US2360468A - Separation of oxygen from air by liquefaction - Google Patents

Description

Oct. 17, 1944. A. c. BROWN 2,360,458

SEPARATION OF OXYGEN FROM AIR BY LIQUEFACTION Filed May 29. 1939 2 Sheets-Sheet 2 Patented Oct. 17, 1944 SEPARATION F OXYGEN FROM AIR BY i LIQUEFACTION Andrew C. Brown, Wilmette, lll., assigner to Compressed Industrial Gases, Inc., a corporation of Delaware Application May 29, 1939, Serial No. 276,325

Claims. I (Cl. 62-122) One of the objects of the invention is the proi vision oi a new and improved pressure operated mechanism for automatically maintaining proper phase equilibrium or proper balance within the 4 system.

Another object of the invention is the provision of new and improved means for automatically controlling the operation of the rectifying apparatus by maintaining a predetermined height ci the crude oxygen bath in the high pressure rectiiying column.

Another object of the invention is the provision oi new and improved pressure operated mechanism for controlling the supply of reflux delivered to thelow pressure column, the mechanism being so constructed that none of the moving parts will be subjected to the low temperature of liquid air or oxygen.

A. further object of the invention is the provision of a new and improved mechanism for automatically maintaining the entire system in vproper equilibrium so that the production of oxygen of a high degree oi purity may be ob` tair'ied in a continuous or uninterrupted process.

Another object of the invention is the provision of new and improved mechanism for removing carbon dioxide from the crude oxygen during the operation ofthe Oxygen producing apparatus. e i

A still further object oi the invention is the provision of a new and improved method of producing oxygen in a continuous process and on an economical'and productive scale.

A.further object of the invention is the provision of a new and improved apparatus for producing oxygen gas and for automatically controlling the operation of the apparatus, that is simple in construction, edicient in operation, and that is composed of a minimum number of moving parts. n y

. Other and further objects and advantages of the invention will appear from the following description,`taken in connection with the accompanying drawings, in whichs Fig. l is a side elevation of a double rectifying column device showing theparts diagrammatica ly; viFig. 2 is a similar view oi' a single column de- Fig. 3 is a vertical section of the expansion valve and the control therefor; and

Fig. 4 is a modified form of expansion valve and control therefor.

In the production of oxygen gas and in the control oi' the reiiux and phase equilibrium, it is the common practice to control the operation of the apparatus, and especially the operation of the expansion valves, by manually operated means. For economical operation, it is necessary that the amount of @uid ilowing through the expansion valve shall be such that at the discharge into the column, the liquid shall be in phase equilibrium with the surrounding fluid in the column.

in the production of oxygen gas and in the control of the reflux and phase equilibrium in the columns, it is the common practice to control the operation of the system by manually operated expansion valves. Such an arrangement is objectionable because of the close attention necessary to the operation of the system. Where the valve is manually controlled, solidied CO2 and other particles whose criticals are above that of oxygen and nitrogen will restrict and even clog the passage thereby necessitating the adjustment of the valve to restore the normal operation of the column. But where automatically controlled, the valve is automatically opened when clogged for permitting the obstruction to pass through' the same.

The present invention seeks to provide novel means for eiecting the control of the system by automatically operated mechanism which in turn is controlled indirectly by the level of the crude oxygen bath Within the rectifying column.

Referring now to Fig. 1 of the drawings, which is a diagrammatic view showing a double column rectiiier, the reference character l0 designates the rectifier column which comprises the high pressure section Il and the lower pressure section l2 vertically above the high pressure section. The two sections are separated by the partition i4 which constitutes the flue sheet of a condenser il. The column is of a well-knowntype, except for the condenser I1, and each section is provided with evaporating trays, those of the lower section being shown diagrammatically at I5 and i 8. These trays are of the conventional construction and need not be described in detail.

