EP0299364A2 - Process and apparatus for air separation by rectification - Google Patents

Abstract

In a process and apparatus for air separation by rectification air 1 is preliminarily separated in a first rectification stage 2 of a two-stage rectification column to obtain a nitrogen-rich fraction 4 and an oxygen-rich fraction 8. These two fractions are fed to the second rectification stage 6 and separated into oxygen and nitrogen fractions. An argon-enriched fraction, containing essentially oxygen and argon, is removed from the second rectification stage at an intermediate point and is separated in a raw argon column 10 by rectification into an argon-rich fraction 18 and a liquid fraction 19 containing essentially oxygen. The liquid fraction 19 is fed back into the second rectification stage. Another fraction 22 is removed from the raw argon column above the bottom thereof and is separated in a high-purity oxygen column 23 to produce a high-purity oxygen fraction 25, 26 and a lighter residual fraction 24. An additional nitrogen-rich fraction is removed from the head of the first rectification stage and separated in a high-purity nitrogen column 31 producing a bottom liquid fraction 29, which is fed back to the head of the first rectification stage, and a residual gas fraction 33. A high-purity nitrogen fraction 34 is removed at a point several plates below the head of the first rectification stage.

Description

The invention relates to a method for air separation by rectification, in which the air is pre-divided into a nitrogen-rich and an oxygen-rich fraction in a first rectification stage and the two fractions are fed to a second rectification stage and decomposed into oxygen and nitrogen. The invention further relates to an apparatus for performing such a method.

Such a process, in which oxygen and nitrogen are obtained by two-stage rectification, has been described in US Pat. No. 4,575,388. The decomposition products oxygen and nitrogen are removed from the bottom or the top of the second rectification stage.

US Pat. No. 4,575,388 also shows a crude argon column into which argon-enriched gas from the second rectification stage is introduced. A liquid bottom fraction consisting essentially of oxygen is returned from the crude argon column to the second rectification stage.

The oxygen-rich liquid that accumulates in the bottom of the crude argon column has a relatively high concentration of impurities, since the fraction enriched with argon from the second rectification stage contains not only oxygen and nitrogen, but also krypton, xenon and hydrocarbons, all of which collect in the bottom of the crude argon column . By returning the bottom liquid to the second rectification stage, the impurities get into the bottom of the second rectification stage and thus into the oxygen removed as a decomposition product.

Because of the impurities in the oxygen, the process does not allow high-purity, in particular liquid, oxygen which is free from krypton, xenon and hydrocarbons to be obtained from the second rectification stage. High-purity oxygen is required, for example, in the electronics industry.

Nitrogen, which is obtained in the known process, also contains traces of other gases, for example of helium, neon, hydrogen and carbon monoxide. However, nitrogen of the highest purity is required for the modern semiconductor industry.

The removal of carbon monoxide can be carried out catalytically. However, a helium discharge usually attached to the head of the first rectification stage can only bring about a slight reduction in helium, neon and hydrogen.

The invention is therefore based on the object of developing a method of the type mentioned at the outset which enables the production of high-purity decomposition products, preferably high-purity oxygen and high-purity nitrogen.

This object is achieved in that a further fraction above the bottom is taken from a crude argon column into which a stream essentially containing oxygen and argon is introduced from the second rectification stage and is broken down into high-purity oxygen and a lighter residual fraction in a high-purity oxygen column.

With the aid of these process steps, oxygen can be produced as a high-purity decomposition product which is essentially free of argon, krypton, xenon and hydrocarbons.

The concentration of krypton, xenon and hydrocarbons in the crude argon column decreases from the bottom of the column. The fraction removed above the bottom of the column therefore only contains the components oxygen, argon and nitrogen, while it is free from krypton, xenon and hydrocarbons. In the high-purity oxygen column, the oxygen is separated by rectification from nitrogen and argon. In this way, oxygen with a purity of less than 10 ppm, preferably less than 5 ppm, most preferably less than 2 ppm of hydrocarbons, krypton, xenon and nitrogen and a content of less than 20 ppm, preferably less than 15 ppm, of argon produce.

The oxygen obtained in the bottom of the ultrapure oxygen column is preferably withdrawn in liquid form. If high-purity gaseous oxygen is to be produced using the method, at least a portion of the high-purity oxygen is removed in gaseous form from the high-purity oxygen column. The removal takes place just above the bottom of the column.

The residual fraction, which essentially contains oxygen, argon and nitrogen, is removed from the top of the high-purity oxygen column and preferably returned to the crude argon column or to the second rectification stage above the removal point of the further fraction.

