Vol. 46 No. 4
SCIENCE IN CHINA (Series B)
August 2003
Effects of magnetic fields on HCFC-141b refrigerant gas
hydrate formation
LIU Yong ( ), GUO Kaihua (), LIANG Deqing ()
& FAN Shuanshi (
)
Guangzhou Institute of Energy Conversion, Guangzhou 510070, China
Correspondence should be addressed to Liang Deqing (email: liangdq@ms.giec.ac.cn)
Received December 20, 2002
Abstract
Low-pressure refrigerant gas hydrates have brilliant prospects as a cool storage medium for air-conditioning systems. Intensive effects of some specific magnetic fields on the formation process of HCFC-141b refrigerant gas hydrate are depicted experimentally. Under influence of
these specific magnetic fields, the orientation and growth region of gas hydrate are altered; induction time of hydrate crystallization can be shortened extremely, and it can be shortened to 40 min
from 9 h; hydrate formation mass can be enhanced considerably, and hydration rate can arrive at
100% in some instances. Meanwhile, the relations of induction time and hydration rate changed
with magnetic field intensity are depicted, and some elementary regulations are found.
Keywords: magnetic fields, gas hydrate, induction time, hydration rate, cool storage.
DOI: 10.1360/02yb0178
Gas hydrates are crystalline compounds formed (usually above 0) by water reacting with
some gases or volatile liquids (hydrate former). Guest molecules, such as gas or volatile liquid
molecules, are enclosed firmly inside the host cavities and act with water molecules in weak van
der Waals force. Gas hydrate usually includes natural gas hydrate, refrigerant gas hydrate and CO2
gas hydrate. Refrigerant hydrates can be formed above 0, and their crystallization is similar to
the ordinary ice, so it is also called “warm ice”. Because the phase change temperature of the refrigerant gas hydrate is between 5 and 12 and with a formation heat which is equal to that of
ice, it can be a substitute of ice as cool storage medium for air-conditioning systems. The performance and applicability of the gas hydrate cool storage system can have a great superiority to
that of the ice[1,2]. To develop a cost-effective cool storage system for air-conditioning, a lowpressure gas hydrate cool storage medium should be employed. Unfortunately, those low-pressure
refrigerants, 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.
Consequently, one has a very long induction time of reaction and a quite slow growth speed, even
with surfactants and nucleate seeds added[3
ü5]
. Therefore, if refrigerant hydrates are employed as a
medium of thermal energy storage in engineering application, a rapid and uniform formation of
gas hydrate will be the key for technical success.
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Electrical and magnetic stationary fields affect significantly the equilibrium formation conditions and growth kinetics of ice from water. Currently, there are no reliable experimental and
theoretical data about the effect of these fields on a hydrate formation. Makogon[6] indicated the
effect of a stationary magnetic field on density and structure of hydrates. Denser hydrates with a
more regular structure formed under the influence of magnetic field. At present, there are no literatures about 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 can
considerably affect the formation of HCFC-141b refrigerant gas hydrate.
1
Experimental
1.1
Experimental apparatus
This experiment was carried out on the experimental system of gas hydrates at a low temperature; and the experimental apparatus is vided in the literature[7,8].
The visualization hydrate reactor in the magnetic field is composed of transparent glass container, airproof lid, magnets, iron wires and stainless steel fixing clip. The schematics of the reactors are shown in fig. 1. Inner diameter of the glass container is 22 mm, and the volume of that is
3.8×10−5 m3. Airproof lid is used to prevent refrigerant from volatilizing and outside impurity
dropping into. Iron wires, 2 mm in diameter and 100 mm in length, which are fixed by stainless
steel fixing clip, are distributed along the circle whose radius is 7 mm in the glass container. There
are two kinds of magnets: one is 20 mm in diameter and 3 mm in thickness; the other is 45 mm in
diameter and the thickness of it changes from 5 to 25 mm. The magnet whose diameter is 45 mm
was put on the top of the reactor. Magnet, diameter in 20 mm, is firstly put on the bottom of the
reactor, and then magnet whose diameter is 45 mm is put below it, and the magnetic intensity is
adjusted by changing the thickness of magnet whose diameter is 45 mm. In fig. 1 (a), the bottoms
of iron wires are attracted to the South Pole of the magnet. In fig. 1 (b), the bottoms of iron wires
are attracted to the North Pole of the magnet. In fig. 1(c), the top of iron wires are attracted to the
North Pole of the magnet and the bottom of iron wires are attracted to the South Pole of the magnet.
