Morphology Investigation on Cyclopentane Hydrate Formation/Dissociation in a Sub-Millimeter-Sized Capillary

crystals

Article Morphology Investigation on Cyclopentane Hydrate Formation/Dissociation in a Sub-Millimeter-Sized Capillary

Qiang Sun 1,*, Mei Du 1, Xingxun Li 1, Xuqiang Guo 2 and Lanying Yang 1 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing 102249, China; [email protected] (M.D.); [email protected] (X.L.); [email protected] (L.Y.) 2 Department of Engineering, China University of Petroleum-Beijing at Karamay, Sinkang 834000, China; [email protected] * Correspondence: [email protected]

Received: 24 April 2019; Accepted: 6 June 2019; Published: 14 June 2019

Abstract: The formation, dissociation, and reformation of cyclopentane (CP) hydrate in a sub-millimeter-sized capillary were conducted in this work, and the morphology of CP hydrate was obtained during above processes, respectively. The influences of the supercooling degree, i.e., the hydrate formation driving force, on CP hydrate crystals’ aspect and growth rate were also investigated. The results demonstrate that CP forms hydrate with the water melting from ice at the interface between the CP and melting water at a temperature slightly above 273.15 K. With the action of hydrate memory effect, the CP hydrate in the capillary starts forming at the CP-water interface or CP–water–capillary three-phase junction and grows around the CP–water interface. The appearance and growth rate of CP hydrate are greatly influenced by the supercooling degree. It indicates that CP hydrate has a high aggregation degree and good regularity at a high supercooling degree (or a low formation temperature). The growth rate of CP hydrate crystals greatly increases with the supercooling degree. Consequently, the temperature has a significant influence on the formation of CP hydrate in the capillary. That means the features of CP hydrate crystals in a quiescent system could be determined and controlled by the temperature setting.

Keywords: morphology; CP hydrate crystals; sub-millimeter-sized capillary; morphology; supercooling degree

1. Introduction Hydrates are a kind of crystalline compound, which is usually formed by water and gas molecules at the proper temperature (T) and pressure (P) [1,2]. Most common gases, such as CH4, CO2,N2,O2, tetrahydrofuran (THF), and cyclopentane (CP), are able to form hydrates at different T-P conditions [3,4]. In hydrates, water molecules (host molecules) build cavities with a specific shape and size through a hydrogen-bond interaction, and gas molecules (guest molecules) are encaged in the cavities and stabilize the cavity structure [5,6]. The interaction between host and guest molecules is Van der Waals force [7]. The molecular size of the gas should be suitable for the size of the cavity in order to form stable hydrates. Consequently, different guest molecules should form hydrates with different structures, i.e., structure I, II, and H [8,9]. Since Hammerschmidt found natural gas pipeline blocking caused by hydrate formation in 1934 [10], gas hydrates have attracted extensive attention in industrial circles. So far, hydrates could potentially be applied to natural gas storage and transportation [11–13], gas mixture separation [14,15], CO2 capture and sequestration [16,17], sea water desalination [18,19], and so on. The study of hydrate-based technologies has obtained considerable development and great progress in the past

Crystals 2019, 9, 307; doi:10.3390/cryst9060307 www.mdpi.com/journal/crystals Crystals 2019, 9, 307 2 of 10 decades. It is believed that the study of CP hydrate formation and dissociation has great significance for the flow assurance of oil/gas [20]. Just like most gaseous guest molecules of hydrates, CP is difficult to dissolve in water. In addition, CP forms hydrates at relatively mild conditions. Therefore, CP is frequently used as a hydrate-former (guest molecule) for hydrate formation and dissociation research [4,19–21]. At the same time, the size of the reactor for hydrate formation also has great influence on the thermodynamics or kinetics of hydrates. For example, the porous media with strong interfacial effect influences the formation/dissociation kinetics of hydrates [22]. Thereby, the study of gas hydrates in porous media has great significance to both the upstream and the downstream of the oil and gas industry. So far, the study of hydrates in porous media is generally carried out in a macroscopic hydrate reactor loaded with porous material [23,24]. The knowledge of small-scale features of hydrates in porous media is relatively limited. In fact, the microscopic observation and study of hydrates enables researchers to study and understand the behavior, characteristics, and rules of hydrate formation/dissociation [25–28]. Consequently, we conducted the formation and dissociation of CP hydrate in a special porous medium, i.e., a transparent sub-millimeter-sized capillary, in this work and investigated the features of CP hydrate in a capillary accordingly.

