US2057804A - Method of separating the constituents of air - Google Patents

Description

Oct. 2 0, 1936. s. TWOMEY 2,057,804

METHOD OF SEPARAI'ING THE CONSTITUENTS OF AIR Filed May 9, 1934 2 Sheets-Sheet 1 FIG].

LEE 5. TWOMEY lNl/E/VTOR awe Q).

A TORNEY 6t 1935- L. s. TWOMEY 2,057,304

' METHOD OF SEPARATING THE CONSTITUEN TS OF AIR Filed May 9, 1934 2 Sheets-Sheet 2 LEE 5. TWOMEY ATTY NE) 7pl/EN TOR Patented Oct. 20,- 1936 PATENT OFFICE METHOD OF SEPARATING ENTS THE CONSTITU- OF AIR Lee S. Twomey, Vista, Calif.

Application May 9, 1934, Serial No. 724,690

24 Claims.

The object or my invention is to provide a I method for the separation of nitrogen, oxygen, and argon from the mixture of gases constituting the terrestrial atmosphere, in any desired state of purity, and also to provide an apparatus suited to the performance of said method.

The attached drawings, Figs. 1 and 1a, are at once a flow sheet of the method and a diagrammatic vertical section of a preferred apparatus. In. reading these drawings the right end of Fig. 1 should be joined to the left end of Fig. 1a to form a single sheet, each figure showing substantially one half of the apparatus.

Fig. 2 is a diagram illustrating pipe connections by means of which the mixed gas stream flowing from the dehydrating interchanger may be cooled by liquid refrigerant to control the temperature at the cold end of the dehydrating interchanger.

Elements of apparatus Referring to the drawings, the apparatus is seen to consist of the following elements:

Element A is the primary air interchanger. .A shell H] adapted to operate at a pressure somewhat above 60 pounds gauge is provided with tube sheets lll I, an air inlet pipe l2 communicating with any suitable air pump not shown, an air outlet pipe l3 communicating with interchanger B, and a plurality of water drains l t-M having valves l5-l5.

A multiplicity of tubes arranged between the tube sheets are divided into separate banks or groups I56, ll, !8, and it by means of upper headers Z0, 2!, 22, and 23 and corresponding lower headers 24, 25, 26, and El. The order and arrangement of these banks is optional, but in practice each bank consists of a large number of tubes of small diameter, closely spaced.

For producing high velocity of air flow over the tubes and angular impingement of the air currents thereon I provide a plurality of the staggered bailles indicated at 28-28.

While the drawings show but a single interchanger in element A, it is desirable, if not necessary, to duplicate this element and to provide branched connecting lines with diversion valves to permit alternated'use for the purpose of defrosting.

of which the general arrangement may be identical with that of the primary interchanger. It consists of a shell 3| having tube sheets 32--32 and three groups of tubes 33, 3t, and 35, these groups being separated by upper headers 36, 31,

Element B is the secondary air interchanger,-

and 38 and lower headers 39, 40, and ll. The shell is also provided with staggered baflies 42-42.

Element C is the refrigerant evaporator, consisting of a shell 43 adapted to a slight super-- atmospheric pressure, tube sheets 4444 and a sufllcient number of tubes 45.

Element D is the nitrogen interchanger, consisting of a shell 46 adapted to a slight superatmospheric pressure, a tube bank 41a with headers Ill-41, and a plurality of baflles 4848.

Elements E and F are nitrogen Vaporizers iorming part of the transfer circuit, and consist of shells 69 and 50 adapted to about 55 pounds gauge pressure, upper headers 5| and 52, lower headers 53 and 54 and tube banks 55 and 56.

Elements G, H, and I are pure oxygen reboilers, consisting of shells 51, 58 and 59 adapted to a low superatmospheric pressure, upper tube headera 60, El and 52, lower tube headers 63, 64 and 65, and tube banks 66, Si and 68.

