Phase behaviour of mixed-gas hydrate systems containing carbon dioxide

We describe a new apparatus suitable for measurements of the phase behaviour and phase properties of fluid mixtures under conditions of high-pressure. We propose a synthetic method for the determination of gas solubility, and present results for the system (CO2 + H2O). In addition, we report new measurements of the hydrate equilibrium curves in aqueous systems containing either pure carbon dioxide or mixed gases including CO2. For hydrates formed in the (CO2 + H2O) system, we find an enthalpy of dissociation of 77 kJ · mol−1. This value was unchanged by the addition of mass fraction 0.043 of NaCl to the water. Compared with pure CO2, mixtures of CO2 with air exhibited markedly different dissociation pressures at given temperature, but were characterised by the same enthalpy of dissociation. However, two mixtures containing either nitrogen or methane and hydrogen both exhibited a higher enthalpy of dissociation, 106 kJ · mol−1, consistent with these systems forming structure II hydrates.