CA1053587B - Process and apparatus for the separation of gas mixtures into component fractions according to their molecular or atomic weight - Google Patents

Abstract

PROCESS FOR THE SEPARATION OF GAS
MIXTURES INTO COMPONENT FRACTIONS
ACCORDING TO THEIR MOLECULAR OR
ATOMIC WEIGHT

ABSTRACT OF THE DISCLOSURE

A process is provided for separation of gas mixtures into component fractions according to their molecular or atomic weight. The mixture is subjected to the centrifugal force applied in a cone-shaped vortex having a diameter of not over 5 mm. at a gas feed absolute pressure from about 5 to about 1000 mm. of mercury, and a pressure ratio within the range from about 1. 5 to about 10. The higher molecular or atomic weight fractions are separated from an outer or peripheral portion of the vortex, and the lower molecular or atomic weight fractions are separated from an inner or core portion of the vortex.
This application is a continuation-in-part of Serial No. 53, 712, filed July 10, 1970, Serial No. 353,148, filed April 20, 1973, and Serial No.
353,288, filed April 23, 1973, all now abandoned.

Description

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05;~58'7 SPECIFICATION

Methods for the separation of gas mixtures into component fractions according to their molecular or atGmic weight are easy to devi6e in principle, but in practice rather difficult to reduce to or embody in the formof a workable apparatus. Because the components 5 differ in molecular weight, they can be separated according to mass.
Because the molecules or atoms aredifferent, it is also possible to separate them by applying an electric or magnetic field~ and taking advantage of the different response of different types of molecules or atoms to such fields. Thus, for example, isotopes of particular 10 nuclear spin can be singled out, provided an atomic beam of the element in question can be produced, and the atoms have a non-zero electronic spin in the ground state. (~ther arrangements acting on ion beams are possible, such as mass filters, which make use of the focusing action on ions of an electric quadrupole field. A superimposed 15 high frequency field brings ions of one particular mass into resonance, and throws them out of the beam. Electric fields are more easily maintained than magnetic fields, and an array of many beams ope~ating in parallel can easily be arranged. However, these are only laboratory methods, and are not suitable for use on a large scale, because of the cost and equipment needed. ^;
The difference in mass gives rise to a difference in diffusion rate, and this has been utilized commercially in the separation or enrichment of iæotopes, especially of uramum. The mi~ture of gases is ca~sed to impinge on a porous membrane in which the pores are small 25 enough for molecular flow to take place, and so the lighter molecules pass more readily through the pores than the heavier ones. The 1. ~ -... ~. .

i~)5351~37 mixture can ~IU8 be separated by flowing it along the membrane surface; the portion that passes through is enriched as to the lighter component, and the portion that does not is enriched as to the heavier component. In the case of U 235 and U 238, the fractional change in the concentration of the two isotopes is very small, æo many separation stages are needed.
The gas centrifuge uses a rotating cylinder with annular entry ports at one end thereof. The gas iæ collected at the other end of the cylinder in two annuli. The heavier components are concentr~ted in the outer exit annulus, and the lighter components in the inner exit annulus.

It is also possible by various techn~ues to cause the gas to circulate twice through the rotor, paæsing in one direction along the periphery and inthe other at a smaller radius. However, Avery, Phyæics Bulletin, 1970, pages 17 to 21 (January, 1970) points out that within current engineering limitations for such equipment the size of the individual machine is æuch that the separative work output is order~ ~ ~
of magnitude less than that of a diffuæion plant ætage, and a centrifuge ~ ~ -enrichment plant for practical purposes will require hundreds of thousands of machines. This means that methodæ muæt be developed for maæs producing the machines and the components at low cost, and this means a major design problem. Avery points out that while a gas centrifuge may be technically feaæible, it remainæ yet to be æeen if it be practical, æince coæt indications are very speculative, since it has not been tested except in small-scale experiments.
It has been proposed to utilize the inertia effects arising from differences in maæs in a gas centrifuge without employing moving parts. One rather obvious method iæ to rotate the gas in a cyclone, in which it would be expected that the heavier molecules would -2 .
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10535~7 difuse outward and the lighter towards the center of the cyclone.
However, in order to maintain a cyclone a certain inward flow of the gas mixture is necessary, and owing to the fixed relation between diffusion and kinematic viscosity, the net reæult is that both 5 components move towards the center, though at a different rate.
N~ller and Mtlrtz, Naturwissenschaten 1958 45 (16), pages 382-3, reported some separation, but had difficulties with turbulence at high Reynolds numbers. The result is that this method has not been attractive, and it has been considered that at best, the ~ -- 10 æeparation obtained should be similar to that of the diffusion method, but since the power consumption is high, the advantage of the centrifuge over gas diffusion is lost. Thus, London has stated in his text Separation of Isotopes (London, George Newnes Limited) that as the .
whole process iæ less straightforward than the diffusion method, it is 15 not likely to offer any advantage~
Avery, Physics Bulletin 1970, pages 17 to 21 points out that the gas centrifuge was tried as part of the U. S. Manhattan Project, but it was the gaseous diffusion method that was adopted as more practical, and the USAEC is indicating its preference still to be ~or the 20 diffusion method, in preference to moving to a new process.
~ accordance with the invention, it has been determined that gaseous mixtures of components differing in molecular or atomic weight can be separated into component fractions according to molecular or atomic weight, if the mixture be subJected to celItrifugal 25 force applied to a cone-shaped vortex having a diameter of not over 5 mm. at a gas feed absolu~e pressure of from about 5 to about 1000 mm.
OI mercury alld a pressure ratio within the range from about 1. 5 to ,~ .