Element J is the low pressure fractionating column, having a shell 69 adapted to a low superatmospheric pressure. This shell is continuous with and supported by the shell 16 of high pressure column K. The two shells are nonleakably separated by a condensing unit consisting of tube sheets 'H H, a head 12 and a plurality of tubes 13. The condenser thus formed is partially submerged in liquid accumulating in the lower end of the upper shell, while the interior of the tubes drains into the upper end of the lower shell.

A substantial part of the height of shell 69 is filled with spaced evaporating plates, which are shown in the drawings in two of the numerous optional types. At M I indicate the conventional disc and doughnut plates, while indicates the well known bubbling plate which is provided with vapor nozzles 16 and bubble caps ill. The use of such plates for condensation and fractional reevaporation of mixed vapors is well known and understood and any type of plate or grid known in the art may be used, though in practice I prefer a bubbling plate.

Element K is the high pressure fractionating column, consisting of a shell 10 provided with disc and doughnut plates 18 or bubbling plates 19 as above described. It is also provided near its upper end with an annular pocket termed the nitrogen receiver.

Element L is the argon rectifying unit, consisting of a shell 8| which is closed at its upper end 82 and open at its lower end into the interior 65 pipe 98.

of column J. This shell is provided with any preferred form of bubbling plates 88.

Element M is the transfer unit comprising a transfer interchanger, a water cooled coil and a gas pump or compressor. The interchanger consists of a shell 85, upper and lower tube headers 8686 and a tube bank 81. The gas compressor 88 should be capable of operating at about 55 pounds discharge head. The lower tube header 86 communicates with the suction side of this compressor, which discharges through cooling coil 89 into the lower end of shell 85. It is desirable to provide this shell with staggered baffles 99-99. The coil is immersed in a water bath 9| whichis supplied with cold water through a pipe 92 and overflows heated water through Functioning of apparatus The above apparatus, when operated in the manner hereinafter described, functions as follows, it being understood that all temperatures are stated in degrees Kelvin (Centigrade degrees absolute) and all pressures in pounds above atmosphere.

Air which has previously been freed from carbon dioxide and dust is introduced into interchanger A through pipe I2, at atmospheric temperature and at such pressure as will cause the flow of air at the desired rate to and into high I pressure column K. The pressure within this column being somewhat under 60 pounds gauge, the pressure at which air is delivered into interchanger A will be column pressure plus pressure drop through intervening apparatus, which will vary with details of construction.

Coming into contact with the various tube banks in this interchanger, which are cooled by the final product gases in a manner to be described, the air is precooled to 200. The water is thus substantially removed from the entering air, a portion of this water being drained through pipes I4 as it accumulates. A portion of the water collects as frost in the portion of the shell which is below the freezing point, from which it is periodically removed by alternating and thawing.

Passing to interchanger B through pipe I3 the air flows over tube banks 33, 34, and 35 and is reduced in temperature to 96. It now passes to the bottom of column K through pipe 94, entering the column in gaseous form at 96 and column pressure. This pressure will be the vapor pressure of the nitrogen condensing in the upper portion of the column plus liquid head on the plates, or more or less 58 pounds gauge at the point of entry of the air flow.

The course of the air feed flow through the apparatus is indicated by the directional arrows I.

A stream of liquid nitrogen at 116 and at a pressure approximating 300 pounds, drawn from a source of supply not shown, is introduced through pipe 95 into nitrogen receiver 80. The quantity admitted is regulated by valve 98 and the evaporationconsequent to reduction of pressure to that of the column reduces the temperature of the nitrogen entering the receiver to 94. The condenser tubes 53 are maintained at a temperature of 92 by the crude oxygen bath, as will be described, and at this temperature and the pressure existing within the tubes, nitrogen is condensed and drains into the nitrogen receiver. A portion of the mixed condensate and nitrogen feed constantly overflows to reflux the fractionating plates below.

The supply of liquid nitrogen may be produced in any desired manner, but I find it highly economical and advantageous to utilize the apparatus and method described in my copending application, Serial No. 724,691, filed May 9, 1934 and entitled Method of producing low temperature refrigeration.

The temperature at the base of column K being 96 while the top of the column has a temperature determined by the boiling point of substantially pure nitrogen at flve atmospheres absolute, or about 94, the fractionating plates function in the well known manner to condense a crude oxygen (concentration 35% to 40%) which collects in a pool 91 in the bottom of the column.

