The air distillation double column (1,2) includes an apparatus for vaporizing oxygen and condensing nitrogen which comprises on one hand a running type main heat exchanger (7) which is partially immersed during stoppage of the equipment and on the other hand an auxiliary heat exchanger (20) which alone is responsible for liquid vaporization when the equipment is restarted.
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9. Equipment for air distillation of the double column type having a low pressure column and a mean pressure column and comprising at least a main heat exchanger disposed in the low pressure column, said exchanger being of the running type and including oxygen ducts for causing oxygen to run in excess in said ducts, means for withdrawing total vaporized oxygen and excess liquid oxygen through an end of said ducts, nitrogen ducts in indirect heat exchange relationship with the oxygen ducts, means for feeding nitrogen gas from the mean pressure column to said nitrogen ducts, and means for returning condensed nitrogen to said mean pressure column, wherein said main heat exchanger is mounted so as to be at least partially immersed during a stoppage in the operation of the double column, the improvement comprising at least an auxiliary heat exchanger for providing, by itself, liquid vaporization when said main exchanger is at least partially immersed in said liquid oxygen.
1. Apparatus for vaporizing oxygen and condensing nitrogen for air distillation equipment of the double column type having a low pressure column containing a vat and a mean pressure column and comprising at least a main heat exchanger disposed in the vat of the low pressure column, said exchanger being of the running type and including oxygen ducts and nitrogen ducts, means for causing oxygen to run in excess in said oxygen ducts, means for withdrawing total vaporized oxygen and excess liquid oxygen through a lower end of said oxygen ducts, nitrogen ducts in indirect heat exchange relationship with the oxygen ducts, means for feeding nitrogen gas originating from the mean pressure column to the nitrogen ducts and means for returning condensed nitrogen to the mean pressure column, wherein the main heat exchanger is mounted so as to be at least partially immersed during a stoppage in the operation of the air distillation equipment, the improvement comprising at least an auxiliary heat exchanger for ensuring, substantially by itself, liquid vaporization when the main exchanger is at least partially immersed.
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1. Field of the Invention
The present invention relates to vaporizers-condensers of air distillation equipments. It first concerns an apparatus for vaporizing oxygen and condensing nitrogen for an air distillation double column of the type comprising at least a main heat exchanger disposed in the vat portion of the low pressure column, this exchanger being of the running type and including oxygen ducts, means for causing oxygen in excess to run in these ducts, means for withdrawing total vaporized oxygen and excess liquid nitrogenupwardly moving liquid upwards, for example by means of a thermosiphon or gas "extraction" comprising an indirect heat exchanger 15A heated with an appropriate fluid, which can be the "rich liquid" originating from the vat of column 1, as it is known in the art. On FIG. 1, this variant has been represented in broken line and a duct 17 for withdrawing oxygen gas from column 2 and a duct 18 for withdrawing liquid nitrogen from column 1 have also been illustrated.
To reduce at a maximum as much as possible the height of the lower pressure column, it is provided that the level N is at a small distance below the exchanger 7, as indicated above. If the equipment is stopped as explained above, the "working charge" of many plates is collected in the vat portion of column 2, and the liquid climbs up to a level N1 in which the exchanger 7 is partially immerse. In particular, there is a certain height of liquid in the lower portion of the oxygen ducts of this exchanger. When the equipment is stared again, a small quantity of oxygen is vaporized, but since the oxygen ducts are opened only towards the bottom, a state of equilibrium is rapidly achieved, and the exchanger cannot continue to operate. FIGS. 2-5, on which the ducts concerning nitrogen have been omitted for better illustration of the drawing, show how the equipment can be modified according to the invention to enable the repriming of the exchanger 7.
In the solution of FIG. 2, the vat of column 2 contains two main heat exchangers 7 disposed in parallel at the same level as FIG. 1, i.e. with their lower ends very close to the bottom 6, just above level N of the bath of liquid oxygen. The holder 8 is common to both exchangers.
The equipment includes an auxiliary cylindrical member 19 containing an auxiliary heat exchanger 20. This exchanger is also of the running type and is similar to exchanger 7. Cylindrical member 19 is closed at the top by an upper partition 21 and at the bottom by a lower partition 22, the latter being above the lever of the holder 8 of exchangers 7. Duct 16 for the upward movement of liquid opens at the top of cylindrical member 19; a duct 23 connects the bottom 22 to holder 8, and ducts 24 and 24A, respectively connect the space located immediately below exchanger 20 and the space located above partition 21 to the region of the cylindrical member 5 located immediately above holder 8.
