Kinetic study of the acid hydrolysis of sugar cane bagasse

Transcription

1 Journal of Food Engineering 55 (2002) Kinetic study of the acid hydrolysis of sugar cane bagasse R. Aguilar a, J.A. Ramırez b, G. Garrote c,m.vazquez c, * a Department of Food Engineering, UAM Mante, Universidad Autonoma de Tamaulipas, Boulevard Enrique Cardenas Gonzalez 1201, Col. Jardın, Cd. Mante, Tamaulipas, 89840Mexico b Department of Food Science and Technology, UAM Reynosa-Aztlan, Universidad Autonoma de Tamaulipas, Apdo. Postal 1015, Reynosa, Tamaulipas, Mexico c Area de Tecnologıa de los Alimentos, Departamento de Quımica Analıtica. Escuela Politecnica Superior, Universidad de Santiago de Compostela (Campus Lugo), Lugo, Spain Received 10 July 2001; accepted 4 March 2002 Abstract Economic interest in xylitol production can be enhanced if the needed xylose solutions can be obtained from the hydrolysis of low-cost lignocellulosic wastes. Sugar cane bagasse is a renewable, cheap and widely available waste in tropical countries. The hydrolysis of sugar cane bagasse to obtain xylose solutions has a double consequence, the elimination of a waste and the generation of a value-added product. The objective of this work was to study the xylose production from sugar cane bagasse by sulphuric acid hydrolysis at several temperatures (100, 122 and 128 C) and concentrations of acid (2%, 4% and 6%). Kinetic models were developed to explain the variation with time of xylose, glucose, acetic acid and furfural generated in the hydrolysis. Optimal conditions found were 2% H 2 SO 4 at 122 C for 24 min, which yielded a solution with 21.6 g xylose/l, 3 g glucose/l, 0.5 g furfural/l and 3.65 g acetic acid/l. In these conditions, 90% of the hemicelluloses was hydrolysed. Ó 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction * Corresponding author. Tel.: x22420; fax: addresses: raguilar@ing-man.uat.mx (R. Aguilar), ramirez@qui-rey.uat.mx (J.A. Ramırez), gilgv@lettera.net (G. Garrote), vazquezm@lugo.usc.es (M. Vazquez). Xylose is the main hemicellulosic sugar. It can be used as a carbon and energy source in fermentation processes. Among others, the bioconversion from xylose to xylitol is of interest as it is a polyol with important applications as a sweetener. Xylitol has important advantages over glucose or saccharose, such as anticarcinogenicity, low caloric value and negative heat of dissolution (Kim, Ryu, & Seo, 1999; Parajo, Domınguez, & Domınguez, 1996; Silva, Felipe, & Mancilha, 1998). Economic interest in xylitol production by fermentation can be enhanced if the required xylose solutions can be obtained from the hydrolysis of low-cost lignocellulosic wastes. Sugar cane bagasse is a renewable, cheap and widely available waste. The hydrolysis of sugar cane bagasse to obtain xylose solutions has a double consequence, the elimination of a waste and the production of a value-added product that enhances the economics of the process (du Toit, Olivier, & van Bijon, 1984). Dilute acids can be used as catalysts of a limited hydrolysis called prehydrolysis. This consists of the hydrolysis of the hemicellulosic fraction, the remaining cellulose and lignin fractions being almost unaltered. H 2 SO 4 (Nguyen et al., 1999; Nguyen, Tucker, Keller, & Eddy, 2000), HCl (Springer, 1966), HF (Franz, Erckel, Riehm, Woernle, & Deger, 1982) or CH 3 COOH (Conner & Lorenz, 1986) are acids commonly employed as catalysts. The above acids release protons that break the heterocyclic ether bonds between the sugar monomers in the polymeric chains formed by the hemicelluloses and the cellulose. The breaking of these bonds releases several compounds, mainly sugars such as xylose, glucose and arabinose. Other compounds released are oligomers, furfural and acetic acid. A quantitative hydrolysis of the hemicelluloses can be performed almost without damage to the cellulose because the bonds in hemicelluloses are weaker than in cellulose. Therefore, a solid waste formed by cellulose and lignin is obtained in the prehydrolysis of sugar cane bagasse. This waste can be used in the production of glucose solutions to obtain lactic /02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S (02)

