Patent Application
20130031931
The invention is directed to an absorption heat pump having an improved degassing range of the working medium.
Classical heat pumps are based on a circuit of a refrigerant via an evaporator and a condenser. In the evaporator, a refrigerant is vaporised and heat is withdrawn from a first medium due to the heat of vaporisation taken up by the refrigerant. The vaporised refrigerant is then brought to a higher pressure with a compressor and condensed in the condenser at a higher temperature than in the evaporation, resulting in the heat of vaporisation being liberated again and heat being passed to a second medium at a higher temperature level. The liquefied refrigerant is subsequently depressurised again to the pressure of the evaporator.
The classical heat pumps have the disadvantage that they consume a large amount of mechanical energy for compression of the gaseous refrigerant. Absorption heat pumps, on the other hand, have a reduced mechanical energy requirement. Absorption heat pumps have, in addition to the refrigerant, the evaporator and the condenser of a classical heat pump, a sorption medium, an absorber and a desorber. In the absorber, the vaporised refrigerant is absorbed in the sorption medium at the pressure of the evaporation and the refrigerant is subsequently desorbed again from the sorption medium in the desorber by introduction of heat at the higher pressure of the condensation. The compression of the liquid working medium composed of refrigerant and sorption medium requires less mechanical energy than the compression of the refrigerant vapour in a classical heat pump; the consumption of mechanical energy is replaced by the heat energy used for desorption of the refrigerant.
The size of the circuit of sorption medium via absorber and desorber required for operation of an absorption heat pump is determined essentially by the degassing range of the working medium of the absorption heat pump; for the present purposes, the working medium is the mixture of sorption medium and refrigerant in the circuit via absorber and desorber of the absorption heat pump and the term degassing range refers to the difference in the refrigerant content between refrigerant-depleted working medium and refrigerant-rich working medium. To be able to construct compact and inexpensive absorption heat pumps, there is therefore a need for absorption heat pumps having a large degassing range.
A large part of the absorption heat pumps used in industry use a working medium which contains water as refrigerant and lithium bromide as sorption medium. However, this working medium has an unsatisfactory degassing range in many applications, since in the case of a working medium composed of water and lithium bromide the water concentration in the working medium must not go below from 35 to 40% by weight because otherwise crystallization of lithium bromide and thus solidification of the working medium can occur.
WO 2005/113702 describes absorption heat pumps which use a working medium having an ionic liquid as sorption medium, with the ionic liquid preferably having unlimited miscibility with the refrigerant. Although the problem of crystallization of the sorption medium can be avoided when using ionic liquid as sorption medium, the degassing range achieved is generally no better than in the case of working media composed of water and lithium bromide, especially not for working media containing water as refrigerant.
WO 2006/134015 describes, in Example VII a), the use of the ionic liquids 1-ethyl-3-methylimidazolium methylsulfonate, 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium hydroxide as additives for a working medium composed of lithium bromide and water in order to reduce the crystallisation temperature of the sorption medium. However, the proportions of water, lithium bromide and ionic liquid which should be present in the working medium are not disclosed and nothing is disclosed about the influence of the additive on the degassing range of the working medium.
In Korean J. Chem. Eng., 23 (2006) 113-116, K.-S. Kim et al. propose working media composed of water, lithium bromide and the ionic liquid 1-butyl-3-methylimidazolium bromide, which working media contain lithium bromide and the ionic liquid in a weight ratio of 4:1 and 7:1. In these working media, the ionic liquid acts as anticrystallisation additive which increases the solubility of lithium bromide and reduces the crystallisation temperature. However, nothing about the degassing range of the working media disclosed can be deduced from the solubilities, viscosities and surface tensions disclosed.
U.S. Pat. No. 6,155,057 describes addition of organic salts containing an alkylammonium ion or a heterocyclic cation to working media containing a lithium halide as sorption medium in order to prevent crystallisation of the sorption medium.
It has now surprisingly been found that the use of a working medium comprising, as sorption medium, a lithium salt and at least one organic salt having an organic cation Q+ having a molar mass of not more than 200 g/mol, where lithium salt and organic salt have the same anion having a molar mass of not more than 200 g/mol, makes it possible to achieve a higher degassing range of the working medium than when using the lithium salt or the organic salt alone.
