IMPROVING T H E SEPARATION EFFICIENCY OF LIQUID T H E R M A L DIFFUSION C O L U M N S T . A . WASHALL AND F. W . MELPOLDER The Atlantic Refining Co., Philadclphio, Pa.
Significant increases in both the rote of separation and separation efficiency of o batch-type thermogravitationol liquid thermal diffusion column were obtained by modification of the convective flow pattern of the liquid.
A tight
fitting wire spiral was placed inside the annular space to modify the vertical paths of ascending and descending liquid and ais0 to reduce the adverse action of possible eddy currents. The beneficial effect of the wire spiral was greater for a 1.25-inch diameter brass column than for a 0.75-inch stainless steel coiumn.
Wire diameter and spacing of the
spiral turns were critical factors in determining optimum conditions for maximum seporation. HERMOORAVITATIONAL, LIQum THERMAL DIFFUSION COLUMNS Tof the batch type have been used extensively for the fractionation of petroleum stocks. Many separations of hydrocarbon mixtures which could not be achieved by conventional methods like distillation, absorption, liquid extraction, and crystallization have been made by liquid thermal diffusion. Consequently, considerable effort has been expended on the modification and improvement of both operating techniques and equipment. I n a review of the latest advances in liquid thermal diffusion, Brown and Jones (2) have pointed out that most of the recent work has been concerned with improving the efficiency of thermagravitational columns. One of the earliest reDorts of imDroved separation of liquid nixtures resulting from modification of the convective flow >attern was made by Alkhazov ( I ) and coworkers. They iivided a short column into sections with rings of cellulose .2LL.L2.L J.. .... 3 -L. iivuuii w i i i c ~ iLCUULCU UIC: a ~ m u u ar rhat point. Debye and Bueche ( 3 ) emdoved a column in which the annular SDacine was packed with glass wool. Later, Powers and Wilke (5) demonstrated that the degree of separation in a flat plate apparatus was improved when the convective flow was reduced by tilting the apparatus a t various angles. Sullivan, Ruppel, and Willingham ( 6 ) tested two columns in which the inner tube was rotated and a third column in which the annulus was packed with glass wool. For a given annular space, the packed column produced better separations than the rotary columns. Both rotary and packed columns gave better separations than the open column with the same annular spacing. I n a more intensive study of packed thermal diffusion columns, Sullivan, Ruppel, and Willingham (7) found that improved separations were obtained in various wide-annulus columns (0.03 to 0.125 inch) when the annular spacing was packed with glass wool. They indicated that the degree of
separation obtained a t the steady state was a function of both the annular spacing and the packing density. For a fured annular spacing, the degree of separation increased with packing density. However, the lowest packing density gave the fastest approach to the steady state. Uniformity of the annulus is particularly critical in columns having an annular space of about 0.012 inch, Since it is reported (8) that the degree of separation is an inverse function of the fourth power of the annular spacing, and the time to reach the steady state is an inverse function of the seventh power of the annular spacing, the uniformity of the annulus width becomes an important function of column efficiency. T h e work reported here was an attempt to increase column efficiency by winding a wire helix within the annulus of the column. The wire spiral was designed to overcome some of the practical problems associated with the construction and operation of thermal diffusion columns. Both the diameter of the wire and the wire spacing were critical factors affecting the maximum separation attainable. Greatest separation was obtained when the diameter of the wire was essentially equal to the annular spacing. The increased column efficiency was achieved because the uniformity of the annulus was improved, the deleterious effects of possible eddy currents or spurious convection currents were minimized, and the rate and flow pattern in which separated components were moved to the top and bottom of the column were more favorable. I n another study, Lorenz and Emery ( 4 ) have indicated that a reduction in convective velocity increased the relative importance of the horizontal flux.
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l & E C P R O C E S S D E S I G N AND DEVELOPMENT
Figure 1. Spiral wire inserted as a spacer in the annulus of a conventional concentric tube thermal diffusion column
Experimental
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The effects of wire spacing, wire diameter, and temperature gradient on degree of separation were studied using a test mixture comprised of cis- and trans-decahydronaphthalenes. Extensive use of this text mixture has shown it to be very satisfactory for evaluating liquid thermal diffusion columns. The separation factor, U , used in evaluating separation efficiency, is normally calculated for equilibrium conditions. However, because of the long residence periods required to reach the steady state. the separation factor. u t , was used in evaluating column performance at nonequilibrium conditions in this work. The latter is defined as follo\vs:
I
50
0
01
0 2
0 3
+(S.SPACING
0 4
05
06
07
I N INCHES)
Figure 2. Separation decreased rapidly when wire spacings were smaller or larger than optimum. In both cases, residence time was 48 hours and maximum wire diameter was used
where
.Va
=
mole fraction of trans-decahydronaphthalene
-I= T mole o fraction of cis-decahydronaphthalene ut
= separation factor for specified conditions of residence
time and temperature gradient The effect of a \vire-\vound spiral on separation was studied with two different coricentric tube thermogravitational columns. The first column, made of brass tubing, had a fractionating section of 36 inches and an annulus of 0.012 inch. Outside diameter of the inner tube was 1.12 inches, while that of the outer tube was 1.2.; inches. The volume of this column was 26 ml. The second column was 72 inches long and was constructed of stainless steel. Outside diameter of the inner tube was 0.63 inch? while that of the outer tube was 0.75 inch. The annular spacing of the 72-inch column was 0.012 inch and the volume was 30 ml. Through the use of packing nuts, the inner tubes of both colurnns were removable. The wires employed were stainless steel and were available commercially. To apply the wire, the inner tube was removed from the column and placed in a horizontal position. After the desired spacing was marked, the wire was wrapped in a spiral manner over a Lfoot section of the inner tube. Then the wrapped section waij inserted carefully into the outer tube, and another 1-foot sectilJn of the inner tube was wrapped with wire. This procedure was repeated until the entire inner tube was wrapped and inserted into the outer tube. -4 small amount of the test mixture was used as lubricant during insertion of the wire-wrapped inner tube. A section of a concentric tube thermal diffusion column with a spiral wire in the annulus is shown in Figure 1. I t should be emphasized that this wire spiral was not used as an electrical heater, but rather as a guide and spacer within the annulus. The test mixture was charged into the bottom port from a glass syringe. The column was heated electrically on the outer wall and cooled by passing tap water through the inner tube. All of the work in the: 3-foot brass column was carried out with a hot wall temperature of 230' F. and an average cold wall temperature of 80' F., giving a mean temperature gradient of 150' F. Two temperature levels were employed in the operation of the 6-foot stainless steel column. At a AT of 150' F. the hot wall temperature \vas 230' F. and for a A T of 200' F. the hot wall temperature was 300' F. Samples of 1 ml. were removed from top and bottom ports for composition determination a t intervals. Material was added through a center port to maintain the liquid level.
