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Dilute Acid Hydrolysis of Cellulose and Cellulosic Bio-Waste Using a Microwave Reactor System
DILUTE ACID HYDROLYSIS OF CELLULOSE AND CELLULOSIC BIO-WASTE USING A MICROWAVE REACTOR SYSTEM A. Orozco, M. Ahmad, D. Rooney and G. Walker School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, Northern Ireland, UK.
Abstract: The dilute acid hydrolysis of grass and cellulose with phosphoric acid was undertaken in a microwave reactor system. The experimental data and reaction kinetic analysis indicate that this is a potential process for cellulose and hemi-cellulose hydrolysis, due to a rapid hydrolysis reaction at moderate temperatures. The optimum conditions for grass hydrolysis were found to be 2.5% phosphoric acid at a temperature of 1758C. It was found that sugar degradation occurred at acid concentrations greater than 2.5% (v/v) and temperatures greater than 1758C. In a further series of experiments, the kinetics of dilute acid hydrolysis of cellulose was investigated varying phosphoric acid concentration and reaction temperatures. The experimental data indicate that the use of microwave technology can successfully facilitate dilute acid hydrolysis of cellulose allowing high yields of glucose in short reaction times. The optimum conditions gave a yield of 90% glucose. A pseudo-homogeneous consecutive first order reaction was assumed and the reaction rate constants were calculated as: k1 ¼ 0.0813 s21; k2 ¼ 0.0075 s21, which compare favourably with reaction rate constants found in conventional non-microwave reaction systems. The kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent ‘hot spot’ formation within the reactor, which would have cause localised degradation of glucose. Keywords: dilute acid hydrolysis; cellulose hydrolysis; microwave reactor; reaction kinetics; bio-ethanol.
INTRODUCTION
Correspondence to: Dr G. Walker, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT9 5AG, Northern Ireland, UK. E-mail: [email protected] DOI: 10.1205/psep07003 0957–5820/07/ $30.00 þ 0.00 Process Safety and Environmental Protection Trans IChemE, Part B, September 2007 # 2007 Institution of Chemical Engineers
Several approaches have been examined for the hydrolysis of waste cellulose to sugars and the subsequent fermentation into bioethanol and other bio-products (Broder et al., 2001). Most research has been focussed on the production of bio-ethanol using simultaneous saccharification and fermentation (SSF), but considerable improvements have been made in the technologies for the production of ethanol from lingo-cellulosic materials (Lark et al., 1997).
Bio-ethanol Production There are several methods to manage solid biowaste such us landfilling, composting, incineration, recycling (Reiht et al., 2002). These methods normally have adverse effects in the environment, e.g., landfill putrescible waste producing methane, leachate and odour. Furthermore, landfill requires a large amount of land causing detrimental of the landscape and local amenities. Incineration has the advantages that it can generate energy from waste and reduce the volume and weight of waste by up to 90% and 70%, respectively. The perceived disadvantage of incineration is that it produces gas pollutants (carbon dioxide and dioxins) and fly ash. Biomass contains five different sugars (xylose, glucose, arabinose, mannose and galactose), and all which require processing (usually fermentation) to form economically viable products (Korte et al., 2002). In the United States 94% of bio-ethanol is produced from corn (Broder et al., 2001), furthermore, Brazil produces most of its liquid vehicular fuel from sugar cane (Ackerson et al., 1991).