2. Experimental Section

2.1. Materials The speciﬁcations and suppliers of experimental materials are presented in Table1. A cuboid-shaped capillary with a side length of 500 µm and a length of 10 cm was manufactured using a micromachining method.

Table 1. Sources and speciﬁcations of the experimental gas and reagents.

Materials Speciﬁcations Suppliers CP 98 wt% Beijing InnoChem Technology Ltd. (Beijing, China) Liquid nitrogen 99 mol% Beijing Yiyangfuli Commercial and Trading Ltd. (Beijing, China) Deionized water 15 106 Ω cm Water distillation unit (SZ-93) (Beijing, China) × ·

2.2. Apparatus The schematic diagram of the experimental apparatus is shown in Figure1. It mainly consists of a vacuum pump, two hand pumps, a water bath with a light source, a capillary, a high-resolution camera, and temperature and pressure transducers. The vacuum pump is used to eliminate gaseous impurities in the apparatus. The two hand pumps are used to maintain the desired pressure and CP–water interface position in the capillary. The water bath provides the reaction system with the desired experimental temperature. The capillary is made of acrylic sheets and completely immersed in the water bath. The temperature and pressure of the reaction system are measured by two transducers with an accuracy of 0.1 K and 0.01 MPa, respectively. The formation and dissociation phenomena of CP hydrate crystals in the capillary are displayed by the camera and recorded by a computer. Crystals 2019, 9, x FOR PEER REVIEW 3 of 10 Crystals 2019, 9, 307 3 of 10 Crystals 2019, 9, x FOR PEER REVIEW 3 of 10

Figure 1. Schematic diagram of the experimental apparatus. Figure 1. SchematicSchematic diagram diagram of of the the experimental experimental apparatus. 2.3. Procedure 2.3. Procedure 2.3. ProcedureDue to the quiescent contact and poor mass transfer effect between CP and water in the capillary,Due toit thetakes quiescent a long time contact for andCP to poor form mass hydrate transfer crystals, effect betweeneven under CP a and high water supercooling in the capillary, degree it Due to the quiescent contact and poor mass transfer effect between CP and water in the takes[28]. However, a long time the for presence CP to form of hydrate hydrate seed crystals, or wa eventer underfrom ice a high melting supercooling could significantly degree [28 ].shorten However, the capillary, it takes a long time for CP to form hydrate crystals, even under a high supercooling degree theinduction presence time of hydratefor hydrate seed reformation or water from [29]. ice This melting could could be attributed significantly to the shorten memory the induction effect of hydrate time for [28]. However, the presence of hydrate seed or water from ice melting could significantly shorten the hydrate[30]. Consequently, reformation [the29]. capillary This could loaded be attributed with CP to and the water memory was eff fiectrst of dipped hydrate into [30 ].liquid Consequently, nitrogen induction time for hydrate reformation [29]. This could be attributed to the memory effect of hydrate the(where capillary ice generated), loaded with and CP then and waterput into was a firstwater dipped bath with into liquid an initial nitrogen temperature (where ice of generated),270.15 K. With and [30]. Consequently, the capillary loaded with CP and water was first dipped into liquid nitrogen thenthe increase put into in a watertemperature, bath with CP an hydrate initial temperatureformed first of at 270.15 the CP–ice K. With interface the increase at a intemperature temperature, of (where ice generated), and then put into a water bath with an initial temperature of 270.15 K. With CPslightly hydrate above formed 273.15 first K [31,32]. at the As CP–ice the temperature interface at aincreased temperature to a value of slightly (Tdis) slightly above 273.15 higher K than [31, 32the]. the increase in temperature, CP hydrate formed first at the CP–ice interface at a temperature of Asphase the equilibrium temperature temperature increased to a(T valueeq ≈ 280.15 (Tdis) slightlyK), CP higherhydrate than completely the phase equilibriumdissociated. temperatureThe above slightly above 273.15 K [31,32]. As the temperature increased to a value (Tdis) slightly higher than the (periodTeq 280.15 for the K ),temperature CP hydrate rise completely is defined dissociated. as dissociation The above time period(tdis) in forthis the work. temperature After that, rise the is phase≈ equilibrium temperature (Teq ≈ 280.15 K), CP hydrate completely dissociated. The above definedtemperature as dissociation of the water time bath (t diswas) in set this to a work. lower Aftervalue that, (supercooled the temperature temperature, of the T watersup) to bathmake was CP period for the temperature rise is defined as dissociation time (tdis) in this work. After that, the setreform to a hydrate lower value with the (supercooled action of a temperature,memory effect T (watersup) to from make hydrate CP reform immediate hydrate dissociation). with the action The temperature of the water bath was set to a lower value (supercooled temperature, Tsup) to make CP ofdriving a memory force e(supercoolingffect (water from degree) hydrate of CP immediate hydrate dissociation).reformation in The a capillary driving forceis the (supercooling temperature reform hydrate with the action of a memory effect (water from hydrate immediate dissociation). The degree)difference of CP(ΔT) hydrate between reformation Teq and T insup a. capillaryThe temperature is the temperature profile of dithefference reaction (∆ T)system between during Teq andthe driving force (supercooling degree) of CP hydrate reformation in a capillary is the temperature Tabovesup. The process temperature is shown profile in Figure of the reaction2. The systemphenomena during of the CP above hydrate process crystal is shown formation in Figure and2. difference (ΔT) between Teq and Tsup. The temperature profile of the reaction system during the Thedissociation phenomena in a capillary of CP hydrate were crystalobserved formation and recorded and dissociation over time. inEach a capillary process were(generation observed of andice, above process is shown in Figure 2. The phenomena of CP hydrate crystal formation and recordedfirst formation, over time. dissociation, Each process and (generationreformation of of ice, CP first hydrate) formation, was dissociation,repeated two and to reformationthree times ofto dissociation in a capillary were observed and recorded over time. Each process (generation of ice, CPreduce hydrate) the experimental was repeated error. two to three times to reduce the experimental error. first formation, dissociation, and reformation of CP hydrate) was repeated two to three times to reduce the experimental error.

