US3064814A - Thermal diffusion method - Google Patents

Description

Nov. 20, 1962 R. GRAssELLI ETA; 3,064,814

THERMAL DIFFUSION METHOD y Filed Jan. 2e, 19Go ROBERT QSSELLI United States @arent @ffice 3,054,814 Patented Nov. 20, 1962 3,664,814 THERMAL DIFFUSEGN METHOD Robert Grasselli, Cleveland, Ghio, and Karl J. Alising,

Arlov, Sweden, assignors, by mes-ne assignments, to

Universal Oil Products Company, Des Plaines, lll., a

corporation of Delaware Filed Jan. 26, 1960, Ser. No. 4,6% 2 Claims. (Cl. 210--72) This invention relates to an improved continuous method for separating materials by liquid thermal diffusion.

It has been known for some time that it is possible to separate by liquid thermal diffusion materials that are incapable of separation by any other known methods or that are separated by other methods with only great difficulty.

Succinctly stated, the process of liquid thermal diffusion consists of confining a liquid mixture, a term intended herein to include mixtures that are liquid under conditions of operation, in a narrow separation chamber and imposing a temperature gradient across the chamber. This is usually accomplished by confining the liquid mixture in a vessel or column having two closely spaced parallel or concentric walls, one of which is maintained at a substantially higher temperature than the other. The walls are vertical or have vertical components and thermal circulation of the contents of the chamber is therefore set up by virtue of the differences in density adjacent the relatively heated and cooled walls. One of the dissimilar fractions having a higher than initial concentration of one component accumulates adjacent the hot wall. Another fraction having a higher than initial concentration of a diderent component preferentially accumulates adjacent the cold wall. The fraction adjacent the hot wall ascends in the chamber as the top product. The fraction adjacent the cold wall descends in the chamber as the bottom product.

It has been found that in order to obtain satisfactory rates of separation by thermal diffusion, the opposed thermal diffusion chamber-forming surfaces of the hot and cold wall should be spaced apart no more than about 1/2 inch, spacings of the order of 0.15 inch or less, and preferably about 0.02 to 0.06 inch, being most effective. It will readily be seen that due to the small spacings involved, considerable diiculty is encountered in obtaining column eiciency which may be defined as the product of the degree of separation between the feed mixture and the desired product and the production rate of the desired product. Any improvement in either or both of these factors would be highly advantageous.

lt has already been found that the degree of separation, particularly at relatively high feed rates, can be significantly improved over that which is obtainable with static thermal diffusion methods by employing a continuous mixed-end feed system for thermal diffusion. Such a thermal diffusion method and apparatus in which such method may be employed is disclosed in Patent No. 2,827,171 which is assigned to our assignee. The thermal diffusion unit disclosed therein offers excellent separation efficiency by providing upper and lower reservoirs communicating with the upper and lower ends of a vertical separation chamber. These reservoirs have sufficient volume to permit mixing of fresh feed with liquids entering the reservoirs from the chamber, as well as preventing a turn-about of the liquid within the separation chamber before it reaches the reservoir; i.e., the liquid ascending the hot wall, for example, does not start down the cold wall but rather is mixed with the feed in the reservoir and the mixtures start down the cold wall. 'The same is true ,at the bottom. Generally, these effects are obtained when the reservoirs are at least about twice as wide and twice as high or deep as the spacing between the walls forming the separation chamber. In conducting this mixed-end feed system, the feed liquid mixture is continuously introduced into the upper and lower reservoirs and is thereby adrnixed in the reservoirs with the ascending and descending fractions; a portion of the liquid enters the separation chamber to replace the fractions entering the reservoirs; and the remainder, enriched with the components in the dissimilar fractions, is withdrawn from the upper and lower reservoirs, respectively.

The present invention offers an improvement over such a mixed-end feed method of thermal didusion by increasing the throughput of material or rate of flow of liquid across one end of the apparatus over the throughput or rate of ow of liquid across the other end. Although any ratio of throughput rates at the opposite ends of the apparatus greater than 1:1 is operable for the present invention to improve separation efficiency, it has been found that ratios from 2:1 to 20:1 are most effective and the preferred range is from 2:1 to 10:1. There is usually no economic advantage to be gained by increasing the ratio over 10:1 since the improvement gradually approaches but never reaches a limited value.

