US2892544A - Thermal diffusion method and apparatus - Google Patents

Description

June 30, 1959 A. JONES THERMAL nzwuszon METHOD AND APPARATUS 3 Sheets-Sheet l wrWA Z'T 'F ONES I ATT RNEY June 30, 1959 A. JONES 2,392,544

THERMAL DIFFUSION METHOD AND APPARATUS Filed Dec. 24, 1956 3 Sheets-Sheet 2 INVENTOR flew/m? L. Jb/vss ATT NEY June 30, 1959 A. L. JONES THERMAL DIFFUSION METHOD AND APPARATUS 3 Sh'eets-Sheet 3 Filed Dec. 24, 1956 ATTO United States Patent THERMAL DIFFUSION METHOD AND APPARATUS Arthur Letchcr Jones, Lyndhurst, Ohio, assignor to The Standard Oil Company, Cleveland, Ohio, a corporation of Ohio Application December 24, 1956, Serial No. 634,393

21 Claims. (Cl. 210-72) This invention relates to an improved method and apparatus for effecting, by thermal diffusion, separations of mixtures of dissimilar components into fractions containing the components in different concentrations.

The method of effecting separations by thermal diffusion basically involves imposing a temperature gradient on a mixture of the components to be separated. This can be accomplished, for example, by providing two closely spaced surfaces, one of which is heated relative to the other and referred to as the hot wall, the other being referred to as the cold wall, and introducing the mixture to be separated into the space between the hot and cold walls. Methods and apparatus that have transformed thermal diffusion from a laboratory curiosity to practical utility are described in a number of patents to Jones et al., including US. Patents 2,541,069; 2,541,- 070; 2,541,071; 2,712,386; 2,720,975; 2,720,978; 2,723,- 033 and 2,742,154, and to Jones, including 2,720,976; 2,720,977; 2,720,979 and 2,723,034.

It is believed that the effectiveness of the thermal diffusive forces exerted on the components of the mixture due to the difference in temperature is related to the distance between the hot and cold walls. For this reason, among others, the tendency has been to utilize a spacing between the hot and cold walls that is quite small, particularly when the mixture to be resolved into different fractions is a liquid under the conditions of operation. It will readily be appreciated, however, that a reduction in the spacing between the opposed walls of a thermal diffusion separation chamber to a minimum of the order of .01 inch, while perhaps effective in obtaining a good separation within the chamber, very greatly increases the difiiculties of physically removing the dissimilar fractions from the chamber without remixing them at least in part. With spacings of the order of 0.01 inch and even as great as 0.15 inch, thin, razor sharp dividers intended to physically separate a fraction preferentially accumulating adjacent the hot wall, referred to generally as the hot wall fraction, from the cold wall fraction tends to create considerable turbulence at reasonable flow rates and therefore remixing of the fractions just prior to withdrawal from the thermal diffusion separation chamber.

This invention is addressed specifically to the problem of effectively separating the fractions into which a mixture of dissimilar materials is resolved within a thermal diffusion separation chamber.

In accordance with the invention, a mixture which is to have a temperature gradient imposed upon it for the purpose of separating the mixture into dissimilar fractions by thermal diffusion is absorbed in a porous and inert carrier material. Inasmuch as the carrier material is porous, the components in the mixture are free to move within the carrier material under the influence of thermal diffusive forces. The mixture is thereby resolved into fractions that accumulate preferentially in the facial strata of the carrier material to which the maximum and minimum temperatures are respectively applied.

r 2,892,544 Patented June 30, 1959 The strata of the carrier material are then physically separated from one another by any suitable means and, while completely avoiding undesired remixing of the different fractions in various strata, the fraction accumulated in at least one of the separated strata is then removed by any suitable means.

