Kinetic Study of Acid Hydrolysis of Rice Straw

Rice straw is a renewable, cheap, and abundant waste in tropical countries. The pentose content of rice straw can be used as a substrate for many types of value-added products such as xylitol and biofuel. Dilute acid hydrolysis mainly releases pentose from rice straw. The objective of the study was to determine the effect of H2SO4 concentration and reaction time on the xylose production. The variation of the main product xylose with the reaction time was described by a kinetic model and kinetic parameters were calculated to describe the variation of the xylose production with time. The optimum yield (19.35 g/L) was obtained at 0.24 mol/L H2SO4 and 30 minutes.


Introduction
Rice straw is one of the most abundant agricultural wastes. For instance, approximately 731 million tons per year rice straw is produced globally (Africa: 20.9 million tons, Asia: 667.6 million tons, Europe: 3.9 million tons, America: 37.2 million tons 61, and Oceania: 1.7 million tons) [1]. Nearly 600 million tons of agricultural residues are produced by India annually; out of which, approximately 300 million tons of it remain unused.
e options for the disposal of rice straw are limited by the great bulk of material, slow degradation in the soil, harboring of rice stem diseases, and high mineral content. ough rice straw is used as animal feed and soil fertilizer, the utilization ratio remains low compared to other straws. Rice straw has low digestibility value as animal feed. Although rice straws contain materials for social bene�t, their apparent value is less than the cost of collection, transportation, and processing for bene�cial use. Open-�eld burning is a major practice for rice straw disposal. It causes air pollution, a threat to human health [2]. Rice straw contains 19-27% of hemicellulose, a heteropolymer composed mainly of xylose followed by arabinose [3,4]. e hemicellulosic and cellulosic contents of rice straw can be hydrolyzed chemically or enzymatically. Chemical hydrolysis includes dilute sulfuric acid hydrolysis that can be used for either the pretreatment before enzymatic hydrolysis or the conversion of hemicellulose to pentose [5] remaining cellulose and lignin fractions being almost unaltered. Lignocellulosic structure as well as hydrolysis reactions of sugar polymers in a dilute acid medium is very complicated. e substrate is in the solid phase and the catalyst in the liquid phase. Various factors (particle size, liquid to solid ratio, type and concentration of acid used, temperature, and reaction time) in�uence monomer yield [6].
In the present work, the effect of acid concentration and reaction time on xylose production was studied and a kinetic model was developed to describe the variation of xylose production with time.

Rice Straw.
Rice straw was used as raw material in the experiments. It was procured from Bankura, India. It was airdried, ground, size fractioned to 0.5 mm, and stored at room temperature for subsequent experiments. Main components of rice straw such as cellulose, hemicellulose, and lignin were determined [7]. e composition of rice straw is shown in Table 1.

Cellulose Estimation.
Acetic/nitric reagent (150 mL of 80% acetic acid and 15 mL of concentrated nitric acid) was added to 0.5 g sample and placed in a water bath at 100 ○ C for 30 minutes. e mixture was cooled and centrifuged for 20 minutes. e supernatant was discarded and the residue was washed with distilled water and mixed with 10 mL of 67% H 2 SO 4 . en it was allowed to stand for 1 hour at room temperature. e solution (1 mL) was diluted to 100 mL and to 1 mL of this diluted solution and 10 mL of freshly prepared anthrone reagent (0.2% anthrone in concentrated H 2 SO 4 ) was added. e mixture was heated in a boiling water bath for 10 minutes and cooled. e color was measured at 630 nm. Cellulose powder (Hi Media, India) was used as a standard.

Hemicellulose Estimation.
In a re�uxing �ask, 10 mL cold neutral detergent solution was added to 1 g powdered sample. e neutral detergent solution was prepared as follows. Disodium ethylenediamine tetraacetate (18.61 g) and sodium borate decahydrate (6.81 g) were dissolved in about 200 mL of distilled water by heating and to this, 100 mL solution containing sodium lauryl sulphate (30 g) and ethoxy ethanol (10 mL) was added. A solution (100 mL) of 4.5% Na 2 HPO 4 was then added to the mixture. e �nal volume was made up to 1 L with distilled water and the pH adjusted to 7.
To the mixture of rice straw sample and cold neutral detergent solution, decahydronaphthalene (2 mL) and sodium sulphite (0.5 g) were added. en the mixture was heated to boiling and re�uxed for 1 h. e contents inside the re�uxing �ask were �ltered through sintered glass crucible (G-2) followed by hot water washing. Finally two washings with acetone were given and the residue was transferred to a crucible. e sample was dried at 100 ○ C for 8 h, cooled in a desiccator, and weighed. e residue was designated as neutral detergent �ber (NDF). e amount of acid detergent �ber (ADF) was subtracted from the amount of neutral detergent �ber (NDF) for the calculation of hemicellulose content. e acid detergent �ber (ADF) was prepared by the following method. 100 mL acid detergent solution (2% cetyl trimethyl ammonium bromide in 1 N sulphuric acid) was added to the powdered sample which was placed in a round bottom �ask. e sample was heated to boil in 5�10 minutes and re�uxed for 1 h aer the onset of boiling. e boiling was adjusted to slow, even level. e contents were �ltered through a preweighed sintered glass crucible (G-2) by suction and washed with hot water twice. e contents were washed with acetone until the �ltrate was colorless. en it was dried at 100 ○ C for overnight, cooled in a desiccator, and weighed. e ADF content was expressed as a percentage, that is, / where was the weight of the �ber and was the weight of the sample.

