NO173230B - AZEOTROP MIXING WITH MINIMUM BOILING POINT AND USE THEREOF - 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 an azeotropic mixture of fluoroalkanes with a low boiling point and which has no or only a small impact on the environment and which can be used to replace chlorofluorocarbons, CFCs, in low-temperature compression refrigeration systems and to replace trifluorobromomethane and difluorochlorobromoethane as fire extinguishing agents.

It has been established that, due to its important effect coefficient on ozone, sooner or later CFCs must be replaced with coolants that contain less chlorine and are therefore less aggressive towards the environment. Due to its low impact on the environment, 1,1,1,2-tetrafluoroethane (R 134a) has already been proposed as a replacement for CFCs. However, due to its high boiling point of -26.5 °C, the use of R 134a has been reserved for applications at medium evaporation temperatures (-25 °C; -26 °C) and cannot be calculated to be used for applications at low boiling temperatures (for example - 40 °C). In reality, in practice, the minimum temperature reached in the evaporator is limited by the value of the normal boiling temperature of the coolant in order to avoid the introduction of air or brine into the installations in the event of discharge from the evaporator, which could cause a disturbance of the normal functioning of the system.

In the area of fire extinguishing and fire fighting, chlorobromofluoroalkanes and bromofluoroalkanes are used essentially, more particularly trifluorobromomethane, difluorochlorobromomethane and 1,1,2,2-tetrafluoro 1,2-dibromoethane. These connections are used in particular in premises where there is electrical equipment or electronics that are sensitive to corrosion (calculators, telephone exchanges, command rooms for ships). However, like CFCs, these compounds have a high ODP (ozone depletion potential).

However, it has now been found that 1,1,1,2-tetrafluoroethane (R 134a) and perfluoropropane (R 218) form an azeotrope with a minimum boiling point equal to about -41.1°C at 1.013 bar and where the amount of R 218 at normal boiling point is about 76 wt-#. Of course, this composition varies as a function of the pressure of the mixture and, at a given pressure, can be easily determined according to known techniques.

According to this, the present invention concerns an azeotropic mixture with a minimum boiling point and this mixture is characterized by the fact that it consists of a mixture of 1,1,1,2-tetrafluoroethane and perfluoropropane and that at its normal boiling point, -41.1 °C below 1.013 bar, contains 76 wt-# perfluoropropane and 24 wt-# 1,1,1,2-tetrafluoroethane.

The invention also relates to the use of the azeotrope as described above as a cooling liquid, aerosol propellant, expansion agent for plastic foam or extinguishing agent.

The azeotrope according to the invention has the advantage of exhibiting an ODP = 0. This means that the azeotrope is free of a destructive effect on the ozone layer of the stratosphere. The ODP value is defined as the ratio between the lowering of the ozone column recorded during the emission of one mass unit of the substance and the same reduction for trichlorofluoromethane which is chosen as a reference, ODP = 1. As an indication, the ODP value for trifluorobromomethane = 10.

Due to its low boiling point, the azeotropic mixture according to the invention can be used as a coolant in applications at low boiling temperatures (-40 °C; -41 °C) in the same way as in the case of commercial low-temperature coolers where R 218 alone has poor thermodynamic properties and where R 134a alone cannot be used for the reasons stated above.

It has also been found that this azeotrope can be used as an extinguishing agent by particularly replacing trifluorobromomethane and difluorochlorobromomethane. It has a "Burner Cup" value of less than 10% and in conclusion exhibits a high extinguishing power. The azeotrope according to the invention can be used to fight fire according to the same techniques as trifluorobromomethane and difluorochlorobromomethane. Thus, it can be advantageously used for the protection of premises by the technique known as total flooding. It can be pressurized with inert gases such as nitrogen, which allows the discharge rate to be increased. It can also be used for portable extinguishers.

Due to the physical properties that are close to those of CFCs, the mixture according to the invention can also be used as an aerosol propellant or as an expansion agent for plastic foam. The following examples are intended to illustrate the invention without limiting it.

EXAMPLE 1

The azeotrope according to the invention has been studied experimentally at different temperatures by analysis by means of gas chromatography of the composition of the liquid phase and of the vapor phase for different mixtures of R 134a and R 218.

The pressures were measured with an accuracy of over 0.02 bar using a HEISE manometer. Temperatures are measured with an accuracy of 0.02 °C using a 1000 ohm platinum probe.

The attached diagram shows the liquid:vapor equilibrium curve for the mixture R 218:R134a, created at a temperature of 20.3 °C. In the diagram, the abscissa shows the weight share R 218 and the ordinate the absolute pressure in bar; the signs indicate the experimental points.

For each temperature, a curve analogous to the one in the diagram is obtained. On successive addition of R 218 to R 134a, the pressure developed by the mixture increases regularly and passes via a maximum and then decreases regularly, which shows that an azeotrope with a minimum boiling point exists.

Table 1 gives the pressure:temperature ratio for this azeotrope, compared to those of the pure compounds.

The vapor pressure of the azeotope remains for a wide temperature spectrum above the vapor pressure of the pure compounds. These values suggest that the mixture remains azeotropic over the entire temperature range.

EXAMPLE 2

The characterization of the azeotrope at normal boiling point takes place directly by means of the boiling temperature of the different mixtures R 218:R 134a using an "ebulio-meter".

Table 2 summarizes the results obtained and allows the azeotrope to be characterized by:

the normal boiling point which is equal to about -41.1 °C, and the amount by weight is 218 which is equal to about 76%.

EXAMPLE 3

This example shows the use of the azeotrope according to the invention as a coolant.

The thermodynamic performance of the azeotropic mixture according to the invention is compared with the performance of the two individual components under conditions close to those encountered in a commercial cooling system, namely the following:

Table 3 summarizes the thermodynamic performances observed under conditions for pure R 134a, pure R 218 and the azeotropic mixture according to the invention.

It can be observed that the azeotropic mixture according to the invention exhibits numerous advantages compared to pure R 134a or pure R 218, namely: a much lower compression ratio and thus an improvement of the compressor volume requirement and as a result lower

installation costs;

an available volumetric cooling capacity that is significantly too high, which for a given cooling capacity allows the use of

a smaller compressor than that defined for the use of pure R 134a or pure R 218.

This increase in the available volumetric cooling power allows, in the case of the azeotrope according to the invention, to increase the available cooling power by 37% for an already existing installation that uses R 134a.

EXAMPLE 4

This example shows the use of the azeotrope according to the invention as an extinguishing agent.

The extinguishing efficiency is generally measured according to the method called "Burner Cup".

This method is described in the standard ISO/DIS 7075-1 as the minimum percentage of extinguishing agent (measured on a volume basis) in a mixture of air plus extinguishing compound that is necessary to extinguish a burning liquid.

The lower the "Burner Cup" value, the more effective the extinguishing connection.

In the present case, ethanol was used as a flammable liquid.

The "Burner Cup" value for the azeotropic mixture R 218:R 134a according to the invention is 9.5 H>.

Claims (5)

1. Azeotropic mixture with a minimum boiling point, characterized in that it consists of a mixture of 1,1,1,2-tetrafluoroethane and perfluoropropane and that at its normal boiling point, -41.1 °C under 1.013 bar, it contains 76 wt-# of perfluoropropane and 24 wt # 1,1,1,2-tetrafluoroethane. 2. Use of the azeotrope according to claim 1 as a coolant. 3. Use of the azeotrope according to claim 1 as an aerosol propellant. 4. Use of the azeotrope according to claim 1 as an expanding agent for plastic foam. 5 . The use of the azeotrope according to claim 1 as an extinguishing agent.