A study of CO2/CO separation by sub-micron b-oriented MFI membranes

Abstract

Separation of CO₂ and CO is of great importance for many industrial applications. Today, CO₂ is removed from CO mainly by adsorption or physical or chemical absorption systems that are energy-intensive and expensive. Membranes are listed among the most promising sustainable and energy-efficient alternatives for CO₂ separation. Here, we study CO₂/CO separation by novel sub-micron b-oriented MFI zeolite membranes in a temperature range of 258–303 K and at a feed pressure of 9 bar. Under all experimental conditions studied, the membranes were CO₂-selective and displayed high CO₂ permeance ranging from 17 000 to 23 000 gpu. With decreasing temperature, the CO₂/CO selectivity was increasing, reaching a maximum of 26 at 258 K. We also developed a mathematical model to describe the membrane process, and it indicated that the membrane separation performance was a result of selective adsorption of CO₂ on the polar zeolite. The heat of adsorption of CO₂ on the zeolite is more negative due to the high quadrupole moment and polarisability of the molecule as compared to CO. At the same time, diffusional coupling (correlation effects) at high adsorbed loadings was found to favour the overall CO₂/CO selectivity of the membranes by reducing the diffusivity of the lighter CO molecule in the ca. 0.55 nm pores in the zeolite. The model also indicated that the separation performance was limited by the mass transfer resistance in the support and concentration polarisation on the feed side of the membrane.