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Step-change variation of acid concentration in a percolation reactor for hydrolysis of hardwood hemicellulose
Bioresource Technology 72 (2000) 289±294
Step-change variation of acid concentration in a percolation reactor for hydrolysis of hardwood hemicellulose Sung Bae Kima,*, Dong Moon Yuma, Soon Chul Parkb a
Division of Applied Chemical Engineering and R I I T, Gyeongsang National University, Chinju 660-701, South Korea b Biomass Research Team, Korea Institute of Energy Research, Taejon, South Korea Received 14 January 1999; received in revised form 23 April 1999; accepted 17 May 1999
Abstract A potential advantage of applying two dierent acid concentrations in a percolation reactor was investigated both theoretically and experimentally. In the proposed mode of operation, a lower acid concentration was applied ®rst to recover the easily hydrolyzable hemicellulose, then a higher acid concentration to recover the remaining hemicellulose. The reaction conditions applied were 160±180°C and 0.05±0.2% of sulfuric acid. The model predicted that the step-change operation of acid concentration increased the product yield by 3±4% over that of uniform acid concentration. The dierence in yield was not fully veri®able experimentally since it was barely over the experimental error range. However, a signi®cant dierence was found in sugar decomposition, the step-change process showing much less furfural formation. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biomass; Hardwood; Percolation; Acid hydrolysis; Pretreatment
1. Introduction Utilization of hemicellulose is an important component in the overall biomass conversion process as it accounts for 10±40% of total biomass. Dilute acid hydrolysis is one of the feasible methods applicable to recover hemicellulose from cellulosic biomass (McMillan, 1992). The major problem of acid hydrolysis is that the decomposition of monomeric sugars produced during the reaction takes place simultaneously with the hydrolysis of the polysaccharides. Because of this problem, it is necessary to choose a proper reaction condition, reactor type and operation mode. A percolation reactor has been a popular choice in hemicellulose processing (Cahela et al., 1983). Use of this reactor can minimize sugar decomposition because the sugar product is removed from the reactor as it is formed. This allows high sugar yields in comparison to a batch reactor (Kim and Lee, 1987). A relatively high sugar concentration is also obtainable because of the high solid-to-liquid ratio that exists in a packed-bed * Corresponding author. Tel.: +82-591-751-5385; fax: +82-591-7531806.
column reactor. As an unsteady-state reactor the percolation reaction process can be designed in such a way that the reaction condition and operation mode can be varied during its operation. It is a relevant point because hemicellulose is regarded as a biphasic substrate, i.e., it is composed of two dierent fractions, easy-to-hydrolyze and dicult-to-hydrolyze fractions. Dierent reaction conditions can thus be applied to each of the two different fractions of hemicellulose. As one of such approaches, a non-uniform temperature regimen has been adopted, in which a lower temperature is applied to the easily hydrolyzable xylan and a higher temperature is subsequently applied to hydrolyze the remaining xylan (Kim et al., 1993, 1994; Chen et al., 1996; Torget and Hsu, 1994). This operation was proven to outperform the conventional uniform-temperature operation. Following the same concept, one can vary the acid concentration during the reactor operation for the purpose of improving the yield of hemicellulose. This method may prove particularly eective when the sugar decomposition reaction is less sensitive to the acid concentration than is the hydrolysis reaction. The main intent of this investigation was to prove the validity of the proposed concept from theoretical and experimental grounds.
0960-8524/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 0 8 1 - 4
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S.B. Kim et al. / Bioresource Technology 72 (2000) 289±294
2. Methods 2.1. Materials Oak wood chips supplied from the Korea Institute of Energy Research were milled using a laboratory knife mill. Milled material was screened to the nominal size of 40±100 mesh. The composition of untreated oak wood was 44.0% (w/v) glucan, 18.7% XMG (xylan + mannan + galactan), 22.1% klason lignin, 2.6% acid soluble lignin, 0.29% ash, and 1.8% extractives. 2.2. Experimental set-up and operation The percolation reactor set-up was quite similar to that previously reported (Kim and Lee, 1996 ). The system consisted of a stock solution reservoir, pump, programmable drying oven, reactor, and liquid holding tank, which also served as a back-pressure vessel. Aqueous sulfuric acid solution was pumped by a metering pump (TPC minipump) to a packed-bed reactor through a preheating coil. The ¯ow rate of sulfuric acid solution was monitored by ¯ow meter. The reactor was constructed out of SS 316 to the dimension of 1.6 ´ 14.7 cm2 (ID ´ L) (internal volume 29.6 cm3 ). The reactor temperature was controlled in a temperature-programmable oven (Fisher Scienti®c) and monitored by a digital thermometer. An autoclave (Parr Instrument) was used as a liquid holding tank to which a nitrogen cylinder was connected to apply back pressure (160 psig), preventing evaporation of liquid. The biomass feed (9.7 g) was presoaked with sulfuric acid solution overnight. To carry out the reaction, oven temperature was initially set at 230°C. After 10 min, inside reactor temperature reached approximately 100°C. At this time sulfuric acid solution was pumped to the reactor. This operation reduced the liquid input as well as the preheating time. After the reaction, the reactor was pumped with water or sulfuric acid to remove the residual sugar trapped in the treated biomass. The euent collected in the holding tank was ®ltered and analyzed for its composition. The wet solid discharged from the reactor was dried at 105°C overnight for measurement of weight loss. It was further subjected to chemical analysis. 2.3. Analytical methods The solid samples were analyzed for sugars, Klason lignin, acid soluble lignin and ash following the procedures described in the NREL Chemical Analysis & Testing Standard Procedure (Ehrman and Magill, 1992). Since the hydrolyzates contained signi®cant amounts of oligomers, a secondary hydrolysis was carried out with 4% sulfuric acid at 121°C for 45 min. Sugars and decomposition products were measured by RI and UV detector, respectively. The column used in HPLC was a
Bio-Rad Aminex HPX-87H. Since this column does not resolve xylose, mannose and galactose, the combined value of xylan + mannan + galactan is expressed as XMG.
