IE904049A1 - An azeotropic mixture with a low boiling point based on fluoroalkanes - Google Patents

Abstract

An azeotrope of minimum boiling point, capable of being employed as refrigerating fluid replacing chlorofluorocarbons or as an extinguishing agent replacing bromofluorocarbons and chlorobromofluorocarbons. The azeotrope according to the invention is a mixture of 1,1,1,2-tetrafluoroethane and perfluoropropane. At normal boiling point (approximately -41.1 DEG C at 1.013 bar) its perfluoropropane content is approximately 76 mass% and that of 1,1,1,2-tetrafluoroethane is 24 %. This azeotrope can also be employed as an aerosol propellant or as a blowing agent for plastic foams.

Description

The present invention relates to a mixture of fluoroalkanes with a low boiling point, which has little or no effect on the environment and is capable of being employed to replace chlorofluorocarbons (CFCs) in low5 temperature compression refrigeration systems and to replace trifluorobromomethane and difluorochlorobromomethane as extinguishing agent.

It is now established that because of their effect on ozone, CFCs will have, in the longer or shorter term, to be replaced with refrigerating fluids containing less chlorine which are, consequently, less aggressive towards the environment. Bearing in mind its very small effect on the environment, 1,1,1,2-tetrafluoroethane (R 134a) has already been proposed as a substitute for CFCs. However, because of its high boiling point (-26.5°C), the use of R 134a by itself is restricted to intermediate evaporation temperature applications (-25°C; -26°C) and cannot be used in low boiling temperature applications (for example -40°C). In fact, the minimum temperature reached in the evaporator is in practice limited by the normal boiling temperature of the refrigerating fluid in order to avoid the entry of air or brine into the plant in case of leakages at the evaporator, as this would present the risk of disturbing the normal operation of the system.

In the field of extinguishing and of firefighting, - 3 use is made principally of chlorobromofluoroalkanes and bromofluoroalkanes, particularly trifluorobromomethane, difluorochlorobromomethane and 1,1,2,2-tetrafluoro-1,2dibromoethane. These compounds are employed especially in premises where corrosion-sensitive electrical and electronic equipment is to be found (e.g. data processing rooms, telephone exchanges, engine rooms aboard ships). However, like the CFCs, these compounds have high ODPs (ozone depletion potentials).

It has now been found that 1,1,1,2tetrafluoroethane (R 134a) and perfluoropropane (R 218) form an azeotrope with a minimum boiling point of approximately -41.1°C at 1.013 bars and whose R 218 content at the normal boiling point is approximately 76 % on a mass basis. This composition naturally varies as a function of the pressure of the mixture and, at a given pressure, can be easily determined by well-known methods. Accordingly the present invention provides a minimum boiling point azeotrope which is a mixture of 1,1,1,2-tetrafluoroethane and perfluoropropane such that at its normal boiling point it contains approximately 76 % on a mass basis of perfluoropropane and 24 % on a mass basis of 1,1,1,2tetrafluoroethane.

The azeotrope according to the invention has the advantage of exhibiting substantially zero ODP. This means that it is devoid of destructive action on the - 4 stratospheric ozone layer. The ODP is defined as the ratio between the lowering of the ozone column recorded during the emission of a unit mass of substance and the same lowering in the case of trichlorofluoromethane, chosen as reference (ODP = 1). By way of indication, trifluorobromomethane has an ODP of 10.

Because of its low boiling point, the azeotropic mixture according to the invention can be employed as a refrigerating fluid in applications at low boiling temperatures (-40°C; -41°C), as in the case of low temperature commercial refrigeration where R 218 by itself has mediocre thermodynamic properties and where R 134a by itself cannot be employed for the reasons set out above.

It has also been found that this azeotrope can be employed as an extinguishing agent, especially to replace trifluorobromomethane and difluorochlorobromomethane. In fact, it has a Cup Burner value lower than 10 % and consequently exhibits a high extinguishing power.

The azeotrope according to the invention can be employed for firefighting according to the same techniques as trifluorobromomethane and difluorochlorobromomethane. Thus, it can be advantaqeously employed for the protection of premises using the so-called complete immersion technique. It can be pressurized with inert gases such as nitrogen, and this allows its discharge speed to be increased. It can also be employed in portable extinguisher 1E 904049 - 5 techniques.

Given its physical properties which are close to those of the CFCs, the mixture according to the invention can also be employed as an aerosol propellant or as a blowing agent for plastic foams.

The following Examples further illustrate the present invention.

EXAMPLE 1 The azeotrope according to the invention was 10 investigated experimentally at various temperatures by analysis of the liquid phase and vapour phase compositions, using gas phase chromatography, for various mixtures of R 134a and R 218.

The pressures were measured with an accuracy 15 greater than 0.02 bar by means of a Heise manometer. The temperatures were measured to within 0.02°C by means of a 1,000-ohm platinum probe.

