Ind. Eng. Chem. Res. 2009, 48, 4587–4591
4587
Hydrothermal Pretreatment of Rubber Wood for the Saccharification Process Phacharakamol Petchpradab,†,‡ Takuya Yoshida,§ Tawatchai Charinpanitkul,| and Yukihiko Matsumura*,† Department of Mechanical System Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan, Hawaii Natural Energy Institute, UniVersity of Hawaii at Manoa, 1680 East-West Road, POST 109, Honolulu, Hawaii 96822, and Center of Excellence in Particle Technology Faculty of Engineering, Chulalongkorn UniVersity, Payathai Road, Patumwan, Bangkok 10330, Thailand
Rubber wood (Hevea brasiliensis) was employed as a biomass raw material for the hydrothermal pretreatment process for the enzymatic hydrolysis. Pulverized rubber wood was treated at 130-280 °C using an autoclave with inner volume of 96 cm3. The glucose content of the treated liquid product was measured. The solid product was analyzed for cellulose content and further treated with cellulase. The product glucose yield indicated the effectiveness of the hydrothermal pretreatment. A reaction network model for the hydrothermal pretreatment was proposed and applied successfully. The reaction parameters were determined. 1. Introduction Countries of Southeast Asia including Thailand are the biggest producers of various rubber wood products in the world. Furniture and particle board industries are the main consumers of the rubber wood. In these industries, generally only 10% of the wood ends up as the end product. In other words, 90% results in residues, which is comprised of 54% from small branches, 32% from wastes at the sawmill, and 4% from furniture factories. This leads to an annual availability of rubber wood residues of 6.9 Tg (6 900 000 t) in the plantation and 4.1 Tg (4 100 000 t) at the sawmill.1 Therefore, the rubber wood chips are excessive waste and can be considered as a promising energy source. Most of the lignocellulosics can be used directly as fuel either by direct combustion or by first gasifying and then burning the gas. However, there is a great deal of interest in utilizing and converting the lignocelluloses fraction as feedstock material for ethanol and other chemicals.2 Thus, in this study, ethanol production from rubber wood residue is studied. Rubber wood is a typical lignocellulosics, and production of ethanol from lignocellulosics has been studied all over the world. The concept is to hydrolyze the cellulose and hemicellulose to recover C6 and C5 sugars and to ferment the obtained sugars into ethanol. Various treatments have been proposed and studied, but to achieve a high sugar yield, twostage hydrolysis is often employed. Lignocellulosics are first treated by various chemicals or under severe condition (pretreatment), and then the product is treated with the enzyme, cellulase.3 For example, the pretreatment included ammonia explosion, aqueous ammonia recycle, controlled pH, dilute acid, flowthrough, and lime approaches. Among the various pretreatments, hydrothermal pretreatment enjoys the advantages of not using toxic or harmful chemicals, of employing relative mild conditions, and of being free from costly treatment of wastewater or byproducts. Hydrothermal pretreatment for cellulase hydrolysis has been studied by many researchers, but one of the pioneers who * To whom correspondence should be addressed. Fax: +81-82-4227193. E-mail:
[email protected]. † Hiroshima University. ‡ Present affiliation: Center of Excellence in Particle Technology, Faculty of Engineering, Chulalongkorn University, Payathai Road, Patumwan, Bangkok 10330, Thailand. § University of Hawaii at Manoa. | Chulalongkorn University.
