US3426543A

US3426543A - Combining pure liquid and vapor nitrogen streams from air separation for crude hydrogen gas washing

Info

Publication number: US3426543A
Application number: US677839A
Authority: US
Inventors: Rudolf Becker
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1963-06-19
Filing date: 1967-10-17
Publication date: 1969-02-11
Anticipated expiration: 1986-02-11
Also published as: DE1258882B

Description

Feb. 11, 1969 R. BECKER 3, 6, 3

COMBINING PURE LIQUID AND VAPOR NITROGEN STREAMS FROM AIR SEPARATIQN FOR CRUDE HYDROGEN GAS WASHING Filed 061;. 1.7, 1967 *[U 1755:. Wig-N 80 m Brigas 58%78 Yic lnvenfor RUDOLF BECKER A #o/nep United States Patent 7 Claims priority, application Gsermany, June 19, 1963,

U.S. Cl. 62-13 Int. Cl. F25j 3/04 21 Claims ABSTRACT OF THE DISCLOSURE Air is separated in an air separation plant employing a double rectification column. An impure nitrogen stream from the high pressure column is recompressed and then cooled by vaporizing oxygen from the low pressure column as well as exiting ammonia synthesis gas and scrubber liquid streams from a crude hydrogen scrubber before returning to the high pressure column. Separate streams of pure nitrogen liquid and vapor are removed from the high pressure column which streams are combined and completely liquefied with part of the liquid nitrogen being used to wash a crude hydrogen gas while another part of the pure liquefied nitrogen is added to the washed hydrogen to form a gas stream for ammonia synthesis.

This is a continuation-in-part of copending US. patent application Ser. No. 373,418, filed June 8, 1964, now abandoned.

This invention relates to a process for air fractionation and is particularly concerned with the use of a highly compressed gas in cold circulation for pressure vaporization of liquid oxygen, and with the use of a group of periodically transposable regenerators for heat exchange between the air to he fractionated and the oxygen.

Besides the production of oxygen, the invention is also concerned with the separation of pure nitrogen, especially for the production of synthetic ammonia.

The invention is carried out in a very simple and economical manner by taking from the rectification column an impure fraction of circulating nitrogen, warming it to the temperature of the surroundings by passage through a regenerator which has been cleansed by scavenging with nitrogen during at least one preceding stage, liquefying the impure fraction by heat exchange after high compression and, preferably, after cleansing in an adsorber, and then, after passage through an expansion device, returning the impure nitrogen to the rectification column at a point near the place from which it was taken, whereby pure nitrogen is obtained from the head section of the column, preferably for independent use.

The impure nitrogen is preferably removed from the middle section of the pressure column and, after being at least partly liquefied, is returned to the same place.

For the production of the required additional cold, a portion of the impure nitrogen fraction that was taken from the middle section of the pressure column, after a preliminary warming, is expanded while doing work and is then used as a scavenging gas for the aforementioned scavenging step for the regenerators. The gas to be expanded can be taken, after sutfi-cient prewarming, from a suitable place on the regenerator through which circulating nitrogen is passed, and/ or if it bypasses this regenerator, is prewarmed by passage through a heat exchanger with circulating nitrogen before being returned to the pressure column.

Patented Feb. 11, 1969 "ice Another feature of this invention is that the pure nitrogen is taken from the head of the pressure column.

If the pure nitrogen is to be used for the synthesis of ammonia, it is advantageous to take the pure nitrogen from the head of the column in a partly liquid and partly gaseous state, in preferred proportions of 1 part liquid to 6-20 parts gas, the most preferred proportion being 1 part liquid to 10 par-ts gas; and after complete liquefaction with further cooling under the same pressure if necessary, it is subjected to heat exchange with possibly expanded products of a synthetic ammonia plant, is then compressed to the pressure of the washing column of that plant, and is used as a washing fluid for hydrogen gas and as a component of the ammonia synthesis gas. In this manner the necessity of compressing the gaseous nitrogen to the normally higher pressure of the washing column of the synthetic ammonia plant is avoided, and the pump for compressing the liquefied nitrogen uses much less energy.

