ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT Metal or Metalloid Containing Lube Additives
Literature Cited
Organic compounds consisting of metal salts or containing certain other heavy elements have been found t o be concentrated very strongly a t the bottom of the column. This includes such materials as zinc alkyldithiophosphates, calcium and barium sulfonates and other compounds commonly added to mineral oils to impart oxidation resistance, antiwear, or dispersing properties. Emission spectra of thermal diffusion fractions of an additive oil are illustrated in Figure 12. The actual concentration in the bottom fraction of elements zinc, phosphorus, calcium, or barium is more than 10,000 times their concentration in fraction 1. The process, therefore, may be applicable as a nondestructive means of concentrating certain compounds which may be present in trace amounts.
(1) Am. Petroleum Inst., Pittsburgh, Pa., Res. Proj. 44 (1950). ( 2 ) Broeder, J. J., and Nes, K. van, Proc. Third World Petroleum Conor., 6, 25 (1951). (3) Clusius, IC.,and Dickel, G., Naturwissenschajten, 26, 546 (1938). (4) Debye, P. J. W., U. S. P a t e n t 2,567,765 (Sept. 11, 1951). ( 3 ) Debye, P. J . W., Ann. Physik, 36, 248 (1939). (6) Farber, M., and Libby, W., J. Chem. Phys., 8 , 965 (1940). ( 7 ) Groot, S. R. de, Physica, 9, 801 (1942). (8) Ibid., p. 923. (9) Haak. F. A , , and Xes, K. v a n , J Inst. Petroleum, 37, 245 (1951) (10) Horsley, L. H., Anal. Chem., 19, 508 (1947). (11) Jones, A, L., Petroleum Processzng, 6, 132 (1951). (12) Jones, A. L., and Foreman, R. TV., IND.ENG. CHEM,, 44, 2249 (1962). (13) Jones, A. L., and Milbergor, E. C., Ibid., 45, 2689 (1953). (14) Kramers, H., a n d Broeder, J. J., A n a l . C h i m . A c t a , 2, 687 (1948). (15) Nes, K. v a n , and Westen, H. A. van, “Aspects of t h e Constitution of Mineral Oils,” Elsevier, Netherlands, 1951. (16) Niini, R., Suomen Kemistilehti, 26B, 42 (1953) (in English). (17) O’Donnell, G., A n a l . Chem., 23, 894 (1951). (18) Tilvis, E., SOC.Sci. Fennica, Commentationes Phya.-Math.. 13, 16 (1947) (in English).
Acknowledgment The authors wish t o express their thanks t o L. L. Withrow and J. M. Campbell of these laboratories for their continued support of this work; t o B. M. Johnson and S. G. Anderson for experimental assistance; and t o A. L. Jones, Standard Oil Co. (Ohio) for helpful advice and discussion.
RECEIVED for review August 4, 1964.
ACCEPTED
November 2d, 1954.
Rotary and Packed Thermal Diffusion Fractionating Columns for Liquids LLOYD J. SULLIVAN, THOMAS C. RUPPEL, AND CHARLES 6. WILLINGHAM MeIIon Institute of Industrial Research, Pittsburgh 73, Pa.
The percentage separation of organic liquid mixtures b y thermal diffusion in a concentric tube fractionator, operated as a batch unit, is found to b e enhanced when the inner member i s rotated, or when the annular space is packed; glass wool is satisfactory as a packing material. Operated as each of three column types, a bench model apparatus gave the higher percentage separation when operated as a packed column. Construction details are given for the bench model apparatus, and for a 5-foot laboratory packed column having a 0.063-inch width b y 1.245-inch inside diameter annulus and a 200-ml. charging capacity. This laboratory packed column produces separations equivalent to a 5-foot Jones-Hughes open annulus column having a 0.01 2-inch width b y 0.625-inch inside diameter annulus and a 25-ml. charging capacity. The test mixtures used were n-hexadecane-decahydronaphthalene, n-hexadecane-1 -methylnaphthalene, and a urea nonadductible microcrystalline wax.
