US2552561A

US2552561A - Method of producing oxygen

Info

Publication number: US2552561A
Application number: US641277A
Authority: US
Inventors: Frank J Jenny; Edward G Scheibel
Original assignee: Hydrocarbon Research Inc
Current assignee: Hydrocarbon Research Inc
Priority date: 1946-01-15
Filing date: 1946-01-15
Publication date: 1951-05-15
Anticipated expiration: 1968-05-15

Description

May 15, 1951 F. J. JENNY ET AL METHOD OF PRODUCING OXYGEN 2 Sheets-Sheet 1 Filed Jan. 15, 1946 lNVENTO-RS FRANK J. JEN NY EDWARD GSCHEIBEI Z174/M JZM/ ATTORNEY y 951 F. J. JENNY E1- AL 2,552,551

METHOD OF PRODUCING OXYGEN Filed Jan. 15, 1945 2 Sheets-Sheet 2 5 mvsmoRs FRANK J. JENNY EDWARD C7. SCHElBEL HTTOR N EY OXYGEN UNIT ED STATES PATENT OFFICE METHOD OFPRODUCING OXYGEN Frank J..Jenny,:New York, N.,Y., and EdWardG.

Scheibel,;-Nutley, N. .L, assignors toHydrocarbonResearch, Inc., New York, N. Y.

Application January 15, 1946,-Serial Dim-641,277

14 Claims.

1 This invention relates to the production a of oxygen'by the'liquefaction and rectificationof air, and more particularly to the operation of the usual two-stage rectification system and associated heat exchangers.

All temperatures hereingiven are in degrees F.-and pressures are in-pounds per square inch gauge.

Oxygen is commonly produced by partial liquefaction of air and rectification at low temperatures; preferably rectification is conducted in twostages at dififerent pressures. The refrigeration necessary for liquefaction is supplied to the air-after it has been compressed and watercooled to approximately room temperature, by

indirect'heat exchange with the efliuent products of rectification. Howeven-an additional amount of refrigeration must be supplied'tocompensate for cold losses resulting from the difference in enthalpy between the incoming air and-the outgoing products of rectification and for heat leaks into thesystem. Methods of supplyingthis refrigeration heretofore used, involve compressing at least a portion of the incoming air to pressures as'high as 3000 pounds andexpanding with or without the performance of Work to product a temperature drop; -or compressing all the incoming air to about 600 pounds and after the air has been partially cooled by the products of rectification expanding a portion, of the air. These methods are wasteful from the standpoint of compressor energy and require agreat 'deal1of equipment in the form of extra compressorsgintercoolers and expanders.

For economicaloperation, it is essential to recover the cold content of the outgoing products :of rectification. This is usually accomplished by passing these products in heat transfer relationship withthe incoming. air. In older systems,=in .order to avoid'deposition of frost and solid carbon dioxide in the tubular counter-current heat exchangers throughwhich the air ispassed in in- ..=.absorbing capacity through which the warm incoming air and thecold products of rectification are alternately passed with periodically; reversed operation so that streams of Warm airoareiflowed through the same packing-filled spacesthatlthe 5 cold separated-oxygenand nitrogen traversed during the previous step in the process thehighboiling impurities deposited in these. spacesduringthe passage of air therethrough'lbeingremoved by sublimation during the subsequent lo flcw inc reversedirection or theproductscoi rectification. Theuse of these reversing heat exchangers. in a process in which the;air is compressed to relatively high pressure results in more costly operation from the standpoint ;of horsepower requirements because upon every reversal, which may take place every three minutes, volume of compressed air'in the heat --ex changers is lost and must be again replaced. Also in the operation of these reversing heat ex- -changers containing packing-filled spaces through which the gases-flow, undue dilution takes, place of (a) a compressed air. stream fio'wing to the rectification system with nitrogen, and (b) the nitrogen stream with air, which dilution necessarily occurs upon each reversal-of flow,- causing the air stream to "flow through' -the packing-filled spaces containing nitrogen, and :the nitrogen stream to flow through the packing-filled spaces containing air, lefttherein from no the streams of nitrogen and air, respectively, passed therethrough'during the preceding step of the process.

Copending application Serial No. 632,858,,filed December 5, 1945, discloses and claims a process for producing oxygen by liquefaction and rectificaticn of air involving-the flow ofair at about 'TOtoabout pounds at-a temperature of'about "MP-to about 110 F. through the heatexch-ange paths of 'two or more reversing heat exchangers n in series,reach exchanger containing two other paths through which are passed, respectively, streams of oxygen and nitrogen products of rectification, in heatexchange relation With-the air .passing therethrough. One-of thestreams-fiow- ;ing between'the first exchangerand a second exchanger .is refrigerated eitheribyan external refrigerant er by expanding a minorportiomof the total :air fintroduced into the process, "say from 5% to 31.0% by volume, preferably about 7%,t0 produce refrigeration Whichisimparted to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system. The temperature conditions in the first exchanger are such that substantially all moisture present in the air is removed therefrom in the form of frost. The temperature conditions in the second exchanger are such as to effect substantially complete removal of carbon dioxide from the air in its passage therethrough.

At the colder end of the second exchanger where the oxygen and nitrogen products of rectification enter and air leaves the exchanger, there is maintained between these products of rectification and the countercurrent stream of air a temperature difference in the range of about 5 to about F., preferably about 6 to about 8 This temperature difierence is the difference between the temperature of the air and the weighted average temperature of the products of rectification, all temperatures being taken at the colder end of the second exchanger. The weighted average temperature of the products of rectification is calculated by multiplying the temperature of the oxygen product stream by the volume percentage of the stream based on th combined volume of the products of rectification and adding thereto the corresponding figure obtained by multiplying the temperature of the nitrogen product stream by its volume percentage. Thus, for example, if the rectification system is operated to produce two streams of substantially pure oxygen and pure nitrogen, the weighted average temperature of the two streams would be approximately the sum of of the oxygen stream temperature and 80% of the nitrogen stream temperature. Periodically the flow or" air and nitrogen through their respective paths in the two exchangers is reversed so that upon reversal the air flows through the paths in the two exchangers through which during the preceding step the nitrogen had passed, and the nitrogen flows through the paths in the two exchangers through which had previously passed the air. The nitrogen removes, by sublimation, the carbon dioxide deposited during the preceding step in the second exchanger and the frost deposited during the preceding step in the first exchanger.

Operating in this manner, complete purging of carbon dioxide is attained upon each reversal of flow. Likewise complete purging of frost is obtained so that the equipment may be operated continuously.

