US2720976A

US2720976A - Thermal diffusion apparatus

Info

Publication number: US2720976A
Application number: US273738A
Authority: US
Inventors: Jones Arthur Letcher
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1952-02-27
Filing date: 1952-02-27
Publication date: 1955-10-18
Anticipated expiration: 1972-10-18

Description

Oct. 18, 1955 JONES 2,720,976

THERMAL DIFFUSION APPARATUS Filed Feb. 27, 1952 SEPARATION An x I0 INVENTOR.

FEED RATE, .h.

ARTHUR LETCHER JONES ATTORNEYS.

United States Patent Ofifice 2,720,976 Patented Oct. 18, 1955 THERMAL DIFFUSIQN APPARATUS Arthur Letcher Jones, Lyndhurst, Ohio, assignor to The Standard Oil Company, Cleveland, Ohio, a corporation of Ohio Application February 27, 1952, Serial No. 273,738 4 Claims. (Cl. 210-525) The present invention relates to a new and useful improvement in apparatus for separating liquid mixtures by continuous thermal diffusion.

It has been known for some time that it is possible to separate liquid mixtures into two or more dissimilar fractions by subjecting a thin film of liquid mixture to a temperature gradient. For many years this was accomplished in apparatus consisting essentially of two closely spaced, parallel walls forming a narrow slit and provided at each end of the slit with a reservoir having a volume considerably in excess of the volume of the slit. By filling such an apparatus with a liquid mixture and maintaining the opposed walls at ditferent temperatures, the liquid in one reservoir becomes enriched in one component of the liquid mixture, and the liquid in the other reservoir becomes impoverished in said component or enriched in another component.

After many hours or several days, the liquids in the two reservoirs are separately withdrawn.

The amount of liquid that can be subjected to thermal diffusion in apparatus of this type is extremely small, being measured in tens of ccs., and the time and heat energy required is so great that the technique of separating liquid mixtures by thermal diffusion remained nothing more than a laboratory curiosity for some eighty years.

More recently it has been proposed, e. g., in Patents 2,541,069-071, to subject liquid mixtures to thermal diffusion by maintaining a temperature gradient across a narrow slit formed by closely spaced concentric or fiat walls, and continuously passing a stream of the liquid mixture through the slit. It was found that by such means the rate of separation obtainable is much improved.

The slit widths of liquid thermal diffusion columns are extremely narrow, i. e., less than about 0.15 and desirably not more than about 0.06". When continuous liquid thermal diffusion is carried out in apparatus designed for concurrent flow, i. e., where the liquid mixture to be subjected to thermal diffusion is introduced at one portion of a vertical, horizontal or inclined slit, and the dissimilar fractions are withdrawn from adjacent the hot and cold walls, respectively, at points remote from the point of introduction, there is, theoretically, no limit to the minimum slit width due to the absence of endwise thermal circulation within the slit. For practical considerations, however, the minimum slit width for concurrent flow is of the order of about 0.01".

There is reason to believe that the withdrawal of dissimilar fractions of a liquid mixture from a thermal diffusion column, particularly when they move through the slit concurrently, causes sufficient turbulence to bring about a rather considerable remixing, immediately prior to Withdrawal, of the fractions separated within the slit. The end result of this is that the degree of separation obtainable is less than the degree of separation actually obtained within the slit, i. e., the efiiciency of withdrawal is not commensurate with the efficiency of separation by thermal diffusion.

The present invention is addressed particularly to improving the efiiciency with which fractions separated by thermal diffusion within a column can be withdrawn. In essence, the improvement with which the present invention is concerned is an improved form of withdrawal port, which comprises an elongated section in the face of one of the walls that is porous and has a longitudinal axis transversely of the direction of flow of liquid within the slit, and a passage for liquid extending to the exterior of the apparatus, one portion of which is substantially coextensive with and contiguous to the porous section.

The porous section may be an integral portion of one of the walls or built up by providing an elongated groove in the face of one of the walls forming the thermal diffusion slit and fitting an elongated stn'p member of porous, inert material in the groove, one face of the strip member being in substantially the same plane as the wall face.

The strip member may be of any suitable material or structure capable of distributing substantially uniformly the flow of liquid through it over the entire area of the member. Thus, for example, it may be, and preferably is in the form of a strip of sintered metal or porous ceramic material, e. g., porcelain, or it may have a composite structure, e. g., it may comprise fine screening material at the surfaces enclosing fibrous packing material, clay or the like.

