Recovery of Dilute Aqueous Butanol By Membrane Vapor Extraction with Dodecane or Mesitylene

A novel isothermal, nonselective-membrane separation process, designated membrane vapor extraction (MVE), efficiently recovers butanol from a dilute aqueous solution, for example, from a fermentation broth. In MVE, the two feed and solvent liquids are not in contact; they are separated by vapor. Therefore, compared to conventional extraction, MVE avoids formation of difficult-to-separate emulsions.

In MVE, a semi-volatile aqueous solute (e.g., butanol) vaporizes at the upstream side of a membrane, diffuses as a vapor through the membrane pores, and subsequently condenses and dissolves into a high-boiling nonpolar solvent, highly favorable to the solute but not to water. Design analysis of an industrial-scale MVE unit for processing 2-wt % aqueous butanol by dodecane solvent at 40 °C indicates over 90 % recovery of the feed butanol with essentially no water loss and with very low energy requirement. However, the published design study gives no experimental evidence for the calculated MVE separation.

Here, we present experimental data to validate the MVE process. We use an omniphobic (i.e., hydrophobic and oleophobic), 0.2-µm pore-diameter Versapore R membrane housed in a small 6-cm wide by 10-cm long plate-and-frame channeled flow cell with 0.8-cm gap thickness. The membrane flow cell is operated between recirculating flow loops.

Since vapor transport across the membrane contributes minimal resistance, MVE performance is governed by mass transfer through feed and solvent boundary layers. Prediction of the experimental-extraction performance determined from Graetz-Lévéque analysis agrees well with experiment using no adjustable parameters. Preliminary analysis of downstream solute recovery from the extract via distillation is shown to be more efficient than that for pervaporation because of insignificant water carry over through the MVE membrane.