US2982108A - Separation of air - Google Patents

Description

May 2, 1961 J. F. GRUNBERG ETAL SEPARATION OF AIR Filed Dec. 14. 1956 j)! cla J11? ass GRUNBERG United States Patet'rtO ce r -Patented May 2, 1961 The nitrogen compressed and liquefied according to this 2,982,193 invention will be preferably introduced at the top of the lowest pressure'column, while liquid nitrogen, coming SEPARATION OF AIR from at least one of the higher pressure columns, will Jacq F, G ()uh-emont, Qubec, and Warren 5 be preferably introduced lower than the said top. This ar Nelson, Montreal, Qu b C d assignol-s t LAi' rangement is particularly advantageous in the case where Liquide, Socit Auonyme Pour lEtude et lExploitatwo nitrogen gas streams at low pressure are to be protion des Procds- George Claude, Paris, France Filed Dec. 14, 1956, ser. No. 628,353 Claims priority, applica tionFrance Dec. 15, 1955 5 Claims. or. 62-28) This invention relates to a air at low temperatures.

Previously compressed air is cooled and purified, at least in part,'in an exchangeapparatus, the gas streams reversed periodically, and the compressed air is then rectified in at least two columns operating at different pressures with mutual heat exchange. A principal purpose is to eliminate beforehand the high boiling impurities 0; air, like water vapor and CO simply by cooling t em.

If a reversing or a regenerating exchanger is used for cooling air beforehand, or if, the air stream going in and the gas stream going out succeed each other periodically in the same pipe, complete evaporation by the separated gas of the impurities deposited as solids by the air can only be assured under certain conditions; For example, the average throughput of gas passing through the cold portion of the exchanger in the opposite direction to air must exceed the average throughput of process for the separation of air, so that the dilference in temperature between the air and the gas at the cold end of the exchanger is not too great.

To accomplish this,.it has already been proposed, in a' two-column rectification plant, to lead a stream of nitrogen through a special pipe,from the upper part of the high-pressure column,'through the exchange apparatus, in a direction opposite to that of the air going in, to compress this nitrogen, to liquefy it by heat transfer,

and to use it as the reflux liquid. However, thisimethod has the disadvantage that it does not improve appreciably the separation carried out in the low pressure column.

A'novel feature column.

This arrangement overcomes the inconvenience-stated above and presents the advantageof increasing the: flows of reflux liquids in1, the;rectification unit thereby innproving the separation. -When'afractidn or the airfleavingthe heat exchange apparatus is sent to an'expansion machine, the compressed nitrogen can be used to" warm this fraction before expanding-it. 1

, The compressed 'nitrogen ;can.also be used -to heat the products of separationff''oxygen ahd nitrogen extracted from"the-"'lo vvest pressure "colu'rn'nibefore they reach the exchange apparatus used for the initial cooling of the air.

Finally, the compressed-nitrogen caii also be used to heat the highest pressure column, andall or at-least a of the present invention isthat the recycled nitrogen gas is withdrawn from the low-pressure duced, one in a relatively impure state to be released as waste to the atmosphere after heating in the proper exchangers, and an'essentially pure nitrogen product, a part of which is compressed and liquefied in accordance with the invention.

The various improvements of this invention are described in the following example and illustrated in the by pipe 4, or in the space surrounding the other seg ments, according to the position of reversing-valves 2 and 2A.

current to the air:

(a) Impure nitrogen flowing eitherin thespace' su rrounding the segments, if the air is flowing through seg-' ment 4, or in segment 4 if the. air is flowing in the spacesurrounding the segments;

('b) Oxygen flowing in the segment represented by.

pipe 5, and

(0) Pure nitrogen flowing inthe segment represented by pipe 6. A part of this pure'nitrogen can leave the exchanger, after passing through its cold section through valves 37 and pipe 37A. 7 a The air, leaving the exchanger at about 171 C flows through reversing-valve 2A and pipe 7. Then it divides into two parts: one, flowing through pipe 8, 40 enters high pressure column 15a few plates above its bottom, under a pressure of about 5.8 kg./cm. the other part flows through valve 9, pipe 9A,. and exchanger 10 'where his warmed by heat. transfer 'with'the pure and compressed nitrogen to about 150 C. Then this airl is expanded in turbine 11 to about'atmospheric pressure. The discharge temperature of the turbine is approximately To iimprove the. operation of low-pressure columnj '16-17, a part of the air leaving turbine 11 can be sent in a variable quantity through' valve 13 into pipe 13A f and, exchanger 14, whereit i'swarmed to about 173 Q substantially the temperature of the cold end of r i the exchanger-by indii'ectheat exchangewith pure nitro gen under pressure. ,'This' airf raction is thenmixed with v .the streamof impure nitrogen at theinlet of exchanger 3; the.remaining air leaving theturbine is sent through pipe. 12 to the centr'al z'one of low-pressure rectification colum 16,'17'whichopefrates at .substantially atmos pheric pressure.

The columm1i5fandt16l 1i7 are arranged in the usual f manner; heat is-exchange'dlindirectly between themby "condenser-vaporiser',18. The bottom of high-pressure column 15 is'heated by coil 44, in which some compressed nitrogen condenses. {The aii liquefied'and en:

riched in ox g n eollects rhe-soni t bt hh column), 5 and flows ith'rough pipe 22, passing th'rou'gh heat 1 exchanger 23 counter-current 'to' impure nitrogen and through expansion valve 24, to the middle part ofcol- 1 pressure column; the other part iswithdrawnthrough pipe 19 enters heat exchangerzfl where it flows counter. I p

In this exchanger, the following gases flow counter-f umn.16. A part of the impure nitrogen condensed in; condenser-vaporiser 18 serves as reflux in the high-f current to gaseous nitrogen. It is then expanded through valve 21 before entering the low pressure column.

