WO1998050738A2 - Polyol-derived esters as a lubricator for co2-based refrigerating machines - Google Patents

Abstract

The invention relates to fuel compounds containing a lubricating agent derived from synthetic polyester oils with unbranched or branched carbon acid groups C5 to C10, which are suitable for lubricating refrigerating machines, heat pumps and similar equipment such as air conditioning systems, which use carbon dioxide as fuel.

Description

Polyol esters as lubricants for CO ₂ refrigeration machines

The invention relates to equipment compositions containing lubricants based on synthetic esters, which are suitable for the lubrication of refrigeration machines, heat pumps and related systems, such as air conditioning systems, which are based on carbon dioxide

Operating resources are suitable.

Carbon dioxide has been used as a refrigeration machine resource since the beginning of modern refrigeration technology. The Linde company built the first compression refrigeration machine in 1881 using carbon dioxide as the refrigerant. Until the middle of this century, carbon dioxide was mainly used in ship refrigeration systems below its critical temperature (critical point T _k = 31.4 ° C / p _k = 72.9 bar). Glycerin was used as the lubricant. Later, with the introduction of CFC refrigerants, carbon dioxide was hardly used anymore.

The phase-out of environmentally harmful chlorofluorocarbons (CFCs) as refrigeration equipment resources has led to the re-consideration of carbon dioxide as a possible equipment for refrigeration machines. Carbon dioxide is easily and cheaply available and has no harmful effect on the ozone layer. As refrigeration equipment, carbon dioxide continues to be characterized by a higher isentropic and volumetric cooling efficiency than CFC equipment.

Development work with the natural refrigerant CO ₂ was resumed only in the 1980s. Initial tests showed the technical possibilities of a transcritical, also called supercritical, carbon dioxide cycle with pressures of over 100 bar.

Various research centers have now taken up this concept and are working on the implementation of suitable refrigeration systems. At the same time, however, work is continuing on the further development of the conventional method in the subcritical area.

Compared to conventional refrigerants such as fluorine / chlorine or fluorine hydrocarbons, increased operating pressures and operating temperatures (or .DELTA.p 's and .DELTA.T' s) are required for carbon dioxide as refrigerating machine operating resources place high demands on sealing materials, moving parts and lubricants. This applies in particular to refrigeration systems that are operated in a transcritical cycle.

Polyol esters have already been used as lubricants for both procedures (see e.g. Lubricants for Carbon Dioxide; U. Hesse, H.O. Spauschus in Research Science and Technology, Proceedings, Aarhus Denmark 3-6 Sept. 1996, ISSN 0 151 163).

In a transcritical cycle, the compression cycle takes place

Go through states in which the carbon dioxide as a resource is in both the subcritical (subcritical) and supercritical (supercritical) range.

CO ₂ behaves very differently in terms of solubility towards esters. Subcritical CO ₂ is polar. For this state, the ester oils according to the invention prove to be soluble in the range of the usual working temperatures of about + 5 ° C. to -50 ° C., while many esters in the transcritical range, where CO _{2 is} largely non-polar, are no longer or only partially soluble. When working in the supercritical area, the esters are only poorly soluble in the evaporation area. The evaporation range of the carbon dioxide at the selected pressure ranges is approximately between -6 ° C and 10 ° C.

The invention is based on the problem of finding a selection of possible polyol esters which meet the following requirements for lubricants for CO ₂ -

Chillers are sufficient:

- a higher polarity

- excellent thermal and chemical stability

- very good hydrolytic stability - very good lubrication properties

- completely miscible with carbon dioxide below 0 ° C

- low hygroscopicity

- high compatibility with sealing materials and

- good viscosity - temperature behavior.

This applies in particular to the difficult conditions of the transcritical refrigeration cycle. Because of the higher pressures and the increased efficiency enz chillers are designed smaller for the transcritical area and often have small diameters in the pipe systems. This requires sufficient mobility of the lubricant in the corresponding temperature range.