The process of this invention will now be described more in detail with reference to the schematic drawing illustrating the process and apparatus of the invention.

The air to be fractionated enters by conduit 1, is passed through compressor 2 to compress it to about 6 atmospheres, and arrives by conduit 3 at regenerator 4, which is one of the series of transposable regenerators 4 to 7, through valve 411, which is one of the series of valves 4a to 7a that control communication between conduit 3 and the regenerators. The deeply cooled air passes through a transposable valve or part of double check valve 4a, which is one of the series of such valves 4d to 7d for delivery through conduit 8 to the lower part of pressure column 9 which, in conjunction with the low pressure column 10, forms a double rectifier tower arrangement.

The liquid in the sump of the pressure column, which is under a pressure of about 5.6 atm., is delivered from the bottom of the column by conduit 11 to adsorber 12, thence to heat exchanger 13 from Where it passes through conduit 14 and expansion valve 15 to the middle portion of the low pressure column 10 in which the pressure is about 1.4 atm.

From the middle portion of the pressure column 9 an impure nitrogen fraction is removed by conduit 16 and is passed through the transposable valve 72, which is one of the series of valves 4e to 7e, for delivery to regenerator 7 of the series 4 to 7. The greater portion of this fraction passes through transposable valve 7c, which is one of the series of valves 40 to 7c, and then through conduit 17 to the compressor 18 for compression to about atm.

The compressed gas leaves the compressor via conduit 19, and preferably passes through an adsorber 20 having a bypass valve 21, to a group of

heat exchangers

23, 24 and 25 and from there passes through the collecting conduit 26 to heat exchanger 27.

From heat exchanger 27, the circulating nitrogen passes on the one side through conduit 28 and through heat exchangers 29 and 30 and through conduit 31 to the collecting conduit 32, while on the other side it passes through conduit 33, heat exchanger 34 and conduit 35 to the same collecting conduit 32.

In the conduit 32 there is a throttle valve 36 through which the nitrogen, which is now at least partl liquefied, passes for return to the same portion of the pressure column 9.

Above the place of entry of the conduit 32 in column 9 there is mounted a liquid-collecting basin 37 from which the conduit 38 leads to the outside, delivering liquid nitrogen through the heat exchanger 39, conduit 40 and expansion valve 41 to the head of the low pressure column 10.

From the head of the low pressure column 10 nitrogen is withdrawn through conduit 42 and through heat exchanger 39, conduit 43, heat exchanger 13 and conduit 44 3 and through the two valves 5d and 6d of the series M to 7d, and passes as scavenging gas through two transposable regenerators 5 and 6 of the series 4 to 7 in which it is warmed to the temperature of the surroundings. The scavenging gas then passes through the two valves 51) and 6b of the series 4/) to 7b and makes its exit through conduit as impure nitrogen gas.

The periodic reversal of the regenerators 4 to 7 is performed in such a manner that each regenerator in a first phase of operation is traversed in a downward direction by air to be cooled therein to a low temperature in preparation for its fractionation, in a second and third phase in an upward direction by scavenging gas which is to be warmed therein while effecting sublimation of the CO and of ice formations, and in a fourth phase in an upward direction by circulating nitrogen that is to be warmed.

A portion of the circulating nitrogen fraction is advantageously diverted from the lower half of the corresponding regenerators through valve 7 f of the series of valves 4] to 7f and is delivered through conduit 46 having valve 47 therein and through conduit 48 to the expansion turbine 49. The expanded gas then passes through conduit 50 to the collection conduit 44 for scavenging gas. Additionally, or alternatively, a portion of the impure nitrogen can be diverted from the circulation conduit 16 by Way of conduit 51 for passage through heat exchanger 34 and conduit 52 having therein valve 53 and then through conduit 48 to the expansion turbine 49. The regulation is determined by the temperature equilibrium which must be maintained during the process.

From the sump of the low pressure column 10 liquid oxygen is removed by conduit 54, is compressed to about 41 atm. by pump 55, and is delivered b conduit 56 to a series of

heat exchangers

29, 27 and 23 in which the liquid oxygen is brought to about the temperature of the surroundings for delivery under about 40 atm. by conduit 57 having therein valve 58 to its destination.

