EP0136926A1 - Process and apparatus for air distillation in a double column - Google Patents

Abstract

To improve the distillation in the low-pressure column (2) when the oxygen produced is surplus, there is provided at the top of this column an additional section (18) fed at the top by liquid oxygen and at the bottom by impure nitrogen. It is thus possible to increase the efficiency of argon extraction of the installation and/or its production of liquid oxygen, of liquid nitrogen and/or of gaseous nitrogen. <IMAGE>

Description

The present invention relates to the technique of distilling air in a double distillation column.

As is well known, double column air distillation installations essentially comprise a medium pressure distillation column operating at about 6 bars, a low pressure distillation column operating a little above atmospheric pressure, and a condenser. -vaporizer. The air is sent, after purification, to the tank of the medium pressure column. The "rich liquid" (oxygen-enriched air) collected in the bottom of the medium pressure column is sent to reflux at an intermediate point in the low pressure column, while the "lean liquid", consisting almost entirely of nitrogen, collected in head of the medium pressure column is sent to reflux at the head of the low pressure column. Below the inlet of the rich liquid, the low pressure column often includes an "argon tapping" for the production of this gas. The low pressure column is generally provided in the tank with gaseous oxygen and liquid oxygen withdrawal pipes, and the medium pressure column is generally provided at the head with gaseous nitrogen and liquid nitrogen withdrawal pipes. The overhead vapor of the low pressure column ("impure nitrogen") consists of nitrogen containing a few% o to a few% oxygen and is generally rejected into the atmosphere.

In installations intended essentially to produce gaseous oxygen supplied directly to a user by pipeline, it sometimes happens that the oxygen is temporarily excess. This is particularly the case during periods of shutdown of the user's factories. With conventional distillation installations, the gaseous oxygen is then put into the atmosphere, and the energy expended for the separation of this oxygen is lost.

The object of the invention is to take advantage of the temporary drop in oxygen demand to increase one or more of the other productions of the installation, that is to say one or more of the productions of argon, liquid oxygen, liquid nitrogen or nitrogen gas.

To this end, the subject of the invention is a process for the distillation of air by means of a double column comprising a first distillation column, called the medium pressure column, operating under a relatively high pressure, and a second distillation column, called a low pressure column, operating under a relatively low pressure, characterized in that: an auxiliary column operating substantially at the pressure of the low pressure column is sent to the scmet a liquid taken from one of the two columns; a gas less rich in oxygen than this liquid and taken from the low pressure column is sent to the base of this auxiliary column; and the liquid collected at the base of the auxiliary column is sent under reflux to the low pressure column, substantially at the level of the sampling of said gas. The term "auxiliary column" means a column having the structure of a distillation column, that is to say comprising a packing or a certain number of plates of the type of those used in distillation.

The invention also relates to an air distillation installation with double column intended for the implementation of such a process. This installation, of the type comprising a first distillation column, called the medium pressure column, operating under a relatively high pressure, a second distillation column, called the low pressure column, operating under a relatively low pressure, a heat exchanger adapted to vaporize the liquid oxygen collected in the tank of the low pressure column by heat exchange with the overhead vapor of the first column, and means for ensuring a reflux in the second column, is characterized in that said means comprise an auxiliary column operating substantially at the pressure of the low pressure column, this auxiliary column being supplied at the top by a liquid taken from one of the two columns and at the bottom by a gas less rich in oxygen than this liquid and taken from the low pressure column, and means for sending back into the low pressure column, substantially at the level of the sampling of said gas, the l liquid collected at the base of the auxiliary column.

Maximum efficiency is obtained when the liquid supplying the auxiliary column is the liquid oxygen collected in the bottom of the low pressure column and said gas is the overhead vapor of this low pressure column.

In a particularly simple embodiment of the installation according to the invention, the auxiliary column is constituted by an additional column section provided at the top of the low pressure column.

• - la figure 1 représente schématiquement la double colonne d'une installation de distillation d'air conforme à l'invention ;
• - la figure 2 est un schéma de principe de la double colonne d'une autre installation suivant l'invention ; et
• - les figures 3 et 4 illustrent deux modes d'intégration du schéma de la figure 2.

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

• - Figure 1 schematically shows the double column of an air distillation installation according to the invention;
• - Figure 2 is a block diagram of the double column of another installation according to the invention; and
• - Figures 3 and 4 illustrate two modes of integration of the diagram in Figure 2.

