| A two-zone bioreactor was developed for simultaneous saccharification and extractive fermentation of lignocellulosic materials into lactic acid. The system is composed of an immobilized cell reactor, a separate column reactor containing the lignocellulosic substrate and a hollow-fiber membrane. It is operated by recirculating the cell free enzyme (cellulase) solution from the immobilized cell reactor to the column reactor through the membrane. The enzyme and microbial reactions thus occur at separate locations, yet simultaneously. This design provides flexibility in reactor operation. It simplifies the separation of the solid substrate from the microorganism. It makes a fed-batch or continuous operation feasible. Most importantly, the simultaneous product removal of inhibitory lactic acid can be easily achieved in this system because the product stream is free of microorganism and solid substrate. This reactor system was tested using pretreated switchgrass as the substrate. It was operated under a fed-batch mode with continuous removal of lactic acid by solvent extraction. The overall lactic acid yield obtainable from this bioreactor system is 77% of the theoretical.; Recent advancement in acid hydrolysis reactor technology has progressed to the point where it can compete with the enzymatic process. The focus of the new development is the emergence of a counter-current shrinking-bed reactor. A theoretical model was developed for a counter-current shrinking-bed reactor. The results of the model strongly support the earlier findings of the NREL's experimental work based on bench scale simulated counter-current reactor system. The simulation results of bed-shrinking reactor predict that glucose yield above 90% and near quantitative pentose yield (97–99%) with sugar concentrations between 2.0–3.0 g/100 mL for both, are attainable under optimum set of operating conditions in acid hydrolysis of cellulose and hemicellulose respectively.; A novel aspect of a well-known Lactobacillus strain was uncovered that it can ferment xylose as efficiently as glucose. This strain is a registered organism, extremely stable upon long term operation. A pH level about 6.0 was found to be the optimum. The yield (g/g) of lactic acid obtained from xylose is in excess of 80% with initial volumetric productivity of 0.38 g/L hr. Very low amounts of side-products, acetate and ethanol are detected. It shows high tolerance for lactic acid as well as extraneous toxins. In addition to xylose, it can ferment all other minor sugars in hemicellulose except arabinose. |