Surprisingly, they have proven to be suitable for solving these tasks:

Equipment compositions for refrigeration machines, heat pumps and related systems, such as air conditioning systems, containing carbon dioxide as the operating medium, the operating medium consisting essentially exclusively of carbon dioxide, and an ester or an ester mixture as a lubricant consisting of: (1) an acid component, the acid component,

(a) based on the total carboxylic acid group concentration, consists of at least 40 mol% of n-pentanoic acid and

(b) the remainder of the residue consists of linear or branched C6 to CIO monocarboxylic acids and / or branched C5 monocarboxylic acids, and (2) an alcohol component with 4 to 8 hydroxyl groups, one, two or three quaternary carbon atoms and 5 to 21, preferably 5 to 15, carbon atoms, the -OH groups of the polyhydroxy alcohol as the alcohol component preferably being connected to those carbon atoms which in turn have only quaternary carbon atoms in the adjacent position. The polyhydroxy alcohol as the alcohol component may further contain 0 to 4, preferably 0, 1 or 2, ether bridges. The alcohol component is particularly preferred: pentaerythritol and / or dipentaerythritol and / or tripentaerythritol.

In another embodiment, the operating agent composition contains an ester or an ester mixture, the alcohol component of which, based on the total talc alcohol group concentration, consists of at least 50 mol% dipentaerythritol and / or tripentaerythritol.

With regard to the acid component, those esters or ester mixtures are also preferred, independently of one another, whose acid component according to (l) (a), based on the total carboxylic acid group concentration, consists of at least 60 mol% of n-pentanoic acid; whose acid component according to (l) (b) consists predominantly of branched monocarboxylic acids, particularly preferably of 5 to 40 mol% of branched monocarboxylic acids, based on the total carboxylic acid group concentration. In the latter case, the acid components remaining for the complete esterification of all hydroxyl groups consist of linear C5 to C10 Monocarboxylic acids. Preferably, all the hydroxyl groups of the alcohol component are completely esterified in the ester or the ester mixture.

For the esters according to the invention, particularly good results with regard to high density and good lubricating properties on the one hand and the properties listed above are achieved if n-pentanoic acid was used as the main component for the esterification. As stated above, their proportion is over 40 mol%, preferably over 60 mol%. To control the viscosity, in addition to the n-pentanoic acid, carboxylic acids in the C number range from 5 to 10 are used for the esterification, where these are branched if additional C5 acids are used. The C5 to C10 carboxylic acids can have both an unbranched and branched structure, as can be seen from the inventive examples in Table 1.

At least 50 mol% of all alcohol components are preferably the esters

Dipentaeryritol or tripentaerythritol groups. Esters with these alcohol groups have proven particularly useful for refrigeration machines operating in the transcritical field.

The esters according to the invention are characterized by densities in excess of 940 kg / m ³

Usually even over 1000 kg / m ³ , and also surprisingly also have the properties listed above, which are required for use as a refrigerating machine oil in combination with carbon dioxide as the operating medium.

It was surprisingly found that the return transport of the ester lubricants to the compressor is only guaranteed if their density is significantly higher than that of the liquid carbon dioxide in this temperature / pressure range. In such a case, an effective phase separation takes place even under the special conditions in CO ₂ refrigeration machines, in which the ester according to the invention, according to its higher density, the lower, the carbon dioxide the upper

Phase forms. A backflow of the ester to the compressor is thus guaranteed with a corresponding system design, as is trouble-free evaporation.

The evaporation temperatures of the systems under development in the supercritical range are in the range from -6 ° C to 10 ° C, one

Area of equal density of mineral oil and carbon dioxide. Mineral oils are therefore unsuitable as the sole lubricant. With a density of over 1000 kg / m ^3, esters according to the invention are not only significantly heavier than mineral oil, but also have the decisive advantage that, depending on the structure, they are completely soluble in carbon dioxide below the range from -6 ° C. to 10 ° C. to be.

Esters with a density of over 940 kg / m ³ are particularly suitable in the evaporation range of the carbon dioxide. Practical tests have shown, however, that those with higher densities, above about 1000 kg / m ^{3, worked} even better. This can be explained by a faster settling of esters according to the invention if complete solubility in the area of the evaporation of the carbon dioxide, for example at 0 ° C., has not yet been given.

