EP3652264A1 - Cryogenic cooling composition and method - Google Patents

Abstract

Cooling composition comprising a mixture of solid particles of CO2 and liquid nitrogen, in which: - the content of solid particles of CO2 is between 70 and 85% by weight and - the solid particles of CO2 have a diameter of less than or equal to 50 μm.

Description

 COMPOSITION AND METHOD FOR COOLING AT CRYOGENIC TEMPERATURE

The present invention relates to a cooling composition, and a cooling method using said cooling composition.

 The need to produce significant cooling powers is found in all branches of industry, in parts of the medical field, to rapidly and deeply cool reaction media, metallic materials, plastics, organic, food, human tissue, and plant material. ..

 In addition to refrigeration machines of all types, cryogenic fluids are widely used because they allow the rapid generation of large refrigeration capacities with simple design equipment.

 Two main technologies are used:

 - immersion of the product to be cooled in a liquid

 - split jet projected on the surface of the product

In almost all industrial cases, the processes are carried out at ambient pressure.

The main media used are liquid nitrogen, liquid argon, carbon dioxide in liquid or solid form.

 Due to being at the liquid-vapor or solid-vapor equilibrium during the implementation, a layer of gas occurs immediately in contact with the material to be cooled between the surface of the material to be cooled and the fluid or solid (calming layer). This layer is of the order of 0.1 to 1 millimeter for liquid nitrogen.

 Within this layer of heating, the conductive heat exchange is limited by the thermal conductivity of the gas which is lower than that of the liquid and which greatly reduces the exchange coefficient.

The thermal conductivity of N ₂ gas is about 17 times less than that of liquid nitrogen, which reflects the fact that the calefaction layer acts as a heat shield inhibiting heat transfer.

 This limits:

Cooling capabilities by simple touch and therefore the use of the media cited for rapid cooling of solid materials, freezing and preservation of food products, plants, plant or human tissues. This phenomenon of calefaction continues as long as the surface temperature is higher than the temperature of Leidenfrost (temperature of heating) variable according to the type and nature of surface.

 Below this temperature, the exchange is by a normal boiling mode (nucleate boiling and transition) and the heat flow increases considerably although the temperature difference becomes small. An example is shown in Figure 1.

Indeed, Figure 1 shows the thermal flux expressed in W.nr ² as a function of the temperature difference at the liquid / solid interface (surface temperature - the temperature of the liquid) for a 4cm diameter brass bar , and of height 10cm and immersed in liquid nitrogen. The initial temperature of the brass is 15 ° C. There are three areas:

a zone A during which boiling is nucleated;

 a zone B during which a transition boiling is observed; and

a zone C during which a film boiling is observed (heating phenomenon).

To try to circumvent this limitation, several devices can be implemented:

- Cause strong turbulence around the material to be cooled. This substantially increases the heat flow but introduces an additional energy expenditure and additional consumption of cold vector.

- For example, subcooling the liquid nitrogen by conducting the process under partial vacuum (by rapid pumping of the gaseous phase). This technique makes it possible to significantly increase the heat flux but to the detriment of a major overconsumption of the cold vector.

 - Disperse divided materials such as silica in the fluid to promote heat exchange by solid-solid contact.

 This significantly increases the heat flow but requires removing the divided materials of the product to be cooled or taking the materials compatible with the material to be cooled.

- Project liquid jets at high speed on the surface of the product to be cooled in order to reduce, or even break, the heating layer. This makes it possible to greatly increase the thermal flux but at the expense of a significant energy expenditure and overconsumption of the fluid. From there, a problem is to provide an improved solution for cooling an element.

 A solution of the present invention is a cooling composition comprising a mixture of solid particles of CO 2 and liquid nitrogen in which:

the content of solid particles of CO 2 is between 70 and 85% by weight and

 the solid particles of CO2 have a diameter less than or equal to 50 μιη.

 The cooling composition according to the invention is preferably manufactured by means of a process comprising:

a) a step of forming the CO 2 particles comprising the expansion of CO 2 gas, preferably in a flash cone; and

b) a step of mixing the CO 2 particles and liquid nitrogen.

