EP0379435B2 - Process and apparatus for the separation of air and the production of highly pure oxygen - Google Patents

Abstract

Oxygen from the bottom of a low pressure column (4) is purified from hydrocarbons in a first auxiliary column (12), and the vapor at the top of this column is distilled in a second auxiliary column (17) heated at the base thereof with air at medium pressure. Ultra-pure oxygen is produced at the bottom of the second auxiliary column. Application in the production of ultra-pure oxygen for the electronic industry.

Description

The present invention relates to a method and an installation for producing oxygen ultra-pure from a main distillation apparatus double column air.

A method and an installation of this type are described in documents US-A-4755202, EP-A-299.364, and SU-757817.

By "ultra-pure oxygen" is meant oxygen practically free of methane (and therefore of hydrocarbons) and argon, for example containing less 0.1 ppm oil and less than 10 ppm argon, these contents being fixed in advance and being able vary depending on the application. Ultra-pure oxygen is especially intended for industries electronic.

The object of the invention is to enable production ultra-pure oxygen at the cost of a modification inexpensive of the main distillation apparatus and, above all, without additional energy expenditure.

The characteristics, to do this, of a process and of an installation according to the invention are set out in claims 1,4 and 7.

Preferential execution allows production substantially free of high purity oxygen of hydrocarbons.

The characteristics, to do this, of a process and of an installation according to the invention are set out in claims 5 and 8.

• la figure 1 représente schématiquement une installation conforme à l'invention; et
• la figure 2 représente schématiquement une variante de cette installation.

Some examples of implementation of the invention will now be described with reference to the accompanying drawings, in which:

• Figure 1 schematically shows an installation according to the invention; and
• FIG. 2 schematically represents a variant of this installation.

The installation shown in Figure 1 is intended to separate air into its constituents to produce : nitrogen gas under oxygen pressure gaseous to about 99.5% purity; and oxygen ultra-pure with a predetermined maximum content methane and argon, for example less than 0.1 ppm methane and less than 10 ppm argon. The production ultra-pure oxygen corresponds to a small fraction, preferably between 5 and 10%, of the oxygen production of the installation.

The installation includes a main unit of air distillation 1 itself comprising a double distillation column 2. The double column includes a medium pressure column 3 surmounted by a low pressure column 4. A condenser-vaporizer 5 connects indirect heat exchange with nitrogen at the top of column 3 and the tank liquid (oxygen at approximately 99.5% purity) from column 4.

The air to be treated, purified and cooled to its point of dew, is mostly introduced under the medium pressure, approximately 6 bar absolute, at bottom of column 3 via line 6. Its condensation rich liquid product "LR, part of which is expanded in an expansion valve 7 and introduced at an intermediate level in column 4, which operates at low pressure, slightly above of atmospheric pressure. Some cash poor "LP, consisting mainly of nitrogen, is taken at the head of column 3 then, after expansion in an expansion valve 8, is introduced at the head of column 4. Double column 2 also includes a line 9 producing gaseous oxygen at 99.5% purity, at the bottom of column 4, a line 10 for producing nitrogen gas under 6 bars, at the top of column 3, and a pipe 11 exhaust gas W (impure nitrogen) leaving from the top of column 4.

A first auxiliary column 12 having a small number n of theoretical plates, with n between 3 and 8, is connected to the tank of column 4 by lines 13 for supplying gas and 14 for returning the liquid, and is equipped a head condenser 15. The latter is supplied by a portion of the rich liquid LR, expanded in an expansion valve 16.

A second auxiliary column 17 is supplied at an intermediate location, via a pipe 18, by the overhead vapor of the column 12. E! It has n + n 1 theoretical plates below the pipe 18. Its tank has a vaporizer 19 and its top a condenser 20. The vaporizer is heated by a line 21, and the condenser 20 is cooled by the rest of the rich liquid LR, expanded in an expansion valve 22. The rich liquid vaporized in the condensers 15 and 20 is returned to column 4 via a common pipe 23. The liquefied fluid leaving the vaporizer 19 can be returned to the corresponding level in column 4 or, as shown, be combined with the rich liquid withdrawn from the tank of column 4, since its flow is low compared to that of this rich liquid.

A pipe 24 connects the top of the column 17 at an intermediate point in column 4.