Copending application Serial No. 632,859, filed December 5, 1945, discloses and claims a process which combines with the process of the aforesaid application Serial No. 632,858 involving the feature of refrigerating one of the streams flowing from the first exchanger to a second exchanger, the amount of cold thus introduced into the process being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system, and maintaining at the colder end of the second exchanger where the oxygen and nitrogen products of rectification enter and air leaves the exchanger, a temperature difference between the air and the weighted average temperature of the nitrogen and oxygen from about 5 to about 10 R, preferably about 6 to about 8 F., the feature of efiecting the removal of incondensibles such as hydrogen, helium and neon from the rectification system without substantial reduction in the yield of oxygen recovered in the process. Thus, in accordance with the invention of said application Serial No. 632,859, a stream of air at about '70 to about pounds and a temperature of about 70 to about 110 F. is passed through the heat exchange path of two heat exchangers in series, each of the heat exchangers containing at least three paths in heat exchange relation with each other, and streams of oxygen and nitrogen products of rectification are passed through two other paths in the heat exchangers in heat exchange relation with the air passing therethrough. At least one of the streams, preferably the air stream, is cooled during its flow from one heat exchanger to the other, the amount of cold thus introduced into the process being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system. The flow of air and nitrogen and oxygen into and from the second heat xchanger is regulated so that the difference between the temperature of the air and the weighted average temperature of the nitrogen and oxygen at the colder end of the second exchanger is about 5 to about 10 F., preferably about 6 to about 8 R, thereby effecting complete removal of carbon dioxide from the air in its passage through the second exchanger.

The air from the heat exchangers fiows to the high pressure stage of a two-stage rectification system in indirect heat exchange relation with the rectification products from the low pressure stage. From about 1% to about 15% by volume of the total nitrogen introduced into the process, which nitrogen contains incondensible gases, is withdrawn from the high pressure stage, preferably about 10% of the nitrogen thus withdrawn is heated and mixed with the remaining thereby yielding a nitrogen stream having a temperature sufficiently high to avoid formation of liquid nitrogen upon the subsequent expansion of this nitrogen stream. After expanding the nitrogen stream to lower itstemperature, the expanded nitrogen stream is passed in heat exchange relation with oxygen and nitrogen streams supplied as reflux to the low pressure stage, and with the air supplied to the high pressure stage, thereby cooling the reflux liquids and improving the efficiency of the operation of the rectification system.

Periodically, the iiow of air and nitrogen through their respective paths in the two heat exchangers is reversed, the air, upon reversal flows through the paths through which had previously flowed the nitrogen and the nitrogen flows through the paths through which had previously flowed the air, so that upon each reversal the nitrogen substantially completely removes the carbon dioxide deposited in the second exchanger and the frost deposited in the first exchanger during the preceding step of the process.

The processes of the aforesaid copending applications represent a substantial and important advance in the art of producing oxygen by the liquefaction and rectification of air, over prior procedures.

This invention is in the nature of an improvement on the processes of these applications. Among the objects of this invention is to still further minimize power losses occasioned by reversal of flow through the heat exchangers,

atszgcci each reversal, as noted above, resulting ina loss of the volume of compressed air in theexchangcm -which must be replaced, and to i also minimize-dilution of the compressedair stream with nitrogen'and-the nitrogen stream with air, and still obtain, upon each reversal-of flow, substantially complete removal of frost and carbon dioxide deposited in the exchangers during the preceding step of the process, without the use of chemical reagents, so that the process may be operated continuously. Other objects and advantages of this invention will-be apparent from the following detailed description thereof.

According to the present discovery, a stream of air at about 70 to about 85 poundsgaugeand a temperature of about 70 to about 110 F. is passed through a reversing heat exchanger zone in heat exchange relation with streams or" oxygen and nitrogen products of rectification, the air being cooled to a temperature at which substantially complete removal of "moisture therefrom is accomplished. The air then flows through a second zone or exchanger of the non-reversing type or one which is reversed only infrequently, say at periods of about a weeks time, passing therethrough in heat exchange relation with oxygen and nitrogen products of rectification which flow from this exchanger to the first-mentioned exchanger. The rate of flow of the air and the oxygen and nitrogen products of rectification through this second or intermediate exchanger zone is controlled so that the temperature of the air at the colder end of this exchanger is within the range of 180 to 210 F., preferably about -l95 to 205 F. One of the streams flowing between the first exchanger zone and this intermediate or second exchanger zone is refrigerated, the amount of cold thus introduced into the process being adequate to compensate for cold losses resulting from the difierence in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system.

The air from the second zone or intermediate exchanger flows through another zone or reversing exchanger in heat exchange relation with the oxygen and nitrogen products of rectification, the flow through this exchanger or zone bein con trolled so that the difference between the temperature of the air and the weighted average temperature of the oxygen and nitrogen at the colder end of this exchanger is in the range of about to about F, preferably about 6 to about 8-F., the temperature conditions in this exchanger being such as to-effect substantially complete removal of carbon dioxide from the air in its passage therethrough.

The air from the second reversing exchanger is passed to the high pressure stage of a two-stage rectification system in indirect heat exchange relation with rectification products from the low pressure stage of this system. Preferably from about 1% to about by volume of the total nitrogen introduced into the process, which nitrogen contains incondensible gases, is withdrawn from the high pressure stage of the system, and after heating, if desired, as hereinabove disclosed, expanded and passed in heat exchange relation with nitrogen and oxygen streams fed to the low pressure stage and air fed to the high pressure stage.

Periodically, the flow of air and nitrogen through their respective paths in thetwo reversing heat exchangers is reversed so that upon reversal, the air flows through the paths in the two t exchangers through which; during-the preceding step, the nitrogen had passed and the nitrogen flows through the paths in-the two exchangers Operating in this manner, complete-purging of carbon dioxide and frost is obtained upon each reversal of flow, so that the equipment may be operated continuously and the volume loss upon each reversal of flow is'less than in the processes of the aforesaid pending applications, by an ill) amount equal to the volume of the intermediate or-second exchanger. Thus power losses are minimized; also dilution of the compressed air stream with nitrogen and the nitrogen stream with air is reduced.

In the preferred embodiments illustrated in the drawings, Fig. 1 shows diagrammatically apreferred layout of apparatus forpracticing the process of this invention in which the air stream flowing from the first reversing heat exchanger to the intermediate exchanger is cooled'byan external refrigerant and provision is made for effecting the removal of incondensibles, such as hydrogen, helium and neon from the rectification system; and Fig. 2 shows diagrammatically a preferred layout of apparatus for practicin the process of this invention in which refrigeration is: supplied to the air stream fiowing from the first reversing exchanger to the second reversing exchanger by expanding a portion or" this air stream and passing the expanded portion of the air-in heat exchange relation with the remainder, and the rectification system is designed to effect removal of incondensible gases. It will be understood, however, that the process of this invention may be carried out in other equipment; for example, the method of Fig. 2 of applying refrigeration to the air stream flowing from the first reversing exchanger to the second may be used instead of that of Fig. l in the layout of equipment shown in Fig. l, and vice versa, i. e., the

external refrigerant method of Fig. 1 may be used instead of that shown in Fig. 2, with the equipment shown in Fig. 2. Moreover, each of the reversing exchangers and the non-reversing ex-- changer may be replaced by two or more smaller exchangers placed in series and/or parallel, if desired, although this is objectionable from the standpoint of increasing construction costs, or the number of heat exchange paths may be increased over the three-path construction shown in the drawings, or other refri eration systems may be employed in lieu of those disclosed in the drawings. Hence, the scopeof theinvention is not confined to the embodiments herein described.