The invention accordingly provides an absorption heat pump comprising an absorber, a desorber, a condenser, an evaporator, a volatile refrigerant and a sorption medium comprising a lithium salt and at least one organic salt having an organic cation Q+, where lithium salt and organic salt have the same anion, the anion has a molar mass of not more than 200 g/mol and is not a halide and the organic cation Q+ has a molar mass of not more than 200 g/mol.
The absorption heat pump of the invention comprises an absorber, a desorber, a condenser, an evaporator, a volatile refrigerant and a sorption medium. The working medium of the absorption heat pump is a mixture of sorption medium and refrigerant. During operation of the absorption heat pump of the invention, gaseous refrigerant is absorbed in refrigerant-depleted working medium in the absorber to give a refrigerant-rich working medium with liberation of heat of absorption. In the desorber, gaseous refrigerant is desorbed from the obtained refrigerant-rich working medium by supply of heat to give refrigerant-depleted working medium which is recirculated to the absorber. The gaseous refrigerant obtained in the desorber is condensed in the condenser to liberate heat of condensation, the liquid refrigerant obtained is vaporised in the evaporator, taking up heat of vaporisation, and the gaseous refrigerant obtained in the process is recirculated to the absorber.
The term absorption heat pump, as used in the invention, encompasses all apparatuses by means of which heat is taken up at a low temperature level and given off again at a higher temperature level and which are driven by supply of heat to the desorber. The absorption heat pumps of the invention thus encompass both absorption refrigeration machines and absorption heat pumps in the narrower sense, in which absorber and evaporator are operated at a lower working pressure than desorber and condenser, and absorption heat transformers, in which absorber and evaporator are operated at a higher working pressure than desorber and condenser. In absorption refrigeration machines, the uptake of heat of vaporisation in the evaporator is used for cooling a medium. In absorption heat pumps in the narrower sense, the heat liberated in the condenser and/or absorber is used for heating a medium. In absorption heat transformers, the heat of absorption liberated in the absorber is used for heating a medium, with the heat of absorption being obtained at a higher temperature level than the heat supplied to the desorber. In a preferred embodiment, the absorption heat pump is operated as absorption refrigeration machine and heat is taken up from a medium to be cooled in the evaporator.
The absorption heat pump of the invention comprises a sorption medium comprising a lithium salt and at least one organic salt having an organic cation Q+. Lithium salt and organic salt have the same anion. The proportion of lithium salt and organic salt in the sorption medium is preferably more than 50% by weight and particularly preferably more than 80% by weight. The sorption medium can contain other lithium salts which have an anion different from that of the organic salt in addition to the lithium salt. The proportion of lithium salts having a different anion in the total amount of lithium salts is preferably less than 20% by weight, particularly preferably less than 10% by weight.
The sorption medium preferably contains lithium salt and organic salt in a molar ratio at which the melting point of the mixture of lithium salt and organic salt is lower than the melting points of the lithium salt and the organic salt. The sorption medium particularly preferably contains lithium salt and organic salt in a molar ratio which deviates by not more than 25% from the molar ratio of a eutectic mixture of lithium salt and organic salt. At the preferred molar ratios of lithium salt and organic salt, a particularly wide degassing range of the working medium is achieved and the working medium can be used in the absorption heat pump in a particularly wide temperature range.
In the organic salt the organic cation Q+ has a molar mass of not more than 200 g/mol and preferably not more than 165 g/mol. The use of one or more organic salts whose organic cation Q+ has a low molar mass according to the invention is essential for achieving a wide degassing range of the working medium.
