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Ind. Eng. Chem. Res. 2000, 39, 34-39
Organosolv Delignification of Eucalyptus globulus: Kinetic Study of Autocatalyzed Ethanol Pulping Mercedes Oliet, Francisco Rodrı´guez,* Aurora Santos, Miguel A. Gilarranz, Fe´ lix Garcı´a-Ochoa, and Julio Tijero Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, Universidad Complutense, 28040 Madrid, Spain
The autocatalyzed delignification of Eucalyptus globulus in 50% ethanol (w/w) was modeled as the irreversible and consecutive dissolution of initial, bulk, and residual lignin. Their respective contributions to total lignin was estimated as 9, 75, and 16%. Isothermal pulping experiments were carried out to evaluate an empirical kinetic model among eight proposals corresponding to different reaction schemes. The calculated activation energy was found to be 96.5, 98.5, and 40.8 kJ mol-1 for initial, bulk, and residual delignification, respectively. The influence of hydrogen ion concentration was expressed by a power-law function model. The kinetic model developed here was validated using data from nonisothermal pulping runs. Introduction
k1
As shown in a previous effort,1 it was shown that the methanol delignification of Eucalyptus globulus can be described as the dissolution of three types of lignin, namely, initial, bulk, and residual. Their respective contents in wood were found to be 10, 69, and 21% of the total lignin. The current paper complements the above-mentioned work and focuses on the ethanol pulping of E. globulus without additives (autocatalyzed). Features such as low cost, low toxicity, and easy recovery make ethanol very interesting as a pulping liquor. For this reason, ethanol is the most used solvent in organosolv pulping. In fact, the Alcell process, which is the only autocatalyzed process that has reached pilot-plant scale, is ethanolbased.2 The aim of this work is to develop an empirical kinetic model capable of describing delignification in autocatalyzed ethanol pulping. The model expression takes into account the influence of both temperature and hydrogen ion concentration on the delignification rate.
ke
LN 98 LD {\} LR k1
k2
} LR LN 98 LD {\ k -2
kb
LNb 98 LD
k1
k1
} LD LN {\ k e
Model I
(1)
Model II
(2)
* To whom correspondence should be addressed. Phone: 34-91-3944246. Fax: 34-91-3944243. E-mail: frsomol@ eucmos.sim.ucm.es.
(4)
Model V
(5)
Model VI
(6)
Model VII
(7)
Model VIII
(8)
kr
LNr 98 ki
LNi {\ } k ei
kb
LNb {\ } LD k eb
kr
} LNr {\ k er
kj
LNj 98 LD ej
LN 98 LD
Model IV
ki
kj
Kinetic Models. A total of eight kinetic models were tested. The reaction schemes considered for model development are displayed in eqs 1-8. They are based on two different approaches to lignin reaction and dissolution. On one hand, in models I-IV it is assumed that lignin is a homogeneous substance. On the other hand, in models V-VIII it is supposed that delignification results from the reaction of three different lignin species, initial, bulk, and residual, which react at different rates.
(3)
LNi 98
} LD LNj {\ k
Experimental Section
Model III
The expressions of the kinetic models were developed considering the lignin reaction as first-order, as shown in eq 9. The reaction rate expressions and their integrated forms were described in a previous work.1
dL ) -kL ) -k0e-E/RTL dt
(9)
The discrimination of the most suitable model for the influence of temperature was made in terms of the sum of square residuals (SQR) and the confidence interval for the estimated parameters. After this discrimination, three expressions were tested to model the influence of the hydrogen ion concentration on the rate constant. The expressions considered were the following:
kj ) k0j[H+]
(10)
kj ) k0j[H+]mj
(11)
10.1021/ie9905005 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/03/1999
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 35 Table 1. Experimental Conditions for Pulping Runs liquor-to-wood ratio (L kg-1)
runs
temp. (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
110 120 130 140 150 160 170 180 190 200 130 130 130 180 180 180
Isothermal 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50
17 18 19
170 185 200
Nonisothermal 7 7 7
kj )
kA0j[H+]aj 1 + kB0j[H+]dj
[H+] (M) 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 1.25 × 10-4 3.3 × 10-3 1.05 × 10-5 8.7 × 10-7 3.5 × 10-3 1.09 × 10-5 7.9 × 10-7 autocatalyzed autocatalyzed autocatalyzed
(12)
Procedure A 4-L autoclave provided with external black liquor circulation, autosampling, and heating and cooling systems was used to carry out the pulping experiments.3 The chipped wood was impregnated in water, placed in the autoclave, and heated to 100 °C. The pulping liquor, whose concentration was corrected for impregnation water to attain a final concentration of 50% ethanol (w/ w), was heated in an auxiliary vessel and transferred to the autoclave. Once the operating temperature was reached, black liquor autosampling was started. Lignin conversion values were calculated from black liquor analysis, which was carried out by UV spectrophotometry at 280 nm. The materials and detailed procedure were described ina former work.1 Table 1 shows the experimental conditions for the runs performed to develop and validate the kinetic model. The isothermal pulping experiments 1-16, which were carried out at a liquor-to-wood ratio of 50 L kg-1 (liters of