The condenser Il comprises the ue sheet i4, a header I9 and a plurality of tubes or ilues 20 between the flue sheet I4 and header I9. These ues or tubes place the upper section of the column in communication with the header i9. The

header I9 and ues or tubes 20 contain almost pure liquid oxygen condensed in the upper section of the column, as will presently be described. v

The upper section I2 of the column is enlarged at its lower end as at I8 to extend below the ue sheetk I44 and condenser I1 and is attached to the lower section II below its upper end to form a reservoir 2I for containing liquid nitrogen precipitated by the condenser I1 in the lower section of the column. In other words, the temperature of the oxygen in the condenser, being below the critical temperature of nitrogen at the pressure obtaining in the lower section or high pressure section of the column, the nitrogen constituent of the iluid evaporated from the bath collected in the lower portion of the column will pass upwardly beyond the trays I and I6 and be condensed by the condenser I1 and collected in the reservoir 2 I, while the oxygen constituent of this fluid will be condensed by the trays I5 and I6 and fall back into the liquid in the bottom of the column thus adding to the richness of the oxygen upper end of the upper section of the column by means of a passage 24 which, when the system is operating, conducts nitrogen gas from the upper end of the column to the lower end of the container 23. This gas'passes through the container and is discharged through the pipe 25.

The air, after it has been compressed to say around 700 or 750 lbs., is conducted through the heat exchange device 22 through a pipe 26 which takes a sinuous path through the heat exchange device for increasing its radiating surface and has its temperature greatly reduced by the cold nitrogen gas ilowing countercurrent through the container and by the rened oxygen which isA conducted counter to the flow of air from the reservoir I9 by a pipe 21.

The pipe 21 is in the form of a coil extending about the pipe 26 in the heat exchange device for assisting in reducing the temperature of the incoming air. As a result', the air has its temperature greatly reduced so that it is liquefied by the time it passes from the heat4 exchange device. This-liquid is conducted from the heat exchange device through the pipe 26 to a heat exchange member in the form of a heating or boiling coil 28 which, inthe form of the construction shown, is a coil that is normally immersed in the bath of crude oxygen 29 in the bottom of the rectifying column,

The temperature of the liquid in the coil 28 is above that of the crude oxygen of the bath 29 surrounding it and as a result, the liquid in the bath will be evaporated and will rise upward in the high pressure column II where the oxygen is condensed and falls back into the bath and the nitrogen is condensed and collected in a separate chamber or reservoir. as is well lmown in rectifying columns.

A suitable expansion valve 3I is provided forvv discharging the liquid air contained in the heat exchange 23 into the high pressure section II of the rectifying column. As shown, the pipe 32 is in communication with the' coil 28 and the valve 3I. The iluid passing the expansion valve 3| is discharged through the nozzle 33 in the lower portion of said column. A hand operated valve 34 is provided for manually controlling the now of liquid through the conduit 32. In the normal operation of the device, the valve 34 is moved to full open position and left that way.

Suitable means are provided for automatIcally controlling the ilow of the liquid air through the expansion valve 3|. Any suitable means may be employed for this purpose. In the form of the construction shown, which is by way of example only, a pressure controlled valve is employed for this purpose. As shown in Fig. 3, this valve comprises a body portion 35 having a hollow head 36 on its outer end. The body portion 35 is provided with an axial bore 31 through which a valve plunger 38 loosely extends. The inner end o! the body portion is provided with a plug 39 having a valve seat 4I thereon which is in alinement with the valve plunger 38 and is adapted to be engaged by said plunger for closing the passage 31 therethrough. The pipe 32 is connected to the body portion 35 of the valve and is adapted to deliver the liquid air to the valve.

The amount of fluid passing through the valve will be determined by the position of the plunger 38. 'I'he operation of the plunger 38 is controlled by a diaphragm 42 which is clamped between the two sections 43 Aand 44 ofthe head 36. The plunger 38 extends through the diaphragm 42 and is clamped to the diaphragm lby suitable nuts and washers 45 threaded on the outer end of the plunger 38 at each side of the diaphragm 42.

'Ihe extent to which the valve may be opened, that is, the range of movement of the plunger 38, may be adjusted as desired. This is accomplished by adjustably connecting the head portion 36 to the body portion 35 of the valve 3I. In the form of the construction shown,` this is accomplished by threading the outer end oi the body portion 35 into a neck 40 of the head portion 36. The neck is provided with a counterbore for receiving packing held byv a gland 60 for forming a gas and liquid tight joint at this point. When it is desired that the Valve shall be capable of `being opened wider, the 4head is partially unscrewed from the body portion which will adjust thediaphragm 42 farther.' from the valve seat 4I thereby permitting the valve to open wider.

The operation of the diaphragm 42, which vcontrols the movement of the valveplunger 3l,

is controlled by a pressure sensitive mechanism 46. This mechanism comprises a container 41 having a conduit 48 connecting the lower portion of the interior thereof with the high pressure section of the columnv beneath the surface of the crude oxygenba'th. A second p ipe or con"- duit 49 places the upper portion of the container 41 in communication with the column abovev the --level of the liquid, the arrangement being such that the liquid in the column may pass intojthe container and be maintained at the same level as the crude oxygen bath. It is so mounted, however, 'that during the normal operation of the system, the level of the liquid in the container 41 will be intermediate between the top and lower portions thereof.