~OS35;87 about 10. The pressure ratio is defined as pllllet . Under core outlet these operating conditions, which are quite important to the separation, it is possible to isolate a higher molecular weight fraction in a peri- -~pheral portion of the vortex, and a lower molecular weight fraction in a 5 core portion of the vortex. This method is simple and straightforward, requires no equipment with moving parts, apart from the gas propulsion equipment, and is practical for operation commercially on a l~rge scale.
If the gas components are quite different in molecular or -atomic weight, it may be possible to effect a good separation in one 10 cyclone stage. If they are rather close in molecular or atomic weight, ~-however, it may be necessary to repeat the process in several stages, recovering the higher molecular weight or lower molecular weight fraction each time from the appropriate portion of the vorte~, and then recycling to a further cyclone stage. ~ the case of the separation of isotopes, such as separating U 235 from U 238, it may be necessary to -apply several hundred cyclone stages for a satisfactory enrichment of the core portion of the ~ortex.
Accordingly, the proceæs in accordance with the invention for separation of gas mixtures into component fractions according to their 20 molecular or atomic weight comprises subjecting the mixture to the centrifugal force applied in a cone-shaped vortex having a diameter of not over 5 mm. at a gas feed absolute pressu~ e within the range from ~;
about 5 to about 1000 mm. of mercury, and a pressure ratio pinlet core outl~
within the range from about 1. 5 to abo lt 10, discharging a higher molec-25 ular weigm fraction from a peripheral portion of the vortex, and separat-ing a lower molecul~r weig~ fraction from a core portion of the vortex.
The apparatus in accordance ~1ith the invention comprises a housing with a separator chamber therein that is circular in cross- ;
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section, has an apex end and a base end, is cone-shaped at least at the apex end, and has a diameter at the base end of at most 5 mm~, and a diameter at the apex end of at least 0. 01 mm.; ~t least one gas inlet through the housing at the base end, arranged for tangential flow of 5 gas from outside the housing into the chamber, to establish a vortical gas f low in the chamber from the base end towards the apex end, with the gaseous components distributed towards the periphery of the vortex with increasing molecular or atomic weight, and towards the core of the vortex with decreasing molecular oratomic weight, the vortex core 10 having a lower gas pressure than the vortex periphery; an outlet through the housing in axial alignment to the chamber at the base end of the chamber; and an outlet through the housing in axial alignment to the chamber at the apex end of the chamber, the apex end outlet receiving peripheral vortical gas flow from the chamber, and the base end outlet 15 receiving core vortical gas flow from the chamber, ~o that lower molecular or atomic weigm components are concentrated in the flow withdrawn via the base outlet, and higher molecuL~r or atomic weight components are concentrated in the flow withdrawn via the apex outlet.
This cyclonic separator is simple and straightforward in construction, 20 has no moving part~, and is practical for commercial gas separation on a large scale despite its small size. -The centrifugal forces in the vortex cause the heavier molecu~r weight or atomic weight particles to diffuse towards the periphery of the vortex, and the lighter molecular or atomic weight particles to diffuse 25 into the central or core portion of the vortex. The core portion of the vortex is at a lower gas pressure than the peripheral portion. Since in the apparatus of the invention the peripheral portion and core portion of the vortex are drawn o~f at opposite ends of the separator ~' 5.
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~ O535~37 chamber, two opposing or countercurrent flows are created within the separator chamber, a peripheral flow, flowing in one direction towards the peripheral portion or apex end outlet, and a core port~on flow flowing in the opposite direction, towards the core portion or base end outlet.
5 This in effect appreciably extends the zone of separation.

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The cyclonic separators of the invention can be formed of any suitable material that is resistaltt to attack or corrosion by the gas mi~ures to be separated under the operating conditions. Metals can be used, such as stainless steel and aluminum, and nickel and chromium 10 alloys. However, unless the metal can be cast, it is difficult to shape it in the very small sizes required in the invention. Ceramic, glass and plastic materials that are strong, resistant to pressure, and capable ~ ~
of retaining their shape under the gas pressures to be encountered, are ~-therefore preferred. Such materials can be shaped or molded by 15 injection or compression molding into the shapes desired, and can be manufactured in quantity without detriment. Materials such as glass, porcelain, nylon, polytetrafluoroethylene, polyesters, polycarbonates, polyethylene, pol~rpropylene, synthetic rubbers, phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde resins are suitable, ~ -20 as well as polyoxymethylene and chlorotrifluoroethylene polymers.
~ the preferred embodiment of cyclonic separator, a tubular baffle extends from the base outlet into the chamber to a point beyond the gas inlet or inlets, to deflect gas flow away from the base outlet, and enhance initiation of a gas vortex at the base end, 25 and thence through the chamber towards the apex end. The tangential - orientation of the one or more gas inlets impart a cyclonic or vortical - ;
flow to the gas mixture being introduced. The inlets should be , j 0 .

