WO1998050499A2 - Polyalkylene glycols as lubricants for co2-based refrigerating machines - Google Patents

Abstract

Disclosed are fuel compounds containing lubricant based on polyalkylene-glycols, which are suitable for lubricating refrigerating machines, heat pumps and related equipment, for which the fuel required is carbon dioxide.

Description

Polyalkylene glycols as lubricants for CO ₂ refrigeration machines

The invention relates to equipment compositions containing lubricants based on polyalkylene glycols, which are suitable for the lubrication of refrigeration machines,

Heat pumps and related systems, such as air conditioning systems that use carbon dioxide as a refrigerant, are suitable.

Carbon dioxide has been used as a refrigeration machine resource since the very beginning of modern refrigeration technology. So the Linde company had been building for years

1881 the first compression refrigeration machine using carbon dioxide as a 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.

'' For carbon dioxide as refrigeration equipment, there is an increased demand for conventional refrigerants such as fluorine / chlorine or fluorine hydrocarbons drive pressures and operating temperatures (or Δp 's and ΔT' s) are required, which place high demands on sealing materials, moving parts and lubricants. This applies in particular to refrigeration systems that are operated in a transcritical cycle.

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

In a transcritical cycle, states are run through in the course of the compression cycle in which the carbon dioxide as refrigerant is located both in the subcritical (subcritical) area and in the supercritical (supercritical) area.

CO ₂ behaves very differently with regard to the solubility properties. Subcritical CO ₂ is polar, while CO ₂ is non-polar in the supercritical range. The polyalkylene glycols according to the invention are soluble in CO ₂ for higher mass fractions of lubricants over the entire temperature range from the critical temperature T _k to below -55 ° C. With smaller proportions of lubricants, the polyalkylene glycols are no longer or only partially soluble in carbon dioxide.

The invention is based on the problem of finding a selection of possible polyalkylene glycols which meet the following requirements for lubricants for CO ₂ cooling machines:

- excellent thermal and chemical stability

- very good hydrolytic stability

- very good lubrication properties

- completely miscible in the mixing ratios necessary for the operating machines

- 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 increased efficiency, chillers for the transcritical area are designed to be smaller and smaller often have small diameters in the pipe systems. This requires sufficient mobility of the lubricant in the corresponding temperature range.

Surprisingly, these solutions have proven to be suitable: equipment compositions for refrigeration machines, heat pumps and related systems, such as air conditioning systems, containing carbon dioxide as the refrigerant, the refrigerant preferably consisting essentially exclusively of carbon dioxide, and polyalkylene glycols or their mixtures consisting of, in each case based on the polymer chain: (a) at least 40%, preferably 60%, of the monomer units from monomer units of the type - (- CH (CH ₃ ) -CH ₂ -O -) - and / or - (- CH ₂ -CH (CH ₃ ) -O -) - and (b) to the remainder essentially from monomer units of the type - (- RO -) -, where R is a linear or branched, saturated alkylene group with 2 to 6 carbon atoms, optionally 1 to Can carry 2 further oxy groups (-O-) and can be different for each monomer unit.

The polyalkylene glycols have hydrogen, alkyl, aryl, alkylaryl, aryloxy, alkoxy, alkylaryloxy and / or hydroxyl end groups (c). The end groups of the alkyl or aryl type preferably have 1 to 8, particularly preferably 1 to 4

Carbon atoms. In the latter case, these are alkyl and / or alkoxy end groups.

For the polyalkylene glycols according to the invention, particularly good results are achieved with regard to high density and good lubricating properties on the one hand and the properties listed above on the other hand if the polyalkylene glycol as the main component, namely at least 60%, contains propylene oxide units (monomer units of type (a)).

