EP0181796A1 - Reservoir for a cryogenic mixture and process for drawing off the liquid - Google Patents

Abstract

The liquid is drawn from this vessel through a coiled tube (20) provided with a calibrated valve (21) at its inlet. The resulting drop in pressure is compensated for by means of a pressure raising circuit (27). Application in the storage of liquefied air.

Description

The present invention relates to a container for liquid cryogenic mixture of the type comprising a reservoir in which is disposed a duct forming a heat exchanger which crosses the neck of the reservoir and the inlet of which is connected to the lower part of the reservoir by means of a member for creating a pressure drop, a line for drawing off the liquid mixture, and a pressure rise circuit.

In conventional cryogenic vessels, the pressure is kept roughly constant despite the inevitable heat inflows by the intermittent elimination of part of the gas phase by means of a calibrated valve. The preservation of liquid cryogenic mixtures in such a container is impossible, since the vaporization of the liquid resulting from the heat inputs is accompanied by a distillation of this liquid, so that it gradually becomes richer in component (s). (s) less volatile.

Containers of the aforementioned type have been proposed to avoid such a change in the composition of the liquid. In fact, when the internal pressure of the tank exceeds the outlet pressure of the heat exchanger by a quantity predetermined by the member creating a pressure drop, a certain quantity of liquid passes through the latter and, being reduced to a lower pressure, vaporizes then heats up by taking the corresponding heat from the container. The heat inputs are thus compensated without venting the gaseous phase, at the cost of a small liquid leak, and the composition of the liquid remains practically constant throughout the emptying of the container. The product leakage is of the same order as that which results from the venting of the gas phase, mentioned above.

An important drawback of this solution lies in the increased complexity of the tank structure, and in particular in the difficulty of adapting it to existing tanks of relatively low capacity and with a narrow neck. Indeed, the tank must include, in addition to the liquid withdrawal line and the cooling circuit comprising the exchanger, at least one filling line and, when the tank is intended to be filled with liquid mixture already prepared, a circuit circulation of an auxiliary refrigerant ensuring sub-cooling of the liquid mixture, and therefore an absence of distillation, during filling.

The invention aims to solve this problem by simplifying the structure of the tank.

To this end, the invention relates to a container of the aforementioned type, characterized in that the line for drawing off the liquid mixture is connected to the outlet of the heat exchanger, so that all of the liquid drawn off passes through this exchanger .

In an advantageous embodiment, the heat exchanger extends over almost the entire height of the tank.

The invention also relates to a process for withdrawing a liquid cryogenic mixture from a container provided with a cooling circuit by expansion of a liquid leak, characterized in that the liquid is drawn off exclusively through the cooling.

• - la figure 1 représente schématiquement en coupe longitudinale un récipient conforme à l'invention ;
• - la figure 2 représente plus en détail l'agencement intérieur du récipient ; et
• - la figure 3 est une vue en plan de la tête de ce récipient.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which:

• - Figure 1 shows schematically in longitudinal section a container according to the invention;
• - Figure 2 shows in more detail the interior arrangement of the container; and
• - Figure 3 is a plan view of the head of this container.

The container shown comprises an internal tank 1 with a capacity of 100 to 200 liters, for example, and an external envelope 2 (not shown in FIG. 1) separated by a vacuum space 3. The tank 1 has an upper neck 4 closed by a removable head 5.

• - une conduite 7 de remplissage en pluie, pourvue d'une vanne 8 et d'un raccord d'entrée 9 et ouverte à son extrémité inférieure ;
• - une conduite 10 d'entrée d'un agent réfrigérant pourvue d'un raccord d'entrée 11 et d'un clapet anti-retour 12 ;
• - une conduite 13 de sortie de cet agent, munie d'un clapet anti-retour 14 ; et
• - une conduite 15 de soutirage de liquide et de fuite de liquide comportant, à l'extérieur du réservoir, une soupape de mise à l'atmosphère 16, une vanne 17 et un raccord de sortie 18.