conducted extensive work is Bobleter. He studied slow hydrothermal pretreatments of various biomass feedstocks and reported discouraging results.4 In 1992, Mok et al.5,6 reported the exciting results of fast hydrothermal pretreatments, and soon thereafter Bobleter’s group confirmed Mok et al.’s findings. Subsequent promising results were reported by Lynd et al. in conjunction with Antal’s group.7,8 The current issues of the field concern inhibition of the saccharification and fermentation reactions by the products of the hydrothermal pretreatment, the recovery and fermentation of the pentosans, the overall heat balance of the system, and the need to achieve a solids loading approaching 20 wt %. Another researcher group who studied hydrothermal pretreatment for cellulase hydrolysis is that of Ballesteros et al.9 They employed an autoclave to treat olive residue. Hydrothermal treatment has also been studied as the only pretreatment without the following cellulase treatment by several researchers.10,11 The aquasolve process is one of the early studies.12,13 In this process, Allen et al. added hot compressed water into the vessel of biomass, and after a few minutes, released the pressure of the vessel and removed the water so that separation of the components was to be achieved. A similar idea was studied by Sakaki et al., and they also reported that inhibition of byproducts on ethanol fermentation was small.14 However, their glucose yield was limited because of the decomposition of product glucose. Thus, only hydrothermal pretreatment cannot achieve complete hydrolysis. Cellulase works at a temperature around 30 °C, and it catalyzes only the hydrolysis. However, without pretreatment, the effectiveness of cellulase is largely reduced because of the presence of intermolecular hydrogen bond linkages at the crystalline region in cellulose.15 Considering the results from these previous studies, we decided to concentrate on the twostage hydrolysis of rubber wood using hydrothermal pretreatment in this study. After the hydrothermal pretreatment in the first stage, the lignocellulosic feedstock is much more easily hydrolyzed by cellulase in the second stage. However, it is also known that hydrothermal pretreatment under conditions too severe results in degradation of carbohydrate, which is to be avoided.16,17 Namely, too high of a temperature or too long of a treatment time is not desirable from the viewpoint of avoiding the
10.1021/ie801314h CCC: $40.75 2009 American Chemical Society Published on Web 03/25/2009
4588 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 1. Conditions for Enzymatic Hydrolysis hydrothermally treated rubber wood volume of buffer fluid added 10 g/dm3 cellulase solution
1g 60 cm3 5 cm3
Table 2. Composition of Rubber Wood Feedstock hemicelluloses and cell wall lignin cellulose ash
Figure 1. Autoclave reactor.
Figure 2. Proposed reaction network.
carbohydrate degradation. Then, there should be an optimum condition for this hydrothermal pretreatment. Unfortunately, in spite of this pile of studies, the optimum condition for the hydrothermal pretreatment has not been determined. This is due to the variety of biomass. In some biomass species, crystallinity of cellulose may not be high, and hydrothermal pretreatment may be of little importance. The optimum condition should also depend on the physical form of the biomass. If the biomass is in the form of fine powder, the reactions taking place in the hydrothermal pretreatment should be faster than when biomass is treated in large pieces. The content of hemicellulose and lignin should also affect the optimum condition, since they attach and cover cellulose and prevent the easy access of cellulase to cellulose, and conditions suitable for removing hemicellulose and lignin should be of importance. Establishment of the guideline to obtain the optimum conditions for the hydrothermal pretreatment is desired. The purpose of this study is to treat rubber wood hydrothermally at 130-280 °C in an autoclave reactor so that the effect of temperature on the reaction of cellulose in the rubber wood during the hydrothermal pretreatment is determined, that the overall reaction model and kinetic parameters are derived, and that the optimum condition for the hydrothermal pretreatment of rubber wood is determined. Usually the severity factor of Overend and Chornet is used as a representation of the time and temperature history of the pretreatment. However, the severity factor is an overall parameter and does not go along with the kinetic analysis addressed here. Thus, in this study, we dare not employ the concept of severity factor. 2. Experimental Section 2.1. Hydrothermal Pretreatment. Figure 1 shows the autoclave employed in this study. The inner volume of the autoclave was 96 cm3. Seven grams of rubber wood powder was loaded in the autoclave with 63 g of deionized water. After starting the agitator, the autoclave was heated up by the embedded heaters. When the temperature reached the target value, the heaters were turned off, and the reactor was cooled with the air fan. The sample after pretreatment was separated
0.29 0.28 0.39 0.04
