EP2539650B1 - Cryogenic cooling method using a gas-solid diphasic flow of co2 - Google Patents

Description

The present invention relates to the field of processes using CO ₂ as a cryogenic fluid, in processes of cooling, freezing and crusting of products, in particular food products, but also as a source of cold in refrigerated trucks carrying fresh products. and / or frozen (so heat-sensitive).

In such processes and applications, the CO ₂ is most often intended to be used in direct injection, with control temperatures of the products to be cooled which typically vary between 0 to -20 ° C in the case of refrigerated transport, and between -40 ° C to -70 ° C in cells and other cooling tunnels.

• la question de la sécurité : il requiert la mise en place de dispositifs permettant d'éviter le risque d'asphyxie (systèmes d'alarme, systèmes d'extraction, capteurs de CO₂), avec les coûts et les contraintes que cela implique ;
• du point de vue thermodynamique : les calories des gaz d'extraction notamment ceux à -40°C/-70°C sont difficilement valorisables car après leur contact direct avec les produits à refroidir, ils deviennent pollués, par la présence de traces d'humidité, de particules de produits, etc...

Although the use of direct injection CO ₂ has undeniable advantages, in particular the absence of a thermal barrier and therefore the guarantee of maximum thermal efficiency, it has disadvantages, among which are:

• the question of safety: it requires the establishment of devices to avoid the risk of asphyxiation (alarm systems, extraction systems, CO ₂ sensors), with the costs and constraints that entails;
• from the thermodynamic point of view: the calories of the extraction gases especially those at -40 ° C / -70 ° C are difficult to recover because after their direct contact with the products to be cooled, they become polluted, by the presence of traces of humidity, particle of products, etc ...

But there are also many applications where CO ₂ is used in indirect injection in an open loop, especially in applications for refrigerated transport but also in freezing tunnels; where a heat exchanger is supplied with liquid CO ₂ which, by evaporating in this exchanger, extracts the heat from the medium to be cooled and thus produces the desired cold (the transfer of the cold to the products passes through a exchange with the internal air tunnel or truck by the intervention of ventilation means associated with each exchanger). A liquid / vapor phase change is therefore implemented here which, with regard to the thermodynamic properties of CO ₂ , is "flanged" at a theoretical pressure of 5.18 bar corresponding to the pressure of the triple point of this fluid. In other words, the temperature at which the phase change is made is limited, and in all cases, it is strictly greater than -56.6 ° C. The demonstration is thus made that the use of CO ₂ indirect injection does not achieve very low temperature levels, unlike liquid nitrogen for example.

The document WO 2006/129034 discloses a method and apparatus for transferring frigories to products using liquid CO ₂ according to the preambles of claims 1 and 5, respectively.

The present invention wishes to propose new conditions of use of CO ₂ as a source of cold in such indirect injection applications.

As will be seen in more detail below, the invention proposes to set up a two-phase gas-solid flow.

The invention relates to a process using liquid CO ₂ as a cryogenic fluid, for transferring frigories to products, a process of the so-called indirect injection type where the liquid CO ₂ is sent to a heat exchanger system where it is evaporates, the transfer of cold products passing through an exchange between the air surrounding the products and the cold walls of the heat exchanger, favored by the intervention of ventilation means associated with the heat exchanger system, the process is characterized in that before reaching the exchanger system, the liquid CO ₂ has undergone an expansion operation, at a pressure chosen to obtain a solid / gas mixture at the outlet of the expansion operation.

According to a preferred embodiment of the invention, before reaching the expansion operation, the liquid CO ₂ has been put into a heat exchange position with the cold gases obtained at the outlet of the system. heat exchanger (resulting from the melting carried out in the heat exchanger system).

This heat exchange between the liquid CO ₂ and the cold gases obtained at the outlet of the heat exchanger system is for example made in a plate heat exchanger.

• on envoie dans l'échangeur de ce procédé d'injection indirecte non pas, comme selon l'art antérieur, du CO₂ liquide, mais un fluide résultant d'une détente, dans lequel il y a une part de solide (c'est un fluide diphasique gaz/solide) ;
• et le mode avantageux de mise en oeuvre de l'invention explicité plus haut, où avant d'être envoyé dans le détendeur, le liquide échange avec la phase gaz extraite du système d'échangeur thermique (ce qui est une manière de sous-refroidir ce liquide), offre un rendement thermique plus élevé puisque la fraction solide dans le liquide sous-refroidi puis détendu est alors plus élevée.

