US2422626A - Recovery of nitrogen and oxygen - Google Patents

Description

F. KOEHLER Filed Ad 24, 1938 2 Sheets-Sheet 1 I .l llllllll l-lIIILII-III'Ir-IIIIIII'II 4 Franz Kathi-r "Hi6 ATTORNEYS June 12?, 194'? macovmmr OF mmoemn AND oxmml Patented June 17, 1947 aacovnnx or NITROGEN AND OXYGEN Franz Koehler, Leuna, Germany; vestedin the Attorney General, of the United States Application August 24, 1938, Serial No. 226,591

. In Germany August 28, 1937 Sections 3am! 14, Public Law 690, August a, 1946. Patent expires August 28, 1957 2 Claims.

The present invention relates to improvements in and apparatus for the recovery of nitrogen and oygen.

It is already known that by liquefaction of air at high pressure and while using counter-current tubular heat-exchangers and by rectification of the, liquefied mixture, practically pure nitrogen and oxygen can be prepared. This method has two considerable drawbacks, however, namely- (1) that a great part of the air to be separated into its constituents has to be brought to high pressure and (2) that the whole of the air must be carefully purified from water and carbon dioxide before the liquefaction in order to avoid stoppages in the counter-current tubular heatexchangers.

By reason of these drawbacks, another process has been adopted for some years in which considerably lower pressures are used and in which it is unnecessary to purify the whole of the air from water vapor and carbon dioxide before the liquefaction. This method differs mainly-from that already mentioned in that the heat-exchange is not efiected by leading the gas to be cooled.

and the gas absorbing heat in counter-current to each other in tubular exchangers but in socalled recuperators which are cooled periodically,

whereby the low heat content necessary to cover the loss of low heat content is produced by releasing from pressure a small amount (from about 4 to 10 per cent of the whole) of air compressed to high pressure (as for example 200- atmospheres) and if necessary by releasing from pressure one of the gases to be recovered in expansion engines. By reason of the advantages attending this method, it has acquired a great importance for the recovery of oxygen, although by the said method it is impossible to obtain pure nitrogen as well as the pure oxygen.

I have now found that by the last-mentioned method both nitrogen and oxygen of high purity can be prepared by withdrawing from the central part of the upper of the two columns used for.

rectification a small amount of a mixture of nitrogen and oxygen and usin its low heat content within the system.

In other processes for separating air into its would be deemed injurious because considerable amounts of nitrogen and oxygen are necessary for the removal of the substances (carbon dioxide and water) separatedin the recuperator from the gases which are not purified according to the said process.

In spite of this, however, pure nitrogen and oxygen are obtained according to this invention by the withdrawal of a certain amount of a mixture of nitrogen and oxygen at a central part of the upper column, when the amount of highly compressed, preliminarily purified air is increased as a substitute for the withdrawn gas mixture and as an additional source for the cooling. This additional highly compressed air provides the additional amount of nitrogen and oxygen necessary for the continuous removal of the impurities from the recuperator. Moreover, by releasing the pressure on the same, so much low heat content is produced that the whole of the nitrogen of the lower column can be used as a trickling liquid for the upper column. In this way there can be recovered in the same apparatus not only oxygen of a high degree of purity but also nitrogen of a high degree of purity.

The production of the additional highly compressed and preliminarily purified air, according to this modification, requires extra energy and therefore involves extra expense. This may be avoided according to another modification of theinvention by combining the measures usual part of the nitrogen of the lower column in exconstituents in which a recuperator is not used,

it has already been proposed to withdraw a mixtureof nitrogen and oxygen at'a central part of thefractionation column in order to obtain purer nitrogen and p'urer oxygen. It could not have been expected, however, that this measure would be of use in' a method using a recuperator in which any removal of gas mixture from the system pansion machines, only a relatively small amount of a mixture of nitrogen and oxygen need be withdrawn from the central part of the upper column and additional large amounts of preliminarily purified, highly compressed'air are not necessary.

There isthus the advantage that-only a simple apparatus, a small expenditure of energy and the use of low pressures for the major portion ofthe air are necessary and that a preliminary purification of the major portion of the air is unneces- The mixture of oxygen and nitrogen can be withdrawn from the rectification column ata point at which it contains from about 5 to 35, preferably from 15 to 25 per cent of oxygen. By withdrawing such a mixture there is produced in the rectification column such equilibrium relation that the amount of trickling liquid (nitrogen) present is sufficient for washing the mixture of nitrogen and oxygen even when a part of the nitrogen has been withdrawn from the lower column for the cooling in pressure-release machines.

The mixture of nitrogen and oxygen withdrawn from the upper rectification column serves in particular to precool the highly compressed air used for the additional refrigeration. This highly compressed air is released to the pressure prevailing in the rectificaton column and introduced into the upper or lower rectification column for utilization of the low heat content thus produced.

