US2946200A - Method of separating gaseous mixtures - Google Patents

Description

July 26, 1960 c. J. SCHILLING V 2,946,200

METHOD OF SEPARATING GASEOUS MIXTURES Filed June 9, 1955 INVENTOR OLARENGE'J. saw/ave ATTORNEY ,fractionating operations.

Unite States Patent 2,946,200 METHOD OF EPARATING GASEOUS MIXTURES Clarence J. Schilling, Allentown, Pa, assignor to Air Products Incorporated, a corporation of Michigan- Filed June 9, 1955, Ser. No. 514,213

7 Claims. (Cl. 62-42) This invention relates to improvements in the separation of mixtures of component gases having different boiling points by low temperature fractionation and more particularly to the generation of liquid reflux for a fractionating operation.

In the separation of mixtures of component gases into a liquid high boiling point fraction and a gaseous low boiling point fraction by a fractionating operation employing a column having a liquid-vapor contact zone, liquid reflux or wash liquid for the column is frequently obtained by liquefying gaseous fraction originating in and flowing upwardly from the liquid-vapor contact zone of the column. This is generally accomplished by means of a condenser structure mounted at the top of the column. Condenser structures provided for this purpose usually include a plurality of spaced vertically disposed elongated tubular members of relatively small diameter positioned with their lower ends discharging into the liquid-vapor contact zone of the column and with their upper ends communicating with a closedchamber formed by the dome of the condenser'structure. -A separate passageway in the form of a conduit of relatively large diameter is provided for conducting gaseous fraction from the column into the closed chamber. The tubular members are enclosed in a chamber containing a relatively cold fluid at a temperature below the boiling point of the gaseous fraction at the existing pressure to effect liquefaction of the gaseous fraction in the tubular members. Liquefied gaseous fraction flows downwardly in the tubular members, due to the action of gravity, and is discharged from their lower ends into the column. This action produces a suction effect drawing gaseous fraction from the closed chamber downwardly into the tubular members for liquefaction by heat exchange with the relatively cold fluid.

Gaseous mixtures usually include component gas or gases having such low boiling points as not to be liquified at the pressure and :temperature existing during normal Such gases are known as incondensables or incondesable gases. The incon densable gases, having relatively low boiling points, are

mixed with the gaseous low boiling point fraction and flow upwardly therewith into the condenser and collect in the closed chamber formed by the condenser dome.

,Such accumulation of incondensables reduces the condenser action and decreases fractionating efficiency due to an insuflicient supply ofreflux liquid. For example, in the separation of air into liquid oxygen and gaseous nitrogen products, the incondensable gases present comprise hydrogen, neon and helium which flow with the gaseous nitrogen fraction into the condenser structure. Although these incondensable gases constitute less than one-tenth of one percent of the air feed mixture, unless they are removed from the condenser structurethe supply of'liquid reflux will be materially impaired. This results from the fact that the incondensable hydrogen, neon and helium gases accumulating in the condenser tubes lower the-partial pressure of the' gaseous nitrogen fraction 27 therein so that the existing pressure and temperature are insutficient to condense the gaseous nitrogen fraction at the required rate, and since the incondensable gases become concentrated in the condenser tubes upon condensation of the gaseous nitrogen fraction. In order to overcome these difliculties it has been the general practice to bleed the dome of the condenser structure and thus remove a suflicient quantity of the incondensable gases for adequate operation.

It has been discovered that the provision of means for bleeding the dome of the condenser structure does not overcome the problems presented by incondensable gases when the incondensable gases constitute more than one percent of the feed mixture. While it cannot be definitely stated why incondensable gases in excess of one percent of the feed mixture are not sufliciently bled from the condenser dome to prevent a reduction in the production of reflux liquid, it has been determined that a proportion of the incondensable gases are drawn into the condenser tubes by the suction or pumping action of the condensing gaseous low boiling point fraction and a decrease in the liquefaction of the gaseous low boiling point fraction results for reasons discussed above. It has also been determined that the provision of means for bleeding the condenser dome is inadequate to overcome the problem presented by incondensable gases in excess of one percent of the feed mixture even when thecondenser dome is bled in such a manner as to continuously withdraw a large proportion of the gaseous low boiling point fraction.

