Kinetics of ASAM and Kraft Pulping of Eucalypt Wood (Eucalyptus globulus)

Transcription

1 I. Miranda and H. Pereira: Kinetics of ASAM and Kraft Pulping 85 Holzforschung 56 (2002) Kinetics of ASAM and Kraft Pulping of Eucalypt Wood (Eucalyptus globulus) By Isabel Miranda and Helena Pereira Centro de Estudos Florestais, Instituto Superior de Agronomia, Lisboa, Portugal Keywords ASAM Kraft pulping Eucalyptus globulus Delignification Summary The kinetics of ASAM and kraft pulping of eucalypt wood (Eucalyptus globulus) were studied in relation to delignification and polysaccharide removal. In comparison to kraft, ASAM pulping had lower mass losses and delignification for the same temperature and reaction times (59.2 % at Kappa 25 vs 50.0% at Kappa 17, at 180 ºC). The ASAM pulps have a higher brightness. ASAM pulping had a short initial period with no mass loss and lignin removal, followed by two reaction phases: a main phase where 61 % of lignin was removed (at 180 ºC) and a subsequent final phase. In comparison to kraft, the main delignification rates of ASAM pulping were approximately 2.5 slower (at 180 ºC, min 1 for ASAM and for kraft pulping), and the calculated Arrhenius activation energies were higher (132.4 kj mol 1 and 83.5 kj mol 1, respectively). The loss of cellulose was relatively small (12.5 %) and lower than in kraft pulps. Introduction ASAM pulping, an alkaline sulfite process with addition of anthraquinone and methanol, has been investigated as an alternative to the kraft process to avoid air pollution problems and high bleaching costs. ASAM pulps combine the strength of kraft pulps with the brightness and easy bleachability of sulfite pulps (Patt and Kordsachia 1986; Patt et al. 1987; Kordsachia and Patt 1988). It is generally agreed that the alkaline delignification of wood is a first order reaction with respect to lignin concentration, the entire process consisting of three phases: initial, bulk and final. The initial phase is a diffusion controlled process, where approximately 20 % of the lignin is removed. The bulk phase, accounting for further removal of 70 % of the lignin, is chemically controlled and delignification proceeds at a useful rate only at temperatures above 150 ºC, mainly by cleavage of β-aryl ether bonds. In the final phase, the remaining lignin is removed at a very low rate, by cleavage of covalent lignin-carbohydrate bonds or others (Olm and Tistad 1979; Gierer 1980; Ljunggren 1980). These processes are satisfactorily modelled considering the simultaneous occurrence of three parallel reactions, with order one with respect to lignin concentration and different rate constants (Gierer 1980; Dolk et al. 1989; Chiang et al. 1990; Labidi and Pla 1992). Xylan and cellulose are depolymerized by peeling and alkaline hydrolysis of glycosidic bonds which are assumed to take place during the consecutive stages in the pulping process as the result of simultaneous reactions, as proposed by De Groot et al. (1994 and 1995). ASAM pulping has been applied successfully to several hardwoods and softwoods, as well as to annual plants. For eucalypt wood, pulp yields between 56.9 % (Kappa number 14.3) and 53.6 % (K 10.0) were reported for E. globulus (Kordsachia et al. 1992) and 46.1 % (K 20.6) for E. camaldulensis wood (Puthson et al. 1997). Kordsachia et al. (1990) obtained 64.5 % (K 24.5) and 51.7 % (K 8.9) yields with different ratios of Na 2 SO 3 /NaOH with Betula verrucosa wood, and Kordsachia and Patt (1988) obtained pulps with yields between 52.9 % and 52.5 % (K ) with Pinus sylvestris. For annual plants, very high yields with low Kappa numbers were obtained: wheat straw 52.9 %, K 7.4; elephant grass 59.9 %, K 10.5; sorghum bagasse 44.5 %, K 8.5; sugarcane bagasse 60.5 %, K 7.9 (Prof. Patt, personal communication). Kinetics of ASAM delignification have not yet been reported. In this paper, we study the kinetics and the influence of temperature on the rate and selectivity of delignification in ASAM pulping using Eucalyptus globulus wood. Materials and Methods Air-dried wood chips of Eucalyptus globulus Labill. from a composite