Study Of Hydrothermal Pretreatment Of Lignocellulosic Biomass And Continuous Effect Membrane Distillation Process

In biochemical conversion process from lignocellulose to ethanol, thepretreatment step, as the core reaction unit in the whole process, plays a decisive rolein the efficiency of subsequent enzymatic hydrolysis and fermentation steps. Theseparation unit for concentrating prehydrolyzate, enzymatic hydrolyzate and processwastewater containing non-volatile solutes generated in the whole process, whoseperformance also plays a vital role in the efficiency of energy utilization and recycleof utility water. Focusing on the above two key unit operations, in this dissertation,how the chemical composition and ultrastructure of plant cell wall change withdifferent pretreatment conditions, and how these changes impact the subsequentbiochemical conversion were studied. Research related to development ofenergy-efficient concentration process in order to achieve high performance ratio wasalso conducted. The objective of this dissertation was aimed at establishing a highlyefficient route and platform that could convert lignocellulose to ethanol.Hydrothermal processing of corn stover, as a pretreatment step, caused a varietyof effects including fractionation of glucan, xylan and lignin from whole biomassmatrix and modification of physicochemical properties in both solid and liquidfractions. A modified severity factor for non-isothermal pretreatment with asymmetrictemperature curve was successfully applied as a reaction ordinate to evaluate differentpretreatment conditions. In light of material balance and multiple characterizationmethods, the influence of process parameters including severity facor, solid loadingand pH on the kinetic behaviors of major components in lignocellulose duringhydrothermal pretreatment and these influences on the subsequent bioconversion werestudied. It was found that a trade-off between glucose and xylose yield existed aftereither aqueous/steam pretreatment followed by enzymatic hydrolysis, but this couldbe eliminated by introduction of the third parameter, pH. High yields in glucose(>90%) and xylose (>85%) were simultaneously obtained. Hydrolysis, degradationand condensation reaction involved in xylan chemistry and hydrolysis of lignin werestrongly pH-dependent. Soluble xylo-oligmers in the prehydrolyzate and somedegradation products were verified as the inhibitory compounds to enzymatichydrolysis but behaved in a different way. Furan derivatives and some aromatic aldehydes/ketones/acids were assumed to be key potential fermentation inhibitors.Aqueous pretreatment controlling its pH at a near neutral value showed advantagesover that without pH control. Based upon the quantitative and qualitative informationon destruction of plant cell wall, a clear and systematic picture of mechanisims ofhydrothermal pretreatment was finally provided.A continuous-effect membrane distillation (CEMD) process was developed byequipping air gap membrane distillation (AGMD)-based and strictly-parallel hollowfiber module with internal heat recovery. Within the studied experimental range, themaximum PR of13.8was obtained. A face-centered central composite experimentaldesign was conducted to investigate the influences of operating variables includingcold-feed temperature, hot-feed temperature, and feed-in flow rate on the performancewhich was indicated by flux, performance ratio and evaporation efficiency. Atheoretical model based on governing transport equations was established to predictthe process performance and the model described the experimental data fairly well. Inlight of model, possible ways to further increase PR were predicted. The diluteaqueous sugar solution was successfully concentrated12-fold to a final concentrationof about20%wt by using CEMD process with a final PR of8.2, and the goal toestablish the high energy-efficiency concentration process was achieved.