Kinetic modelling of corncob autohydrolysis

Process Biochemistry 36 (2001) 571 – 578 www.elsevier.com/locate/procbio

Kinetic modelling of corncob autohydrolysis Gil Garrote, Herminia Domı´nguez, Juan Carlos Parajo´ * Department of Chemical Engineering, Uni6ersity of Vigo (Campus Ourense), Edificio Polite´cnico, As Lagoas, 32004 Ourense, Spain Received 16 June 2000; accepted 3 October 2000

Abstract Corncobs were reacted with water and treated at temperatures in the range 145 – 190°C during 0 – 12.3 h at a liquor to solid ratio of 8 or 10 kg/kg (autohydrolysis treatments), to hydrolyse the hemicellulose fraction to xylooligomers (useful as food ingredients) and xylose (a carbon source for further fermentation stages). The time-courses of xylan and xylan-degradation products (including xylooligomers, xylose, furfural and other degradation products) were established. The kinetics of xylan degradation was modelled by means of a mechanism involving sequential, first order, pseudohomogeneous kinetics. The values of the kinetic coefficients were calculated, and their dependence on temperature was established using Arrhenius-type equations. The proposed model provides a satisfactory interpretation of the experimental data, allowing the selection of optimized operational conditions. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Autohydrolysis; Corncobs; Lignocellulosic materials; Xylan; Xylooligosaccharides

1. Introduction Agricultural wastes are renewable, costless and widespread sources of chemicals. In Spain, about 16× 106 metric tons of agricultural wastes are produced yearly [1]. Even if in most cases the lignocellulosic materials (LCM) are burned (an application with low added value causing environmental impact) [2], the lignocellulosic nature of the agricultural wastes makes them suitable for chemical processing to obtain end products with high added value. The chemical fractionation of LCM into products derived from their structural components (cellulose, hemicellulose and lignin) is an attractive possibility leading to an integral benefit to the feedstocks. Since the separation of the structural fractions requires the degradation of some of them, a possible first step in a process aiming at the fractionation of LCM can be based on the selective degradation of hemicelluloses, by means of prehydrolysis (carried out in acidic media) or autohydrolysis (in which water is reacted with the raw material). This operational way allows the production of soluble, valuable chemicals from hemicelluloses [3], * Corresponding author. E-mail address: jcparaxcParajo´).

leaving a solid residue (made up of cellulose and lignin) which can be valorised by further processing Xylan is the main component of the hemicellulose of non-woody LCM, particularly in the case of the raw material chosen in this work (see below). Araban and acetyl groups are also components of hemicellulose. Autohydrolysis cause the hydrolysis of xylan, leading to xylooligosaccharides (XO) and xylose as major reaction products. Acetic acid (from acetyl groups), other monosaccharides and sugar-degradation products can be also generated in amounts dependent on both the composition of the raw material and the operational conditions. Autohydrolysis proceeds through hydronium-catalyzed reactions. In the initial reaction stages, the hydronium ions come from water autoionization, whereas the acetic acid generated in the media is the main source of catalyst in further reaction stages, leading to liquors with pH in the range 3–4 [4]. In comparison with prehydrolysis, autohydrolysis shows advantages such as: (i) the corrosion problems are avoided, since no mineral acid is added to the reaction media; (ii) no neutralization sludges are to be handled; (iii) the mild operational conditions allow excellent selectivity towards cellulose degradation; (iv) limited concentrations of unwanted sugar-degradation products are found in the media [5]; (v) the soluble

