Effect of CH3COOH and K2CO3 on Hydrothermal Pretreatment of

Mar 1, 2013 - ABSTRACT: Excessive amounts of fast-growing water hyacinth have densely invaded many canals, lakes, and rivers throughout. Thailand ...
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Effect of CH3COOH and K2CO3 on Hydrothermal Pretreatment of Water Hyacinth (Eichhornia crassipes) Phacharakamol Petchpradab Phothisantikul,† Ranisorn Tuanpusa,‡ Minoru Nakashima,† Tawatchai Charinpanitkul,‡ and Yukihiko Matsumura§,* †

Department of Mechanical Systems Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527 Japan Center of Excellence in Particle Technology, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Payathai Road, Patumwan, Bangkok 10330 Thailand § Division of Energy and Environmental Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527 Japan ‡

ABSTRACT: Excessive amounts of fast-growing water hyacinth have densely invaded many canals, lakes, and rivers throughout Thailand, damaging the ecology of local waterways. Water hyacinth is a typical lignocellulosic material and is recognized as a potential source of renewable energy. In this study, hydrothermal pretreatment accompanied by enzymatic hydrolysis of dried water hyacinth is investigated in the temperature range 160−220 °C, using a hydrothermal ball-mill reactor; this enables hydrothermal pretreatment and ball-mill pulverization to be conducted simultaneously. The effects of CH3COOH and K2CO3 on the liquid composition were investigated experimentally (CCH3COOH = 0.5−1.0 wt %, CK2CO3 = 0.5 wt %). In the absence of CH3COOH and K2CO3 at 220 °C, a glucose yield of 0.267 was obtained. The highest glucose yield of 0.855 was achieved at 200 °C with CCH3COOH = 0.75 wt % and water hyacinth intake = 10 wt %. In the presence of 0.5 wt % K2CO3, a glucose yield of 0.195 was obtained at 220 °C. The addition of K2CO3 did not suppress hydrolysis in the hydrothermal pretreatment. The autocatalytic effect of acid production in the hydrothermal pretreatment is therefore not large. A pseudofirst-order kinetic model with regard to cellulose content was developed to explain the conversion mechanism of cellulose to glucose in the hydrothermal pretreatment process.

1. INTRODUCTION Bioethanol is a form of a renewable energy obtained by the conventional fermentation of sugars obtained from sugar and starch crops. Cellulosic biomass is also used as an agricultural feedstock for ethanol production. Ethanol can be used as a fuel for vehicles in its pure form. However, it is widely used as a gasoline additive to increase the octane number and improve vehicle emissions. Bioethanol has a number of advantages over conventional fuels. It comes from a renewable resource rather than from a finite resource. Another benefit over fossil fuels is with respect to greenhouse gas emissions. Biomass is an alternative renewable fuel that helps to solve greenhouse gas problems. It is an excellent way to make use of agricultural wastes and other biodegradable wastes and to create fuel sources and energy.1,2 It is also cheaper than fossil fuels and does not pollute the environment. One important fact is that the use of lignocellulosic biomass, for instance, wood or straw, does not compete with the food chain. However, current processes for converting conventional lignocellulosic biomass to useful materials are highly inefficient;2,3 and available biomass wastes such as water hyacinth are good alternative sources. Water-hyacinth growth has become an environmental problem in many parts of the world. Water hyacinth is an aquatic plant that can be found in every river in Thailand. It affects biogeochemical processes, fisheries, recreational activities on waterways, and spoils the scenery of various rivers.4 Water hyacinth is therefore considered to be a waste material. However, it is a promising energy source because of its carbon content. Most lignocellulosic materials can be used directly as © 2013 American Chemical Society

