Segregation effects in adsorption of CO 2-containing mixtures and their consequences for separation selectivities in cage-type zeolites

Abstract

For separation of CO 2 from gaseous mixtures with CH 4, N 2, 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 CO 2 is dictated by both adsorption and diffusion characteristics. For adsorption of CO 2/CH 4 mixtures, Grand Canonical Monte Carlo (GCMC) simulations show that the window regions of cage-type zeolites have a significantly higher proportion of CO 2 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 CO 2/N 2, CO 2/Ar, and N 2/CH 4 segregation effects are also present but their impact is far less severe than for CO 2/CH 4 mixtures. For CO 2-bearing mixtures, the preponderance of CO 2 in the window regions has important consequences for mixture diffusion. Molecular dynamics (MD) simulations for self-diffusion in binary mixtures demonstrate that the CO 2 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 CO 2/CH 4, CO 2/N 2, and CO 2/Ar mixtures. For N 2/CH 4 separation, DDR and ERI are the best choices.