EP0410832B1 - Evaporator-condenser for a double column air separation apparatus - Google Patents

Description

The present invention relates to double column air distillation installations comprising an oxygen vaporization and nitrogen condensation apparatus of the type comprising at least one main heat exchanger disposed in the tank of the low pressure column, this exchanger being of the trickle type and comprising oxygen passages, means for causing excess liquid oxygen to flow in these passages, means for evacuating all of the vaporized oxygen and the excess oxygen liquid through the lower end of the same passages, nitrogen passages in indirect heat exchange relationship with the oxygen passages, means for supplying the nitrogen passages with nitrogen gas coming from the medium pressure column, means for returning the condensed nitrogen to the medium pressure column.

In air distillation systems of the double column type, the liquid oxygen which is in the bottom of the low pressure column is vaporized by heat exchange with the nitrogen gas taken off at the head of the medium pressure column. For a given operating pressure of the low pressure column, the temperature difference between oxygen and nitrogen made necessary by the structure of the heat exchanger imposes the operating pressure of the medium pressure column. It is therefore desirable that this temperature difference is as small as possible, in order to minimize the expenses linked to the compression of the air to be treated injected into the medium pressure column.

Drip type vaporizers / condensers are very advantageous for their excellent heat exchange performance, and can be produced reliably and economically thanks to the technology described in EP-A-130 122 in the name of the applicant.

However, the following problem arises.

During a shutdown of the air distillation installation following an incident (momentary power cut, machine incident, etc.) or scheduled, the liquids stored on the trays of the upper column (lower column pressure) and possibly in the argon mixture column associated with the double column, or even the liquids stored on the trays of the lower column (medium pressure column) if no action is taken regarding the operation of the liquid ascent valve rich, will end up poured into the tank of the low pressure column, precisely where the vaporizer-condenser is installed.

With units for which high purities and high extraction yields are required, the number of trays is considerable and the "charge in use" or "hold-up" of liquid thus suddenly poured into the tank of the low pressure column during a stop, will represent a height of several meters. When the exchanger is placed in the tank of the low pressure column during a stop, will represent a height of several meters. When the exchanger is placed in the tank of the low pressure column and when the exit of oxygen, both gaseous and liquid, can only take place from the bottom of the exchanger, the latter then being at least partially submerged , is unable to reboot when restarting the installation.

Putting the unit back into service after a few moments, a few hours, or even a few days of shutdown therefore requires a preliminary purging of the liquid still present in the tank, then that this liquid is welcome since it makes it possible to instantly recharge the trays of the various columns of which it constituted the "charge in use".

To be able to re-prime the condenser vaporizer without draining the liquid collected in the tank, one could think either of installing the exchanger at a sufficient height from the bottom of the column tank so that the collected liquid does not reach the lower part of this exchanger , either to install outside the column, or as an appendage or wart to the column tank, a capacity for retaining this liquid. However, these solutions would require realizing a space of large dimensions which is useless in normal operation, which would represent an excessive cost of investment.

The object of the invention is to solve the problem of re-initiating the heat exchanger relatively economically.

According to the invention, the main heat exchanger is arranged so as to be at least partially submerged during a stop of operation of the double column, and the apparatus comprises at least one auxiliary heat exchanger adapted to ensure alone the vaporization of liquid when the main exchanger is at least partially submerged.

In a first embodiment, the auxiliary exchanger is a flow type exchanger comprising oxygen passages, means for causing excess liquid oxygen to flow in these passages, nitrogen passages in relation to indirect heat exchange with oxygen passages, means for supplying passages from nitrogen to nitrogen gas coming from the medium pressure column, and means for returning the condensed nitrogen to the medium pressure column, the auxiliary exchanger being situated entirely above the maximum level of the liquid in the tank of the column low pressure, and means are provided for raising this liquid 'to the top of the oxygen passages of the auxiliary exchanger as well as means for returning liquid from the lower end of the auxiliary exchanger to the top of the passages d oxygen from the main exchanger.

