Gas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tube

Science in China ser. B Chemistry 2005 Vol 48 No 175-8Gas hydrate fast nucleation from melting ice and quiescentgrowth along vertical heat transfer tubeXIE Yingming", GUo Kaihua, LIANG Deqing, FAN Shuanshi, GU JianmingChen jinggui1. Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai 200030, China;2. Guangzhou Institute of Energy Conversion, The Chinese Academy of Sciences, Guangzhou 510640, ChinaCorrespondenceshouldbeaddressedtoLiangDeqing(email:liangdq@msgiecac.cn)Received July 9, 2004Abstract During the observation of HCFC141b gas hydrate growth processes outside a vertical heat transfer tube, two exciting phenomena were found: fast nucleation of gas hydrate frommelting ice, and the spontaneous permeation of water into the guest phases along the surface ofheat transfer tube to form gas hydrate continuously. These two phenomena were explained withZhou& Sloan s hypothesis and the theory of surface free energy respectively, and a novel meth-d of gas hydrate formation was presented- gas hydrate fast nucleation from melting ice andquiescent growth along heat transfer tube. There is no mechanic stirring in this method, theformed gas hydrates are comthe ratio of unreacted interstitial water is little. which overcomethe drawback of high energynd high ratio of unreacted interstitial water among the formedgas hydrates in the system with mechanic stirring. This finding will benefit the gas hydrate application technologies such as natural gas storage technology or cool storage technology with gashydrateKeywords: gas hydrate, natural gas storage, cool storage fast nucleation from melting ice, permeation, surface free enerDOI:10.136004yb0061Gas hydrates, or clathrate hydrates, are ice-like and transportation of natural gas 2, Some refrigerantcrystal, composed of host lattice(cavities) formed by gas hydrates can be formed in a range of 277-293Khydrogen-bonded water molecules, and other guest and 0-40 atms, with large fusion heat (270-430molecules called guest molecules. The guest molecu- kJ/kg), so they have been considered as one of theles act with host lattice in weak van der Waals force. a most promising phase change materials for cool stor-large variety of gases or volatile liquids can form gas age in air conditioning systemhydrates when they come into contact with water un-For engineering application of natural gas storageder certain conditions of temperature and pressure, with hydrate, or cool storage with hydrate, how tosuch as natural gases cO, and many kinds of freonmake gas hydrate form compactly is a key problemefrigerants!ll.Since naturaV凵中国煤化工 miscible w1One volume of natural gas hydrate can contain water. In mote film180+ volumes(standard temperature and pressure)of at the interfar.=w drophobic guestnatural gas. This property is appealing for the storage phases without mechanical stirring. Although satisCopyright by Science in China Press 2005Science in China ser. B Chemistryfactorily high hydrate growth rates can be achievedThe reactor is composed of a cylindrical borosilwith a stir system, the following problems make it cate glass tube (50 mmX200 mm)and two pieces ofimpractical: stirring needs additional energy costs; the stainless steel made flanges. They joined togethermechanical stirring system will also increase the initial through four screws. A straight copper tube (10 mm)investment and maintenance costs; stirring will in- passed through the reactor along axes center as a heatcrease the ratio of unreacted interstitial water in the transfer tube. Coolant (purified ethanol) flowed fromformed gas hydrates, therefore decrease the compacinside the heat transfer tube to cool the reactor Twohich leads to thepieces of platinum resistance thermometers penetrateddecrease of unit gas storage volume or cool storage into the reactor, from upside flange and downsideflange respectively, to measure the temperatures ofYutakal5I found that CO2 gas hydrate can growdifferent places in the reactor, shown as fig. 2along the solid surface of high polar surface free ener-gy without stirring. Based on this finding, a set ofvisual gas hydrate reaction apparatus was configuredthe gas hydrate formation process outside a verticalheat transfer tube in a quiescent reactor was observedthe reaction materials were HCFC 141b and water. Itwas found that gas hydrate can grow continuouslyalong the heat transfer tube compactly without stirring,from which a novel method of gas hydrate formation-gas hydrate fast nuclemelting ice and quiescent growth along heat transfertube1 Experimental apparatus and procedure1. 