Thermodynamic properties of semiclathrate hydrates formed from the TBAB+TBPB+water and CO2+TBAB+TBPB+water systems

Abstract : Semiclathrate hydrates formed from binary systems of tetra-n-butylammonium bromide (TBAB) + water and tetra-n-butylphosphonium bromide (TBPB) + water have been investigated for their promising application in the refrigeration system. In the present work, semiclathrate hydrates in the ternary system of TBAB + TBPB + water were studied for the first time, in the presence and absence of CO2 pressure. Using DSC technique, the dissociation temperatures of semiclathrate hydrates were determined at the mass fraction of salt mixture ranges from 0.05 to 0.60 with variable composition from pure TBAB to pure TBPB. At any salt concentration tested, the dissociation temperature of semiclathrate hydrates increased regularly with TBAB composition increase. Applying CO2 pressure in the TBAB + TBPB + water system resulted in further increase of dissociation temperature due to the gas enclathration in the mixed hydrate structure. The phase transition enthalpy of semiclathrate hydrates were investigated at wsalts = 0.40 and variable TBPB compositions, without CO2 and with pCO2=1.0 MPapCO2=1.0 MPa. It was found that the dissociation enthalpies of CO2-free and CO2-containing semiclathrate hydrates are almost independent of the TBPB salt composition. From our current results, TBAB + TBPB + CO2 hydrates appear as valuable phase change materials to be used as hydrate slurries for cold storage and transportation in applications requiring variable working temperatures.