Note: Descriptions are shown in the official language in which they were submitted.
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Glycol-free Heat Transfer Fluid
FIELD OF THE INVENTION
[0ow] The present invention relates to a heat transfer fluid, and more
specifically, the present
invention relates to a glycol-free heat transfer fluid including corrosion
inhibitors
BACKGROUND OF THE INVENTION
[0002] Explosion-proof electric heater is a closed loop heat exchanger (HE)
partially filled with
a heat transfer fluid in which an electrical element is immersed. The
explosion-proof
heaters are designed to be used in hazardous environments, where an explosive
mixture of petroleum gases or vapors, or flammable dust (for example but not
limited to:
aluminum/magnesium dust, carbon black, coal, coke, flour, starch, grain dusts)
may
exist under normal operating conditions.
[0003] Figure 1 illustrates an exploded view of an exemplary explosion-proof
heater 100. The
explosion-proof heater 100 includes a closed loop heat exchanger 102 which is
initially
kept under vacuum. The electrical element heats up a glycol-based heat
transfer fluid
and produces steam, which travels upwards through a number of heat exchange
columns 104. Collectively, the exchange columns 104 may be considered as the
tube
portion of the closed loop heat exchanger 102. The heat exchange columns 104
generally have a carbon steel core and aluminum fins. The closed loop heat
exchanger
102 also have a can portion 110 which contains a liquid phase of the heat
transfer fluid.
The can portion 110 is made of carbon steel. An external fan 106, installed on
an
explosion-proof motor 108, blows ambient air onto the heat-exchange columns
104
causing steam to condense. The generated heat is forcibly convected into the
environment.
[0004] Referring to Figure 2, the closed loop heat exchanger 102 contains a
pressure relieve
valve (PRV), preferably at the top of the a closed loop heat exchanger 102.
The
pressure relieve valve comprises a PRV body 202, a plug 204, a gasket 206, a
gasket
holder 208, a spring 210, a locking nut 212 and an adjustment knob 214. The
pressure
relieve valve releases the heat transfer fluid when the closed loop heat
exchanger
malfunctions. The malfunction of the heat exchanger may be caused by
inadequate
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cooling. Generation of steam, constant heating and lack of cooling will
dramatically
increase pressure inside the heat exchanger 102.
[0005] Commercially available heat transfer fluids generally contain ethylene
or propylene
glycol solutions in a concentration of about 10% or more, and a corrosion
inhibitor, for
example, di-potassium phosphate (K2HPO4) in a concentration of about 2% or
more. In
general, ethylene glycol is toxic and not environment friendly. Propylene
glycol is less
toxic compared to ethylene glycol, it nevertheless pollutes the environment.
[0006] During the long term operation of the heat exchanger 102, a deposit
inside the pressure
relieve valve was observed. The deposit mainly contained K2HPO4 upon analysis.
In
certain occasions, the deposit would totally obstruct the pressure relieve
valve. A totally
plugged PRV and malfunctioning unit, as described above, would lead to a
rupture of
the heat exchanger or the entire explosion-proof electric heater. As the
heaters are
located in hazardous locations, any of the above events are highly
undesirable.
[0007] When lower K2HP0.4 concentration was used, it may not provide adequate
corrosion
protection without glycol. However, the presence of glycol, e.g. propylene
glycol in
exchange heater does not add any benefit to exchange heater start-up or its
performance during regular operation. The propylene glycol is a flammable
fluid. The
10% propylene glycol in water may not pose any fire hazard, however, in case
of
leakage in the heat exchanger 102, the heat exchanger 102 may lose steam and
may
cause concentration of propylene glycol in the heat exchanger 102 to increase,
which
may cause a potential fire hazard.
[mos] The presence of heater elements may also decompose glycol into organic
acids or
formaldehyde, resulting in excessive generation of gas which in turn may cause
a high
pressure that will engage the pressure relieve valve.
[0009] In addition, the K2HPO4 based inhibitor dissolves only in liquid-phase
and does not
protect the vapor phase of the heat exchanger.