0032-9592/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 5 3 - 3

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reaction products are useful for a variety of purposes [6,7], and (vi) autohydrolysis can improve the susceptibility of the solid residues to further processing (including enzymatic hydrolysis of the cellulosic fraction or chemical delignification), owing to the improvements in cellulose accessibility and to the partial hydrolytic degradation of lignin [3,8,9]. Autohydrolysis can be oriented towards the production of xylooligomers, which can account for about 60 – 70% of the initial xylan [3]. Xylooligomers can be used as food additives owing to their favourable effects on the intestinal flora [10 – 12] or in the manufacture of fermentation media for healthy bacteria [13]. Alternatively, xylose is the main reaction product when the reaction is allowed to proceed, leading to solutions that can be used as fermentation media. A rigorous study of LCM processing in acid containing- or in water containing-media should be complex, owing to its characteristics (heterogeneous, catalytic reaction, with multi-stage mechanism involving steps such as pore diffusion of catalyst, chemical reaction, transportation of reaction products, etc.) [14]. Because of this, simplified approaches have been used in the literature to study this type of problem. The mathematical interpretation of hemicellulose degradation has been carried out by two alternative methods: studies based on the utilisation of severity factors [15] or kinetic modelling based on pseudohomogeneous reactions. In this study, we have chosen the latter approach for its superior ability to reproduce the experimental data. This work deals with the kinetic modelling of the autohydrolysis of corncobs, an agricultural waste. The temperature range tested (145 – 190°C) and the liquor to solid ratios employed (8 or 10 g/g) were selected ac-

cording to literature data and our own experience, whereas the duration of reactions was fixed to provide an overview of both generation and decomposition of oligomers. The effects of treatments on xylan and on xylan-reaction products have been established, and a kinetic model (based on consecutive, pseudohomogeneous, first-order reactions) has been developed to describe the time-course of xylan degradation and the composition of liquors.

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 LCM Aliquots from the homogenised sample were subjected to moisture determination (drying at 105°C to constant weight) and to quantitative acid hydrolysis with 72% sulphuric acid following standard methods [16]. 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 [3]. The results determined for the composition of the raw material (expressed as weight percent, o.d. basis) is as follows: glucan, 31.7%; xylan, 30.9%; arabi-

Table 1 Operational conditions considered in this work Experiment

Temperature (T, °C)

Time (t, h)

1 2 3 4 5 6 7 8

145 160 175 190 145 160 175 190

0–12.1 0–6.1 0–2.0 0–0.67 0–12.3 0–6.0 0–2.0 0–0.67

Fig. 1. Kinetic model proposed in this work.

Liquor/solid ratio (LSR, kg/kg) 8 8 8 8 10 10 10 10

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Fig. 2. Calculation scheme developed to determine the values of the regression parameters.

nan, 3.77%; acetyl groups, 3.78%; Klason lignin, 20.3%; others (by difference), 9.58.

2.3. Hydrothermal processing of corncobs Milled corncob samples and water were mixed in the desired proportions and reacted in a stainless steel Parr reactor (model 4563 M) fitted with a two six-blade turbine impellers. The vessel was heated with an external fabric mantle, and cooled by an internal stainless steel loop. Temperature was monitored using an inner thermocouple, and controlled by a PID module. The

moisture of LCM was considered as water in the material balances. The reaction media were heated to the desired temperature (heating time, 15–20 min) and time zero was set at beginning of the isothermal reaction stage. Since a fraction of the feedstock can have reacted at t= 0, a sample was withdrawn at this moment in order to measure the initial conversion of the substrate.

2.4. Analysis of solid residues and liquors from hydrothermal treatments At the end of treatments, the solid residues were

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Table 2 Results obtained for a, kinetic coefficients and R 2 (see text for definition and units) Experiment

a (dimensionless) k1 (h−1) k2 (h−1) k3 (h−1) k4 (h−1) k5 (h−1) k6 (h−1) R 2 for var. PRXn R 2 for var. OEP R 2 for var. XEP R 2 for var. FEP

1

2

3

4

5

6

7

8

0.859 0.331 0.353 0.209 0.0761 0.286 0.144 0.988 0.929 0.952 0.900

0.868 1.12 0.751 0.478 0.214 0.672 0.273 0.987 0.954 0.986 0.889

0.882 5.00 1.99 1.27 0.692 2.44 0.724 0.994 0.965 0.972 0.867

0.897 11.1 6.54 3.65 1.52 7.09 2.89 0.988 0.968 0.967 0.711

0.846 0.371 0.303 0.178 0.0382 0.170 0.125 0.984 0.914 0.973 0.873

0.852 1.21 0.682 0.450 0.134 0.342 0.232 0.990 0.979 0.992 0.962

0.874 4.14 1.76 1.18 0.488 1.22 0.589 0.993 0.985 0.989 0.964

0.891 11.5 5.25 3.74 2.21 3.41 1.51 0.990 0.973 0.965 0.857

recovered by filtration, washed, air-dried, milled to a particle size B0.5 mm, subjected to quantitative saccharification to measure their contents in cellulose, hemicellulose and lignin using same methods as for raw material analysis. The data concerning the composition of the raw material are shown in the Methods section. In autohydrolysis treatments, a sample of liquors was filtered through 0.45 mm membranes and used for direct HPLC determination of monosaccharides, furfural and acetic acid. A second sample of liquors (20 ml) was subjected to quantitative posthydrolysis (with 4% H2SO4 at 121°C during 60 min) before HPLC analysis. The increase in monosaccharide concentration caused by posthydrolysis provided a measure of the oligomer concentration [3].

