OPERATING MEDIUM FOR AN ABSORPTION REFRIGERATION DEVICE

The invention is directed to a working medium for an absorption refrigeration machine giving reduced friction.

Classical refrigeration machines are based on a circuit in which a refrigerant is vaporized and cooling is achieved by the heat of vaporization taken up by the refrigerant. The vaporized refrigerant is then brought to a higher pressure by means of a compressor and condensed at a higher temperature than that at which vaporization has occurred, with the heat of vaporization being liberated again. The liquefied refrigerant is subsequently depressurized again to the pressure of the vaporizer.

Classical refrigeration machines have the disadvantage that they consume a great deal of mechanical energy for compression of the gaseous refrigerant. In contrast, absorption refrigeration machines have a reduced mechanical energy requirement. Absorption refrigeration machines have a sorption medium, an absorber and a desorber in addition to the refrigerant, the vaporizer and the condenser of a classical refrigeration machine. The vaporized refrigerant is absorbed at the pressure of the vaporization in the sorption medium in the absorber and is subsequently desorbed again from the sorption medium in the desorber at the higher pressure of the condensation by supply of heat. Compression of the liquid working medium composed of refrigerant and sorption medium requires less mechanical energy than compression of the refrigerant vapour in a classical refrigeration machine; the consumption of mechanical energy is replaced by the thermal energy used for desorption of the refrigerant.

The major part of absorption refrigeration machines used industrially employs a working medium containing water as refrigerant and lithium bromide as sorption medium. However, this working medium has a series of disadvantages which stand in the way of wide use of absorption refrigeration machines. To achieve a high efficiency, a maximum portion of the refrigerant has to be desorbed from the working medium in the desorber. However, for a working medium composed of water and lithium bromide, the water concentration must not fall below 35-40% by weight since crystallization of lithium bromide and thus solidification of the working medium can otherwise occur. A working medium composed of water and lithium bromide also has a corrosive effect on many materials and results in high friction and thus accelerated wear on moving parts of the pump required for conveying the working medium between absorber and desorber.

WO 2006/134015 describes, in Example VII a), the use of the ionic liquids 1-ethyl-3-methylimidazolium methylsulphonate, 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methyl-imidazolium hydroxide as additives for a working medium composed of lithium bromide and water in order to reduce the crystallization temperature of the sorption medium. However, what proportions of water, lithium bromide and ionic liquid should be present in the working medium is not disclosed.

In Korean J. Chem. Eng., 23 (2006) 113-116, K.-S. Kim et al. propose working media which are composed of water, lithium bromide and the ionic liquid 1-butyl-3-methyl-imidazolium bromide and in which the lithium bromide and the ionic liquid are present in a weight ratio of from 4:1 to 7:1. In these working media, the ionic liquid acts as anticrystallization additive which increases the solubility of lithium bromide and reduces the crystallization temperature.

However, the working media proposed by Kim et al. give an increased coefficient of friction compared to a working medium composed of water and lithium bromide, so that the problem of high friction cannot be solved thereby.

It has now surprisingly been found that working media comprising water, lithium bromide and an ionic liquid having a significantly reduced coefficient of friction compared to a working medium composed of water and lithium bromide can be produced when the water content and the weight ratio of ionic liquid to lithium bromide are selected in a suitable range.

The invention accordingly provides a working medium for an absorption refrigeration machine, which comprises from 5 to 30% by weight of water and from 65 to 95% by weight of a sorption medium consisting of lithium bromide and at least one ionic liquid, where the sorption medium contains ionic liquid and lithium bromide in a weight ratio of from 0.5:1 to 5:1.

The invention also provides an absorption refrigeration machine comprising an absorber, a desorber, a vaporizer, a condenser, a circulation pump and a working medium which is conveyed in a circuit via absorber and desorber by means of the circulation pump, wherein the working medium fed to the desorber has the composition according to the invention.

