Lubricant including a blend of an ester of pyromellitic acid and an ester of trimellitic acid for refrigeration systems

Abstract

A working fluid comprising a lubricant and a refrigerant, the lubricant comprising a blend of a tetra-alkyl ester of pyromellitic acid or pyromellitic anhydride and a tri-alkyl ester of trimellitic acid or trimellitic anhydride.

Description

LUBRICANT INCLUDING A BLEND OF AN ESTER OF PYROMELLITIC ACID AND AN ESTER OF TRIMELLITIC ACID FOR REFRIGERATION SYSTEMSBACKGROUND

[0001] The exemplary embodiment relates to a working fluid for a compressor of a cooling system and finds particular application in connection with a lubricant for a working fluid which includes a hydrofluoroolefin-based refrigerant.

[0002] Cooling systems are widely used for cooling air in domestic and commercial refrigerators, automobiles, refrigerated transport vehicles, heat pumps, and air conditioners. Such systems generally include a compressor for pressurizing the gaseous refrigerant before it enters a condenser. The compressor is lubricated by a lubricant, which should be compatible with the refrigerant being used. Due to the potential damage to the ozone layer by chlorofluorocarbon refrigerants (CFCs), refrigerants with low or no chlorine content, such as hydrofluoroolefin refrigerants (HFOs), are being considered as replacements. HFOs are composed solely of hydrogen, fluorine and carbon atoms and contain at least one double bond between the carbon atoms.

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

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

[0005] JP5546726B2, published July 9, 2014, entitled REFRIGERATOR OILAND 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 dihydric alcohol having 2 to 15 carbon atoms, and a 2,3,3,3-tetrafluoropropene.

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

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

[0008] EP0461435, published entitled APPLICATION OF AROMATICCARBOXYLIC ESTERS AS LUBRICANT IN REFRIGERANT COMPRESSORS, describes esters produced from aromatic carboxylic acids and monohydric alcohols as lubricants for refrigerant compressors which are operated 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 straight-chain or branched, primary monohydric alcohols having 4 to 20 carbon atoms.BRIEF DESCRIPTION

[0009] The present invention provides a working fluid which includes a lubricant and a refrigerant, wherein the lubricant is a blend of a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid. In the lubricant, each alkyl group in the tetra-alkyl ester of pyromellitic acid is independentlyselected from alkyl groups containing 4 to 14 carbon atoms and each alkyl group in the tri-alkyl ester of trimellitic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms. The alkyl groups in both the tetra-alkyl ester of pyromellitic acid and the tri-alkyl ester of trimellitic acid may be linear or branched.

[0010] The blend of components in the lubricant may contain at least 50 wt% of the tetra-alkyl ester of pyromellitic acid and at least 2 wt% or at least 5 wt% up to 50 wt% of the tri-alkyl ester trimel litic acid.

[0011] The lubricant may also include 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.

[0012] The refrigerant in the working fluid of any preceding claim may include at least one hydrofluoroolefin (“HFO”).

[0013] The working fluid as described in any of the embodiments herein may find use in a refrigeration system comprising a compressor, condenser, and an evaporator.

[0014] 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 including a lubricant and a refrigerant, the lubricant comprising a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid, the refrigerant comprising a hydrofluoroolefin.

[0015] In the method, the forming of 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.

[0016] In the method in the compressor, the lubricant may reach a maximum concentration in the working fluid 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. %.

[0017] In this method the four alkyls of the tetra-alkyl ester of pyromellitic acid may be selected from linear and branched alkyl groups comprising 4 to 14 carbon atoms, and mixtures thereof. Each of the alkyls is independently selected from linear and branched alkyl groups comprising at least 4, carbon atoms, or at least 6 carbon atoms, or at least 8 carbon atoms, or up to 14 carbon atoms, or up to 13 carbon atoms, or up to 12 carbon atoms, or up to 10 carbon atoms. In this method the three alkyls of the tri-alkyl ester of trimellitic acid may be selected from linear and branched alkyl groups comprising 4 to 14 carbon atoms, and mixtures thereof. Each of the alkyls is independently selected from linear and branched alkyl groups comprising at least 4, carbon atoms, or at least 6 carbon atoms, or at least 8 carbon atoms, or up to 14 carbon atoms, or up to 13 carbon atoms, or up to 12 carbon atoms, or up to 10 carbon atoms.

[0018] In another aspect of the exemplary embodiment, a method of improving solubility of hydrofluoroolefin refrigerant in a working fluid comprising supplying a lubricant to the working fluid, the lubricant comprising a blend of a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid.

[0019] In another aspect of the exemplary embodiment, a method of reducing the occurrence of lubricant foam in an hydrofluoroolefin refrigerant includes supplying a lubricant to the hydrofluoroolefin refrigerant, the lubricant comprising a blend of a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid.

[0020] In the method, the reduction of foam may result from improved release of refrigerant vapor from the lubricant.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIGURE 1 is a block diagram of a refrigeration system in accordance with one aspect of the exemplary embodiment.DETAILED DESCRIPTION

[0022] Aspects of the exemplary embodiment relate to a lubricant which includes a blend of tetra-alkyl ester of pyromellitic acid (1 ,2, 4, 5- benzenetetracarboxylic acid) (or its dianhydride) and a tri-alkyl ester of trimellitic acid (benzene-1 ,2,4-tricarboxylic acid) and to a working fluid which includes a refrigerant and the lubricant described herein, and to methods of lubricating a refrigeration system.

[0023] The exemplary lubricants tend to have improved miscibility with low or no chlorine refrigerants (LCRs), such as HFO refrigerants, and can provide improved wear and lower foaming properties, when compared to existing lubricants. However, the exemplary lubricants are not limited to use with LCRs.

