Experimental investigation on microscopic decomposition process of

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Experimental investigation on microscopic decomposition process of natural gas hydrate particles Shupeng Yao, Yuxing Li, Wuchang Wang, Guangchun Song, Zhengzhuo Shi, Xiaoyu Wang, and Shuai Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01003 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on June 2, 2019

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Experimental investigation on microscopic decomposition process of natural gas hydrate particles Shupeng Yao, Yuxing Li *, Wuchang Wang, Guangchun Song, Zhengzhuo Shi, Xiaoyu Wang, Shuai Liu

Shandong Key Laboratory of Oil-Gas Storage and Transportation Safety, China University of Petroleum, Qingdao 266580, Shandong, China

Abstract: Hydrate decomposition is an essential part of hydrate mining and has important research significance. In this paper, the hydrate thermal decomposition experiments were carried out in the self-designed high-pressure stirring hydrate reaction system, and the microscopic behaviors of the decomposition were captured by a high-speed camera. The main content of the study is the microscopic morphology of hydrate decomposition. According to the experimental data, three kinds of hydrate particles, including granular, flake and block hydrate particles, were defined, and their decomposition behaviors were analyzed. Finally, based on the independent physical model of three hydrate particles, the physical model of hydrate particle flow decomposition was established. Key words: hydrate decomposition; microscopic behaviors; physical model; hydrate particles

1. Introduction Natural gas hydrate is a kind of cage-like crystal formed by partial components of natural gas with water under high pressure and low temperature environment (1, 2). Since the first discovery of hydrates in the laboratory in 1810(3), it has received attention and research from scientists all over the world. Especially in recent years, the problem of hydrate exploitation has attracted the attention of more and more scientific research institutions and scholars (4, 5). It is estimated that the reserves of hydrate are equivalent to twice the total carbon equivalent of currently known oil, natural gas and coal, of which about 95% exist under the seabed, and their successful mining can effectively alleviate the world energy shortage (6). Hydrate decomposition is an indispensable part of hydrate mining. Therefore, research on hydrate decomposition and understanding the mechanism of hydrate decomposition are of great significance for hydrate mining. Meanwhile, because hydrate decomposition has an important

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application in oil and gas pipeline transportation, research on hydrate decomposition has been extensively studied. At present, the hydrate decomposition methods mainly include three methods: thermal decomposition, pressure reduction decomposition and injection chemical decomposition. In the case of hydrate thermal decomposition, Selim et al. (7) regarded the hydrate thermal decomposition process as a mobile interface ablation problem, and based on the heat conduction law of one-dimensional semi-infinite long flat wall, established a mathematical model for describing the heat transfer law of hydrate decomposition process. Kamath et al. (8) studied the thermal decomposition rate of methane and propane hydrate, and concluded that the process of thermal decomposition of hydrate is similar to the nucleate boiling process of fluid. Li et al. (9) analyzed the energy efficiency of thermal decomposition through a 2D reaction device. The results show that the energy efficiency value increases with the increase of initial temperature, background permeability and brine concentration of methane hydrate deposit. In the case of pressure reduction decomposition, Kim et al. (10) used a semi-batch stirred kettle to carry out the pressure reduction decomposition test of methane hydrate. It is considered that the kinetic process without mass transfer control can be used to describe the decomposition process of hydrate. It is mainly divided into two steps: ① the rupture of the water molecule crystal lattice on the surface of the hydrate particle; ② hydrate particle shrinks, and the gas molecules escape from the surface. Jamaluddin et al. (11) established a hydrate decomposition kinetic model containing mass transfer and heat transfer based on the Kim model. Matthew et al. (12) removed the effects of mass transfer and heat transfer on hydrate decomposition based on Kim model, and studied the decomposition intrinsic kinetics of methane and ethane hydrate more accurately. Kazunari et al. (13) studied the decomposition of CO2 hydrate, CH4 hydrate, and hydrate formed by the mixture of CO2 and CH4, and established a mathematical model to predict the changes of gas-phase components during the decomposition process. Sun et al. (14) studied the kinetics of methane decomposition using a selfdesigned hydrate device, established a hydrate decomposition kinetics model and calculated the decomposition rate of methane hydrate. Feng et al. (15) carried out decomposition experiments on