The head 5 is constituted by a flange 6 crossed vertically by four pipes which are rigidly connected to it:

• - a rain filling pipe 7, provided with a valve 8 and an inlet connection 9 and open at its lower end;
• - a coolant inlet pipe 10 provided with an inlet connector 11 and a non-return valve 12;
• - an outlet pipe 13 for this agent, fitted with a non-return valve 14; and
• a pipe 15 for withdrawing liquid and for leaking liquid comprising, outside the tank, a venting valve 16, a valve 17 and an outlet fitting 18.

Line 7 ends at an intermediate level in the neck 4, while the other three lines 10, 13 and 15 extend downwards approximately to the level of the connection of the neck 4 and the tank 1. At this location, line 10 is connected to the inlet of a heat exchange coil 19, the outlet of which is in turn connected to the inlet of line 13. At the same level, the line 15 is connected to the upper end of a second heat exchange coil 20 which extends to the bottom of the tank 1, where its lower end is fitted with a calibrated valve 21. Over the entire height occupied by the coil 19, the coil 20 has a reduced diameter and is disposed inside thereof. Below the coil 19, the coil 20 takes a larger diameter, substantially equal to that of the coil 19 and slightly less than the inside diameter of the neck 4.

Thus, the head 5, comprising the flange 6, the pipes 7, 10, 13 and 15 and the two coils 19 and 20, forms a removable monobloc assembly which can be fixed on the tank by threading the two coils in the neck and in fixing the flange 6 with screws on a flange 22 provided at the entrance to the neck 4, with the interposition of a seal 23.

To immobilize the two coils well, the valve 21 rests on the bottom of the tank 1 by slightly compressing the coil 20.

• - un disque de rupture 24 relié au col 4 par une tubulure équipée d'un manomètre ;
• - une tubulure 25 de purge, reliée au col 4, équipée d'une vanne et normalement condamnée par un bouchon 26 ; et
• - un circuit 27 de remontée en pression, comprenant une crosse 28 de soutirage de liquide au fond du réservoir 1, un vaporiseur 29 et une conduite 30 qui relie ce dernier au col 4 par l'intermédiaire d'un régulateur de pression 31.

Cam shown in Figure 1, the container also includes:

• - a rupture disc 24 connected to the neck 4 by a tube fitted with a pressure gauge;
• - A bleed pipe 25, connected to the neck 4, equipped with a valve and normally blocked by a plug 26; and
• a circuit 27 for raising the pressure, comprising a stock 28 for withdrawing liquid from the bottom of the reservoir 1, a vaporizer 29 and a pipe 30 which connects the latter to the neck 4 by means of a pressure regulator 31.

A cryogenic liquid mixture to be stored, for example a mixture of oxygen and nitrogen with 22% oxygen (that is to say practically liquid air), is introduced in rain, under the storage pressure. (for example 10 to 20 bars), via line 7. Before and during this filling, liquid nitrogen circulates in line 10 and the coil 19, and escapes in the gaseous state via line 13, to ensure that the pressure during these filling operations, and in particular if the tank is initially hot, does not exceed the value maximum tolerated, without the need to vent some of the vapor phase or liquid phase of the mixture.

During storage, the inevitable heat inputs cause some vaporization of the liquid and an increase in pressure. When the latter exceeds the opening pressure of the valve 16 by a predetermined value corresponding to the setting of the valve 21 (for example 2 bars), the latter opens and a small amount of liquid passes through the coil 20.

This leakage of liquid vaporizes then heats up in the coil 20, taking heat from the liquid and from the vapor phase which overcomes it, and the heated gas escapes through the valve 16. Thus, the liquid is under- cools, the vapor partially recondenses, and the pressure drops to a value for which the valve 21 and the valve 16 close. This allows, at the cost of a small loss of liquid, to keep the liquid with an almost constant composition, since the vapor phase is never put into the atmosphere.

Among the two phenomena explained above, it is the recondensatian of the vapor phase which constitutes the most efficient use of the cold produced by the expansion of the liquid. However, due to the extension of the coil 20 over the entire height of the tank, the heat exchange surface contained in the vapor phase increases in proportion to the volume of this phase. The efficiency of the vapor reoondensation is thus maximum.

When it is desired to draw off liquid, the valve 17 is opened, which puts the line 15 in communication with the operating circuit (not shown), connected to the connector 18. The operating circuit is assumed to be at a pressure lower than the storage pressure by an amount greater than the pressure drop imposed by the valve 21.