The present invention overcomes the foregoing problem by providing a novel method of producing liquid reflux for a fractionating operation by which any desired proportion ofcondensable gaseous low boiling point fraction may be continuously liquefied to provide a constant supply of liquid .reflux for the fractionating operation irrespective of the presence of incondensable gases in the gaseous low boiling point fraction in excess of one percent of the feed mixture. According to the disclosed method the gaseous fraction of the fractionating opera tion, comprising a condensable gaseous low boiling point component and incondensable gas or gases of still lower boiling point, is circulated downwardly through the condenser tubes by a force, in addition to the pumping or suction'eflect produced by the action of gravity on the condensed gaseous low boiling point fraction, to prevent the accumulation of incondensable gases in the condenser tubes and a concomitant drop incondenser efiiciency.

This circulating force is established by withdrawing a stream of gaseous fraction from a zone located below the condenser tubes in communication with the lower ends of the condenser tubes and isolated from the fractionating zone with respect to vapor. The withdrawn stream of gaseous mixture includes the incondensable gases and may comprise a portion of the condensable gaseous low boiling point component of the gaseous fraction. i

The foregoing will be more fully understood with reference to the following detailed description considered in connection with the accompanying drawing which discloses a fraetionating column for producing pure argon which is designed to operate according to the principles of the present invention. It is to be expressly understood however that the novel method provided by the present invention is not restricted to the environment of argon production but has broad application to the fractionation of gaseous mixtures includingincondensablegases exceeding one percent of the mixture, such as in theseparation of hydrocarbons by low temperature fractionation for example, and the disclosed environment is not intended to define the limits of the invention, reference for the latter purpose being hadto the appended claims.

The single figure of the drawing is a diagrammatic presentation of a fractionating apparatus for producing a substantially pure argon designed in such a manner as to operate in accordance with the principles of the present invention. I a

In a patent application of George Fedorko, Serial Number 509,449, filed May 19, 1955, now Patent No. 2,909,410, for Argon Recovery, there is disclosed a sys tem for obtaining substantially pure argon in which an argon fraction containing oxygen is obtained from a side column fed trom a conventional two stage air fractionating column and in which the argon fraction containing oxygen is mixed with ammonia in critical proportions to produce a reaction product comprising a mixture of argon, nitrogen, hydrogen and water vapor. The reaction product may comprise a mixture including from 10% to 25% nitrogen, from 1% to 5% hydrogen and the balance argon, depending upon the oxygen concentration in the argon fraction and upon other operating conditions. The reaction mixture after drying is'fractionated, such as in a fractionation column 10, to obtain a substantially pure argon product. The reaction mixture is introduced by way of a conduit 11 to the mid point of the fractionating column which presents a fractionating zone provided with a stack of vertically spaced iractionating trays 12 of conventional construction. In the column 10 the reaction product is separated into a liquid high boiling point fraction comprising substantially pure argon which collects in a pool 13 in the base of the column, and a gaseous fraction comprising the low boiling point components, nitrogen and hydrogen, of the reaction product stream as well as a proportion of the argon which flows upwardly in the column.

Liquid reflux for the column 10 is obtained by liquefying a portion of the nitrogen components of the gaseous fraction in a condenser structure 14 provided at the upper end of the column. The condenser structure includes a centrally positioned tube 15 of relatively large diameter and a plurality of relatively small diameter tubes 16 positioned in spaced relation about the tube 15. The tubes 15 and 16 are vertically disposed with their upper ends communicating with a chamber 17 formed by a closed dome 18 of the condenser structure. The lower end of the tube 15 communicates with the fractionating zone of the column, while the lower ends of the small diameter tubes 16 open into an annular chamber 19 formed by a horizontally disposed plate member 20 extending between the outer surface of the tubular member 15 and the inner walls of the column in a'plane spaced abstracted from the nitrogen causes the liquid argon to boil and provide a vapor source for column operation. From the boiling coil 30, the nitrogen stream is conducted through a conduit 31 to an expansion valve 32 by which its pressure is reduced with a further decrease in temperature to a sufiiciently low level to effect liquefaction of the nitrogen component in the tubes 16; from the expansion valve the nitrogen stream is conducted by way of a conduit 33 to the inlet conduit 28 of the chamber 21.

