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Autohydrolysis of corncob: study of non-isothermal operation for xylooligosaccharide production
Journal of Food Engineering 52 (2002) 211–218 www.elsevier.com/locate/jfoodeng
Autohydrolysis of corncob: study of non-isothermal operation for xylooligosaccharide production Gil Garrote, Herminia Domınguez, Juan C. Parajo
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Department of Chemical Engineering, University of Vigo (Campus Ourense), Polytechnical Building, As Lagoas s/n, 32004 Ourense, Spain Received 16 March 2001; accepted 15 May 2001
Abstract Corncob samples were treated with water (autohydrolysis reaction) under non-isothermal conditions (reaction temperature up to 216°C) using a liquid to solid ratio of 8:1 kg/kg. The experimental variables considered were solid residue yield, solid residue composition (defined by the contents of cellulose, xylan, araban, acetyl groups and lignin in the solid residues) and composition of liquors (characterised by the concentrations of xylooligomers, xylose, glucose, arabinose, acetic acid and furfural). Most of the cellulose was retained in the solid phase, whereas partial delignification (up to 26% lignin removal) was achieved. Up to 94% of xylan was removed, producing xylooligomers (useful as food additives) and other reaction byproducts (including xylose and furfural). The decomposition of xylan into xylooligomers, with further generation of xylose, furfural and degradation products, was modelled by a series of parallel and consecutive reactions, which were assumed to be first order with coefficients showing an Arrhenius-type dependence on temperature. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Autohydrolysis; Corncob; Kinetics; Xylooligosaccharides
1. Introduction Agricultural wastes are widely available in Spain, were about 16 million metric tons are generated yearly (Jimenez, Perez, Garcıa, L opez, & Ariza, 2000). As the agricultural wastes are of lignocellulosic nature (with cellulose, hemicelluloses and lignin as structural components), their chemical processing can be undertaken according to the ‘‘biomass refining’’ philosophy (Myerly, Nicholson, Katzen, & Taylor, 1981), which is based on the separation of ‘‘fractions’’ according to their chemical properties. This operational method allows a complete utilisation of the raw materials by means of sequential treatments. In this field, autohydrolysis (in which the feedstock and water are the only reagents) causes a selective solubilisation of hemicelluloses, leading to liquors containing sugar oligomers, sugars and sugar-decomposition products and to a solid phase enriched in cellulose and lignin, which can be subjected to further processing.
*
Corresponding author. Tel.: +34-9-8838-7047; fax: +34-9-88387001. E-mail address: [email protected] (J.C. Paraj o).
The hemicellulose fraction of corncobs has a comparatively high content of acetylated xylan (a polymer made up of xylose units with acetyl substituents). When corncobs are contacted with water and heated, the hydronium ions coming from water autoionisation cause both xylan depolymerisation (to give xylooligomers and xylose) and cleavage of acetyl groups (to give acetic acid, which increases the hydronium concentration in the reaction medium). Under harsh operational conditions, xylose can be dehydrated to furfural, and furfural can be converted into degradation products. Under the operational conditions usually used in autohydrolysis (temperatures below 230°C), cellulose is not significantly affected (Abatzoglou, Koeberle, Chornet, Overend, & Koukios, 1990; Aoyama, Seki, & Saito, 1995; Biermann, Schultz, & McGinnis, 1984; Carrasco et al., 1994; Garrote, Domınguez, & Paraj o, 1999; Saska & Ozer, 1995), whereas the lignin fraction can be depolymerised to give soluble compounds and a non-soluble fraction with increased susceptibility towards further processing (Heitz et al., 1991; Lora & Wayman, 1978). According to the above ideas, the autohydrolysis of corncobs can be oriented towards the production of xylooligosaccharides, which can be substituted with acetyl groups (Kabel, Schols, & Voragen, 1999) of arabinose
0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 1 ) 0 0 1 0 8 - X
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Nomenclature EaFAST ; EaSLOW ; Ea2 ; . . . ; Ea6 activation energies F percent of initial xylan converted into furfural kFAST 0 ; kSLOW 0 ; k20 ; . . . ; k60 pre-exponential factors (h1 ) T temperature (K) t reaction time (h) X percent of initial xylan converted into xylose Xn percent of total xylan remaining in the substrate
units. Xylooligomers are currently used as food additives for functional foods, owing to their favourable effects on intestinal flora (Ishihara, Nagao, & Shimizu, 1995; Okazaki, Koda, Izumi, Fujikawa, & Matsumoto, 1991), and to the fact that healthy bacteria (and not detrimental bacteria) can use this type of compounds as a carbon source (Kontula, Wright, & Mattila-Sandholm, 1998) The objective of this study is to provide a kinetic assessment on the non-isothermal autohydrolysis of corncobs. A rigorous study of hemicellulose decomposition is difficult, owing to the complex reaction mechanism (which includes the diffusion of catalyst, reagents and products through the lignocellulosic matrix). Because of this, the degradation of xylan is carried out using pseudohomogeneous kinetics, which provides the information needed for further developments. The change in variables affecting the solid phase from the treatments (solid residue yield and solid residue composition) with time were also included in this study in order to obtain information useful for the further utilisation of this fraction.
XnFAST percent of fast-reacting xylan remaining in the substrate XnSLOW percent of slow-reacting xylan remaining in the substrate XOH percent of initial xylan converted into highmolecular weight xylooligosaccharides XOL percent of initial xylan converted into lowmolecular weight xylooligosaccharides a weight fraction of fast-reacting xylan in the untreated substrate
cellulose, hemicelluloses and Klason lignin was performed by extraction with 96% ethanol (for 24 h) followed by quantitative acid hydrolysis (Browning, 1967). The solid residue after hydrolysis was recovered by filtration and considered as Klason lignin. The monosaccharides and acetic acid contained in hydrolysates were determined by HPLC as reported elsewhere (Garrote et al., 1999). The moisture of the wood was considered as water in the material balances. 2.3. Hydrothermal processing of corncob samples Milled corncob samples and water were mixed in the desired proportions and reacted in stainless steel Parr reactors with 3.75 l total volume fitted with a double sixblade turbine impellers. The vessels were heated with external fabric mantles, and cooled by internal stainless steel loops. Temperature was monitored using inner thermocouples, and controlled by PID modules. Time zero was taken when temperature reached 60°C. The reaction media were heated during the desired time, and then cooled. After achieving normal pressure, the reactor was opened and samples of the suspensions were withdrawn.