In a second embodiment, the auxiliary exchanger is an exchanger of the same type as the main exchanger and is disposed substantially at the same level as the latter in the tank of the low pressure column, the top of the oxygen passages of the 'auxiliary exchanger being supplied exclusively by a pipe for raising the liquid contained in this tank.

In a third embodiment, the auxiliary heat exchanger is a bath type exchanger arranged below the main exchanger in the tank of the low pressure column.

• la Fig. 1 représente schématiquement la structure et le fonctionnement d'un échangeur de chaleur du type à ruissellement et à sortie d'oxygène exclusivement par le bas ; et
• les Fig. 2 à 5 représentent schématiquement une partie d'une installation de distillation d'air suivant l'invention, selon plusieurs modes de réalisation différents de l'appareil de vaporisation-condensation.

A few embodiments of the invention will now be described with reference to the appended drawings, in which:

• Fig. 1 schematically shows the structure and operation of a heat exchanger of the trickle type and with oxygen outlet exclusively from the bottom; and
• Figs. 2 to 5 schematically represent a part of an air distillation installation according to the invention, according to several different embodiments of the vaporization-condensation apparatus.

We see in each of the figures the top of the medium pressure column 1 and the tank of the low pressure column 2 of a double column air distillation installation, each column comprising distillation plates 3 or an equivalent structure of heat and material exchange. Column 1, which operates at around 6 bar absolute, is limited by a cylindrical ferrule 4, and column 2, which operates a little above atmospheric pressure, by a cylindrical ferrule 5. The two columns are separated by a bottom 6 curved upwards. The head nitrogen of column 1 is condensed by vaporizing liquid oxygen reaching the tank of column 2, by means of an indirect heat exchanger 7 of the trickle type.

The exchanger 7 is essentially constituted by a large parallelepiped block, for example 1 to 1.5 square meters of horizontal section and 3 to 6 meters in height, formed of a stack of a large number of parallel vertical plates in aluminum which define between them flat passages. Each of these passages contains aluminum waves forming spacers and fins and is delimited by vertical or horizontal bars. Part of these passages, for example one passage in two, is an oxygen passage, and the remaining passages are nitrogen passages. The oxygen passages are supplied from above with liquid oxygen by means of a liquid retention 8 formed at the top of the exchanger, laterally closed and open down. The nitrogen passages are closed on all sides and are supplied laterally with gaseous nitrogen, near their upper end, by means of a semi-cylindrical box 9 with a horizontal axis, which communicates with the top of the column 1 by 1 'through a pipe 10. The condensed nitrogen is collected laterally at the bottom of the same passages by another semi-cylindrical box 11 with a horizontal axis and, from there, is returned to column 1 by a pipe 12. The latter opens out in a channel 13 which ensures a guard of liquid nitrogen. The block of the exchanger 7 is assembled by brazing in the furnace.

In normal operation, a liquid oxygen bath 14 is present in the tank of the column 2, and its level N is located below the lower end of the exchanger 7, at a small distance from the latter. A pump 15 raises via a line 16 a flow D of liquid oxygen in the reservoir B, which also receives a flow D of liquid oxygen from the plates of the column 2. A flow D of oxygen is vaporized in the exchanger 7 , so that a flow D of excess liquid oxygen falls into the bath 14. The flows can deviate more or less from the value D in practice.

Other details concerning the structure and operation of such a trickle vaporizer-condenser are described in the abovementioned EP-A-130 122.

As a variant, the pump 15 can be replaced by any other means for raising the liquid, for example by a thermosyphon or "gas-lift" constituted by an indirect heat exchanger 15A heated by a suitable fluid, which can be "rich liquid " from the tank in column 1, as is conventional in the art. In Fig. 1, this variant is shown in dashed lines, and there is also shown a pipe 17 for withdrawing gaseous oxygen from column 2 and a pipe 18 for withdrawing liquid nitrogen from column 1.