tdis T CP hydrate dissociation dis t T dis T eq △T=Teq-Tsup Tdis CP hydrate dissociation Tsup CP hydrate reformation 273.15 eqK △T=T -T T eq sup sup CP hydrateCP hydrate formation reformation 273.15 K CP hydrate formation Temperature

Temperature Ice formation

Ice formation Time Figure 2.2. TheThe temperature temperature proﬁle profile of cyclopentaneof cyclopentane (CP)Time hydrate(CP) hydrate formation formation and dissociation and dissociation in a capillary. in a capillary. 3. ExperimentalFigure 2. The Results temperature and Discussionprofile of cyclopentane (CP) hydrate formation and dissociation in a capillary. 3.1.3. Experimental First Formation Results of CP Hydrateand Discussion Crystals in Capillary 3. Experimental Results and Discussion 3.1. FirstCP andFormation water of were CP Hydrate ﬁrst introduced Crystals in into Capillary the water-wet capillary at atmospheric pressure and room temperature. An interface appeared between CP and water, as shown in Figure3a. The interface 3.1. First Formation of CP Hydrate Crystals in Capillary

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CrystalsCP2019 and, 9, 307water were first introduced into the water-wet capillary at atmospheric pressure4 and of 10 room temperature. An interface appeared between CP and water, as shown in Figure 3a. The interfacelooked shiny looked because shiny because of the di ofﬀ erentthe different refractive refr indexactive ofindex light of between light betw CPeen and CP water.and water. After After the thecapillary capillary was was contacted contacted with liquidwith liquid nitrogen, nitrogen, liquid waterliquid changedwater changed to solid ice,to solid as shown ice, as in shown Figure3 inb. TheFigure calorimetric 3b. The calorimetric study results study show results that show CP hydrate that CP could hydrate not formcould readily not form at thereadily current at the low current T [33]. Thislow T could [33]. also This be could reﬂected also by be the reflected gap between by the the gap solid between edge and the capillary solid edge inner and wall capillary (box in inner Figure wall3b). (boxIt indicates in Figure that 3b). ice andIt indicates CP coexisted that steadilyice and inCP a coexisted quiescent steadily system atin a a temperature quiescent system of 240.15 at K.a temperatureCP could not of form 240.15 hydrate K. CP with could ice atnot the form current hydr condition.ate with ice On at the the contrary, current CP condition. formed hydrateOn the withcontrary, the waterCP formed from icehydrate melting with at theirthe water interface from at ice a temperature melting at their slightly interface above at 273.15 a temperature K (shown slightlyin Figure above4c). Meanwhile, 273.15 K (shown the meniscus in Figure direction 4c). Meanwhile, of the interface the meniscus between direction CP and iceof inthe Figure interface3b wasbetween opposite CP and that ice of in CP Figure and water3b was in opposite Figure3 thata, which of CP resulted and water from in Figure icing and 3a, which the related resulted volume from icingexpansion. and the Consequently, related volume the solidexpansion. mixture Consequently, was dominated the by solid ice atmixture this moment. was dominated by ice at this moment.

