JPH04226193A - Method of use of aromatic carboxylic acid as lubricant of refrigerating compressor - Google Patents

Abstract

According to the invention, esters of aromatic carboxylic acids and monohydric alcohols are used as lubricants for refrigerant compressors which are operated with chlorine-free, partially fluorinated hydrocarbons as refrigerant.

Description

[Detailed description of the invention]

0001

FIELD OF INDUSTRIAL APPLICATION The present invention relates to the use of aromatic carboxylic acid esters as refrigerants and as lubricants for refrigeration compressors operated with chlorine-free, partially fluorinated hydrocarbons. Regarding.

0002

BACKGROUND OF THE INVENTION Refrigeration compressors are used extensively for refrigeration cooling in industrial, commercial, and domestic equipment. The device works with a mechanical compressor, which compresses the refrigerant, condenses it with air, water or another medium by cooling in a condenser, and absorbs heat from the medium to be cooled in an evaporator. evaporate. As refrigeration agents, primarily ammonia is used in large-scale installations and chlorofluorocarbons such as dichlorodifluoromethane and chlorotrifluoromethane in large-scale installations, industrial refrigeration installations and domestic installations.

For lubrication of refrigeration compressors, mineral oils, generally based on naphthenes, are used, similar to highly refined white oils. Fully synthetic oils for refrigeration compressors include alkyl aromatic and poly-alpha-olefins.

The function of lubricating oil is to lubricate moving compressor parts, remove heat from hot compressor parts, and seal compression chambers and valves. This function also determines the properties that the lubricant must satisfy. The lubricating oil must resist heat loads and must also remain fluid at the evaporator temperature. Furthermore, it should be taken into account that the lubricating oil is conveyed from the compression chamber to the refrigeration circulation and cannot be completely removed by the subsequently connected oil separator. Lubricating oils must therefore be miscible with refrigerants over a wide temperature and concentration range;
As a result, it is ensured in the compressor that the lubricating oil that has entered the refrigeration circulation is returned.

[0005]

BACKGROUND OF THE INVENTION Chlorofluorocarbons have been suspected for some time of depleting the ozone layer of the Earth's atmosphere. Efforts are therefore being made to limit its use where it cannot be replaced. Moreover, attempts are being made to replace it with substances that have the same effect, but are harmless. In the future, chlorine-free, partially fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane, 1,1,1,2,3,3, 3-heptafluoropropane, pentafluoroethane, 1,1,1,
3,3,3-hexafluoropropane and trifluoromethane will be used. The materials are distinguished by high thermal stability and associated thermodynamic properties in the temperature range encountered during operation of refrigeration compressors.

However, these chlorine-free, aliphatic fluorohydrocarbons have the disadvantage that they exhibit very little miscibility with the lubricants used to date. Over a wide concentration range, the two substances form a two-phase mixture, so that the recovery of the lubricant carried away from the compressor is severely hampered, especially at low steam temperatures. Known lubricants can therefore be used together with chlorine-free refrigerant alternatives in installations.
Generally no longer in use.

Attempts have been made to circumvent this difficulty by using lubricants based on polyalkylene glycols. Most polyalkylene glycols are completely miscible with the above-mentioned chlorine-free lubricants between -40 and +50°C. A mixing gap appears only at temperatures above about 50° C., and the mixing gap widens over a wide concentration range as the temperature increases.

Nevertheless, it is expected that polyalkylene glycols can only be used to a limited extent as lubricants due to their strong hygroscopic properties. Drying polyalkylene glycols to <100 ppm residual moisture requires high costs. Furthermore, care must be taken to avoid additional moisture absorption by complete air exclusion. Polyalkylene glycols can therefore only be used in small, hermetically sealed refrigeration systems, such as domestic refrigerators. In large-scale refrigeration installations where maintenance operations require occasional opening of the refrigerant circulation, significant water uptake can be expected in the process. Moisture bound to the polyalkylene glycol can no longer be removed even by filter dryers. Therefore, the problem with raw materials in this system is the result. moreover,
The thermal stability of polyalkylene glycols does not meet all requirements. Already at temperatures above about 180° C. (such temperatures can occur in the pressure valve of the compressor at the highest working conditions), the decomposition of the polyalkylene glycol begins.

The object of the invention is therefore to create a refrigeration system which does not have the above-mentioned disadvantages and which can be used whenever chlorine-free, partially fluorinated hydrocarbons are used as refrigerating agents. An object of the present invention is to provide a lubricant for compressors.