[0024] As used herein, the term “refrigeration system” refers generally to any system, or any part or portion of such a system, which employs a refrigerant to provide cooling and / or heating. Such refrigeration systems include, for example, air conditioners, electric refrigerators, chillers, heat pumps, organic Rankine cycle systems, and the like. The exemplary lubricant finds particular use in a compression refrigeration system, such as an air conditioning system, heat pump, or an organic Rankine cycle system, in which a refrigerant is circulated. The refrigerant is a fluid which is used in the refrigeration cycle of the refrigeration system. It generally undergoes a repeated phase transition from a liquid to a gas and back again. The lubricant combines with the refrigerant to form a working fluid. A ratio by weight of the lubricant to the refrigerant in the working fluid typically varies throughout the refrigeration cycle, with a maximum weight ratio of lubricant: refrigerant of at least 1 :99, or at least 5:95, or at least 10:90, or up to 40:60, or up to 30:70, apart from in the compressor itself, where the lubricant: refrigerant ratio may be higher, such as up to 99:1 , or higher.The Lubricant

[0025] The lubricant used in the working fluid of the present invention contains a blend of a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimelletic acid.

[0026] The tetra-alkyl ester of pyromellitic acid (which may also be formed from pyromellitic anhydride) used in the lubricant herein may the general formula (I):where each of R1, R2, R3, and R4is independently an alkyl group, e.g., a C4-C14 alkyl group, which can be branched or unbranchedR5is a C1-C5 hydrocarbyl group; andN is from 0 to 2.

[0027] R1, R2, R3, and R4can be the same or different. In one embodiment, they are the same.

[0028] In some embodiments, one or more (or all) of R1, R2, R3, and R4is a C4 or higher, or a C7 or higher alkyl group. In some embodiments, one or more of R1, R2, R3, and R4is a C14 or lower, or a C10 or lower alkyl group. In one embodiment, each alkyl group is selected from linear and branched alkyl groups of 4 to 14 carbon atoms, and mixtures thereof. In one embodiment, the alkyl group is selected from linear and branched alkyl groups containing 6 to 10 carbon atoms, and mixtures thereof. In one embodiment, the alkyl groups are selected from branched alkyl groups of 8 to 10 carbon atoms or 8 carbon atoms, and mixtures thereof. In one embodiment, the alkyl groups comprise one or more linear alkyl groups having 10 or fewer carbon atoms.

[0029] In one embodiment, one or more (or all) of R1, R2, R3, and R4is derived from linear or branched alcohols having 4 to 14 carbon atoms including butyl alcohols, pentyl alcohols, hexyl alcohols, heptyl alcohols, octyl alcohols, nonyl alcohols, decyl alcohols. Example alcohols include 1 -butanol (n-butanol), 2-butanol (sec-butanol), 2-methylpropan-1 -ol (isobutanol), 2- methylpropan-2-ol (tert-butyl alcohol), pentan-1 -ol (n-pentanol), pentan-2-ol(sec-pentanol), pentan-3-ol (sec-pentanol), 2-methylbutan-1 -ol, 2- methylbutan-2-ol, 3-methylbutan-1 -ol, 3-methylbutan-2-ol, 2,2- dimethylpropan-1 -ol, hexan-1 -ol, hexan-2-ol, hexan-3-ol, 2-Methylpentan-1 - ol, 3-Methylpentan-1 -ol, 4-Methylpentan-1 -ol, 2-Methylpentan-2-ol, 3- Methylpentan-2-ol, 4-Methylpentan-2-ol, 2-Methylpentan-3-ol, 3- Methylpentan-3-ol, 2,2-Dimethylbutan-1 -ol, 2,3-Dimethylbutan-1 -ol, 3,3- Dimethylbutan-1 -ol, 2,3-Dimethylbutan-2-ol, 3,3-Dimethylbutan-2-ol, 2- Ethylbutan-1 -ol, 2-ethylhexanol, 2-propylheptanol, iso-octanol, isononyl alcohol, isodecanol, and isotridecanol. In one embodiment, one or more (or all) of R1, R2, R3, and R4is 2-ethylhexyl.

[0030] Example hydrocarbyl groups suited to use as R5include C1-C5 alkyl groups and C2-C5 alkenyl groups, which can be linear or branched. In some embodiments, the hydrocarbyl groups may include heteroatoms and heteroatom substituents which, in the context of this invention, do not alter the predominantly hydrocarbon nature of the substituent (e.g., halo (especially chloro and fluoro), hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, and sulfoxy). Representative alkyl and alkenyl groups include methyl, ethyl, propyl, butyl, butyl, butyl, and pentyl groups, unsaturated equivalents thereof, and the like.

[0031] In one embodiment, n is 0, i.e. , R5is absent.

[0032] Other example tetra-alkyl esters of pyromellitic acid are those in which R1, R2, R3, and R4are independently linear C7 alkyl groups, linear or branched C8 alkyl groups, linear or branched C9 alkyl groups, linear or branched C9 alkyl groups, wherein R1, R2, R3, and R4may be the same or different.

[0033] The tri-alkyl ester of trimel letic acid (which may also be formed from trimellitic anhydride) may have the general formula (II):

[0034] R1, R2, and R3can be the same or different. In one embodiment, they are the same.

