Preferential enclathration of CO2 into tetra-n-butyl phosphonium bromide semiclathrate hydrate in moderate operating conditions: Application for CO2 capture from shale gas

This work presents an investigation of using tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrate for CO2 capture from simulated shale gas that is composed of 40% CO2 and 60% CH4 in mole fraction. The experiments were performed in a stirred tank reactor with the mass fraction of TBPB (wTBPB) varying from 5.0% to 33.2%. The phase equilibrium data of TBPB semiclathrate hydrate formed in the presence of the 40% CO2/CH4 gas mixture were measured and reported. The effects of TBPB concentration on gas solubility, hydrate formation kinetics, and CO2 separation efficiency were investigated in the temperature range of 278.1-284.2 K with the initial pressure fixed at 2.8 MPa. The results indicated that at a fixed driving force (overpressure), the gas uptake obtained at the induction time was nearly constant, despite the variation in TBPB concentration, while the rate of hydrate formation increased with the increase in TBPB concentration. Compared to the results obtained at a mole fraction of 1% THF, it was found that enhanced hydrate formation kinetics, a higher CO2 separation factor (31.7 ± 3.3), and a higher operating temperature (284.2 K) were obtained at the stoichiometric TBPB concentration (wTBPB = 33.2%). Therefore, TBPB may be used as a promising promoter for hydrate-based CO2 capture from the simulated shale gas (CO2/CH4 gas mixture).