Cellulose Hydrolysis The main challenges in producing ethanol from renewable lingo-cellulosic biomass (e.g., MSW) have been found in hydrolysis stage of the process. The hydrolysis of cellulose to glucose only occurs at economically viable yields when a catalyst is used. The three main catalyst classifications are: enzymatic, concentrated acid and dilute acid catalysts (Badger, 2000). The main advantages in using enzymatic catalysts are the high specific characteristic of enzymes (i.e., no by-products), enzymes operate under mild conditions, are environmentally friendly and 446
Vol 85 (B5) 446–449
HYDROLYSIS OF CELLULOSE USING REACTOR SYSTEM a small amount of enzyme results in high yields. In using enzymatic hydrolysis however, pre-treatment is necessary to open up the structure and to provide access for the enzyme to the active sites. Pre-treatment is usually preformed by energy intensive physical methods, high temperature and pressure or the use of a chemical solvent e.g., dilute acid. Also, the presence of lignin can be inhibitory to the enzyme hydrolysis. Other disadvantages of enzyme hydrolysis include the long reaction times, large reactors and the high cost of enzymes. Concentrated acid processes use relatively mild temperatures and the only pressure involved is that created in pumping materials from vessel to vessel. These low temperatures and pressures minimize the degradation of sugars to undesirable by-products. Concentrated acid disrupts the hydrogen bonding in the cellulose chain converting it to an amorphous state. The cellulose, once de-crystallized, forms a homogeneous gel with the acid, which allows hydrolysis reactions. Water can then be added at low temperatures to dilute the solution, providing conditions to form glucose. Waste grass clippings have the potential to be a biomass feedstock for ethanol production due to the high content of cellulose and hemi-cellulose (approximately 60% and 70% w/w dry mass). Hemi-cellulose can decompose at temperatures of approximately 1608C to form xylose and other sugars. However, for the decomposition of crystalline cellulose temperatures of 200 –4008C are usually required, with this generating problems with sugar degradation. At these temperatures, cellulose degrades into hydroxymethyl furfural and xylose degrades into furfural (Wyman, 1994; Zaldivar et al., 2001). The aim of this research, is to study dilute acid hydrolysis of cellulose and waste cellulosic biomass (grass clippings) with phosphoric acid (1–10%v/v), employing a microwave reactor. Process parameters investigated include variation in reaction temperature (1508C to 2008C) and acid catalyst concentration. The product sugars from the reaction, principally glucose, are intended for fermentation in bio-ethanol production.
MATERIALS AND METHODS Acid Hydrolysis of Grass and Cellulose Mixtures of 5% (w/w) grass clippings (or cellulose) with distilled water were prepared. To analyse the effect of acid concentration phosphoric acid at 1.0, 2.5, 5.0, 7.5 and 10.0%(v/v) were used at 1758C. To study of effect of temperature on the system, the reactions were carried out at 150, 160, 175 and 2008C with 1–2%(v/v) phosphoric acid. The acid was added before starting the reaction in order to avoid pre-hydrolysis. The grass and cellulose hydrolysis experiments were undertaken in a 100 ml microwave reactor system (Explorer PLS, Focused Synthesis Instrumentation).
447
Table 1. Characterisation of grass cuttings. Sample % Carbon % Hydrogen % Nitrogen % Ash % Cellulose Grass
43.93
6.34
0.76
13.01
21.6
column with: isocratic elution at a flow rate of 0.8 ml min21; 85 : 15 acetonitrile : water as the mobile phase; an evaporative light scattering detector (ELSD) Sedex 55. The operation conditions of the ELSD were 408C, 2 bar. Nitrogen was used as a carrier gas. An ADC 12 single channel Pico logger with 12 bits resolution connected to a PC with Picolog data acquisition software was used for recording data. Quantification of the sugars was carried out using Origin Pro 7.5 SR 5 software and calibration curves were developed for sugar standards of L-arabinose, D-mannose, D-glucose, D-galactose, D-xylose and D-cellobiose and correlations coefficients for each standard was r 2 . 0.99. The total reducing sugar concentration in the sample was determined after according to procedure ‘Determination of Structural Carbohydrates and Lignin in Biomass’ described by the NREL (2004). All the experiments were carried out in triplicate with the average concentrations and yields reported.
RESULTS AND DISCUSSION Material Characterization Initially the grass cuttings were analysed for suitability in the ethanol production process, analyses included moisture content, chemical composition (CHN analysis), ash content, cellulose content and calorific value (see Table 1).
