US2062537A - Method of separating gases by freezing - Google Patents

Description

Dec. 1, 1 936. s. TWOMEY 2,062,537

METHOD OF SEPARATING' GASES BY FREEZING Filed May 'Q, 1934 2 Sheets-Sheet l nflu FIGJ LE E TWOMEY IN VENTOR Dec. 1, 1936.

1.. s. TWOMEY 2,062,537

METHOD OF SEPARATING GASES BY FREEZING Filed May 9, 1934 2 Sheets-Sheet 2 MEY LEE 5. T\/N pheric pressure.

Patented Dec. 1, 1936 UNITED STATES PATENT OFFICE DIETHOD OF SEPARATING GASES BY FREEZING Lee S. Twomey, Vista, Calif.

Application May 9, 1934, Serial No. 724,697

, 29 Claims.

An object of my invention is to separate gases having a relatively low condensing temperature from other gases which can be caused to freeze at higher temperatures.

A further object of my invention is to provide a means for cooling, liquefying, and/ or solidifying gases, in which the required refrigeration is applied in steps, at successively lowered temperatures.

The structure and operations comprising the invention may best be seen with reference to the attached drawings, in which Figs. 1 and 2, when placed together end to end on the line A-A, illustrate in a highly diagrammatic'form, one modification of the invention, and

Figs. 1 and 3, when joined in the same manner, similarly illustrate another modification thereof.

Referring to Figs. 1 and 2 of the drawings, the refrigeration system consists of a succession of compressors III, II, II, and I3, handling anhydrous ammonia, ethylene, methane, and nitrogen respectively and delivering these compressed gases into coolers ll, I5, l6, and I! which are provided with separate supplies of cold water. These coolers are shown as tubular units but any preferred means for bringing compressed and heated gases back to atmospheric temperature may be used.

Condensers for ethylene, methane, and nitrogen respectively are indicated at I8, I8, and 20, and receivers for liquid ammonia, ethylene, methane, and nitrogen respectively at 2 I, 22, 23, and 24.

Anhydrous ammonia under a pressure approximating atm.- absolute is cooled to atmospheric temperature in water cooler I4 and is there liquefied, the liquid ammonia passing through pipe 25 into receiver 2| at substantially atmospheric temperature. From this receiver a stream of liquid ammonia passes through pipe 26 and expansion valve 21 to the shell of condenser I8, within which it is permitted to evaporate and to expand to substantially atmos- The expanded ammonia gas returns through pipe 28 to the suction of compressor I0, thus completing a closed ammonia cycle.

Gaseous ethylene under a pressure aproximating 22 atm. absolute is cooled to atmospheric temperature in water cooler I5, the cooled gas passing through pipe 29 into condenser I8 in which it is liquefied by the evaporation of liquid ammonia. The liquefied gas passes through pipe 30 into receiver 22 in which it collects at the temperature of evaporating ammonia. From this receiver a stream of liquid ethylene passes through pipe 3| and expansion valve 32 to the shell of condenser I9, within whichit is permitted to evaporate and to expand to substantially atmospheric pressure. The expanded ethylene gas returns through pipe 33 to the suction of compressor I I, thus completing a closed ethylene cycle.

Gaseous methane under a pressure approximating 28 atm. absolute is cooled to atmospheric temperature in water cooler I6, the cooled gas passing through pipe 34 into condenser I9 in which is is liquefied by the evaporation of liquid ethylene. 35 into receiver 23 in which it collects at the temperature of evaporating ethylene. From this receiver a stream of liquid methane passes through pipe 36 and expansion valve 31 into the shell of condenser 20, within which it is permitted to evaporate and to expand to substantially atmospheric pressure. The expanded methane gas returns through pipe 38 to the suction of compressor I2, thus completing a closed methane cycle.

The liquefied gas passes through pipe Gaseous nitrogen under a pressure approximating 25 atm. absolute is cooled to atmospheric temperature in water cooler H, the cooled gas passing through pipe 39 into condenser 20 in which it is liquefied by the evaporation of liquid methane.