3. Model development Hemicellulose is in general more easily hydrolyzable in dilute acids than cellulose. But it is also known that a certain fraction of hemicellulose is quite dicult to hydrolyze. The easily hydrolyzable fraction of hemicellulose in various cellulosic materials has been reported to be 60±80% of the total pentosan (Conner, 1984; Grohmann et al., 1985; Springer et al., 1963; Springer and Zoch, 1968). Under the reaction conditions adopted in this study, a signi®cant amount of oligomers were formed at the initial stage of reaction. The kinetic model was therefore set into the pattern described below:
where He and Hd are the easy-to-hydrolyze and hardto-hydrolyze hemicellulose fractions, respectively. O is soluble oligomers; and X is XMG as monomer. Dierential equations and analytical solutions for each component are described elsewhere (Chen et al., 1996). The kinetic parameters determined in the previous study (Kim et al., 1997) were used, and are listed in Table 1. Each rate constant, ki , is expressed by the Arrhenius equation with the addition of an acid term such that ki koi (Ac)Ni exp(ÿEi/RT), where koi : frequency factor; Ac: acid concentration; Ni: acid concentration exponent; Ei: activation energy. The percolation reactor was a packed-bed, ¯owthrough-type reactor. In the modeling procedure, it was assumed that the internal and external diusion eects were negligible. The details of the modeling procedure are described elsewhere (Chen et al., 1996). Table 1 Kinetic parameters of hemicellulose hydrolysis ki koi (Ac)Ni exp (ÿEi / RT)a
a
i
koi (minÿ1 (% w/v)ÿNi )
Ni
Ei (cal/g-mol)
1 2 3 4
1.22 ´ 1014 6.03 ´ 1013 8.99 ´ 1013 4.82 ´ 1013
0.66 1.18 1.37 0.71
28 400 28 000 27 100 30 700
Data coverage: 150±190°C, 0.05±0.2% H2 SO4 .
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3.1. Operation with uniform acid concentration The total yield of sugar including XMG monomer and its oligomers is expressed as YT uniform concentration Fe Yoe Yxe Fd Yod Yxd ;
2
where Fe and Fd are the easy-to-hydrolyze and hard-tohydrolyze hemicellulose fractions, respectively. Y is the yield for uniform acid concentration operation: subscripts o and x identify oligomers and XMG; and subscripts e and d are easy and hard fractions, respectively. 3.2. Step-change acid concentration The term s1 represents the dimensionless reaction time up to the acid concentration shifting point, and s2 represents the rest of the reaction time. Total yield consists of four parts, i.e., easy fraction reacting for the duration of s1 , hard for s1 , easy for s2 , and hard for s2 . The total yield can be obtained by multiplying exp(ÿbe s1 ) and exp(ÿbd s1 ) to the yield for the duration of s2 and is expressed as YT step Fe Yoe1 Yxe1 Re Yoe2 Yxe2 Fd Yod1 Yxd1 Rd Yod2 Yxd2 ;
3
where Re exp ÿbe s1 ; Rd exp ÿbd s1 ; be k1 L=Uo ; bd k2 L=Uo , where L is reactor length; Uo is velocity inside percolation reactor.
Fig. 1. Eect of reaction time on theoretical yield under step-change operation (reaction condition: 170°C, acid shifting time 10 min).
To determine the optimum acid shifting point, the eect of acid shifting time on yield was investigated as shown in Fig. 2. At the early phase of the process, high yield values were obtained as the acid shifting time was decreased. But at the latter phase of the process, the maximum yield reached was lower as the acid shifting time was decreased. Since the acid concentration inside the reactor was higher as the acid shifting time was decreased, higher yield values can be obtained at the early phase, but lower ones at the latter phase due to more
4. Results and discussion 4.1. Model prediction The simulation of the percolation reactor with a stepchange in acid concentration was conducted following the procedure described in the previous section. Three sets of acid-concentration ranges were studied: 0.05± 0.1%, 0.05±0.2%, and 0.1±0.2%. Fig. 1 shows the eect of the reaction time on product yield. The product yield means the total yield of XMG including monomer and oligomers. In all cases, the maximum yield was found to be about 96%. With shifting of the acid concentration from 0.05% to 0.1%, a longer reaction time was required to reach the maximum point. In the other two cases (0.05±0.2% and 0.1±0.2%), about the same level of yields were obtained with a reaction time of 25±30 min. After this point, the yield of the 0.1±0.2% case was slightly lower than that of the 0.05±0.2% case since sugars are decomposed more quickly under higher acid conditions than lower acid conditions.