2. Experimental 2.1. Raw material Locally collected corncob samples were milled to pass an 8 mm screen, since in preliminary studies no diffusional limitations were observed for this particle size. Samples were air-dried, homogenised in a single lot to avoid compositional differences among aliquots, and stored. 2.2. Analysis of raw material Aliquots from the homogenised wood lot were subjected to moisture determination (drying at 105°C to constant weight) and to ash content determination (by calcination at 575°C for 4 h). The determination of
2.4. Analysis of solid residues and liquors from hydrothermal treatments At the end of treatments, the solid residues were recovered by filtration, washed, air-dried and milled to a particle size <0.5 mm in order to ensure quantitative polysaccharide hydrolysis. Milled samples were assayed for cellulose, hemicelluloses, araban, lignin and acetyl groups using same methods as for raw wood analysis. An aliquot of liquors was filtered through 0.45 lm membranes and used for direct HPLC determination of monosaccharides, furfural, and acetic acid. A second aliquot of liquors (20 ml) was subjected to quantitative posthydrolysis under optimised conditions (4% H2 SO4 at 121°C for 60 min) before HPLC analysis. The increase in the concentrations of monosaccharides and
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acetic acid caused by posthydrolysis measured the concentrations of oligomers and acetyl groups bound to oligosaccharides. 2.5. Fitting of data An empirical equation (selected from the built-in functions of the TableCurve software, Jandel Scientific, USA) was used to fit the temperature profile, which was used in the numerical solution of the differential equations describing the time course of xylan and xylandegradation products. The set of differential equations was solved by the fourth-order Runge–Kutta method. The pre-exponential factors and the activation energies were calculated by minimising the sum of the squares of deviations between experimental and calculated data using a commercial optimisation routine dealing with Newton’s method, according to the philosophy reported by Garrote, Domınguez, and Paraj o (2001).
3. Results and discussion The compositional data of the corncob samples used as a raw material (as the average of six replicate analysis) are shown in Table 1. The results are expressed as weight percent in ethanol extractives, ashes, cellulose, xylan, araban, lignin and acetyl groups. Other nondetermined fractions include uronic acids, soluble lignin and other fractions with minor importance for the purposes of this work. The non-isothermal autohydrolysis of corncobs followed the standard heating profile of the Parr reactor employed in this work (see Fig. 1). The solid to liquor ratio employed (8 kg/kg) was the minimum value leading to a satisfactory degree of mixing. The reaction was stopped when the reaction medium reached the desired temperature (in the range 170–216°C, where the main effects on hemicelluloses were observed), and both the liquors and the solid phase from the treatments were analysed. The overall set of experiments included 20 assays. Table 1 Composition of corncob (as the average of six replicate determinations) Fraction
Weight percent of wood o.d. basis
Cellulose (as glucan) Xylan Araban Acetyl groups Klason lignin Ash Ethanol extractives Others (by diff.)
34.3 31.1 3.01 3.07 17.7 1.3 2.7 6.8
Fig. 1. Heating profile employed in this study.
3.1. Effects of the treatments on the solid residue Table 2 shows the experimental data on solid residue yield and solid residue composition achieved in the experiments. A continuous decrease in solid residue yield was observed in experiments, to reach values below 60% for experiments with final temperatures in the range 206–216°C. The fraction of dissolved solids corresponded mainly to extractive removal and xylan solubilisation. Even if in some cases the percent of lignin in treated samples was higher than in the raw material (a fact due to the selective removal of hemicelluloses and extractives), the treatments resulted in decreased total amounts of lignin. Under harsh conditions (experiments 19 and 20), the percents of lignin removal were 22.8– 26.1% of the amount of initial lignin. On the other hand, the cellulose percent of solid residues increased steadily with the severity of treatments, owing to the resistance of this polymer to hydrothermal processing. Cellulose was mostly found in the solid phase (in some experiments, cellulose recoveries slightly higher than 100% were determined owing to the influence of the experimental error). As expected, the contents of araban and acetyl groups in the solid phases decreased steadily with the course of treatments, according to a typical exponential pattern. Even if these compounds are important for specific reasons (for example, for the contribution of acetyl groups to the acidification of the reaction medium), their low concentrations limited their interest for the general objectives of this study. The variation with time of xylan degradation is described in Section 3.3. 3.2. Effects of the treatments on the composition of liquors Fig. 2(a) shows the concentration of arabinose bound to oligomers as well as the free arabinose (both variables expressed in g of equivalent arabinose/l) during the heating process. The arabinose oligomer concentration increased with time to reach a maximum concentration
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Table 2 Chemical composition of solid residues from hydrothermal treatments (results expressed as weight percent, o.d. basis) Experiment
Final temperature (°C)
Solid residue yield (%)
Cellulose (%)
Xylan (%)
Araban (%)
Acetyl groups (%)
Klason lignin (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
170 175 180 183 186 188 190 192 194 196 198 200 202 204 206 208 210 212 214 216
98.1 97.1 92.9 89.1 82.0 81.5 79.3 77.8 75.2 69.3 65.2 66.6 62.8 61.6 59.7 58.8 57.9 57.5 55.5 56.4
– – – 34.4 40.2 43.5 39.8 43.9 47.4 49.0 52.8 56.8 57.1 63.8 61.2 63.2 64.7 65.6 67.7 68.0
28.5 28.3 27.9 26.4 25.7 23.2 21.4 20.5 20.2 18.9 15.8 12.6 11.8 10.3 9.61 8.71 5.95 4.94 3.99 3.10
2.47 1.88 1.76 1.91 1.82 1.50 1.23 1.27 0.888 1.16 0.861 0.510 0.640 0.347 0.403 0.396 0.228 0.160 0.089 0.049
2.86 2.67 2.69 2.41 2.46 2.15 1.96 1.88 1.90 1.69 1.40 1.10 1.09 0.928 0.879 0.836 0.601 0.502 0.405 0.257
– – – 24.2 20.0 24.1 26.0 23.5 21.0 21.8 22.0 27.4 23.0 24.7 27.9 25.7 26.9 26.5 25.1 25.8
at 195°C (about 2 g/l). Comparatively, the free arabinose was generated with slower kinetics, to reach a maximum concentration of 1.7 g/l at 206°C, and then decreased owing to the participation of degradation reactions. The joint contribution of both arabinose and arabinose oligomers reached a maximal value of 3.7 g/l at 202°C.
Fig. 2. Time variation of concentrations of soluble byproducts obtained in autohydrolysis: (a) Arabinose (free and bound to oligomers), (b) Glucose.
Fig. 2(b) shows that the glucose concentration increased steadily with temperature to reach a maximum concentration of 0.8 g/l. This fact confirms that cellulose was unaffected by treatments, a desirable feature of the studied process, which seeks a selective fractionation of the raw material. Fig. 3 shows the dependence of both acetyl groups bound to oligomers and free acetic acid on temperature. The maximum concentration of acetyl groups linked to oligomers (2.5 g/l) corresponded to about 50% of the initial acetyl groups, and was reached at 202°C, whereas the acetic acid concentration increased steadily during the heating process to reach maximum concentrations of about 3.7 g/l. The evolution of the concentrations of xylan-derived species (xylooligomers, xylose, furfural and degradation products) is discussed in Section 3.3. It can be emphas-
Fig. 3. Time variation of the concentrations of free acetic acid and acetyl groups bound to oligomers.