liquor per kg of wood, on a dry basis), were used to discriminate the most suitable model and to estimate the value of the model parameters. In experiments 1-10 acetic acid was added to the pulping liquors to maintain a hydrogen ion concentration identical to that of autocatalyzed pulping. In these experiments the pulping temperature was varied to study its influence on the delignification rate for the tested models. Experiments 3, 8, and 11-16 were designed to study the influence of the hydrogen ion concentration. In runs 11 and 16 sulfuric acid was added to the pulping liquors to achieve a higher hydrogen ion concentration than that in autocatalyzed pulping. On the contrary, in runs 12, 13, 15, and 16 acetic acid/sodium acetate buffer was employed to attain a hydrogen ion concentration below the autocatalyzed conditions. Finally, runs 17-19 were performed at a liquor-to-wood ratio of 7 L kg-1 and nonisothermal conditions (heating rate, 3 °C min-1) to validate the kinetic model.
Figure 1. Experimental lignin conversion vs cooking time. [H+] ) 1.25 × 10-4 M; liquor-to-wood ratio ) 50 L kg-1; isothermal runs; temperature ) 110-200 °C (runs 1-10).
Results and Discussion The lignin conversion data at different pulping temperatures are plotted in Figure 1. The data trend in the runs at 120-140 °C shows two different slopes for delignification in each experiment. This behavior was previously reported for kraft pulping3-5 and autocatalyzed methanol pulping.1 The first slope can be attributed to initial delignification and the second to bulk delignification. The transition point between these stages can be observed for a lignin conversion value of about 9%. This value is nearly identical to the 10% calculated for methanol pulping1 and significantly lower than that for kraft pulping where it amounts to about 20%.3-5 In the plots at 170-200 °C two well-delimited delignification stages can be appreciated. In this case, the first of them corresponds to bulk delignification and the second to residual delignification. A dependence of the conversion at the transition point on the pulping tem-
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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
Table 2. Influence of Temperature on the Delignification Rate (Sum of Square Residuals (SQR) Obtained by Experimental Data Fitting (Runs 1-10) to Models I-VIII) model
SQRa
I, eq 1 II, eq 2 III, eq 3 IV, eq 4 V, eq 5 VI, eq 6 VII, eq 7 VII, eq 7 VII, eq 7 VII, eq 7 VIII, eq 8 VIII, eq 8 VIII, eq 8 VIII, eq 8
0.633 0.487 0.519 0.559 0.891 0.604 0.007i 0.379b 0.002r 0.388t 0.006i 0.365b 0.002r 0.373t
confidence interval includes the zero value? no no yes yes yes yes no no no yes yes no
a The superscripts “i”, “b”, “r”, and “t” represent the dissolution of initial, bulk, residual, and total lignin, respectively.
perature can be appreciated. Thus, the lignin conversion reaches values of 82 and 86% at 170 and 200 °C, respectively. Therefore, the transition from the bulk to the residual stage can be observed for a lignin conversion mean value of 84%. On one hand, the calculated value is slightly higher than that previously reported for the autocatalyzed methanol pulping1 (79%). On the other hand, it is a rather low value for lignin conversion when compared to that found in kraft pulping3-5 (9097%). The earlier transitions to the bulk and residual stages observed in autocatalyzed alcohol pulping can be due to lignin condensation and precipitation. It was reported by Paszner and Cho6 that in the autocatalyzed ethanol pulping the condensation of solvolitically liberated lignin is of importance and gives rise to solvent-insoluble and low-solubility condensation products. On one hand, under these circumstances, the net amount of lignin dissolved at the transition point is lower than that in other pulping media, such as kraft, where the amount of condensation is lesser. On the other hand, the different stages of delignification can be related to changes in the lignin composition and accesibility.7 The first lignin portions removed would correspond to lowcomplexity structures located at the middle lamella and the cell corner, whereas residual delignification would be controlled by the dissolution of the most cross-linked molecules and condensation products and low-accessible lignin placed in the cell wall. To develop a suitable kinetic model, the delignification reaction rate can be interpreted as the dissolution of a single species whose reaction mechanism changes as delignification proceeds (models II-IV) or as the dissolution of different lignin species with different reaction rates (models V-VIII). In the last case, the dissolution of the lignin species can be simultaneous (models VI and VIII) or consecutive (models V and VII). In model I a change in the reaction mechanism cannot be predicted; therefore, a poor correlation is expected. Nonlinear regression8 was used to fit experimental data from runs 1-10 to the integrated expressions of the proposed models. Table 2 summarizes the results of these regressions. It is obvious that model VII is the most suitable of those tested. Its SQR value is very low and it fulfills the statistical criterion; that is to say, the confidence interval estimated for the parameters does
Figure 2. Reproduction of experimental data from isothermal runs 1-10. Values calculated from eq 13.
