A general mathematical model for performance evaluation of acetone-butanol continuous flash extractive fermentation system was formulated in the terms of productivity, energy requirement (energy utilization efficiency) and product purity. The simulation results based on experimental data showed that the most pronounced performance improvement could be achieved by using highly concentrated substrate as feed and the increase in solvent dilution rate could only improve the total productivity at the expense of energy utilization efficiency. A two-vessel partial flash system, with the first vessel of two to three plates and the second vessel as a complete flash vessel, is required to ensure high product purity. This method of mathematical formulation and analysis could be used and extended as a general tool to other bioprocesses equipped with multiple in siturecovery systems.