US2000992A

US2000992A - Separation of constituents of gaseous mixtures

Info

Publication number: US2000992A
Application number: US709633D
Authority: US
Inventors: Joseph L Schlitt
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1934-01-31
Filing date: 1934-01-31
Publication date: 1935-05-14
Anticipated expiration: 1952-05-14

Description

- May 5 J, a... SCHLETT 2,60,,92

SEPARATION OF C ONSTITUENTS OF GASEOUS MIXTURES Filed Jan. 31, 1934 mVENToR 1m, 4324/1? ATTORNEYS Parental-May l4,

I :um'rso STATES PATENT OFFICE.

- anus:

v ssrsna'rron or conmruanrs or GASEOUS Mnrronas Jofiph L. Bchlitt. Stamford, Coma, to

Reduction Company, lmcorpera York, N. Y., a corporation of New York ted, New

Application .lanuary 31, 1934, Serial No. 709,083 I 1e o1 (on. eke-115s) I This invention relates to the liquefaction and separation of gases in gaseous mixtures and particularly to the separation in a highly economical manner of air into its two main constituents 5 for the purpose of recovering the oxygen content.

In order that the distinctive features of my inventlon may be appreciated I shall outline briefiy the normal operation of a commercial oxygen apparatus according to the well-known Claude method. In this method, atmospheric air is compressed to a pressure of substantially 30 atmospheres in a multi-stage compressor provided with water intercoolers and aftercoolers, and the air is thereafter cooled in heat exchangers by indirect contact with cold gases resulting iron: the separation. A portion of the cooled air is then conveyed to a so-called liquefier and is cooled and condensed therein by indirect contact with the cold separated gases.

I The liquid produced in the liquefier is conducted through a pressure-reducing valve to a bowl or receptacle at the base of the rectification column. The other portion of the cooled air is delivered to an expansion engine where it is cooled by ex.- pansion with external work to a pressure of 4 or 5 atmospheres, this being the pressure necessary for selective liquefaction in the tubular backward return condenser at the baseof the rectification column. The cold air from the engine passes into the bowl above referred to and thence upwardly through the tubes of the condenser just men tioned. wherein it partially-condenses by thermal contact with liquid oxygen resulting from-rectification, as hereinafter described, and which is evaporating from around the tubes at a pressure only slightly above atmospheric. I

The portion of the air liquefied in the tubes flows downwardly against the ascending stream of imcondensed vapor and this results in a substantialenrichment in oxygen of the liquid portion accumulating in the bowl below the tubes of the tubular condenser, whereas the uncondensed residual gas leaving the top of the con-g from. the second condenser to the upper level of the rectification column wherein it cascadw over trays of the usual design and es the oxygen from the vapors by reciprocal condensation and evaporation.

It should be noted in the above ription of the Claude method that the portion ofthe incoming fluid passing to the expansion engine is expanded from the initial or head pressure only down tothe pressure prevailing in the tubes of lo the tubular condenser above described, that is, down to an exhaust pressure of 4 or 5 atmospheres. In order to produce the necessary amount of refrigeration the initial or head pressure is thus much higher than would be necessary were the i5 .bet'ween pans or trays of the rectifier, is smallest at the top and greatest at the bottom or the rectifier. This condition of pressure range is due to the total integrated head of liquid on the various trays which a vapor. as it ascends through 25 the rectifier, must overcome when the rectifier is functioning P p rly. In actual liquid air rectifiers in use today; we find that this pressure runs around 300 grams/sq. cm. at the top. At the bottom the pressure will be around .150 so grams/sq. cm. higher. The top pressure is necessary-in order to cause the outgoing vapors leaving the rectifier to fiow outward into either a gas container, a compressor or freely to the atmosphere. We must bear in mind that pressures of 85 such magnitude are present in every normal rectification column even though we are accustoigeid to say that the rectifier pressure is atmos-' ph c.

The present invention has for its primary ob 40 Ject the perfection of a method of separation in which the gasconstituting the working fluid of the expansion machine is at an initial pressure equal to or only very slightly less than the pressure prevailing at the top of the rectifier and after 45 expansion in the expansion machine with the production of external work it is exhausted at a pressure substantially lower than one atmosphere absolute. This sub-atmospheric pressure of the exhaust of the expansion machine may be conveniently produced by a turbo-vacuum pump.

. Another object of this invention is to provide a method and apparatus for separation in which the outgoing nitrogen. for example, is expanded in a turbine expansion machine, such machines I! of the rectifier I3. 65

for the purpose of clarity. It will be'understood that all pressures herein referred to are absolute pressures, and that the expressions indirect'contact and "thermal contact" refer'to heat exchanging relation such as found in surface condensers in which the fluids are separated by the walls of the tubes and exchange heat by conduction through these walls.