2 310 R. Aguilar et al. / Journal of Food Engineering 55 (2002) Nomenclature [ ] concentration a ratio for the xylan fractions in raw material (g of susceptible xylan/g of total xylan) a G ratio for the glucan fractions in raw material (g of susceptible glucan/g of total glucan) a regression parameter Ac 0 potential acetic acid concentration (g/l) AcH acetic acid concentration (g/l) C acid concentration (%) CXn 0 initial composition of xylan E a activation energy (kj/mol) F furfural concentration (g/l) F 0 potential concentration of furfural (g/l) G glucose concentration (g/l) initial glucose concentration (g/l) G 0 G n glucan concentration (g/l) HMF 5-hydroxymetil-2-furfural k 1 rate of the generation reaction (min 1 ) k 2 rate of the decomposition reaction (min 1 ) k i kinetic coefficient (min 1 ) k i0 pre-exponential factor (min 1 ) M monomer concentration (g/l) M 0 initial monomer concentration (g/l) n regression parameter P polymer concentration (g/l) P 0 initial polymer concentration (g/l) R gas constant (8: kj/(mol K)) T temperature (Kelvin) t time (min) WSR water/solid ratio acid or ethanol by fermentation or for the production of paper pulp (David, Fornasier, Greindl-Fallon, & Vanlautem, 1985; Grethlein & Converse, 1991). The hydrolysates obtained in the prehydrolysis of sugar cane bagasse can be used after neutralization for conversion to xylitol (Silva et al., 1998) or single cell protein (Nigam, 1998) due to the high xylose content. For the fermentation process, the presence of acetic acid and/or furfural in hydrolysates can hinder or prevent a subsequent fermentation step. Therefore, hydrolysates with low concentrations of inhibitors are required. Hydrolysis reactions of sugar polymers in a diluteacid medium are very complex. The substrate is in a solid phase and the catalyst in a liquid phase. The mechanism of the hydrolysis reaction includes (Carrasco, 1991; Fengel & Wegener, 1984; Harris, 1952): (i) diffusion of protons through the wet lignocellulosic matrix; (ii) protonation of the oxygen of a heterocyclic ether bond between the sugar monomers; (iii) breaking of the ether bond; (iv) generation of a carbocation as intermediate; (v) solvation of the carbocation with water; (vi) regeneration of the proton with cogeneration of the sugar monomer, oligomer or polymer depending on the position of the ether bond; (vii) diffusion of the reaction products in the liquid phase if it is permit for their form and size; (viii) restarting of the second step. Therefore, due to the difficult in modelling these processes, empirical models have been developed. This work deals with the acid prehydrolysis of sugar cane bagasse with sulphuric acid. Kinetic models were developed to explain the variation with time of the main products generated. The prehydrolysis was optimised to obtained xylose solutions with a low concentration of growth inhibitors. 2. Materials and methods The raw material used, sugar cane bagasse, was collected in a local industry (Ingenio Azucarero de Mante, Tamaulipas, Mexico). It was air dried, milled, screened to select the fraction of particles with a size lower than 0.5 mm, homogenized in a single lot and stored until needed. Analyses of the main fractions (cellulose, hemicelluloses and Klason lignin) were carried out using a quantitative acid hydrolysis under standard conditions (Garrote, Domınguez, & Parajo, 1999). The main composition of the sorghum straw is shown in Table 1. Treatments were performed in the range C in media containing 2, 4 or 6 g H 2 SO 4 /100 g liquor using a charge of 1 g sugar cane bagasse/10 g liquor on dry basis. Samples were collected at several reaction times in the range min. The experiments were performed in nine sets. The operational conditions of the sets are shown in Figs At given reaction times, samples of liquors were taken from the reaction media and analysed. The samples were diluted with water (1/10 v/v), centrifuged to separate the water-insoluble phenolic fraction and analysed by UV vis spectroscopy at 280 nm for furfural and by HPLC Table 1 Main components of sugar cane bagasse used in this study Components Percentage dry weight Glucan Xylan Araban Klason lignin Others

