Studies On Isolation Of Components From Rice Straw With Recyclable Mixed Solvents

With the increasing depletion of fossil resources, more and more attentions have been paid to utilization of biomass resources. The straw, as large output and short growth cycle has become truly inexhaustible and renewable resources. In order to make a high value use of this kind of biomass, it is quite necessary to separate the three main components, cellulose, hemicelluloses and lignin. In this thesis, we focus on the components separation of rice straw (RS) with recyclable mixed organic solvents.At first, we studied the influence of a water pre-treating process, by cooking and extraction, on the components in rice straw, and the results showed that the contents of both the hemicelluloses and lignin decrease, demonstrating the components could be initially separated with the pre-treatment of water. The influences of the catalysts in the system of ethylene glycol (EG)／water mixture on the efficiency of components separation were investigated. The experimental results indicated that the hemicelluloses are readily to be hydrolyzed, and the lignin is easily dissolved simultaneously under catalyzed by 0.1 mol/L sulfuric acid. It is efficient at delignification but fails to removal hemicelluloses with 0.1 mol/L sodium hydroxide as catalyst. Neither weak alkali nor Lewis acid can act as catalyst in removal of hemicelluloses and lignin from rice straw in the EG/water mixed solvent. The catalyzing mechanism is supposed as:under acidic condition, hemicelluloses are readily hydrolyzed because of the decreasing of the chemical bonds between lignin and hemicelluloses, as well as lignin becomes more soluble. The ester of lignin can be cleaved under alkali condition, and the condensation of a-C of lignin can be prevented by the methylation of phenylpropanol which is medium active group.In order to increase the efficiency of components separation, we combined the treatment of the solvents of ethylene glycol (EG)-water mixture with steam explosion (SE) pretreatment. The optimized conditions determined through orthogonal experiments are cooking with 80% concentration of EG aqueous solution under 100℃for 90 min. The components of the sample obtained under such conditions are holocellulose (includeα-,β-andγ-cellulose) of 94.15%, acidic insoluble lignin of 1.97%, ash of 3.88%, and the degree of polymerization (DP) is 287.45. We further attempt to put the two processes of components separation and bleaching in one stage for consideration of convenient operating. The SE pretreated rice straw was cooked with mixed solvent of ethanol-water-hydrogen peroxide to isolate cellulose and the by-products had also been recycled efficiently. The optimized conditions decided by the orthogonal experiments are temperature of 80℃, hydrogen peroxide of 2.0 g/120 ml, ethanol 80%(v%), reaction time of 10 h. The sample obtained under such conditions has been analyzed to contain holocellulose (includeα-,β-andγ-cellulose) of 95.61%, acidic insoluble lignin of 2.02%, ash of 2.37%, and its DP value is 1150.64.The mixture of EG-water-catalyst after using was recovered by vacuum distillation, with a recovery up to 96%, and it could be recycling application in separating components of RS without any additional agent or treatment, getting a result of better effect for hydrating hemicelluloses and similar effect for removal lignin as compared to the original mixture solvent. The ethanol was reclaimed from the used mixture of ethanol-water-hydrogen peroxide, with a recovery up to 90% by vacuum distillation, and it was found to be re-utilized to separate the RS in the event of adding proper water and hydrogen peroxide.