EP4446582A1 - Improving delivery of high pressure co2 to a user station by adding a liquefier to the point of use - Google Patents

Abstract

An installation for supplying a user station with pure or substantially pure liquid CO₂, at high pressure, high pressure preferably being in the range from 50 to 60 bar, from a source of liquid CO₂, the installation comprising a means for compressing the liquid CO₂ located between the source and the user station, characterized in that there is a liquefier (C) for the fluid circulating between the source and the compression means, said liquefier being positioned at the inlet of the compression means.

Description

The present invention relates to the field of installations for supplying liquid _CO2 to a user station.

In the context of the present invention, we are particularly interested in being able to provide a liquid that is called "clean" or substantially clean, a concept well known to those skilled in the art, highly sought after by many industries for well-known reasons of better "cryogenic quality" in terms of available frigories.

Such a "clear" or subcooled liquid is liquid at lower pressure, and at a lower temperature than when it was at higher pressure.

As an illustration, let us consider the example of machining metal parts under liquid nitrogen spraying: the higher the spray pressure in the machining zone, the better the heat exchange coefficients. However, when the cryogen, for example liquid nitrogen, is sprayed, gas is created - due to its expansion - at the outlet of the spray nozzle. The quantity of gas generated is directly proportional to the temperature of the liquid nitrogen and its pressure upstream of the nozzle. We therefore understand the interest in focusing on having a sub-cooled liquid.

The following therefore focuses on being able to supply a user station with liquid _CO2 that is frank, or substantially frank, below the critical point but in the vicinity of this point, typically around 50-60 bar and at room temperature or close to room temperature. Such an application can be called a “High Pressure _CO2 ” application.

We know that many industrial applications require so-called "high pressure" _CO2 . These include all supercritical _CO2 applications, i.e. beyond the critical point of _CO2 (around 32°C and 74 bar).

The operating pressure is very high, in particular to enable cleaning actions, or extraction of aromas, or even complex chemistries, by modulating the properties of the fluids according to the operating conditions (therefore essentially pressure and temperature).

• 15-20 bar abs (et donc -30 à -20°C environ) dans un stockage refroidi. Il règne à l'intérieur du stockage un équilibre liquide-gaz de CO₂ qui est prédit par une courbe d'équilibre que l'on peut bien visualiser sur un diagramme thermodynamique de Mollier du CO₂ par exemple. Pour garder l'ensemble froid, l'emballage ou le stockage est alors isolé pour limiter les entrées de chaleur (de l'ambiance qui règne autour du stockage) entrées qui provoquent une montée en pression du stockage par évaporation.
• ou bien 45-55 bar abs (et à température ambiante) pour un stockage à température ambiante. Il règne là aussi un équilibre liquide-vapeur de CO₂ mais à une pression supérieure étant donné que le stockage ou emballage n'est pas isolé ou maintenu en froid.

However, _CO2 is most often stored in lower temperature conditions than those required by the application at the end user station, generally namely:

• 15-20 bar abs (and therefore -30 to -20°C approximately) in a cooled storage. There is a liquid-gas equilibrium of _CO2 inside the storage which is predicted by an equilibrium curve that can be clearly visualized on a Mollier thermodynamic diagram of _CO2 for example. To keep the whole thing cold, the packaging or storage is then insulated to limit the heat inputs (from the atmosphere around the storage) which cause a rise in pressure in the storage by evaporation.
• or 45-55 bar abs (and at room temperature) for storage at room temperature. Here too, there is a liquid-vapor equilibrium of _CO2 but at a higher pressure since the storage or packaging is not insulated or kept cold.

• Une première étape de montée en pression du liquide cryogénique pour atteindre une pression en dessous du point critique du CO₂ mais tout de même relativement proche de ce point (50 à 60 bar par exemple). C'est ce que nous pouvons appeler du CO₂ liquide Haute Pression.
• Puis une seconde étape pour atteindre la pression cible demandée au point d'utilisation dans le cas par exemple d'une application supercritique. On dépasse alors les conditions du point critique et on entre donc dans le domaine supercritique via cette seconde étape.