Fig. 2. Eect of acid shifting time on theoretical yield under stepchange operation (reaction condition: 170°C, 0.05 ® 0.2% H2 SO4 , t1 acid shifting time).
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sugar decomposition. As shown in Fig. 2, it may be very important to select a proper acid shifting time. Since sugar concentration is inversely related to reaction time and yield in a percolation reactor, the reaction time should be reasonable for both sugar concentration and yield. In this case the optimum acid shifting time was found to be 10 min. Fig. 3 shows the eect of the reaction temperature on product yield at a ®xed reaction time. The maximum yield at 0.05±0.1% was lower than the other two cases at 160°C and 170°C. At 170°C the acid level had little eect on yield in the cases of 0.05±0.2% and 0.1±0.2%, but the yields of both cases were about 5% higher than that of the 0.05±0.1% case. At 180°C, however, the yield at 0.05±0.2% was about 3% higher than that of the 0.1± 0.2% case. The modeling results here collectively indicated that the acid concentration range of 0.05±0.2% was the best acid level policy of the three in terms of the yield and acid consumption. Applying two dierent liquid velocities along with varying acid concentration may have a similar eect on the process. In order to verify this, the velocity ratio, Uo2 =Uo1 (the ratio of velocity in high acid-concentration phase to the velocity in low acid-concentration phase) was introduced in the percolation reactor simulation program. The results in Fig. 4 indeed prove that the concept was valid because the product yield increased as the ¯ow-rate ratio was increased. As the ¯ow ratio was increased, the yield increased but then the sugar concentration decreased. Therefore, it was necessary to decide the optimum ¯ow-rate ratio. An example is shown here: Suppose that the allowable sugar concentration was 2.5%, the optimum ratio was about 1.5 for
Fig. 3. Eect of reaction temperature on theoretical yield under stepchange operation at 30 min of reaction time (reaction condition: acid shifting time 10 min).
Fig. 4. Eect of ¯ow-rate ratio on theoretical yield and sugar concentration under step-change operation (reaction condition: 170°C, acid shifting time 10 min, total reaction time 30 min).
the 0.05±0.2 case. At this ratio, the product yield obtained was 97%. 4.2. Comparison between the step-change method and the uniform acid method A direct comparison between the dierent modes of operation is shown in Fig. 5. At 30 min of reaction time, the yields of two step-change modes (0.05±0.2% and 0.1± 0.2%) were 97% and 96%, respectively. These values are 3±4% higher than those of two uniform modes (0.1%
Fig. 5. Eect of acid concentration on theoretical yield under uniform and step-change operations (reaction condition: 170°C, acid shifting time 10 min).
S.B. Kim et al. / Bioresource Technology 72 (2000) 289±294
293
4.3. Experimental comparison In order to support the theoretical results discussed in the preceding part, an experimental veri®cation was attempted. In the percolation process, a relatively short reaction time was applied since the reaction time is inversely related with product concentration. Thus the liquid amount introduced during the operation often was not sucient to recover all the hemicellulose sugars. Since the sulfuric acid concentration applied in this work was extremely low, a signi®cant amount of oligomers were produced during the hydrolysis reaction. Because of the large molecular size, the oligomers were not easily removed from the solid biomass matrix. To overcome this problem, a sulfuric acid solution was used as a leaching solvent in this experiment. Leaching was applied to recover the remaining sugars and oligomers trapped within the biomass after the percolation process. Table 2 shows the eect of leaching solvent on XMG recovery at the acid shifting time of 5 and 10 min in the 0.05±0.2% case. Water and dilute sulfuric acid (0.2%) were used as the leaching solvents. The XMG yield with acid shifting time of 5 min was slightly higher than that with acid shifting time of 10 min because of the high acid condition in the reactor. It was also found that dilute sulfuric acid as leaching solvent was more eective than water. Our test proved that the XMG yield with sulfuric acid was 0.5±0.6% higher than that with water. For comparison purpose, an identical leaching condition was applied for both cases (Table 3). The dierence between the two cases in XMG yields, however, was within the experimental error range. But the furfural production of the step change operation was about 40% less than that with uniform acid operation. Since the furfural amount produced in the 0.05±0.2% case was equivalent to a 3% XMG yield, it was considered that approximately 5% of XMG was still trapped inside the solid matrix. This residual XMG could be recovered if extra leaching solvent was used. Our results were almost identical to the ®ndings of Torget and Hsu (1994), in which the xylose yield was 92% and the product concentration was about 2.5% at the reaction condition of
Fig. 6. Theoretical yield vs. sugar product concentration under uniform and step-change operations (reaction condition: 170°C, t reaction time, acid shifting time 10 min).
and 0.2%). Considering that the maximum yield of a batch reactor is only 85% under the identical experimental conditions (Kim and Lee, 1987; Grohmann et al., 1985), these yields were indeed signi®cantly higher than that of a batch reactor, approximately 8% for uniform mode and 11% for step-change mode. In the percolation process, the product yield was inversely related to product concentration. Therefore, the yield value alone was rather meaningless. Fig. 6 shows the yield vs. sugar concentration curves obtainable under uniform and step-change operations. The yield values followed the inverse relationship with product concentration. When the yield was compared at a ®xed reaction time (t 30 min), the yield and sugar concentration of the 0.05±0.2% case were 96% and 3.2%, respectively. These values were certainly higher than those obtainable from the uniform acid method, 93% and 3.1%. In view of the fact that the sugar concentration level in the actual percolation process was in the range of 2.5±3.5%, the data in Fig. 6 indeed proved that the step-change operation produced higher yields than uniform acid operation.
