Efficient purification of biogas using ionic liquid as absorbent: Molecular thermodynamics, dynamics and experiment

Biogas, as a cheap and easily available clean biomass energy, has attracted extensive attention. In this work, the simultaneous decarburization, desulfurization and dehydration of biogas with ionic liquids (ILs) was proposed. Based on the COSMO-RS model, the separation performance of 400 ILs composed of 20 cations and 20 anions was screened by using Henry's law constant (HLC) and selectivity coefficient as separation performance indexes. Among them, 1,3-dimethylimidazolium methylsulfate [C1MIM][MeSO4] was considered to be the most promising candidate absorbent. Furthermore, the HLC and selectivity of CH4, CO2 and H2S in [C1MIM][MeSO4] were predicted by COSMO-RS at different temperatures and pressures. The results show that the selected [C1MIM][MeSO4] has excellent selective absorption performance for CO2 and H2S. The regeneration experiment of [C1MIM][MeSO4] showed that the absorption effect of CH4, CO2 and H2S was almost unchanged (i.e., HLC was almost unchanged) after 6 absorption-desorption cycles. Molecular surface electrostatic potential (ESP) analysis was performed to obtain the binding sites of intermolecular interactions to reveal the separation mechanism of simultaneous decarbonization, desulfurization and dehydration of ILs. Interaction region indicator (IRI) analysis showed that IL [C1MIM][MeSO4] mainly interacted with the components to be separated in the form of hydrogen bonds. The spatial distribution function (SDF) reveals the spatial distribution of different gases around [C1MIM][MeSO4] from the perspective of cluster macromolecules. This provides theoretical insights into molecular thermodynamics and dynamics for the development of new ILs for biogas purification.