2.5. Fitting of data The experimental data were fitted to the proposed kinetic models by minimization of the sum of squares using commercial software with a built-in optimization routine dealing with the Newton’s method (see below).

3. Results and discussion

3.1. Operational conditions and definition of 6ariables Table 1 shows the operational conditions considered in this work: temperature (T, °C), reaction time (t, h) and liquor to solid ratio (LSR, kg water/kg oven-dry corncob). The maximum reaction times were selected to allow a complete observation of the time-course of XO concentration, and the temperature range covers the usual values used in mild autohydrolysis processes. The dependent variables selected to follow the autohydrolysis process and their nomenclature are as follows:

Fig. 3. Experimental and calculated time courses of residual xylan and xylan degradation products for experiments 1, 2 and 3 of Table 1.

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Fig. 5. Arrhenius’ plot for k1 to k6.

Fig. 4. Experimental and calculated time courses of residual xylan and xylan degradation products for experiments 6, 7 and 8 of Table 1.

1. variable measuring the recovery of solid phase after treatments: SY= solid residue yield, g solid recovered after treatments/100 g untreated LCM, oven dry basis (o. d. b.); 2. variable measuring the xylan content of the substrate: %Xn=xylan content of treated samples, g xylan/100 g treated LCM, o. d. b.; 3. variables measuring the composition of liquors: XoC=xylooligomer concentration, g xylooligomers/kg liquor (xylooligomers expressed as xylose equivalent); XyC=Xylose concentration, g xylose/kg liquor; FuC=Furfural concentration, g furfural/kg liquor. The concentrations of xylan, XO, xylose and furfural can be expressed in terms of ‘equivalent percents of the initial xylan’ using the following expressions (in which the superscript RM refers to data of to the raw material):

Table 3 Arrhenius’ parameters determined for the various kinetic coefficients (k0i, preexponential factors; Eai, activation energies) Kinetic coefficient

ln (k0i ) (k0i in h−1)

Eai (kJ/mol)

(a) Experiments carried out with LSR = 8 kg/kg k1 36.2 130 k2 28.8 104 102 k3 27.8 k4 28.8 109 k5 32.3 117 k6 28.4 106 (b) Experiments carried out with LSR = 10 kg/kg k1 34.7 124 k2 28.0 102 k3 29.3 108 k4 38.1 144 k5 29.7 110 k6 23.7 89.9

R2

0.991 0.983 0.993 0.997 0.991 0.962 0.999 0.991 0.993 0.995 0.983 0.985

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PRXn=

%Xn · SY %XnRM





SY · 10 100 132 · OEP= RM 150 %Xn SY XyC · LSR +1 − · 10 100 132 XEP = · RM %Xn 150 SY FuC · LSR +1 − · 10 100 132 FEP = · RM 96 %Xn XoC · LSR +1 −





(1)



(2)



(3)

(4)

where: PRXn=percent of xylan remaining in solid phase after treatments (calculated respect to the xylan contained in raw wood); OEP = equivalent oligomer percent (calculated respect to the xylan contained in raw LCM); XEP = equivalent xylose percent (calculated respect to the xylan contained in raw LCM); FEP =equivalent furfural percent (calculated respect to the xylan contained in raw LCM); and the terms (132/ 150) and (132/96) are the stoichiometric factors giving the interrelationship between xylose or furfural and xylan, respectively.

