With regard to the behavior of eucalyptus wood in other pulping media, in the soda pulping of Eucalyptus diversicolor, the transition from bulk to residual delignification takes place for a lignin conversion value that ranges from 86% to 96%.18 The lower conversion values in soda pulping shows the promotion of delignification by sodium sulfide. Thus, the hydrogen sulfide ion promotes fragmentation of a quinone methyde intermediate rather than condensation with a nucleophile or the formation of enol ethers, and the amount of lignin to be removed in the residual stage is lower.6 In autocatalyzed organosolv pulping of E. globulus with ethanol-water and methanol-water mixtures, the transition takes place for a lignin conversion between 90% and 91%.19,20 In acetic acid pulping, the delignification rate was found to decrease from a lignin conversion of 90%.21 In the aforementioned Organosolv pulping processes, the combination of the earlier transition to a residual phase and the enhanced condensation and precipitation of lignin results in a more difficult delignification when compared to kraft pulping. From the foregoing, the amount of residual lignin is not only a characteristic of wood species but also depends on the pulping process and the cooking conditions. Alkali Consumption. Delignification stages are characterized by different alkali consumption, as can be observed in Figure 2. On the basis of the lignin conversion of 18% estimated for initial delignification, the alkali consumption at initial stage would be 1.7-3 mol of OH/kg of wood, which amounts to 54-60% of the total consumption. In the initial period, the alkali is consumed in the neutralization of acidic reaction prod-
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Figure 2. Alkali consumption in the kinetic runs.
Figure 3. Calculated versus experimental lignin conversion data.
ucts that are formed when the extractives are dissolved, and some carbohydrates are split.22 During the bulk stage, alkali consumption decreases, and 1.1-1.4 mol of OH/kg of wood are removed. In this period, alkali consumption is mainly caused by lignin macromolecule cleavage and polysaccharide reactions.23 The alkali consumption starts again to decrease rapidly after the transition from bulk to residual delignification, and a removal of 0.3-0.8 mol of OH/kg of wood is observed. Taking into account the three stages, the overall consumption is 3-4.9 mol of OH/kg of wood. The values obtained are in agreement with literature. Thus, Chiang et al.24 reported that about 3.8 mol of OH/kg of wood are consumed during the initial and bulk stages of softwood delignification, whereas slightly lower values of about 3.4 mol of OH/kg of wood were found for hardwoods. In the kraft pulping of southern pine, Vanchinathan and Krishnagopalan4 reported an alkali consumption as high as 4.4 mol of OH/kg of wood in the initial stage and 1.5 mol of OH/kg of wood in the bulk stage. The alkali consumption is higher for the runs carried out at a higher alkali charge. The availability of alkali promotes the side reaction of carbohydrates, which gives rise to the occurrence of acidic reaction products. Thus, in the runs where the alkali charge is 0.7 M, a consumption of about 1.7 mol of OH/kg of wood during initial and bulk delignification can be observed. This value rises to 2.2 and 3.0 mol of OH/kg of wood for the runs carried out with alkali charges of 1.2 and 1.6 M, respectively. This effect can be more clearly appreciated in the initial and residual stages. A rise in alkali consumption can also be seen in the runs carried out at a higher temperature. The experimental data shown in Figure 2 were employed to establish the relationship between hydroxide ion concentration and lignin conversion in the three stages of delignification. Data were fitted to the following expressions:
initial stage bulk stage residual stage
d(1 - XL) d[OH] ) 2.7 dt dt
(4)
d(1 - XL) d[OH] ) 0.92 dt dt
(5)
d(1 - XL) d[OH] ) 0.55 (6) dt dt
