Effects of magnetic fields on HCFC-141b refrigerant gas hydrate formation

Vol. 46 No.4SCIENCE IN CHINA (Series B)August 2003Effects of magnetic fields on HCFC-141b refrigerant gas hy-drate formationLIU Yong (刘勇), GUO Kaihua (郭开华), LIANG Deqing (梁德青)& FAN Shuanshi (樊栓狮)Guangzhou Institute of Energy Conversion, Guangzhou 510070, ChinaCorrespondence should be addressed to Liang Deqing (email: liangdq@ms.giec.ac.cn)Received December 20, 2002AbstractLow-pressure refrigerant gas hydrates have billiant prospects as a cool storage me-dium for air-conditioning systems. Intensive effects of some specific magnetic fields on the forma-tion process of HCFC-141b refrigerant gas hydrate are depicted experimentally. Under influence ofthese specific magnetic fields, the orientation and growth region of gas hydrate are altered; induc-tion time of hydrate crystallization can be shortened extremely, and it can be shortened to 40 minfrom 9 h; hydrate formation mass can be enhanced considerably, and hydration rate can arrive at100% in some instances. Meanwhile, the relations of induction time and hydration rate changedwith magnetic field intensity are depicted, and some elementary regulations are found.Keywords: magnetic fields, gas hydrate, induction time, hydration rate, cool storage.DOI: 10.1360/02yb0178Gas hydrates are crystalline compounds formed (usually above 0C) by water reacting withsome gases or volatile liquids hydrate former). Guest molecules, such as gas or volatile liquidmolecules, are enclosed firmly inside the host cavities and act with water molecules in weak vander Waals force. Gas hydrate usually includes natural gas hydrate, refrigerant gas hydrate and CO2gas hydrate. Refrigerant hydrates can be formed above 0'C, and their crystallization is similar tothe ordinary ice, so it is also called“warm ice". Because the phase change temperature of the re-frigerant gas hydrate is between 5 and 12C and with a formation heat which is equal to that ofice, it can be a substitute of ice as cool storage medium for air-conditioning systems. The perfor-mance and applicability of the gas hydrate cool storage system can have a great superiority to thatof the icel1.21. To develop a cost-effective cool storage system for air-conditioning, a low- pressuregas hydrate cool storage medium should be employed. Unfortunately, those low-pressure refriger-ants, such as those of R11 and HCFC-141b, are comparatively inactive in gas hydrate formation.The diffusing speed of the liquid interface is very slow and the phases are scarcely mixed. Conse-quently, one has a very long induction time of reactionm中国煤化工surfactants and nucleate seeds added3- 5I!. Therefore,CNMHGemployed as amedium of thermal energy storage in engineering applauun,"a Tapiu aiu uiuurm formation ofgas hydrate will be the key for technical success.408SCIENCE IN CHINA (Series B)Vol. 46Electrical and magnetic stationary fields affect significantly the equilibrium formation condi-tions and growth kinetics of ice from water. Currently, there are no reliable experimental and theo-retical data about the effect of these fields on a hydrate formation. Makogonl61 indicated the effectof a stationary magnetic field on density and structure of hydrates. Denser hydrates with a moreregular structure formed under the influence of magnetic field. At present, there are no literaturesabout the effects of magnetic fields on refrigerant gas hydrate formation.In this paper, it is found through a set of experiments that some specific magnetic fields canconsiderably affect the formation of HCFC- 141b refrigerant gas hydrate.1 Experimental1.1Experimental apparatusThis experiment was carried out on the experimental system of gas hydrates at a low tem-perature; and the experimental apparatus is vided in the literaturel7.81.The visualization hydrate reactor in the magnetic field is composed of transparent glass con-tainer, airproof lid, magnets, iron wires and stainless steel fixing clip. The schematics of the reac-tors are shown in fig. 1. Inner diameter of the glass container is 22 mm, and the volume of that is3.8x10-5 m3. Airproof lid is used to prevent rfrigerant from voltilizing and outside impuritydropping into. Iron wires, 2 mm in diameter and 100 mm in length, which are fixed by stainlesssteel fixing clip, are distributed along the circle whose radius is 7 mm in the glass container. Thereare two kinds of magnets: one is 20 mm in diameter and 3 mm in thickness; the other is 45 mm indiameter and the thickness of it changes from 5 to 25 mm. The magnet whose diameter is 45 mmwas put on the top of the reactor. Magnet, diameter in 20 mm, is firstly put on the bottom of