Rice Straw Tetrahydrofurfuryl Alcohol Pulping and Its Kinetics Studies for Biomaterial Application Development

• Main Authors: Chen-Lung Ho, 何振隆
• Other Authors: 蘇裕昌
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/52556160316162805702

博士 === 國立中興大學 === 森林學系所 === 100 === This study proceeded to pulp an agroforestry waste material, rice straw, using an atmospheric digestion by tetrahydrofurfuryl alcohol (THFA) and a hydrochloric acid catalyst. The pulp properties, delignification kinetics, cellulose and hemicellulose dissolution kinetics and derivation of empirical equations for the delignification, dissolution of cellulose and hemicellulose etc. The ultimate purposes of the study were to provide a basis for industrial pulping control of the system, and additionally to convert the pulp thus obtained to biomaterials. Henceforth, the optimal pulping conditions could be employed to produce pulp, and then the pulp was saccharified and fed to a microbial fermentation to produce lactic acid which eventually was synthesized into a biodegradable plastic of polylactic acid. And optimal conditions for synthesizing polylactic acid were sought. Firstly, the digestion characteristics, chemical properties of the pulp and handsheet physical properties of the THFA/HCl pulping were evaluated. As for the pulp yields, the method has high delignification specificity, at kappa number 20, the yield was ca. 60%, about 15-20% higher than the traditional alkaline pulping method. Furthermore, with increasing THFA concentrations, efficacies of delignification also increased. Increasing the catalyst dosage also caused an increase in delignification. During the cooking the dissolution of carbohydrate were low, at most 23%, consisted of mostly hemicelluloses, which was as high as 78% of the dissolved carbohydrates. The physical properties of the THFA pulp handsheets were inferior to those of the kraft pulp. The main reason was the damage to cellulose sustained during the acidic cooking condition. As for the delignification reaction kinetic study of THFA/HCl pulping of rice straw, 2 phases of different delignification rates were observed which were closely related to the amount of HCl and temperature. The cooking showed very low activation energy, which, in phase I and phase II, were only 26.5 kJ mol-1 and. 32.2 kJ mol-1, respectively. At phase I pulping, delignification was the main reaction with minimal dissolution of carbohydrate fractions. Consequently, the yield loss was low. In phase II, however, when the residual lignin content was less than about 5%, the dissolution of the carbohydrate was greater than the lignin removal, causing a marked reduction in pulp yield. Comparisons of KL/KC (ratio of delignification rate and rate of carbohydrate dissolution) and ΔL/ΔC (ratio of lignin removed and carbohydrate dissolved during pulping) values in phase I and II, were made. Results showed that both values were greater in phase I, and, both values reached their maxima at 120°C, and catalyst dosage of 0.020 mol L-1. Furthermore, in order to understand the behavior of carbohydrate fractions during the THFA/HCl pulping of rice straw, the dissolution kinetics of cellulose and hemicellulose were investigated. For both cellulose and hemicellulose fractions, the dissolution during pulping could be separated into two phases each. In the initial stage, or phase I, of cellulose dissolution, scarce amounts were solubilized; and in phase II, when delignification reached a level of approximately 85-90%, along with the increases in [HCl] concentration and cooking temperature, cellulose dissolution accelerated. The dissolution rate of hemicellulose, by contrast, with both increases in catalyst [HCl] concentration and cooking temperature, also accelerated; but the phase I dissolution rate was faster than the phase II rate. Comparing the experimental data with the predicted data, the pulp compositions, regardless of lignin, hemicellulose, or cellulose contents, all showed a high degree of correlation (with r > 0.99); thus proving that the derived kinetic equations were applicable to the process rationalization of THFA/HCl pulping of rice straw and in the control of pulp chemical compositions. The aforementioned reaction kinetics were complicated and tedious undertakings, however, therefore, simplified empirical equations were individually derived for the yield, delignification, and dissolution of cellulose and hemicellulose reactions. During the process, it was noted that except for the case of cellulose content which had a lower coefficient of correlation, with r > 0.80, in equations of yield, delignification and dissolution of hemicellulose, the r-values were higher (r > 0.90). In addition, the derived empirical equation produced values that all were within ±5% of the actual experimental values regardless of the yield, lignin, cellulose, or hemicellulose content. Thus, there were minor differences between the empirical and actual values, and the derived equations were adequate for use in the control of THFA/HCl pulping on rice straw. Finally, the best process control conditions were used to pulp rice straw, and the pulp was saccharified as a raw material to ferment into lactic acid. The optimal conditions of lactic acid production were found to entail a 60 g L-1 of hydrolyzed sugar solution using a microbe of Lactobacillus paracasei sub sp. paracase for 72 h. The high lactic acid production rate of this microbial strain indicated a potentially low purification cost. As for the synthesis of polylactic acid, the synthetic conditions entailed a reaction temperature of 140℃, and a catalyst stannous octanoate (Sn(Oct)2) concentration of 0.3 wt%.