(575e) Multi-Objective Optimization of Biobutanol Production: Continuous Fermentation Coupled With Vacuum Separation Unit

Butanol produced via acetone-butanol-ethanol (ABE) fermentation of biomass, is a liquid biofuel that offers many advantages over other biofuels such as bioethanol and is targeted to partly replace gasoline. In ABE fermentation process, a strain of bacteria such as Clostridium acetobutylicumconsumes sugar to produce ABE solvents. Due to the inhibition of butanol, ABE fermentation is known for its low solvent concentrations in the broth. Thus, to produce butanol more economically, it is required to operate the fermentation process under optimal conditions. Multi-objective optimization has been used to determine Pareto-optimal solutions where optimal decisions are made in the presence of two or more conflicting objectives. Based on an available kinetic model and a set of related mass balances for ABE fermentation, a continuous fermentation system coupled with a vacuum separation unit was simulated and a large number of Pareto-optimal solutions were obtained based on 4 decision variables (feed sugar concentration, dilution rate, evaporation rate and biomass concentration), and 3 objective functions (specific productivity, butanol concentration and sugar conversion) in which were maximized.

The optimal solutions were then ranked using the Net Flow Method and were compared to the continuous fermentation without the vacuum separation unit. It was found that for the best solution, the selective removal of solvents via the vacuum separation unit enhanced the specific productivity of the ABE fermenter by 51% for the same substrate conversion. The final average butanol concentration in the vacuum fermentation was 14.8 g/L which represents an increase of 70% compared to that in the standard continuous fermentation.  Moreover, for both fermentation cases, it was shown that the sugar loss is extremely high which has an adverse effect on profitability of the process. In addition to sugar loss, butanol yields over sugar consumption for the best points in standard continuous system and continuous system coupled with vacuum were 10.1% and 13.9%, respectively.