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Gas hydrate technology is known as an alternative method for natural gas storage. Carbon dioxide hydrate has recently received growing attention due to its many applications. Additives such as cyclohexane (CH) (non soluble in water) and 1,3 dioxolane (DX) (soluble in water) are able to tackle thermodynamic and kinetic imitations of hydrate formation. This study attempts to study the synergic effect of the (DX)/(CH) mixture on the formation conditions of CO 2 hydrate systems. These two additives have a promoting effect; however, there is no experimental data on the effect of their mixture in this regard. All experiments were performed by pursuing a constant-volume−pressure search technique with a step heating approach in the pressure, temperature, and the DX mass fraction (w) ranges of (1−55) bar, (280−310) K, and (4.1, 10.1%), respectively. The mixture of DX and CH significantly improved the stability of hydrate formation compared to individual promoters. A thermodynamic framework based on the van der Waals−Platteeuw model was developed to predict the DX/CH−CO 2 hydrate phase equilibrium. The Peng− Robinson equation of state was applied to acquire CO 2 properties in the gas phase. The DX and CH activity coefficient was obtained through the NRTL model. The results of the proposed model were found to be in satisfactory accordance with the experimental phase equilibrium data with a low discrepancy of 2.83%.

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Gas hydrate, or clathrate hydrate, technology is considered to be one of the promising solutions for natural gas storage and transportation. Tetrabutylphosphonium bromide (TBPB) is capable of sorting out stability issues of hydrate formation effectively. On the other hand, using water from natural resources such as sea, river, or well instead of pure water is essential for hydrate formation in the industrial scale. The current study was organized to investigate the hydrate phase behaviors of CH4 + TBPB formed in saline solutions, which are necessary for potential industrial applications. For this purpose, we first generated hydrate dissociation conditions data of the CH4 + TBPB aqueous solution system over TBPB mass fractions of 0.05 and 0.2. It should be mentioned that there is some discrepancy between hydrate dissociation condition data of the CH4 + TBPB + water system reported in literature, which necessitates generation of accurate and reliable data in this work. Afterward, the effects of the presence of NaCl, MgCl2, and NaCl + MgCl2 in aqueous solutions on the hydrate dissociation conditions of the CH4 + TBPB + water system were studied. All equilibrium points were obtained using a reliable constant-volume pressure-search method in the pressure and temperature ranges of 1–5.5 MPa and 280–292 K, respectively. The experimental results show that the aforementioned salts have dual effects on the hydrate phase stability, depending on TBPB concentration in the aqueous solution. In the case of 0.05 mass fraction of TBPB in the aqueous solution, the presence of NaCl (0.05 mass fraction), MgCl2 (0.05 mass fraction), and NaCl + MgCl2 (0.05 + 0.05 mass fractions) can boost the hydrate stability conditions of the CH4 + TBPB + water system and can act as thermodynamic promoters. However, by increasing the TBPB mass fraction in aqueous solution to 0.20 mass fraction, the aforementioned salts can shift the hydrate stability conditions to high pressures/low temperatures and can act as thermodynamic inhibitors. Furthermore, the experimental results show that the thermodynamic inhibition effects of the mineral salts are higher when the pressure of hydrate formation increases. A thermodynamic model based on the van der Waals–Platteeuw (vdW–P) solid solution theory was also developed for the aforementioned systems. The model employs the Soave–Redlich–Kwong equation of state and the Bromley equations for the gas phase and the aqueous phase, respectively. Results show that the thermodynamic model is capable of predicting the hydrate dissociation conditions with acceptable accuracy.