Apparatus And Method For Separating Air By Cryogenic Distillation

Abstract

An apparatus and to a method for producing krypton and xenon by separating air by cryogenic distillation and in particular to a method for separating gasses from air using internal compression (in which the gaseous oxygen is produced by vaporization under pressure in the exchange line) where a krypton/xenon mixture is extracted is provided.

Description

The present invention relates to an apparatus and to a method for producing krypton and xenon by separating air by cryogenic distillation and in particular to a method for separating gasses from air using internal compression (in which the gaseous oxygen is produced by vaporization under pressure in the exchange line) where a krypton/xenon mixture is extracted.

In a unit for separating gasses from air comprising a double column and where the gaseous oxygen is produced by vaporization under pressure in the exchange line (internal compression apparatus) in which provision is made to extract a krypton/xenon mixture, it is observed that the efficiency of this extraction decreases significantly because the pumped oxygen extracted from the low pressure distillation column brings with it a non-negligible proportion of components to be valorized, such as krypton and xenon.

For this type of internal compression apparatus, in the case of the production of krypton/xenon associated with a production of argon, several methods making it possible to overcome this problem have been considered.

The invention can summarized as two essential steps:

• • 1. Impoverishment of the gaseous air entering the column for separation into components to be valorized (krypton, xenon) by transfer of material with a reduced proportion of the rich liquid. This step gives a lower rich liquid, rich in krypton and xenon, preferably with 100% of the xenon thus contained in the gaseous air being extracted as well as more than 80% of the krypton.
  • 2. Concentration of the lower rich liquid rich in krypton and in xenon, by partial vaporization in a vaporizer/condenser. The heat source used for vaporizing the lower rich liquid is preferably a fluid of the column from which comes said lower rich liquid, a fluid coming from a higher distillation stage which, on giving up its heat, condenses in the vaporizer/condenser. The liquid thus produced feeds the low pressure column at an appropriate distillation level. The lower rich liquid itself produces a vaporized phase which feeds the low pressure column at an appropriate distillation level, and a purge where preferably approximately 100% of the xenon contained in the feed is extracted, whereas only about 50% of the krypton is present there.

According to a variant, the low pressure column is fed by the purge rich in krypton and in xenon produced by concentration. The purge rich in krypton and in xenon produced above will feed the low pressure column, at a distillation stage lower than the tapping of the liquid oxygen which is intended to be pumped in order to then be vaporized in the main exchanger. The final mixture, rich in krypton and in xenon, is extracted from the bottom of the low pressure column in order to be subsequently treated there.

Optionally, in the case where the krypton and xenon mixture is in liquid form, this return is injected at the intake of the pump in order to minimize the OL tapping from the column and thus to increase the extraction yield of the krypton and xenon.

Such a method makes it possible to extract about 90% of the xenon present in the air where a conventional method only allows a yield of about 70% at best. With regard to the extraction of the krypton, this reaches about 75% whereas the yield of a conventional apparatus at best only allows a yield of the order of 60%.

According to one purpose of the invention, there is provided a method for producing krypton and xenon by separation of air by cryogenic distillation in a double column comprising a medium pressure column and a low pressure column in which:

• • i) cooled and purified compressed gaseous air is sent to a first level of the medium pressure column
  • ii) cooled and purified compressed liquid air is sent to a second level of the medium pressure column, above the first level
  • iii) a liquid, enriched in oxygen, krypton and xenon is tapped from the bottom of the medium pressure column
  • iv) a liquid is tapped from the medium pressure column, at a level between the first and second levels, and sent to the low pressure column
  • v) the liquid enriched in oxygen, krypton and xenon is partially vaporized in order to form an intermediate gas and an intermediate liquid
  • vi) a first liquid stream rich in oxygen is tapped from a third level of the low pressure column
  • vii) the intermediate liquid is sent from the vaporizer to the low pressure column at an intermediate level of the low pressure column, lower than the third level, or it is mixed with a second liquid stream rich in oxygen
  • viii) a/the second oxygen-rich liquid stream is tapped from the low pressure column at a fourth level, lower than the third level, and, if applicable, at the intermediate level.

According to optional features:

• • the intermediate gas is sent from the vaporizer to the low pressure column at a level higher than the intermediate level;
  • the liquid enriched in oxygen, krypton and xenon is vaporized by heat exchange with a gas tapped from the medium pressure column;
  • the gas tapped from the medium pressure column is tapped from above the first level;
  • the intermediate liquid is sent from the vaporizer to an intermediate level of the low pressure column, lower than the third level;
  • the second oxygen-rich liquid stream (11) is tapped from the low pressure column at a fourth level, lower than the intermediate level;
  • the intermediate liquid is mixed with a second oxygen-rich liquid stream.