3.2. Kinetics of xylan autohydrolysis The mechanism of xylan degradation during acid prehydrolyis proposed by Mehlberg and Tsao [17] included the assumption that two xylan fraction exist in the feedstock (with different susceptibility towards hydrolytic degradation). Xylan is converted into xylooligosaccharides, which give xylose and this pentose is dehydrated to furfural. Other mechanisms for acid prehydrolysis neglect the step of xylooligosaccharide formation [18] with little loss of accuracy [19]. Alternatively, both xylooligosaccharides and xylose can be measured together by a single variable [20]. Under harsh conditions, the participation of furfural-decomposition reactions has been proposed [21]. Autohydrolysis and prehydrolysis have a similar theoretical background, because both of them are hydronium-mediated hydrolytic reactions involving the heterocyclic ether bonds of hemicellulose polysaccharides. The main difference between autohydrolysis and prehydrolysis is the availability of catalyst in the reaction media: as a result of the milder reaction conditions used in autohydrolyiss, the xylooligosaccharide concentrations are much higher in this case. During autohydrolysis, the molecular weight of xylooligosaccharides is progressively reduced, leading to XO with low polymerisation degrees in the late reaction stages. Since the low-molecular weight XO are more likely to give xylose upon hydrolysis than high-molecular weight XO, Conner and Lorenz [22] proposed a time-dependent kinetic coefficient to consider the different probability of obtaining xylose from XO through the process.

A different approach has been used: the progressive process of molecular weight reduction by XO hydrolysis has been simplified to a mechanism involving two sequential stages of DP reduction. The hypotheses assumed in the formulation of the kinetic mechanism are as follows: 1. corncob xylan (Xn) is made up of two fractions, one of them being unreactive under the operational conditions assayed and the another (called susceptible xylan, and denoted XnS) is able to give XO; 2. the relative proportion of the xylan fraction susceptible to hydrolysis is related to the total xylan of LCM (Xn) by means of the ‘susceptible fraction’ a(0BaB 1); 3. the xylan fraction susceptible to hydrolysis is first degraded to high-DP XO (XOH); 4. in a further reaction steps, the high-DP XO are hydrolysed to low-DP XO (XOL), and these are converted into xylose (X), which renders furfural (F) upon dehydration, and furfural is degraded to decomposition products; 5. XOL can give furfural directly; 6. all the reactions follow pseudohomogeneous, firstorder kinetics. Fig. 1 shows the proposed mechanism, which was based on the main experimental trends observed for the concentration profiles. For example, the reaction step leading from XOL to furfural was introduced because consistently low furfural concentrations were predicted by the model when this step was omitted, whereas the decomposition of furfural was evident from material balances concerning xylan and xylan-degradation products. The direct generation of furfural from the terminal monosaccharide units of hemicelluloses has been considered in literature for the mathematical interpretation of hemicellulose degradation in media containing organic acid [23,24]. By integration of the kinetic model derived from the above mechanism in terms of the selected variables, it can be inferred that: PRXn= C1 ·exp(− k1 · t)+ C2

(5)

XOH = C3 ·exp(− k1 · t)+ C4 ·exp(− k2 · t)

(6)

XOL = C5 ·exp(− k1 · t)+ C6 ·exp(− k2 · t) +C7 ·exp[− (k3 + k6)· t]

(7)

XEP= C8 ·exp(− k1 · t)+ C9 ·exp(− k2 · t) +C10 ·exp[− (k3 + k6)· t]+ C11 ·exp(− k4 · t) (8) FEP= C12 ·exp(− k1 · t)+ C13 ·exp(− k2 · t) + C14 ·exp[− (k3 + k6)· t]+ C15 ·exp(− k4 · t) + C16 ·exp(− k5 · t)

(9)

A conservation equation to xylan and xylan-degradation products leads to:

G. Garrote et al. / Process Biochemistry 36 (2001) 571–578

DPEP = 100-PRXn-OEP-XEP-FEP

(10)

The integration constants can be calculated by the equations: C1 = b · PRXn0

(11)

C2 = (1−b)· PRXn0

(12)

k1 · C 1 k2 −k1

(13)

C4 = OEP0 − C3

(14)

C3 =

C5 =

k2 · C3 k3 +k6 −k1

(15)

C6 =

k2 · C4 k3 +k6 −k2

(16) (17)

C7 = − C5 − C6 C8 =

k3 · C 5 k4 −k1

(18)

C9 =

k3 · C 6 k4 −k2

(19)

C10 =

k3 · C7 k4 − k3 − k6

(20)

C11 = XEP0 −C8 − C9 −C10

(21)