In Fig. 1, reference character it indicates a heatexchanger which may be of any well known type- In the embodiment shown on the drawings, it consists of a single shell in which are provided three paths, namely, interior path i i through which flows in one and thesame direction throughout the operation of theexchanger the oxygen product of rectification. ,Paths i2 and 53 are provided within the shell of exchanger ill through which periodically flow air and the nitrogen product of rectification. in heat exchange-relation with each other and with the oxygen. The

heat exchanger has in each of the paths suitable fins of heat-conducting material, e. g., copper, promoting rapid and efficient heat exchange between the gaseous media fiowing therethrough. As the construction of the heat exchanger per se does not form part of this invention and as it may be of any well known type, it is believed further description thereof is unnecessary.

The flow of the air and nitrogen through their respective paths is periodically reversed so that during one step of the process air flows through path l2 and nitrogen through path l3, and upon reversal, during the succeeding step, air flows through path l3 and nitrogen through path l2.

Reversal of flow is accomplished by suitably positioning the compound reversing valves l4 and |5 which may be of any well known type. Valve is disposed in the pipe line system consisting of air inlet pipe l3 leading into valve l4,

and pipe lines IT and i8 leading from the valve b to paths l2 and i3, respectively. At the other end of the heat exchanger l3, lines it and are positioned leading from paths i2 and l3, respectively, to the valve |5.

A substantially non-reversing heat exchanger is provided in the form of a shell having therein paths 2, 3 and 4 provided with fins to promote heat exchange, as in the case of the exchanger [0. Path 2 is the path through which the oxygen product of rectification flows. Paths 3 and 4 are for the flow of air and the nitrogen product of rectification, respectively, in heat exchange relation with each other and with the oxygen.

A refrigeration system 32 of any well known type for supplying a refrigerating medium such as ethylene or carbon tetrafiuoride is provided for cooling either the nitrogen or oxygen flowing from heat exchanger to heat exchanger ID, or the air flowing from heat exchanger I3 to heat exchanger i. This refrigerating system operates to cause the fiow of the refrigeratin medium in indirect heat exchange relation with the nitrogen, oxygen or air to be cooled, the rate of flow and temperature of the various media being so controlled that enough cold is introduced by refrigeration at this point in the process to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the system.

Preferably, the air leaving the heat exchanger H: is refrigerated to cause a drop of about 5 to about 10 F. in its temperature; this has been found adequate for the purposes above stated. Refrigeration of the air is accomplished by causing it to how through pipe line 33, which passes through the refrigerator 32 in indirect heat exchange relation with the refrigerant; pipe line 33 communicably connects valve l5 with path 3 of heat exchanger I through which path the air flows. Line 90!. is the nitrogen line connecting the nitrogen path 4 of heat exchanger I with the valve l5 and a is the oxygen line connecting oxygen path 2 of heat exchanger With the oxygen path ll of heat exchanger It.

A second reversing heat exchanger 2| is provided in the form of a shell having therein

paths

22, 23 and 24 provided with fins to promote heat exchange as in the case of exchangers and I0. Path 24 is the path through which the oxygen product of rectification fiows from the rectification system, hereinafter described, to pipe line 25, which communicates with oxygen path 2 of heat exchanger At one end,

paths

22 and 23 of heat exchanger 2| communicate with

pipe lines

26 and 21, respectively, which are communicably connected with a compound valve 28 provided with a line 33a leading from path 3 of heat exchanger and a second line 9 leading into the path 4 of heat exchanger At the other end,

paths

22 and 23 communicate, respectively, with lines 29 and 30, which in turn communicate with a compound reversing valve 3| which may be of the same type as the other reversing valves. Reversing exchangers I3 and 2| and the substantially non-reversing exchanger may be placed in vertical, horizontal or any other desired position. Moreover, when these exchangers are arranged vertically, the colder end may be above or below the warmer end, i. e., the arrangement of flow therethrough may be reversed so that instead of the air flowing downwardly and the oxygen and nitrogen upwardly, as in the case of exchanger II], the air may flow upwardly and the oxygen and nitrogen downwardly, similar reversal of flow taking place in the other exchangers.

With the arrangement of valves and piping shown, fiow of nitrogen and air through heat exchangers l0 and 2| may be periodically reversed, say every three minutes, while no reversal of flow takes place through heat exchanger I, so that during an initial period of operation, air fiows through heat exchange path |2, through line l9, valve l5, refrigeration system 32 by way of line 33, path 3 in heat exchanger line 33a, valve 28, line 26, cooling path 22 in heat exchanger 2|, pipe line 23, valve 3|, and thence to line 34 leading through the non-reversing heat exchanger 35 to the rectification system hereinafter described. At the same time, nitrogen fiows through pipe line 36 leading from the nonreversing heat exchanger 35 into valve 3|, line 30, through path 23 in heat exchanger 2|, through line 21, valve 28, line 9, path 4 in heat exchanger line So, valve l5, line 29, path [3 in heat exchanger IU, leaving this path through pipe line l8 and passing through valve I4 to the atmosphere or other suitable disposal point. Upon reversal (as shown by dotted arrows and valve settings), the air flows through valve l4, line I8, path |3 in heat exchanger l0, line 26, valve |5, refrigeration system 32 by way of line 33, path 3 in heat exchanger line 33a, valve 28, line 27, path 23 in heat exchanger 2|, line 39, valve 3| and pipe line 34, into the non-reversing heat exchanger 35. At the same time, the nitrogen flows from heat exchanger 35 to pipe line 36 into valve 3|, line 29, path 22 in heat exchanger 2|, line 25, valve 28, line 9, path 4 in heat exchanger line 9a, valve l5, line l3, into path l2 in heat exchanger Ill, thence through line H into valve l4, and thence to the atmosphere or other suitable disposal point.

The rectification ystem comprises a two-stage rectification column 37, the lower section 33 of which is operated at a pressure of about '12 pounds gauge and the upper section 39 of which is operated at a pressure of from about 4 pounds to about 10 pounds gauge, preferably at about 5 pounds gauge. This column as is customary is provided with rectification plates of the bubblecap or other desired type. The lower section 38 of the column 31 communicates with a condenser 40 and has a liquid collecting shelf 4| disposed immediately below the condenser 4!] for collecting liquid nitrogen. Pipe line 42 leads from this shelf 4| to a non-reversing heat exchanger 43 which in turn communicates through a pressure reducing valve 44 with the top portion of the upper section 39, as indicated by the reference character Q5. Condensendil acts as a reboiler-for the upper section 39 of the column 31.

From the base portion of the lower section 38, a pipe line for the flow of crude oxygen (containing approximately 40% oxygen) passes to a non-reversing heat exchanger 4! which communicates through pipe line 68 having a pressure reducing valve 59 therein with the low-pressure section 59 at an intermediate point indicated by the reference character 56. A line 5! having a pressure reducing valve 52 therein, leads from condenser lil to a nitrogen line 58 leading to the nonreversing heat exchanger 35. A. line 5t leads from the of low-pressure column 39 to the heat exchanger 43, the nitrogen flowing through this line passing through the heat exchanger 53 then through line 55 and heat exchanger 41 into line 5% which communicates with heat exchanger An oxygen line 5i leads from the lower part of the low-pressure section 39 of column 31 to path of heat exchanger 2|. The heat exchangers and ll and the two-stage fractionatin column 3i may be of any conventional type. Two separate fractionating columns, suitably interconnected, may be used in place or the two-stage column 3! shown. It will be understood that the equipment throughout is heat insulated to minimize loss of cold.