Dilute Acid Hydrolysis of Waste Cellulosic Material (Grass) to Sugars Figure 1 illustrates a chromatogram of the products of the hydrolysis of grass cuttings, indicating that xylose, glucose and arabinose were the principal sugars formed. Figure 2 illustrates a plot of sugar yield versus phosphoric acid concentration for the hydrolysis of grass cuttings. The maximum sugar yield occurred at 2.5% of phosphoric acid, with a total sugar yield of approximately 190 mg sugar per g of dry grass cuttings. The maximum yield achieved for xylose was, 67 mg g21 dry grass corresponding to 24% of the total theoretical yield. For arabinose, the maximum yield achieved was also at 2.5% phosphoric acid at 24.8 mg g21 of dry grass of 72% of the total theoretical yield. Generally, an increase in the acid concentration decreased the sugar yield due to
Sugar Analysis After the hydrolysis reaction the solution was filtered (Whatman filter paper #54), the pH of the liquid was measured and adjusted with calcium hydroxide to pH 4.00, centrifuged at 13 000 rpm for 15 min and then filtered through 0.45 mm syringe filter (after Agblevor et al., 2004). Sugars in the hydrolysate were measured using a Supelcosil LC-NH2
Figure 1. Chromatogram of grass hydrolysis at 1758C, 2.5 v/v% phosphoric acid, 15 min reaction time.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 446 –449
448
OROZCO et al.
Figure 2. Grass hydrolysis at different phosphoric acid concentration, glucose, xylose and arabinose yield in mg g21 dry grass (microwave reactor).
possible sugar degradation, with xylose most affected by an increase in acid concentration. The acid concentration did not have a significant effect on glucose formation (as opposed to xylose or arabinose), with a yield of 61 mg g21 dry grass corresponding to 20% of the total theoretical yield at phosphoric acid concentration of 2.5%. Analysing the relationship between sugar degradation and acid concentration, suggests that pentoses (arabinose and xylose) are prone to degradation with increased acid concentration, which may indicate that the acid also acts as a catalyst for sugar decomposition. Figure 3 illustrates the effect of temperature on grass hydrolysis with dilute phosphoric acid. The maximum sugar yield was achieved at a temperature of 1758C for each of the sugars analysed. Reaction temperature is an important factor for sugar degradation. The data in Figure 3 indicate that with a reaction temperature of 2008C, the total sugar yield decreases by approximately 30%. The sugar yield data for this higher temperature indicate that pentose sugars are more susceptible to degradation than hexose sugars.
Dilute Acid Hydrolysis of Cellulose to Glucose The kinetics of the hydrolysis reaction using 2%, 4% and 7.5% phosphoric acid as the catalyst at a temperature of
Figure 3. Grass hydrolysis with 1 v/v% phosphoric acid at different temperatures, glucose, xylose and arabinose yield in mg g21 dry grass (microwave reactor).
1608C was investigated, with the experimental data illustrated in Figure 4. The data indicate that the reaction proceeded quite rapidly to give a maximum yield of approximately 50% after 5 min, however, the yield decreased with further increase in reaction time. This may indicate that the reaction follows a sequential scheme. For extended reaction times, it was also noted that when the samples were removed from the reactor, a black solid had formed, which was attributed to charring or carmelisation of glucose and other sugars, which would evidently decrease the recorded glucose yield. These data also indicate that acid concentration affects the reaction and product yield, with more rapid glucose formation found with increased acid concentration. However, the highest glucose yield and thus optimum reaction conditions was found to be 3–5 min with 7.5% acid concentration. In order to describe the reaction kinetics, a pseudo-homogeneous consecutive first order reaction was used, that employs a two-step irreversible reaction mechanism to describe the formation and degradation of sugars. The model, represented in equation (1), has been successfully used by other researchers for the hydrolysis of lignocellulosic materials (Ga´mez et al., 2006; Te´llez-Luis et al., 2002). k1
k2
Lignocellulosic material ! Reducing sugars ! Degradation products
(1)
where k1 is the rate constant for sugar formation; and k2 is the rate constant for sugar degradation. Based on this reaction model the differential equations that define the reaction rate equations are as follows: dP ¼ k1 ½P dt dS ¼ k1 ½P k2 ½S dt
(2) (3)
Equation (2) describes the monomerization reaction rate and equation (3) describes the sugar formation rate, where P is the polymer concentration and S is sugar concentration. Based on this model, an Excelw solver was then used to minimize the error value by altering the values of k1 and k2 with the constraints that k1 and k2 must be greater than zero.