While it is desirable to introduce the nitrogen supply into receiver as above described, it is also permissible to introduce it into nitrogen outlet pipe 98, as through pipe 95a and valve 96a. The advantage in introducing it into the tower as shown is that the vapor produced by reduction of pressure from 300 pounds to that of the column is recondensed and a quiet flow through pipe 98 is produced.

All of the nitrogen of the entering air save that retained in the crude oxygen, plus that introduced through pipe 95 as above, is ultimately collected as a substantially oxygen-free liquid in receiver 88, from which it passes through pipe 98 to and through the tubes of reboiler I where it is cooled against liquid oxygen to 92. From this unit is passes through pipe 99 to the tubes f interchanger D where it is cooled against gaseous final product nitrogen to 80, thence through pipe I to the tubes of vaporizer E where it is cooled against vaporizing nitrogen in the transfer circult to 79, thence through pipe Iill to a point in low pressure column J above the stack of fractionating plates. Valve I92 in pipe Illl maintains the pressure in the above system at that of the high pressure column, about 58 pounds. On passing this valve the pressure is reduced to that of the low pressure column, about 3 pounds. At this pressure and a temperature of 79 the nitrogen feed is retained substantially in liquid form until it reaches the fractionating plates in this column.

The above described flow of nitrogen through the apparatus is indicated in the drawings by directional arrows 2.

The crude oxygen collecting in pool 91 passes through pipe I03 to the tubes of reboiler H where it is cooled against liquid oxygen to 93, thence through pipe I94 to the tubes of vaporizer F where it is cooled against vaporizing nitrogen in the transfer circuit to 81, thence through pipe I95 to a medial point in low pressure column J. A pressure equivalent to that of the high pressure column is maintained in this system by valve I96 and on passing this valve the pressure is reduced to that of the low pressure column. At the temperature and pressure of introduction, 81 and 3 pounds, the crude oxygen enters the column as a liquid.

Attention is here directed to the importance of maintaining both the nitrogen and oxygen feeds to the column substantially entirely in liquid phase, for which due provision is made in the apparatus herein described.

It should be noted that to obtain the best results the crude oxygen feed to the low pressure column should be introduced at a point in its height where the constitution of the feed is substantially identical with that of the liquid on the entering plate.

by the directional arrows 3.

The temperature at the base of column J being" maintained at 91 /92, the boiling point of pure oxygen at 0.2 atmosphere pressure, and that of the top of the column being held at 78 by the reflux nitrogen admitted through valve I02, a close fractionation is effected on the plates and a pool of pure oxygen I01 collects at the base of the column. This pool is in liquid communication through pipes I08, I09, and III! with similar pools maintained around the tubes in reboilers I, H, and G, the slightly higher temperatures of the tube banks in these reboilers supplementing the reboiling effect of condenser tubes I3, the evolved gas returning to the column through pipes III, H2, and H3 in series.

' Pure oxygen is withdrawn from the column through pipe 6, passing thence to header 40 and tube bank 3d in interchanger B where it absorbs heat from the precooled air, thence through pipe I I5 to header 22 and tube bank I8 of interchanger A where it is brought to a temperature say 3 C. below atmospheric in precooling the warm air feed, being finally delivered through pipe H6 at substantially atmospheric pressure to a pure oxygen storage vessel, not shown.

The flow of pure oxygen through the apparatus is indicated by the directional arrows s.

Pure nitrogen in gaseous form leaves column J at 79 through pipe Ill andpasses into the shell of interchanger D, in which it is heated to in withdrawing heat from the liquid nitrogen fiowing from the high pressure to the low pressure column, thence through pipe H8 to header 2| and tube bank 35 in interchanger B where it is heated to 200' by interchange against precooled air, thence through pipe H9 to the tubes 45 of refrigerant evaporator C, where it is cooled to more or less by the evaporation of refrigerant around the tubes, and finally through pipe I26 to header 23 and tube bank IQ of interchanger A, where it is heated to say 3 below atmosphere by interchange against the warm air feed. The pure nitrogen is finally delivered at substantially atmospheric pressure through pipe I2I to a pure nitrogen storage vessel, not shown. The above described fiow follows directional arrows 5.