3 R. Aguilar et al. / Journal of Food Engineering 55 (2002) Fig. 1. Experimental and predicted dependence of the xylose concentration on time at several H 2 SO 4 concentration and temperature. Fig. 2. Experimental and predicted dependence of the glucose concentration on time at several H 2 SO 4 concentration and temperature. for glucose, xylose and acetic acid. The HPLC analyses were carried out using a Transgenomic ION- 300 column (oven temperature ¼ 45 C) with isocratic elution (flow rate ¼ 0:4 ml/min; mobile phase: H 2 SO N). All experiments were carried out in triplicate and means are given. Non-linear regression analyses of experimental data were performed with a commercial optimisation routine using Newton s method (Solver, Microsoft Excel 2000, Microsoft Corporation, Redmont, WA, USA) by minimizing the sum of the squares of deviations between experimental and calculated data according to the philosophy reported by Garrote, Domınguez, and Parajo (2001a). 3. Results The composition of the sugar cane bagasse used in this study is shown in Table 1. The main fractions of sugar cane bagasse were in the same range as other herbaceous materials, such as rice, barley straw (Garrote et al., 1999) and sorghum straw (Tellez-Luis, Ramırez, & Vazquez, 2002). The high content of xylan (20.6%) makes this waste adequate for xylose production. Hydrolysates were obtained using H 2 SO 4 at 100, 122 and 128 C. Table 2 shows the solubilized fractions and Figs. 1 4 show concentration of xylose, furfural, glucose and acetic acid released. For the solid/liquid ratio used, the potential concentration of xylose (corresponding to

4 312 R. Aguilar et al. / Journal of Food Engineering 55 (2002) Fig. 3. Experimental and predicted dependence of the acetic acid concentration on time at several H 2 SO 4 concentration and temperature. Fig. 4. Experimental and predicted dependence of furfural concentration on time at several H 2 SO 4 concentration and temperature. the quantitative conversion of xylan to xylose) was 23.4 g/l. The xylose concentration reached up to g/l. It was obtained in the experiment performed at 122 C during 20 min using 2% H 2 SO 4, corresponding to more than 92% of the potential concentration. It was observed that the xylose concentration reached a maximum value and then decreased with the reaction time in the experiments performed at 122 and 128 C. This suggests that decomposition reactions exist, conducting probably to furfural. During the hydrolysis of sugar cane bagasse, other sugars are released to liquors, mainly glucose. This sugar can be proceeded from the cellulosic fraction or from some heteropolymers of the hemicellulosic fraction. It is important to determine the glucose concentration because this sugar is the main carbon source for most microorganisms. The glucose concentration was affected by the sulphuric acid concentration. The maximum value was 8.86 g/l in the experiments performed with 6% H 2 SO 4 at 128 C during 180 min. In experiments performed in severe conditions, a slight decrease was observed in the glucose concentration over a long time. This fact suggests that decomposition reactions can exist, for example to hydroxymethylfurfural. The acetic acid derives from the hydrolysis of the acetyl groups bound to the hemicellulosic monomers.

5 R. Aguilar et al. / Journal of Food Engineering 55 (2002) Table 2 Solubilized fraction obtained in the sulphuric acid hydrolysis of sugar cane bagasse H 2 SO 4 Temperature ( C) Time (min) % % % % % % % % % Acetic acid at relative high concentrations is an inhibitor of microorganism growth. It can hinder or prevent a subsequent fermentation step because acetic acid can go through the cellular membranes and decreases the intracellular ph. Therefore, the metabolism of the microorganism is affected (Maiorella, Blanch, & Wilke, 1983; van Zyl, Prior, & Du Preez, 1991). The toxicity of acetic acid depends on the concentration of the dissociated isomer (Beck, 1986; Bj orling & Lindman, 1989). In turn, the ratio of dissociated/non-dissociated isomer depends on its concentration and the ph of the media. The effect of acetic acid on the growth of microorganisms is not clear. Ferrari, Neirotti, Albornoz, and Saucedo (1992) reported that 10.5 g acetic acid/l hindered the growth of Pichia stipitis. But Palmqvist, Almeida, and Hahn-H agerdal (1999) found that 9 10 g acetic acid/l enhanced the growth and productivity of Saccharomyces cerevisiae. In our study, 5.1 g acetic acid/l was the maximum reached. It was obtained in experiments using 