Figure 4. Cellulose hydrolysis kinetics—glucose formation in microwave reactor: effect of catalyst concentration (phosphoric acid catalyst, temperature ¼ 1608C).
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 446– 449
HYDROLYSIS OF CELLULOSE USING REACTOR SYSTEM
Figure 5. Cellulose hydrolysis—glucose formation kinetics in microwave reactor (catalyst ¼ 4% phosphoric acid, temperature ¼ 1608C).
The optimum solution was found to be: k1 ¼ 0.0813 s21 and k2 ¼ 0.0075 s21 (see Figure 5). Malester and Green (1992) working on the kinetics of dilute acid hydrolysis of cellulose originating from MSW with sulphuric acid at 2258C and pH of 0.42 quoted the rate constants as k1 ¼ 0.0773 s21 and k2 ¼ 0.0445 s21. The value of k1 in this study is higher but that the value of k2 is much lower, which indicates that sugar/glucose degradation is less predominant under the reaction conditions in this study. This difference could be primarily attributed to the catalyst used and the effect of microwaves on the rate of glucose degradation (temperature difference between the two systems was 2258C versus 1608C in this study). The absorbed microwave power within the system is a function of the dielectric loss factor and relative dielectric constant of the materials within the reactor. However, these parameters essentially remain constant under the experimental conditions in this study. More significantly, microwave fields usually affect hydrogen-bonding networks which may lead to an increase in the rate of breakage of the cellulosic structure, additionally microwaves are known to increase the rate of reactant and product diffusion. These factors may account for the increase in reaction rate constant, k1, in the microwave reactor in comparison with conventional systems. Moreover, the kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent ‘hot spot’ formation within the reactor, which would have caused localized degradation of glucose.
CONCLUSIONS The optimum conditions for grass hydrolysis in the microwave reactors were 2.5% phosphoric acid at a temperature of 1758C, which resulted in a maximum yield for xylose 67 mg g21, arabinose 25 mg g21 and glucose of 61 mg g21 (dry grass). It was found that sugar degradation occurred at acid concentrations greater than 2.5%(v/v) and temperatures
449
greater than 1758C and that pentosas were more susceptible to degradation than glucose. Kinetic analysis of the dilute acid hydrolysis of cellulose (to glucose) indicated that the use of microwave technology can successfully facilitate acid hydrolysis allowing high yields of glucose in short reaction times. The optimum conditions gave a yield of 90% (w/w) glucose. A pseudo-homogeneous consecutive first order reaction was assumed and the reaction rate constants were calculated as: k1 ¼ 0.0813 s21; k2 ¼ 0.0075 s21, which compare favourably with reaction rate constants found in conventional non-microwave reaction systems. The kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent ‘hot spot’ formation within the reactor, which would have cause localised degradation of glucose.
REFERENCES Ackerson, M.D., Clausen, E.C. and Gaddy, J.L., 1991, Production of ethanol from MSW, IECEC-91. Proceedings of the 26th Intersociety Energy Conversion Engineering Conference, 4– 9 August 1991, ANS, Boston, MA, USA, pp. 61. Agblevor, F.A., Murde, A. and Hames, B.R., 2004, Improved method of analysis of biomass sugars using high-performance liquid chromatography, Biotechnology Letters, 26: 1207–1210. Badger, P.C., 2000, Ethanol from cellulose: A general review. P.17– 21. Trends in Crops and New Uses (ASHS Press, Alexandria, VA). Broder, J.D., Harris, R.A. and Ranney, J.T., 2001, Using MSW and industrial residues as ethanol feedstocks, Biocycle, 42(10): 23–26. Ga´mez, G., 2006, Study of the hydrolysis of sugar cane bagasse using phosphoric acid, Journal of Food Engineering, 74(1): 78–88. Korte, N.E., West, O.R., Liang, L., Gu, B., Zutman, J.L. and Fernando, Q., 2002, The