Lubricants comprising esters of pyromellitic acid for refrigeration systems

Abstract

The working fluid contains a lubricant and a refrigerant. The lubricant may include a tetraalkyl ester of pyromellitic acid (1, 2, 4, 5-pyromellitic acid), and the refrigerant may include a hydrofluoroolefin. A method of lubricating a compressor of a refrigeration system includes forming a working fluid in the refrigeration system. The working fluid contains a lubricant and a refrigerant. The lubricant includes a tetraalkyl ester of pyromellitic acid, and the refrigerant may include a hydrofluoroolefin.

Description

Background Art

[0001] Exemplary embodiments relate to working fluids for compressors of cooling systems and find particular application in combination with working fluids comprising a lubricant containing a tetraalkyl ester of pyromellitic acid and a hydrofluoroolefin-based refrigerant.

[0002] Refrigeration systems are widely used to cool air in household and commercial refrigerators, automobiles, refrigerated transport vehicles, heat pumps, and air conditioners. Such systems typically include a compressor that pressurizes the gaseous refrigerant before it enters the condenser. The compressor is lubricated by a lubricant that should be compatible with the refrigerant being used. Because chlorofluorocarbon refrigerants (CFCs) may damage the ozone layer, refrigerants with low or no chlorine content, such as hydrofluoroolefin refrigerants (HFOs), are being considered as alternatives. HFOs consist only of hydrogen, fluorine, and carbon atoms, and contain at least one double bond between the carbon atoms.

[0003] Conventional lubricants for compressors, such as those based on polyol esters (POE), often do not provide the miscibility / solubility properties required to enable these new refrigerants to perform satisfactorily and meet the system performance requirements set by hardware manufacturers. Aromatic esters have been considered as potential components of lubricants that can be used with hydrofluorocarbon-based refrigerants. For example, U.S. Publication No. 20200318023A1, entitled "AROMATIC ESTER LUBRICANTFOR USE WITH LOW GLOBAL WARMING POTENTIAL REFRIGERANTS," published on October 8, 2020 by Bujouves et al., describes a working fluid for a refrigeration system comprising a refrigerant and a lubricant comprising at least one aromatic ester and a polyol ester.

[0004] Another gallic acid-based aromatic ester is described in U.S. Publication No. 20150307762A1, published on October 29, 2015 by Saito et al., entitled “REFRIGERATING MACHINE OIL AND WORKING FLUID COMPOSITION FOR REFRIGERATING MACHINE.”

[0005] JP5546726B2, published on July 9, 2014 and entitled “REFRIGERATOR OIL AND WORKING FLUID COMPOSITION FOR REFRIGERATOR,” describes a working fluid composition for a refrigerator, which contains an ester of an aromatic carboxylic acid selected from phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, and pyromellitic acid, an aliphatic diol having 2 to 15 carbon atoms, and 2,3,3,3-tetrafluoropropylene.

[0006] GB2216541A, entitled "WORKING FLUID / LUBRICANT COMBINATION," published on October 11, 1989, describes a working fluid / lubricant combination for a mechanical vapor recompression heat transfer device. The working fluid comprises a hydrofluorocarbon, a hydrochlorofluorocarbon, or a chlorofluorocarbon, and the lubricant comprises an ester having a molecular weight greater than 250, such as tetrabutyl pyromellitate.

[0007] WO200174977A2 published on October 11, 2001 and entitled "LUBRICANT AND FLUSHING COMPOSITIONS" describes a lubricant composition comprising an ester derivable from the reaction of an aromatic monocarboxylic acid with a monovalent aliphatic alcohol having 1 to 15 carbon atoms.

[0008] EP 0 461 435, entitled "APPLICATION OF AROMATIC CARBOXYLIC ESTERS AS LUBRICANTIN REFRIGERANT COMPRESSORS," describes esters prepared from aromatic carboxylic acids and monohydric alcohols for use as lubricants in refrigerant compressors operating with chlorine-free, partially fluorinated hydrocarbons as refrigerants. The esters can be derived from aromatic carboxylic acids, such as trimellitic acid and pyromellitic acid, and linear or branched primary monohydric alcohols having from 4 to 20 carbon atoms. Summary of the Invention

[0009] According to one aspect of the exemplary embodiment, a working fluid comprises a lubricant and a refrigerant. The lubricant comprises a tetraalkyl ester of pyromellitic acid.

[0010] In the working fluid, the tetraalkyl ester of pyromellitic acid may be at least 80 wt%, at least 90 wt%, or at least 95 wt%, or at least 98 wt% of the lubricant.

[0011] The alkyl group in the tetraalkyl ester of pyromellitic acid can be selected from the group consisting of straight and branched chain alkyl groups containing at least 5 carbon atoms, and mixtures thereof. Each alkyl group in the alkyl group can be selected from the group consisting of straight and branched chain alkyl groups containing at least 6 carbon atoms, or at least 8 carbon atoms, or at most 14 carbon atoms, or at most 12 carbon atoms, or at most 10 carbon atoms.

[0012] In the working fluid, the lubricant can be greater than 10 wt%, or not greater than 5 wt%, or not greater than 2 wt%, or not greater than 1 wt%, or not greater than 0.1 wt%, or 0 wt% of polyol esters, polyol ethers, and mixtures thereof.

[0013] The working fluid may contain a total of no more than 10 wt%, or no more than 1 wt%, or no more than 0.5 wt%, or at least 0.005 wt%, or at least 0.01 wt%, or at least 0.1 wt% of at least one additive selected from the group consisting of corrosion inhibitors, foam inhibitors, lubricity additives, surfactants, and combinations thereof.

[0014] The ratio of fluorine atoms to chlorine atoms in the refrigerant may be at least 1:1, or at least 2:1, or at least 3:1, or at least 4:1, or at least 5:1, or at least 6:1, or at least 10:1, or at least 99:1.

[0015] The refrigerant in the working fluid of any preceding claim may comprise at least one hydrofluoroolefin.

[0016] In the working fluid, the at least one hydrofluoroolefin may be selected from the group consisting of: 2,3,3,3-tetrafluoropropene; 1,3,3,3-tetrafluoropropene; 3,3,3-trifluoropropene; 1,2,3,3,3-pentafluoropropene; 1,1,1,4,4,4-hexafluorobut-2-ene; 1,1,1,4,4,4-hexafluorobut-2-ene; 1,1,1,4,4,5,5,5-octafluoropent-2-ene; and mixtures thereof.