6% H 2 SO 4 at 122 C during a reaction time of 40 min. The concentration of this compound in almost all experiments reached values in the range g acetic acid/l. Through material balances, the initial acetyl groups can be determined. It was g acetyl groups/100 g dry sugar cane bagasse. These values are in accordance with those reported for similar agricultural materials (Tellez- Luis et al., 2002). Furfural was generated as a degradation product from pentoses like xylose. Furfural and soluble phenolic compounds increased with the reaction time and the concentration of the catalyst, sulphuric acid. The temperature lightly affects the decomposition reactions. The higher value reached was 5.83 g furfural/l in the experiment with 6% H 2 SO 4 at 128 C during a reaction time of 300 min. It can be remarked that the hydrolysed fraction quickly increased to values higher than 40% of the initial mass (44.5% in experiments performed with 6% H 2 SO 4 at 122 C during 180 min). Small decreases in the soluble fraction were observed in experiments performed over long times. This is because a small fraction goes to gaseous phase. The sum of the hemicellulosic fraction and the fraction called others in Table 1 (corresponding mainly with extractable compounds, easily released with these treatments) is about 40%. This value is slightly lower than the value for the hydrolysed fraction. It can be inferred that the treatment with sulphuric acid hydrolyses mainly the extractable compounds and the hemicellulosic fraction, the cellulose and the lignin remaining in the solid phase. Once more, the selectivity of the treatment can be seen. 4. Discussion Due to the difficulty in finding a strict mechanism for hydrolysis reactions, it is usual to use simplified models to determine the kinetics of the hydrolysis of lignocellulosic materials. The models proposed in the literature use pseudohomogeneous irreversible first-order reactions. The first model used successful was proposed by Saeman (1945): Cellulose! Glucose! Decomposition products ð1þ This model was designed for the hydrolysis of cellulose from fir using sulphuric acid. The model was also applied to the hydrolysis of the hemicellulosic fraction (Grant, Han, Anderson, & Frey, 1977; Tellez-Luis et al., 2002). Therefore, it can be generalized in Eq. (2): Polymers! k 1 Monomers! k 2 Decomposition products ð2þ where k 1 is the rate of the generation reaction and k 2 is the rate of the decomposition reaction (min 1 ). Solving the differential equations, the following model predicts the concentration of monomers: M ¼ M 0 e k2t k 1 þ P 0 e k 1t e k 2t ð3þ k 2 k 1 where M and P are concentrations of monomer and polymer expressed in g/l, t is time and subscript 0 indicates initial conditions. In this work, Eq. (3) has been applied to model the hydrolysis of sugar cane bagasse.

6 314 R. Aguilar et al. / Journal of Food Engineering 55 (2002) Kinetic modelling of xylose concentration Table 3 Kinetic and statistical parameters of xylose released for the H 2 SO 4 hydrolysis of sugar cane bagasse Operational set a (g/g) k 1 (min 1 ) k (min 1 ) 2% H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C R 2 Xylose is the main product of the hydrolysis of sugar cane bagasse. M 0 was 0 g/l and P 0 was calculated as follow: P 0 ¼ 150 CXn 0 10 ¼ 23:4 geq: xylose=l 132 WSR ð4þ where CXn 0 is the initial composition for xylan (20.6 g xylan/100 g sugar cane bagasse on dry basis), WSR is the water/solid ratio (10 g water/g sugar cane bagasse) and 150/132 is the ratio of the stoichiometric factors. The fitting of experimental data to Eq. (3) was not enough successful (data not shown). Therefore, the kinetic model was modified to include the existence of two hemicellulosic fractions, one easy to hydrolyse and the other difficult to hydrolyse. This fact was previously described by other authors (du Toit et al., 1984). Kobayashi and Sakai (1956) proposed the inclusion of the two fractions in the kinetic models, although, it is frequent to find that one fraction does not react in some experimental conditions (Garrote et al., 1999, 2001a). The parameter a represents the ratio between the fractions (g of susceptible xylan/g of total xylan) in the raw material. Usual values of a are in the range g/g. Eq. (3) was modified to include a as is shown