[0010] US patent application 2012/0061611 describes a heat transfer fluid
comprising water,
glycerol from 30% to 60% (w/w), and a surfactant. The surfactant is added to
reduce the
viscosity of the fluid caused by the high concentration of glycol. The
surfactant has
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further characteristics for corrosion inhibition, a high thermal capacity and
helps to
protect the fluid from degradation at high temperatures.
[0m] US patent application 2006/0163528 describes an aqueous antifreeze
composition
comprising 10 to 50% by weight of one or more dicarboxylic acids, thus
providing
protection against corrosion.
[0012] Therefore, there is a need of a glycol-free heat exchange fluid. There
is further a need
to a glycol-free heat exchange fluid whereby only a small amount of corrosion
inhibitor
is able to protect the internals of the heat exchanger. There is further a
need to a glycol-
free heat exchange fluid as it protects both the liquid phase and the vapor
phase of the
heat exchanger (heater) .
SUMMARY OF THE INVENTION
[0013] The glycol-free heat transfer fluid according to the present invention
provides effective
corrosion protection, using only a small amount (1% (w/w) or less) of
corrosion inhibitors
to provide proper corrosion protection. The glycol-free heat transfer fluid
according to
the present invention is able to protect the internals of the heat exchanger
that is in
contact with liquid (bottom part) and steam/vapor (upper part), thus providing
a two-
phase protection. In addition, the absence of glycols makes the heat transfer
fluid more
stable. The electric heater starts faster in the absence of glycol. The
absence of glycol
also eliminates any glycol decomposition products such as organic acids,
formaldehyde
or formation of gases.
[mug In accordance with one aspect of the present invention there is provided
a glycol-free
heat transfer fluid comprising: sebacic acid; benzotriazole; morpholine, and
at least one
of sodium nitrite and sodium molybdate dihydrate; wherein a sum of
concentrations of
sodium molybdate dihydrate, sodium nitrite, sebacic acid, benzotriazole and
morpholine
is equal to or less than 1% (w/w).
[0015] In accordance with another aspect of the present invention there is
provided a method
of producing a glycol-free heat transfer fluid comprising: providing an
aqueous solution;
and adding sebacic acid, benzotriazole and morpholine, at least one of sodium
nitrite
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and sodium molybdate dehydrate so that a sum of concentrations thereof is
equal to or
less than 1% (w/w). The concentration of morpholine is preferably up to 0.812%
(w/w).
[cxyle] In accordance with another aspect of the present invention there is
provided a use of
the glycol-free heat transfer fluid.
[0017] Preferably, the sum of concentrations of sodium molybdate dihydrate,
sodium nitrite,
sebacic acid, benzotriazole and morpholine is less than 0.65% (w/w).
[0018] Preferably, the concentration of sodium nitrite is up to 0.134 % (w/w).
[0019] Preferably, the concentration of sodium molybdate dihydrate is up to
0.134 % (w/w).
[0020] Preferably, the concentration of sebacic acid is up to 0.028% (w/w).
[0021] Preferably, the concentration of benzotriazole is up to 0.028% (w/w).
[0022] Preferably, the pH of the glycol-free heat transfer fluid is between
9.0 ¨ 10Ø
[0023] This summary of the invention does not necessarily describe all
features of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] These and other features of the invention will become more apparent
from the following
description in which reference is made to the appended drawings wherein:
Figure 1 is an exploded view of an exemplary explosion-proof heater where the
glycol-
free heat transfer fluid of the present invention may be used; and
Figure 2 indicates the location of a pressure relieve valve on the closed loop
heat
exchanger with an exploded view of the pressure relieve valve.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] Aqueous glycol-free heat transfer fluids provide efficient and cost
effective heat transfer.
The water-based heat transfer fluids are generally stable and nontoxic during
operation
of the heat exchanger. However, the aqueous heat transfer fluids come in
contact with
different parts of the heat exchanger, and may cause corrosion. Therefore, a
corrosion
inhibitor or a composition of corrosion inhibitors is needed. Advantageously,
the
corrosion inhibitor or the composition of corrosion inhibitors comprises only
a small
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weight percentage of the glycol-free heat transfer fluids so that the deposit
after a long
operation can be minimized.