In a preferred development of the method according to the invention, the further fraction is removed in liquid form and added to the ultrapure oxygen column as reflux liquid.

According to a preferred development of the method according to the invention, the further fraction is removed from several trays above the column bottom of the crude argon column.

The bottoms between the column sump and the tapping point act as a barrier to the undesirable proportions of krypton, xenon and hydrocarbons. Preferably three to five rectification trays are provided as a barrier.

In a preferred development of the method according to the invention, the high-purity oxygen is removed from a plurality of trays above the bottom of the ultrapure oxygen column (23).

The only fraction that is introduced into the ultrapure oxygen column generally contains such impurities only in orders of magnitude far below ppm. Nevertheless, such fractions can get into the bottom of the ultrapure oxygen column by enriching the smallest traces or by penetrating from the outside, for example through leaks in the bottom heater. For this reason, the removal of a few, preferably three to five trays above the bottom of the ultra-pure oxygen column is particularly advantageous since, as in the crude argon column, these trays serve as a barrier for undesirable traces of krypton, xenon and hydrocarbons. The high-purity oxygen can be withdrawn at this point both in liquid and gaseous form. In order to avoid a slow accumulation of impurities in the sump during operation, it is advantageous if a small part of the sump liquid is led out of the high-purity oxygen column and discarded or returned to the second rectification stage.

In a further preferred embodiment of the process according to the invention, the column bottom of the further column is heated by nitrogen from the top of the first rectification stage.

The heating is preferably carried out by heat exchange in a condenser evaporator arranged in the bottom of the ultrapure oxygen column. It is advantageous here if the nitrogen condenses at least partially during the heating and the condensate is returned to the pressure stage.

An apparatus for carrying out the process according to the invention comprises a two-stage rectification column and a crude argon column connected to the second stage thereof and is characterized by a high-purity oxygen column which is connected to the crude argon column by means of a side removal line.

In a further development of the device according to the invention, the lateral removal line is arranged a plurality of rectifying trays above the bottom of the crude argon column.

The object on which the invention is based is further achieved in that a further nitrogen-rich fraction is introduced from the top of the first rectification stage into a high-purity nitrogen column and is broken down there into a bottom liquid and into a residual gas fraction.

By using these process steps, nitrogen can be produced as a very pure decomposition product. For this purpose, helium, neon, hydrogen and carbon monoxide are rectified from the nitrogen in the high-purity nitrogen column and removed in a residual gas fraction. The residual gas fraction can, for example, be admixed with impure nitrogen, which is usually taken from the second rectification stage and used to regenerate molecular sieve adsorbers.

The bottom liquid of the high-purity nitrogen column is preferably returned to the top of the first rectification stage.

With the help of this measure, the nitrogen purity increases in the head of the first rectification stage and nitrogen with high purity can also be obtained. The high-purity nitrogen is preferably removed in liquid form from the top of the first rectification stage.

In particular, it proves to be advantageous if, in a further embodiment of the subject of the invention, a liquid fraction from high-purity nitrogen is removed from some trays below the head of the first rectification stage.

The intermediate plates below the top condenser retain residues of light gases such as helium, neon or hydrogen, which despite helium discharge can accumulate at the top of the first rectification stage even when additional rectification is used in the high-purity nitrogen column. The liquid high-purity nitrogen has a purity of 99.999% and still contains argon, helium, neon, hydrogen and carbon monoxide.

In a favorable further development of the method according to the invention, the liquid high-purity nitrogen is at least partially supercooled. This makes it easier to store the liquid product portion in a tank. The cooling is preferably carried out in indirect heat exchange with nitrogen from the second rectification stage. The nitrogen can then be led into a separator and removed from it as a liquid.

If part of the high-purity nitrogen is to be obtained in gaseous form, it proves to be advantageous if the liquid high-purity nitrogen is evaporated from the top of the first rectification stage at least partially in heat exchange with condensing nitrogen.

An apparatus for performing the high purity nitrogen recovery process includes a high purity nitrogen column connected to the first rectification stage by a gas line and a liquid line, and a high purity nitrogen bleed line attached to the first rectification stage some trays below the head.

In a preferred development, the process steps from one of claims 1 to 5 and the process steps from one of claims 8 to 12 are used together in the process according to the invention and, moreover, the bottom liquid of the ultrapure oxygen column is heated by heat exchange with the gas in the top of the ultrapure nitrogen column.

In this way, both oxygen and nitrogen can be produced as decomposition products of the highest purity. The energy consumption is particularly low due to the heat exchange between the high-purity oxygen and high-purity nitrogen columns.