fuel either by direct combustion or by gasifying the material and burning the obtained gas. There is also a great deal of interest in using and converting lignocellulose fractions as feedstock materials for the production of ethanol and other chemicals.5 Cellulose and hemicellulose in water hyacinth are first hydrolyzed to obtain sugars, which can then be converted to yield ethanol. Various processes have been proposed and studied to achieve a high yield of lignocellulosic conversion to glucose and/or ethanol.6−10 In most of the processes, lignocellulosic materials are first pretreated with various chemicals under severe conditions and then subjected to the key process, namely enzymatic hydrolysis. Hydrothermal pretreatment has also been studied as the sole pretreatment, without the subsequent cellulase treatment by several researchers.11−13 Mishima et al.14 also reported that the glucose concentration obtained by enzymatic hydrolysis was comparable to or higher than that obtained by acid hydrolysis. Studies have shown that dilute acid is effective in improving ethanol production.15 Xu et al.16 investigated acetic-acidcatalyzed hydrothermal pretreatment of corn stover. The water-insoluble solid obtained from pretreatment of 15 g/kg of raw corn stover was subjected to enzymatic hydrolysis and easily converted to ethanol using baker’s yeast, giving an ethanol concentration of 33.72 g/L, higher than that of 22.04 g/L obtained Received: Revised: Accepted: Published: 5009

September 10, 2012 February 27, 2013 March 1, 2013 March 1, 2013 dx.doi.org/10.1021/ie302434w | Ind. Eng. Chem. Res. 2013, 52, 5009−5015

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using a noncatalyzed liquid hot water pretreatment (195 °C, 15 min). Acetic acid is therefore a good catalyst for hydrothermal pretreatment of lignocellulosic materials. Glucose decomposition is catalyzed by acids formed during the hydrothermal pretreatment,17 but a thorough study of the effects of acid addition during hydrothermal pretreatment has not been reported. It should also be noted that the hydrothermal pretreatment produces organic acids, without the addition of a catalyst. It is expected that these organic acids increase the final glucose yields by functioning as catalysts. The possibility of this kind of autocatalytic effect has been pointed out, but the actual extent of this effect has not been elucidated. One way to measure this effect may be to examine the effect of alkali addition; this will neutralize the effect of the autocatalytic organic acid, and clarify its effect. The purpose of this study is therefore to conduct hydrothermal pretreatments with an acid or alkali to clarify the effects of acid addition, as well as to determine the autocatalytic effect expected from organic acid production during the hydrothermal pretreatment of lignocellulosic materials.

Figure 2. Experimental procedure.

Table 1. Chemical Component of Water Hyacinth

2. METHODS 2.1. Experimental Setup and Operation. The experimental apparatus is shown in Figure 1. The reactor was an

compositions

weight fraction [kg/kg-dry]

hemicelluloses lignin cellulose other

0.62 0.02 0.28 8

Figure 3. Plausible reaction pathways.5 Figure 1. Ball mill pulverization simultaneous reactor.

autoclave made of stainless steel (SS316), and the reactor volume was 800 cm3, with an inner diameter of 9 cm. ZrOr2 balls of diameter 10 mm and feedstock with water were loaded into the ball-mill reactor. The reactor was rotated by a motor controlled by an inverter. The reactor was heated by an electric furnace. The temperature was measured every 30 s. 2.2. Experimental Procedure. Figure 2 shows an overview of the experimental work reported in this study. The experimental work can be divided into two parts: (i) hydrothermal pretreatment of water hyacinth and (ii) enzymatic hydrolysis of cellulose. A summary of the procedure for each part is provided in the figure. The liquid product was analyzed by HPLC for the glucose product. 2.3. Hydrothermal Pretreatment. The target temperature of the hydrothermal pretreatment ranged from 160 to 230 °C. Slurry (100 g) with a feedstock concentration of 10 wt % and 0.5 kg of balls were loaded, and the rotation speed of the reactor was set at 200 rpm for all runs. When CH3COOH is added, its concentration ranged from 0 to 1 wt %. The concentration of K2CO3 was 0.5 wt % when added. CH3COOH and K2CO3 were added into the feedstock before the reaction. 2.4. Substrate. Dried water hyacinth from Thailand was used. Analysis of the water hyacinth used in this study gave a composition as shown in Table 1. The glucose yield was calculated based on the original amount of cellulose. The waterhyacinth composition under each set of conditions was determined using the USDA method.18 Although it does not affect

Figure 4. Representative temperature.