To reduce the height of the low pressure column as much as possible, level N is provided a short distance below the exchanger 7, as indicated above. In the event of a shutdown of the installation, as explained above, the "load in use" of numerous trays collects in the tank of column 2, and the liquid rises to a level N1 for which the exchanger 7 is partially submerged. In particular, a certain height of liquid is present in the lower part of the oxygen passages of this exchanger. When the installation restarts, a small amount of oxygen is vaporized, but since the oxygen passages are only open downwards, a state of equilibrium is quickly reached, and the exchanger cannot continue to operate. Figs. 2 to 5, on which the pipes relating to nitrogen have been omitted for the sake of clarity of the drawing, show how the installation can be modified according to the invention to allow re-ignition of the exchanger 7.

In the solution of FIG. 2, the tank of the column 2 contains two main heat exchangers 7 arranged in parallel at the same level as in FIG. 1, that is to say with their lower end very close to the bottom 6, just above the level N of the liquid oxygen bath. The reservoir 8 is common to the two exchangers.

The installation includes an auxiliary ferrule 19 containing an auxiliary heat exchanger 20. This exchanger is also of the trickle type and has the same constitution as the exchanger 7. The shell 19 is closed at the top by an upper bottom 21 and at the bottom by a lower bottom 22, which is located above the level of the reservoir 8 of the exchangers 7. The liquid ascent pipe 16 opens out at the top of the shell 19; a pipe 23 connects the bottom 22 to the retainer 8, and pipes 24 and 24A respectively connect the space located just below the exchanger 20 and the space located below the bottom 21 to the region of the shell 5 located just above the reservoir 8.

In normal operation, the pump 15 rises liquid oxygen from the bath 14 to the top of the shell 19 to maintain an auxiliary retention 25 of liquid at the top of the exchanger 20. About half of this liquid flow is vaporized in this exchanger, and the excess of liquid oxygen as well as the vaporized oxygen pass into the ferrule 5 via the lines 23 and 24. The excess of liquid oxygen is added to the liquid oxygen falling from the plates of the column 2 in the reservoir 8, and approximately half of the total flow of liquid oxygen supplying the latter is vaporized in the exchangers 7, the excess liquid being taken up by the pump 15.

When the installation is stopped, the tank liquid in column 2 rises to level N1 as in Fig. 1. To restart the installation, the pump 15 rises the liquid at the top of the auxiliary exchanger 20, which, by its position, has remained in operating condition. Part of the liquid flow is therefore vaporized by the only exchanger 20, and the excess liquid as well as the vaporized liquid passes as previously into the shell 15, via the lines 23 and 24. As a result, the level of the liquid gradually decreases in column 2, and when the level N is almost restored, the exchangers 7 can operate again. The exchanger 20 is dimensioned so as to allow the installation to process the air flow rate necessary for priming the plates so that their "charge in use" is reconstituted, this air flow rate being less than the flow rate corresponding to the normal operation of the installation.

Thus, the additional shell 19 and the auxiliary exchanger 20 are constantly used as an additional heat exchange surface, which improves the thermal performance of the installation.

As a variant, the exchanger 20 could be arranged at a level lower than the reservoir 8 or even than the level N1, with an additional pump fitted to the pipe 23. Furthermore, the shell 19 can be constituted by the exchanger block itself in its current part.

In the installation of FIG. 3, the exchangers 7 are three in number and are arranged as in FIG. 2, side by side and just above the bath 14, with a common retainer 8. The auxiliary exchanger consists of three exchangers 20A identical to the exchangers 7 and arranged in column 2, just above these. The pipe 16 comprises a branch 16A opening into the retaining 25A of the exchangers 20A, and a branch 16B opening into the retaining 8 of the exchangers 7. These pipes are equipped with respective stop valves 26A, 26B.

In normal operation, the liquid oxygen bath 14 is at level N. The valve 26A is closed and the valve 26B is open. Auxiliary exchangers 20A are supplied with liquid oxygen only by the plates of column 2, vaporize approximately half of this flow and supply the rest to the reservoir 8. A flow of the same order is raised by the pump 15 to the reservoir 8, half of the total flow is vaporized in the exchangers 7, and the rest falls into the bath 14.