Thus, to go from the storage of liquid _CO2 to the point of use, we will most often find the intervention of the following means:

• A first step of increasing the pressure of the cryogenic liquid to reach a pressure below the critical point of _CO2 but still relatively close to this point (50 to 60 bar for example). This is what we can call High Pressure liquid _CO2 .
• Then a second step to reach the target pressure requested at the point of use in the case of a supercritical application, for example. We then exceed the conditions of the critical point and therefore enter the supercritical domain via this second step.

This pressurization of liquid _CO2 is often carried out by means of volumetric pumps called cryogenic. These pumps, which are based on pistons, do not admit or only very little gas. This problem could be compared to cavitation for centrifugal pumps.

It is therefore necessary to feed them with liquid only, which is called a pure liquid.

However, it may happen, during a more or less long shutdown of the final application (these may be transient phenomena), that part of the liquid trapped in the pipes evaporates a little. This is caused by heat inputs which are more or less significant depending on the conditions prevailing around the consuming installation (between the source of liquid and its point of use, in the connection pipes of the various equipment which allow the implementation).

It can be noted that in continuous operation (therefore continuous use of high pressure liquid _CO2 ), the cryogenic liquid taken from the storage provides enough cold to compensate for the heat inputs and avoid the parasitic evaporation mentioned above.

But if nothing is put in place to get rid of the gas resulting from the evaporation of liquid _CO2 , this gas will inevitably go towards the consuming application when consumption resumes and will be pumped by the pump, causing a disruption in the operation of the pumps or even damage to these pumps.

It could be counter-argued that the evaporation of part of the liquid will increase the pressure which would tend to promote the liquefaction of the gas phase (the opposite effect of the previous one). However, if the evaporation is significant and there is no way to evacuate it (point below), the gas will be evacuated at the next use and will disturb the user.

For example, we can use a device that allows us to manage this quantity of unwanted gas by letting it return upstream, i.e. into storage or packaging, but they cannot be used in many cases: no return to the sources to avoid pollution (use of non-return valves for example).

And so in summary, to the extent that heat enters the installation, in particular in the piping which is not generally insulated (which has (in a way a delaying effect of the phenomenon), is inevitable in the event of a shutdown, the evaporation of a little cryogenic liquid cannot be avoided or prevented.

This is why an expansion valve is always installed and vented to the atmosphere on a pipe, part of which can trap cryogenic liquid that is always ready to evaporate. This is immediate because the liquid is always in equilibrium with its gas phase under storage conditions, unless a liquid subcooler is installed at the outlet of the liquid _CO2 source (we therefore cool more than the equilibrium that prevails in the source so as to avoid evaporation during heat inputs).

In other words, it takes little heat to evaporate a little liquid at equilibrium and therefore little frigories or cold to re-liquefy it.

It can also be noted, still according to the Mollier thermodynamic diagram of CO ₂ , that this phenomenon is all the more marked as we approach the critical point. This is the case for a high pressure CO ₂ application since we are typically between 50 and 60 bar, which then allows the end user to reach the conditions of use, in particular beyond the critical point of CO ₂ , in a single compression step.

Indeed, the same quantity of heat produces more gas as the pressure of the liquid at equilibrium increases (rule of "lever arms", simple expression of a mass balance, to calculate the relative quantity of gas and liquid during heating or cooling in the gas-liquid phase.

