428Science in China Ser. B Chermistry 2004 Vol.47 No.5 428- -433Influences of different types of magnetic fields onHCFC-141b gas hydrate formation processesSHU Bifen, MA Xiaolin, GUO Kaihua & LI JianhongGuangzhou Institute of Energy Conversion, The Chinese Academy of Sciences, Guangzhou 510640, ChinaCorrespondence should be addressed to Shu Bifen (email: shubf@ ms.giec.ac.cn)Received March 17, 2004Abstract In this study, visualizations and experiments are carried out on the influence of staticand rotating magnetic fields on the characteristics of HCFC-141b gas hydrate formation, such ascrystallization form, formation temperature and induction time. It has been found that a properrotating magnetic field can considerably improve the low-pressure gas hydrate formation process,especially in increasing the formation temperature and shortening the induction time. The mor-phology of the gas hydrate formation appears rather complex and compact. However, a properstatic magnetic field can make the gas hydrate crystal more organized, which will be benefit toheat transfer.Keywords: magnetic field, gas hydrate, erystallization, formation temperature, induction time.DOI: 10.1360/03yb0202Gas hydrates are crystalline compounds formedwater- refrigerant liquid-liquid and gas-liquid inter-when gas molecules or volatile liquid molecules comefaces are the most important factors for the gas hydratein contact with water molecules through weak van derformation process.Waals force at favourable pressure and temperature.It has been known that magnetic fields can pro-Refrigerant gas hydrates can be effectively formed atmote phases permeation and emulsification of theappropriate temperature (5- -12C) with a high reac-paramagnetism or diamagnetism liquor, and thereforetion heat (320- -380 kJ/kg). Because of their particularcan speed up crystals growth velocity, improvingthermodynamic properties, refrigerant gas hydrate,crystals modality" 0. The character and action modeespecially low pressure refrigerant gas hydrate, hasof the magnetic field are the main factors influencingbeen considered as one of the most promising cool .the crystal growth processes' 8. For gas hydrates,storage media for air conditioning systems". Gener-there are a few reliable experiment and theoretical re-ally, refrigerant and water cannot be dissolved in eachsearch reports about the effect of magnetic fields onother, and it usually takes a long time for formation ofthe hydrate formation process. Makogon indicated thatthe gas hydrate due to the resistance to phase permea-a stationary magnetic field can make the natural gastion and mass diffusion through the interfaces andhydrate crystal more regular and increase its density'hard hydrate crusts. General methods, such as addingLiu Yong observed that the stationary magnetic fieldadditive or churning up, have an effect but have some中國(guó)煤化工phase permeation of hy-limitation at the same time'"s . Therefore, enhancingHCNMHG By now, there is no re-the phase permeation and mass diffusion through theport aoout tne erect oI une rotating magnetic field onCopyright by Science in China Press 2004Influences of different types of magnetic fields on HCFC- 141b gas hydrate formation processes429the gas hydrate formation process. In this paper,mm. The intensity of the magnetic field is changed byexperiments are carried out on the influence ochanging the thickness of the magnet, which is 5, 10,different magnetic fields, including static and rotating15, 20 and 25 mm respectively. The interior diametermagnetic fields with different intensity, orientation andof the glass container is 22 mm, and the height is 100speed of rotation, on the characteristics of HCFC-141bmm. Some iron wires, with 2 mm in diameter and 95gas hydrate formation process, such as crystallizationmm in height, were fixed inside the reactor by theform, formation temperature and induction time.metal hoop.1 ExperimentalThe schematic experiment apparatus of rotatingGenerally there are several kinds of magneticmagnetic field is shown in fig. 1(c). The intensity andfields according to the distribution of the lines ofthe rotate speed of the rotating magnetic field can bemagnetic force, such as the perpendicular magneticchanged from 95 to 230 mT and 200 to 1200 r/minfield, horizontal magnetic field and rotating magnetrespectively. The reactor in