The transfer circuit is a closed cycle which may be charged with any suitable gas, assumed in the present instance to be nitrogen. Compressor 83 takes the gas at more or less atmospheric pressure and. temperature and delivers it at say 55 pounds pressure into the watercooled coil 89, where the heat of compression is removed. From the coil the gas passes through pipe I22 into the shell 85 of the transfer interchanger where it is cooled against returning cold gas and issues from the shell at 96. Passing now through pipe I23 it enters the tubes of reboile'r G where it is cooled and condensed by the pure oxygen bath to 92. From these tubes it passes through pipe I26 to the U- pipe I25 where the flow is divided'by valves I26 and I2? between the shells t9 and 5b of Vaporizers E and F. Valve I26 reduces the pressure in the shell of vaporizer E to substantially atmospheric and the temperature to 77, at which temperature a portion of the nitrogen is liquid and forms a bath around the tubes. Valve I21 delivers the liquid into the shell of vaporizer F against a back pressure of 6 pounds, which is maintained by regulation of outlet valve I28, the temperature of the liquid bath so formed being 81.

The gas vented from these shells is collected by U-pipe I29 and returned through pipe I33 to the tube bank 81 of the transfer interchanger.

The returned gas enters this tube bank at a tern perature between 77 and 81 (depending on the relative regulation of valves I26, I21, and I28), absorbs heat from the warm gas delivered from the compressor through the water cooled coil,

and passes through pipe I30 to the suction of the compressor at approximately atmospheric temperature.

This completes the closed transfer cycle, the flows through which are indicated on the drawings by directional arrows 6.

The refrigerant evaporator C is suppliedwith a suitable liquefied gas, as for example ethylene, through pipe I3I, at a temperature (in the case of ethylene) of 170, from any source of supply not shown. The quantity admitted is so controlled by expansion valve I32 that evaporation of the liquid refrigerant will reduce the temperature of the pure nitrogen passing through the tubes 65 of this evaporator to more or less The gaseous refrigerant then passes through pipe I33 to tube bank l6 of interchanger A, where it assists in precooling the warm air supply, and is finally delivered through pipe I34 to be returned to the liquefying means not shown. The flow through the refrigerant circuit, in so far as this cycle is shown in the drawings, is indicated by directional arrows 7.

While the use of ethylene in this circuit is preferable, it is permissible and may in some cases be desirable to utilize other liquefied gases, as for example liquid methane or nitrogen drawn from the source which supplies the high pressure column.

In the modification illustrated in Fig. 2 the stream of air leaving dehydrating interchanger A is cooled by the evaporation of a liquid refrigerant to control the temperature of the air column J that its open lower end is at the level where the greatest concentration of argon occurs in the vapor. The upper end 82 of shell 8i being surrounded by an atmosphere of lower temperature than the condensation point of oxygen, the bubbling plates within the argon column are constantly refluxed with a condensate which, as it fiows downwardly from plate to plate, becomes constantly richer in oxygen. this condensate finally being returned to the plates of column J.

The gas escaping from the top of column L consists of argon in a commercially useful admixture with other atmospheric gases. This mixture passes through pipe I35 to header 39 and tube bank 33 of interchanger B, where its heat absorbing capacity is utilized in further cooling the precooled air supply. From this tube bank it passes through pipe I36 to header 20 and tube bank ll of interchanger A, where it assists in precooling the warm air supply, finally issuing from pipe I37 at substantially atmospheric temperature as the argon product. The course of this flow is indicated by the directional arrows 3.

Elements of control As substantially all of the refrigeration required to offset. leakage of heat into the apparatus is produced by the introduction of liquefied and precooled nitrogen into the high pressure column, the capacity of the apparatus is limited solely by the fractionating capacity of the two columns and by the area of interchanging surface, both being fixed in the original design. Below this predetermined limit the throughput of the apparatus may be varied at will by balancing the discharge of the air supply pump against the quantity of liquid nitrogen admitted through valve 96 into the high pressure tower.