[0026] The aqueous heat transfer fluids of the present invention may include
sodium
molybdate. Sodium molybdate (Na2Mo04) is a member of molybdate. Molybdate is a
compound containing an oxoanion with molybdenum in its highest oxidation state
of 6.
Molybdenum can form a very large range of oxoanions. Molybdate is thought to
create a
protective monomolecular film over internal surfaces of closed circulation in
the heat
exchanger as the aqueous glycol-free heat transfer fluid circulates. The film
is an anodic
coating which inhibits corrosive attack on the metal parts.
[0027] The aqueous heat transfer fluids of the present invention may further
include sebacic
acid. Sebacic acid (HOOC)(CH2)8(COOH) is a naturally occurring member of
dicarboxylic acid. Organic acids, including mono- or dicarboxylic acids, have
also been
used as corrosion inhibitors, for example in automobile antifreeze/coolant
formulations.
The mono- or dicarboxylic acids are generally used in high concentrations, for
example,
US Patent 4,946,616 describes a coolant composition including 2-5.5% (w/w) of
at least
two C7-14 dicarboxylic acids.
[0028] The aqueous heat transfer fluids of the present invention may further
include
benzotriazole (C6H8N3). Benzotriazole is mainly used as a corrosion inhibitor
for copper
and its alloys by preventing undesirable surface reactions. A passive layer
with a
complex between copper and benzotriazole is formed when copper is immersed in
a
solution containing benzotriazole. The passive layer is insoluble in aqueous
solutions.
[0029] The aqueous heat transfer fluids of the present invention may further
include
morpholine. Morpholine is an organic chemical compound having the chemical
formula
0(CH2CH2)2NH. Morpholine may be used for pH adjustment and corrosion
protection.
Morpholine decomposes reasonably slowly in the absence of oxygen at high
temperatures and pressures.
[0030] The aqueous heat transfer fluids of the present invention may further
include sodium
nitrite. Sodium nitrite is an effective corrosion inhibitor and is used as an
additive in the
closed loop cooling systems. Alternatively, the heat transfer fluids of the
present
invention may contain sodium nitrite instead of sodium molybdate.
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[0031] Examples
[0032] Numerous experiments were performed as the effectiveness of a corrosion
inhibitor or a
composition of corrosion inhibitors depends on fluid composition, quantity of
water, and
flow regime. In the following, some embodiments are described.
[0033] The experimental setup includes an 800 ml glass beaker filled with 600
ml solution of
heat transfer fluid containing corrosion inhibitors. The balance fluid in
solution was
deionized water. The formulations (CCI-0, CCI-1, CCI-2, CCI-3, CCI-3-2, CCI-3-
3, CCI-
3-4 and CCI-4-2) added in the test were 2.4 ml, the dilution factor is
therefore 1:250.
Coupons were taken from can and tube side of the heater.
[0034] The can side of the heat exchanger 110, as shown in Figure 1, is
exposed to liquid
phase of the heat transfer fluid, while the tube side 104 of heat exchanger is
exposed to
vapour phase of the heat transfer fluid.
[0035] The can and tube coupons represent the exposure of heater metal to
liquid and vapour
phase, respectively. A flame arrestor was also placed in a solution to observe
the
effects on copper. The can side coupon was placed in liquid and the tube side
coupon
was held just above the liquid level to represent the vapour phase of the
heater. The
beaker was placed on a hot plate and a temperature around 80-90 C was
maintained to
avoid any boiling. The top of the beaker was covered with a plastic wrap to
minimize the
loss of fluid due to evaporation. Unless otherwise specified, all tests were
conducted for
7 days.