thereactor, and then magnet whose diameter is 45 mm is put below it, and the magnetic intensity isadjusted by changing the thickness of magnet whose diameter is 45 mm. In fig. 1 (a), the bottomsof iron wires are attracted to the South Pole of the magnet. In fig. 1 (b), the bottoms of iron wiresare attracted to the North Pole of the magnet. In fig. 1(c), the top of iron wires are attracted to theNorth Pole of the magnet and the bottom of iron wires are attracted to the South Pole of the mag-net.Airproof lid-. Glass- GlassNorth pole. Airproof lidcontainerFixing clip-IronFixing clip一 Tron-Iron wiresSouth pole -wires North polewiresSouth pole(ab)(C)Fig. 1. Schematics of the visualization reactors in the magnetic field.1.2 Reagent and apparatus中国煤化工HCFC-141b was bought from AlliedSignal Con.MYHCNMHG%.Doublydis-tilled water was used in this paper. Digital magnetic intensity meter was bought from Shanghai4th Ammeter factory, and its precision is +1 %. Magnetic material is ND-FE-B, and it was manu-No.4EFECTS OP MAGNETIC FIELDS ON HCFC-141b REFRIGERANT GAS HYDRATE FORMATION 409factured by Guangzhou Heshun Magnetic Material Factory. Glass container was made from GG17material by Guangzhou Qianhui Glass Instruments Co., Ltd. Electronical balance was purchasedfrom Beijing Satorius Instruments Co., Ltd., and its precision is 0.01 g.1.3 Experimental processAn important physical characteristic of hydrate, which differs from that of ice, is that it canbe formed above 0C, therefore, the air bath has been set at 1C. 12.5 g HCFC- 141b refrigerantand 10 g water were respectively fed into the reactor, and then the reactor was put into the air bath.A continual focus digital video camera ( Panasonic NV-DX 100EN) was used to observe theHCFC-141b hydrate formation process. Induction time is defined as a period time that systemgoes through from the equilibrium to appearance of the first visual hydrate, and it is a very ordi-nary method for studying hydrate formation via visualization in the static state experiments!9. Andhydrate is an ice-like solid, so it can be departed from liquid. According to the practical sense ofcool storage for air-conditioning, the mass of hydrate was weighed after it has been put into the airbath for 8 h.2 HCFC-141b hydrate formation photos2.1 Morphology of R141b' s formation process under no influence of magnetic fieldZhao et al.17.8] have studied the HCFC- 141b hydrate formation under no influence of themagnetic field at 0.5C. It is found that the hydrate growth orientation is that hydrate crystals takeprecedence to grow into the water, and there is almost no hydrate in refrigerant region.In this paper, hydrate formation in no magnetic field is observed when there are only fouriron wires and no magnets in the glass container. It is found that the characteristics of hydrateformation are similar to that presented by Zhao et al."), as shown in fig. 2. The density of HCFC-141b is larger than that of water, thus, water can float on the HCFC- 141b liquid, as shown in fig.2(a). A slim layer of hydrate is firstly formed on the surface of iron wires and interface between中国煤化工(a)0h(b)8 h47 minTYHCNMH GhFig. 2. Morphology of R141b s formation process under no influence of magnetic field.410SCIENCE IN CHINA (Series B)Vol. 46water and refrigerant after about 9 h, then refrigerant molecules diffuse into water to form newhydrate through the formed hydrate layer. Finally, hydrate only grows in water phase in about 16 h,as shown in fig. 2(b) - +d). .2.2 Morphology of R141b' s formation process under influence of magnetic fieldIt is observed through experiments that the hydrate can extend its growth region in the mag-netic field, and it can grow not only in water but also in refrigerant region, as shown in fig. 3(a). Aslim layer of hydrate is firstly formed on the surface of iron wires and interface between water andrefrigerant after an induction time, and the hydrate growth orientation is that it takes precedence togrow into water region. After 1 h 35 min, the formed hydrates have occupied the whole waterregion in the main. In the later processes, hydrate crystals then continually grow into the refriger-ant phase. Because water can diffuse into the refrigerant phase through the hydrate layer, whichilluminates that the growth region of hydrate will not be restricted in water phase, and hydratecrystals can also grow in refrigerant phase in the magnetic field.45 min1hs minIh35min8h55 min1h20minsha)(b)47 min2h5min 3 h55 minh1 h5 minIh 10 min1h 17 min(C(d)Fig. 3. Morphology of R141B s formation process under influence of magnetic field.It can be observed from fig. 3(b) that hydrate takes precedence to grow