The highly compressed air, after the precooling by the withdrawn mixture of nitrogen and oxygen, may be further cooled by heat exchange with that part of the air or of the nitrogen which is withdrawn from the rectification column in the manner known in the low pressure process in order to be released from pressure in an expansion engine to about the temperature of liquid air.

A specially advantageous modification of the present invention consists in adding the mixture of nitrogen and oxygen withdrawn from the middle of the upper rectificationcolumn, after subjection to heat exchange, to the air to be separated into its constituents. Since this added mixture of nitrogen and oxygen is free from water and carbon dioxide, the content of water and carbon dioxide in the air to be separated into its constituents is reduced. It is then readily possible for these substances, which are deposited by the cooling of the air in the recuperators, to be further removed in the next working operation by the pure oxygen and the pure nitrogen without it being necessary to increase the amount of 'highly compressed air purified from water and carbon dioxide supplied for the further production of low heat content to the mixture to be separated into its constituents in the rectification column, in order to render certain the sublimation of the deposited substances in the recuperators.

In some cases it may be advantageous to compress the gas mixture withdrawn from the central part of the upper rectification column to high pressure after the heat exchange and to use it instead of the highly compressed air serving for the cooling by releasing it from pressure if desired in expansion engines. In this way the amount of air which has to be freed from water and carbon dioxide before its cooling is reduced.

Another way in which the low heat content of the gas mixture withdrawn from the upper rectification column may be utilized consists in using the mixture for the cooling of one or more additional recuperators which are arranged in parallel with the pair of recuperators through which the nitrogen and oxygen flow and also serve for the precooling of the air under low pressure to be separated into its constituents, whereby the valves "may be actuated by the existing regulation device of the pair of recuperators.

Three embodiments of apparatus in accordance with this invention will now be described with reference to the accompanying drawings, but

the invention is not restricted to the particular embodiments shown.

A, B, C and D are recuperators which are operated alternately in known manner, E is a twocolumn apparatus for separting air into its constituents, F is a counter-current heat exchanger, J is a low pressure air turbo-compressor, R is a pressure release valve and K is a high pressure air compressor.

The air compressed by the compressor J to about 4.5 atmospheres is cooled in the recuperators A, B, C and D almost to, the liquefaction temperature and introduced through a pipe a into the lower column of the apparatus E for rectification. Furthermore a certain amount of air compressed to about 200 atmospheres and freed from Water and carbon dioxide is led through a pipe b and the heat exchanger F and released from pressure through the pressurerelease valve R into the lower rectification column. The cold constituents (oxygen and nitrogen) are led through pipes c (nitrogen) and 11 (oxygen) into the cold accumulators A, B, C,

D from which they escape through c and d after absorbing heat. From about the middle of the upper rectification column, there is withdrawn through e a small amount of a mixture of nitrogen and oxygen which is led to the counter-current heat exchanger F to the highly compressed air introduced at b.

The embodiment according to Figure 2 differs from that according to Figure 1 in that for the production of additional low heat content nitrogen under a pressure of about 4.5 atmospheres is withdrawn from the lower rectification column and. supplied through a pipe I to a counter-current exchanger G in which it absorbs part of the heat of the highly compressed air precooled in F, This nitrogen is then released from pressure in the expansion engine H and led into the cold accumulators A and B together with the nitrogen escaping from the upper rectification column. In Fig. 3 the mixture of oxygen and nitrogen withdrawn through line e from the middle part of the upper rectification column in this embodiment is compressed by compressor K, after flowing through heat exchanger F, and used instead of the highly compressed air serving for the cooling by releasing it from pressure with the aid of pressure release valve R.

What I claim is:

1. Process of rectifying air into its major constituents which comprises refrigeratingair without liquefaction at a slightly elevated pressure, passing the refrigerated air into a rectifying column, introducing a relatively small quantity of highly compressed air into said column while releasing the pressure thereon to thereby efi'ect rectification of the air into its major constituents, separately withdrawing said constituents, passing said constituents in heat exchange relationship to the air undergoing refrigeration, withdrawing a relatively small amount of a mixture of nitrogen and oxygen from said rectifier, utilizing its low heat content and recycling the same.

2. Apparatus for rectifying air into its major constituents which comprises a plurality of refrigeration chambers, a two-column rectifying vessel, means for forcing air through said refrigeration chambers and into said rectification vessel, means for forcing a comparatively sma .1 quantity of highly compressed air into the rectification vessel, means for reducing the pressure of said highly compressed air upon its introduction into the rectification vessel. means for sepa- 5 6 rately removingsubstantially pure nitrogen and REFERENCES CITED oxygen from the rectification vessel, means for conducting the same in heat eirchange relation- The .followmg ref rences are of record in th ship through the refrigeration chambers, means file of this Patent! for withdrawing a mixture of nitrogen and oxy en 5 UNITED STATES PATENTS from the rectification vessel and means for re- 'cycling said mixture. Number Name Date a FRANZ KOEHLER v 2,009,08 Gomonet July 23, 1935 2,048,076 Linde July 21, 1936

Images