Characterization of mixed CO2 + TBPB hydrates for refrigeration applications

The present work investigates the use of semiclathrate hydrates, formed from CO2 + tetra-nbutylphosphonium bromide (TBPB) + water mixtures, as appropriate media for cold storage and distribution in refrigeration applications. Previous studies show that these hydrates are able to trap molecules of carbon dioxide resulting in mixed hydrates. Calorimetry devices were used for determining the dissociation enthalpies of mixed CO2 + TBPB hydrates under various stability conditions (P, T) and salt concentrations. The results reveal that mixed CO2 + TBPB hydrates can be considered as good candidates for air-conditioning, due to positive melting temperatures (between 282 to 289 K) at moderate CO2 pressures (between 0.5 to 2 MPa). A hydrate solid fraction model was developed based on a CO2 mass balance taking into account CO2 solubility in aqueous tetrabutylphosphonium salt solution. The salt effect parameter was evaluated in order to estimate the influence of TBPB on the CO2 solubility. Finally, in order to characterize the flow behavior of mixed CO2 + TBPB hydrate slurries, a rheological study was carried out in a dynamic loop and an Ostwald-de Waele model was obtained.

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