sample made up of 20 trees harvested at 12 years of age from commercial pulpwood plantations in different locations in Portugal were used in this study. The chemical composition of the raw material was determined according to standard methods: total extractives by a soxhlet extraction sequence with dichloromethane, ethanol and water, Klason lignin by Tappi standard method 222 om 93, acid soluble lignin by Tappi useful method UM 250 and monosaccharides by HPLC after total hydrolysis. The chemical composition was the following: total extractives 3.7 %, Klason lignin 24.0 %, acid soluble lignin 4.1 % and polysaccharides 69.0 % (including: glucose 49.6 %, xylose 15.4 %, arabinose 1.0 %, mannose 1.6 %, galactose 1.1 %, rhamnose 0.3%). Small wood chips with average dimensions of 2 cm 0.2 cm 0.2 cm were manually prepared. The delignification experiments were carried out in 100 ml rotating stainless reactors which were immersed in a thermostated oil bath. Two ASAM pulpings with different chemical charges (17.5 and 19.4 % Na 2 O, referred to as Copyright 2002 Walter de Gruyter Berlin New York

2 86 I. Miranda and H. Pereira: Kinetics of ASAM and Kraft Pulping Table 1. Experimental conditions for the ASAM and kraft pulping of eucalypt wood ASAM I ASAM II kraft Chemical charge, % (as Na 2 O) (based on o.d. wood) Na 2 SO 3 /NaOH 80/20 80/20 Sulfidity, % 30 Methanol, vol % (based on total cooking liquor) AQ, % (based on o.d. wood) Liquor to wood ratio (ml:g) 4.0:1 4.0:1 4.5:1 ASAM I and ASAM II, respectively) and a reference kraft pulping were carried out under the conditions listed in Table 1. Three cooking temperatures of 160, 170 and 180 ºC were selected and the reaction time at temperature varied from 5 to 150 min. The heating time of the reaction mixture to the cooking temperature was 5 min. After reaction the reactors were cooled in an ice bath. The resulting pulps (or remaining solids) were thoroughly washed with water, the suspension was suction filtered and the solids air-dried over night at room temperature, followed by 48 h at 70 ºC. Yields were determined based on the oven-dry weight of wood chips charged to the reactor. The solids were analysed for Kappa number (Tappi standard method T 236 cm-85), Klason lignin and carbohydrate composition by HPLC, after total hydrolysis. For kinetic modelling, the rate of delignification in each pulping phase was described mathematically by a first order reaction with respect to the lignin remaining in the lignocellulose matrix, calculated as: kraft processes at 170 ºC and Table 2 shows the results obtained at the three temperatures for 150 min pulping time. In ASAM pulping there was an initial phase where little or no lignin is removed corresponding to low mass losses: e.g. 90 % total yield was obtained after 10 min at 180 ºC, as compared to 70 % in the kraft process. This initial phase was longer when the chemical charge was lower: approximately 20 min in ASAM I and under 10 min in ASAM II. This phase was not observed in the kraft process. The slow pulping in the initial phase may be explained by the very short heating time (5 min), which did not allow i=3 L/L 0 = a i exp ( k i t) i=1 where L/L 0 is the fraction of wood lignin remaining in the residue, L 0 the lignin in the wood, a i (experimental value) is the fraction of lignin susceptible to solubilization by the process, and k i is the corresponding rate constant, with i representing the reaction phase (i = 1,2,3). The values of a i were calculated from the L/L 0 values at the beginning and at the end of the corresponding i phase. Therefore, a plot of residual lignin as in L/L 0 versus time gives a straight line with the slope representing the value of k i (min 1 ). The experimental activation energy of the pulping reaction was determined using the Arrhenius equation: E ai k i = (A) exp ( ) RT where k i is the rate constant for phase i, A the Arrhenius constant, E ai the activation energy (kj mol -1 ), R the gas constant (8.314 kj K - 1 mol -1 ) and T the absolute temperature (K). A plot of ln(k i ) versus 1/T should be a straight line with the slope