in Eq. (5). M ¼ M 0 e k2t k 1 þ ap 0 e k 1t e k 2t ð5þ k 2 k 1 Data was fitting applying Eq. (5) and Fig. 1 shows the experimental and predicted data for xylose concentrations. The fitting was performed separately for each set. Table 3 shows the kinetic and statistical parameters of the fitting. The statistical parameters R 2 corroborate that the two-fraction model fits very well. Comparing the values of k 1 with k 2, it can be observed that the kinetic coefficients of generation reactions of xylose are 80-fold higher than those of the degradation reactions. Generalizing, the values of kinetic coefficients increase with temperature and concentration of catalyst. The value of a is in the range The average, 0.80 g/g, is in accordance with values reported for diluteacid hydrolysis. Kim and Lee (1987) reported values of a in the range g/g for oak hydrolysis. Eken- Saracßoglu, Ferda, Dilma, and Cß avsßuoglu (1998) found values of 0.84 g/g for corn cob and 0.86 g/g for sunflower seed hulls. Using bagasse waste, Nee and Yee (1976) obtained a value of 0.65 g/g. Conner and Lorenz (1986) obtained values in the range g/g for oak wood. It is normal to find that a also varies with the operational conditions. Hydrolysing Pinus pinaster (Parajo, Santos, & del Rıo, 1995a) at atmospheric pressure with dilute sulphuric acid, a was in the range g/g, however, it was g/g if the hydrolysis is at higher pressure (Parajo, Santos, & del Rıo, 1995b). For sugar cane bagasse, we have also found that a varies with the operational conditions. In experiments performed at 100 C, a was increased from g/g to g/g. In experiments performed at 122 C, the easy fraction was almost all the xylan (0.97 g/g) but at 128 C the value of a was lower (0.7 g/g), indicating that it is not always possible to increase a by increasing the temperature. The kinetic coefficients can be correlated with temperature by applying the Arrhenius equation: k i ¼ k i0 e Ea RT ð6þ where k i is the kinetic coefficients (i ¼ 1 or 2), k i0 is the pre-exponential factor (same units that k 1 ), E a is the activation energy (kj/mol), R is the gas constant, 8: (kj/(molk)) and T is the temperature in Kelvin. The fitting of k 1 was performed for each concentration of sulphuric acid (2, 4 or 6%). The constant k 1 correlated with temperature as shown in Table 4. The values of k 2 were too small because the degradation reactions are not important. Therefore, k 2 did not give a good fit because experimental error effects hinder the analysis. This behaviour is usual in acid hydrolysis (Garrote, Domınguez, & Parajo, 2001b). For k 1, the parameter R 2 shows a good agreement between experimental and predicted data for all regressions. The average values obtained (E a ¼ 109 kj/mol and ln k 10 ¼ 31:6) are similar to those reported for other lignocellulosic materials. Using two-fractions models, E a values of 127 kj/mol for birch wood (Maloney, Chapman, & Baker, 1985) and 120 kj/mol (Kim, Yum, & Park, 2000) and 96.3 kj/mol (Kim & Lee, 1987) for Table 4 Parameters obtained in the fitting using the Arrhenius equation for the xylose released in the H 2 SO 4 hydrolysis of sugar cane bagasse [H 2 SO 4 ] (%) lnðk 10 Þ (k 1 in min 1 ) E a =R (K 1 ) R , , , Average ,080

7 R. Aguilar et al. / Journal of Food Engineering 55 (2002) several hardwoods were found. For agricultural wastes, 80.3 and 92.3 kj/mol were found (Eken-Saracßoglu et al., 1998). The value of E a obtained in our work also compared well with those reported using other kinetic models. For instances, E a was 65.4 kj/mol (Veeraraghavan, Chambers, Myles, & Lee, 1982) and 172 kj/mol (Bhandari, McDonald, & Bakhshi, 1984) using other kinetic models. The values of k 10 vary greatly between raw materials. Ranganathan, McDonald, and Bakhshi (1985) suggest that these differences can be due to the structure and composition of the material, which can neutralize the acid. Therefore, it is not possible compare our result with others of the literature. It is common to modify the Arrhenius equation to model the effect of the acid concentration (C) as follow (Brennan, Hoagland, & Schell, 1986; Carrasco & Roy, 1992; Ranganathan et al., 1985): k 1 ¼ ac n eð Ea RTÞ ð7þ where a and n are the regression parameters and C is in % (w/w). Using the values previously obtained for k 1 and applying