稻草四氫呋喃醇製漿及動力學研究在生物材料上之開發應用

Abstract

本論文進行農林廢棄物-稻草以有機溶劑製漿法中之常壓四氫呋喃醇(tetrahydrofurfuryl alcohol, THFA)製漿法，分別探討漿料性質、脫木質素反應動力學、纖維素及半纖維素的溶出反應動力學及脫木質素、纖維素及半纖維素溶出量之經驗模式(empirical model)推導等項目，其最終目的為作為工業上製漿製程控制之依據。再者，為使所得漿料，予以轉化為生質材料，故利用製漿動力學所得之製漿製程控制最佳條件(即所得最多纖維素含量之漿料)予以製漿，再將所得漿料糖化後，利用微生物進行醱酵並生成乳酸，以評估微生物醱酵條件及乳酸生成量等。並使用所產生乳酸，進行可生物分解性塑膠--聚乳酸合成，以尋求其合成聚乳酸最適條件。 首先評估此製漿法蒸解特性、漿料化學性質及手抄紙物理性質。於漿料收率方面，此法脫木質素效率高且其於卡巴值為20時，所得收率為60%左右，較一般傳統鹼性製漿法高出15-20%或以上，且隨THFA濃度及催化劑HCl添加量的增高，脫木質化效率亦呈增加之趨勢。而此蒸煮法於蒸煮初期脫木質素效率非常高，可達75%以上。蒸煮時碳水化合物溶出率低，最高僅達23%，而其中以半纖維素較纖維素有較多的溶出量，最高可達78%。但THFA稻草漿於手抄紙之物理性質較傳統硫酸鹽製漿法(kraft pulping, KP)紙漿手抄紙之性質為差，其原因為紙漿中之單纖維強度在酸性蒸解下，導致強度較弱所造成。 於THFA/HCl製漿法脫木質素反應動力學之探討方面，稻草的THFA/HCl蒸煮過程中，可分為二相。此THFA/HCl蒸煮稻草漿活化能非常低，於phase I僅為26.5 kJ mol-1，故顯示其易脫木質素特性。於蒸煮初期，主要為脫除木質素，對於碳水化合物溶解較少，故使收率降低幅度非常少，但於phase II，即當殘餘木質素含量約< 5%時，其碳水化合物之溶出量大於脫除木質素量，導致收率降低幅度非常大。此法較一般化學製漿法具有較快脫木質素反應，且低活化能等特性，顯示本製漿法具有相當優良之蒸煮性。由KL/KC (脫木質素反應速率/碳水化合物溶出速率)及ΔL/ΔC (脫木質素量/碳水化合物溶出量)值的比較方面，均為phase I大於phase II，且於phase I時，蒸煮溫度120℃，催化劑濃度為0.020 mol L-1二值均為達最大值，顯示在蒸煮溫度為120℃時，蒸煮藥液(催化劑)其主要為攻擊木質素，無論是脫木質素速度及脫去木質素量均大於碳水化合物。 再者，為瞭解此製漿法之碳水化合物溶出舉動，故探討纖維素及半纖維素的溶出反應動力學。纖維素及半纖維素的溶出反應，均可分成二相，於纖維素phase I，纖維素溶出率非常之少，於phase II方面，當脫木質素量約為85-90%左右，隨著催化劑[HCl]濃度及蒸煮溫度增加時，纖維素溶出率有增多之現象。半纖維素溶出反應，隨著催化劑[HCl]濃度及蒸煮溫度增加時，半纖維素溶出率有增快之傾向，phase I之溶出速率均較phase II為快。再者，將所推導之動力學方程式與試驗數據相較，均呈現高度相關性(r > 0.99) ，故所推導之動力學方程式可應用於此THFA/HCl之蒸煮控制。 然而，上述所推導反應動力學方程式為一複雜及繁瑣工程，因此以簡易方式，分別推導出收率、脫木質素、纖維素及半纖維素溶出量之經驗式。於推導中得知，除纖維素含量之經驗式中，其r值呈現較低(r > 0.80)之現象之外，其餘經驗式中，其r值均呈現較高之現象(r > 0.90)。再者，將所推導之經驗式與試驗數據相較，均為在 ± 5%誤差範圍內，故由此可知，顯示二者差異非常少，故所推導之公式可應用此THFA/HCl之蒸煮控制。 最後，將製漿動力學所得之最佳條件製漿作為生物材料的前處理方法。所得漿料，經糖化後所產生醣類為原料醱酵成乳酸最佳條件為水解糖濃度60 g L-1，菌株為Lactobacillus paracasei subsp. paracase，醱酵時間72 hr。於聚乳酸合成最佳條件為反應溫度140℃，催化劑辛酸亞錫(Sn(Oct)2)濃度0.3 wt%為最佳。

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%.