An equation of state and a phase diagram for gas-hydrate-forming slurries in gas-shortage

Abstract

Slurries of gas hydrates, especially mixed hydrates of CO2 and TBPB (Tetra-n-Butyl Phosphonium Bromide), are currently under consideration for secondary refrigeration. Secondary refrigeration circuits contain wide sections where the two-phase slurry (solid + liquid), separated from the gas phase, undergoes transformations that would if gas were present, lead to both hydrate formation and gas dissolution. The absence of gas hinders these processes: the slurry is in gas shortage. The analysis demonstrates that, although thermodynamically distant from three-phase equilibria, slurries in gas-shortage are in equilibrium. For describing their thermodynamic state, the notion of CO2 partial pressure in a gas mixture is introduced, which leads to an equation of state and a new phase diagram with temperature or enthalpy versus CO2 mass-fraction. This diagram complements those commonly used for three-phase systems. The thermal behavior of slurries in gas shortage is then investigated, leading to the possibility of future experimental validation of the present analysis.