A device for carrying out this method additionally has a condenser-evaporator which is fitted between the high-purity oxygen and high-purity nitrogen column. In this way, high-purity oxygen and high-purity nitrogen columns can be manufactured as one unit, which brings further savings in production and capital costs.

The invention and further details of the invention are explained in more detail with reference to schematically illustrated exemplary embodiments:

• Figur 1 ein Verfahrensschema einer Ausführungsform des erfindungsgemäßen Verfahrens zur Erzeugung von hochreinem Sauerstoff,
• Figur 2 eine schematische Darstellung eines Details aus einer weiteren Ausführungsform zur Erzeu­gung von hochreinem Sauerstoff,
• Figur 3 eine Ausführungsform des erfindungsgemäßen Verfahrens zur Erzeugung von hochreinem Stick­stoff und
• Figur 4 ein Verfahrensschema einer weiteren Ausfüh­rungsform des erfindungsgemäßen Verfahrens zur gleichzeitigen Erzeugung von hochreinem Sauer­stoff und hochreinem Stickstoff.

Here show:

• FIG. 1 shows a process diagram of an embodiment of the process according to the invention for producing high-purity oxygen,
• FIG. 2 shows a schematic illustration of a detail from a further embodiment for producing high-purity oxygen,
• FIG. 3 shows an embodiment of the method according to the invention for producing high-purity nitrogen and
• FIG. 4 shows a process diagram of a further embodiment of the process according to the invention for the simultaneous production of high-purity oxygen and high-purity nitrogen.

Identical or analogous process steps or plant parts have the same reference symbols in the figures.

From impurities such as CO₂ and H₂O pre-cleaned in the usual way and compressed to a pressure of about 6.3 bar, air is fed via a line 1 to the first stage 2 of a two-stage rectification column 3. At a temperature of approx. -177 ° C, the air is split into a nitrogen-rich fraction in the head and an oxygen-rich fraction in the swamp. A portion of the nitrogen-rich fraction is removed in liquid form (line 4), subcooled in a heat exchanger 5, decompressed and fed at a temperature of about -193 ° C. as reflux to the second stage 6 of the rectification column 3. The two stages 2, 6 of the rectification column 3 are in heat-exchanging connection with one another via a common condenser-evaporator 7.

The oxygen-rich fraction from the bottom of the first stage 2 is removed via a line 8, subcooled in the heat exchanger 5 and removed therefrom at an intermediate point which is at a higher temperature level than the nitrogen-rich fraction 4 supplied. A portion of the oxygen-rich fraction, which is at a temperature of about -182 ° C, is fed to the second stage 6 at an intermediate point during the rest is fed as a coolant to a condenser-evaporator 9 in the top of a crude argon column 10.

Another pre-cleaned air stream, which has been compressed to generate cold and then expanded, is fed via a line 11 to the second stage 6 approximately at the level of the supply of the oxygen-rich fraction 8. In the second stage 6, which is operated at a temperature of approximately -179 ° C. and a pressure of approximately 1.6 bar, the pre-separation fractions from the first stage are converted into pure oxygen, which is obtained in the column bottom and pure nitrogen, which is obtained in the top of the column, disassembled. The oxygen typically has a purity of 99.5% and contains about 0.5% argon and additionally all the krypton and xenon as well as the hydrocarbons in the ppm range that are present in the air.

The oxygen is withdrawn in gaseous form above the column bottom via a line 12 and / or in liquid form from the column bottom via a line 13. The liquid oxygen is subcooled in the heat exchanger 5.

Liquid nitrogen (line 14) with a purity of 99.995% is led out of the top of the second stage 6. Gaseous pure nitrogen with a purity of 99.995% is removed from the top of the second stage 6 via a line 15. These two nitrogen fractions are still contaminated by the usual components such as oxygen, argon, helium, neon, hydrogen and carbon monoxide.

Impure gaseous nitrogen (about 0.15% O₂ content) is removed via a line 16 from the upper third of the column. The two gaseous nitrogen streams are heated in the heat exchanger 5 and removed from the system.

The argon concentration is highest in the second stage 6 somewhat below the middle of the column, for example between the 35th and 36th plate with a total plate number of 96. At this point, a fraction from the second stage is removed via a line 17, which contains up to 91% O₂ a few ppm N₂ to 9% argon and traces of xenon, krypton, hydrocarbons in the ppm range. This fraction is fed to the crude argon column 10 at its lower end and is broken down there by rectification into a gaseous crude argon fraction which is taken from the top of the crude argon column via a line 18 and a liquid bottom fraction which is returned to the second stage via a line 19. The crude argon fraction preferably has a composition of 2% O₂, 97% argon and 1% N₂, the bottom liquid has a composition of 94% O₂, 6% argon.