Fig. 1. Schematics of the visualization reactors in the magnetic field.
1.2 Reagent and apparatus
HCFC-141b was bought from AlliedSignal Company, and its purity is 99.5%. Doubly distilled water was used in this paper. Digital magnetic intensity meter was bought from Shanghai 4th
Ammeter factory, and its precision is ±1%. Magnetic material is ND-FE-B, and it was manufac-
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EFFECTS OF MAGNETIC FIELDS ON HCFC-141b REFRIGERANT GAS HYDRATE FORMATION
409
tured by Guangzhou Heshun Magnetic Material Factory. Glass container was made from GG17
material by Guangzhou Qianhui Glass Instruments Co., Ltd. Electronical balance was purchased
from Beijing Satorius Instruments Co., Ltd., and its precision is 0.01 g.
1.3
Experimental process
An important physical characteristic of hydrate, which differs from that of ice, is that it can
be formed above 0, therefore, the air bath has been set at 1. 12.5 g HCFC-141b refrigerant
and 10 g water were respectively fed into the reactor, and then the reactor was put into the air bath.
A continual focus digital video cameraPanasonic NV-DX100ENwas used to observe the
HCFC-141b hydrate formation process. Induction time is defined as a period time that system
goes through from the equilibrium to appearance of the first visual hydrate, and it is a very ordinary method for studying hydrate formation via visualization in the static state experiments[9]. And
hydrate is an ice-like solid, so it can be departed from liquid. According to the practical sense of
cool storage for air-conditioning, the mass of hydrate was weighed after it has been put into the air
bath for 8 h.
2 HCFC-141b hydrate formation photos
2.1 Morphology of R141b’s formation process under no influence of magnetic field
Zhao et al.[7,8] have studied the HCFC-141b hydrate formation under no influence of the
magnetic field at 0.5. It is found that the hydrate growth orientation is that hydrate crystals take
precedence 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 four
iron wires and no magnets in the glass container. It is found that the characteristics of hydrate
formation are similar to that presented by Zhao et al.[7], as shown in fig. 2. The density of HCFC141b 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
Fig. 2. Morphology of R141b’s formation process under no influence of magnetic field.
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water and refrigerant after about 9 h, then refrigerant molecules diffuse into water to form new
hydrate 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 field
It is observed through experiments that the hydrate can extend its growth region in the magnetic field, and it can grow not only in water but also in refrigerant region, as shown in fig. 3(a). A
slim layer of hydrate is firstly formed on the surface of iron wires and interface between water and
refrigerant after an induction time, and the hydrate growth orientation is that it takes precedence to
grow into water region. After 1 h 35 min, the formed hydrates have occupied the whole water region in the main. In the later processes, hydrate crystals then continually grow into the refrigerant
phase. Because water can diffuse into the refrigerant phase through the hydrate layer, which illuminates that the growth region of hydrate will not be restricted in water phase, and hydrate crystals can also grow in refrigerant phase in the magnetic field.
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 refrigerant
region after it is firstly formed on the surface of iron wires and interface between water and refrigerant. Then refrigerant can diffuse into water to form new hydrate through hydrate layer after
the formed hydrates have occupied the whole liquid HCFC-141b region in the main, but there is
little hydrate in water phase in 8 h. Contrasted to the result of the above-mentioned experiments in
this paper and experiment in literature[7], it is found that hydrate growth orientation can be altered
under the influence of magnetic field.
Crystal growth is a process of discontinuity and nonuniformity in space. Crystallization takes
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EFFECTS OF MAGNETIC FIELDS ON HCFC-141b REFRIGERANT GAS HYDRATE FORMATION
411
place only in interface between solid and liquid. When crystals grow from rich environmental
phase, solidification does not take place until the phase change heat is transported from the interface. While crystals grow from the thin environmental phase, the reaction does not proceed until
growth element is transported to the interface from other places of the growth system, which is
called the transportation effect of crystal growth[10]. There are such characteristics in the hydrate
formation, namely crystal nucleuses can be induced only while external molecules dissolved in
water is saturated. Refrigerant molecules are not dissolved in water, so hydrate should be usually
formed in the interface of water and refrigerant phase (rich environmental phase) where their
molecules can sufficiently contact. Clathrate crystallization can be formed while refrigerant
molecules are enclosed by water molecules in weak van der Waals force, then refrigerant molecules can diffuse 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 of
water and refrigerant (rich environmental phase) at first, and they are firstly formed in the bottom
of refrigerant phase in the magnetic field, as shown in fig. 3(c). Water molecules can diffuse into
the bottom of the refrigerant in the magnetic field, and hydrates are formed firstly on the surface
of the fixing clip and iron wires in the bottom of refrigerant, then hydrates grow along the iron
wires. Hydrates are also formed in the interface of the water and refrigerant after about 4 h, then
they grow in both water and refrigerant phase, and hydrates in the bottom of refrigerant do not
grow obviously at this time.