Figure 3. Formation of CP hydrate and ice in the capillary, the scale bars in the images correspond to Figure 3. Formation of CP hydrate and ice in the capillary, the scale bars in the images correspond to 50 µm. 50 μm. 3.2. Melting of Ice, First Formation, and Dissociation of CP Hydrate in the Capillary 3.2. Melting of Ice, First Formation, and Dissociation of CP Hydrate in the Capillary Figure4 illustrates the melting of ice, first formation, and dissociation of CP hydrate in the capillary at atmosphereFigure 4 illustrates pressure. the CP andmelting ice wereof ice, first first put formation, into a water and bath dissociation with a temperature of CP hydrate of 270.15 in the K, capillaryas shown at in atmosphere Figure4a and pressure. then passed CP and through ice were a temperature-rise first put into a water period. bath The with state aof temperature the reactants of 270.15in the capillaryK, as shown hardly in Figure changed 4a untiland then the temperaturepassed through reached a temperature-rise 273.65 K, when period. the shape The ofstate CP–solid of the reactantsinterface distortedin the capillary obviously, hardly as shownchanged in until Figure the4b. temperature It indicates reached that CP began273.65 toK, formwhen hydrate the shape with of CP–solidthe water interface from the meltingdistorted ice. obviously, When the as temperature shown in Figure rose to 4b. 274.15 It indicates K, the protuberance that CP began shape to ofform the hydrateCP–solid with interface the almostwater flattened,from the as melting shown inice. Figure When4c. Thethe gapstemperature between solidrose edgeto 274.15 and capillary K, the wallsprotuberance disappeared shape and of filledthe CP–solid with new interface solids, almost which wasflattened, believed as shown to be CP in hydrate.Figure 4c. This The couldgaps betweenbe inferred solid from edge the and appearance capillary walls difference disappeared between and the filled newly with formed new solidssolids, andwhich the was pre-existing believed toones be onCPthe hydrate. central This axis could of capillary. be inferred The from new solids,the appearance i.e., CP hydrate, difference looks between homogenous the newly with formed a fine solidstexture. and By the contrast, pre-existing the pre-existing ones on solid,the central composed axis of ice,capillary. appeared The irregularly new solids, with i.e., uneven CP hydrate, grains. Thelooks solid homogenous body extended with a toward fine texture. the CP By phase, contrast, which the also pre-existing demonstrates solid, that composed CP was of consuming, ice, appeared and irregularlyCP hydrate crystalswith uneven were generating.grains. The At solid thesame body time, extended the outline toward of the the ice CP phase phase, bulk which around also the demonstratescentral axis in thethat capillary CP was shrunkconsuming, gradually. and CP However, hydrate when crystals the temperaturewere generating. reached At abovethe same 275.15 time, K, the CPoutline hydrate–ice of the ice phase phase looked bulk around transparent, the central and colloidal axis in substancethe capillary appeared shrunk in gradually. the phase, However, as shown whenin Figure the4 d–f.temperature The pre-existing reached protuberanceabove 275.15 K, of thethe CP–solidCP hydrate–ice interface phase disappeared looked transparent, and trended and to colloidalbecome a substance meniscus, appeared and some in dark the smallphase, particles as shown existed in Figure in the 4d–f. hydrate–ice The pre-existing phase. The protuberance existence of ofcolloidal the CP–solid substance interface and the disappeared dark unordered and particlestrended demonstratesto become a meniscus, that CP hydrate and some was dissociating.dark small Thisparticles occurred existed because in the the hydrate–ice current temperature phase. The was existence too high forof CPcolloidal to form substance hydrate with and the the melting dark unordered particles demonstrates that CP hydrate was dissociating. This occurred because the water [32]. When the temperature rose to a higher value (Tdis), the quantity of colloidal substance current temperature was too high for CP to form hydrate with the melting water [32]. When the decreased distinctly, as shown in Figure4g. After a period at constant T dis, the colloidal form looked temperaturetransparent. rose The to interface a higher between value (T CPdis), and the waterquantity arose of colloidal more clearly, substance and bright decreased and well-ordereddistinctly, as shownCP droplets in Figure appeared 4g. After in the a water period bulk at phase,constant as shownTdis, the in colloidal Figure4 h.form Finally, looked CP dropletstransparent. became The interfacesmaller and between fewer, andCP theand color water halo arose of the more CP–water clearly, interface and bright emerged and again,well-ordered as shown CP in Figuredroplets4i. appearedIt indicates in that the thewater CP hydratebulk phase, crystals as shown dissociated in Figure completely. 4h. Finally, CP droplets became smaller and fewer, and the color halo of the CP–water interface emerged again, as shown in Figure 4i. It indicates that the CP hydrate crystals dissociated completely.