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ised that araban reacted faster than acetyl groups and xylose-containing species. This fact is in agreement with reported data on the relative reaction rate of arabinose and xylose (Fengel & Wegener, 1984), as well as on the relative reaction rate of xylan and acetyl groups (Maloney, Chapman, & Baker, 1986). 3.3. Kinetics of xylan degradation Similarly to prehydrolysis reactions, the degradation of xylan in hydrothermal treatments goes through the hydronium-catalysed cleavage of heterocyclic ether bonds. Because of this, both prehydrolysis and hydrothermolysis follow the same kinetic principles. Previous works on hydrothermolysis and/or prehydrolysis of lignocelluloses (Conner, 1984; Jacobsen & Wyman, 2000; Kim, Yum, & Park, 2000; Kim & Lee, 1987) showed that the hydrolysis of hemicelluloses leads to sugar oligomers, which behave as reaction intermediates to give sugar and sugar-degradation products. Fig. 4 shows several models proposed in the literature for hemicellulose degradation. Model 1 of Fig. 4, proposed by Mehlberg and Tsao (1979), assumes the existence of two xylan fractions having different susceptibility to hydrolysis (fast-reacting xylan and slow-reacting xylan). The existence of two xylan fractions can be justified on the
Fig. 4. Kinetic models proposed for the degradation of xylan in acidic media.
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basis of differences in structure and/or accessibility (Carrasco & Roy, 1992). Both xylan fractions are able to give oligosaccharides. These compounds are hydrolysed to xylose, which is decomposed in acidic medium. Model 2 of Fig. 4, proposed by Conner and Lorentz (1986), complements the former one by introducing a time-dependent kinetic coefficient for the decomposition of oligomers into xylose (in order to take into account the fact that low-molecular weight oligmers are more likely to give xylose upon hydrolysis than high-molecular weight oligomers), as well as by modelling the xylose decomposition by consecutive reactions of dehydration to furfural and furfural consumption. In a related approach developed for the isothermal processing of corncobs, Garrote et al. (2001) simplified the continuous reduction of DP in oligosaccharide hydrolysis by two-consecutive reaction steps to give highmolecular weight oligosaccharides and low-molecular weight oligosaccharides (see Model 3 of Fig. 4). Under the conditions tested, the contribution of the slowreacting xylan to hydrolysis was negligible, and an additional reaction (generation of furfural from low molecular-weight oligosaccharides) was necessary to reproduce the variation with time of xylose concentrations. In order to give a quantitative interpretation of xylan degradation under non-isothermal conditions, a modification of the last model is proposed in this work. As can be seen from the experimental data, up to 94% of the initial xylan was removed under harsh conditions, suggesting that some degradation of the slow-reacting fraction occurred at high temperatures. It can be noted that the upper temperature employed by Garrote et al. (2001) in the isothermal processing of corncobs was 190°C, below the maximum temperature reached in this work, and that the relative proportions of fast- and slow-reacting xylans (measured by the parameter a, weight fraction of fast-reacting xylan) is known to be a function of temperature (Carrasco & Roy, 1992). Assuming that all the reactions participating in Model 4 of Fig. 4 are first order and present an Arrhenius-type depence on temperature, the mechanism is described by the following set of differential equations: dXnFAST EaFAST ¼ kFAST0 exp ð1Þ XnFAST ; dt R TðtÞ dXnSLOW EaSLOW ¼ kSLOW0 exp ð2Þ XnSLOW ; dt R TðtÞ dXOH EaFAST ¼ kFAST0 exp XnFAST dt R TðtÞ EaSLOW þ kSLOW0 exp XnSLOW R TðtÞ Ea2 k20 exp ð3Þ XOH ; R TðtÞ
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dXOL Ea2 Ea3 ¼ k20 exp XOH k30 exp dt R TðtÞ R TðtÞ Ea6 þ k60 exp ð4Þ XOL ; R TðtÞ dX Ea3 Ea4 ¼ k30 exp XOL k40 exp X; ð5Þ dt R TðtÞ R TðtÞ dF Ea4 Ea6 ¼ k40 exp X þ k60 exp XOL dt R TðtÞ R TðtÞ Ea5 k50 exp F; ð6Þ R TðtÞ where TðtÞ is the temperature (K) achieved at the considered reaction time, XnFAST is the percent of fast-reacting xylan remaining in the substrate at the considered reaction time, XnSLOW is the percent of slow-reacting xylan remaining in the substrate at the considered reaction time, XOH is the percent of initial xylan converted into high-molecular weight xylooligosaccharides, XOL is the percent of initial xylan converted into lowmolecular weight xylooligosaccharides, X is the percent of initial xylan converted into xylose and F is the percent of initial xylan converted into furfural. The concentration of this compound was determined by HPLC as explained in Section 2.2, and the percent of initial xylan converted into decomposition products (denoted DP) is calculated by difference: DP ¼ 100 XnFAST XnSLOW XOH XOL X F:
Table 3 Values of preexponential factors and activation energies Parameter
Result
(a) Results reported for isothermal operation with Model 3 of Fig. 4 (Garrote et al., 2001) a (dimensionless) 0.876 36.2 ln k01 (k01 in h1 ) ln k02 (k02 in h1 ) 28.8 27.8 ln k03 (k03 in h1 ) ln k04 (k04 in h1 ) 28.8 ln k05 (k05 in h1 ) 32.3 28.45 ln k06 (k06 in h1 ) Ea1 (kJ/mol) 130 Ea2 (kJ/mol) 104 Ea3 (kJ/mol) 102 109 Ea4 (kJ/mol) Ea5 (kJ/mol) 117 Ea6 (kJ/mol) 106 (b) Results found in this work (Model 4 of Fig. 4) a (dimensionless) 0.865 50.6 ln kFAST0 (kFAST0 in h1 ) ln kSLOW0 (kSLOW in h1 ) 18.5 ln k02 (k02 in h1 ) 28.8 ln k03 (k03 in h1 ) 27.8 28.8 ln k04 (k04 in h1 ) ln k05 (k05 in h1 ) 32.3 ln k06 (k06 in h1 ) 28.45 EaFAST (kJ/mol) 186 125 EaSLOW (kJ/mol) Ea2 (kJ/mol) 104 Ea3 (kJ/mol) 102 Ea4 (kJ/mol) 109 117 Ea5 (kJ/mol) Ea6 (kJ/mol) 106