not include the zero value. This model describes delignification as three consecutive stages, corresponding to the dissolution of initial, bulk, and residual lignin. The reaction is irreversible for all stages. Methanol pulping was also modeled as three consecutive stages, but bulk delignification was found to be better described as a reversible reaction.1 The reproduction of data by model VII is shown in Figure 2, where a good agreement for the majority of data can be appreciated. The most important deviations are observed for the run at 140 °C. This is not a serious drawback because the major part of delignification in nonisothermal industrial pulping takes place at high temperatures (180-200 °C), where the model reproduces experimental data satisfactorily. Therefore, the dependence of delignification on temperature can be described in all stages by an expression of the form
dXL E (1 - XL) ) k0 exp dt RT
(
)
(13)
With this expression and those proposed in eqs 1012 the fitting of data from the runs at different hydrogen ion concentrations (runs 3, 8, and 11-16) was carried out. The results obtained can be seen in Table 3. Equation 12 was only applied to bulk delignification because it is the only stage where enough data were available to fit such a complex model. In fact, the estimated confidence interval for the parameters is large and it includes the zero value as a consequence of the high number of parameters considered. According to the SQR values obtained, the expression shown in eq 11 is the most suitable to model the dependence of the delignification rate constant on the hydrogen ion concentration. The expression selected is identical to the
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 37 Table 3. Influence of the Hydrogen Ion Concentration on the Delignification Rate (Sum of Square Residuals (SQR) Obtained by Experimental Data Fitting (Runs 11-16) to Models in Eqs 10-12) lignin species
dependence on [H+]
initial bulk residual total
eq 10 eq 10 eq 10
initial bulk residual total
eq 11 eq 11 eq 11
0.003 0.136 0.001 0.1396
no no no
bulk
eq 12
0.137
yes
SQR 0.061 10.74 0.034 10.835
confidence interval includes the zero value? no no no
Table 4. Parameters for the Model Developed (Fitting of Data from Runs 1-16 to Eq 14; SQR ) 0.7472) lignin species
parameters
values
validity range
initial
Ln k0i mi Ei/R
23.96 ( 0.32 0.306 ( 0.034 11618 ( 171
XL < 0.09 XL < 0.09 XL < 0.09
bulk
Ln k0b mb Eb/R
23.44 ( 0.31 0.163 ( 0.0053 11851 ( 132
0.09 < XL < 0.84 0.09 < XL < 0.84 0.09 < XL < 0.84
residual
Ln k0r mr Er/R
12.05 ( 5.29 0.694 ( 0.1176 4909 ( 2288
XL > 0.84 XL > 0.84 XL > 0.84
The final expression of the kinetic model for all the delignification stages considered is as follows:
dXL E ) k0 exp [H+]m(1 - XL) dt RT
(
Figure 3. Reproduction of experimental data from isothermal runs 3, 11-13 (130 °C), and 8 and 14-18 (180 °C). Values calculated from eqs 13 and 11.
