Natural Gas Dehydration with Ionic-Liquid-Based Mixed Solvents

In this work, a mixed absorbent of an ionic liquid (IL) and triethylene glycol (TEG) is first proposed for natural gas ( NG) dehydration. The hydrophilic [EMIM][BF4] is selected as an appropriate IL composition from 240 potential candidates by calculating the thermodynamic separation factors (i.e., solubility and selectivity). The separation mechanism of NG dehydration with the mixed absorbent of [EMIM][BF4] + TEG at the microscopic molecular level is systematically unraveled by employing the COSMO-RS model and quantum chemical calculations. The solubility of CH4 in pure [EMIM][BF4] and TEG, as well as in binary mixtures of [EMIM][BF4] + H2O and [EMIM][BF4] + TEG, is experimentally measured and predicted by the modified (mod.) UNIFAC-Lei model. It is proved that the mod. UNIFAC-Lei model can well predict the gas-liquid equilibrium (GLE) and be successfully extended from binary to ternary systems. In the dehydration experiment, the mixed absorbent shows an excellent dehydration performance (i.e., the water content of the product gas is decreased to 172 ppm, in mole fraction). The equilibrium (EQ) stage model embedded with the mod. UNIFAC-Lei model parameters is built to carry out the process simulation of CH4 dehydration at the laboratory and industrial scales. The results show that the total annual cost (TAC) of the optimized process with mixed solvents is reduced by 33.63 and 15.98%, respectively, at the same separation conditions when compared to that of pure TEG and pure IL processes. This confirms that the IL-based mixed absorbent is a promising alternative to pure ILs or conventional solvents for applications in the field of gas separation and purification.