Kinetic Modeling of Residual Delignification. Data of fractional remaining lignin (1 - XL) at the residual stage have been fitted to a model similar to that showed in eq 2. Data were fitted by a nonlinear regression (Marquardt algorithm)25 coupled to a RungeKutta fourth-order method of integration. The kinetic model obtained was
d(1 - XL) 10 350 ( 463 × ) exp 18.54 ( 1.02 dt T [OH]0.59(0.06(1 - XL) (7)
(
)
The value of 0.59 found for the kinetic order of hydroxide ion concentration is lower than the unity value assumed in some kraft pulping literature3,6,11 and close to the value of 0.7 reported by Norden and Teder.10 On the other hand, it is in the 0.2-0.8 range reported by Garland et al.18 in the soda pulping of E. diversicolor. The activation energy has a value of 86.1 kJ/mol, which is in the range between 83 and 146 kJ/mol reported for the kraft pulping of some wood species and annual plants.4,6-8,10-12,18 The reproduction of experimental data was carried out using the model proposed by Santos et al.9 for initial and bulk delignification
d(1 - XL) ) dt
initial
(
exp 5.53 bulk
4533 0.5 T (1 - XL) (8) T
)
d(1 - XL) ) dt 12 750 [OH]0.15[SH]0.16(1 - XL) (9) exp 26.47 T
(
)
and the model developed in the current work for residual delignification in eq 7. To simulate the change of [OH] with lignin conversion, eqs 4-6 were applied. The reproduction of lignin conversion data is shown in Figure 1 as lines. In general, a reasonably good agreement can be observed. The suitability of the model can also be appreciated in the plot of experimental versus calculated data (Figure 3). Conclusions The transition from bulk to residual delignification was observed for a lignin conversion of about 95%. The
Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1959
conversion at transition was found to increase with pulping temperature and alkali concentration. An alkali consumption of 1.7-3, 1.1-1.4, and 0.30.8 mol of OH/kg of wood was measured for initial, bulk, and residual stages, respectively. The consumption of alkali is higher for the runs carried out at higher alkali charge. The kinetic model obtained for residual delignification yielded an activation energy of 86.1 kJ/mol and a kinetic order of 0.59 for the hydroxide ion concentration. Nomenclature a ) kinetic order in eqs 1 and 2 b ) kinetic order in eqs 1 and 2 c ) kinetic order in eq 1 k ) kinetic constant L ) lignin content (% in wood) [OH] ) effective alkali concentration as NaOH (M) [SH] ) hydrosulfide concentration (M) t ) time (min) T ) temperature (°C, K) XL ) lignin conversion XLt ) lignin conversion at phase transition
Literature Cited (1) Varma, V.; Krishnagopalan, G. A. Kinetics of Extended Delignifiction Using Alkali Profiling and On-Line Liquor Analysis. Appita J. 1998, 51 (1), 50-56. (2) Gullichsen, J. Process Internal Measures to Reduce Pulp Mill Pollution Load. Water Sci. Technol. 1991, 24 (3-4), 45-53. (3) Giudici, R.; Park, S. Kinetic Model for Kraft Pulping of Hardwood. Ind. Eng. Chem. Res. 1996, 35 (3), 856-863. (4) Vanchinatan, S.; Krishnagopalan, G. A. Dynamic Modeling of Kraft Pulping of Southern Pine Based on On-Line Liquor Analysis. Tappi J. 1997, 80 (3), 123-133. (5) Juvekar, P.; Ransdell, J.; Cole, B.; Genco, J. Kraft Pulping Kinetics of Eastern White Cedar. AIChE Symp. Ser. 1995, 307 (91), 1-18. (6) Lindgren, C. T.; Lindstro¨m, M. E. The Kinetics of Residual Delignification and Factors Affecting the Amount of Residual Lignin During Kraft Pulping. J. Pulp Pap. Sci. 1996, 22 (8), J290J295. (7) Gustafson, R. R.; Sleicher, Ch. A.; McKean, W. T.; Finlayson, B. A. Theoretical model of the kraft pulping process. Ind. Eng. Chem. Process Des. Dev. 1983, 22 (1), 87-96. (8) Chiang, V. L.; Yu, J.; Eckert, R. C. Isothermal Reaction Kinetics of Kraft Delignification of Douglas Fir. J. Wood Chem. Technol. 1990, 10 (3), 293-310. (9) Santos, A.; Rodrı´guez, F.; Gilarranz, M. A.; Moreno, D.; Garcı´a-Ochoa, F. Kinetic Modeling of Kraft Delignification of Eucalyptus globulus. Ind. Eng. Chem. Res. 1997, 36 (10), 41144125.
