GAS EXTRACTION KINETICS OF VOLATILE ORGANIC-SPECIES FROM WATER WITH A HOLLOW FIBER MEMBRANE

Two mathematical descriptions of the solvent-free extraction process occurring in a hollow fiber membrane system are described. A model solving the diffusion equation and considering the concentration distribution across the diameter and along the axis of the fiber has an analytical solution. Mass transfer through the aqueous phase, membrane, and stripping phase has a semiempirical solution. These two solutions were compared to each other and to experimental data for typical extraction conditions when gas flow rates are much higher than sample flow. Excellent agreement was observed, confirming the assumption that at these conditions diffusion through the aqueous phase is the rate-limiting step. Extraction recoveries were investigated by varying several parameters-geometry of the fiber, its length, diameter, and thickness, linear velocity if the aqueous phase, gas/sample flow ratios, analyte Henry constant, and stripping-phase diffusion coefficients. The velocity of the stripping phase must be al least 20 times faster than that of the aqueous phase otherwise it will adversely affect the removal of volatile organic compounds (VOCs). The porous membrane thicknesses used in this study have no effect on the removal of VOCs from water; however, when the membrane wall is 1 order of magnitude thicker than the inner diameter of the fiber It becomes a factor in the rate of overall mass transfer. Spiralling the hollow fiber increases the removal efficiency of the system at higher flow rates. This is caused by increased mixing of the components in the aqueous phase. The porous membrane extraction method is effective for organic compounds which have Henry constants above 0.1. The investigations were extended to a nonporous silicone hollow fiber membrane. It was found, again, that for the typical experimental conditions, extraction rate is controlled by the diffusion of the analyte in the aqueous phase.