to liquid sample and solid sample. The solid sample was used for enzymatic hydrolysis and measured for the content of cellulose after pretreatment. The liquid sample was analyzed by HPLC with the SUGAR KS-802 (Shodex) column operated at 60 °C with water at 0.8 cm3/min as an elute. The 10 target temperatures were 130, 140, 150, 170, 190, 200, 210, 240, 260, and 280 °C. The pretreatment agitator was set at 500 rpm. 2.2. Enzymatic Hydrolysis. Cellulase from Aspergillus niger powder (g0.3 units/mg-solid) was used in this study. The samples after the hydrothermal pretreatment were used as reactant in this step. The pretreated sample, cellulase, and buffer fluid were placed in a conical flask as shown in Table 1, and the flask was shaken at 250 rpm at 37 °C for 2 days. In each run, the liquid product was sampled every 24 h and was analyzed by HPLC. Buffer fluid was prepared from acetic acid, sodium hydroxide, and deionized water, whose pH was set at 5. 2.3. Component Analysis. The cellulose content of the product of hydrothermal pretreatment was determined following the procedure recommended by the United States Department of Agriculture.18 2.4. Feedstock. The rubber wood chip was pulverized with the ball mill equipment into rubber wood powder and used in all experiments. The average particle size was 28 µm on average. Table 2 shows the components of the rubber wood powder (dry weight basis) determined by the USDA’s method.18 3. Apparent Reaction Network for the Hydrothermal Pretreatment Our group is working to establish the guideline for the optimum conditions for the hydrothermal pretreatment. However, the phenomena that are taking place in the hydrothermal reactor are very complex. Thus, as the first step, we have proposed the network of the “apparent” reactions taking place in the hydrothermal reactor as shown in Figure 2. In Figure 2, C denotes cellulose that cannot be converted into glucose by cellulase treatment. This “cellulase-resistant” cellulose is the starting material, because without hydrothermal pretreatment, almost no glucose is obtained even if the rubber wood feedstock is treated with cellulase. Thus, we can consider cellulose in the rubber wood as a practically nonreactive form in terms of cellulase treatment. This may be due to the high crystallinity, due to the lignin and hemicellulose covering the cellulose, or due to the large size of the pieces of biomass being treated. By the hydrothermal pretreatment, a part of the “cellulaseresistant” cellulose is converted into “cellulase-hydrolyzable” cellulose, because after the hydrothermal pretreatment, the treated rubber wood produces glucose by applying cellulase in the second stage. In Figure 2, C* denotes cellulose that can be converted into glucose by cellulase treatment, produced by hydrothermal pretreatment. This change can be treated as a kind of reaction. Of course, this change is a mixture of physical and chemical phenomena: dissolution of hemicellulose and part of lignin in the hot compressed water, reduction in crystallinity of
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4589
Figure 3. Change of cellulose in the hydrothermal pretreatment and enzymatic hydrolysis processes.
the cellulose, destruction of cell structure, introduction of water molecules into the piece of biomass, and so forth. However, these phenomena are too complex to be treated as they are. Reaction engineering tells us that overall treatment is effective when complex phenomena are to be discussed. This is the approach we take here. In addition to this change, part of the cellulose is directly hydrolyzed into glucose hydrothermally. In Figure 2, this reaction is shown by the arrows from C and C* to G, where G denotes glucose. Part of the produced glucose is further decomposed in the hydrothermal reactor. In Figure 2, this reaction is shown by the arrow from G to D. D denotes decomposition products, which is a mixture of various compounds. It is known that hydrothermal decomposition of glucose results in production of various compounds but that the reaction rate can be expressed by a single overall reaction rate equation.19 Glucose decomposition is catalyzed by acids formed during the hydrothermal pretreatment process as was pointed out by Antal. There is a high possibility of autocatalysis taking place as shown in Antal’s findings.20 Unfortunately, the pH of the liquid was not measured in this study, but the reaction rate parameters obtained in this study are the overall value including this effect. To help readers to understand what is taking place in hydrothermal pretreatment and enzymatic hydrolysis processes, the change of cellulose in the feedstock is shown in Figure 3. The reaction network shown in Figure 2 and the reaction kinetics discussed in the following part of this paper are for the hydrothermal pretreatment step in Figure 3. For the optimal design of the saccharification process, the kinetics of this network of the “apparent” reactions that are taking place in the hydrothermal pretreatment are to be elucidated. In this study, rubber wood is treated hydrothermally at 130-280 °C in an autoclave reactor, to check the validity of the proposed reaction model shown in Figure 2, to determine the kinetic parameters, and to predict the optimal condition for the hydrothermal pretreatment. In the analysis of the experimental results, the yield of each compound shown in Figure 2 was determined as follows. Glucose yield, XG, was determined by dividing the glucose amount in the liquid phase obtained from HPLC nG by the theoretical amount of glucose in the cellulose in the feedstock nC0. XG )
nG nC0
(1)
Figure 4. Temperature change inside the reactor with time.