ð7Þ In Eq. (6), kFAST0 ; kSLOW0 and k20 . . . k60 denote the preexponential factors, and EaFAST ; EaSLOW ; Ea2 . . . Ea6 denote the activation energies. The numerical integration was carried out considering the following equations: Xn ¼ XnFAST þ XnSLOW ;
ð8Þ
XO ¼ XOH þ XOL ;
ð9Þ
a ¼ Xn0FAST =Xn0 ;
ð10Þ
where Xn is the percent of total xylan remaining in the substrate at the considered reaction time and a is the weight fraction of fast-reacting xylan in the untreated substrate. For calculation purposes, a was considered to be independent of temperature in the range of operational conditions tested. The set of Eqs. (1)–(10) has been solved numerically using a fourth-order Runge–Kutta method coupled with an optimisation algorithm based on Newton’s method to minimise the squares of deviations between experimental and calculated values of the experimental variables. The overall calculation scheme was carried out taking as initial values for a and kinetic parameters the results reported by Garrote et al. (2001) for the isothermal autohydrolysis of corncobs (see Table 3(a)). As
expected, the fitting for residual xylan was poor owing to the participation of the slow-reacting xylan fraction in the overall degradation scheme. When the values of the regression parameters were allowed to vary using the optimisation algorithm, the results reported in Table 3(b) were obtained. In comparison with the reported results, the value calculated for the ‘‘susceptible xylan fraction’’ ða ¼ 0:808Þ is close to the results found for isothermal operation (in the range 0.85–0.90), and lies in the range reported for other lignocellulosic raw materials. Reported data for EaFAST are 142 kJ/mol (Mehlberg & Tsao, 1979), 136 kJ/mol (Conner & Lorentz, 1986), 127 kJ/mol (Maloney et al., 1986), 126 kJ/mol (Chen, Lee, & Torget, 1996) and 117 kJ/mol (Kim et al., 2000); whereas for EaSLOW the results are 176 kJ/mol (Mehlberg & Tsao, 1979), 157 kJ/mol (Conner & Lorentz, 1986), 127 kJ/mol (Maloney et al., 1986), 129 kJ/mol (Chen et al., 1996) and 119 kJ/mol (Kim et al., 2000). It can be seen that the values determined in this work (125 and 177 kJ/mol, respectively) are in the range reported in literature. Similar values have been reported for models involving a single xylan fraction. Contrarily, the results reported for the pre-exponential factors showed a wide variation range, depending on the mechanism proposed,
G. Garrote et al. / Journal of Food Engineering 52 (2002) 211–218
on the substrate utilized and on the possibility of expressing the pre-exponential factors as a function of the concentration of hydronium ions (Ranganathan, MacDonald, & Bakhshi, 1985). In the case of hydrothermal treatments, where the concentration of catalyst is low, the presence of neutralizing substances can be very influential on the results determined for the kinetic parameters (Carrasco & Roy, 1992). The variation range of the pre-exponential factors reported are 35:8– 45:3 h1 for the fast xylan fraction, and 36:5–62:2 h1 for the slow-reacting xylan fraction. Again, the results determined in this work (34:5 and 46:6 h1 , respectively) are in the range reported in literature. Once the time variation of xylan was properly modelled by using the optimised kinetic parameters cited previously, the values reported for the rest of the regression parameters ðk20 . . . k60 ; Ea2 . . . Ea6 Þ gave a close interpretation of the experimental data. For comparative purposes, Fig. 5 shows the experimental and calculated data for the chemical species involved in the autohydrolysis of corncob. The best operational conditions are considered to correspond to experiment 13, in which a high conversion of xylan into xylooligosaccharides was achieved (65.6%), with a joint contribution of xylooligosaccharides and xylose accounting for 72.2% of the initial xylan. These results are in the range 55–80% reported in literature for other re-
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search groups in experiments with a variety of feedstocks (Aoyama, 1996; Conner & Lorentz, 1986; Heitz et al., 1991).
4. Conclusions The non-isothermal autohydrolysis of corncobs is a fast and effective method to produce simultaneously solutions containing sugar oligomers and a solid residue mainly made up of cellulose and lignin. Autohydrolysis caused only limited delignification, whereas cellulose was almost entirely retained in solid phase. The overall process of xylan degradation (including the conversion of xylan into high-molecular weight xylooligomers, conversion of high-molecular weight xylooligomers into low-molecular weight xylooligomers, conversion of low-molecular weight xylooligomers into xylose and dehydration of xylose into furfural, and generation of furfural from low-molecular weight xylooligomers) can be modelled by means of first-order, pseudohomogeneous reactions assuming the existence of two xylan fractions with different susceptibility towards hydrolytic degradation. The values calculated for the regression parameters (‘‘susceptible’’ xylan fraction, activation energies and pre-exponential factors of the several reactions involved in the whole mechanism) allowed a close reproduction of the experimental data, and provided basic information for design and economic evaluation of the studied process.
Acknowledgements Authors are grateful to the Commission of the European Communities for the financial support of this work (in the framework of the European Project ‘‘Xylophone’’, reference FAIR-CT98-3811-DG12) as well as to the Spanish ‘‘Ministry of Education’’ for complementary funding (Ref. ALI98-1625-CE).
References
Fig. 5. Experimental and calculated variation of variables Xn (percent of xylan remaining in wood after treatments), XO (percent of initial xylan converted into xylooligosaccharides with high- and low-molecular weight), X (percent of initial xylan converted into xylose), F (percent of initial xylan converted into furfural) and DP (percent of initial xylan converted into degradation products).
