Jialu Zhang, Xiang Liu, Shuai Liu, Yuxing Li, Qihui Hu, Wuchang Wang,*

1 Shandong Key Laboratory of Oil-Gas Storage and Transportation Safety,College of Pipeline and Civil Engineering,China University of Petroleum(East China),Qingdao 266580,China

2 State Key Loboratory of Natural Gas Hydrates, Beijing 100028, China

Keywords:Tetrahydrofuran hydrate Growth Agglomeration Morphology Microstructure Microstructure model

ABSTRACT The evolution of the hydrate particle structure during growth and agglomeration under flowing condition affects the particle as well as flow characteristic, which plays an important role in the flow assurance as well as heat transfer in refrigeration systems.Therefore, this article conducts experiments to study and observe the growth and agglomeration process in the main forming stage of hydrate.It was found that the growth of tetrahydrofuran hydrate was anisotropic and in a layered growth pattern.Single crystals generally transformed from octahedral structure to octahedral skeleton structure with growth, however some single crystals also deformed into plate type particles.The thickness of the plate type particles increased gradually during growth, and the edge part increased earlier than the middle part.During agglomeration, the hydrate particles contacted and sintered together.Sand as the impurity didn’t serve as the nucleation center but affected the agglomeration of hydrate particles by collisions.In addition,the effect increased as the sand size decreased.Finally, a microstructure model for hydrate growth and agglomeration was proposed,which showed the hydrate structure evolution in these processes and could lay a foundation for studying the flow assurance of hydrate slurry.

1.Introduction

As a kind of clean energy, natural gas has been paid attention,and its reserve has also attracted attention.The formation of natural gas hydrates (solidified natural gas (SNG) technology) is a way to store large amounts of natural gas [1-3].Gas hydrate is a nonstoichiometric cage-like crystalline substance formed by small gas molecules such as CH4or CO2and H2O molecules.The crystal structure of the hydrate includes three types: sI, sII and sH.However, its restrict conditions of high pressure and low temperature and formation rate limit the implementation of SNG technology[4-8].The addition of tetrahydrofuran (THF) can not only moderate the formation conditions but also conduce to the natural gas storage[3,9-12].At the same time,THF is liquid phase which could be completely miscible with water and form sII hydrateunder ambient pressure[13-17],and the formation of THF hydrate could be effected by the initial THF: H2O ratios [18].

The study of crystal morphology and structure plays an important role in the growth and agglomeration of hydrates.At the same time, growth and agglomeration also affect the flow safety of hydrates.Due to the moderate formation conditions of THF hydrate,many scholars use THF to study the hydrate crystal structure and growth.THF hydrate single crystal was generally octahedral structure, and octahedral skeleton, hexagonal plate, needles and solid plates could also be formed under different subcoolings[12,19].Besides,Makogonet al.[12]found that the addition of poly(vinylpyrrolidone)(PVP),poly(vinylcaprolactam)(PVCap)or a random terpolymer of the previous two with dimethylaminoethyl methacrylate caused the single crystals to grow as hexagonal plates rather than the octahedral structure.Liet al.[20]used the microscopic manipulating apparatus to observe and study the effects of cyclic structure inhibitors on the morphology and growth of THF hydrate crystals.They found that morphological patterns between each hydrate crystal growth from hydrate-liquid interface into droplet were different.Without inhibitor lamellar structure growth of hydrate crystal was observed, while with PVP was featheriness-like, poly (N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)(PVPP)was like long dendritic crystal,poly(2-vinyl pyridine-co-Nvinylcaprolactam) (PVPC) was Mimosa pudica leaf-like and PVCap was like weeds.Suzukiet al.[21]described foreign particle behavior at the growth interface of THF hydrates in a THF-water solution with a small amount of silica beads.It was found that the growth interface pushed the beads at lower growth rates but encapsulated them into the crystal region at higher growth rates.Chenet al.[22]studied the effect of a weak electric field on the crystal generation and growth of THF hydrate.They found that a lamellar hydrate surface under the weak electric field was eventually formed.Sunet al.[23]studied the influence of PVP with different molecular weights on THF hydrate formation and growth.Different molecular weights of PVP had different inhibitory effects on the growth,and the presence of PVP enhanced the agglomeration strength.However,studies on crystal morphology and structure of THF hydrate under flow conditions are limited.