Suitable organic cations are, in particular, cations of the general formulae (I) to (V):
R1R2R3R4N+ (I)
R1R2R3R4P+ (II)
R1R2R3S+ (III)
R1R2N+═C(NR3R4)(NR5R6) (IV)
R1R2N+═C(NR3R4)(XR5) (V)
where
R1, R2, R3, R4, R5, R6 are identical or different and are each hydrogen, a linear or branched aliphatic or olefinic hydrocarbon radical, a cycloaliphatic or cycloolefinic hydrocarbon radical, an aromatic hydrocarbon radical, an alkylaryl radical, a linear or branched aliphatic or olefinic hydrocarbon radical terminally functionalised by OH, OR′, NH2, N(H)R′ or N(R′)2 or a polyether radical of the formula —(R7—O)n—R8, where in the case of cations of formula (V) R5 is not hydrogen,
R′ is an aliphatic or olefinic hydrocarbon radical,
R7 is a linear or branched alkylene radical containing 2 or 3 carbon atoms,
n is from 1 to 3,
R8 is hydrogen or a linear or branched aliphatic or olefinic hydrocarbon radical,
X is an oxygen atom or a sulphur atom, and
where at least one and preferably each of the radicals R1, R2, R3, R4, R5 and R6 is different from hydrogen.
Cations of the formulae (I) to (V) in which the radicals R1 and R3 together form a 4- to 10-membered, preferably 5- to 6-membered, ring are likewise suitable.
Further suitable cations are heteroaromatic cations having at least one quaternary nitrogen atom in the ring, said atom bearing a radical R1 as defined above, preferably derivatives of pyrrole, pyrazole, imidazole, oxazole, isoxazole, thiazole, isothiazole, pyridine, pyrimidine, pyrazine, indole, quinoline, isoquinoline, cinnoline, quinoxaline or phthalazine which are substituted on the nitrogen atom.
The organic cation Q+ preferably contains a quaternary nitrogen atom. The organic cation Q+ is preferably a 1,3-dialkylimidazolium ion, 1,3-dialkylimidazolinium ion, N-alkylpyridinium ion, N,N-dialkylpyrrolidinium ion or an ammonium ion having the structure R1R2R3R4N+, where R1, R2 and R3 are each, independently of one another, hydrogen, alkyl or hydroxyethyl and R4 is an alkyl radical.
The organic cation Q+ is particularly preferably a 1,3-dialkylimidazolium ion in which the alkyl groups are selected independently from among methyl, ethyl, n-propyl and n-butyl. The organic cation Q+ is particularly preferably 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-(n-butyl)-3-methylimidazolium or 1-(n-butyl)-3-ethylimidazolium.
In a further particularly preferred embodiment, the organic salt is a choline salt.
The organic salt is preferably thermally stable up to a temperature of 150° C. Preference is given to using organic salts which have unlimited miscibility with water and are stable to hydrolysis. Hydrolysis-stable organic salts display less than 5% decomposition by hydrolysis in a mixture with 50% by weight of water when stored at 80° C. for 8000 hours.
Lithium salt and organic salt contain the same anion which has a molar mass of not more than 200 g/mol and is not a halide. The anion can be singly, doubly or multiply negatively charged and is preferably singly negatively charged, with particular preference being given to the anion of a monovalent acid. The restriction of the molar mass of the anion to less than 200 g/mol improves the degassing range of the working medium.
Anions of monovalent inorganic acids, preferably nitrate, nitrite and cyanate, and also anions of monovalent organic acids, preferably of carboxylic acids, such as formate, acetate, propionate and benzoate, are suitable as shared anion of the lithium salt and the organic salt. Monoanions and dianions of divalent inorganic acids, preferably sulphate, hydrogensulphate, carbonate and hydrogencarbonate and also monoanions and dianions of divalent organic acids, preferably oxalate, succinate and malonate, are likewise suitable. Further suitable anions are monoanions, dianions and trianions of trivalent inorganic acids, preferably phosphate, hydrogenphosphate and dihydrogenphosphate. In addition, partial esters of divalent and trivalent inorganic acids, preferably methylsulphate, methylphosphate and dimethylphosphate, are suitable. Especially preferred are nitrate, formate, acetate, cyanate, sulphate, methylsulphate, carbonate, oxalate, succinate, methylphosphate, dimethylphosphate, hydrogenphosphate and dihydrogenphosphate, in particular acetate.