uniformly spaced if there is more than one, for initia~ion of a uniform vo~tical flow. Usually, from two to 9iX ga9 inlets are sufficient. Then, when the gas is introduced into the chamber at h;gh velocity, it is constrained by the cu~ed walls of the separator chamber 5 into a vortex which flows helically towards the apex end or peripheral portion outlet end of the chamber.
It is important that the vortex defined within the cyclone separator chamber (and therefore the separation chamber) have a diameter of not over 5 mm, and preferably 2 mm. or less. The lower 10 limit on diameter is imposed only ~y ~he practicality of manufacture of small cyclones. A practical lower limit appears to be 0.1 mm,.
The len~h of the separator chamber is not critical, but it should not be greater than 2û0 mm. nor less than 5 mm. in length, and if the shape is conical, it should be at least 0-1 mm. in diameter 15 at the apex end.
It has been determined in accordance with the invention that it is not possible to effectively separate gas components according to their molecular or atomic weight, if the chamber has a larger diam~erthan 5 mm., a~d since cyclone chambers heretofilre have 20 been considerably larger, this is probably one of the reasons why cyclonic separators have not heretofore been employed for this purpose. If the vortex is larger in diameter than 5 mm., both components move towards the center of the vortex at too great a rate to permit effective separation, and $he problems noted by London 25 begin to be encountered. ~Ience, the small size overcomes the difficulties that previous worl~ers in the field have encountered with cyclone gas separators.
The cone shape of the separator chamber (and vortex) is 1~5358'7 quite significant in improv~ng separation efficiency. The chamber must decrease in diameter towards the apex end, redu~ing the radius of the vortex and increasing centrifugal force. A cone shape is therefore essential. The chamber can be in the form of a straight-5 sided rigm angle cone from base end to apex end. N can also bepartly cylindrïcal, and cone-shaped only at the apex end. The cone shape need not be uniform or straight sided. Convexly and concavely curved sides can be used, of uniform or increasing or decreasing curvature. The diameter can decrease continuou~ly towards the 10 apex end, or in stages. Thuæ, a variety of cone shapes are possible, and the shape chosen will depend on the particular conditions of the separation to be carried out~ and may be determined by trial-and-error experimentation.
It is also important to have a pressure drop between the gas 15 inlet and the apex and core gas outlets sufficie~; to cause an accelera-tion in the velocity of the gas as it ~pproaches the region of lesser diameter in the cone. This gives ma~imum centrifugal force for separation in the region of smallest radius. As pressure drops, pressure energy or pressure head is converted to velocity head, and 20 velocity increases. Thus, the energy neededto accelerate the particles is obtained from the pressure drop, and is used to increase separation efficiency. It is for this reason that the pressure ratio is quite critical to the process of the inven~ionO
This means also that the maximum separating effectiveness 25 can be at a region within the vortex where the radius is small, instead of at the periphery aE the vortex, and this region is where the maximum separating effect is needed, at the boundary between the core and peripheral regions, where the gas flow towards the apex and core ~05;~ 87 outlets is in different directions. This means that the core region is the region where the heavier particles have the greatest chance of being thrown out, if they have managed to get that far, and this aids in ensuring that they do not remain with the core flow at the core region outlet~
Consequently, the apex ou~let and core outlet diameters are selected so that the pressure ratio pmlet is within the raIlge from core outlet about 1. 5 to about 10, at the operating gas feed absolute pressur e within the range from about 5 to about 1000 mm. of mercury. ~ effect, this means that the gas pressure at the gas inlet is at least 1. 5 times 10 up to about 10 times the pressure at the core outlet of the chamber.
Preferably, for optimum separation efficiency, the pressure ratio is within the range from about 2 to about 6.
The gas can be admitted through the gas inlet via a nozzle, jet or orifice, which partially converts pressure head into velocity 15 head. This is particularly advantageous in initiating the vortex. In fact, high gas velocities at the gas inlet are preferred because this increases the rate of separation.
The inlet velocity ~f the gas mixture can be at least the velocity of sound, at the temperature ~ oper~tion, and it can be several times tbis 20 velocity, if desired, but this requires special gas inlet equipment. It is possible to use lower velocities than this, depending on the gas, and this can be determined b~ experimentation for each gas. The separating effect for a given pressure drop is also to some extent dependent on the inlet~
themselves, their shape, number, and spacing. If the inlets provide a 25 perfectly uniform flow around the periphery, then it is possible to operate at reL~tively low gas inlet v~locities, below the speed of sound.
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105358'7 The process can be operated at any convenient temperat~re.
Small variation~ in temperature are not critical. The operating temperature will normally be selected as the temperature a~ which all of the components to be separated are in the gas phase. In the case of 5 some materials, this may require relatively high temperatures, while in the case of materials which are normally gaseous at norm~l room temperature, normal room temperature can be used. In some cases, very low operating temperatures may be preferableD The range of operating temperatures is thus from about -50 to about 500C., and preferably from about -20 to abou~ 300 C.
In the case where the gas mi~ture is to be subjected to a number of vortex stages, it is advantageouæ to employ an array of vortex separators or cyclones, arranged in two series, in cascade.
A typical cascade serieæ which can be used is described by Avery, Physics Bulletin (1970~, page 18. The core portion from each cyclone ætage is æeparated and combined in serieæ with the apex portion from a later cyclone stage, and thiæ repeated at each stage to the end of the æeries, while in the other series, the apex portions are æeparated and sent through with the core portions from a later stage.
20 Any arrangement of the cyclones and the feedback can be used. ~ this way, no part of the material need be wasted, a~d eventually all of the components separated can be recovered, if desired.
FI~URE 1 æhows in longitudinal æection a typical conical ` ~;
cyclonic æeparator of the invention, which can be uæed in the process 25 of the invention.
EIGURE 2 is a view taken along the lines 2-2 of ~IGURE 1, ~ -and shows the cyclonic separator thereof in cross-sec~ion, with the peripheral portial s and the core portions of the vortex ~low being 10.

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lOS35~37 delineated.
FIGURE 3 shows diagrammatically a typical array of cyclone separators, arranged in a twin cascading series a and b of ape~ portion cyclones and core portion cyclones, showing the flow of 5 the apex portions and the core portions through each series, to the final separation of the components of the gaseous mixture at the end of each series.
The cyclone of ~ggures 1 and 2 has a housing 1 with æix gas inlets 2 arranged tangentially at the base end 3 of the conical 10 separation chamber 5. The gas outle~ 4 for the peripheral portion of the vortex is at the apex end 9 of the conical separation chamber 5, -and the gas outlet 6 for the core portion of the vortex is at the base end of the chamber. The inner end of the tube 7 projects inwardly from the base of the cone, and defines an annulus 8 into which the gas inlets 2 15 open. The gas inletæ 2 because they are placed tangentially initiate a vortical flow of gas shown ~y the helical arrow, about the annulus 8 --defined by the inner end of the tube 7. The vortical flow thus created ;~
proceeds along the periphery of the cone towards the ou~let 4, and in the course of this travel the components of heavier molecular weight 20 or atomic weigm are thrown to the peripheral port~on of the vortex, while the components of lower molecular weigm or atomic weight are drawn towards the core por~ion of the vortex. The core portion of the vortex is drawn in the opposite direction, towards outlet 6. There is thus a countercurrent flow of the inner and outer voffices, and this 25 promotes efficient separation, besides appreciably increasing the zone of separation.

11.

