Cyclic Formation Stability of 1,1,1,2-Tetrafluoroethane Hydrate in Different SDS Solution Systems and Dissociation Characteristics Using Thermal Stimulation Combined with Depressurization

Abstract

Cold storage using hydrates for cooling is a high-efficiency technology. However, this technology suffers from problems such as the stochastic nature of hydrate nucleation, cyclic hydrate formation instability, and a low cold discharge rate. To solve these problems, it is necessary to further clarify the characteristics of hydrate formation and dissociation in different systems. First, a comparative experimental study in pure water and sodium dodecyl sulfate (SDS) solution systems was conducted to explore the influence of SDS on the morphology of the hydrate and the time needed for its formation under visualization conditions. Subsequently, the cyclic hydrate formation stability was investigated at different test temperatures with two types of SDS solution systems—with or without a porous medium. The induction time, full time, and energy consumption time ratio of the first hydrate formation process and the cyclic hydrate reformation process were analyzed. Finally, thermal stimulation combined with depressurization was used to intensify hydrate dissociation compared with single thermal stimulation. The results showed that the growth morphology of hydrate and the time required for its formation in the SDS solution system were obviously different than those in pure water. In addition, the calculation and comparison results revealed that the induction time and full time of cyclic hydrate reformation were shorter and the energy consumption time ratio was smaller in the porous medium. The results indicated that a porous medium could improve the cyclic hydrate formation process by making it more stable and by decreasing time and energy costs. Thermal stimulation combined with depressurization at different backpressures (0.1, 0.2, 0.3, and 0.4 MPa) effectively promoted the decomposition of hydrates, and with the decrease in backpressure, the dissociation time decreased gradually. At a backpressure of 0.1 MPa, the dissociation time was reduced by 150 min. The experimental results presented the formation and dissociation characteristics of 1,1,1,2-tetrafluoroethane hydrates in different systems, which could accelerate the application of gas hydrates in cold storage.

Effects of Salinity on Hydrate Phase Equilibrium and Kinetics of SF6, HFC134a, and Their Mixture

“…Nowadays, SF 6 gas is being replaced by alternative protection gases with a low global warming potential such as HFC134a or SF 6 /HFC134a mixture for magnesium melt protection . Because HFC134a costs only about a third of SF 6 and its global warming potential is 18 times lower, , it can serve as a suitable guest molecule because of its lower hydrate equilibrium compared to other gases ,− and its relative low solubility (0.11% at 27 °C) . Therefore, a hydrate-based desalination process using SF 6 /HFC134a mixture as the hydrate-forming agent is a more promising option in terms of energy consumption and capital cost.…”

Rapid Growth of the CO2 Hydrate Induced by Mixing Trace Tetrafluoroethane

“…Subsequently, the mixture gas was slowly injected to reach the preset pressure, followed by adjusting the water bath temperature to the experimental temperature. After maintaining constant pressure and temperature for approximately 10 h (with a pressure drop of less than 0.01 MPa/h), indicating no further gas consumption, it was considered that the hydrate formation was complete, and the experiment concluded . During the experiment, the change in hydrate formation in the reaction process was recorded by a camera, and temperature and pressure data in the reactor were automatically recorded by a data recorder every 30 s.…”

“…Lee et al showed that the equilibrium pressure of 2–8 mol % R134a/CO 2 mixed (CO 2 + R134a) hydrate was 3–5 times lower than that of pure CO 2 hydrate. Recent studies have found that the equilibrium pressures of HFCs, such as 1,1-difluoroethane (HFC-152a) and 1,1,1,2-tetrafluoroethane (HFC-134a), are relatively low, and these gases are good hydrate formation promoters (<1 MPa) compared with the conventional hydrocarbon gas hydrates. , Additionally, the research by Nesterov et al . demonstrated that R134a, as a gas promoter, can significantly reduce the CO 2 phase equilibrium pressure.…”

“…Recent studies have found that the equilibrium pressures of HFCs, such as 1,1difluoroethane (HFC-152a) and 1,1,1,2-tetrafluoroethane (HFC-134a), are relatively low, and these gases are good hydrate formation promoters (<1 MPa) compared with the conventional hydrocarbon gas hydrates. 27,28 Additionally, the research by Nesterov et al 29 demonstrated that R134a, as a gas promoter, can significantly reduce the CO 2 phase equilibrium pressure. They also discovered that R134a can form SII-type hydrates.…”

Gas–Liquid–Solid Migration Characteristics of Gas Hydrate Sediments in Depressurization Combined with Thermal Stimulation Dissociation

“…The dissociation of hydrates produces free water and gas, and the flow of free water and gas results in irregular hydrate sediment structure (including hydrate, free water, gas, and porous media). The dissociated gaseous methane significantly increases the pressure inside the pore space, and the methane also increases the interactive force between the fluid and hydrate-bearing porous media; these phenomena may result in the reformation of hydrate − and the deformation of hydrate-bearing sediments. , Hydrate reformation could be caused by free-flowing gas in sediments under local temperature. Hydrate reformation can reduce the permeability of sediments and result in the clogging of the penetration path; − the deformation of hydrate-bearing sediments counteracts gas–liquid–solid migration.…”