Figure 5. Amount of glucose generated with time of enzymatic hydrolysis.
The yield of cellulose that can be hydrolyzed by cellulase, XC*, was determined by dividing the amount of glucose increased by the cellulase treatment, nG*, by the theoretical amount of glucose in the cellulose in the feedstock nC0. XC* )
nG* nC0
(2)
The yield of cellulose that cannot be hydrolyzed by cellulase XC was determined by XC )
nC - nG* nC0
(3)
where nC denotes the amount of glucose unit it the cellulose left after hydrothermal pretreatment. Then, the yield of decomposed product XD was determined by XD ) 1 - XG - XC* - XC
(4)
4. Results and Discussion Figure 4 shows the temperature profiles in the reactor. On the basis of this temperature profile, the reaction parameters were determined in the following section. Figure 5 shows the glucose yield with respect to the cellulase reaction time. The glucose yield after hydrothermal pretreatment is shown at the time of cellulase reaction of 0 s. It is clear that glucose yield is less than 0.01 after the hydrothermal pretreatment. The glucose yield increases with time during the cellulase treatment. In this study, we employ the yield after a 48-h
4590 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009
Figure 6. Effect of target temperature of hydrothermal pretreatment on each yield (symbol, experiment; line, calculation value). Table 3. Rate Parameters for the Hydrothermal Reactions
k1 k2 k3 k4
preexponential factor [1/s]
activation energy [kJ/mol]
1.87 × 105 2.02 × 107 1.80 × 1018 2.88 × 102
62.0 87.7 222 14.4
treatment with cellulase as the final yield, since in the previous study, yield at this time showed the maximum value. The effect of temperature of hydrothermal pretreatment on the final yields is shown in Figure 6. It is shown that most of the cellulose is overdecomposed under the conditions employed here from the high value of XD. The final glucose yield that is expressed by the sum of XG and XC* becomes higher when the target temperature of the hydrothermal pretreatment is increased from 130 to 240 °C, but at 280 °C the glucose is further decomposed in the autoclave. The highest value for the glucose yield is as low as 0.1, and this low value indicates that produced glucose is easily decomposed in the autoclave. Mok et al. showed that only about 1 min is needed at 220 °C to extract all of the hemicellulose and half of the lignin from biomass.5,6 Considering this result, time required to heat up the temperature to 280 °C may be too long. However, as long as the autoclave reactor is employed, this slow heating rate is unavoidable, and our result agrees with Mok et al.’s results. Assuming the first order reaction for each reaction in Figure 2, the following set of equations is obtained d[C] ) -k1[C] - k2[C] dt
(5)
d[G] ) k3[C*] + k1[C] - k4[G] dt
(6)
d[C*] ) k2[C] - k3[C*] dt
(7)
d[D] ) k4[G] dt
(8)
We also assumed that the Arrhenius rate law is applicable to each reaction rate constant and integrated the differential equations numerically, following the temperature profiles shown in Figure 4. By fitting the parameters k1, k2, k3, and k4 so that the integrated result reproduces the experimental yields using the least-squares method, the reaction rate parameters are determined as shown in Table 3. Figure 6 compares the experimental and calculated yields of each constituent in this calculation. Figure 7 shows the corresponding parity plot. Good agreement is obtained, and the effectiveness of applying the “apparent” reaction network to hydrothermal pretreatment of
Figure 7. Parity plot.