[0017] The at least one hydrofluoroolefin may be at least 80 weight percent, or at least 90 weight percent, or at least 95 weight percent, or 100 weight percent of all halogenated hydrocarbons in the working fluid.

[0018] The combination of the tetraalkyl ester of pyromellitic acid and the hydrofluoroolefin may be at least 90 weight percent, or at least 95 weight percent, or up to 100 weight percent of the working fluid.

[0019] In the working fluid, a weight ratio of the tetraalkyl ester of pyromellitic acid to the refrigerant in the working fluid may be 1:99 to 99:1.

[0020] The working fluid as described in any embodiment herein may be used in a refrigeration system comprising a compressor and an evaporator.

[0021] In another aspect of the exemplary embodiment, a method of lubricating a compressor of a refrigeration system includes forming a working fluid in the refrigeration system, the working fluid comprising a lubricant and a refrigerant, the lubricant comprising a tetraalkyl ester of pyromellitic acid and the refrigerant comprising a hydrofluoroolefin.

[0022] In the method, forming the working fluid includes supplying a lubricant to a compressor of the refrigeration system, the lubricant mixing with the refrigerant in the compressor to form the working fluid.

[0023] In the method in a compressor, the lubricant may be present in the working fluid at a maximum concentration of at least 1 wt%, or at least 5 wt%, or at least 10 wt%, or at least 15 wt%, or at least 20 wt%, or up to 25 wt%.

[0024] In the method, the four alkyl groups of the tetraalkyl ester of pyromellitic acid can be selected from linear and branched alkyl groups containing at least 5 carbon atoms, and mixtures thereof. Each alkyl group in the alkyl group is independently selected from linear and branched alkyl groups containing at least 6 carbon atoms, or at least 8 carbon atoms, or at least 9 carbon atoms, or at most 14 carbon atoms, or at most 12 carbon atoms, or at most 10 carbon atoms.

[0025] In another aspect of the exemplary embodiment, a method of improving the solubility of a hydrofluoroolefin refrigerant in a working fluid includes supplying a lubricant comprising a tetraalkyl ester of pyromellitic acid to the working fluid.

[0026] In another aspect of the exemplary embodiment, a method of reducing the occurrence of lubricant foam in a hydrofluoroolefin refrigerant includes supplying a lubricant comprising a tetraalkyl ester of pyromellitic acid to the hydrofluoroolefin refrigerant.

[0027] In this method, foaming may be reduced by improving the release of refrigerant vapor from the lubricant. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a block diagram of a refrigeration system according to one aspect of the exemplary embodiment;

[0029] Figure 2 is a graph of kinematic viscosity versus temperature for a working fluid containing tetra-pyromellitic acid according to an exemplary embodiment; and

[0030] Figure 3 is a graph of kinematic viscosity versus temperature for working fluids containing comparative polyol ester / trimellitate blends. DETAILED DESCRIPTION

[0031] Aspects of the exemplary embodiments are directed to lubricants comprising tetraalkyl esters of pyromellitic acid (1,2,4,5-pyromellitic acid) (or its dianhydride), to working fluids comprising a refrigerant and the exemplary lubricant, and to methods of lubricating a refrigeration system.

[0032] The exemplary lubricants tend to have improved miscibility with low or no chlorine refrigerants (LCRs), such as HFO refrigerants, and may provide improved wear and lower foaming characteristics compared to existing lubricants. However, the exemplary lubricants are not limited to use with LCRs.

[0033] As used herein, the term "refrigeration system" generally refers to any system that employs a refrigerant to provide cooling and / or heating, or any component or portion of such a system. Such refrigeration systems include, for example, air conditioners, refrigerators, chillers, heat pumps, organic Rankine cycle systems, and the like. Exemplary lubricants are particularly suitable for use in compression refrigeration systems, such as air conditioning systems, heat pumps, or organic Rankine cycle systems, in which a refrigerant circulates. The refrigerant is a fluid used in the refrigeration cycle of a refrigeration system. It typically undergoes repeated phase changes from liquid to gas and back again. A lubricant is combined with the refrigerant to form a working fluid. The weight ratio of lubricant to refrigerant in the working fluid typically varies throughout the refrigeration cycle, with the maximum lubricant:refrigerant weight ratio being at least 1:99, or at least 5:95, or at least 10:90, or at most 40:60, or at most 30:70, except in the compressor itself, where the lubricant:refrigerant ratio can be higher, such as at most 99:1 or higher.

[0034] lubricant

[0035] The tetraalkyl esters of pyromellitic acid (which may also be formed from pyromellitic anhydride) used in the lubricants herein may have the general formula (I):

[0036]

[0037] where R 1 、R 2 、R 3 and R 4 Each of the C5 to C 14 an alkyl group, which may be branched or unbranched;

[0038] R 5 is a C1 to C5 hydrocarbon group; and

[0039] n is 0 to 2.

[0040] R 1 、R 2 、R 3 and R 4They may be the same or different. In one embodiment, they are the same.

[0041] In some embodiments, R 1 、R 2 、R 3 and R 4 One or more (or all) of R is a C6 or higher, or C7 or higher alkyl group. In some embodiments, R 1 、R 2 、R 3 and R 4 One or more of them is C 12 or lower, or C 10 or lower alkyl groups. In one embodiment, each alkyl group is selected from straight chain and branched chain alkyl groups having 5 to 14 carbon atoms, and mixtures thereof. In one embodiment, the alkyl group is selected from straight chain and branched chain alkyl groups having 6 to 10 carbon atoms, and mixtures thereof. In one embodiment, the alkyl group is selected from straight chain and branched chain alkyl groups having 8 to 10 carbon atoms, and mixtures thereof.

[0042] Suitable for use as R 5 Examples of hydrocarbyl groups include C1 to C5 alkyl groups and C2 to C5 alkenyl groups, which can be straight or branched. In some embodiments, the hydrocarbyl group can include heteroatoms and heteroatom substituents (such as halo (particularly chlorine and fluorine), hydroxyl, alkoxy, sulfhydryl, alkylthiol, nitro, nitroso and sulfoxyl groups) that do not change the main hydrocarbon properties of the substituent in the context of the present invention. Representative alkyl and alkenyl groups include methyl, ethyl, propyl, butyl, butyl, butyl and amyl, their unsaturated equivalents, etc.

[0043] In one embodiment, n is 0, ie, R 5 Does not exist.