into the refrigerantregion after it is firstly formed on the surface of iron wires and interface between water and refrig-erant. Then refrigerant can diffuse into water to form new hydrate through hydrate layer after theformed hydrates have occupied the whole liquid HCFC-141b region in the main, but there is lttlehydrate in water phase in 8 h. Contrasted to the result中国煤化工periments in thispaper and experiment in literature!", it is found that hyYHCNMHGnbeateredun-der the influence of magnetic field.Crystal growth is a process of discontinuity and nonuniformity in space. Crystallization takesNo.4EFECTS OP MAGNETIC FIELDS ON HCFC-141b REFRIGERANT GAS HYDRATE FORMATION 411place only in interface between solid and liquid. When crystals grow from rich environmentalphase, solidification does not take place until the phase change heat is transported from the inter-face. While crystals grow from the thin environmental phase, the reaction does not proceed untilgrowth element is transported to the interface from other places of the growth system, which iscalled the transportation effect of crystal growth! 101 . There are such characteristics in the hydrateformation, namely crystal nucleuses can be induced only while external molecules dissolved inwater is saturated. Refrigerant molecules are not dissolved in water, so hydrate should be usuallyformed in the interface of water and refrigerant phase (rich environmental phase) where their mo-lecules can sufficiently contact. Clathrate crystallization can be formed while refrigerant molecu-les are enclosed by water molecules in weak van der Waals force, then refrigerant molecules candiffuse into water to form new hydrate through the lacuna of the formed hydrate.However, it is found through experiments that hydrates do not crystallize in the interface ofwater and refrigerant (rich environmental phase) at first, and they are firstly formed in the bottomof refrigerant phase in the magnetic field, as shown in fig. 3(c). W ater molecules can diffuse intothe bottom of the refrigerant in the magnetic field, and hydrates are formed firstly on the surfaceof the fixing clip and iron wires in the bottom of refrigerant, then hydrates grow along the ironwires. Hydrates are also formed in the interface of the water and refrigerant after about 4 h, thenthey grow in both water and refrigerant phase, and hydrates in the bottom of refrigerant do notgrow obviously at this time.Meanwhile, it is observed that there are the same regulations for the hydrate formation whenmagnets are distributed according to fig. 1(b). It can be observed from fig. 3(d) that hydrate crys-tals can be formed not only in the interface but also on surface of iron wires in water and refriger-ant phase, and crystal nucleuses manifolded in the latter processes, then these dispersive crystalnucleuses grow continuously on the primary foundation. Although there are many crystal nucle-uses, but hydrate formation mass is very lttle at last, and it can be seen in the hydrate formationcurve in the latter of this paper.3 Effects of magnetic intensity on induction time and hydration rateThe chemical reaction relation of water and HCFC- 141b refrigerant is thatR+17 H2O <→R: 17 H2O+NH (formation heat)Molecular weight of HCFC-141b is 116.95 and that of water is 18, and 12.5 g refrigerant and10 g water are put into the reactor in each experiment, therefore, the theoretical formation mass ofthe hydrate is 13.82 g according to the above-mentioned chemical reaction relation. The ratio ofpractical hydrate formation mass weighed in the experiment and theoretical hydrate formationmass calculated by the above-mentioned chemical中国煤化工hydration rate.Whereas, the practical hydrate formation mass is a lit[:YHC N M H Gydrate formationmass in very few instances, but the error between them is very small, and the hydration rate isconsidered as 100% in these instances.412SCIENCE IN CHINA (Series B)Vol. 46If there are no magnets and only four iron wires in the reactor, it can be measured that the in-duction time is 8 h 47 min, and the hydration rate is 25.11% in 8 h. Meanwhile, the induction timemeasured by Zhao et al.!7] is 8 h 45 min. Whereas, under the influence of magnetic field, it isfound that the induction time can be shortened to about 40 min and the hydration rate can arrive at100% in many instances.The relations that induction time and hydration rate changed with magnetic intensity areshown in fig. 4. And No. 1, No. 2 and No. 3 curves represent that magnets are put onto the reactorrespectively according to fig. 1(a), (b) and (c). Magnetic field intensities on the bottom of the re-actor all change from 32.0x10-2 to 55.0x10-2, and that on the top of reactor in fig. 1(c) is always31.2x10-2. And there are four iron wires fixuped by fixing clip in the reactor.100 r400 t90350主80|0300上60身250-吕.20040-15030100 t0t(b).35 4015~5055 60 X10~2 304550SS60 X10°2Magnetic intensityTFig. 