1 Experimental apparatusThe experiments were done on a self-designedvisual gas hydrate reaction apparatus, which consistsof an air bath, a water bath, a gas hydrate reactor of Fig. 2. Gas hydrate reactor of indirect contact heat transfer. 1,Glassindirect-contact heat transfer(abbr. reactor) which was tube; 2, up-side flange; 3, down-side flange; 4, left-side screw, 5, rightplaced vertically, a digital photo system and a daside screw, 6, heat transfer tube; 7, platinum resistance thermometer Ne1:8, platinum resistance thermometer No. 29, injection port; 10,acquisition system. The apparatus is shown in fig. Iejection portuangzhou district locates in the tropic areaRefrigeratorHewhere the temperature and moisture of air are high. Ifthe reactor is placed directly in the air, the cooliperiod will be long and dew will condense on the eactor surface to affect the observation of gas hydrateformation process, so the reactor will be placed intoWater温the closed aiThe air hath consists of a framemade of dor中国煤化工 bent to insulatCNMH Ge, the temperaFig.I.Experimental setup-visual gas hydrate reaction apparatus. ture and moisture of the air bath are controlled by anas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tubeair coil connected to a cooling system, and a heating time of gas hydrate formation, it was found that gassystem. The deviation of air bath temperature is +0.5 hydrate could nucleate and form quickly near the in-C under controlterface of ice and hcFc 14 1b when the ice was meltedThis phenomenon was called fast nucleation of gaThe coolant is supplied by the water bath (DChydrate from melting ice, and would be explained006, made in Ningbo Tianheng Instrument Factory), theoretically by the hypothesis of Zhouli&Sloan!ints temperature range Isthe latter part of this articledegree is +0.1C under controlAccording to these informal experiment results, ae values are logged by Agilent novel method of quick compact gas hydrate forma-34970A Data Acquisition/Switch Unittion without mechanic stirring was presented, calledThe discrete digital photos are caught bymethod of gas hydrate fast nucleation from meltincamera(Panasonic NV-DX100EN) andin theand quiescent growth along heat transfer tube. Thecomputer through a photo acquisitionformal experimental procedure was as follows30PlusFirst 250 g.03% Sds water solution was conThe purity of HCFC141b and sodium dodecyl fected with double distilled water and SDS, andsulfate (abbr. SDS) are 99.5%0. The water is doubleed into the reactor then the air in the reactor wasdistilled waterdrawn out with the vacuum pump, and 956g liquidHCFC141b was injected into the reactor, so that the1.2 Experimental proceduremole ratio of HcFc 14 lb and water was 1: 17. theBefore the beginning of the formal experiment, ratio corresponding to the stoichiometric compositionmany informal experiments were done and some basic of gas hydrate of structure Il--HCFC141b 17H20characteristics were found as followsThen the reactor was placed into the air bath, the(i) There was only a small amount of gas hy-efrigerator and heater in the air bath were turneddrates formed near the interfaces between water andon to regulate the temperature of air bath and reactorHCFC 141b if there is no surfactant in the water, while to a temperature of 8+0.5C. which was close towhen sds was added into the water at the concentra- the decomposition temperature 8 4C of HCFC 141btion of 0.03%, the water will permeate into the 17H20HCFC141b phase along the heat transfer tube spontaNext. the coolant of -5C in the water bath wasneously, which leads to the continuous growth of gas pumped into the heat transfer tube to cool the reactorhydrate along the heat transfer tube in the HCFC141bIf there was a thin layer of ice formed on the surfaceof the heat transfer tube, the temperature was going tohydrate quiescent growth along heat transfer tube. Theincrease to IC immediately, so that the ice wouldlatter part of this article will give a detailed description melt gradually, which could initiate the fast nucleationof this phenomenon and the related mechanism analyand continuous growth of gas hydrate. During thSISexperimental pre(i)When the temperature of air bath was set to were logged continuously and the photos were taken8C and the temperature of coolant was set to 1C, the every several minutesinduction time of the gas hydrate formation would2 Results and discussionexceed 16 h, so the induction time of gas hydrate foration would be very long in a quiescent reaction 2.1 Descrip中国煤化工 tion and growthsystem when its temperature was above 0CprocessHCNMHG(iii)In the tests aimed at shortening the inductionThe photos of the whole process of gas hydrateScience in China ser. B ChemistryHCFC4lb一15 min20 min30 min35 min45 min75 min160 minFig. 