Gevo’s plant in Luverne, Minnesota, is situated among the cornfields. After all, it’s an ethanol plant that was built to capitalize on the government regulations that encourage the use of ethanol blendstock in gasoline. We’ve modified this plant to produce isobutanol.
At an existing dry-mill corn ethanol plant, the corn mash stream coming out of the front end of the process that feeds the ethanol fermentation is the same stream used for isobutanol fermentation. This stream is prepared for isobutanol fermentation, added to large non-sterile fermentation tanks, and where proprietary yeast is added to convert the sugars to isobutanol.
As with any alcohol fermentation, isobutanol has a stress effect on yeast as the fermentation proceeds and the concentration increases. Unlike ethanol, isobutanol is much more stressful on the fermentation organism, which typically would result in reduced fermentation rate and/or reduced batch size. As one biofuel producer, Gevo has solved for this issue by using its GIFT® (Gevo Integrated Fermentation Technology) process for continuously removing the product during fermentation. Despite the fact that isobutanol’s boiling point is eight degrees Celsius higher than water, the GIFT process is able to remove isobutanol continuously during fermentation to maintain its concentration in the fermenter at target levels to optimize process rates and cost. This is accomplished because isobutanol-water is azeotropic, meaning the vapor has the same composition as the liquid. The fermentation broth is circulated through the GIFT process where it is subject to low-pressure evaporation and isobutanol flashes off the broth resulting in a vapor concentration nearly twenty times greater than what was in the fermenter. When the vapor is condensed, the concentration of isobutanol is now above its solubility limit in water and phase separation occurs, leaving a relatively pure isobutanol-rich phase and a water-rich phase. The isobutanol-rich phase is sent to a purification step and the water-rich phase goes to stripping distillation step where residual isobutanol is removed and the water returned to the fermenter. All of the biomass remains in the fermenter-GIFT system, thereby greatly simplifying purification of the isobutanol.