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HE separation of fluids, particularly organic liquids, by thermal diffusion has recently been reported by several laboratories (4, 6-8). Furry and Jones ( 5 ) in a column theory have shown t h a t the separation obtainable varies as t h e inverse fourth power of the annular spacing, and t h a t equilibrium time of separation is approximately a n inverse seventh power function of the annulus. One type of apparatus used is the concentric tube column introduced by Jones and Hughes ( 7 ) . Jones-Hughes columns ”4 inch in diameter and 5 or 6 feet long with approximately a 0.01-inch annular space have charge capacities of 25 t o 30 ml. In order to separate larger amounts of material batchwise, several columns have been designed. These units retain the narrow open annulus of 0.01 t o 0.03 inch, and the columns have been modified by addition of reservoirs a t the ends ( 4 )or over the length of the unit (9). The time necessary for separation increases in proportion to the size and number of the reservoirs. 208
TWOconcentric tube batch fractionators without reservoir8 have been developed that will separate larger volumes of liquids within a reasonable time. One is a rotary thermal diffusion column, and the other is a column in which the annular space is packed. Glass wool is a satisfactory packing. A thermal diffusion unit with rotating members has been reported in the literature (11). However, it differs from the concentric tube columns in that it was a converted cream separator in which the heated and cooled cones were rotated to increaae the gravitational force field of the system. Debye (8,8 ) has reported and patented the separation of polymers by thermal diffusion. H e used a 10-cm. column packed with glass wool to show the enhanced separation. No other application of a similar column t o the separation of mixtures by thermal diffusion is known to the authors. The theory of separation of organic liquids by thermal diffusion is not as yet adequate t o predict the extent of separation
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 47, No. 2
HYDROCARBON SEPARATIONS possible. There are no reliable data on steady-state separation factors of test mixtures on which the characteristics of multistage apparatus can be based. Therefore, columns are compared on the basis of the percentage separation obtained using binary mixtures or. for comdex mixtures,, by" the spread in some characteristic property of -the fractions.
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Steam Out
-
Steam In
Sleeve Bearing Sampling Tube Y
Leveling Bulb
7
Rotor -Water
Out
Water Jacket Annular Space
Bench Model Apparatus Investigations were made using a small concentric tube bench model thermal diffusion apparatus as an open annulus column, a rotary column, and a packed column. This bench model apparatus is shown diagrammatically in Figure 1. The inner tube which also Herves as the rotor, is a straight round brass tube, 2.5 feet long, closed at the bottom. A section of brass rod, '/d-inch diameter and I/&ch long, extends from the bottom. This rod centers the tube in the bottom of the column by means of a nonrestricted sleeve bearing. This bearing of bronze makes a sliding fit with the inside glass wall and is drilled to permit free circulation of the liquid. The tube is also centered, with a circular sleeve bearing, a t the top of the column. A full-length metal bayonet heater, centered within the inner tube and using SAE 10 lubricating oil as the heat-transfer medium, supplies heat t o t h e inner tube. Two tubes or rotors of different diameters, 0.750 and 0.789 inch, were used for varying the width of the annular space. As rotors they are driven by a variable speed motor and a belt drive. The outer tube, 0.844-inch inside diameter, is a 2-foot borosilicate glass tube sealed flat a t the bottom and with a water jacket similar to a Liebig condenser. Taps for feeding and withdrawing material are located a t the top, middle, and bottom of the unit.
70
60-
-
Water In
Sleeve Bearing Sampftng Tu6e
Figure 1.
50
Bench model rotary thermal diffusion column
C
$40-
.-+6 0
Z e r o R.P.M.
The percentage separation of binary mixtures with rotary and packed columns in small models has been compared with the open annulus column. On the basis of the results, a 200-ml. charge capacity packed unit has been constructed and tested. This unit is introduced as a workable and valuable addition to present apparatus.
Test Mixtures Two binary test mixtures were used in this study. They were n-hexadecane-decahydronaphthalene and n-hexadecane-l-methylnaphthalene. The commercial stock materials, with the exception of the 1-methylnaphthalene, were purified by passing them over activated silica gel. The 1-methylnaphthalene was used as received. Refractive indices a t 25' C. checked, within a few units, the literature values for n-hexadecane and l-methylnaphthalene. The decahydronaphthalene after silica gel purification appeared to be a 50 volume % mixture of the cis and trans isomers. The decahydronaphthalene processed in a 30-ml. Jones-Hughes unit gave no evidence of any tetrahydronaphthalene by refractive index analysis of the fractions. A urea nonadductible microcrystalline wax from petroleum was used for comparing the separating power of the 200-ml. capacity packed column having an 0.063-inch annulus with a 25-ml. capacity Jones-Hughes open annulus column of 0.012-inch width annulus. February 1955
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c
0 0
4
12
8
16
0.028"
20
I
Time (hours)
Figure 2.