In our previous work [24], the morphology of three kinds of microstructures in the THF hydrate formation under flow conditions were observed and described, and the influence of different factors on the morphology, size and proportion of them was analyzed.It was found that stirring and the volume concentration of THF (4%,6%,8%,and 10%)had a great influence on the agglomeration of hydrates.The formation and agglomeration process of THF hydrate were preliminarily described, focusing on the state of agglomeration.However, there were few analyses of the structure evolution in growth and agglomeration under flowing conditions.Therefore, on the basis of the previous studies, the evolution process of the microstructure of THF hydrate during growth and agglomeration was further analyzed in this article.In addition,due to the occasional existence of impurities during hydrate formation,the effect of sand as the impurity on THF hydrate was analyzed.What’s more, the microstructure model of hydrate growth and agglomeration was also established.

2.Experimental

2.1.Experimental apparatus and materials

The experimental apparatus is shown in Fig.1.The selfdesigned high pressure reactor for gas hydrate was used,including visual windows, magnetic agitator, a low temperature thermostat and a circulating water bath jacket andet al.Besides,the FASTCAM SA-X2 high-speed camera (Photron, Japan) directed perpendicular to the reactor was used to capture the morphology through the visual circular window.The work mode of the high-speed digital camera is shown in Fig.2.For more details about the apparatus,refer to the Ref.[24].

Fig.1. Experimental apparatus.

Fig.2. The work mode of the high-speed digital camera [25].

The experimental materials included THF, quartz sand and water.And the particle size of sand is 150, 106, 75 and 44 μm.The purity of THF is more than 99% (volume) and the density at room temperature is 0.8860-0.8890 g·ml-1.

2.2.Experimental procedure

First the reactor was washed with deionized water.Then 8%(volume) THF solution was prepared with deionized water and THF reagent and sand were weighed with a mass fraction of 0.5%.Next, the prepared THF solution and sand were mixed and added to the reactor.

After the solution with sand was added to the reactor,the magnetic agitator was turned on with the initial speed 100 r·min-1to make sand particles flowing with the solution instead of depositing on the bottom of the reactor.Then,the water bath was turned on to cool the liquid temperature to the experimental setting temperature.

Next,the high-speed camera was set up at the suitable position.The inside of the reactor was observed from the visual window every 10 min for confirming whether hydrates were formed.Once hydrates were formed,the morphology was recorded by the highspeed camera every 5 min at the beginning of formation, then every 60 min.Until the hydrate formation was stable,the temperature of the water bath was increased to decompose the hydrate and the morphology was also observed and recorded by the high-speed camera every 5 min.

The software ProAnalyst?(Photron, Japan) was not only used for particle tracking processing, but also used to measure the particle size with the given ruler.

The effect of sand size was also considered and five sets of experiments were carried out and the experimental condition parameters are shown in Table 1.

Table 1Experimental condition parameters

3.Results and Discussion

In previous study[24],THF hydrate was a polyhedral structure,which could be divided into three categories: single crystal, plate type particles and agglomeration.

Although the entire formation process had been carried out for several hours, the formation and growth of tetrahydrofuran hydrate was very rapid and almost completed within ten minutes.The particle concentration was higher when hydrates were formed in large quantities in the later period.In order to clearly observe the evolution of the particle growth and agglomeration process,the corresponding evolution of particles during the main forming stage within ten minutes was focused through the video taken(mainly referred to the three videos taken at the time of the beginning, 5 and 10 min).Due to the limitation of observation, the change of a certain particle couldn’t be observed in the whole process, so the situation of the whole particles was mainly studied.Some hydrate particles whose morphology could be clearly observed in the three videos was selected and the statistical analysis was further completed.