The double distillation column illustrated in FIG. 1 comprises a lower distillation column or medium pressure column 1, an upper distillation column or low pressure column 2, and, between these two columns, a condenser-vaporizer 3. The air at distill, suitably purified, is injected at about 6 bars through a pipe 4 in the tank of column 1 to give rise to a tank liquid 5 enriched with oxygen, called rich liquid, and to a top vapor consisting almost entirely of nitrogen .

This overhead vapor can be partially withdrawn through a line 6; the rest is condensed in condenser 3 and collected at the head of column 1 in a trough 7. Part of this liquid, known as lean liquid, rises through a pipe 8, is expanded to a pressure slightly above atmospheric pressure in a valve 9 and is injected in liquid form substantially at the top of the column 2 to flow in reflux of this column; another part of the lean liquid can be drawn off through a line 10, and the rest of this lean liquid overflows from the trough 7 and flows under reflux in column 1.

Part of the rich liquid rises through a pipe 11, is expanded in a valve 12 fitted to this pipe and is injected in liquid form in column 2 below the injection level of the lean liquid. The rest of the rich liquid is sent via a pipe 11A into the head condenser of a column (not shown) for producing argon, then, after vaporization in this condenser, is returned to 11B in column 2, a few trays below from the point where the pipe emerges 11.

Column 2 also comprises, at a level lower than the injection of rich liquid, two pipes 13, 14 going towards the argon production column and returning therefrom, respectively; a line 15 for withdrawing gaseous oxygen and a line 16 for withdrawing liquid oxygen leave from the bottom of column 2; a first waste gas, consisting of nitrogen containing a few% of oxygen (impure nitrogen), can be evacuated by a line 17 located substantially at the same level as the injection of lean liquid.

Column 2, which can be considered as limited above the level of line 17, is extended upwards by an additional column section 18 of smaller diameter which communicates by its base with column 2 and which comprises plates or a packing of the same type as those used in distillation and corresponding to a small number of theoretical plates, for example to six theoretical plates. A pipe 19 provided with a lifting pump 20 starts from the bottom of the column 2 and ends at the top of this section 18. The top of the section 18 is equipped with a pipe 21 serving for the evacuation of the head vapor from this section, which constitutes a second waste gas from the installation.

In normal operation, the pump 20 is stopped. The desired flow rates of gas and liquids are drawn off through lines 6, 10, 15 and 16, and the impure nitrogen is evacuated through line 17 and / or through line 21, this by an appropriate adjustment of the valves (not shown ) with which these pipes are provided. At the head of column 2, the reflux is ensured solely by the ascent of lean liquid.

It may happen that, for a limited period of time, the gaseous oxygen becomes surplus, for example due to the temporary shutdown of a consuming factory directly connected by a pipe to the pipe 15. In this case, the operates the pump 20 so as to raise via the line 19 a liquid oxygen flow rate equal to the excess oxygen flow rate, the valve of the line 15 is correspondingly closed, and the valves of the lines 17 and 21 are operated so as to pass an appropriate flow of impure nitrogen through the section 18.

Consequently, liquid oxygen at approximately -180 ° C. is poured onto the upper plate of the additional section 18 and undergoes a countercurrent exchange with the impure nitrogen arriving at approximately -196 ° C at the lower plate of this section 18. This leads to the evacuation of nitrogen charged with oxygen via line 21 at approximately -190 ° C, and to the drop below the section 18 of a liquid to about -196 ° C consisting of nitrogen containing a few% o to a few% oxygen. It is easy to calculate the minimum flow rate of impure nitrogen making it possible to obtain the oxygen content of this liquid which is suitable for equilibrium to be achieved at the injection of lean liquid.

• - en cuve de la colonne 2, on a remplacé un soutirage gazeux par un soutirage liquide, ce qui revient à augmenter le chauffage de cette colonne ;
• - au niveau de l'injection de liquide pauvre, on a superposé à ce liquide un débit supplémentaire de liquide de reflux, de sorte que les moyens assurant le reflux en tête de la colonne 2 sont constitués par la remontée de liquide pauvre et par le tronçon additionnel 18.