(10) Norden, S.; Teder, A. Modified Kraft Processes for Softwood Bleached-grade Pulp. Tappi J. 1979, 62 (7), 49-51. (11) Lindgren, C. T.; Lindstro¨m, M. E. Kinetics of the Bulk and Residual Delignification in Kraft pulping of birch and Factors Affecting the Amount of Residual Phase Lignin. Nord. Pulp Pap. Res. J. 1997, 12 (2), 124-134. (12) Epelde, G.; Lindgren, C. T.; Lindstro¨m, M. E. Kinetics of Wheat Straw Delignification in Soda and Kraft Pulping. J. Wood Chem. Technol. 1998, 18 (1), 69-82. (13) Agarwall, N.; Gustafson, R. A Contribution to the Modeling of Kraft Pulping. Can. J. Chem. Eng. 1997, 75 (1), 8-15. (14) Trinh, D. T. The Measurement of Lignin in Kraft Pulping Liquors Using an Automatic Colorimetric Method. J. Pulp Pap. Sci. 1988, 14 (1), J19-J22. (15) Kim, H.; Hill, M. K.; Fricke, A. L. Preparation of Kraft Lignin from Black Liquors. Tappi J. 1987, 70 (12), 112-116. (16) Schild, G.; Mu¨ller, W.; Sixta, H. Prehydrolysis Kraft and ASAM Paper Grade Pulping of Eucalypt Wood. A Kinetic Study. Das Papier 1996, 50 (1), 10-22. (17) Twimasi, K. A. The Kinetics of the Sulphate Pulping of Gmelina arborea from Ghana. J. Wood Chem. Technol. 1997, 17 (1/2), 179-185. (18) Garland, C. P.; James, F. C.; Nelson, P. J.; Wallis, F. A. A Study of the Delignification of Eucalyptus diversicolor During Soda Pulping. Appita J. 1987, 40 (2), 116-120. (19) Gilarranz, M. A.; Rodrı´guez, F.; Santos, A.; Oliet, M.; Garcı´a-Ochoa, F.; Tijero, J. Kinetics of Eucalyptus globulus Delignification in a Methanol-Water Medium. Ind. Eng. Chem. Res. 1999, 38 (9), 3324-3332. (20) Oliet, M.; Rodrı´guez, F.; Santos, A.; Gilarranz, M. A.; Garcı´a-Ochoa, F.; Tijero, J. Organosolv Delignification of Eucalyptus globulus: Kinetic Study of Ethanol Pulping. Ind. Eng. Chem. Res. 2000, 39 (1), 34-39. (21) Va´zquez, G.; Antorrena, G.; Gonza´lez, J.; Freire, S.; Lo´pez, S. Acetosolv Pulping of Pine Wood. Kinetic Modelling of Lignin Solubilization and Condensation. Bioresour. Technol. 1997, 59 (2/ 3), 121-127. (22) Rekunen, S.; Jutila, E.; Lo¨nnberg, B.; Virkola, N. E. Examination of Reaction Kinetics in Kraft Cooking. Pap. Puu 1980, 62 (2), 80-90. (23) Sjo¨stro¨m, E. Wood Chemistry: Fundamentals and Applications; Academic Press Inc.: New York, 1981. (24) Chiang, V. L.; Puumala, R. J.; Takeuchi, H. Comparison of Softwood and Hardwood Kraft Pulping. Tappi J. 1988, 71 (9), 173-176. (25) Marquadt, F. W. An algorithm for least-squares estimation of nonlinear parameters. J. Soc. Ind. Appl. Math. 1963, 2, 431442.
Received for review November 6, 2001 Revised manuscript received January 23, 2002 Accepted January 28, 2002 IE0108907
