Patent ID: 8304588

Claim:
A method for producing isobutanol in a retrofit ethanol production plant comprising: a. pretreating a feedstock to form fermentable sugars in a pretreatment unit; b. culturing a microorganism capable of producing isobutanol in a fermentation medium comprising the fermentable sugars in a fermentation unit to produce isobutanol; c. distilling a portion of the fermentation medium comprising isobutanol and viable microorganisms, thereby removing at least some of the isobutanol therefrom; d. returning the isobutanol-depleted portion of the fermentation medium from step (c) comprising viable microorganisms to the fermentation unit; and e. transferring the fermentation medium from the fermentation unit to a beer still; wherein said distilling forms a vapor phase comprising isobutanol and water, and said method further comprises: i. condensing the vapor phase to form an isobutanol-rich liquid phase and a water-rich liquid phase; and ii. separating the isobutanol-rich liquid phase from the water-rich liquid phase; and iii. returning said water-rich liquid phase to the fermentation unit.