Lignin
Estimation. 25 mL of 72% H 2 SO 4 and 1 g asbestos were added to acid detergent �ber (ADF). e mixture was kept for 3 h at room temperature and intermittently stirred. Aer that the mixture was diluted and �ltered with preweighed Whatman no. 1 �lter paper. en it was dried at 100 ○ C, cooled in a desiccator and weighed. en the �lter paper was transferred to a preweighed silica crucible and kept in a muffle furnace at 550 ○ C for 3 h. e crucible was cooled in a desiccator, and weighed for ash content

Xylose Estimation.
Xylose content was determined using the phloroglucinol assay [9] with minor modi�cations. Brie�y, the color reagent consisting of 0.5 g of phloroglucinol, 100 mL of glacial acetic acid, and 10 mL of conc. HCl was freshly prepared and used within 4 days. Stock of standard xylose solution (10 g/L) was prepared by dissolving D-xylose (Himedia, India) powder in saturated benzoic acid and used for preparation of the calibration curve. Fiy microliters of sample was mixed with 5 mL color reagent and subsequently heated at 100 ○ C for 6 min. e reaction mixture was rapidly cooled down to room temperature in a water bath and the absorbance at 554 nm was recorded.

Results and Discussion
For acid hydrolysis, H 2 SO 4 concentration was varied from 0.093 mol/L to 0.28 mol/L and reaction time was varied from 15 to 60 minutes. Xylose concentration obtained from the hydrolysate at various H 2 SO 4 concentrations and reaction times is given in Table 2. It was observed that maximum xylose was released at 0.24 mol/L H 2 SO 4 concentration and 30 minutes. Xylose concentration decreased with reaction time. is may be due to sugar decomposition and production of inhibitor. e level and composition of the sugar released depend on the type of the acid in the reaction  [11]. e variation of the main product xylose with reaction time may be described by a kinetic model. Saeman, 1945 [12], proposed a simpli�ed model for hydrolysis of lignocellulosic materials using pseudo-homogeneous irreversible �rst-order reactions [13,14]. e model was used for hemicellulosic fraction [13]: (2) where 1 and 2 are the rate constants (min −1 ) for generation and decomposition reaction, respectively.
Predicted model for concentration of monomers is given below by solving the differential equation [12]: where is concentration of monomers (g/L), is concentration of polymers (g/L), 0 is initial monomer concentration (= 0), and is reaction time. Equation (3) is used for kinetic modeling of xylose concentration.
where 0 initial xylan concentration was assumed as 20.6 g xylan per 100 g rice straw on dry basis [15]. WSR is the water/solid ratio and 150/132 is the ratio of the stochiometric factors. Aguilar et al. (2002) [13] modi�ed (3) for better �tting of the experimental data to the kinetic model. e modi�ed equation is given by is the ratio between fractions (g of susceptible xylan/g of total xylan). e usual range of is 0.5-1 g/g. Experimental data of xylose concentration in acid hydrolysate was �tted applying (5). e plots of predicted and experimental data of xylose concentration of each set are shown in Figure 1. e kinetic parameters of each set are given in Table 3. Values of correlation coefficient 2 for each set were 0.99 which indicate the �tness of the model. Chi-square analysis of the experimental data at each H 2 SO 4 concentration was carried out. e chi-square statistic test ( 2 ) is the sum of the squares of the differences between the experimental data and predicted data obtained by calculating from models, with each squared difference divided by the corresponding predicted data obtained by calculating from models. e ( 2 ) will be a small number if the experimental data and predicted data from the model are similar and vice versa. erefore, the model that gives the smallest chi-square value is considered the best �t [16]: e range of the 2 value signi�es the good �t of the model at each acid concentration (Table 4). From Table 3, it is observed that values varied from 0.528 to 0.999 depending on H 2 SO 4 concentration. Values of generally vary with operational conditions. Different values ranging from 0.5 to 1 g/g have been reported [13].
Values of 1 and 2 values also increased with H 2 SO 4 concentration and 1 values were higher compared to 2 values for each set. is indicates that generation rate is higher than degradation rate.
For zero initial monomer, rate of monomer formation / can be obtained by differentiating (5) and putting Now for heterogeneous solid-liquid noncatalytic reactions, where is mass transfer constant, is interfacial area, and is acid concentration. Comparing (7) and (8), the following equation can be obtained: A plot of 0 ( 1 �( 2 − 1 )) (− 1 − 1 + 2 − 2 ) (denoted by "A") versus (acid concentration) should give a straight line. e results obtained were in agreement with this as shown in Figure 2. Linear regression of (9) gave an 2 of 0.98 assuming and to be nearly constant.
A short discussion of the practical utility of the present work may be relevant here. e optimized xylose production obtained in the present study (19.35 g/L) was higher than in previous reports [14,17]. For this level of production at the laboratory scale, the raw material cost was computed to be approximately $ 1.5 per kg xylose. is cost is quite low enough to indicate the economic feasibility of the process on the large scale. Now acid hydrolysis breaks down mainly the hemicellulose component of rice straw to release the pentose, mainly xylose, in the hydrolysate. Acid hydrolysis can also bring about deligni�cation of the straw to make the cellulosic component amenable to sacchari�cation to release hexoses such as glucose [15]. is sacchari�cation may be carried out enzymatically or by second step acid hydrolysis [15]. Now the �rst step of acid hydrolysis cannot be prolonged to release both pentoses and hexoses together as excessive treatment time decomposes the pentoses which are released �rst.
Regarding the utilization of rice straw, an agricultural waste, for production of value-added products, both pentose and hexose sugars are useful substrates for bioethanol production. Also, xylose can be used for xylitol production. Hence, xylose production is an important component in the complete utilization of rice straw.