~)535~7 In operatîon, the gas mixture entering via the inlets 2 describes a vortex about the periphery of the chamber, and there is a peripheral flow of gas towards the outlet 4. ~t the same time, a core portion is formed at the center of the vortex, and the gas flow 5 in this portion is in the opposite direction, towards the sore portion outlet 6. The pressure in this core portion is reduced compared to the pressure at the inlet 2. In this way, the lower molecular weight portion leaves the vortex separator via the ~utlet 6, and the heavier molecular weight portion leaves the separator via the outlet 4.
An array of these cyclones in two cascading series a and b to provide a plurality of centrifugal separation stages can take the form shown in E~gure 3. The gas mi~Lure enters the inlet via compressor C to the first cyclonic separator I, blended in the compressor C, with apex portion from cyclonic separator IIa, and core portionfrom cyclonic æeparator IIb. Series a concentrates or ;
enriches with respect to a ligmer component. The core portion is drawn off at the base of I, and passed to the next separator IIa of the series, blended with apex portion from cyclonic separator ma; thence ~ .
as core portion to cyclonic separator ma, blended with apex portion ..
20 from separator I~Ta; thence as core portion to cyclonic separator IYa, . .
- blended with apex portion from Va; thence as core portion to cyclonic separator Va, blended with apex portion from separator Vb, thence as core portion to cyclonic separator VIa.
In this way, the core portions become successively more and 25 more concentr~ted in the lighlter component, and finally at VI~, the end OI the series, the lighter componen~ is withdraYlm from the system.
Series b concentrates with respect to a heavier component.
The apex portion from I is drawn off at the apex, and passed via 12.

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1~535~7 compressor C to cyclonic separator IIb, blended urith core portion from IIIb; the apex portion drawn off and fed to the separator lIlb, after blending with core portion from IVb; the apex portion drawn off from IIIb and fed to IVb, after blending with core portion from Vb; the 5 apex portion drawn off and fed to Vb~ after blending with core portion from VIb, the apex portion from Vb drawn off and fed to VIb. Thus, the apex portions become successively concentrated in the heavier component.
The following Examples in the opinion of the inventor represent preferred embodimen~s of the invention. -Example 1 The cyclones used in this test were of the type shown in Figures 1 and 2. These cyclones were used to separate carbon dioxide from air in mixtures of approximately constant composition, 15 containing 8. 5 volume percent CO2. The gas flowed from a receptacle through a reducing valve, an excess pressure guard, a fil~er, a control valve and a venturi flow meter, to the container with the cyclones.
The two fractions from the cyclones were passed through venturi meters and control ~lv~s to a vacuum pump, and a portion drawn off through the 20 valves to a gas analyzer for a~alysis. The gas pressures in the cyclone container compartments were measured with absolute mercury mano-meters to an accuracy of approximately 0. 5 mm. of mercury.
The difference in CO2 content between the two fractions from the cyclones was registered by means of an infrared analyzer and a 25 connected potentiometric recorder.
The following data were obtained with the 2 mm. cyclone.
This cyclone had a tapering o~ the cone (defined as the diameter of the , 13.
.'' ~053587 base divided by the height to the apex) of 1:10 and three or six uniformly spaced inlet openings of rectangular cross sec~ion, 0. 6 mm.
by 0. 3 mm.

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~0535187 The separation factor is defined by the equation Xapex - core where x is the mole fraction, in this ca~e, of Xin (l-xin) carbon dioxide, in the gas. The greater the separation factor, the more effective the separation. The flow distribution factor is defined as the molar flow fraction of the incoming gas leaving through the apex outlet. It is apparent from the data that good separation is obtained.
The cyclones of the invention are useful in the separation of gas mixtures intotheir components according to molecular weight ;~
or atomic weight. It iæ possible in this way to separate in vapor form isotopes of various elements in the form of gaseous compounds, which after separ~ion can be treated to recover the element in any desired form, including the elemental metal. It ie possible, for example, to æeparate U 235 from U 238 in the mixtures thereof as uranium hexa-iluoride.

Example 2 The isotope U 235 is separated from U 238 in uranium hexa- -;
iluoride gas, in accordance with the following procedure.
The apparatus used is composed of 310 stages in an array whose through flow is in accordance with the diagram shown in P9gure 3.
Each separator chamber has a 2 mm. maximum diameter at the inlet port, and a 1. 0 mm. diameter at the core portion outlet and a~ the apex port. The separator chambers are conical, as shown in Figures 1 and 2, with a length of 10 mm. The inlet pressure is 90 mm. Hg.; the core and apex gas outlet pressure are 15 mm. Hg. The pressure drop ratio is 4. The gas entrance velocity in the separation chamber is the velocity of sound.

18.

The gas mixture ir~roduced in the first cyclone separator stage contains 99. 3~ U 238 and 0. 7~ U 235. In each OI the a series cyclone stages, the core fraction is em~iched in U 235. The gas emerging from the 250 cyclone stage (counted from the feed stage) of the core portion 5 series is enriched in U 235 to 3~c, and the gas obtained from the apex portion series of cyclones contains nearly all U 238 and a very small amount, û. 2%, of U 235.
The cyclones of the inven~on are also useful as the molecule separator stage in gas chromatography-mass spectrometry systems.
10 Such systems combine two physicochemical methods, using the molecule separator of the invention as the key function. Gas chromato- ~`
graphy is a most efficient technique for separating components of organic compounds with sufficient volatility and thermostability, while ~
mass spectrometry is a uniqu0 method for identifying those ` -15 components. The direct introduction of separated gaseous components from the gas chromatography column via the cyclones of the invention tothe mass spectrometer reduces deadvolume to a minimum, achieves a fast flow rate, and provides high sample-to-carrier gas enrichment.
The gas chromatography and mass spectrometers employed can be 20 conventional. A suitable system, for instance, is the LKB gas chromatography-mass spectrometry system, which includes a single-focusing mass spectrometer equipped with a 60 sector, 20 cm. radius magnetic analyzer, and sweep generator for fast scanning of spectra, a rhenium filament to provide an ion source of the electron bombardment 25 type, and a measuring system including a 14-stage electron multiplier, electrometer, and a wide band amplifier feeding a direct-writing W
oscillograph. A helium carrier gas is used, and the sample is injected through the gas chroma~ographic column with helium.
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19.