From the head of the pressure column 9 pure gaseous nitrogen is removed by conduit 59 while liquid nitrogen is removed by conduit 60 under a pressure of about 5.5 atm., both of these conduits delivering their nitrogen to the heat exchanger 61 in which most of the remaining gaseous nitrogen is liquefied. The largely liquefied nitrogen is then delivered by conduit 62 to heat exchanger 63 where it is further cooled and is delivered by conduit 64 to pump 65. Pump 65 then delivers the liquid nitrogen through conduit 66 having therein valve 67 to the head of the Washing column 68 under a pressure of about 25 atm.

A portion of the liquid nitrogen is diverted from conduit 66 by conduit 69 with valve 70 directly to conduit 71 which draws gas from the head of the washing column 68 to produce a stoichiometric mixture for ammonia synthesis. The conduit 71 then delivers the hydrogen-nitrogen mixture through

heat exchangers

61 and 72,

conduits

73 and 74,

heat exchangers

75 and 25, and then through

conduits

76 and 77 to the collecting conduit 78 having therein valve 79 for delivery of the hydrogen-nitrogen mixture to an ammonia synthesis plant under a pressure of about 24 atm.

A hydrogen rich gas such as converter gas is delivered by conduit 80 to compressor 81, and thence by conduit 82 and heat exchanger 83, which can be water-cooled, and by conduit 84 to heat exchanger 75, then through conduit 85 to heat exchanger 72, and from there through conduit 86 to heat exchanger 87. From heat exchanger 87 the gas is delivered by conduit 88 to the cooling coil 89 in heat exchanger 63 and from there by conduit 90 to the lower portion of the washing column 68.

The liquid nitrogen with impurities consisting mainly of CO is taken from the sump of the washing column 68 by conduit 91 with expansion valve 92 therein for passage through

heat exchangers

63 and 87, conduit 93, heat exchanger 30, conduit 94, heat exchanger 24, and then to conduit 95 for delivery to the Outside.

Referring to the drawing, the following table is illustrative of quantities found in the various conduits:

C onduit Fluid description Quantity 80,000 N11L /l1. 30,000 Nm. /l1. 12,500 Nmfi/h. 35,000 Nmfi/h. 42,000 Nmfi/h. 10,000 Nmfi/h. 1,000 Nmn' /h.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

What is claimed is:

1. In a process of fractionating air and producing ammonia synthesis gas by washing raw hydrogen containing carbon monoxide with pure liquid nitrogen wherein said air is fractionated in a double column having a high pressure section and a low pressure section to produce said pure liquid nitrogen, the improvement which comprises withdrawing pure nitrogen fluid from the top zone of the high pressure section of the double column in two separate streams, one being substantially completely in the liquid phase, and the other being substantially completely in the vapor phase in proportions of 6-20 parts vapor to 1 part liquid, combining and completely liquefying said combined fluid streams by heat exchange with one or more streams from the ammonia synthesis gas production, said streams being (1) pure ammonia synthesis gas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbon monoxide.

2. A process as defined by claim 1 wherein streams (1), (2), and (3) are heat exchanged with the pure nitrogen fluid.

3. In a process of fractionating air and producing ammonia synthesis gas by washing raw hydrogen containing carbon monoxide with pure liquid nitrogen wherein said air is fractionated in a double column having a high pressure section and a low pressure section to produce said pure liquid nitrogen, the improvement which comprises withdrawing pure nitrogen fluid from the top zone of the high pressure section of the double column in two separate streams, one being substantially completely in the liquid phase, and the other being substantially completely in the vapor phase in a proportion of 10 parts vapor to 1 part liquid, combining and completely liquefying said combined fluid streams by heat exchange with one or more streams from the ammonia synthesis gas production, said streams being (1) pure ammonia synthesis gas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbon monoxide.