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Jimenez, L., Perez, I., Garcıa, J. C., L opez, F., & Ariza, J. (2000). Influence of oxygen bleaching variables of enzyme-treated soda pulp from wheat straw on the quality of black liquor. Process Biochemistry, 35, 685–691. Kabel, M. A., Schols, H. A., Voragen, A. G. J. (1999). Use of enzymes and Maldi-TOF-MS in the structural characterisation of xylooligosaccharides. In 3rd Carbohydrathe bioengineering meeting, Newcatsle Upon Tyne, UK. Kim, S. B., Yum, D. M., & Park, S. C. (2000). Step-change variation of acid concentration in a percolation reactor for hydrolysis of hardwood hemicellulose. Bioresource Technology, 72, 289–294. Kim, S. B., & Lee, Y. Y. (1987). Kinetics in acid-catalyzed hydrolysis of hardwood hemicellulose. Biotechnology and Bioengineering Symposium, 17, 71–84. Kontula, P., Wright, A., & Mattila-Sandholm, T. (1998). Oat bran b-gluco- and xylo-oligosaccharides as fermentative substrates for lactic acid bacteria. International Journal of Food Microbiology, 45, 163–169. Lora, J. H., & Wayman, M. (1978). Delignification of hardwoods by autohydrolysis and extraction. Tappi, 61, 47–50. Maloney, M. T., Chapman, T. W., & Baker, A. J. (1986). An engineering analysis of the production of xylose by dilute acid hydrolysis of hardwood hemicellulose. Biotechnology Progress, 2(4), 192–202. Mehlberg, R., Tsao, G.T. (1979). Low liquid hemicellulose hydrolysis of hydrochloric acid. In 178th ACS national meeting, Washington DC, USA. Myerly, R. C., Nicholson, M. D., Katzen, R., & Taylor, J. M. (1981). The forest refinery. Chemtech, 11, 186–192. Okazaki, M., Koda, H., Izumi, R., Fujikawa, S., & Matsumoto, N. (1991). In vitro digestibility and in vivo utilization of xylobiose. Journal of Japan Society for Nutrition and Food Science, 44, 41–44. Ranganathan, S., MacDonald, D. G., & Bakhshi, N. (1985). Kinetic studies of wheat straw hydrolysis using sulphuric acid. Canadian Journal of Chemical Engineering, 63, 840–844. Saska, M., & Ozer, E. (1995). Aqueous extraction of sugarcane bagasse hemicellulose and production of xylose syrup. Biotechnology and Bioengineering, 45, 517–523.
Autohydrolysis of corncob: study of non-isothermal operation for xylooligosaccharide production Gil Garrote, Herminia Domınguez, Juan C. Parajo
*
Department of Chemical Engineering, University of Vigo (Campus Ourense), Polytechnical Building, As Lagoas s/n, 32004 Ourense, Spain Received 16 March 2001; accepted 15 May 2001
Abstract Corncob samples were treated with water (autohydrolysis reaction) under non-isothermal conditions (reaction temperature up to 216°C) using a liquid to solid ratio of 8:1 kg/kg. The experimental variables considered were solid residue yield, solid residue composition (defined by the contents of cellulose, xylan, araban, acetyl groups and lignin in the solid residues) and composition of liquors (characterised by the concentrations of xylooligomers, xylose, glucose, arabinose, acetic acid and furfural). Most of the cellulose was retained in the solid phase, whereas partial delignification (up to 26% lignin removal) was achieved. Up to 94% of xylan was removed, producing xylooligomers (useful as food additives) and other reaction byproducts (including xylose and furfural). The decomposition of xylan into xylooligomers, with further generation of xylose, furfural and degradation products, was modelled by a series of parallel and consecutive reactions, which were assumed to be first order with coefficients showing an Arrhenius-type dependence on temperature. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Autohydrolysis; Corncob; Kinetics; Xylooligosaccharides
1. Introduction Agricultural wastes are widely available in Spain, were about 16 million metric tons are generated yearly (Jimenez, Perez, Garcıa, L opez, & Ariza, 2000). As the agricultural wastes are of lignocellulosic nature (with cellulose, hemicelluloses and lignin as structural components), their chemical processing can be undertaken according to the ‘‘biomass refining’’ philosophy (Myerly, Nicholson, Katzen, & Taylor, 1981), which is based on the separation of ‘‘fractions’’ according to their chemical properties. This operational method allows a complete utilisation of the raw materials by means of sequential treatments. In this field, autohydrolysis (in which the feedstock and water are the only reagents) causes a selective solubilisation of hemicelluloses, leading to liquors containing sugar oligomers, sugars and sugar-decomposition products and to a solid phase enriched in cellulose and lignin, which can be subjected to further processing.
*
Corresponding author. Tel.: +34-9-8838-7047; fax: +34-9-88387001. E-mail address: [email protected] (J.C. Paraj o).
The hemicellulose fraction of corncobs has a comparatively high content of acetylated xylan (a polymer made up of xylose units with acetyl substituents). When corncobs are contacted with water and heated, the hydronium ions coming from water autoionisation cause both xylan depolymerisation (to give xylooligomers and xylose) and cleavage of acetyl groups (to give acetic acid, which increases the hydronium concentration in the reaction medium). Under harsh operational conditions, xylose can be dehydrated to furfural, and furfural can be converted into degradation products. Under the operational conditions usually used in autohydrolysis (temperatures below 230°C), cellulose is not significantly affected (Abatzoglou, Koeberle, Chornet, Overend, & Koukios, 1990; Aoyama, Seki, & Saito, 1995; Biermann, Schultz, & McGinnis, 1984; Carrasco et al., 1994; Garrote, Domınguez, & Paraj o, 1999; Saska & Ozer, 1995), whereas the lignin fraction can be depolymerised to give soluble compounds and a non-soluble fraction with increased susceptibility towards further processing (Heitz et al., 1991; Lora & Wayman, 1978). According to the above ideas, the autohydrolysis of corncobs can be oriented towards the production of xylooligosaccharides, which can be substituted with acetyl groups (Kabel, Schols, & Voragen, 1999) of arabinose
0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 1 ) 0 0 1 0 8 - X
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Nomenclature EaFAST ; EaSLOW ; Ea2 ; . . . ; Ea6 activation energies F percent of initial xylan converted into furfural kFAST 0 ; kSLOW 0 ; k20 ; . . . ; k60 pre-exponential factors (h1 ) T temperature (K) t reaction time (h) X percent of initial xylan converted into xylose Xn percent of total xylan remaining in the substrate
units. Xylooligomers are currently used as food additives for functional foods, owing to their favourable effects on intestinal flora (Ishihara, Nagao, & Shimizu, 1995; Okazaki, Koda, Izumi, Fujikawa, & Matsumoto, 1991), and to the fact that healthy bacteria (and not detrimental bacteria) can use this type of compounds as a carbon source (Kontula, Wright, & Mattila-Sandholm, 1998) The objective of this study is to provide a kinetic assessment on the non-isothermal autohydrolysis of corncobs. A rigorous study of hemicellulose decomposition is difficult, owing to the complex reaction mechanism (which includes the diffusion of catalyst, reagents and products through the lignocellulosic matrix). Because of this, the degradation of xylan is carried out using pseudohomogeneous kinetics, which provides the information needed for further developments. The change in variables affecting the solid phase from the treatments (solid residue yield and solid residue composition) with time were also included in this study in order to obtain information useful for the further utilisation of this fraction.