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Figure 4. SnapshotsSnapshots of of the CP–ice interface with an increase in temperature.temperature. The The scale scale bars bars in the imagesimages correspond correspond to 50 μµm. 3.3. Reformation Process and Appearance of CP Hydrate Crystals in Capillary 3.3. Reformation Process and Appearance of CP Hydrate Crystals in Capillary After CP hydrate dissociated in a time of tdis (20 minutes) at a temperature of Tdis (282.15 K), the After CP hydrate dissociated in a time of tdis (20 minutes) at a temperature of Tdis (282.15 K), the temperature of reactants in the capillary was cooled to 277.15 K (Tsup) to make CP reform hydrate with temperature of reactants in the capillary was cooled to 277.15 K (Tsup) to make CP reform hydrate the action of a hydrate memory eﬀect, as shown in Figure5. The reformation of CP hydrate started with the action of a hydrate memory effect, as shown in Figure 5. The reformation of CP hydrate from the joint of the CP–water–capillary wall, where a tiny crystal particle appeared, as indicated by started from the joint of the CP–water–capillary wall, where a tiny crystal particle appeared, as the white arrow in Figure5a. This occurred because the temperature there was lowest in the capillary, indicated by the white arrow in Figure 5a. This occurred because the temperature there was lowest which is the closest to that of the outside surroundings (Tsup). It indicates that CP hydrate generally in the capillary, which is the closest to that of the outside surroundings (Tsup). It indicates that CP starts forming very near or at the water–CP interface, although hydrate nucleation is a stochastic hydrate generally starts forming very near or at the water–CP interface, although hydrate nucleation process. The hydrate particle grew afterwards in the CP phase and slid along the CP–water interface to is a stochastic process. The hydrate particle grew afterwards in the CP phase and slid along the the central apex, as shown in Figure5b–k. During this period, the shape of the CP–water meniscus CP–water interface to the central apex, as shown in Figure 5b–k. During this period, the shape of the hardly changed. After reaching the apex of CP–water interface, CP hydrate crystals began to grow CP–water meniscus hardly changed. After reaching the apex of CP–water interface, CP hydrate towards the water phase and the CP phase simultaneously, as shown in Figure5l,m. The CP–water crystals began to grow towards the water phase and the CP phase simultaneously, as shown in interface was covered with a polycrystalline crust, resulting in the deformation of CP–water meniscus, Figure 5l,m. The CP–water interface was covered with a polycrystalline crust, resulting in the as shown in Figure5n. The CP–water meniscus at the apex gradually faded away, and a new CP deformation of CP–water meniscus, as shown in Figure 5n. The CP–water meniscus at the apex gradually faded away, and a new CP hydrate–water interface emerged. In addition, the appearance

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Crystals 2019, 9, x FOR PEER REVIEW 6 of 10 hydrate–water interface emerged. In addition, the appearance of the CP hydrate crystals at this moment wasof the not CP flat hydrate but bumpy. crystals It at indicates this moment that thewas aggregation not flat but extentbumpy. of It the indicates CP hydrate that the was aggregation increasing. Afterwards,extent of the theCP polycrystallinehydrate was increasing. crust kept Afterwards, growing over the the polycrystalline capillary wall, crust which kept has growing been referred over the to capillaryas a “halo” wall, [25 ,which34,35], has as shown been referred in Figure to5 o,p.as a “halo” The degree [25,34,35], of unevenness as shown of in CP Figure hydrate 5o,p. looked The degree more ofintensive. unevenness The morphologyof CP hydrate of CP looked hydrate more with inte thensive. temperature The morphology profile at di offferent CP hydrate times is shownwith the in temperatureFigure6. profile at different times is shown in Figure 6.