The polyalkylene glycols according to the invention are thus either homopolymers, namely polypropylene glycol (or polypropylene oxide), or copolymers, terpolymers etc., furthermore containing monomer units of type (b). In this case, the monomer units can have a statistical distribution or a block structure. In a further embodiment of the invention, the monomer units of type (b) are at least 5% contained in the polymer chain. The

Monomer units of type (b) are preferably ethylene oxide and / or butylene oxide units. The polyalkylene glycols according to the invention can be prepared by reacting alcohols, including the polyalcohols, as starter compounds with alkylene oxides such as ethylene oxide, propylene oxide and / or butylene oxide. After the reaction, these only have a free hydroxyl group as the end group.

Polyalkylene glycols with only one hydroxyl group are preferred over those with two free hydroxyl groups.

Particularly preferred in terms of stability, hygroscopicity and compatibility are polyalkylene glycols, which e.g. after a further etherification step no longer have any free hydroxyl groups.

With regard to the molecular weights, preference is given to those polyalkylene glycols which have average molecular weights from 400 to 3000 g / mol, particularly preferably from 1000 to 2000 g / mol.

The polyalkylene glycols according to the invention are distinguished by densities of more than 940 kg / m ³ , and preferably even more than 1000 kg / m ³ , and moreover surprisingly also have the properties listed above which are suitable for use as refrigerating machine oils in combination with carbon dioxide Refrigerants are required.

Surprisingly, it was found that the return transport of the polyalkylene glycol lubricants to the compressor with less good solubility is particularly well ensured 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 polyalkylene glycol according to the invention forms the lower phase according to its higher density, and carbon dioxide forms the upper phase. A backflow of the polyalkylene glycol to the compressor is thus guaranteed with the appropriate system design. This is important for the mixing ratios of CO ₂ - refrigerant / polyalkylene glycol lubricant, for which the lubricant is no longer completely soluble in CO ₂ .

Polyalkylene glycols with a density of over 940 kg / m ³ are particularly suitable under these conditions. 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 the polyalkylene glycols according to the invention when complete solubility in the carbon dioxide refrigerant is no longer ensured.

In addition, additives customary in the equipment compositions such as wear improvers, anti-foaming agents, antioxidants such as alkylated phenolic antioxidants such as e.g. in particular Irganox® L101, viscosity index improvers, anti-corrosion agents or high-pressure additives, such as, in particular, phosphate esters.

Furthermore, the polyalkylene glycols according to the invention can be used together with neopentyl polyol esters as lubricants. Such esters are identified, for example, in German patent application DE 197 19 132 of RWE-DEA Aktiengesellschaft für Mineralöl und Chemie, filed on May 7, 1997. This ester or the ester mixture preferably consists of neopentyl polyol esters of the following type:

(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 consists of linear or branched C6 to C10 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 alcohol component preferably being linked to those carbon atoms which in turn only have 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 a further embodiment, the operating agent composition contains an ester or an ester mixture, the alcohol component of which, based on the total alcohol group concentration (the ester), consists of at least 50 mol% dipentaerythritol and / or tripentaerythritol. Esters with these alcohol groups have proven particularly useful for refrigeration machines that are operated in the transcritical range. 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. The neopentyl glycol esters according to the invention preferably have densities above 940 kg / m ³ , generally even above 1000 kg / m ³ .

According to a further embodiment of the invention, the polyalkylene glycols according to the invention can be used together with neopentyl polyol esters as lubricants, which predominantly, i.e. greater than 90%, preferably about 100%, have branched C5 to C10 acid groups, particularly preferably branched C8 to C10 acid groups. With regard to the definition of the preferred alcohol groups of these neopentyl polyol esters, reference is made to the paragraph above.

The proportion of the polyalkylene glycols according to the invention is preferably at least 20% by weight, particularly preferably at least 40% by weight, based on the proportion of lubricant (i.e. the lubricants without refrigerants and additives) in the operating agent composition. The percentage of neopentyl polyol esters as

When using lubricant mixtures of different classes of compounds, lubricant is preferably 20 to 60% by weight, particularly preferably 40 to 60% by weight, in each case based on the lubricant content in the operating agent composition.