the conclusion of this study, it is to be noted that drying may have made a substrate less accessible or susceptible to hydrolysis, 2.5. Enzymatic Hydrolysis. The solid samples obtained by hydrothermal pretreatment were used as the reactants in this process. The pretreated sample (35 g) was enzymatically hydrolyzed. Cellulase from Aspergillus niger powder (≥0.3 units/mg-solid) was used in this study. Cellulase powder 0.05 g were dissolved in 5 mL of water and used per sample. Buffer solution (60 mL) 5010

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no glucose could be obtained, even if the water-hyacinth feedstock is treated with cellulase. We can therefore consider the cellulose in the water hyacinth to be in a practically nonreactive form in terms of cellulase treatment. This is ascribed to high crystallinity and to lignin and hemicellulose covering the cellulose. On hydrothermal pretreatment, part of the cellulase-resistant cellulose is converted into cellulasehydrolyzable cellulose. As a result, the treated water hyacinth can provide glucose when hydrolyzed by cellulase in the second step. C* represents the cellulose that can be converted into glucose by cellulase treatment, produced by hydrothermal pretreatment. In the hydrothermal reactor, parts of C and C* are hydrolyzed to produce glucose, G. Part of the produced glucose is further decomposed to various products, represented in total by D. For a batch reactor setup, based on the reaction pathway in Figure 3, the concentration of each individual species as a function of time can be derived from the set of ordinary differential eqs 1−4.

Figure 5. Change of cellulose in the hydrothermal pretreatment and enzymatic hydrolysis processes.

was also added; the buffer solution was prepared from acetic acid, sodium hydroxide, and deionized water. Its pH was set at 5. The mixture was shaken at 250 rpm at 37 °C for 48 h; the glucose concentration in the liquid phase was then analyzed using high-performance liquid chromatography (HPLC). 2.6. Pseudofirst-Order Kinetic Model. The rate constants were calculated based on the reaction pathway shown in Figure 3. C represents cellulose that cannot be converted into glucose by cellulase treatment. This cellulase-resistant cellulose is the starting material; without hydrothermal pretreatment, almost

d[C] = −k1[C] − k 2[C] dt

(1)

Figure 6. Effect of reaction temperature on the yield of G, C*, C, and D, (a) CH3COOH 0.5 wt %, (b) K2CO3 0.5 wt %, (c) without catalyst. 5011

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Figure 7. Final glucose yield in liquid fraction at difference pretreatment conditions: 0.5 wt % K2CO3 (chain-link pattern), Without solvent (verticle line pattern), 0.5 wt % CH3COOH (dotted pattern), 0.75 wt % CH3COOH (black), 1.0 wt % CH3COOH (white).

d[G] = k 2[C] + k 3[C*] − k4[G] dt

(2)

d[C*] = k1[C] − k 3[C*] dt

(3)

d[D] = k4[G] dt

(4)

Figure 6 shows the effect of target temperature on the solid sample composition after enzymatic hydrolysis. Only the representative cases of acetic acid and potassium carbonate catalysts at concentrations of 0.5 wt %, along with the case without a catalyst, are shown. For the same concentrations of CH3COOH catalyst and K2CO3 catalyst (0.5 wt %), the final glucose yield, XC* + XG, is higher for the CH3COOH catalyst. The effectiveness of the acid catalyst is clearly demonstrated. The final glucose yield with the K2CO3 does not greatly differ from the case with no catalyst, showing that the effect of 0.5 wt % K2CO3 is not significant in this study. 3.3. Effect of Acetic Acid Concentration. Figure 7 shows the effect of catalyst concentration on the final glucose yield for each pretreatment temperature. At a pretreatment temperature of 160 to 200 °C, an increase in CH3COOH concentration from 0.5 to 1.0 wt % resulted in a sharp increase in the glucose yield. However, a slight decrease in the glucose yield was observed above 200 °C. The highest glucose yield, 0.86, was achieved under conditions of 200 °C with 0.75 wt % CH3COOH. In the absence of a catalyst, a glucose yield of 0.27 was obtained at 220 °C, and with 0.5 wt % K2CO3, the glucose yield was only 0.20 at 220 °C. The highest glucose yield was obtained by adding CH3COOH at 0.75 wt % at 200 °C. Further increases in CH3COOH concentration lowered the yield. The K2CO3 catalyst did not improve the final glucose yield. Again, the effectiveness of CH3COOH is clear. Xu et al.16 stated that when CH3COOH is introduced in a liquid hot water pretreatment, there is a positive effect because CH3COOH can loosen the structure by removing hemicelluloses, increasing the convertibility of the cellulose. Gong et al.19 showed that as a result of lignin removal, the internal surface area gradually increased with increasing CH3COOH concentration from 6 wt % to 18 wt %; this implies that CH3COOH acts as a solvent and forms acetyl groups with hemicellulose.12 For alkali catalysts, Toor et al.20 reported that the addition of alkaline salts has a positive influence on hydrothermal processes and improves gasification and accelerates the water-gas shift reaction, but