The additional refrigerating effects produced by the passage of a refrigerant through evaporator C and by the expansion of nitrogen in the transfer circuit are negligible in point of quantity and are designed solely to afford controls through which optimum operating conditions may be maintained.

It will be obvious that to maintain constant conditions throughout the apparatus, not only the volume of air entering the high pressure column but also its temperature must be held level, and that the attempt to compensate variations in the temperature of the warm air feed by regulation of the liquid nitrogen supply would require simultaneous readjustment of all the controls.

The introduction into the system of the refrigerant evaporator provides a ready and simple means for compensating variations in temperamm or moisture of the warm air feed, a single manual or temperature-responsive valve controlling the amount of refrigerant evaporated in heat exchange relation with the stream of pure nitrogen prior to'its entry to the primary air interchanger and thus directly controlling the heat absorbing capacity of this interchanger. This completely independent control permits the regulation of the temperature of the air stream in the precooling'stage and without disturbance to the remaining adjustments.

This regulation is preferably applied to the pure nitrogen stream because of its materially greater volume and heat carrying capacity as compared to other of the final products, but by providing for a greater temperature range in the control it could be as well applied to the final product oxygen, or less desirably it might be applied to the air supply itself.

A material advantage attendant on the introduction of a minor and closely controllable'refrigerating effect into the system at the upper end of the primary interchanger is the retention of all material formation of ice within this interchanger, which is designed for the removal of ice whenever it accumulates to such point as to be objectionable. If the temperature at the upper or air-outlet end of the primary interchanger is allowed to fluctuate materially, the zone of material ice formation is at times liable to advance into the secondary interchanger, from which frost cannot be removed without shutting down the entire apparatus.

The purpose of a controlled refrigerating effect, in whatever manner applied, not only compensates changes in the temperature of .the entering air due to seasonal changes in the temperature of cooling water used to remove the heat of compression, but also provides controllable heat absorbing capacity for condensing and freezing a variable moisture content which is also seasonal.

The transfer circuit has three distinct functions. First, it adds a material amount of heat to the pure oxygen pool in reboiler G and thus augments the reboiling action of condenser tubes 13. This reboiling effect may be quantitatively varied by controlling the combined openings of valves I26, I21 and I28, thus regulating the amount of the warmer gas passing through the coil of reboiler G.

Second, it affords a means for controlling the heat gradient within the low pressure column by varying the relative temperatures of the liquid nitrogen and the crude oxygen passing toward expansion valves I02 and I06 respectively. This temperature variation is produced by diverting a portion of the transfer fluid from one to the other of vaporizers E and F, by which diversion the cooling effect applied to one of the streams is increased at the expense of the other.

The third and perhaps the most important function of the transfer circuit is to effect a supercooling of the nitrogen reflux, in its passage through interchanger E, by which it is maintained in a wholly liquid condition after it passes through valve I02 into the lower pressure zone within column J. By preventing ebullition of the reflux at the point of pressure release, spattering and entrainment of the liquid inthe outflowing pure nitrogen stream are avoided, the operation of the column is steadied and its capacity materially increased, and the purity of the final product nitrogen is greatly enhanced. The same effect on the crude oxygen feed is produced in the same-manner in interchanger F.

Reservations It should be noted that both the apparatus and the methods above described may be varied inseveral particulars without departing from the spirit of the invention.

The interchangers, for example, are shown in a vertical position and reference is made to the top and the bottom of these elements in describing the various flows through them. It will be understood that the descriptions are so framed for convenience only, and that with due regard to feasible construction these elements may be placed either vertically or horizontally and that, with the possible exception of the primary interchanger A, the flow directions may be reversed so long as counterflow is maintained. This reservation, however, will not apply to reboilers or vaporizers in which a pool of liquid is maintained.

In the drawings the oxygen reboilers are shown connected serially. These elements have no interdependent function and each may be independently attached to the column by a liquid pipe and a vapor pipe, at substantially the level indlcated.