[0035] In some embodiments, one or more (or all) of R1, R2, and R3is a C4 or higher, or a C7 or higher alkyl group. In some embodiments, one or more of R1, R2, R3, and R4is a C14 or lower, or a C10 or lower alkyl group. In one embodiment, each alkyl group is selected from linear and branched alkyl groups of 4 to 14 carbon atoms, and mixtures thereof. In one embodiment, the alkyl group is selected from linear and branched alkyl groups containing 6 to 10 carbon atoms, and mixtures thereof. In one embodiment, the alkyl groups may be independently selected from branched alkyl groups of 8 to 14 carbon atoms or 10 to 13 carbon atoms, and mixtures thereof. In one embodiment, the alkyl groups may be independently selected from linear alkyl groups having 10 or fewer carbon atoms.

[0036] In one embodiment, one or more (or all) of R1, R2, and R3is derived from linear or branched alcohols having 4 to 14 or 4 to 10 carbon atoms including butyl alcohols, pentyl alcohols, hexyl alcohols, heptyl alcohols, octyl alcohols, nonyl alcohols, decyl alcohols. Example alcohols include 1 -butanol (n-butanol), 2-butanol (sec-butanol), 2-methylpropan-1 -ol (isobutanol), 2- methylpropan-2-ol (tert-butyl alcohol), pentan-1 -ol (n-pentanol), pentan-2-ol (sec-pentanol), pentan-3-ol (sec-pentanol), 2-methylbutan-1 -ol, 2- methylbutan-2-ol, 3-methylbutan-1-ol, 3-methylbutan-2-ol, 2,2- dimethylpropan-1 -ol, hexan-1 -ol, hexan-2-ol, hexan-3-ol, 2-Methylpentan-1 - ol, 3-Methylpentan-1 -ol, 4-Methylpentan-1 -ol, 2-Methylpentan-2-ol, 3- Methylpentan-2-ol, 4-Methylpentan-2-ol, 2-Methylpentan-3-ol, 3- Methylpentan-3-ol, 2,2-Dimethylbutan-1 -ol, 2,3-Dimethylbutan-1 -ol, 3,3- Dimethylbutan-1 -ol, 2,3-Dimethylbutan-2-ol, 3,3-Dimethylbutan-2-ol, 2-Ethylbutan-1 -ol, 2-ethylhexanol, 2-propylheptanol, iso-octanol, isononyl alcohol, isodecanol, and isotridecanol. In one embodiment, R1, R2, and R3are selected from mixtures of branched alkyl groups having 8 to 10 or 10 to 13 carbon atoms. For example, in one embodiment, R1, R2, and R3may be independently isodecyl and isotridecyl groups. In another embodiment, R1, R2, and R3may all be 2-ethylhexyl groups.

[0037] The tetra-alkyl ester of pyromellitic acid and tri-alkyl ester of trimelletic acid may the only components of the lubricant (other than impurities). The blend of the tetra-alkyl ester of pyromellitic acid and tri-alkyl ester of trimelletic acid may contain at least 50 wt% of the tetra-alkyl ester of pyromellitic acid and at least 2 wt% or at least 5 wt% up to 50 wt% or up to 20 wt% or up to 30 wt% of the tri-alkyl ester trimellitic acid. In one embodiment, blend of the tetra-alkyl ester of pyromellitic acid and tri-alkyl ester of trim el I itic acid contains 30 wt% or less or 20 wt% or less of the tri-alkyl ester of trimellitic acid.

[0038] The In other embodiments, the lubricant may further include one or more other lubricant oils and / or one or more additives. However, care should be used when adding other lubricant oils and / or aditives so that the desirable properties of the blend of the tetra-alkyl ester of pyromellitic acid and tri-alkyl ester of trimellitic acid relative to its use with the refrigerant is not unduly diminished.

[0039] Examples of other lubricant oils which may be used in the lubricant include a polyol ester, a polyol ether, a polyalkylene glycol a hydrocarbon oil, or a mixture thereof. Specific examples of polyol esters and polyol ethers which may be employed in the lubricant include (i) an aromatic ester comprising the reaction product of an aromatic hydrocarbon having at least one carboxylic functional group and a (mono)alkylalcohol and / or a glycol ether; (ii) a polyolester oil, wherein the polyolester oil comprises a polyol esterified with at least one (mono)carboxylic acid that has at least 5 carbon atoms; (iii) a polyol esterified with a mixture of (mono)carboxylic acids or their anhydrides, wherein the (mono)carboxylic acids or anhydrides, individually, have 5 to 13 carbon atoms; and mixtures thereof. Suitable polyols include trimethylolpropane, dipentaerythritol, neopentylglycol, monopentaerythritol,polypentaerythritol, and combinations thereof. In some embodiments, the polyol ester may comprise esters and / or complex esters of aromatic polycarboxylic acids or their anhydrides. The complex ester may be composed of polyol oligomeric units (such as trimethylolpropane, dipentaerythritol, neopentylglycol, monopentaerythritol, and / or polypentaerythritol), and a polyacid or acid anhydride (such as succinic, glutaric, adipic, citric, trimellitic, and / or pyromellitic). The complex ester may be fully or partially capped with functional (mono)carboxylic acids or (mono)alkylalcohols or singly-capped glycol ethers, or a mixture thereof.

[0040] In one embodiment, the lubricant contains, in total, 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 polyol ester(s) and polyol ether(s). In one embodiment, the lubricant is free or substantially free of polyol ester oils, wherein the polyol ester oil comprises a polyol esterified with at least one (mono)carboxylic acid that has at least 5 carbon atoms. In one embodiment, the lubricant contains no more than 5 wt. %, or no more than 1 wt. %, or no more than 0.1 wt. % of polyol ester oils, or no polyol ester oil.

[0041] Example hydrocarbon oils include C9-C16 alkanes and mixtures theeof, e.g., petrolleum distillates, such as mineral oil, vegetable oils, and mixtures thereof. Where present, hydrocarbon oils, in total, may be 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 of the lubricant.