The chamber 41 has mounted therein a coil of pipe one end of which extends beyond the container'and is provided with a valve 62. for closing the pipe. 'I'he other end of the coil 5| extends through the wall of the container and is in communication with the hollow head 38 outwardly of the diaphragm 42. The coil 5i, including the external portion thereof, is illled with a suitable gas such as hydrogen or helium that will not liquefy under the temperatures and pressures used in the rectifying column. By means of this arrangement, the pressure .within the air duct 32 tending tov force the diaphragm 42 and with it the valve plunger 26 outwardly, will be ccunterbalanced by the pressure of the gas within the pressure control device 5|. It will thus be seen that when, for any reason, the liquid in the bath'29- becomes colder than it should be, the pressure within the conduit 32 will move the diaphragm 42 outwardly, due to the diminished pressure of the gas within the coil 5|, for opening the valve wider and permitting a greater flow of liquid air through the pipe 26 which, in turn, will warm the crude oxygen and cause the evaporation of the liquid in the bath 29. Should solidified COa or other solidi- :lied constituents of the air restrict the valve opening, the pressure in the air duct or conduit .32 will increase and this increased pressurefon the inner side of the diaphragm 42 will move the plunger or valve stem 39 outwardly thus opening the valve and permitting the obstruction to pass into the column.

If the level of the liquid in the bath 29' rises, more of the coil 5| will be immersed thus cooling the gas within the pressure controlled device causing it to contract thereby opening the valve 3| wider and permitting more compressed air to ow through the tube. On the other hand,the

lowering of the level of the liquid in the bath 29 will lower the level of the liquid within which the coil 5| is immersed thereby causing the gas to expand by added heat and moving the valve 3| toward closed position.

The column I0, heat exchanger 22, filter 53, and control mechanism 46 are preferably provided With suitable heat insulating materialv 16. The control mechanism 46 is without the col- -umn and between the column and the exterior of the insulation. It is positioned without the column because there is not suilicient heat differential between the liquid crude oxygen and its vapor to cause it to properly operate. It is placed within the insulation but near the surface so that it will be suillciently sensitive to function properly and yet not absorb so much heat as to .be uneconomical.

Alow pressure section l2 of the column, as will now be described.

The liquid air, in passing through the coil 29, will cause the crude oxygen in the bath 29 to boil and as the vapor rises, the oxygen willbe condensed in the evaporating trays I5 and I6 will also be washed by the liquid air discharged by the nozzle 33. The nitrogen gas will rise in the lower chamber and be condensed in the condenser |1 by the liquid oxygen contained therein and will be collected in the receptacle 2|. as will presently appear.

This crude oxygen, less carbon dioxide, which is solidified in crystals within the liquid, will be conveyed through a pipe 52 to a filter 53. 'I 'he pipe enters the lower end which will contain more orl of the lter chamber l54 and terminates adjacent the upper end thereof and the chamber is lled around the pipe with suitable filter material 55. A pipe 56 leads from the upper end of the cham- Iber 54 into the heat exchanger 22 for conducting the carbon dioxide gas through the heat exchanger and discharges the same through a valve 5l in the upper end of the pipe 56 that extends beyond the heat exchange. The filtered crude oxygen is conducted from the lower part of the chamber 54 through a pipe 56 through an expansion valve 59 to the nozzle 6i in the upper section of the column.

The expansion valve 59 is of the same'type as that shown in Fig. 3 and its description need not be repeated.

The regulating pressure for counteracting the pressure in the pipe 58 on the diaphragm 42 of this valve is obtained from the compressed air pipe 26. A branch pipe 62 conducts compressed air to the head 36 of the valve 59. The pipe 62 is provided with a pressure regulator 64 which controls the pressure on the outer side ofthe diaphragm of the valve 59. This pressure remains constant and is comparatively low as for instance about lbs. Since the pressure regulator 64 is of the conventional type, its description is not thought necessary.

The crude 4oxygen from the bath 29 andthe nitrogen from the reservoir 2| are forced through the valves 36 and 69 by the pressure developed by the vapor from the boiling crude oxygen in the bath 29 and this pressure depends on the operation of the valve 3|. For instance, if the amount of liquid air owing through the coil 28 is small, the boiling of the crude oxygen will likewise be small and the pressure in the section of the column I0 will be reduced and this will result in the pressure inthe pipes 1| and 12 partially or entirely closing the valves 36 and 69.