The working medium of the invention comprises from 5 to 30% by weight of water and from 65 to 95% by weight of a sorption medium consisting of lithium bromide and at least one ionic liquid. The working medium of the invention preferably comprises from 10 to 30% by weight of water. Water and sorption medium together preferably make up more than 90% by weight of the working medium, particularly preferably more than 95% by weight.

The sorption medium of the working medium of the invention comprises lithium bromide and at least one ionic liquid. Here, the term ionic liquid refers to a salt or a mixture of salts of anions and cations, with the salt or the mixture of salts having a melting point of less than 100° C. The term ionic liquid here refers to salts or mixture of salts which are free of nonionic materials or additives. The ionic liquid preferably consists of one or more salts of organic cations with organic or inorganic anions. Mixtures of a plurality of salts having different organic cations and the same anion are particularly preferred.

Suitable organic cations are, in particular, cations of the general formulae (I) to (V):

R¹R²R³R⁴N⁺  (I)

R¹R²N⁺=CR³R⁴  (II)

R¹R²R³R⁴P⁺  (III)

R¹R²P⁺=CR³R⁴  (IV)

R¹R²R³S⁺  (V)

where

R¹,R²,R³,R⁴are identical or different and are each hydrogen, a linear or branched aliphatic or olefinic hydrocarbon radical having from 1 to 30 carbon atoms, a cycloaliphatic or cycloolefinic hydrocarbon radical having from 5 to 40 carbon atoms, an aromatic hydrocarbon radical having from 6 to 40 carbon atoms, an alkylaryl radical having from 7 to 40 carbon atoms, a linear or branched aliphatic or olefinic hydrocarbon radical which has from 2 to 30 carbon atoms and is interrupted by one or more —O—, —NH—, —NR'—, —O—C(O)—, —(O)C—O—, —NH—C(O)—, —(O)C—NH—, —(CH₃)N—C(O)—, —(O)C—N(CH₃)—, —S(O₂)—O—, —O—S(O₂)—, —S(O₂)—NH—, —NH—S(O₂)—, —S(O₂)—N(CH₃)— or —N(CH₃)—S(O₂)— groups, a linear or branched aliphatic or olefinic hydrocarbon radical which has from 1 to 30 carbon atoms and is terminally functionalized by OH, OR′, NH₂, N(H)R′ or N(R′)₂or a polyether radical of the formula —(R⁵—O)_n—R⁶having a block or random structure,

R′ is an aliphatic or olefinic hydrocarbon radical having from 1 to 30 carbon atoms,

R⁵is a linear or branched hydrocarbon radical having from 2 to 4 carbon atoms,

n is from 1 to 200, preferably from 2 to 60,

R⁶is hydrogen, a linear or branched aliphatic or olefinic hydrocarbon radical having from 1 to 30 carbon atoms, a cycloaliphatic or cycloolefinic hydrocarbon radical having from 5 to 40 carbon atoms, an aromatic hydrocarbon radical having from 6 to 40 carbon atoms, an alkylaryl radical having from 7 to 40 carbon atoms or a radical —C(O)—R⁷,

R⁷is a linear or branched aliphatic or olefinic hydrocarbon radical having from 1 to 30 carbon atoms, a cycloaliphatic or cycloolefinic hydrocarbon radical having from 5 to 40 carbon atoms, an aromatic hydrocarbon radical having from 6 to 40 carbon atoms or an alkylaryl radical having from 7 to 40 carbon atoms,

where at least one and preferably each of the radicals R¹, R², R³and R⁴is different from hydrogen.

Cations of the formulae (I) to (V) in which the radicals R¹and R³together form a 4- to 10-membered, preferably 5- to 6-membered, ring are likewise suitable.

Heteroaromatic cations having at least one quaternary nitrogen atom bearing a radical R¹as defined above in the ring, preferably derivatives of pyrrole, pyrazole, imidazole, oxazole, isoxazole, thiazole, isothiazole, pyridine, pyrimidine, pyrazine, indole, quinoline, isoquinoline, cinnoline, quinoxaline or phthalazine substituted on the nitrogen atom, are likewise suitable.

