Bioconversion of lignocellulosic substrate into lactic acid: Pretreatment and extractive fermentation.

摘要

This research work was focused on the improvement in pretreatment of biomass, and the direct bioconversion of pretreated biomass into lactic acid. The kinetic study on dilute acid pretreatment has proven that hemicellulose in corn cobs/stover mixture (CCSM) is biphasic. For the first time, the soluble xylose-oligomer was considered in the kinetics of dilute acid pretreatment. The kinetic parameters were determined covering temperature of 120-150{dollar}spcirc{dollar}C, and sulfuric acid concentration of 0.44-1.90%. With incorporation of the kinetic data, the modeling on a non-shrinking percolation reactor operated under various modes was conducted. The results affirmed that temperature step-change operation increased the product yield by 2.9-10.9% over that of uniform temperature operation. Further improvement was seen with the application of a two-stage, reverse-flow operation which has essentially doubled the sugar concentration over that of the temperature step-change operation.; The reactor simulation was extended to cover a shrinking bed, two-stage reverse-flow operation. The results have shown that this operation increases the sugar yield over a non-shrinking bed operation by about 5% at {dollar}tau=1.0,{dollar} and 2.8% at {dollar}tau=1.5.{dollar} An optimum flow rate of liquid exists at a given {dollar}tau.{dollar} It was found that fast portion of hemicellulose is almost completely hydrolyzed after the first stage reaction. Most of the slow portion of hemicellulose is hydrolyzed in the second stage. For {dollar}tau=1.5,{dollar} the bed shrinkage was about 27% after completion of the hydrolysis of hemicellulose in CCSM. About three quarters of the total shrinkage occurred after the first stage. The decomposition of sugar product during the hydrolysis took place primarily in the high temperature stage.; The bioprocess section of this study was focused on lactic acid production from cellulosic biomass by cellulase and Lactobacillus delbrueckii via simultaneous saccharification and fermentation (SSF). The optimal conditions were found to be pH 5.0 and temperature 46{dollar}spcirc{dollar}C for the SSF. The feasibility of incorporating a microporous hollow fiber membrane as an extractor into the SSF reactor was studied. The bioreactor system was operated under a fed-batch mode with continuous removal of lactic acid by an in-situ extraction. With the consideration of solvent extraction capacity, toxicity and viscosity, a solvent mixture of 20% Alamine 336, 40% oleyl alcohol, and 40% kerosene was found to serve as an effective extractant. Progressive change of pH (from 5.0 to 4.3) has significantly improved the overall performance of the extractive fermentation. The mass transfer resistance on shell side was found to be dominant in the extraction process.