Bioconversion of corn stover into value-added chemicals: Dilute sulfuric acid pretreatment, xylo-oligosaccharides production, and lactic acid fermentation.

摘要

In the first part of this dissertation, dilute-acid pretreatment of corn stover was investigated under high-solids condition using a percolation reactor. High-solids condition in a pretreatment process can lead to high productivity and low liquid throughput. The latter reduces the cost of process energy as well as the load of wastewater treatment. The effects of temperature, acid concentration, acid flow rate, and reaction time on the glucan digestibility and recovery of hemicellulose sugars were investigated. The experimental results were assessed to determine the optimum ranges of reaction conditions. The xylose yield was affected sensitively by the flow rate under a given reaction condition. This behavior appears to be related to sugar decomposition, mass transfer resistance and the fact that acid is neutralized by the buffering components of the biomass. Further improvement of this process was sought with modifications in the operation mode of the percolation reactor. The reactor was preheated under atmospheric pressure to remove moisture that may cause autohydrolysis and subsequent sugar decomposition. In addition, liquid throughput was minimized to the extent that only one reactor void volume of liquid was collected. This was done to attain high xylose concentration. For the substrates treated under the optimum reaction and operating conditions, near quantitative glucan digestibility was obtained while the decomposition of carbohydrates was suppressed to an extremely low level.; The second part of this dissertation dealt with enzymatic production of xylo-oligosaccharides (XOs, also known as xylo-oligomers) from corn stover and cobs. The process started with a pretreatment known as soaking in aqueous ammonia (SAA). The pretreated feedstock containing digestible xylan was then directly subjected to selective enzymatic hydrolysis for the production of XOs. The complex substrate purification step required in the conventional method is not necessary in this process. This simplifies the overall process, and more importantly, improves the process economics. In the subsequent stage, fractionation and refining of XOs were accomplished by charcoal adsorption followed by ethanol elution, with XOs being collected in relatively high yields. Xylanolytic hydrolysis of the SAA treated corn stover has shown high digestibility of the remaining glucan. As a feedstock for production of XOs, corn cobs are superior to corn stover because of high xylan content and density. In the last part of the dissertation, production of lactic acid by simultaneous saccharification and co-fermentation (SSCF) was investigated. (Abstract shortened by UMI.)