It will therefore be understood from the foregoing:

• this indirect injection process is sent into the exchanger not, as in the prior art, liquid CO ₂ , but a fluid resulting from a relaxation, in which there is a solid part (this is a two-phase gas / solid fluid);
• and the advantageous mode of implementation of the invention explained above, where before being sent into the expander, the liquid exchanges with the gas phase extracted from the heat exchanger system (which is a way of sub-cooling this liquid), offers a higher thermal efficiency since the solid fraction in the subcooled and then expanded liquid is then higher.

• la figure 1 est une représentation schématique partielle d'un mode de mise en oeuvre de l'invention ;
• la figure 2 présente des courbes de différence d'enthalpie, permettant de visualiser la différence d'enthalpie entre les points 2 et 3 de la figure 1, incluant les chaleurs latente et sensible, ceci pour deux niveaux de pression, 5.18 bar (pression du point triple) et 1 bar.
• la figure 3 est une représentation schématique partielle d'un mode avantageux de mise en oeuvre de l'invention mettant en oeuvre un sous-refroidissement du CO₂ liquide avant son arrivée dans le détendeur.

Other features and advantages of the present invention will thus become more clearly apparent in the following description, given by way of illustration but in no way limiting, with reference to the accompanying drawings for which:

• the figure 1 is a partial schematic representation of an embodiment of the invention;
• the figure 2 presents curves of difference of enthalpy, allowing to visualize the difference of enthalpy between points 2 and 3 of the figure 1 , including latent and sensible heat, for two pressure levels, 5.18 bar (triple point pressure) and 1 bar.
• the figure 3 is a partial schematic representation of an advantageous embodiment of the invention implementing an undercooling of the liquid CO ₂ before its arrival in the expander.

The figure 1 allows to visualize in a simple and clear way the flow of liquid CO ₂ in a process according to the invention. We can refer if necessary, to better follow the following, but it is in no way an obligation, to a diagram of Mollier, a diagram well known to those skilled in the art, but that the Applicant has chosen not to be included here for readability purposes.

As we can read on the figure 1 , the liquid CO ₂ (point 1) withdrawn from storage, for example under standard conditions of 20 bar / -20 ° C. (or 45 ° C./8 bar depending on the country concerned), is expanded to a lower pressure than that of the triple point, for example at 5.18 bar (point 2), before reaching the exchanger system.

The exchanger system is implemented in a so-called indirect injection process: for example in a cooling, freezing or crusting operation of products, in particular foodstuffs (the exchanger system is then for example present at inside a cryogenic cell or tunnel), or in a refrigerated truck carrying perishable thermosensitive products.

Thus, at point 2, a diphasic gas / solid mixture whose solid fraction varies as a function of the pressure at point 2. As an illustration, it is typically 52% at 5.18 bar / -56.6 ° C. and 47% at 1 bar / -80 ° C.

This two-phase mixture is then circulated inside the exchanger system where the mixture gives up its latent heat of fusion in addition to a portion of its sensible heat. The design of the exchanger and in particular its exchange surface, as well as the CO ₂ flow rate, will define the cooling capacity delivered as well as the exit temperature of the gas at point 3.

The figure 2 presents curves of difference of enthalpy, allowing to visualize the difference of enthalpy between points 2 and 3 of the figure 1 , including latent and sensible heat, for two pressure levels after expansion of liquid CO ₂ , 5.18 bar (ie the pressure of the triple point) and 1 bar.

This figure 2 shows well the available energy (expressed in variation of enthalpy) contained in a kilogram of CO ₂ when this one is relaxed from 20 bar to 5.18 bar representing the limit of the change of phase liquid / vapor (low curve in the figure), or from 20 bar to 1 bar (high curve in the figure), to obtain in accordance with the invention a two-phase solid / gas mixture. Note that in both cases, the change in enthalpy is all the greater as the exit temperature of the gas is also higher, and the fact that this enthalpy change is even higher than the pressure after relaxation is low. Hence the undeniable energy interest of what the present invention proposes by the implementation of a solid fluid / gas instead of a liquid / gas fluid as in the prior art.