3. Photos of gas hydrate nucleation and growth processicleation and growth are shown in fig. 3, and thetemperature trends in the reaction system are shown inBefore the reaction of the system, there existedthree phases in the reactor: vapor phase(mainly filledwith vapor HCFC141b), 0.03% SDS water solution gphase and liquid HCfC141b phase. Platinum resis2tance thermometer No. l(abbr. Ptl)was in the watersolution phase and platinum resistance thermometero 2(abbr. Pt2) was in the liquid HCFC141b phaseas shown by the first photo of fig 3. When the temc IiR中国煤化工2014016operature of air bath and the reactor was stabilized atCNMHG8.0C, coolant of -5C was pumped from the waterFig 4. Temperature trends of reaction systemas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tubebath into the heat transfer tube to cool the reactor, was high. furthermore values of Ptl and Pt2 werewhich also means the beginning of the experiment 5 different with each other in most cases and the valuesmin later, ice emerged at the surface of the heat trans- of Pt2 were higher than the values of Ptl in generalfer tube. 10 min later, the thickness of the formed ice because the gas hydrate formed mainly in the liquidlayer was about I mm, just as shown by the second HCFC141b phase, not in the Sds water solutionphoto of fig 3. This process was ice formation process. phaseThen the temperature of coolant was increased to IC, 2.2 Mechanism explanation of fast hydration fromthe ice fusion gas hydrate nucleation process began. melting ice5 min after the beginning of this process, gas hydrateAccording to the observed phenomenon from fig 3emerged at the surface of heat transfer tube of vapor the process of gas hydrate fast nucleation from meltingphase and liquid HCFC141b phase, as shown by thethird photo of fig 3. This meant the end of the nuclea- ice can be divided into two processes as follows: (I)Icetion process and the beginning of the growth process. formation process: According to Zhou's hypothesis 6,TIonhen the temperature of water is below OC, the waterhydrate nucleation process took only 15 min totally,molecules formed great plenty of dynamic and half-they were called the process of gas hydrate fast nu- baked polyhedron units through hydrogen bonding, mostcleaton from melting ice totallyof which are pentagonal dodecahedraThe residual photos(from the fourth photo to thealso including some other polyhedron units consisting oflast photo of fig 3)illustrated the process of gas hy- kaidecahedron(56) units and pentakaidecahedrondrate quiescent growth along heat transfer tube. At the(526)units. When these units were polymerizedinitial and the middle period of the growth process, thegas hydrate mainly grew upwardsthe liquid gether, the ice crystals formed. (2) Ice fusion gas hydrate nucleation process: According to the hypothesis ofHCFC141b phase along the heat transfer tube, theliquid-liquid interface (interface between water soZhoul& Sloan", when the temperature was above 0Ction and liquid HCFC141b) was raised gradually bya part of hydrogen bonding in the ice crystal(about 15%)the formed gas hydrate, while the vapor-liquid inter-broke, which changed the ice crystal into ice fragments,face (interface between vapor phase and the waterand these ice fragments immediately changed into polhedron units, mainly dodecahedron units(5 2).Whenalmost ththese polyhedron units met HCFC141b molecules, manyWhen the liquid-liquid was close to the liquid-vaporinterface, gas hydrate began to grow largely in the pentakaidecahedron(56) cages formed (it is just avapor phase near the vapor-iquid interface along the supposition of the author, it needs further confirmation byheat transfer tube. 160 min after the beginning of thisaccurate experiments). These empty cages would com-prise the nearly HCFC141b molecules to form moreexperiment, the growth process finished, liquid reac- stable unit gas hydrate crystals, and release formationtion materials(SDs water solution and HCFC141b) heat. The released formation heat was partially absorbedwere reacted completely and changed into solid gashydrate at 100%. The reactor space filled with white by the heat transfer tube, and partially absorbed by theand granular solid gas hydrate, which surrounded the nearby