Table 2 Eect of acid shifting time and leaching solvent on glucan and XMG recovery in residual solid and hydrolyzatea Acid conc. shifting time (min)
Leaching solvent
Solid remaining (%)
Untreated 5 5 10 10
Glucan (%) Solid
Liquid
44.0 H2 O H2 SO4 H2 O H2 SO4
59.6 59.2 59.6 59.5
40.4 41.1 41.3 41.0
1.7 1.9 1.5 1.9
XMG (%) Total
Solid
44.0
18.7
42.1 43.0 42.8 42.9
0.0 0.0 0.0 0.0
Liquid
Total
Furfural (%)
18.7 16.7 17.3 16.5 17.0
16.7 17.3 16.5 17.0
0.45 0.51 0.55 0.46
a Reaction condition: 170°C, 0.05 ® 0.2% sulfuric acid, 1 ml/min, 15 min. Leaching condition: 2 ml/min, 20 min, 170 ® 158°C. Note: all sugars and furfural content based on the original oven-dry untreated biomass.
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Table 3 Comparison of uniform and step-change modes concentration on glucan and XMG recovery in residual solid and hydrolyzatea Reaction condition
Solid remaining (%)
Untreated 0.05% 0.1% 0.2% 0.05±0.2%
Glucan (%) Solid
XMG (%) Liquid
44.0 62.3 60.8 59.3 60.5
41.5 41.2 40.8 41.2
1.5 1.6 2.1 1.9
Total
Solid
44.0
18.7
43.0 42.8 42.9 43.1
0.0 0.0 0.0 0.0
Furfural (%) Liquid
Total 18.7
16.4 17.1 17.0 17.2
16.4 17.1 17.0 17.2
0.43 0.72 0.73 0.51
a Reaction condition: 170°C, sulfuric acid, 1 ml/min, 10 min, acid shifting time 5 min. Leaching condition: 0.2% sulfuric acid, 2 ml/min, 20 min, 170 ® 158°C. Note: all sugars and furfural content based on the original oven-dry untreated biomass.
0.73% sulfuric acid and temperature step-change operation (140±170°C). From the theoretical work, about 3% increase of product yield was anticipated by way of the acid stepchange operation. However, the experimental work showed an yield increase of only about 1%, which was short of full veri®cation of the theoretical prediction. The non-ideal behavior in the reactor operation such as dispersion and diusion eect of sugars as well as acid may have minimized the dierence predicted by the model. A distinct advantage, however, was that sugar decomposition was signi®cantly lower in the case of the step-change method. Acknowledgements This research was supported by the Ministry of Commerce, Industry, and Energy, South Korea. References Cahela, D.R., Lee, Y.Y., Chambers, R.P., 1983. Modeling of percolation process in hemicellulose hydrolysis. Biotechnol. Bioeng. 25, 3±17. Chen, R., Lee, Y.Y., Torget, R., 1996. Kinetics and modeling investigation on two-stage reverse ¯ow reactor as applied to dilute-acid treatment of agricultural residues. Appl. Biochem. Biotechnol. 57/58, 133±146.
Conner, A.H., 1984. Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis. Wood Fiber Sci. 16 (2), 268±277. Ehrman, T., Magill, K., 1992. NREL ± CAT Standard Procedure No.001±003, Golden, CO. Grohmann, K., Torget, R., Himmel, M., 1985. Optimization of dilute acid pretreatment of biomass. Biotechnol. Bioeng. Symp. 15, 59± 80. Kim, B.J., Lee, Y.Y., Torget, R., 1993. An optimal temperature policy of percolation process as applied to dilute-acid hydrolysis of biphasic hemicellulose. Appl. Biochem. Biotechnol. 39/40, 119±129. Kim, B.J., Lee, Y.Y., Torget, R., 1994. Modi®ed percolation process in dilute-acid hydrolysis of biphasic hemicellulose. Appl. Biochem. Biotechnol. 45/46, 113±129. Kim, S.B., Kim, S.J., Yum, D.M., Park, S.C., 1997. Kinetics in dilute acid hydrolysis of wood hemicellulose. Korean J. Biotechnol. Bioeng. 12 (4), 402±409. Kim, S.B., Lee, Y.Y., 1996. Fractionation of herbaceous biomass by ammonia-hydrogen peroxide percolation treatment. Appl. Biochem. Biotechnol. 57/58, 147±156. Kim, S.B., Lee, Y.Y., 1987. Kinetics in acid-catalyzed hydrolysis of hardwood hemicellulose. Biotechnol. Bioeng. Symp. 17, 71±84. McMillan, J., 1992. Processes for Pretreating Lignocellulosic Biomass: A Review, NREL/TP-421-4978, National Renewable Energy Laboratory, Golden, CO. Springer, E.L., Harris, J.F., Neill, W.K., 1963. Rate studies of the hydrotropic deligni®cation of aspen wood. Tappi 46, 551±555. Springer, E.L., Zoch, L.L., 1968. Hydrolysis of xylan in dierent species of hardwood. Tappi 51, 214±218. Torget, R., Hsu, T., 1994. Two-temperature dilute-acid prehydrolysis of hardwood xylan using a percolation process. Appl. Biochem. Biotechnol. 45/46, 5±22.
