Gas Adsorption and Permeation in MFI Zeolites and Membranes

Doktorsavhandling, 2009

The single and multi component gas adsorption behavior on silicalite-1 and cation exchanged MZSM-5 zeolites (M = H, Na, and Ba) were investigated by a step change response method. Temperature programmed desorption (TPD) and in-situ fourier transform infrared (FTIR) spectroscopy were also performed to study the type of adsorbed species and their thermal stabilities. It was found that CO2 was adsorbed on silicalite and HZSM-5 as one type of adsorbed species, and on NaZSM-5 and BaZSM-5 zeolites as at least two types of adsorbed species over the temperature range studied. The CO2 adsorption behavior for silicalite and HZSM-5 was adequately described by a single site Langmuir adsorption model but for NaZSM-5 and BaZSM-5 a dual site Langmuir model was required. FTIR-spectra revealed that CO2 was adsorbed on MZSM-5 and silicalite in several ways and formed carbonate bands, which seemed to be caused by different adsorption sites. The multi-component adsorption behavior is adequately predicted by an extended Langmuir model, but failed to describe the CO2 adsorption on BaZSM-5 for a H2/CO2 gas mixture. Single and binary H2/CO2 gas permeation was studied through a silicalite-1 composite membrane over a temperature range 25 to 300oC. In general, single component fluxes decreased with increasing temperature whereas binary component fluxes showed a maximum followed by a continuous decrease. A complete mass transport model including surface diffusion and gas translational diffusion in zeolite crystals, Knudsen diffusion in defects, as well as viscous flow and Knudsen diffusion in the support material was developed to simulate the single and binary gas permeation measurements. The modeling results showed that surface diffusion was the dominant mass transport mechanism, and also the transport resistance of the support material was not negligible. The feasibility of a water gas shift (WGS) membrane reactor was evaluated in a modeling study. A Silicalite-1 membrane was considered to be integrated into a WGS reactor operating under conditions favorable for selective CO2 permeation in the membrane. The modeling study showed that both the WGS reaction rate and the CO2/H2 permeation played an important role on the overall reactor performance. Lower outlet CO concentration then the equivalent equilibrium composition could be achieved, but not levels required for a PEM fuel cell with an acceptable H2 recovery. This is due to the fact that sufficiently high CO2/H2 selective permeation cannot be achieved with a silicalite-1 membrane.