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
