Pervaporative dehydration of concentrated aqueous solutions of selected polar organics by a perfluoropolymer membrane

A perfluoropolymer membrane known as CMS-3 has been applied to separation of aqueous mixtures containing high concentrations of volatile organic compounds (VOCs), such as acetone, n-butanol and ethanol by pervaporation. A concentrated aqueous solution of much less volatile ethylene glycol was also investigated. The feed compositions for mixtures of water-ethylene glycol were varied between 5 and 30 wt% water. For mixtures containing VOCs encountered in biofuel production, acetone, n-butanol and ethanol, the composition was fixed and temperatures between 30 and 50 °C were explored. Separation factors ranging from 2420 to 12,800 were observed for water over ethylene glycol as the ethylene glycol content in the feed increased from 70% to 95%. In aqueous mixtures of the three VOCs, the separation factors for water over n-butanol, ethanol and acetone at 30 °C were respectively 7180, 900, and 235. These results seem to support evidence of the size-exclusive nature of the CMS-3 polymer membrane; smaller molecular solvents such as water are more selectively transported when compared to larger ones such as n-butanol even though the membrane is highly hydrophobic. Flux values were modest due to the relative thickness of the membrane used. In this study, the maximum water flux was found to be 33 g/(m² h) at 50 °C for quaternary mixtures of n-butanol, ethanol, acetone and water using a 25 μm thick, flat sheet membrane. For this membrane, the permeability of a variety of species e.g., permanent gases, vapors, volatile organic liquids, appears to be strongly correlated with the molecular diameter and shows a sharp drop-off as the molecular diameter increases.