In this study, general models were developed to predict the solubility of CO2, H2S, CH4 and C2H6 in ionic liquids (ILs) where no experimental data are available. These models use fugacity functions based on asymmetric activity coefficients calculated using the fundamental COSMO-RS method, the Peng-Robinson equation of state (PR-EOS), and an empirical Henry’s constant of the solute in the IL. The Henry’s constant has been correlated with the temperature and pressure of the system and physical properties of the ILs. It was found that, for CO2 and CH4, the molecular weight (MW) of the ILs and for H2S and C2H6, the surface area of the ILs are the best choice to correlate solubilities. 425 ILs were ranked based on their absorption capacity and selectivity of H2S and CO2 versus CH4 and C2H6 absorption using the newly developed procedure. The top eight ILs were selected and characterized for use in a commercial simulator. EOS’s binary interaction parameters between solutes and IL were determined using the proposed model.
The performance of the ILs as solvents in gas sweetening plants is compared to Morphysorb (a physical solvent) and MDEA (a selective chemical solvent) at similar gas feed and product specifications. Among the candidate ILs, pmim-L appears to be the best option for gas sweetening. ILs show better performance over MDEA and Morphysorb when operating at high H2S compositions, that is for bulk removal of acid gases. In a case study, the H2S concentration was reduced from 13% to 5% and the total heating and pumping duty required for the pmim-L gas plant was 23 times less than the MDEA and 9 times less than the Morphysorb. Also, pmim-L required 89 times less cooling than MDEA and 13 times less than a Morphysorb for the simulated gas plant. Furthermore, IL gas plants require negligible makeup solvent whereas MDEA plant requires 4.8 kg/hr pure MDEA and 13 m3/hr makeup water. A Morphysorb plant required 84 kg/hr makeup solvent. It was also shown that ILs are hydroscopic and can reduce the water content of natural gas. With a few percent additional energy, the pmim-L gas plant can be converted to a gas sweetening-dehydration plant which is able to meet the water content specifications for natural gas pipelines. Alkanolamine plants require an additional dehydration unit to produce sales gas. Based on the partial pressure of H2S in the feed and product, guidelines have been provided to choose between MDEA and pmim-L gas plants.