The liquefied gas passes through pipe 40 into receiver 24 in which it collects at the temperature of evaporating methane and is stored for use in operations which are to be described.

The above apparatus and method are described and claimed in my copending application Serial No. 724,691, filed May 9, 1934, under the title Method of producing low temperature refrigeration. The apparatus and method produce continuing supplies of four refrigerants having successively lower boiling points and the present application is concerned, not with the manner in which these refrigerants are produced, but with a specific manner in which they are used.

The gas which is to be subjected to the freezing treatment herein described may be any mixed 4 gas containing one or more constituents which pass into the solid state at a temperature producible with the refrigeration steps described, together with one'or more constituents which remain in the gaseous form at the temperature required for the solidification of the first named.

As an example, and without in anywise limiting the invention thereto, it will be described in connection with the purification of hydrogen by the a removal therefrom of nitrogen, carbon monoxide and such other gases of higher condensing temperature as may occur in the original gaseous mixture.

Such original gas may, for example, be the product of water gas manufacture and to avoid irrelevant description it will be assumed that the carbon dioxide present in most water gas has been scrubbed out or otherwise removed by any of the well known means. Such gas may have a composition approximating the following:

Per cent Hydrogen 49 Carbon monoxide 46 Methane 1 Nitrogen 4 discharge. As this pressure drop will vary widely with details of construction, the actual discharge pressure cannot be specified.

The compressed gas passes into any cooler 43 adapted to remove the heat of compression, the

drawings showing a tubular cooler supplied with cold water. From this cooler the gas passes through pipe 44 to the warm end of dehydrating interchanger 45. This interchanger, which as illustrated is of the well known horizontal type, is provided with bailles 46 for accelerating the flow of gas over the tubes and with drain valves 41 for withdrawing liquid water which may collect in the pockets formed by the baffles. It is further provided with various bundles or concentric rows of tubes having individual headers, through which various cold gases resulting from later stages of operation are passed without interniixing. The cold end of this interchanger is maintained at such temperature that substantially all of the water originally present in the gas is removed, in part as a liquid, in part as frost on the tubes, and in order to remove this frost and free the interchanger from ice, it is necessary either to provide this unit in duplicate, with appropriate diversion pipes and valves, or to warm it up at intervals to melt out the ice.

The water-free gas passes through a succession of connecting pipes numbered 48 and through the shells of a succession of interchangers 5|, 52, 53, 54, and 55, the gas entering the upper end and leaving the lower end of each shell. Each interchanger is provided with bailles as just described and with a plurality of banks or groups of tubes through which cold gases and other cooling media are separately passed in the reverse direction, that is, from bottom to top; so that the lower end of each interchanger is the cold end. At the lower end each'interchanger is provided with a trap or separator 49 from which any condensate may be withdrawn through a drain pipe 50 while the uncondensed gas passes forward through pipe 48 to the next interchanger.

From receiver 2| a pipe 60 conducts a stream of liquid ammonia to an expansion valve 6| by which it is admitted to one of the tube groups of interchanger 5|. In'these tubes it is permitted to evaporate and to expand to atmospheric pressure, the expanded gas passing then through various pipe sections numbered 62 to the cor responding tube group of interchanger 45 and thence to the suction of ammonia compressor II at substantially atmospheric temperature and pressure From receiver 22 a pipe 63 conducts a stream of liquid ethylene to an expansion valve 44 by which it is admitted to one of the groups of tubes of interchanger 52. In these tubes it is permitted to evaporate and to expand to atmospheric pressure, the expanded gas passing through various pipe sections numbered 65 to tube groups 0! interchangers 5| and 45 and thence to the suction of ethylene compressor II at substantially atmospheric temperature and pressure.