effect of solvent concentration on the use of palladized-iron for the step-wise dechlorination of polychlorinated biphenyls in soil extracts, Waste Management, 22(3): 343– 349. Lark, N., Xia, Y., Qin, C., Gong, C.S. and Tsao, G.T., 1997, Production of ethanol from recycled paper sludge using cellulase and yeast, Kluveromyces marxianus, Biomass and Bioenergy, 12(2): 135 –143. Malester, I.A., Green, M. and Shelef, G. 1992, Kinetics of dilute acid hydrolysis of cellulose originating from municipal solid-wastes, Industrial & Engineering Chemistry Research, 31(8): 1998– 2003. NREL, Biomass Program, 2004, Determination of Structural Carbohydrates and Lignin in Biomass, Version 2004. Reiht, J.H., Uil, H., Veen, H., Laat, W.T.A.M., Niessen, J.J., Jong, H.W. and Elbersen, H.W.B., 2002, Co-production of bio-ethanol, electricity and heat from biomass wastes: potential and R&D issues. First European Conference on Agriculture and Renewable Energy, 17– 21 July. Te´llez-Luis, S.J., Ramı´rez, J.A. and Va´zquez, M., 2002, Mathematical modelling of hemicellulosic sugar production from sorghum straw, J Food Eng, 52(3): 285– 291. Wyman, C.E., 1994, Alternative transportation fuels from biomass, Proceedings of 29th Intersociety Energy Conversion Engineering Conference—IECEC’94, 7 –11 August AIAA, Monterey, CA, USA, pp. 1090. Zaldivar, J., Nielsen, J. and Olsson, L., 2001, Fuel ethanol production from lignocellulose: a challenge for metabolic engineering and process integration, Applied Microbiology and Biotechnology, 56(1– 2): 17– 34. The manuscript was received 10 January 2007 and accepted for publication after revision 5 June 2007.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 446 –449
Abstract: The dilute acid hydrolysis of grass and cellulose with phosphoric acid was undertaken in a microwave reactor system. The experimental data and reaction kinetic analysis indicate that this is a potential process for cellulose and hemi-cellulose hydrolysis, due to a rapid hydrolysis reaction at moderate temperatures. The optimum conditions for grass hydrolysis were found to be 2.5% phosphoric acid at a temperature of 1758C. It was found that sugar degradation occurred at acid concentrations greater than 2.5% (v/v) and temperatures greater than 1758C. In a further series of experiments, the kinetics of dilute acid hydrolysis of cellulose was investigated varying phosphoric acid concentration and reaction temperatures. The experimental data indicate that the use of microwave technology can successfully facilitate dilute acid hydrolysis of cellulose allowing high yields of glucose in short reaction times. The optimum conditions gave a yield of 90% glucose. A pseudo-homogeneous consecutive first order reaction was assumed and the reaction rate constants were calculated as: k1 ¼ 0.0813 s21; k2 ¼ 0.0075 s21, which compare favourably with reaction rate constants found in conventional non-microwave reaction systems. The kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent ‘hot spot’ formation within the reactor, which would have cause localised degradation of glucose. Keywords: dilute acid hydrolysis; cellulose hydrolysis; microwave reactor; reaction kinetics; bio-ethanol.
INTRODUCTION
Correspondence to: Dr G. Walker, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT9 5AG, Northern Ireland, UK. E-mail: [email protected] DOI: 10.1205/psep07003 0957–5820/07/ $30.00 þ 0.00 Process Safety and Environmental Protection Trans IChemE, Part B, September 2007 # 2007 Institution of Chemical Engineers
Several approaches have been examined for the hydrolysis of waste cellulose to sugars and the subsequent fermentation into bioethanol and other bio-products (Broder et al., 2001). Most research has been focussed on the production of bio-ethanol using simultaneous saccharification and fermentation (SSF), but considerable improvements have been made in the technologies for the production of ethanol from lingo-cellulosic materials (Lark et al., 1997).