4. Effects of magnetic intensity on the induction time and hydration rate. (a) Effects on induction time; (b) effects onhydration rate. No.1, No.2 and No.3 curves: Magnets are distributed according to fig. 1(a), (b) and (C) respectively.Relations that induction time changes with the magnetic field intensity are shown in fig. 4(a).It can be found from No.1 and No.2 curves in fig. 4(a) that regulations that induction time changeswith the magnetic field intensity are the same although different magnetic poles are put on thebottom of the reactor. The induction time decreases with the increasing of magnetic intensitywhile magnetic intensity is less than 36.5x10-2 T, then it changes evenly when the magnetic inten-sity is between 36.5x10 2 and 52.0x10-2 T, and it will increase when the magnetic field intensity islarger than 52.0x10-2 T. The less induction time should be better for the hydrate formation. There-fore, magnetic intensity should not be larger or smaller.Meanwhile, it can be observed from the curves in fig. 4(a) that the induction time of eachpoint in No.1 curve is much smaller than that in No.2 curve, which shows that the South Pole ofmagnet has better effects on the hydrate formation than the North Pole. And it can be seen fromNo.3 curve that the induction time on the first point of the curve is relatively large, but that onother points of curve always fluctuate between 40 min中国煤化工that it will havebetter effects on the hydrate formation while magnets ard:MYHCNMHGreactor.The relations that hydration rate changes with the magnetic field intensity are shown in fig.4(b). No.2 curve shows that the hydration rate decreases with increasing of the magnetic intensityNo.4EFECTS OP MAGNETIC FIELDS ON HCFC-141b REFRIGERANT GAS HYDRATE FORMATION 413when the North Pole of magnet is put on the bottom of the reactor. By contrasting No. 1 curve toNo.2 curve, it can be seen that the hydration rate of each point in No.1 curve is much larger thanthat in No.2 curve, which shows that the South Pole of magnet has better effects on the hydrateformation than the North Pole. Meanwhile, it can be seen that the hydration rate of each point inNo.3 curve is much larger than that in No. 1 and No. 2 curves, which ilustrates that magnets puton both ends of the reactor have better effects on the formation process of hydrate than that put onthe bottom of the reactor. According to the practical sense of cool storage for air conditioning,induction time should be shorter and the hydration rate should be much more. It can be observedfrom fig. 4 (a) and (b) that if magnets are put on both ends of the reactor, they have better effects .on both induction time and hydration rate; and the South Pole of magnet has better effects on thehydrate formation than the North Pole.Relations that the induction time and hydration rate change with the number of iron wires areshown in fig. 5. No. 1 and No.2 curves represent that magnets are put onto the reactor accordingto fig. l(a). And No.3 curve represents that magnets are put onto the reactor according to fig.1(c).Magnetic intensity is 41.0x102T in No.1 curve, that is 52.0x10-2 T in No. 2 curve, and that onthe top and bottom of the reactor is respectively 31.2x10-2 and 52.0x10-2 T in No. 3 curve. In the :above conditions, we can find the relations that induction time and hydration rate change with thenumber of iron wires while the magnetic intensity is invariable.39F ()00[ (b)200主N83345678910112Number of iron wiresFig. 5. Effects of the number of iron wires on the induction time and hydrate formation mass. (a) Effects on induction time,(b) efcts on hydrate formation mass. No.1: Magnets are distributed according to fig. 1(a), and its intensity is41.0x10-2 T; No.2: magnets are distibuted according to fig. l(a), and its intensity is 52.0x10-2 T; No.3: magnets are distributed according to fig.1(c), and its intensity on the top and bottom of the reactor is respectively 31.2x10-2 and 52.0x10-TIt can be seen from No.1 and No.2 curves in fig. 5(a) that the induction time fluctuates withthe increasing of the number of iron wires according to the same regulation although the magneticintensity on the bottom of reactor is different. And the induction time at each point of No. 3 curveis obviously shorter and fluctuates evenly with the increasing of number of the iron wires con-trasting to that of No.1 and No.2 curves.中国煤化工。