Oxygen permeation through functionalized hydrophobic tubular ceramic membranes

The aim of our research is to identify improved means for the production of an O2 saturated single-phase aqueous stream at the inlet to a fixed-bed catalytic reactor for the oxidative decomposition of organic contaminants within wastewaters produced aboard manned spacecraft. The method of oxygenation must be capable of stable long-term operation at reactor temperature (90–127 °C) in the absence of gravity. Membrane mediated O2 saturation of the reactor influent has been used to eliminate the potential for two-phase flow related problems arising from the domination of gas–liquid phase separation in microgravity by surface (capillary) forces. However, current systems employing organic polymers lack high temperature operational capability and mechanical resilience. In the current study, we have investigated the use of hydrophobic ceramic membranes as improved media for the controlled dissolution of O2 in a flowing aqueous stream. To improve oxygen selective mass transfer and inhibit water loss, the surface properties of tubular microporous α-Al2O3 membranes were modified by silanization reactions to attach functional groups covering a range of hydrophobicity. Three straight-chain hydrocarbon functionalities were employed: butyl, dodecyl, and octadecyl silanes. In addition, a 10-carbon fluoroalkyl moiety was examined. O2 permeation was monitored across the variously functionalized membranes using a coaxial tube-in-shell membrane contactor with a constant pressure of flowing gas on the shell side, and a recirculating water stream on the tube side. Oxygen mass transfer coefficients were derived from the time-dependent rise in O2 saturation in the recirculating stream. The best performance was observed for the perfluorinated C10 functionalized alumina membrane, with respect to both oxygen transfer rates and minimal water loss. Operational capability at elevated temperature was demonstrated.