Bio-ethanol Production There are several methods to manage solid biowaste such us landfilling, composting, incineration, recycling (Reiht et al., 2002). These methods normally have adverse effects in the environment, e.g., landfill putrescible waste producing methane, leachate and odour. Furthermore, landfill requires a large amount of land causing detrimental of the landscape and local amenities. Incineration has the advantages that it can generate energy from waste and reduce the volume and weight of waste by up to 90% and 70%, respectively. The perceived disadvantage of incineration is that it produces gas pollutants (carbon dioxide and dioxins) and fly ash. Biomass contains five different sugars (xylose, glucose, arabinose, mannose and galactose), and all which require processing (usually fermentation) to form economically viable products (Korte et al., 2002). In the United States 94% of bio-ethanol is produced from corn (Broder et al., 2001), furthermore, Brazil produces most of its liquid vehicular fuel from sugar cane (Ackerson et al., 1991).
Cellulose Hydrolysis The main challenges in producing ethanol from renewable lingo-cellulosic biomass (e.g., MSW) have been found in hydrolysis stage of the process. The hydrolysis of cellulose to glucose only occurs at economically viable yields when a catalyst is used. The three main catalyst classifications are: enzymatic, concentrated acid and dilute acid catalysts (Badger, 2000). The main advantages in using enzymatic catalysts are the high specific characteristic of enzymes (i.e., no by-products), enzymes operate under mild conditions, are environmentally friendly and 446
Vol 85 (B5) 446–449
HYDROLYSIS OF CELLULOSE USING REACTOR SYSTEM a small amount of enzyme results in high yields. In using enzymatic hydrolysis however, pre-treatment is necessary to open up the structure and to provide access for the enzyme to the active sites. Pre-treatment is usually preformed by energy intensive physical methods, high temperature and pressure or the use of a chemical solvent e.g., dilute acid. Also, the presence of lignin can be inhibitory to the enzyme hydrolysis. Other disadvantages of enzyme hydrolysis include the long reaction times, large reactors and the high cost of enzymes. Concentrated acid processes use relatively mild temperatures and the only pressure involved is that created in pumping materials from vessel to vessel. These low temperatures and pressures minimize the degradation of sugars to undesirable by-products. Concentrated acid disrupts the hydrogen bonding in the cellulose chain converting it to an amorphous state. The cellulose, once de-crystallized, forms a homogeneous gel with the acid, which allows hydrolysis reactions. Water can then be added at low temperatures to dilute the solution, providing conditions to form glucose. Waste grass clippings have the potential to be a biomass feedstock for ethanol production due to the high content of cellulose and hemi-cellulose (approximately 60% and 70% w/w dry mass). Hemi-cellulose can decompose at temperatures of approximately 1608C to form xylose and other sugars. However, for the decomposition of crystalline cellulose temperatures of 200 –4008C are usually required, with this generating problems with sugar degradation. At these temperatures, cellulose degrades into hydroxymethyl furfural and xylose degrades into furfural (Wyman, 1994; Zaldivar et al., 2001). The aim of this research, is to study dilute acid hydrolysis of cellulose and waste cellulosic biomass (grass clippings) with phosphoric acid (1–10%v/v), employing a microwave reactor. Process parameters investigated include variation in reaction temperature (1508C to 2008C) and acid catalyst concentration. The product sugars from the reaction, principally glucose, are intended for fermentation in bio-ethanol production.
MATERIALS AND METHODS Acid Hydrolysis of Grass and Cellulose Mixtures of 5% (w/w) grass clippings (or cellulose) with distilled water were prepared. To analyse the effect of acid concentration phosphoric acid at 1.0, 2.5, 5.0, 7.5 and 10.0%(v/v) were used at 1758C. To study of effect of temperature on the system, the reactions were carried out at 150, 160, 175 and 2008C with 1–2%(v/v) phosphoric acid. The acid was added before starting the reaction in order to avoid pre-hydrolysis. The grass and cellulose hydrolysis experiments were undertaken in a 100 ml microwave reactor system (Explorer PLS, Focused Synthesis Instrumentation).
