Energy Requirements for Butanol Recovery Using the Flash Fermentation Technology

Acetone-butanol-ethanol (ABE) facilities have traditionally presented unattractive economics because of the large energy consumption during recovery of the products from a dilute fermentation broth (similar to 13 g/L butanol). This problem results from the high toxicity of butanol to microorganisms that catalyze its production. Flash fermentation is a continuous fermentation system with integrated product recovery. The bioreactor is operated at atmospheric pressure and the broth is circulated in a closed loop to a vacuum chamber where ABE is continuously boiled off at 37 degrees C and condensed afterward. With this technology the beer achieved a concentration of butanol as high as 30-37 g/L. This paper studies the energy requirements for butanol recovery using the flash fermentation technology and its effect on the energy consumption by the downstream distillation system. Compressors are used to remove the vapors from the flash tank, thus maintaining the desired vacuum. The heat recovery technique of vapor recompression is used to reduce energy requirements. With this technique the heat generated by the compression and partial condensation of the vapors provides the energy for boil up (heat of vaporization) in the flash tank. Thus the energy requirement for the flash fermentation is essentially the electrical power demanded by compressors. Energy for recirculation pumps accounts for approximately 0.5% of the total energy consumption. Small increments in butanol concentration in the beer can have important positive impacts on the energy consumption of the distillation unit. Nonetheless, the energy use of the recovery technology must be included in the energy balance. For a fermentation with a wild-type strain, the total energy requirement for butanol recovery (flash fermentation + distillation) was 17.0 MJ/kg butanol, with 36% of this value demanded by the flash fermentation. This represents a reduction of 39% in the energy for butanol recovery in relation to the conventional batch process. In the case of a fermentation with a hyper-butanol producing mutant strain, the use of the flash fermentation could reduce the energy consumption for butanol recovery by 16.8% in relation to a batch fermentation with the same mutant strain.