ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ~~
-
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 J a n . 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) J o h n , 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., Anal. Chem., 23, 894-8 (1951). (IO) Taylor, G. I., Phil.Trans., 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
HYDROCARBON SEPARATIONS
Table 1.
Properties of Charge Materials for Thermal Diffusion Study
Refractive index, 25O C. Specifio gravity, looo p. Average molecular weight Viscosity looo F. (S.U.S.) ~iscosjty:.2100 F. (s.u.s.) Viscosity index Distillation data Initial b.p., F.
1.0000
Distillate 1.4922 0.8800 375 163.6 42.9 77.4
Furfural Extracted Raffinate 1.4678 0.8617 380 142.2 42.4 93.7
Furfural Extract 1,5393 0.9580 370 546 53.7 5.4
668 738 780 830
710 756 793 833
624 692 724 762
Paraffin
E% 90%
I ;
a
.
u Y (0
Table II.
Operating Data on Furfural Extraction Unit
Solvent ratio Furfursl/oil Tower temperatures, TOD
3:l F.
250 170 60 110 19
5-ft. nests of 1-inch Raschig rings
The solvent extraction operating conditions and yield of the raffinate are presented in Table 11. Equipment and Procedure
Portions of the feed stock, raffinate, and extract were charged separately t o laboratory batch-type thermal diffusion columns. Each of these oil samples was separated into 10 thermal diffusion fractions. The thermal diffusion apparatus used was of a concentric, metal tube type, 5 feet long, with an annular spacing of 0.0115 inch, as described in a previous publication ( 4 ) . The total volume of annular spacing is 22.5 ml. The column is equipped with withdrawal ports located a t 6-inch intervals along the length of the column, and the entire contents of it were withdrawn in 10 separate fractions after a separation had been established within the apparatus. The hot wall was maintained
1.5800
I.JBO0
t 14400L'I ' 2 ' 3 ' 4
Figure 1.
February 1955
' 5 ' 6 ' 7 ' 8 ' 9 ' IO FRACTION
Refractive index versus fraction for paraffin distillate stocks
2
3
4
5
6
7
8
9
IC
FRACTION
Figure 2. Specific gravity (100" F.) versus fraction for paraffin distillate stocks
at a temperature of 210' F., while the average cold fluid temperature was 100" F. Each sample was processed under these conditions for a period of 1 week. The contents were removed from the column by withdrawing the uppermost fraction first and then successively working from the top to the bottom of the apparatus. Discussion
Figure 1 shows the refractive indices of the fractions obtained from the three paraffin distillate stocks. The fraction occupying the uppermost position in the column is Fraction 1and the lowermost fraction is Fraction 10. No values are shown for Fractions 1 and 2 of the paraffin distillate feed stock and raffinate, because these two fractions contain large amounts of solid wax and their refractive index values could not be measured at 25 C. The pattern of the refractive index values indicates t h a t a significant degree of separation was produced by thermal diffusion in the case of each of the three oils. Every thermal diffusion fraction is different. There is a regular increase of values from the top t o the bottom of the column for each of the three stocks. The widest spread of refractive index values is found among the fractions from the furfural extract. The level of the values shows the extract fractions t o be principally aromatic in character. The range of refractive index values among the fractions from the raffinate is smaller than that from the extract. These values suggest that the raffinate composition is predominantly naphthenes. The feed stock values fall in intermediate positions. T h e density pattern of the fractions is quite similar t o that of refractive index and is shown in Figure 2 . I n order t o make viscosity measurements on the small volume (2.25 ml.) of each thermal diffusion fraction it was necessary to fabricate a special viscometer of the Ostwald type. Calibration of this miniature type viscometer revealed that viscosty values could be measured with a degree of error not exceediing 0.2%. The viscosity data for the paraffin distillate stock and the 10 thermal diffusion fractions from it are presented in Table 111. Viscosity measurements could not be made on Fractions 1 and 2 a t 100" F., because they were solid with wax a t this temperature. For all liquid fractions, the viscosity increases progressively from the top of the column t o the bottom. The uppermost liquid fractions from the thermal diffusion column have high viscosity index values. The heaviest and most viscous fraction is 10. This fraction and t h e one immediately above it, 9, have viscosity values comparable t o those of bright stocks, Average molecular weights of all 10 fractions
INDUSTRIAL AND ENGINEERING CHEMISTRY
213
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT were determined experimentally. W7ithin experimental error, all fractions showed the same molecular weight. Mair and Rossini (6) have shown that mass spectrographic examination of thermal diffusion fractions from lubricating oils reveals no detectable separation between fractions with respect t o molecular
-J
0
I:
;1
3-
U
W
a VI
0
f
a
2-
B
Figure 3 shows the calculated number of aromatic and naphthenic rings for the 10 fractions from the paraffin distillate rafiinate. Values for Fractions 1 and 2 are not shown, because the necessary properties for the calculations were not measured. These fractions obviously have a very low ring content. The calculations for the remaining 8 fractions show that the aromatic content of the raffinate averages less than '/z ring per molecule. An independent analysis by infrared shows the whole raffinate to contain approximately 8% aromatics. Since the average molecular weight of the r d n a t e is approximately 400, excellent agreement is indicated between the two analyses. The molecular types concentrating toward the bottom of the column contain more rings per molecule than those occupying the upper part of
a W
m I.