A desirable operating range involves the intro duction of air at a pressure of '70 to 85 pounds gauge and at a temperature of 70 to 110 F. into path H2 or E3, as the case may be, in heat exc anger it, the air leaving this path at a temperature of l2 3 to -150 F. and substantially the same pressure. The. nitrogen and oxygen products of rectification enter the heat exchanger iii at a temperature of 127 to -162 F. and leave at a temperatureof 60" to 100 F. The air, in passing from heat exchanger It to heat exchanger 5, is refrigerated from 5 to P. so that it enters heat exchanger l at a temperature of l25 to l59 F. The air leaves this heat exchanger 5 at a temperature of 180 to -210 E; the oxygen enters heat exchanger l at a temperature of -183 to '2l3 and leaves at a temperature of -127 to 162 F.; the nitrogen enters this heat exchanger at a temperature of -l83 to -213 F. and leaves at a temperature of -12? to 162 F. The air enters heat exchanger Zi at a temperature of 180 to 210 F. leaves at a temperature of 260 to 280 F. The oxygen enters heat exchanger 2! at a temperature of 288 to 293 F. and leaves at a temperature of -183 to 213 F.; the nitrogen enters heat exchanger 2i at a temperature of -25 to 285 F. and leaves at a temperature f -183 to 213 F. The several streams sufier only a small pressure drop in flowing through the exchangers ill, I and 2!.

One example of the operation of the process of this invention in the layout of equipment shown in Fig. l is described below. It will be understood this example is given for purposes of exeinplification only, and the invention is not limited thereto.

Air under pressure of about '75 pounds and at a temperature of about 160 F. is supplied through line valve i and line H to heat exchanger g through path E2 in which it is cooled .a'. heat exchange relation with ethylene in oration system 32 and is cooled thereby to a temperature of li2 lit, passes through the path 3 of the heat exchanger 1, leaving this ti temperature of 290 F., and then 1. 1." J. at

passes through path 22 of theheat exchanger 2 i leaving this path at a temperature of 273 F. Substantially all moisture is removed in the form of frost in path 12 of heat exchanger ill and all carbon dioxide is removed in solidified form in path 22 of heat exchanger 2i; no appreciable removal of condensible material takes place in heat exchanger I. From heat exchanger 21 the air flows through the non-reversing heat exchanger in heat exchange relation with nitrogen and enters high pressure column 38 at a temperature of -275 and a pressure of 72 pounds.

Crude oxygen at a temperature of 278 F. a pressure of 72 pounds leaves the base of column flows through the heat exchanger M where its temperature is reduced to 286 F. and, upon flow through the pressure reducing valve 39, is flashed, entering low pressure column 39 at a temperature of -31G to 315 F. and at a pressure of 5 pounds. Pure oxygen is withdrawn through line til at a temperature of 292 F. and a pressure of 5 pounds, and flows through path 24, its temperature being increased to 203 F. The oxygen. at this temperature passes through path 2 of heat exchanger l where its temperature is increased to 144 F., flows through path ll of heat exchanger ill and is withdrawn from this path at a temperature of F. and at a pressure of one pound.

Nitrogen, at a temperature of about -286 F. and a pressure of '72 pounds, is withdrawn through line 5! and passes through valve 52, its temperature being reduced to about -315 F. as a result of the expansion through the pressure reducing valve 52. Nitrogen, at a temperature of 315 F. and a pressure of 5 pounds, is also withdrawn from the top of low pressure column 33 through line 54' and flows through heat exchanger 43 where its temperature is raised to about 304 F. The nitrogen flows from heat exchanger through heat exchanger 4! and mixes with that from line 5!. The nitrogen stream thus produced at a temperature of 294 F. flows through line 56 into the heat exchanger 35 where the temperature of the nitrogen is raised to -278 F. The nitrogen at this temperature and a pressure of about 5 pounds enters path 23, flowing therethrough in heat exchange relation with the oxygen which enters at a temperature of '292 F. and the air which enters the opposite end of heat exchanger 2! at a temperature of -2(lil F. The nitrogen leaves path 23 at a temperature of 2Q3 F. At the colder end of the heat exchanger 2!, the nitrogen and the oxygen streams have a weighted average temperature of about 281 F., while at this point the air is at a temperature of 2'73 F. A temperature difference of about 8 F. is, therefore, maintained. From the heat exchanger 2!, the nitrogen'fiows through line 21, reversing valve iii, line 9, entering path 4 of heat exchanger l at a temperature of -203 F. and leaving at a temperature of 14l F. It flows through line Qa, valve !5, into and through heat exchanger ill, entering path 53 of this heat exchanger at a temperature of about -14l F. and leaving at a temperature of 90 F. and a pressure slightly above atmospheric, say one pound.

Upon reversal (as shown by dotted arrows and valve settings), which may take place every three minutes through the heat exchangers it and 25 the air flows through the paths i3 and'23, respectively, of heat exchangers it and 2 l, and nitrogen flows through paths i2 and 22, respectively, of heat exchangers it and 2!; theair and nitrogen flow, respectively, through paths 3 and 4 of heat exchanger no reversal of flow through these two paths taking place. The flow is otherwise substantially the same, and the temperature and pressure conditions remain the same. The nitrogen in its flow through path 22 of heat exchanger 2| removes by sublimation the carbon dioxide deposited in this path by the air during the preceding step. Likewise, the nitrogen in its flow through path l2 of heat exchanger it removes from this path the frost deposited therein by the air during the preceding step. Thus, in the continued operation, upon each reversal, the nitrogen effects removal of the frost and carbon dioxide deposited in the paths of heat exchangers l and 2| through which the air has passed during the preceding step of the process.

The layout of apparatus shown in Fig. 2 differs from that of Fig. 1 chiefly in that, instead of the refrigerator 32, an expander and associated piping is employed to impart refrigeration to one of the streams flowing between heat exchanger l0 and heat exchanger 2|, and the rectification system 31 is designed to effect removal from the high pressure stage of incondensible gases by expansion of a minor portion of the nitrogen containing such incondensible gases, and employing the expanded nitrogen to supply refrigeration to the nitrogen and oxygen streams employed as reflux in the low pressure stage of the rectification system, preferably also to supply refrigeration to the air introduced in the high pressure stage of the rectification system, thus increasing the efiiciency of the operation of the rectification system.

Corresponding parts in Figs. 1 and 2 have been given like reference characters, and it is believed the construction and operation of these parts will be apparent from the above description thereof.