Suitable inorganic anions are, in particular, tetrafluoroborate, hexafluorophosphate, nitrate, sulphate, hydrogensulphate, phosphate, hydrogenphosphate, dihydrogen-phosphate, hydroxide, carbonate, hydrogencarbonate, thiocyanate, dicyanamide and the halides, preferably chloride or bromide.

Suitable organic anions are, in particular R^aOSO₃⁻, R^aSO₃⁻, R^aOPO₃²⁻, (R^aO)₂PO₂⁻, R^aPO₃²⁻, R^aCOO⁻, (R^aCO)₂N⁻, (R^aSO₂)₂N⁻ and C(CN)₂⁻, where R^ais a linear or branched aliphatic hydrocarbon radical having from 1 to 30 carbon atoms, a cycloaliphatic hydrocarbon radical having from 5 to 40 carbon atoms, an aromatic hydrocarbon radical having from 6 to 40 carbon atoms, an alkylaryl radical having from 7 to 40 carbon atoms or a linear or branched perfluoroalkyl radical having from 1 to 30 carbon atoms.

In a preferred embodiment, the ionic liquid comprises one or more 1,3-dialkylimidazolium salts, where the alkyl groups are particularly preferably selected independently from among methyl, ethyl, n-propyl, n-butyl and n-hexyl. Particularly preferred ionic liquids are salts of one or more of the cations 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-(n-butyl)-3-methylimidazolium, 1-(n-butyl)-3-ethylimidazolium, 1-(n-hexyl)-3-methylimidazolium, 1-(n-hexyl)-3-ethylimidazolium, 1-(n-hexyl)-3-butylimidazolium with one of the anions chloride, bromide, acetate, methylsulphate, ethylsulphate, dimethylphosphate or methylsulphonate, in particular with chloride or bromide.

In a further preferred embodiment, the ionic liquid comprises one or more quaternary ammonium salts having a monovalent anion and cations of the general formula (I) in which

R¹is an alkyl radical having from 1 to 20 carbon atoms,

R²is an alkyl radical having from 1 to 4 carbon atoms,

R³is a radical (CH₂CHRO)_n—H where n is from 1 to 200 and R=H or CH₃and

R⁴is an alkyl radical having from 1 to 4 carbon atoms or a radical(CH₂CHRO)_n—H where n is from 1 to 200 and R=H or CH₃.

Particularly preferred anions are chloride, acetate, methylsulphate, ethylsulphate, dimethylphosphate or methylsulphonate.

Processes for preparing the ionic liquids are known to those skilled in the art from the prior art.

Preference is given to using ionic liquids which are miscible in any ratio with water, stable to hydrolysis and thermally stable up to a temperature of 150° C. Hydrolysis-stable ionic liquids show less than 5% degradation by hydrolysis in a mixture with 50% by weight of water on storage at 80° C. for 8000 h.

The sorption medium of the working medium of the invention contains ionic liquid and lithium bromide in a weight ratio of from 0.5:1 to 5:1. The weight ratio is preferably from 0.5:1 to 2:1 and particularly preferably from 1:1 to 2:1.

The ionic liquid and the weight ratio of ionic liquid to lithium bromide in the sorption medium are preferably selected so that the working medium remains as a single phase at a desorption of water down to a water content of the working medium of 10% by weight and at an absorption of water up to a water content of the working medium of 30% by weight. The ionic liquid and the weight ratio of ionic liquid to lithium bromide in the sorption medium are particularly preferably selected so that the working medium remains as a single phase at a desorption of water down to a water content of the working medium of 5% by weight and at an absorption of water up to a water content of the working medium of 30% by weight.

The working medium can contain further additives in addition to the sorption medium and water.

The working medium preferably contains one or more corrosion inhibitors as additive. Here, it is possible to use all nonvolatile corrosion inhibitors which are known from the prior art as being suitable for the materials used in the absorption refrigeration machine.

Further preferred additives are wetting-promoting additives which can be used in an amount of from 0.01 to 10% by weight based on the working medium. Preference is given to using the wetting-promoting additives described in WO 2009/097930 on page 6, line 14 to page 8, line 6, particularly preferably the compounds described in WO 2009/097930 on page 7, line 7 to page 8, line 6.

