Using transient breakthrough experiments for screening of adsorbents for separation of C2H4/CO2 mixtures

The recovery of C2H4 from gaseous reactor effluents from processes such as oxidative coupling of methane (OCM), and biomass gasification (at relatively low temperatures (770-880 degrees C) is becoming of increasing industrial and economic importance. The reactor effluents are N-2/H-2/CO/CO2/CH4/C2H6/C2H4 mixtures, typically containing less than 15% C2H4. For recovery of C2H4, pressure swing adsorption (PSA) technology using a selective adsorbent offers an energy-efficient alternative to the more conventional separation schemes such as amine absorption and cryogenic distillation The major objective of this investigation is to screen commercially available cation exchanged zeolites (13X, CaX, NaY, 5A, 4A) and activated carbon (AC) to determine the most suitable adsorbent. For all these materials, the adsorption strengths of CO2, and C2H4 are significantly higher than that of other gaseous constituents; consequently, the C2H4/CO2 separation selectivity is the key to the efficacy of any adsorbent. The variety of adsorbents were screened using transient breakthrough experiments with feed mixtures using different C2H4/CO2 ratios. On the basis of the breakthroughs, the adsorbents could be distinguished in three different categories: (i) 13X and 4A are selective to CO2, (ii) CaX, NaY, and 5A are virtually non-selective, and (iii) AC is selective to C2H4 over the entire range of feed compositions and is therefore the adsorbent of choice. The experimental breakthrough results are also compared with simulations using published unary isotherm data, along with the Ideal Adsorbed Solution Theory (IAST) for determination of mixture adsorption equilibrium. This comparison demonstrates that screening adsorbents solely on the basis of IAST calculations is likely to be misleading. This article underscores the need for performing transient breakthrough experiments with realistic feed gas mixtures for process modelling and development purposes.