C12 =

k4 · C8 + k6 · C5 k5 − k1

(22)

C13 =

k4 · C9 + k6 · C6 k5 − k2

(23)

C14 =

k4 · C10 + k6 · C7 k5 −k3 − k6

(24)

C15 =

k4 · C11 k5 −k4

(25)

C16 = FEP0 −C12 − C13 −C14 −C15

(26)

In the above equations, the superscript 0 correspond to the concentration of susceptible unreacted substrate at t = 0, the parameter b corresponds to the soluble fraction of unreacted xylan at t =0, which is related with the ‘susceptible fraction’ a by the equation: b = 1 −(1 −a)·

PRXnRM PRXn0

(27)

Since XOH and XOL were measured together as xylose equivalents, their joint contribution was measured by OEP: OEP=XOH +XOL

(28)

For each experiment, the series of experimental data concentration/time were fitted to the above equations according to the calculation scheme shown in Fig. 2, to determine the optimal values of a and k1 to k6.

577

Table 2 shows the values determined for the regression parameters, and for the statistical coefficient R 2. Figs. 3 and 4 show the time-course of experimental and calculated data concerning xylan and xylan degradation products in representative experiments. The reasonable agreement between experimental results and model predictions observed under all the operational conditions confirmed the validity of the model. The ability of the model for quantitative predictions in the experimental domain corresponding to XO concentrations near the maximum particularly important considering the applications and the high added-value of this type of compounds. The results corresponding to the maximum percent of xylan recovery as XO varied from 52.6 (experiment 1) to 68.9 (experiment 7), and increased first with temperature to reach a plateau (about 70% xylan conversion into XO) at 175–190°C. The maximum conversion of xylan into XO was slightly affected by the liquor to solid ratio, even if improved XO percents were determined for the highest liquor to solid ratio tested. These data are close to those reported for XO generation from Eucalyptus wood, which led up to 65% xylan conversion into xylooligomers. In the same way, the results lie in the range reported in literature, which ranged from the 55% for bamboo grass [6] up to 69.3% in hardwoods [22]. The dependence of the kinetic coefficients on temperature has been established by means of the Arrhenius equation. Fig. 5 shows the linear interrelationship found between ln(ki ) and 1/T. Table 3 shows the preexponential factors, the activation energies and the values of R 2 determined for all the cases considered. Little effects of the liquor to solid ratio on preexponential factors and activation energies were observed, as it was observed in a previous study on the autohydrolysis of Eucalyptus wood [3]. The differences between the values of the regression parameters listed in Table 3 are in the range acceptable for the joint contribution of the experimental and fitting errors. Several studies on hemicellulose processing in acidic media have been reported. In autohydrolysis experiments, the activation energies reported for the degradation of hardwood xylan are in the range 103–143 kJ/mol [25]. From the data reported by Conner and Lorentz [22] for oak wood, it can be calculated that the activation energy for the autohydrolysis of suceptible xylan is in the vicinity of 110 kJ/mol. In acid prehydrolysis, the reported values for activation energies lie in a broader range, typically 110–150 kJ/mol, depending on the assumptions of the models (xylan made up of one or two fractions). The variety of hypotheses in which the models are based makes it difficult to compare of the preexponential factors, which in some cases have been considered to be a function of the hydronium ion concentration and are affected by the composition of the raw materials [26] as well as by the presence of neutralising compounds in the feedstock [18].

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4. Conclusions The hydrothermal processing of corncobs, leading to solutions containing xylan-degradation products (xylooligosaccharides, xylose and furfural), was assessed by means of a kinetic model based on parallel and consecutive, pseudohomogeneous, first-order reactions. The reaction steps included the degradation of susceptible xylan to high-molecular weight xylooligosaccharides, the hydrolysis of these compounds to low-molecular weight xylooligosaccharides (which can give either xylose of furfural), the dehydration of xylose to furfural, and the generation of decomposition products from this compound. The kinetic coefficients for isothermal operation were calculated through an optimisation algorithm, allowing a close interpretation of the experimental data. The influence of temperature on the kinetic coefficients was interpretated by means of the Arrhenius% equation, allowing the calculation of both preexponential factors and activation energies for the several reactions involved in the mechanism, providing a theoretical background for further process analysis.

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).

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