According to another purpose of the invention, there is provided an apparatus for producing krypton and xenon by separation of air by cryogenic distillation comprising a double column constituted by a medium pressure column and a low pressure column and a vaporizer comprising:

• • i) means for sending cooled and purified compressed gaseous air to a first level of the medium pressure column
  • ii) means for sending cooled and purified compressed liquid air to a second level of the medium pressure column, above the first level
  • iii) means for tapping a liquid, enriched in oxygen, krypton and xenon from the bottom of the medium pressure column
  • iv) means for tapping a liquid from the medium pressure column, at a level between the first and second levels, and it is sent to the low pressure column
  • v) means for sending at least a portion of the liquid enriched in oxygen, krypton and xenon to the vaporizer where it is partially vaporized to form an intermediate gas and an intermediate liquid
  • vi) means for tapping a first liquid stream rich in oxygen from a third level of the low pressure column
  • vii) means for sending the intermediate liquid from the vaporizer to the low pressure column at an intermediate level of the low pressure column or for mixing it with a second liquid stream rich in oxygen and
  • viii) means for tapping a/the second oxygen-rich liquid stream from the low pressure column at a fourth level, lower than the third level, and, if applicable, at the intermediate level.

The apparatus possibly comprises:

• • means for sending the intermediate gas from the vaporizer to the low pressure column at a level higher than the intermediate level;
  • means for sending a gas tapped from the medium pressure column to the vaporizer;
  • means for tapping the gas from the medium pressure column is tapped from above the first level;
  • means for pressurizing and vaporizing the first oxygen-rich stream, preferably by heat exchange with the air to be distilled
  • means for sending the intermediate liquid from the vaporizer to an intermediate level of the low pressure column, lower than the third level;
  • means for tapping the second oxygen-rich liquid (11) from the low pressure column at a fourth level, lower than the intermediate level
  • means for mixing the intermediate liquid with a second oxygen-rich liquid stream.

The invention will be described in more detail with reference to the FIGURE.

A stream of medium-pressure gaseous air 1 is sent to the medium pressure column 20 of a double air separation column. The double column also comprises a low pressure column 21 thermally coupled with the medium pressure column 20 by means of a vaporizer 24, which vaporizes the bottom liquid of the low pressure column by heat exchange with the gaseous nitrogen at the top of the medium pressure column.

A high-pressure liquid air stream is expanded in a valve and then divided into two. The stream 3 thus formed is sent to the medium pressure column 20 a few theoretical trays above the entry point of the stream 1. The rest of the liquid air 4 is cooled in the subcooler 26, expanded and then sent to the low pressure column 21.

A lower stream of rich liquid 5, enriched in oxygen, krypton and xenon, is tapped from the bottom of the medium pressure column 20, cooled in the subcooler 26 and then expanded and sent to a vaporizer 22 where it partially vaporizes by heat exchange with the nitrogen-enriched gaseous air 8 coming from the medium pressure column 20. The gaseous air is condensed in order to form a stream 9 which is then sent to the low pressure column 21. The vaporized rich liquid 16 is expanded as an intermediate gas and sent to the low pressure column whilst the remaining intermediate liquid 7 is sent to the low pressure column 21 a few trays above the bottom. A higher stream of rich liquid 6 is tapped between the air intake 1 and the air intake 3 and/or the tapping of the stream 8. This stream 6 is cooled in the subcooler 26, mixed with the vaporized rich liquid 16 and sent to the low pressure column 21. Impoverished liquid, enriched with nitrogen 15, is tapped at the top of the medium pressure column 20 and sent to the top of the low pressure column 21 after cooling and expansion.

Liquid oxygen 10 is tapped from the low pressure column 21 a few trays above the bottom and preferably above the liquid intake 7. This liquid is pumped by the pump 23 in order to serve as an oxygen-rich product.

Liquid oxygen 11, enriched with krypton and xenon, is tapped from the bottom of the low pressure column 21 and can be purified subsequently in order to produce a mixture of krypton and xenon. The intermediate liquid 7 can be mixed directly with this liquid oxygen stream instead of being sent to the low pressure column 21.

Low pressure nitrogen 13 is tapped at the top of the low pressure column 21, heated in the subcooler and heated against the air streams 1, 2 (heating not shown).

Items of equipment:20 Medium pressure column, 21 Low pressure column, 22 Lower rich liquid vaporizer, 23 Liquid oxygen pump.