From the heat exchanger ill, the compressed air flows through valve l and line 33, the major portion of the air flowing through this line into the zone 3 of heat exchanger A minor portion, say from 5% to by volume, preferably about 7%, the exact amount being controlled by the setting of valve 6 in line 5, flows through line 5 into an expension engine or turbine l hereinafter referred to as an expander. The air leaving the expander I at a lower pressure and temperature flows through line 8 into line 9 where it mixes with the nitrogen stream flowing through this line from valve 28, the resultant mixture flowing through path 4 of heat exchanger 5, the cold thus introduced into the system by the expansion of the minor portion of the air being imparted to the major portion of the air flowing through path 3 of heat exchanger in heat exchange relation with the oxygen and the mixture of nitrogen and expanded air.

Line 5| leads from the top of the condenser or reboiler M) and has a regulating valve 5| a therein. This line communicates with an expander 53 which discharges by way of line 53a, into line 54, which as above described leads from the top of the low pressure stage 39 of the rectification system for the flow of nitrogen therethrough. Preferably, there is also provided a branch line 58- having a regulating valve 59 therein and leading from line 5i into a path 59 disposed in heat exchanger 2| in indirect heat exchange relation with the oxygen, nitrogen and air passing through the other three paths in this exchanger 2|. A line 6| leads from path 63 back to line 5|. Regulating valves 5| a and 59, disposed in lines 5| and 58, respectively, regulate the portions of the nitrogen stream flowing from the condenser 40 which are passed directly to expander 53 and indirectly through path 60 of exchanger 2|. By the arrangement of lines hereinabove described, a minor portion of the total nitrogen introduced into the process passes from line 5|, and, preferably, of the portion thus Withdrawn, a minor portion, say about 10%, passes through line 58, path 60, and line 6|, entering line 5| where it mixes with the remainder of the nitrogen withdrawn from the condenser 40. The portion of nitrogen passing through path 6|] is warmed up by indirect heat exchange, and by mixing with the remainder of the nitrogen, the stream entering expander 53 is at a temperature sufficient to avoid condensation or formation of liquid nitrogen in the expander. In a preferred embodiment of the invention, from about 1% to about 15% by volume of the total nitrogen introduced into the process and containing incondensibles, such as hydrogen, helium and neon, is passed through line 5|, and of this quantity about 10% by volume passesthrough heating path and about 90% by volume continues through line 5|.

The nitrogen stream, refrigerated as a result of the expansion, flows from the expander 53 to a line 53a which meets line 54 conveying the nitrogen stream leaving the top of the low pres sure section 39. The mixture then flows through heat exchanger 43 in indirect heat exchange rela-' tion with the nitrogen passing through this ex-- changer and thereafter flowing through reducing valve 44 into the top of low pressure sectiorr 39. From heat exchanger 43, the mixed nitrogen stream flows through line 55 into and throughheat exchanger 41 where it flows in indirect heat exchange relation with the crude oxygen flowing therethrough to low pressure section 39. From the heat exchanger 47, the mixed nitrogen stream passes through line 56 into and through heat exchanger 35 where it passes in indirect heat exchange relation with air flowing into and from this exchanger by way of line 34. From the heat exchanger 35, the nitrogen stream containing incondensible gases removed from the high pressure stage, as hereinabove described, flows through line 36 into a compound valve 3|, thence through

path

22 or 23, as the case may be, of heat exchanger 2|, through compound valve 26, through path 4 of heat exchanger I, through compound valve l5, then through path l2 or |3 of heat exchanger IE), and finally through compound valve M to the atmosphere, the flow through

paths

22 or 23 of heat exchanger 2|, and I2 or |3 of heat exchanger 59, depending upon the setting of the valves associated with these exchangers.

One example of the operation of the process in the equipment shown in Fig. 2 is described below. It will be understood this example is given for purposes of exempliflcation only, and the invention is not limited thereto.

Air under pressure of about pounds gauge and at a temperature of about 100 F. is supplied through line I6, valve I4, and line l'i to heat exchanger iii, flowing through path l2 in which it is cooled to a temperature of F. Of the air flowing from valve |5, 7% by volume is expanded in engine 7, the pressure of the expanded air being about 5 pounds and its temperatureabout 215 F. This cold expanded air discharges into line 9 conveying nitrogen at a temperature of 200 F. from exchanger 2| to exchanger and mixes with the nitrogen. The remainder of the air at a temperature of 80 F. passes through path 3 of exchanger leaving thispath ata temperature-.of..195 F. It thenhflows: through path 22 of exchangerlz'l, leavingthisi. path at a temperature or -275 F. Substantially all moisture is removed :ir'rtheiorm ofzfrostin path IZ-of exchanger id.- Little onnocondensation. takesplaceinpath 3+of.exchanger- I, and. substantially all:carbon dioxide is. removed. in. solidified. form in cooling path. 2 2 of. heat. exchanger 2i. The airfrom exchangerdl flows.- throughheatexchanger 35 in heat-exchange relation..with nitrogen. and enters .high. pressure columnuafi ata temperature of-2789 E; and a: pressure .of 72 pounds.

Crude oxygen at a temperature. of .280 F.. and.-a=.-pressure of 72 pounds leaves the base-of column 38, flows through heat. exchanger- 4'!- whereits. temperature is:.reduced.'to.-289" F. and upon flow through the pressurereducing valve 49 is. flashed, entering low pressure column 39- at atemperatureof- -310 to '3'1 5.- F. and :a pressureof 5 pounds. Pure oxygen is. withdrawn. through line 5'5 at a temperature ctr-292.5 F. and .-a;.pressure of 5 pounds and flows. through path 24; its temperature being increasedto .--200:

F. The. oxygenat this temperature enters .path 25 2 .of heat exchar-iger. l andleavesthis pathnata temperature of 92 at which-temperature it enters path i i oi-heat exchangerlllancl is :dischargedzfrom this path at a'temperature. of.90 F; and at a pressure of one pound.

Nitrogen at a temperature of about .--286.5 F. and a-wpressure of 72tpounds in amount equal to 12.5% :by volume of the-total nitrogen introduced into the processis withdrawn. throughline 5|. Of-the nitrogen. flowing through line. 5|, 10% passes through line 58- and heatingpath. 6E, its temperaturebeing. increasedIto-200 F. The remaining 90 of thenitrogen. flows through valve 5min linefil and ismixed with. the. other 10% nitrogen, thetemperature of-the mixture being. about 280 F. The'nitrogen streamleavingthe expander 53-"isat a pressure of 5.pounds -and a. temperatureoi 315 F. The expanded-nitrogen. flows through line 53a. and :becomes mixed-with. nitrogen at a temperature of 31'5.5 F. and at a;-.45 pressure. of 5 pounds. flowing through... line. 54.' The resultant nitrogenstreampasses through exchangeriin. indirect heat exchange relation. withnitrogen employed.as.refiux in column .39; itstemperature beingthereby increased .to 306 whilethe temperature of the nitrogen flbwing. through IineJiZ'Jpressure of '72 pounds) and exchangeriiil is reducedto 300 F. This nitrogen, by expansion through valve 45, has itspressure reduced to 5' pounds and its temperature. torts -315I5 F. The nitrogen product of rectification then flows throughh'eat.exchangerATwhere its temperature is increased to 293."F.'" The crude oxygen stream fiowingthrough exchanger 41' is thereby cooled from a temperature off-280 to a temperature of 239 F. The nitrogen then flows through exchanger. 35' in heatexch'ange" relationiwiththeair; the nitrogen stream tem perature beingthereby increased to 279 F., at which-temperature it enters theheat'exc'hange'r65 2i, flows th'erethrough and is 'th'erebyrheated 'to 200 At the colder'endlof th'e'heatexchanger. 2i, thenitrogen andoxygenstreams have a, weighted average temperature "of 'lnearly" 282 while'at this pointtheair'is'at'a'term To peraturecf' -275 A temperature"difference" of about "7 F. is; therefore, maintained? Fiorn" heat exchanger 2!, the nitrogen-flows"through heat 'exchangenl where it is 'heated"to"-92 F.