On the other hand, if the valve 3| is openedito admit 'the passage of more liquid air, this air passing through the bath 29 will evaporate more crude oxygen,and result in greater pressure in the section li. It will thus be seen that the automatic valves 36 and 69 depend, fortheir operation, 'on the proper operation of the automatic valve 3|, thereby maintaining a proper balance in the operative conditions obtaining in the two'sections of the column.

The crude oxygen passing through the pipe 58 is discharged through the nozzle 6| for reuxing the liquid. 'I'his iiuid will pass over the evaporating pans 65 andl 66 which are similar to the pans |5-and I6 for condensing the oxygen. v'l'he condensed liquid falls on the condenser and is collected yin the header or reservoir I9 as oxygen comparatively free from other gases.

The oxygen liquid is removed through the pipe 21 which conducts the same through the-heat exchanger 22 where the oxygen absorbs heat .from the incoming compressed air and is convertedlto a'gas at a comparatively low pressure. The upper 'end of the pipe r21 extends outwardly through the wall of the heatexchanger 22 as shown at 61. AFrom thence the oxygen, in the form of a gas, is delivered to compressors or containers, not

The Acondenser |1 is similarI to the iiash type of steam boilers and functions `very eiectively to condense the nitrogen gas in the high pressure section of the rectifying column. It will be noted that the pure oxygenis removed from the bottom of the reservoir vand as a consequence;4 vany amount of oxygen removed resultsln a movement of the entire mass of liquid oxygen in the condenser whereby not only fresh liquid oxygen is continually supplied to the reservoir, but the liquid is brought more eiectively into contact with the walls o1' the condenser and as a result. the condenser functions very efliciently for condensing the nitrogen in the lower section of the column. The liquid nitrogen that collects in the receptacle 2| is utilized for washing down or condensing the oxygen which rises from the condenser an-d is evaporated from said reflux.

The temperature within the lower pressure section of the column is regulated by the automatic controls and is maintained between the criticals of oxygen and nitrogen at the pressure obtaining therein so that the oxygen will be condensed and the nitrogen collected as a gas. in the upper portion of the column. As shown, a. pipe 68 conducts the liquid nitrogen from the reservoir 2| through an expansion valve 69 which is substantially the same as that shown in Fig. 3 and its description need not be repeated. The pressure on the outer side of the diaphragm of this valve is obtained through the pipe 62 and pressure regulator 84 as was the valve 59 previously described.

The pipe 62 is provided with branches 1| and 12 which conduct the compressed air to the valves 89 and 59, respectively. The liquid nitrogen is discharged at the upper end of the column by a nozzle 13 and will condense the oxygen in the vapors rising in the upper section of the column and will in turn be vaporized. 'Ihe nitrogen vapor will be carried oi by the pipe 24 and delivered to the heat exchange 23 as previously described.

In order that the system shall operate economically, the nozzles for the expansion valves are so located within the rectifying column that the iluid discharged therefrom will be in phase equilibrium with the surrounding fluid.

Instead of employing the pressure of the compressed air in the pipes 26 and 62, the expansion valve shown in Fig. 4 may be employed. This valve is substantially like the valve shown in Fig. 3 except that the diaphragm 42a, which operates the plunger 39a, is held againstv outward pressure by an adjusting screw 14 which engages a follower 15 between which and a washer 16 is interposed a compression spring 11. The pressure applied to the diaphragm 42a may be adjusted to any desired amount by the screw 14.

It will thus be seen that even though there is not quite 12 C. diierence between the boiling points of oxygen (-182.5 (-194" C.) at ordinary pressures, the automatic controls may be so adjusted that both sections of the column maybe maintained in phase equilibrium and the oxygen separated from the nitrogen by rectification.

