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Hui, L., Xiong, H., and Du, K. (2014). "Optimum bioconversion process of wheat straw to glucose: Ethanol pretreatment," BioRes. 9(2), 3163-3173.

Abstract

The research aimed at optimizing the conditions of pretreatment of wheat straw and of enzymatic hydrolysis of the cellulose (ethanol pulp). The ethanol pretreatment process involving acid-catalytic and enzymatic hydrolysis were evaluated for bioconversion of wheat straw to glucose. The influence of the independent process variables on cellulose yields, cellulose contents, lignin contents, and the rate of lignin removed were analyzed over a broad range by the response surface methodology (RSM). The results of the factorial experiment showed that the significant external factors affecting acid-catalytic ethanol pretreatment of wheat straw were ethanol concentration, maximum temperature, acid dosage, and time at maximum temperature. By analyzing the response surface plots, the optimum process parameters for pretreatment were obtained as follows: ethanol concentration 65%, maximum temperature 180 ºC, acid dosage 1.2%, and time at maximum temperature 60 min. Pulps with residual klason lignin ranging from 9.27% to 13.56% (w/w) were prepared from wheat straw using the acid-catalytic ethanol pretreatment process and were evaluated for bioconversion using enzymatic hydrolysis of the cellulose fraction to glucose. The effects of temperature, pH value, time of enzymolysis, and cellulase dosage on the hydrolysis yield of cellulose were separately examined.


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Optimum Bioconversion Process of Wheat Straw to Glucose: Ethanol Pretreatment

Lanfeng Hui,* Huangwei Xiong, and Kunlun Du

The research aimed at optimizing the conditions of pretreatment of wheat straw and of enzymatic hydrolysis of the cellulose (ethanol pulp). The ethanol pretreatment process involving acid-catalytic and enzymatic hydrolysis were evaluated for bioconversion of wheat straw to glucose. The influence of the independent process variables on cellulose yields, cellulose contents, lignin contents, and the rate of lignin removed were analyzed over a broad range by the response surface methodology (RSM). The results of the factorial experiment showed that the significant external factors affecting acid-catalytic ethanol pretreatment of wheat straw were ethanol concentration, maximum temperature, acid dosage, and time at maximum temperature. By analyzing the response surface plots, the optimum process parameters for pretreatment were obtained as follows: ethanol concentration 65%, maximum temperature 180 ºC, acid dosage 1.2%, and time at maximum temperature 60 min. Pulps with residual klason lignin ranging from 9.27% to 13.56% (w/w) were prepared from wheat straw using the acid-catalytic ethanol pretreatment process and were evaluated for bioconversion using enzymatic hydrolysis of the cellulose fraction to glucose. The effects of temperature, pH value, time of enzymolysis, and cellulase dosage on the hydrolysis yield of cellulose were separately examined.

Keywords: Response surface methodology; Wheat straw; Acid-catalytic ethanol pretreatment; Cellulase; Enzymatic hydrolysis

Contact information: Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science and Technology, Tianjin 300222, China; *Corresponding author: hlfeng@tust.edu.cn

INTRODUCTION

During the past few decades, global warming has emerged as a major political and scientific issue. It is likely that this is due primarily to increased emissions of greenhouse gases formed by burning fossil fuels, concurrent with increased energy consumption (Dickerson and Johnson 2004). Various studies have shown that ethanol or ethanol-blended transportation fuels produce less harmful emissions, while the production of ethanol from biomass has the advantage of displacing transportation fuels derived from oil with a fuel obtained from a renewable resource (Bergeron 1996; Galbe and Zacchi 2002). Lignocellulose is an abundant and renewable feedstock. There is an estimated annual worldwide production of 10 to 50 billion dry tonnes, accounting for about half of the global biomass yield (Claassen et al. 1999).

Wheat straw is an important source of lignocellulosic biomass. However, in China, it is typically incinerated for power generation or used to make other low value-added products.

Pretreatment is an important stage for practical cellulose conversion processes. It is required to change the structure of cellulosic biomass and to make cellulose more accessible to the enzymes, so that hydrolysis of the carbohydrate fraction to monomeric sugars can be achieved more rapidly and with high yield. Various pretreatment options have been used to fractionate, solubilize, hydrolyze, and separate cellulose, hemicellulose, and lignin moieties. These different processes usually exploit a combination of chemical and physical treatments that serve to render the lignocellulosics more receptive to subsequent enzymatic hydrolysis (McMillan 2011).

Hydrolysis of cellulose to glucose can be achieved using either inorganic acids or cellulolytic enzyme. Chemical hydrolysis of biomass is relatively efficient and economic. However, it generates fermentation inhibitors (Leathers 2003). On the other hand, enzymatic hydrolysis, despite its relatively slow rate, is a biocompatible and environmentally friendly option. Thus enzyme hydrolysis is more attractive than others. The research described below is concerned with the evaluation of the organosolv process for the bioconversion of wheat straw. In this study, an acid-catalytic ethanol pretreatment process was used to pretreat the wheat straw. The process parameters were studied by using response surface methodology (RSM). Enzymatic hydrolysis was used to convert the cellulose to glucose.