[0044] Examples of tetraalkyl esters of pyromellitic acid are those wherein R 1 、R 2 、R 3 and R 4 independently selected from a linear or branched C7 alkyl group, a linear or branched C8 alkyl group, a linear or branched C9 alkyl group, and a linear or branched C10 alkyl group, wherein R 1 、R 2 、R 3 and R 4 Can be the same or different.

[0045] An example of an ester of pyromellitic acid is tetrakis(2-ethylhexyl)benzene-1,2,4,5-tetracarboxylate ("pyromellitic acid ester"), which has formula (II):

[0046]

[0047] The tetraalkyl ester of pyromellitic acid may be the sole component of the lubricant (aside from impurities). In other embodiments, the lubricant may further comprise one or more other lubricating oils and / or one or more additives. However, care should be taken when adding other lubricating oils and / or additives so that the desired properties of the tetraalkyl ester when used with the refrigerant are not unduly impaired.

[0048] Examples of other lubricating oils that can be used in lubricants include polyol esters, polyol ethers, polyalphaolefins, polyalkylene glycols, hydrocarbon oils, or mixtures thereof. Specific examples of polyol esters and polyol ethers that can be used in lubricants include (i) aromatic esters comprising the reaction product of an aromatic hydrocarbon having at least one carboxyl functional group with a (mono) alkyl alcohol and / or glycol ether; (ii) polyol ester oils, wherein the polyol ester oils comprise a polyol esterified with at least one (mono) carboxylic acid having at least 5 carbon atoms; (iii) polyols esterified with a mixture of (mono) carboxylic acids or their anhydrides, wherein the (mono) carboxylic acids or anhydrides each have 5 to 13 carbon atoms; and mixtures thereof. Suitable polyols include trimethylolpropane, dipentaerythritol, neopentyl glycol, monopentaerythritol, polypentaerythritol, and combinations thereof. In some embodiments, the polyol ester may comprise an ester and / or complex ester of an aromatic polycarboxylic acid or its anhydride. Complex esters can be composed of oligomeric units of polyols (such as trimethylolpropane, dipentaerythritol, neopentyl glycol, monopentaerythritol and / or polypentaerythritol) and polyacids or anhydrides (such as succinic acid, glutaric acid, adipic acid, citric acid, trimellitic acid and / or pyromellitic acid). Complex esters can be fully or partially end-capped with functional (mono)carboxylic acids or (mono)alkyl alcohols or mono-end-capped glycol ethers or mixtures thereof.

[0049] In one embodiment, the lubricant contains no more than 10% by weight, or no more than 5% by weight, or no more than 2% by weight, or no more than 1% by weight, or no more than 0.1% by weight of polyol esters and polyol ethers in total. In one embodiment, the lubricant is free of or substantially free of polyol ester oil, wherein the polyol ester oil comprises a polyol esterified with at least one (mono)carboxylic acid having at least 5 carbon atoms. In one embodiment, the lubricant contains no more than 5% by weight, or no more than 1% by weight, or no more than 0.1% by weight of polyol ester oil, or is free of polyol ester oil.

[0050] The polyol esters and / or polyol ethers, when present, may have a neat viscosity of at least 4 cSt or at most 400 cSt, as measured according to ASTM D445-21 at 40° C. In other embodiments, the neat viscosity may be at least 5 cSt, or at least 10 cSt, or at least 30 cSt, or at least 100 cSt, or at least 170 cSt, or at least 200 cSt, or at most 350 cSt, or at most 200 cSt, or at most 170 cSt, as measured according to ASTM D445-21 at 40° C. Example ranges as measured according to ASTM D445-21 at 40° C. include 200 cSt to 400 cSt, 200 cSt to 350 cSt, 170 cSt to 200 cSt, 100 cSt to 170 cSt, 32 cSt to 120 cSt, 46 cSt to 68 cSt, or 5 cSt to 30 cSt.

[0051] Exemplary hydrocarbon oils include C9 to C 16 Alkanes and mixtures thereof, for example petroleum distillates, such as mineral oil, vegetable oil and mixtures thereof. When present, the hydrocarbon oil may comprise no more than 10 wt %, or no more than 5 wt %, or no more than 2 wt %, or no more than 1 wt %, or no more than 0.1 wt % of the lubricant.

[0052] In one embodiment, the lubricant contains no more than 10 wt%, or no more than 5 wt%, or no more than 2 wt%, or no more than 1 wt%, or no more than 0.1 wt% total of lubricating oils other than tetraalkyl esters of pyromellitic acid or anhydride.

[0053] The lubricant (and / or refrigerant) may further comprise one or more additional additives selected from antioxidants, corrosion inhibitors, antiwear agents, extreme pressure (EP) additives, friction modifiers, foam inhibitors, viscosity modifiers, tackifiers, lubricity additives, surfactants, and combinations thereof. In one embodiment, the additional additives total at least 0.005% or at most 0.5% by weight of the lubricant (and / or refrigerant).

[0054] Suitable antioxidants include butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), phenyl-a-naphthylamine (PANA), octylated / butylated diphenylamine, high molecular weight phenolic antioxidants, hindered bisphenolic antioxidants, di-α-tocopherol, di-tert-butylphenol, and mixtures thereof.

[0055] In some embodiments, the antioxidant includes one or more of the following: (i) hexamethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), CAS Reg. No. 35074-77-2, commercially available from BASF; (ii) N-phenylaniline, reaction product with 2,4,4-trimethylpentene, CAS Reg. No. 68411-46-1, commercially available from BASF; (iii) phenyl-a-naphthylamine and / or phenyl-b-naphthylamine, such as N-phenyl-ar-(1,1,3,3-tetramethylbutyl)-1-naphthylamine, commercially available from BASF; (iv) tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate] )] methane, CAS Reg. No. 6683-19-8; (v) thiodiethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), CAS Reg. No. 41484-35-9, which is also listed as thiodiethylenebis(3,5-di-tert-butyl-4-hydroxy-hydro-cinnamate); (vi) butylated hydroxytoluene (BHT); (vii) butylated hydroxyanisole (BHA); (viii) bis(4-(1,1,3,3-tetramethylbutyl)phenyl)amine, commercially available from BASF; (ix) 3,5-bis(1,1-dimethylethyl)-4-hydroxy-phenylpropionic acid thiodi-2,1-ethanediyl ester, commercially available from BASF.

[0056] The antioxidant may be present in the lubricant at 0.01 wt % to 6.0 wt % or 0.02 wt % to 1 wt %.

