Kinetic studies on acid hydrolysis of Meranti wood sawdust for xylose production
Highlights
► Meranti wood sawdust can be used as a potential source of xylose. ► Kinetic parameters were obtained to predict the concentration of products released. ► The yield of xylose was more than 89% of the potential concentration. ► Results are validated against experimental literature data.
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 (Parajó 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 the food, pharmaceutical and odontological industries (Roberto et al., 1995, Roberto et al., 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, Hyvönen and Koivistoinen, 1982, Mäkinen, 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, Parajó et al., 1994, Téllez-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 (Parajó 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 (Parajó 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.
Section snippets
Raw material
The raw material, Meranti wood sawdust (MWS), used in this study was collected from local sawmill (Seng Peng Sawmills Sdn Bhd, Malaysia). The MWS was screened to remove oversized particles, sun dried and then passed through a 0.5 mm sieve using a vibratory sieve shaker (Analysette 3 Pro, Fritsch, Germany) to select the particle size less than 0.5 mm (>0.5 mm retained). The screened MWS was homogenized in a single lot to avoid compositional differences among aliquots and stored in polypropylene
Composition of MWS
The analyses of MWS were done in order to determine the principal structural components using quantitative acid hydrolysis under standard methods. The main components of MWS are shown in Table 1. The major polymeric components of MWS were in the range of those of other wood materials stated in the literature (Balat et al., 2008, Sinağ et al., 2009). The xylan content of the MWS biomass (29.22%) fell within the range 11–35% that has been reported for hardwoods and agricultural residues (Nigam
Conclusion
MWS contains 29.22% xylan, which is a promising source of xylose. Acid hydrolysis of MWS was accomplished in batch mode at 130 °C with several sulfuric acid concentrations and residence time. Hydrolysis reaction was assessed with the proposed kinetic models based on pseudohomogeneous irreversible first-order series reactions. The time course of the concentration of components in the hydrolysate was determined and the results were interpreted through the kinetic model which allowed a close
Acknowledgments
The authors wish to thank the Graduate Research Scheme (Grant no. GRS 090310), UMP for the financial assistance and the FRIM and UTM for the chemical analyses of the raw material. The authors are also grateful to Prof. M.R. Karim for his helpful revision of this manuscript.
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