In operation, the oxygen conveyed through line 13 contains argon and methane as impurities. Methane separates from oxygen and argon in column 12 with n theoretical plates, all the more completely as the number n is large. The calculation shows that a number less than or equal to 8 is sufficient for the usual applications of ultra-pure oxygen.

So, it's a mixture made up practically only oxygen and argon which enters the column 17 via line 18. Ultra-pure liquid oxygen having the maximum desired argon content and in methane is withdrawn through a pipe 25 in tank from column 17.

In addition, given the reflux rates required in column 17 and the flows to be involved, it is necessary to concentrate the vapor in argon returned to the main distillation apparatus via the conduct 24. This is why column 17 includes above his diet a number of additional trays, reflux being ensured throughout the column by the head condenser 20.

The invention applies in the same way to main air distillation apparatuses producing oxygen with a purity of less than 99.5%, for example 95 or 97%. Indeed, the gas drawn off through line 13 then contains nitrogen, which easily separates from the oxygen in the auxiliary column 17. As indicated at 21 in FIG. 1, a vaporizer 19 is used to heat oxygen-depleted gas taken from the lower or intermediate part of column 3. The oxygen content of the heating gas must, however, remain sufficient to ensure, by the condensation of this gas, the vaporization of ultra-pure oxygen . In fact, this vaporization takes place at a pressure higher than that of the tank of column 4 due to the presence of the n1 additional plates of column 17.

The variant shown in Figure 2 shows how the invention can be applied to a device main air distillation 1A provided with a column 26 of oxygen-argon separation. We will use same reference numbers, possibly followed with the suffix A, to designate the corresponding elements to those in Figure 1.

For the production of argon, a pipe 13A called "argon tapping" starts from an intermediate location of the column 4, N theoretical plates above the tank. This pipe opens into the tank of column 26 and conveys a gas essentially consisting of oxygen and argon, and a return pipe 14A starts from the lowest point of column 26 and opens into column 4 at approximately the level 13A argon stitching. The column 26 is equipped with a head condenser 15A supplied by the portion of the rich liquid LR which is not expanded in the valve 17, this liquid being expanded in an expansion valve 16A. The rich liquid vaporized leaving the condenser 15A is returned to the column 4, a little below the rich liquid coming from the valve 17. The raw argon produced at the top of the column 26 is discharged through a pipe 27.

The main distillation apparatus 1A is modified in the following way to produce ultra-pure oxygen.

At a level corresponding to a small number n of theoretical plates above the tank of the column 26 ( n between 3 and 8), liquid is drawn off through a line 18A and sent to the head of an auxiliary column 17A. A line 28 returns the overhead vapor from this column to the same level as column 26. The lower part of column 26 defined under lines 18A and 28 will be designated by 12A, this part 12A corresponding to the first auxiliary column 12 of the figure 1, as it will appear below.

A 19A vaporizer is placed in the tank column 17A. This steamer is heated as previously by the line 21 and combined with the rich liquid LR after condensation.

In operation, the oxygen-argon gas mixture conveyed through line 13A contains methane as an impurity. Methane separates from oxygen and argon in the lower section 12A with n theoretical plates of column 26, all the more completely as the number n is large. The calculation shows that a number n less than or equal to 8 is sufficient for the usual applications of ultra-pure oxygen.

Thus, it is a mixture consisting almost entirely of oxygen and argon which enters the column 17A. By choosing the reflux rate at the top of column 17A roughly equal to that at the bottom of column 4, the level 99.5% of the oxygen produced in the tank of column 4 is found in line 29 , and column 17A has n 1 theoretical plates below this pipe, which makes it possible to obtain ultra-pure liquid oxygen in the tank having the maximum desired argon content. This ultra-pure oxygen is drawn off through line 25.

As a numerical example, we can choose N of the order of 30 to 40 and n 1 of the order of 15 to 30.

It should be noted that, as shown in FIG. 2, a line 29 for drawing off liquid oxygen at approximately 99.5% purity can, as shown, from an intermediate location of the column 17A situated substantially N + n trays theoretical below the top of this column 17A. This oxygen is practically free of hydrocarbons and can therefore be used for certain applications where hydrocarbons are undesirable, for example in the medical field.

