Membrane Characterization Based on the Upstream Pressure Decay in a Dynamic Gas Permeation Test

Abstract

The time-lag method is believed to be the most usable method for extracting the membrane properties via a simple dynamic permeation experiment; however this method suffers from major drawbacks that limit its use for some materials and under certain conditions. One of the major drawbacks of the time-lag method is that it relies on monitoring the pressure rise downstream from the membrane due to gas permeation, whereas the method is derived by assuming that the pressure downstream from the membrane is maintained at zero during the entire permeation experiment. To rectify this problem, it is proposed to characterize the membrane based on the pressure decay upstream from the membrane during a conventional time-lag experiment, while continuously evacuating the downstream side of the membrane. This modification allows for a better adherence to the boundary conditions on which the time-lag method relies. Right after initiation of the gas permeation experiment, the membrane behaves as a semi-infinite solid, and the rate of pressure decay is directly proportional to the square root of time. Once the permeating gases emerge downstream from the membrane, the membrane no longer behaves as a semi-infinite solid and the pressure decay becomes a non-linear function of the square root of time. In the proposed new method, the membrane properties are extracted based on the deviation of the recorded pressure decay from the semi-infinite behavior in the square root of the time domain. In this paper, we present the mathematical bases of the new method along with preliminary experimental results. Results indicate that diffusivity, solubility and permeability of nitrogen in polyphenylene oxide (PPO) membrane used in this study are very close to the literature values.