• De sous refroidir, par exemple par passage sur un échangeur, le liquide sortant à l'équilibre du stockage: mais ceci représente un coût certain (opératoire et investissement), et un caractère superflu en marche continue (qui représente la majeure partie des utilisations du CO₂ liquide en général) et malgré tout une évaporation à terme si la chauffe se prolonge dans le temps.
• Ou d'évacuer le gaz par une soupape, ou un séparateur diphasique, à positionner entre l'endroit où on produit le gaz et le poste utilisateur final. Ici encore ceci a un coût certain et la mise à l'atmosphère du CO₂ va produire une grande quantité de froid et de neige. En effet, en dessous de 5,3 bar, le liquide relâché, en même temps que le gaz, va générer un mélange tri phasique (gaz, liquide et solide (neige dite « sèche »)) qu'il est difficile de maitriser. On peut même assister à un bouchage de l'évacuation en cas d'accumulation de neige et de givrage avec l'humidité ambiante du point de rejet. On peut aussi noter que l'évacuation d'une partie du fluide (donc mélange gaz-liquide) perturbera immanquablement le flux principal de liquide (on a une arrivée par la canalisation et deux sorties). On pourrait en résumer positionner par exemple un pot dégazeur après toutes les entrées de chaleur, donc au plus proche du point d'utilisation.

To limit or prevent this parasitic vaporization, we could consider:

• To sub-cool, for example by passing through an exchanger, the liquid leaving the storage at equilibrium: but this represents a certain cost (operation and investment), and is superfluous in continuous operation (which represents the majority of uses of liquid _CO2 in general) and despite everything an evaporation in the long term if the heating continues over time.
• Or to evacuate the gas through a valve, or a two-phase separator, to be positioned between the place where the gas is produced and the final user station. Here again this has a certain cost and the release of _CO2 into the atmosphere will produce a large quantity of cold and snow. Indeed, below 5.3 bar, the liquid released, at the same time as the gas, will generate a three-phase mixture (gas, liquid and solid (snow) called "dry")) which is difficult to control. We can even see a blockage of the evacuation in the event of accumulation of snow and icing with the ambient humidity of the discharge point. We can also note that the evacuation of a part of the fluid (therefore gas-liquid mixture) will inevitably disrupt the main flow of liquid (there is an arrival through the pipe and two exits). We could summarize by positioning for example a degasser pot after all the heat inputs, therefore as close as possible to the point of use.

One of the objectives of the present invention is therefore to propose a technical solution to the problems developed above, distinguishing itself from and improving the solutions proposed so far in the literature.

As will be seen in more detail below, it is proposed according to the present invention to implement a liquefier, as close as possible to the point of use.

As an illustration to better understand the present invention, let us consider the case of a user station supplied from a bottle of liquid _CO2 with a dip tube. We know that between the bottle and its compressor, the piping can be subjected to heat inputs leading to evaporation of the liquid _CO2 . The gas cannot be evacuated and returned to the bottle because the line is equipped with a non-return valve. Also, if a liquid gas mixture arrives in the compressor (most often a piston compressor), it will malfunction and may even break (the compressor only accepts pure liquid).

According to one of the embodiments of the invention, it is therefore proposed to add a liquefier to the compressor inlet itself so as to limit as much as possible the connection length between the liquefier and the compressor, and thus limit as much as possible the reformation of gas between the liquefier and the compressor.

• Quoi qu'il se produise en amont (donc du stockage du CO₂ liquide jusqu'au point d'obtention du point CO₂ Haute Pression, à savoir typiquement de 50 à 60 bar de CO₂ liquide), le gaz généré au sein du liquide sera liquéfié avant d'être utilisé. On évite ainsi de laisser progresser le gaz formé vers l'aval de l'installation, i.e. vers le poste d'utilisation du CO₂ Haute Pression.
• Le besoin de frigories à apporter pour liquéfier le gaz formé est très mesuré, très raisonnable. Autrement dit, étant assez proche du point critique l'effort est limité. En conséquence, une installation simple, peu onéreuse et peu consommatrice d'électricité, permet d'assurer cette liquéfaction. On peut rester dans la plupart des cas sur une température finale de 5 à 15°C, largement suffisante.

