MFI membranes for separation of carbon dioxide from synthesis gas at high pressures

• MFI membranes were evaluated for separation of CO2 from model synthesis gas.
• MFI membranes with different counter-ions and Si/Al ratios were used.
• Silicalite-1 membranes showed better performance than NaZSM-5 and BaZSM-5 membranes.
• Highest CO2/H2 separation factor was 35 for a silicalite-1 membrane at 273 K.
• The separation performance was controlled by the shape of the adsorption isotherm.

Membranes are considered to be one of the most promising technologies for simple and energy efficient removal of carbon dioxide from gas mixtures. In the present work, MFI membranes with different Si/Al ratios and counter-ions were evaluated for separation of carbon dioxide from synthesis gas, i.e. a mixture of carbon dioxide and hydrogen. Single gas permeation experiments were also carried out. These membranes consisted of a 0.5 µm thick MFI film grown on graded alumina supports. The feed pressure was varied between 3 and 8 bar and the permeate pressure was kept constant at 1 bar, while the temperature was varied in a range of 273–350 K. The silicalite-1 membrane showed the best overall carbon dioxide separation performance, in terms of flux and separation factor, when compared with NaZSM-5 and BaZSM-5 membranes. The silicalite-1 membrane displayed a CO2/H2 separation factor of 31, with a carbon dioxide flux of ca. 560 kg m−2 h−1 at 8 bar feed pressure and a temperature of 273 K. The higher performance of the silicalite-1 membrane was attributed to a more suitable CO2 adsorption isotherm, which resulted in larger difference in fractional surface loading of CO2 between the feed and permeate side for this type of membrane, and consequently higher CO2 flux and CO2/H2 separation factor. Accordingly, the difference in membrane performance was larger at low temperature (273 K), while at elevated temperatures, the CO2/H2 separation factor decreased for all membranes and the difference between the membrane types diminished, as a result of decreased carbon dioxide adsorption.