Date of Award

5-31-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering - (Ph.D.)

Department

Chemical Engineering

First Advisor

Sirkar, Kamalesh K.

Second Advisor

Xanthos, Marino

Third Advisor

Mitra, S.

Fourth Advisor

Knox, Dana E.

Fifth Advisor

Iqbal, Zafar

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. Liquid membranes have high selectivities for removing solvents from aqueous solutions compared with polymeric membranes or ceramic membranes; however 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 but highly hydrophobic 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 butanol achieved were 275, 220, and 80 respectively with 11.0, 5.0, and 1.2 g/m2-hr for the mass fluxes of butanol, acetone and ethanol respectively at a temperature of 54 0C 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 5 times with similar selectivity of solvents when an ultrathin liquid membrane about 5 times thinner was used. However, acetic acid in the feed solution reduced the selectivities of the solvents somewhat without reducing the solvent fluxes due to the 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 and not in the pore of the coating 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.

Adding small amounts of n-butanol increased considerably the selectivity of ethanol for removing ethanol from an aqueous solution representing a yeast-based fermentation broth. This happens primarily due to the increase in the concentrations of organics in the permeate. Additional possible reasons are: n-butanol can increase the solubility of ethanol and n-butanol in TOA, increase the diffusivity of solvents in TOA, and increase the effective partial pressures of all solvents. The solubility of all solvents in TOA obtained by Headspace-Gas Chromatography was decreased as the temperature was increased; however, solubilities of all solvents do not decrease sharply, compared with the increasing value of the diffusivity predicted by the Wilke-Chang equation; therefore, permeability, a product of solubility and diffusivity, increased at elevated temperatures. The predicted species fluxes may be estimated from the permeability, the vapor pressure, the mole fraction and the activity coefficient which was estimated by UNIFAC method; however, this method was found to be unsatisfactory for n-butanol.

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