Fundamental Study on Kinetics of Hemicellulose Hydrolysis and Bioconversion of Hemicellulose Hydrolysate Mixture into Lactic Acid

Abstract

In the first part of this dissertation, the kinetics of hemicellulose hydrolysis in dilute-acid hydrolysis of Kramer Corn Stover (KCS) under high-solid conditions was investigated. Dilute-acid pretreatment is one of the most advanced and widely accepted pretreatment technologies. The National Renewable Energy Laboratory (NREL) has developed a continuous auger-driven pretreatment reactor that can be operated with high-solid charge. It operates at high temperatures and with short residence times resulting in high productivity and sugar concentrations above 10 wt%. We investigated the kinetics of the reactions occurring in dilute-acid pretreatment of KCS, covering the reaction conditions similar to those of the NREL reactor. For this purpose, batch experiments were performed under high solid conditions and the data were put into kinetic model. The conventional bi-phasic kinetic model was initially adopted: two different fragments of hemicellulose to oligomers, to xylose, and to decomposition products. In the model verification process, we observed consistent discrepancies in which xylose and its oligomers were under-estimated during the latter phase of the reactions. It was then speculated that side reactions may have occurred, likely the recombination of xylose and its oligomers with soluble lignin forming lignin-carbohydrate complex (LCC). In subsequent investigations, we positively identified the presence of LCC in the pretreatment liquid by Nuclear Magnetic Resonance (NMR) and by Fourier Transform Infrared (FTIR). The kinetic model was therefore modified incorporating the side reactions. The revised model has shown close and consistent agreement with the experimental data. Although it is a simplified empirical model, the experimental data yielded reliable degree of confidence in the Arrhenius plots for all of the rate constants. This model can thus serve as a useful tool for optimal design and operation of dilute-acid pretreatment reactors in NREL. Although the continuous reactor is designed to behave as a plug flow reactor (PFR), the residence time distribution (RTD) data obtained from tracer tests have shown that the flow pattern in the reactor deviates significantly from PFR. The kinetic information was put into the reactor performance model incorporating the RTD information. The results were further analyzed to assess the effects of dispersion (RTD) on the performance of the NREL-continuous reactor. The kinetic data, the model procedure, and the output of reactor performance (yield of xylose and oligomers) as it relates to the degree of dispersion are presented, and the strategy to improve the reactor performance is discussed. In the second part of this dissertation, secondary hydrolysis of hemicellulose liquor from dilute-acid pretreatment of KCS was investigated. Dilute-acid pretreatment often generates substantial amounts of xylose oligomers. Those oligomers need to be hydrolyzed to monomer for it to be effectively utilized in the bioconversion process. A post treatment, secondary hydrolysis, is therefore necessary. Because of the unique features of the NREL-continuous reactor, namely high solid loading, low acid content, and short residence time, the hydrolysis of hemicellulose was limited. The pretreatment liquor (often termed as pre-hydrolysate) obtained from this reactor thus contains relatively high amounts of xylose oligomers, typically in the range of 10-40% of the total sugar. In order to fully utilize the sugars in the pre-hydrolysate, oligomers need to be hydrolyzed to monomers. However, the secondary hydrolysis of this liquor was found to be much more difficult than hydrolysis of xylose oligomers in a clean environment. This investigation was undertaken to verify the factors inhibiting the acid-catalyzed oligomer hydrolysis. For this purpose, the secondary hydrolysis was experimentally studied using rapid-heating small-scale batch reactors. The experiments were done for pre-hydrolysate and for clean xylose oligomers. The comparison of the results indicated that a significant fraction of oligomers may be bound to lignin fragments, and some of it may also exist in branched form. Formation of LCC was also proven to be a significant factor in oligomer hydrolysis and xylose degradation. The third part of the dissertation deals with the production of lactic acid from the mixture of pre-hydrolysate and paper mill sludge by simultaneous saccharification and co-fermentation (SSCF). Paper mill sludge is a solid waste material composed of pulp residues and ash generated from pulping and paper making process. The carbohydrate portion of the sludge from a Kraft mill has chemical and physical characteristics similar to those of commercial-grade wood pulp. Because of its high carbohydrate contents and well-dispersed structure, the sludge can be biologically converted to value-added products without pretreatment. In bioconversion of solid feedstock such as paper mill sludge, a certain amount of water must be added to attain fluidity. In this study, hemicellulose pre-hydrolysate, in place of water, was added to the sludge to increase the concentration of the final product. Pre-hydrolysate was obtained by hot-water treatment of pine wood in which the total sugar concentration reached 4 wt. %. The mixture was subjected to SSCF using enzymes (cellulase and pectinase) and Lactobacillus delbrueckii (ATCC-10863). Pectinase was added to convert mannose oligomers in the prehydrolysate to monomers. During the SSCF of the mixture, calcium carbonate existing in the paper sludge acted as a buffer keeping the pH to near optimum. The overall product yield on the basis of total carbohydrate content of the initial feed ranged 80-90% of the theoretical maximum. Use of the mixture of pre-hydrolysate and pulp mill sludge as the fermentation feed also increased the product concentration to 60 g of lactate/L.