Preparation Of Pine Dissolving Pulp And Study On The Prehydrolysates

The demand for dissolving pulp has been increasing for the past10years. Dissolving pulp prices have appreciated with the demand, attracting some producers to convert paper grade pulp to dissolving pulp. In the production of high purity dissolving pulps, both lignin and hemicelluloses must be extensively removed, in order to ensure high purity of alpha-cellulose; the extracted hemicellulose can be separated and conversed into high value-added products. In the work described in this thesis, the hot water pre-hydrolysis kraft process was used to produce pine dissolving pulp. The chemical compositions of pine wood, constituents of carbohydrates and inhibitors in prehydrolysates were analyzed; the extraction process of hemicelluloses, cooking and bleaching processes of pre-hydrolyzed wood chips were studied; the degradation of carbohydrates was discussed and its degradation kinetic models were established.The pine wood chips were pre-hydrolyzed in hot water, and degradation of carbohydrates was explored in different temperature and time. It has been found that the fast-to-slow order of polysaccharides dissolution is araban, galactosan, mannan, xylan and glucan. Elevating temperatures and extending time could improve removal efficiency of hemicelluloses, but inhibitors will have a substantial increase in prehydrolysates. Besides, holocellulose content in pre-hydrolyzed wood chips remains same essentially, instead, lignin content increase after90min at170℃. Therefore, the pre-hydrolysis conditions were chosen as:temperature of170℃, time of90min and liquid to solid ratio of4to1. Under this circumstance,50～60%of mannan and xylan are still retained in pine wood chips.After analysis of carbohydrates contents both in raw material and pre-hydrolyzed wood chips, it has been found that manan and xylan are the main hemicelluloses. The first-order degradation kinetic models were established for arabinose, galactose, glucose, xylose and mannose, whose apparent activation energy is57.34,79.30,97.18,106.03and98.01KJ mol-1, respectively.Analysis of carbohydrates in prehydrosates showed that arabinose mainly exists in form of monosaccharide, while galactose, xylose, mannose and glucose are in form of oligosaccharides. In prehydrosates, there are inhibitors like furfural, hydroxymethylfurfural (HMF), acetic acid and acid-soluble lignin. Furfural and HMF are products from dehydration of pentose and hexose, and acetic acid comes from acetyl shedding off from hemicelluloses.The prehydrolyzed wood chips were cooked in Kraft method. Effects of alkali dosage, cooking time and cooking temperature on cooking results were discussed. According to requirements of Kappa values, viscosity, yield and residual alkali concentration for cooking pulps, the bestcooking conditions were identified. Effects of prehydrolysis conditions on cooking results were discussed, and the results showed that Kappa value of cooking pulp, residual alkali concentration of black liquor increased gradually with prehydrolysis time, while pulp yield, pentosan content and viscosity decreased gradually with prehydrolysis time, alpha-cellulose content of pulp increased firstly and then decreased slightly along with prehydrolysis time. Analysis of the cooking pulp carbohydrate content showed that the main substance in the pulp is glucose, arabinose and galactose have been completely eliminated, and there are a small amount of xylose and mannose.A two-stage oxygen delignification and a D0EPD1D2A sequence were performed to cooking pulp. It is found that the efficiency of two-stage oxygen delignification can reach60to70%. After ECF bleaching, a dissolving pulp with high brightness, high alpha-cellulose content, low viscosity and low iron ion content has been prepared, and all the quality specifications are superior to the softwood dissolving pulp industry standard of Grade A.