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.

Thermodynamic properties of TBAB + CO2 + water system are important for using TBAB + CO2 semiclathrate hydrates in some promising hydrate-based techniques. This work is an experimental and modeling study of two fundamental properties in vapor–liquid phase equilibrium of TBAB + CO2 + water, that is, CO2 solubility in TBAB solution and density of CO2-saturated TBAB solution. With the use of a PVT system, CO2 solubility was measured; thus, Henry’s constant was obtained at temperatures from 288.1 K to 303.1 K and TBAB mass fraction from 0.10 to 0.40. Our results showed that CO2 is either salted-in or salted-out by the presence of TBAB, depending upon the temperature and TBAB concentration. For the density, we present experimental data for TBAB + water system at T = 283.15 K to 303.15 K with pressures up to 5 MPa. The density of CO2-saturated TBAB aqueous solution was calculated with the help of Duan’s model, which takes into account the effect of pressure and temperature to calculate the apparent molar volume of dissolved CO2 in pure water.

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