3.1.Crystal structure evolution in THF hydrate growth

The focus was mainly the growth of the single threedimensional structure and the plate-like structure, that is, the growth of single crystal particles and plate type particles.

3.1.1.Single three-dimensional structure

As the observation in previous experiments,most single crystals had an octahedral structure or an octahedral skeleton structure,belonging to the single three-dimensional structure.As shown in Fig.3(a),the transverse and longitudinal size of single crystals both increased during growth (the longitudinal direction here referred to the connection direction between the two apexes of the octahedral structure or the octahedral skeleton structure which was shown as the red ruler in the figure; the transverse direction was perpendicular to the longitudinal direction shown as the blue ruler), which indicated that the single crystals grew in all directions.And when the size increased to a certain extent, the single crystal began to change from the octahedral structure to the octahedral skeleton structure.It was further observed that generally the skeleton structure of the octahedron with larger particle size was more obvious.So it could be inferred that the size of single crystals with octahedral structure first increased after formation,and then particles continued to grow in the layered growth pattern, transforming from the original octahedral structure to the octahedral skeleton structure.The single crystals with the initial octahedral skeleton structure continued to grow in the layered growth pattern.So the larger the size was, the more layers were and the more obvious the octahedral skeleton structure was.In addition, during the main forming stage 65 single crystal particles at different time were selected and their transverse and longitudinal size were measured.Then these particles were ranked by longitudinal size and their size distributions were shown in Fig.4.It was found that the longitudinal size of the single crystal particles was larger than the transverse size,and the growth rate of longitudinal size was also higher.The transverse size fluctuated more obviously when growing.This showed that the growth of single crystal was anisotropic.

In addition to the conventional growth as above, the structure of the single crystal would deform during growth.This kind of deformation occurred not only in the small-sized octahedral structure, but also in the large-sized octahedral skeleton structure.The crystal shown in Fig.3(b) could be observed just after the initial formation,which looked like a folded flat plate.As it grew,the size increased and when it increased to a certain extent, it also grew into the layered growth pattern so there were many layers on the surface of the particle.Observing this particle from another angle, it was found that the structure was very similar to that of single crystal, plus the similarity of the growth mode, so this type of particles should be deformed from single crystals.The single crystal in Fig.3(c)deformed into the plate-like structure.The octahedral three-dimensional structure of the single crystal disappeared slowly, and the plate-like structure with the hexagonal cross-section was beginning to take shape.In addition, as shown in Fig.3(d), the octahedral skeleton structure also deformed causing the shape irregular.

Fig.3. Morphology of single crystal: (a) single crystal with different size: the first three particles forming octahedral structure, the others forming octahedral skeleton structure; (b-d) deformation of single crystal.

3.1.2.Plate-like structure

The plate-like structures that appeared in the research of previous scholars were all hexagonal in cross-section [12,19,24], but some plate-like structures with different cross-sectional shapes were observed in the experiment,shown in Fig.5(a)and(b).Therefore, it was speculated that the size growth of plate-like structure with a hexagonal cross-sectional shape was anisotropic,that is,the growth rate in each direction was inconsistent.This caused the growth rate in some directions to be so slow that the side length of the hexagon was inconsistent, and sometimes changed the cross-sectional shape from the original hexagon to quadrilateral or pentagon or even triangle.This was consistent with our previous study [24]that different growth speed caused the cross-sectional shape of the plate type particles change.In addition, the thickness of plate type particles with the initial structure of thin plate gradually increased in a layered growth pattern during growth so that the layered structure became more and more obvious.All in all,the size and thickness of the plate type particles increased during growth but there was no obvious correspondence between size and thickness.Particles with layered structures and different cross-sectional shapes might be obtained from the growth of corresponding plate-like particles, or they might be deformed from particles with other cross-sectional shapes due to inconsistent growth rates of size.What’s more, it was clearly observed that the thicknesses of the edge and the middle part were different in Fig.5(c),which meant that the thicknesses of the edge and middle part didn’t grow at the same time.The activity of different part was different which led to depressed region in the central region of the face and also reflected the anisotropy of the plate-like structure during growth.Besides, the structure of some particles was damaged with some gaps on the surface, as shown in Fig.5(d), which was caused by collisions between particles under the shear force.