It then" enters-and flowsthrough heat exchanger the aforesaid paths 3 and 4.

the temperature and pressure conditions remain the same. The nitrogen, in its flow through path 22 .ofheat exchanger2l, removes by sublimation, the carbon dioxide deposited in this path by the airduringthe preceding step. Likewise, the nitrogen, in its flow through path I2 of heat ex-. changer Iii, removes from this path the frost deposited therein by the air during preceding step.- ihus, in the continued operation, upon each reversal, the nitrogen effects removal of the carbon dioxide and frost deposited inthe paths through which the air has passed during the preceding step of the process.

Operating in accordance with. this invention, whichinvolves the intermediate non-reversing heat exchanger l, power requirements for each. reversal period are diminished by an amount equalto the power required to compress to a pressure of '70 to pounds gauge a volume of air corresponding to the volume of path 3 or 4 of heat exchanger 1; also, dilution of the compressed air stream with nitrogen and the nitrogen stream with air is minimized by an amount corresponding to the volume of gaseous. fluid in These results are made possible by maintaining theinlet temperature of the air to the intermediate exchanger I within the range of from 75 to F., preferably from 75 to 85 F., and the exit temperature Within the range of from. to -21'0 Fl, preferably from to 205 F. Operating with these temperature conditions and the..other conditions above pointed out, substantially. allifrostis removed from the air in exchanger. liLYallQcarbon. dioxide in exchanger 2! andwlittleor no condensation takes place in exchanger. It will be understood that, while in thehnormalwoperation no reversal otfiow takes. placein exchanger E, as aprecautionary matter, to..insure that the path through which the air flows remains in. unobstructed condition, occasionally, say about once a week or month, flow through. pathstend lin exchanger I may be reversed, so that the nitrogen stream. passes through the path through which. has previously passed the air, and vice versa.

The expressions .reversing theflow of air and nitrogen and -reversal are used herein in the sense .commonlyemployed in this art, namely, to meanvthe:switchingofithe flow of two streams, for: example, the air and the nitrogen streams, so that upon; each. reversal the air flows through: the path: through which :had previously fiowe'd the nitrogen; and the nitrogen flows. through thepethilthrough which had previously fldwed'theair;

Since certain changes be medein carrying. out the *above process without departing from the scopeof the invention, it isintended that all ma+-- te'r contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

l A process for producing oxygen by the liquefaction and rectification of air, which comprises, passing a stream of air through a path in a heat exchange zone containing at least three paths in heat exchange relation with each other, passmg, respectively, streams of oxygen and nitrogen products of rectification through the other paths in said zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature such that substantially all moisture is removed therefrom, passing the air from the first zone through a path in a second zone in heat exchange relation with streams of oxygen and nitrogen products of rec-- tification, coolin at least one of said streams flowing between said first zone and said second zone, the amount of cold thus introduced being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system, passing the air from said sec- 0nd zone through a path in a third zone in heat exchange relation with streams of oxygen and nitrogen products of rectification, maintaining temperature conditions within said second and third zones such as to effect substantially no corn densation of condensible constituents in the air in its passage through said second zone and the substantially complete removal of carbon dioxide from the air in its passage through said third zone and periodically reversing the flow of air and nitrogen through their respective paths in the first and third zones, the air upon reversal flowing through the paths through which had previously fiowed the nitrogen and the nitrogen flowing through the paths through which had previously flowed the air, whereby upon each reversal the nitrogen substantially completely removes the carbon dioxide deposited in said third zone and the frost deposited in said first zone during the preceding step of the process.

2. A process for producing oxygen by the liquefaction and rectification of air, which comprises passing a stream of air at about '70 to about 85 pounds gauge and a temperature of about 70 to about 110 F. through a path in a heat exchange zone containing at least three paths in heat exchange relation with each other, passing, respectively, streams of oxygen and nitrogen products oi rectification through two other paths in said zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature such that substantially all moisture is removed therefrom, passing the air from the first zone through a path in a second zone in heat exchange relation with streams of oxygen and nitrogen products of rectification, cooling at least one of said streams flowing between said first zone and said second zone, the amount of cold thus introduced being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system, passing the air from said second zone through a path in a third zone in heat exchange relation with streams of oxygen and nitrogen products of rectification, maintaining temperature conditions within said second and third zones such as to effect substantially no condensation of condensible constituents in the air in its passage through said second zone and the substantially complete removal of carbon dioxide from the air in its passage through said third zone, maintaining the temperature difference between the temperature of the air leaving and the weighed average temperature of the nitrogen and oxygen entering said third zone so that it falls within the range of about 5 to about 10 F., and periodically reversing the fiow of air and nitrogen through their respective paths in the first and third zones, the air upon reversal fiowing through the paths through which had previously flowed the nitrogen and the nitrogen flowing through the paths through which had previously flowed the air, whereby upon each reversal the nitrogen substantially completely removes the carbon dioxide deposited in said third zone and the frost deposited in said first zone during the preceding step of the process.

3. A process as defined in claim 1 in which the air stream fiowing between the first zone and the second zone is cooled.

4. A process as defined in claim 1 in which the air stream flowing between the first zone and the second zone is cooled and said cooling is effected by passing said air stream in heat exchange relation with a refrigerant.

5. A process for producing oxygen by the liquefaction and rectification of air, which comprises passing a stream of air through a path in a heat exchange zone containing at least three paths in heat exchange relation with each other, passing respectively streams of oxygen and nitrogen products of rectification through the other paths in said zone in heat exchange relation with the air passing therethrough thereby cooling the air to a temperature such that substantially all moisture is removed therefrom, passing a major portion of the air from the first zone through a path in the second zone in heat exchange relation with streams of oxygen and nitrogen products of rectification, expandin the remaining minor portion of the air from the first zone, imparting the refrigeration thus produced to one of the streams flowing through the second zone to introduce an amount of cold adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system, passing said major portion of the air from said second zone through a path in a third zone in heat exchange relation with streams of oxygen and nitrogen products of rectification, maintaining temperature conditions within said second and third zones such as to efiect substantially no condensation of condensible constituents in the air in its passage through said second zone and the substantially complete removal of carbon dioxide from the air in its passage through said third zone, and periodically reversing the flow of air and nitrogen through their respective paths in the first and third zones, the air upon reversal flowing through the paths through which had previously flowed the nitrogen and the nitrogen flowing through the paths through which had previously flowed the air, whereby upon each reversal the nitrogen substantially completely removes the carbon dioxide deposited in said third zone and the frost deposited in said first zone during the preceding step of the process.

