Modelling and Optimisation of Integrated Biotechnical Processes -Application to the Fermentative Production of Lactic Acid from Wheat Flour

Sammanfattning: Biotechnical processes are often integrated to increase productivity and yield. It is difficult to predict the effects of process integration and to find optimum conditions for the process. A valuable tool for the evaluation of a complete process is mathematical modelling. In this thesis, a computer program developed for simulation and optimisation of integrated biotechnical processes is presented. The computer program can be used for simulation, parameter fitting, optimisation, economical evaluation and Monte Carlo simulation. The computer program has been used to evaluate a process for the fermentative production of lactic acid from wheat flour. In this process, wheat starch is hydrolysed to glucose in two steps: liquefaction and saccharification. The glucose is fermented to lactic acid by lactic acid bacteria, using components from the wheat flour as nutrients. Fermentation is performed at a pH higher than the pKa for lactic acid and the end product of the fermentation step is thus mainly lactate. In the downstream processing the lactate is separated from cells and wheat flour particles using centrifugation and membrane techniques. The lactate is converted to lactic acid by electrodialysis and is concentrated by evaporation. The work presented in this thesis comprises the development of kinetic models for the saccharification of wheat starch and for the fermentation of glucose to lactic acid. The computer program has been used to determine the values of the parameter in the kinetic models, using experimental data. These models have been used to study the integration of the saccharification and fermentation steps. The computer program has also been used to perform an economical evaluation of the entire process for the production of lactic acid with a concentration of 70 wt-%. A number of process configurations have been suggested and compared, and the optimal wheat flour concentration has been determined. To investigate the influence of variations in operational and investment costs on the lactic acid production cost, a Monte Carlo simulation has been performed.