equal to E ai /R. A stepwise calculation method was used, first the values for the fractions a i are estimated, then the reaction rate k i for each temperature curve is calculated, followed by calculation of the activation energy E ai from the calculated k i at each temperature. The kinetics of cellulose and xylan degradation were also modelled using the same approach and assuming that both degradation reactions are of first order. Cellulose was calculated based on the glucose content after total hydrolysis minus the hemicellulosic glucose, estimated assuming a 2:1 mannose-to-glucose ratio. Results and Discussion The variation in mass yield as a function of pulping time is illustrated in Figure 1 for the ASAM II and the reference Fig. 1. Mass yield during ASAM and kraft pulping of eucalypt wood at 170 ºC. Table 2. Pulp yield, Kappa number and residual lignin obtained with 150 min pulping at three temperatures of eucalypt wood with ASAM (17.5 % and 19.5 % Na 2 O) and kraft processes Pulp yield Kappa Residual lignin (% o.d. wood) number (% o.d. pulp) ASAM I 160 ºC ºC ºC ASAM II 160 ºC ºC ºC Kraft 160 ºC ºC ºC

3 I. Miranda and H. Pereira: Kinetics of ASAM and Kraft Pulping 87 for a complete wood impregnation, cell wall swelling and AQ homogeneous distribution. In fact, it is known that a significant part of the delignification already occurs during the heating up phase of eucalypt ASAM pulpings (Kordsachia et al. 1992). Most of the solubilization occurs in the following phase, where the bulk of lignin is removed, corresponding to approximately 60 min pulping time and to the highest mass losses. In the subsequent stage the mass loss proceeds at a much slower rate. ASAM pulping was more influenced by reaction temperature than kraft pulping and only occurred at a satisfactory rate above 160 ºC. The differences between chemical charges in both ASAM pulpings were not a determinant in the pulping yield, even if the stronger reacting conditions (ASAM II) allowed for a faster delignification. In comparison to kraft, ASAMpulping shows lower delignification and mass losses for the same reaction times (59.2 % at K 25 vs 50.0 % at K 17, at 180 ºC). The ASAM and kraft pulps differed in colour, with ASAM pulps having a higher brightness. These results are in accordance with former investigations on ASAM pulping of E. globulus (Kordsachia et al. 1992) even if these authors carried out the pulping to pulps with lower Kappa number and tested various modifications, such as methanol content of the cooking liquor, kind of additional alkali source and alkali ratio. The yields from ASAM cookings carried out by Kordsachia et al. (1992) with Eucalyptus globulus wood ranged between 53.3 % with Kappa number 14.0 and 51.7 % with Kappa number It was found that the optimum methanol charge depends on the wood species and that the ratio of sodium sulfite to total alkali also influences the results in relation to pulp yield, delignification degree and brightness. There is, therefore, scope to optimise the process parameters for E. globulus, which was not the aim here. Previous studies have successfully modelled delignification kinetics in terms of two (or three) simultaneous first order processes with different reaction rates and supposed that there are two different fractions of lignin which are solubilized at different rates: one easily soluble and the other with more difficulty. Therefore, the corresponding kinetic model consists in two (or three) parallel first-order reactions, with lignin in solution as the product (Gierer 1980; Dolk et al. 1989; Chiang et al. 1990; Labidi and Pla 1992). The same was found here, where two successive delignification phases are involved in the ASAM and kraft pulpings, corresponding to a main delignification and a final delignification stage (Fig. 2). The occurrence of an initial phase was not seen in this work probably due to the fact that the lowest temperature used was 160 ºC. The plots of the adjusted logarithmic residual lignin content against pulping time at the three different temperatures for the ASAM and kraft processes were