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The present work investigates equilibrium conditions and dissociation enthalpy of hydrates formed from CO 2 -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 CO 2 pressure range from 0.3 to 2.5 MPa. The results showed that the presence of TBAB allowed decreasing the formation pressure of CO 2 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 2.51CO 2 •TBAB•38H 2 O 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 CO 2 -TBAB-water system.

The present work investigates the formation conditions and the latent heat of dissociation of hydrates formed from tetrahydrofuran (THF)-CO 2 -water mixtures. The conditions investigated are 3.8-15 wt % for THF concentration and 0.2-3.5 MPa for the CO 2 partial pressure range, conditions that are adapted to the use of the corresponding hydrate slurries as secondary refrigerants. Both differential thermal analysis (DTA) and differential scanning calorimetry (DSC) methods were used for the experimental determinations. Experimental values were compared with modeling, combining the van der Waals and Platteeuw approach with the Redlich-Kwong equation of state associated to a modified Huron-Vidal (MHV2) mixing rule. At fixed temperature, adding THF to the systems results in a drastic reduction of CO 2 equilibrium pressure. For instance, at 280 K, a 78.9% decrease of CO 2 pressure is experimentally observed if the solution contains 3.8 wt % of THF. Furthermore, a dissociation enthalpy of (CO 2 + THF) hydrates roughly two times higher that that of CO 2 hydrates was calculated from measured and predicted data of hydrate formation.

The present work investigates equilibrium conditions and dissociation enthalpies of semiclathrate hydrates formed from CO 2 + tetra-n-butylammonium chloride (TBACl) + water, CO 2 + tetra-n-butylammonium nitrate (TBANO 3 ) + water, and CO 2 + tetra-n-butylphosphonium bromide (TBPB) + water mixtures. Differential scanning calorimetry (DSC) was used for the determination of hydrate-liquid-vapor (H-L-V) equilibrium conditions in the presence of TBACl, TBANO 3 , and TBPB solutions at ammonium salt mass fractions of 0.3618, 0.3941, and 0.3707, respectively, and at CO 2 pressure in the range of (0.5 to 2.0) MPa. Results reveal that semiclathrate hydrates of TBACl, TBANO 3 , and TBPB are able to incorporate carbon dioxide in their structure and that the resulting mixed hydrates have significantly lower formation pressures than those of pure CO 2 hydrate. The dissociation enthalpies of semiclathrate hydrates of TBACl, TBANO 3 , and TBPB with CO 2 were determined by both DSC and the Clausius-Clapeyron equation. The DSC experiments demonstrate that mixed hydrates of TBANO 3 , TBACl, and TBPB with CO 2 have higher melting enthalpies than single hydrates. From our measurements, it appears that mixed TBPB + CO 2 hydrate has appropriate stability conditions (p, T) and latent heat content for secondary refrigeration applications. DSC measurements combined with the Clausius-Clapeyron equation show that mixed TBPB + CO 2 hydrate can store large amounts of CO 2 and thus could be attractive for gas capture and storage applications.

The phase behavior of simple and mixed semiclathrate hydrates formed from CO2 + tetrabutylphosphonium bromide (TBPB) + water mixtures was investigated by pressure-controlled differential scanning calorimetry (DSC) at TBPB concentrations in the range of 0 to 0.073 mole fraction and at CO2 pressure in the range of (0 to 2.0) MPa. In a previous article we demonstrated that TBPB + CO2 mixed hydrates present high dissociation enthalpies and could be used as phase change material, covering the range of temperature from (284.6 to 289.0) K, for secondary refrigeration applications. The present work investigates a broader domain of compositions, resulting in x–T phase diagrams at atmospheric conditions and at various CO2 pressures. These data are required to model the potential latent heat of hydrate slurries as a function of gas pressure and aqueous phase composition over the whole range of interest for refrigeration purposes. The results presented show that adding TBPB to the water at low concentration (0.0058 mole fraction) decreases the pressure of formation of CO2 hydrates to 0.5 MPa at 281.6 K, instead of 3.5 MPa at the same temperature in the absence of a promoter. Crystallization of CO2 + TBPB hydrate could therefore offer an attractive means of capture for CO2.