It is to be understood that the invention is contemplated for use with a mixed-end feed system to either a` single thermal diffusion chamber such as is disclosed in Patent No. 2,827,171 or to a series of thermal diffusion chambers such as disclosed in Patent No. 2,827,172 wherein the flow is across the individual chambers and the ow at the top of the chambers may be either countercurrent or concurrent in relation to the ilow at the bottom of the chambers. More detailed information relating to the apparatus and the method of these systems may be obtained from the foregoing cited references, and it is intended here to include such disclosure from these patents as may be necessary for a complete and clear understanding of the invention.

Several important advantages are to be gained over the conventional mixed-end feed system by the improvement of the present invention. lt has been found, for example, that a higher quality product may be produced at the same rate. Furthermore, the rate at which a given quality product is produced may be increased. It has also been found that for any given level of througput to a chamber, a higher quality product may be produced by increasing the throughput at one end in relation to the other. Moreover, from an .operational viewpoint, the method of the invention provides for better separation of viscous materials since this method inherently decreases the likelihood of thermal siphoning and bypassing.

A better understanding of the invention will be gained from the following discussion taken together with the accompanying patent drawing which is a schematic illustration of a preferred embodiment for practicing the present invention utilizing the apparatus shown in Patent No. 2,827,172. ln the drawing, annular thermal diffusion chambers 10, 11, 12, and n, are formed by an outer surface of inner tube 13 and an inner surface of outer tube 14. Suitable means not forming a part of this invention are provided for relatively heating and cooling the chamber walls 13- and 14, as indicated by the symbols H and C representing the hot and cold walls, respectively. Hence, tube 13 may be relatively heated by passing steam therethrough as indicated by the arrows, and outer tube 14 may be relatively cooled by circulating water through a cooling jacket 17 in the Amanner indicated by the arrows. The symbols FT and FB represent top feed liquid and bottom feed liquid, respectively, and the symbols PT and PB represent top product and bottom product,

respectively. The direction of flow across the series of separation chambers is indicated by the arrows, and flow across the top is countercurrent to ow across the bottom of the chambers.

In operation, top feed liquid FT is continuously'introduced into upper reservoir of separation chamber 10 and bottom feed liquid FB is continuously introduced into lower reservoir 16 of separation chamber n, while the inner and outer tubes forming the annular separation chambers 10, 11, 12, and n are relatively heated and cooled. The fractions that accumulate adjacent the hot walls ascend to the upper reservoirs, become mixed with the feed therein, and are transferred to the next upper reservoir in the series until the feed enriched in that particular fraction PT is withdrawn from the upper reservoir V15-of separation chamber n at a rate equivalent to FT. The fractions that accumulateadjacent the cold walls descend into the lowervreservoirs and likewise become mixed with the liquid therein. This liquid, enriched with the fraction descending into the lower reservoir, is passed intoY the next lower reservoir in the series until finally liquid that has been enriched with the cold wall fraction PB is withdrawn from the lower reservoir 16 of separation chamber 10 at a rate equivalent to FB. in accordance with the invention, the flow rates across the top and bottom are unequal and Vary from 2 to 20, preferably from 2 to l0. Expressed mathematically:

-;; ;=%0 to it and from 2 to 2o The ladvantages offered by the method of the present 'invention will become further apparent from the discussion of results obtained from a series of tests conducted with a 4-column thermal diffusion unit operated on a countercurrent mixed-end feed system as schematically illustrated in the patent drawing. Each'of the columns was six feet in height, having an equidistant annular separation chamber ofA 0.032 inch, a hot wall temperature of 450 F., and a'cold walltemperature of 150 F. The feed stock for thesetests'was a lubricating oil stock obtained by solvent extraction 'of an oil reii-fefi from a Pennsylvania crude having a viscosity of -150 SSU at 100 F. and a viscosity index of 100. The feed was 'divided equally between the two end reservoirs in one run and then for comparison the feed was unequally divided in varying ratios. The runs were repeated for varying levels of throughput, and the results are reported in the following table:

It may be noted from the above table that as the throughput increases, the effectiveness of the separation decreases, as is understood in the thermal diffusion art,

but that at any given level of throughout to the thermal diifusion unit the top product quality in terms of viscosity index numbers improves (i.e., the viscosity index number increases) as the ratio of bottom throughput to top throughput is increased. There is a marked improvement in changing the ratio from 1:1 to 2:1. Obviously from the results shown, the major improvement is gained from increasing this ratio to about 9: 1, and the subsequent improvement gained by increasing this ratio from 9:1 to 19:1 is nominal and generally not justifiable economically. Y

Furthermore, it will be observed from the foregoing table that if t e top product quality is held constant while the bottom to top throughput-'ratio is increased, the rate of top product production will be increased. Compare, for example, Run No. 1 where 12 ml./hr. of V1`18"-VI top product is produced at a bottomto top throughputratio of 1:1 withRun Nofll where 48-ml./hr.-of 118 -Vljtop product is produced when this ratio is increased to 9:1. "fhus the amount of 1185171` product is'increased fourfold without any increase inthe 'consumption of heat.