Figure 8. Arrhenius plot.
rubber wood is validated. Figure 8 presents the Arrhenius plot for the reaction rate constants. Since k1 is smaller than k2, cellulose (C) is mainly converted to glucose (G), not to the hydrolyzable form (C*). However, k4 is greater than k1, and the produced glucose is further decomposed. The reaction rate coefficients for hydrothermal degradation of cellulose and glucose4 are also shown in Figure 8. The reaction rate coefficient k3 (C* f G) well agrees with previous study. The reaction rate coefficient (C f G) is much higher than in the previous study. This may be because lignin and hemicellulose is preventing cellulase to hydrolyze cellulose. Hydrothermal pretreatment washes away the lignin and hemicellulose from the surface of cellulose, but part of the cellulose is connected to hemicellulose and lignin. This part cannot be crystalline because of the interaction with hemicellulose and lignin and, once they are washed away, is easily hydrolyzed under the hydrothermal condition. Previous study employed pure cellulose, and thus its reactivity is much lower. However, we do not have any evidence for this assumption, and this mechanism is to be addressed in the future work. Meanwhile, the reaction rate coefficient k4 (G f D) is much higher than in previous study of glucose degradeation, which implies the effect of autocatalysis. The obtained parameters can be used to determine the optimum condition of continuous hydrothermal treatment of rubber wood powder. For the autoclave reactor, the optimum target temperature is found to be 235.5 °C, by setting the heating rate to 8 °C/min and cooling rate to 4 °C/min. The expected glucose yield under this condition is 0.183. This result indicates that the autoclave, which only allows limited, slow heating rates and cooling rates, is not a suitable reactor for hydrothermal pretreatment. This is because product glucose is readily decomposed in the heating and cooling step, whose duration cannot be shortened due to the limited temperature change rate
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4591
of the autoclave. Reactors that allow different heating and cooling rates are needed. Reaction rate parameters determined here can be employed to predict the optimum condition for this kind of reactor, which, however, is beyond the scope of this study. Finally, the reaction rate parameters obtained here are only for rubber wood and are not applicable to other biomass feedstock. In the previous study, we have treated other biomass species and have found the reaction rate parameters differ from species to species, while effectiveness of the reaction network was maintained.21 However, the reason for this is again beyond the scope of this study. 5. Conclusions Rubber wood was hydrothermally treated as a pretreatment of the cellulase hydrolysis for ethanol production. The network shown in Figure 2 is effective to express the result of this hydrothermal pretreatment. The reaction rate parameters were determined for the hydrothermal pretreatment of rubber wood. The optimum conditions for the hydrothermal pretreatment were predicted, and the highest yield of 0.183 is expected. This result shows the limitation of the hydrothermal pretreatment using the autoclave where only a slow heating and cooling rate is available. Acknowledgment The authors are grateful for the JASSO scholarship that allowed P.P.’s stay and work in Hiroshima University. Part of this work was supported by the JST Project. Literature Cited (1) Prasertsan, S.; Sajjakulnukit., B. Biomass and Biogas Energy in Thailand: Potential, Opportunity and Barriers. Renewable Energy 2006, 31, 599. (2) Sreenath, H. K.; Koegel, R. G.; Moldes, A. B.; Jeffries, T. W.; Straub, R. J. Enzymic Saccharification of Alfalfa Fibre after Liquid Hot Water Pretreatment. Process Biochem. 1999, 35, 33. (3) Wyman, C. E.; Dale, B. E.; Elander, R. T.; Holtzapple, M.; Ladisch, M. R.; Lee, Y. Y. Coordinated Development of Leading Biomass Pretreatment Technologies. Bioresour. Technol. 2005, 96, 1959. (4) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Froling, M.; Antal, M. J.; Tester, J. W. Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies. Energy EnViron. Sci. 2008, 1, 32. (5) Mok, W. S. L.; Antal, M. J. Uncatalyzed Solvolysis of Whole Biomass Hemicellulose by Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1992, 31, 1157.