XnFAST percent of fast-reacting xylan remaining in the substrate XnSLOW percent of slow-reacting xylan remaining in the substrate XOH percent of initial xylan converted into highmolecular weight xylooligosaccharides XOL percent of initial xylan converted into lowmolecular weight xylooligosaccharides a weight fraction of fast-reacting xylan in the untreated substrate
cellulose, hemicelluloses and Klason lignin was performed by extraction with 96% ethanol (for 24 h) followed by quantitative acid hydrolysis (Browning, 1967). The solid residue after hydrolysis was recovered by filtration and considered as Klason lignin. The monosaccharides and acetic acid contained in hydrolysates were determined by HPLC as reported elsewhere (Garrote et al., 1999). The moisture of the wood was considered as water in the material balances. 2.3. Hydrothermal processing of corncob samples Milled corncob samples and water were mixed in the desired proportions and reacted in stainless steel Parr reactors with 3.75 l total volume fitted with a double sixblade turbine impellers. The vessels were heated with external fabric mantles, and cooled by internal stainless steel loops. Temperature was monitored using inner thermocouples, and controlled by PID modules. Time zero was taken when temperature reached 60°C. The reaction media were heated during the desired time, and then cooled. After achieving normal pressure, the reactor was opened and samples of the suspensions were withdrawn.
2. Experimental 2.1. Raw material Locally collected corncob samples were milled to pass an 8 mm screen, since in preliminary studies no diffusional limitations were observed for this particle size. Samples were air-dried, homogenised in a single lot to avoid compositional differences among aliquots, and stored. 2.2. Analysis of raw material Aliquots from the homogenised wood lot were subjected to moisture determination (drying at 105°C to constant weight) and to ash content determination (by calcination at 575°C for 4 h). The determination of
2.4. Analysis of solid residues and liquors from hydrothermal treatments At the end of treatments, the solid residues were recovered by filtration, washed, air-dried and milled to a particle size <0.5 mm in order to ensure quantitative polysaccharide hydrolysis. Milled samples were assayed for cellulose, hemicelluloses, araban, lignin and acetyl groups using same methods as for raw wood analysis. An aliquot of liquors was filtered through 0.45 lm membranes and used for direct HPLC determination of monosaccharides, furfural, and acetic acid. A second aliquot of liquors (20 ml) was subjected to quantitative posthydrolysis under optimised conditions (4% H2 SO4 at 121°C for 60 min) before HPLC analysis. The increase in the concentrations of monosaccharides and
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acetic acid caused by posthydrolysis measured the concentrations of oligomers and acetyl groups bound to oligosaccharides. 2.5. Fitting of data An empirical equation (selected from the built-in functions of the TableCurve software, Jandel Scientific, USA) was used to fit the temperature profile, which was used in the numerical solution of the differential equations describing the time course of xylan and xylandegradation products. The set of differential equations was solved by the fourth-order Runge–Kutta method. The pre-exponential factors and the activation energies were calculated by minimising the sum of the squares of deviations between experimental and calculated data using a commercial optimisation routine dealing with Newton’s method, according to the philosophy reported by Garrote, Domınguez, and Paraj o (2001).
3. Results and discussion The compositional data of the corncob samples used as a raw material (as the average of six replicate analysis) are shown in Table 1. The results are expressed as weight percent in ethanol extractives, ashes, cellulose, xylan, araban, lignin and acetyl groups. Other nondetermined fractions include uronic acids, soluble lignin and other fractions with minor importance for the purposes of this work. The non-isothermal autohydrolysis of corncobs followed the standard heating profile of the Parr reactor employed in this work (see Fig. 1). The solid to liquor ratio employed (8 kg/kg) was the minimum value leading to a satisfactory degree of mixing. The reaction was stopped when the reaction medium reached the desired temperature (in the range 170–216°C, where the main effects on hemicelluloses were observed), and both the liquors and the solid phase from the treatments were analysed. The overall set of experiments included 20 assays. Table 1 Composition of corncob (as the average of six replicate determinations) Fraction
Weight percent of wood o.d. basis
Cellulose (as glucan) Xylan Araban Acetyl groups Klason lignin Ash Ethanol extractives Others (by diff.)
34.3 31.1 3.01 3.07 17.7 1.3 2.7 6.8
Fig. 1. Heating profile employed in this study.
3.1. Effects of the treatments on the solid residue Table 2 shows the experimental data on solid residue yield and solid residue composition achieved in the experiments. A continuous decrease in solid residue yield was observed in experiments, to reach values below 60% for experiments with final temperatures in the range 206–216°C. The fraction of dissolved solids corresponded mainly to extractive removal and xylan solubilisation. Even if in some cases the percent of lignin in treated samples was higher than in the raw material (a fact due to the selective removal of hemicelluloses and extractives), the treatments resulted in decreased total amounts of lignin. Under harsh conditions (experiments 19 and 20), the percents of lignin removal were 22.8– 26.1% of the amount of initial lignin. On the other hand, the cellulose percent of solid residues increased steadily with the severity of treatments, owing to the resistance of this polymer to hydrothermal processing. Cellulose was mostly found in the solid phase (in some experiments, cellulose recoveries slightly higher than 100% were determined owing to the influence of the experimental error). As expected, the contents of araban and acetyl groups in the solid phases decreased steadily with the course of treatments, according to a typical exponential pattern. Even if these compounds are important for specific reasons (for example, for the contribution of acetyl groups to the acidification of the reaction medium), their low concentrations limited their interest for the general objectives of this study. The variation with time of xylan degradation is described in Section 3.3. 3.2. Effects of the treatments on the composition of liquors Fig. 2(a) shows the concentration of arabinose bound to oligomers as well as the free arabinose (both variables expressed in g of equivalent arabinose/l) during the heating process. The arabinose oligomer concentration increased with time to reach a maximum concentration
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Table 2 Chemical composition of solid residues from hydrothermal treatments (results expressed as weight percent, o.d. basis) Experiment
Final temperature (°C)
Solid residue yield (%)
Cellulose (%)
Xylan (%)
Araban (%)
Acetyl groups (%)
Klason lignin (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
170 175 180 183 186 188 190 192 194 196 198 200 202 204 206 208 210 212 214 216
98.1 97.1 92.9 89.1 82.0 81.5 79.3 77.8 75.2 69.3 65.2 66.6 62.8 61.6 59.7 58.8 57.9 57.5 55.5 56.4
– – – 34.4 40.2 43.5 39.8 43.9 47.4 49.0 52.8 56.8 57.1 63.8 61.2 63.2 64.7 65.6 67.7 68.0
28.5 28.3 27.9 26.4 25.7 23.2 21.4 20.5 20.2 18.9 15.8 12.6 11.8 10.3 9.61 8.71 5.95 4.94 3.99 3.10
2.47 1.88 1.76 1.91 1.82 1.50 1.23 1.27 0.888 1.16 0.861 0.510 0.640 0.347 0.403 0.396 0.228 0.160 0.089 0.049
2.86 2.67 2.69 2.41 2.46 2.15 1.96 1.88 1.90 1.69 1.40 1.10 1.09 0.928 0.879 0.836 0.601 0.502 0.405 0.257
– – – 24.2 20.0 24.1 26.0 23.5 21.0 21.8 22.0 27.4 23.0 24.7 27.9 25.7 26.9 26.5 25.1 25.8
at 195°C (about 2 g/l). Comparatively, the free arabinose was generated with slower kinetics, to reach a maximum concentration of 1.7 g/l at 206°C, and then decreased owing to the participation of degradation reactions. The joint contribution of both arabinose and arabinose oligomers reached a maximal value of 3.7 g/l at 202°C.