To further promote substantially uniform distribution of flow of liquid through the porous section or strip member, the portion of the passage that is substantially coextensive with and contiguous thereto and the portion of the passage extending to the exterior of the apparatus are sufiiciently large in volumetric proportions in relation to volumetric flow capacity of the porous strip member to assure substantially uniform flow through all of the pores but not so large as to delay unduly the displacement from the passage of unseparated liquid mixture with which it is initially filled upon commencement of the liquid thermal diffusion process, or hold up more than necessary of the separated fraction during the continued operation. The passage, particularly the portion contiguous to the porous section, may be tapered to assist in uniform withdrawal, especially if the volumetric proportions are relatively small.

One of the primary advantages of the withdrawal port of this invention is that it increases enormously the chiciency, probably due to a minimization of turbulence, with which fractions separated within a thermal diffusion slit can actually be removed from the slit.

This and other advantages, as well as the utility of the invention, will become further apparent from the following detailed description made with reference to the accompanying drawing, wherein:

Figure l is a cross-sectional view of a liquid thermal diffusion apparatus, provided with two withdrawal ports illustrative of the invention;

Figure 2 is a cross-sectional line 22 of Figure l; and

Figure 3 is a graph demonstrating the superior efficiency of withdrawal ports constructed in accordance with this invention as compared with that of a groove type take-off port.

Referring now to Figures 1 and 2 illustrating schematically a liquid thermal diffusion apparatus utilizing two Withdrawal ports constructed in accordance with the present invention, the opposed walls 10 and 11 having wall faces 12 and 14, respectively, are substantially parallel, liquid impervious and stationary. The wall faces 12 and 14 are separated from one another a distance of less than view taken along section about 0.15", desirably not more than 0.06" and preferably from about 0.01 to about 0.035" apart by one or more gaskets 16 to form a narrow slit 17. The walls and Y11 may be maintained in their ,position relative of the wall 10 and a'passage through which the liquid mixture :can'be introduced from outside the apparatus by any suitable means and to the slit 17 by way of groove 19.

The

apparatus ures

1 and 2 is provided, at the end of the slit 17- remote from the

inlet port

19, 20, with two withdrawal ports, oneiin wall 10 and the other in wall 11. The withdrawal port in wall 10, for example, comprises an elongated groove in the face12 and an elongated strip member 21 of porous material fitted in the groove. The face 22 of the strip member 21 is in substantially the same plane as 1 k the wall face 12. Under, adjacent and substantially coextensive With the str'm member 21 there is an elongated flow-equalizing portion 24 of a passage for liquid that communicates with a passage portion 26 extending to the exterior of the wall 10.

A series of 'testswere run on apparatus essentially similar to that illustrated .in Figures 1 and 2 wherein the Width of the slit 17 was 0.035", the slit was in the vertical position shown, and the effective length and breadth of the slit were both 10'. One wall was maintained at a temperature of 270 F. and the other was maintained at a temperature of 70F. In one series of tests the two withdrawal ports were constructed as shown in the drawing, the strip member 21 being a porous, sintered metal strip.

In a comparative series of test runs all conditions were identical, except that instead of the withdrawal ports illustrated in Figures 1 and 2, grooves having a depth of oneeig'hth of an inch and a width of between two and three sixteenths of .an inch, connected by means of equally spaced holes in the bottom of the groove to a passage similar to passage 26 of the apparatus illustrated, but extending the entire length of the groove, were used. I

In each test a 50/50 mixture of cetane and methylnaphthalene was fed into the column by way of

inlet port

19, 20 at various rates of flow, and fractions from adjacent the hot and cold walls, respectively, were with- I drawn through the oppositely disposed withdrawal ports at equal rates. To measure the degree of 'separationobtained'in 'each instance, the index of refraction at C.

of the two fractions were obtained, the difference in indices of refraction between the hot wall product and the cold wall product being an accurate indicator of the extent to which separation took place. The results of these tests are illustrated "-in the graph of Figure 3, wherein the curve A represents the degree of separation, in terms of difierence between-the indices of refraction of the hot wall and cold wall products obtained with the thermal diffusion column having the withdrawal ports of this invention, andcurve B represents the degree of separation, expressed similarly, obtained under identical conditions but with asimple groove type of withdrawal port. These curves reveal that with the withdrawal ports of the present invention, the degree of separation rises rapidly as the feed rate is increased to about one liter per hour and remains practically level at higher feed rates, whereas with the simple groove type withdrawal ports the degree of separation is less than one-fifth as great at feed rates ranging from about 1 to 6 liters per hour.