6. A process for producing oxygen by the liquefaction and rectification of air, which comprises, passing air at about to about pounds gauge and a temperature of about70 to about 110 F. through a path in a heat exchange. zone containing three paths in heat exchange relation with each other, fiowing oxygen and. nitrogen products of rectification, respectively, through the other two paths in said zone in heat exchange relation with the air, the air thus being cooled to a temperature of about 120.." to about 150 F., the moisture being thereby removed from, the air, refrigerating the air leaving the. first zone to a temperature of about. -125 to about 16 .0. F. and passing the air thus refrigerated through a second zone in heat exchange relation with oxygen and nitrogen products. of rectification, thereby cooling the air to. a temperature of about. 180 to about 210 F., thereafter pass.- in the air through a path in a third zone in heat exchange relation with oxygen and nitrogen products. of rectification, thereby cooling the air to. a temperature of about 260 to about "80 F., the diiferential between the temperature of the air and the weighted average temperature of the oxygen and nitrogen at the colder end or said third zone being within the range of about to about F., and periodically ree versing the flow of air and nitrogen through their respective paths in the first and third zones, the air uponreversal flowing through the paths in the said third and first zones through which had previously flowed the nitrogen and the nitrogen flowing through the paths in the said third and first zones through which had previously flowed the air.

7. A process as. defined in claim 6 in which the air flowing from the first zone to the second zone is refrigerated by passin in heat exchange. relation with ethylene.

8. A process as defined in claim 6 in which the reversal of fiow of nitrogen and air through their respective paths in the first and third mentioned zones takes place at approximately three minute intervals.

9. A process for producing oxygen by the liquea faction and rectification of air which comprises passing a stream of air at about 70 to about 85 pounds gauge and a temperature of about 70 to about 110 F. through a path in a heat exchange zone containing at least three aths in heat exchange relation with each other, passing, respectively, streams of oxygen and nitrogen products of rectification through twoother paths in said zone in heat exchange relation with the air passing therethrough, thereby cooling the. air to a temperature such that substantially all mois-. ture is removed therefrom, passing the air from the first zone through a path in a second zone in heat exchange relation with streams of oxygen and nitrogen products of rectification, cooling t least one of said streams, the amount of cold thus introduced being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system, passe ing the air from the second zone through a path in a third zone in heat exchange relation with.

streams of oxygen and nitrogen products of rec-1 tification, regulating the flow of air, oxygen and nitrogen into and from said second and third zones so that the temperature within said second zone is such that substantiall no condensation occurs and the temperature within the third zone is such as to effect substantially complete removal of carbon dioxide from the air in its passage through said third zone, passing the air 18' from the third zone to the high pressure stage of a two-stage rectification system, withdrawing. a minor portion or the. nitrogen containing incondensible gases from the. high pressure stage, 8X:- panding the nitrogen thus withdrawn to. cool the same, passing the expanded nitrogen in heat. exchange relation with nitrogen and oxygen sup.- plied as reflux to. the low pressure sta e and with air supplied to the. high pressure stage, and periodically reversing the flow of air and nitrogen through their respectiv paths in the first and third zones, the air upon reversal flowing through the paths through which had previously flowed the nitrogen the nitrogen flowing through the paths. through which had previously flowed the. air, whereby upon. each reversal the nitrogen substantially comp etely removes. the carbon dioxide deposited in. said third zone and the frost deposited in said first zone. during, the preceding step of the process...

10. A pro e s for. produ ing o y en by h li u faction and rectifi ation of air, which comprises passin air at. about '70 to about 8 p n s ga e at a temperature or about 70 to about F. r u h a p th. n. a h at exchang zone containe ins. three p t in eat excha g relation with ach o her, flowi oxygen and nitro en nrod ucts of rectification, respectively, through the other two paths in said zone in heat ex han e re ion with the air, the a r time being cooled to a temperature of about 12 Q to about 15Q F., the moisture being thereby removed from the air, refrigerating the air leaving the first zone to a temperature of about l..2. t about l6o F., and passing the air thus refrigerated through a path in a second zo e in, heat exchange relation with oxygen and nitrogen products, of rectification, thereby cooling the air to a temperature of about 180 to about 2j1 0 F., thereafter passing the air through a path a third zone in heat exchange relation with oxygen and nitrogen products of rectification, thereby cooling the air to a temperature of about 260 to about -280 F., the differential between the temperature of the air and the weighted average temperature of the oxygen and nitrogen at the colder end of said third zone being within the range of about 5 to about 10 F., passing the air from the third zone to the high pressure stage of a two-stage rectification system, withdrawing a minor portion of the nitrogen containing in-. condensible gases in the high pressure stage, expanding the nitrogen thus withdrawn to cool the same, passing theexpanded nitrogen in heat ex}. change relation with nitrogen and oxygen supplied as reflux to the low pressure stage and with air supplied to the high pressure stage, and periodically reversing the flow of air and nitrogen through their respective. paths in the first and third mentioned zones, the air upon reversal flowing through the paths in the said third; and first zones through which had previously flowed the nitrogen and thenitrogen flowing through the paths in the said third and first zones through which had previously flowed the ai 11. A process for producing oxygen by the liquefaction and rectification of air which comprises passing air at about 70 to about 35 pounds gauge and a temperature of. about 70 to about 110 F. through a path in a heat exchange zone containing three paths in heat exchange relation with each other, flowing ox gen and nitrogen products of rectification, respectively, through the other two paths in said zone in heat exchange relation with the ai the air thus being cooled to a temperature of about -120 to about 150 F., the moisture being thereby removed from the air, refrigerating the air leaving the first zone to a temperature of about 125 to about 160 F. and passing the air thus refrigerated through a path in a second zone in heat exchange relation with oxygen and nitrogen products of rectification, thereby cooling the air to a temperature of about 180 to about 2l0 F., thereafter passing the air through a path in a third zone in heat exchange relation with oxygen and nitrogen products of rectification, thereby cooling the air to a temperature of about -260 to about 280 F., the differential between the temperature of the air and the weighted average temperature of the oxygen and nitrogen at the colder end of said third zone being within the range of about 5 to about F., passing the air from the third zone to the high pressure stage of a two-stage rectification system, withdrawing from the high pressure stage about 1% to about of the total nitrogen introduced into the process, said nitrogen containing incondensible gases, heating approximately 10% of the nitrogen thus withdrawn, mixing the heated nitrogen with the remaining 90% of the nitrogen thus withdrawn, thereby yielding a nitrogen stream having a temperature suificiently high to avoid the formation of liquid nitrogen upon expansion of the nitrogen stream, expanding said nitrogen stream, passing the expanded nitrogen in heat exchange relation with oxygen and nitrogen supplied as refiux to the low pressure stage and with the air supplied to the high pressure stage, and periodically reversing the flow of air and nitrogen through their respective paths in the first and third zones, the air upon reversal flowing through the paths in the said third and first zones through which had previously fiowed the nitrogen and the nitrogen flowing through the paths in the said third and first zones through which had previously flowed the air.