In one embodiment, the carrier material comprises two or more strips of any material such as paper, cloth, asbestos, or an open-celled elastic material, that is inert to the components of the mixture to be subjected to thermal diffusion and that is porous or absorbent in having connecting interstices or open cells that permit limited convective flow of the mixture and relatively unlimited molecular movement of the mixture components. Thus, for example, the carrier may consist of two strips of absorbent paper placed face-to-face to form a composite strip that is saturated with the mixture to be subjected to thermal diffusion, passed through a thermal diffusion zone in which the opposed faces of the composite strip are subjected to different temperatures to impose a temperature gradient across the mixture in the strips and, after leaving the thermal diffusion zone, the two strips are readily separated and one or both of the dissimilar fractions are separately removed. If desired, the strips may be recycled by replacing them face-to-face and resaturating them with fresh mixture.

A considerable number of modifications of the embodiment specifically described are also within the scope of the invention. Thus, for example, the thermal diffusion zone may be vertical, horizontal or inclined and it may be formed by plane surfaces, the peripheral surfaces of drums or combinations thereof, and may even comprise a chamber having a relatively heated portion and a relatively cooled portion separated by nothing more than the carrier for the mixture subjected to thermal diffusion. Any of these modifications and others are permissible so long as they are capable of imposing a temperature gradient across the mixture in the carrier material.

The distance between the outside surfaces of the carrier while in the thermal diffusion zone, and therefore also between opposed hot and cold walls, should be of the order of about 0.01 to 0.15 inch and advantageously as little as 0.002 inch or even somewhat less for liquid mixtures, and may be somewhat greater for gaseous mixtures. For optimum separation, the difference between the temperature of the hot wall and that of the cold wall should be as great as possible. It is important, however, to bear in mind that the temperature of the hot wall or walls should not be so high as to cause decomposition of the mixture or any of its components under the conditions of operation, including residence time in the thermal diffusion zone, or to cause a change in state, e.g., liquid to vapor or gas. Likewise, the temperature of the cold wall should be high enough to avoid a change of state of the mixture or its components from gas to liquid or solid or from liquid to solid, and further to avoid precipitation, congealing and preferably also significant increases in viscosity, if the mixture is a liquid, that would tend to reduce the rate of thermal diffusion.

While the web-type of carrier is preferred because of its ready availability, other types such as an absorbent powder that is pushed or fed through a thermal diffusion zone by any suitable means such as a plunger or a screw and split into halves or several strata by one or more knife edges located at the end of the zone intermediate the chamber-forming walls would also be operable.

Web-type of carriers may be in the form of sheets, strips or endless belts and may be used in various combinations depending upon the rate of production, degree of separation and number of fractions desired. Thus, for example, if as many as six different fractions are do 2,892,544 I i i A sired, six carrier strips may be saturated with the liquid mixture, passed through the thermal diffusion zone together and thereafter separated for individual removal of, the different fractions. It is also within the scope of the invention to use a carrier member that serves only to convey the mixture to the thermal diffusion zone and to transfer dissimilar fractions from it to auxiliary carrier members.

The means for removing the various fractions individually from the separated carrier members following thermal diffusion may also be varied considerably depending upon the fraction to be removed and the characteristics of the carrier material, as will readily be appreciated. Thus, for example, if the carrier material is a resilient elastomer that readily recovers its original dimensions after being distorted, it can be squeezed by passage through nip rollers or equivalent means to remove the fraction absorbed in it and bring it into condition for absorbing another such fraction. The separated c'arrier strip can also be passed through a volatile solvent for the fraction, or through a heated zone in which the fraction absorbed in it is distilled. Other means of removal will readily occur to those skilled in the art.

In all of these variations there is the underlying concept of eifecting separation of the mixture within an absorbent carrier material and then, instead of attempting to separate liquids or gases per se, it is the carrier material that is divided into two or more strips or strata, each containing a dissimilar fraction.

The method and apparatus of the invention are adaptable to the separation of liquid and gaseous mixtures, particularly mixtures that are extremely difficult or impossible to separate by other means such as distillation, solvent extraction, and the like. Thus, by way of illustration, separation by thermal dilfusion has been effected between such difiicultly separable components as the alpha and beta isomers of monomethylnaphthalene, the cisand trans-stereo isomers of LZ-dimethylcyclohexane', orthoand para-xylene, to name but a few.