4. In a process of fractionating air and producing ammonia synthesis gas by washing raw hydrogen with liquid nitrogen wherein said air is fractionated in a double column having a high pressure section and a low pressure section, the improvement which comprises withdrawing impure nitrogen from the high pressure section, heating said impure nitrogen by indirectly transferring heat between said impure nitrogen and raw air, compressing resultant heated impure nitrogen, cooling resultant compressed impure nitrogen in indirect heat exchange with one or more streams from the ammonia synthesis gas production, said streams comprising (a) a mixture of nitrogen and carbon monoxide, and (b) pure ammonia synthesis gas; and recycling resultant cooled impure nitrogen to the double column.

5. A process as defined by claim 4 wherein said impure nitrogen is withdrawn from a middle section of said high pressure section.

6. A process as defined by claim 4 wherein said indirect heat exchange of compressed impure nitrogen is conducted with streams (a) and (b).

7. A process as defined by claim 4 wherein said indirect heat exchange of compressed impure nitrogen is conducted with streams (a) and (b).

8. A process as defined by claim 4, further comprising withdrawing pure nitrogen fluid from the high pressure section of the double column, partly in the vapor and partly in the liquid state, and completely liquefying said fluid by heat exchange with one or more streams from the ammonia synthesis gas production, said streams being (1) pure ammonia synthesis gas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbon monoxide.

9. A process as defined by claim 8 wherein streams (1), (2), and (3) are heat exchanged with the pure nitrogen fluid.

10. A method of fractionating air comprising: compressing air and passing the compressed air in one direction through a first regenerator means to the high pressure part of a double rectifying column near the bottom thereof, withdrawing liquid from the bottom of said high pressure column and delivering it to an intermediate point in the low pressure part of said double rectifying column, withdrawing nitrogen from the top of said low pressure column and passing said nitrogen through at least a second regenerator means to be scavenged in the direction opposite to said one direction, withdrawing impure nitrogen from a middle section of said high pressure column at a point wherein said impure nitrogen is not of sufficient purity for ammonia synthesis, and passing the impure nitrogen through a further regenerator means in the direction opposite to said one direction, compressing at least a part of the impure nitrogen after passing the same through said further regenerator means, drawing liquid oxygen from the bottom of said low pressure column compressing said liquid oxygen and exchanging heat between the evaporating compressed oxygen and the compressed impure nitrogen, the latter being thereby at least partly liquefied, and returning the at least partly liquefied impure nitrogen to said high pressure column near the point of withdrawal of the impure nitrogen from the column, and withdrawing pure gaseous nitrogen from the top of a condenser-reboiler in the high pressure column, the purity of said pure gaseous nitrogen being suflicient for ammonia synthesis gas.

11. A process as defined by claim 10, further comprising withdrawing liquid nitrogen of sufiicient purity for ammonia synthesis from the top zone of the high pressure column, said withdrawing being at a point above the point for said withdrawing of impure nitrogen.

12. A method as defined by claim 10 further comprising the steps of withdrawing pure condensed liquid nitrogen from the top zone of the high pressure column, and combining said liquid with said pure gaseous nitrogen for flow to ammonia synthesis.

13. A process as defined by claim 12, wherein both said pure gaseous nitrogen and said pure condensed liquid are of suflicient purity for ammonia synthesis gas, the point of withdrawing said pure condensed liquid being above the point of withdrawing said impure nitrogen, said pure gaseous fraction and said pure liquid fraction being passed to the ammonia synthesis plant without any further rectification.

14. A method of fractionating air comprising: compressing air and passing the compressed air in one direction through a first regenerator means to the high pressure part of a double rectifying column near the bottom thereof, withdrawing liquid from the bottom of said high pressure column and delivering it to an intermediate point in the low pressure part of said double rectifying column, withdrawing nitrogen from the top of said low pressure column and passing said nitrogen through at least a second regenerator means to be scavenged in the direction opposite to said one direction, withdrawing impure nitrogen from a middle section of said high pressure column and passing the impure nitrogen through a further regenerator means in the direction opposite to said one direction, compressing at least a part of the impure nitrogen after passing the same through said further regenerator means, drawing liquid from the bottom of said low pressure column, compressing said liquid oxygen and exchanging heat between evaporating compressed oxygen and the compressed impure nitrogen thereby at least partly liquefied, and returning the impure nitrogen to said high pressure column near the point of withdrawal of the impure nitrogen from the column, withdrawing pure nitrogen from the head of said high pressure column partly as a liquid and partly as a gas, completely liquefying the withdrawn pure nitrogen by heat exchange with one or more streams from an ammonia synthesis gas plant, said streams being (1) pure pure ammonia synthesis gas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbon monoxide. pumping the resultant liquefied nitrogen to an increased pressure, washing said hydrogen gas with a part of the nitrogen, and adding another part of the nitrogen to the washed gaseous hydrogen to produce ammonia synthesis gas.