From receiver 23 a pipe 66 conducts a stream of liquid methane to an expansion valve 61 by which it is admitted to one of the tube groups of interchanger 53. In these tubes it is permitted to evaporate and to expand to atmospheric pressure, the expanded gas passing through various pipe sections numbered 68 to tube groups of interchangers 52, 5|, and 45 and thence to the suction of methane compressor l2 at substantially atmospheric temperature and pressure.

It is possible for a condensate of methane to be produced in interchanger 54 and if this condensate is of suilicient purity to introduce into the methane cycle, it is withdrawn from trap 46 of interchanger 54 through pipe 69 and expansion valve Ill and introduced into the methane tube group of interchanger 53, from which the expanded gas returns to methane compressor l2 through pipes 66 and interchangers 52, 5|, and 45. If not of suillcient purity for this purpose, or if the quantity is in excess of makeup in the methane cycle, it may be withdrawn through the drain valve 50 as a liquid or an expanded gas.

A mixture of carbon monoxide and nitrogen which may condense to liquid form in interchanger 55 is withdrawn from trap 49 through pipe II and expansion valve I2 to a tube group of interchanger 54. In these tubes the liquid is permitted to evaporate and to expand to atmospheric pressure, the expanded gas passing through various pipe sections numbered 12 to interchang- -ers 53, 52, 5|, and 45 and leaving the system through pipe 14 at substantially atmospheric temperature and pressure. If desired. the liquid may be withdrawn as such through valve Ila.

From receiver 24 a pipe 15 conductsa stream of liquid nitrogen to an expansion valve 16 by which it is admitted to one of the tube groups of interchanger. In these tubes it is permitted to evaporate and to expand to a pressure which will vary with the degree to which the hydrogen is to be purified and with other variables to which reference will be made. The expanded gas passes through various pipe sections numbered 11 to interchangers 54, 53, 52, 5|, and 45 thence to the suction of nitrogen compressor l3 at substantially atmospheric temperature.

The gas which has passed through the shell and around the tubes of sections 5| to 55 has already been cooled to a very low temperature and has been deprived of all but a small residue of carbon monoxide and nitrogen. It next passes to an interchanger 56 which, preferably, is horizontally disposed as shown.

In this interchanger the gas passes through the tubes instead of through the shell as in the preceding units 5|-55 and as the cross-sectional area of the gas passage is thereby much reduced, the gas passes through these tubes at a high velocity. In these tubes the gas isfurther cooled, to a temperature below the freezing point of carbon monoxide, which is precipitated in the form of snow and in very large part carried out of the tubes by the rapidly moving gas and deposited in the next unit.

The tubes of interchanger 56 direct the snowladen gas into an extension 18 of the interchanger shell, this extension tangentially entering the side wall of a small cyclone separator 19, which is of conventional construction except in that it is proportioned for'the working pressure applied to the gas, as for example 20 atm. In this separator snow particles are separated by centrifugal force in the well known manner and collect in a mass 86 which may be removed through the vent valve 8| as will be described.

From the top of separator 19 a pipe 82 conducts the gas stream under pressure to an expansion valve 83 by which it is admitted to a second cyclone separator 84 which is maintained at substantially atmospheric pressure. By this expansion the temperature of the gas is further reduced and a second crop of solidified particles of nitrogen and carbon monoxide obtained. These solids collect in a mass 85 which maybe removed through the vent valve 86 as will be described.

'From the top of separator 84 the gas stream, which now consists of highly purified hydrogen, passes through a series of pipes numbered 81, first to the shell of interchanger 56, then to tube groups of interchangers 55, 54, 53, 52, and 45 and thence out of the system through pipe 88 at substantially atmospheric temperature and pressure.

The temperatures available in the various stages of the above process are, with one exception, determined by the boiling temperatures of the refrigerant liquids used at the pressures to which they are expanded, with an allowance for the temperature head required to make an interchanger operative.

Assuming that the ammonia, ethylene, and methane compressors are operated at exactly 1 atm. suction pressure, the boiling temperatures for the three liquids are: for ammonia, 240 K., for ethylene, 168 K., and for methane, 112 K.