In a preferred embodiment, the shared anion of lithium salt and organic salt has a pKb of more than 5 and preferably more than 7. The choice of salts having an anion having such a pKb allows the stability of the organic salt against decomposition during operation of the absorption to be improved.
The absorption heat pump of the invention preferably contains water, methanol, ethanol, 2-propanol, trifluoroethanol, sulphur dioxide, carbon dioxide or ammonia, particularly preferably water, ethanol, 2-propanol or trifluoroethanol and most preferably water, as refrigerant.
In a preferred embodiment, the organic salt and the refrigerant are selected so that a solution of the lithium salt in the refrigerant which is saturated at 35° C. has a higher vapour pressure than a mixture of the organic salt and the refrigerant having the same proportion by weight of refrigerant. Preference is given to using water, methanol, ethanol, 2-propanol or trifluoroethanol and particularly preferably water as refrigerant. Examples of such an embodiment are 1-ethyl-3-methylimidazolium nitrate as organic salt, lithium nitrate as lithium salt and water as refrigerant, and 1-(n-butyl)-3-methylimidazolium acetate as organic salt, lithium acetate as lithium salt and water as refrigerant. A further-improved degassing range of the working medium is achieved by a corresponding selection of organic salt and refrigerant.
The absorption heat pump of the invention can contain further additives, preferably corrosion inhibitors and/or additives which promote wetting, in the working medium in addition to sorption medium and refrigerant. The proportion of corrosion inhibitors is preferably from 10 to 50 000 ppm, particularly preferably from 100 to 10 000 ppm, based on the mass of the sorption medium. The proportion of additives which promote wetting is preferably from 10 to 50 000 ppm, particularly preferably from 100 to 10 000 ppm, based on the mass of the sorption medium.
All non-volatile corrosion inhibitors, which are known from the prior art to be suitable for the materials used in the absorption heat pump, can be used as corrosion inhibitors.
Preferably one or more surfactants from the group consisting of nonionic surfactants, zwitterionic surfactants and cationic surfactants are used as additive which promotes wetting.
Suitable nonionic surfactants are alkylamine alkoxylates, amidoamines, alkanolamides, alkylphosphine oxides, alkyl-N-glucamides, alkyl glucosides, bile acids, alkyl alkoxylates, sorbitan esters, sorbitan ester ethoxylates, fatty alcohols, fatty acid ethoxylates, ester ethoxylates and polyethersiloxanes.
Suitable zwitterionic surfactants are betaines, alkylglycines, sultains, amphopropionates, amphoacetates, tertiary amine oxides and silicobetaines.
Suitable cationic surfactants are quaternary ammonium salts having one or two substituents having from 8 to 20 carbon atoms, in particular corresponding tetraalkylammonium salts, alkylpyridinium salts, ester quats, diamidoamine quats, imidazolinium quats, alkoxyalkyl quats, benzyl quats and silicone quats.
In a preferred embodiment, the additive which promotes wetting comprises one or more nonionic surfactants of the general formula R(OCH2CHR′)mOH where m is from 4 to 40 and R is an alkyl radical having from 8 to 20 carbon atoms, an alkylaryl radical having from 8 to 20 carbon atoms or a polypropylene oxide radical having from 3 to 40 propylene oxide units and R′ is methyl or preferably hydrogen.
In a further preferred embodiment, the additive which promotes wetting comprises a polyether-polysiloxane copolymer containing more than 10% by weight of [Si(CH3)2O] units and more than 10% by weight of [CH2CHR—O] units, where R is hydrogen or methyl. Particular preference is given to polyether-polysiloxane copolymers of the general formulae (VI) to (VIII):
(CH3)3Si—O—[SiR1(CH3)—O]n—Si(CH3)3 (VI)
R2O-Ap-[B-A]m-Aq-R2 (VII)
R2O-[A-Z]p—[B—Si(CH3)2—Z—O-A-Z]m—B—Si (CH3)2[Z—O-A]qO1-q/R2 (VIII)
where
A is a divalent radical of the formula —[CH2CHR3—O]r—,
B is a divalent radical of the formula —[Si(CH3)2—O]s—,
Z is a divalent linear or branched alkylene radical having from 2 to 20 carbon atoms, preferably —(CH2)3—,
n is from 1 to 30,
m is from 2 to 100,
p, q are each 0 or 1,
r is from 2 to 100,
s is from 2 to 100,
from 1 to 5 of the radicals R1 are radicals of the general formula —Z—O-A-R2 and the remaining radicals R1 are each methyl,
R2 is hydrogen or an aliphatic or olefinic alkyl radical or acyl radical having from 1 to 20 carbon atoms and
R3 is hydrogen or methyl.
