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.

Introduction

The unbalanced utilization of electricity, especially in the summer for air-conditioning and refrigeration processes, appears to be an irrepressible situation. Peak–valley electricity price is an effective policy to solve the unbalanced utilization of electricity. The larger the peak-to-valley electricity price difference, the more economical the energy storage. Power supply-side peak adjustment and user-side peak adjustment can be adopted to utilize the peak-to-valley difference and create more value. Cold storage, a high-efficiency user-side peak adjustment technology, can make the difference between the peak and valley electricity prices more cost effective, whereby cold storage occurs during the valley price period and is used at the peak price time.[1,2] However, conventional cold storage technologies have inevitable shortcomings: low cold storage density for water cold storage, low cold storage efficiency for ice cold storage, and equipment aging and failure for eutectic salt cold storage.[3] Therefore, cold storage using gas hydrates (with a latent heat of phase change that is close to that of ice (335 kJ/kg), a phase change temperature of 0–15 °C and no corrosion of metal equipment[4−7]) has attracted much attention for solving these shortcomings. Gas hydrates are ice-like crystals that host water molecules to form lattices via hydrogen bonding to engage gas molecules at a certain temperature and pressure.[8−11] According to the selection principle of the cold storage medium, it is also very important to choose suitable hydrates as the cold storage medium.[5] Among hydrates, 1,1,1,2-tetrafluoroethane (R134a) hydrates match well with the cold storage system due to the lower phase equilibrium pressure (0.05–0.41 MPa[12]), higher phase equilibrium temperature (1–10 °C[12]), and larger phase change latent heat (358 kJ/kg[13]). Nevertheless, R134a hydrates in cold storage applications are limited by the large stochastic nature of hydrate nucleation, the instability of cyclic hydrate formation (including first hydrate formation and several repeated hydrate reformations), and the low hydrate dissociation rate.

It is well known that the first hydrate formation that reflects the stochastic nature of nucleation possesses a long induction time, implying that more time and energy must be consumed,[14−16] which goes against the principle of cold storage at cost.[17,18] Therefore, many efforts, such as the use of surfactants and porous media, have been made to shorten the induction time.[19−21] Sodium dodecyl sulfate (SDS), as a perfect surfactant for enhancing hydrate formation, is widely used,[22] but there is no consensus on the characteristics of cyclic hydrate reformation (excluding the first hydrate formation from cyclic hydrate formation). The induction time of hydrate reformation is dramatically reduced owing to the presence of the memory effect.[23−25] Many studies have proven that the memory effect could effectively promote hydrate reformation.[26−30] These studies found that porous media can highlight the memory effect in hydrate reformation.[29] However, it remains unclear and controversial as to whether the memory effect maintains stability upon cyclic hydrate reformation, and related research is not comprehensive, especially in SDS systems or porous media.

The hydrate dissociation rate is the key to determining the cold discharge efficiency. Different dissociation methods always cause significant differences in the hydrate dissociation rate. Popular methods for triggering hydrate dissociation are thermal stimulation, inhibitor injection, and depressurization,[31−38] and the addition of inhibitors is not applied in cold storage because the inhibitors will hinder hydrate reformation. Thermal stimulation is used to investigate the kinetics of hydrate dissociation.[39−42] Depressurization is applied to explore gas hydrate exploitation and its geological effects.[43−50] Both thermal stimulation and depressurization achieve hydrate decomposition by adjusting the temperature and pressure.[51] Therefore, the driving force (temperature rise and pressure drop) of the hydrate phase transition is the main controlling factor that determines the cold discharge mode.[52,53] At present, single thermal stimulation is often used to decompose hydrates in cold discharge, which cannot effectively and rapidly extract the cold amount.[54] However, our previous studies have proven that thermal stimulation combined with depressurization can accelerate hydrate dissociation.[51] There is no related experiment in the research of hydrate cold storage to interpret the effect using the combined method in cold discharge.