Meanwhile, it is observed that there are the same regulations for the hydrate formation when
magnets are distributed according to fig. 1(b). It can be observed from fig. 3(d) that hydrate crystals can be formed not only in the interface but also on surface of iron wires in water and refrigerant phase, and crystal nucleuses manifolded in the latter processes, then these dispersive crystal
nucleuses grow continuously on the primary foundation. Although there are many crystal nucleuses, but hydrate formation mass is very little at last, and it can be seen in the hydrate formation
curve in the latter of this paper.
3
Effects of magnetic intensity on induction time and hydration rate
The chemical reaction relation of water and HCFC-141b refrigerant is that
R+17 H2O ↔R17 H2O+∆H (formation heat)
Molecular weight of HCFC-141b is 116.95 and that of water is 18, and 12.5 g refrigerant and
10 g water are put into the reactor in each experiment, therefore, the theoretical formation mass of
the hydrate is 13.82 g according to the above-mentioned chemical reaction relation. The ratio of
practical hydrate formation mass weighed in the experiment and theoretical hydrate formation
mass calculated by the above-mentioned chemical relation is defined as the hydration rate.
Whereas, the practical hydrate formation mass is a little larger than theoretical hydrate formation
mass in very few instances, but the error between them is very small, and the hydration rate is
considered as 100% in these instances.
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If there are no magnets and only four iron wires in the reactor, it can be measured that the
induction time is 8 h 47 min, and the hydration rate is 25.11% in 8 h. Meanwhile, the induction
time measured by Zhao et al.[7] is 8 h 45 min.Whereas, under the influence of magnetic field, it is
found that the induction time can be shortened to about 40 min and the hydration rate can arrive at
100% in many instances.
The relations that induction time and hydration rate changed with magnetic intensity are
shown in fig. 4. And No. 1, No. 2 and No. 3 curves represent that magnets are put onto the reactor
respectively according to fig. 1(a), (b) and (c). Magnetic field intensities on the bottom of the reactor all change from 32.0×10−2 to 55.0×10−2, and that on the top of reactor in fig. 1(c) is always
31.2×10−2. And there are four iron wires fixuped by fixing clip in the reactor.
Fig. 4. Effects of magnetic intensity on the induction time and hydration rate. (a) Effects on induction time; (b) effects on hydration 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 changes
with the magnetic field intensity are the same although different magnetic poles are put on the
bottom of the reactor. The induction time decreases with the increasing of magnetic intensity
while magnetic intensity is less than 36.5×10−2 T, then it changes evenly when the magnetic intensity is between 36.5×10−2 and 52.0×10−2 T, and it will increase when the magnetic field intensity is
larger than 52.0×10−2 T. The less induction time should be better for the hydrate formation. Therefore, 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 each
point in No. 1 curve is much smaller than that in No.2 curve, which shows that the South Pole of
magnet has better effects on the hydrate formation than the North Pole. And it can be seen from
No. 3 curve that the induction time on the first point of the curve is relatively large, but that on
other points of curve always fluctuate between 40 min and 70 min, which shows that it will have
better effects on the hydrate formation while magnets are put onto both ends of the reactor.
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 intensity
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EFFECTS OF MAGNETIC FIELDS ON HCFC-141b REFRIGERANT GAS HYDRATE FORMATION
413
when the North Pole of magnet is put on the bottom of the reactor. By contrasting No. 1 curve to
No. 2 curve, it can be seen that the hydration rate of each point in No. 1 curve is much larger than
that in No. 2 curve, which shows that the South Pole of magnet has better effects on the hydrate
formation than the North Pole. Meanwhile, it can be seen that the hydration rate of each point in
No. 3 curve is much larger than that in No. 1 and No. 2 curves, which illustrates that magnets put
on both ends of the reactor have better effects on the formation process of hydrate than that put on
the 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 observed
from 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 the
hydrate formation than the North Pole.