In total :

• - In the tank of column 2, a gas withdrawal was replaced by a liquid withdrawal, which amounts to increasing the heating of this column;
• - At the injection of lean liquid, an additional flow of reflux liquid has been superimposed on this liquid, so that the means ensuring the reflux at the head of column 2 are constituted by the rise in lean liquid and by the additional section 18.

In other words, we increased both the reflux R = L / V at the top of column 2 and the reverse of this tank ratio of the same column. The efficiency of the distillation in column 2 is therefore improved, and this improvement can be used to increase the extraction yield of argon. For example, this yield can drop from 70% to 88% by recycling 85% of the oxygen separated by line 19.

On the other hand, if we consider the section of column 2 located at the level of the pipe 17, that is to say just below the additional section 18, we see that the latter has the overall effect of liquefy at about -196 ° C an impure nitrogen flow rate equal to the flow rate of recycled liquid oxygen, by means of liquid oxygen at about -180 ° C. Thus, the section 18 provides a refrigeration transfer of the heat pump type, which results from the energy released by mixing in this section, under conditions close to reversibility, oxygen and nitrogen.

• - On peut augmenter le soutirage d'azote gazeux en moyenne pression par la conduite 6. En effet, ceci diminue la quantité d'azote condensée et donc le chauffage de l'oxygène liquide, ce qui a pour conséquence une diminution simultanée de la vaporisation en cuve de la colonne 2 et du reflux de liquide pauvre en tête de cette même colonne, mais ce phénomène défavorable peut être compensé par l'effet expliqué plus haut du tronçon 18 ;
• - L'azote gazeux ainsi soutiré peut soit être utilisé en tant que tel, soit détendu, par exemple dans une turbine (non représentée) dont est pourvue l'installation, afin de produire du froid. Ce froid permet alors d'augmenter la production de liquide (azote ou oxygène) de l'installation ;
• - L'augmentation de la production de liquide de l'installation peut être obtenue d'une autre manière, dans les installations à insufflation d'air dans la colonne basse pression, en augmentant le débit d'air turbiné, car, comme précédemment, la réduction consécutive du chauffage et du reflux de la colonne basse pression peut être compensée par l'effet du tronçon 18.

This refrigeration transfer can also be used to increase the other productions of the installation:

• - It is possible to increase the withdrawal of nitrogen gas at medium pressure via line 6. In fact, this reduces the amount of nitrogen condensed and therefore the heating of liquid oxygen, which results in a simultaneous reduction in the vaporization in the tank of column 2 and the reflux of poor liquid at the top of this same column, but this unfavorable phenomenon can be compensated by l 'effect explained above of the section 18;
• - The nitrogen gas thus withdrawn can either be used as such, or expanded, for example in a turbine (not shown) which is provided with the installation, in order to produce cold. This cold then increases the production of liquid (nitrogen or oxygen) of the installation;
• - The increase in the production of liquid in the installation can be obtained in another way, in installations with air insufflation in the low pressure column, by increasing the flow of turbined air, because, as before, the subsequent reduction in heating and reflux of the low pressure column can be compensated for by the effect of section 18.

Of course, in the cases which have just been mentioned, the extraction efficiency of the argon is increased to a lesser extent, or even is reduced to the value it would have in the absence of the section 18.

FIG. 2 shows how the principle of the invention can be applied to an installation of the so-called "minaret" type, that is to say in which pure nitrogen is produced at low pressure by distillation of the impure nitrogen at the head of the low pressure column. In this figure, the various withdrawal lines have not been shown, for the sake of clarity, and the elements corresponding to elements of FIG. 1 have been designated by the same references.

We thus find the medium pressure 1 and low pressure 2 columns. the condenser-vaporizer 3, the air inlet 4, the ascent 11 for the rich liquid 5, with its expansion valve 12, the argon tappings 13 and 14, the pipes 11A and 11B associated with the condenser at the head of the column of argon production, and line 17 escape of impure nitrogen.

The double column 1-2 is equipped with an additional column 22, known as a "minaret", for producing low pressure pure nitrogen. This column is fed at the bottom with nitrogen impure, by a pipe 23, and above by the lean liquid 7 taken from the top of the column 1, by a pipe 24 provided with an expansion valve 25. The pure nitrogen leaves at the top of the column 22 by a pipe 26, and the liquid from the bottom of this column is sent to reflux at the top of column 2 by a pipe 27. Furthermore, as is conventional in this type of installation, the reflux at the top of column 2 is also ensured by an ascent of "lower lean liquid" taken from column 1 lower than the liquid 7, by a pipe 28 provided with an expansion valve 29.