a non-linear regression analysis to the model of Eq. (7), k 1 correlated with H 2 SO 4 concentration and temperature as shown in Eq. (8). This equation can be considered well fitted (r 2 ¼ 0:97). k 1 ¼ e 30:7 C 0:734 eð T Þ ð8þ The value of the regression parameter (a) is in accordance with the average of the pre-exponential values shown in Table 4 (31.6). The value of E a =R also coincides with those shown in Table 4. The value of other regression parameters (n) are in the range reported in the literature for similar lignocellulosic materials: n ¼ 1:55 (Eken-Saracßoglu et al., 1998), n ¼ 0:80 (Veeraraghavan et al., 1982) and n ¼ 0:66 (Kim et al., 2000) Kinetic modelling of glucose concentration Glucose is a by-product obtained in the acid hydrolysis of sugar cane bagasse. The glucose released in the hydrolysis can proceed from both hemicellulosic heteropolymers and cellulose. The glucose from cellulose is not usually hydrolysed in the range of operational conditions commonly used for the acid hydrolysis. Therefore, it is probable that the released glucose proceed almost quantitatively from hemicelluloses. The Saeman s model described in Eq. (3) can be used but the value of P 0 cannot clearly be determined. In this case, the decomposition product is mainly 5-hydroxymetil-2-furfural (HMF). This is formed in acid medium due to the release of three molecules of water from hexose. In a first approach, P 0 was fixed at 43.2 g/l, a value corresponding to a quantitative conversion from glucan to glucose. The fitting was not successful (data not shown) and the model did not predict the experimental data. In a second approach, the potential concentration of glucose was used as a parameter of regression. It was considered that there is a glucan fraction that it is susceptible to react and other not susceptible. The parameter a G represents the ratio between the glucan fractions (g of susceptible glucan/g of total glucan) in the raw material. Consequently, the glucose concentration can be calculated by Eq. (9). G ¼ G 0 e k2t k 1 þ a G Gn 0 e k 1t e k 2t ð9þ k 2 k 1 where G is the glucose concentration (g/l); G 0 is the initial glucose concentration (0 g/l); Gn 0 is the glucan concentration corresponding with the quantitative conversion to glucose (43.2 g/l); k 1 is the rate of the generation reaction from glucan to glucose (min 1 ); k 2 is the rate of the decomposition reaction from glucose to HMF (min 1 ). Table 5 shows the kinetic and statistical parameters fitting the model of Eq. (9) for glucose generated. Fig. 2 shows the experimental and predicted data in these hydrolyses. R 2 showed a good agreement between experimental and predicted data. At 100 or 122 C, kinetic parameters of decomposition reactions of glucose (k 2 ) are small or almost 0. However, k 2 is in the range of k 1 at 128 C. It can be supposed that this is due to the high E a of the reactions of glucose degradation to HMF. The parameter a G was affected by temperature and acid concentration, although the effect of the temperature was more intense. Using 2% H 2 SO 4 at 100 C, only 5% of glucan was susceptible to hydrolysis. This fraction proceeds from hemicellulosic heteropolymers. In the strictest condition (6% H 2 SO 4 at 128 C), 51% was susceptible to hydrolysis. However, in experiments at 100 or 122 C it was always below 18.2%. It can be concluded that 18.2% represents the glucose in the hemicelluloses of sugar cane bagasse and that at 128 C 32.8% of the cellulose fraction is hydrolysed. This Table 5 Kinetic and statistical parameters of glucose released for the H 2 SO 4 hydrolysis of sugar cane bagasse Operational set a G (g/g) k (min 1 ) k (min 1 ) 2% H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C R 2