 These particles can be either dispersed in liquid nitrogen with slight agitation or liquid nitrogen is poured onto the particles contained in a container. Note that the order of implementation does not affect the size of the CO2 particles obtained.

 In the cooling composition, the liquid nitrogen must completely wet the mass of the particles.

 The amount of liquid nitrogen relative to the amount of solid CO2 should be as close as possible to the amount necessary for:

 - the liquid nitrogen wets all the solid C02 particles and

- There is a surplus of liquid nitrogen sufficient to prevent very rapid drying of the mass of solid CO 2 (paste) which occurs during the first seconds of quenching of the object to be cooled and which is difficult to compensate by injection compensatory liquid nitrogen on the surface.

In this configuration, the solid CO 2 particles cooled at the temperature of the liquid nitrogen are the main vector of the heat exchange participating in the heat exchange by the direct solid / solid contacts and minimize the heating effect. This cooling composition shows a heat exchange capacity greatly increased compared to liquid nitrogen under the same conditions.

The cooling composition according to the invention makes it possible to obtain a heat exchange coefficient equal to or> 230 WM-2.K-1 in the cauld zone. approximately twice that of liquid nitrogen under the same conditions and up to 210 WM-2.K-1 depending on the conditions in the nucleate boiling zone or 10 times that of liquid nitrogen under the same conditions .

 This cooling composition is sufficiently fluid and manipulable to constitute immersion baths for deep cooling of metals, plastics, food products, plant and human tissues. This implies a very low temperature cooling in English we speak of "deep freezing". The composition is transferable and "pumpable" by the usual means for the transfer of cryogenic fluids.

 The present invention also relates to a method of cooling an element to be cooled, implementing a cooling composition as defined in claim 1 comprising the following successive steps:

a) stirring the composition at a speed less than 1 revolution per second,

c) immersing and maintaining the element to be cooled in the composition,

with during the entire duration of step c):

the stirring of step b) is maintained, and

 the proportion of liquid nitrogen in the composition is measured and is kept constant at plus or minus 5% by the addition of liquid nitrogen.

 Thanks to the cooling method according to the invention a cryogenic temperature cooling of the element to be cooled is made possible

 Preferably, step c) is carried out at a pressure between 1 bar absolute and 10 bar absolute.

 Note that the cooling time depends on the size of the element to be cooled, its shape, the type of material and also its temperature. It can be said that under the same conditions and for the same object, the cooling time gain to achieve a target temperature obtained by implementing the method according to the invention is at least 30%.

For example, for a bar of diameter 40mm and height 100mm, brass (70% Cu / 30% Zn), the bar must be immersed for about 3 minutes and 30 seconds for that its surface temperature (measured by thermal probe Pt100 at 3mm from the edge of the bar) goes down from 13 ° C to -196 ° C.

 Note that the stirring of the composition allows the maintenance of particles in homogeneous suspension.

Claims

claims

A cooling composition comprising a mixture of solid particles of CO 2 and liquid nitrogen in which:

the content of solid particles of CO 2 is between 70 and 85% by weight and

 the solid particles of CO2 have a diameter less than or equal to 50 μιη.

A method of manufacturing a cooling composition as defined in claim 1, comprising:

a) a step of forming the solid particles of CO 2 comprising the expansion of CO 2 gas, preferably in an expansion cone; and

b) a step of mixing the CO 2 particles and liquid nitrogen.

3. A method of cooling an element to be cooled, using a cooling composition as defined in claim 1 comprising the following successive steps:

a) stirring the composition at a speed less than 1 revolution per second,

c) immersing and maintaining the element to be cooled in the composition,

with during the entire duration of step c):

the stirring of step b) is maintained, and

 the proportion of liquid nitrogen in the composition is measured and is kept constant at plus or minus 5% by the addition of liquid nitrogen.

4. Method according to claim 3, characterized in that step c) is carried out at a pressure between 1 bar absolute and 10 bar absolute.