Fig. 2. Time variation of concentrations of soluble byproducts obtained in autohydrolysis: (a) Arabinose (free and bound to oligomers), (b) Glucose.
Fig. 2(b) shows that the glucose concentration increased steadily with temperature to reach a maximum concentration of 0.8 g/l. This fact confirms that cellulose was unaffected by treatments, a desirable feature of the studied process, which seeks a selective fractionation of the raw material. Fig. 3 shows the dependence of both acetyl groups bound to oligomers and free acetic acid on temperature. The maximum concentration of acetyl groups linked to oligomers (2.5 g/l) corresponded to about 50% of the initial acetyl groups, and was reached at 202°C, whereas the acetic acid concentration increased steadily during the heating process to reach maximum concentrations of about 3.7 g/l. The evolution of the concentrations of xylan-derived species (xylooligomers, xylose, furfural and degradation products) is discussed in Section 3.3. It can be emphas-
Fig. 3. Time variation of the concentrations of free acetic acid and acetyl groups bound to oligomers.
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ised that araban reacted faster than acetyl groups and xylose-containing species. This fact is in agreement with reported data on the relative reaction rate of arabinose and xylose (Fengel & Wegener, 1984), as well as on the relative reaction rate of xylan and acetyl groups (Maloney, Chapman, & Baker, 1986). 3.3. Kinetics of xylan degradation Similarly to prehydrolysis reactions, the degradation of xylan in hydrothermal treatments goes through the hydronium-catalysed cleavage of heterocyclic ether bonds. Because of this, both prehydrolysis and hydrothermolysis follow the same kinetic principles. Previous works on hydrothermolysis and/or prehydrolysis of lignocelluloses (Conner, 1984; Jacobsen & Wyman, 2000; Kim, Yum, & Park, 2000; Kim & Lee, 1987) showed that the hydrolysis of hemicelluloses leads to sugar oligomers, which behave as reaction intermediates to give sugar and sugar-degradation products. Fig. 4 shows several models proposed in the literature for hemicellulose degradation. Model 1 of Fig. 4, proposed by Mehlberg and Tsao (1979), assumes the existence of two xylan fractions having different susceptibility to hydrolysis (fast-reacting xylan and slow-reacting xylan). The existence of two xylan fractions can be justified on the
Fig. 4. Kinetic models proposed for the degradation of xylan in acidic media.
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basis of differences in structure and/or accessibility (Carrasco & Roy, 1992). Both xylan fractions are able to give oligosaccharides. These compounds are hydrolysed to xylose, which is decomposed in acidic medium. Model 2 of Fig. 4, proposed by Conner and Lorentz (1986), complements the former one by introducing a time-dependent kinetic coefficient for the decomposition of oligomers into xylose (in order to take into account the fact that low-molecular weight oligmers are more likely to give xylose upon hydrolysis than high-molecular weight oligomers), as well as by modelling the xylose decomposition by consecutive reactions of dehydration to furfural and furfural consumption. In a related approach developed for the isothermal processing of corncobs, Garrote et al. (2001) simplified the continuous reduction of DP in oligosaccharide hydrolysis by two-consecutive reaction steps to give highmolecular weight oligosaccharides and low-molecular weight oligosaccharides (see Model 3 of Fig. 4). Under the conditions tested, the contribution of the slowreacting xylan to hydrolysis was negligible, and an additional reaction (generation of furfural from low molecular-weight oligosaccharides) was necessary to reproduce the variation with time of xylose concentrations. In order to give a quantitative interpretation of xylan degradation under non-isothermal conditions, a modification of the last model is proposed in this work. As can be seen from the experimental data, up to 94% of the initial xylan was removed under harsh conditions, suggesting that some degradation of the slow-reacting fraction occurred at high temperatures. It can be noted that the upper temperature employed by Garrote et al. (2001) in the isothermal processing of corncobs was 190°C, below the maximum temperature reached in this work, and that the relative proportions of fast- and slow-reacting xylans (measured by the parameter a, weight fraction of fast-reacting xylan) is known to be a function of temperature (Carrasco & Roy, 1992). Assuming that all the reactions participating in Model 4 of Fig. 4 are first order and present an Arrhenius-type depence on temperature, the mechanism is described by the following set of differential equations: dXnFAST EaFAST ¼ kFAST0 exp ð1Þ XnFAST ; dt R TðtÞ dXnSLOW EaSLOW ¼ kSLOW0 exp ð2Þ XnSLOW ; dt R TðtÞ dXOH EaFAST ¼ kFAST0 exp XnFAST dt R TðtÞ EaSLOW þ kSLOW0 exp XnSLOW R TðtÞ Ea2 k20 exp ð3Þ XOH ; R TðtÞ
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dXOL Ea2 Ea3 ¼ k20 exp XOH k30 exp dt R TðtÞ R TðtÞ Ea6 þ k60 exp ð4Þ XOL ; R TðtÞ dX Ea3 Ea4 ¼ k30 exp XOL k40 exp X; ð5Þ dt R TðtÞ R TðtÞ dF Ea4 Ea6 ¼ k40 exp X þ k60 exp XOL dt R TðtÞ R TðtÞ Ea5 k50 exp F; ð6Þ R TðtÞ where TðtÞ is the temperature (K) achieved at the considered reaction time, XnFAST is the percent of fast-reacting xylan remaining in the substrate at the considered reaction time, XnSLOW is the percent of slow-reacting xylan remaining in the substrate at the considered reaction time, XOH is the percent of initial xylan converted into high-molecular weight xylooligosaccharides, XOL is the percent of initial xylan converted into lowmolecular weight xylooligosaccharides, X is the percent of initial xylan converted into xylose and F is the percent of initial xylan converted into furfural. The concentration of this compound was determined by HPLC as explained in Section 2.2, and the percent of initial xylan converted into decomposition products (denoted DP) is calculated by difference: DP ¼ 100 XnFAST XnSLOW XOH XOL X F:
Table 3 Values of preexponential factors and activation energies Parameter
Result