8 316 R. Aguilar et al. / Journal of Food Engineering 55 (2002) deduction was confirmed by the values obtained for the fractions of susceptible xylan (a), which were clearly not affected by temperature. However, temperature clearly increased a G. This was because some cellulose is hydrolysed at high temperature. In the cellulose, crystalline and amorphous fractions exist. The crystalline fraction can be the fraction that reacts at 128 C Kinetic modelling of acetic acid concentration The acetic acid is generated for the hydrolysis of the acetyl groups present in the hemicellulosic heteropolymers. Based on the experimental data, degradation reactions of the acetic acid was not observed in accordance with the results for sorghum straw and other lignocellulosic materials (Garrote et al., 1999, 2001b; Tellez-Luis et al., 2001). Therefore, the model of Eq. (10) describes the generation of acetic acid in the hydrolysis of sugar cane bagasse. Acetyl groups! k 1 Acetic acid ð10þ Based on this reaction model and solving the differential equation leads to Eq. (11), which expresses the acetic acid concentration (AcH) as a function of time (t). AcH ¼ Ac 0 1 e k 1t ð11þ where Ac 0 is the potential concentration of acetyl groups and k 1 the rate of acetic acid generation (min 1 ). The Ac 0 was expressed as acetic acid and introduced as a regression parameter. The acetyl groups are in the hemicelluloses. Therefore, a behaviour similar to xylan is expected as reported Garrote, Domınguez, and Parajo (2001c). Table 6 shows the kinetic and statistical parameters obtained in the fitting of the acetic acid generated in the hydrolysis of sugar cane bagasse. R 2 showed that all the equations obtained are well fitted and Fig. 3 confirms the good agreement between experimental and predicted data. Ac 0 varied from 2.64 g/l using 2% H 2 SO 4 at 100 C to 4.53 g/l using 6% H 2 SO 4 at 122 C, the average being Table 6 Kinetic and statistical parameters of acetic acid released for the H 2 SO 4 hydrolysis of sugar cane bagasse Operational set Ac 0 (g AcH/l) k (min 1 ) R 2 2% H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C g/l. Ac 0 was not clearly affected by temperature and H 2 SO 4 acid concentration Kinetic modelling of furfural concentration In the hydrolysis of sugar cane bagasse, furfural (a decomposition product of pentoses) and phenolic compounds (decomposition products of lignin) are generated. The analytical method used in this work for the determination of furfural cannot distinguish them. However, dilute-acid hydrolysis affects hemicelluloses and, only in strict conditions, cellulose. Therefore, the amount of phenolic compounds in the hydrolysates of sugar cane bagasse is negligible and the results reported by the method can be considered as furfural. Based on furfural results from Fig. 4, a similar model to those used for acetic acid can be considered. Eq. (12) expresses the furfural concentration (F) as a function of time (t). F ¼ F 0 1 e k 1t ð12þ where F 0 is the potential concentration of furfural and k 1 the rate of furfural generation (min 1 ). F 0 was introduced as a regression parameter. Table 7 shows the kinetic and statistical parameters obtained in the fitting of the furfural generated in the hydrolysis. Fig. 4 shows the comparison between experimental and predicted data. It can be observed that k 1 and F 0 are affected by temperature and H 2 SO 4 concentration. F 0 varied from 0.74 g/l using 2% H 2 SO 4 at 100 C to 5.59 g/l using 6% H 2 SO 4 at 128 C. The same behaviour was observed for the hydrolysis of sorghum straw but in that case the highest concentration reached was only 3.33 g/l (Tellez- Luis et al., 2002) Overall optimisation It is important to obtain sugar solutions (xylose and glucose) with low concentrations of inhibitor (furfural and acetic acid) if the hydrolysates of sugar cane bagasse are going to be used as fermentation media. Table 7 Kinetic and statistical parameters of furfural generated for the H 2 SO 4 hydrolysis of sugar cane bagasse Operational set F 0 (g/l) k (min 1 ) R 2 2% H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 100 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 122 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C % H 2 SO 4 at 128 C

