GB2306497A - Working fluid (refrigerant) composition comprising carbon dioxide and a polyol ester - Google Patents

Abstract

A working fluid (refrigerant) composition comprises: (i) carbon dioxide; (ii) a lubricant comprising a polyester base oil having at least one polyol ester of molecular weight of greater than 250.

Description

WORKING FLUID COMPOSITIONS This invention relates to working fluid compositions comprising a refrigerant and a lubricant and to the use of such compositions in heat transfer devices, particularly heat transfer devices of the transcritical compression type.

Hitherto, heat transfer devices have tended to use fully and partially halogenated chlorofluorocarbon refrigerants such as trichlorofluoromethane (refrigerant R-ll), dichlorodifluoromethane (refrigerant R-12), chlorodifluoromethane (refrigerant R-22) and the azeotropic mixture of chlorodifluoromethane and chloropentafluoroethane (refrigerant R-115); the azeotrope being refrigerant R-502. In particular, refrigerant R-12 has found widespread use in automobile air conditioning systems.

However, the fully and partially halogenated chlorofluorocarbons have been implicated in the destruction of the earth's protective ozone layer and as a result the use and production thereof has been limited by international agreement.

Whilst heat transfer devices of the type to which the present invention relates are essentially closed systems, loss of refrigerant to the atmosphere can occur due to leakage during operation of the equipment or during maintenance procedures. It is important, therefore, to replace fully and partially halogenated chlorofluorocarbon refrigerants by materials having low or zero ozone depletion potentials.

In addition to the possibility of ozone depletion, it has been suggested that significant concentrations of chlorofluorocarbon refrigerants in the atmosphere might contribute to global warming (the so-called greenhouse effect). It is desirable, therefore, to use refrigerants which have relatively short atmospheric lifetimes as a result of their ability to react with other atmospheric constituents such as hydroxyl radicals.

Heat transfer devices of the transcritical compression type are known in the art. It would be desirable to use carbon dioxide as the refrigerant in such devices, since it has both a zero ozone depletion potential and a low global warming potential. Moreover, carbon dioxide is non-toxic, non-flammable and has good thermodynamic properties.

In a heat transfer device, the refrigerant forms part of a working fluid composition which also comprises a lubricant. The lubricant circulates around the device along with the refrigerant and provides for continual lubrication of the compressor. In addition to possessing good lubricity in the presence of the refrigerant, the properties desired of a lubricant include good hydrolytic stability and good thermal stability. Moreover, in order to provide for the return of the lubricant to the compressor, the lubricant should be compatible with the refrigerant, which in practice means that the lubricant and refrigerant should possess a degree of mutual solubility, i.e. the lubricant and the refrigerant should be at least partially soluble in one another.

The present invention provides a working fluid composition for use in heat transfer devices, particularly heat transfer devices of the transcritical compression type, which comprises a refrigerant based on carbon dioxide and a suitable lubricant.

According to the present invention there is provided a composition comprising: (1) a refrigerant consisting essentially of carbon dioxide (CO2); and (2) a lubricant comprising a polyol ester base oil comprising at least one polyol ester compound having a molecular weight greater than 250.

The refrigerant (component (1)) consists essentially of carbon dioxide (CO2) which exhibits a low temperature refrigeration action.

In stating that the refrigerant consists essentially of CO2, we are intending to exclude any refrigerant which comprises a substantial quantity of one or more further refrigerant compounds in addition to the C02. Thus, except for the possible presence of small amounts of impurities, the refrigerant component of the present composition consists solely of CO2.

The lubricant (component (2)) comprises a polyol ester base oil comprising at least one polyol ester compound having a molecular weight greater than 250. Such a base oil is conveniently obtained from the reaction of at least one polyol and at least one monocarboxylic acid or an esterifiable derivative thereof.

It will be appreciated from the above, that the polyol ester base oil may comprise a mixture of two or more polyol ester compounds having a molecular weight greater than 250. When the base oil is a mixed polyol ester composition comprising two or more polyol ester compounds, this may be prepared directly by reacting two or more polyols with one or more monocarboxylic acids (or esterifiable derivatives thereof) or by reacting one or more polyols with two or more monocarboxylic acids (or esterifiable derivatives thereof).

Alternatively, mixed polyol ester compositions may be prepared by combining two or more different polyol ester compounds each of which is the reaction product of a particular polyol and a particular monocarboxylic acid. Other ways of preparing mixed polyol ester compositions will be apparent to those skilled in the art.

A minor proportion of a polycarboxylic acid, e.g. an aliphatic dicarboxylic acid, may also be used in the synthesis of the polyol ester base oil in order to increase the viscosity of the lubricant.

Usually, the amount of the carboxylic acid(s) which is used in the synthesis of the base oil will be sufficient to esterify all of the hydroxyl groups contained in the polyol, although residual hydroxyl functionality may be acceptable.

A preferred lubricant is one comprising a neopentyl polyol ester base oil obtained from the reaction of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof. Suitable neopentyl polyols for the formation of such a base oil include pentaerythritol, polypentaerythritols such as di- and tripentaerythritol, trimethylol alkanes such as trimethylol ethane and trimethylol propane, and neopentyl glycol. The base oil may be formed from linear and/or branched aliphatic carboxylic acids, such as linear and/or branched alkanoic acids. Preferred acids are selected from the C5 8' particularly the C5 7, linear alkanoic acids and the C5 10' particularly the C5 9, branched alkanoic acids.

The lubricant may also comprise one or more of the additives which are conventionally used in refrigeration lubricants. Specific mention may be made of oxidation resistance and thermal stability improvers, corrosion inhibitors, metal deactivators, viscosity index improvers, anti-wear agents and extreme pressure resistance additives. Such additives are well known to those skilled in the art.

When the lubricant comprises one or more additives, such additives may be present in the amounts conventional in the art. Preferably, the cumulative weight of all the additives will not be more than 8 %, e.g. 5 %, of the total weight of the lubricant.

The composition of the invention will typically comprise a major amount of the CO2 refrigerant and a minor amount of the ester lubricant. Preferably, the composition of the invention will comprise from 50 to 99 % by weight, more preferably from 70 to 99 % by weight, of the refrigerant and from 1 to 50 X by weight, more preferably from 1 to 30 % by weight, of the lubricant.

The composition of the present invention may be used in a transcritical compression type heat transfer device to provide cooling, e.g. for automobile air conditioning.

Claims (1)

1. Claims:

   1. A composition comprising: (1) a refrigerant consisting essentially of carbon dioxide (CO2); and (2) a lubricant comprising a polyol ester base oil comprising at least one polyol ester compound having a molecular weight greater than 250.