[0042] In one embodiment, the lubricant contains, in total, 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 lubricant oils other than the tetra-alkyl ester(s) of pyromellitic acid or anhydride.

[0043] The lubricant (and / or the refrigerant) may further include one or more additional additives selected from antioxidants, corrosion inhibitors, anti-wear agents, friction modifiers, foam inhibitors, viscosity modifiers, lubricity additives, and combinations thereof. In one embodiment the additional additive(s), in total, are at least 0.005 wt. % or up to 0.5 wt. % of the lubricant (and / or the refrigerant).

[0044] Suitable antioxidants include butylated hydroxytoluene (BHT), butylatedhydroxyanisole (BHA), phenyl-a-naphthyl amine (PANA), octylated / butylated diphenyl amine, high molecular weight phenolic antioxidants, hindered bis-phenolic antioxidant, di-alpha-tocopherol, di- tertiary butyl phenol, and mixtures thereof.

[0045] In some embodiments, the antioxidant includes one or more of: (i) hexamethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), CAS registration number 35074-77-2, available commercially from BASF; (ii) N- phenylbenzenamine, reaction products with 2,4,4-trimethylpentene, CAS registration number 6841 1 -46-1 , available commercially from BASF; (iii) phenyl-a-naphthylamine and / or phenyl-b-naphthylamine, for example N- phenyl-ar-(1 , 1 ,3,3-tetramethylbutyl)-1 -naphthalenamine, available commercially from BASF; (iv) tetrakis [methylene(3,5-di-tert-butyl-4- hydroxyhydrocinnamate)] methane, CAS registration number 6683-19-8; (v) thiodiethylenebis (3,5-di-tert-butyl-4-hydroxyhydrocinnamate), CAS registration number 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, available commercially from BASF; (ix) benzenepropanoic acid, 3,5-bis(1 ,1 -dimethylethyl)-4-hydroxy-, thiodi-2,1 - ethanediyl ester, available commercially from BASF.

[0046] The antioxidant(s) may be present in the lubricant at from 0.01 wt.% to 6.0 wt. %, or from 0.02 wt.% to 1 wt. %.

[0047] Suitable corrosion inhibitors include (i) triazoles or substituted triazoles, such as tolyltriazole (5-methyl-1 H-benzotriazole); / V, / V-bis(2- ethylhexyl)-ar-methyl-1 H-benzotriazole-1 -methanamine, CAS registration number 94270-86-70, sold commercially by BASF under the trade name Irgamet™ 39; (ii) fatty acids derived from animal and / or vegetable sources, and / or the hydrogenated forms of such fatty acids, for example Neo-Fat™ which is commercially available from Akzo Nobel Chemicals, Ltd.; (iii) / V- methyl- / V-(1 -oxo-9-octadecenyl)glycine, CAS registration number 110-25-8; (iv) dodecanoic acid; (v) Triphenyl phosphorothionate, CAS registrationnumber 597-82-0; and

[0070] (vi) phosphoric acid, mono- and dihexyl esters, compounds with tetramethylnonylamines and C11-14 alkylamines.

[0048] One useful additive is the N-acyl derivative of sarcosine, such as an N-acyl derivative of sarcosine. One example is N-methyl-N-(1 -oxo-9- octadecenyl) glycine. This derivative is available from BASF under the trade name SARKOSYL™ O. Another additive is an imidazoline such as Amine O™ commercially available from Ciba-Geigy.

[0049] The corrosion inhibitors, where used, may be present in the lubricant at a concentration of at least 0.01 wt. %, or at least 0.02 wt. %, or to 6.0 wt. %, or up to 0.1 wt.%, or up to 0.05 wt. %.

[0050] To inhibit wear on the metal surfaces of the compressor, the lubricant may include an anti-wear agent. Anti-wear agents are polar additives that attach to frictional metal surfaces. They react chemically with the metal surfaces when metal-to-metal contact occurs in conditions of mixed and boundary lubrication and are activated by the heat of contact to form a film that minimizes wear. Friction modifiers are typically used to alter the friction between moving parts and can operate at lower loads that are not activated by contact temperatures.

[0051] The anti-wear agent, and friction modifiers may be from 0.1 wt. % to 4 wt. % of the lubricant and may be used separately or in combination.

[0052] In some embodiments, the lubricant includes a viscosity modifier. Example viscosity modifiers include ethylene vinyl acetate, polybutenes, polyisobutylenes, polymethacrylates, olefin copolymers, esters of styrene maleic anhydride copolymers, hydrogenated styrene-diene copolymers, hydrogenated radial polyisoprene, alkylated polystyrene, and complex ester. The addition of a viscosity modifier improves the viscosity and viscosity index of the lubricant. When used, the viscosity modifier may be at least 0.01 wt. %, or at least 0.05 wt. % or at least 0.1 wt. % of the lubricant, or up to 10 wt. %, or up to 5 wt. % of the lubricant. Example viscosity modifiers include a complex ester available from Index Chemical Co. Philadelphia, Pa, as CG 5000™, which can also serve as a pour point depressant.The Working Fluid

[0053] The working fluid includes the blend of a tetra-alkyl ester of pyromellitic acid or pyromellitic dianhydride and a tri-alky ester of trimellitic acid or trimellitic anhydride, a refrigerant, and optionally other components of the lubricating composition. A ratio by weight lubricant blend to the refrigerant in the working fluid may be from 1 :99 to 99: 1 , or at least 5:95, or at least 10:90, at least within the compressor of a refrigerant system.