Considering the advantages of surfactant-SDS systems and porous media for hydrate formation, the cold storage characteristics of three different systems (pure water system, SDS solution system, and SDS solution with a porous medium system) were interpreted from the perspective of the cyclic hydrate formation stability. The induction time, full time, and energy consumption time ratio of the first hydrate formation and cyclic hydrate reformation in the SDS solution with or without a porous medium system at 1, 3, and 5 °C were analyzed. Thermal stimulation combined with depressurization was applied to analyze the hydrate dissociation characteristics for the cold discharge process, and the backpressure was 0.1, 0.2, 0.3, and 0.4 MPa. The results provide some basic data for cold storage and cold discharge, along with a reference for the application of hydrate cold storage technology.

Materials and Methods

Experimental Apparatus and Materials

Figure a presents a schematic of the experimental apparatus. This apparatus consisted of a stainless steel cylindrical vessel with a design pressure of 10 MPa, two 70 mm diameter sapphire windows, and an effective volume of 1650 mL; a gas supply system with an R134a gas cylinder and a gas flowmeter; a data acquisition system to record the pressure and temperature; a backpressure gas gathering system with a backpressure regulator; an image acquisition system with a digital camera (EOS 6D, Canon Company, lens model EF24-105 mm f/4L IS USM); and a thermostatic bath system. The temperature of the vessel during hydrate formation and dissociation is controlled by the thermostatic bath system (XT5718RC-E800L, Xutemp, Hangzhou, Co., Ltd. with an accuracy of ±0.1 K and a temperature varying from −15 to 50 °C). Two temperature transducers (Pt-1000 with an accuracy of ±0.2%) and a pressure transducer (Unik 5000 with a pressure limit of 25 MPa and precision of 0.25% FS) were connected to the vessel.

Figure 1

(a) Schematic diagram of the experimental apparatus; (b) gas and liquid distributions in the pure water system and SDS solution system (without a porous medium); (c) gas, liquid, and porous medium distributions in the SDS solution with a porous medium system.

In this study, three systems, including a pure water system, SDS solution system, and SDS solution with a porous medium system, were used for hydrate formation and dissociation. As shown in Figure b,c, the experimental materials in the three systems were 600 g deionized water, 600 g 300 ppm SDS solution, and 250 g 300 ppm SDS solution and 900 g glass beads, respectively. The amounts of water in the three systems were in excess for hydrate formation, which meant that some of the water remained after hydrate formation. The charge amount of the R134a gas was approximately 0.94 mol, as measured by the flowmeter. Because of the low solubility of R134a gas (25 °C, 0.15%) in water,[55] when the initial temperature was 17 °C, the pressure was basically maintained at 0.49 MPa. The 99.99% R134a gas in this study is supplied by Shandong Dongyue Chemical Co., Ltd. Glass beads with particle sizes ranging from 0.6 to 0.8 mm were used to fill the vessel to simulate the porous medium layer.

Procedures

Hydrate Formation

The experimental materials were first placed in the vessel. Subsequently, the vessel was purged with R134a gas to a pressure of 0.49 MPa three times to ensure that the gas occupying the void space within the vessel was purely R134a. Then, the vessel temperature was set at 17 °C. The vessel was then pressurized to 0.49 MPa with R134a gas, and the system was left to stabilize. After the temperature and pressure of the system stabilized, the vessel was cooled to the test temperature to induce hydrate formation. During this period, a pressure drop was observed along with a temperature spike which meant that a large amount of gas was consumed and a lot of heat was released, indicating that R134a hydrates were formed in large quantities. After that, the hydrate formation was considered to be finished when the vessel temperature and pressure decreased at rates of <0.1 K h–1 and <0.01 MPa h–1, respectively.

In this work, cyclic hydrate formation was studied in the SDS solution system and SDS solution with a porous medium system. The process of cyclic hydrate formation was defined as follows: the first-formed hydrates at the test temperature were dissociated using thermal stimulation, and then the vessel was cooled to the same test temperature to start the second hydrate formation. This process was repeated several times to complete the cyclic hydrate formation process. The process of cyclic hydrate formation was performed under closed conditions (the vessel was kept closed during cyclic hydrate formation and decomposition and no gas and liquid were discharged). In this study, the storage period of dissociated solution in the vessel was kept the same (200 min) at 17 °C. The storage period of dissociated solution was defined as the period between the end of hydrate dissociation and the start of hydrate reformation. Uchida et al. have found that the storage period of 200 min is most conducive to the hydrate reformation.[24,56] The test temperatures of hydrate formation were 1, 3, and 5 °C.