Referring to the drawing, the incoming gaseous mixture enters the apparatusin three portions. The procedure is described for convenience in reference to the separation of air, although the procedure is applicable to other gaseous mixtures. The first portion, compressed to the pressure necessary for subsequent condensation in the tubular backward return condenser hereinafter described, is supplied by acompressor 5, preferably of the turbo type, and passes through pipe 6 to a heat exchanger A. This portion is cooled in the exchanger A bysindirect contact with the outgoing waste nitrogen product as hereinafter described. The pressure of the air thus entering the exchanger A may be, for example, about three atmospheres. This portionleaves the exchanger A in substantially saturated vapor condition and passes through pipe I to thebowl 3 below the tubes 9 of a tubular condenser-at the bottom of a column In. The air passes up through the tubes 9 and is partially liquefied therein; the liquid returning downwardly in contact with the,

ascending vapor and. becoming thus enriched with oxygen, whereas the uncondensed vapor portion leaving the top of the tubes is substantially pure nitrogen.

The enriched liquid portion flows from the bowl 8 through a pipe II and pressure reducing valve I! to an intermediate. level=ofthe rectifier 13, which constitutes the upper portion of the column I8 and is provided with the usual trays l4 over which the liquid flows in contact with. vapors rising through the column to effect a further enrichment of the liquid in oxygen and a correspending enrichment of ,the ,vapors in nitrogen.

The uncondensed residueleaving the top of the tubes. 9 passes through pipe II and thence through tubes l6 to a condenser which is immersed in liquid resulting from the rectification and accumulated in a receptacle H. The residue passing through the tubes I8 is liquefied in toto and leaves the condenser-through a'pipe I8 and pressure reducing valve l9 and is delivered at the top It flows downwardly over the trays 14 .in contact with the rising vapors, and washes therefrom substantially the remaining portions of oxygen. The resulting vapor escapes through a pipe 28 as the effluent from the rectification.

A second portion of the incoming air is supplied by a' compressor 2|, preferably of the turbo type, and enters the exchanger A through a pipe 22 where it is cooled'by indirect contact with the outgoing nitrogen product as hereinafter described. From the exchanger A this portion of the incoming air at a pressure only slightly above atmospheric and in an unliquefled condition enters a suitable intermediate level of the rectifier 13. In the event that the rectification column is operated in its entirety at sub-atmospheric pressure, in which case compressor 2| would be omitted, it may be desirable to introduce a turboexpander 23 between the exchanger A and the rectifier 13. The expansion of the entering air as it enters the rectifier results in further cooling thereof and definitely improves the results obtainable by the operation as herein described. A pipe 24 delivers the air to the expander 23. If the expander 23 is omitted, as in the case where the rectifier I3 is operated at atmospheric pressure or above, pipe 24 will deliver the air directly to the rectifier l3.

A third portion of the incoming air compressed by the compressor 5 is delivered by a pipe 25 including avalve 26 to an exchanger B, wherein it is cooled by indirect contact with the oxygen product from the column ID. This oxygen product leaves the column H) at the base thereof through a pipe 21 which delivers it to the exchanger B. It escapes through a pipe 28. In the event that the rectifier I3 is operated at sub-atmospheric pressures, a pump 29, preferably of the turbo type, is connected to the pipe 28 to exhaust the oxygen product from the system. It will be understood that the oxygen product thus delivered may be expanded in the expander 30, preferably of the turbo type, which is connected to separate compartments of the exchanger C by pipes 3| and 32.

The cooling of the air in the liquefier C causes a part of the air to liquefy. The air leaves the liquefier C through a pipe 33 which delivers it to a separator 34 wherein the liquid portion is separated, and delivered thence through a pipe 35 and pressure reducing valve 36, to an intermediate level of the rectifier 13. This liquid joins the downwardly fiowing liquid in the rectifier I3 and 'is subjected to rectification as hereinbe fore described. The unliquefied vapor from the separator 34 is delivered through a pipe 31 to the bowl 8 at the bottom of the column in and passes upwardly through the tubes 9 with the air entering through the pipe 1.