The low pressure column 1617 is really composed oftwo parts havingdifierent diameters: lower end 16 and upper end 17.

The lower section of the low-pressure column. 16 produces at its top impure gaseous nitrogen. It is withdrawn through valve 30, pipe 30A, exchangers 20 and 23, pipe 31 and exchanger 3. It then leaves the apparatus. At the bottom of column 16 oxygen is collected as follows and under the following conditions:

(a) Gaseous oxygen, which flows through valve 26, pipe 26A, exchanger 27 to pipe 28, and segment of exchanger 3, before it is discharged through pipe 29; and

(b) Liquid oxygen, which flows through valve 25 and pipe 25A into segment 5 of exchanger 3, without passing through its colder section, and joins the stream of gaseous oxygen already mentioned. The 01 ject of this arrangement is to get additional cooling of the air before it flows through the cold section in accordance wth the process described in French Patent No. 821,949 of May 18, 1937, and US. Patent No. 2,355,660 of August 15, 1944.

Upper end 17 of the low pressure column is a zone for purifying nitrogen. As the quantity of pure nitrogen produced from the top is considerably less than that rising in lower end 16, it may have a smaller diameter, as shownon the drawing. The pure gaseous nitrogen at about 193 C. is removed-through pipe 34, while the reflux of liquid nitrogen, expanded through valve 49, enters the top of the column. 4

'The cold, gaseous, pure nitrogen in pipe 34 passes through exchanger 35, where it is warmed to about 173 C. by indirect heat exchange with liquid nitrogen under pressure. Then it flows through pipe 36 to reversingexchanger 3. It passes through the cold end of this exchanger and then it is removed totally or partially, through valve 37 and pipe 37A at about 130 C. It is warmed by the compressed nitrogen to about 24 C., and, after combining with the nitrogen that has passed through the warm end of exchanger 3, it is compressed by compressor 39 to a pressure of about 15 to 30 kg./cm.

A part of the compressed nitrogen can be removed through valve 40 and pipe 40A to be used in an adjacent plant, for example a liquid nitrogen scrubbing unit. The other part goes through pipe 41 into exchanger 38 where it is cooled to about -l12 C. After combining with nitrogen at about 149 C. coming from the adjacent plant through pipe 40B, it passes through pipe 42 to exchanger counter-current with the air going to expansion turbine 11 and which cools the Weclaim:

1. A process for the purification and separation of air at low temperatures, comprising the steps of compressing said air, cooling it and depositing its high-boiling impurities in a heat exchange apparatus which reverses periodically the gaseous streams of air and of at least part of the separated nitrogen, .liquefying and rectifying part of said compressed air in at least two rectification columns under successively lower pressures, expanding with external work another part of said air, warming the separated components of air by heat exchange with the incoming compressed air and reevaporating the deposited high-boiling impurities into at least said part of the separated nitrogen, warming another stream of nitrogen under the lowest pressure by counter-current heat exchange with the incoming compessed air in the colder part of said heat exchange apparatus, compressing said other stream of nitrogen, cooling the compressed stream of nitrogen by heat exchange with said other part of air to be-expanded, at least partly liquefying it and introducing it into a rectification colur. n as a reflux liquid.

2. A process according to claim 1, wherein at least part of the expanded air is blown into the rectification column under the lowest pressure.

3. A process according to claim 1, wherein part of the expanded air is warmed by heat exchange with said other stream of liquefied nitrogen, then is added to the separated gaseous nitrogen before it is warmed by counter-current heat exchange with the incoming compressed air.

4. A process according to claim 1, wherein said other stream of gaseous nitrogen under the'lowest pressure, before being warmed by heat exchange with the compressed incoming air, is warmed by heat exchange with said stream of at least partly liquefied nitrogen.

5. In a process for the purification and separation of air at low temperatures, wherein compressed air is cooled counter-currently to its separated components and its nitrogen to -156 C. It leaves exchanger 10 through pipe 43, passes through exchanger 32 counter-current to impure nitrogen, exchanger 27 counter-currentto oxygen, and finally through coil 44 in the bottom of high-pressure column 15 where it liquefies. The liquid nitrogen produced goes through pipe 45 to exchanger 14 counter current to the part of the expanded air which joins the impure nitrogen, then through pipe 46 to exchanger35 counter-current to cold pure nitrogen, which cools 'it to about 188 C. -A portion of this liquid nitrogen is sent to the adjacent plant, for example to a nitrogen high-boiling impurities are deposited in a heat exchange apparatus which reverses periodically the gaseous streams of compressed air and of at least part of the separated nitrogen, the said compressed air is at least partly liquefied and rectified in at least two rectification columns under successively lower pressures, and said part of separated nitrogen taken under the lowest pressure is warmed by counter-current heat exchange with the incoming compressed air, thereby re-evaporating said impurities, the provement of warming another stream of gaseous nitrogen under the lowest pressure by counter-current heat exchange with said compressed incoming air in the colder part of said heat exchange apparatus, compressing said other stream of nitrogen and at least partly liquefying it, cooling it byheat exchange with said other stream of gaseous nitrogen under the lowest presure to be warmed in the colder part of said heat exchange apparatus, and introducing it into at least the uppermost part of the rectification column under the lowest pressure as a reflux liquid.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,478

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