CO₂/CH₄ and H₂S/CO₂ Selectivity by Ionic Liquids in Natural Gas Sweetening

CO 2 and H 2 S in natural gas usually lead to low caloric value and corrosion of the transportation pipeline, and hence, separating CO 2 /CH 4, H 2 S/CH 4, and H 2 S/CO 2 is essential for natural gas purification, desulfurization, as well as gas regeneration and reutilization. Trapping H 2 S, CO 2, and CH 4 not only eliminates their environmental contamination, but also increases feedstocks for industrial production. In order to avoid the contamination and causticity problems caused by the conventional alkali absorbents, ionic liquids have been proposed as alternatives to absorb and separate different gases for decades. In this paper, the investigations of pure ionic liquids in the selective absorption of CO 2 /CH 4, H 2 S/CH 4, and H 2 S/CO 2 are reviewed. Important influencing factors, including temperature, pressure, functionality, and properties of gas and ionic liquids, were analyzed. Ionic liquids with alkali groups on the cation and anion are promising CO 2 and H 2 S solvents and competent separators for removing CO 2 or/and H 2 S from CH 4 . However, this requires more careful structure and property adjustments in order to efficiently separate CO 2 from the acidic H 2 S. It was observed that ionic liquids with moderate basicity exhibit much better selectivity because strong alkali groups could strongly bind with both H 2 S and CO 2, leading to low H 2 S/CO 2 selectivities. A statistic work was carried out and revealed that ionic liquids with small molecular weights and compact structures usually have better selectivity; low temperature and pressure are favorable for increasing separation performance in physically selective absorption. As for the selectivity involving chemisorption, increasing temperature possibly enhances H 2 S/CO 2 selectivity, which is caused by the reduction of CO 2 capacity, dominated by physisorption, rather than an increase of H 2 S capacity. In comparison with the ideal selectivity, real selectivity makes more application sense because it usually involves competitive absorption of binary gas mixtures rather than a pure gas component. According to the reported real selectivities combining experimental results and molecular dynamic simulation, it is concluded that feeding gas ratio, ionic liquid dosage, temperature, and pressure are significant factors required to be adjusted to approach great real gas selectivity.