Part of the crude argon is condensed from the first rectification stage 2 to form reflux liquid in the condenser-evaporator 9 by heat exchange with previously relaxed oxygen-rich liquid. The oxygen-rich liquid is partially evaporated. The evaporated portion is removed via a line 20 and passed into the second stage 6 together with liquid removed from the evaporator chamber (line 21). Three to five trays above the bottom liquid, a liquid fraction is removed from the crude argon column via a line 22 and fed to a high-purity oxygen column 23. The fraction 22 consists only of the components O₂, argon and N₂ and is free of krypton, xenon and hydrocarbons. The reason for this is that the impurities (krypton, xenon, hydrocarbons are retained in the bottom of the column between the column bottom and the removal point in the column bottom and are returned to the second stage 6 via line 19.

In the high-purity oxygen column 23, which is operated at a temperature of -179 ° C. and a pressure of 1.5 bar, nitrogen and argon are separated from the oxygen and are gas-free Residual fraction removed from the top via a line 24 and passed back into the crude argon column 10 above the point of withdrawal of the liquid fraction 22 or back into the column 6 (dashed line 42) above line 17. High-purity liquid oxygen with a purity of 99.999% is removed from the bottom of the ultrapure oxygen column 23 via a line 25. The oxygen typically has the following impurities: hydrocarbons, krypton, xenon, nitrogen each less than 1 ppm, argon less than 10 ppm. The high-purity liquid oxygen is subcooled in the heat exchanger 5 and then removed from the system. If necessary, in addition or as an alternative, high-purity gaseous oxygen can be removed via a line 26 above the bottom of the column.

The column sump is heated by nitrogen, which is removed from the top of the first stage 2 and fed via a line 27 to a condenser-evaporator 28 arranged in the column sump. During the heat exchange, the nitrogen condenses and is returned to the head of the first stage 2 via a line 29. Part of the gaseous nitrogen is branched off from line 27 and removed via line 30.

FIG. 2 shows a modification of the method of FIG. 1. Since the major part of the modified method is identical to that of FIG. 1, only the ultrapure oxygen column 23 is shown in FIG.

The removal of the liquid high-purity oxygen via line 25 or the gaseous high-purity oxygen via line 26 takes place here some floors above the sump. The preferably three to five rectification trays retain undesirable fractions such as krypton, xenon and hydrocarbons, which can get into the bottom of the ultrapure oxygen column 23 by enriching traces or by penetrating through a less dense point on the condenser-evaporator 28.

Line 43 is used to discharge a small amount of bottom liquid, which is either discarded or returned to the second rectification stage. In this way, the accumulation of undesirable fractions in the bottom of the ultrapure oxygen column 23 can be largely prevented.

FIG. 3 shows a further embodiment of the method according to the invention, in which high-purity nitrogen is generated. Analog system parts are provided with the same reference numerals as in Figure 1.

Gaseous nitrogen from the top of the first rectification stage 2 is fed via line 27 into a high-purity nitrogen column 31, which is operated at approximately the same pressure as the first rectification stage, and is separated there into a liquid bottom fraction and a residual gas fraction. The residual gas fraction contains undesirable fractions such as helium, neon and carbon monoxide and is drawn off via line 33 and admixed with the impure nitrogen fraction 16 from the second rectification stage 6.

The bottom fraction flows back via line 29 to the first rectification stage, from which high-purity nitrogen is withdrawn via line 34. Between the top condenser and the tapping point for line 34 are two to five rectification trays, which act as a barrier for helium, neon and carbon monoxide, the concentration of which is greatest in the top of the column. The high-purity nitrogen 34 has a purity of 99.999%, the rest consists essentially of argon.

In addition to the line 27, another line 32 opens into the high-purity nitrogen column 31. The line 32 is connected directly to the condenser 7 and is also referred to as a helium drain. The air components helium, neon and carbon monoxide accumulate in this area. These components are removed together with the nitrogen via line 32 from the first stage 2 and with the top fraction of the ultrapure stick column of material 31 removed.

The top of the high-purity nitrogen column 31 is cooled with oxygen-rich liquid 46, which comes from the bottom of the first rectification stage 2. The oxygen-rich liquid is partially evaporated, leaves the top condenser of the high-purity nitrogen column 31 via line 44 or 45 and is then introduced into the second rectification stage 6.