[0054] The refrigerant is a compound or compounds capable of maintaining a fluid state at temperatures over a range of at least -30 to170 °C and which is able to undergo a repeated phase transition from a liquid to a gas and back again. The refrigerant is one which is sufficiently miscible with the tetra-alkyl ester of pyromellitic acid and / or dianhydride

[0055] The refrigerant in the working fluid may comprise at least one halogenated carbon compound (a “halocarbon”). As used herein, a halocarbon can include any carbonaceous compound that has one or more carbon atoms that are bonded with one or more halogens. In one embodiment, the halocarbon in the refrigerant may comprise at least one of a hydrofluorocarbon (HFC), 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 (HFO), a chlorofluoroolefin(CFO), a hydrochloroolefin(HCO), a hydrochlorofluoroolefin, and a hydroolefin, or a mixture thereof.

[0056] As used herein, a low (or no) chlorine refrigerant (LCR) contains no more than 2 atomic percent of chlorine, or no more than 1 atomic percent of chlorine. A no chlorine refrigerant contains no more than 0.01 atomic percent of chlorine (i.e. , any chlorine present results from impurities).

[0057] 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 predominantly, but not entirely LCR (at least 50 wt. % LCR, or at least 60 wt. % LCR, or at least 70 wt. % LCR, or at least 80 wt. % LCR, or at least 90 wt. % LCR, or up to 99 wt.% LCR). Put another way, a ratio of fluorine atoms to chlorine atoms in the refrigerant may be atleast 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 up to 99: 1 .

[0058] Suitable halocarbons for use in LCRs include hydrofluorocarbons, particularly HFOs (hydrofluoroolefins). The hydrofluorolefin may be selected from 1 ,1 ,1 ,4,4,4-hexafluorobut-2-ene (mixture of isomers) (HFO-1336mzz or R-1336mzz); (Z)-1 ,1 ,1 ,4,4,4-hexafluorobut-2-ene (HFO-1336mzz-(Z) or R- 1336mzz(Z)); (E)-1 ,1 ,1 ,4,4,4-hexafluorobut-2-ene (HFO-1336mzz-(E) or R- 1336mzz(E)); (E)-1 ,2,3,3, 3-pentafluoropropene (HFO-1225ye-(E) or R- 1225ye(E)) 2,3,3,3-tetrafluoropropene (HFO-1234yf or R-1234yf); 1 , 3,3,3- tetrafluoropropene (mixture of isomers) (HFO-1234ze or R-1234ze); (Z)- 1 ,3,3,3-tetrafluoropropene (HFO-1234ze-(Z) or R-1234ze(Z)); (E)-1 , 3,3,3- tetrafluoropropene (HFO-1234ze-(E) or R-1234ze(E)); trifluoroethylene (HFO-1123 or R-1123); 3,3,3-trifluoropropene (HFO-1243zf or R-1243zf); or any mixture thereof.

[0059] In one embodiment, the hydrofluoroolefin(s) constitute at least 80 wt. %, or at least 90 wt. %, or at least 95 wt. %, or 100 wt. % of all halocarbons in the refrigerant.

[0060] In some embodiments, the refrigerant may include at least one HFO and at least one hydrochloroolefin (HCO). HCOs are composed solely of hydrogen, chlorine and carbon atoms but contain at least one double bond between the carbon atoms. In one aspect of this embodiment, a ratio of HFO: HCO in the refrigerant is at least 1 :1 , or at least 2:1 , or at least 2.5:1 , or up to 99:1. Examples of HCOs include 1 ,2-dichloroethene (R-1130, with the optional further designation of E or Z for the trans or cis isomer). One example HFO: HCO mixture is designated R-514A, which is an azeotropic olefin blend comprising 74.7% cis-1 ,1 ,1 ,4,4,4-hexafluoro-but-2-ene and 25.3% trans-1 ,2- dichloroethene (R-1130(E)), and which is sold under the tradename Opteon™ XP30 by Chemours.

[0061] In some embodiments, the refrigerant may include at least one HFO and at least one hydrochlorofluoroolefin (HCFO) or hydrofluorocarbon (HFC). HCFOs are composed solely of hydrogen, chlorine, fluorine and carbon atoms but contain at least one double bond between the carbon atoms. In one aspectof this embodiment, a ratio of HFO: HCFO in the refrigerant is at least 2:1 , or at least 3:1 , or up to 99:1 (or higher). Examples of HCFOs include 1 -chloro-3.3.3-trifluoropropene (HCFO-1233zd or R-1233zd, with the optional further designation of E or Z for the trans or cis isomer). HFCs may be selected from difluoromethane (R-32), pentafluoroethane (R-125), 1 ,1 , 1 ,2-tetrafluoroethane (R-134a), 1 ,1 -difluoroethane (HFC-152a), 1 ,1 ,2,2-tetrafluoroethane (HFC- 134), and 1 ,1 ,1 ,2,3,3, 3-heptafluoropropane (HFC-227ea), or any mixture thereof.

[0062] The refrigerant may further include one or more saturated halogenated carbon compounds (halocarbons), for example hydrofluorocarbons and / or hydrochlorocarbons. Exemplary saturated halocarbons 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), 1 ,1 ,1 ,3,3,3-hexafluroropropane(R-236fa), 1 ,1 ,1 ,3,3-pentafluoropropane (R-245fa), dichloromethane, trichlorofluoromethane, bromochlorodifluoromethane, dichlorodifluoromethane, chlorotrifluoromethane, trifluoroiodomethane, 1 ,1 ,2- trich loro-1 ,2,2-trifluoroethane, chloropentafluoroethane, 1 -chloro-1 , 1 - difluoroethane, octafluorocyclobutane, and mixtures thereof.

