Separation and purification of biobutanol during bioconversion of biomass

Biofuels from biomass are becoming increasingly more important, due to the need for reduction in greenhouse gas (GHG) emissions, energy independence, and limited global availability and increasing demand and costs of fossil fuels. Butanol has several advantages over ethanol as a drop-in biofuel such as higher energy content, potential for higher blending percentage with gasoline, lower vapor pressure, and lower hygroscopy. It can be used in existing transportation fuel distribution infrastructure. Butanol can be produced by fermentation of carbohydrates derived from biomass using Clostridium acetobutylicum or C. beijerinckii under anaerobic conditions. There are still many unsolved challenges for making biobutanol technically, and economically viable. The unsolved challenges lie in severe product (especially butanol) inhibition during bioprocessing, which leads to low butanol yield and productivity, and very low final product concentration (<3 wt%), causing expensive downstream processing (product separation) costs. There are two ways for solving these problems. One is the modification of microorganisms for ABE (Acetone, Butanol and Ethanol) fermentation by genetic engineering, which could keep the microorganisms alive and active under higher concentration of products in the broth. This could significantly increase the product yield, productivity, and concentration and hence reduce the production costs. However, this is still an unrealized long term goal. Another approach is the development of efficient separation and purification processes for product recovery. And, even if the modification of microorganisms becomes a reality, product separation and purification will still remain a major critical challenge. In this study, an extensive review of separation and purification of butanol from fermentation broth is provided, including gas stripping, vacuum flash, liquid-liquid extraction, membrane solvent extraction or perstraction, membrane pervaporation, membrane distillation, thermopervaporation, reverse osmosis, adsorption, and integrated bioprocessing with various separation methods. It is concluded that membrane pervaporation, solvent extraction, and adsorption are the most energy-efficient approaches for removal of butanol from the ABE fermentation broths. It should be noted that this is not a strict comparison and it is suggested that each separation process should be optimized before comparison. Integration of bioreactors with these energy-efficient separation methods could significantly increase the product yield, productivity and concentration and hence lower the production cost. Butanol dehydration is also discussed. This review could be helpful in the research and development and commercialization of biobutanol as renewable drop-in biofuels and biochemicals.