An experimental study on the formation enthalpy of CO2 semi-clathrate hydrates with TBAB and TBPB additives

Abstract

CO2 hydrates has been proven by many studies as a promising phase change material (PCM).Tetra-nbutylammonium 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 semiclathrate 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 semiclathrates 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.