fig. 1(c) is the same as thatfield!o. Considering that refrigerant and water cannotin fig. 1(a). HCFC-141b and water were used as thebe dissolved in each other and they are stratified nor-gas hydrate formation medium. The purity ofmally, the perpendicular and the rotating magneticHCFC-141b is over 99.5% and water was quadraticdistilled.fields are adopted in this experiment.1.2 Experiment process1.1 Experiment apparatusThe experiment was carried out in a low tem-Visualizations and experiments were carried outperature visualization experimental systeml".on gas hydrates formation process. Before the experi-ments began, the temperature of the constant-tempera-The visualization hydrate reactor in magneticture chamber was set at 1C, and 15 g distilled waterfield is composed of permanent magnet, rotating ap-and 5 g HCFC-141b were charged into the bydrateparatus, transparent glass container, iron wires, metalreactor. When the temperature of the chamber becamehoop, airproof cover and clamp. Its schematic issteady, the hydrate reactor was put into the chambershown in fig. 1.and fixed as in fig. 1. Then, the temperature in the re-The schematic experiment apparatus of staticactor was measured by the temperature sensor andmagnetic field is shown in fig. 1(a) and 1(b). The Nrecorded by data acquisition and control system. Thepole of magnet is upward in fig. 1(a), and the S pole isexperiment began and the morphology of the gas hy-upward in fig. 1(b). The diameter of the magnet is 45drate formation was recorded by the camera.AirproofζScoverClampcover .<5MetalThermalhoopresistance hoopresistanceI ronresistancewiIron .Iron wirewireGlassRotating,vesselmagnetNpoleS poleMagneta)(b)中國(guó)煤化工Fig. 1. Schematic diagram of experiment apparatus. (a) The N pole of magnet is.MYHCNMHGoleofmagnetisupwardinstatic magnet; (C) the schematic of rotating magnet experiment apparatus.430Science in China Ser. B Chemistry2 Results and discussionthe nucleation points are centralized, and the macro-2.1 The morphology of gas hydrates formationstructure of the crystals is dense.processFig. 2(c) and 2(d) show the pictures of the gasThe pictures shown in fig. 2 are about the gashydrate formation process in rotating magnetic fields.hydrate formation process. The time of the beginningIt can be seen that the rotate speed of the magneticof the thermodynamic balance is defined as the zerofields can affect the gas hydrates formation area.point of the process.When the rotate speed 0 was 500 r/min, crystals wereformed at the liquid-liquid interface of HCFC-141bIt is known that the process of crystal growth is aand water and grew up into the water firstly, then grewspecially discrete and nonuniform process. Nucleationdown into the HCFC-141b, as shown in fig. 2(c).always appears first at the place where the concentra-However, when the rotate speed 0 was 1150 r/min,tion of crystallization medium is the strongest, and thecrystals were first formed at the liquid-liquid interfacecrystallization heat must be carried away immediatelyof HCFC-141b and water and then grew up into theso that the crystal can be formedl21. The pictures ofwater and down into the HCFC- 141b at the same time,crystal growth in static magnetic fields are shown inas shown in fig. 2(d). This phenomenon indicates thatfig. 2(a) and 2(b), and their experiment apparatus area higher rotate speed of rotating magnetic field is bet-shown in fig. 1(a) and 1(b). In fig. 2(a) and 2(b), it canter than a lower one in promoting the phase permea-be seen that crystals were formed first both at thetion and mass transfer between the two phases, thusHCFC141b-water interface and the surfaces of thecausing a higher crystal growth rate. Compared withirons, after then crystals began to grow upward alongthe static magnetic field, crystals in rotating magneticwith the surface of the iron, and finally grew down-field are more centralized, denser and homogeneous.ward along with the surface of the iron. After about 7h, crystals were distributed in the whole space of theIn utilization, the more compact the crystals are,reactor. Fig. 2(a) also shows that the nucleation pointsthe more compact the cool storage system will be, buare dispersed when the N pole of the magnet is upward,the heat transfer must be strengthened at the same time.and the macrostructure of the crystals is loose. BuOn the other hand, when crystals are looser, the coolwhen the S pole is upward, as it shows in fig. 2(b),storage system will be larger, and the heat transfer37 min 48 min83 min 215 min 437 min28 min40 mir112 min412 min 485 min(a) The N pole of magnet is upward in static magnet(b) The S pole of magnet is upward in static magnctH= 435 mT51 min60 min74 min88 min103 min中國(guó)煤化Inin38 min .