Permeation Mechanism Of Supercritical Carbon Dioxide In Macroporous Membranes And Modeling Analysis

Processes coupling membrane with supercritical fluid have shown great application potential in various areas such as supercritical solvent recovery, membrane separation, and even membrane reactions under supercritical conditions. However, only a limited number of studies have been reported concerning the permeation mechanism of supercritical fluids through membranes. In fact, the permeation mechanism of supercritical fluids in porous membrane and modeling analysis are important from both fundamental and practical points of view. It will help to gain a thorough understanding of this technique, design better coupled processes and guide operating in large-scale applications. This work investigated the permeation of CO₂ through three kinds of macroporous ceramic membranes with different pore diameter at 300³90 K and 1¹5 MPa. The use of macroporous membranes was to eliminate or at least minimize the effect of surface diffusion, allowing reliable verification of permeation models developed in this work. Based on the dusty-gas model, two improved permeation models named as Model I and Model II were developed to quantitatively predict the permeation of supercritical CO₂ in porous membranes. The results showed that the viscous flow plus Knudsen diffusion could explain the permeation mechanism of supercritical CO₂ through macroporous membranes, where the surface diffusion was negligible. The viscosity and density of CO₂ had a substantial change in the studied range of pressure and temperature, especially in the near-critical region. So Model II taking these into consideration could depict its permeation much more satisfactorily than either the traditional model or Model I. This was verified by our own measured permeances and literature data. The results also showed that the accurate structural parameters of porous membrane could be obtained through the permeation of gas CO₂ in porous membrane when the temperature was about 383 K. The variation of permeation mechanism of supercritical fluids at high pressure should also be given enough attention.