The effluent nitrogen product, after expansion as hereinbefore described, escapes from the liquefier C through a pipe 38 and is delivered to the exchanger A, where it serves to cool the incoming air. It is delivered thence through a pipe 39 to a pump 48, preferably of the turbo type, which maintains a sub-atmospheric pressure in the eflluent system which includes the liquefier C, the exchanger A and the connecting pipes. The operation of the exhaust pump 40 may be regulated so that the rectifier I3 operates at atmospheric pressure or above or below atmospheric pressure as may be desired. It is not essential that sub-atmospheric pressures be employed in the rectifier, but it is extremely advantageous to maintain a sub-atmospheric pressure in the emuent system in the manner herein described, and

7 irigerative eifect in the cycle.

to expand the emuent nitrogen in the expander for the purpose set forth.

This expansion of the nitrogen in the expander 30 cools the nitrogen to a lower temperature before it again enters the heat exchanger or liquefier C. In re-entering the liquefier, the temperature of the expanded nitrogen is increased, and it is further increased by passing through the exchanger A so that it leaves the eliluent system at substantially atmospheric temperature but at sub-atmospheric pressure.

The expander Ill may be designed to operate with an initial pressure less than atmospheric, and the apparatus arranged so that the pressure in the column "I, either in the upper portions or throughout its entire length, may be less than atmospheric. This low pressure in the column Ill can be maintained by producing a sufllcient vacuum with the pump 40, and by using an expander 30 in which the pressure drop is less than the vacuum maintained by the pump 40.

The method as described provides for a re- 'duction of the pressure in the rectification column to a very low pressure which may be only slightly above or even below atmospheric pressure, while the pressure in the efiiuent system is maintained below atmospheric pressure by a vacuum pump on the exhaust side of the turbine.

One important advantage of the present invention is that by reason of the method of expansion of the efliuent, for example nitrogen product in a turbo-expansion machine, the efliciency of the production of refrigerative effect is relatively high. This is true for the reason that a relatively large quantity of working fluid entering the expansion machine at a relatively low pressure is an ideal condition for turbine expansion efiiciency. This condition, combined with the fact that the exhaust pressure is relatively very low, makes it possible to secure good thermo dynamic efficiency in the production of the necessary re- Another advantage of my invention is that since a partial vacuum is produced in the eiiiuent product, for examplenitrogen, a vacuum pump which receives the eiiiuent from the main rectification column through the turbine, it is possible, by regulating the nozzle openings leading to the turbine, to regulate to a considerable extent the pressure conditions prevailing in the main rectification column. In fact, by means of the expansion turbine combined with the vacuum pump, it is possible to reduce the'pressure in the main rectifier to a point even somewhat below that of the surrounding atmosphere, particularly in the upper parts of said rectifier. This reduction of pressure may be sufflcient to make it possible to introduce into an intermediate level of the rectifier any desired quantity of gaseous refrigerated and unseparated initial mixture such as air without the use of any compressor or blower whatever.

This degree of pressure reduction in the main rectifier may or may not be desirable. In case we wish to operate the rectifier at a pressure slightly above atmospheric, a turbo-compressor or blower maybe employed by means of which to introduce the proper amount of unseparated gaseous mixture. The amount of such gaseous mixture possible to be added in the process forming the subject matter of the present application is very greatly in excess of that possible in any known process for reasons to be stated immediately.

Consider first the turbo-expander Ill. The

pressure of the fluid entering this expander is only that of the fluid leaving the top of the main rectifier; that is, a pressure as we shall see at which we are able to operate at will and only a small amount greater than, .or equal to, or even less than, atmospheric; whereas the pressure of the exhaust is subatmospheric and equal to that pressure produced by the vacuum pump. These conditions of low pressure and large volume are those recognized as most ideal for turbine expansion and the advantage of the resulting highly eflicient expansion in turbine 30 is additonal to all those to be cited hereinafter.

On account of the fact that in operating this process, we are able to expand at least 80% of the entering air under conditions entailing high efilciency, the degree of vacuum, and thus the power necessary to be expended is, therefore, very small.

At the reduced pressure, the rectification in the main rectifier is much more favorable than in.

cases where the pressure is higher, for the reason that the composition of liquid and vapor in phase equilibrium with each other differ more widely as the pressure is lowered. For this reason, the number of pans" or trays required to accomplish a given degree of separation is substantially reduced. 7

Since the reduced pressure conditions in the main rectifier result in a lowered temperature of the oxygen-enriched liquid. reaching the bottom of the rectifier, this in turn requires much less pressure in the incoming air entering the tubes oi the tubular condenser at the base of the 'rectifier in order that said entering air may be selectively liquefied by thermal contact with the substantially the quantity of gaseous refrigerated,

unseparated air possible to be added at an intermediate level of said rectifier.

