Pervaporation of dilute aqueous streams: Transport mechanisms and membrane design

Since the mid 1980's there has been a dramatic increase in industrial and academic research in pervaporation technology. The advantages provided by pervaporation separations are: no solvents are necessary for the final separation, reduced energy demand and lower capital costs.; Pervaporation experiments were conducted for aqueous streams containing methyl ethyl ketone, diethyl ketone and acetone using polydimethylsiloxane (PDMS) membranes. The membrane permeabilities to both the ketone and water and the selectivity of the ketone over water were measured. Pervaporation experiments were also conducted for acetic acid solutions using control and modified PDMS membranes. The modified membranes incorporated tertiary amine groups and alcohols into PDMS as a means of improving membrane selectivity of acetic acid over water.; To understand the transport mechanism, Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR) was used to study unsteady state diffusion. In addition to diffusion coefficient measurements, the technique can determine the molecular state of a diffusing component. Combining FTIR-ATR and pervaporation results, a model was proposed to estimate the number of water molecules solvating the organic constituents within the membrane and provide elucidation of the transport mechanism.; PDMS membranes enhance the selectivity of MEK over water by factors of 4.8 1, 3.87 and 2.81 above vapor-liquid equilibrium alone for 1, 2, and 3 mole% solutions of MEK, respectively. The number of water molecules in solvated MEK clusters was approximately 2 for all three MEK concentrations. Furthermore, estimated solubilities showed that the increase in selectivity for decreasing MEK concentrations, was the result of decreasing water solubility with lower MEK concentrations. Results for acetone and DEK provided additional validity to the proposed model and the observed trends were as expected.; Pervaporation results for 3 mole% acetic acid solutions using 3 different modified PDMS membranes were compared with the results for PDMS membranes. The polydimethylsiloxane-dimethylpropylamine (PDMS-DMAP) and polydimethylsiloxane-dimethylpropylamine-silanol (PDMS-DMAP-PDMSOH) membranes had a slight enhancement of the selectivity of acetic acid over water. However, the formation of a second phase within the membrane limited the membranes selectivity, even though the mass uptake of the modified membranes increased by approximately 13% over PDMS. The polydimethylsiloxane-silanol (PDMS-PDMSOH) membranes increased the selectivity and permeability by 40 and 58%, respectively.