105~587 SPECI~ICATION OF THE SUPPLEMENTARY DISCLOStJRE
~ . .
In accordance with the invention of the principal disclosure, gaseous m~xtures of components differing in molecular or atomic weight are separated into component fractions according ; -to molecular or atomic weight, by subjecting the mixtures to centrifugal force applied to a cone-shaped vortex having a diameter of not over 5 mm. at a gas feed absolute pressure of from about 5 to about 1000 mm.
of mercury and a pressure ratio within the range from about 1. 5 to about ;
10. The pressure ratio is defined as inlet mm. Hg. . Under Pcore outlet mm. Hg.
these operating conditions, which are quite important to the separation, -U is possible to isolate a higher molecular weight fraction in a peripheral portion of the vortex, and a lower molecular weight fraction in a core portion of the vortex. This method is simple and straight- ~-forwaxd, requires no equipment with moving parts, apart from the gas propulsion equipment, and is practicalfor operation commercially on a large scale.
It has now been determined, in accordance with this supple-mentary disclosure, that the vortex should have a cone angle, i. e. the angle at the ape2c end of the conical chaniber (by extrapolation of the cone sides to their melting point), within the range from 1 to 90, preferably from about 3 to about 30. Any cone of a cyclone separator chamiber is of course truncated. Good results have been obtained at a ratio of the diameter at the base of the cone to the diameter at the apex of the cone D base from D apex outlet 1.3to3.5~

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105;~587 and at a ratio of the base diameter to core outlet diameter Dbase of from 1. 3 to 3. 5.
core outlet Accordingly, the process in accordance with the invention for separation of gas mixtures into component fractions according to their molecular or atomic weight comprises subjecting the mixture to the centrifugal force ap~lied in a cone-shaped vortex having a diameter of not over 5 mm. and a cone angle within the range from 1 to 90 at a gas feed absolute pressure within the range from about 5 to about 1000 mm. of mercury, and a pressure ratio inlet mm. Hg. within the range from a~out 1. 5 to about 10, core outlet mm. Hg.
discharging a higher molecular weight fraction from a peripheral portion of the vortex, and separating a lower molecular weight fraction from a core portion of the vortex.
Apparatus for carrying out the process in accordance with the invention comprises a housing with a separator chamber therein that is circular in cross-section, has an apex end and a base end,- is cone-shaped at least at the apex end, and has a diameter at the base end of at most 5 mm., a diameter at the apex end of at least 0. 01 mm., and a cone ~gle within the range from 0 to 90; at least one gas inlet through the housing at the base end, arranged for tangential flow of gas from outside the housing into the chamber, to establish a vortical gas flow in the chamber from the base end towards the apex end, with the gaseous components distributed towards the periphery of the vorte~ with increasing molecular or atomic weight, and towards the core of the vortex with decreasing molecular or .. . . , . :: . .

1053'~87 atomic weight, the vortex core having a lower gas pre~sure thaII the vortex periphery; an outlet through the housing in axial alignment to the chamber at the base end of the chamber; and an outlet through the housing in axial alignment to the chamber at the apex end of the chamber, the apex end outlet receiving peripheral vortical gas flow from the chamber, and the base end outlet receiving core vortical gas flow from the chamber, so that the lower molecular or atomic weight components are concentrated in the flow withdrawn via the base outlet, and higher molecular or atomic weight components are concentrated in the flow withdrawn via the apex outlet. This cyclonic separator is simple and straightforward in construction, has no m~ving parts, and is practical - for commercial gas separation on a large scale despite its small size.
It is important, as noted in the principal disclosure, that the vortex defined within the cyclone separator chamber (and therefore the separating chamber) have a diameter of not over 5 mm., and preferably a mm. or less. In accordance with this supplementary disclosure, the vortex diameter should be more preferably between 1 mm and 0.1 mm. The lower limit on diameter is imposed by the practicality of manufacture of small cyclones. A practical lower limit appears to be 0.1 mm.
The length of the separator chamber together with the diameter determines the ~olume of the separator chamber, and the ~rolume in turn determines the residence time of the gases therein, which of course must be sufficient for the desired separation.
Accordingly, the length and diameter are selected to give a chamber of the determined volume for the separation. Thus, the length should 1~53'~87 not be greater than 200 mm. nor leæs than 0.1 mm., and if the chamber is coniFal in shape, it should be at least 0.1 mm. in diameter at the apex end.
The shape of the separator chamber (and vorte2~) is quite significant. It has been found that a high separation efficiency is obtained in conically shaped chambers. The chamber must decrease in diameter towards the apex end, reducing the radius of the vortex and increasing centrifugal force.
The terms "cone shaped" or "conically shaped" as used herein refer to the effective cone shape of the chamber, i. e., to the decreased diameter of the chamber at the apex outlet end as compared to the diameter at the inlet end or base outlet end. The chamber is in effect a cone if the chamber at the apex outlet has a lower diameter than the chamber at the inlet. If this is the case, the vortex in the chamber will be reduced in diameter towards the apex end, even if the shape of the chamber in between the ends is not a straight-sided conebut, for example, a cylinder.
The chamber can be in the form of a straight-sided right angle cone from base end to apex end. It can also be partly cylindrical, 7`
and cone-shaped only at the ap~ end. The cone shape need not be uniform or straight-sided. Convexly and concavely curved sides can be used, of uniform or increasing or decreasing curvature. The ~ ~ -diameter can decrease continuously towards the apex end, or in stages. A cone with straight sides but with varying cone angles can be used. Thus, a variety of cone shapes are possible, and the shape chosen will depend on the particular conditions ~f the separation to be carried out~ and may be determined by trial-and-error experimentation.

~7' -105;3587 It ~I~S b~en Iound advantageous to blend the ~as mixture to be separated with an incrt ~as o~ lower molecular or atomic wei6ht. An lncreased degree o~ separation is obtained, and the effect is particularly marked at amounts o inert gas in excess of about 25~/c by volume of the blend, and especially in excess of a~out 60~C by volume of the blend. This effect is believed to be due to an increase in the sonic velocity of the gas, because of the reduced average m~lecular or atomic weight of the blend, and some influence on the overall diffusion mechanism.
While any gas can be used that is inert to, i. e. does not react appreciably with, the gas mixture to be separated, it must have a molecular or atomic weight less than the gas mixture, and preferably as low a molecular or atomic weight as p~ssible, so that hydrogen and helium are preferred.
However, nihogen, carbonmonoxide, carbondioxide, andwateralsocanbeuse In the case where an inert gas is used, the process can be operated at a gas inlet absolute pressure higher than 10~ mm. Hg.