Figure 5. ReformationReformation of CP hydrate hydrate crystals in the capillary capillary.. The scale bars in the images correspond to 50 μµm.

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Crystals 2019, 9, 307 7 of 10 Crystals 2019, 9, x FOR PEER REVIEW 7 of 10 Figure 6. Morphology of CP hydrate with the temperature profile at different times.

3.4. Influence of Supercooling Degree on CP Hydrate Crystals Reformation

After CP hydrate dissociated completely at Tdis in capillary, it is definite that the temperature (Tsup) for CP hydrate reformation influenced the aspect and growth rate of the hydrate halo, i.e., the supercooling degree (△T) influenced the morphology and formation kinetics of the CP hydrate. Figure 7 displays the snapshots and time (△t) of CP hydrate halos grown from the CP–water interface to the same distance (≈ 985 μm) in the CP phase at different △T. At a low supercooling degree (△T = 5 K in Figure 7a), CP hydrate crystals appeared obviously layered and sheet-like. A long gap appeared and existed between CP hydrate and capillary inner wall along with the CP hydrate formation, as shown in the box of Figure 7a. The substance in the gap was considered to be water, which could be inferred by the incomplete shining CP–water interface, similar to Figure 3a. It indicates CP hydrate formed deficiently at a supercooling degree, and it seems thin and crispy. As supercooling degree increased (△T = 7 K in Figure 7b), the water gap between CP hydrate and the capillary wall could hardly be observed. However, there was a clear wetting angle between the CP bulk phase and the capillary wall, as shown in the box of Figure 7b. CP hydrate FigureFigurecrystals 6. MorphologyalsoMorphology looked of oflayered CP CP hydrate hydrate and with withflaky the the but temperature temperature more compact profile profile at than different different at △ times.T = 5 K. As Tsup decreased further (△T = 9 K in Figure 7c), the wetting angle between the CP phase and the capillary almost3.4. Influence Influence disappeared, of of Supercooling and CP Degree hydrate on CP crystals Hydrate looked Crystals like Reformation slender strips. When the supercooling △ degreeAfter reached CP hydrate 11 K, shown dissociated in Figure completely 7d, CP hydrate at Tdis appearedin capillary, more it isuniform definite and that denser the temperature than at T After CP hydrate dissociated completely at Tdis in capillary, it is definite that the temperature =(T 9sup K.) forIt seems CP hydrate that the reformation surface of influencedthe CP hydrate the aspect crystals and hardly growth had rate lacuna. of the In hydrate conclusion, halo, i.e.,CP (Tsup) for CP hydrate reformation influenced the aspect and growth rate of the hydrate halo, i.e., the hydratethe supercooling had a high degree aggregation ( T) influenced degree theand morphology a good regularity and formation at a high kinetics supercooling of the CP degree. hydrate. It supercooling degree (△T)4 influenced the morphology and formation kinetics of the CP hydrate. lookedFigure7 solid, displays dense, the snapshotsand compact. and In time other ( t)words, of CP the hydrate morphology halos grown and fromproperty the CP–waterof the CP interfacehydrate Figure 7 displays the snapshots and time4 (△t) of CP hydrate halos grown from the CP–water could be controlled by the supercoolingµ degree. interfaceto the same to the distance same (distance985 m) (≈ in985 the μm) CP in phase the CP at diphasefferent at differentT. △T. ≈ 4 At a low supercooling degree (△T = 5 K in Figure 7a), CP hydrate crystals appeared obviously layered and sheet-like. A long gap appeared and existed between CP hydrate and capillary inner wall along with the CP hydrate formation, as shown in the box of Figure 7a. The substance in the gap was considered to be water, which could be inferred by the incomplete shining CP–water interface, similar to Figure 3a. It indicates CP hydrate formed deficiently at a supercooling degree, and it seems thin and crispy. As supercooling degree increased (△T = 7 K in Figure 7b), the water gap between CP hydrate and the capillary wall could hardly be observed. However, there was a clear