linear, indicating first-order processes (Fig. 2). The parameters a i (the fraction of initial lignin susceptible to solubilization by the process) and k i (estimated value for the rate constant) were calculated after a best straight-line fit to the experimental data by linear regression. The results for the kinetic parameters for the isothermal lignin removal during ASAM and kraft pulping are shown in Table 3. Fig. 2. Kinetics for ASAM and kraft delignification of eucalypt wood. For the ASAM pulping, rate constants of min 1 (r 2 = 0.938) for the main phase and min 1 (r 2 = 0.980) for the final delignification phase (at 180 ºC) were obtained. The rate constants for the main delignification reactions in the ASAM process were approximately 2.5 times smaller than the corresponding rate constants for the kraft process confirming the slower delignification and mass loss in ASAM pulping at the three temperatures. The delignification in the final stage proceeds at a rate approximately 10 times slower than in the main phase. At 160 ºC, during the studied time period, the ASAM delignification proceeded much slower and only the main phase was present. A comparison of the amounts of lignin removed from the wood was made on the basis of the proportions of total lignin removed (a i ). The proportion of lignin removed in the main phase (a 1 ) increases with temperature, e.g. 55 % and 61% removal, respectively, at 170 ºC and 180 ºC in ASAM pulping and 66 % and 74 %, respectively, in kraft. The comparison of both processes for the same temperature shows less lignin solubilization in ASAM. The values obtained for the rate constants on the main phase at each temperature were used to calculate the acti- Holzforschung / Vol. 56 / 2002/ No. 1

4 88 I. Miranda and H. Pereira: Kinetics of ASAM and Kraft Pulping Table 3. Kinetic parameters (rate constant and activation energy) for the lignin (for main phase) and polysaccharide removal during ASAM and kraft pulping (i = 1 for the main phase and i = 2 for the final phase of delignification) Temp. (ºC) L/L 0 = a 1 exp ( k 1 t) + a 2 exp ( k 2 t) Activation energy (kj mol 1 ) Lignin removal exp ( t) ASAM exp ( t) exp ( t) E 1 = exp ( t) exp ( t) exp ( t) exp ( t) Kraft exp ( t) exp ( t) E 1 = exp ( t) exp ( t) E 2 = 74.6 Glucose removal exp ( t) exp ( t) ASAM exp ( t) exp ( t) E 1 = exp ( t) exp ( t) E 2 = exp ( t) exp ( t) Kraft exp ( t) exp ( t) E 1 = exp ( t) exp ( t) E 2 = Xylose removal exp ( t) exp ( t) ASAM exp ( t) exp ( t) E 1 = exp ( t) exp ( t) E 2 = exp ( t) exp ( t) Kraft exp ( t) exp ( t) E 1 = exp ( t) exp ( t) E 2 = vation energy as E ASAM = kj mol 1 for the ASAM and E kraft = 83.5 kj mol 1 for the kraft process. The activation energy for the ASAM delignification of eucalypt wood is higher than the value found for the kraft pulping in the present work but similar to the range of values reported by Hubbard et al. (1992) for the alkaline hydrolysis of nonphenolic β-aryl ethers of lignin models (75.8 to kj mol 1 depending on the substituent), and to the 126 kj mol -1 and 132 kj mol 1 reported by Garland et al. (1987) for the alkaline delignification during soda pulping of young Eucalyptus diversicolor and E. regnans wood, respectively. Studies of the alkaline delignification of the hemp woody core (De Groot et al. 1994), western hemlock (Dolk et al. 1989) and Populus trichocarpa (Labidi and Pla 1992) woods have found for the bulk delignification values of 143.6, 129 and kj mol 1, respectively. For kraft pulping of southern pine chips, a value of kj mol 1 was reported (Vanchinathan and Kishnagolapan 1995). It is interesting to note that the activation energy of kj mol 1 found in the present work for the ASAM delignification is within the range of values reported for AQ-sulphite reactions (155 kj mol 1 ) and for sulphite only reactions (122 kj mol 1 ) of loblolly pine (Eagle and McDonough 1988). For neutral sulphite pulping of hardwoods, Eagle and McDonough (1988) reported an average value of 110 kj mol 1 (values ranged from 103 to 117 kj mol 1 ). In order to assess the effects of the pulping conditions on the other