Fluid dynamics and transport phenomena effects of gas mixtures and mercury vapour in microporous membranes

Abstract

High temperature gas separation is attractive in chemical engineering processes, as it reduces the energy penalties associated with cooling down requirements with conventional gas separation technologies. Among potential novel technologies, inorganic membranes based on molecular sieve silica have performed well at high temperatures. The majority of the work in this field has been carried out for small laboratory membrane modules, under special conditions where fluid flow effects are negligible. However, fluid flow has a significant effect in the performance of large multitube membrane modules, which in turn tends to affect the industrial performance of these systems in terms of gas production and separation. Therefore, to addresses this gap in knowledge, Matlab was used to develop a 2D CFD cylindrical model based on the Navier-Stokes equation. However, CFD does not account for discontinuity caused by the membrane. In order to take into consideration the driving force for gas permeation and fluid flow dynamics on both feed and permeate domains, the CFD model was coupled with microporous transport phenomena Maxwell-Stefan formalism. A cylindrical 2D CFD model combined with membrane mass transfer was applied to simulate H2/Ar separation, and validated for a large module. Results show that the H2 molar fraction was the most influential factor affecting the driving force. High temperature conferred high H2 fluxes/permeance in the case of single gas permeation, thus complying with activated transport. However, H2 fluxes were greatly affected in gas mixture process, attribute to the reduction of the H2 driving force where temperature played a minor role only. Nevertheless, the H2 purity was not significantly affected due to the molecular sieving characteristics of the membranes. The 2D model simulation results clearly indicate that the concentration polarization is negligible in these operating conditions. Therefore, 2D model simulation can be satisfactorily represented by 1D model simulation.A transient 1-D model was also developed to investigate the effect of operating conditions such as feed flow rate, feed pressure, and flow configuration. Changing these parameters led to different flows and H2 fraction distributions in the feed domain, thus altering the driving forces for the preferential permeation of H2. Confinement changes associated with altering the module radius did not significantly affect H2 yield, purity and recovery. In addition, it was found that the counter-current were effectively the same as the co-current flows. The introduction of diffusion in this transient 1D model differed from other 1D models reported in the literature for liquid and gas separation at room temperature. Diffusion was paramount and the transient 1D model was validated against experimental work, thus confirming the major role played by gas to gas diffusion for high temperature gas separation. This model proved to be useful in engineering design, as it was further applied to examine the separation performance by varying membrane length and membrane arrangements for a multi-membrane system under several operating conditions. Further work was carried out to understand the microporous mass transfer using the Oscillator model, which accounts for the energetic potential distribution of a gas molecule in micropores. This model takes into consideration the microporous information (i.e. membrane material, pore size, gas molecule size) to the macroscopical results (i.e. permeability, apparent activation energy and heat of adsorption). Due to the positive apparent activation energy, it was found that He with a LennardJones kinetic diameter of 2.6A could diffuse through pore sizes of 2.1A. This was attributed to the kinetic energy supplied by the high temperature gas permeation tests, thus providing the He molecule to diffuse through pore sizes smaller than its kinetic diameter. In addition, the oscillator model was also used to validated further experimental work was carried out at the Imperial College (UK) to determine the effect of mercury (Hg) on gas permeation in microporous membranes. Industrial gas separation, such as syngas in coal gasification, contains Hg in ppm concentrations. It was found the Hg changed the transport phenomena of N2 diffusion. The effect media theory (EMT) combined with the Oscillator model was used to investigate this phenomenon. It was found that Hg strongly adsorption/condensed in the micropores at 100˚C and therefore greatly reduced the N2 permeation, contrary to the minor effect at 300˚C as Hg adsorption/condensation was no longer prevalent.