[0057] Suitable resists include (i) triazoles or substituted triazoles such as tolyltriazole (5-methyl-1H-benzotriazole); N,N-bis(2-ethylhexyl)-ar-methyl-1H-benzotriazole-1-methylamine, CAS Reg. No. 94270-86-70, marketed by BASF under the trade name Irgamet TM (ii) fatty acids derived from animal and / or vegetable sources and / or hydrogenated forms of such fatty acids, such as Neo-Fat commercially available from Akzo Nobel Chemicals, Ltd. TM (iii) N-methyl-N-(1-oxo-9-octadecenyl)glycine, CAS Reg. No. 110-25-8; (iv) mono- and di-isooctyl phosphate, reacted with a tertiary alkyl group and a (C12 to C14) primary amine, CAS Reg. No. 68187-67-7; (v) dodecanoic acid; (vi) triphenylphosphorothioate, CAS Reg. No. 597-82-0; and

[0070] (v) mono- and di-hexyl phosphate, reacted with tetramethylnonylamine and C 11-14 Alkylamine compounds.

[0058] A useful additive is an N-acyl derivative of sarcosine, such as an N-acyl derivative of sarcosine. An example is N-methyl-N-(1-oxo-9-octadecenyl)glycine. This derivative is available under the trade name SARKOSYL TM O is available from BASF. Another additive is an imidazoline, such as Amine O available from Ciba-Geigy. TM .

[0059] When used, the resist may be present in the lubricant at a concentration of at least 0.01 wt%, or at least 0.02 wt%, or at most 6.0 wt%, or at most 0.1 wt%, or at most 0.05 wt%.

[0060] To inhibit wear on the metal surfaces of the compressor, the lubricant may contain one or more of an antiwear agent, an extreme pressure (EP) additive, and a friction modifier. In some cases, a compound or combination may provide two or more of these functions. Antiwear agents are polar additives that adhere to the rubbing metal surfaces. When metal-to-metal contact occurs under conditions of mixed and boundary lubrication, they chemically react with the metal surface and are activated by the heat of contact to form a film that minimizes wear. EP additives chemically react with the metal (iron) surface to form a sacrificial surface film that reduces the likelihood of welding and seizure of opposing surface micro-asperities formed by metal-to-metal contact. They are activated under high loads and by the high contact temperatures generated. Example EP additives are sulfur compounds, phosphorus compounds, and boron compounds. Friction modifiers are typically used to modify the friction between moving parts and can operate at lower loads that are not activated by contact temperatures. One product that can provide antiwear, EP, reduced friction, and corrosion inhibition is a phosphamidon salt, such as Irgalube TM 349, which is commercially available from BASF. Exemplary antiwear / EP inhibitors / friction modifiers are phosphorus compounds such as triphenylthiophosphate (TPPT), which can be Irgalube TM TPPT is available from BASF; tricresyl phosphate (TCP), which can be purchased from Kronitex TM TCP was purchased from Chemtura; and tert-butylphenyl phosphate, which can be Syn-O-Ad TM 8478 was purchased from ICL Industrial Products.

[0061] Antiwear agents, EP additives, and friction modifiers may comprise 0.1 wt% to 4 wt% of the lubricant and may be used alone or in combination.

[0062] In some embodiments, the lubricant comprises a viscosity modifier and / or a tackifier. Example viscosity modifiers include esters of ethylene vinyl acetate, polybutene, polyisobutylene, polymethacrylate, olefin copolymers, styrene maleic anhydride copolymers, hydrogenated styrene-diene copolymers, hydrogenated star polyisoprene, alkylated polystyrene, fumed silica, and complex esters. Example tackifiers include natural rubber dissolved in oil. Adding a viscosity modifier and / or a tackifier can provide adhesion and improve the viscosity and viscosity index of the lubricant. Some applications and environmental conditions may require an additional sticky surface film to protect equipment from corrosion and wear. When used, the viscosity modifier and / or tackifier can be at least 0.01% by weight of the lubricant, or at least 0.05% by weight, or at least 0.1% by weight, or at most 10% by weight, or at most 5% by weight of the lubricant. Example viscosity modifiers / tackifiers include Functional V-584. TM Natural rubber available from Functional Products, Inc., Macedonia, Ohio; and CG 5000 TM Complex esters available from Inolex Chemical Co., Philadelphia, Pa., also serve as pour point depressants.

[0063] In some embodiments, chlorine-containing halocarbons can provide lubricity to the working fluid.These refrigerants can chemically react with metals in refrigeration systems to form a protective surface film composed of metal chlorides.

[0064] Example surfactants include fluorosurfactants such as those sold by 3M under the trade name Novec TM Those sold by FC-4434.

[0065] Exemplary foam modifiers include dimethyl polycyclohexane, polyacrylates, and mixtures thereof.

[0066] Exemplary demulsifiers include condensation polymeric alcohols, fatty acid esters, fatty alcohols alkoxylated with alkylene oxides, and mixtures thereof.

[0067] Working fluid

[0068] The working fluid comprises a tetraalkyl ester of pyromellitic acid or pyromellitic dianhydride, a refrigerant, and optionally other components of a lubricating composition. At least within the compressor of the refrigeration system, the weight ratio of at least one of the tetraalkyl ester of pyromellitic acid and the tetraalkyl ester of pyromellitic anhydride to the refrigerant in the working fluid may be 1:99 to 99:1, or at least 5:95, or at least 10:90.

[0069] The refrigerant is one or more compounds capable of remaining in a fluid state at a temperature in the range of at least -30°C to 20°C and capable of undergoing repeated phase changes from liquid to gas and back to liquid. The refrigerant is a refrigerant that is substantially miscible with the tetraalkyl ester of pyromellitic acid and / or dianhydride.

[0070] The refrigerant in the working fluid may comprise at least one halocarbon compound ("halocarbon"). As used herein, a halocarbon may include any carbon-containing compound having one or more carbon atoms bonded to one or more halogens. In one embodiment, the halocarbon in the refrigerant may comprise at least one of a hydrofluorocarbon, a hydrochlorocarbon, a hydrochlorofluorocarbon, and a chlorofluorocarbon, or a mixture thereof. In some embodiments, the halocarbon in the refrigerant may comprise at least one of a hydrofluoroolefin, a hydrochloroolefin, a hydrochlorofluoroolefin, a chlorofluoroolefin, or a mixture thereof.

[0071] As used herein, a low (or no) chlorine refrigerant (LCR) contains no more than 2 atomic % chlorine, or no more than 1 atomic % chlorine. A chlorine-free refrigerant contains no more than 0.01 atomic % chlorine (ie, any chlorine present is derived from impurities).

