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

CO2 and H2S in natural gas usually lead to low caloric value and corrosion of the transportation pipeline, and hence, separating CO2/CH4, H2S/CH4, and H2S/CO2 is essential for natural gas purification, desulfurization, as well as gas regeneration and reutilization. Trapping H2S, CO2, and CH4 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 CO2/CH4, H2S/CH4, and H2S/CO2 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 CO2 and H2S solvents and competent separators for removing CO2 or/and H2S from CH4. However, this requires more careful structure and property adjustments in order to efficiently separate CO2 from the acidic H2S. It was observed that ionic liquids with moderate basicity exhibit much better selectivity because strong alkali groups could strongly bind with both H2S and CO2, leading to low H2S/CO2 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 H2S/CO2 selectivity, which is caused by the reduction of CO2 capacity, dominated by physisorption, rather than an increase of H2S 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.