cell wall components, the mass fraction of cellulose and xylan remaining in the lignocellulose matrix was plotted against pulping time at the three different temperatures for the ASAM and kraft processes (Figs. 3 and 4). The cellulose is resistant to alkaline hydrolysis and the losses are assumed to be determined by peeling reactions (Rydholm 1965). Figure 4 confirms that the cellulose removal proceeds slowly in both ASAM and kraft processes. After 150 min of cooking at 180 ºC, the extent of cellulose loss reached the maximum values of 12.5 % with the ASAM process and 20 % with the kraft process. The activation energies found in the present work were 96.3 kj mol 1 and kj mol 1 for the ASAM and kraft processes, respectively, within the range of those reported for peeling reactions (102.8 kj mol 1, Haas et al. 1967; 88.6 kj mol 1,Lai and Sarkanen 1967; kj mol 1, Young and Liss 1978). Report of higher activation energy (141.5 kj mol 1 ) was also given by Agarwal et al. (1992) for cellulose degradation in alkaline pulping. Yield losses of xylan by alkaline degradation are assumed to be determined by dissolution, followed by both hydrolysis and peeling reactions (Fengel and Wegener 1984). The removal of xylan fragments in the initial stage for both ASAM and kraft processes proceeds very quickly with a high reaction rate, and at 180 ºC, respectively, and corresponds to 66 % and 64 % xylan removal. For ASAM pulping, the rates of xylan removal are similar to the delignification rates, with a higher reactivity for the lower temperature. In this phase approximately 60 %

5 I. Miranda and H. Pereira: Kinetics of ASAM and Kraft Pulping 89 Fig. 3. Polysaccharidic glucose removal from eucalypt wood during ASAM and kraft pulping (fraction of initial component mass). Fig. 4. Polysaccharidic xylose removal from eucalypt wood during ASAM and kraft pulping (fraction of initial component mass). of xylan is removed (a 1 = 0.66 for ASAM and a 1 = 0.64 for kraft) and the calculated activation energy is low (E (1)ASAM = 74.8 kj mol 1 ; E (1)kraft = 91.5 kj mol 1 ), which is close to the 89.1 kj mol 1 found by De Groot et al. (1995) for the first phase of xylan removal in alkaline pulping of hemp woody core, and within the range kj mol 1 found for peeling reactions by Young and Liss (1978). For the second reaction phase a lower rate (approximately 10 times slower) and slightly higher activation energy were found (E (2)ASAM = 84.0 kj mol 1 ; E (2)kraft = kj mol 1 ). These values are under the values of kj mol 1 and kj mol 1 calculated by De Groot et al. (1995) for the second reaction phase of xylan removal in alkaline delignification of hemp woody core and by Lai and Sarkanen (1967) for the secondary peeling of cotton cellulose degradation. The graphic presentation of the removal of lignin, cellulose and hemicellulose from eucalypt wood during ASAM and kraft processes at 180 ºC allows comparison of the selectivity of both processes in relation to the main cell wall components (Fig. 5). A higher cellulose content is present in the ASAM pulps in relation to kraft pulps, e.g. at 180 ºC, and 150 min. pulping, 88 % and 76 % of glucose was retained in ASAM and kraft pulping, respectively. However, in the conditions used, the ASAM process showed a lower selectivity in relation to lignin. For instance, the ratio of lignin:cellulose fractions remaining after 150 min pulping at 180 ºC was 0.4 and 0.25, respectively, for ASAM and kraft pulping, and the ratio lignin:xylan was 1.6 and 1.1. Conclusion ASAM delignification kinetics can be modelled as a firstorder process in two successive reactive phases, following a short induction period without lignin solubilization. Delignification proceeds at a satisfactory rate at temperatures of 170 ºC or above and the activation energies are higher than for kraft pulping. Xylan loss is fast in the early stage of the cooking and appears to be equivalent for both ASAM and kraft processes. The loss of cellulose was relatively small, and lower than in kraft pulping. Holzforschung / Vol. 56 / 2002/ No. 1