"Remembering that the pressure in the rectifier may be reduced, if we wish, to a figure substantially below atmospheric, it will be understood that it is often desirable to operate the rectifier at a pressure substantially below atmospheric, inasmuch as under these conditions a turbine 23,

operated on the incoming air, materially increases the total refrigeration present in the cycle The refrigerative effect obtained in this manner as-- sists the turbine 30 in producing the total necessary refrigerative effect, so that the degree of vacuum necessary in the exhaust of turbine 30 is reduced below the figure necessary were turbine 23 not in operation.

On account of the combined advantages of chiclent expansionand low pressure necessary for the initial condensation of the air. the pressure necessary to be generated in compressor I2 is very much reduced and is brought into the range of eflicient turbo-compression. This entails the very important practical advantage that no oil or other compressor impurities are introduced into the rectifier. I

Finally, in addition to the advantages already listed, we may mention the fact that in this process all transformations are made to be more nearly reversiblefrom athermodynamic standpoint. This applies particularly to the exchanger system, wherein we have several counter-current streams of-fiuid in thermal contact and at pressures not widely different. This condition, as is well known among students of thermodynamics, results in the heat transfer from one fluid to another being accomplished with only small temperature differences, that is to say, the heat transfer is more nearly reversible and results in conservation of cold with its attendant economies.

With the above description in mind it should be noted at this point that the various advantages of the. low pressure or, subatmospheric pressure proposed in the specification are cumulative. They are accomplished by reason of the fact that the reduction of pressure is produced in a manner which causes the various advantages above pointed out to be cumulative, that is, the presence of each improved condition in turn increases the advantages obtainable by all the others. The various improvements I have pointed out render this process the .most efficient from the standpoint of cost that has ever been proposed.

Various changes may be made in the details of the apparatus as well as in the procedure as described, without departing from the invention or sacrificing any of the advantages thereof.

I claim:

1. The method of separating the constituents of gaseous mixtures which comprises subjecting the compressed and cooled gaseous mixture to progressive liquefaction and rectification to separate an effluent consisting substantially of a constituent of lower boiling point, expanding the effluent from the pressure of rectification to a subatmospheric pressure and exhausting the effluent by suction.

2. The method of separating the constituents of gaseous mixtures which comprises subjecting the compressed and cooled gaseous mixture to progressive liquefaction and rectification to separate an effluent consisting substantially of a constituent of lower boiling point, expanding the eflluent from the pressure of rectification to a sub-atmospheric pressure, utilizing the cold expanded effluent at sub-atmospheric pressure to cool the entering gaseous mixture, and exhausting the effluent by suction.

3. The method of separating the constituents of gaseous mixtures which comprises initially compressing and cooling the gaseous mixture, subjecting the mixture to progressive condensation, rectifying the resulting liquids, withdrawing and expanding the gaseous effluent from the rectification to sub-atmospheric pressure, maintaining the sub-atmospheric pressure by exhausting the effluent system and utilizing the effluent at the temperature attained byexpansion and at subatmospheric pressure 'to cool a part of the cycle.

4. The method of separating .the'- constituents of gaseous mixtures which comprises initially compressing and cooling a portion of the gaseous mixture, subjecting the mixture to progressive condensation, rectifying the resulting liquids, withdrawing and expanding the gaseous effluentfrom the rectification tosub-atmospheric pressure by exhausting the effluent system and utilizing the effluent at .the temperature attained by expansion and-at sub'-atmospheric pressure to cool another portion of the gaseous mixture subjecting the latter portion after cooling to rectification in both liquid and gaseous phases.

6. The method of separating the constituents of gaseous mixtures which comprises initially compressing and cooling a portion of the gaseous mixture, subjecting the mixture to progressive condensation, rectifying the resulting liquids, withdrawing and expanding the gaseous effluent from the rectification to sub-atmospheric pressure by exhausting the effluent system and utilizing the effluent at the temperature attained by expansion and at sub-atmospheric pressure to cool a part of the cycle, cooling another portion of the gaseous mixture and subjecting it thereafter directly to rectification.

7. The method of manufacturing oxygen which comprises partially liquefying air by backward return condensation through thermal contact with previously-liquefied oxygen, to produce a liquid enriched in oxygen, rectifying the enriched liquid by reciprocal evaporation and condensation at sub-atmospheric pressure, and maintaining the sub-atmospheric pressure by exhausting the nitrogen effluent from the rectification.

8. The method of manufacturing oxygen which comprises partially liquefying air by backward return condensation throughthermal contact with previously-liquefied oxygen, to produce a liquid enriched in oxygen, rectifying the enriched liquid by reciprocal evaporation and condensation at sub-atmospheric pressure, maintaining the sub-atmospheric pressure by exhausting the nitrogen effluent from the rectification, expanding the nitrogen effluent and utilizing the resulting cold nitrogen at a sub-atmospheric pressure to cool a portion of the cycle.