Fluids:

1 Medium pressure gaseous air, 2 High pressure liquid air, 3 Expanded liquid air feeding the medium pressure column, 4 Expanded liquid air feeding the low pressure column, 5 Lower rich liquid, enriched in krypton and xenon, 6 Vaporized lower rich liquid, impoverished in krypton and xenon, 7 Purge rich in krypton and xenon, 8 Impoverished air extracted from the medium pressure column, 9 Liquid impoverished air, 10 Liquid oxygen extracted from the low pressure column, 11 Liquid oxygen extracted from the low pressure column rich in krypton and xenon.

Claims

1-15. (canceled)

16. A method for producing krypton and xenon by separation of air by cryogenic distillation in a double column comprising a medium pressure column and a low pressure column, said method comprising: i) sending cooled and purified compressed gaseous air to a first level of the medium pressure column;ii) sending cooled and purified compressed liquid air to a second level of the medium pressure column, wherein said second level is above the first level;iii) withdrawing a liquid, enriched in oxygen, krypton and xenon from the bottom of the medium pressure column;iv) withdrawing a liquid from the medium pressure column, at a level between the first and second levels, and sent to the low pressure column;v) partially vaporizing the liquid enriched in oxygen, krypton and xenon in order to form an intermediate gas and an intermediate liquid;vi) withdrawing a first liquid stream rich in oxygen from a third level of the low pressure column;vii) sending the intermediate liquid from the vaporizer to the low pressure column at an intermediate level of the low pressure column, lower than the third level, andviii) a second oxygen-rich liquid stream is tapped from the low pressure column at a fourth level, lower than the third level, and, lower than the intermediate level.

17. The method of claim 16, wherein the vaporized intermediate gas is sent to the low pressure column at a level higher than the intermediate level.

18. The method of claim 16, wherein the liquid enriched in oxygen, krypton and xenon is vaporized by heat exchange with a gas tapped from the medium pressure column.

19. The method of claim 18, wherein the gas tapped from the medium pressure column is tapped from above the first level.

20. The method of claim 16, wherein the intermediate liquid is sent from the vaporizer to an intermediate level of the low pressure column, lower than the third level.

21. The method of claim 20, wherein the second oxygen-rich liquid stream is withdrawn from the low pressure column at a fourth level, lower than the intermediate level.

22. The method of claim 16, wherein the intermediate liquid is mixed with a second oxygen-rich liquid stream.

23. An apparatus for separation of air by cryogenic distillation comprising a double column constituted by a medium pressure column and a low pressure column and a vaporizer comprising: i) means for sending cooled and purified compressed gaseous air to a first level of the medium pressure columnii) means for sending cooled and purified compressed liquid air to a second level of the medium pressure column, above the first leveliii) means for tapping a liquid, enriched in oxygen, krypton and xenon from the bottom of the medium pressure columniv) means for tapping a liquid from the medium pressure column, at a level between the first and second levels, and it is sent to the low pressure columnv) means for sending at least a portion of the liquid enriched in oxygen, krypton and xenon to the vaporizer where it is partially vaporized to form an intermediate gas and an intermediate liquidvi) means for tapping a first liquid stream rich in oxygen from a third level of the low pressure columnvii) means for sending the intermediate liquid from the vaporizer to the low pressure column at an intermediate level of the low pressure column or for mixing it with a second liquid stream rich in oxygen andviii) means for tapping a/the second oxygen-rich liquid stream from the low pressure column at a fourth level, lower than the third level, and lower than the intermediate level.

24. The apparatus of claim 23, further comprising means for sending the intermediate gas from the vaporizer to the low pressure column at a level higher than the intermediate level.

25. The apparatus of claim 23, further comprising means for sending a gas tapped from the medium pressure column to the vaporizer.

26. The apparatus of claim 25, further comprising means for tapping the gas from the medium pressure column from above the first level.

27. The apparatus of claim 23, further comprising means for pressurizing and vaporizing the first oxygen-rich stream,

28. The apparatus of claim 27, wherein said means for pressurizing and vaporizing the first oxygen-rich stream is by heat exchange with the air to be distilled.

29. The apparatus of claim 23, further comprising means for sending the intermediate liquid from the vaporizer to an intermediate level of the low pressure column, lower than the third level.

30. The apparatus of claim 29, further comprising means for tapping the second oxygen-rich liquid stream from the low pressure column at a fourth level, lower than the intermediate level.

31. The apparatus of claim 23, further comprising means for mixing the intermediate liquid with a second oxygen-rich liquid stream.