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

Abstract

Gasification of biomass at relatively low temperatures (770–880 °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.