Pervaporation membranes highly selective for solvents present in fermentation broths

Abstract

Pervaporation is an energy-efficient alternative to distillation for removing volatile organic compounds (VOCs) from water especially solvents from their dilute solutions in a fermentation broth. Polymeric, ceramic and liquid membranes have been used in the pervaporation process for removing such solvents; however, polymeric and ceramic membranes have rather poor solvent selectivity compared to liquid membranes even though they achieve reasonable solvent mass fluxes. Liquid membranes have stability problems due to various losses. The loss of liquid membrane (LM) to the feed solution leads to toxicity for the organisms in a fermentation broth. A new liquid membrane-based pervaporation technique has been developed to achieve high selectivity, ensure stability and prevent contamination of the fermentation broth. Trioctylamine (TOA) as a liquid membrane was immobilized in the pores of a hydrophobic hollow fiber substrate having a nanoporous coating on the broth side and studied for pervaporation-based removal of solvents (acetone, ethanol, and butanol) from their dilute aqueous solutions. The LM of TOA in the coated hollow fibers demonstrated high selectivity and reasonable mass fluxes of solvents in pervaporation. The selectivities of butanol, acetone, and ethanol achieved were 275, 220, and 80, respectively with 11.0, 5.0, and 1.2 g/m 2 h for the mass fluxes of butanol, acetone and ethanol, respectively at a temperature of 54 °C for a feed solution containing 1.5 wt% butanol, 0.8 wt% acetone, and 0.5 wt% ethanol. The mass fluxes were increased by as much as five times with similar selectivity of solvents when an ultrathin liquid membrane was used. However, acetic acid in the feed solution reduced the selectivities of the solvents without reducing the solvent fluxes due to coextraction of water which increases the rate of water permeation to the vacuum side. The TOA-based LM present throughout the pores of the coated substrate demonstrated excellent stability over many hours of experiment and essentially prevented the loss of liquid membrane to the feed solution and the latter's contamination by the liquid membrane.