447
Table 1. Characterisation of grass cuttings. Sample % Carbon % Hydrogen % Nitrogen % Ash % Cellulose Grass
43.93
6.34
0.76
13.01
21.6
column with: isocratic elution at a flow rate of 0.8 ml min21; 85 : 15 acetonitrile : water as the mobile phase; an evaporative light scattering detector (ELSD) Sedex 55. The operation conditions of the ELSD were 408C, 2 bar. Nitrogen was used as a carrier gas. An ADC 12 single channel Pico logger with 12 bits resolution connected to a PC with Picolog data acquisition software was used for recording data. Quantification of the sugars was carried out using Origin Pro 7.5 SR 5 software and calibration curves were developed for sugar standards of L-arabinose, D-mannose, D-glucose, D-galactose, D-xylose and D-cellobiose and correlations coefficients for each standard was r 2 . 0.99. The total reducing sugar concentration in the sample was determined after according to procedure ‘Determination of Structural Carbohydrates and Lignin in Biomass’ described by the NREL (2004). All the experiments were carried out in triplicate with the average concentrations and yields reported.
RESULTS AND DISCUSSION Material Characterization Initially the grass cuttings were analysed for suitability in the ethanol production process, analyses included moisture content, chemical composition (CHN analysis), ash content, cellulose content and calorific value (see Table 1).
Dilute Acid Hydrolysis of Waste Cellulosic Material (Grass) to Sugars Figure 1 illustrates a chromatogram of the products of the hydrolysis of grass cuttings, indicating that xylose, glucose and arabinose were the principal sugars formed. Figure 2 illustrates a plot of sugar yield versus phosphoric acid concentration for the hydrolysis of grass cuttings. The maximum sugar yield occurred at 2.5% of phosphoric acid, with a total sugar yield of approximately 190 mg sugar per g of dry grass cuttings. The maximum yield achieved for xylose was, 67 mg g21 dry grass corresponding to 24% of the total theoretical yield. For arabinose, the maximum yield achieved was also at 2.5% phosphoric acid at 24.8 mg g21 of dry grass of 72% of the total theoretical yield. Generally, an increase in the acid concentration decreased the sugar yield due to
Sugar Analysis After the hydrolysis reaction the solution was filtered (Whatman filter paper #54), the pH of the liquid was measured and adjusted with calcium hydroxide to pH 4.00, centrifuged at 13 000 rpm for 15 min and then filtered through 0.45 mm syringe filter (after Agblevor et al., 2004). Sugars in the hydrolysate were measured using a Supelcosil LC-NH2
Figure 1. Chromatogram of grass hydrolysis at 1758C, 2.5 v/v% phosphoric acid, 15 min reaction time.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 446 –449
448
OROZCO et al.
Figure 2. Grass hydrolysis at different phosphoric acid concentration, glucose, xylose and arabinose yield in mg g21 dry grass (microwave reactor).
possible sugar degradation, with xylose most affected by an increase in acid concentration. The acid concentration did not have a significant effect on glucose formation (as opposed to xylose or arabinose), with a yield of 61 mg g21 dry grass corresponding to 20% of the total theoretical yield at phosphoric acid concentration of 2.5%. Analysing the relationship between sugar degradation and acid concentration, suggests that pentoses (arabinose and xylose) are prone to degradation with increased acid concentration, which may indicate that the acid also acts as a catalyst for sugar decomposition. Figure 3 illustrates the effect of temperature on grass hydrolysis with dilute phosphoric acid. The maximum sugar yield was achieved at a temperature of 1758C for each of the sugars analysed. Reaction temperature is an important factor for sugar degradation. The data in Figure 3 indicate that with a reaction temperature of 2008C, the total sugar yield decreases by approximately 30%. The sugar yield data for this higher temperature indicate that pentose sugars are more susceptible to degradation than hexose sugars.
Dilute Acid Hydrolysis of Cellulose to Glucose The kinetics of the hydrolysis reaction using 2%, 4% and 7.5% phosphoric acid as the catalyst at a temperature of
Figure 3. Grass hydrolysis with 1 v/v% phosphoric acid at different temperatures, glucose, xylose and arabinose yield in mg g21 dry grass (microwave reactor).
