Much attention has been made to solidified natural gas (SNG) technology via clathrate hydrates in recent years. Tetra-n-butyl ammonium bromide (TBAB) is known as a promising promoter to tackle hydrate technology limitations. This research focuses on investigating the effect of NaCl, MgCl2, and the mixture of NaCl + MgCl2 (two major soluble salts in naturally occurring water are NaCl and MgCl2) on hydrate stability conditions of methane in the presence of TBAB aqueous solution. An isochoric pressure search method was employed to generate the dissociation/equilibrium data in the temperature, pressure, and TBAB composition ranges of 275–291 K, 0.5–5.5 MPa, and 5–20 wt %, respectively. The experimental results reveal that in the case of wTBAB = 5%, NaCl and MgCl2 with the low concentration of 5% have a promotion effect for the systems of CH4 + TBAB + NaCl + H2O, CH4 + TBAB + MgCl2 + H2O, and CH4 + TBAB + NaCl + MgCl2 + H2O and shift the dissociation curve toward milder region (higher temperature and lower pressure). However, in the case of wTBAB = 20%, NaCl and MgCl2 play an inhibition role in all of the aforementioned systems. A thermodynamic model was developed based on the van der Waals–Platteeuw (vdWP) solid solution theory, to predict the behavior of methane in the presence of the promoter in saline water. The Peng–Robinson equation of the state (PR EOS) is used to describe the thermodynamic properties of the gas phase, and the electrolyte non-random two-liquid (e-NRTL) activity coefficient model is employed to determine the activity coefficient of water and promoter in the electrolyte solution. The presented model results are in satisfactory agreement with the experimental data generated in this work. The discrepancy of the model results with experimental is 10.78%.

In this communication, the promoting effects of two additive mixtures (cyclopentane + TBANO3) on CH4 hydrate phase equilibrium were experimentally investigated using the isochoric method. The mass fractions of TBANO3 employed in this paper were 5, 10, 15, and 20. The results showed that cyclopentane is a good promoter in comparison with pure water + CH4 systems, and TBANO3 has a promoting effect but not like cyclopentane in compassion with pure water + CH4 systems. Additionally, the mixture of cyclopentane + TBANO3 has an important promoting effect in comparison with pure water and TBANO3 + water + CH4 systems, but it has a less promoting effect in comparison with cyclopentane + water + CH4 systems. Furthermore, a new thermodynamic model was investigated to predict the equilibrium conditions of hydrate dissociation (pressure and temperature). The hydrate phase was described by the van der Waals–Platteeuw model, and the aqueous phase was predicted with the E-NRTL model. In addition, the NRTL model was used to predict the organic phase, and the gas phase was described with SRK EOS. The results revealed that the experimental hydrate dissociation data’s agreement with the proposed model is acceptable and possesses an average of 2.43%. Finally, an analogy was generated with the literature data. This study gives a better comparison of hydrate phase equilibrium combined with CH4 capture.

Available water for the formation of natural gas hydrate at the industrial scale is normally saline water in which the dissolved salts inhibit the hydrate formation. To overcome this challenge, thermodynamic promoters can be used. In the present work, hydrate phase equilibria of methane + mixed promoters of tetrahydrofuran (THF) and tetra-n-butyl ammonium bromide (TBAB) were investigated in the absence and presence of NaCl, MgCl2, and NaCl + MgCl2 aqueous solutions. The phase equilibrium data, which were generated using an isochoric pressure-search method with a step heating technique, are reported in the pressure and temperature ranges of 1.09–5.16 MPa and 286.0–299.0 K, respectively. Equilibrium data show that an aqueous mixture of THF + TBAB (0.15 + 0.05 mass fractions) in the presence of NaCl systems is a more effective promoter compared to a single promoter TBAB or THF even at a high concentration of about 0.2 mass fraction.

Gas hydrate technology has a great potential for natural gas storage and transportation on the industrial scale. The required water for hydrate formation could be supplied from the sea and river water, in which the dissolved salts inhibit the formation of hydrate. Some additives like tetra-n-butyl ammonium chloride (TBAC), which can form semiclathrate structures, can promote the thermodynamic stability conditions of hydrate formation. Although sufficient phase equilibrium data of TBAC semiclathrate hydrates of methane seem to be available, there are some discrepancies in the phase equilibrium data, particularly for the 0.05, 0.3, and 0.34 mass fractions of TBAC. Furthermore, the phase equilibrium data of TBAC in the presence of NaCl, MgCl2, and mixed NaCl + MgCl2 have not been reported in the literature yet. In this work, first, the hydrate phase equilibrium data of TBAC + CH4 were obtained accurately. In the next phase, the effects of NaCl and/or MgCl2 on the phase stability conditions of TBAC + CH4 hydrate systems were studied. The experiments were conducted with the TBAC aqueous solution at two concentrations of 0.05 and 0.20 mass fractions in the absence and presence of NaCl (0.05 mass fraction), MgCl2 (0.05 mass fraction), and NaCl + MgCl2 (0.05 + 0.05 mass fractions). The phase equilibrium data were reported in the pressure and temperature ranges of (1.27–5.46 MPa) and (282.2–291.8 K), respectively. Equilibrium data show that the presence of NaCl and/or MgCl2 (0.05 mass fraction) + TBAC (0.05 mass fraction) in an aqueous solution has synergetic effects on the promotion of the stability conditions of methane hydrate as compared to that of TBAC (0.05 mass fraction). At a higher concentration of TBAC (0.20 mass fraction), the mineral salts play the role of an inhibitor and shift the methane + TBAC hydrate phase equilibrium curve to the left side. The hydrate dissociation data generated in this work show that TBAC can promote the stability conditions and improve the stability of methane in the presence of the mineral salts effectively.

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.