[0063] Where present, the halogenated carbon compounds(s) may be 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. % of the refrigerant, 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, in total.

[0064] The refrigerant may further include one or more halogen-free organic compounds (organic compounds that include no halogen atoms). Suitable organic compounds are fluids at the operating temperature of the refrigeration system and may be selected from an alkane, an alkene, an alcohol, a glycol, an ether, a glycol ether, an oil of mineral origin, a silicone oil, a paraffin of natural origin, a naphthene, a synthetic paraffin, an alkylbenzene, a polyalphaolefin, a polyalkene glycol, a polyol ester, apolyvinyl ether, and mixtures thereof. Example alkanes include C2-C8 alkanes, such as propane, a butane, a pentane, a hexane, or a mixture thereof. Example alkenes include C2-C8 alkenes, such as a propene, a butene, a pentene, a hexene, or a mixture thereof. Example alcohols include C2-C8 alcohols, such as ethyl alcohol, a propyl alcohol, a butyl alcohol, a pentyl alcohol, a hexyl alcohol, or a mixture thereof. Example ethers include C2-C8 ethers, such as diethyl ether and ethylene glycol monobutyl ether (DGME).

[0065] Where present, the halogen-free organic compound(s) may be 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. % of the refrigerant, 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, in total.

[0066] In some embodiments, the refrigerant may comprise carbon dioxide.

[0067] In some embodiments, the refrigerant may include a hydrofluoroolefin (HFO) and carbon dioxide. For example, refrigerant R463A is a mixture of hydrofluorocarbons, hydrocarbons, and carbon dioxide.

[0068] The working fluid may further include one or more additives which do not fall within the categories described above. Example additives include nanoparticles, stabilizers, surfactants, tracing agents, fluorescent agents, odorants and solubilizers.

[0069] In some embodiments, the tetra-alkyl ester of pyromellitic acid and hydrofluoroolefin, in combination, may be at least 90 wt. %, or at least 95 wt. %, or up to 100 wt. % of the working fluid.

[0070] Refrigerants useful herein may have a low Global Warming Potential (GWP). GWP is the heat absorbed by any greenhouse gas in the atmosphere. GWP values are calculated as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (CO2), which has been attributed a GWP value of 1. Example refrigerants and mixtures thereof 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. By comparison, conventional HFC refrigerants, such as R-410A and R-404A have values of nearly 2,000 and 4,000 GWP, respectively.

[0071] The exemplary lubricant is miscible with the selected hydrofluorocarbon refrigerant or refrigerant blend over the operational temperatures of the compression refrigeration system. Miscibility of the lubricant with the refrigerant over operational temperatures ensures that the lubricant that enters into the refrigeration system out of the compressor andean be carried through the heat transfer equipment, which includes the evaporator and the condenser and back to the compressor where it functions as a lubricant and that a non-miscible lubricant portion is not present as blockage in the system restricting refrigerant movement through the system. It also assures that minimal lubricant oil films exist on the heat transfer equipment where it might interfere with the efficiency of heat transfer by acting as a thermal insulating film. While the necessary extent of miscibility of the lubricant and the refrigerant may vary depending upon application in embodiment, a desired range is from -20 °C to 80 °C, or from -10°C to 70° C weight ratios of lubricant: refrigerant of 5:95, or 10:90, and / or 20:80.

[0072] The amount of each chemical component described is presented exclusive of any solvent or diluent oil, which may be customarily present in the commercial material, that is, on an active chemical basis, unless otherwise indicated. 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.

[0073] It is known that some of the materials described above may interact in the final formulation, so that the components of the final formulation may be different from those that are initially added. For instance, metal ions (of, e.g., a detergent) can migrate to other acidic or anionic sites of other molecules. The products formed thereby, including the products formed upon employing the lubricant / working fluid in its intended use, may not be susceptible of easy description. Nevertheless, all such modifications and reaction products are included within the scope of the present invention; thepresent invention encompasses the composition prepared by admixing the components described above.Method of Preparing the Lubricant

[0074] The tetra-alkyl ester of pyromellitic acid or pyromellitic anhydride and the tri-alkyl ester of trimellitic acid or trimellitic anhydride may be prepared by methods now known to those skilled in the art or hereafter discovered. In general, the lubricant components may prepared by (i) reacting pyromellitic acid and / or pyromellitic dianhydride or trimellitic acid and / or trimellitic anhydride with a branched or unbranched monohydric alkyl alcohol having from 4 to 14 carbon atoms, in a sufficient amount to form the tetra-alkyl ester or tri-alkyl ester respectively, and optionally (ii) combining the tetra-alkyl ester of pyromellitic acid and / or pyromellitic dianhydride with the tri-alkyl ester of trimellitic acid or trimellitic anhydride, and / or one or more other lubricants or additives as described above.

[0075] The esterification can be performed by heating the pyromelltic acid (or anhydride) or the trimellitic acid (or anhydride) with the alkyl alcohol at a temperature of about 200 to 250 °C, under reflux. The alky alcohol may be in excess over the stoichiometric amount, such as an excess of about 10%, or more. An esterification catalyst, such as sulfuric acid, may be employed, if desired. Water generated in the reaction and any residual alcohol are removed.