An absorption refrigeration machine according to the invention comprises an absorber, a desorber, a vaporizer, a condenser, a circulation pump and a working medium which is conveyed in a circuit via absorber and desorber by means of the circulation pump, wherein the working medium fed to the desorber has the abovementioned composition according to the invention.

The working media of the invention show a significant reduction in the coefficient of friction, especially in the region of mixed friction in the Stribeck curve, compared to working media composed of water and lithium bromide or of water, lithium bromide and 1-butyl-3-methylimidazolium bromide which are known from the prior art. The working media of the invention thus allow the use of pumps whose bearings come into contact with the working medium without special materials e.g. ceramic materials, being necessary for this purpose. The absorption refrigeration machines of the invention display reduced wear of the circulation pump compared to absorption refrigeration machines of the prior art.

The working media of the invention can be used not only in absorption refrigeration machines in which the uptake of heat by the coolant in the vaporizer is utilized for cooling but also in absorption heat pumps in which the heat liberated in the absorber and in the condenser is used for heating.

The following examples illustrate the invention but do not restrict the subject matter of the invention.

Coefficients of friction μ (traction coefficient) were determined for working media according to the invention and working media according to the prior art at 40 and 70° C. with a friction testing instrument MTM2 from PCS Instruments at different speed differences between the friction surfaces. Measurements were carried out using a ball having a diameter of 19.05 mm and a disc having a diameter of 46 mm as friction partners, both made of polished stainless steel 1.3505 having a surface roughness of 0.01 μm, at a force of 30 N, a contact pressure of 947.8 MPa and a sliding/rolling ratio of 50%.

Table 1 shows the composition of the working media investigated in Examples 1 to 6. Example 1 is a working medium composed of water and lithium bromide without a proportion of ionic liquid, as is used in commercial absorption refrigeration machines. Examples 2 and 3 are working media according to the prior art from K.-S. Kim et al., in Korean J. Chem. Eng., 23 (2006) 113-116, having a weight ratio of lithium bromide to ionic liquid of 4. Examples 4 to 6 are working media according to the invention with 1-butyl-3-methylimidazolium bromide as ionic liquid.

In FIG. 1, the Stribeck curve (coefficient of friction μ versus speed difference v between the friction surfaces in mm/s) at 40° C. is plotted for each of the working media of Examples 1 to 3 (Example 1: open circles; Example 2: open triangles; Example 3: open squares). FIG. 2 shows the corresponding Stribeck curves at 70° C.

Both figures show that the working media of Examples 2 and 3 according to the prior art from K.-S. Kim et al. give a higher coefficient of friction than the working medium without ionic liquid as per Example 1.

In FIG. 3, the Stribeck curve (coefficient of friction μ versus speed difference v between the friction surfaces in mm/s) at 40° C. is plotted for each of the working media of Examples 3 to 6 (Example 3: open squares, Example 4: solid circles, Example 5: solid triangles, Example 6: asterisks). FIG. 4 shows the corresponding Stribeck curves at 70° C.

Both figures show that the working media according to the invention of Examples 4 and 5 give a coefficient of friction which is lower than that for the working media known from K.-S. Kim et al. and is also below the coefficient of friction of the working medium without ionic liquid as per Example 1 and below the coefficient of friction of the working medium without lithium bromide as per Example 6.

In FIG. 5, the Stribeck curve (coefficient of friction μ versus speed difference v between the friction surfaces in mm/s) at 40° C. is plotted for the working media of Examples 1, 7 and 8 (Example 1: open circles; Example 7: solid squares, Example 8: solid diamonds). FIG. 6 shows the corresponding Stribeck curves at 70° C.

Both figures show that a working medium according to the invention gives a low coefficient of friction in the region of mixed friction, i.e. at low speed differences between the friction surfaces, which is below the coefficient of friction of the working medium without ionic liquid as per Example 1 even at a water content of 5% by weight or 30% by weight.