Computer-Aided Solvent Design for Extractive Fermentation Processes with Cell-recycling

• Main Authors: Hou-Chieh Cheng, 程厚捷
• Other Authors: none
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/12297184056097072527

博士 === 國立中正大學 === 化學工程所 === 96 === In this study, a systematic framework for the design and analysis of solvents in extractive fermentation-distillation systems is developed using a computer-aided molecular design (CAMD) technique. The article framework consists of performances of fermentation process, solvent evaluation, and solution strategy approaches. Bioethanol is a bulk chemical and must carry out continuous fermentation to achieve economic and beneficial production. Achieving high productivity of ethanol continuous fermentation requires retaining high cell density culture and maximizing the dilution rate. Continuous fermentation with cell recycling can increase ethanol productivity; however, such a high ethanol concentration may poison viable microorganisms and abrogate the fermentation process. Extractive fermentation is a technique that solvent can be used to reduce the end product inhibition through the use of water-immiscible phase that removes the fermentation product in situ. However, the toxicity of the organic solvent used to remove the end product in reaction media is always a problem. A good selection of the solvent will definitely benefit the fermentation process. In this study, crisp and fuzzy optimization approaches are respectively introduced to design an optimal biocompatible solvent for an extractive fermentation process. The optimal design problem is formulated as a mixed-integer nonlinear programming model in which performance requirements of the compounds are reflected in the objective and the constraints. In general, the requirements for the objective and constraints are not rigid; consequently, the fuzzy optimization approach is applied to soften the rigid requirement for maximization of the extraction efficiency and to consider the mass flow rate of solvent as the softened inequality constraints to the solvent design problem. Having elicited the membership function for the objective function and the constraint, the optimal solvent design problem can be formulated as a fuzzy goal attainment problem. Mixed-integer hybrid differential evolution is applied to solve the problem in order to find a satisfactory design. When designing a biocompatible solvent for the extractive fermentation process, many issues, such as extractive efficiency, conversion, amount of solvent utilized and so on, have to be considered. An interactive multiple-goal design procedure is introduced to determine a trade-off result in order to satisfy such contradicted goals. In this study, crisp and fuzzy multiple-goal optimization approaches are respectively introduced to design an optimal biocompatible solvent to a two-stage extractive fermentation with cell recycling for ethanol production. Both approaches could be iterated to solve the interactive multiple goal design problem in order to yield a trade-off result. However, the crisp optimization design is a tedious task that requires the designer to provide various pairs of the upper bounds for the design problem to obtain the corresponding solution. The fuzzy optimization approach is able to be trade-off several goals simultaneously and to yield the overall satisfactory grade for the product/process design problem. For achieving high ethanol productivity, we compare the process performance between single and double stage extractive fermentation with cell-recycling. The double stage process could use a smaller amount of fresh solvent to increase more ethanol productivity than that of the single stage process, but this would also decrease overall conversion. Comparing the two designs for the double stage process, process design and the simultaneous process/solvent design, the simultaneous design could yield higher overall ethanol productivity than the process design. Moreover, the maximum ethanol production rate of the double-stage extractive fermentation with cell recycling was about ten-fold higher than that of the continuous fermentation and about two-fold higher than the continuous fermentation with cell recycling. Based on anhydrous ethanol production of great bulk, besides raising the ethanol production rate by extractive fermentation technique, it is still a great challenge how to purify fermentation product that mainly consist of ethanol/water mixture forms a minimum-boiling azeotrope. For the standpoint of solvent characteristics, the use of an extractive solvent similarly could also enhance the volatility difference between the components to be separated in distillation column. In the other word, extractive distillation is an alternative technique for purifying ethanol fermented product in order to achieve anhydrous ethanol production. In this study, using CAMD technique is proposed to design suitable solvents for an integrated extractive fermentation and extractive distillation process in order to achieve anhydrous ethanol production.