'Ihe form of the construction shown in Fig. 2 differs from that just described in that it is a single column apparatus. In this form of construction, the upper end 18 of thecolumn 19 constitutes a heat exchanger and the compressed air enters the section 18 through a pipe 8| which is formed into a coil within the section 18 as shown in Fig. 2 of the drawings. From the heat exchanger 18, the compressedr air is conducted by a continuationof the pipe 8| to the lower portion of the column which, after the device is in operation, has, in the bottom` thereof, an oxygen bath 82. The pipe-8| is in communication with a heat exchange coil-88 immersed in this bath. From the coil 83, the liquid air is'conducted through rst the manually operated valve 84 and C.) and nitrogen` then the automatic pressure regulated valve 8l and is discharged in the upper portion of the lower section 86 of the column. The valve 8l is controlled by a mechanism 81 similar to the device shown at 46 and 41 in Fig. 1. In other Words, the mechanism 81 is connected to the lower section of the column above the level of the liquid by a pipe 9| and below it by a pipe 92 so that the level of the liquid in the device 81 will be the same as that in the column. The raising or lowering of the liquid in the device 81 will operate the diaphragm in the same man,- ner as already described.

The liquid oxygen is removed from the bath 82 by a pipe 93 which conducts the liquid oxygen through the heat exchanger 18 where it absorbs heat from the compressed air coming in through the pipe 8|. The pipes 8| and 93 are, in practice, in close contact so that there will be the proper interchange of heat. 'I'he oxygen. which, due to the absorption of heat, will be in the form of gas, may be delivered through the valve 94.

The temperature of the lower portion of the column will be maintained above the critical temperature of nitrogen gas by the automatic valve 85 so that the nitrogen constituent of the air will pass as a gas at low temperature through the heat exchanger 18 for lowering the temperature of the compressed air entering the system through the conduit or pipe 8|. The nitrogen gas escapes through a conduit into the air or into a container, if it is desired to collect the nitrogen.

In the form of the device shown in Fig. '1, the nitrogen passing along vthe pipe 25 may be, and preferably is, conducted to suitable retainers or'collectors and likewise the carbon dioxide gas passing through the valve 51 may be discharged into the air or, if desired, it may be conducted to suitable receivers or suitable storage tanks.

While the apparatus is disclosed as being adapted for separating oxygen, nitrogen and carbon dioxide from the air, it is understood that the invention is not so limited as it may be used as an apparatus for separating the other constituents of the air, or it may be used in separating and rectifying elements or compounds of other mixtures that may be reduced to gaseous form.

It willthus be seen that the control mechanism for the expansion valves depends for its operation, eitherdirectly orindirectly,on the operation of the mechanism for operating the expansion valve 3|. None of the operating mechanism, or at least the moving parts, is so exposed to low temperatures as to affect its operation. This is oonsidered an important feature of the invention.

It is thought from the foregoing, taken in connection with the accompanying drawings. that the construction and operation of my device willbe apparent to those skilled -in the art an'd 'that'cha-nges in size, shape, proportion and details of construction may be made without departing from the spirit and scope of the appended claims.

I claim as my invention: 'j'

l. In a system for producing oxygen from air, a rectifying column adapted to contain a bath of crude oxygen in its lower end, heat interchange means within said bath, a heat exchange device, means for conducting nitrogen uid at low temperature to and from said device, means for conducting air under high pressure through said device for liquefying the air and for delivering the liquid air to said heat interchange means, means for conducting the compressed air fluid to a reduction valve for discharging said fluid into said column above said bath, and means controlled by the height of the liquid in said bath for controlling the operation of said valve.

2. A method of separating oxygen from air that comprises compressing the air, cooling the air continuously by passing a stream of cooled compressed air through a bath of crude oxygen for evaporating nitrogen gas therefrom in a continuous process, condensing the nitrogen gas, and controlling the amount of air flowing in said stream by pressure responsive means controlled by the height of the level of the liquid bath.

3. An expansion valve for use in rectifying columns comprising a head, a diaphragm in said head, a valve member secured to said diaphragm, a chamber, means .for placing the chamber in communication with the lower portion of a rectifying column, and a coil containing a highly expansible fluid in said chamber, said coil having one end in communication with said head outwardly of said diaphragm.

4. In a system for producing oxygen from air, a rectifying column having a high pressure portion and a lower pressure portion, the lower end of the high pressure portion of said column being adapted to contain a crude oxygen bath, means for conducting compressed air through said bath and for discharging the same into said high pressure portion above said bath, an expansion valve for controlling the discharge through said means, pressure responsive means exterior of said bath but aiected by the temperature thereof for automatically controlling the operation of said valve, conduit means for conducting liquid from said bath to the low pressure portion of said column, and means for automatically controlling the amount of liquid flowing through said conduit means.

