Typically, the production of fuel-grade chemicals, such as butanol, from biomass fermentation processes involves low-yield batch processes and high energy costs. Now, researchers from Imperial College London (ICL; www.imperial.ac.uk), in partnership with bp plc (London; www.bp.com), have demonstrated a new membrane-based extraction process for biofuels that helps overcome these hurdles, reportedly boasting a 25% reduction in energy use and a tenfold increase in production yield. In typical biomass fermentation processes, the produced butanol contains water that must be removed, either through heterogeneous distillation (requiring an energy-intensive phase change) or batch liquid-liquid extraction, explains Ji Hoon Kim, ICL researcher and co-author of the study. “Our new membrane-extraction process can extract the produced biofuel from a fermentation broth using an extractant with a high partition coefficient, providing high recovery rates through a membrane without requiring phase change,” adds Kim. Since the extractant — an organic solvent based on 2-ethyl-1-hexanol — has around half the heat capacity of water and a higher boiling point than butanol, it does not require evaporating any water or extractant, meaning that the separation process requires significantly less energy. The ultra-thin-film composite membrane is highly selective for butanol, blocking the transport of water and extractant. Due to the extractant’s fast recovery rate and high recovery capacity, the volume ratio of extractant to fermentation broth required to operate the system is very small, avoiding potential toxicity issues with the fermentation microorganisms.
Furthermore, since the membrane quickly extracts the butanol from water into the organic solvent, the butanol level in the fermentor is kept low, and microbes can remain more active, increasing productivity and enabling more streamlined, continuous operation, adds Andrew Livingston, ICL professor of chemical engineering and lead author on the project. The membrane extraction process has been demonstrated in the laboratory using a 2-L reactor, and Livingston says that the team is now looking at scaling up membrane production and investigating the effects of biofouling on the system.