CO2 concentration in aqueous solution from gas–liquid equilibrium system to gas–liquid–hydrate coexistence system

Abstract

Hydrate-based CO2 sequestration is a new CCUS technology. The main concern for this method is the safe and long-term storage of CO2, which is related to the gas concentration in the surrounding aqueous solution. This work conducted a series of experiments to measure the saturated CO2 concentration in aqueous solution from gas-liquid (V–Lw) two-phase equilibrium system, gas-liquid-hydrate (V–Lw–H) three-phase equilibrium system up to hydrate stability region. The CO2 solubility curves in wider temperature and pressure range from V–Lw two-phase equilibrium system to V–Lw–H coexistence system were achieved in this work. When decreasing the temperature from V-Lw two-phase region into hydrate stability region under the constant pressure, the CO2 solubility in liquid phase would firstly increase with the decrease of temperature, and reach the maximum value at the V–Lw–H three-phase equilibrium temperature. After that, it would conversely turn to decrease with the decrease of temperature in hydrate stability region. When increasing the pressure from V–Lw two-phase region into hydrate stability region at constant temperature, the CO2 solubility would increase with the increase of pressure up to the point where the system pressure is equal to V–Lw–H three-phase equilibrium pressure. After that, it barely changes with the increase of pressure in hydrate stability region, indicating that the intrinsic formation reaction that the dissolved gas combines with water molecules to form gas hydrate is fast step, and the gas molecules dissolve at gas-liquid interface is the rate-determining step for hydrate formation process. The CO2 solubility in brine under V–Lw–H three-phase equilibrium conditions is higher than that in deionized water, which may contribute to revealing the mechanism that salt can promote hydrate formation rate at low concentrations.