The advantages of the method and apparatus of the invention, particularly in effecting separation of a liquid mixture into fractions containing the components in different than initial concentrations, are considerable. One of the most important advantages is that the interstitial structure of the absorbent carrier material efiectively avoids any turbulence or remixing of the fractions when the strata in which they have become concentrated are physically separated from other strata of the remainder of the carrier material. Another very important and ancillary advantage is that the effective width or thickness of the mixture at the time of application of the temperaure gradient can be reduced very appreciably Without encountering any separation ditficulties and that hence very significant increases in temperature gradient and therefore efiiciency of separation can be obtained.

Another advantage is that the method and apparatus of the invention provide means for reaping the full benefit of the rapidity with which thermal diffusion takes place when a temperature gradient is imposed across a mixture.- This is manifested in the short residence time that is required in the thermal diffusion separation zone and in the facility with which the dissimilar fractions can be physically separated from one another without promoting turbulence.

It is to be understood, of course, that while it is possible to work with higher temperature gradients and smaller spacings between hot and cold walls, it is entirely within the scope of the invention to utilize temperature gradients and wall spacings such as those described in the prior art.

The advantages referred to, as well as other advantages and the utility of the invention, will become more .apjaarentfrom the following descriptionof the accompanyingffigures of drawing and from the examples ineluded to illustrate the best modes now contemplated for practicing theinvention.

In the drawing:

Figure 1 is a schematic illustration of an apparatus in which two superimposed strips of absorbent material are used to pick up and become saturated with a liquid mixture, carry the mixture through a thermal diffusion zone, separate the different fractions they contain, and convey the separated fractions to individual removal zones;

Figures 2 and 3 are schematic views illustrating the use of a multiplicity of superimposed carrier strips;

Figures 4 and 5 illustrate embodiments in which a carrier strip is saturated with the liquid mixture to be subjected to thermal diffusion and two auxiliary strips are employed, one on each side, to absorb dissimilar fractions from the carrier strip;

Figure 6 illustrates an embodiment in which two contiguous saturated strips are passed vertically through a thermal diffusion separation chamber, then separated for individual removal of the fractions accumulated therein and recycled for resaturation with the initial mixture; and

Figure 7 illustrates an embodiment in which two carrier members are mounted on rotatable drums.

v Referring now to Figure 1, a composite carrier strip 10, 11 is saturated with a liquid mixture 12 in a vessel 14 and guided by means of suitable rollers to pass between a hot plate 16 and a cold plate 17, said plates being relatively heated and cooled by any suitable means forming no part of this invention and being spaced apart a distance approximately equal to the combined thickness of the carrier strips 10, 11, i.e., suflicient to avoid binding of the carrier strip but afford maximum heat transfer between the strip members and the plates.

The thermal diffusive forces generated by the temperature gradient across the liquid mixture absorbed in the composite carrier strip 10, 11 effect a preferential accumulation of one of the components adjacent the hot plate and in a preferential accumulation of another of the components adjacent the cold plate with the result that the liquid mixture in the composite strip 10, 11 is resolved into dissimilar fractions, one in strip 10 and the other in strip 11. After leaving the thermal diffusive zone between the hot and cold plates 16 and 17, the strips 10 and 11 are separated, as shown, strip 11 passing around a guide roller 19 through a removal chamber 20 and strip 10 passing between two pressure rollers 21 for removal of the fractions represented by the symbols P and P respectively. The strips 10 and 11 are then guided back to the vessel 14 for resaturation with the liquid mixture.

Figure 2 illustrates means of separating a mixture into more than two fractions, e.g., six fractions, wherein the fractions accumulating in the strip members 22, 24 and 26 are progressively more concentrated in the component preferentially accumulating adjacent the hot plate H and the strip members 27, 29 and 30 are progressively more concentrated in the components preferentially accumulating adjacent the cold plate C. If desired, only the fractions in strips 26 and 30 are removed and the rest of the strips can be recycled.