[0076] Pyromellitic acid and trimelltic acid and the corresponding anhydrides are widely available commercially.Refrigeration System

[0077] As illustrated in FIGURE 1 , in an exemplary refrigeration system 10, a refrigerant is compressed by a compressor 12 and directed, in a compressed gaseous state, from an outlet 14 of the compressor, through a first fluid line 16, to a condenser 18, where the compressed refrigerant gas is liguified, by cooling it. The liguified refrigerant passes through a second fluid line 20 to an expansion device 22, such as a valve, where the pressure of the refrigerant is reduced, lowering its temperature. The refrigerant then enters an evaporator 24, where the liguified refrigerant serves to remove heat fromthe surrounding atmosphere and, as a result, returns 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 the lubricant from a reservoir 32 to the moving parts of the compressor 12. The refrigerant picks up some of the lubricant while in the compressor, forming the working fluid, which carries some of the lubricant towards the condenser, a process referred to as entrapment. Generally, at least some of the lubricant is separated out from the compressed refrigerant. For example, a receptacle 34 collects the lubricant from the first fluid line 16 between the compressor and condenser. The collected lubricant may be returned to the lubricant inlet 30 of the compressor via a return line 36. Accordingly, other than within the compressor itself, the concentration of the lubricant in the working fluid is at its highest at a location 38 in the first fluid line 16 adjacent the outlet 14. At this location, the ratio of lubricant : refrigerant may be at least 1 :99, or at least 5:95, or at least 10:90, or up to 40:60, or up to 30:70.

[0078] In another embodiment, a method of lubricating a compressor is disclosed. The method may include supplying to the compressor a working fluid including (a) a lubricant comprising (i) at least one tetra-ester of pyromellitic acid or pyromellitic dianhydride, and (b) a refrigerant.

[0079] In one embodiment, the components (a) and (b) are introduced separately to the compressor, for example, by introducing component (a) to the compressor while the refrigerant is passing through the compressor.

[0080] Methods of improving the working viscosity of a refrigerant for a refrigeration system are also disclosed. The method may comprise adding a lubricant, as described herein, to a refrigerant.

[0081] The present methods, systems and compositions are adaptable for use in connection with a wide variety of heat transfer systems in general and refrigeration systems in particular, such as air-conditioning (including both stationary and mobile air conditioning systems), refrigeration, heat-pump systems, and the like.

[0082] Without intending to limit the scope of the exemplary embodiments, the following examples demonstrate the advantages of the lubricantcontaining the blend of tetra-alkyl esters derived from pyromellitic acid or its anhydride and tri-alkyl ester of trimellitic acid or its anhydride.EXAMPLES

[0083] Multiple lubricant samples with combinations of pyromellitic acid tetra-ester and trimellitic acid tri-ester were prepared and evaluated for high temperature miscibility with a variety of refrigerants. Lubricant blends were characterized by measuring kinematic viscosity at 40 C (ASTM D445) and foam tendency (ASTM D892-18). Samples are summarized below (Table 1 ). Table 1 : Lubricant Compositions1Tri (isodecyl / isotridecyl) trimellitate2Pyromellitic acid, tetra-2-ethylhexylester

[0084] Lubricant blends of the present invention may also be evaluated using the same tests as set forth in Table 1 for foaming performance when combined with other lubricants, such as polyol esters.

[0085] Miscibility of lubricant and refrigerant is carried out according to ANSI / ASHRAE Standard 218-2019. This procedure is designed to determine the critical solution locus of miscible properties of a lubricant and refrigerant mixture. The oil blends from above are combined with trans-1 ,3,3,3- tetrafluoropropene (R-1234ze(E) (Table 2).Table 2: Miscibility

[0086] Lubricant blends of the present invention may also be evaluated using the same tests as set forth in Table 2 for miscibility when combined with other lubricants, such as polyol esters.

[0087] As will be appreciated under the operating conditions of a compressor in a refrigeration system, the results may differ.

[0088] Each of the documents referred to above is incorporated herein by reference. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts of materials, reaction conditions, molecular weights, number of carbon atoms, and the like, are to be understood as modified by the word “about.” 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 arenormally understood to be present in the commercial grade. However, the amount of each chemical component is presented exclusive of any solvent or diluent oil, which may be customarily present in the commercial material, unless otherwise indicated. It is to be understood that the upper and lower amount, range, and ratio limits set forth herein may be independently combined. Similarly, the ranges and amounts for each element of the invention may be used together with ranges or amounts for any of the other elements.

[0089] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

WE CLAIM:

1. A working fluid comprising a lubricant and a refrigerant, the lubricant comprising a blend of a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid.

2. The working fluid of claim 1 , wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms wherein at least one alkyl group is branched.

3. The working fluid of claims 1 or 2, wherein each alkyl group in the tetraalkyl ester of pyromellitic acid is derived from a linear or branched alcohol having 4 to 14 carbon atoms.

4. The working fluid of any preceding claim, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is a 2-ethylhexyl group.

5. The working fluid of any preceding claim, wherein each alkyl group in the tri-alkyl ester of trimellitic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms wherein at least one alkyl group is branched.

6. The working fluid of any preceding claim, wherein each alkyl group in the tri-alkyl ester of trimel litic acid is derived from a linear or branched alcohol having 4 to 14 carbon atoms.

7. The working fluid of claim any preceding claim, wherein each alkyl group in the tri-alkyl ester of trimellitic acid is a mixture of isodecyl and isotridecyl groups.

8. The working fluid of any preceding claim, wherein the lubricant contains at least 50 wt% of the tetra-alkyl ester of pyromellitic acid.

9. The working fluid of any preceding claim, wherein the lubricant contains at least 2 wt% or at least 5 wt% up to 50 wt%, or up to 30 wt% or up to 20 wt% of the tri-alkyl ester of trimellitic acid.

10. The working fluid of claim 9, wherein the lubricant contains 30 wt% or less or 20 wt% or less of the tri-alkyl ester of trimellitic acid.