one for autocatalyzed methanol pulping.1 The suitability of the model to reproduce the data at high temperatures can be seen in Figure 3 (runs 8 and 14-16). At low temperature a poor data reproduction is attained for long cooking times (runs 3 and 11-13). However, in nonisothermal pulping, only wood chips are subjected to low temperatures during the heating period. From Figure 3 it can also be seen that there is a significant dependence of the delignification rate on the hydrogen ion concentration. A similar behavior was described by Tirtowidjojo et al.9 for acid-catalyzed pulping of cottonwood in a methanol-water medium. These results support the idea that delignification is caused by acid-catalyzed cleavage of such linkages as R-ether and β-ether bonds in lignin the macromolecule.10 In acid pulping the cleavage of R-ether has been reported to be predominant, but the role of β-ether must not be ruled out in strongly acidic pulping of hardwoods.11
)
(14)
Once the most suitable expression to model the pulping system was selected, all data (runs 1-16) were fitted simultaneously to it to recalculate the model parameters. The estimated values are summarized in Table 4. The activation energy for initial delignification has a value of 96.5 kJ mol-1, very close to that of the 93.1 kJ mol-1 found for autocatalyzed methanol pulping1 and significantly higher than that for kraft pulping3,5 of hardwoods and softwoods (40 kJ mol-1). In bulk delignification the estimated activation energy, 98.5 kJ mol-1, agrees with the value for methanol pulping of E. globulus1 (98.4 kJ mol-1) and it is near the value reported by Kleinert12 for the autocatalyzed ethanol pulping of spruce wood (117.5 kJ mol-1). Significantly lower values can be found in other organosolv pulping media. Thus, activation energies of 66-78 kJ mol-1 were found for acetic acid pulping of E. globulus,13 and around 56 kJ mol-1 for alkaline alcohol pulping of hardwoods.14,15 With regard to kraft pulping, bulk activation energies ranging from 105 kJ mol-1 for E. globulus3 to 127-144 for softwoods4,5 were reported. In the residual stage, the estimated value (40.8 kJ mol-1) is higher than that in methanol pulping.1 This difference can explain the different pulp lignin contents observed in the autocatalyzed ethanol and methanol pulping of E. globulus.16,17 On the other hand, the activation energy is slightly lower than those for other hardwoods in organosolv media14,15 (56.3-63.5 kJ mol-1) and much lower than those in kraft pulping of softwoods3-5 (90-146 kJ mol-1). The kinetic order of the hydrogen ion concentrations for initial, bulk, and residual delignification is 0.31, 0.16, and 0.69, respectively. The estimated values are in good agreement with those obtained for autocatalyzed methanol pulpin ie9902582g,1 although they are lower than unity, which is the value assumed in other works.9,13,18 The plot of experimental vs calculated values for runs 1-16, shown in Figure 4, indicates the suitability of the model for data reproduction. Validation Data from the three nonisothermal runs (17-19) were reproduced to validate the model. In nonisothermal runs the hydrogen ion concentration in the pulping liquor increases as the experiment progresses. The dependence of the black liquor pH on delignification is shown in Figure 5 for experimental data from runs 17-19. The hydrogen ion concentrations in the pulping liquors drop
38
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
Figure 4. Calculated vs experimental lignin conversion (isothermal runs 1-16) for the kinetic model in eq 14 and parameters in Table 4.
Figure 5. Black liquor pH vs lignin conversion. Data from nonisothermal runs 17-19.
dramatically during the initial stage of delignification, especially at the beginning of this stage as can be seen from Figure 5. For lignin conversion values larger than 40%, the hydrogen ion concentrations level off. The data trend shows a stoichiometric relationship between delignification and hydrogen ion concentration; besides, this relationship does not depend on temperature. The fitting of experimental data shown in Figure 5 yields the following expression: -XL/0.001784
pH ) 3.97 + 15.29 exp
-XL/0.0846
+ 2.39 exp
(15) Equation 15 predicts high pH values in the vicinity of XL ) 0 because of the asymptotic trend of the data. For the sake of simplicity the pH value was assumed to be 8.75 in the range 0 < XL < 0.0035 and that given by eq 15 for XL > 0.0035. Thus, eq 15 was used, together with eq 14 and the parameter values in Table 4, to reproduce data from runs 17-19. The results are plotted in Figure 6, where a good agreement with experimental data can be observed. Conclusions An empirical kinetic model for the delignification of E. globulus in 50% ethanol (w/w) has been discriminated and validated. It was assumed that lignin is composed of initial, bulk, and residual lignin, their respective contents in wood being 9, 75, and 16% of the total lignin. The delignification takes place according to three irreversible and consecutive reactions, corresponding to
Figure 6. Validation of the model. Reproduction of data from nonisothermal runs 17-19 by the model in eq 14 and parameters in Table 4.
the above-mentioned lignin species. The expressions developed to model the system are the following:
dXL 96.5 + 0.31 ) 2.54 × 1010 k0 exp [H ] (1 - XL) dt RT XL < 0.09 (16)
(
)
dXL 98.5 + 0.16 ) 1.51 × 1010 k0 exp [H ] (1 - XL) dt RT 0.09 < XL < 0.84 (17)
(
)
dXL 40.8 + 0.69 [H ] (1 - XL) ) 1.71 × 104 k0 exp dt RT XL > 0.84 (18)
(
)
Acknowledgment The authors are grateful to the Comisio´n Interministerial de Ciencia y Tecnologı´a for financial support (Project AMB94-0012-C02-01).