Accordingly, in the process in accordance with the invention, for separation of gas mixtures into component ~ractions according to their molecular 4 or atomic weight, when an inert gas is used the gas fe~d absolute pressure is at least about 5 mm. of mercury, with no upper limit, and the pressure ratio inlet mm. Hg. is up to about 10, with no lower limit.
.
core outlet mm. Hg. . -After the separation has been effected, the inert gas can be separated by conventional techniques such as con~ensation of the heavier or less volatile separated components.
Figure 4 is a graph of observed values of the separation factor E
a~ainst the uranium distribution ratio ~ of the data in Example 7, Table IX.

The cyclones used in this test were of the type shown in Figures 1 and 2. These cyclones were used to separate carbon dioxide from air in mixture~ of approximately constant composition, containing 8. 5 volume percent CO2. The gas flowed from a receptacle through a reducing valve, a filter, a control valve and a venturi flow meter, to the cont~iner with the cyclones. The two fractionæ from the cyclones were passed through venturi meters and control valves to a Yacuum pump, and a portion drawn off through the valves to a gas analyzer for analysis. The gas pressures in the cyclone container compartments were measured with absolute mercury manometers to an accuracy of approximately 0. 5 mm. of mercury.
The difference in COz content between the two fractionæ
from the cyclones was registered by meanæ of an infrared analyzer and a connected potentiometric recorder.
The following data, Tables IV, V and VI, were obtained with the 2 mm. cyclone. This cyclone had a cone angle of 5. 7 and three or six uniformly spaced inlet openings of rectangular cross ;
section, O. 6 mm. by 0. 3 mm.

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105;~587 The separation factor E is defined by the equation E ~ Xapex~xcor~3 xcore(l - xapex ) where x is the mole fraction, in this case, of carbon dioxide, in the gas. The greater the separation factor, the more effective the separation. The flow distribution factor is defined as the molar flow fraction of the incoming gas lea~ring through the apex outlet. It i~
apparent from the data that good separation is obtained. ~ ;
The cyclones of the invention are useful in the æeparation of gas mixtures into their components according to molecular weight or atomic weight. It is possible in this way to separate isotopes of various elements in the form of gaseous compounds, which after separation can be treated to recover the element in any desired form, - -including the elemental metal. It is possible, for e~aInple, to separate U 235 from U 238 in the mL~tures thereof as uranium hexafluoride. ~ -EX~PLE 4 The isotope U 235 is separated from U 238 in uranium he~afluoride gas, in accordance with the following procedure.
The apparatus used is composed of 813 stages in an array whose through flow is in accordance with the diagram shown ln Figure 3. Each separator chamber has a 2 mm. base diameter, and a 1. 2 mm. diameter at the core and the apex outlets. The separator chambers are conical, as shown in Fi~ures 1 and 2, with a length of 10 mm. The inlet pressure is 90 mm. Hg.; the core and apex gas .. ;~,' , . ~:

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iOs,3ss7 outlet pressures are 15 mm. Hg. The preæsure ratio is 6. The gas entrance velocity in the separation chamber is the velocity of sound.
The gas mlxture introduced in the first cyclone separator stage contains 99. 3~Zc U 238 and 0. 7~c U 235. In each of the Series a cyclone stages, the core fraction is enriched in U 235. The gas emerg-ing ~rom the last cyclone stage (counted from the feed stage) of the core portion Series _ is enriched in U 235 to 3%, and the gas obtained from the apex portion Series _ of cyclones contains nearly all U 238 and a very small amount, 0. 2%, of U 235.

The cyclones used in this test were of the type shown in Figures 1 and 2. The cyclones were used to separate carbon monoxide from carbon dioxide in a mixture containing about 25~C CO, and about 75% C2~ and blended with the amounts of helium shown in Tables VII
and VIII. The gas blend flowed through a system as in Example 3 and the composition of the two f~actions from the cyclones measured by infrared analyzers in the same way.
The data obtained using a 2 mm. cyclone is given in Table VII. The data obtained using a 1 mm. cyclone is given in Table VIII. The cone angle of the cyclone in both cases was 5. 7.

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''' '' . ', ' ' : ---lOS~5~7 The separation factor E i~ defined by the equation E = xapex - Xcore xcore (l~Xapex) where x is the mole fraction of carbon dioxide in the rnixture carbon mono~ide-carbon dioxide (excluding the helium). The flow distribution factor is defined as the molar flow fraction OI the incoming gas lea~ing through the apex outlet. It îs apparent from the data that good separation is obtained, and that the separation factor increases as the amount of helium increases.

:- , The isotope U 235 is separated from U 238 in uranium hex2fluoride gas blended with helium gas, in accordance with the following procedure. ~
The apparatus uæed is composed of 200 stages in an ~ -a~ray whose through flow is in accordance with the diagram in Figure 3.
Each separator chamber ha~ a 2 mm. ma~imum cone diameter and a 1 mm. diameter at the core outlet and at the apex outlet. The separator chambers are conical, as shown in Figures 1 and 2, with a length of 10 mm. The inlet preæsure is 300 mm. Hg. The core and apex gas outlet pressures are 60 mm. Hg. The pressure ratio is 5.
The gas entrance velocity in the separation chamber is the velocity of sound.
The uranium hexafluoride introduced into the first cyclone separation stage contains 99. 3~c U 238 and 0. 7~c U 235. The , ,.~,.. :.~ . ..
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feed to each cyclone separator stage i~ regulated to contain 90% helium, the same in each, by blending appropriate selected fraction from subse~quent Series a stageæ as shown in Figure 3. In each of the Serie~
a cyclone stages, the core fraction is enriched in U 235, the gas emerging from the last stage of the core portion Serieæ a enriched in U 235 to 3~c, and the gas obtained for the apex portion Series b of cyclones contains nearly all U 238 and a very small amount 0. 2%, of U 235.

