Modeling of the dissociation conditions of H2Â +Â CO2 semiclathrate hydrate formed with TBAB, TBAC, TBAF, TBPB, and TBNO3 salts. Application to CO2 capture from syngas

Syngas produced from coal gasification or methane reforming contains a significant amount of carbon dioxide (CO2) that has to be removed. Semiclathrate hydrates have been considered as a possible medium for gas separation and CO2 capture. Here, we apply Paricaud's approach [J. Phys. Chem. B 2011, 115, 288-299] to model the dissociation conditions of hydrogen (H2) + CO2 semiclathrate hydrates formed with tetra-n-butyl ammonium bromide (TBAB), tetra-n-butyl ammonium chloride (TBAC), tetra-n-butyl ammonium fluoride (TBAF), tetra-n-butyl phosphonium bromide (TBPB), and tetra-n-butyl phosphonium nitrate (TBANO3). A very good description of the melting temperatures of H2 semiclathrate hydrates is obtained over wide ranges of pressure and salt composition. It is predicted that the H2 storage capacities of semiclathrate hydrates is rather low (about 0.05 wt% at 4 MPa), showing that semiclathrate hydrates are not good candidates to store H2. However, high separations factors are predicted at moderate salt concentrations, confirming the possibility of using semiclathrate hydrates to separate H2 and CO2. It is predicted that the dissociation conditions of semiclathrates in presence of gas mixture significantly depend on the global composition of the mixture: in particular, the melting point is changed by varying the initial amount of water even if the global mole fraction ratio between H2 and CO2 is kept constant. It is shown that an increase of the salt concentration gives rise to a decrease of the separation factor, and the sudden drop of CO2 capture observed for high salt concentrations is explained by a change of hydrate structure.