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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.

International audienceA new equation of state for electrolyte solutions has been developed from an expression of the Helmholtz free energy containing a nonelectrolyte part and a part relative to ions. The nonelectrolyte part is taken from the equations of state (EOS) of Schwartzentruber et al. (1989). The ionic part is composed of an MSA long-range term to account for electrostatic interactions and a short-range interaction term specific to ions. Using correlations between parameters and experimental ionic diameters, the model reduces to a one-parameter model. It has been applied to numerous strong electrolyte systems and extended to ternary systems to test its predictability without mixing parameters for ions. Its results compare well to the results reported for other one-parameter models (electrolyte EOS). Furthermore, it was found that the cation-anion interaction parameter could also be correlated to experimental ionic diameters. Then, the osmotic coefficients of 28 alkaline and alkaline-earth halide systems may be represented with a root mean square relative deviation of 2.9% using only six correlation parameters. This result has been extended to other systems, with the conclusion that the model with all parameters correlated may also be applied to systems other than halide solutions. The resulting model is predictive. The quality of the prediction was tested by determining osmotic coefficients relative to six systems without any parameter adjustment. The deviations of the predicted values range from 2.0 to 5.4%. The quality of the representation of mixed salts systems without mixing parameters was evaluated using experimental osmotic coefficients of 30 ternary systems

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.

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