, Sa~ple~ of purified uranium hexafluoride CUF6) and helium (92% He, 8% UF~ by volume) were prepared as follows. The vessel containing the purified UF6 was connected to an evacuated sample container. The vessel was kept at 18~ C in a constant temperature bath, and the containers cooled by liquid nitrogen. The amount of UF6 thus transferred to the container was determined by weighing. The difference between the UF~-contents of sarnple pairs was kept at less than 2%. The UF6 in the containers was then disæolved in 3 g carbon tetrachloride (tetrachloromethane, CCl4) which was likewise transferred to the container by distillation, to give a homogeneouæ sample.
The sample containers were made of aluminum, which -~
has a relatively low absorption of gamma-rays. The wall thiclmess was 1. 75 + 0. 01 mm.
A well-type sodium iodide (NaI)-detector coupled to a multichannel analyzer was used to measure the gamma-ray emission from the samples for the range from 50 to 210 keV, at 3 ke~J per channel.

~053587 The number of COUlltS belonging to the characteristic peak at 185 keV, corrected for the inf luence of Thorium 634 (Th-234) peaked at 94 keV, w was taken as a measure of the U 235 content of the sample. The back-ground radiation proved to be of negligible importance. The containers fitted very closely into the detector well.
The apparatus used was composed of four cyclone separators, in parallel, constituting one stage of a cascade in accordance with the diagram in Fi~ure 3. Each cyclone separator chamber had a 2 mm. maximum cone diameter, and a 1 mm. diameter at the core outlet and at the apex outlet. The separator chambers were conical, as shown in Figures 1 and 2, with a length of 10 mm The inlet pressure used was 180 mm. Hg. The pressure in the enriched stream ~base outlet) from the cyclones waæ
fixed at 36 mm. Hg. while the pressure in the depleted stream (apex outlet) was varied between 36 and 65 mm. Hg.
Several run8 were made under these conditions, and -the separation efficiency determined by way of the separation factor.
The following results were obtained:
TABLE IX

(E ~10~ ) DISTRIBUTION ;-RATIO (~ ) -. , 0. 20 2 20 0.33 23 ~ 0. 50 4 13 0. 67 , - -.- ' ' ' ,. . .

locj;~5~7 The observed separation factor is plotted against the uranium distribution ratio ~ for four of the runs in the graph shown in Fi~ure 4.
The uranium distribution ration ~ is defined as:
Molar flow rate uranium _ through apex outlet Total molar flow rate uranium through apex and base outlets As the curve shows, the separation factor E reaches a maximum value of about 23 x 103 at a value of ~ of about 0. 5.
This is quite significant to power consumption; U means that an equivolume distribution of effluent flow at the apex and base ;`
outlets of the cyclones can be used in the simplest form of cascade, of the type shown in Figure 3, and will give minimum power consumption.
Collsequently, in the operation of the cascade equal flow volumes can be maintained in the return and forward flows from the apex and base ends of cyclone I, and thence through every cyclone in the a and b series II to VI, etc., which is a very convenient way to operate, and in this way when operating at maximum separation efficiency the power con-~umption will be at a minimum Also the whole installation is simple and les~ expensive.
Accordingly, in the light of the above result~, the isotope U 235 is separated from U 238 in uranium hexafluoride gas blended with helium gas, in accordance with the following procedure.
The apparatus used is composed of 200 stages in an array whose through flow is in accordance with the diagram in Figure 3.
Each separator chamber has a 2 mm. maximum cone diameter and a 1 mm. diameter at the core outlet and at the apex outlet. The `

1~)5;~587 separator chambers are conical, as shown in Fi~ures 1 and 2, with a length OI 10 mm. The inlet pressure is 180 mm. Hg. The core and apex gas outlet pressures are 36 mm. Hg. The pressure ratio is 5.
The gas entrance velocity in the separation chamber is the velocity of sound. Equal flow volumes are maintained in the effluent gas at the apex outlet and base outlet of each cyclone in each series.
The uranium hexafluoride introduced into the first cyclone separation stage contains 99.3% U 238 and 0. 7% U 235. The feed to the first cyclone sepaxator stage contains 92~c helium, and 8~c UF8 by volume, and proceeds to subse~uent stages in the series by -blending equal volumes of feed and fraction selected from a subsequent ~;
Series a stage as shown in Figure 3. ~ach stage is operated at a uranium distribution ratio ~ of about 0.5. In each of the Series a cyclone stages, the core fraction is enriched in U 235, the gas emerging from the last stage of the core portion Series a being enriched in U 235 to 3 3Zc~ and the gas obtained from the final stages apex poxtion of the Series _ of cyclones containing nearly all U 238, and a very small amount, 0.2~C of U 235.

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Claims (42)

1. A process for separation of a gas mixture into component fractions according to their molecular or atomic weight, comprising subjecting the mixture to the centrifugal force applied in a vortex having a diameter of not over 5 mm. at a gas inlet absolute pressure of from 5 to 1000 mm. Hg., and a pressure ratio within the range from 1. 5 to 10; discharging a higher molecular weight fraction from a peripheral portion of the vortex, and separating a lower molecular weight fraction from a core portion of the vortex.

2. A process according to claim 1 in which the fractions thus separated from the mixture are subjected to successive vortical separa-tion stages under the stated conditions to further enrich them in higher or lower molecular weight fractions.

3. A process according to claim 1 in which isotopes in a mixture thereof are separated.

4. A process according to claim 1 in which the vortex is conical.

5. A process according to claim 1 in which the vortex has a diameter of not over 2 mm.

6. A process according to claim 1 in which the gas mixture at the temperature of operation has a velocity at least that of the velocity of sound.

7. A vortex separator for separating gaseous mixtures into component fractions according to their molecular or atomic weight at feed pressures from 5 to 1000 mm. Hg. absolute pressure, comprising a housing having a separator chamber therein that is circular in cross-section, has an apex end and a base end, is cone-shaped at least at the apex end, and has a diameter at the base end of at most 5 mm.; and a diameter at the apex end of at least 0.01 mm.; at least one gas Inlet through the housing at the base end of the chamber, arranged for tangential flow of gas from outside the housing into the chamber, to establish a vortical gas flow in the chamber from the base end towards the apex end, with the gaseous components distributed towards the periphery of the vortex with increasing molecular or atomic weight, and towards the core of the vortex with decreasing molecular or atomic weight; an outlet through the housing in axial alignment to the chamber at the base end of the chamber;
and an outlet at the apex end of the chamber, the apex end outlet receiving peripheral vortical gas flow from the chamber, and the base end outlet receiving core vortical gas flow from the chamber, so that lower molecular or atomic weight components are concentrated in the flow withdrawn via the base outlet, and higher molecular or atomic weight components are concentrated in the flow withdrawn via the apex outlet.