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water-saturated hydrates by the h&p method (above or below the equilibrium pressure of hydrate dissociation) method. It has been found that in the water-saturated hydrate reservoir, the conventional h&p method is not applicable and should be used in conjunction with the reduced pressure method. Li et al. (16) used a self-designed device to perform methane hydrate formation and dissociation experiments under different conditions by a pressure reduction method in a simulated porous medium. It was found that when the early production pressure is very low, the hydrate decomposition is inhibited, which leads to a decrease in the temperature in the separation zone. In case of injection chemical decomposition, Fan et al. (17) conducted an experiment of methane hydrate decomposition under low concentration ethylene glycol conditions, and found that ethylene glycol can reduce the heat required for hydrate decomposition and accelerate the decomposition of hydrate. Reed et al. (18) conducted an experimental study of a brine/mixed gas/condensate system in a large self-designed loop, indicating that dynamic inhibitors can significantly increase the supercooling value of the system. Behar et al. (19) conducted a study on the plugging decomposition of hydrates in oil-water systems in a self-designed loop. The results show that the use of dynamic inhibitors and anti-polymerization agents is significantly less than that of thermodynamic additives. Leporcher et al. (20) studied the effect of a new dynamic inhibitor and found that it can effectively delay the growth of hydrate crystals. When the supercooling condition is higher than 8 °C, the dynamic inhibitor can replace methanol. Zheng et al. (21) studied the formation and decomposition characteristics of methane hydrate in the presence of magnesium chloride (MgCl2) and potassium chloride (KCl). It is found that MgCl2 and KCl are both kinetic inhibitors of hydrate formation on the basis of thermodynamic inhibition. Compared with MgCl2, KCl has weaker inhibition effect on hydrate. In addition, there are some hydrate decomposition studies that combine different methods. Zheng et al. (22) analyzed the sensitivity of the hydrate decomposition front under the conditions of pressure reduction and wellbore heating. It is found that the average decomposition rate of the decomposition front has a close relationship with the overall thermal conductivity, initial hydrate saturation and bottom hole pressure, and has little


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relationship with the intrinsic hydrate reaction rate and the intrinsic permeability. Wang et al (23) studied the hydrate decomposition of hydrates, ice, water and methane at quadruple point in a pilotscale hydrate simulator by thermal stimulation assisted depressurization. It is found that the thermal stimulation below the quadruple point has a weak effect on the decomposition of hydrate. Misyura et al. (24) conducted a comparative study on the decomposition characteristics of natural and artificial hydrates, and found that the density and size of pores have a significant effect on the dissociation kinetics. However, there are still few studies on the microscopic properties of hydrate decomposition, which needs further experimental research to supplement. In this paper, the natural gas hydrate decomposition experimental study was carried out by a self-designed high-pressure stirring hydrate reaction system. The method of thermal decomposition was applied in this study, and the high-speed camera was used to capture the microscopic properties of hydrate decomposition. After studying and analyzing the microscopic decomposition behaviors of hydrate particles, a physical model of flow decomposition of hydrate slurry was established. The research content of the article can provide theoretical support for the study of hydrate slurry flow safety and hydrate mining technology.

2 Experimental section 2.1 Apparatus All of the work in this experiment is based on a self-designed high-pressure hydrate reaction system. The system consists of a high-pressure cell with a visible window, a gas supply system, a temperature, pressure control system, and a data acquisition system. The system structure is shown in Fig. 1.


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Fig.1 Schematic diagram of the high-pressure visual cell system.

The high-pressure cell with visible window is made of stainless steel with a total volume of 950ml. The design pressure is 10MPa, and the design temperature range is -20°C to 100°C. Meanwhile, in order to facilitate the experimental observation, there is a visible window with a diameter of 65 mm in front of and behind the middle of the cell body. The gas supply system and the pressure control system are composed of a high pressure gas cylinder, a gas pressure reducing valve and an exhaust valve, which can provide the gas and pressure required for the experiment. In addition, the temperature is controlled by a water bath/chiller with a temperature control accuracy of ±0.05 °C. What’s more, to study the microscopic state of hydrates, the PHOTRON FASTCAM SA-X2 high-speed camera was used to record the flow behaviors and microscopic morphology of hydrated particles. Finally, parameters such as temperature and pressure are collected and recorded through the PC-based data acquisition system. 2.2 Experimental Materials The materials used in the experiment mainly include natural gas and deionized water. Among them, natural gas is provided by Beijing Nanfei Industry and Trade Co., Ltd., and the composition details are shown in Table 1. Meanwhile, deionized water is made in the laboratory. Table1 Natural gas component parameters Compositions