Hydrate Dissociation

Hydrate Dissociation Using Single Thermal Stimulation

After hydrate formation was completed at different test temperatures, the vessel temperature was adjusted back to 17 °C under the closed condition until the R134a hydrates decomposed completely (pressure difference <0.01 MPa h–1).

Hydrate Dissociation Using Thermal Stimulation Combined with Depressurization

After hydrate formation at 1 °C, hydrate dissociation using thermal stimulation combined with depressurization was conducted. Prior to the combined dissociation, the backpressure pipe needed to be purified with R134a gas three times. The temperature and pressure in the vessel were at the phase equilibrium point (1 °C and 0.05 MPa[12]) before the combined dissociation. The pressure was lower than atmospheric pressure. Therefore, during the combined dissociation, the thermostatic bath temperature was set at 17 °C. When the vessel temperature was 7 °C (the vessel pressure was higher than atmospheric pressure to avoid the backflow of gas into the vessel), the backpressure regulator was opened at this time to perform the combined dissociation. The dissociation was considered to be finished when no gas was discharged from the backpressure equipment, and the temperature and pressure in the vessel had stabilized (pressure decrease <0.01 MPa h–1).

Results and Discussion

R134a Hydrate Formation Characteristics

Extremely Slow Hydrate Formation in the Pure Water System

The typical formation of R134a hydrates in pure water is revealed by the pressure–temperature–time (P–T–t) plots in Figure . The corresponding formation images are shown in Figure . The whole formation process was sustained for 19 days, and the hydrate morphology was peculiar. During R134a hydrate formation, R134a liquefaction was recorded at 32 min. The liquid R134a clumped together on the surface of the water and sank to the bottom as a droplet because the liquid R134a was denser than water. After the vessel had cooled down to the test temperature, the temperature was kept almost stable at 1 °C, and the pressure very slowly decreased within the next 19 days, indicating continuous and slow R134a hydrate formation. During the process of hydrate formation, hydrates were formed from the gas–liquid interface toward water and gradually grew as time went on. At 4.53 h, dendritic hydrates germinated at the inner surface of the vessel and the gas–water interface that produced an armor effect (a film-like layer of solid hydrates formed at the gas–liquid interface[57]). Approximately 1 day later, there was a cluster of hydrates formed from the water surface to the water center. The hydrate cluster gradually increased and extended into the water phase over the next 1.22 days. From 2.44 to 3 days, there were many loose hydrates around the hydrate cluster. Although the hydrate morphology change was inconspicuous after 12 days, hydrates were forming at all times.

Figure 2

Pressure and temperature histories during hydrate formation in the pure water system.

Figure 3

R134a hydrate formation morphology in the pure water system.

The extremely slow hydrate formation was attributed to the smaller contact area of water and R134a. Although R134a liquefaction increased the contact area, it could not effectively promote the formation of hydrates. Meanwhile, the hydrate film at the gas–liquid interface impeded the gas–liquid contact. In addition, the gradually thickening hydrate film in the process of hydrate formation prevented the R134a gas from diffusing into the aqueous phase, further hindering hydrate formation. On the basis of the P–T–t outlines and images, most hydrate formation locations are in the water phase, and the hydrate formation duration lasts 19 days, which is too long for cold storage applications.