The argon column L may be omitted if it is not desired to remove the argon, without any fur ther change in the apparatus than the omission of the corresponding headers and tube banks from interchangers A and B.

The temperatures and pressures disclosed are intended to be illustrative, though they are an accurate disclosure of desirable conditions for the simultaneous production of pure nitrogen and pure oxygen in the apparatus shown. If either or both of the main products should be desired in a less pure state, the apparatus is fully capable of regulation to that end, usually with some increase in its throughput capacity, with a corresponding variation from thedescribed temperatures and pressures.

The method of fractionating shown may be used for the separation of any mixture of liquefled gases in which the boiling point of the higher boiling constituent at the pressure carried on the low pressure column (which may be either above or below atmospheric) lies below the critical temperature of the lower boiling. An example is' the fractionation of an argon-nitrogen mixture, in which a substantially complete separation may be made in the described apparatus but by the use of other temperatures.

Advantages claimed The ease and completeness with which the apparatus may be regulated and the most desirable operating conditions maintained has already been pointed out.

By reason of the application of the main refrigerating effect in the form of a liquefied and supercooled gas (nitrogen) of very low boiling point, it is possible to utilize a highly economical method of producing this refrigeration, such as that described in my copending application Serial No. 724,691, thus greatly reducing the power consumption per unit of product. An actual apparatus on a large working scale is rectifying air to substantially pure oxygen and nitrogen with a power consumption of 175 H. P. per 1,000 cubic feet of free air per minute, whereas the best previous practice of which I am aware requires 289 H. P. forthe same amount of air and produces a decidedly less closefractionation.

By reason of the introduction of the main reirigerant supply directly into the high pressure column, the large amount of interchange surface required for the initial condensation of the air is avoided and the apparatus is greatly reduced in first cost. For the same reason, the time required for starting the apparatus from room temperature is much reduced as, assuming a suificient supply of the refrigerant to be available, the reduction in cooling down time is practically only limited by the ability of the apparatus to withstand sudden cooling without damage.

The most important advantage of the apparatus and method over'the disclosures of the prior art lies in the simultaneous production of the two main products, oxygen and nitrogen, in a state of purity. It has heretofore been possible to obtain pure nitrogen and impure oxygen, or vice versa, but to the best of my knowledge no method or apparatus shown in the prior art has simultaneously yielded both of these products in a commercially pure state. The nitrogen yield of the above apparatus, in actual operation under the described conditions, has a purity of 99.8% to 100% whentested by the most approved methods, while the oxygen yield shows a purity of 99.8%, the impurity being a trace of argon. The argon yield is in the neighborhood of 70% of the quantity existing in the air supplied, and the purity is within commercial requirements, the principal contaminating body being oxygen.

The term binary mixture used in some of the attached claims is intended to include mixtures containing two major constituents which it is desired to separate, together with relatively small quantities of various other gases which in the fractionation steps pass into one or the other of the major constituents. Thus air is a binary mixture in the spirit of the claims which call only for the separation of oxygen from nitrogen.

I claim as my invention:

1. In' anair fractionating process including the separation of substantially pure nitrogen from crude oxygen in a first fractionating zone maintained at relatively high pressure and the refractionation of said products in a second fractionating zone maintained at a relatively low pressure, the steps comprising: maintaining a third fractionating zone in direct heat exchange relationship with-said second zone, said third zone communicating with said second zone at only the lower end of said third zone; withdrawing from a medial position in said second zone a stream of mixed gases including oxygen and argon; cooling said third zone by direct heat transfer to said secand zone to produce a condensate containing argon and oxygen; subjecting said condensate to repeated fractionation whereby gaseous argon is separated from said condensate; withdrawing. gaseous argon from the upper part of said third zone, and returning to said second zone a condensate containing less argon than the gas withdrawn into said third zone.