'The degree of separation that takes place within a thermal diffusion slit is determined, for any given liquid mixture, by such conditions as temperature gradient, slit width, wall surface area and residence time of the liquid, all of which were the same in the comparative tests. It isself-evident', therefore, that the results obtained in these illustrated by way of example in Fig.

tests are indicative of the ,great superiority of the withdrawal ports of this invention over the groove type of withdrawal port. The withdrawal ports of this invention make it possible to separate physically the fractions concentrated adjacent the hot and cold walls within the thermal diffusion slit with far greater efficiency than has heretofore been possible.

It is to be understood that various changes and modifications will readily occur to those skilled in the art upon reading this description. All such changes and modifications are intended to be included within the scope of the invention as defined in the appended claims.

I claim:

1. Liquid thermal diffusion apparatus comprising two tially uniform narrow slit, means for relatively heating and cooling said walls to impose a temperature gradient across said slit an inlet communicating with the slit and at least two outlets communicating with the slit, wherein at least one of the outlets comprises an elongated porous section in the face of one of the walls, one face of the porous section being in substantially the same plane as the wall face, and a passage for liquid extending to the exterior of the apparatus, one portion of said passage being substantially coextensive with and contiguous to the porous section.

2. The apparatus defined in claim 1 wherein the slit forming walls are vertical and two outlets are disposed, one in each wall, at one end of the slit.

3. Liquid thermal diffusion apparatus comprising two substantially parallel, liquid-impervious stationary walls, the opposed faces of which are smooth and spaced apart a distance less than about 0.15 inch to form a substantially uniform narrow slit, means for relatively heating and cooling said walls to impose a temperature gradient across said slit an inlet communicating with the slit and at least two outlets communicating with the slit, wherein at least one of the outlets comprises an elongated groove in the face of one of the walls; an elongated strip member of porous material fitted in the groove, one face of the strip member being in substantially the same plane as the wall face; and a passage for liquid extending to the exterior of the apparatus, one portion of said passage being substantialy' coextensive with and contiguous to the strip member.

4. Liquid thermal difiusion apparatus comprising two substantially parallel, liquid-impervious stationary walls, the opposed faces of which are smooth and spaced apart a distance less than about 0.15" to form a substantially uniform narrow slit, means for relatively heating and cooling said walls to impose a temperature gradient across said slit wherein at least one of the walls is provided with an elongated porous section in the face there of, said elongated porous section having its greatest dimension extending in a direction transverse to the direc-- tion of fiow of liquid within'the slit and one face of the porous section being in substantially the same plane as the wall face, and with a passage for liquid extending to the exterior of the apparatus, one portion of said passage being substantially coextensive with and contiguous to the porous section, and at least one of said walls having an inlet for supplying liquid to said slit, said inlet being spaced from said porous section.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. LIQUID THERMAL DIFFUSION APPARATUS COMPRISING TWO SUBSTANTIALLY PARALLEL, LIQUID-IMPERVIOUS STATIONARY WALLS, THE OPPOSED FACES OF WHICH ARE SMOOTH AND SPACED APART A DISTANCE LESS THAN ABOUT 0.15 INCH TO FORM A SUBSTAN TIALLY UNIFORM NARROW SLIT, MEANS FOR RELATIVELY HEATING AND COOLING SAID WALLS TO IMPOSE A TEMPERATURE GRADIENT ACROSS SAID SLIT AN INLET COMMUNICATING WITH THE SLIT AND AT LEAST TWO OUTLETS COMMUNICATING WITH THE SLIT, WHEREIN AT LEAST ONE OF THE OUTLETS COMPRISES AN ELONGATED POROUS SECTION IN THE FACE OF ONE OF THE WALLS, ONE FACE OF THE POROUS SECTION BEING IN SUBSTANTIALLY THE SAME PLANE AS THE WALL FACE, AND A PASSAGE FOR LIQUID EXTENDING TO THE EXTERIOR OF THE APPARATUS, ONE PORTION OF SAID PASSAGE BEING SUBSTANTIALLY COEXTENSIVE WITH AN CONTIGUOUS TO THE POROUS SECTION.