3. RESULTS AND DISCUSSION 3.1. Temperature Changes. The temperature changes in the hydrothermal pretreatment reactor are shown in Figure 4. The reactor was heated using an electric furnace and when it reached the target temperature, the heating process was stopped and the reactor was cooled by water. The heating rate was 5 °C/min. 3.2. Effect of Target Temperature on Glucose Yield. The changes in cellulose in the feedstock are shown in Figure 5; C denotes cellulose that cannot be hydrolyzed to glucose by cellulase treatment, and C* denotes hydrolyzed cellulose, which can be converted to glucose by cellulase treatment. G and D denote glucose and decomposition products of glucose, respectively.3 The details of glucose yield determination for hydrothermal pretreatment were described in our previous work.5 Please note that all the yield is for the product of hydrothermal pretreatment. The glucose yield, XG, was determined by dividing the amount of glucose in the liquid phase, nG, obtained using HPLC, by the theoretical amount of glucose in the cellulose in the feedstock, nG0. The yield of C*, XC*, was determined by dividing the difference between the amount of glucose before and after the cellulase treatment, nG* by nG0. The yield of cellulose that cannot be hydrolyzed by cellulase, XC, was determined by dividing the amount of cellulose after the hydrothermal pretreatment by nG0. The yield of decomposed product, XD, was determined by subtraction. 5012

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Figure 8. Comparison between calculated [lines] and experimental product yields [symbols] obtained from an experiment with (a) Noncatalyst, (b) CH3COOH 0.5%wt, (c) CH3COOH 0.75 wt %, (d) CH3COOH 1.0 wt %, (e) K2CO3 0.5%wt.

the experimental yield and the calculated yield using the leastsquares method, the reaction rate parameters were determined. The calculated results are compared with the experimental yields in Figure 8. It was observed that with increasing CH3COOH concentration, the hydrolyzable cellulose yield increased significantly leading to an increase in the final glucose yield. Taherzadeh et al.21 reported that almost 100% hemicellulose removal is possible using dilute-acid pretreatment. The pretreatment is not effective in dissolving lignin, but it can disrupt lignin and increase the susceptibility of cellulose to enzymatic hydrolysis. The effect of CH3COOH concentration on each reaction rate constant is shown in Figure 9. The effect of K2CO3 addition is also shown. This is for treatment at 220 °C, which is a typical example. Addition of acid increases the values of

this was at a much higher temperature than those used in this study. On the basis of these reports, a plausible sequence of changes is as follows: The CH3COOH causes swelling of the cellulose increasing its internal surface area and possibly making it less crystalline. This in turn leads to breaking of the structural chain between lignin and the major structures, so lignin dissolves in the CH3COOH solution. It should be further noted that, under acidic conditions, the crystalline structure of cellulose would possibly be damaged by interaction with H+ ions. However, the presence of anions would not lead to significant breakage of the cellulosic structure.21 3.4. Determination of Reaction Rate Parameters. On the basis of the pseudofirst-order model, the Arrhenius rate law was applied to each reaction. By fitting the correlation between 5013

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(3) Reaction rate constant analysis showed that CH3COOH addition enhances conversion of cellulose to hydrolyzable cellulose and glucose, and greatly enhances conversion of hydrolyzable cellulose to glucose, but suppresses glucose decomposition. (4) K2CO3 addition does not affect cellulose conversion to hydrolyzable cellulose and glucose, nor does it affect the decomposition of glucose, but it increases the conversion rate of hydrolyzable cellulose to glucose.