Fast Hydrate Formation in the SDS Solution System

To shorten the R134a hydrate formation time, SDS was added to the pure water system to cause the hydrate film to become rough, soft, or to break to promote hydrate formation. The R134a hydrate formation P–T–t plots and images in the 300 ppm SDS solution system are shown in Figures and 5, respectively. On the whole, the full time of hydrate formation was approximately 140 min, which was significantly shorter than that in the pure water system, indicating that the effect of the surfactant, SDS, on enhancing the rate of hydrate formation was notable. According to the observations, the same R134a liquefaction was recorded in the SDS solution system. However, there was a significant difference due to the presence of SDS for the hydrate formation morphology. At 88 min, R134a hydrates rapidly formed along the vessel window. From 88 to 94 min, flocculent hydrates were formed and gathered into a cluster in the solution phase and increased gradually along the direction from the solution surface to the bottom. Between 94 and 140 min, the hydrates continued to form and settle toward the bottom and took on a completely formed morphology at 140 min. Meanwhile, the P–T–t outline shows a temperature peak that corresponds to a steep pressure descent from 88 to 140 min, indicating that R134a hydrates had formed in a certain quantity that was accompanied by gas consumption, which gave off much heat due to the large phase change heat of hydrates (358 kJ/kg[13]).

Figure 4

Pressure and liquid temperature histories during hydrate formation in the SDS solution system.

Figure 5

R134a hydrate formation morphology in the SDS solution system.

Compared with the formation process of R134a hydrates in the pure water system, the role of SDS in the solution was to destroy the hydrate film so that the R134a gas in the upper vessel remained in contact with the water phase until the hydrates had completely formed. That is, SDS could indirectly increase the gas–liquid contact area. The result of hydrate formation in the SDS solution shows a rapidly elapsed time. Therefore, the SDS solution system has an application value for R134a hydrate cold storage compared with the pure water system. However, according to the observations, the low hydrate density caused by the flocculent hydrates will restrict the application of hydrate cold storage, and the production of a hydrate slurry or the addition of a polymerization accelerant may solve such problems.

Cyclic R134a Hydrate Formation Stability

As previously discussed, SDS can significantly promote the formation of hydrates. Porous media have a special function that shortens the induction time in hydrate reformation. Therefore, the roles of SDS and a porous medium in cyclic hydrate reformation need to be discussed. A total of six groups of experiments were performed in this section, in which three groups were in the SDS solution system (without a porous medium) and the other groups were in the SDS solution with a porous medium system. The hydrate formation temperatures (test temperature) of each of the three groups were 1, 3, and 5 °C. In each group, cyclic hydrate formation was performed, including one first formation and four reformations under closed conditions. The experimental parameters are shown in Table .

Table 1

Experimental Parameters of Hydrate Formation

exp. group	A		B		C		D		E		F
exp. no	A1	A(2–5)	B1	B(2–5)	C1	C(2–5)	D1	D(2–5)	E1	E(2–5)	F1	F(2–5)
test temperature	1 °C	1 °C	3 °C	3 °C	5 °C	5 °C	1 °C	1 °C	3 °C	3 °C	5 °C	5 °C
first formation	√		√		√		√		√		√
reformation		√		√		√		√		√		√
SDS solution (g)	600	600	600	600	600	600	250	250	250	250	250	250
glass beads (g)							900	900	900	900	900	900

The induction time, tind, of gas hydrate formation is an evaluation parameter for the working medium cold storage efficiency and indicates the duration time of the total system kept in the hydrate formation environment (low temperature and high pressure). In addition, both time and energy costs are associated with the induction time, and the shorter the induction time is, lower are the time and energy costs, which is beneficial to the cold storage system, and vice versa. Meanwhile, this article evaluated the timeliness of the hydrate formation environment by the energy consumption time ratio (rECT), which is defined as followswhere ta is the time when the system reaches phase equilibrium in cooling; tb is the time corresponding to the end of supercooling (hydrate nucleation); and tfull is the full time of hydrate formation from before the temperature fall to the completely formed point, which are marked in Figure .

The average value (E(tind)) and standard difference (σ) were used to evaluate the central tendency and dispersion degree of the reformation induction time to judge the cyclic hydrate reformation stability under different conditions.where N is the exp. group of hydrate formation, and Ni is the exp. no (i = 2, 3, 4, and 5).

Cyclic R134a Hydrate Formation in the SDS Solution System

Figure shows the curves of the temperature and pressure of cyclic hydrate formation in the SDS solution system. The effects of the test temperatures (1, 3, and 5 °C) are discussed in detail, and the experimental parameters and results are shown in Table . First, it appeared that R134a hydrates formed in a relatively short time at all test temperatures. In each hydrate formation episode, there was a temperature peak when the pressure dropped sharply at the same time, and these temperature peaks were dispersed. In contrast, the temperature peaks at 5 °C were relatively low because less heat was released during hydrate formation and the temperature decreased when the amount of heat released was less than the cold input from outside.