Relations that the induction time and hydration rate change with the number of iron wires are
shown in fig. 5. No. 1 and No. 2 curves represent that magnets are put onto the reactor according
to fig. 1(a). And No. 3 curve represents that magnets are put onto the reactor according to fig.1(c).
Magnetic intensity is 41.0×10−2 T in No. 1 curve, that is 52.0×10−2 T in No. 2 curve, and that on
the top and bottom of the reactor is respectively 31.2×10−2 and 52.0×10−2 T in No. 3 curve. In the
above conditions, we can find the relations that induction time and hydration rate change with the
number of iron wires while the magnetic intensity is invariable.
Fig. 5. Effects of the number of iron wires on the induction time and hydrate formation mass. (a) Effects on induction time,
(b) effects on hydrate formation mass. No. 1: Magnets are distributed according to fig. 1(a), and its intensity is 41.0×10−2 T; No.
2: magnets are distributed according to fig. 1(a), and its intensity is 52.0×10−2 T; No. 3: magnets are distributed according to fig.
1(c), and its intensity on the top and bottom of the reactor is respectively 31.2×10−2 and 52.0×10−2 T.
It can be seen from No. 1 and No. 2 curves in fig. 5(a) that the induction time fluctuates with
the increasing of the number of iron wires according to the same regulation although the magnetic
intensity on the bottom of reactor is different. And the induction time at each point of No. 3 curve
is obviously shorter and fluctuates evenly with the increasing of number of the iron wires contrasting to that of No. 1 and No. 2 curves.
It can be observed from three curves in fig. 5(b) that the hydration rate increases with the increasing of the number of the iron wires. And hydration rate at each point of No. 3 curve is much
more than that of No. 1 and No. 2 curves. It can be seen from the contrast of three curves in fig. 5
that there are more effects on both the induction time and hydration rate when magnets are dis-
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Vol. 46
tributed according to fig. 1(c), and the induction time will be shorter and hydration rate will be
much more.
4
Effects of other magnetic fields on the hydrate formation
Other different magnetic fields are imposed to the reactor and their schematics are respectively 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 the
interface of water and refrigerant, as shown in fig. 6(c). Schematic of annular magnet is shown in
fig. 6(d), and the reactor is put into the inner of a set of annular magnets. There are only four
pieces of iron wires in the reactor, as shown in fig. 6(e). There are four pieces of brass wires in the
reactor and magnets are imposed to both ends of the reactor, as shown in fig. 6(f). It is found
through a set of experiments that the hydrate cannot be formed in 8 h under the effects of these
magnetic fields. Magnetic fields in fig. 6(a)—(d) illustrate that magnetic fields cannot affect the
formation process in all conditions. Magnetic fields in fig. 6(e)—(f) show that there is little effect
on 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 is
only a kind of magnet whose diameter is 45 mm on the bottom of the reactor, and iron wires are
put into the reactor, it is found that the hydrate can be formed in few instances in 8 h and experiments cannot be repeated basically. At present, it has been tested that only when magnets on the
bottom of the reactor are combination of different diameter magnets and then iron wires are magnetized 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 can be formed when iron wires are magnetized by magnets, which can affect the formation
process of HCFC-141b gas hydrate intensively.
Fig. 6. Schematics of other magnetic fields on the reactor.
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5 Conclusions
(i) In the specific magnetic field, hydrate growth orientation can be altered; growth region
can be extended into both water and refrigerant phases; hydrate can be formed firstly in the bottom of refrigerant phase.
(ii) The induction time of hydrate crystallization can be shortened extremely to 40 min from
9 h in the magnetic field. Meanwhile, magnetic pole, magnetic intensity and the number of iron
wires all can affect the hydrate formation.
(iii) The hydrate formation mass can be enhanced considerably in the magnetic field, and the
hydration rate can arrive at 100% in some instances. Meanwhile, magnetic pole, magnetic intensity 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 affect the
hydrate formation.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.
59836230), the Major State Basic Research Program (Grant No. G2000026306) and Superintendent Fund of Guangzhou Institute
of Energy Conversion, the Chinese Academy of Sciences (Grant No. 07-20406).
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