Rapid Growth of the CO2 Hydrate Induced by Mixing Trace Tetrafluoroethane.

Rapid formation of the CO2 hydrate can be significantly induced by the gaseous thermodynamic promoter 1,1,1,2-tetrafluoroethane(R134a) due to the mild phase equilibrium conditions, although the formation mechanism and dynamic behavior are not clear. Therefore, a visual experimental system was developed to study the effects of different concentrations of R134a on the induction time, gas consumption, and growth morphology of the CO2 hydrate. At the same time, the combined effects under stirring and sodium dodecyl sulfate (SDS) systems were also studied. In addition, visualization and experimental model diagrams were combined to explain the fast formation mechanism of the R134a/CO2 hydrate. The results show that the CO2 hydrate average conversion rate was increased by more than 63% with the addition of mixed trace R134a(7%). A special phenomenon is found that two temperature peaks appear on the hydrate formation temperature curve, corresponding to two different stages of hydrate formation when stirring or SDS is added to the mixed gas reaction system. Furthermore, the gas consumption in stirring and SDS systems increases by 9 and 44%, respectively. Finally, it is also found that the R134a/CO2 mixed hydrate formed under the action of SDS has a "capillary" mechanism, which provides a gas-liquid phase exchange channel and a large number of nucleation sites for CO2 hydrate, thus promoting the formation of CO2 hydrate. This paper provides a novel, simple, and efficient method for CO2 hydrate gas storage technology.

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

The exploitation of natural gas hydrate is always hindered by the migration of water and sands due to gas production. Depressurization combined with thermal stimulation is an effective method for hydrate dissociation. This paper reported the influence of gas-liquid-solid migration on morphological change of hydrate sediments in natural gas production using visualization method. Different backpressures combined with thermal stimulation methods were applied to simulate natural gas hydrate exploitation. Pressure compensation was first employed to study sediment recovery features. The expansion rate of a porous medium layer under combined dissociation and different backpressure (4.5, 3.5, 2.5, 1.5, and 0.1 MPa) was discussed. A 176% hydrate sediment expansion rate was found after the combined dissociation at 0.1 MPa. In addition, it was observed that the height of the water layer above the porous media after pressure compensation was gradually reduced with a decrease in backpressure and eventually disappeared at 0.1 MPa. It was also found that the disappearing water layer caused an anomalous memory effect phenomenon. Expansion and subsidence of sediments provide a better reference for hydrate exploitation and geological safety.

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

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.

A novel apparatus for in situ measurement of thermal conductivity of hydrate-bearing sediments.

An experimental apparatus was developed to synthesize natural gas hydrates and measure the thermal conductivity of hydrate-bearing sediments in situ. The apparatus works over a temperature range varying from -20 °C to 50 °C and up to a maximum pressure of 20 MPa. This apparatus is mainly composed of a thermal conductivity test system and a reaction cell, into which a lab-fabricated thermistor probe is inserted. This thermistor has excellent temperature sensitivity and can work at high pressures. The basic principles of this apparatus are discussed, and a series of experiments were performed to verify that the apparatus can be practically applied in chemical engineering. The thermistor-based measuring method was applied successfully in a high-pressure environment both with and without porous media.

In situ observations by magnetic resonance imaging for formation and dissociation of tetrahydrofuran hydrate in porous media.

Tetrahydrofuran (THF) hydrate has long been used as a substitute for methane hydrate in laboratory studies. This article investigated the formation and dissociation characteristics of THF hydrate in porous media simulated by various-sized quartz glass beads. The formation and dissociation processes of THF hydrate are observed using magnetic resonance imaging (MRI) technology. The hydrate saturation during the formation is obtained based on the MRI data. The experimental result suggests that the third surface has an effect on hydrate formation. THF hydrate crystals lean to form on the glass beads and in their adjacent area as well as from the wall of the sample container firstly. Furthermore, as the pore size diminishes, or as the formation temperature decreases, the nucleation gets easier and the formation processes faster. However, the dissociation rate is mostly dependent on the dissociation temperature rather than on the pore size.