2. The method of operating a multiplate frac tionating column for the separation of the lowerboiling constituent from a mixture of gases, which comprises: introducing said mixture, in substantially wholly gaseous form and at a temperature approximating its point of incipient liquefaction, into said column at a point below the plates therein; maintaining the upper end of said column at the condensation temperature of the pure lower-boiling constituent and thereby producing a condensate; refluxing a portion of said con densate over said plates to effect condensation of said mixture; subjecting the mixed condensates to repeated fractionations on said plates whereby a condensate rich in the higher-boiling constituent is collected in the lower end of said column and the pure lower-boiling constituent passes in gaseous form into the upper end of said column; withdrawing last said condensate in liquid form from the lower end of said column, and condensing, collecting and withdrawing pure lower-boiling constituent from the upper part of said column.

3. In the operation of a two stage air fractionating column in which the stages operate at different pressures, the step of introducing the air feed in substantially wholly gaseous form below the plates in the higher pressure stage in said column.

4. The method of controlling cold-end temperature in the dehydration step of a mixed-gas separating operation which comprises: passing a stream of said mixed gas successively through a zone in which the temperature of said gas is reduced to a point below the freezing point of water and through a zone in which the temperature of said stream is still further reduced; returning a stream of a separated gas from said separating operation into heat exchange relation with said mixed gas stream successively in last said zone and in first said zone, and evaporating a controlled supply of a liquid refrigerant in heat ex change relation with one of said streams at a point intermediate said zones.

5. A method substantially as and for the purpose set forth in claim 4, in whichthe mixed gas is air and the refrigerant is liquefied ethylene.

6. A method substantially as and for the purpose set forth in claim 4, in which the refrigerant is applied to the mixed gas stream.

7. A method substantially as and for the purpose set forth in claim 4, in which the refrigerant is applied to the returning gas stream.

8. In an air fractionating operation involving the collection of an oxygen-rich condensate in a streams of condensate to difierent temperatures and to such temperatures as to maintain each said stream substantially wholly liquid as it passes into said low pressure zone.

10. A step substantially as and for the purpose described in claim 9, in which'the refrigeration required for supercocling said stream is supplied by a liquid refrigerant introduced into said fractionating. operation from a source extraneous to said fractionating operation.

11. In a mixed-gas fractionating operation involving the feeding of a stream of condensate maintained at relatively high pressure into a fractionating zone maintained at relatively low pressure with attendant release of the pressure on said stream, the steps comprising: supercooiing said stream of condensate by heat exchange against boiling liquid nitrogen, and so controlling the pressure on said boiling nitrogen as to reduce said stream to a temperature at which its pressure may be reduced to that of said low pressure zone without material ebullition of said stream at its point of entry into said low pressure zone.

multiple fractionating column, the steps comprising: introducing said air in gaseous form into the lower end of said column and introducing a stream of liquid nitrogen into the upper end of said column to act as refiux therein, the nitrogen so introduced being additional to any nitrogen produced by the fractionation of said air and being at least sufficient in quantity to compensate infiltration of heat into said column.

13."Ihe method of separating the constituents of a binary mixture of gases, which comprises: introducing said mixture in precooled gaseous form into a first fractionating zone maintained at a relatively high pressure; separately condensing and collecting in said first zone a crude condensate rich in the higher-boiling constituent of said mixture and a pure condensateconsisting of the lower-boiling constituent in substantially pure form; introducing said crude condensate into a second fractionating zone maintained at a relatively low pressure; transferring heat from said first zone to said second zone to produce condensation in said first zone; introducing said pure condensate into the upper part of said second zone to act as reflux therein and simultaneously introducing from an extraneous source into the upper part of said second zone a further quantity of a liquid substantially identical with said pure condensate, said further quantity being at least sufiicient to compensate infiltration of heat into both said zones.

14. A method substantially as and for the purpose set forth in claim 13, in which the further quantity of liquid is intermixed with said pure 12. In the continuous fractionation of air in a condensate inlsaid first zone and travels with said condensate into 'said second zone.

15. A method substantially as and for the purpose set forth in claim 13, in which the further quantity of liquid is intermixed with said pure condensate at a point between said first zone and said second zone and travels with said condensate into said second zone.

16. A method substantially as and for the purpose set forth in claim 13, in which the further quantity of liquid andsaid pure condensate are separately and simultaneously introduced into the upper part of said second zone.

