Pervaporation as a means of recovering ethanol from lignocellulosic bioconversions

Fermentation of AFEX-treated and enzymatically hydrolyzed forages and agricultural residues resulted in weak ethanol concentration in broths (maximum 5 g/L). A porous ethanol-selective membrane was evaluated on a model solution (2 wt/wt% ethanol) using pervaporation process. When used under permeate-side pressure of 20 mmHg, at feed temperature of 30°C and at a feed Re = 2000, a porous polytetrafluoroethylene (PTFE) membrane (0.2 μm pore diameter) produced a maximum ethanol flux of 36 g/m2 h with selectivity near 1.66. Increasing feed temperature to 60°C resulted in 465 g/m2 h for the ethanol flux but the selectivity was around 2.25. Decreasing the permeate-side pressure to 10 mmHg at 30°C increased the ethanol flux to 86 g/m2 h and the selectivity to near 2.85. Two non-porous membranes are presently being investigated. Preliminary results with 10 μm thick polydimethylsiloxane and polyoctylmethylsiloxane membranes showed an increased selectivity to a value near nine. However, the ethanol and total fluxes are greatly reduced, even at a working pressure of 1 mmHg.