(262bc) Solubility of Gases in Conventional Solvents and an Ionic Liquid for Synthesis Gas and Natural Gas Sweetening from Monte Carlo Simulations

The aim of the present work is to study the removal of acid gases from the synthesis and natural gases using physical solvents. Synthesis gas (syngas) is made from the gasification of a fuel to produce a mixture of carbon monoxide (CO) and hydrogen (H₂). However, the syngas also contains several impurities like carbon dioxide (CO₂), nitrogen (N₂), and sulfur compounds (mainly, hydrogen sulfide, H₂S). Similar impurities are also present in raw natural gas. These sour gases should be removed to meet customer and pipeline specifications. CO₂ should be removed to avoid dry ice formation during the liquefaction of the gas and due to the low caloric value of the gas, while H₂S is highly corrosive for the pipelines. The removal of the acid gases is typically performed in an absorber-stripper configuration using either a physical, a chemical solvent or a mixture of both (hybrid) solvents. The choice of the solvent is case specific and depends on many factors like the type and concentration of the impurities, the composition, the temperature, the pressure and ultimately the product specifications. However, chemical solvents (e.g., amines) are most widely used in the natural gas industry. Although amines are very effective in reducing the acid gas concentration, especially at low acid gas partial pressures, it has several serious drawbacks. These include their high energy requirement for solvent regeneration, volatility, corrosivity and the low CO₂/H₂S selectivity. Since the natural gas and syngas sweetening operates at high pressures, physical solvents can be applied for acid gas removal. In the past decade, ionic liquids (ILs) have emerged as a promising physical solvent for acid gas removal. ILs are defined as salts with melting point lower than 100 ^oC and characterized by a very low vapor pressure, high thermal and chemical stability.

The solubility data of acid gases are extremely important for the optimal design of absorption columns. However, experimental data of poorly soluble gases (e.g., CH₄, N₂ and H₂) and toxic gases (e.g., CO and H₂S) are scarcely reported in the literature for a wide range of temperatures and pressures. Moreover, experimental data for the solubility of gas mixtures in solvents, which requires an increased experimental effort, are even more scarce. Here we use force field based Monte Carlo simulations to compute the solubility of the pure gases CH₄, C₂H₆, H₂, CO, N₂, H₂S, and CO₂ in several conventional solvents (namely Selexol, Purisol, Rectisol and Fluor solvent) and an ionic liquid [bmim][Tf₂N]. The Henryâ??s coefficients of the gases in the solvents are calculated from the solubility data. Both the calculated solubilities and Henryâ??s coefficients of the gases from Monte Carlo simulations are in good agreement with available experimental data. The results show that molecular simulation can be a powerful tool, in the absence of experimental data, to obtain gas solubilities in complex solvents. Furthermore, the ideal selectivity of relevant gases is calculated from the Henryâ??s coefficients. The computed selectivities show the potential of the investigated solvents in removing the acid gases, namely CO₂ and H₂S. In conclusion, Monte Carlo simulations can be employed to obtain the required solubility data for designing natural gas or synthesis gas sweetening processes when no experimental data is available.

We have submitted a paper to the special issue of Fluid Phase Equilibria honoring Theo de Loos.