It should nevertheless be mentioned that for certain applications of food cryogenics, for example for certain products in freezing tunnel applications, the effect of cryogenic temperature is strongly sought. Thus, in such applications, it will be difficult to obtain gases at the outlet of the exchanger at such a high temperature since the temperature of the air surrounding the products, sought in such processes, must typically reach -60 ° C. to -80 ° C. .

For such specifications, as for other applications, it will then be particularly advantageous to implement the advantageous embodiment of the invention which is illustrated in FIG. figure 3 below.

This advantageous mode aims to maximize the calories still present in the gas extracted at the outlet of the exchanger system.

Let's look at the embodiment of the figure 3 .

• le CO₂ liquide (point 1) soutiré du stockage, par exemple dans des conditions standard déjà évoquées plus haut dans le cadre de la figure 1, passe, avant d'atteindre le détendeur, dans un échangeur à plaques où il échange thermiquement avec les gaz issus du système d'échangeur (point 4), système d'échangeur présent dans le tunnel, ou le camion etc.....;
• on voit donc que circulent à contre-courant dans l'échangeur à plaques le CO₂ liquide venant du stockage (point 1), et les gaz extraits du système d'échangeur thermique (point 4), ce qui permet le sous-refroidissement du courant de CO₂ liquide avant que celui-ci n'atteigne le poste de détente (point 2) ;
• entre les points 1 et 2, le liquide reste donc à pression sensiblement constante mais subit un refroidissement ;
• à la sortie du poste de détente (point 3) le mélange solide/gaz obtenu est dirigé vers le système d'échangeur thermique ;
• les gaz extraits du système d'échangeur thermique (point 4), une fois passés dans le sous-refroidisseur, sont évacués (point 5) ;
• la production de froid se fait donc au niveau du système d'échangeur entre les points 3 (après détente) et 4 (sortie d'échangeur).

This figure shows the presence of an additional means, it is a means for performing a heat exchange, in this case a subcooler, for example constituted as is the case here of a plate heat exchanger, the medium of which we will here explain the intervention:

• the liquid CO ₂ (point 1) withdrawn from the storage, for example under the standard conditions already mentioned above in the context of the figure 1 passes, before reaching the expander, in a plate heat exchanger where it exchanges thermally with the gases from the exchanger system (point 4), exchanger system present in the tunnel, or the truck etc .... .
• it is thus seen that the liquid CO ₂ coming from the storage (point 1) and the gases extracted from the heat exchanger system (point 4) are circulating countercurrently in the plate heat exchanger, thus allowing the subcooling of the liquid CO ₂ flow before it reaches the expansion station (point 2);
• between points 1 and 2, the liquid therefore remains at substantially constant pressure but undergoes cooling;
• at the outlet of the expansion station (point 3) the solid / gas mixture obtained is directed towards the heat exchanger system;
• the gases extracted from the heat exchanger system (point 4), once passed into the subcooler, are evacuated (point 5);
• the production of cold is therefore at the level of the exchanger system between points 3 (after relaxation) and 4 (exchanger outlet).

As mentioned above, the temperature in this point 4 will be dictated by the technical constraints of the user application of the cold, which leads to a higher or lower level.

Two examples of conditions and constitutions of phases at the different points 1, 2, 3, 4 and 5 of the figure 3 .

First example:

Point T (° C) P (bar) h (kJ / kg) Thermodynamic state 1 -20 20 40.8 Liquid-vapor equilibrium 2 -34 20 13.1 Subcooled liquid 3 -80 1 13.1 Gas-solid mixture 4 -60 1 323.5 Overheated gas 5 -25 1 351.2 Overheated gas

Second example:

Point T (°) P (bar) h (kJ / kg) Thermodynamic state 1 -20 20 40.8 Liquid-vapor equilibrium 2 -52 20 -22.4 Subcooled liquid 3 -80 1 -22.4 Solid gas mixture 4 -80 1 288.0 Gas solid mixture 5 -25 1 351.2 Overheated gas

This second example illustrates a case where if the user application of cold requires a temperature of the medium to cool as cold as possible, it is possible to consider a partial exploitation of the heat of fusion in the exchanger system (between points 3 and 4 ), the total melting of the mixture and its overheating then being done in the subcooler with a recovery of calories.