Claims (9)

1. Process for producing ultra-pure oxygen from a principal air distillation apparatus (1) with a double column (2) comprising a medium pressure column (3) and a low pressure column (4), consisting of the following steps :

   conveying to the bottom of a first auxiliary column (12) a first gas taken from the bottom of the low pressure column (4), returning to the latter the liquid produced at the bottom of the first auxiliary column, distilling in a second auxiliary column (17) a fluid withdrawn at a first level at the top of the first auxiliary column, the fluid being conveyed to a second level of the second auxiliary column, ultra-pure oxygen being produced at the bottom of this second auxiliary column,

   heating the bottom of the second auxiliary column (17) at a higher pressure than that of the bottom of the low pressure column, by reason of the presence of additional theoretical plates in the second auxiliary column, owing to the fact that the number of theoretical plates between the bottom of the second auxiliary column and the second level is greater than the number of theoretical plates between the bottom of the first auxiliary column and the first level, by condensing there a heating gas available at the medium pressure at the level of the lower or intermediate part of the medium pressure column (3), and

   returning the condensate to the principal air distillation apparatus (1).
2. Process according to claim 1, characterized in that the second auxiliary column (17) has a top condenser (20) and is fed at an intermediate point with vapour from the top of the first auxiliary column (12), vapour from the top of the second auxiliary column being returned to the low pressure column (4).
3. Process according to either of claims 1 or 2,
   characterized in that the fluid condensed at the bottom of the second auxiliary column (17) is added to the rich liquid (LR) produced at the bottom of the medium pressure column (3).
4. Process for producing ultra-pure oxygen from a principal air distillation apparatus (1A) with a double column (2) comprising a medium pressure column (3) and a low pressure column (4), consisting of the steps of conveying to the bottom of a first auxiliary column (12A) a first gas taken from the argon tapping point of the low pressure column (4), returning to the latter the liquid produced at the bottom of the first auxiliary column, distilling in a second auxiliary column (17A) a liquid produced in the first auxiliary column at a first level and conveyed to the top of the second auxiliary column, ultra-pure oxygen being produced at the bottom of this second auxiliary column,
   characterized in that the bottom of the second auxiliary column (17A) is heated at a higher pressure than that of the bottom of the low pressure column, by reason of the presence of additional theoretical plates in the second auxiliary column, owing to the fact that the total number of theoretical plates between the bottom of the low pressure column and the argon tapping point and between the bottom of the first auxiliary column and the first level is less than the total number of theoretical plates in the second auxiliary column, by condensing there a heating gas available at the medium pressure at the level of the lower or intermediate part of the medium pressure column (3), and of returning the condensate to the principal air distillation apparatus (1A).
5. Process according to claim 4, characterized in that liquid oxygen practically free from hydrocarbons is withdrawn at an intermediate level (29) of the second intermediate column (17A).
6. Process according to claim 4 or 5, characterized in that the fluid condensed at the bottom of the second auxiliary column (17A) is added to the rich liquid (LR) produced at the bottom of the medium pressure column (3).
7. Plant for separating air and producing ultra-pure oxygen, of the type comprising a principal air distillation apparatus (1A) with a double column (2) having a medium pressure column (3) and a low pressure column (4), a first auxiliary column (12A) the bottom of which is connected to the argon tapping zone of the low pressure column (4) by a first gas conduit (13A) and by a second liquid conduit (14A) and a second auxiliary column (17A) the top of which is connected by a third conduit (18A) to a first level of the first auxiliary column and the bottom of which has an indirect heat exchanger (19A),
   characterized in that it includes means (21) for withdrawing at the level of the lower or intermediate part of the medium pressure column (3) a heating gas under the medium pressure and introducing it into the heat exchanger (19A) of the second auxiliary column, the bottom of the second auxiliary column being at a higher pressure than that of the bottom of the low pressure column, by reason of the presence of additional theoretical plates in the second auxiliary column, owing to the fact that the total number of theoretical plates between the bottom of the low pressure column and the argon draw-off point and between the bottom of the first auxiliary column and the first level is less than the total number of theoretical plates in the second auxiliary column, and means for returning the condensate coming from the heat exchanger to the low pressure column (4) of the principal air distillation apparatus (1A).
8. Installation according to claim 7, characterized in that the second auxiliary column (17A) is equipped at an intermediate level with a conduit (29) for withdrawing oxygen practically free from hydrocarbons.
9. Installation according to claim 7 or 8, characterized in that it includes means for adding the fluid condensed in the heat exchanger (19A) to the rich liquid (LR) produced at the bottom of the medium pressure column (3).

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