9. The method of separating the constituents of gaseous mixtures which comprises subjecting the compressed and cooled gaseous mixture to progressive liquefaction and rectification to separate an effluent consisting substantially of a constituent of lower boiling point, expanding the effluent and exhausting the effluent by suction sufficient to afford rectification at sub-atmospheric pressure.

10. The method of separating the constituents of gaseous mixtures which comprises subjecting the compressed and cooled gaseous mixture to progressive liquefaction and rectification to separate an effluent consisting substantially of a constituent of lower boiling point, expanding the emuent, exhausting the effluent by suction sufflcient to afford rectification at sub-atmospheric pressure and exhausting the constituent of higher tion and condensation, at-sub-atmospheric pressure, to obtain substantially pure oxygen.

12. The method of manufacturingv oxygen which comprises partially. liquefying air by backward return condensation, through thermal contact with previously-liquefied oxygen, to produce liquid air enriched in oxygen, delivering the enriched liquid to a rectification column in which .the liquid is rectified, by reciprocal evaporation and condensation, to form substantially pure oxygen, withdrawing the eiiiuent nitrogen from the rectification column to maintain asub-atmospheric pressure in the column, expanding the -withdrawn nitrogen to lower the temperature thereof, and refrigerating a portion of the cycle with the expanded nitrogen at sub-atmospheric pressure. v

13. An apparatus for separating the constituents of gaseous mixtures by liquefaction and rectification .comprising a column having means for, subjecting the gaseous mixture to selective liquefaction with backward return and the gaseous residue to non-selective liquefaction, and

means for rectifying liquids produced, turbocompressor means for compressing the gaseous mixture prior to its entrance to the column, and means for cooling and delivering a portion of the compressed unliquefied gaseous mixture directly to the rectifying means, including an exchanger, means for supplying a cold effluent product of the rectification to the exchanger, and means for maintaining such cold efiiuent in the exchanger at sub-atmospheric pressure. 14. An apparatus for separating the constituents of gaseous mixtures by liquefaction and rectification comprising a column having means for subjecting the gaseous mixture to selectivev liquefaction with backward return and the gaseous residue to non-selective liquefaction, and means for rectifying liquids produced, turbocompressor means ,for compressing the gaseous mixture prior to its entrance to the column, means for cooling a portion of the gaseous mixture including an exchanger, means for expanding an efliuent product of the rectification, means for delivering the expanded product to they exchanger, means for withdrawing the efiluent product at sub-atmospheric pressure from the exchanger, and means for cooling and delivering another portion of the compressed unliquefie'd gaseous mixture directly to the rectifying means.

15. An apparatus for separatingthe constituents of gaseous mixtures by liquefaction and? rectification comprising a column having means for subjecting the gaseous mixture to selectiveliquefaction with backward return and the gaseous residue to non-selective liquefaction and means for rectifying the liquids produced, turboous residue to non-selective liquefaction and. means for rectifying the liquids produced, turbocompressor means for compressing the gaseous mixture prior to its entrance to the column, means for cooling a portion of the gaseous mixture including an exchanger, means for expandingan eflluent product of the rectification, means for delivering the expanded product to the exchanger, means for withdrawing the effluent.

product at sub-atmospheric pressure from the exchanger, and a second exchanger in the path of the expanded efliuent adapted to cool another portion of the gaseous mixture.

17. An apparatus for separating theconstituents of gaseous mixtures by liquefaction and rectification comprising a column having means for subjecting the gaseous mixture to selective liquefaction with backward return and the. gaseous residue to non-selective liquefaction and means for rectifying the liquids produced, turbocompressor means for compressing the. gaseous.

mixture, means for distributing the gaseous mixture in three parts, including means for cooling each part by heatexchange with products of the separation and for delivering two of the parts to the means for selective liquefaction and the third part to the rectifier, means for expanding an efliuent product of the rectification. and for delivering the expanded product to the cooling means and means for withdrawing such expanded product at sub-atmospheric pressure;

18. An apparatus for separating the constituents of gaseous mixtures by liquefaction and rectification comprising a column having means for subjecting the gaseous mixture to selective liquefaction with backward return and the gaseous residue to non-selective liquefaction and compressor means for compressing the gaseous mixture, and means for positively withdrawing an eflluent product of-the rectification, thereby maintaining a sub-atmospheric pressure in the efliuent outlet system. 1

Y JOSEPH LPSCHLITI.

1 means for rectifying'the liquids produced, turbo- 1