Separation as function of elapsed time using bench model columns
With the rotor stationary the apparatus is a conventional open annulus column, or, if the annular space is packed, a packed column. The annular space is packed with glass wool by removing the inner tube and cementing layers of glass wool to it with Duco cement. The apparatus is then reassembled and the cement removed by washing with acetone. The model column a5 first assembled using the 0.789-inch outside diameter rotor had a n annular width of 0.028 inch, equivalent to the largest annular space generally used in open annulus col-
umns. This column was operated a5 an open annulus unit with the inner tube carefully centered and maintained stationary. The wall of the inner tube was heated by steam circulated through the bayonet heater, and the outer wall cooled by a flow of water
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING. DESIGN, AND PROCESS DEVELOPMENT
Graphite -Asbestos
The motion of the fluid is critically related to mechanical stability of the column and to centering of the rotor. Stable motion in the fluid was easily destroyed by poor centering of the rotor. This fluid motion is thoroughly discussed (1) in two-phase systems. A mathematical treatment is given by Taylor (IO). The effect of packing in the annulus was studied in the 0.047inch annulus using a double layer of Pyrex glass wool (Corning Catalog KO. 800). The data are set forth in Figure 2, in which the percentage separation is plotted as a function of elapsed time. Percentage separation is calculated from the empirical relationship Percentage separation
Gloss Woo/ Packmg
=
nzg (bottom) - nz; (top) nz2 (decahydronaphthalene)-nzg(n-hexadecane) x 100
r0
1 Figure 3.
Water In
Laboratory packed thermal diffusion column
Initial concentration in all cases is a 50 volume mixture. Curves A , and BI are the results of experiments with the unit operating as an open annulus apparatus for spacing of 0.028 and 0.047 inch, rebpectively. Curves A2 and Bz represent the same units with the inner tube rotating a t an optimum speed, 260 and 150 r.p.m., respectively; Curve C is for the unit with annulm spacing of 0.047 inch operated as a packed column. For a given time of operation at the same annular spacing, the packed column, C, gave the maximum separation. The rotary column, 132, fell below the packed column and the unmodified column was far less efficient than either column. Predictions of the effect of the annular spacing of steady-state conditions cannot be made from these curves, but the general shape mould indicate that the narrower the annulus the more efficient the unit. That is, As operated under optimum conditions was a more efficient unit than Bz operated under optimum conditions for BP. At the completion of each experiment the column was cut into
I476
through the jacket. S o difference in separation was found when the temperature gradient was reversed by cooling the inner tube and passing steam through the jacket. The column was filled with the test mixture from a leveling bulb attached by flexible plastic tubing to the center tap. This bulb was also used to maintain a constant level in the apparatus during operation. To follow the course of the separation, samples sufficient for refractive index measurements were taken from the top and bottom taps a t intervals of 1 to 3 hours over a 24hour period. Kithout changing the apparatus, but with a fresh charge of test mixture for each experiment, the column JT-as also tested as a rotary concentric tube apparatus. Upon completion of these teste, the 0.750-inch diameter rotoi (0.047-inch annular width) was installed and the previous series of experiments repeated. When the inner tube of the column is rotated and the thermal gradient established, motion of the liquid is visible. At very slow speeds, the liquid in the annulus appears t o follo\J the rotoi sluggishly. As the speed is increased, moving spirals which have a secondary motion that gives them the appearance of a coil develop first in sections of the column. These join to resemble the threads of a screw, extending over the length of the column. At optimum speed of rotation the motion of the liquid is in the form of layers of rings with each resembling two circular coils wrapped in opposite directions Between the layers of rings is a space of clear, apparently motionless, fluid. Further increase in the speed of rotation flattens the rings until the motion becomes erratic. The optimum speed of rotation for separation produces the most stable rings and varies with annular spacing. The speed of rotation necessary for stable motion increases with a decrease in annular space and with an increase in the viscosity of the liquid.
210
1
I
I
1
Froc t to n
Figure 4. Refractive index of fractions from laboratory packed column as function o f fraction number for column packed with 2 and 3 layers of glass wool
__ 2 Layers of glass wool - _ _3 Layers of glass wool
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 2
HYDROCARBON SEPARATIONS
'462 I460
1 (Top)
Per Cent by Volume
LBottornl
Figure 5. Comparative separation of urea nonadductible microcrystalline wax Refractive index plotted per cent by volume for fractions trom Jones-Hughes 25-1111.open annulus column operated for 125 hours and from 200-ml. packed column operated for 300 hours
10% fractions by withdrawal through the bottom of the column. The concentration of the mixture changed regularly from the top to the bottom of the column. The second test mixture, n-hexadecane-1-methylnaphthalene, gave identical results with the first. For any elapsed time in one of the columm, B for example, the ratio of percentage separation for the rotary column to the percentage separation of the open annulus column was very nearly a constant. The packed column shows good separation, and the separation is very reproducible. The centering of the apparatus is critical but much less than in the rotary unit. Trapped gases in the packing decrease the separation obtained. The rotary and packed columns of 0.047-inch annular spacing were both found to give better separations than a similar column with a 0.028-inch open annulus.