[0072] In one embodiment, the halocarbon component of the refrigerant is an LCR or a mixture of LCRs. In another embodiment, the halocarbon component of the refrigerant is primarily, but not entirely, an LCR (at least 50% by weight LCR, or at least 60% by weight LCR, or at least 70% by weight LCR, or at least 80% by weight LCR, or at least 90% by weight LCR, or at most 99% by weight LCR). In other words, the ratio of fluorine atoms to chlorine atoms in the refrigerant can be at least 1:1, or at least 2:1, or at least 3:1, or at least 4:1, or at least 5:1, or at least 6:1, or at least 10:1, or at most 99:1.

[0073] Suitable halogenated hydrocarbons for LCRs include hydrofluorocarbons, particularly HFOs (hydrofluoroolefins). HFOs consist solely of hydrogen, fluorine, and carbon atoms, but contain at least one double bond between the carbon atoms. Example HFOs include propylene-based HFOs, such as 2,3,3,3-tetrafluoropropene (chemical formula CH2=CFCF3), and which have the designation HFO 1234yf (or R-1234yf), with the optional additional designation E or Z for the trans or cis isomer, and are marketed by Chemours as Opteon TM YF sold and sold by Honeywell as Solstice TMYF; 1,3,3,3-tetrafluoropropene (chemical formula CHF=CHCF3), which may also be called 1,1,1,3-tetrafluoropropene and which has the name HFO-1234ze (or R-1234ze), with the optional additional designation E or Z for the trans or cis isomer; 3,3,3-trifluoropropene (chemical formula CH2=CHCF3) with the name HFO-1243zf (or R1243zf), 1,2,3,3,3-pentafluoropropene (chemical formula HFC=C(F)CF3) with the name HFO-1225ye, with the optional additional designation E or Z for the trans or cis isomer Isomers, with optional additional designation E or Z; butene-based HFOs such as 1,1,1,4,4,4-hexafluorobut-2-ene (HFO-1336mzz, with optional additional designation E or Z for the trans or cis isomer) and 1,1,1,4,4,4-hexafluorobut-2-ene (HFO-1336mzz or R-1336mzz, with optional additional designation E or Z for the trans or cis isomer); and pentene-based HFOs such as 1,1,1,4,4,5,5,5-octafluoropent-2-ene (HFO-1438mzz). Mixtures of HFOs can be used in refrigerants.

[0074] In one embodiment, the hydrofluoroolefins comprise at least 80 weight percent, or at least 90 weight percent, or at least 95 weight percent, or 100 weight percent of all halogenated hydrocarbons in the refrigerant.

[0075] In some embodiments, the refrigerant may comprise at least one HFO and at least one hydrochloroolefin (HCO). HCO is composed solely of hydrogen, chlorine, and carbon atoms, but contains at least one double bond between the carbon atoms. In one aspect of this embodiment, the ratio of HFO:HCO in the refrigerant is at least 1:1, or at least 2:1, or at least 2.5:1, or at most 99:1. Examples of HCOs include 1,2-dichloroethylene (R-1130, with the optional additional designation E or Z for the trans or cis isomer). An exemplary HFO:HCO mixture is designated R-514A, which is an azeotropic olefin blend comprising 74.7% cis-1,1,1,4,4,4-hexafluorobut-2-ene and 25.3% trans-1,2-dichloroethylene (R-1130(E)), and is marketed by Chemours under the trade name Opteon. TM XP30 for sale.

[0076] In some embodiments, the refrigerant may comprise at least one HFO and at least one hydrochlorofluoroolefin (HCFO). HCFOs are composed solely of hydrogen, chlorine, fluorine, and carbon atoms, but contain at least one double bond between the carbon atoms. In one aspect of this embodiment, the ratio of HFO to HCFO in the refrigerant is at least 2:1, or at least 3:1, or at most 99:1 (or higher). Examples of HCFOs include 1-chloro-3,3,3-trifluoropropene (HCFO-1233zd or R-1233zd, with the optional additional designation E or Z for the trans or cis isomer).

[0077] The refrigerant may also contain one or more saturated halocarbon compounds (saturated halocarbons), such as hydrofluorocarbons and / or hydrochlorocarbons. Exemplary saturated halocarbons include trifluoromethane (R-23), difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1,2-tetrafluoroethane (R-134a), 1,1,1-trifluoroethane (R-143a), 1,1-difluoroethane (R-152a), 1,2-difluoroethane, 1,1,1,2,3,3,3-heptafluoropropane (R-227ea), ...-tetrafluoroethane (R-134a), 1,1,1,2-trifluoroethane (R-143a), 1,1,1,2- , 1,1,3,3,3-hexafluoropropane (R-236fa), 1,1,1,3,3-pentafluoropropane (R-245fa), dichloromethane, trichlorofluoromethane, bromochlorodifluoromethane, dichlorodifluoromethane, chlorotrifluoromethane, trifluoroiodomethane, 1,1,2-trichloro-1,2,2-trifluoroethane, chloropentafluoroethane, 1-chloro-1,1-difluoroethane, octafluorocyclobutane and mixtures thereof.

[0078] When present, the halocarbon compounds may total at least 0.01 wt %, or at least 0.05 wt %, or at least 0.1 wt %, or at least 1 wt %, or at least 2 wt %, or at least 3 wt %, or up to 90 wt %, or up to 50 wt %, or up to 20 wt %, or up to 10 wt %, or up to 5 wt % of the refrigerant.

[0079] Refrigerant can also comprise one or more halogen-free organic compounds (organic compounds that do not comprise halogen atoms).Suitable organic compound is fluid at the operating temperature of refrigeration system, and can be selected from paraffin, cycloalkane, synthetic paraffin, alkylbenzene, polyalphaolefin, polyalkylene glycol, polyol ester, polyvinyl ether and their mixture from the oil, silicone oil, natural origin of alkane, alkene, alcohol, glycol, ether, glycol ether, mineral origin.Example alkane comprises C2 to C8 alkane, such as propane, butane, pentane, hexane or their mixture.Example alkene comprises C2 to C8 alkene, such as propylene, butylene, pentene, hexene or their mixture.Example alcohol comprises C2 to C8 alcohol, such as ethanol, propyl alcohol, butanol, amyl alcohol, hexanol or their mixture.Example ether comprises C2 to C8 ether, such as diethyl ether and ethylene glycol monobutyl ether (DGME).