(6) Mok, W. S. L.; Antal, M. J.; Varhegyi, G. Productive and Parasitic Pathways in Dilute Acid-Catalyzed Hydrolysis of Cellulose. Ind. Eng. Chem. Res. 1992, 31, 94. (7) Lynd, L. R.; Weimer, P. J.; van Zyl, W. H.; Pretorius, I. S. Microbial Cellulose Utilization: Fundamentals and Biotechnology. Microbiol. Mol. Biol. ReV. 2002, 66, 506. (8) Zhang, Y.-H. P.; Lynd, L. R. Toward an Aggregated Understanding of Enzymatic Hydrolysis of Cellulose: Noncomplexed Cellulase Systems. Biotechnol. Bioeng. 2004, 88, 797. (9) Ballesteros, I.; Oliva, J. M.; Negro, M. J.; Manzanares, P.; Ballesteros, M. Ethanol Production from Olive Oil Extreaction Residue Ppretreated with Hot Water. Appl. Biochem. Biotechnol. 2002, 98-100, 717. (10) Kumagai, S.; Hayashi, N.; Sakaki, T.; Nakada, M.; Shibata, M. Fractionation and Saccharification of Cellulose and Hemicellulose in Rice Hull by Hot-Compressed-Water Treatment with Two-Step Heating. J. Jpn. Inst. Energy 2004, 83, 776. (11) Sasaki, M.; Adschiri, T.; Arai, K. Kinetics of Cellulose Conversion at 25 MPa in Sub- and Supercritical Water. AIChE J. 2004, 50, 192. (12) Allen, S. G.; Kam, L. C.; Zemann, A. J.; Antal, M. J., Jr. Fractionation of Sugar Cane with Hot, Compressed, Liquid Water. Ind. Eng. Chem. Res. 1996, 35, 2709. (13) Laser, M.; Schulman, D.; Allen, S. G.; Lichwa, J.; Antal, M. J., Jr.; Lynd, L. R. A Comparison of Liquid Hot Water and Steam Pretreatments of Sugar Cane Bagasse for Bioconversion to Ethanol. Bioresour. Technol. 2002, 81, 33. (14) Sakaki, T.; Shibata, M.; Miki, T.; Hirosue, H.; Hayashi, N. Decomposition of Cellulose in Near-Critical Water and Fermentability of the Products. Energy Fuels 1996, 10, 648. (15) Adschiri, T.; Hirose, S.; Malaluan, R.; Arai, K. Noncatalytic Conversion of Cellulose in Supercritical and Subcritical Water. J. Chem. Eng. Jpn. 1993, 26, 676. (16) Yoshida, T.; Nonaka, H.; Matsumura, Y. Hydrothermal Treatment of Cellulose as a Pretreatment for Ethanol Fermentation: Cellulose Hydrolysis Experiments. J. Jpn. Inst. Energy 2005, 84, 544. (17) O’Dwyer, J. P.; Zhu, L.; Granda, C. B.; Holtzapple, M. T. Enzymatic Hydrolysis of Lime-Pretreated Corn Stover and Investigation of the HCH-1 Model: Inhibition Pattern, Degree of Inhibition, Validity of Simplified HCH-1 Model. Bioresour. Technol. 2007, 98, 2969. (18) United States Department of Agriculture. Forage Fiber Analyses (Apparatus, Reagents, Procedures, and Some Applications). Agric. Handb. 1970, 379. (19) Matsumura, Y.; Yanachi, S.; Yoshida, T. Glucose Decomposition Kinetics in Water at 25 MPa in the Temperature Range of 448-673 K. Ind. Eng. Chem. Res. 2006, 45, 1875. (20) Antal, M. J.; Leesonboom, T.; Mok, W. S.; Richards, G. N. Mechanism of formation of 2-furaldehyde from D-xylose. Carbohydr. Res. 1991, 217, 71. (21) Matsumura, Y.; Yoshida, T., Okahisa, D.; Tolan, J. S. Hydrothermal pretreatment of lignocellulosics for ethanol fermentation. Presented at ISHR&ICSTR, Aug. 5-9, 2006, Sendai, Japan; OB-2-05.
ReceiVed for reView August 30, 2008 ReVised manuscript receiVed January 30, 2009 Accepted February 6, 2009 IE801314H