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 39
Nomenclature a ) kinetic order for in eq 12 d ) kinetic order for H+ in eq 12 E ) activation energy (kJ mol-1) H+ ) hydrogen ion (mol L-1) k ) kinetic constant ke ) equilibrium constant k0 ) frequency factor L ) lignin content (g L-1) m ) kinetic order for H+ in eq 11 R ) gas constant (kJ mol-1 K-1) SQR ) sum of squares residuals: Σ[(1 - XL)exp - (1 XL)calc]2 t ) time (min) T ) temperature (°C, K) XL ) lignin conversion (LD/L0) H+
Subscripts b ) related to bulk lignin D ) related to dissolved lignin i ) related to initial lignin j ) delignification stage: initial, bulk, or residual N ) related to native lignin r ) related to residual lignin R ) related to redepositated lignin 0 ) value at initial time
Literature Cited (1) Gilarranz, M. A.; Rodrı´guez, F.; Santos, A.; Oliet, M.; Garcı´aOchoa, F.; Tijero, J. Kinetics of Eucalyptus globulus Delignification in a Methanol-Water Medium. Ind. Eng. Chem. Res. 1999, 38 (9), 3324-3332. (2) Stockburger, P. An Overview of Near-Commercial and Commercial Solvent-Based Pulping Processes. Tappi J. 1993, 76 (6), 71. (3) 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), 4114. (4) 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.
(5) 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. (6) Paszner, L.; Cho, H. J. Organosolv Pulping: Acidic Catalysis Options and Their Effect on Fiber Quality and Delignification. Tappi J. 1989, 72 (2), 135-142. (7) Paszner, L.; Behera, N. C. Topochemistry of Softwood Delignification by Alkali Earth Metal Salt Catalysed Organosolv Pulping. Holzforschung 1989, 43 (3), 159-168. (8) Marquadt, F. W. An Algorithm for Least-Squares Estimation of Nonlinear Parameters. J. Soc. Ind. Appl. Math. 1963, 2, 431. (9) Tirtowidjojo, S.; Sarkanen, K. V.; Pla, F.; McCarthy, J. L. Kinetics of Organosolv Delignification in Batch and Flow-through Reactors. Holzforschung 1988, 42 (3), 177. (10) Sarkanen, K. V. Chemistry of Solvent Pulping. Tappi J. 1990, 73 (10), 215-219. (11) McDonough, T. J. The Chemistry of Organosolv Delignification. Tappi J. 1993, 76 (8), 186-193. (12) Kleinert, T. N. Ethanol-Water Delignification of Wood. Rate Constants and Activation Energy. Tappi J. 1975, 58 (8), 170. (13) Va´zquez, G.; Antorrena, G.; Gonza´lez, J. Kinetics of AcidCatalysed Delignification of Eucalyptus globulus Wood. Wood Sci. Technol. 1995, 29 (4), 267. (14) Park, J. K.; Phillips, J. A. Ammonia Catalyzed Organosolv Delignification of Polar. Chem. Eng. Commun. 1988, 65, 187. (15) Faass, G. S.; Roberts, R. S.; Muzzy, J. D. Buffered Solvent Pulping. Holzforschung 1989, 43 (4), 245. (16) Gilarranz, M. A.; Rodrı´guez, F.; Oliet, M.; Tijero, J. Ethanol-Water Pulping: Cooking Variables Optimization. Can. J. Chem. Eng. 1998, 76 (2), 253-260. (17) Gilarranz, M. A.; Oliet, M.; Rodrı´guez, F.; Tijero, J. Methanol-Based Pulping of Eucalyptus globulus. Can. J. Chem. Eng. 1999, 77 (3), 515-521. (18) Springer, E. L.; Harris, J. F.; Neill, W. K. Rate Studies of the Hydrotropic Delignification of Aspenwood. Tappi J. 1963, 46 (9), 551.
Received for review July 12, 1999 Revised manuscript received October 5, 1999 Accepted October 23, 1999 IE9905005