Figure 6

Changes in the temperature and pressure over time during cyclic hydrate formation at different test temperatures in the SDS solution system (A: 1 °C, B: 3 °C, and C: 5 °C).

Table 2

Experimental Results of Hydrate Formation under Different Conditions

	A1	A2	A3	A4	A5	B1	B2	B3	B4	B5
tind (min)	58.5	26	66.5	55	34.5	226.5	94	17.5	68.5	16.5
tfull (min)	141.5	113	163.5	96.5	124.5	322	190.5	138	163.5	135.5
rECT	0.4134	0.2301	0.4067	0.5699	0.2771	0.7034	0.4934	0.1268	0.4190	0.1218

The corresponding experimental results of the different test temperatures were discrepant. At 1, 3, and 5 °C, with the same initial temperature and pressure of 17 °C and 0.49 MPa, respectively, the final pressures after hydrate formation were lower than atmospheric pressure at 1 and 3 °C, but that at 5 °C was higher than atmospheric pressure. The pressures before reformation that corresponded to the pressures after the closed dissociation were all lower than the initial pressure. That is, some R134a gas exists in the water phase in some form after hydrate dissociation. Because of the short standing time, the existing gas in the water phase before reformation cannot completely strip the water phase.[56] The stored gas would promote hydrate reformation.[58] Combining Figure and Table , it can be found that the induction time at the 1 °C test temperature in the first formation was shorter than those at 3 and 5 °C. Meanwhile, the reformation induction times (A3, C2, and C5) at 1 and 5 °C were longer than the first formation induction time, and the reformation induction time was chaotic at all test temperatures in the SDS solution system, which implied that the memory effect that could promote hydrate reformation did not always play a notable role every time.[59] The reason may be that the gas presented an uneven distribution in the SDS solution after hydrate dissociation, resulting in the dispersion of temperature peaks in the cyclic hydrate reformation.

Cyclic R134a Hydrate Formation in the SDS Solution with a Porous Medium System

The results presented by the SDS solution with a porous medium system were different from those without the porous medium. Figure shows the outlines of the temperature and pressure during cyclic hydrate formation in the SDS solution with a porous medium system. There is no first formation at 5 °C in Figure C. Three repeat experiments for the first hydrate formation at 5 °C are shown in Figure S1 in the Supporting Information. It was found that the time of the supercooling stage was longer than one day for hydrate formation at 5 °C, which was longer than 2000 min. Therefore, the first hydrate formation at 5 °C was induced by a temperature shock in which the temperature of the vessel was set to 17 °C again at the supercooling stage and then cooled again to 5 °C to achieve the first hydrate formation.

Figure 7

Changes in the temperature and pressure over time during cyclic hydrate formation at different temperatures in the SDS solution with a porous medium system (A: 1 °C, B: 3 °C, and C: 5 °C).

As was discussed for the temperature peak, the distributional difference between the first formation and reformation was distinct. For all test temperatures, the occurrence times of the temperature peaks at first formation were longer than those of the reformations. As shown in Figure and Table , the induction time at first formation gradually increased with increasing test temperature, indicating that the low temperature was beneficial for promoting first hydrate formation. In addition, relative to the SDS solution system, the induction time at first formation in the SDS solution with a porous medium system was longer, showing that the porous medium did not promote or even inhibit the first hydrate formation. However, the induction times of hydrate reformation were maintained within 40 min, which were shorter than that of the first formation at all test temperatures, and the full times at reformation were shorter than 150 min. These results verified that cyclic hydrate reformation in the presence of porous media was more stable than in the absence of porous media. Meanwhile, it was found that the pressures before the reformation that corresponded to the pressures after the closed dissociation were all lower than the initial pressure, which was the same as that in the SDS solution system. However, the SDS solution with the porous medium system was more stable during cyclic hydrate reformation because of the additional role of the porous medium. The additional role can be summarized as follows: the porous medium could distribute the stored gas in the aqueous phase in a relatively uniform manner, and the larger specific surface area of the porous medium provided a larger gas–liquid contact area for the hydrate reformation.