0010

SUMMARY OF THE INVENTION Therefore, the present invention utilizes an ester made from an aromatic carboxylic acid and a monohydric alcohol as a refrigerating agent and a chlorine-free, partially fluorinated hydrocarbon. For use as a lubricant for refrigeration compressors operated by

Carboxylic acid esters derived from aromatic carboxylic acids and monohydric alcohols are widely used as plasticizers for polyvinyl chloride. however,
The profile of properties required for this use of carboxylic esters is fundamentally different from the characteristics that lubricants for compression refrigerators must exhibit. It could therefore not have been foreseen that compounds useful in particular in the plastics area could be used equally effectively within a completely different technical field.

The esters used according to the invention are derived from aromatic mono-, di- and polycarboxylic acids. Examples of such acids are benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, pyromellitic acid and trimesic acid.

These carboxylic acids or their acid anhydrides are reacted with linear or branched monohydric primary alcohols, alcohols having 2 to 20 carbon atoms being preferred. Branched primary alcohols with at least 4 carbon atoms have proven particularly preferred. Examples of such alcohols are n-butanol,
Isobutanol, n-pentanol, 2-methylpentanol, 3-methylbutanol, 2-ethylbutanol, 2-methylpentanol, n-hexanol, heptanol (in the form of a mixture of isomers), octanol (in the form of a mixture of isomers) 2-ethylhexanol, n-octanol, i-nonanol, i-decanol, i-tridecanol, i-hexadecanol and i-octadecanol.

The alcohols mentioned above are obtained primarily from olefins by oxo synthesis and subsequent hydrogenation of the corresponding aldehydes or by aldol condensation. For oxo synthesis, all olefins are suitable. Oxo synthesis generally yields a mixture of isomeric alcohols. For the production of the ester, it is not necessary to separate this mixture.

Esters suitable for use as lubricants include, for example, di-2-ethylhexyl phthalate, di-2-ethylhexyl phthalate,
These are isononyl phthalate, di-isodecyl phthalate, di-n-butyl phthalate, di-2-ethylhexyl terephthalate, and di-isodecyl terephthalate. Of particular interest are esters of phthalic acid, such as di-2-ethylhexyl phthalate, di-isononyl phthalate, di-isodecyl phthalate.

Esters are prepared in known manner from the acids and alcohols mentioned above in the presence of acidic catalysts. Suitable catalysts are mineral acids, such as sulfuric acid, phosphoric acid, and their acid salts, as well as trialkyl or triaryl phosphates and p-toluenesulfonic acid. In order to achieve a reaction as complete as possible, it is possible to use an excess of the reaction components and/or to distill off the water of reaction, optionally using those forming azeotropes, for example benzene or toluene. Recommended. The special measures to achieve high color stability that must be taken when using esters as plasticizers are unnecessary when used as lubricants.

[0017]

Effects of the Invention The lubricant used according to the invention can be combined with a chlorine-free, partially fluorinated hydrocarbon used as a refrigerant, still at -40°C, i.e. in a compression refrigeration installation. It has good miscibility over the possible temperature range. Its viscosity is between about 10 and 150 cST at 40°C and meets the requirements placed on lubricants for the abovementioned application ranges. The lubricant furthermore exhibits excellent thermal stability, excluding atmospheric oxygen and moisture, ie under the conditions that must be met in the refrigerant circulation. The ester is not hygroscopic. The ester can therefore be dried without great expense. German standard DIN
51503, a residual moisture not exceeding 35 ppm can be easily achieved.

The esters used as lubricants can be used in pure form, as mixtures of isomeric compounds and as mixtures of two or more esters of different chemical composition.

[0019]

EXAMPLE In the following example, the preparation of an ester is described,
In addition, we will list physical properties that are important for using esters as lubricants. Of course, the scope of the invention is not limited to the described embodiments.

Example 1: Di-(2-ethylhexyl)-
Preparation of Phthalate (DOP) 148 g (1 mol) of phthalic anhydride are mixed with 292 g (2.2 mol) of 2-ethylhexanol and 65 g of cyclohexane as entraining solvent and 1.0 g of water as catalyst.
0.59 g (0.006 mol) of concentrated sulfuric acid diluted with 48 g
Esterification at 135° C. for 7 hours in the presence of. The water of reaction is removed as an azeotrope. For work-up, the crude ester is neutralized with a 5% strength by weight caustic soda solution and then in the presence of a 5% strength by weight caustic soda solution for 1 hour.
Distill with steam for 3 hours at 35° C. and 150 mbar (15 kPa) and wash with water. 150mb
The ester is dried in a nitrogen stream for 3 hours at a pressure of ar (15 kPa) and at 135°C. After filtration, 362 g of product were obtained, corresponding to a yield of 93%.