8. A vortex separator according to claim 7 wherein a tubular baffle extends from the base outlet into the chamber to a point beyond the inlet, to deflect gas inlet flow away from the base outlet.

9. A vortex separator according to claim 7, in which the chamber is at most 200 mm. long.

10. A vortex separator according to claim 7, in which the sides of the chamber define a straight-and smooth-sided conical space.

11. A vortex separator according to claim 7, in which the gas inlets are at least two in number, and uniformly spaced.

12. A vortex separator according to claim 7, in which the gas inlets are constricted to give a supersonic inlet flow velocity.

13. A vortex separator according to claim 7, in which the chamber is conical at least in a major portion.

14. A vortex separator according to claim 7, in which the maximum chamber diameter is less than 2 mm.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE

15. A process for separation of a gas isotope mixture into at least two gaseous component fractions according to their molecular or atomic weight, comprising applying centrifugal force by rotating the mixture in a conical vortex having peripheral and core vortices and a diameter of not over 5 mm. and a cone angle within the range from 1° to 90°, at a gas inlet absolute pressure of at least 5 mm. Hg., and a pressure ratio of up to 10; recovering a higher molecular or atomic weight gaseous isotope fraction from the peripheral vortex, and recovering a lower molecular or atomic weight gaseous isotope fraction from the core vortex.

16. A process according to claim 15 in which said gas inlet absolute pressure is within the range from 5 to 1000 mm. Hg. and said pressure ratio is within the range from 1.5 to 10.

17. A process according to claim 16, in which the fractions thus separated from the mixture are subjected to successive vortical separa-tion stages under the stated conditions to further enrich them in at least one member selected from the group consisting of higher molecular or atomic weight fractions, lower molecular or atomic weight fractions, and both.

18. A process according to claim 17 in which the successive vortical separation stages are operated at a uranium distribution ratio of about 0.5.

19. A process according to claim 15 or 16 in which isotopes of differing atomic weight in a mixture of gaseous compounds thereof are separated.

20. A process according to claim 15 or 16 in which the cone angle is within the range from 3° to 30°.

21. A process according to claim 15 or 16 in which the vortex has a diameter of not over 2 mm.

22. A process according to claim 15 or 16 in which the vortex has a diameter of from 1 to 0.1 mm.

23. A process according to claim 15 or 16 in which the gas mixture velocity is at least at the velocity of sound in the gas mixture at the temperature of operation.

24. A process according to claim 15, in which the gas isotope mixture is blended with an inert gas of lower molecular or atomic weight than the said mixture, and the blend is then rotated in the vortex.

25. A process according to claim 24, in which the inert gas is hydrogen or helium.

26. A process according to claim 24, in which the gas mixture velocity is at least at the velocity of sound in the gas mixture at the temperature of operation.

27. A process according to claim 24, 25 or 26, in which the amount of inert gas is about 25% by volume of said blend.

28. A process according to claim 24, 25 or 26, in which the amount of inert gas is about 60% by volume of said blend.

29. A process according to claim 15 or 24, in which the gas isotope mixture comprises a mixture of uranium isotopes in the form of gaseous uranium compounds.

30. A process according to claim 15 or 24, in which the gas isotope mixture comprises a mixture of uranium isotopes in the form of gaseous uranium hexafluorides.

31. A vortex separator for separating gaseous mixtures into component fractions according to their molecular or atomic weight, comprising a housing having a separator chamber therein that is circular in cross-section, has an apex end and a base end; has a diameter at the base end of at most 5 mm. and a diameter at the apex end of at least 0.01 mm; and has a cone angle within the range from 0°
to 90°; at least one gas inlet through the housing at the base end of the chamber, arranged for tangential flow of gas from outside the housing into the chamber, to establish a vortical gas flow in the chamber from the base end towards the apex end, with the gaseous components dis-tributed towards the periphery of the vortex with increasing molecular or atomic weight, and towards the core of the vortex with decreasing molecular or atomic weight; and outlet through the housing in axial alignment to the chamber at the base end of the chamber; and an outlet at the apex and of the chamber, the apex end outlet receiving peripheral vortical gas flow from the chamber and the base end outlet receiving core vortical gas flow from the chamber.

32. A vortex separator according to claim 31, wherein the cone angle is within the range from 1° to 90°.

33. A vortex separator according to claim 31 or 32, wherein a tubular baffle extends from the base outlet into the chamber to a point beyond the inlet, to deflect gas inlet flow away from the base outlet.

34. A vortex separator according to claim 31 or 32, in which the chamber is at most 200 mm. long.

35. A vortex separator according to claim 31 or 32, in which the sides of the chamber defined a straight- and smooth-sided conical space.

36. A vortex separator according to claim 31 or 32, in which the gas inlets are at least two in number, and uniformly spaced.

37. A vortex separator according to claim 31 or 32, in which the gas inlets are constricted to give a supersonic inlet flow velocity.

38. A vortex separator according to claim 31 or 32, in which the chamber is conical in a major portion.

39. A vortex separator according to claim 31 or 32, in which the maximum chamber diameter is less than 2 mm.

40. A vortex separator according to claim 31 or 32, in which the maximum chamber diameter is from 1 to 0.1 mm.

41. A vortex separator according to claim 31 or 32, in which the angle of the apex end of the conical portion of the chamber is within the range from 3° to 30°.

42. A vortex separator according to claim 31 or 32, in which the ratio of the diameter at the base of the cone to the diameter at the apex of the cone is within the range from about 1. 3 to about 3. 5, and the ratio of the diameter at the base of the cone to the diameter at the core outlet is within the range from about 1. 3 to about 3. 5.