In other words, by playing on the exchange surface of the heat exchanger, it is possible to realize in the exchanger a partial melting, thus leaving at point 4 a solid / gas mixture, it then takes place in the heat exchanger a change of state of fusion, which operates at a constant temperature for a pure fluid such as CO ₂ (in the mode illustrated here it is not the temperature that changes but the mass fraction of the solid which decreases as and when measure to turn into vapor).

• d'augmenter la puissance frigorifique de l'échangeur du système d'injection indirect puisque le sous-refroidissement du CO₂ liquide permet de gagner jusqu'à 12% d'énergie disponible ;
• d'améliorer l'échange thermique car un fluide sous-refroidi, une fois détendu donne lieu à une fraction solide plus élevée, ce qui est bénéfique pour le coefficient de transfert.
• Si l'application exige une température du milieu la plus froide possible, on peut envisager une exploitation partielle de la chaleur de fusion dans le procédé (entre les points 3 et 4), la fusion totale du mélange et sa surchauffe se faisant alors dans le sous-refroidisseur avec une récupération des calories.

It will be understood from reading all the explanations given above, the method according to the invention in its mode of figure 3 allows:

• to increase the cooling capacity of the exchanger of the indirect injection system since the subcooling of the liquid CO ₂ makes it possible to save up to 12% of available energy;
• to improve the heat exchange because a sub-cooled fluid, once relaxed gives rise to a higher solid fraction, which is beneficial for the transfer coefficient.
• If the application requires the coldest possible medium temperature, partial exploitation of the heat of fusion in the process (between points 3 and 4) can be envisaged, the total melting of the mixture and its overheating then taking place in the process. subcooler with a recovery of calories.

Claims (6)

1. Process using liquid CO₂ as a cryogenic fluid for transferring cold to products, a process of the "indirect injection" type where liquid CO₂ is conveyed into a heat exchanger system where it evaporates, the transfer of cold to the products taking place by means of an exchange between the air surrounding the products and the cold walls of the heat exchanger, promoted by the involvement of ventilation means associated with the heat exchanger system, characterised in that, before reaching the exchanger system, the liquid CO₂ has been subjected to an operation for reducing pressure to a pressure chosen in order to obtain, at the output of the pressure-reducing operation, a solid/gas mixture.
2. Process according to claim 1, characterised in that before reaching the pressure-reducing operation, the liquid CO₂ exchanges heat with the cold gases obtained at the output of the heat exchanger system, in a means which allows for such a heat exchange.
3. Process according to claim 2, characterised in that the exchange surface of the exchanger system is sized so as to carry out, in the exchanger, only a partial melting of the entering gas/solid mixture, the complete melting of the mixture then taking place in said means which allows for heat exchange.
4. Process according to either claim 2 or claim 3, characterised in that said means which allows for heat exchange is a plate exchanger.
5. Plant for transferring cold to products using liquid CO₂, the plant using a process of the "indirect injection" type and comprising:

   - a heat exchanger system capable of passing liquid CO₂ therethrough; and

   - ventilation means associated with the heat exchanger system, capable of bringing the air surrounding the products into contact with the cold walls of the heat exchanger system,

   characterised in that it comprises a pressure-reducing system, positioned upstream of the exchanger system, thus capable of reducing the pressure of the liquid CO₂, before it arrives in the exchanger system, to a pressure chosen in order to obtain a solid/gas mixture at the output of the pressure-reducing operation.
6. Plant according to claim 5, characterised in that it additionally comprises a subcooling system, for example a plate exchanger, positioned in the plant according to the following arrangement:

   - the subcooling system is positioned between the source of liquid CO₂ and the pressure-reducing system, in order to make it possible for the liquid CO₂ to be able to pass through a first pathway of this subcooling system, before reaching the pressure-reducing system;

   - said arrangement is furthermore such that it makes it possible to pass the cold gases extracted from the heat exchanger system through a second pathway of the subcooling system.

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