I:
4.
I3
3l 1
TOTAL R I N G S
O'
I ' 2
'
3
'
4
5 ' 6 ' 7 ' 8 ' FRACTION '
9
'
IO'
Figure 3. Paraffin distillate raffinate number of rings versus fractions
weight. This means that the large viscosity difference produced between fractions is a result of the sorting out of different niolecular shapes or configurations by thermal diffusion. The volatility of different thermal diffusion fractions of lubricants is essentially the same ( 3 ) . This is not surprising in light of the absence of molecular weight fractionation. However, a fraction having a viscosity of 15,000 Saybolt Universal Seconds (S.U.S.) a t 100" F. (Fraction 10) will distill just as readily as one with a viscosity of 75 S.U.S. a t 100" F. (Fraction 3). This has been confirmed experimentally by distillation of these fractions. The viscosity data for the fractions from the furfural raffinate are listed in Table 111. The thermal diffusion fractions from the raffinate occupying the upper half of the column are not greatly different with respect to viscosity or viscosity index properties from the corresponding fractions produced from the feed stock. The contents of the lower half of the column are significantly different. I n the case of the raffinate, only one fraction was obtained having a negative viscosity index. This was the bottommost fraction, 10, from the thermal diffusion column. Less viscous bottom fractions were obtained from the raffinate than from the feed stock. Table I11 contains the viscosity data for the fractions obtained from the furfural extract. The three uppermost fractions from t h e thermal diffusion column have viscosity index values considerably higher than those for t h e remaining fractions. T h e uppermost Fraction, 1, has the remarkably high viscosity index of 176. T h e three uppermost fractions from the apparatus were combined] and the measured viscosity index of this composite sample was 120. I n this case, 30% of the extract has higher measured viscosity index values than the finished furfural raffinate (95 viscosity index). Boelhouwer, van Steenis, and Waterman (1) have shown how the viscosity properties of mineral oils vary with respect t o the structure of the constituent hydrocarbons. They show that kinematic viscosities are related t o the chemical composition of the oils as expressed by the average number of rings in the molecule. I n order t o relate the properties of these thermal diffusion fractions with structure, the method of ring analysia, as proposed by Hazelwood (&') was used in this study t o calculate the average number of aromatic and naphthenic rings per molecule for the feed, raffinate, and extract thermal diffusion fractions. 214
O' I
'
2 ' 3 ' 4
'
5 ' 6 ' 7 FRACTION
'
8
'
9 '10
Figure 4. Paraffin distillate extract number o f rings versus fractions
Table 111. Tlieriiial Diffusion Fraction Feed 11
2 31 4 5 6
7 8
9 10 Feed 1
2 3 4 5 6
7 8 9 10 Feed 1 2 3 4 5 6
7
8
9 10
Viscosity Data
Viscosity (S .U .S.) 210° F. Paraffin Distillate 163.8 43.18 Solid 34.86 Solid 37.48 74.46 38.22 88.00 38.50 115.7 40.40 180.0 43.76 330.2 47.97 63.70 ?,3?8.0 0,800.0 95.72 15,680.0 165.6 100° F.