Crystalswetting 2019 angle, 9, x FORbetween PEER REVIEWthe CP bulk phase and the capillary wall, as shown in the box of Figure8 of 7b. 10 CP hydrate crystalsΔT also= 5 K, looked Δt = 587 layered s and flaky but more ΔcompactT = 7 K, Δthant = 420 at s△ T = 5 K. As Tsup decreased further (△T = 9 K in Figure 7c), the wetting angle between the CP phase and the capillary almost disappeared, and CP hydrate crystals looked like slender strips. When the supercooling degree reached 11 K, shown in Figure 7d, CP hydrate appeared more uniform and denser than at △T = 9 K. It seems that the surface of the CP hydrate crystals hardly had lacuna. In conclusion, CP hydrate had a high aggregation degree and a good regularity at a high supercooling degree. It looked solid, dense, and compact. In other words, the morphology and property of the CP hydrate could be controlled by the supercooling degree.

ΔT = 9 K, Δt = 227 s ΔT = 11 K, Δt = 158 s

Figure 7. TheThe influence inﬂuence of of the the supercooling supercooling degree degree on on the the CP CP hydrate hydrate halo halo grown to a similar length (≈ 985985 µμm).m). TheThe scalescale barsbars inin thethe imagesimages correspondcorrespond toto 5050 µμm.m. ≈ At a low supercooling degree ( T = 5 K in Figure7a), CP hydrate crystals appeared obviously In addition, the supercooling degree4 also influenced the growth rate of CP hydrate crystals. It tooklayered less and time sheet-like. for CP hydrate A long gapto grow appeared to the and sa existedme length between at a higher CP hydrate supercooling and capillary degree, inner which wall along with the CP hydrate formation, as shown in the box of Figure7a. The substance in the gap was means the growth rate of CP hydrate markedly in creased with the supercooling degree, as shown in Figureconsidered 8. to be water, which could be inferred by the incomplete shining CP–water interface, similar to Figure3a. It indicatesΔT = 5 CP K, hydrateΔt = 587 formed s deﬁciently at a supercoolingΔT = 7 K, degree, Δt = 420 and s it seems thin and 7 crispy. As supercooling degree increased ( T = 7 K in Figure7b), the water gap between CP hydrate /s 6 4 μm 5

1 Average formation rate, rate, formation Average 0 4681012 Degree of supercooling, K

Figure 8. The growth rate of CP hydrate crystals at different supercooling degrees.

4. Conclusions We focused on CP hydrate crystal formation, dissociation, and reformation in a sub-millimeter-sized capillary in this work. The morphology of CP hydrate was obtained respectively, and the influence of the supercooling degree on CP hydrate reformation was investigated. The results demonstrate that CP formed hydrate with the water melting from ice at the CP–water interface at a temperature slightly above 273.15 K. Consequently, the temperature significantly influenced the transformation of CP–water–ice–CP hydrate. CP hydrate started to form at the CP–water–capillary three-phase junction and grew on the CP–water meniscus. The supercooling degree influenced both the aspect and growth rate of CP hydrate. CP hydrate has a high aggregation degree and a good regularity at a high supercooling degree. The growth rate of CP hydrate crystals strongly increased with the supercooling degree. It indicates that the formation of CP hydrate and the corresponding features are determined by the supercooling degree.

Author Contributions: Experimental operation—M.D.; Supervision and writing—Q.S.; Design—X.L. and X.G.; Instrumental analysis—L.Y.

Funding: This work was supported by Science Foundation of China University of Petroleum, Beijing (2462017BJB05, 2462016YJRC005, and 2462018BJC004), National Natural Science Foundation of China (21306226, 21808238), Science Foundation of CUPBK (RCYJ2017A-02-001, RCYJ2017A-03-001), which are greatly acknowledged.

Conflicts of Interest: The authors declare that they have no conflict of interest.