[0036] Following formulations are prepared to test glycol-containing
(propylene glycol, PG) and
glycol-free compositions:
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CC1-0 CC-1 CCI-3-2 CCI-3-3 CCI-3-4 CCI-2 CCI-4-2 CCI-3
Triethanolamine 20% -
morpholine
20% 20% 20% 20% 20% 20% 20%
benzotriazole 1% 1% 0.33% 1% 1% 1% 0.5% 1%
Na2Mo04.2H20 3% 3% 3% 3% 3%
1.5%
NaNO2 3%
8.75% 1.5%
Sebacic acid 1% 1% 1% 0.5% 0% 1% 0.5% 1%
Balance DI H20
[0037] Test 1: a heat transfer fluid including 10% (w/w) propylene glycol,
0.08% (w/w)
triethanolamine, 0.004% (w/w) benzotriazole, 0.012 A (w/w) Na2Mo04*2H20_and
0.004% (w/w) sebacic acid was tested.
[0038] Result: Tube coupon was corroded, indicating that triethanolamine was
not protecting
the vapor phase. Triethanolamine was replaced with morpholine in formulation.
Morpholine was further added to adjust the pH of the solution to 9.0 - 10Ø
[0039] Test 2: a heat transfer fluid including 10% (w/w) propylene glycol,
including 0.41% (w/w)
morpholine, 0.004% (w/w) benzotriazole, 0.012% (w/w) Na2Mo0.4.2H20_and 0.004%
(w/w) sebacic acid was tested for 28 days.
[0040] Result: slight corrosion was observed on tube coupon.
[0041] Test 3: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004% (w/w)
benzotriazole, 0.012 % (w/w) Na2Mo04.2H20_and 0.004% (w/w) sebacic acid were
tested for 28 days.
[0042] Result: insignificant corrosion.
[0043] Test 4: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.0013%
(w/w) benzotriazole, 0.012 % (w/w) Na2Mo04*2H20_and 0.004% (w/w) sebacic acid
was
tested for 28 days.
[0044] Result: Slight corrosion was observed on the tube side.
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[0045] Test 5: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004% (w/w)
benzotriazole, 0.012 % (w/w) Na2Mo04.2H20_and 0.002% (w/w) sebacic acid was
tested for 28 days.
[0046] Result: very little corrosion was observed.
[0047] Test 6: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004% (w/w)
benzotriazole, 0.012 % (w/w) Na2Mo04 .2H20_and 0.004% (w/w) sebacic acid was
tested for 28 days.
[0048] Result: no corrosion was observed, one of the best results.
[0049] Test 7: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004% (w/w)
benzotriazole, 0.012 % (w/w) NaNO2 and 0.004 (w/w) sebacic acid was tested for
28
days and 56 days, respectively.
[0050] Result: insignificant corrosion on tube coupon was observed, solution
was slightly hazy.
[0051] Test 8: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.002% (w/w)
benzotriazole, 0.035% (w/w) NaNO2 and 0.002% (w/w) sebacic acid was tested for
28
days.
[0052] Result: slight corrosion on tube coupon was observed, solution was
slightly hazy.
[0053] Test 9: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004% (w/w)
benzotriazole, 0.006 % (w/w) Na2Mo04 =2H20_, and 0.006 % (w/w) NaNO2 and
0.004%
(w/w) sebacic acid were tested for 28 days.
[0054] Result: insignificant corrosion on tube coupon was observed, solution
was slightly hazy.
[0055] Test 10: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004%
(w/w) benzotriazole, 0.012 % (w/w) Na2Mo04 *2H20, 0.004% (w/w) benzotriazole,
was
tested for 28 days.
[0056] Result: slight corrosion on tube coupon was observed, solution was
slightly hazy.
[0057] Test 11: a glycol-free heat transfer fluid including 0.33% (w/w)
morpholine, 0.004%
(w/w) benzotriazole, 0.012 % (w/w) Na2Mo04*2H20_and 0.004% (w/w) sebacic acid
was
tested for 56 days.
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[0058] Result: no corrosion was observed, the best result.
[0059] Test 12: a glycol-free heat transfer fluid, including 0.33% (w/w)
morpholine, 0.004%
(w/w) benzotriazole, 0.012% (w/w) NaNO2 and 0.004% (w/w) sebacic acid was
tested
for 56 days.
