Studies on separation performance of interfacially formed membranes

Jennifer R. Du, Department of Chemical Engineering, University of Waterloo

29 April 2010 at 11:00

Location: JHE 342

Interfacial polymerization technique is based on a reaction between two monomers (e.g., polyamines and polyacyl chlorides) dissolved separately in two immiscible solvents. The aqueous polyamine solution can impregnate a microporous substrate, and upon contact with the other solution, a crosslinked film forms at the interface. Such membranes may not yield sufficient permselectivity, especially when the membrane is dried. As a result, the conventional liquid-liquid interfacial polymerization process is mainly limited to reverse osmosis and nanofiltration of liquid mixtures. This work presents a novel method for preparing thin-film composite membranes via an improved interfacial polymerization procedure. Side-chain active polymers were employed instead of monomeric amines. A solid-liquid or solid-vapor interfacial reaction, occurring between the preformed solid polymeric layer and a solution or vapor of crosslinking agent, was exploited for the film formation, instead of the conventionally used liquid-liquid interfacial reaction. This procedure has several advantages: 1) due to existing backbones provided by the polymer, a defect-free membrane can be easily obtained, 2) due to the employment of a polymer instead of monomers, the penetration of aqueous solution into the pores of the substrate is minimized, resulting in a substantial reduction in the mass transport resistance through the membrane, 3) the microvoids caused by the solvents involved in the liquid-liquid interfacial reaction is eliminated, 4) uniform distribution of fixed charges from the functional groups can be achieved, and 5) by controlling the crosslinked networks, the membrane can be finely-tuned for use in gas separation, pervaporation and nanofiltration.

Poly(N,N-dimethylaminoethyl methacrylate) interfacially formed membranes were prepared by above method. Considering the weak acid-base interactions between CO2 and amino groups, the membrane exhibited a facilitated transport to CO2. A CO2 permeance of 85 GPU and a CO2/N2 ideal separation factor of 50 were achieved at 23C and 0.4 MPa of CO2 feed pressure, and such performance is superior to most gas separation membranes reported for flue gas separation. Due to high hydrophilicity, the membrane was also exploited for natural gas dehydration. At 25C, the permeance of water vapor was 5350 GPU and the water vapor/methane selectivity was 4735 when methane was completely saturated with water vapor. The membranes also showed good permselectivity for ethylene glycol dehydration. At 30C, a permeation flux of ~1 mol/(m2.h) and a permeate concentration of 99.7 mol% water were achieved at a feed water content of 1 mol%. The membrane performance compares very favorably with those reported in the literature, including commercial pervaporation membranes. Due to the presence of amino groups, the membrane was positively charged. When tested in nanofiltration process, a good permeation flux of 30 L/(m2.h) and a MgSO4 rejection ratio of 90 % were obtained at 0.8 MPa and 30C with an aqueous solution containing 0.1 wt% MgSO4. The membrane was shown to be resistant to oxidants, especially free chlorine, which exists in most water systems.

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