EXPERIMENTAL

Materials and Methods

Pretreatment

The wheat straw used in this study was collected from the field at the harvest of the wheat crop growing at the farm of Hebei province, China. After being stored for three months, the wheat straw was cut into smaller pieces of approximately 30 mm length and sieved using screen of 14 mesh size. The cooking liquor was prepared by mixing water with industrial ethyl alcohol (concentration, 93% to 95%) from the market.

Wheat straw pulp was attained by acid-catalytic ethanol pretreatment process in a 15 L cooking reactor. A range of processing conditions (ethanol concentration, 45% to 65%; maximum temperature, 175 to 195 ºC; time at maximum temperature, 45 to 75 min; sulfuric acid dosage 0.8% to 1.2%; the ratio of liquor to wood, 6:1) were chosen to provide pulps with residual lignin contents varying from 9.27% to 13.56% (w/w).

Design of experiments

In this study, a Box-Behnken design (BBD) was used to evaluate the main and interaction effects of the factors: ethanol concentration (X1), cooking temperature (X2), catalyst dosage (X3), and time at maximum temperature (X4). The range and levels of the variables investigated are given in Table 1, whereas the experimental designs are presented in Table 2. A polynomial quadratic equation was fitted to evaluate the effect of each independent variable to the response:

 (1)

In this expression Y is the response; xi and xj are the coded independent variables, and β0βiβii, and βij are intercept, linear, quadratic, and interaction constant coefficients, respectively. Response surfaces and contour plots were developed using the fitted quadratic polynomial equation obtained from regression analysis.

Table 1. Independent Variables Used in the BBD and Actual Factor Levels Corresponding to Coded Factor Levels

Table 2. Experimental Design

Enzymatic hydrolysis

Commercial cellulase was provided by Novozyme (Tianjin, China). Cellulase activity was determined using the filter paper assay and was expressed as filter paper activity (FPA). Batch hydrolysis experiments were routinely conducted at 2% (w/w) cellulose consistency in 50 mL of 100 mM sodium acetate buffer, pH 4.8. Reaction mixtures were incubated at 160 rpm in a rotary shaker and sampled periodically for sugar analysis using SBA-40C Biosensing analyzer.

Analytical Methods

The lignin content of the wheat straw before and after pretreatment was determined using the following TAPPI standards: T 17 wd-70 for cellulose content, T 222 om-02 for lignin content.

The following equation was used to evaluate the cellulosic yield:

 (2)

RESULTS AND DISCUSSION

Ethanol Pretreatment

The response surface plots of each of the two factors and their interactive effect on the cellulose contents, cellulose yield, lignin contents, and the rate of lignin removal were obtained by use of an SAS package (Fig. 1(a-d) and Fig. 2(a-d) ). The four independent variables designated as X1, X2, X3, and X4, respectively, are ethanol concentration (X1), cooking temperature (X2), catalyst dosage (X3), and time at maximum temperature (X4).

Fig. 1. (a) Response surface of ethanol concentration (X1) vs. maximum temperature (X2) on the cellulose contents

Fig. 1. (b) Response surface of ethanol concentration (X1) vs. maximum temperature (X2) on the cellulose yield

Fig. 1. (c) Response surface of ethanol concentration (X1) vs. maximum temperature (X2) on the lignin contents

Fig. 1. (d) Response surface of ethanol concentration (X1) vs. maximum temperature (X2) on the extent of lignin removed

Effect of maximum temperature and ethanol concentration

From Fig. 1(a-d), it follows that changes in the cellulose contents, cellulose yield, lignin contents, and the rate of lignin removed with maximum temperature and ethanol concentration can be estimated by keeping acid dosage and time at maximum temperature constant. Thus, with a center acid dosage (1.0%) and a center time at maximum temperature (60 min) the maximum temperature and the ethanol concentration both had a great impact on all of the response variables.

As Fig. 1 shows, the cellulose contents, cellulose yields, and the rate of lignin removal increased and the lignin contents decreased quickly at the beginning of the increasing maximum temperature. However, this tendency of cellulose yield stopped and started to go to in the opposite direction when the maximum temperature reached approximately 180 ºC. However the cellulose contents increased with the maximum temperature. All the figures above show that the higher the ethanol concentration was, the better the values of respond variables were. So the ethanol concentration was determined at 65% as an optimum condition in the next experiment. In this case it could be supposed to accelerate the removal of lignin and the dissolution of carbohydrate.

From the foregoing, it thus follows that all the responding variables depended markedly on ethanol concentration and maximum temperature. Accordingly, the optimum maximum temperature and ethanol concentration were judged to be 180 ºC and 65%, respectively.

Effect of time at maximum temperature and acid dosage

From Fig. 2, it follows that changes in the cellulose contents, cellulose yield, lignin contents, and the extent of lignin removed with acid dosage and time at maximum temperature can be estimated by keeping ethanol concentration and maximum temperature constant. Thus, with a center maximum temperature (185 ºC) and a center ethanol concentration (55%), the acid dosage and time at maximum temperature had little effect on all of the response variables.

Fig. 2. (a) Response surface of acid dosage (X3) vs. time at maximum temperature (X4) on the cellulose contents