It can be observed from three curves in fig. 5(b) thases with the in-creasing of the number of the iron wires. And hydratior:MYHcNMHG3curveismuchmore than that of No.1 and No.2 curves. It can be seen from the contrast of three curves in fig. 5that there are more effects on both the induction time and hydration rate when magnets are dis-414SCIENCE IN CHINA (Series B)Vol. 46tributed according to fig. 1(c), and the induction time will be shorter and hydration rate will bemuch more.4 Effects of other magnetic fields on the hydrate formationOther different magnetic fields are imposed to the reactor and their schematics are respec-tively shown in fig. 6. There are only magnets on both ends of the reactor, as shown in fig. 6(a).Crushed magnets are put into the reactor, as shown in fig. 6(b). Magnets are put vertically onto theinterface of water and rfrigerant, as shown in fig. 6(c). Schematic of annular magnet is shown infig. 6(d), and the reactor is put into the inner of a set of annular magnets. There are only fourpieces of iron wires in the reactor, as shown in fig. 6(e). There are four pieces of brass wires in thereactor and magnets are imposed to both ends of the reactor, as shown in fig. 6(f). It is foundthrough a set of experiments that the hydrate cannot be formed in 8 h under the effects of thesemagnetic fields. Magnetic fields in fig. 6(a) - (d) illustrate that magnetic fields cannot affect theformation process in all conditions. Magnetic fields in fig. 6(e) - +f) show that there is lttle effecton the hydrate formation although iron wires and brass wires can be served as the inducing nucleation factor. Meanwhile, magnets are put onto the reactor according to fig. 1, but when there isonly a kind of magnet whose diameter is 45 mm on the bottom of the reactor, and iron wires areput into the reactor, it is found that the hydrate can be formed in few instances in 8 h and experi-ments cannot be repeated basically. At present, it has been tested that only when magnets on thebottom of the reactor are combination of different diameter magnets and then iron wires are mag-netized by them in the reactor, the hydrate formation can be affected intensively. Maybe HCFC-141b molecules and water molecules are both pole molecules, and the specific magnetic field canbe formed when iron wires are magnetized by magnets, which can affect the formation process ofHCFC-141b gas hydrate intensively.Magnets, GlasscontainerGlassCrushedmagnets(a)(C, AnnularIron. Brasswires一Glass-Glass中国煤化工rainer(d)e)MYHCNMHGFig. 6. Schematics of other magnetic fields on the reactor .No.4EFFECTS OF MAGNETIC FIELDS ON HCFC- 141b REFRIGERANT GAS HYDRATE FORMATION 4155 Conclusions(i) In the specific magnetic field, hydrate growth orientation can be altered; growth regioncan be extended into both water and refrigerant phases; hydrate can be formed firstly in the bot-tom of refrigerant phase.(i) The induction time of hydrate crystallization can be shortened extremely to 40 min from9 h in the magnetic field. Meanwhile, magnetic pole, magnetic intensity and the number of ironwires all can affect the hydrate formation.(ili) The hydrate formation mass can be enhanced considerably in the magnetic field, and thehydration rate can arrive at 100% in some instances. Meanwhile, magnetic pole, magnetic inten-sity and the number of iron wires all can affect the hydration rate.(iv) Magnetic fields cannot affect the formation process of gas hydrate in all conditions.Specific magnetic field, which is formed when iron wires are magnetized by magnets, can affectthe hydrate formation.Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nc59836230), the Major State Basic Research Program (Grant No. G2000026306) and Superintendent Fund of Guangzhou Instituteof Energy Conversion, the Chinese Academy of Sciences (Grant No.07 20406).References1. Guo, K. H, Gas hydrate and advanced uses in cool storage systems, Refrigeration (in Chinese), 1994. 47(2): 22- -28.2. Guo, K. H, Shu, B. F., Yang, W. J.. Advanced and applications of gas hydrate in thermal energy storage technology, inProc. 1st Trabzon Int. Energy and Environment Symp, Vol.1 (ed. Ayhan, T), Trabzon: Karadeniz Technical UniversityPress, 1996, 381- -386.3. 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S. et al, Formation process and fractal growth model of HCFC- 141b refrigerant gas hy-drate, Science in China, Series B, 2002, 45(2): 216 -224.9. Skovborg, O., Ng, H. J.. Rasmussen, P. et al, Measurement of induction times for the formation of methane and ethanegas hydrates, Chemical Engineering Science, 1993, 48(3): 445 -453.10. Zhang, K. C., Zhang, L. H., Crystal Growth, Beijing: Science Press, 1982.中国煤化工MHCNMHG