So :

• Whatever happens upstream (i.e. from the storage of liquid _CO2 to the point where the High Pressure _CO2 point is reached, i.e. typically 50 to 60 bar of liquid _CO2 ), the gas generated within the liquid will be liquefied before being used. This avoids allowing the gas formed to progress downstream of the installation, i.e. towards the High Pressure _CO2 utilization station.
• The need for frigories to be provided to liquefy the gas formed is very measured, very reasonable. In other words, being quite close to the critical point, the effort is limited. Consequently, a simple, inexpensive and low-power-consuming installation can ensure this liquefaction. In most cases, we can remain at a final temperature of 5 to 15°C, which is largely sufficient.

• Un groupe froid permettant d'assurer la production d'une eau froide, à une température avantageusement dans l'intervalle allant de 5°C à 15°C, via un serpentin.
• Une cuve contenant de l'eau et deux serpentins :
  • ∘ Un premier où circule le liquide frigorigène en provenance du groupe froid, permettant de refroidir l'eau dans laquelle est immergé ce serpentin, eau qui servira d'intermédiaire, de media, pour conduire les frigories vers le fluide CO₂ à refroidir (échange thermique indirect).
  • ∘ Un second permettant le passage du fluide CO₂ Haute Pression, mélange diphasique gaz-liquide, le gaz ayant été généré suite à une entrée de chaleur. Au fur et à mesure de la progression le long du serpentin, les frigories, transférées par l'eau froide environnante, seront transférées au mélange gaz-liquide et permettront la liquéfaction du gaz.
  • ∘ La conception et le choix des conditions opératoires permettra le bon transfert des frigories générées par le groupe frigorifique (efficacité de transfert), la liquéfaction du gaz, voir même le sous-refroidissement du liquide CO₂ Haute Pression. Ceci va aussi permettre d'améliorer la compression de ce liquide (ou plus exactement sa montée en pression).

According to one of the embodiments of the invention, there is an implementation installation comprising the following elements:

• A cold group ensuring the production of cold water, at a temperature advantageously in the range of 5°C to 15°C, via a coil.
• A tank containing water and two coils:
  • ∘ A first one where the refrigerant liquid from the cold unit circulates, allowing the water in which this coil is immersed to be cooled, water which will serve as an intermediary, a media, to conduct the frigories towards the _CO2 fluid to be cooled (indirect heat exchange).
  • ∘ A second one allowing the passage of the High Pressure _CO2 fluid, a two-phase gas-liquid mixture, the gas having been generated following a heat input. As you progress along the coil, the frigories, transferred by the surrounding cold water, will be transferred to the gas-liquid mixture and will allow the gas to liquefy.
  • ∘ The design and choice of operating conditions will allow the proper transfer of the frigories generated by the refrigeration unit (transfer efficiency), the liquefaction of the gas, and even the sub-cooling of the High Pressure _CO2 liquid. This will also improve the compression of this liquid (or more precisely its pressure increase).

Other means of producing this liquefier can be imagined, provided that sufficient transfer of frigories from a cold source (glycol water) is ensured. of a network could replace a refrigeration unit) towards the so-called “hot” fluid (namely High Pressure _CO2 which is a two-phase gas-liquid mixture).

So if the user site already has cold water, for example through a chilled water network for example), the small tank liquefier mentioned above can be replaced by a simple exchanger. The cold fluid will be the chilled water from the network and the hot fluid (heat source) will be the _CO2 to be cooled/liquefied.

Let us illustrate in the following an example of application and calculation of implementation of the invention.

Let's take the example of a required High Pressure _CO2 flow rate of D=60 kg/h. We must therefore deliver between 55 and 60 bar.

Consider the case where _CO2 is at a temperature of 15 to 20°C, which corresponds to a point on the gas-liquid (or liquid-vapor) equilibrium of _CO2 (Mollier diagram).

We retain a thermal capacity or specific heat of _CO2 under these conditions at 5 J/kg/K.

