Vapour permeation for ethanol recovery from fermentation off–gas

In ethanol fermentations, about 2% of the ethanol leaves the fermenter with the off–gas. Conventionally, this is recovered by absorption in water. As alternative, vapour permeation was investigated conceptually for ethanol recovery from fermentation off–gas. A preliminary techno–economic evaluation of this system using hydrophobic membrane was carried out. The results were compared with conventional absorption. For the assumed membrane, concentrated ethanol (∼66 mass%) might be achieved using vapour permeation whereas absorption achieves 2 mass%, and needs much more distillation to achieve ∼93 mass%. The ethanol recovery costs for base case absorption and for hydrophobic vapour permeation were calculated to be 0.211 and 1.389 US $/kg, respectively. The ethanol recovery cost decreases with increase in membrane permeability in hydrophobic vapour permeation but the base case cost was not achieved. In the vapour permeation process, membrane cost dominates at lower membrane permeabilities whereas at the permeabilities 3 times higher than original, the costs for vacuum on permeate side of membrane governs the ethanol recovery cost.