12. A process for producing oxygen by the liquefaction and rectification of air, which comprises passing a stream of air through a path in a heat exchange zone containing at least one other path in heat exchange relation with the air path, passing a stream of rectification product through the said other path in said zone in heat exchange relation with the air passing therethrough thereby cooling the air, passing the air from the first zone through a path in a second zone containing at least one other path in heat exchange relation with the air path, passing a stream of rectification product through said other path in said second zone, cooling at least one of said streams flowing between said first zone and said second zone, the amount of cold thus introduced being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the process and for heat leaks into the system, passing the air from the second zone through a path in a third zone containing at least one other path in heat exchange relation with the air path, passing a stream of rectification product through said other path in said third zone, maintaining temperature conditions within said second and third zones such that substantially no condensation occurs in said second zone and the substantially complete removal of carbon dioxide from the air takes place in its passage through said third zone, and periodically reversing the flow of air and rectification product through their respective paths in the first and third zones, the air upon reversal flowing through the paths through which had previously fiowedthe rectification product, and the rectification product flowing through the paths through which had previously flowed the air, whereby upon each reversal the rectification product substantially eompletely removes carbon dioxide deposited in said third zone during the preceding step of the process.

13. A process for producing oxygen by the liquefaction and rectification of air, which comprises passing a stream of air through a path in a heat exchange zone containing at least one other path in heat exchange relation with the air path, passing a stream of rectification product through the said other path in said zone in heat exchange relation with the air passing therethrough thereby cooling the air to a temperature such that substantially all moisture is removed therefrom, passing the air from the first zone through a path in a second zone containing at least one other path in heat exchange relation with the air path, passing a stream of rectification product through said other path in said second zone, cooling at least one of said streams during its flow, the amount of cold thus introduced being adequate to compensate for cold losses resulting from the difference in enthalpy between the air introduced into and the products of rectification withdrawn from the proces and for heat leaks into the system, passing the air from the second zone through a path in a third zone containing at least one other path in heat exchange relation with the air path, passing a stream of rectification product through said other path in said third zone, maintaining temperature conditions within said second and third zones such that substantially no condensation occurs in said second zone and the substantially complete removal of carbon dioxide from the air takes place in its passage through said third zone, and periodically reversing the flow of air and rectification product through their respective paths in the first and third zones, the air upon reversal flowing through the paths through which had previously flowed the rectification product, and the rectification product flowing through the paths through which had previously flowed the air, whereby upon each reversal the rectification product substantially completely removes carbon dioxide deposited in said third zone and the frost deposited in said first zone during the preceding step of the process.

14. In apparatus for the separation of a gas mixture at low temperatures involving a reversing heat exchanger for removin by condensation from said gas mixture two condensible components thereof at different temperature separated by an intermediate temperature range in which no component of said gas mixture is condensed, the improvement of said apparatus wherein said reversing heat exchanger comprises a series of three separate heat exchanger units serially connected for the flow of said gas mixture through said series of three units to effect the removal by condensation of one of said two condensible components in each of the two terminal units of said series, and valve arrangements associated with said terminal unit adapted to alternate the fluid flows in said terminal units without alternating the fluid flows in the intermediate unit of said series in which intermediate unit no component of said gas mixture is condensed.

FRANK J. JENNY. EDWARD G. SCHEIBEL.

(References on following page) REFERENCES CITED Number The following references are of record in the 23 3 file of this patent: 2,4 l 59 UNITED STATES PATENTS 5 Number Name Dat mb r 1,626,345 Le Rouge Apr. 26, 1927 373,918 469,943

22 Name Date Twomey Oct. 20, 1936 Trumpler Feb. 8, 1949 FOREIGN PATENTS Country Date Great Britain June 2, 1932 Great Britain Aug. 3, 1937

Claims

1. A PROCESS FOR PRODUCING OXYGEN BY THE LIQUEFACTION AND RECTIFICATION OF AIR, WHICH COMPRISES, PASSING A STREAM OF AIR THROUGH A PATH IN A HEAT EXCHANGE ZONE CONTAINING AT LEAST THREE PATHS IN HEAT EXCHANGE RELATION WITH EACH OTHER, PASSING, ESPECTIVELY, STREAMS OF OYGEN AND NITROGEN PRODUCTS OF RECTIFICATION THROUGH THE OTHER PATHS IN SAID ZONE IN HEAT EXCHANGE RELATION WITH THE AIR PASSING THERETHROUGH, THEREBY COOLING THE AIR TO A TEMPERATURE WUCH THAT SUBSTANTIALLY ALL MOISTURE IS REMOVED THEREFROM, PASSING THE AIR FROM THE FIRST ZONE THROUGH A PATH IN A SECOND ZONE IN HEAT EXCHANGE RELATION WITH STREAMS OF OXYGEN AND NITROGEN PRODUCTS OF RECTIFICATION, COOLING AT LEAST ONE OF SAID STREAMS FLOWING BETWEEN SAID FIRST ZONE AND SAID SECOND ZONE, THE AMOUNT OF COLD THUS INTRODUCED BEING ADEQUATE TO COMPENSATE FOR COLD LOSSES RESULTING FROM THE DIFFERENCE IN ENTHALPY BETWEEN THE AIR INTRODUCED INTO AND THE PRODUCTS OF RECTIFICATION WITHDRAWN FROM THE PROCESS AND FOR HEAT LEAKS INTO THE SYSTEM, PASSING THE AIR FROM SAID SECOND ZONE THROUGH A PATH IN A THIRD ZONE IN HEAT EXCHANGE RELATION WITH STREAMS OF OXYGEN AND NITROGEN PRODUCTS OF RECTIFICATION, MAINTAINING TEMPERATURE CONDITIONS WITHIN SAID SECOND AND THIRD ZONES SUCH AS TO EFFECT SUBSTANTIALLY NO CONDENSATION OF CONDENSIBLE CONSTITUENTS IN THE AIR IN ITS PASSAGE THROUGH SAID SECOND ZONE AND THE SUBSTANTIALLY COMPLETE REMOVAL OF CARBON DIOXIDE FROM THE AIR IN ITS PASSAGE THROUGH SAID THIRD ZONE, ND PERIODICALLY REVERSING THE FLOW OF AIR AND NITROGEN THROUGH THEIR RESPECTIVE PATHS IN THE FIRST AND THIRD ZONES, THE AIR UPON REVERSAL FLOWING THROUGH THE PATHS THROUGH WHICH HAD PREVIOUSLY FLOWED THE NITROGEN AND THE NITROGEN FLOWING THROUGH THE PATHS THROUGH WHICH HAD PREVIOUSLY FLOWED THE AIR, WHEREBY UPON EACH REVERSAL THE NITROGEN SUBSTANTIALLY COMPLETELY REMOVES THE CARBON DIOXIDE DEPOSITED IN SAID THIRD ZONE AND THE FROST DEPOSITED IN SAID FIRST ZONE DURING THE PRECEDING STEP OF THE PROCESS.