Volume fraction /%

C1

95.9


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C2

3.1

C3

1

2.3 Experimental condition In this study, three repeated experiments were performed to analyze the microscopic characteristics of gas hydrate decomposition. The experiment was carried out by means of thermal decomposition. In order to better analyze the microscopic morphology of the hydrate, the hydrate concentration should not be too high. Therefore, at the time of hydrate formation, a lower initial pressure was selected to start the experiment. In addition, in the experiment, it is necessary to clearly observe the decomposition and breakage process of the hydrate, and also to ensure that the hydrate slurry has a strong fluidity. After comparison, the stirring speed was selected to be moderately 100 r/min. The specific condition parameters are shown in Table 2. Table 2 Experimental condition parameters Initial pressure of experiment /MPa 3.2

Chiller temperature 12.3

Stirring speed

repeat times

100

3

2.4 Experimental method The focus of the experiment is on the study of microscopic properties when hydrates decompose. Therefore, in this experiment, less attention can be paid to the process of hydrate formation, etc., mainly focusing on the decomposition process after hydrate particles stabilization. The specific experimental process is as follows: The high-pressure cell is cleaned by means of distilled water washing and nitrogen purging to ensure cleanness in the cell and transparency of the visible window. (2) The cell is tested for tightness via high pressure nitrogen. (3) 650 ml of distilled water is injected into the cell. (4) Start stirring and adjust the stirring to 100 r/min. (5) Adjust the water bath and cool the cell to the experimental operating temperature (3℃). (6) Supply natural gas to the experimental system ACS Paragon Plus Environment

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through the gas supply system until the pressure reaches the experimental condition pressure. (7) The formation of the hydrate is carried out until the parameters tend to be stable. (8) Adjust the water bath temperature to the heat exchange temperature of the design working condition to decompose the hydrate. During the decomposition process, the hydrate microscopic behaviors were recorded by a high speed camera. During the shooting process, different shooting speeds should be selected for different time periods to ensure the sharpness of the shooting. In this work, the shooting speed of the high-speed camera ranged from 250 to 2000 fps. 3 Results and discussion 3.1 Microscopic morphology and characteristics of hydrates in decomposition In this experiment, similar to the hydrate aggregation experiment conducted by Song et al. (2527), the hydrate particles initially had three types of granulated hydrate particles, flaky hydrate particles and block hydrate particles. Therefore, this chapter will also be analyzed based on these three types. The granulated hydrate particles have a small particle size and are the most numerous one among the three types, which are irregularly shaped three-dimensional structures. There are two main sources of granulated hydrate particles throughout the decomposition process. The first one is a granulated hydrate particle naturally formed during hydrate formation, as shown in Fig. 2(a). Another granulated hydrate particles is broken from the flaky hydrate particles and the block hydrate particles. Fig. 2(b) shows the flaky hydrate particles which are about to be broken into granulated hydrate particles. Compared to the other two types of hydrate particles, the granulated hydrate particles have a relatively large heat exchange area, and thus the decomposition speed is relatively fast. Fig. 3 shows the morphology of the typical granulated hydrate decomposition at different stages. At the beginning, the granulated hydrate particles have relatively sharp edges (Fig. 3(a)). As the environmental conditions change, the surface hydrate of the particles first decomposes under decomposition conditions, and then decomposes into the interior of the particles. At this time,


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the edges of the hydrate particles become smoother (Fig. 3 (b), (c)). After further decomposition, the hydrate particles will leave only the nucleus veins (Fig. 3(d)) and eventually all decompose into water and natural gas. Based on the above analysis, the physical model of the entire decomposition process is shown in Fig. 4.

