Membrane separations in ionic liquid assisted processing of lignocellulosic biomass

摘要

2nd generation biofuels currently hold a significant market share. With increasing impact of biofuel its production routes have to be optimized in terms of CO2 emissions, competition with the food chain and utilization of the whole plant. The cluster of excellence "Tailor-made Fuels from Biomass" investigates processing of lignocellulosic biomass to next generation biofuels. Complete utilization of the raw material is achieved by initial separation of its constituents cellulose, hemicellulose and lignin under mild conditions. These three fractions can be chemically or biocatalytically converted under independently optimized conditions. This allows for the complete conversion of the plant material to valuable fuel compounds or side-products such as itaconic acid. The fractionation of wooden biomass is performed with ionic liquid. Ionic liquid allows for the disintegration of the strong ligno-cellulose bonds, resulting in the dissolution of the raw material. A feasible fractionation of the constituents is part of the process design. Cellulose, for instance, is separated from the solution via precipitation with water. The cellulose fraction is than hydrolysed to glucose, which is fermented to itaconic acid. Itaconic acid serves as intermediate for the formation of 2-MTHF (2-methyltetrahydrofurane), a prospective fuel candidate. In this thesis the conversion of cellulose to glucose downstream of the wood dissolution process in Ionic Liquid is discussed. This sub-process comprises the pretreatment of cellulose with ionic liquid to reduce its crystallinity. The amorphous cellulose is than enzymatically hydrolysed to glucose. The separation of the intermediate glucose from the reaction mixture is performed via multiple membrane seperation processes, namely ultrafiltration, nanofiltration and electrodialysis. The separation of glucose from residual saccharides in ionic liquid/water mixtures is carried out with nanofiltration. The mass transport of glucose across solvent-resistant nanofiltration membranes is modeled with the Maxwell-Stefan approach. The semi-empirical model, based on systematic experimental results, allows for the prediction of the glucose yield from the nanofiltration as a unit operation. The complete subprocess is economically evaluated with respect to a nearly complete recycle and dehydration of ionic liquid. In the outlook nanofiltration of highly concentrated ionic liquid solutions stemming directly from the wood dissolution process is discussed as well as the dissolution of wooden biomass with alternative solvent systems.