[0080] When present, the non-halogen containing organic compounds may total at least 0.01 wt %, or at least 0.05 wt %, or at least 0.1 wt %, or at least 1 wt %, or at least 2 wt %, or at least 3 wt %, or at most 90 wt %, or at most 50 wt %, or at most 20 wt %, or at most 10 wt %, or at most 5 wt % of the refrigerant.

[0081] In some embodiments, the refrigerant may comprise carbon dioxide.

[0082] In some embodiments, the refrigerant may comprise a hydrofluoroolefin (HFO) and carbon dioxide. For example, refrigerant R463A is a mixture of hydrofluorocarbons, hydrocarbons, and carbon dioxide.

[0083] The working fluid may also contain one or more additives that do not fall into the above categories. Example additives include nanoparticles, stabilizers, surfactants, tracers, fluorescent agents, odorants, and solubilizers.

[0084] In some embodiments, the combination of the tetraalkyl ester of pyromellitic acid and the hydrofluoroolefin may be at least 90 weight percent, or at least 95 weight percent, or up to 100 weight percent of the working fluid.

[0085] Refrigerants useful herein may have a low global warming potential (GWP). GWP is the amount of heat absorbed by any greenhouse gas in the atmosphere. The GWP value is calculated as the multiple of the amount of heat that the same mass of carbon dioxide (CO2) would absorb, where the GWP value of carbon dioxide is defined as 1. Example refrigerants and their mixtures may have a GWP of less than 50, or less than 10, or less than 5. For example, HFO-1234yf has a GWP of less than 1. In contrast, conventional HFC refrigerants, such as R-410A and R-404A, have GWP values ​​approaching 2,000 and 4,000, respectively.

[0086] The exemplary lubricant is miscible with the selected hydrofluorocarbon refrigerant or refrigerant blend at the operating temperature of the compression refrigeration system. The miscibility of the lubricant with the refrigerant at the operating temperature ensures that the lubricant entering the refrigeration system from the compressor can be transported back to the compressor through the evaporation orifices and heat transfer equipment where it serves as a lubricant, and that the immiscible lubricant portion does not exist in the system as a blockage that restricts the movement of the refrigerant through the system. It also ensures that there is a minimal lubricating oil film on the heat transfer equipment where it can interfere with the heat transfer efficiency by acting as an insulating film. Although the necessary degree of miscibility of the lubricant and refrigerant may vary depending on the application in the embodiment, the desired range is -20°C to 80°C, or -10°C to 70°C, with a lubricant:refrigerant weight ratio of 5:95, or 10:90, and / or 20:80.

[0087] The working fluid comprising the lubricant and the refrigerant may have an operating viscosity at 323 K of at least 40 centistokes (cSt = mm2) at 3 bar. 2 / s), or at least 8 cSt at 7 bar, or at least 8 cSt at 10 bar, or at least 3 cSt at 20 bar. The working fluid is capable of withstanding a range of temperatures, such as the high temperatures in the discharge area of ​​the compressor.

[0088] Unless otherwise indicated, the amount of each chemical component described does not include any solvent or diluent oil, which may be customarily present in the commercial material, i.e., on an active chemical basis. However, unless otherwise indicated, each chemical or composition referred to herein should be interpreted as being a commercial grade material which may contain the isomers, by-products, derivatives, and other such materials which are normally understood to be present in the commercial grade.

[0089] It is known that some of the above substances may interact in the final formulation so that the components of the final formulation may be different from those initially added. For example, metal ions (e.g., metal ions of detergents) may migrate to other acidic or anionic sites of other molecules. The products thus formed, including those formed when the lubricant / working fluid is used in its intended use, may not be easy to describe. However, all such modifications and reaction products are included within the scope of the present invention; the present invention includes compositions prepared by mixing the above components.

[0090] Method for preparing lubricant

[0091] In one embodiment, the method of preparing a lubricant comprises (i) reacting pyromellitic acid and / or pyromellitic dianhydride with a sufficient amount of a branched or unbranched monohydric alkyl alcohol having 5 to 14 carbon atoms to form a tetraalkyl ester, and optionally (ii) combining the tetraalkyl ester of pyromellitic acid and / or pyromellitic dianhydride with one or more lubricating oils (i.e., different from the reaction product of step (i)) and / or one or more additives as described above.

[0092] The esterification can be carried out by heating pyromellitic acid with an alkyl alcohol at a temperature of about 200° C. to 250° C. under reflux. The alkyl alcohol may be present in a stoichiometric excess, such as an excess of about 10% or more. If desired, an esterification catalyst, such as sulfuric acid, may be used. Water produced during the reaction and any residual alcohol are removed.

[0093] Pyromellitic acid is widely commercially available from Sigma-Aldrich and the like.

[0094] Refrigeration system

[0095] like Figure 1As shown, in the exemplary refrigeration system 10, refrigerant is compressed by a compressor 12 and directed from the compressor's outlet 14 in a compressed gaseous state via a first fluid line 16 to a condenser 18, where the compressed refrigerant gas is liquefied by cooling. The liquefied refrigerant then passes through a second fluid line 20 to an expansion device 22, such as a valve, where the refrigerant's pressure is reduced, thereby lowering its temperature. The refrigerant then enters an evaporator 24, where the liquefied refrigerant is used to remove heat from the surrounding atmosphere and thereby return to a gaseous state. A third fluid line 26 carries the gaseous refrigerant back to the compressor via a compressor inlet 28. A lubricant inlet 30 supplies lubricant from a reservoir 32 to the moving parts of the compressor 12. While in the compressor, the refrigerant picks up some lubricant, forming a working fluid that carries some lubricant toward the condenser, a process known as entrapment. Generally, at least some lubricant is separated from the compressed refrigerant. For example, a receiver 34 collects lubricant from the first fluid line 16 between the compressor and the condenser. The collected lubricant may be returned to the lubricant inlet 30 of the compressor via a return line 36. Thus, the concentration of lubricant in the working fluid, other than within the compressor itself, is highest at a location 38 in the first fluid line 16 adjacent the outlet 14. At this location, the lubricant:refrigerant ratio may be at least 1:99, or at least 5:95, or at least 10:90, or at most 40:60, or at most 30:70.

[0096] In another embodiment, a method of lubricating a compressor is disclosed. The method may include supplying a working fluid to the compressor, the working fluid comprising (a) a lubricant comprising (i) pyromellitic acid or at least one tetraester of pyromellitic dianhydride, and (b) a refrigerant.

[0097] In one embodiment, components (a) and (b) are introduced separately into the compressor, for example by introducing component (a) into the compressor as the refrigerant passes through the compressor.