ice, which melt the residual ice ulteriorly, andheat transfer tube completely, as shown by the last melting ice would produce more polyhedron units(mainly dodecahedron units 52), and these new polyhephoto of fig 3dron units would comprise the nearly HCFC141b mo-From the temperature trends illustrated by fig. 4, lecules ulteriorly so that more unit gas hydrate crystalsit was found that the temperatures kept decreasing formed. Th中国煤化工 antil the formeduring the process of fast nucleation from melting ice, unit crystalsCNMHGthe gas hydratewhile kept increasing during the process of quiescent nuclei with the critical size. This time the nucleationgrowth, which meant that the gas hydrate growth rate process finished and the growth process began. TheScience in China ser. B Chemistryprocess from ice fusion to gas hydrate growth is illus- 3, we found that water can permeate spontaneouslytrated in fig. 5. The time taken was less than 5 min, so along the heat transfer tube into the liquid HCFC141bthe ice fusion process and the gas hydrate growth pahse or vapor phase so that gas hydrate can formprocess almost began at the same time. From the abo- continuously in the vapor or liquid HCFC141b phaseve analysis, the reason that the gas hydrate can nucle- along heat transfer tube compactly without stirring,ate from melting ice quickly lies in that the water from which was called gas hydrate quiescent growth alongmelting ice comprises abundant 5 2 cavities, which can heat transfer. Yutaka 5I also found analogous phetransform into 526 easily and quicken the nucleation nomenon in his experiment. He considered that underprocess greatly. On the contrary, there exists only the effect of surface tension the water can permeatesparse 5 2 cavities in the ordinary water, so it will take into the liquid CO, phase through between the hydratea long induction time for the water molecules in the film on the liquid-liquid interface and the surface ofordinary water to form gas hydrates with the guest glass so that massive gas hydrate can form in the liq-moleculesuid CO2 phase without stirringQ1+Q2=, (HydrateIce(n)Here the theory of surface free energy is alsoformation heat)used to explain the above experiment phenomenon inthis article. But we considered that the hydrate film on(Ice fusion heat)the liquid-liquid interface is a kind of porous medials,so the water can easily permeate into the other side ofH leat transferthis hydrate film, and after this, the fact that the waterCavities(512,5"6)can permeate from between the surface of heat transfertube and the liquidHCFC141b phase is threal reason for gas hydrate continuous growth alongNucleationgrowth processheat transfer tube compactly without stirring, just asGas hydrate(HCFC141b 17H,O)illustrated by the photos of figs. 3 and 6. The detailedd by theFig. 5. Illustration of fast hydration from melting ice.rowheads in fig 6It was found from a series further experiments2.3 Mechanism analysis of quiescent growth along that the gas hydrate quiescent growth along heat transheat transfer tubefer tube can still happen when the concentration ofAccording to the observed phenomenon from fig. SDS was as low as 0.005%, from which it was conHeat transfcrHcat transfertubepWaterhydrateHCFC14IbWater中国煤化工CNMHGFig. 6. Gas hydrate quiescent growth along heat transfer tube. (a) Permeation of water into vapor phase to form gas hydrate; (b)permeation of water into liquid HCFC141b to form gas hydrateas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tubesidered that the Sds concentration was higher at theTable I Surface free energies of liquids and solids(mJ. m)liquid-liquid interface or vapor-liquid interface than inSurface free energy(m/m)the center of bulk water solution phase because of theWater 9 SDS75.652.3strong adsorption of the surface of heat transfer tubeSDS. Generally speaking, when the SDS concentration Liquid HCFC141b 33.633.60at the interfaces is exactly at the critical micelle con-Superscripts d and p indicate dispersion and polar components,centration (abbr. CMC), the surface freerespectively. The surface free energy of SDS water solution is equal toenergy (or that of the solution at CMC concentration, and the values of its dispersurface tension) of the water solution is at the smallest sion and polar components were calculated in proportion to that ofvalue, the permeation of water along the heat transfer water. The polar component of liquid R141b is assumed to be zerotube is at the most possibility, so in the following cal-of no polar moculation and analysis the SDs