- Allowing 5 C. heat head for interchange, the

minimum temperature to which the compressed gas may be brought by the ammonia cooled tubes in interchanger 5i is 245 K., by the ethylene cooled tubes in interchanger 52, 173 K., and by the methane cooled tubes in interchanger 53, 117 K.

The temperature of the gas issuing from interchanger 54 will vary with the composition and quantity of the liquid condensed in interchanger 55. In the specific case cited in which the gas originally contained 46% carbon monoxide and 4% nitrogen, the condensate will consist in round figures of 3 parts nitrogen to 45 parts carbon monoxide and will have a boiling temperature would be considered good practice in the other units, it is possible to reduce the heat head to as little as 2 C. Making this allowance, the temperature available in interchanger 55 would be lowered to 66 K. (the freezing point of carbon monoxide) at an expansion pressure of 0.15 atm. absolute. As no freezing should take place in this section, which should liquefy as much as possible of the carbon monoxide and nitrogen contents, the temperature should not be carried lower at this point but should closely approach the freezing point.

At 20 atm. pressure on the gas being treated, the proportions of these two gases in. the partially purified gas passing out of interchanger 55 will be as their partial pressures at say 66 K. to the applied pressure. The vapor pressure of carbon monoxide at this temperature is very close The heat evolved in condensing and freezing these residualquantities of carbon monoxide and nitrogen must be absorbed by the hydrogen expanded into the second separator 84. By reason of the J oule-Thompson effect this expanded gas passes into the shell of interchanger 56' at a temperature materially below that of the compressed gas in separator 19, and as is well known, the magnitude of the cooling effect thus produced increases as the temperature from which the gas is'expanded is decreased and also as the pressure prior to expansion is increased.

The Joule-Thompson effect is the sole cooling effect after the-temperature of the compressed gas has been fixed at a desired point, by external refrigeration, at the cold end of interchanger 55. This effect must at least balance the input of heat from all sources. The sources of heat input are: latent heat of condensation and congelation and sensible heat of the impurities; heat infiltration to the two separators; the excess heat capacity of the compressed gas in the tubes of interchanger 56 over that of the same gas after expansion, and imperfection of interchange in unit 56.

If the Joule-Thompson effect as confined to the functioning of the unit comprising the two separators and interchanger 56 be insuflicient to compensate this total heat introduction, the temperature will tend to progressively rise and the unit will fail to function. such case is to raise the applied pressure, by which the Joule-Thompson efi'ect is increased. If on the other hand, the J oule-Thompson effect is in excess, the result of the interchange in unit 56 is to progressively lower the temperature of the gas passing into separator 19 and to expansion valve 83, and this progressive reduction may go so far that the hydrogen itself will liquefy or even freeze. If this condition should be too nearly approached, the temperature may be .raised by decreasing the applied pressure or by diminishing the refrigeration of preceding interchangers to such an extent as to materially raise the temperature of the gas leaving unit 55.

On the other hand, increased purity is produced by increasing the applied pressure, and with reasonable care in insulating the cold end of the apparatus it is possible to reach a tempera- The remedy in ture K.) at which neglible traces (of the order of 0.1% nitrogen and 0.05% carbon monoxide) may be obtained at 20 atm. applied pressure and correspondingly smaller quantities at higher presures.

The frozen gases accumulate slowly in separators l9 and 04 but in time will require removal. This may be accomplished by temporarily withdrawing the supplies of liquid refrigerants and opening the vent indicated at I! in the drawings and fully opening expansion valvev ll, thlll removing the applied pressure. when the gas temperature rises to 66 K., the snow collected in the separators will melt and may be withdrawn through drains 0| and I, or at 82 the liquid resulting from this melting will be rapidly evaporated and vented through valve ll.

It will be evident that, as the returned product gases are interchanged against a substantially equa weight of entering gas. the amount of heat liberated in cooling the gas is substantially equal to that absorbed in heating the products back to exit temperature. It is not exactly equal becauseheat is liberated in condensing and partially freezing water, and because the heat content of the compressed entering gas is less than that of the discharged products at atmospheric pressure, and because a small proportion of the original gas has been frozen and discarded from the system.