(C) Rotating magnetic field0= 500 r/min, H= 145 mTYHCNMHGir.Fig. 2. Pictures of gas hydrate formation process.Influences of dfferent types of magnetic fields on HCFC-141b gas hydrate formation processes431efficiency will be better too.speed of magnetic field with the lower one, it can be2.2 The temperature curves of gas hydrates forma-seen firstly that under the higher rotate speed condi-tion processestion, the hydrate formation temperature was about 4"C,higher than that in the lower one (about 2C). Sec-Fig. 3(a) shows the temperature curves of the gasondly, the crystal-formed process was greatly short-hydrate formation processes in static magnetic fields.ened (shown as BC period) and the induction time ofThe typical gas hydrate formation process can be di-nucleation was about zero, much shorter than that invided into four periods, called cooling period, nuclea-the lower one (about 60 min), resulting in the crystaltion period, crystal growing period and ending period,growth speed up and the process shortened (shown asrespectively. The medium for forming gas hydrate isCD period). It is concluded that the higher the rotatingcooled from point A. When the medium is at the de-speed of magnetic field is, the more effectively thecomposition temperature (8.4"C), no nucleation ap-magnetic field works in improving the gas hydratespears until the temperature goes down to point B. Theformation processes.temperature difference between decomposition pointand point B is called the supercooling degree. During2.3 The induction time varied with different mag-the nucleation period, the temperature almost does notnetic fieldschange until the nucleation is finished at point (It is known that the crystal morphology and crys-where the temperature rises up suddenly. The time costtal growth temperature are the two important charac-for nucleation during point B and C is called inductionteristics of crystal growth process. Another importanttime.characteristic is the induction time which is one of theimportant factors affecting the crystal growth rate.hydrate formation processes in the static magneticFig. 4(a) shows the curve of induction time withfield. In fig. 3(a), comparing the temperature curves ofvariation of intensity of the static magnetic fields. Se-the N pole of magnet upward with that of the S pole ofries 1 is the induction time curve with the S pole of themagnet upward, it can be seen that there was no ap-magnet upward; series 2 is the induction time curveparent difference between them. This indicates that thewith the N pole of the magnet upward. It can be seenorientation of the lines of the magnetic force does notthat the relation between the induction time and thehave notable influence on the temperature curves ofintensity is quadratic, and there is an optimal pointthe gas hydrate process.corresponding to a minimum induction time when theFig. 3(b) shows the temperature curves of the gasintensity of static magnet is H = 435 mT. The mini-hydrate formation processes in the rotating magneticmum induction time is 28 and 37 min respectivelyield. In this figure, comparing the higher rotatingwhen the S pole or the N pole of magnet is upward.AH= 435 mT(aA(A)H= 145 mT(b)0 = 1150 r/minI ,N pole uptured只@= 500 r/min2 B(C)2 s pole uptured50 10015020025030035040010150200/min中國(guó)煤化工Fig.3. The temperature curves of the gas hydrate formation processes. (a) The tenMHc N M H Grelds; () he empeatrecurves in rotating magnetic fields.432Science in China Ser. B Chemistry母I. S pole upward100 r70 r+1. H=9S mT0t士2. N pole upward60一2 H= 145 mT+3. H= 230 mT旨6040-30 t20a)10(b200 300 400 500600500 7009001100H/mTw/t. min-'Fig. 4. The induction time curves. (a) In static magnetic field; (b) in rotating magnetic field.Fig. 4(b) shows the variation of the induction(ii) In static magnetic fields with differenttime with different rotating speeds of the magneticorientation of magnetic lines of force, the temperaturefield at the different magnetic intensity. In this figure,curves of crystal growth processes are similar. But init can be seen that although the intensity of the rotatingrotating magnetic fields the crystals growth tempera-magnetic fields can affect the induction time, it is notture is much higher than that in static magnetic fields.the main factor. The main factor is the rotating speed.