The equilibrium condition measurements were performed in the cyclopentane clathrate hydrate forming systems coexisting with sodium chloride as a technological basis for the development of seawater desalination technology using clathrate hydrates. The experiments were performed in the system under vacuum condition in addition to the system under atmospheric pressure. The equilibrium conditions in the system under the vacuum condition are essential to accurately construct the phase diagram in the cyclopentane clathrate hydrate forming system without the effects of the surrounding air. The equilibrated phases were sodium chloride aqueous solution + cyclopentane clathrate hydrate + liquid cyclopentane + vapor cyclopentane in the system under the vacuum condition. The concentration of sodium chloride investigated in this study, wNaCl, was 0 ≤ wNaCl ≤ 0.210 in mass fraction. The equilibrium temperature in both systems decreased with increasing wNaCl. The equilibrium temperature at a fixed concentration of sodium chloride under the vacuum condition was lower than that under the atmospheric pressure. It was attributed to the compression by the atmospheric pressure in addition to the incorporation of molecules constituting the atmospheric air.
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