Step-change variation of acid concentration in a percolation reactor for hydrolysis of hardwood hemicellulose Sung Bae Kima,*, Dong Moon Yuma, Soon Chul Parkb a
Division of Applied Chemical Engineering and R I I T, Gyeongsang National University, Chinju 660-701, South Korea b Biomass Research Team, Korea Institute of Energy Research, Taejon, South Korea Received 14 January 1999; received in revised form 23 April 1999; accepted 17 May 1999
Abstract A potential advantage of applying two dierent acid concentrations in a percolation reactor was investigated both theoretically and experimentally. In the proposed mode of operation, a lower acid concentration was applied ®rst to recover the easily hydrolyzable hemicellulose, then a higher acid concentration to recover the remaining hemicellulose. The reaction conditions applied were 160±180°C and 0.05±0.2% of sulfuric acid. The model predicted that the step-change operation of acid concentration increased the product yield by 3±4% over that of uniform acid concentration. The dierence in yield was not fully veri®able experimentally since it was barely over the experimental error range. However, a signi®cant dierence was found in sugar decomposition, the step-change process showing much less furfural formation. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biomass; Hardwood; Percolation; Acid hydrolysis; Pretreatment
1. Introduction Utilization of hemicellulose is an important component in the overall biomass conversion process as it accounts for 10±40% of total biomass. Dilute acid hydrolysis is one of the feasible methods applicable to recover hemicellulose from cellulosic biomass (McMillan, 1992). The major problem of acid hydrolysis is that the decomposition of monomeric sugars produced during the reaction takes place simultaneously with the hydrolysis of the polysaccharides. Because of this problem, it is necessary to choose a proper reaction condition, reactor type and operation mode. A percolation reactor has been a popular choice in hemicellulose processing (Cahela et al., 1983). Use of this reactor can minimize sugar decomposition because the sugar product is removed from the reactor as it is formed. This allows high sugar yields in comparison to a batch reactor (Kim and Lee, 1987). A relatively high sugar concentration is also obtainable because of the high solid-to-liquid ratio that exists in a packed-bed * Corresponding author. Tel.: +82-591-751-5385; fax: +82-591-7531806.
column reactor. As an unsteady-state reactor the percolation reaction process can be designed in such a way that the reaction condition and operation mode can be varied during its operation. It is a relevant point because hemicellulose is regarded as a biphasic substrate, i.e., it is composed of two dierent fractions, easy-to-hydrolyze and dicult-to-hydrolyze fractions. Dierent reaction conditions can thus be applied to each of the two different fractions of hemicellulose. As one of such approaches, a non-uniform temperature regimen has been adopted, in which a lower temperature is applied to the easily hydrolyzable xylan and a higher temperature is subsequently applied to hydrolyze the remaining xylan (Kim et al., 1993, 1994; Chen et al., 1996; Torget and Hsu, 1994). This operation was proven to outperform the conventional uniform-temperature operation. Following the same concept, one can vary the acid concentration during the reactor operation for the purpose of improving the yield of hemicellulose. This method may prove particularly eective when the sugar decomposition reaction is less sensitive to the acid concentration than is the hydrolysis reaction. The main intent of this investigation was to prove the validity of the proposed concept from theoretical and experimental grounds.
0960-8524/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 0 8 1 - 4
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S.B. Kim et al. / Bioresource Technology 72 (2000) 289±294
2. Methods 2.1. Materials Oak wood chips supplied from the Korea Institute of Energy Research were milled using a laboratory knife mill. Milled material was screened to the nominal size of 40±100 mesh. The composition of untreated oak wood was 44.0% (w/v) glucan, 18.7% XMG (xylan + mannan + galactan), 22.1% klason lignin, 2.6% acid soluble lignin, 0.29% ash, and 1.8% extractives. 2.2. Experimental set-up and operation The percolation reactor set-up was quite similar to that previously reported (Kim and Lee, 1996 ). The system consisted of a stock solution reservoir, pump, programmable drying oven, reactor, and liquid holding tank, which also served as a back-pressure vessel. Aqueous sulfuric acid solution was pumped by a metering pump (TPC minipump) to a packed-bed reactor through a preheating coil. The ¯ow rate of sulfuric acid solution was monitored by ¯ow meter. The reactor was constructed out of SS 316 to the dimension of 1.6 ´ 14.7 cm2 (ID ´ L) (internal volume 29.6 cm3 ). The reactor temperature was controlled in a temperature-programmable oven (Fisher Scienti®c) and monitored by a digital thermometer. An autoclave (Parr Instrument) was used as a liquid holding tank to which a nitrogen cylinder was connected to apply back pressure (160 psig), preventing evaporation of liquid. The biomass feed (9.7 g) was presoaked with sulfuric acid solution overnight. To carry out the reaction, oven temperature was initially set at 230°C. After 10 min, inside reactor temperature reached approximately 100°C. At this time sulfuric acid solution was pumped to the reactor. This operation reduced the liquid input as well as the preheating time. After the reaction, the reactor was pumped with water or sulfuric acid to remove the residual sugar trapped in the treated biomass. The euent collected in the holding tank was ®ltered and analyzed for its composition. The wet solid discharged from the reactor was dried at 105°C overnight for measurement of weight loss. It was further subjected to chemical analysis. 2.3. Analytical methods The solid samples were analyzed for sugars, Klason lignin, acid soluble lignin and ash following the procedures described in the NREL Chemical Analysis & Testing Standard Procedure (Ehrman and Magill, 1992). Since the hydrolyzates contained signi®cant amounts of oligomers, a secondary hydrolysis was carried out with 4% sulfuric acid at 121°C for 45 min. Sugars and decomposition products were measured by RI and UV detector, respectively. The column used in HPLC was a
Bio-Rad Aminex HPX-87H. Since this column does not resolve xylose, mannose and galactose, the combined value of xylan + mannan + galactan is expressed as XMG.
