Dynamic separation of binary ethene/ethane mixtures by Ba-ETS-4

Abstract

The dynamic adsorptive separation performance of the small pore Ba²⁺-exchanged ETS-4, titanosilicate zeotype material, for mixtures of ethene and ethane was investigated. Na-ETS-4 was ion-exchanged with Ba ²⁺ to form microporous Ba-ETS-4, and characterized using XRD, SEM and EDX spectroscopy. Adsorption isotherms and fractional uptakes were recorded at three different temperatures of 30, 50, and 70°C to evaluate the effect of temperature on adsorption capacity and kinetics. Time-dependent fractional uptake recorded at different temperatures confirmed that ethene diffuses significantly faster than ethane in Ba-ETS-4, which can be attributed to ethene’s smaller molecular size, i.e. at 30°C, the time-constant diffusivity of ethene was 4.75 times higher than ethane. Though diffusivity increases for both gases with temperature, ethane rises more, reducing the ethene/ethane diffusivity ratio from 3.58 to 3.09 between 50 °C and 70 °C. Moreover, a high ethene/ethane kinetic selectivity of 12.5 was observed at 30 °C. Breakthrough experiments further highlighted the impact of diffusion limitations on separation. A comprehensive analysis has been performed for ethene/ethane mixtures between 30 and 90°C, ethene/ethane compositions varying from 10/90 (vol/vol%) - 90/10 (vol/vol%) and different regeneration conditions. Ethane exhibited a sharp breakthrough front, as it was excluded from adsorption in mixture conditions. A broader profile was observed for ethene, which its adsorption is affected by diffusion limitations. At higher temperatures, ethene’s breakthrough became sharper and more delayed, indicating reduced diffusion limitations. Optimal separation was achieved at 70 °C based on ethene uptake and breakthrough time difference. Given the strong adsorption in Ba-ETS-4, a TSA process is required for complete desorption of the adsorbed hydrocarbons.