Very high flux MFI membrane for CO2 separation

Membrane separation of CO2 from synthesis gas, natural gas or biogas may be a simple and energy-efficient alternative to other separation techniques. In this work, membranes comprised an about 0.7 μm thick MFI film on a graded alumina support was used to separate mixtures of CO2/H2, CO2/CO/H2 and CO2/CH4 gas at high pressure. The membranes were prepared on masked supports.The single gas permeance of H2 was larger than that of CO2, but for a binary CO2/H2 mixture, the H2 permeability was blocked and the membrane was CO2 selective. A CO2 flux as high as 657 kg m−2 h−1 was observed at room temperature, 3000 kPa feed pressure and 600 kPa permeate pressure. Under these conditions, the CO2/H2 separation factor was 16.2. The maximum CO2/H2 separation factor was 32.1, and was observed at 1000 kPa feed pressure, a permeate pressure of 200 kPa and a temperature of 275 K. Under these conditions, the CO2 flux was still as high as 332 kg m−2 h−1. The highest measured CO2 permeance for the binary CO2/H2 mixture was 93 × 10−7 mol m−2 s−1 Pa−1. The membrane was also CO2 selective for a CO2/CO/H2 mixture. However, both the CO2 flux and the CO2/H2 separation factor were reduced slightly in the presence of CO, probably as a result of competing adsorption between CO and CO2. The CO2/CH4 separation factor was much lower than the CO2/H2 separation factor, probably as a result of competing adsorption between CO2 and CH4 in the first case. The highest measured CO2/CH4 separation factor was 4.5.The results show that MFI membranes are promising candidates for separation of CO2 from synthesis gas, natural gas or biogas. The very high CO2 fluxes likely result from a combination of several factors such as: low film thickness, a fully open graded support, high CO2 pressure in the feed resulting in saturation of the zeolite at the feed side, relatively high CO2 diffusivity in MFI pores, relatively high molecular weight, and high pressure drop. The separation is likely caused by efficient blocking of H2 transport by CO2 adsorption.

► An MFI membrane was evaluated for CO2 separation at high pressures.
► High CO2 flux was assigned to low film thickness and fully open graded support.
► Selectivity resulted from CO2 adsorption, which blocked H2 transport.