3. Model development Hemicellulose is in general more easily hydrolyzable in dilute acids than cellulose. But it is also known that a certain fraction of hemicellulose is quite dicult to hydrolyze. The easily hydrolyzable fraction of hemicellulose in various cellulosic materials has been reported to be 60±80% of the total pentosan (Conner, 1984; Grohmann et al., 1985; Springer et al., 1963; Springer and Zoch, 1968). Under the reaction conditions adopted in this study, a signi®cant amount of oligomers were formed at the initial stage of reaction. The kinetic model was therefore set into the pattern described below:
where He and Hd are the easy-to-hydrolyze and hardto-hydrolyze hemicellulose fractions, respectively. O is soluble oligomers; and X is XMG as monomer. Dierential equations and analytical solutions for each component are described elsewhere (Chen et al., 1996). The kinetic parameters determined in the previous study (Kim et al., 1997) were used, and are listed in Table 1. Each rate constant, ki , is expressed by the Arrhenius equation with the addition of an acid term such that ki koi (Ac)Ni exp(ÿEi/RT), where koi : frequency factor; Ac: acid concentration; Ni: acid concentration exponent; Ei: activation energy. The percolation reactor was a packed-bed, ¯owthrough-type reactor. In the modeling procedure, it was assumed that the internal and external diusion eects were negligible. The details of the modeling procedure are described elsewhere (Chen et al., 1996). Table 1 Kinetic parameters of hemicellulose hydrolysis ki koi (Ac)Ni exp (ÿEi / RT)a
a
i
koi (minÿ1 (% w/v)ÿNi )
Ni
Ei (cal/g-mol)
1 2 3 4
1.22 ´ 1014 6.03 ´ 1013 8.99 ´ 1013 4.82 ´ 1013
0.66 1.18 1.37 0.71
28 400 28 000 27 100 30 700
Data coverage: 150±190°C, 0.05±0.2% H2 SO4 .
S.B. Kim et al. / Bioresource Technology 72 (2000) 289±294
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3.1. Operation with uniform acid concentration The total yield of sugar including XMG monomer and its oligomers is expressed as YT uniform concentration Fe Yoe Yxe Fd Yod Yxd ;
2
where Fe and Fd are the easy-to-hydrolyze and hard-tohydrolyze hemicellulose fractions, respectively. Y is the yield for uniform acid concentration operation: subscripts o and x identify oligomers and XMG; and subscripts e and d are easy and hard fractions, respectively. 3.2. Step-change acid concentration The term s1 represents the dimensionless reaction time up to the acid concentration shifting point, and s2 represents the rest of the reaction time. Total yield consists of four parts, i.e., easy fraction reacting for the duration of s1 , hard for s1 , easy for s2 , and hard for s2 . The total yield can be obtained by multiplying exp(ÿbe s1 ) and exp(ÿbd s1 ) to the yield for the duration of s2 and is expressed as YT step Fe Yoe1 Yxe1 Re Yoe2 Yxe2 Fd Yod1 Yxd1 Rd Yod2 Yxd2 ;
3
where Re exp ÿbe s1 ; Rd exp ÿbd s1 ; be k1 L=Uo ; bd k2 L=Uo , where L is reactor length; Uo is velocity inside percolation reactor.
Fig. 1. Eect of reaction time on theoretical yield under step-change operation (reaction condition: 170°C, acid shifting time 10 min).
To determine the optimum acid shifting point, the eect of acid shifting time on yield was investigated as shown in Fig. 2. At the early phase of the process, high yield values were obtained as the acid shifting time was decreased. But at the latter phase of the process, the maximum yield reached was lower as the acid shifting time was decreased. Since the acid concentration inside the reactor was higher as the acid shifting time was decreased, higher yield values can be obtained at the early phase, but lower ones at the latter phase due to more
4. Results and discussion 4.1. Model prediction The simulation of the percolation reactor with a stepchange in acid concentration was conducted following the procedure described in the previous section. Three sets of acid-concentration ranges were studied: 0.05± 0.1%, 0.05±0.2%, and 0.1±0.2%. Fig. 1 shows the eect of the reaction time on product yield. The product yield means the total yield of XMG including monomer and oligomers. In all cases, the maximum yield was found to be about 96%. With shifting of the acid concentration from 0.05% to 0.1%, a longer reaction time was required to reach the maximum point. In the other two cases (0.05±0.2% and 0.1±0.2%), about the same level of yields were obtained with a reaction time of 25±30 min. After this point, the yield of the 0.1±0.2% case was slightly lower than that of the 0.05±0.2% case since sugars are decomposed more quickly under higher acid conditions than lower acid conditions.