When the installation is stopped, the rise of the liquid to the level N1 partially immerses the exchangers 7. On restarting, the valve 26B is closed, the valve 26A is open, and the pump 15 rises the liquid in the reservoir. upper 25A. Part of this flow vaporizes, the liquid gradually drops in the column tank, and when it has returned to approximately level N, the exchangers 7 operate again. The advantage of this solution lies in the fact that it is possible to have auxiliary exchangers having a much larger heat surface in the shell of the column itself, which makes it possible to further improve the exchange performance. heat in normal operation, for example reaching a temperature difference of about 0.5 ° C between medium pressure nitrogen and liquid oxygen. It is further noted that the pipe 17 for withdrawing gaseous oxygen can be placed at any location between the top of the exchangers 7 and the plates of the column 2 without risking conveying liquid.

It should be noted that the exchangers 20 of FIG. 2 and 20A of FIG. 3 could be made so as to allow the evacuation of the liquid vaporized from above, as described in the abovementioned EP-A.

In the embodiment of FIG. 4, two main exchangers 7 and two auxiliary exchangers 20B are provided side by side in the shell 5. The four exchangers have their lower ends located a short distance above level N; they are all identical, with one difference: the two exchangers 7 have a common retainer 8 open upwards as in the previous examples, while the two exchangers 20B have a common retainer 25B hermetically covered by a horizontal horizontal feed box -cylindrical 27 into which the pipe 16 opens. A pipe 27A starts from the top of the box 27, leaves the ferrule 5, is fitted outside of the latter with a valve 27B and opens into the ferrule 5, at - above level N.

In normal operation of the installation, the valve 27B is open. The same flow reaches the reservoir 8 coming from the plates and the reservoir 25B via the pipe 16. Each exchanger vaporizes approximately a quarter of this flow, and the excess liquid falls into the bath 14 to be raised by the pump 15.

When the installation stops, the liquid rises to level N1 and partially immerses the four exchangers. To restart, the valve 27B is closed; the pump rises the liquid in the box 27 and develops in it a pressure which allows the oxygen vaporized in the exchangers 20B to overcome the thrust of the liquid bath in the lower part. The liquid gradually decreases in the column tank, the pressure in the box 27 also decreases progressively, and when the level N is almost found, the exchangers 7 start to operate again, and the valve 27B is opened.

The advantage of this solution is that no additional height of the shell 5 or any auxiliary space outside the column is necessary.

Fig. 5 shows a solution which can be considered as a variant of FIG. 2: the ferrule 19 is at a lower level than in FIG. 2, the bottom 22 being approximately at the level of the bottom 6 of the double column. A pipe 28 fitted with a valve 29, replacing the pipe 23, connects the tanks of the ferrules 5 and 19. The pipe 24 connects as in FIG. 2 the space located just below the exchanger 20 at the region of the shell 5 located above the reservoir 8. The pipe 24A is equipped with a valve 24B.

In normal operation, the valves 29 and 24B are open, and the level N is established in the two ferrules 5 and 19. The exchanger 20 constitutes an additional evaporator-condenser supplied with liquid oxygen by line 16 while the exchanger 7 is supplied with liquid oxygen by the plates 3 only.

As soon as the installation is stopped, the valve 29 is closed simultaneously with the pump stopping, which prevents immersion of the exchanger 20. During a restart, liquid is vaporized by the only exchanger 20 , and it is two-phase fluid which returns to column 2 via line 24.

Another possibility is to leave the valve 29 open. The exchanger 20 is then partially submerged like the exchanger 7 during the stops of the installation, and the restart is carried out by closing the valve 24B and creating by means of the pump 15 an overpressure in the upper bottom of the shell. 19, analogously to what has been described with reference to FIG. 4. This restart mode with the submerged exchanger 20 can also be carried out with the valve 29 closed.

In the embodiment of FIG. 6, there are provided three exchangers 7 and, just below these and just above the bottom 6, several, for example three, auxiliary exchangers 20C of the bath or thermosyphon type. These exchangers differ from the exchangers 7 in that the upper retainer 8 does not exist, the oxygen passages being freely open upwards. Such exchangers, conventional in the air distillation technique, can operate while being completely submerged. Furthermore, the pipe 16 is deleted.