17. In a method of fractionating air involving the production of an oxygen-rich condensate and liquid nitrogen in a first fractionating zone maintained at a relatively high pressure and the production of substantially pure gaseous nitrogen and gaseous oxygen in a second fractionating zone maintained at a relatively low pressure; a closed heat transfer cycle comprising: compressing a stream of gaseous nitrogen and removing the heat of compression; cooling said gaseous nitrogen by heat exchange against returning gaseous nitrogen within said cycle; liquefying said gaseous nitrogen by heat exchange against pure liquid oxygen condensing in said second zone, whereby said pure oxygen is reboiled; evaporating said liquid nitrogen under a pressure less than the compression pressure by heat exchange against a liquid moving from said first zone to said second zone, whereby said liquid is cooled, and returning the evaporated nitrogen to be again compressed.

18. In a method of fractionating a mixture of gases involving the production of condensates of different boiling points in a first fractionating zone maintained at relatively high pressure, the separate transfer of said condensates to a second fractionating zone maintained at relatively low pressure and the production in said second zone of a condensate consisting of only the higherboiiing component of said mixture, the heattransfer steps comprising: passing a compressed and cooled gaseous refrigerant in heat exchange relation with last said condensate, whereby said gaseous refrigerant is liquefied and last said condensate is reboiled; reducing the pressure on said liquefied refrigerant and passing said liquefied refrigerant in heat exchange relation to first said condensates, whereby first said condensates are cooled and said liquefied refrigerant is returned to the gaseous state.

19. In the continuous fractionation of air in a multiplate fractionating column, the step of applying a refrigerating effect to said column by the evaporation of liquid nitrogen additional to any nitrogen produced by the fractionation of said air. a

20. In the continuous fractionation of a mixed gas in a multiplate fractionating column, the step of applying a refrigerating effect to said column by the evaporation of a liquid refrigerant of substantially the same composition as the lowest boiling constituent of said mixture, said refrigerant being additional to any quantity of said constituent produced by said fractionation.

21. In a method of fractionating a mixture of gases involving the production of condensates of different boiling points in a first fractionating zone maintained at relatively high pressure, the separate transfer of said condensates to a second fractionating zone maintained at relatively low pressure and the production in said second zone of a condensate consisting of only the higherboiling component of said mixture, the heattransfer steps comprising: passing a compressed and cooled gaseous refrigerant in heat exchange relation with last said condensate, whereby said gaseous refrigerant is liquefied and last said condensate is'reboiled; reducing the pressure on said liquefied refrigerant and passing said liquefied refrigerant in heat exchange relation to one of first said condensates; whereby last said condensate is cooled and said liquefied refrigerant is returned to the gaseous state.

22. In a mixed-gas fractionating operation involving the feeding of a stream of condensate maintained at relatively high pressure into a fractionating zone maintained at relatively low pressure with attendant release of the pressure on said stream, the steps comprising: supercooling said stream of condensate by heat exchange against a boiling liquid refrigerant, and so controlling the pressure on said boiling refrigerant as to reduce said stream to a temperature at which its pressure may be reduced to that of said low pressure zone without material ebulliti'on of said stream at its point of entry into said low pressure zone.

23. Ina mixed-gas fractionating operation involving the feeding of a stream of condensate maintained at relatively highpressure into a fractionating zone maintained at relatively low pressure with attendant release of the pressure on said stream, the step of supercooling said stream of condensate to a temperature at which its pressure may be reduced to that of said low pressure zone without material ebullition of said stream at its point of entry into said low pressure zone.

24. The method of controlling cold-end temperature in the dehydration step of a mixed-gas separating operation which comprises: passing a stream of said mixed gas successively through a zone in which the temperature of said'gas is reduced to a point below the freezing point of water and through a zone in which the temperature of said stream is still further reduced; returning a stream of a separated gas from said separating operation into heat interchange relation with said mixed-gas stream successively in last said zone and in first said zone, and applying a cooling step to one of said streams at a point intermediate said zones.

LEE S. "I'WOMEY.

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