Distillation of Lubricating Oils Using a Single-Stage Centrifugal Molecular Still W. LL. THOMAS Analytical Branch, The flritish Petroleum Co., ltd., BP Research Centre, Chertsey Road, Sunbory-on-Thames, Middlesex, England
b This paper describes a method which has been developed for deriving true boiling point curves for lubricating oils with a CVC 54nch centrifugal molecular still. Reconstitution of the distillation products indicates that cracking does not occur at temperatures up to 300" C. Comparison of the results obtained with the molecular still and with a high-vacuum pot still shows that there is good agreement between the property /yield relationships for the two units to the stage where thermal cracking commences in the high-vacuum still. Thereafter there i s increasing divergence between the properties of the distillates as cracking in the high-vacuum still becomes more severe. Some properties of the fractions are pre!;ented, together with carbon number distribution data determined by mass spectrometry, and the possibility of simulating distillation curves from carbon number data is discussed.
L
on the application of molecular distillation in the petroleum adustry, and, in view of increasing interest in high molecular weight lubiicating oils, an investigation has been made concerning the possibility of developing analyticaltype molecular distilbtion procedures for high-boiling petroleum fractions. The object of this work is to improve existing high-vacuum distillation techniques and to extend them to higher ITTLE HAS BEEN PUBLISHED
Table 1.
Feedstock Inspection Data
Light lubricating oil
Heavy lubricating oil
Density at 20" C., grams/ml. 0 8717 0 9051 Molerular wt. 368 694 Sulfur content, wt. 7. 1.03 1.56 Refractive index, nD20 1 4809 1 4973 Kinematic viscosity at 100" F., cs. 35.2 566.7 Kinematic viscosity a t 210" F., C8. 5.51 33.0 Viscosity index 102 96
was then returned to the base of the bell jar, the evaporator temperature was increased by a suitable interval, and the residue was distilled to give a further 10% distillate fraction on the original charge. The temperature was maintained constant, as before. This procedure was repeated until a total of 9 or 10 fractions had been collected. Distilland temperature was measured a t a point near the periphery of the rotor by a11 iron-constantan thermocouple connected to a Cambridge temperature indicator. Still pressure was measured with a Pirani gauge. After the distillation, the still was allowed to cool and the residue was drained out of the bell jar. The still was cleaned with solvent and the holdup was recovered by distilling off the solvent in a stripping flask in a stream of nitrogen. The distillate fractions, residue, and hold-up were weighed to the nearest 0.1 gram, and a weight balance was made covering the distillation. The small loss incurred was EXPERIMENTAL allocated proportionately to the distillation products according to weight. Apparatus. A full description of The hold-up was not combined with the CVC 5-inch centrifugal molecular the residue since it comprised a mixture still used in the present work has been of distillate and residue. given previously (1). The 1-liter highThe distillates and residues were vacuum still was a pot still of convenexamined for density, kinematic vistional design operated with the minicosity, sulfur content, and molecular mum reflux. The mass spectrometer weight, and the refractive indices of employed for the examination of the the distillates were also measured. lubricating oil distillates was a modified Ring-type analyses were calculated MS2 instrument, manufactured by from these results by the n-d-M method ilssociated Electrical Industries, Ltd., ( 7 ) . In addition, the two feedstocks Manchester, England (8). Materials. LIGHT LUBRICATING and those distillates which had carbon numbers less than about 70 were anOIL. This oil corresponds to one of alyzed by mass spectrometer and their the less viscous refinery grades of carbon number distributions in the lubricating oil obtained by processing alkylbenzene (C,H2,-6) series were dea distillate cut from Kuwait crude oil. termined. Hydrocarbons beyond this HEAVYLUBRICATING OIL. This is carbon number could not be examined the most viscous of the refinery grades because of pumping-out difficulties of lubricating oil and is produced from encountered in the mass spectrometer residual stocks by propane deasphalting with substances of such low vapor followed by the usual refining treatpressure. ments. This oil was derived from Normal boiling points were derived mixed Kuwait/Iraq crude oil. Propfrom the observed still temperatures and erties of these feedstocks are given in pressures using Maxwell and Bonnell's Table I. vapor pressure correlation (6). They Procedure. About 500 grams of were also estimated from the measured the feedstock were charged t o the molecular weights of the distillate still under vacuum and the material fractions using the relationship between was degassed by circulation over the molecular weight and boiling point for evaporator. When a still pressure of n-alkanes and narrow petroleum fracapproximately 5 microns Hg mas attions given in Figure 1, constructed tained, the evaporator temperature was from data available a t the BP Research adjusted so as to give an approximately Centre. lOyo distillate fraction on the charge, The t w o feedstocks were also disand was maintained constant during the actual distillation. The residue tilled in the I-liter high-vacuum pot that remained in the residue receiver still and a number of fractions were
molecular weight regions than is practicable a t present. This work was done with a 5-inch Consolidated Vacuum Corporation centrifugal molecular still which was developed by Biehler, Hickman, and Perry (I), and which has been used successfully for the assay of crude oil residues (4). Finished lubricating oils were used for the experiments described in this paper and the results presented are of particular interest because they are the first known published data for the yie!d/product quality relationships for lubricating oil distillates in this high molecular weight range. Carbon number distribution data determined by mass spectrometry are also included. For comparison, results are also given for the high-vacuum distillation of the same feedstocks using a 1-liter highvacuum pot still of conventional design.