Graph 1 of the accompanying Figure shows the liquid/vapour equilibrium curve for R 218/R 134a mixtures, established at a temperature of 20.3°C. On this graph the abscissa shows the mass fraction of R 218 and the ordinate the absolute pressure in bars; the - signs correspond to the experimental points.

A curve similar to that of Graph 1 can be obtained for each temperature. On successively adding R 218 to R 134a the pressure developed by the mixture increases - 6 steadily, then passes through a maximum and decreases steadily, which demonstrates the existence of the azeotrope with a minimum boiling point.

Table 1, which follows, gives the pressure5 temperature relationship for this azeotrope, compared with that for the pure substances.

TABLE 1 Temperature (°C) Absolute pressure (bars) R 218/R 134a azeotrope Pure R 134a Pure R 218 -40.0 0.3 20.3 39.9 1.10 4.92 9.08 15.10 0.53 2.94 5.78 10.26 0.87 4.20 7.66 12.98 The vapour pressure of the azeotrope remains higher 20 than the vapour pressure of the pure substances over a wide range of temperature. These data show that the mixture remains azeotropic throughout this temperature range.

EXAMPLE 2 The characterization of the azeotrope at the normal 25 boiling point was carried out by direct measurement of the boiling temperature of various R 218/R 134a mixtures by means of an ebulliometer.

Table 2 summarizes the results obtained and enables the azeotrope to be characterized by: - its normal boiling point which is equal to approximately -41.1°C, - its mass content of R 218, which is equal to approximately 76 %.

TABLE 2 Temperature (°C) Mass content of R 218 -26.5 0 -40.4 70.9 -40.8 74.6 -41.0 75.2 -41.0 76.4 -40.9 78.3 -36.7 100.0 EXAMPLE 3 This Example illustrates the use of the azeotrope according to the invention as a refrigerating fluid.

The thermodynamic performance of the azeotropic 25 mixture according to the invention was compared with the - 8 performance of the two constituents by themselves, under conditions close to those encountered in commercial refrigeration systems, namely the following: - condensation temperature : +30°C - evaporation temperature : -30°C - liquid supercooling : - 5®C - vapour superheating at the compressor : +15°C Table 3 summarizes the thermodynamic performance 10 observed under these conditions for pure R 134a, pure R 218 and the azeotropic mixture according to the invention.

TABLE 3 R 218/R 134a azeotrope Pure R 218 Pure R 134a Evaporation pressure (bars) 1.69 1.36 0.85 Condensation pressure (bars) 11.58 10.1 7.70 Compression ratio 6.85 7.43 9.06 20 Refrigerating capacity (kJ/m3) 877 710 640 Coefficient of performance 2.7 2.4 3.1 It can be seen that the azeotropic mixture according to the invention offers a number of advantages over pure R 134a or R 218, especially: - 9 - a lower compression ratio and therefore an improvement in the volumetric efficiency of the compressor and consequently lower plant running costs - a considerably higher available volumetric refrigerating capacity which, in practice, for a given refrigerating capacity, makes it possible to employ a smaller compressor than that required for pure R 134a or R 218.

In the case of the azeotrope according to the 10 invention, this increase in available volumetric refrigerating capacity also makes it possible to increase by 37 % the available refrigerating capacity of an existing plant using R 134a.

EXAMPLE 4 This Example illustrates the use of the azeotrope according to the invention as an extinguishing agent.

The extinguishing efficiency is generally measured by the so-called Cup Burner method.

This method, described in draft standard 20 ISO/DIS 7075-1, shows the minimum percentage of extinguishing compound (measured by volume) in a mixture of air plus extinguishing compound needed to extinguish a flaming liquid fuel.

The lower the Cup Burner value, the more effective 25 is the extinguishing compound.

We employed ethanol as the liquid fuel. 9.5 %. - 10 The Cup Burner value for the R 218/R 134a azeotropic mixture according to the invention is equal to

Claims (7)

1. A minimum boiling point azeotrope which is a mixture of 1,l,l,2-tetrafluoroethane and perfluoropropane such that at its normal boiling point it contains 5 approximately 76 % on a mass basis of perfluoropropane and 24 % on a mass basis of 1,1,1,2-tetrafluoroethane.

2. Use of the azeotrope according to Claim 1 as a refrigerating fluid.

3. Use of the azeotrope according to Claim 1 as 10 an aerosol propellant.

4. Use of the azeotrope according to Claim 1 as a blowing agent for plastic foams.

5. Use of the azeotrope according to Claim 1 as an extinguishing agent.

6. A minimum boiling point azeotrope according to Claim 1, substantially as hereinbefore described and exemplified.

7. Use according to any one of Claims 2-5, substantially as hereinbefore described and exemplified.