The additives which promote wetting are known to those skilled in the art from the prior art as additives for aqueous solutions and can be prepared by methods known from the prior art.
In a preferred embodiment of the absorption heat pump of the invention, the refrigerant-containing vapour phase and the sorption medium-containing liquid phase are separated from one another by a semipermeable membrane in the absorber and/or desorber, with the semipermeable membrane being permeable to the refrigerant and impermeable to the sorption medium.
The semipermeable membrane is preferably a solution diffusion membrane. A solution diffusion membrane has virtually no pores. For a solution diffusion membrane the selective permeability of the membrane for the refrigerant is due to the refrigerant dissolving in the material of the membrane and diffusing through the membrane, while the sorption medium is insoluble in the material of the membrane. The suitability of a solution diffusion membrane for the absorption heat pump of the invention can therefore be determined by a person skilled in the art by simple tests on the solubility of refrigerant and sorption medium in the material of the membrane.
For the embodiment using water as refrigerant, it is possible to use any pore-free membrane, known to those skilled in the art from the technical fields of dialysis, reverse osmosis and pervaporation as being suitable for the removal of salts from aqueous salt solutions, as solution diffusion membrane.
The material used for the solution diffusion membrane is preferably a hydrophilic or hydrophilically functionalised polymer containing polyvinyl alcohol, polyimide, polybenzimidazole, polybenzimidazolone, polyamide hydrazide, cellulose ester, cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose butyrate, cellulose nitrate, polyurea, polyfuran, polyethylene glycol, poly(octylmethylsiloxane), polysiloxane, polyalkylsiloxane, polydialkylsiloxane, polyester-polyether block copolymer, polysulphone, sulphonated polysulphone, polyamide, in particular aromatic polyamide, polyether, polyether ether ketone, polyester, polyether-urea composite, polyamide-urea composite, polyether sulphone, polycarbonate, polymethyl methacrylate, polyacrylic acid or polyacrylonitrile. It is likewise possible to use mixtures or copolymers of two or more of these polymers. Particular preference is given to solution diffusion membranes composed of cellulose acetate, crosslinked polyethylene glycol, crosslinked polydimethylsiloxane or a polyester-polyether block copolymer.
In a further preferred embodiment, the semipermeable membrane is a microporous membrane. For the purposes of the invention, microporous membranes are membranes which have pores extending through the membrane, the pores having a minimum diameter in the range from 0.3 nm to 100 μm. The membrane preferably has pores in the range from 0.3 nm to 0.1 μm.
Preference is given to using a microporous membrane which is not wetted by the working medium composed of sorption medium and refrigerant. Here, the term wetting refers to a contact angle between working medium and microporous membrane of less than 90 degrees, which leads to penetration of working medium into pores of the membrane as a result of capillary forces. The contact angle between working medium and microporous membrane is preferably greater than 120 degrees, particularly preferably greater than 140 degrees. The use of a nonwetting microporous membrane can also prevent flow of the liquid working medium through the pores of the membrane to the vapour side of the membrane in the case of a pressure on the side of the liquid working medium which is higher than that on the vapour side. The suitability of a microporous membrane for the absorption heat pump of the invention can therefore be determined by a person skilled in the art by determining the contact angle between the working medium and the membrane.
For the embodiment using water as refrigerant, preference is given to using a hydrophobic microporous membrane as semipermeable membrane. Suitable hydrophobic microporous membranes are known to a skilled person in the technical field of functional clothing as membranes which are watertight and permeable to water vapour.