Liquid argon product may be withdrawn from the column through a conduit 34.

The gaseous fraction produced upon fractionation of the reaction mixture, comprising a mixture of condensable argon and nitrogen, and incondensable hydrogen, flows upwardly through the central tube 15 into the chamber 17, and from the chamber 17 downwardly into the tubes 16 in heat exchange relation with the relatively colder fluid circulating through the chamber 21. Since the fluid in the chamber 17 is below the boiling point of nitrogen at the existing pressure, nitrogen and argon components in the tubes 16 will be condensed, and the liquid nitrogen and argon components will flow downwardly through the tubes 16 and collect in a pool 35 in the chamber 19. However, due to the relatively low boiling point of the hydrogen, the hydrogen component of the gaseous mixture remains in gaseous phase. The downward flow of the liquid nitrogen and' argon components in the tubes 16, due to the action of gravity, produces a suction or pumping action which draws gaseous fraction, including the incondensable hydrogen component, from the chamber 17 into the tubes 16. However, for reasons mentioned above, the pumping action of the liquid components will not cause the gaseous hydrogen component to flow downwardly in the tubes 16 and into the chamber 19.

In accordance with the principles of the present invention the accumulation of 'incondensable hydrogen component in the tubes 16 is prevented by establishing a below the lower ends of the tubes 16 and above the uppermost fractionating tray. The tubes 16 are housed in a chamber 21 formed by a cylindrical side wall member 22, and transverse partition members 23 and '24 which separate the chamber 21 from the chambers 17 and 19. ,The annular chamber 19 is provided with a gaseous withdrawal conduit 25 communicating with the chamber at a level below the lower ends of 'the small diameter tubes 16, and with liquid distributing conduits 26 extending from the plate member 20 downwardly into the fractionating zone of the column. Conduits27 and 28 com municate with the chamber 21 for circulating a stream of cold fluid through the chamber in out-of-eontact heat exchange relation with the fluid flowing through the tubes 15 and-16. In order'to liquefy the nitrogen component of the gaseous fraction, the circulating fluid is at a temperature below the boiling point of nitrogen at the existing pressure. According to the above-mentioned Fedorko application, by operating the column 10 at the proper pressure, a suitable fluid for this purpose may comprise a stream of gaseous nitrogen withdrawn from the high pressure section of the two stage air fractionating column. As shown,a stream of high pressure gaseous nitrogen is conducted through a conduit 29 to a boiling coil immersed in the pool of liquid argon 13. The liquid argon is at a relatively low temperature due to the column pressure, and the high pressure nitrogen stream is cooled upon passing through the boiling coil while the h forced circulation of the gaseous fraction downwardly through the small diameter tubes 16 into the chamber 19. This is accomplished by withdrawing a stream of gaseous mixture, including the incondensable hydrogen component, from the chamber 19 through the conduit 25. This manner of flowing the gaseous fraction through the condenser prevents the accumulation of incondensable gases in the chamber 17 and in the small diameter tubes 16 and assures uniform heat exchange relationship between the condensable components of the gaseous mixture and the relatively colder fluid in the chamber 21 and provides a co'ntinous supply of reflux liquid in accordance with column requirements.

The liquid reflux requirement of the fractionating col umn 10 does not necessitate liquefying the total nitrogen component or the gaseous fraction and the unliquefied po'rtion of the nitrogen component is withdrawn through the conduit 25 together with the incondensable hydro- -gren component and gaseous argon.