In Figure 3 a plurality of composite strips 10, 11 are shown moving upwardly through a multiple slit thermal diffusion separation chamber comprising alternate hot and cold plates designated by symbols H and C, respectively.

The apparatus illustrated in Figure 4 comprises a main carrier strip 31 of absorbent and inert material saturated with a liquid mixture by passage under the sprays indicated-schematically at 32 and passing between auxiliary strips34 and 35 carried by the drums 36 and 37. The peripheral-surfaces of the drums 36 and 37 are relatively cooled and heated by any suitable means and are spaced apart a distance equal to the combined thickness of the strips31, 34 and 35. The speed of rotation of the drums, and therefore of the movement of the main-strip ,31and the auxiliary. strips ,34 and 35, is adjustedso'as to permit sufiicient time of contact between the auxiliary strips and the main strip to impose a thermal gradient across the main carrier strip 31 and to permit absorption by the auxiliary strips 34 and 35 of the fractions preferentially accumulating adjacent the relatively hot and cold surfaces of the drums. If desired, the thermal diffusive effect of the relatively heated and cooled drums 36 and 37 may be augmented by precooling and preheating, respectively, the opposed faces of the carrier strip 31 e.g.,' by direct contact with cooled and heated drums 38 and 39. The fraction accumulating in the auxiliary strip 34 is removed from the strip by any suitable means such as the pinch rollers 40 for collection of a product designated as P and the fraction accumulating in the auxiliary strip 35 is shown as being removed therefrom by passage through a chamber 41 heated by a coil or other equivalent means 42, the volatilized product being represented by the symbol P In Figure there is shown an apparatus comprising vertically disposed hot and cold plates 44 and 46 between which there is passed a main carrier strip 31 and two auxiliary carrier strips34 and 35. The auxiliary strip 34 is shown as alternately passing over the cold plate in intimate contact with one face of the main carrier strip 31 and through a cold wall product removal zone 47. The other auxiliary strip 35 is shown as passing alternately over the hot plate 44 while in intimate contact with the other face of the main carrier strip 31 and passing through a hot wall product removal zone 49. The main carrier strip 31, after having given up portions of its absorbed mixture to the auxiliary strips 34 and 35, is resaturated with liquid mixture 50 by passage through the vessel 51 and is thereupon recycled for repeated passage through the thermal diffusion separation chamber between the auxiliary strips.

Figure 6 illustrates an embodiment generally similar to that illustrated in Figure 1 except that the saturated composite strip 10, 11 passes upwardly through a thermal diffusion zone comprising a heated chamber portion 52 separated from a cooled chamber portion 54 by the composite strip so that the heat of the chamber portion 52 is applied directly to the exposed face of the strip member and the exposed face of the strip member 11 is exposed directly to the relatively low temperature in the chamber portion 54. After leaving the thermal diffusion zone, the strip members 10 and 11 are parted and directed to suitable zones for removal of their preferentially absorbed fractions. Thus, for example, the fraction in the strip member 10 may be heated by a coil 56 or other suitable means in a chamber 57 and the other fraction can be removed by passing the strip member 11 through a vessel 59 through which a solvent for the fraction is continuously circulated. After removal of the fractions, the strip members 10 and 11 are recycled for resaturation with the initial liquid mixture 12 in the vessel 14, reunited into a composite strip and again passed through the thermal diffusion zone 52, 54.