Fig.2 Two different sources of granulated hydrate particles


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Fig.3 Morphology of the typical granulated hydrate particles decomposition at different stages

Fig.4 Physical model of granulated hydrate particles decomposition

In this experimental study, as shown in Fig. 5, the flaky hydrates are mainly divided into two types, one is crisp hydrate particles (Fig. 5(a)), and the other is snowflake hydrate particles (Fig. 5(b)). This is due to the different formation patterns of hydrates in the formation, Fig. 5 (a) is the main trunk non-directional growth, and Fig. 5 (b) is the main branch growth (28).


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Fig.5 Two types of flaky hydrate particles

Fig. 6 shows the morphology of the typical crisp hydrate decomposition at different stages. As shown in Fig. 6(a)-(b), when the ambient temperature is changed, the surface hydrate of the crisp hydrate is decomposed under the decomposition condition first. The decomposition of the hydrate surface layer will absorb the heat around, so that the hydrate particle structure inside the hydrate particles remains relatively stable, and the overall framework of the particles is relatively complete. As the decomposition proceeds further, the weak connection between the large nucleus in the edge region of the hydrate particle will first decompose, and the interconnection inside the hydrate particles will change from the solid phase to the liquid phase. Subsequently, it gradually disengages from the body under the action of the flow shearing force, as shown in Fig. 6(c). Fig. 6(d)-(f) shows that the hydrate particles are further decomposed, and the hydrates between the crystal nucleuses of the particles are gradually decomposed, and finally the hydrate particles are decomposed into small granulated hydrate particles by the crisp hydrate particles. Fig. 7 shows the morphology of the typical snowflake hydrate particles decomposition at different stages. Compared with crisp hydrate particles, the thickness of snowflake hydrates is generally relatively thin, the transparency is relatively high, and the decomposition rate is relatively fast. However, similar to crisp hydrate particles, the snowflake hydrate particles are also first decomposed from the surface, making the snowy veins of the hydrate particles clearer, as shown in Fig. 7(a)-(b). As the decomposition progresses further, the weak connection at the edge of the


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hydrate particle is also decomposed first, and gradually disengages from the particle body under the action of flow shear, as shown in Fig. 7(c)-(d). Eventually, the hydrate particles will break into granulated hydrate particles for further decomposition, as shown in Fig. 7(e)-(f). Based on the above analysis, the physical models of the entire decomposition process of crisp hydrate particles and snowflake hydrate particles are shown in Fig. 8.

Fig.6 Morphology of the typical crisp hydrate decomposition at different stages


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Fig.7 Morphology of the typical snowflake hydrate particles decomposition at different stages

Fig.8 Physical model of flaky hydrate particles decomposition

Block hydrate particles are grown by a large amount of hydrates, and their shapes are irregular and the particles are thick, so they have a stable structure and a long decomposition process. In this experiment, the block hydrates were the least in the three types of hydrates, but the decomposition time was the longest. Fig. 9 shows the morphology of the typical block hydrate particles decomposition at different stages. Since the block hydrate is thick, in the decomposition of the hydrate, in addition to the decomposition of the hydrate particles themselves, peeling of the outer layer hydrate particles first occurs, as shown in Fig. 9(b). As the decomposition proceeds, the internal connections of the block hydrate particles are continuously decomposed until the interaction force between the particles is less than the flow shear force, and then broken into two or more small block hydrate particles, as shown in Fig. 9 (c). Eventually, the small block hydrate particles will be continuously decomposed into flaky hydrate particles to be further decomposed as shown in Fig. 9(d). Based on the above analysis, the physical model of the entire decomposition process is shown in Fig. 10.


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Fig.9 Morphology of the typical block hydrate particles decomposition at different stages

Fig.10 Physical model of block hydrate particles decomposition

3.2 Change in particle size during hydrate decomposition In this experimental study, as described above, the hydrate particles are non-spherical irregular three-dimensional structures. Therefore, in the description of the particle size, it is difficult to accurately express the particles by the average particle diameter. After analysis, the maximum particle size is selected to describe the particle size of the hydrate particles. In this paper, the particle size of hydrate particles is calculated by ProAnalyst, a processing software of high-speed cameras. First, the hydrate particles are captured using image filtering and particle tracking. Then, based on the captured hydrate, the maximum particle size of the hydrate particles was calculated. At the same time, in order to more accurately reflect the behaviors of the particle size distribution during the decomposition of hydrate, a large number of videos and images were taken in the experiment, thus ensuring the accuracy of the experimental conclusions.