Fig. 2. Eect of acid shifting time on theoretical yield under stepchange operation (reaction condition: 170°C, 0.05 ® 0.2% H2 SO4 , t1 acid shifting time).
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sugar decomposition. As shown in Fig. 2, it may be very important to select a proper acid shifting time. Since sugar concentration is inversely related to reaction time and yield in a percolation reactor, the reaction time should be reasonable for both sugar concentration and yield. In this case the optimum acid shifting time was found to be 10 min. Fig. 3 shows the eect of the reaction temperature on product yield at a ®xed reaction time. The maximum yield at 0.05±0.1% was lower than the other two cases at 160°C and 170°C. At 170°C the acid level had little eect on yield in the cases of 0.05±0.2% and 0.1±0.2%, but the yields of both cases were about 5% higher than that of the 0.05±0.1% case. At 180°C, however, the yield at 0.05±0.2% was about 3% higher than that of the 0.1± 0.2% case. The modeling results here collectively indicated that the acid concentration range of 0.05±0.2% was the best acid level policy of the three in terms of the yield and acid consumption. Applying two dierent liquid velocities along with varying acid concentration may have a similar eect on the process. In order to verify this, the velocity ratio, Uo2 =Uo1 (the ratio of velocity in high acid-concentration phase to the velocity in low acid-concentration phase) was introduced in the percolation reactor simulation program. The results in Fig. 4 indeed prove that the concept was valid because the product yield increased as the ¯ow-rate ratio was increased. As the ¯ow ratio was increased, the yield increased but then the sugar concentration decreased. Therefore, it was necessary to decide the optimum ¯ow-rate ratio. An example is shown here: Suppose that the allowable sugar concentration was 2.5%, the optimum ratio was about 1.5 for
Fig. 3. Eect of reaction temperature on theoretical yield under stepchange operation at 30 min of reaction time (reaction condition: acid shifting time 10 min).
Fig. 4. Eect of ¯ow-rate ratio on theoretical yield and sugar concentration under step-change operation (reaction condition: 170°C, acid shifting time 10 min, total reaction time 30 min).
the 0.05±0.2 case. At this ratio, the product yield obtained was 97%. 4.2. Comparison between the step-change method and the uniform acid method A direct comparison between the dierent modes of operation is shown in Fig. 5. At 30 min of reaction time, the yields of two step-change modes (0.05±0.2% and 0.1± 0.2%) were 97% and 96%, respectively. These values are 3±4% higher than those of two uniform modes (0.1%
Fig. 5. Eect of acid concentration on theoretical yield under uniform and step-change operations (reaction condition: 170°C, acid shifting time 10 min).
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4.3. Experimental comparison In order to support the theoretical results discussed in the preceding part, an experimental veri®cation was attempted. In the percolation process, a relatively short reaction time was applied since the reaction time is inversely related with product concentration. Thus the liquid amount introduced during the operation often was not sucient to recover all the hemicellulose sugars. Since the sulfuric acid concentration applied in this work was extremely low, a signi®cant amount of oligomers were produced during the hydrolysis reaction. Because of the large molecular size, the oligomers were not easily removed from the solid biomass matrix. To overcome this problem, a sulfuric acid solution was used as a leaching solvent in this experiment. Leaching was applied to recover the remaining sugars and oligomers trapped within the biomass after the percolation process. Table 2 shows the eect of leaching solvent on XMG recovery at the acid shifting time of 5 and 10 min in the 0.05±0.2% case. Water and dilute sulfuric acid (0.2%) were used as the leaching solvents. The XMG yield with acid shifting time of 5 min was slightly higher than that with acid shifting time of 10 min because of the high acid condition in the reactor. It was also found that dilute sulfuric acid as leaching solvent was more eective than water. Our test proved that the XMG yield with sulfuric acid was 0.5±0.6% higher than that with water. For comparison purpose, an identical leaching condition was applied for both cases (Table 3). The dierence between the two cases in XMG yields, however, was within the experimental error range. But the furfural production of the step change operation was about 40% less than that with uniform acid operation. Since the furfural amount produced in the 0.05±0.2% case was equivalent to a 3% XMG yield, it was considered that approximately 5% of XMG was still trapped inside the solid matrix. This residual XMG could be recovered if extra leaching solvent was used. Our results were almost identical to the ®ndings of Torget and Hsu (1994), in which the xylose yield was 92% and the product concentration was about 2.5% at the reaction condition of
Fig. 6. Theoretical yield vs. sugar product concentration under uniform and step-change operations (reaction condition: 170°C, t reaction time, acid shifting time 10 min).
and 0.2%). Considering that the maximum yield of a batch reactor is only 85% under the identical experimental conditions (Kim and Lee, 1987; Grohmann et al., 1985), these yields were indeed signi®cantly higher than that of a batch reactor, approximately 8% for uniform mode and 11% for step-change mode. In the percolation process, the product yield was inversely related to product concentration. Therefore, the yield value alone was rather meaningless. Fig. 6 shows the yield vs. sugar concentration curves obtainable under uniform and step-change operations. The yield values followed the inverse relationship with product concentration. When the yield was compared at a ®xed reaction time (t 30 min), the yield and sugar concentration of the 0.05±0.2% case were 96% and 3.2%, respectively. These values were certainly higher than those obtainable from the uniform acid method, 93% and 3.1%. In view of the fact that the sugar concentration level in the actual percolation process was in the range of 2.5±3.5%, the data in Fig. 6 indeed proved that the step-change operation produced higher yields than uniform acid operation.
