The aim of this work is to study how additives can improve formation conditions, thermal efficiency and flowing of clathrate hydrate slurries. In the case of PCM storage applications for refrigeration, hydrates must have a high dissociation enthalpy. In addition, the stability conditions must be adapted, the temperature being consistent with the application (between 273.15 and 298.15 K) and the pressure being suitable for an industrial facility. In the present study, different quaternary salts are used to reduce the formation pressure of CO_2 hydrate: TBACl (tetra-n-butylammonium chloride), TBANO_3 (tetra-n-butylammonium nitrate), and TBPB (tetra-n-butylphosphonium bromide). Firstly the influence of additives on the thermodynamic properties of the CO_2-H_2O system is studied. Results acquired by Differential Scanning Calorimetry, DSC, on formation temperatures and dissociation enthalpies show that each of the studied quaternary salts may form a mixed hydrate with CO_2. These mixed hydrates are more stable than without additives, resulting in a significant decrease of the CO_2 pressure needed for the hydrate formation. They also reveal that mixed TBPB ＋ CO_2 hydrate has suitable (P, T) conditions and latent heat content and therefore seems very appropriate as PCM for cold storage applications. After the previous thermodynamic study, a preliminary rheological characterisation of the TBPB hydrate slurry was also performed using a dynamic loop allowing pressure drop and flow rate measurements.
展开▼