5. In an apparatus for separating oxygen and nitrogen from the air, a rectifying column, a condenser dividing the column into an upper low pressure section and a lower high pressure section, a heat exchanger device, means for conducting compressed air through said exchanger device to said high pressure section, said condenser comprising a partition wall having apertures therein, tubes secured in said apertures and extending downwardly therefrom, a chamber in communication with the lower ends of said tubes,

lil

said column being enlarged about said condenser and provided with an annular reservoir below said condenser, means, including said chamber, for condensing nitrogen gas in said high pressure portion of said column and for collecting the condensed nitrogen gas in said reservoir, means for liquefying oxygen and collecting the same in said condenser for assisting in condensing ni' trogen gas in the high pressure portion of said column, means for conducting liquid nitrogen from the high pressure `to the low pressure section of said column and for spraying the same into the upper portion of said column, and means for conducting liqueed oxygen from the lower portion of said condenser through said heat exchanger device for cooling the incoming compressed air.

6. In a ysystem for producing oxygen from air, a rectifying column adapted to contain a bath of crude oxygen in its lower end, a heat exchange coil in said bath for evaporating the liquid of said bath, a conduit for conducting compressed air to said coil, a conduit for conducting liqueed air from said coil to a point above said bath within said column, means anterior of said bath for cooling said air within said first-named conduit, an expansion valve in said last-named conduit above said bath, and temperature responsive means including mechanism operated by differential pressure for automatically controlling the amount of fluid discharged through said valve into said column above said bath.

7. In a system for producing oxygen from air, a rectifying column, insulation extending about said column, said column adapted to contain a bath of crude oxygen in its lower end, a heat exchange coil in said bath for evaporating the liquid of said bath, a conduit for conducting compressed air to said coil, means anterior of said coil for cooling said air, and means affected by the level of the bath in said column for controlling the operation of said column, said means comprising a receptacle in communication with the lower portion of said column both above and below the level of the liquid in said column for receiving a liquid from said bath said receptacle being within said insulation outside of said column, an expansion valve above the liquid level within said column, adapted to discharge liquid from said coil into said column above the level of said bath, and heat responsive means affected by the temperature of the liquid within said receptacle for controlling the operation of said valve.

8. In a system for producing oxygen from air, a rectifying column having a high pressure portion and a low pressure portion, a crude oxygen bath in the lower end of the high pressure portion, means for conducting compressed air through said bath and for discharging the same into said high pressure portion above said bath, a passage for conducting liquid from the high to the low pressure side of said column and for discharging the same into said low pressure side of the column above the lower portion thereof, an expansion valve in said means for controlling the discharge through said means, and differential pressure responsive means for controlling the operation of said valve.

9i' In an apparatus for separating the component parts of gas mixture by rectification comprising'a rectifying column having a high pressure section adapted to have a bath of liqueed gas in its lower portion, a heating coil within said bath, a heat exchanger exterior of said column and including a conduit for conducting said` gas mixture in compressed condition through said heat exchanger to said heating coil, an expansion valve, a conduit for conducting the compressed gas from said heating coil to said valve for discharging the same into said column above said bath for lowering the temperature of said bath below the critical temperatures of certain only of said gases and for lowering the temperature of certain other of said gases below the congealing point whereby crystals of said other gas will form in the mass of said bath of liquid, a filter casing, iilter material within said casing, a conduit for conducting liquid and crystals therein to the upper portion of the interior of said casing above said filter material, a conduit for conducting said other gas through said heat exchanger for cooling the mixed gas flowing therethrough and for heating said other gas, a valve for controlling the discharge of said other gas from said heat exchanger, a conduit for conducting the liquefied portion of said gas from said lter to the upper portion of said column after the same has passed through said filter.

10. An apparatus for separating oxygen and nitrogen from air comprising a low and high pressure rectifying column, the high pressure portion of the column having partially rectified lliquid oxygen therein, a heat exchanger, a con- "Zduit for conducting compressed air through said heat exchanger for liquefying said air and through said partially rectified oxygen for evaporating the latter, a plurality of expansion valves for discharging said liquid air and partially rectified oxygen into said column at different heights, a conduit tor conducting partially rectied liquid oxygen. from the high 'to the low pressure side of said column, a filter within said conduit for removing solidified particles of carbon dioxide from said crude oxygen, and a conduit for conducting said carbon dioxide from said lter through said heat exchanger for heating the carbon dioxide and ior cooling the compressed air entering through said heat exchanger, and a manually operated valve for said last named conduit through which said carbon dioxide gas is discharged.

ANDREW C. BROWN.

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