Optimization of a Fed-Batch Simultaneous Saccharification and Cofermentation Process from Lignocellulose to Ethanol

This paper presents the design of optimal control policies for a fed-batch process of simultaneous saccharification and cofermentation to produce ethanol from lignocellulose using enzymes and a recombinant yeast. The goal of the optimal design herein is to find the optimal feed rate, temperature, feed lignocellulosic concentration, and fermentation time that maximizes the ethanol production rate under the constraints of operational restrictions. We applied an Arrhenius relationship to each rate constant to express the temperature effect on the kinetic model for both saccharification and fermentation. Good bioreactor performance not only is defined as the maximum production rate yielded but also considers follow-up separation specifications as the constraints for the optimization problem. Additional restrictions were also considered in the optimization problem to cope with operating costs and environmental impact. Both crisp and fuzzy optimization methods are applied to solve the problem. The fuzzy optimization approach more flexibly obtains a compromised design compared with the crisp optimization method.