It was also found in previous studies the structure of gas hydrate would transform during growth:The hydrate crystals that grew download the hydrate film transformed from needlelike to dendritic structures and the floating crystals transformed from simple geometric shape (octahedral and triangular or hexagonal platelets) into equiaxed dendritic shape.The transition was due to the constitutional undercooling (Mullins Sekerka instability)caused by the interaction between temperature and concentration distribution in front of growing hydrate/liquid interface.Though the structure of THF and gas hydrate was indeed different, the structure transition observed during the growth of tetrahydrofuran hydrate in the experimental process was similar to the abovementioned transition.Therefore, the reason for the transition of tetrahydrofuran hydrate was also due to the constitutional undercooling.Besides, the mass transfer process was also important for hydrate growth [26-29].

3.2.Crystal structure evolution in THF hydrate agglomeration

Fig.5. Morphology of plate type particle: (a) thin plate; (b) plate-like structure with layers on the thickness; (c) plate-like structure with thicker edges than the middle marked in the black circle; (d) plate-like structure with gaps circled in black circles.

Fig.6. Morphology of agglomerations:(a)touching state;(b) sintered state;(c) chain-like structure;(d)cluster-like structure;(e)agglomeration of plate type particles and single crystals; (f) agglomeration of plate type particles.

After the initial formation, there was mutual agglomeration between single crystals.As shown in Fig.6(a), two agglomerated single crystals touched first with arbitrary position, and the existence of the touching interface (the dotted line) could be clearly observed (See Video 1 in Supplementary Material).Then the two particles slowly sintered together with the touching interface disappearing in Fig.6(b)(See Video 2 in Supplementary Material).The two particles formed a small agglomeration and agglomerated with the third particle,and then with more other particles.Finally due to the different touching position two forms of agglomerations were formed, which were the chain-like structure in Fig.6(c) and the cluster-like structure in Fig.6(d).Besides, some smaller plate type particles also participated in the agglomeration process of single crystals.In addition to the mutual agglomeration of single crystals,single crystals also agglomerated with plate type particles.The process was similar but the agglomeration just occurred on the edge of plate particles.The single crystal first touched and then sintered on the edge of the plate type particles.Then more single crystal particles agglomerated at the edge of the plate type particles or agglomerated with the single crystal particles already existing on the edge shown in Fig.6(e).Plate type particles also agglomerated with each other in Fig.6(f).

Based on the particles size distribution in previous experiments,agglomeration of particles was mainly achieved by the formation of THF hydrate between particles to connect the two particles together.Therefore, the agglomeration between the THF hydrate particles was related to the content of THF in the liquid phase,which meant the agglomeration stopped when the THF content in the liquid phase was too low to continue to form hydrates [24].

3.3.Effects of sand as the impurity on THF hydrate

Fig.7. Sand particles of different size stirred together with hydrates.(The darker color in black circles was the sand and the lighter in blue circles was the hydrate.)

Fig.8. The size distribution of agglomeration: (a) the size distribution curves of agglomeration with different size sand; (b) the measurement of the size of agglomeration.

