Highlighting non-idealities in C₂H₄/CO₂ mixture adsorption in 5A zeolite

Gasification of biomass at relatively low temperatures (770-880 degrees C) yields producer gas mixtures containing H₂, CO and CO₂, along with CH₄, C₂H₆, and C₂H₄. It is of economic importance to recover C₂H₄ by selective adsorption using cation-exchanged zeolites such as LTA-5A. However, calculations of mixture adsorption equilibrium for LTA-5A using the Ideal Adsorbed Solution Theory (IAST) predict that the mixture adsorption equilibrium is selective to CO₂, and recovery of C₂H₄ is not feasible. In sharp contrast to IAST predictions, transient breakthrough experiments with H₂/CH₄/C₂H₆/CO/CO₂/C₂H₄ feed mixtures of different compositions show that selective recovery of C₂H₄ from the producer gas is feasible provided the C₂H₄/CO₂ molar ratio in the feed mixture is below unity; for C₂H₄/CO₂ molar ratios exceeding unity, the selectivity is in favor of CO₂. The selectivity reversal phenomena signifies strong non-idealities in mixture adsorption. Such non-idealities can be quantified using the Real Adsorbed Solution Theory (RAST). This article underscores the need for performing transient breakthrough experiments with realistic producer gas mixtures for process modelling and development purposes.