The equipment essentially consists exclusively, i.e. more than 95% by weight, preferably about 100% by weight, based on the operating medium (i.e. the refrigerant without lubricant and additive content), made of carbon dioxide.

The lubricant consists either preferably of more than 90% by weight, particularly preferably about 100% by weight, of the ester according to the invention, in each case based on the lubricant without any refrigerant or additive content; or is, according to another embodiment of the invention, a mixture containing the esters according to the invention in addition to other lubricants and then preferably consists of 20 to 60% by weight of the esters according to the invention, in each case based on the lubricant without any refrigerant and additive content, the remaining one Part of the lubricant are preferably polyalkylene glycols (eg polypropylene oxide / polyethylene oxide copolymers) or alkylbenzenes or dialkylbenzenes.

In addition, additives customary in the equipment compositions, such as wear improvers, anti-foaming agents, antioxidants, such as, in particular, Irganox® L101, viscosity index improvers, corrosion inhibitors or also

High-pressure additives, such as in particular phosphate esters, may be included. Table 1

example

Alcohols mol%:

Pentaerythritol 100 66

Dipentaerythritol 100 100 100 100 34 80 80

Tripentaerythritol 20 20

Carboxylic acids mol%: n-C5 100 100 70 50 50 70 65 50 n-C8 40 n-C9 25 25 10 n-CIO 10 i-C8 5 50 5 25 50

Characteristics:

Density kg / m ³ at 15 ° C 1019 1037 1012 1003 1010 1005 1013 1004

Viscosity mmVsec: at 40 ° C 15.93 48.0 56.4 81.1 52.5 33.5 70.61 87.49 at 100 ° C 3.69 8.3 9.0 10.2 8.7 6.2 9.90 10.87

Viscosity index 119 148 138 107 143 136 123 110

Claims

claims 1. Operating agent composition containing carbon dioxide as the operating medium, the operating medium consisting essentially exclusively of carbon dioxide, and an ester or an ester mixture as a lubricant, characterized in that the ester or the ester mixture consists of (1) an acid component, the acid component, (a) based on the total carboxylic acid group concentration, consists of at least 40 mol% of n-pentanoic acid and (b) to the remaining radical consists of linear or branched C6 to CIO monocarboxylic acids and / or branched C5 monocarboxylic acids, and (2) an alcohol component having 4 to 8 hydroxyl groups, one, two or three quaternary carbon atoms, 5 to 21 carbon atoms and 0 to 4 ether linkages. 2. Operating agent composition according to claim 1, containing an ester or an ester mixture, the alcohol component of which is pentaerythritol and / or dipentaerythritol and / or tripentaerythritol. 3. Equipment composition according to one of the preceding claims, characterized in that the equipment composition contains an ester or an ester mixture, the alcohol component, based on the total alcohol group concentration, consists of at least 50 mol% of dipentaerythritol and / or tripentaerythritol. 4. Operating agent composition according to one of the preceding claims, characterized in that the operating agent composition contains an ester or an ester mixture whose acid component according to (l) (a), based on the total carboxylic acid group concentration, consists of at least 60 mol%> n-pentanoic acid. 5. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains an ester or an ester mixture whose acid component according to (l) (b) consists exclusively of branched monocarboxylic acids. 6. Operating fluid composition according to one of claims 1 to 4, characterized in that the operating fluid composition contains an ester or an ester mixture whose acid component according to (l) (b), based on the total carboxylic acid group concentration, consists of 5 to 40 mol% of branched monocarboxylic acid. 7. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains an ester or an ester mixture, the density of which is above 940 kg / m ³ , preferably above 1000 kg / m ³ . 8. Resource composition according to one of the preceding claims, characterized in that the resource composition contains an ester or an ester mixture, the hydroxyl groups of which are completely esterified. 9. Equipment composition according to one of the preceding claims, characterized in that the equipment is performed in a transcritical cycle. 10. Equipment composition according to one of claims 1 to 8, characterized in that the equipment is operated exclusively in a subcritical range. 11. Use of the esters according to one of the preceding claims in refrigeration machines which are operated with carbon dioxide.