9 R. Aguilar et al. / Journal of Food Engineering 55 (2002) Table 8 Composition of the hydrolysates obtained for the optimum conditions of each set Temperature ( C) [H 2 SO 4 ] (%) Time (min) Xylose (g/l) Glucose (g/l) Acetic acid (g/l) Furfural (g/l) For comparative purposes, the optimum values for each set are those that result in a high concentration of xylose and a low concentration of potential microorganism growth inhibitors like acetic acid and furfural. Figs. 1 4 show that it is possible to obtain solutions with xylose concentrations of about 20 g/l, a furfural concentration lower than 1 g/l and an acetic acid concentration lower than 3 g/l. Furthermore, the glucose concentration is normally lower than 3 g/l in the optimum conditions, indicating a small degradation of the cellulosic fraction. This is favourable if a later use of the solid residue is desired. For instance, applications for the solid residue are the production of cellulosic paste or the generation of glucose solutions by enzymatic hydrolysis. The kinetic models developed permit the prediction of the reaction time for the maximum values of xylose. Table 8 shows the optimum conditions found for each set and the values obtained of xylose, glucose, acetic acid and furfural in those conditions. 2% H 2 SO 4 at 122 C was selected because it resulted in solutions with 21.6 g xylose/l, 3 g glucose/l and low concentrations of furfural (0.52 g/l) and acetic acid (3.65 g/l). In these conditions, 90% of the hemicellulosic sugars was hydrolysed with a small concentration of by-products and negligible degradation of the cellulose fraction. Acknowledgements The authors are grateful to Consejo Nacional de Ciencia y Tecnologıa (CONACyT), Mexico, for the financial support of this work (Proj. J28897B). References Beck, M. J. (1986). Factors affecting efficiency of biomass fermentation to ethanol. Biotechnology and Bioengineering Symposium, 17, Bhandari, N., McDonald, D. G., & Bakhshi, N. N. (1984). Kinetic studies of corn stover saccharification using sulphuric acid. Biotechnology and Bioengineering, 26, Bj orling, T., & Lindman, B. (1989). Evaluation of xylose-fermenting yeasts for ethanol production from spent sulfite liquor. Enzyme and Microbial Technology, 11, Brennan, A., Hoagland, W., & Schell, D. J. (1986). High temperature acid hydrolysis of biomass using an engineering-scale plug flow reactor: results of low solids testing. Biotechnology and Bioengineering Symposium, 17, Carrasco, F. (1991). Fundamentos de la produccion de furfural. Afinidad, 48, Carrasco, F., & Roy, C. (1992). Kinetic study of dilute-acid prehydrolysis of xylan-containing biomass. Wood Science Technology, 26, Conner, A. H., & Lorenz, L. F. (1986). Kinetic modelling of hardwood prehydrolysis. Part III: Water and dilute acetic acid of southern red oak prehydrolysis. Wood Fiber Science, 18, David, C., Fornasier, R., Greindl-Fallon, C., & Vanlautem, N. (1985). Enzymatic hydrolysis and bacterian hydrolysis-fermentation of Eucalyptus wood pretreated with sodium hypochlorite. Biotechnology and Bioengineering, 26, du Toit, P. J., Olivier, S. P., & van Bijon, P. L. (1984). Sugar cane bagasse as a possible source of fermentable carbohydrates I. Characterization of bagasse with regard to monosaccharide, hemicellulose, and amino acid composition. Biotechnology and Bioengineering, 26, Eken-Saracßoglu, N., Ferda, D., Dilma, G., & Cß avsßuoglu, H. (1998). A comparative kinetic study of acidic hemicellulose hydrolysis in corn cob and sunflower seed hulls. Bioresource Technology, 65, Fengel, D., Wegener, G., (1984). Wood: Chemistry, ultrastructure, reactions. Berlın: Walter de Gruyter. Ferrari, M. D., Neirotti, E., Albornoz, C., & Saucedo, E. (1992). Ethanol production from Eucalyptus wood hemicellulose hydrolysate by Pichia stipitis. Biotechnology and Bioengineering, 40, Franz, R., Erckel, R., Riehm, T., Woernle, R., & Deger, H. M. (1982). Lignocellulose saccharification by HF. In Energy from biomass (pp ). London: Applied Science Publishers. Garrote, G., Domınguez, H., & Parajo, J. C. (1999). Mild autohydrolysis: an environmentally friendly technology for xylooligosaccharide production from wood. Journal of Chemical Technology and Biotechnology, 74, Garrote, G., Domınguez, H., & Parajo, J. C. (2001a). Kinetic modeling of corncob autohydrolysis. Process Biochemistry, 36, Garrote, G., Domınguez, H., & Parajo, J. C. (2001b). Generation of xylose solutions from Eucalyptus globulus wood by autohydrolysisposthydrolysis processes: posthydrolysis kinetics. Bioresource Technology, 79, Garrote, G., Domınguez, H., & Parajo, J. C. (2001c). Study on the deacetylation of hemicelluloses during hydrothermal processing of Eucalyptus wood. Holz als Roh- und Werkstoff, 59, Grant, G. A., Han, Y. W., Anderson, A. W., & Frey, K. L. (1977). Kinetics of straw hydrolysis. Developments in Industrial Microbiology, 18,

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