Evaluation and screening of adsorbents for the separation of carbon dioxide from natural gas

© 2020 Lefu TaoReplacing coal with natural gas is one of the feasible approaches to mitigate CO2 emission at present. To satisfy the increasing global demand for natural gas, more research and development efforts are being put in converting the highly sour natural gas into applicable energy sources. Adsorption processes offer a promising method to remove the bulk sour component, CO2, from high pressure sour natural gas. Adsorptive properties of six zeolites and one silica gel (CBV-760, CBV-780, HSZ-320HOA, HSZ-350HUA, HSZ-385HUA, HSZ-390HUA, and Sorbead WS) were characterised by various methods. Their adsorption isotherms of CO2 and CH4 in the low-pressure regime (0 – 10 bar) were used to determine the isosteric heat of adsorption, while the high-pressure adsorption isotherms (0 – 50 bar for CO2 and 0 – 100 bar for CH4) were fitted to the Toth and the Langmuir adsorption isotherm models. The model fitting parameters were further used in a rigorous simulation model to evaluate the performance of the selected adsorbents in separating a gaseous mixture with 30% CO2 and 70% CH4. The simulated CH4 recovery were used as a major adsorbent performance metric. Besides, another traditional adsorbent performance indicator, IAST (Ideal Adsorbed Solution Theory) selectivity, was employed for the screening of the adsorbents. Both evaluation metrics gave different results about the performance ranking for the same CO2/CH4 separation application. Although HSZ-320HOA was suggested to be the best adsorbent among selected materials by the IAST selectivity, the process simulation preferred Sorbead WS which achieved the highest CH4 recovery (71%) among selected adsorbents. To push the limitations of the traditional adsorbent performance indicators, a new indicator was proposed. The new indicator, adiabatic column working selectivity, contained all features that other indicators have, and also involved a new factor, the components remaining in the void gas phase. The adsorbent evaluation results of the adiabatic column working selectivity were compared with the results of other indicators and the rigorous simulation model. It was found that only the new indicator could match the results of simulations well, while other indicators deviated more or less from the simulation results. The new indicator was proved effective for screening the adsorbents for the application of CO2 removal from natural gas, but to apply this indicator, two process variables, the temperature and bed composition at the end of the desorption step must be determined beforehand. To overcome this difficulty, two simplified simulation models were developed to facilitate the calculation of all critical process variables. The newly developed simplified models could determine the comparable desorption conditions as well as the rigorous simulations, but within much less computation time. A pressure swing adsorption (PSA) plant was operated in the CO2CRC Otway CO2 Capture Research Facility applying Sorbead WS as the adsorbent to separate CO2 from gas mixtures with various compositions (28 mol% to 78 mol% CO2 balanced with CH4) under different conditions from May 2017 to May 2019. Using the recorded process conditions, the process performance predicted by the simplified models could agree well with the actual process performances. This outcome verified the validation of the simplified models. Overall, we found a novel adsorbent performance indicator, adiabatic column working selectivity to evaluate the performance of adsorbents for the application of CO2 removal from natural gas at high-pressure. At the same time, with the help of a newly developed simplified verified by a real PSA plant, the calculation results of the adiabatic column working selectivity can more precisely predict and screen adsorbents for the targeted application