Modeling of the dissociation conditions of H₂ + 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.