Paraffin Distillate Raffinate 142.2 Solid Semi-Solid 73.97 78.64 97.58 123.5 174.4 331.0 397.6 3,257.0 Paraffin Distilrate Extract 53.94 545,7 37.09 63.60 39.00 95.60 42.05 156.2 47.40 333.5 58.87 1,096 67.33 1,986 83.44 4,868 97.33 8,840 122.1 13,400 168.0 100,500
INDUSTRIAL AND ENGINEERING CHEMISTRY
Viscosity Index 77.4
...
i59 120
88.6 69.0
16.4 - 103 -265 - 196 92.7
... ...
153 139 112
89.6 (1.0 61 0
52.3 .lo6 5.4 176
112 5E.5
-
10; - 177 -330 -465
- 409 - 1960 Vol. 41, No. 2
HYDROCARBON SEPARATIONS the apparatus. The average number of aromatic rings per molecule is essentially the same for each of the thermal diffusion fractions from the raffinate. The average number of naphthenic rings per molecule increases from the top to the bottom of the column. The bottommost fraction, 10, consists of molecules having an average of nearly 4 naphthenic rings per molecule. As shown in Table 111, this fraction has a higher viscosity than any of the other raffinate fractions.
1
10,000
/"""
500
type for liquid petroleum products. The viscosity values for the thermal diffusion fractions plot in straight lines on this chart at both temperatures. Since the average number of rings is computed from properties of molecular weight and refractive index, a similar relationship may be obtained by substituting refractive index values for the average number of rings. Different fractions yield a straight line plot only a t constant molecular weight values (1). Fractionation techniques such as distillation which produce fractions of different molecular weight will not plot in a straight line on this chart. Figure 6 is the same type of plot for the extract fractions. I n this case, however, when the average number of rings per molecule reaches a value of 4, a break appears in the relationship a t both 100" F. and 210" F. The significance of this break is not understood. I t shows that the rate of increase of viscosity with the increasing average number of rings is much greater after the total number of rings per molecule becomes greater than four.
200 I00
Iopoo
5000
2000 1000
40
3 ?
35
o
Figure 5.
3 200 I
2 3 4 TOTAL RINGS PER MOLECULE
5
Paraffin distillate raffinate viscosity versus number of rings
Figure 4 shows a corresponding treatment of the properties of fractions obtained from the furfural extract of paraffin distillate. The extract fractions show a response to thermal diffusion different from those of the raffinate. I n this case, both aromatic and naphthenic rings increase in number per molecule from the top to the bottom of the column while in the raffinate only the naphthenic rings concentrated downward. The relative numbers of aromatic and naphthenic rings in all fractions are about equal. As shown in Table 111, the uppermost fraction from the extract has the very high viscosity index value of 176. The composite of the upper three fractions had a value of 120 viscosity index. Single ring aromatics with long paraffinic side chains are known to have viscosity index values among the highest of any that have been synthesized. Condensed ring compounds appear to predominate in the bottommost fractions. The calculated average number of rings per molecule for the thermal diffusion fractions from the raffinate is plotted versus viscosity in Figure 5. The viscosity scale is the standard ASTM
February 1955
500
g
100
UJ
0
u 2 5
50
I
0
I
2
3
4
I
5
TOTAL RINGS PER MOLECULE
Figure 6.
Paraffin distillate extract viscosity versus number of rings literature Cited
(1) Boelhouwer, C., van Steenis, J., and Waterman, H. I., Fuel, 33, 60-5 (1954). (2) Hazelwood, R. N., Anal. Chem., 26, 1073 (1954). (3) Jones, A. L., Petroleum Processing, 6, 132 (1951). (4) Jones, A. L., and Milberger, E. C., IND.ENQ.CHEM..45, 2689 (1953). ( 5 ) Mair, B. J., and Rossini, F. D., Ibid., in press. RECEIVED for review August 6,1954.
A C C ~ P T EDecember D 13, 1954.
INDUSTRIAL AND ENGINEERING CHEMISTRY
215
