Experimental and numerical study of the flux of isobutane vapors near saturation through multi-layered ceramic membranes

Abstract

The transport of vapors of isobutane near saturation through multi-layered asymmetric membranes is investigated experimentally and theoretically. The influence of the upstream state of the vapor, whether far or close to saturation, and of the orientation of the membrane on the mass flow rate is investigated. For a membrane with five layers, the mass flux increases from about 0.25 kg m⁻²s⁻¹ for a vapor further from saturation to about 0.45 kg m⁻²s⁻¹ for a vapor close to saturation. Also, close to saturation the mass flux in the flow direction from the separation layer to the support is up to 50% larger than in the opposite direction. The membranes consist of three to five layers, the support has a pore size of 3 μm, the finest separation layer has a pore size of 20 nm. Plane, circular membranes were tested in steady-state permeation experiments. The upstream pressure varied between about 0.3 times the saturation pressure and a value a few percent smaller than the saturation pressure, which is about 3.5 bar. Pressure differences between 0.1 and 0.5 bar were applied. Theoretical descriptions of the flow process are given, assuming that condensation may take place. For one description any heat transfer is neglected and the flow is assumed to be isothermal while for two other descriptions heat transfer and temperature variations due to condensation and evaporation are considered. For the experiments presented here the mass fluxes predicted by these three descriptions do not differ by a wide margin, e.g., the predictions vary between 1.02 and 1.25 kg m⁻²s⁻¹. Qualitatively, the increase of the mass flux for a vapor close to saturation and the dependence of the mass flux on the flow direction is recovered by all three descriptions.