Phase equilibrium and dissociation enthalpy for semi-clathrate hydrate of CO2+TBAB

The present work investigates equilibrium conditions and dissociation enthalpy of hydrates formed from CO2-TBAB(tetra-n-butylammonium bromide)-water mixtures. Differential Thermal Analysis (DTA) was used for Hydrate-Liquid-Vapour (H-L-V) equilibrium condition determination in a TBAB concentration range from 4.43 to 9.01 wt% and in a CO2 pressure range from 0.3 to 2.5 MPa. The results showed that the presence of TBAB allowed decreasing the formation pressure of CO2 hydrate by approximately 74 and 87% at 283 and 279 K, respectively. Moreover, pressure reductions were dependent on the TBAB concentration. The dissociation enthalpy and the composition of double hydrate formed from 9.01 wt% TBAB solution were determined by both the DTA and Clapeyron equation. The DTA method resulted in 313.2 kJ per kg of hydrate for the dissociation enthalpy and 2.51CO2∙TBAB∙38H2O for composition of the double hydrate. For the use of Clapeyron equation, the volume change was defined by taking into account the gas solubility in the liquid phase. The calculation results showed a discrepancy with the experimental data obtained by DTA, suggesting the limited application of Clapeyron equation in the CO2-TBAB-water system.