Figure 7 illustrates an embodiment in which a relatively cooled drum 60, having on its peripheral surface an absorbent and inert carrier member 61, and a relatively heated drum 62, having on its peripheral surface an absorbent and inert carrier member 64, are mounted for rotation in such manner that the carrier members 61 and'64 on the drum's'are immersed in the liquid mixture 12 in the vessel 14 for saturation therewith. While the carrier members 61 and 64 are in contact with one another, a thermal gradient is imposed across the liquid they contain to establish thermal diffusive forces resulting in a preferential accumulation of one fraction in the carrier member 61 and of another fraction in the carrier member 64. The fraction absorbed in the carrier member 61 is removed therefrom by pressing with a pinch roller 66 whereas the fraction absorbed in the carrier member 64 is volatilized within the hood 67 for removal as P;,.

Example I In an apparatus of the type-shown in Figures 1 and 2, five strips of paper having a combined thickness of about 0.0066 inch, were used as the absorbent carrier. They were saturated with a 50:50 (by volume) mixture of cetane and dibutyl phthalate. The refractive indexes of the components were 1.4331 and 1.4908, respectively, and that of the mixture was 1.4602.

The mixture in the combined strips was subjected to thermal diffusion by contact of the exterior members with surfaces at 50 F. and F., so that the strips were exposed to these temperatures for a residence time of three minutes.

The strips were then separated and examined individually to determine the refractive indexes of the fractions absoorbed. The results were as follows, strips Nos. 1 and 5 being the strips immediately adjacent the hot and cold surfaces, respectively:

Refractive Index with Residence Time of Three Minutes Strip N o.

Example 2 The procedure of Example 1 was repeated with eight strips of paper having a combined thickness of 0.0106 inch, and a residence time of ten minutes. The average results are tabulated immediately below:

Refractive Index with Residence Time of Ten Minutes Strip N0.

aoqmcnheato- The data in these examples demonstrate the eflicacy of the invention in separating an initial mixture into fractions having varying concentrations of the mixture components.

It is to be expected that numerous modifications will readily become apparent to those skilled in the art upon reading this description. All such modifications are intended to be included within the scope of the invention :as defined in the appended claims.

I claim:

1. Method for separating from a mixture of dissimilar materials a fraction containing said materials in a different concentration which comprises imposing a temperature gradient across a thin layer of an absorbent and inert carrier containing the mixture, whereby the concentration of one component of the absorbed mixture is increased adjacent one face of the carrier layer and decreased adjacent the other face, and separating a facial stratum of the carrier layer from the remainder of'the carrier for physically removing the fraction of themixture absorbed therein from the remainder of the mixture.

2. Method for separating a mixture of dissimilar materials into fractions containing the materials in different concentrations which comprises imposing a temperature gradient across a thin layer of an absorbent and inert carrier containing the mixture, whereby the concentration of one component of the absorbed mixture is increased adjacent one face of the carrier layer and de creased adjacent the other face, and dividing the carrier layer into stratafor physically removing the fractions of the mixture absorbed therein. I

3. Method for separating a mixtureof dissimilar materials into fractions containing the materials in different concentrations which comprises imposing a temperature gradient across a thin layer of an absorbent and inert carrier containing the mixture, whereby the concentration of one component of the absorbed mixture is increased adjacent one face of the carrier layer and decreased adjacent the other face, dividing thecarrier layer into two facial strata for physically separating the fractions of the mixture absorbed therein, and removing the fractions from said strata. H v '4. Method for separating a mixture'of dissimilar ma{ terials into fractions containing the materials in different concentrations which comprises imposing a temperature gradient across a thin assemblage of super-imposed absorbent and inert carrier strips containing the mixture, whereby the concentration of one component of the absorbed mixture is increased in one of said carrier strips and diminished in another, separating the carrier strips from one another, and separately removing from at least one of said carrier strips the fraction of the mixture absorbed therein.

5. Method for separating a mixture of dissimilar materials into fractions containing the materialsjndifferjent concentrations which comprises imposing atemperature gradient across two thin superimposed carrier strips of an absorbent and inert material, whereby the concentration of one component of the absorbed mixture isincreased in one of the strips and diminished in the other strip, sepa: rating the carrier strips from one anothe'rfor physically removing a fraction, and separately removing frornat least one of the carrier strips the fraction of the mixture absorbed therein.