concentration at theTable 2 Interfacial free energies(m] m-2)interfaces was presumed to be at the Cmc concentra了sRtion, so that the surface free energy of water at the326y1-104h#-7h+397-116+336interfaces was also presumed to be equal to that ofThe data were calculated from Fowkes method:Y1=Ysolution at cmc concentrationY2-2iY2-2Yr?, where Y was the interfacial free energyAccording to the principle of the spontaneousbetween materials 1 and 2process, the following relationship between the interThe material of the heat transfer tube in this ex-facial free energy must holdperiment is copper, a kind of metal, so its surface free(i SDS water solution can permeate downside energy is greater than 100 m J/m, and its polar surfacealong the surface of heat transfer tube into the liquid free energy is greater than its dispersion surface freeHCFC141b phaseenergy generally, so the polar surface free energy ofthe heat transfer tube in this experiment is greater thar50 m//m in general, which accords with the deduced(ii) SDs water solution can permeate upsideonclusion in this article: yT >26.8 mJ/along the surface of heat transfer tube into the vaporYutakal5used the theory of surface free energy tocalculate and analyze the phenomena in his experiYs+7s<竹(2) ments and concluded that the higher the polar surface(ii) Liquid HCFC141b cannot permeate upside free energy of solid(such as those solid made ofalong the surface of heat transfer tube into the SDsquartz glass, metal or some other inorganic materials ),ater solution phasethe easier the permeation of water along the surface ofsolid (or the heat transfer tube in this experiment) intoYTR+ yrs> YTs(3) the guest phase(the quiescent growth of gas hydrateHere, subscripts T, S, and R are heat transfer tube, along the heat transfer tube ), while this phenomenonSDS water solution and liquid HCFC141b, respec- can hardly happen at the surface of solid made of rtively Surface free energies of several materials rele- ganic materials such as polytetrafluoroethylene bevant to our experiments and their interfacial free enercause its polar surface free energy is low in generalgies(values or expressions)are listed in tables I and 2, His conclusion also exactly accords with the conclu-respectively. Although the surface free energy of heat sion deduced in this article as eq(4)transfer tube cannot be known from the existing lit- 3 Conclusioneratures. it can be deduced from the above formulaH中国煤化工tistions(1F(3)as followss experimentsome usefulCNMHGollows>268mJ/m2(i) The method of gas hydrate fast nucleationScience in China ser. B Chemistryfrom melting ice can greatly shorten the induction and the initial cost. This finding will benefit gas hytime of gas hydrate formation. The reason lies in that drate application technologies such as natural gas hythe water from melting ice comprises abundant 5 2 drate storage technology, or cool storage technologycavities. which can transform into 5264 under the ef-fect of the guest molecules and form the unit gas hy- Acknowledgements This work was supported by the Guangdongdrate crystals quickly, so that the time needed for the and the State Key Development Program for Basic Research of Chinaformation of gas hydrate nuclei with critical size can (Grant No. G2000026306)be shortened greatlReferences() Gas hydrate can grow continuously along the 1. Davidson, D w. Clathrate Hydrates, Water---A Comprehe nheat transfer tube compactly without stirring. Thesive Treatise Franks. F. Eds. vol II. New York: Plenum Pressreason lies in that under the effect of surface tension1973,115-148.water can permeate spontaneously into the guest 2. Mori, Y.H., Recent advances in hydrate-based technologies forphases from between the surface of heat transfer tubenatural gas storage-a review, Journal of Chemical Industryneering, 2003, 54(Suppl ): 1-17.and the liquid or vapor HCFC141b. This phenomenon 3. Guo, K.H., Shu, B F, Gas hydrate and advanced uses in coolcalled the quiescent growth of gas hydrate along theheat transfer tube1) Only at the surface of solid which has high4. Zhong, Y, Rogers, R. E, Surfactant effects on gas hydrate formation, Chemical Engineering Science, 2000, 55: 41754187polar surface free energy(such as those solid made of5. Yutaka, T, Shuichiro, H, Ken, O, Massive CO, clathrate hydrategrowth at a high-polar-energy surface, Journal of Crystal Growth,can the permeation of water along the surface of solid2000,220180-184(or heat transfer tube in this experiment)into the guest 6. Zhou, D. 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