In normal operation, pump, 15 moves liquid oxygen upwardly bath 14 above cylindrical member 19 to maintain an auxiliary liquid holder 25 above the exchanger 20. About half of this flow of liquid is vaporized in the exchanger, and the excess of liquid oxygen as well as vaporized oxygen pass through cylindrical member 5 via ducts 23 and 24. The excess liquid oxygen is added to the liquid oxygen falling from the plates of column 2 in the holder 8, and about half of the total flow of liquid oxygen feeding the latter is vaporized in the exchangers 7, the excess liquid being taken over by the pump 15.
During a stoppage of the equipment, the vat liquid of column 2 moves upwardly to level N1 as in FIG. 1. To reprime the equipment, pump 15 pushes some liquid up to the top of the auxiliary exchanger 20, which through its position, remains operational. A portion of the flow of liquid is therefore vaporized through the sole action of the exchanger 20, and the excess liquid as well as the vaporized liquid is sent, as previously, into cylindrical member 5, via ducts 23 and 24. Then, the liquid level progressively decrease in column 2, and when the level N returns about to normal, the exchangers 7 can resume their operation. The dimensions of exchanger 20 are such as to permit the equipment to treat a flow of air which is required for priming the plates in order that their "working charge" be reconstituted, this flow of air being lower than the flow corresponding to the normal operation of the equipment.
Thus, the additional cylindrical member 19 and the auxiliary exchanger 20 are constantly used as additional heat exchange surface which improves the thermic performance of the equipment.
As a variant, the exchanger 20 may be disposed at a level which is lower than the holder 8 or even lower than level N1 with an additional pump provided on the duct 23. On the other hand, cylindrical member 19 can be made of the exchanger block per se in its running portion.
In the equipment of FIG. 3, there are three exchangers 7 and they are disposed Sin FIG. 2, side by side and immediately above bath 14, with a common holder 8. The auxiliary exchanger is made of three exchangers 20A which are identical to exchanger 7 and are disposed in column 2 immediately above the latter. Duct 16 comprises a branch 16A opening in the holder 25A of exchangers 20A, and a branch 16B opening in the holder 8 of exchangers 7. These ducts are provided with respective stop valves 26A and 26B.
In normal operation; the bath 14 of liquid oxygen is at level N. Valve 26A is closed and valve 26B is opened. The auxiliary exchangers 20A are supplied with liquid oxygen exclusively by means of the plates of column 2, they vaporize just about half this flow and send the remainder to holder 8. A flow of the same order is moved upwardly to holder 8 by means of pump 15, half of the total flow is vaporized in the exchangers 7 and the remaining portion falls into bath 14.
During a stoppage of the equipment, the upward movement of the liquid to level N1 causes the exchangers 7 to be partially immersed. When the equipment is again started, valve 26B is closed, valve 26A is opened, and pump 15 moves liquid upwardly to upper holder 25A. A portion of this flow is vaporized, the liquid progressively lowers in the vat of the column, and when the liquid returns to approximately the level N, the exchangers 7 resume their operation. The advantage of this solution resides in the fact that it is possible to dispose auxiliary exchangers having a much higher heat surface than the cylindrical member of the column per se, which enables further improvement in the heat exchange performance in normal operation, for example to reach a temperature difference on the order of 0.5°C between mean pressure nitrogen and liquid oxygen. It will be noted, moreover, that duct 17 for withdrawing oxygen gas can be disposed anywhere between the top of exchangers 7 and the plates of column 2 without running the risk of removing some liquid.
It should be noted that the exchangers 20 of FIG. 2 and 20A of FIG. 3 could be designed so as to permit the withdrawal of the vaporized liquid from the top as described in the European Patent Application mentioned above.
In the embodiment of FIG. 4, there are provided, side by side in cylindrical member 5, two main exchangers 7 and two auxiliary exchangers 20B. The four exchangers have their lower end located at a short distance above level N; they are all identical, with the exception that the two exchangers 7 have a common holder 8 which is opened at the top as in the previous examples while the two exchangers 20AB have a common holder 25B which is sealingly covered with a semi-cylindrical horizontal feeding box 27 to which duct 16 opens. A duct 27A starts from the top of the box 27, exits from cylindrical member 5, is provided outside the latter with a valve 27B and opens in cylindrical member 5, above level N.