laboratory Packed Column The 200-ml. charge capacity packed column is shown diagrammatically in Figure 3. Constructed from standard sizes of stainless steel tubing, its dimensions are Outer tube, inside diameter Inner tube, outside diameter Width of annular space Length of fractionating section
Inches 1.370 1.245 0.063
60
The annular space is sealed above the top port and below the bottom port with a Teflon ring and graphite-asbestos packing held in place by the packing nuts as shown. A lock nut is fastened to the top of the column to prevent creeping of the inner tube with alternate heating and cooling of the unit. The inner tube is centered in the outer tube by the end-seals and by three sets of small threepoint spacers welded to the surface at equal distances along its length. Standard copper tubing fittings at either end of the inner tube are used t o connect with the cold water supply. February 1955
The outer wall is heated using three spiral-wound insulated Nichrome elements. These elements each heat individually the column length. Each heater is controlled by a separate 220-volt variable autotransformer. A copper-constantan thermocouple is located a t each end of the column in the water flow to measure the cold wall temperature; six copper-constantan thermocouples are located on the surface of the column, a t each port. An iron-constantan couple is fixed to the hot wall a t porta 1 and 2. These couples connect through a selector switch to a temperature-limiting control for automatic safety shutoff and permit operation of the column as a 60-inch or a 48-inch unit. The ports are 3 X 1/1 inch stainless steel rod with a '/,&-inch hole and are equipped with stainless steel needle valves. Each section betxeen ports has a 40-ml. capacity under packed conditions with approximately a 2-ml. drainage holdup. The column is easily taken apart for packing, which is done as described for the bench model unit. Several test runs with the column repacked in the same n a y for each run showed little variation in separation. Preliminary tests of several types of packing are given in Table I. The only variable changed during operation was the packing. A single layer of steel wool, two layers of steel wool, and a single layer of glass wool are comparable and showed better separation than an open annulus column. The Pyrex wool, steel wool, and copper screen were easily packed uniformly in the column. The stainless steel helices, however, were very difficult to distribute uniformly in Hot Wall 0 the curved annulus. -0 This fact may exrn z plain the failure of z > the helices to inz crease separation. c E m Of the materials teated, glass wool was the only material to give significant increase in Hot Wall separation with additional packing. B Figure 4 shows that 0 % u m there is little differD ence in the separaz z tion of n-hexadecC r a n e-d e c a h y d r om naphthalene test mixture using 2 or Cold W a l l 3 layers of glass Figure 6. Schematic drawing of wool. flow patterns, open annulus column The column is (top), packed and rotary filled by attaching a (bottom) reservoir to the bottom port, evacuating the column with a vacuum pump, and allowing external pressure on the reservoir to fill the column. This procedure was used to prevent trapping of air in the glass wool packing. In order to remove the material from the column in separate fractions, a suitable container is attached to the bottom port of each section beginning with the top. The needle valves are opened for both ports and the section allowed to drain. For viscous materials, draining may be forced by nitrogen pressure. The pressure to be used will depend on the viscosity of the material being separated. The comparative separations of the urea nonadductible wax in the 0.063-inch annulus packed column and in a 0.012-inch Jones-Hughes open annulus column are shown in Figure 5. A temperature difference of 70' C., with the cold-wall temperature 70" C., was used for these separations. The packed column was operated for 300 hours and the open annulus column for 125 hours.
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ~~
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Table I. Effect of Different Packing Materials in Separating 50 Volume % Mixture of n-Hexadecane-Decahydronaphthalene Packing Material None Stainless steel helices 3/18 inch 2 Layers copper scree’n, 440-micron pore 1 Layera steel wool, medium weight 2 Layers steel wool, medium weight 1 Layere glass wool, Corning Catalog Ko. 800 2 Layers glass ~ 0 0 1 Corning , Catalog No. 800 3 Layers glass wool, Corning Catalog No. 800
Separation, % b 3 3 13 30 25 28
92 88
5 Single thickness of material as received from supplier. b Columns operated for 100 hours with hot-wall temperature of 100” C., and cold-wall temperature of 20’ C.