15. A process as defined by claim 14, further comprising exchanging heat between said compressed impure nitrogen and one or more streams from a plant for producing ammonia synthesis gas by scrubbing crude hydrogen with liquid nitrogen, said streams comprising (a) a mixture of nitrogen and carbon monoxide, and (b) pure ammonia synthesis gas.

16. A process as defined by claim 15 wherein both streams (a) and (b) are heat exchanged with said compressed impure nitrogen.

17. A process as defined by claim 16 wherein streams (l), (2), and (3) are heat exchanged with pure nitrogen.

18. A process as defined by claim 14 wherein streams (1), (2), and (3) are heat exchanged with pure nitrogen.

19. A method of fractionating air comprising: compressing air and passing the compressed air in one direction through a first regenerator means to the high pressure part of a double rectifying column near the bottom thereof, withdrawing liquid from the bottom of said high pressure column and delivering it to an intermediate point in the low pressure part of said double rectifying column, withdrawing nitrogen from the top of said low pressure column and passing said nitrogen through at least a second regenerator means to be scavenged in the direction opposite to said one direction, withdrawing impure nitrogen from a middle section of said high pressure column and passing the impure nitrogen through a further regenerator means in the direction opposite to said one direction, compressing at least a part of the impure nitrogen after passing the same through said further regenerator means, exchanging heat between said compressed impure nitrogen and one or more streams from a plant for producing ammonia synthesis gas by scrubbing crude hydrogen with liquid nitrogen, said streams comprising (a) a mixture of nitrogen and carbon monoxide, and (b) pure ammonia synthesis gas, drawing liquid oxygen from the bottom of said low pressure column compressing said liquid oxygen and exchanging heat between the evaporating compressed oxygen and said compressed impure nitro gen thereby at least partly liquefied, and returning the latter impure nitrogen to said high pressure column near the point of withdrawal of the impure nitrogen from the column, withdrawing pure nitrogen from the top zone of the high pressure column for separate use.

20. A process as defined by claim 19 wherein both streams (a) and (b) are heat exchanged with said compressed impure nitrogen.

21. The combination, in an apparatus for fractionating air; first regenerator means, means to compress air and pass it through said first regenerator means to chill the air, a double rectification column with a high pressure and a low pressure portion connected near the bottom of the high pressure portion to said first regenerator means to receive the chilled air therefrom, at least a second regenerator means, means for withdrawing nitrogen from the head of said low pressure column and for passing said nitrogen through said at least second regenerator means to scavenge said regenerator means, means for withdrawing impure nitrogen from a middle section of said high pressure rectifying column and for passing said impure nitrogen through a further regenerator means to be warmed, means for compressing the warmed impure nitrogen employed for cooling the further regenerator means, means for at least partly liquefying the compressed nitrogen, and means including a throttle valve for returning the at least partly liquefied nitrogen to said column near the point thereof from which it was withdrawn from the column, conduit means leading from the head of the high pressure portion of said column for drawing pure nitrogen therefrom, said conduit means comprising two separate pipes, one for withdrawing liquid-phase pure nitrogen, and the other for withdrawing vapor-phase pure nitrogen, heat exchanger means to which said conduit means lead operable for effecting complete liquefication of said nitrogen, pump means connected to the heat exchanger means to References Cited UNITED STATES PATENTS 2,895,304 7/1959 Wucherer et al 6213 2,936,593 5/1960 Grunberg 62-20 2,962,867 12/1960 Seidel 62-13 XR 3,216,206 11/1965 Kessler 6213 2,526,996 10/1950 Crawford 62--13 XR NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

US. Cl. X.R.