The withdrawn gaseous stream may comprise from 50% to nitrogen, from 8% to 12% hydrogen and the balance argon. Therate of flow of the gaseous mixture, through the conduit 25 is determined in accordance with column operating conditions and the condenser structure -14 is designed to provide the required reflux production. Thus the rate of flow of the gaseous mixture withdrawn from beneath the condenser tubes 16, in the disclosed example, is determined by column operating conditions and is in excess of the rate of flow required to establish the forced downward circulation through the condenser tubes 16. For the latter purpose it is only necessary to withdraw the stream including incondensables at a rate suflicient to establish a pressure drop along the tubes 16 for efiecting the downward flow. Therefore, in cases in which the total condensable component of the gaseous mixture is required t9 lee-liquefied liquid reflux for the column esteem or for other fractionating zones or for other purposes, only theincondensable component of the gaseous fraction or the incondensable portion and the necesary proportion of the condensable component to establish the required pressure drop, are withdrawn from the zone be- 'neath the condenser tubes 16.

The mere withdrawal of a relatively large volume of gaseous mixture from the condenser structure do'es not result in overcoming the loss of condensing action due to the incondensable gases. It has been determined from actual experimentation with a fractionating column con structed according to the example described above that the withdrawal of a gaseous stream including from 50% to 80% nitrogen, from 8% to 12% hydrogen and the balance argo'n from the chamber 17 through the upper endof the dome -18 does not prevent the incondensable gases from entering the condenser tubes 16. Therefore the novel method provided by the present invention which eliminates problems resulting from the presence of incondensables in the gaseous fraction includes the combination of steps for maintaining a pressure drop across the condenser tubes 16 to establish a forced downward flow of fluid in the condenser tubes by withdrawing a stream of gaseous mixture, including the incondensables, from a zone communicating with the lower ends of the condenser tubes and isolated from the fractionating zone with respect to gas.

The method provided by the present invention is applicable to the fractionation of all gaseous mixtures including incondensables in excess of one percent of the gaseous mixture in which at least a portion of the gaseous fraction of the fractionating operation is required to be liquefied. Thus the present invention has application to the separation of a wide range of hydrocarbons. for example, in addition to the separation of a reaction mixture comprising argon, nitrogen and hydrogen as described above. Reference therefore will be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. The method of producing liquid reflux for a fractionating operation, in which operation a mixture of component gases comprising from to nitrogen, from 1 to 5% hydrogen and the balance argo'n is fed to a fractionating zone producing a liquidargon fraction and a gaseous fraction including nitrogen and hydrogen components, the method comprising the steps of conducting a stream of gaseous fraction to a first chamber communicating with the upper ends of a plurality of spaced vertically disposed elongated heat exchange passageways, withdrawing a stream of gaseous fraction from a second chamber communicating with the lower ends of the heat exchange passageways and from heat exchange relationship with relatively cold fluid surrounding the heat exchange passageways so that the gaseous fraction in the first chamber flows downwardly thro'ugh the heat exchange passageways into the second chamber, passing a stream of cold fluid at a temperature below the boiling point of the nitrogen component at the existing pressure in out-o'f-contact heat exchange relation with the gaseous fraction flowing downwardly in the heat exchange passageways to provide liquefied nitrogen component, the liquefied nitrogen component flowing downwardly in the heat exchange passageways and collecting in the second chamber, and withdrawing a stream of liquefied nitrogen component from the second chamber and introducing the withdrawn liquefied nitrogen component into the fractionating zone as liquid reflux, the stream of gaseous fraction withdrawn from the second chamber including the hydrogen component comprising from 8% to 12% of the withdrawn stream of gaseous fraction.

'2. The method defined by claim 1 in which the stream of gaseous fraction withdrawn from the second chamher includes from 50% to nitrogen and from 8% to 12% hydrogen.