Moreover, if the top product rate is held constant while the bottom to top throughput ratio is' increased, Athe quality of the'top product will be improved. Compare, forexample, Run No. 5 where a 113 VI top productis obtained at a rate of 48 ml./hr. at a ratio'of 1:1 with Run No. 11 of the table where a118 VI top product is obtained at the same rate of 4S mL/hr. when the ratio is increased to 9:1. In this case theVI can'be raised l5 numbers in accordance with the invention with no increase in heat consumption.

It is to be expected that various modifications of the method of the present invention will suggest themselves to those skilled in the art upon a reading of the foregoing description. All such modifications are intended to be included as are deiined in the appended claims.

We claim:

l. in a thermal diffusion method which comprises conining a liquid mixture in a vertical thermal diffusion separation chamber comprising opposed walls and terminating in reservoirs at its upper and lower ends, said reservoirs having a width and height greater than the distmce between said opposed walls, imposing a temperature gradient across the liquid in the chamber, whereby dissimilar fractions are formed, one of which contains a higher than initial concentration of one component and ascends to enter the upper reservoir and another of which contains a higher than initial concentration of kanother component and descends to enter the lowerV reservoir, continuously feeding fresh liquid into the upper and lower reservoirs for admixture with the dissimilar 'fractions Vfron said thermal diffusion separation chamber, directing portions of the admixed liquid in the respective reservoirs into the upper and lower ends of the thermal diffusion separation chamber, and continuously and Yseparately withdrawing dissimilar fractions from the upper and lower reservoirs, the improvement providing a higher rate of separation and an improved quality enriched fraction from one of said reservoirs in which the rate of the fresh feed to the upper reservoir FT is approximately equal to the rate of withdrawalfrom the upper reservoir PT and in which the rate of fresh feed to the lower reservoir FB is approximately equal Vto the rate of Withrawal from the lower reservoirY PB and in which from 2 to 20, and the greater rate of feed is introduced through the reservoir at the end of the chamber opposite that from which an improved quality enrichedfractionis being withdrawn.

2. In a thermal diffusion method for continuouslyseparating a liquid mixture into fractions enriched with dissimilar components which comprises imposing a temperature gradient across a plurality of separation zonesV defined by opposed, substantially vertical surfaces substantially equidistantly spaced apart and communicating at their upper and lower ends with reservoirs having dimensions greater than the distance between said vertical surfaces and constituting a series of mixing zones; continuously introducing liquid mixture containing said dissimilar components into a first member of said series of mixing zones -in said -upper yand-lowerreservoirs, whereby the separation zones are filled with liquid mixture for separation into ascending fractions containing higher than initial concentrations of one of the dissimilar components and descending fractions containing higher than initial concentrations of another of said dissimilar components; admixing introduced liquid in said upper and lower reservoirs with the ascending and descending fractions entering said reservoirs from the separation zones; and continuously and separately withdrawing fractions enriched with dissimilar components by admixture with said ascending and descending fractions from a member of said series of mixing zones remote from the place of introduction of said liquid mixture in said upper and lower reservoirs, respectively, the improvement in which the rate of the fresh feed to the upper reservoirs FT is approximately equal to the rate of withdrawal from the upper reservoirs PT and in which the rate of fresh feed to the lower reservoirs FB is approximately equal to the rate of withdrawal from the lower reservoirs PB and in which F T FB- where X has a value within the range from /o to 1/2 and from 2 to 2G, and the greater rate of feed is introduced through the reservoirs in the ends of the separation zones opposite those from which an improved quality enriched fraction is being withdrawn.

References Cited in the le of this patent UNITED STATES PATENTS 2,541,071 Jones et al Feb. 13, 1951 2,723,033 Jones et al. Nov. 8, 1955 2,723,034 Jones Nov. 8, 1955 2,734,633 Jones et a1 Feb. 14, 1956 2,827,171 Frazier Mar. 18, 1958 2,827,172 Frazier Mar. 18, 1958 2,827,173 Jones Mar. 18, 1958 FOREIGN PATENTS 736,134 Great Britain Aug. 31, 1955

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