UTILISATION DE SOLUTIONS AQUEUSES D'ALCOOLS INFERIEURS COMME MILIEU DE TRANSFERT DU FROID

USE AS COLD TRANSFER MEDIA OF AQUEOUS SOLUTIONS OF LOWER ALCOHOLS

Abrégé français

On divulgue un milieu de transfert du froid pour utilisation dans un système de réfrigération ayant des circuits séparés pour le réfrigérant et le milieu de transfert du froid. Le milieu de transfert du froid est une solution aqueuse d'un alcool inférieur additionnée (1) d'un acide polycarboxylique de faible poids moléculaire, (2) d'un ester ou d'un sel de l'acide sorbique, (3) d'un polyalcool, (4) d'un acide aliphatique ou d'un sel d'un acide aliphatique, (5) d'un surfactant anionique et/ou (6) d'un composé de triazole sans soufre. On divulgue également une solution aqueuse contenant de la silice finement divisée comme inhibiteur de sédimentation.

Abrégé anglais

Disclosed is a cold transfer medium for use in a
refrigeration system having separate circuits for a refrigerant
and the cold transfer medium. The cold transfer medium is an
aqueous solution of a lower alcohol with the addition of (1) a
low molecular weight polycarboxylic acid, (2) an ester or salt
of sorbic acid, (3) a polyhydric alcohol, (4) an aliphatic acid
or a salt of an aliphatic acid, (5) an anionic surfactant
and/or (6) a sulfur-free triazole compound. Also disclosed is
such an aqueous solution containing finely divided silica as a
sedimentation inhibitor.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A cold transfer medium for a refrigeration system
consisting essentially of an aqueous solution of a water-soluble
lower alcohol with at least one additive selected
from the group consisting of:
(1) a low molecular weight polycarboxylic acid,
(2) an ester or salt of sorbic acid,
(3) a polyhydric alcohol,
(4) an aliphatic acid or salt of an aliphatic acid,
(5) an anionic surfactant, and
(6) a sulfur-free triazole compound, or with finely
divided silica.
2. The cold transfer medium as claimed in claim 1,
wherein the additive used is the following combination of two
substances from two different classes of substances:
(1) and (2), (1) and (3), (1) and (4),
(1) and (5), (1) and (6), (2) and (3),
(2) and (4), (2) and (5), (2) and (6),
(3) and (4), (3) and (5), (3) and (6),
(4) and (5), (4) and (6) or (5) and (6).
3. The cold transfer medium as claimed in claim 1,
wherein the additive used is the following combination of
three substances from three different classes of substances:
(1), (2) and (3), (1), (2) and (4), (1), (2) and (5),
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(1), (2) and (6), (1), (3) and (4), (1), (3) and (5),
(1), (3) and (6), (1), (4) and (5), (1), (4) and (6),
(1), (5) and (6), (2), (3) and (4), (2), (3) and (5),
(2), (3) and (6), (3), (4) and (5), (3), (4) and (6) or
(3), (5) and (6).
4. The cold transfer medium as claimed in claim 1,
wherein the additive used is the following combination of four
substances from four different classes of substances:
(1), (2), (3) and (4), (1), (2), (3) and (5),
(1), (2), (3) and (6), (1), (3), (4) and (5),
(1), (3), (4) and (6), (1), (3), (5) and (6),
(2), (3), (4) and (5), (2), (3), (4) and (6),
(2), (3), (5) and (6) or (3), (4), (5) and (6).
5. The cold transfer medium as claimed in claim 1,
wherein the additive used is the following combination of five
substances from five different classes of substances, namely:
(1), (2), (3), (4) and (5),
(1), (2), (3), (4) and (6),
(1), (2), (3), (5) and (6),
(1), (2), (4), (5) and (6),
(1), (3), (4), (5) and (6) or
(2), (3), (4), (5) and (6).
6. The cold transfer medium as claimed in claim 1,
wherein the additive used is a combination of six substances
from six different classes of substances (1) to (6).
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7. The cold transfer medium as claimed in claim 1,
wherein the aqueous solution contains finely divided silica
as a sedimentation inhibitor.
8. The cold transfer medium as claimed in any one of
claims 1 to 6, wherein finely divided silica is also added.