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AUTHOR INFORMATION

Corresponding Author

Figure 9. Effect of acetic acid concentration on each reaction rate constant at 220 °C.

*Fax: +81-82-422-7193, e-mail: [email protected]. Notes

k1 and k2, which both lead to higher final glucose yields. The effect of acid is much larger on k3 and k4. When the concentration of CH3COOH is higher than 0.5 wt %, the value of k3 suddenly increases; this indicates the rate of glucose production from C*. In contrast, the value of k4 is suppressed with increasing CH3COOH concentration. The decomposition of glucose is suppressed by CH3COOH. This is interesting since it has been reported that acid enhances the decomposition of xylose.17 As 0.001−0.02 M sulfuric acid at 250 °C was used in the xylose study, it is concluded that a strong acid catalyzes sugar decomposition, but weak acids such as CH3COOH do not have this this effect and instead suppress the decomposition of sugars. A reaction analysis such as that conducted in this study for the case of sulfuric acid addition should give some insights into this interesting topic; this is beyond the scope of this study but is our next target. The effect of adding K2CO3 on the reaction rate coefficient k4 is a good measure of the effectiveness of autocatalysis. Unfortunately, the pH values have not been determined. However, the amount of 0.5 wt % potassium carbonate is sufficiently high to cancel out the effect of 0.5 wt % of CH3COOH, or acid from the hydrolysis if any. As can be seen in Figure 9, the values of k1, k2, and k4 are not affected. The reaction rate constant k3 increases indicating that conversion of C* to G is enhanced. This results in increased yields of G and D for the case of K2CO3 addition compared with the case without a catalyst, as can be seen in Figure 8. On the basis of these experimental results, the addition of K2CO3 does not suppress the hydrolysis reaction in hydrothermal pretreatment, which indicates that the autocatalytic effect of acid production in the hydrothermal reaction is not large in water-hyacinth treatment.

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS Hydrothermal pretreatment and enzymatic hydrolysis were conducted to investigate the effects of CH3COOH and to evaluate the autocatalytic effect in the hydrothermal treatment of water-hyacinth feedstock. The final yields of glucose obtained with and without a catalyst were compared, and the reaction rate constants were determined based on a pseudofirstorder reaction model. The following conclusions were obtained. (1) CH3COOH addition is effective in increasing the final glucose yield. The amount of hydrolyzable cellulose and the glucose yield are both increased. (2) Addition of K2CO3 does not greatly affect the final glucose yield, indicating that the autocatalytic effect in hydrothermal pretreatment is not large. 5014

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(15) Emmel, A.; Mathias, A. L.; Wypych, F.; Ramo, L. P. Fractionation of Eucalyptus grandis chips by dilute acid-catalysed steam explosion. Bioresour. Technol. 2003, 86, 105. (16) Xu, J.; Thomsen, M. H.; Thomsen, A. B. Investigation of acetic acid-catalyzed hydrothermal pretreatment on corn stover. Appl. Microbiol. Biotechnol. 2010, 86, 509. (17) 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. (18) United States Department of Agriculture. Forage Fiber Analyses. Apparatus, Reagents, Procedures, and Some Applications. Agric. Handb. 1970, 379. (19) Gong, G.; Liu, D.; Huang, Y. Microwave-assisted organic acid pretreatment for enzymatic hydrolysis of rice straw. Biosystems Engineering 2010, 107, 67. (20) Toor, S. S.; Rosendahj, L.; Rudolf, A. Hydrothermal liquefaction of biomass: A review of subcritical water technologies. Energy 2011, 36, 2328. (21) Taherzadeh, M. J.; Karimi, K. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: A review. Int. J. Mol. Sci. 2008, 99, 1621. (22) Girisuta, B.; Danon, B.; Manurung, R.; Janssen, L. P. B. M.; Heeres, H. J. Experimental and kinetic modelling studies on the acidcatalysed hydrolysis of the water hyacinth plant to levulinic acid. Bioresour. Technol. 2008, 99, 8367.

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