1608C was investigated, with the experimental data illustrated in Figure 4. The data indicate that the reaction proceeded quite rapidly to give a maximum yield of approximately 50% after 5 min, however, the yield decreased with further increase in reaction time. This may indicate that the reaction follows a sequential scheme. For extended reaction times, it was also noted that when the samples were removed from the reactor, a black solid had formed, which was attributed to charring or carmelisation of glucose and other sugars, which would evidently decrease the recorded glucose yield. These data also indicate that acid concentration affects the reaction and product yield, with more rapid glucose formation found with increased acid concentration. However, the highest glucose yield and thus optimum reaction conditions was found to be 3–5 min with 7.5% acid concentration. In order to describe the reaction kinetics, a pseudo-homogeneous consecutive first order reaction was used, that employs a two-step irreversible reaction mechanism to describe the formation and degradation of sugars. The model, represented in equation (1), has been successfully used by other researchers for the hydrolysis of lignocellulosic materials (Ga´mez et al., 2006; Te´llez-Luis et al., 2002). k1
k2
Lignocellulosic material ! Reducing sugars ! Degradation products
(1)
where k1 is the rate constant for sugar formation; and k2 is the rate constant for sugar degradation. Based on this reaction model the differential equations that define the reaction rate equations are as follows: dP ¼ k1 ½P dt dS ¼ k1 ½P k2 ½S dt
(2) (3)
Equation (2) describes the monomerization reaction rate and equation (3) describes the sugar formation rate, where P is the polymer concentration and S is sugar concentration. Based on this model, an Excelw solver was then used to minimize the error value by altering the values of k1 and k2 with the constraints that k1 and k2 must be greater than zero.
Figure 4. Cellulose hydrolysis kinetics—glucose formation in microwave reactor: effect of catalyst concentration (phosphoric acid catalyst, temperature ¼ 1608C).
Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 446– 449
HYDROLYSIS OF CELLULOSE USING REACTOR SYSTEM
Figure 5. Cellulose hydrolysis—glucose formation kinetics in microwave reactor (catalyst ¼ 4% phosphoric acid, temperature ¼ 1608C).
The optimum solution was found to be: k1 ¼ 0.0813 s21 and k2 ¼ 0.0075 s21 (see Figure 5). Malester and Green (1992) working on the kinetics of dilute acid hydrolysis of cellulose originating from MSW with sulphuric acid at 2258C and pH of 0.42 quoted the rate constants as k1 ¼ 0.0773 s21 and k2 ¼ 0.0445 s21. The value of k1 in this study is higher but that the value of k2 is much lower, which indicates that sugar/glucose degradation is less predominant under the reaction conditions in this study. This difference could be primarily attributed to the catalyst used and the effect of microwaves on the rate of glucose degradation (temperature difference between the two systems was 2258C versus 1608C in this study). The absorbed microwave power within the system is a function of the dielectric loss factor and relative dielectric constant of the materials within the reactor. However, these parameters essentially remain constant under the experimental conditions in this study. More significantly, microwave fields usually affect hydrogen-bonding networks which may lead to an increase in the rate of breakage of the cellulosic structure, additionally microwaves are known to increase the rate of reactant and product diffusion. These factors may account for the increase in reaction rate constant, k1, in the microwave reactor in comparison with conventional systems. Moreover, the kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent ‘hot spot’ formation within the reactor, which would have caused localized degradation of glucose.
CONCLUSIONS The optimum conditions for grass hydrolysis in the microwave reactors were 2.5% phosphoric acid at a temperature of 1758C, which resulted in a maximum yield for xylose 67 mg g21, arabinose 25 mg g21 and glucose of 61 mg g21 (dry grass). It was found that sugar degradation occurred at acid concentrations greater than 2.5%(v/v) and temperatures
449
greater than 1758C and that pentosas were more susceptible to degradation than glucose. Kinetic analysis of the dilute acid hydrolysis of cellulose (to glucose) indicated that the use of microwave technology can successfully facilitate acid hydrolysis allowing high yields of glucose in short reaction times. The optimum conditions gave a yield of 90% (w/w) glucose. A pseudo-homogeneous consecutive first order reaction was assumed and the reaction rate constants were calculated as: k1 ¼ 0.0813 s21; k2 ¼ 0.0075 s21, which compare favourably with reaction rate constants found in conventional non-microwave reaction systems. The kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent ‘hot spot’ formation within the reactor, which would have cause localised degradation of glucose.
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Trans IChemE, Part B, Process Safety and Environmental Protection, 2007, 85(B5): 446 –449