6 90 I. Miranda and H. Pereira: Kinetics of ASAM and Kraft Pulping Fig. 5. Variation of total lignin, cellulose and hemicelluloses content with time at 180 ºC for E. globulus wood delignified by ASAM and kraft processes (as a percentage of starting material). Acknowledgements The work was supported by Centro de Estudos Florestais under financing from Fundação para a Ciência e Tecnologia (Portugal). We thank Prof. Rudolf Patt for the useful suggestions and comments. References Agarwal, N., W.T. McKean and R.R. Gustafson Cellulose degradation kinetics in alkaline pulping. Appita 45 (3), Chiang, V.L., J. Yu and R.C. Eckert Isothermal reaction kinetics of kraft delignification of Douglas fir. J. Wood Chem. Technol. 10 (3), De Groot, B., J.E.G. van Dam, R.P. van der Zwan and K. van t Riet Simplified kinetic modelling of alkaline delignification of hemp wood core. Holzforschung 48 (3), De Groot, B., J.E.G. van Dam, R.P. and K. van t Riet Alkaline pulping of hemp woody core: Kinetic modelling of lignin, xylan and cellulose extraction and degradation. Holzforschung 49 (4), Dolk, M., J.F. Yan and J.L. McCarthy Lignin 25. Kinetics of delignification of Western hemlock in flow-through reactors under alkaline conditions. Holzforschung 43 (2), Eagle, A.J. and T.J. McDonough A kinetic study of high yield AQ-sulphite pulping of loblolly pine. Appita 41(2), Fengel, D. and G. Weneger Wood: Chemistry, Ultrastructure, Reactions. Walter De Gruyter, Berlin. Garland, C.P., F.C. James, P.J. Nelson and A.F.A. Wallis A study of the delignification of Eucalyptus diversicolor wood during soda pulping. Appita 40 (2), Gierer, J Chemical aspects of kraft pulping. Wood Sci. Technol. 14 (4), Haas, D.W., B.F. Hrutfiord and K.V. Sarkanen Kinetic study on the alkaline degradation of cotton hydrocellulose. J. Appl. Polymer Sci. 11, Hubbard, T.F., Jr., T.P. Schultz and T.H. Fisher Alkaline hydrolysis of nonphenolic β-o-4 lignin model dimmers: Substituent effects on the leaving phenoxide in neighboring group vs. direct nucleophilic attack. Holzforschung 46 (4), Kordsachia, O. and R. Patt Full bleaching of ASAM pulps without chlorine compounds. Holzforschung 42 (3), Kordsachia, O., B. Reipschläger and R. Patt ASAM pulping of birch wood and chlorine free pulp bleaching. Paperi Puu 72 (1), Kordsachia, O., B. Wandinger and R. Patt Some investigations on ASAM pulping and chlorine free bleaching of Eucalyptus from Spain. Holz Roh-Werkstoff 50, Labidi, A. and F. Pla Délignification en milieu alcalin de bois feuillus à l aide d un réacteur à lit fixe et à faible temps de passage. Partie II. Développements cinétiques. Holzforschung 46 (2), Lai, Y.-Z. and K.V. Sarkanen Kinetics of alkaline hydrolysis of glycosidic bonds in cotton cellulose. Cellulose Chem. Technol. 1 (5), Ljunggren, S The significance of aryl ether cleavage in kraft delignification of softwood. Svensk Papperstidn. 83 (13), Olm, L. and G. Tistad Kinetics of the initial stage of kraft pulping. Svensk Papperstidn. 82 (15), Patt, R. and O. Kordsachia Herstellung von Zellstoffen unter Verwendung von alkalischen Sulfitlösungen mit Zusatz von Anthrachinon und Methanol. Papier 40 (10A), V1 V8. Patt, R., O. Kordsachia and J. Knoblauch The ASAM process Alkaline sulfite, anthraquinone, methanol pulping. In: International Symposium on Wood and Pulping Chemistry, Paris, Proceedings. pp Puthson, P., O. Kordsachia, J. Odermatt, M. Zimmermann and R. Patt ASAM pulping of Eucalyptus camaldulensis and TCF Bleaching of the resulting pulps. Holzforschung 51 (3), TAPPI Test Methods TAPPI Press, Atlanta, USA. TAPPI Useful Methods TAPPI Press, Atlanta, USA. Rydholm, S.A Pulping Processes. Interscience Publishing, New York. Vanchinathan, S. and G.A. Kishnagolapan Kraft delignification kinetics based on liquor analysis Tappi J. 78 (3), Young, R.A. and L. Liss A kinetic study of the alkaline endwise degradation of gluco- and galactomannans. Cellulose Chem. Technol. 12, Received July 28 th 2000 Isabel Miranda Helena Pereira Centro de Estudos Florestais Departamento de Engenharia Florestal Instituto Superior de Agronomia Universidade Técnica de Lisboa Tapada da Ajuda Lisboa Portugal Tel.: Fax.: