Phase Equilibrium and Dissociation Enthalpies for Cyclopentane plus Methane Hydrates in NaCl Aqueous Solutions

The phase equilibrium data of cyclopentane (CP) + methane hydrates in brine water with NaCl mass fractions of w = 0.0.035, 0.070, and 0.100 were measured in the temperature range (284.4 in 301.3) K using a visual high-pressure phase equilibrium apparatus. The dissociation enthalpies of CP + methane hydrates in hot brine water were determined via the Clausius-Clapeyron equation based on these phase equilibrium data. The effect of help gas and salinity on the phase equilibrium of CP hydrate and CP + methane hydrates was studied, respectively. Liquid CP forms hydrates with small-molecule help gas at a temperature region much higher than the quadruple point of pure CP hydrate. Methane partially occupied the small cages of structure II hydrate, which accelerates the nucleation and growth of CP + methane hydrates and increases the stability of the hydrates. The phase equilibrium pressure of CP + methane hydrates increases with the increase in temperature, and it increases linearly with the increase in NaCl concentration in solutions. The higher the temperature, the more remarkable effect the temperature and salinity has on the phase equilibrium pressure. The dissociation enthalpy decreases with the increase in temperature and the NaCl concentration.