The effect of sand as the impurity on THF hydrate was studied.Sand particles did not become the nucleation center of hydrates as that was previously found in the methane hydrate with sand experiment[30].Most of the sand particles were dispersed among the hydrate particles and stirred together with the hydrate shown in Fig.7 (See Video 3 in Supplementary Material) and rarely wrapped in hydrate agglomerations.It could be clearly observed that the addition of sand affected the agglomeration of single crystals, causing the significant reduction of the agglomeration size.For this reason, when the hydrate formation was stable, dozens of hydrate agglomerations whose complete structure could be observed clearly were randomly selected in the experiment with and without sand, measuring the size of these agglomerations and calculating the average size (size here referred to the farthest distance between two points in the visible area of the particle, as shown in Fig.8(b)).As shown in Fig.8(a) and Table 2, the size of agglomerations was significantly larger when there weren’t sand particles and the size of agglomerations was reduced with the size of sand.This was because sand particles collided with hydrate particles in the stirring process,thereby affecting their agglomeration.Moreover, for the same quality of sand, the number increased as the sand size decreased,so the frequency of collision with hydrate increased and its effect on hydrate agglomeration was intensified.What’s more,few sand particles were wrapped in hydrate agglomerations, which indicated that the THF hydrate particles didn’t agglomerate so quickly that the sand particles could be carried in at once.

4.Microstructure Model of THF Hydrate Growth and Agglomeration

4.1.Microstructure model of growth

Through the above analysis, the microstructure model of the growth process was proposed, as shown in Fig.9.During growth,the size of single crystals with octahedral structure increased after they were formed, but the octahedral structure was still maintained.When the size reached a certain level,they began to transform to the octahedral skeleton structure and further grew.There were also some single crystals that deformed into a plate type particle with a hexagonal cross-sectional shape,which further grew as plate type particles.The initial plate type particles were thin plates and the size and thickness of them increased with growth.However, due to the difference growth rate of the size increasing, the cross-sectional shape might change.

Fig.9. Microstructure model of growth.(Red arrows represented the thickness of edge first grew.)

4.2.Microstructure model of agglomeration

The microstructure model of the agglomeration process was proposed in Fig.10.During the agglomeration of single crystals,the two particles first touched and sintered together, and then agglomerated with more particles to form two types of agglomerations, which were chain-like structure and cluster-like structure.The agglomeration between the plate type and single crystal particles was similar, but the single crystals only agglomerated at the edge of the plate type particles.When sand existed, the collision among sand particles and hydrate particles results in decreasing the size of the agglomerations and few sand particles could be wrapped in hydrate agglomerations.

Fig.10. Microstructure model of agglomeration.(The red ones represented single crystals that didn’t sinter and the black dotted lines represented the disappearing contact interface between hydrate particles after sintering.)

What’s more, the microscopic morphology evolution of the crystal structure could affect macroscopic flow characteristics including flow pattern,slurry viscosity as well as hydrates deposition in pipeline.So controlling the size of hydrate agglomerations was critical to the safety of flow in pipelines.In addition, during hydrates deposition, sand was usually wrapped in the deposition layer in single and layered form,which not only affected the structure and mechanical properties of the deposition but also affected the decomposition mechanism of sediments during the plugging removal process.

5.Conclusions

This article mainly studied the structural evolution of THF hydrate particles during the growth and agglomeration process in the main forming stage of hydrate.At the same time, the effect of sand as the impurity on THF hydrate was also studied.The conclusions are drawn as follows:

(1) The growth of THF hydrate particles was anisotropic and in a layered growth pattern.For single crystals,they transformed from the octahedral structure to octahedral skeleton structure or deformed into the plate type.For plate type particles,the thickness of the edge increased earlier than the middle part and different growth speed caused the cross-sectional shape change.

(2) During agglomeration, the particles first touched and then sintered together to form different kinds of agglomerations.Large-size plate type particles only had agglomeration phenomenon at the edges.Moreover, the agglomeration of THF hydrate was not very fast.

(3) When micro sand existed as the impurity, it didn’t serve as the nucleation center but affected the agglomeration.With sand size decreasing, the collision frequency increased and the effect on agglomeration was intensified.

(4) The model of the morphological structure was proposed,showing the evolution of the particle structure during the growth and agglomeration process.Microscopic morphology evolution of the crystal structure could further affect macroscopic phenomena, including hydrate agglomeration, deposition and so on.So the model can help investigate hydrate flow safety issues.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51991363 (Major Program), 51974349,U19B2012), State Key Laboratory of Natural Gas Hydrates(443CCL2020RCPS0225ZQN) which are gratefully acknowledged.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.07.002.