Example 2: Preparation of di-(isononyl)-phthalate (DINP) A mixture of 148 g (1 mol) of phthalic anhydride and 322 g (2.23 mol) of isononyl alcohol was mixed with 70 g of cyclohexane and used as catalyst. , water 1.48g
Esterification is carried out at 135° C. for 12 hours in the presence of 0.59 g (0.006 mol) of concentrated sulfuric acid diluted with The water of reaction is removed as an azeotrope.

For post-treatment, first 5% by weight of the crude ester
It is neutralized with a concentrated caustic soda solution and then distilled with steam for 3 hours at 150 mbar (15 kPa) and 135° C. in the presence of a 5% strength by weight caustic soda solution.

After washing with water until the neutralization point is reached, 150 m
4 at a pressure of bar (15 kPa) and a temperature of 135 °C.
Dry in a nitrogen stream for an hour. After filtration, 388g of DINP
was obtained, corresponding to a yield of 93%.

Example 3: Preparation of di-(n-butyl)-phthalate (DBP) A mixture of 148 g (1 mol) of phthalic anhydride and 152 g (2.05 mol) of n-butanol was added to 50 g of cyclohexane and as a catalyst. p-Toluenesulfonic acid5.
After addition of 7 g (0.03 mol), the reaction is carried out for 6 hours at 130°C. The water of reaction liberated during esterification is removed as an azeotrope.

The acidic catalyst contained in the crude ester is first neutralized by adding NaOH at a concentration of 5% by weight. After repeated washing with water, the product was then heated to 150 mbar for 4 hours.
Dry at a pressure of (15 kPa) and a temperature of 135°C. After filtration, 262 g of di-(n-butyl)-phthalate are obtained, corresponding to a yield of 94%.

Characteristics of the Esters Described in Examples 1 to 3 Important criteria for the use of the aforementioned esters as lubricants are their thermal behavior, their viscosity and their miscibility with cryogens--the original 1,1 , 1,2-tetrafluoroethane and 2H-heptafluoropropane.

Testing of the thermal stability of esters was carried out using DIN51
593 in a U-tube at 250/40°C and 9
It was held for 6 hours. 250 branches of U-tube filled with oil
°C and is in contact with the vapor phase of the refrigerant, which is condensed into the second branch and then adjusted to 40 °C. Evaluation is carried out optically (oil discoloration). Supplementally, the aqueous extract in the vapor phase is tested for fluoride ions by means of an ion-sensitive electrode.

Gas chromatographic analysis of the gas phase provides conclusions regarding possible gaseous decomposition products of the ester or cryogen. Comparison of chromatograms is performed in a blank test.

The viscosity of the ester was measured by Ubbelohd.
e at 40° C. in a viscometer.

To test the miscibility of the ester with the freezing agent, a certain amount of the ester (approximately 0.2-3 g) is placed in a small glass tube of approximately 10 ml capacity. After immersion in liquid nitrogen, 6-3 g of cryogen are condensed therein, depending on the adjusted concentration. The small glass tube is then evacuated, sealed by melting, and the mixture of specific composition is passed through a temperature range of -40°C to +80°C. The decomposition point, ie the point on the limit curve of the miscibility graph, can be determined when two phases are formed or when it becomes cloudy. The totality of identified decomposition points thus gives the limit curve of the mixing gap.

Di-(2-ethylhexyl)-phthalate (DOP) Thermal stability: No decomposition Viscosity: ν = 27.3 mm2 /s Miscibility: Mixing gap di-(isononyl)-phthalate beyond the usual working range (DINP) Thermal stability: No decomposition Viscosity: ν = 37.1 mm2 /s Miscibility: Mixing gap beyond normal working range Di-(n-butyl)-phthalate (DBP) Viscosity: ν = 8.76 mm
2/s Miscibility: Completely miscible

Claims (5)

[Claims] 1. Esters prepared from aromatic carboxylic acids and monohydric alcohols as refrigerants and as lubricants for refrigeration compressors operated with chlorine-free, partially fluorinated hydrocarbons. How to use. 2. A process according to claim 1, wherein the ester is derived from a monocarboxylic acid, in particular benzoic acid. 3. Process according to claim 1, wherein the ester is derived from an aromatic dicarboxylic acid, in particular phthalic acid, isophthalic acid or terephthalic acid. 4. A process according to claim 1, wherein the esters are derived from aromatic carboxylic acids, in particular trimellitic acid and pyromellitic acid. 5. A process according to claim 1, wherein the ester is derived from a linear or branched monohydric primary alcohol having from 4 to 20 carbon atoms.