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W I R E ANGLE I D E G R E E S ]
Figure 3. Maximum separation occurred a t wire angle of 40" in the 3-foot column and 64" in the 6-foot column
ANNULUS W I D T H
5 O r
"
40-
3-
P
COLUMN 3 F T X I 25 INCH O D
c
a Y
1
30-
sa
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-c Y L
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WIRE D I A M E T E R I I N C H E S )
Figure 4. Maximum separation was obtained at a wire diameter essentially equal to the dimension of the annular space
Results and Discussion An optimum wire spacing existed for both columns when a wire having the same diameter as the annular spacing was employed as shown in Figure 2. Maximum separation was obtained at a spacing of 3 inches for the 3 foot X 1.25 inch column, while the greatest separation factor was obtained at a 4-inch spacing for the 6 foot X 0.75 inch column. Figure 3 indicates the effect of wire angle on the degree of separation for the same wire spacings seen in Figure 2. MaxiVOL. 1
NO. 1
JANUARY
1962
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100
UNWRAPPED I 5 INCH SPACING
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UNWRAPPED 2 0 INCH SPACING
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Figure 5. Columns with a wire spiral gave much greater rates and degrees of separation than open annulus columns. Maximum wire diameter was used in both %foot (above) and 6-foot (below) columns
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R E S I D E N C E TIME ( H O U R S )
mum separation in the %foot column occurred at a wire angle of 40°, while the maximum for the 6-foot column was found at an angle of 64 '. The diameter of the wire had a very pronounced effect on the degree of separation attainable at a given residence period. Figure 4 shows that for a given annular spacing, maximum separation was obtained when the wire diameter was increased to essentially the same dimension as the annular space. The effect of wire spacings on the rate and degree of separation for short residence periods is shown in Figure 5. In both the 3-foot and the 6-foot columns. the rates and degrees of separation were greatest at the optimum wire spacings. The effect of temperature gradient on the separation obtained before and after introducing a wire spiral into the 6-foot open annulus column was studied for short residence times (Figure 6). An improved rate of separation at a higher AT was observed for the wire spiral column. but the reverse occurred for the open annulus column. The decreased separation at the higher temperature level in the open annulus column may have been due to the formation of eddy currents or spurious convection currents. The convective flow in a column with a wire spiral was considerably different than that in a bare column. In addition to the horizontal transfer caused by the temperature gradient
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l&EC PROCESS DESIGN A N D DEVELOPMENT
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R E S I O E N C E T I M E (HOURS)
Figure 6. Improved separation rate resulted at the higher temperature level and gradient when the 6-foot column was fitted with a spiral of maximum diameter wire and 4-inch spacing
and the convective flow in the vertical direction, a modified column had ascending and descending component streams along the wire spiral. Components that tended to concentrate at the hot vall traveled u p ~ a r dalong the underside of the wire, irhile components that tended to concentrate at the cold wall traveled downward along the topside of the vire. A concentric tube column vith a wire spiral in the dnnulus may be comparable to a narrow. flat plate column inclined on edge. literature Cited
(1) Alkhazov, D. G., Murin, -4,. N., Ratner, A. P., Zmest .4kad. 'Vauk S.S.S.R., Otdel. Khim. Nauk 1943, pp, 3-7. ( 2 ) Brown, G. R., Jones, A. L., Petrol. Refiner 39, 156 (1960). (3) Debye, P., Bueche, .4.M.,in "High Polymer Physics, A Symposium," H. A. Robinson. ed., p. 497, Chemical Publishing Co., Brooklyn, N. Y . , 1948. (4) Lorenz, M., Emery, A. H.: J r , Chern. En!. Sci. 11, 16 (1959) (5) Powers, J. E., Wilke. C. R., A.Z.Ch.E. Jourrial3, 213-22 (1957). (6) Sullivan, L. .J., Ruppel, T. C., \l'illingham, C. B., IND.END. CHEM.47, 208 (1955). (7) Zbid., 49, 110 (1957). (8) Trevoy, D. J., Drickamer. H. G., J . Cheni. Phys. 17, 1120 (1949).
RECEIVED for review .March 30, 1961 ACCEPTEDhugust 1. 1961 Symposium on Less Common Separation Methods, 139th Meeting, ACS, St. Louis, Mo., March 1961.