9. The cold transfer medium as claimed in any one of
claims 1 to 6, wherein the additive of class (1) is selected
from dicarboxylic and tricarboxylic acids containing a hydroxyl
group, the additive of class (2) is selected from methyl
sorbate, ethyl sorbate and sodium sorbate, the additive of
class (3) is selected from glycol, dipropylene glycol,
triethylene glycol, glycerol and pentaerythritol, the additive
of class (4) is selected from acetic acid, propionic acid and
lactic acid as well as their alkali metal salts, the additive
of class (5) is selected from long-chain alkyl sulfates or
sulfonates and long-chain alkylarylsulfonates.
10. A cold transfer medium for a refrigeration system
having separate circuits for a refrigerant and a cold transfer
medium, the cold transfer medium being an aqueous solution of
a water-soluble lower alkanol and containing at least one
additive selected from the group consisting of:
(1) a low molecular weight polycarboxylic acid,
(2) an ester or salt of sorbic acid,
(3) a polyhydric alcohol,
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(4) a low molecular weight aliphatic monocarboxylic
acid or hydroxy-monocarboxylic acid or a salt thereof,
(5) an anionic surfactant, and
(6) a sulfur-free triazole compound,
wherein the additive is chosen and employed within
an amount of 0.005 to 5% by weight based on the aqueous
solution such that the cold transfer medium, when cooled in
an ice generator, forms a readily pumpable liquid ice containing
fine ice crystals having a mean crystal size smaller than
that when the additive is not employed.
11. The cold transfer medium as claimed in claim 10,
wherein the aqueous solution contains 5 to 30% by volume of
ethanol and 70 to 95% by volume of water.
12. The cold transfer medium as claimed in claim 10 or
11, wherein the additive (1) is malic acid or tartaric acid;
the additive (2) is methyl sorbate, ethyl sorbate or sodium
sorbate; the additive (3) is ethylene glycol, propylene glycol,
dipropylene glycol, triethylene glycol, glycerol or penta-erythritol;
and the additive (4) is acetic acid, propionic
acid, lactic acid or an alkali metal salt of these acids.
13. A cold transfer medium for a refrigeration system
having separate circuits for a refrigerant and a cold transfer
medium, the cold transfer medium being an aqueous solution of
a water-soluble lower alkanol and containing finely divided
silica having a particle size in the range of from about 5 to
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about 20 nm in an amount effective to inhibit sedimentation of
ice crystals when the cold transfer medium is cooled in an ice
generator to form a readily pumpable liquid ice containing the
ice crystals.
14. The cold transfer medium as claimed in claim 13,
wherein the aqueous solution contains 5 to 30% by volume of
ethanol and 70 to 95% by volume of water.
15. A refrigeration system having separate circuits for
a refrigerant and a cold transfer medium, wherein the cold
transfer medium is as defined in any one of claims 1 to 14.
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Description

CA 02210902 1997-07-21
USE AS. COLD.TRANSFER MEDIA OF AQUEOUS
SOLUTIONS OF LOWER kLCOHOLS
The invention relates to a cold transfer medium
consisting essentially of an aqueous solution of a lower
alcohol containing a property-improving additive which may
be a single substance or a combination of substances of two
to six different classes of substances or containing finely
divided silica as a sedimentation inhibitor.
Chlorofluorocarbons (CFCs) such as difluorodichloro-
methane and trifluorochloromethane were previously widely usedas refrigerants, but have fallen into disrepute after it
became known that they contribute to the destruction of the
ozone layer around the earth and to the so-called greenhouse
effect. The engineering measures which seek to reduce the
amounts of CFCs, hydrochlorofluorocarbons (H-CFCs) or fluoro-
carbons (FCs) required for a particular cooling performance
include the separation of a refrigeration system into a
refrigerant circuit and a circuit containing a cold transfer
medium. In the first-mentioned circuit, the temperature of
the refrigerant is still reduced by partial evaporation and
indirect heat exchange takes place between the refrigerant
and the cold transfer medium which circulates in the second
circuit and provides its cooling effect at the desired
location by taking up heat. For economic reasons, particularly
in relatively large facilities such as air-conditioned
buildings, supermarkets having a plurality of refrigerated or
freezer chests or cool stores, this principle of separating
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the refrigerant circuit and the circuit containing the cold
transfer medium is also employed when one of the refrigerants
mentioned are used.