So, the total heat to be exchanged to go from 20 to 12°C is / $$\mathrm{Q}=666.7\mathrm{W}=\mathrm{D}.\mathrm{Cp}.\left(20-10\right)$$

We seek to subcool this flow to 12°C using a coil which is immersed in cold water maintained at 10°C.

• Le liquide froid : l'eau du bain thermostaté (donc maintenu) à 10°C.
• Le liquide « chaud » : le CO₂ diphasique liquide-gaz entrant à environ 20°C et sortant à environ 12°C.

So, this coil is indeed a heat exchanger where:

• Cold liquid: bath water thermostatically controlled (therefore maintained) at 10°C.
• The “hot” liquid: the two-phase liquid-gas _CO2 entering at around 20°C and exiting at around 12°C.

We consider a counter-current exchange model and then the so-called logarithmic temperature difference ΔT = 4.97 ° C which corresponds to the difference between the temperatures between the two fluids which exchange at the two ends of the coil. We took care to have an outlet temperature of the CO ₂ flow slightly higher than that of the cold fluid (12 > 10 ° C). This avoids the "pinching" of the heat exchange, therefore the stopping of the transfer in this counter-current model retained.

We can then deduce an overall heat transfer coefficient U~4843 W/m ² /K by applying the Mc Adams formula. This known formula is applicable in the case of a tube, in which turbulence is ensured with a constant external wall temperature (here cold fluid).

avec A qui est la surface totale d'échange du tube, à savoir la surface totale moyenne (entre diamètre externe et interne) du tube dont la longueur est L.If we take a pipe diameter of 4 mm internal diameter (in order to ensure turbulence and therefore good heat transfer), we can deduce a minimum necessary length which will be Lmin=2.2 m. We then apply the classic general formula for heat exchanges on the coil with: $$\mathrm{Q}=\mathrm{U}.\mathrm{HAS}.\Delta \mathrm{T}$$

with A which is the total exchange surface of the tube, namely the average total surface (between external and internal diameter) of the tube whose length is L.

• Une cuve de 20 litres qui contient un groupe froid et un serpentin en cuivre pour maintenir l'eau du bain à une température cible entre 5 et 15°C.
• Un serpentin en inox de diamètre interne de 4 mm environ, avec une longueur de 10 m sachant que le minimum requis pour un bon transfert a été calculé à 2,2 m.

For this application, which consists of maintaining a flow of high-pressure _CO2 at 55-60 bar at a maximum of 12°C, we implement the following:

• A 20-litre tank that contains a cooling unit and a copper coil to maintain the bath water at a target temperature between 5 and 15°C.
• A stainless steel coil with an internal diameter of approximately 4 mm, with a length of 10 m, knowing that the minimum required for good transfer was calculated at 2.2 m.

1. 1. Une température de bain de 5°C permettrait de réduire encore la longueur minimale d'échange à 1,5 m environ. On a donc une bonne souplesse d'utilisation du liquéfacteur ainsi dimensionné pour assurer une liquéfaction du gaz qui aurait pu se former dans l'amont de l'utilisation.
2. 2. L'équipement peut être facilement adapté à d'autres conditions de débit, de température cible au point d'utilisation.

It can be noted that:

1. 1. A bath temperature of 5°C would allow the minimum exchange length to be further reduced to around 1.5 m. This provides good flexibility in using the liquefier, sized in this way to ensure liquefaction of the gas that may have formed upstream of use.
2. 2. The equipment can be easily adapted to other flow conditions, target temperature at the point of use.

