Modeling of the dissociation conditions of H-2 + CO2 semiclathrate hydrate formed with TBAB, TBAC, TBAF, TBPB, and TBNO3 salts. Application to CO2 capture from syngas

摘要

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 (H-2) + 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 (TBANO(3)). A very good description of the melting temperatures of H-2 semiclathrate hydrates is obtained over wide ranges of pressure and salt composition. It is predicted that the H-2 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 H-2. However, high separations factors are predicted at moderate salt concentrations, confirming the possibility of using semiclathrate hydrates to separate H-2 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 H-2 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. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.