Overlooking these differences, the cycle, once established, would be self-sustaining and no re frigeration would be required were it not for leakage of heat into the apparatus; Even with the most effective insulation such leakage cannot be avoided, and at the very low temperatures attained in this process the provision of refriger-,

ation to offset this heat leakage calls for a large expenditure of power, this being in fact the major element of cost attending the operation.

The cost of refrigeration increases rapidly as the temperature is lowered. Referring to a quantity of each of the refrigerants which, when expanded to atmospheric temperature and pressure, is equivalent to one ton of ice melting capacity per 24 hours, the theoretical horsepower consumption for adiabatic compression of the gaseous refrherant to the stated pressures is as follows: for ammonia, 1.36 H. P.; for ethylene, 3.0 H. P.; for methane, 4.40 H. P.; and for nitrogen, 4.'70 H. P.

It will be evident that these power consumptions are cumulative, inasmuch as to condense a ton-equivalent of nitrogen requires the expansion of a ton-equivalent of ethylene, to condense which has required the expansion of a ton-equivalent of ammonia. The accumulated figures for a ton equivalent are therefore: for ammonia, 1.36

- H. P.; for ethylene, 4.36 H. P.; for methane, 8.76

H. P.; and for nitrogen 13.46 H. P.

It will be understood that these figures are relative only as on the one hand they overlook the factor of efficiency in compressing machinery and, on the other, they are based on single stage adiabatic compression.

, The described input of heat occursin all parts -:'of the apparatus and, if interchange of returning products against entering gas be the sole cooling means prior to the introduction of nitrogen to interchanger, the total external refrigerative eii'ect (equivalent to heat input from all sources less the availablecooling effect in the returning gas) must be supplied by the addition of nitrogen, the most expensive refrigerating liquid.

It-is therefore a source of material power economy to introduce into each stage of the refrigeration such supply of a liquid refrigerant as will reduce the temperaturein that stage to a minimum. For example, to supply external refrigeration to interchangers BI and 45 by expanding ammonia in interchanger il in such quantity and tosuchpressureastoreducethetemperaturecf thegastointerchangeri2to245'Kora lower temperature, and in similar manner to reduce the cold ends of interchanger: l2 and II to 168 and 112 (or lower) by expanding suilicient quantities of ethylene and methane.

Because of the difference in specific heats between compressed and expanded gases, the heat capacity of the compressed gas between any selected limits of temperature (as for example, the temperatures shown by the compressed gas as it enters and as it leaves interchanger BI) is greater than the heat capacity of the expanded products through the same temperature range. For this reason and others above stated, it is possible to introduce into interchanger ii a quantity of extraneous ammonia refrigeration sufiicient to remove this excess heat and to prevent it from being carried forward into the next interchanger, bringing the temperature of the compressed gas leaving interchanger SI to substantially that of the expanded gas entering it. In like manner external ethylene refrigeration may be applied in interchanger 52 and methane refrigeration in interchanger 53, to produce exit temperaturesof the compressed gas substantially equal to the entering temperatures of the expanded products.

By virtue of these steps. which at once compensate heat input to each individual cooling stage and also take advantage of the greater heat capacity of the compressed gas in each stage as it is reached, the greatest possible proportion of the total refrigeration required is applied in the first stage and with the cheapest refrigerant, with a corresponding reduction in the amount of refrigeration required in the next stage, and this step being applied in each stage, the effect is cumulative not only as regards the intermediate refrigerants but also as regards the quantity of nitrogen refrigerant required to bring the com-' pressed gas to the predetermined temperature at which it must leave interchanger 55.