Eight fractions representing 12.5% by volume of charge were collected from the 25-ml. column, and 5 fractions representing 20% by volume of the charge were collected from the packed column. While individual differences are found from fraction to fraction in the two columns, the separation is readily seen to be equivalent. For the 25-1111. column each fraction is approximately 3 ml., while for the packed column each fraction is 40 ml. Thus each fraction from the packed column is an adequate charge for reprocessing in a 25-ml. column.
Summary Two rotary columns and one packed column were tested to determine whether the annulus could be modified to give good separations for larger volumes of mat,erial. The packed column yielded the best results for a given annular space. Modified columns of 0.047-inch annular spacing produced better separations than a similar column with an 0.028-inch open annulus. A packed thermal diffusion column with an 0.063-inch annular space showed a 92% separation of a test mixture compared with
only 3% separation for the same column without packing. Of the packing materials tested, glass wool gave the best separation, Using a partly refined petroleum microciystallirie wax, equivalent separations were obtained in a 25-ml. open annulus column with a 0 012-inch width annulus and in a 200-ml. packed unit having a 0.063-inch width annulus. The improvement in separation with both rotary and packed concentric tube thermal diffusion columns over that in open annulus columns of the same dimensions probably results largely from changes in the flow pattern of the convecting streams. I n Figure 6, an idealized flow pattern for an open annulus column is given in the schematic drawing a t the top where the maximum velocities occur a t about 1/4 the distance between the walls, and the two streams shear near the center of the annulus. Theschematic drawing a t the bottom in Figure 6 is suggested for the flow pattern in the packed and rotary columns where the maximum velocities are shown near each wall. Here the enrichment of each stream would be higher, and the possibility of remixing near the center of the annulus is less. Literature Cited (1) British P a t e n t Specification, 615,425, accepted Jan. 0, 1949. (2) Debye, P., U. S. P a t e n t 2,567,765 (April, 1946). (3) Debye, P., a n d Bucche, A . M., “High Polymer Physics,” p. 532, Chemical Publishing Co., Brooklyn, N. Y., 1948. (4) Drickamer, H. G., a n d Trevoy, D. J., J. Chem. Phys., 17,1120-4 (1949). ( 5 ) Furry, W. H., and Jones, R. C . , Rev. Mod. Phys., 18, No. 2, 151-224 (1946). (6) John, H. F., Dissertation Abstr., 13, 1003 (1963). (7) Jones, A. L., IND.ENG.CHEnf., 45, 2659-96 (1953). (5) Rlelpolder, F. W., presented at Analytical Conference, Pittsburgh, Pa., 1952. (9) O’Donnell, G., A n a l . Chem., 23, 894-8 (1951). (IO) Taylor, G. I., Phil.T r a n s . , A223,289 (1923). (11) Tilvis, E., Sac. Sci. Fennica, Commentationes, Phgs-Math., 13, 1-59 (1947). RECEIVED f o r review September 7, 1954.
ACCBPTEDDecember 1, 1954.
lubricating Oil Fractions Produced A. LETCHER JONES The Stundard Oil Co. (Ohio), 2127 Cornell Road, Clevelund, Ohio
A mid-continent paraffin distillate and a furfural extract and raffinate from it have been separated into 10 fractions each b y thermal diffusion. These fractions have widely different viscosity properties without being changed with respect to molecular weight or volatility. O f the furfural extract 30% o f the volume had viscosity index of 120; 10% of the raffinate had a viscosity index of -106 and was in the viscosity range of a bright stock, although the whole raffinate i s a lightweight oil with a viscosity index of 95. The viscosity of the fractions appears to be related to the composition through the average number o f rings per molecule.
T
HERMAL diffusion is capable of separating lubricating oil stocks into fractions of widely different physical properties with density, refractive index, viscosity, viscosity index, and color usually being quite different ( 3 ) . On the other hand, the fractions are remarkably similar with respect t o both molecular weight and volatility (3, 5 ) . I n an effort t o learn more about the nature of petroleum lubricant fractions produced by thermal 212
diffusion and also in order to make a comparison between thermal diffusion and furfural solvent extraction, a study has been made of a mid-continent paraffin distillate stock. Samples of the paaffin distillate, a furfural extract, and a furfural rafEnate of it were selected for the study. Xone of these stocks was dewaxed. Some of the physical properties of these materials are listed in Table I.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 2
