Developing an efficient sorbent and adopting an ethylene adsorption separation unit in downstream of an oxidative coupling of methane (OCM) process were the main focus of the present study. Since the mole fraction of the generated ethylene in the OCM reactor outlet is relatively low, the processing cost of the accompanying components and thereby the separation cost per ton of ethylene using the conventional cryogenic separation are insupportable. Zeolite 13X was modified in this research, demonstrating outstanding ethylene adsorption capacity and selectivity. The conducted adsorption experiments at a miniplant-scale unit enabled monitoring the adsorption breakthrough times and measuring the sorbents' capacity under different operating pressures in the range of 1-5 bar while processing different feed flow and feed compositions, representing the attachment of the adsorption unit to different parts of the OCM process. The modified zeolite 13X showed superior performance than the reference sorbents such as zeolite 4A and activated carbon. Physical treatment of zeolite 13X, by calcining it at 550 °C, proved to be efficient in increasing its adsorption capacity. Chemically treating zeolite 13X via copper exchange on the other side increased its ethylene adsorption selectivity in competition to CO2 adsorption. In processing the CO2-rich feed streams (with the CO2 content 2.25 times of its C2H4 content), the Cu-exchanged zeolite 13X showed a promising ethylene adsorption capacity of 0.46 molC2H4·kgs-1 combined with an adsorption selectivity of 0.45 molC2H4·molCO2. In processing the CO2-free feed streams (with the C2H4 content 2.75 times of its C2H6 content), using the calcined zeolite 13X secured the highest adsorption capacity of 1.4 molC2H4·kgs-1 along with an adsorption selectivity of 3.8 molC2H4·molC2H6 under 5 bar adsorption pressure. These indicate the promising potentials of the developed sorbents and the designed adsorption unit for processing the OCM reactor outlet gas stream before and after removal of its CO2 content.
Bibliographical noteFunding Information:
Dr. Babak Mokhtarani thanks Alexander von Humboldt (AvH) Foundation for financial support to this work.