In normal operation of the installation, the level N is such that the exchangers 20C are almost entirely submerged. The reservoir 8 of the exchangers 7 is supplied only with liquid oxygen coming from the plates. About half of the flow is vaporized in these exchangers, and the rest falls into the bath 14. The exchangers 20C vaporizing this excess flow, it is therefore not in principle necessary to flow the liquid back to the reservoir 8. As a variant however, since bath vaporizers have a lower yield than trickle vaporizers, it may be preferable to size the 20C exchangers so that they vaporize only a small fraction of the flow of liquid oxygen, the excess flow then being raised in reservoir 8 as before.

When the installation is stopped, the liquid rises to level N1, so that the exchangers 20C are completely submerged and the exchangers 7 partially submerged. Restarting is carried out without difficulty, first only by the vaporization provided by the 20C exchangers, then, when the level N is almost restored, also by exchangers 7.

Due to the presence of bath exchangers, the solution of FIG. 6 is more particularly suitable for cases where relatively moderate heat exchange performance is acceptable, for example a temperature difference of the order of 1 ° C. between the medium pressure nitrogen and the liquid oxygen.

Claims (8)

1. Double column (1,2) air distillation plant having an apparatus for oxygen evaporation and nitrogen condensation comprising at least one main heat exchanger (7) arranged in the interior of the low pressure column (2), this exchanger being of the trickle type and comprising oxygen passages, means (8) for causing an excess of liquid oxygen to trickle into these passages, means for carrying away all of the evaporated oxygen and the excess of liquid oxygen by way of the lower ends of the same passages, nitrogen passages in contact by indirect thermal exchange with the oxygen passages, means (9, 10) for supplying the nitrogen passages with gaseous nitrogen produced by the medium pressure column (1), and means (11, 12) for returning condensed nitrogen to the medium pressure column, characterised in that the main heat exchanger (7) is arranged so as to be at least partly immersed when there is a halt in the operation of the double column, and in that the apparatus comprises at least one auxiliary heat exchanger (20 ; 20A ; 20B ; 20C) suitable for ensuring by itself the evaporation of liquid when the main exchanger is at least partly immersed.
2. Plant according to claim 1, characterised in that the auxiliary exchanger (20, Figure 2; 20A) is an exchanger of the trickle type comprising oxygen passages, means (25; 25A) for causing an excess of liquid oxygen to trickle into the passages, nitrogen passages in contact by indirect thermal exchange with the oxygen passages, means for supplying the nitrogen passages with gaseous nitrogen produced by the medium pressure column (1), and means for returning condensed nitrogen to the medium pressure column, and in that there are provided means (15, 16) for raising the liquid contained in the vessel of the low pressure column to the tops of the oxygen passages of the auxiliary exchanger as well as means (23) for returning liquid from the lower end of the auxiliary exchanger to the tops of the oxygen passages of the main exchanger (7).
3. Plant according to claim 2, characterised in that the auxiliary exchanger is located entirely above the maximum level (N1) of the liquid in the vessel of the low pressure column (2).
4. Plant according to claim 3, characterised in that the auxiliary exchanger (20 ; 20A) is arranged entirely above the top of the main exchanger (7).
5. Plant according to any one of claims 2 to 4, characterised in that the auxiliary exchanger (20) is arranged outside the low pressure column (2).
6. Plant according to claim 4, characterised in that the auxiliary exchanger (20A) is arranged in the tube (5) of the low pressure column (2), above the main exchanger (7).
7. Plant according to claim 1, characterised in that the auxiliary exchanger (20, Figure 5 ; 20B) is an exchanger of the same type as the main exchanger (7) and is arranged substantially at the same level as the latter, the tops of the oxygen passages of the auxiliary exchanger being covered by a sealed supply box (21, Figure 5 ; 27) supplied exclusively by a pipe (16) for raising the liquid contained in the said vessel.
8. Plant according to claim 1, characterised in that the auxiliary heat exchanger (20C) is an exchanger of the type having a bath arranged below the main exchanger (7) in the vessel of the low pressure column (2).

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