'3. The method of producing liquid reflux in a fractionating operation, in which operation a mixture of component gases comprising from 10% to 25% nitro gen, from 1% to 5% hydrogen and the balance argon is fed to a fractionating zo'ne producing a liquid argon fraction and a gaseous fraction including nitrogen, hydrogen and argon components, the method comprising the steps of conducting a stream of gaseous fraction to a first chamber communicating with the upper ends of a plurality of spaced vertically disposed elongated heat exchange passageways, withdrawing a stream of gaseous fraction from a second chamber in communication with the lower ends of the heat exchange passageways and from heat exchange relationship with relatively cold fluid surrounding the heat exchange passageways so' that the gaseous fraction in the first chamber'flows downwardly through the heat exchange passageways into the second chamber, passing a stream of cold fluid 'at 'a temperature below the boiling point of the nitrogen com ponent at the existing pressure in out-of-conta'ct heat exchange relation with the gaseous fraction flowing downwardly in the heat exchange passageways to provide liquefied nitrogen component, the liquefied nitrogen component flowing downwardly 'in the. heat exchange passageways-and collecting in the second chamber, and withdrawing a stream of liquefied nitrogen component from the second chamber and introducing the withdrawn stream of liquefied 'nitrogent component into the fractionating zone as liquid reflux, the stream of gaseous fraction withdrawn from the secon'd'cha'mber including the hydrogen component and comprising-from 50% to 80% nitrogen, from 8% to 12%"hydrogen and the balance argon. i

4. In operation of a condenser adapted to liquefy gaseous mixture including a condensable component and an incondensable component comprising at least one percent of the gaseous mixture, the condenserbeing of the type including an upper chamber'to which the gaseous mixture is fed and a lower chamber connected to the upper chamber by a pluralityof vertically disposed tubes surrounded-by a relatively cold fluid at a temperature below the boiling point of the condensable component at the existing pressure, and in which the gaseous mixture fed to the upper chamber flows downwardly into the tubes and condensable component is liquefied in the tubes by heat interchange with the relatively cold fluid and liquefied condensable component flows downwardly in the tubes to the lower chamber producing a suction eflect drawing additional gaseous mixture from the upper chamber downwardly into the tubes, the method of maintaining continuous liquefaction of condensable component of the gaseous mixture in the tubes which comprises withdrawing a gaseous stream from the lower chamber and from heat exchange relationship with the relatively cold fluid, and controlling the rate of flow of the gaseous stream to establish a pressure drop across the tubes sufficient to prevent blocking of the tubes by accumulation of incondensable component of the gaseous mixture in the tubes, the gaseous stream including incondensable component of the gaseous mixture.

5. In operation of a condenser adapted to liquefy gaseous mixture including a condensable component and an incondensable component comprising at least one percent of the gaseous mixture, the condenser being of the type including an upper chanmber to which the gaseous mixture is fed and a lower chamber connected to the upper chamber by a plurality of vertically disposed tubes surrounded by a relatively cold fluid at a temperature below the boiling point of the condensable component at the existing pressure, and in which the gaseous mixture fed to the upper chamber flows downwardly into the tubes and condensable component is liquefied in the tubes by heat interchange with the relatively cold fluid r '7 and liquefied condensable component flows downwardly in the tubes to the lower chamber producing a suction eilect drawing additional gaseous mixture from the upper chamber downwardly into the tubes, the method of maintaining continuous liquefaction of the condensable component of the gaseous mixture in the tubes which comprises withdrawing a gaseous stream from the lower chamber and from heat exchange relationship with the relatively cold fluid, and controlling the rate of flow of the gaseous stream to establish a pressure drop across the tubes sufiicient to prevent blocking of the tubes by accumulation of incondensable component of the gaseous mixture in the tubes, the gaseous stream including incondensable compo'nent and a portion of the condensable component.

