Segregation effects in adsorption of CO₂ containing mixtures and their consequences for separation selectivities in cage-type zeolites

Abstract

For separation of CO₂ from gaseous mixtures with CH₄, N₂, or Ar, cage-type zeolites such as DDR, CHA, LTA, and ERI are of practical interest. These zeolites consist of cages separated by narrow windows and the selectivity of separation of CO₂ is dictated by both adsorption and diffusion characteristics. For adsorption of CO₂/CH₄ mixtures, Grand Canonical Monte Carlo (GCMC) simulations show that the window regions of cage-type zeolites have a significantly higher proportion of CO₂ than within the cages. Due to the segregated nature of mixture adsorption, the ideal adsorbed solution theory is unable to predict the mixture loadings accurately. For adsorption of CO₂/N₂, CO₂/Ar, and N₂/CH₄ segregation effects are also present but their impact is far less severe than for CO₂/CH₄ mixtures. For CO₂-bearing mixtures, the preponderance of CO₂ in the window regions has important consequences for mixture diffusion. Molecular dynamics (MD) simulations for self-diffusion in binary mixtures demonstrate that the CO₂ slows down the partner molecules far more than anticipated by the Maxwell-Stefan diffusion theory using pure component data inputs. The GCMC and MD simulation results also lead to the conclusion that DDR and CHA yield the highest permeation selectivities for membrane-based separation of CO₂/CH₄, CO₂/N₂, and CO₂/Ar mixtures. For N₂/CH₄ separation, DDR and ERI are the best choices.