Preference is given to using hydrophobic microporous membranes composed of polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride or fluoroalkyl-modified polymers. It is likewise possible to use mixtures or copolymers of two or more of these polymers. Further suitable membranes are inorganic hydrophobic microporous membranes or composite membranes comprising an inorganic hydrophobic microporous material, for example membranes whose pores are formed by silicalite or hydrophobised silica.
The semipermeable membrane is preferably arranged on a porous support layer. Arrangement on a porous support layer makes it possible to achieve a mechanically stable membrane unit even when using a thin semipermeable membrane. This allows more rapid mass transfer through the membrane and thus a smaller and more compact construction of the absorber. The support layer is preferably arranged on the side of the semipermeable membrane adjoining the vapour phase. Such an arrangement of the support layer leads to a lower resistance to mass transfer than arrangement of the support layer on the side of the membrane facing the liquid working medium.
The porous support layer can comprise either inorganic or organic materials. The membrane is preferably arranged on a porous support layer composed of a hydrophobic polymer, in particular a polyolefin, a polyester or polyvinylidene fluoride. The support layer can additionally contain reinforcements, e.g. by means of layers of fabric.
In a preferred embodiment, the semipermeable membrane is arranged in the form of hollow fibres. The embodiment of the membrane in the form of hollow fibres allows a particularly compact construction of absorber and/or desorber and operation with a higher pressure difference between the vapour phase and the liquid phase.
The absorption heat pump of the invention preferably has a two-stage or multistage, particularly preferably two-stage, construction, as described, for example, for absorption refrigeration machines in F. Ziegler, R. Kahn, F. Summerer, G. Alefeld “Multi-Effect absorption chillers”, Rev. Int. Froid 16 (1993) 301-311.
The absorption heat pump of the invention preferably has an additional heat exchanger by means of which heat is exchanged between the refrigerant-depleted working medium which is fed from the desorber to the absorber and the refrigerant-rich working medium which is fed from the absorber to the desorber. A countercurrent heat exchanger is particularly preferably used for this purpose.
In a preferred embodiment, at least one of the system components absorber, desorber, condenser and evaporator has a wall surface made of a polymeric material via which heat is exchanged with the surroundings. The polymeric material is in this case preferably a polyamide, a polyimide or polyether ether ketone. As polyamide, preference is given to using polyamide 12. As polyimide, preference is given to using a polyimide of benzophenonetetracarboxylic dianhydride and a mixture of tolylene diisocyanate and methylenedi(phenyl diisocyanate), which can be obtained under the trade name P84 from Evonik Fibres. Corrosion of the heat-exchanging surfaces can be avoided by the use of a polymeric material. At the same time, a high heat transfer coefficient can be achieved by the use of a polyamide, a polyimide or polyether ether ketone, allowing a compact construction of the absorption heat pump.
The following examples illustrate the invention without limiting the subject matter of the invention.
The maximum achievable degassing range was determined for operation of an absorption refrigeration machine at a pressure in the evaporator and absorber of 10 mbar, a temperature in the absorber of 35° C., a pressure in the desorber and condenser of 70 mbar and a maximum temperature in the desorber of 80° C. For this purpose, the sorption medium was mixed with various amounts of water, the vapour pressure was measured in each case and in this way the content of water was determined at which the mixture with sorption medium had a vapour pressure of 10 mbar at 35° C. or a vapour pressure of 70 mbar at 80° C. respectively. The mixture having a vapour pressure at 35° C. of 10 mbar corresponds to the most refrigerant-rich working medium which can be used in operation of the absorption refrigeration machine. The most refrigerant-depleted working medium which can be used in operation of the absorption refrigeration machine is the mixture having a vapour pressure of 70 mbar at 80° C. The degassing range is calculated as the difference in the mass fraction of the refrigerant water between the most refrigerant-rich working medium and the most refrigerant-depleted working medium. The results are shown in Table 1.
The data in Table 1 show that, according to the invention, a higher degassing range is achieved with a sorption medium of an organic salt and a lithium salt having the same anion if the anion is not a halide ion. An improved degassing range is also achieved compared to the industrially used sorption medium lithium bromide.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10160434.6 | Apr 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/056104 | 4/18/2011 | WO | 00 | 10/16/2012 |