If the high-purity nitrogen is desired in liquid form, the nitrogen removed via line 34 is subcooled in heat exchanger 5 and then expanded. The flash gas produced during the expansion is separated off in a separator 35 and mixed with the nitrogen in line 15 via a line 36. Liquid nitrogen of the highest purity can be removed via line 37.

If, in addition or as an alternative, high-purity nitrogen is required as a gas, the high-purity liquid nitrogen (line 38) is partially or completely expanded without prior cooling and fed to an evaporator 39. The evaporator is heated by a partial flow of the gaseous nitrogen in line 30, which is branched off via a line 40 and, after the heat exchange in the evaporator 39, is admixed with the nitrogen-rich fraction 4. The high-purity gaseous nitrogen is discharged from the evaporator 39 via a line 41.

An embodiment of the method according to the invention, in which both high-purity oxygen and high-purity nitrogen can be produced, is shown in FIG. Here the high-purity oxygen column 23 and the high-purity nitrogen column 31 are combined into one unit and are in heat-exchanging connection via a condenser-evaporator 28 now common. This allows the heating of the bottom of the ultrapure oxygen column 27 and the cooling of the top of the High purity nitrogen column 31 can be carried out with the help of only one heat exchange apparatus.

Claims (15)

1.A method for air separation by rectification, in which the air is pre-divided into a nitrogen-rich and an oxygen-rich fraction in a first rectification stage and the two fractions are fed to a second rectification stage and broken down into oxygen and nitrogen, characterized in that from a crude argon column (10 ), into which a stream (17) essentially containing oxygen and argon from the second rectification stage (6) is introduced, a further fraction (22) above the bottom is removed and in a high-purity oxygen column (23) in high-purity oxygen (25, 26) and a lighter residual fraction (24) is broken down. 2. The method according to claim 1, characterized in that the further fraction (22) is removed in liquid form and is added as reflux liquid to the ultrapure oxygen column (23). 3. The method according to claim 1 or 2, characterized in that the removal of the further fraction (22) takes place several trays above the column bottom of the crude argon column (10). 4. The method according to any one of claims 1 to 3, characterized in that the high-purity oxygen (25, 26) is removed from a plurality of trays above the bottom of the ultrapure oxygen column (23). 5. The method according to any one of claims 1 to 4, characterized in that the column bottom of the ultrapure oxygen column (23) is heated by nitrogen (27) from the top of the first rectification stage (2). 6. Apparatus for carrying out the method according to claim 1 with a two-stage rectification column and with a crude argon column connected to its second stage, characterized by a high-purity oxygen column (23) which is connected to the crude argon column (10) by means of a lateral removal line (22). 7. The device according to claim 6, characterized in that the lateral removal line (22) has a plurality of rectifying trays above the bottom of the crude argon column (10). 8.A method for air separation by rectification, in which the air is pre-divided into a nitrogen-rich and an oxygen-rich fraction in a first rectification stage and the two fractions are fed to a second rectification stage and broken down into oxygen and nitrogen, characterized in that a further nitrogen-rich fraction ( 27) from the top of the first rectification stage (2) into a high-purity nitrogen column (31), where it is broken down into a bottom liquid (29) and a residual gas fraction (33). 9. The method according to claim 8, characterized in that the sump flow (29) is returned to the head of the first rectification stage (2). 10. The method according to claim 9, characterized in that some trays below the head of the first rectification stage (2) a liquid fraction (34) is taken from high-purity nitrogen. 11. The method according to claim 10, characterized in that the liquid high-purity nitrogen (34) is at least partially supercooled. 12. The method according to claim 10 or 11, characterized in that the liquid high-purity nitrogen (34) is evaporated at least partially in heat exchange with condensing nitrogen (40) from the top of the first rectification stage (2). 13. An apparatus for performing the method according to any one of claims 8 to 12, characterized by a high-purity nitrogen column (31) which is connected to the first rectification stage (2) by a gas line (27) and by a liquid line (29), and by a Extraction line (34) for high-purity nitrogen, which is attached some floors below the head at the first rectification stage (2). 14. A method for air separation, in which the process steps from one of claims 1 to 6 and the procedural steps from one of claims 9 to 12 are applied together, characterized in that the bottom liquid of the ultrapure oxygen column (23) by heat exchange with the gas in the top of the Ultrapure nitrogen column (31) is heated. 15. The apparatus for performing the method according to claim 14, characterized by a condenser-evaporator (28) which is mounted between the ultra-pure oxygen and ultra-pure nitrogen column.

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