Computational Screening of Physical Solvents for CO2 Pre-combustion Capture.

Abstract

A computational scheme was used to screen physical solvents for CO2 pre-combustion capture by integrating the commercial NIST database, an in-house computational database, chem-informatics, and molecular modeling. A commercially available screened hydrophobic solvent, diethyl sebacate, was identified from the screening with favorable physical properties and promising absorption performance. The promising performance to use diethyl sebacate in CO2 pre-combustion capture has also been confirmed from experiments. Water loading in diethyl sebacate is very low, and therefore, water is kept with H2 in the gas stream. The favorable CO2 interaction with diethyl sebacate and the intermediate solvent free volume fraction leads to both high CO2 solubility and high CO2/H2 solubility selectivity in diethyl sebacate. An in-house NETL computational database was built to characterize CO2, H2, N2, and H2O interactions with 202 different chemical functional groups. It was found that 13% of the functional groups belong to the strong interaction category with the CO2 interaction energy between -15 and -21 kJ/mol; 62% of the functional groups interact intermediately with CO2 (-8 to -15 kJ/mol). The remaining 25% of functional groups interact weakly with CO2 (below -8 kJ/mol). In addition, calculations show that CO2 interactions with the functional groups are stronger than N2 and H2 interactions but are weaker than H2O interactions. The CO2 and H2O interactions with the same functional groups exhibit a very strong linear positive correlation coefficient of 0.92. The relationship between CO2 and H2 gas solubilities and solvent fractional free volume (FFV) has been systematically studied for seven solvents at 298.2 K. A skewed bell-shaped relation was obtained between CO2 solubility and solvent FFV. When an organic compound has a density approximately 10% lower than its density at 298.2 K and 1 bar, it exhibits the highest CO2 loading at that specific solvent density and FFV. Note that the solvent densities were varied using simulations, which are difficult to be obtained from the experiment. In contrast, H2 solubility results exhibit an almost perfect positive linear correlation with the solvent FFV. The theoretical maximum and minimum physical CO2 solubilities in any organic compound at 298.2 K were estimated to be 11 and 0.4 mol/MPa L, respectively. An examination of 182 experimental CO2 physical solubility data and 29 simulated CO2 physical solubilities shows that all the CO2 physical solubility data are within the maximum and minimum with only a few exceptions. Finally, simulations suggest that in order to develop physical solvents with both high CO2 solubility and high CO2/H2 solubility selectivity, the solvents should contain functional groups which are available to interact strongly with CO2 while minimizing FFV.