[0098] Also disclosed is a method of improving the operating viscosity of a refrigerant used in a refrigeration system. The method may include adding a lubricant as described herein to the refrigerant.

[0099] Generally speaking, the methods, systems and compositions of the present invention can be adapted for use in conjunction with various heat transfer systems, particularly refrigeration systems, such as air conditioning (including stationary and mobile air conditioning systems), refrigeration, heat pump systems, and the like.

[0100] Blends of trimellitate esters with POE have previously been shown to have reduced miscibility and solubility in HFO refrigerants. However, such mixtures tend to result in poor foaming properties and moderate wear characteristics. In contrast, tetraalkyl esters derived from pyromellitic acid or anhydride exhibit improved miscibility with HFO refrigerants, as well as improved wear resistance and foaming properties, compared to POE and trimellitate / POE blends.

[0101] Without intending to limit the scope of the exemplary embodiments, the following examples demonstrate the advantages of tetraalkyl esters derived from pyromellitic acid or its anhydride.

[0102] Example

[0103] Blending of tetra(2-ethylhexyl)benzene-1,2,4,5-tetracarboxylate with polyol esters and polyol ester / trimellitate Comparison of Things

[0104] Tetrakis(2-ethylhexyl)benzene-1,2,4,5-tetracarboxylate ("pyromellitic acid ester") was obtained from Zimmer Schwarz.

[0105] Triisodecyltridecyl trimellitate is available as TruVis TM TM2200 was obtained from Teknor Apex Company.

[0106] ISO VG 220 synthetic polyol ester (POE) lubricant formulated for use in refrigeration and air conditioning compressors using HFC refrigerants (Emkarate TM RL220H) was obtained from CPI Fluid Engineering. RL220H POE had a kinematic viscosity of 215 cSt at 40°C as determined according to ASTM D445-21.

[0107] Second polyol ester TM RL170H) was also obtained from CPI Fluid Engineering. RL170H POE had a kinematic viscosity of 170 cSt at 40°C as determined according to ASTM D445-21.

[0108] The first refrigerant, R-1234ze(E) (trans-1,1,1,3-tetrafluoropropene), is manufactured by Honeywell and commercially available from Aspen Refrigerants.

[0109] The second refrigerant, R-514A, an azeotropic olefin blend comprising 74.7% cis-1,1,1,4,4,4-hexafluoro-RLbut-2-ene and 25.3% trans-1,2-dichloroethylene (R-1130(E)), is manufactured by Chemours and commercially available from Aspen Refrigerants.

[0110] The lubricant compositions evaluated are shown in Table 1. These compositions contained no additives.

[0111] Table 1: Lubricant (wt%)

[0112]

[0113] The physical properties of the neat lubricants were determined and are shown in Table 2, including foaming tendency and wear characteristics.

[0114] Table 2: Test results

[0115]

[0116]

[0117] For reference, lower blister tendencies and four-ball wear scar diameters are generally more suitable for this application.Example A (pyromellitic acid ester, no trimellitic acid ester) performed very well in both tests.

[0118] Table 3 shows the miscibility of the lubricant tetra(2-ethylhexyl)benzene-1,2,4,5-tetracarboxylate with refrigerant R-1234ze(E) and refrigerant R-514A under various conditions. In bench tests, miscibility was determined by combining the lubricant and refrigerant in a tube, sealing the tube under vacuum, stirring the tube, and observing the mixture at different temperatures. In Table 3, 1P represents a single phase, HZ represents hazy (translucent), and CL represents turbid (opaque). A single phase and hazy are considered miscible, while turbidity is considered immiscible.

[0119] Table 3: Miscibility results

[0120]

[0121] The one phase (IP) results indicate that the lubricant is miscible in the refrigerant. Cloudiness (CL) indicates that there is a lower miscibility. Ideally, miscibility is good at the temperature of operation in the evaporator, but miscibility is poor at lower temperatures. The results for Example A are particularly good when the lubricant is about 20% by weight of the working fluid, especially at the low temperatures expected in refrigeration systems. The data in Tables 2 and 3 show that Example A has an overall better foaming tendency than Examples B and C, while having comparable or improved solubility compared to Examples B, C, and D.

[0122] The working fluid is tested using a pressure, viscosity and temperature ("PVT") apparatus. The PVT apparatus exposes the working fluid to various temperatures and pressures and provides solubility and Daniel curves. The procedures for using a PVT apparatus and generating solubility and Daniel curves are known in the art and can generally be summarized as follows. The working fluid is gravity-fed into the reservoir of the PVT apparatus. The temperature and pressure of the reservoir are varied and controlled by transducers. Once the fluid is loaded, a pump circulates the fluid through various measurement stages where various fluid properties such as liquid density, solubility, circulating mass flow rate and liquid viscosity (ASTM D7483-21) as well as evaporation will change. The PVT apparatus may also have an observation window to allow the user to observe the working fluid during the test. The test conditions are controlled and data is recorded throughout the test with the help of software. The software then uses the recorded data to generate solubility and Daniel curves. Additional information on PVT equipment testing can be found in Christopher J. Seeton and Pedrag Hrnjak, "Thermophysical Properties of CO2-Lubricant Mixtures and Their Affect on 2-Phase Flow in Small Channels (Less than 1 mm)", International Conference on Refrigeration and Air Conditioning, Paper 774, pp. 1-8 (2006).

[0123] The Daniel curve shows the effect of different refrigerant concentrations on lubricant viscosity at various temperatures and pressures. Figure 2

[0045] These are the viscosity and vapor pressure Denier curves for a working fluid containing a pyromellitic acid ester (Example A) in an HFO refrigerant (R-1234ze). Figure 3

[0045]

[0046] Are the viscosity and vapor pressure Denier curves for a working fluid comprising a polyol ester / trimellitate blend (Example B) and the same HFO refrigerant (R-1234ze). Figure 2 and Figure 3A comparison of the results shows that the working fluid with pyromellitic acid ester (Example A) has a higher kinematic viscosity than the working fluid with the polyol ester / trimellitate blend (Example B). Based on these curves, it can be expected that pyromellitic acid ester is a more effective lubricant.

[0124] Figure 2 and Figure 3 The PVT curve shown in and similar curves for Examples C and D were used to determine the dilution and working viscosity under the conditions of interest.