The section 18 of FIG. 1 is replaced by an auxiliary column 18A supplied at the bottom by impure nitrogen by a pipe 30 and at the top by liquid oxygen taken from the tank of column 2, by the pipe 19 equipped with its lifting pump 20. This column 18A operates in the same way as the section 18 of FIG. 1; the gaseous mixture of nitrogen and oxygen produced at the head is evacuated via line 21, and the tank liquid is sent under reflux to the top of column 2 via line 31. The same results are thus obtained in this context advantages than in the case of Figure 1.

In practice, the four columns 1, 2, 18A and 22 can be integrated by superimposing them. Thus, in the example of FIG. 3, the column 18A has been placed at the top of the column 2, constituting a section 18 similar to that of FIG. 1, while the column 22, of smaller diameter than the section 18, has been arranged above the latter, without communication with it. It is not necessary to describe in detail this arrangement, which results clearly from the foregoing explanations. It will only be noted that, to simplify the installation, the conduits 27 and 28 were joined at 32 before reaching column 2.

In FIG. 4, the column 18A is integrated in the form of a section 18B into the shell of the column 22, at the top of the latter and without communication with this column, which communicates directly by its base with the top of the column 2. Apart from the fact that it is now column ₂ 2 and no longer column 18A (that is to say section 18) which is supplied directly with impure nitrogen and that, consequently, it is the pipe 31 and no longer the pipe 27 which is joined at 32 to the pipe 28, the arrangement and the operation feels completely similar to those of Figure 3. In Figure 4, the diameter of the section 18B is equal to that of the minaret 22; alternatively, it could be lower.

Of course, in some cases, the columns 1, 2, 18A and 22 can all have the same diameter and, therefore, all be integrated into a single cylindrical shell.

Claims (9)

1. - Process for the distillation of air by means of a double column comprising a first distillation column (1), called the medium pressure column, operating under a relatively high pressure, and a second distillation column (2), called the column low pressure, operating under a relatively low pressure, characterized in that: the liquid of a column taken from the auxiliary column (18; 18A; 18B) operating substantially at the pressure of the low pressure column (2) is sent one (2) of the two columns (1, 2); a gas less rich in oxygen than this liquid and taken from the low pressure column (2) is sent to the base of this auxiliary column; and the liquid collected at the base of the auxiliary column is sent under reflux to the low pressure column, substantially at the level of the sampling of said gas. 2. - Method according to claim 1, characterized in that the liquid is taken from the low pressure column (2). 3. - Method according to claim 2, characterized in that the liquid taken from the low pressure column is liquid oxygen collected in the tank of this column. 4. - Method according to any one of claims 1 to 3, characterized in that said gas is the top gas of the low pressure column (2). 5. - Double column air distillation installation, of the type comprising a first distillation column (1), called the medium pressure column, operating under a relatively high pressure, a second distillation column (2), called the low pressure column , operating under a relatively low pressure, a heat exchanger (3) adapted to vaporize the liquid oxygen collected in the bottom of the low pressure column by heat exchange with the overhead vapor of the first column, and means (8, 11, 18; 18A, 22; 18B) to ensure reflux in the second column, characterized in that said means comprise an auxiliary column (18; 18A; 18B) operating substantially at the pressure of the low pressure column (2), this auxiliary column being supplied at the top by a liquid taken from one (2) of the two columns and at the bottom by a gas less rich in oxygen than this liquid and taken from the low pressure column, and means (31) for send the liquid collected at the base of the auxiliary column to reflux into this low pressure column, substantially at the level of the sampling of said gas. 6. - Installation according to claim 5, characterized in that the liquid supplying the auxiliary column (18; 18A; 18B) is taken from the low pressure column (2). 7. - Installation according to claim 6, characterized in that the liquid supplying the auxiliary column (18; 18A; 18B) is the liquid oxygen collected in the bottom of the low pressure column (2). 8. - Installation according to any one of claims 5 to 7, characterized in that said gas is the top vapor of the low pressure column (2). 9. - Installation according to any one of claims 5 to 8, characterized in that the auxiliary column (18) is constituted by an additional column section provided at the top of the low pressure column (2).

Images