Rethinking the design of a hybrid extraction-distillation process for purification of biobutanol from ABE fermentation broth

Abstract

There is an increase in fuel demand as a result of the rapid growth of the world population and shortage of fossil fuels, and it enhances environmental concerns about fossil fuel usage. Thus, usage of alternative energy sources instead of conventional fuels will play an important role in the future. One of the promising alternatives is biofuels, and among alternative biofuels, biobutanol comes forward with its higher fuel value and lower volatility compared with bioethanol. In several studies, acetone-butanol-ethanol (ABE) fermentation is used for biobutanol production. However, a dilute fermentation broth with a very low concentration is obtained because of the butanol toxicity to microorganisms. In addition, there are homogeneous (ethanol-water) and heterogeneous (butanol-water) azeotropes in the fermentation broth making the purification of biobutanol a challenging work. Several process configurations have been studied in the literature to separate the components of ABE fermentation broth. One of the studied configurations is the hybrid extractor-distillation process, where it is claimed that all the water can be removed from the raffinate phase of extractor using a solvent called hexyl acetate. Thus, the extract phase includes only ABE mixture and hexyl acetate, and they can be easily separated using conventional distillation columns since there is no more azeotrope in the process. However, the preliminary results of this study show that although most of the water is removed from the raffinate of extractor, there is still a significant amount of water leaving the extractor in the extract phase along with ABE mixture and hexyl acetate. Thus, separation of this extract phase using a chain of conventional distillation columns is impossible. This study aims to propose a new process configuration including extraction and distillation stages to purify biobutanol from the fermentation broth. In this novel configuration, the extract phase of the extractor is fed into an extractive distillation column to separate the remaining water from the ABE mixture by using hexyl acetate as the entrainer. The distillate of the extractive column consists of water and entrainer are separated using a decanter. The bottoms of this column are sent to a recovery column to remove the entrainer from the bottoms and recycle back to the upstream units. The distillate stream including anhydrous ABE mixture is then sent to a chain of conventional distillation columns to purify butanol, acetone, and ethanol.