[0125] Table 4 shows the data obtained from the curve, where Dil. (%) is the dilution percentage and WV is the working viscosity. Two sets of conditions are shown. Condition 1 corresponds to a pressure of 12.4 bar and a temperature of 66°C, while Condition 2 corresponds to a pressure of 4.5 bar and 40°C.

[0126] Table 4: PVT Dilution and Working Viscosity under Conditions of Interest

[0127]

[0128]

[0129] The results show that at lower dilutions, equal or higher working viscosities can be achieved with the pyromellitic acid ester (Example A).

[0130] It will be appreciated that under the operating conditions of the compressor in the refrigeration system, the results may be different.

[0131] Each file mentioned above is incorporated herein by reference.Unless in the examples or otherwise clearly indicated, all numerical values ​​for the amount of substance, reaction conditions, molecular weight, number of carbon atoms, etc., in this specification sheet should be understood to be modified by the word "about".Unless otherwise indicated, each chemical or composition mentioned herein should be interpreted as commercial grade material, which may contain isomers, by-products, derivatives, and other such materials generally understood to be present in commercial grade. However, unless otherwise indicated, the amount of each chemical component does not include any solvent or diluent oil, which may be present in commercial material generally. It should be understood that the upper and lower limits of amount, scope, and ratio as herein described may be independently combined.Similarly, the scope and amount of each element of the present invention may be used together with the scope or amount of any other element.

[0132] It will be appreciated that variations of the features and functions disclosed above and others, or alternatives thereof, may be combined into many other different systems or applications, wherein various currently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art and are intended to be encompassed by the appended claims.

Claims

1. A working fluid comprising a lubricant and a refrigerant, wherein the lubricant comprises a tetraalkyl ester of pyromellitic acid.

2. The working fluid of claim 1, wherein the tetraalkyl ester of pyromellitic acid is at least 80 wt%, at least 90 wt%, or at least 95 wt%, or at least 98 wt% of the lubricant.

3. The working fluid of claim 1 or 2, wherein each alkyl group in the tetraalkyl ester of pyromellitic acid is selected from linear and branched alkyl groups containing at least 5 carbon atoms, and mixtures thereof.

4. A working fluid according to any preceding claim, wherein at least some of the alkyl groups in the tetraalkyl ester of pyromellitic acid are selected from linear and branched alkyl groups containing at least 6 carbon atoms, or at least 8 carbon atoms, or up to 14 carbon atoms, or up to 12 carbon atoms, or up to 10 carbon atoms.

5. A working fluid according to any preceding claim, wherein the lubricant comprises no more than 10 wt%, or no more than 5 wt%, or no more than 2 wt%, or no more than 1 wt%, or no more than 0.1 wt%, or 0 wt% of polyol esters, polyol ethers, and mixtures thereof.

6. The working fluid of any preceding claim, further comprising a total of no more than 10 wt%, or no more than 1 wt%, or no more than 0.5 wt%, or at least 0.005 wt%, or at least 0.01 wt%, or at least 0.1 wt% of at least one additive selected from the group consisting of corrosion inhibitors, foam inhibitors, lubricity additives, surfactants, and combinations thereof.

7. A working fluid according to any preceding claim, wherein the ratio of fluorine atoms to chlorine atoms in the refrigerant is at least 1:1, or at least 2:1, or at least 3:1, or at least 4:1, or at least 5:1, or at least 6:1, or at least 10:1, or at least 99:

1.

8. A working fluid according to any preceding claim, wherein the refrigerant comprises at least one hydrofluoroolefin.

9. The working fluid of claim 8, wherein the at least one hydrofluoroolefin is selected from the group consisting of: 2,3,3,3-tetrafluoropropene; 1,3,3,3-tetrafluoropropene; 3,3,3-trifluoropropene; 1,2,3,3,3-pentafluoropropene; 1,1,1,4,4,4-hexafluorobut-2-ene; 1,1,1,4,4,4-hexafluorobut-2-ene; 1,1,1,4,4,5,5,5-octafluoropent-2-ene; and mixtures thereof.

10. The working fluid of claim 8 or 9, wherein the at least one hydrofluoroolefin is at least 80 wt%, or at least 90 wt%, or at least 95 wt%, or 100 wt% of all halogenated hydrocarbons in the working fluid.

11. The working fluid of any one of claims 8 to 10, wherein the tetraalkyl ester of pyromellitic acid and the hydrofluoroolefin together comprise at least 90 wt%, or at least 95 wt%, or at most 100 wt% of the working fluid.

12. A working fluid according to any preceding claim, wherein the weight ratio of the tetraalkyl ester of pyromellitic acid to the refrigerant in the working fluid is from 1:99 to 99:

1.

13. Use of a working fluid according to any preceding claim in a refrigeration system comprising a compressor and an evaporator.

14. A method of lubricating a compressor of a refrigeration system, the method comprising forming a working fluid in the refrigeration system, the working fluid comprising a lubricant and a refrigerant, the lubricant comprising a tetraalkyl ester of pyromellitic acid, and the refrigerant comprising a hydrofluoroolefin.

15. The method of claim 14, wherein forming the working fluid comprises supplying the lubricant to a compressor of the refrigeration system, the lubricant mixing with the refrigerant in the compressor to form the working fluid.

16. A method according to claim 14 or claim 15, wherein in the compressor, the lubricant reaches a maximum concentration of at least 1 weight %, or at least 5 weight %, or at least 10 weight %, or at least 15 weight %, or at least 20 weight %, or at most 25 weight % in the working fluid.

17. The process according to any one of claims 14 to 16, wherein the four alkyl groups of the tetraalkyl ester of pyromellitic acid are selected from linear and branched alkyl groups containing at least 5 carbon atoms, and mixtures thereof.

18. The method of any one of claims 14 to 17, wherein each alkyl group in the alkyl group is independently selected from straight chain and branched chain alkyl groups comprising at least 6 carbon atoms, or at least 8 carbon atoms, or at least 9 carbon atoms, or up to 14 carbon atoms, or up to 12 carbon atoms, or up to 10 carbon atoms.

19. A method of improving the solubility of a hydrofluoroolefin refrigerant in a working fluid, the method comprising supplying a lubricant to the working fluid, the lubricant comprising a tetraalkyl ester of pyromellitic acid.

20. A method of reducing the occurrence of lubricant foam in a hydrofluoroolefin refrigerant, the method comprising supplying a lubricant to the hydrofluoroolefin refrigerant, the lubricant comprising a tetraalkyl ester of pyromellitic acid.

21. The method of claim 20, wherein the reduction in foaming is due to improved release of refrigerant vapor from the lubricant.

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