11. The working fluid of any preceding claim wherein the lubricant further comprises a polyol ester.

12. The working fluid of any preceding claim, wherein the lubricant further comprises 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.

13. The working fluid of any preceding claim, wherein the refrigerant comprises at least one hydrofluoroolefin.

14. The working fluid of claim 13, 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-hexafluoro-but-2-ene; 1 ,1 ,1 ,4,4,4-hexafluoro-but-2-ene;1 ,1 ,1 ,4,4,5,5,5-octafluoropent-2-ene; and mixtures thereof.

15. The working fluid of claims 13 or 14, 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 halocarbons in the working fluid.

16. The working fluid of claims 13 to 15, wherein the refrigerant consists of a hydrofluoroolefin.

17. The working fluid of any of claims 13 to 15, wherein the refrigerant comprises at least one of a hydrofluorocarbon, a hydrochlorocarbon, a hydrochlorofluorocarbon, a chlorofluorocarbon or a mixture thereof.

18. The working fluid of claim 17, wherein the refrigerant comprises a blend of a hydrofluoroolefin and a hydrofluorocarbon.

19. The working fluid of any of claims 1 to 12, wherein the refrigerant comprises at least one of a hydrofluoroolefin, a chlorofluoroolefin, a hydrochloroolefin, a hydrochlorofluoroolefin, a hydroolefin, or a mixture thereof.

20. The use of the working fluid of any preceding claim in a refrigeration system comprising a compressor and an evaporator.21 . A method of lubricating a compressor of a refrigeration system comprising forming a working fluid of any of claims 1 to 19 in the refrigeration system.

22. A method of improving solubility of a hydrofluoroolefin refrigerant in a working fluid comprising suppling a lubricant to the working fluid wherein the lubricant a blend of a tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid.

23. The method of claim 22, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms wherein at least one alkyl group is branched.

24. The method of claim 22 or 23, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is derived from a linear or branched alcohol having 4 to 10 carbon atoms.

25. The method of claim 22 to 24, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is a 2-ethylhexyl group.

26. The method of any of claims 22 to 25, wherein each alkyl group in the trialkyl ester of trimel litic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms wherein at least one alkyl group is branched.

27. The method of any of claims 22 to 26, wherein each alkyl group in the trialkyl ester of trimellitic acid is derived from a linear or branched alcohol having 4 to 14 carbon atoms.

28. The method of any of claims 22 to 27, wherein each alkyl group in the trialkyl ester of trimellitic acid is a mixture of isodecyl and isotridecyl groups.

29. The method of any of claims 22 to 28, wherein the lubricant contains at least 50 wt% of the tetra-alkyl ester of pyromellitic acid.

30. The method of any of claims 22 to 29, wherein the lubricant contains at least 2 wt% or at least 5 wt% up to 50 wt% or up to 30 wt% or up to 20 wt% of the tri-alkyl ester of trimellitic acid.31 . The method of any of claims 22 to 30, wherein the lubricant contains 30 wt% or less or 20 wt% or less of the tri-alkyl ester of trimel litic acid.

32. The method of any of claims 22 to 31 , wherein the lubricant further comprises a polyol ester.

33. The method of any of claims 22 to 32, wherein the hydrofluoroolefin refrigerant is selected from 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-hexafluoro-but- 2-ene; 1 ,1 ,1 ,4,4,4-hexafluoro-but-2-ene; 1 ,1 ,1 ,4,4,5,5,5-octafluoropent-2-ene, or mixtures thereof.

34. A method of reducing the occurrence of lubricant foam in a hydrofluoroolefin refrigerant, comprising suppling a lubricant to the working fluidwherein the lubricant a blend of a tetra-alkyl ester of pyromellitic acid and a trialkyl ester of trimel litic acid.

35. The method of claim 34, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms wherein at least one alkyl group is branched.

36. The method of claim or 35, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is derived from a linear or branched alcohol having 4 to 10 carbon atoms.

37. The method of claim 34 to 36, wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is a 2-ethylhexyl group.

38. The method of any of claims 34 to 37, wherein each alkyl group in the trialkyl ester of trimel litic acid is independently selected from alkyl groups containing 4 to 14 carbon atoms wherein at least one alkyl group is branched.

39. The method of any of claims 34 to 38, wherein each alkyl group in the trialkyl ester of trimellitic acid is derived from a linear or branched alcohol having 4 to 10 carbon atoms.

40. The method of any of claims 34 to 39, wherein each alkyl group in the trialkyl ester of trimellitic acid is a mixture of isodecyl and isotridecyl groups.41 . The method of any of claims 34 to 40, wherein the lubricant contains at least 50 wt% of the tetra-alkyl ester of pyromellitic acid.

42. The method of any of claims 34 to 41 , wherein the lubricant contains at least 2 wt% or at least 5 wt% up to 50 wt% or up to 30 wt% or up to 20 wt% of the tri-alkyl ester of trimellitic acid.

43. The method of any of claims 34 to 42, wherein the lubricant contains 30 wt% or less or 20 wt% or less of the tri-alkyl ester of trimel litic acid.

44. The method of any of claims 34 to 43, wherein the lubricant further comprises a polyol ester.

45. The method of any of claims 34 to 44, wherein the hydrofluoroolefin refrigerant is selected from 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-hexafluoro-but- 2-ene; 1 ,1 ,1 ,4,4,4-hexafluoro-but-2-ene; 1 ,1 ,1 ,4,4,5,5,5-octafluoropent-2-ene, or mixtures thereof.

46. The method of any of claims 34 to 45, wherein the reduction of foam results from improved release of refrigerant vapor from the lubricant.

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