Tests have demonstrated the effectiveness of the polyalkylene glycols according to the invention as a lubricant in CO ₂ refrigeration machines. For example, FIG. 1 shows the viscosity / vapor pressure / temperature behavior of a mixture of carbon dioxide and the polyalkylene glycol Triton GL-220 from DEA Mineraloel AG (butanol-started polypropylene oxide).

Claims

claims 1. Operating agent compositions for refrigeration machines containing carbon dioxide as refrigerant and polyalkylene glycols or mixtures thereof, the Polyalkylene glycols based on the polymer chain consist of (a) at least 40%, preferably at least 60%, of the monomer units from monomer units of the type - (- CH (CH ₃ ) -CH ₂ -O -) - and / or - (- CH ₂ -CH (CH ₃ ) -O -) - and (b) to the remaining residue from monomer units of the type - (- RO -) -, where R is a linear or branched, saturated alkyl group having 2 to 6 carbon atoms, optionally 1 to 2 carries further oxy groups (-O-) and can be different for each monomer unit. 2. Equipment composition according to claim 1, characterized in that the equipment composition contain polyalkylene glycols or mixtures thereof, (c) have the end groups which (i) hydrogen and / or hydroxyl end groups and / or (ii) alkyl, aryl, Alkylaryl, aryloxy, alkoxy, and / or alkylaryloxy End groups with 1 to 8, preferably 1 or 4 carbon atoms. 3. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contain polyalkylene glycols or mixtures thereof, the monomer units of which Type (b) make up at least 5% of the monomer units of the molecule. 4. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contain polyalkylene glycols or mixtures thereof, the monomer units of which Type (b) are ethylene oxide and / or butylene oxide units or have been produced therefrom. 5. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains polyalkylene glycols or mixtures thereof which have only one free hydroxyl group per molecule. 6. Operating fluid composition according to one of claims 1 to 4, characterized in that the operating fluid composition contains polyalkylene glycols or mixtures thereof which have no free hydroxyl group per molecule. 7. Operating agent composition according to one of the preceding claims, characterized in that the operating agent composition contains polyalkylene glycols or mixtures thereof, which have an average molecular weight of 400 to 3000 g / mol, particularly preferably from 1000 to 2000 g / mol. 8. Resource composition according to one of the preceding claims, characterized in that the resource composition further contains esters or an ester mixture, the esters being neopentyl polyol esters. 9. Operating composition according to claim 8, characterized in that the neopentyl polyol esters consist of (1) an acid component, where the acid component, (a) based on the total carboxylic acid group concentration, consists of at least 40 mol% n-pentanoic acid and (b) the remaining remainder consists of linear or branched C6 to C10 monocarboxylic acids and / or branched C5 - monocarboxylic acids, and (2) an alcohol component having 4 to 8 hydroxy groups, one, two or three quaternary carbon atoms, 5 to 21 carbon atoms and 0 to 4 Ether bonds. 10. Resource composition according to claim 8, characterized in that the neopentyl polyol esters consist of (1) a C5 to CIO acid component, the acid component, based on the total carboxylic acid group concentration, consisting of at least 90 mol% of branched carboxylic acids, and (2) an alcohol component having 4 to 8 hydroxy groups, one, two or three quaternary carbon atoms, 5 to 21 carbon atoms and 0 to 4 ether linkages. 11. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains polyalkylene glycols or polyalkylene glycol / ester mixtures, in each case according to one of the preceding claims, which have a density of over 940 kg / m ³ , preferably over 1000 kg / m ³ exhibit. 12. Operating agent composition according to one of the preceding claims, characterized in that the operating agent composition contains only polyalkylene glycols according to one of claims 1 to 7 or polyalkyl englycol / ester mixtures according to one of claims 8 to 11 as a lubricant. 13. Equipment composition according to one of the preceding claims, characterized in that the equipment is carried out in a transcritical cycle. 14. Equipment composition according to one of claims 1 to 12, characterized in that the refrigerant is operated only in a subcritical range. 15. Use of the polyalkylene glycols or polyalkylene glycol / ester Mixtures according to one of the preceding claims as lubricants in refrigeration machines which are operated with carbon dioxide.