This introduction of refrigerants of graduated boiling temperatures into the various interchange stages has not only the advantage of economy, tending to move refrigerative efiect out of zones of low temperature and costly refrigeration into zones of higher temperature and cheaper refrigeration, but it has also the advantage of enabling the compressed gas exit temperature of each interchanger unit to be positively controlled. This control is sometimes most desirable, as for the removal of intermediate constituents such as methane.

While the above description refers to the fractionation of a specific gas mixture, the process maybeusedinanycaseinwhichthefreezing point of one of the constituents of a gaseous mix ture is materially above the condensing temperature of other constituents at the pressure applied.

The apparatus by which the final expansion andcoolingof the gas is accomplishedmaybe modified in the form shown in Fig. 3 which, in combination with Fig. 1, shows a complete apparatus of the modified form.

In this form the pipe 40 leaving interchanger leads to an expansion valve 00 by which the gas is reduced to atmospheric pressure on enter- 15 ing a cyclone separator 9|. In this separator of this interchanger a part of the gas, approximately three-quarters, passes through pipe 94 to the lower end of an interchanger 95 in which it is heated to 283 K. in cooling warm cycled gas. The remaining-i oneequarter of the gas at 61 K. passes throughpipelttp a tube group of interchanger .55 from 'iwhl'ch it-'is returned through the series" of interchangers to the hydrogen vent 88 as previously described.

From the upper end of interchanger 95, a pipe 91 leads to a compressor 90 bywhich the gas is raised to a low superatmospheric pressure, say

4 atm. or thereabouts. From the compressor pipe 99 delivers the gas to a water cooled interchanger I by which the heat of compression is removed. The gas then passes through pipe IM to the warm end of interchanger 95 in which I it is counterflowed against the cold gas entering through pipe 94 and cooled to 70 K. The cooled gas then passes through pipe I02 to the warm end of interchanger 93, in which it is cooled to about 32 K., then through pipe I03 to an expansion engine I0 doing external work, and thence through pipe I to separator 9|. In expanding from 4 atm. to 1 atm. the temperature of the cycled gas is reduced to about 25 K.

The two gas streams from valve 90 and pipe I05 are so directed toward each other in entering separator 9| that this intermixture is completed by the whirling motion in the separator. The temperatures of the two gases are thus equalized at about 33 K., assuming the proportions used to beas above described.

At this temperature the vapor pressures of nitrogen and carbon monoxide are unknown, so far-as I am aware, but straight line projections of curves terminating at 57 K. for nitrogen and at 52 K. for carbon monoxide indicate pressures materially below 0.0001 atm. for both nitrogen and carbon monoxide, corresponding to less than 0.01% each of nitrogen and carbon monoxide in the gas leaving separator 9|.

=-This temperature would not in fact be reached by intermixing the volumes above stated, as the heat leaking into the separator and the heat liberated in condensing and freezing must be withdrawn. This, however, is a matter of quantity of heat rather than of ultimate temperature, and the temperature of the mixture in the separator may be reduced as far as may be desired, within limits, by increasing the proportion of the colder cycled gas and/or by lowering the temperature of the expansion engine exhaust to not below the liquefying temperature of hydrogen at 1 atm. pressure or about 203 K.

Neither modification of the method above described is limited to the use of, the form of separator shown. The conditions existing in this step lend themselves particularly well to the application of the well known Cottrell method of electrostatic precipitation of finely divided suspended solids, and it is also entirely feasible to utilize the principle of the bag-house, either in its conventional form or in such modification as adapts it to extreme low temperature conditions. It is also possible, and in some cases desirable, to" eil'ect the separation of gas from solids by mere retardation and settling, as for example by introducing pipe 82 radially instead of tangentially into chamber 84.

Figs. 2 and 3 also include certain details which are interchangeable and alternative. Thus in Fig. 2, expansion valve 83 may be replaced by an expansion engine as indicated at I04 in Fig. 3. Likewise, in Fig. 3, an interchange against a colder fluid, as in int'erchanger 55 or 56 of Fig.

2, may be used to replace the compression of tle cycled stream and its expansion in engine I 4.