Consequently, this valve opens, and the withdrawn liquid passes entirely through the coil 20, the valve 16 remaining closed. All the liquid withdrawn therefore produces cold, which results on the one hand in a sub-cooling of the liquid remaining in the tank, and on the other hand a recondensation of the vapor phase which overcomes it.

As soon as the pressure drops below a predetermined value, the pressure regulator 31 opens, liquid passes through the butt 28 into the vaporizer 29, and the vapor thus produced is returned by the pipe 30 in the neck 4 from tank, until restored seed of the set pressure. Due to the speed of circulation of the fluid in the circuit 27, the vapor reinjected into the neck 4 has the same composition as the liquid withdrawn by the butt 28.

Thus, when the liquid is drawn off, there is simultaneously a sub-cooling of the liquid and a significant stirring, through the pressure rise circuit 27, which tends to keep the vapor phase at the same position as the liquid phase. These two phenomena oppose the distillation of the mixture, and it is thus possible to withdraw almost all of the liquid without disturbing variation in its composition.

It is also noted that the fact of withdrawing the liquid through the coil 20 minimizes the number of pipes to be provided, which allows all the necessary pipes to pass through the neck of the tank, even if it is narrow. The invention can thus easily be adapted to existing small containers of relatively reduced capacity.

Alternatively, the valve 21 can be replaced by another member capable of creating a pressure drop, for example by a capillary tube or a sintered element.

The container described above has circuit 10-19-13 because it was assumed that it was filled with the liquid mixture already prepared. However, the mixing can also be carried out in the container itself, in the following manner: the necessary quantity of the most volatile liquid (for example liquid nitrogen) is poured, which ensures that the container is cooled and the equilibrium of nitrogen under atmospheric pressure at 77 K; in an external condenser, which may be common to a set of receptacles, the other constituent (s) of the mixture (for example liquid oxygen) is sub-cooled to the same temperature (for example in a coil immersed in nitrogen liquid), then pour it (them) into the container. In this case, the circuit 10-19-13 is no longer necessary, and as a result the removable head 5 of the reservoir 1 no longer has more than two pipes passing through the flange 6, namely the pipes 7 and 15, and the structure of the tank is further simplified.

Claims (9)

1. - Container for liquid cryogenic mixture, of the type comprising a reservoir (1) in which is disposed a duct forming a heat exchanger (20) which passes through the neck (4) of the reservoir and the inlet of which is connected to the lower part of the reservoir by means of a member (21) for creating a pressure drop, a pipe (15) for drawing off the liquid mixture, and a circuit (27) for raising the pressure, characterized in that the pipe (15) for withdrawing the liquid mixture is connected to the outlet of the heat exchanger (20), so that all of the liquid withdrawn passes through this exchanger. 2. - Container according to claim 1, characterized in that the heat exchanger (20) extends over almost the entire height of the tank (1). 3. - Container according to one of claims 1 and 2, characterized in that it also comprises a filling pipe (7) passing through a flange (6) adapted to be removably attached to the inlet of the neck (4 ) from the tank (1). 4. - Container according to claim 3, characterized in that it also comprises two pipes (10, 13) inlet and outlet, respectively, of an auxiliary refrigerant fluid connected together by an auxiliary heat exchanger (19 ), these two pipes (10, 13, 15) passing through said flange (6). 5. - A container according to claim 4, characterized in that the two heat exchangers (19, 20) are nested one inside the other, the transverse dimension of the assembly being less than the diameter of the neck (4) of the tank (1). 6. - Container according to any one of claims 1 to 5, characterized in that the or each heat exchanger (19, 20) is constituted by a coil, in particular helical. 7. - Container according to any one of claims 1 to 6, comprising a valve (16) for venting, characterized in that this valve is mounted on the liquid withdrawal line (15). 8. - Container according to any one of claims 1 to 7, characterized in that it comprises a rupture disc (24) connected to the upper part (4) of the reservoir (1). 9. - Method for withdrawing a cryogenic liquid mixture in a container provided with a cooling circuit by expansion of a liquid leak, characterized in that the liquid is drawn off exclusively through the cooling circuit.

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