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In the previous studies on the particle size of natural gas hydrate particles, the particle size of hydrate particles is often less than 100 μm (29, 30), which is due to the fact that hydrates are often formed in W/O media in previous studies of hydrates. Most of the hydrate particles formed in the W/O system are converted from micron-sized water droplets dispersed in a continuous oil phase. As a result, the particle size is generally small. For other types of hydrates (such as carbon dioxide, R11 hydrate, etc.)(31, 32), the particle size of hydrate particles is often small as well. In this study, hydrated particles are formed in the water phase, with a large number of large flaky and block hydrate particles formed at the water-air interface and by natural gas bubbles. The maximum particle size can be up to 1509μm. Therefore, this experiment can explore the change of particle size during the decomposition of hydrate from a larger particle size range. Taking one repetition as an example for analysis, Fig. 11 shows the change of particle size of hydrate slurry with time (after 50 minutes of decomposition, the number of hydrate particles is small, and no more statistics are made here). As shown in the figure, we can see that as the decomposition progresses, the average particle size of the hydrate gradually decreases with time. In the initial stage of the decomposition stage, due to the structure is very stable, at the same time, there are many large hydrate particles and the relative heat exchange area is small, so the hydrate decomposition is slow, so the average particle size changes little. As the decomposition progresses, the large hydrate particles are continuously decomposed and broken, so that the relative heat exchange area is increased. Therefore, the decomposition rate of the hydrate increases, and the average particle diameter of the hydrate rapidly decreases. However, according to Fig. 11, we can find that the maximum particle size and the minimum particle size of the hydrate remain substantially unchanged during the decomposition of the hydrate. This is because the large block hydrate particles structure are very stable, so the decomposition process is slow. As for the granulated hydrate particles, although they are continuously decomposed, the flaky and block hydrates will be continuously decomposed and broken to form small granulated hydrates as well, so the minimum particle size will remain stable.


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Fig. 12 shows the particle size distribution for different time periods (taking one repetition as an example). It can be seen from the figure that the hydrate particle size is mainly below 1000 μm. Moreover, as the decomposition time increases, we can find that the proportion of hydrate particles with large particle size has a tendency to decrease gradually, while the proportion of hydrate particles with small particle size has an increasing trend. Compared with large hydrate particles, small hydrate particles have a relatively large heat exchange area and are more easily decomposed into gas and water. Therefore, the increase in the proportion of small particle size hydrate particles is mainly due to the breakage of the flaky and block hydrate particles. Fig. 13 shows the hydrate decomposition process (taking one repetition as an example). To better reflect the number of hydrate particles, no adjustments were made to the shooting speed and shutter speed during this experiment. Therefore, the degree of lightness and darkness in the picture can well reflect the transmittance of the hydrate slurry, and the better the transmittance, the less the number of hydrate particles. From the figure, we can see that the transmittance of the hydrate slurry increases rapidly after 21 minutes, which means that the hydrate particles are rapidly decomposed, which also proves the conclusion in Fig. 12.The black ball appearing in the figure after 28 minutes of decomposition is a natural gas bubble, which reflects an increase in the rate of hydrate decomposition. And on the other hand, the generation of bubbles also increases the disturbance of the slurry, increases the heat exchange rate, and promotes the decomposition of hydrates. By the time of decomposition for 56 min, the number of hydrate particles is already small, and only a small amount of block hydrate particles and granulated hydrate particles that have not been decomposed.


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Maximum particle size The average particle size Minimum particle size

1600 1400

Hydrate particle size/μm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1200 1000 800 600 400 200 0

0

10

20

30

40

50

Time after hydrate decomposition/min

Fig.11 Particle size of hydrate particles at different times

Fig.12 Curves of hydrate particle size distribution


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Fig.13 Variation in the micromorphology of hydrate particles with time in one typical experiment.

