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One of the limitations in the process of hydrate formation to benefit its positive application is high pressure and low temperature conditions. Design and construction of a unit with the aforementioned conditions is therefore expensive and unsafe. Thus, an investigation of methods for moderation of hydrate formation conditions seems to be very important. As mentioned in literature, utilization of ammonium salts in water normally promotes the hydrate formation conditions. One of these salts is tetra-n-butylammonium fluoride (TBAF). In this research, the dissociation data of semiclathrate hydrates for the systems of methane + TBAF + water, carbon dioxide + TBAF + water, and nitrogen + TBAF + water have been measured and reported. Experimental measurements were performed at three concentrations of TBAF, that is, (0.02, 0.05, and 0.15) mass fraction. A comparison of hydrate dissociation data in the presence or absence of TBAF shows the promotion effect of TBAF on methane, carbon dioxide, and nitrogen hydrate formation. By increasing the concentration of TBAF from (0.02 to 0.15) mass fraction, its promotion effect increases, and the p−T curves of the double gas + TBAF semiclathrate systems shift to the low pressure and high temperature regions (moderate conditions). Results of the experiments show that, contrary to clathrate hydrates, a small increase in temperature of semiclathrate hydrates, studied herein, leads to a noticeable increase in dissociation pressure.

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Serious issues regarding the scarcity of freshwater encouraged researchers to work on various methods for seawater desalination. Hydrate-based desalination (HBD) is a novel method that has received investigators’ attention due to its outstanding properties. According to the literature, refrigerant hydrates have great potential to be employed in the HBD process because of their highly moderate phase equilibrium. The main purpose of the current study is to investigate the influence of different concentrations of NaCl (0, 1, 3.5, 5, and 8 wt %) on the major kinetic parameters of R410a refrigerant hydrate formation in the presence and absence of cyclopentane with the concentration of 0.5 mol % as a coformer. The experiments were performed in a stirred laboratory reactor at an initial temperature of 278.15 K and initial pressures of 0.9 and 1 MPa. The measurements of induction time and mole of gas consumption show that NaCl increases the induction time and decreases the amount of gas consumption. However, the addition of 0.5 mol % cyclopentane decreases the induction time slightly and increases the final amount of gas consumed. Besides, despite the fact that cyclopentane decreases the initial rate of gas uptake, it could promote the amount of gas consumption during hydrate formation. The results of the water-to-hydrate conversion ratio illustrated that NaCl with concentrations up to 5 wt % exhibits an insignificant decrease of water-to-hydrate conversion, while 8 wt % NaCl decreases this parameter down to 51.48%. Moreover, the positive effect of cyclopentane on the growth stage was revealed by determination of the apparent rate constant of hydrate growth. This study provides useful data for designing and understanding the HBD process via R410a hydrate formation.

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The effect of synthesized nanostructures, including graphene oxide, chemically reduced graphene oxide with sodium dodecyl sulfate (SDS), chemically reduced graphene oxide with polyvinylpyrrolidone, and multi-walled carbon nanotubes, on the kinetics of methane hydrate formation was investigated in this work. The experiments were carried out at a pressure of 4.5 MPa and a temperature of 0 °C in a batch reactor. By adding nanostructures, the induction time decreases, and the shortest induction time appeares at certain concentrations of reduced graphene oxide with SDS and graphene oxide, that is, at a concentration of 360 ppm for reduced graphene oxide with SDS and 180 ppm for graphene oxide, with a 98% decrease in induction time compared to that in pure water. Moreover, utilization of carbon nanostructures increases the amount and the rate of methane consumed during the hydrate formation process. Utilization of multi-walled carbon nanotubes with a concentration of 90 ppm showes the highest amount of methane consumption. The amount of methane consumption increases by 173% in comparison with that in pure water. The addition of carbon nanostructures does not change the storage capacity of methane hydrate in the hydrate formation process, while the percentage of water conversion to hydrate in the presence of carbon nanotubes increases considerably, the greatest value of which occurres at a 90 ppm concentration of carbon nanotubes, that is, a 253% increase in the presence of carbon nanotubes compared to that of pure water.

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