Extractive pervaporation to separate ethanol from its dilute aqueous solutions characteristic of ethanol-producing fermentation processes

Abstract

The addition of a third component to a binary distillation system in extractive distillation leads to a much better separation between the two principal components via modification of the liquid phase activity coefficients and consequently the vapor–liquid equilibria. A small amount of n-butanol added to the solution containing 5–10 wt% ethanol enhances the separation performance in extractive pervaporation using tri- n-octylamine (TOA) as the liquid membrane with coated porous hollow fibers. The selectivity of ethanol increased considerably to as much as 113 at 54 °C for the feed containing ∼10 wt% ethanol when 2.5 wt% n-butanol was added to the feed compared with the selectivity of ethanol without any n-butanol added, namely, 38. The mass flux of ethanol for ∼10 wt% ethanol in feed obtained was 16.2 g/(m 2 h), which is considerably higher (by 73%) than the flux obtained for the experiment without n-butanol added to the feed; further for 2.5 wt% n-butanol in a feed of ∼10 wt% ethanol, the total concentration of solvents in the permeate exceeded 95%. The added n-butanol increases somewhat the solubility of ethanol in the liquid membrane. The solubility measurements of solvents show that the solubility is decreased with increased temperatures. The fluxes of the solvents were increased by a factor of five by reducing the thickness of the TOA layer in the porous substrate wall of the coated fibers. As a result, ethanol flux and selectivity achieved were around 59.8 g/(m 2 h) and 100 respectively for a mixture containing ∼10 and 2 wt% ethanol and n-butanol respectively at 54 °C. The TOA-based LM present throughout the pores of the coated substrate demonstrated excellent stability over many hours and essentially prevented the loss of liquid membrane to the feed solution.