KINETIC MODELING OF THE ACID HYDROLYSIS OF WOOD SAWDUST

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KINETIC MODELING OF THE ACID HYDROLYSIS OF WOOD SAWDUST

A B S T R A C T

Meranti wood sawdust (MWS) is a lignocellulosic waste of sawmill which can be used as a promising source of xylose. The main application of xylose is its bioconversion to xylitol, a high value product. The growing interest in biotechnological production of biofuels and specialty chemicals from lignocellulosic waste is justifiable as these materials are low priced, renewable and widespread sources of sugars. The aim of this research was to study xylose production from MWS by acid hydrolysis. Batch hydrolysis was conducted at 130 1C using several concentrations of H2SO4 (2–6%) and residence time (0–120 min). The kinetic parameters of mathematical models were determined to predict xylose, glucose, furfural, and acetic acid concentration in the hemicellulosic hydrolysate. Optimum sulfuric acid concentration and residence time obtained were 6% and 20 min, respectively. These conditions yielded a hydrolysate containing 18.5, 4.2, 0.4, and 4.1 g/l xylose, glucose, furfural, and acetic acid, respectively. The yield of xylose was more than 89% of the potential concentration.

1. Introduction

Biotechnological production of biofuels, specialty chemicals or food ingredients from lignocellulosic biomass has attracted considerable attention because the biomass is renewable, widespread and an inexpensive source of polysaccharides (Parajo´ et al., 1996). Sawdust is a lignocellulosic waste of sawmill that is available at low cost throughout the year. It is produced in huge quantities by sawmills and the economical disposal of them is a serious problem to the wood industries. Sawdust is commonly used as fuel in manufacturing plants and in local utilities. Other uses of sawdust are: as litter and bedding material in livestock and poultry structures, for the production of fiberboards and paper pulp (Arends and Donkersloot-Shouq, 1985; Harkin, 1969). Sawdust from red Meranti species was chosen as raw material in this study because it is one of the most common and popular hardwood species in Malaysia. The utilization of MWS to produce xylose has a dual consequence, the elimination of waste and the generation of high value product.

Xylose is a hemicellulosic sugar which can be an economical starting raw material for the production of a wide variety of compounds or fuels by chemical and biotechnological processes. One of these compounds is xylitol that is extensively utilized in

n Corresponding author. Tel.: þ60 9 549 2825; fax: þ60 9 549 2889. E-mail address: [email protected] (A.M. Mimi Sakinah).

doi:10.1016/j.ces.2011.11.007

the food, pharmaceutical and odontological industries (Roberto et al., 1995, 2003). The most significant application of xylitol is its use as an ideal sweetener for diabetic patients because of its insulin independent metabolism (Pepper and Olinger, 1988; Ylikahri, 1979). Other potential uses of xylitol are: as an anticariogenic agent in toothpaste formulations, as thin coatings on vitamin tablets, in chewing gum, ice cream, mouthwashes, beverages and in bakery products (Emodi, 1978; Hyvonen and¨ Koivistoinen, 1982; Makinen, 1992¨ ).

Numerous investigations on dilute acid hydrolysis of different lignocellulosics such as corncobs, sugarcane bagasse, brewer’s spent grain, Eucalyptus wood, and sorghum straw have been performed by several research groups (Dominguez et al., 1997; Lavarack et al., 2002; Mussatto and Roberto, 2005; Parajo´ et al., 1994; Te´llez-Luis et al., 2002). Xylose was produced as the main sugar from hemicellulose and at the same time, other byproducts such as glucose, furfural, acetic acid, etc., were also generated in low amounts during hydrolysis (Dominguez et al., 1997; Mussatto and Roberto, 2005). It was also demonstrated that the amount of sugar released during hydrolysis depended on the type of raw material and operating conditions of the experiment such as temperature, acid concentration and residence time (Pessoa et al., 1996). During hydrolysis, acid concentration was found to be the most important parameter affecting the sugar yield while temperature showed the highest impact on the formation of sugar degradation products

(Neureiter et al., 2002). Under controlled experimental conditions, the dilute acid hydrolysis of lignocellulosics mainly produces xylose from hemicellulose, leaving a solid residue containing the cellulose and lignin fractions almost unaltered. Hemicelluloses are more susceptible to mild acid due to its amorphous, branched structure compared to cellulose, which needs severe treatment conditions due to its crystalline nature (Parajo´ et al., 1998a).

The main chemical components of MWS are cellulose, hemicellulose and lignin. The hemicellulosic fraction can be easily and selectively extracted with dilute sulfuric acid under mild conditions to obtain xylose-rich hydrolysate which can be used as a substrate to produce xylitol by bioconversion. Dilute acid hydrolysis is still preferable to enzymatic hydrolysis as it is low cost, simple, faster method and commonly used for the hydrolysis of lignocellulosic biomass (Carvalheiro et al., 2005; Rivas et al., 2006). However, the major disadvantage of acid hydrolysis is that it generates a hydrolysate that contains not only the sugar needed for bioconversion but also sugar and lignin degradation products as well as acetic acid that could slow down or prevent the bioconversion of xylose (Parajo´ et al., 1995). Therefore, it is important to choose less severe conditions that will maximize the yield of xylose while minimizing the undesired products. In order to evaluate experimental conditions for optimizing xylose yields, it is necessary to know the kinetic characteristics of products released during acid hydrolysis of MWS. There is no report available on the acid hydrolysis of wood sawdust to extract xylose. The aim of this research was to determine the kinetic parameters of mathematical models for predicting the production of xylose, glucose, furfural, and acetic acid during hydrolysis of MWS by sulfuric acid and optimize the process.

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