3.3 Hydrate decomposition physical model During the flow of the hydrate slurry, the hydrate particles may collide, aggregate and break. As pointed out in Song et al. (25), the main trend of the particle size of the hydrate particles will increase before stabilization. When the environmental conditions reach the hydrate decomposition conditions, hydrate particle decomposition will be added in the original behaviors. It is found that the decomposition of natural gas hydrate during the flow of hydrate slurry can be divided into two types: large hydrate particles broken into small pieces of hydrate and hydrate particles selfdissolved. At this point, the flow of the hydrate slurry will be collision, aggregation, breakage and decomposition. Moreover, the breakage and decomposition will dominate in it, so the particle size of the hydrate particles will gradually decrease until it disappears. ACS Paragon Plus Environment

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Fig. 14 shows the curve of pressure change of hydrate decomposition, and the physical model of hydrate flow decomposition is shown in Fig. 15. As shown in Fig. 14, the decomposition of hydrate particles is mainly divided into three stages. The first stage is the initial stage of hydrate decomposition, which has a larger number of larger and thicker hydrate particles with a particle size of up to the millimeter range. There are two decomposition states of the large hydrate particles in this stage. One is that the surface hydrate of the large hydrate particles gradually decomposes, the thickness gradually becomes thinner, and the crystallization vein of the hydrate particles becomes more and more clear. The other is that the weaker internal connections of the large hydrate particles are decomposed and disappear, resulting in the breaking of the large hydrate particles into two or more smaller hydrate particles. At the same time, small hydrates are undergoing a similar decomposition state, but compared to large blocks, small hydrates generally do not break, gradually become needle-like and gravel-like particles, and even completely decompose. In the second stage, the amount of particles in the hydrate slurry is increased due to the large amount of hydrated particles being broken. At this time, the hydrate particle thickness is reduced and the transparency is increased. Meanwhile, the small hydrate particles are also greatly increased, and the edges and corners are more distinct as well. At this stage, due to the breakage of the hydrate particles, the heat exchange area of the particles is increased, and the hydrate particles are rapidly decomposed. What’s more, at this stage, due to the accelerated decomposition rate, many natural gas bubbles are generated. The generation of bubbles will increase the disturbance of the slurry and enhance the heat transfer, thereby further promoting the decomposition of hydrate particles. The third stage is the end stage of decomposition. At this time, decomposition is basically completed, and the large particles disappear substantially. Only a small amount of large hydrate particles with large initial thickness are still decomposing. The amount of hydrate particles at this stage is significantly reduced, and the texture of the hydrate particles tends to be blurred. The edges of the particles are more rounded as well. As a result, the rate of decomposition of the hydrate is gradually reduced until the decomposition is completed (33).


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3.1 3.0 2.9

Pressure/MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.8 2.7

Stage 2

Stage 1

2.6

Stage 3

2.5 2.4 2.3

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Time after hydrate decomposition/s

Fig.14 Curve of pressure change of hydrate decomposition

Fig.15 Physical model of hydrate flow decomposition

4 Conclusions In this paper, the hydrate thermal decomposition experiment was carried out by a self-designed high-pressure stirring reaction system, and the microscopic behaviors of hydrate particles were captured and studied by a high-speed camera. According to the experimental data, the physical model of hydrate particle flow decomposition was established and the following conclusions were obtained:


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(1) In this study, the hydrate particle were mainly classified into three types: granulated hydrate particles, flaky hydrate particles, and block hydrate particles. (2) There are two main sources of granulated hydrate particles throughout the decomposition process. The first one is a granulated hydrate particle naturally formed during hydrate formation. Another granulated hydrate particles is detached from the flaky hydrate particles and the block hydrate particles. (3) The flaky hydrate particles are divided into two types: crisp and snowflakes. These hydrate particles are large in size but thin in thickness. The decomposition process of flaky hydrates is often accompanied by the breakage of hydrate particles. (4) Block hydrate particles are large and thick, and have strong stability. Therefore, the decomposition process of the block-like hydrate particles is the longest. The decomposition process of the block hydrate particles is often accompanied by the continuous breakage of the hydrate particles as well.

Acknowledgments This work was supported by Shandong Provincial Natural Science Foundation, China (Grant No. ZR2017MEE057), the Fundamental Research Funds for the Central Universities (14CX02207A, 17CX05006), which are gratefully acknowledged.

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