6. In a fractionating operation in which a stream of gaseous mixture is fed to a fractionating zone producing a liquid high boiling point fraction and a gaseous low boiling point fraction including a condensable component and an incondensable component comprising at least one percent of the gaseous mixture, and in which gaseous low boiling fraction is fed to an upper chamber connected to a lower chamber by a plurality of vertically disposed tubes surrounded by a relatively cold fluid at a temperature below the boiling point of the condensable component of the gaseous low boiling point fraction, and in which the gaseous low boiling point fraction fed to the upper chamber flows downwardly into the tubes and condensable component is liquefiedin the tubes by heat exchange with the relatively cold fluid and the liquefied condensable component flows downwardly in the tubes to the lower chamber producing a suction efiect drawing additional gaseous low'boiling point fraction into thetubes, the method of maintaining continuous liquefaction of condensable component of the gaseous low boiling point fraction to provide reflux for the fractionating zone which comprises withdrawing a gaseous stream from the lower chamber and from heat exchange relationship with the relatively cold fluid, controlling the rate of flow of the withdrawn gaseous stream to establish a pressure drop across the tubes sufiicient to prevent blocking of the tubes by accumulation of incondensable component of the low boiling fraction in the tubes, and withdrawing a stream of liquefied condensable component from the second chamber and introducing the withdrawn stream into the fractionating zone as reflux; the withdrawn gaseous stream including incondensable component of the gaseous mixture.

7. In a fractionating operation in which a stream of gaseousmixture is fed to a fractionating zone producing a liquid high boiling point fraction and a gaseous 10W 'boiling'point fraction including a condensable component and an incondensable component comprising at least one percent of the gaseous mixture, and in which gaseous low boiling fraction is fed to an upper chamber connected to a lower chamber by a plurality of vertically disposed tubes surrounded by a relatively cold fluid at a temperature below the boiling point of the condensable component of the gaseous low boiling point fraction, and in which the gaseous low boiling point fraction fed to the upper chamber flows downwardly into the tubes and condensable component is liquefied in the tubes by heat exchange with the relatively cold fluid and liquefied condensable component flows downwardly in the tubes to the lower chamber producing a suction effect drawing additional gaseous low boiling point fraction into the tubes, the method of maintaining continuous liquefaction of condensable component of the gaseous mixture to provide reflux for the fractionating zone l-which comprises withdrawing a gaseous stream from the'lower chamber and from heat exchange relationship with the relatively cold fluid, controlling the rate of flow of the withdrawn gaseous streamto establish a pressure drop across the tubes sufiicient to prevent blocking of the tubes by accumulation of incondensable component of the low boiling fraction in the tubes, and withdrawing a stream of liquefied condensable component from the second chamber and introducing the withdrawn stream into the fractionating zone as reflux, the gaseous stream including incondensable component and a portion of the condensable component. 1

References Cited in the file of this patent I UNITED STATES PATENTS 1,512,268

Claims (1)

1. THE METHOD OF PRODUCING LIQUID REFLUX FOR A FRACTIONATING OPERATION, IN WHICH OPERATION A MIXTURE OF COMPONENT GASES COMPRISING FROM 10% TO 25% NITROGEN, FROM 1% TO 5% HYDROGEN AND THE BALANCE ARGON IS FED TO A FRACTIONATING ZONE PRODUCING A LIQUID ARGON FRACTION AND A GASEOUS FRACTION INCLUDING NITROGEN AND HYDROGEN COMPONENTS, THE METHOD COMPRISING THE STEPS OF CONDUCTING A STREAM OF GASEOUS FRACTION TO A FIRST CHAMBER COMMUNICATING WITH THE UPPER ENDS OF A PLURALITY OF SPACED VERTICALLY DIPOSED ELONGATED HEAT EXCHANGE PASSAGEWAYS, WITHDRAWING A STREAM OF GASEOUS FRACTION FROM A SECOND CHAMBER COMMUNICATING WITH THE LOWER ENDS OF THE HEAT EXCHANGE PASSAGEWAYS AND FROM HEAT EXCHANGE RELATIONSHIP WITH RELATIVELY COLD FLUID SURROUNDING THE HEAT EXCHANGE PASSAGEWAYS SO THAT THE GASEOUS FRACTION IN THE FIRST CHAMBER FLOWS DOWNWARDLY THROUGH THE HEAT EXCHANGE PASSAGEWAYS INTO THE SECOND CHAMBER,

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