6. Method defined in claim wherein the removal is performed by applying pressure to the carrier strip.

7. Method defined in claim 5 wherein the removal is performed by passing the carrier strip through a solvent for the fraction absorbed therein.

8. Method defined in claim 5 wherein'the removal is performed by heating the carrier strip for distilling the fraction absorbed therein. H v, g I

9. Method defined in claim 5 wherein the carrier strips, after removal, are recycled for reabsorbingthe mixture'o'f dissimilar materials before being again subjected to a tem perature gradient.

10. Method for separating a mixture of dissimilar materials into fractions containing the materials in different concentrations which comprises saturating a thin carrier strip of absorbent and inert material with said mixture, bringing absorbent and inert auxiliary strips into intimate contact with the sides ofthe carrier strip for forming a composite strip, imposing a temperature gradient across the composite strip whereby one component of the absorbed mixture preferentially accumulates in one of the auxiliary strips and another component preferentially accumulates in another of the auxiliary strips, separating the auxiliary strips from the carrier strip, separately removing dissimilar fractions of the initial mixture absorbed from the auxiliary strips, and recycling the carrier strip and auxiliary strips.

11. Apparatus for separating a mixture of dissimilar materials into fractions containing the materials in different concentrations comprising means for relatively heating and cooling the opposed surfaces of a thin layer of an absorbent and inert carrier containing saidmixture, means for separating from the carrier layer a thinner facial stratum of substantially uniform thickness, and means for removing from said separated stratum a fraction containing one of the materialstherein in a dilferentthan initial concentration.

12. Apparatus defined in claim 11 wherein the carrier comprises a plurality of superimposed strips and a facial stratum is an exterior one of said superimposed strips.

13. Apparatus defined in claim 11 wherein the means for relatively heating andcooling the carrier comprises closely and substantially equidistantly spaced surfaces maintained at different temperatures.

14. Apparatus defined in claim 11 wherein the means for relatively heating and cooling the carrier comprises closely and substantially equidistantly spaced plane surfaces maintained at different temperatures.

15. Apparatus defined in claim 11 wherein the means for relatively heating and cooling the carrier comprises rotatable drums having heated and cooled peripheral surfacesfor simultaneously coming into intimate contact with the opposed surfaces of the carrier.

16. Apparatus defined in claim 11 wherein the means for relatively heating and cooling the carrier comprises adjacent heated and cooled chambers separated from one another by the carrier.

'17. Apparatus defined in claim 11 wherein the removing means comprises a pressure roller for squeezing the fraction from the separated stratum.

18. Apparatus defined in claim 11 wherein the removing means comprises a vessel containing a solvent for the fraction in the separated stratum.

19. Apparatus defined in claim 11 wherein the removing means comprises a heater for distilling the fraction in the separated fraction.

20. Apparatus for separating a mixture of dissimilar materials into fractions containing the materials in difierent concentrations comprising means for saturating two endless strip members of absorbent and inert material with the mixture; means. for passing the saturated strip members, whilejin intimate face-to-face contact with one another, through a thermal diffusion zone comprising means for relatively heating and cooling the exterior surfaces of said strip members, whereby the concentration of one component of the absorbed mixture is increased in one of the strip members and diminished in the other; and means for removing from at least one of the strip members a fraction containing one of the dissimilar materials therein in a different than initial concentration.

21. Thermal diffusion apparatus for separating a mixture of dissimilar materials into fractions containing the materials in different concentrations, comprising a carrier material that is inert to, and capable of absorbing, the mixture, and means for moving the carrier material through a thermal diffusion zone comprising means for relatively heating and cooling different portions of said carrier material whereby the concentration of at least one of the dissimilar materials of the mixture absorbed in said carrier material-is increased in one portion thereof and diminished in another portion thereof, and means for removing from said carrier material a fraction containing one of said dissimilar materials.

2,521,112 Beams Sept. 5, 1950

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