A Novel Technique for the Separation of Dilute Butanol from Aqueous Fermentation Broths

Butanol is a promising biofuel which can be manufactured by fermentation. Irrespective of whether n-butanol or iso-butanol is produced, the alcohol is generally expressed at low concentrations (~1 wt%) in these fermentations. It also inhibits the organisms, and so removal of the butanol from the fermentation vessel as it is produced can improve the productivity of the fermentation. However, separation of dilute butanol from aqueous fermentation broths requires substantial amounts of energy. In this dissertation, a novel separation technique has been devised, employing the solvent extraction of butanol from aqueous broths by volatile hydrocarbons. This separation technique performs the separation of butanol selectively and efficiently. It is investigated theoretically in this dissertation. The use of volatile hydrocarbons allows the extracted butanol and the hydrocarbon to be separated by distillation, employing waste heat from the fermenter or other similar lowgrade heat sources. Such heat is often abundant on fermentation plants. Therefore, minimal high-grade heat (heat at temperatures higher than the fermenter) would be required for the process. The equilibria of butanol and side-products with C4 – C5 hydrocarbons were investigated using vapour-liquid equilibria and excess enthalpy measurements from the literature. Models of the extraction and distillation processes were then built using these analyses. A flowsheet simulation predicted that under 2 MJ/kg butanol of high-grade heat (mostly at only ~50°C) was required by the separation scheme provided that sufficient low-grade waste heat was available (~40 – 20 MJ/kg butanol for extraction of 1 – 2 wt% broths). This high-grade heat requirement is around 5% of the heat required for distillation of butanol from aqueous broths at 1 wt% Various configurations of flowsheet and solvent were investigated to improve the process. Distribution coefficients of butanol in C4 – C6 hydrocarbons were found to triple between 37°C and 100°C, and therefore extraction performed at elevated temperatures was found to significantly reduce the low-grade heat requirements of the system. The formation of butanol-gasoline blends via extraction of butanol into volatile hydrocarbons could also eliminate high-grade heat requirements.EPSRC Doctoral Training Gran