(a) Results reported for isothermal operation with Model 3 of Fig. 4 (Garrote et al., 2001) a (dimensionless) 0.876 36.2 ln k01 (k01 in h1 ) ln k02 (k02 in h1 ) 28.8 27.8 ln k03 (k03 in h1 ) ln k04 (k04 in h1 ) 28.8 ln k05 (k05 in h1 ) 32.3 28.45 ln k06 (k06 in h1 ) Ea1 (kJ/mol) 130 Ea2 (kJ/mol) 104 Ea3 (kJ/mol) 102 109 Ea4 (kJ/mol) Ea5 (kJ/mol) 117 Ea6 (kJ/mol) 106 (b) Results found in this work (Model 4 of Fig. 4) a (dimensionless) 0.865 50.6 ln kFAST0 (kFAST0 in h1 ) ln kSLOW0 (kSLOW in h1 ) 18.5 ln k02 (k02 in h1 ) 28.8 ln k03 (k03 in h1 ) 27.8 28.8 ln k04 (k04 in h1 ) ln k05 (k05 in h1 ) 32.3 ln k06 (k06 in h1 ) 28.45 EaFAST (kJ/mol) 186 125 EaSLOW (kJ/mol) Ea2 (kJ/mol) 104 Ea3 (kJ/mol) 102 Ea4 (kJ/mol) 109 117 Ea5 (kJ/mol) Ea6 (kJ/mol) 106
ð7Þ In Eq. (6), kFAST0 ; kSLOW0 and k20 . . . k60 denote the preexponential factors, and EaFAST ; EaSLOW ; Ea2 . . . Ea6 denote the activation energies. The numerical integration was carried out considering the following equations: Xn ¼ XnFAST þ XnSLOW ;
ð8Þ
XO ¼ XOH þ XOL ;
ð9Þ
a ¼ Xn0FAST =Xn0 ;
ð10Þ
where Xn is the percent of total xylan remaining in the substrate at the considered reaction time and a is the weight fraction of fast-reacting xylan in the untreated substrate. For calculation purposes, a was considered to be independent of temperature in the range of operational conditions tested. The set of Eqs. (1)–(10) has been solved numerically using a fourth-order Runge–Kutta method coupled with an optimisation algorithm based on Newton’s method to minimise the squares of deviations between experimental and calculated values of the experimental variables. The overall calculation scheme was carried out taking as initial values for a and kinetic parameters the results reported by Garrote et al. (2001) for the isothermal autohydrolysis of corncobs (see Table 3(a)). As
expected, the fitting for residual xylan was poor owing to the participation of the slow-reacting xylan fraction in the overall degradation scheme. When the values of the regression parameters were allowed to vary using the optimisation algorithm, the results reported in Table 3(b) were obtained. In comparison with the reported results, the value calculated for the ‘‘susceptible xylan fraction’’ ða ¼ 0:808Þ is close to the results found for isothermal operation (in the range 0.85–0.90), and lies in the range reported for other lignocellulosic raw materials. Reported data for EaFAST are 142 kJ/mol (Mehlberg & Tsao, 1979), 136 kJ/mol (Conner & Lorentz, 1986), 127 kJ/mol (Maloney et al., 1986), 126 kJ/mol (Chen, Lee, & Torget, 1996) and 117 kJ/mol (Kim et al., 2000); whereas for EaSLOW the results are 176 kJ/mol (Mehlberg & Tsao, 1979), 157 kJ/mol (Conner & Lorentz, 1986), 127 kJ/mol (Maloney et al., 1986), 129 kJ/mol (Chen et al., 1996) and 119 kJ/mol (Kim et al., 2000). It can be seen that the values determined in this work (125 and 177 kJ/mol, respectively) are in the range reported in literature. Similar values have been reported for models involving a single xylan fraction. Contrarily, the results reported for the pre-exponential factors showed a wide variation range, depending on the mechanism proposed,
G. Garrote et al. / Journal of Food Engineering 52 (2002) 211–218
on the substrate utilized and on the possibility of expressing the pre-exponential factors as a function of the concentration of hydronium ions (Ranganathan, MacDonald, & Bakhshi, 1985). In the case of hydrothermal treatments, where the concentration of catalyst is low, the presence of neutralizing substances can be very influential on the results determined for the kinetic parameters (Carrasco & Roy, 1992). The variation range of the pre-exponential factors reported are 35:8– 45:3 h1 for the fast xylan fraction, and 36:5–62:2 h1 for the slow-reacting xylan fraction. Again, the results determined in this work (34:5 and 46:6 h1 , respectively) are in the range reported in literature. Once the time variation of xylan was properly modelled by using the optimised kinetic parameters cited previously, the values reported for the rest of the regression parameters ðk20 . . . k60 ; Ea2 . . . Ea6 Þ gave a close interpretation of the experimental data. For comparative purposes, Fig. 5 shows the experimental and calculated data for the chemical species involved in the autohydrolysis of corncob. The best operational conditions are considered to correspond to experiment 13, in which a high conversion of xylan into xylooligosaccharides was achieved (65.6%), with a joint contribution of xylooligosaccharides and xylose accounting for 72.2% of the initial xylan. These results are in the range 55–80% reported in literature for other re-
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search groups in experiments with a variety of feedstocks (Aoyama, 1996; Conner & Lorentz, 1986; Heitz et al., 1991).
4. Conclusions The non-isothermal autohydrolysis of corncobs is a fast and effective method to produce simultaneously solutions containing sugar oligomers and a solid residue mainly made up of cellulose and lignin. Autohydrolysis caused only limited delignification, whereas cellulose was almost entirely retained in solid phase. The overall process of xylan degradation (including the conversion of xylan into high-molecular weight xylooligomers, conversion of high-molecular weight xylooligomers into low-molecular weight xylooligomers, conversion of low-molecular weight xylooligomers into xylose and dehydration of xylose into furfural, and generation of furfural from low-molecular weight xylooligomers) can be modelled by means of first-order, pseudohomogeneous reactions assuming the existence of two xylan fractions with different susceptibility towards hydrolytic degradation. The values calculated for the regression parameters (‘‘susceptible’’ xylan fraction, activation energies and pre-exponential factors of the several reactions involved in the whole mechanism) allowed a close reproduction of the experimental data, and provided basic information for design and economic evaluation of the studied process.
Acknowledgements Authors are grateful to the Commission of the European Communities for the financial support of this work (in the framework of the European Project ‘‘Xylophone’’, reference FAIR-CT98-3811-DG12) as well as to the Spanish ‘‘Ministry of Education’’ for complementary funding (Ref. ALI98-1625-CE).
References
Fig. 5. Experimental and calculated variation of variables Xn (percent of xylan remaining in wood after treatments), XO (percent of initial xylan converted into xylooligosaccharides with high- and low-molecular weight), X (percent of initial xylan converted into xylose), F (percent of initial xylan converted into furfural) and DP (percent of initial xylan converted into degradation products).
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