Table 2 Eect of acid shifting time and leaching solvent on glucan and XMG recovery in residual solid and hydrolyzatea Acid conc. shifting time (min)
Leaching solvent
Solid remaining (%)
Untreated 5 5 10 10
Glucan (%) Solid
Liquid
44.0 H2 O H2 SO4 H2 O H2 SO4
59.6 59.2 59.6 59.5
40.4 41.1 41.3 41.0
1.7 1.9 1.5 1.9
XMG (%) Total
Solid
44.0
18.7
42.1 43.0 42.8 42.9
0.0 0.0 0.0 0.0
Liquid
Total
Furfural (%)
18.7 16.7 17.3 16.5 17.0
16.7 17.3 16.5 17.0
0.45 0.51 0.55 0.46
a Reaction condition: 170°C, 0.05 ® 0.2% sulfuric acid, 1 ml/min, 15 min. Leaching condition: 2 ml/min, 20 min, 170 ® 158°C. Note: all sugars and furfural content based on the original oven-dry untreated biomass.
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Table 3 Comparison of uniform and step-change modes concentration on glucan and XMG recovery in residual solid and hydrolyzatea Reaction condition
Solid remaining (%)
Untreated 0.05% 0.1% 0.2% 0.05±0.2%
Glucan (%) Solid
XMG (%) Liquid
44.0 62.3 60.8 59.3 60.5
41.5 41.2 40.8 41.2
1.5 1.6 2.1 1.9
Total
Solid
44.0
18.7
43.0 42.8 42.9 43.1
0.0 0.0 0.0 0.0
Furfural (%) Liquid
Total 18.7
16.4 17.1 17.0 17.2
16.4 17.1 17.0 17.2
0.43 0.72 0.73 0.51
a Reaction condition: 170°C, sulfuric acid, 1 ml/min, 10 min, acid shifting time 5 min. Leaching condition: 0.2% sulfuric acid, 2 ml/min, 20 min, 170 ® 158°C. Note: all sugars and furfural content based on the original oven-dry untreated biomass.
0.73% sulfuric acid and temperature step-change operation (140±170°C). From the theoretical work, about 3% increase of product yield was anticipated by way of the acid stepchange operation. However, the experimental work showed an yield increase of only about 1%, which was short of full veri®cation of the theoretical prediction. The non-ideal behavior in the reactor operation such as dispersion and diusion eect of sugars as well as acid may have minimized the dierence predicted by the model. A distinct advantage, however, was that sugar decomposition was signi®cantly lower in the case of the step-change method. Acknowledgements This research was supported by the Ministry of Commerce, Industry, and Energy, South Korea. References Cahela, D.R., Lee, Y.Y., Chambers, R.P., 1983. Modeling of percolation process in hemicellulose hydrolysis. Biotechnol. Bioeng. 25, 3±17. Chen, R., Lee, Y.Y., Torget, R., 1996. Kinetics and modeling investigation on two-stage reverse ¯ow reactor as applied to dilute-acid treatment of agricultural residues. Appl. Biochem. Biotechnol. 57/58, 133±146.
Conner, A.H., 1984. Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis. Wood Fiber Sci. 16 (2), 268±277. Ehrman, T., Magill, K., 1992. NREL ± CAT Standard Procedure No.001±003, Golden, CO. Grohmann, K., Torget, R., Himmel, M., 1985. Optimization of dilute acid pretreatment of biomass. Biotechnol. Bioeng. Symp. 15, 59± 80. Kim, B.J., Lee, Y.Y., Torget, R., 1993. An optimal temperature policy of percolation process as applied to dilute-acid hydrolysis of biphasic hemicellulose. Appl. Biochem. Biotechnol. 39/40, 119±129. Kim, B.J., Lee, Y.Y., Torget, R., 1994. Modi®ed percolation process in dilute-acid hydrolysis of biphasic hemicellulose. Appl. Biochem. Biotechnol. 45/46, 113±129. Kim, S.B., Kim, S.J., Yum, D.M., Park, S.C., 1997. Kinetics in dilute acid hydrolysis of wood hemicellulose. Korean J. Biotechnol. Bioeng. 12 (4), 402±409. Kim, S.B., Lee, Y.Y., 1996. Fractionation of herbaceous biomass by ammonia-hydrogen peroxide percolation treatment. Appl. Biochem. Biotechnol. 57/58, 147±156. Kim, S.B., Lee, Y.Y., 1987. Kinetics in acid-catalyzed hydrolysis of hardwood hemicellulose. Biotechnol. Bioeng. Symp. 17, 71±84. McMillan, J., 1992. Processes for Pretreating Lignocellulosic Biomass: A Review, NREL/TP-421-4978, National Renewable Energy Laboratory, Golden, CO. Springer, E.L., Harris, J.F., Neill, W.K., 1963. Rate studies of the hydrotropic deligni®cation of aspen wood. Tappi 46, 551±555. Springer, E.L., Zoch, L.L., 1968. Hydrolysis of xylan in dierent species of hardwood. Tappi 51, 214±218. Torget, R., Hsu, T., 1994. Two-temperature dilute-acid prehydrolysis of hardwood xylan using a percolation process. Appl. Biochem. Biotechnol. 45/46, 5±22.