Coolants based on ethylene or propylene glycol are
at present widely used as a cold transfer medium. Although
these coolants have a high specific heat and therefore a
corresponding cooling performance, they display a temperature
behaviour directly dependent on the cooling performance. For
this reason, increasing use is being made more recently of a
so-called liquid ice mixture comprising a water-soluble
alcohol, generally ethanol, and water as a cold transfer
medium which has a favourable heat transfer behaviour. This
mixture is cooled in an ice generator specifically constructed
for this purpose until part Gf the water freezes out in the
form of fine ice crystals. A heat exchange surface is kept
free mechanically so that growth of the ice crystals deposited
there is prevented. The resulting two-phase system, also
knGwn as liquid ice, is conveyed in a closed circuit and is
readily pumpable. The mean particle diameter of the ice
crystals in the liquid ice is about 100 ~m in the system based
on ethanol/water which is customary nowadays. A particular
problem in the liquid ice system is the deposition of an
organic slime on the heat exchange surface and/or in pipes,
which reduces heat exchange, increases the pressure drop in
the system and makes frequent cleaning necessary.
It has now been found that the aqueous solution of
a lower alcohol with an addition of (1) a low molecular weight
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polycarboxylic acid, (2) an ester or salt of sorbic acid,
(3) a polyhydric alcohol, (4) an aliphatic acid or a salt
of an aliphatic acid, (5) an anionic surfactant and/or (6) a
sulfur-free triazole compound can be advantageously used as
a cold transfer medium.
The additive employed according'to the invention
reduces the mean particle size of the ice crystals. A typical
mean particle size is between 20 and 50 ~m. A smaller mean
particle size is desirable from two points of view. On the
one hand, it improves heat transfer at the location of the
cooling effect required so that a smaller volume of the cold
transfer medium has to be conveyed per unit time. On the
other hand, the rheological properties of the liquid ice are
ccnsiderably better owing to the smaller mean particle size
of the ice crystals. The pressure drop in the system is
therefore lower, which saves pumping energy. In addition,
the above-mentioned formation of an organic slime is
suppressed.
The lower alcohol suitable for the cold transfer
medium has to be soluble in water (or miscible with water)
to at least such an extent that a single-phase system having
an indicated proportion of the two substances is formed.
Preference is given to a lower alkanol miscible in all
proportions with water, for example methanol, ethanol and
isopropanol, or a mixture of such alkanols. The preferred
alcohol is ethanol which can be used in the commercial
denatured form. The water can be, but does not have to be,
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deionized. A preferable cold transfer medium contains from 5
to 30% by volume of the alcohol and from 70 to 95% by volume
of water.
A suitable additive is selected from the above-
mentioned classes of substances (1) to (6). Among the low
molecular weight polycarboxylic acids (1), examples of
preferred additives are dicarboxylic and tricarboxylic acids
which contain a hydroxyl group and are therefore sufficiently
soluble in the aqueous solution of the alcohol, for example
malic acid and tartaric acid. The sorbic acid derivatives
(2) which can be used include methyl sorbate, ethyl sorbate
and sodium sorbate. Examples of preferred polyhydric alcohols
(3) which may be mentioned are low molecular weight polyhydric
alcohols such as ethylene glycol, propylene glycol,
dipropylene glycol, triethylene glycol, glycerol and penta-
erythritol. Aliphatic acids (4) or salts thereof which can
be used include, for example, low molecular weight aliphatic
monocarboxylic acids such as acetic acid and propionic acid,
and low molecular weight hydroxy-monocarboxylic acids, lactic
acid as well as their alkali metal salts. Anionic surfactants
(5) which can be used as additive are, for example~ long-chain
alkyl sulfates or s~fonates and also long-chain alkylaryl-
sulfonates.