• i) C : une cuve « tampon » contenant un liquide L (eau, ou eau glycolée par exemple)
  • La cuve est munie de deux serpentins qui vont permettre les transferts thermiques :
    • Entre le CO₂ et le liquide média ;
    • Entre le fluide frigorigène (provenant du groupe froid GF) et le liquide média.
  • (La référence S fournit un exemple de serpentins pouvant être mis en oeuvre dans la cuve C)
• j) GF : un groupe froid
• k) FG-E : du CO₂ à refroidir, à liquéfier (donc chaud), circulant de la source de CO₂ vers la cuve C.
  • Il s'agit de CO₂ Haute Pression, se situant en général à 50-60 bar et à température ambiante.
  • C'est donc du CO₂ liquide comportant une faible portion de gaz parasite dû aux entrées de chaleur s'étant produites en amont.
• l) FG-S :
  On a ici du CO₂ refroidit, circulant de la cuve C vers le poste utilisateur, un liquide franc (le gaz parasite a été liquéfié).
• m) FF-F :
  Le fluide frigorigène, froid, circulant du groupe froid vers la cuve.
• n) FF-C :
  Le fluide frigorigène réchauffé, circulant de la cuve vers le groupe froid.

There Figure 1 attached provides a partial schematic representation of an installation suitable for implementing the present invention, in which the following installation elements are recognized:

• i) C: a “buffer” tank containing a liquid L (water, or glycolated water for example)
  • The tank is equipped with two coils which will allow thermal transfers:
    • Between _CO2 and the media liquid;
    • Between the refrigerant (coming from the GF refrigeration unit) and the media liquid.
  • (Reference S provides an example of coils that can be used in tank C)
• j) GF: a cold group
• k) FG-E: _CO2 to be cooled, to be liquefied (therefore hot), circulating from the _CO2 source to tank C.
  • This is High Pressure _CO2 , generally at 50-60 bar and at room temperature.
  • It is therefore liquid _CO2 with a small portion of parasitic gas due to the heat inputs occurring upstream.
• l) FG-S:
  Here we have cooled _CO2 , circulating from tank C to the user station, a pure liquid (the parasitic gas has been liquefied).
• m) FF-F:
  The cold refrigerant fluid circulates from the refrigeration unit to the tank.
• n) FF-C:
  The heated refrigerant fluid circulates from the tank to the refrigeration unit.

The invention then relates to an installation for supplying a user station with pure or substantially pure liquid _CO2 , at high pressure, high pressure preferably being in the range from 50 to 60 bar, from a source of liquid _CO2 , the installation comprising a means for compressing the liquid _CO2 located between the source and the user station, characterized in that there is a liquefier for the fluid circulating between the source and the compression means, which liquefier is positioned at the inlet of the compression means.

Claims (3)

Installation for supplying a user station with liquid _CO2, either pure or substantially pure, at high pressure, high pressure preferably being in the range from 50 to 60 bar, from a source of liquid _CO2 , the installation comprising a means for compressing the liquid _CO2 located between the source and the user station, characterized in that there is a liquefier for the fluid (C) circulating between the source and the compression means, said liquefier being positioned at the inlet of the compression means. Installation according to claim 1, characterized in that the liquefier implements a heat exchange between a cold fluid (FF-F) and the high-pressure two-phase gas-liquid _CO2 considered as a “hot” fluid (FG-E), the cold fluid being for example constituted by: - glycolated water, or - cold water from a chilled water network present on the site housing said user station. Installation according to claim 1, characterized in that the liquefier is produced by the implementation of the following means: • a cold group (GF) for ensuring the production of cold water, at a temperature advantageously located in the range from 5°C to 15°C, via a coil; • a tank (C) containing water or another media, and two coils (S): ∘ A first coil through which a refrigerant liquid (FF-F) circulates from the cooling unit, making it possible to cool the water or the media in which this coil is immersed, water thus capable of serving as an intermediary to conduct frigories towards the _CO2 fluid (FG-E) to be cooled or liquefied, coming from said _CO2 source; ∘ A second coil (S) capable of accommodating circulation of the high pressure two-phase gas-liquid _CO2 fluid (FG-E), transferred frigories by the cold water or the surrounding cold media being transferred to the _CO2 fluid circulating in this second coil and making it possible to cool this fluid and liquefy the proportion of gas phase which composes it, before directing this fluid thus cooled (FG-S) towards said user station.

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