Cyclic R134a Hydrate Formation Stability Assessment

To further evaluate the cyclic hydrate formation stability in the different systems, the rECT values in the first formation and reformations were calculated and compared. Figure shows the trend of the first formation rECT in the presence of porous media. Figure presents the trend of the reformation rECT under different conditions. As shown in Figure , in the case of first hydrate formation with a porous medium at 5 °C, rECT = 1. The induction time (>2000 min) of the first formation was too long (as shown in Figure S1 in the Supporting Information), so that the difference between the induction time and full time was pretty small. In order to conduct the hydrate reformation at 5 °C, the first formation of hydrate was achieved by temperature shock after a long period of supercooling stage. The induction time and full time may last longer and be more similar without a temperature shock, which is going to make rECT approach 1.

Figure 8

Comparison of the first formation rECT with or without porous media.

Figure 9

Comparison of the cyclic hydrate reformation rECT under different conditions. (a) Single rECT; (b) total rECT.

Because of the diverse test temperatures and experimental materials, there were different rECT trends. In the SDS solution system, the first formation rECT at 1 °C was lower than those at 3 and 5 °C. In the SDS solution with and without a porous medium system, the first formation rECT gradually increased with increasing test temperature. Comparing the different experimental materials, the first formation rECT in the presence of porous media was higher than that in the absence of porous media at every test temperature. These results indicated that a higher test temperature and the presence of a porous medium would increase the time and energy costs of first formation, and the lower the temperature was, the lower rECT would be in terms of the overall performance.

The reformation rECT differed from that of the first formation. As shown in Figure a, the rECT of a single reformation fluctuated significantly in cases A–C and peaked at 0.7484 in C2. In the cases of D–F, the rECT values during cyclic reformation were much more stable and maintained below 0.35. For the cyclic reformation process in Figure b, the total rECT in cases A–C was greater than those in cases D–F. Although the total rECT of B was not very different from those of D and E, its single reformation rECT fluctuated significantly. Hence, the above results show that the single rECT was more uniform and the total rECT in the presence of porous media was smaller during cyclic reformation.

The average value and standard deviation of the reformation induction time under different conditions were calculated, as shown in Figure . Overall, the average value and standard deviation in case C were much larger than those of the other cases, and the average values and standard deviations in cases D–F were both smaller than those in cases A–C. In particular, the standard deviations in cases D–F were 1 order of magnitude smaller. Therefore, the average values in cases D–F were smaller, and the dispersion degree around the average value was lower.

Figure 10

Comparison of the average value and standard deviation of the reformation induction time under different conditions.

Overall, at first formation, the time cost and energy cost of the system without a porous medium was lower than those with a porous medium. However, in the process of cyclic reformation, based on not only the stability of the single rECT and the total rECT but also the central tendency and dispersion degree in reformation induction time, the SDS solution with a porous medium system was more applicable to cold storage systems.

R134a Hydrate Dissociation Characteristics

Hydrate Dissociation Characteristics Using Single Thermal Stimulation

Six experiments were carried out to investigate the R134a hydrate dissociation characteristics under different hydrate formation conditions. Figure shows the temperature variation during dissociation by thermal stimulation. The heat absorbed by hydrate decomposition dominated the temperature change in the vessel. At the beginning of dissociation, the smaller amount of heat required for hydrate dissociation would not affect the temperature rise in the vessel. When the temperature in the vessel increased to 11–14 °C, the hydrates decomposed in large quantities, resulting in the hydrates absorbing the sensible heat of the system to supplement its own decomposition. The temperature would no longer rise or even fall, which was called temperature buffering. The same mechanism of temperature buffering was proposed by Chong et al.[45] In the presence of porous media, temperature buffering lasted longer, reaching approximately 50 min, whereas the buffering lasted only approximately 20 min in the absence of porous media. It can be concluded that the presence of porous media impedes the transfer of heat, so the time of temperature buffering in the SDS solution with a porous medium system was longer. In addition, the full decomposition stage was slower than that of the system without a porous medium due to the presence of the porous medium, which increased the hydrate dissociation time.