The additives can consist of one or more represent-
atives from any one of the classes of substances mentioned or
be combinations of a plurality of substances from a plurality
of different classes of substances. These include combinations
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of two substances from two different classes of substances,
namely
(1) and (2), (1) and (3), (1) and (4),
(1) and (5), (1) and (6), (2) and (3),
(2) and (4), (2) and (5), (2) and (6),
(3) and (4), (3) and (5), (3) and (6),
(4) and (5), (4) and (6) or (5) and (6);
combinations of three substances from three different classes
of substances, namely
0 (1), (2) and (3), (1), (2) and (4), (1), (2) and (5),
(1), (2) and (6), (1), (3) and (4), (1), (3) and (5),
(1), (3) and (6), (1), (4) and (5), (1), (4) and (6),
(1), (5) and (6), (2), (3) and (4), (2), (3) and (5),
(2), (3) and (6), (3), (4) and (5), (3), (4) and (6),
(3), (5) and (6) or (4), (5) and (6);
combinations of four substances from four different classes
of substances, namely
(1), (2), (3) and (4), (1), (2), (3) and (5),
(1), (2), (3) and (6), (1), (3), (4) and (5),
(1)~ (3)~ (4) and (6)~ (1), (3), (5) and (6),
(2), (3), (4) and (5), (2), (3), (4) and (6),
(2), (3) , (5) and (6) or (3), (4), (5) and (6)i
and also combinations of six substances from six different
classes of substances (1) to (6).
The aqueous solution whose use is subject matter of
the invention generally contains the additive in an amount of
from 0.005 to 5% by weight, preferably from 0.1 to 0.5% by
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weight, based on the weight of the total aqueous solution
including the additive. The optimum amount depends, inter alia,
on the desired mean particle size, the lower alcohol used, its
concentration in the aqueous solution and also on the desired
cooling temperature and can be readily determined by guideline
tests.
The aqueous solution may contain one or more further
additives customary for a cold transfer medium, for example
an additional flow improver or a foam inhibitor. The favour-
able property profile of the cold transfer medium is notimpaired thereby.
It has also been found that the addition of a small
amount of finely divided silica counteracts the sedimentation
of the ice crystals which is not infrequently observed,
particularly in the unagitated state. Silica suitable for
this purpose is, for example, pyrogenic silicon dioxide having
a mean particle size in the range from about 5 to about 20 nm.
A hydrophilic product of this type is preferred. Examples of
pyrogenic silicon dioxide which can be used are, inter alia,
commercially available under the trade-marks Aerosil~ and
Cabot-Sil~. However, other non-pyrogenic finely divided
silicas having a similar mean particle size are also an
effective sedimentation inhibitor. Furthermore, the sediment-
ation inhibition occurs in the case of all known aqueous
solutions cf lower alcohols which are used as a liquid ice
mixture, regardless of whether the liquid ice mixture contains
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the additives according to the invention and/or other
additives. A preferred amount of the finely divided silica
is from about 0.01 to 1% by weight based on the cold transfer
medium.
The cold transfer medium according to the invention
may be used in any customary refrigeration plants having
separate circuit for a refrigerant and a cold transfer medium.
Temperatures below zero which can be achieved depend on the
alcohol used and on the ratio of the alcohol to water. In
the case of from 10 to 20% by weight of ethanol, about -15~C
can be achieved without the proportion of ice crystals which
becomes ever greater with falling temperature substantially
increasing the energy requirement for the circuit containing
the cold transfer medium.
The following example illustrates the invention but
should not be interpreted to restrict its scope as defined in
the claims.
Example
In a conventional refrigeration plant using ammonia
as refrigerant, a mixture of 10% by volume of commercial
ethanol denatured with methyl ethyl ketone, 90% by volume of
water and 0.2% by weight of a mixture comprising one substance
from each of the six classes of substances (1) to (6)
(obtainable from IWC International, Marl, Federal Republic of
Germany as Corrogard~ 36) is used as a cold transfer medium.
The pressure drop in the system is 0.45 bar, the specific
throughput of the cold transfer medium needed for the required
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cooling performance is 92 l/min. After 3 months of
uninterrupted operation, no corrosion and/or accumulation of
organic slime was apparent on the heat exchange surfaces of
V2A steel and in the copper pipes.
The addition of 0.1% by weight of finely divided
silica (Cabot-Sil~ EH5) reliably suppresses the sedimentation
tendency displayed by the liquid ice, particularly in the
static state.
When an otherwise identical cold transfer medium
without additive was used, the pressure drop was 0.51 bar and
the specific throughput of the cold transfer medium was 103
l/min under otherwise identical conditions. ~he additional
energy requirement for the circuit containing cold transfer
medium is thus about 20%.
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