Assessment of ionic liquids as H2S physical absorbents by thermodynamic and kinetic analysis based on process simulation

A comprehensive evaluation of ionic liquids (ILs) as potential H2S absorbents was performed using both molecular and process simulation. First, the Conductor-like-Screening MOdel for Real Solvents (COSMO-RS method) was applied to select promising ILs absorbents and to understand the H2S gas solubility from a molecular point of view. The ILs screening more than 700 ionic combinations determines that H2S physical absorption is mainly controlled by the hydrogen-bond acceptor capacity of the anion, due to the easily hydrogen bond formation when mixed with the acidic solute. Based on molecular simulation analysis, 6 ILs of different nature were evaluated in a typical industrial packed absorption column using COSMO-based/Aspen Plus methodology. Equilibrium based simulations demonstrated higher H2S separation efficiency (Le. lower solvent expenses and smaller equipment sizes) when increasing H2S absorption capacity of the IL solvent. In contrast, rigorous process simulation analysis (including kinetic equations) reveals a strong mass transfer kinetic control in the H2S absorption in commercial packed column, which severely limits the maximum H2S absorption given by thermodynamics. As a result, ILs that present the best performance in the thermodynamic aspect, become worse for the operation. In fact, it was found that H2S recovery at given operating conditions increases when decreasing the viscosity of IL, being 1-ethyl-3-methylimidazolium dicyanamide, the one that presents the best absorbent performance, requiring the lowest operating temperatures and liquid volume flows. Lastly, the absorption operation was designed to achieve fixed H2S recovery using different liquid/gas feed ratios, resulting in column heights and diameters inside the typical range marked by heuristic rules for usual industrial packed columns. In sum, current prospective study based on COSMO-RS and Aspen Plus have been proved as a useful tool to analyze the potential industrial application of ILs in the H2S capture and to select the most adequate ILs, before starting with experimental tests, highly demanding in cost and time.