I claim as my invention:

1. The method of fractionating a substantially water-free mixture of hydrogen with nitrogen and/ or carbon monoxide which includes the steps of cooling said mixture to a temperature be-' low the freezing point of nitrogen and above the temperature at which hydrogen begins to liqueiy under the existing conditions and parting the gaseous hydrogen from constituents which have assumed, other than the gaseous form during said cooling step.

2. The method of fractionating a substantially water-free mixture of hydrogen with nitrogen and/ or carbon monoxide which includes the steps of cooling said gas to a temperature above the freezing point of carbon monoxide and at which a portion of said nitrogen and/or carbon monoxide is condensed; removing said condensate; i'urther cooling said mixture to a temperature below the freezing point of nitrogen and above the temperature at which hydrogen begins to liquefy under the existing conditions, and parting the gaseous hydrogen from the frozen nitrogen and/or carbon monoxide.

3. A method substantially as and for the purpose set forth in claim 1, in which said mixture is initially compressed to a material superatmospheric pressure and partially cooled and in which a further cooling effect is produced by expansion of said partially cooled compressed gas.

4. A method substantially as and for the purpose set forth in claim 1, in which said mixture is partially cooled and is brought to said freezing point by heat interchange with a colder fluid.

5. The method of fractionating a substantially water-free mixed gas which comprises: coolin said gas to a temperature above the freezing point of a material constituent thereof; further cooling said gas by admixture with a colder gas to a temperature below said freezing point and at which another constituent of the original gas retains the gaseous form, and parting the gaseous phase of the admixture from constituents which have assumed other than the gaseous form during said further cooling step.

6. A method substantially as and for the purpose set forth in claim 5, including the production of said colder gas by compression of said gas to a material superatmospheric pressure, partial cooling of the compressed gas and expansion of the partially cooled gas.

7. A method substantially as and for the purpose set forth in claim 5, including the production of said colder gas by compression of said gas to a material superatmospheric pressure, partial cooling of the compressed gas and expansion of the partially cooled gas under conditions in which said expansion does external work.

8. A method substantially as and .for the purpose set forth in claim 5, including the production of said colder gas by steps including a heat interchange with a colder fluid.

. 9. The method of fractionating a substantially water-free mixed gas which comprises; cooling said gas to ateaiperatin-e above the freezing point of a material constituent thereof further coolingsaidgasbyadmixturewithacoldergastoa temperature below said freezing point and at which another constituent of the original gas retains the gaseous form; parting the gaseous phase of the admixture from constituents whichhave assumed other than the gaseous form during said cooling'steps; cooling a portion of the gaseousphaseandreturningsaidportiontosupply said colder admixed gas.

10. A method substantially as and for the purposesetforthinclaimihinwhichthecooling of said returned portion is produced by compressing said portion to a substantial" superatmospheric pressure. partially cooling the compressed portion and expanding the partially cooled portion.

11. A method substantially as and for the purposesetforthinclaim9,inwhichthecooling oi the returned portion is produced by compressing said portion to a substantial superatmospheric pressure, partially cooling the compressed portion and expanding the partially cooled portion under conditions in which said expansion does external W0l'k..

vposesetf 12. A mzthod substantially as and for the purrthinclaimitinwhichthecooliug of the returned portion is produced by steps including a heat interchange with a colder fluid.

13. A method substantially as and for the pur- 'posesetforthinclaimLinwhichthepartingof the gaseous from the nongaseous phase is produced by submitting "the mixed gases to a centrif fl effect.

14. A method substantially as and for the purpose set forth in claim 1, in which the parting of the gaseous from the nongaseous phase is produced by passing the mixed phases into a zone of relative quiescence wherein the nongaseous phases may subside, and withdrawing the gaseous phase from said zone.

gaseous form during said cooling step; further cooling the parted gaseous constituent to a temperature at which further quantities of said material constituent are'frozen, and again parting the remainder of the gaseous constituent from frozen matter.