(li) In static magnetic field, magnetic intensity isAn increase of rotating speed 0 will shorten the in-n important factor affecting the induction time. Theduction time remarkably. When 0 was 500 r/min andthe intensity changed between 95 and 230 mT, the in-optimal intensity is 435 mT, and the effect is betterduction time varied between 48 and 65 min. When wwhen the S pole is upward. In rotating magnetic field,was 1150 r/min and at the same situation of magnetican increase of rotating speed 0 will shorten the induc-fields as before, the induction time was nearly zero. Ittion time remarkably. When w= 1150 r/min, the in-is well known that the shorter the induction time is, theduction time is nearly zero.better the performance of energy storage system willIn short, it can be seen that a proper rotatingbe.magnetic field can considerably improve thelow-pressure gas hydrate formation process, especially3 Conclusionsin increasing the formation temperature and shortening( i ) Based on the visualization results about thethe induction time. The morphology of the gas hydrateinfluence of static and rotating magnetic fields on theseems rather complex and compact. So enhancementcharacteristics of HCFC-141b gas hydrate formation,of the heat transfer is required in utilization. And theit can be found that in static magnetic fields, crystalseffect of static magnetic field on the crystal growth isare firm and close-gathered when the S pole of theweaker than that of rotating magnetic field on themagnet is upward, but when the N pole is upward,crystal growth, with better system heat transfer.crystals are loose and porous. It also can be found thatAcknowledgements This work was supported by the National Natu-when the rotating speed is low, the crystals growthral Science Foundation of China (Grant Nos. 20076046 and 59836230).process in rotating magnetic field is similar to that inReferencesstatic magnetic fields. But when the rotating speed ishigh, crystals are first formed at the liquid-liquid in-1. Guo, K. H, Shu, B. F, Yang, w. K., Advances and applications ofterface of HCFC-141b and water, and then grow upgas hydrate thermal energy storage technology, Proc. 1st TIEES,into the water and down into the HCFC-141b at the中國(guó)煤化Iabhai p D. etal. Catmresame time, resulting in a notable enhance of the crys-CYHC NM H G&. Sci. 1987, 421); 2647-tals growth speed.Influences of different types of magnetic fields on HCFC-141b gas hydrate formation processes4333. Isobe, F, Mori, Y. H., Formation of gas hydrate or ice by di-12- -26.rect-contact evaporation of CFC altermatives, Int. J. Refrig, 1992,8. Lan, M.J, Ye, s. C., Use Bridgman method for crystal growth in!5(13): 137-142.horizontal magnetic field, Chinese Journal of Materials Research4. Kakimoto, K. Effects of rotating magnetic fields on temperature(in Chinese), 2000, 14(1): 76- -81.and oxygen distributions in silicon mel, J. Crystal Growth, 2002,. Makogon, Y. F, Hydrate of Hydrocarbons, Oklahoma: Pennwell237-239: 1785- -1790.Books, 1997, 103.， Kaddeche, S, Hadid , H. B., Putelat, T. et al, Instabilities in lig-0. Liu, Y, Guo, K. H. Liang, D. Q. et al, Effects of magnetic fieldsuid metals cotolled by constant magnetic field-Part II: borizon-on HCFC-141b refrigerant gas hydrate formation, Science intal magnetic field, J. Crystal Growth, 2002, 242: 501- -510.China, Series B, 2003, 46(4): 407- -415.6. Liang, X. A., Jin, W. Q, Pan, Z. L.. Utilization of the magneticfield in crystal growth, Joumal of Inorganic Materials (in Chinese),11. Zhao, Y. L, Guo, K. H, Liang, D. et al, Formation process and1999, 14(6): 833- -839.fractal growth model of HCFC- 141b refrigerant gas hydrate, Sci-. Liua, Y. C., Okano, Y., Dosta, S.. The effect of applied magneticence in China, Series B, 2002, 45(2): 216- -224.ield on flow structures in liquid phase electroepiaxy--2. Zhang, K. C., Zhang, L. H, Crytals Growth (in Chinese), Beijing:three-dimensional simulation model, J. Crystal Growth, 2002, 244:Science Press, 1982.中國(guó)煤化工MYHCNMHG