An Experimental Study on the Formation Enthalpy of CO2 Semi-Clathrate Hydrates with TBAB and TBPB additives
CO2 hydrates has been proven by many studies as a promising phase change material (PCM).Tetra-n-butylammonium bromide (TBAB) and tetra-n-butylphosphonium bromide (TBPB) are usually used to moderate CO2 hydrate formation conditions. A challenge to determine the gas hydrate enthalpy is the calorimeter needs to beoperable and accurate underhigh pressures. T-history method offers an easy and accurate way to characterise PCMs. Using this method, a T-history reaction tube was designed that stands 10 bar pressure.In a previous study the formation enthalpy of CO2–TBAB semi-clathrate hydrate was found 318.5 kJ•kg-1(hydrate) and that of CO2–TBPB semi-clathrate hydrate was 351.7 kJ•kg-1(hydrate). The enthalpy of CO2semi-clathrates are high compared to other PCMs, such as TBAB salt hydrate (193.2 kJ•kg-1), C14–C18 (158.3 kJ•kg-1), hexadecane-tetradecane (147.7–182.7 kJ•kg-1), and capric acid and lauric acid (115.1 kJ•kg-1). In this work, influencing factors of formation enthalpy and specific heat of CO2 semi-clathrate hydrate were determined using the reaction tube under various conditions. It was found that higher TBAB/TBPB salt mass fraction in the solution and higher feed pressure were favourable for larger latent heat storage capacity and specific heat of liquid phase when using these semi-clathrates in practical conditions (with insufficient CO2 and overpressure driving force). Adding TBAF and SDS also enlarged the latent heat of semi-clathrates and their effect at various concentrations is different.