References

Crystals 2019, 9, x FOR PEER REVIEW 8 of 10

Crystals 2019, 9, 307 8 of 10 and the capillary wall could hardly be observed. However, there was a clear wetting angle between the CP bulk phase and the capillary wall, as shown in the box of Figure7b. CP hydrate crystals also looked layered and ﬂaky but more compact than at T = 5 K. As Tsup decreased further ( T = 9 K 4 4 in Figure7c), the wetting angle between the CP phase and the capillary almost disappeared, and CP hydrate crystals lookedΔT = 9 likeK, Δ slendert = 227 s strips. When the supercoolingΔT = 11 degree K, Δt reached= 158 s 11 K, shown in Figure7d, CP hydrate appeared more uniform and denser than at T = 9 K. It seems that the surface of Figure 7. The influence of the supercooling degree on the CP hydrate4 halo grown to a similar length the CP hydrate crystals hardly had lacuna. In conclusion, CP hydrate had a high aggregation degree (≈ 985 μm). The scale bars in the images correspond to 50 μm. and a good regularity at a high supercooling degree. It looked solid, dense, and compact. In other words,In theaddition, morphology the supercooling and property degree of the also CP hydrate influenced could the be growth controlled rate by of the CP supercooling hydrate crystals. degree. It tookIn less addition, time for the CP supercooling hydrate to degree grow alsoto the inﬂuenced same length the growth at a higher rate of supercooling CP hydrate crystals. degree, Itwhich took lessmeans time the for growth CP hydrate rate of to CP grow hydrate to the markedly same length increased at a higher with supercooling the supercooling degree, degree, which as means shown the in growthFigure 8. rate of CP hydrate markedly increased with the supercooling degree, as shown in Figure8. 7

/s 6 μm 5

1 Average formation rate, rate, formation Average 0 4681012 Degree of supercooling, K Figure 8. The growth rate of CP hydrate crystals at different supercooling degrees. Figure 8. The growth rate of CP hydrate crystals at different supercooling degrees. 4. Conclusions 4. Conclusions We focused on CP hydrate crystal formation, dissociation, and reformation in a sub-millimeter-sizedWe focused on capillary CP hydrate in this work. crystal The morphologyformation, ofdissociation, CP hydrate wasand obtained reformation respectively, in a andsub-millimeter-sized the influence of the capillary supercooling in this degree work. on CP The hydrate morphology reformation of wasCP investigated.hydrate was The obtained results demonstraterespectively, thatandCP the formed influence hydrate of the with supercooling the water melting degree from on iceCP at hydrate the CP–water reformation interface was at ainvestigated. temperature The slightly results above demonstrate 273.15 K. that Consequently, CP formed hydrate the temperature with the water significantly meltinginfluenced from ice at thethe transformationCP–water interface of CP–water–ice–CP at a temperature hydrate. slightly CP above hydrate 273.15 started K. to Consequently, form at the CP–water–capillary the temperature three-phasesignificantly junction influenced and the grew transformation on the CP–water of CP–water–ice–CP meniscus. The supercooling hydrate. CP degreehydrate influenced started to bothform theat the aspect CP–water–capillary and growth rate ofthree-phase CP hydrate. junction CP hydrate and hasgrew a high on aggregationthe CP–water degree meniscus. and a good The regularitysupercooling at a degree high supercooling influenced both degree. the Theaspect growth and rategrowth of CP rate hydrate of CP crystalshydrate. strongly CP hydrate increased has a withhigh theaggregation supercooling degree degree. and a Itgood indicates regularity that at the a formationhigh supercooling of CP hydrate degree. and The the growth corresponding rate of CP featureshydrate arecrystals determined strongly by increase the supercoolingd with the degree. supercooling degree. It indicates that the formation of CP hydrate and the corresponding features are determined by the supercooling degree. Author Contributions: Experimental operation—M.D.; Supervision and writing—Q.S.; Design—X.L. and X.G.; InstrumentalAuthor Contributions: analysis—L.Y. Experimental operation—M.D.; Supervision and writing—Q.S.; Design—X.L. and X.G.; Instrumental analysis—L.Y. Funding: This work was supported by Science Foundation of China University of Petroleum, Beijing (2462017BJB05, 2462016YJRC005,Funding: This work and 2462018BJC004), was supported National by Science Natural Foundation Science Foundation of China of ChinaUniversity (21306226, of Petroleum, 21808238), ScienceBeijing Foundation of CUPBK (RCYJ2017A-02-001, RCYJ2017A-03-001), which are greatly acknowledged. (2462017BJB05, 2462016YJRC005, and 2462018BJC004), National Natural Science Foundation of China Conflicts(21306226, of 21808238), Interest: The Science authors Foundation declare thatof CUPBK they have (RCY noJ2017A-02-001, conflict of interest. RCYJ2017A-03-001), which are greatly acknowledged.

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