17. A method substantially as and for the purin which the gas is pose set forth in claim 16, initially compressed to a material superatmospheric pressure and in which a cooling of said gas-is produced by expansion to less than compression pressure.

18. A method substantially as and for the purpose set forth in claim 16, in which the gas is initially compressed to a material superatmospheric pressure and in which said further cooling effect is produced by the expansion of said gas to less than compression pressure between first said parting and second said parting.

aooassfl I 19.2 The method of separating condensible constituents from a-mixture of gases which comprises: cooling a stream of said gas in stages by her. interchange with a succession of expanded and evaporating liquid refrigerants having successively lower boiling points; separating and withdrawing from said stream any condensate produced in each stage, and returning the gaseous product from each said expansion and evaporation through a heat interchange step in which it is brought to substantially atmospheric temperature in cooling said stream.

20. A method substantially as and for the purpose set forth in claim 19, in which the successive liquid refrigerants in-the order of their boiling points are anhydrous ammonia, ethylene, methane, a mixture of carbon monoxide and nitrogen produced in one of the cooling stages, and nitrogen.

21. A method substantially as and for the purpose set forth in claim 19, in which liquid anhydrous ammonia. liquid ethylene, liquid methane,'and liquid nitrogen are used as refrigerants in the order named; in which the gaseous products of the expansion and evaporation of said refrigerants are brought to substantially atmospheric temperature by heat interchange with warmer fiuids, and in which said gaseous refrigerants are again liquefied by compression and cooling and cycled each through its respective step of expansion and evaporation.

22. The method of cooling a stream of gas which comprises: introducing into and intermixing with said stream a second stream of gas having a temperature lower than that of first said stream withdrawing a portion of -the mixed stream; lowering the temperature of the remainder of said mixed stream and returning said cooled remainder to supply said second stream.

23. The method of fractionating a gas stream which comprises: introducing into and intermixing with said stream a second stream of gas having a temperature lower said stream; removing from portion of said mixed stream after said removal: lowering the temperature of the remainder of said mixed stream and returning said cooled remainder to supply said second stream.

24. The method of fractionating a substantially water-free mixed gas which includes the steps of initially compressing said gas to a material superatmospheric pressure; cooling said gas to a temperature below the freezing point of a material constituent thereof and at which another constituent remains in gaseous form, the final step in said cooling being a heat interchange between said compressed gas and a colder expanded gas; P rting the compressed gaseous constituent from any constituents which have solidified during said cooling, and expanding said parted gaseous constituent to supply said colder gas. I

25. The method of cooling a gaseous stream which comprises: passing said stream in heat interchange relation with asuccession of expanded and evaporating liquid refrigerants comprising anhydrous ammonia, ethylene, methane and nitrogen, and returning the gaseous product of each said evaporation through a heat interchange step in whichit is brought to substantially atmospheric temperature in cooling said stream.

26.' The method of cooling a stream of gas to a temperature below the critical temperature of than that of firstthe mixed stream a: constituent of first said stream; withdrawing a temperature of nitrogen; expanding said gas' stream and thereby lowering its temperature, and intermixing with said expanded stream a second stream of gas having a temperature lower than that of said expanded stream.

28. The method of cooling a fluid stream which comprises: producing a series of liquid refrigerants of progressively lower boiling points; utilizing the expansion and evaporation of one of said refrigerants to effect a first cooling of said stream and also to effect the liquefaction of a refrigerant of said series having a lower boi ing point, and utilizing the expansion and evaporation of a refrigerant of said series having a boiling point lower than that of first said cooling refrigerant to effect a second cooling of said stream.

29. The method of cooling a gaseous stream which comprises: passing said stream in heat interchange relation with a succession ofexpanded and evaporating liquid refrigerants having succemively lower atmospheric pressure boiling points and returning the gaseous product of each said expansion and evaporation, through all the preceding stages and in heat interchange relation with said stream, to its respective source, and reliquefying each said product to produce renewed supplies of said refrigerants.

LEE s. TWOMEY. 2o

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