[0060] Result: no corrosion was observed, one of the best results.
[0061] Test 13: a glycol-free heat transfer fluid, including 0.33% (w/w)
morpholine, 0.004%
(w/w) benzotriazole, 0.006% (w/w) Na2Mo04=2H20, 0.006% (w/w) Na2NO2 and 0.004%
(w/w) sebacic acid was tested for 56 days.
[0062] Result: increased corrosion was observed when comparing to tests 11 and
12.
[0063] Test 14: a heat transfer fluid with 10% (w/w) propylene glycol,
including 0.33% (w/w)
morpholine, 0.004% (w/w) benzotriazole, 0.012 % (w/w) Na2Mo0.4.2H20_and 0.004%
(w/w) sebacic acid was tested for 56 days.
[0064] Result: adding 10% (w/w) propylene glycol increases corrosion.
[0065] Tests 11 - 13 indicate that glycol-free heat transfer fluid containing
Na2Mo04.2H20
and/or NaNO2 produced the best results for water based heat transfer fluid.
[0066] Test 15: The long term performance of the aqueous glycol-free heat
transfer fluid was
tested in an explosion-proof heater under elevated ambient conditions for 3 to
6 months.
The compositions used are as follows: 800 mg/L (0.08(w/w)%) morpholine, 40
mg/L
(0.004(w/w)%) benzotriazole, 120-192 mg/L (0.012-0.0192(w/w)%) Na2Mo04.2H20,
40
mg/L (0.004(w/w)%) sebacic acid, and 2.5 ml/L (0.25 (v/v)/o) morpholine to
adjust pH
between 9 and 10.
[0067] The aqueous glycol-free heat transfer fluid performed very well. No
visible sign of
corrosion was observed. Inductively coupled plasma (ICP) analysis indicates
molybdenum deposition on steel, forming a protective layer and a very low rate
of
corrosion.
[0068] Based on 4.5L of the aqueous glycol-free heat transfer fluids used in
Tests 6 and 11,
after evaporation the amount of solids remains at 0.33 to 0.5 g. By
comparison, 57g ¨
90 g solids remain after evaporation of 4.5 L of 2% di-potassium phosphate
(K2HPO4).
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=
This result shows that the solids were about 100-200 times less than prior art
glycol
based heat transfer fluid with di-potassium phosphate as an inhibitor. The
amount
remaining as solids clearly indicates that the aqueous glycol-free heat
transfer fluids of
the present invention reduces the risk of obstructing the pressure relieve
valve.
Furthermore, due to faster formation of vapour phase, the aqueous glycol-free
heat
transfer fluids provide faster start-up of the electric heater, and will
eliminate any
possible fire hazard due to the absence of glycol and its decomposition
products. The
heat transfer fluid of the present invention can also be used both under
vacuum and no
vacuum. Heaters and heat exchangers are initially under vacuum, however, in
field
condition vacuum may be lost. Corrosion inhibitors are more in need when
vacuum is
lost.
[0069] While the patent disclosure is described in conjunction with the
specific embodiments, it
will be understood that it is not intended to limit the patent disclosure to
the described
embodiments. On the contrary, it is intended to cover alternatives,
modifications, and
equivalents as may be included within the scope of the patent disclosure as
defined by
the appended claims. In the above description, numerous specific details are
set forth in
order to provide a thorough understanding of the present patent disclosure.
The present
patent disclosure may be practiced without some or all of these specific
details. In other
instances, well-known process operations have not been described in detail in
order not
to unnecessarily obscure the present patent disclosure.
The terminology used herein is for the purpose of describing particular
embodiments
only and is not intended to be limiting of the patent disclosure. As used
herein, the
singular forms "a", "an" and "the" are intended to include the plural forms as
well, unless
the context clearly indicates otherwise. It will be further understood that
the terms
"comprises" or "comprising", or both when used in this specification, specify
the
presence of stated features, integers, steps, operations, elements, and/or
components,
but do not preclude the presence or addition of one or more other features,
integers,
steps, operations, elements, components, and/or groups thereof.
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