Figure 11

Temperature variation with respect to time after thermal stimulation dissociation under different formation conditions.

Hydrate Dissociation Characteristics Using Thermal Stimulation Combined with Depressurization

As mentioned before, the stability of cyclic hydrate formation in the SDS solution with a porous medium system was higher, but the dissociation rate was not as high as that of the SDS solution system. To shorten the dissociation time, thermal stimulation combined with depressurization was performed to promote hydrate dissociation in the SDS solution with a porous medium system.

Figure shows the variations in the temperature and pressure in the combined dissociation. Temperature buffering was also found in the combined dissociation. The time and temperature range during temperature buffering varied due to the different backpressures—0.1, 0.2, 0.3, and 0.4 MPa. As shown in Figure a, the same range of temperature buffering occurred for the combined dissociation at 0.4 MPa, and that of the thermal stimulation dissociation was 12–14 °C. The difference was that the time range of the 0.4 MPa backpressure dissociation occurred slightly earlier (at 200 min). After hydrate formation at 1 °C, the pressure in the vessel was negative after hydrate formation. To eliminate the influence of negative pressure, when the temperature rose to 7 °C during the combined dissociation, the backpressure regulator was opened. Therefore, temperature buffering occurred immediately after the backpressure regulator was opened during the combined dissociation at 0.1 MPa, and the sharp decrease in pressure was accompanied by a temperature drop. The temperature buffering during the combined dissociation at 0.1 MPa lasted 70–110 min, with a temperature range of 5–7 °C. However, the temperature buffering during the combined dissociations at 0.2 and 0.3 MPa occurred at 150 and 175 min, respectively. Therefore, the time range of temperature buffering gradually advanced with a decrease in the backpressure, the temperature stage decreased, and the dissociation time was shortened. At 0.1 MPa backpressure, the dissociation time could be reduced by 150 min.

Figure 12

Changes in temperature and pressure over time during thermal stimulation combined with depressurization dissociation. (a) Temperature curve during the combined dissociation; (b) pressure curve during the combined dissociation.

The common drivers of the pressure and temperature during the combined dissociation enabled the hydrates to decompose faster, which accelerated the decomposition rate of the hydrates. As a result, the hydrate dissociation time could be notably shortened. That is, the cold discharge efficiency can be improved by thermal stimulation combined with depressurization, and the lower the backpressure is, shorter is the hydrate dissociation time.

Conclusions

The cyclic hydrate formation stability and dissociation characteristics of R134a hydrates were investigated using different systems. In the study of the hydrate formation characteristics, it is found that the destruction of the hydrate film can accelerate the formation of hydrates, and the water phase can be rapidly filled with hydrate clusters to complete hydrate formation rapidly under the action of SDS. In cyclic hydrate formation, a high test temperature and the presence of a porous medium would increase the time and energy costs of first hydrate formation. However, the porous medium could distribute the stored gas evenly to the liquid phase and provide a larger contact area during cyclic hydrate reformation, which would result in induction times of hydrate reformation of within 40 min and full times of hydrate reformation that are shorter than 150 min. In addition, through the calculation and comparison of rECT, the average value and the dispersion degree verified that cyclic hydrate reformation in the presence of a porous medium was more stable than that in the absence of a porous medium.

For the cold discharge stage, the hydrate dissociation time of the SDS solution system was shorter than that of the SDS solution with a porous medium system. In addition, a temperature buffering process would be generated within 11–14 °C during the thermal stimulation dissociation process. In the combined dissociation, temperature buffering occurred earlier, and different backpressures corresponded to various temperature buffering ranges. With the decrease in backpressure, the hydrate dissociation time became shorter. Hydrate dissociation can be completed faster by the driving forces of both the temperature and pressure, resulting in a reduction in the dissociation time by 150 min at a backpressure of 0.1 MPa. The findings of this study can provide some insight for designing and implementing optimal industrial applications for the use of hydrate cold storage.