In normal operation of the equipment, valve 27B is opened. The same flow reaches holder 8 as originating from the plates and the holder 25B via duct 16. Each exchanger vaporizes about a quarter of this flow, and the excess liquid falls in bath 14 to be sent upwardly via pump 15.
During a stoppage of the equipment, the liquid moves upwardly to the level N1 and partially immerses the four exchanges. To restart the installation, valve 27B is closed; the pump pushes some liquid upward in box 27 and introduces into the latter an overpressure which enables vaporized oxygen in the exchangers 20B to act against the thrust of the liquid bath in lower portion. The liquid progressively lowers in the vat of the column, the pressure in box 27 also decrease gradually and when the level N returns about to normal, the exchangers 7 start again to operate and valve 27B is opened.
The advantage of this solution is that no additional height of the cylindrical member 5 nor any auxiliary space outside the column are required.
FIG. 5 represents a solution which may be considered as a variant of FIG. 2: the cylindrical member 19 is at a lower level than FIG. 2, the bottom 22 is substantially at the level of bottom 6 of the double column. A duct 28 provided with a valve 29, replacing the duct 23, connects the vats of the cylindrical members 8 and 19. As in FIG. 2 duct 24 connects the space located immediately below exchanger 20 to the region of cylindrical member 5 located above holder 8. Duct 24A is provided with a valve 24B.
In normal operation, the valves 29 and 24B are opened and level N is established in the two cylindrical members 5 and 19. Exchanger 20 constitutes an additional vaporizer-condenser which is supplied with liquid oxygen through the duct 16 while the exchanger 7 is supplied with liquid oxygen exclusively by means of the plates 3.
As soon as the equipment stops valve 29 is closed at the same time as the pump stops, which prevents the immersion of the exchanger 20. When restarting, liquid is vaporized through the sole action of the exchanger 20, and it will be relised it is a diphasic fluid which returns to column 2 via duct 24.
Another possibility is that valve 29 remains opened. The exchanger 20 is then partially immersed as is the case with the exchanger 7 during stoppages of the equipment, and restarting is carried out by closing valve 24B and by providing an overpressure at the upper end of the cylindrical member 19 by means of pump 15, in a similar manner as that which has been described with respect to FIG. 4. This way of restarting with the immersed exchanger 20 can, on the other hand still be carried out with valve 29 closed.
In the embodiment of FIG. 6, there are provided three exchangers 7 and, immediately below these exchangers and immediately above bottom 6, there are a plurality of, for example three auxiliary exchangers 20C of the bath or thermosiphon type. These exchangers differ from exchangers 7 in that there is no upper holder 8, the oxygen ducts being upwardly freely opened. Such exchangers, which are well known in the art of air distillation, may operate by being completely immersed. On the other hand, there is no duct 16.
In normal operation of the equipment, level N is such that the exchangers 20C are nearly entirely immersed. The holder 8 of the exchangers 7 is supplied with liquid oxygen originating solely from the plates. About half the flow is vaporized in these exchangers and the remaining portion falls into bath 14. Since the exchangers 20C vaporize this excess flow, it is therefore not necessary in principle to send some liquid towards holder 8. However as a variant, since the bath type vaporizers have a lower yield than the running type vaporizers, it may be preferable to adjust the size of the exchangers 20C so that they vaporize only a small fraction of the flow of liquid oxygen, the excess flow then being sent into holder 8 as previously mentioned.
During a stoppage of the equipment, the liquid climbs to level N1, so that the exchangers 20C are totally immersed and the exchangers 7 are partially immersed Restarting is carried out without difficulty, first exclusively through the vaporization provided by the exchangers 20C, then when the level N is substantially reestablished also by means of exchangers 7.
Due to the presence of bath type exchangers, the solution of FIG. 6 is more particularly suitable to those cases where relatively moderate heat exchange performance is acceptable, for example, a difference of temperature of the order of 1°C between mean pressure nitrogen and liquid oxygen.
Petit, Pierre, Grenier, Maurice
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