VOL. 36, NO. 6, M A Y 1964
e
1047
1000
900 ,800
loo Ld
300
400
500
602
700
000
900 200
NORMAL B O I L I N G POiNTsC
Figure 1. Relationship between molecular weight and normal boiling point for narrow petroleum fractions
removed a t successively higher temperatures with the minimum reflux. The high-vacuum distillates were submitted to the same examination as the molecular still fractions. To determine whether thermal cracking was occurring a t the higher temperatures employed in the centrifugal molecular still, the distillates and residue from each of two distillations were blended together to form reconstituted heavy lubricating oil. Normal inspection data were determined with these reconstituted feedstocks, as well as n-d-M analysis and carbon number distribution. Boiling points, in addition to being derived by the methods described above, were also estimated from the feedstock carbon number data as follows. The carbon numbers were first converted into molecular weights by the formula C,,HZ,-s for the alkylbenzene series of hydrocarbons. Then, with the molecular weight/normal boiling point relationship presented in Figure 1, the normal boiling point that corresponded to each molecular weight was interpolated. The quantities of the alkylbenzene hydrocarbon corresponding to each carbon number, expressed as molar percentages of the total alkylbenzene content, were next converted into the weight percentages by multiplying by the appropriate molecular weights, and then normalizing and multiplying by 100. The cumulative weight yo yields were finally plotted against the estimated normal boiliig points to give simulated distillation curves. This procedure does not take into account the possible presence of isobaric hydrocarbons-i.e., hydrocarbons of the same molecular weight but of different typee.g., members of the C,H2,-20 series. It also ignores the possibility of sulfur compounds occurring a t the same mass numbers. When distillation curves are simulated as described above, the carbon number distribution is converted to a weight composition basis with molecular weights derived for alkylbenzenes. However, the boiling points are obtained from a relationship which is applicable to n-alkanes and narrow boiling fractions. This procedure is
1048
ANALYTICAL CHEMISTRY
rl
Table 111.
Feedstock, frac-
tion No.
Table IV.
High-Vacuum Distillation Product Yields
Heavy lubricating oil Light lubricating oil CumuCumulative lative distildistilCumulate late Cumulative on lativtr on Yield on yield ctn charge, Yield on yield on charge, charge, charge, mid. charge, charge, mid. wt. % wt. % wt. % wt. 96 wt. % wt. % 9.9 9.8 9.9 9.9 10.0 10.0 10.0 10.1 10.2
1 2 3 4 5 6
49-
...
10 11 12
...
9.9 19.7’ 29.t; 39.5 49.5 59.5 69.5 79.6 89.8 ...
...
4.9 14.8 24.6 34.6 44.5 54.5 64.5 74.6 84.7
... ...
...
.
Residue 10.’2
lO0:O
I
.
6.7 13.3 20.0 26.9 33.5 40.3 47.1 53.9 60.7 67.6 74.6 81.7 100.0
6.7 6.6 6.7 6.9 6.6 6.8 6.8 6.8 6.8 6.9 7.0 7.1 18.3
justified because the niolecular weights calculated from the alk ylbenzene carbon numbers show good agreement with the experimental values (8). Alkylbenzenes, which have the formula C,H2,-e, have a position which is approximately midway between the extreme compositions represented on the one hand by the n-paraffins, wit’+ the formula CIHgn+2, and on the other hand by the hydrocarbons, dejignated by the formula CnH2n--10. RESULTS
Operating conditions and product yields for the molecular distillation of the light and heavy lubricating oils are given in Table 11. This table also includes the normal boiling points estimated from hlaxwell and Bonnell’s vapor pressure correlation and from Figure 1. The produ1:t yields for the high-vacuum distillation of the two feedstocks are shown in Table 111. Distillation curves and properties of the distillates are presented in Figures 2 to 14.
3.3 10.0 16.7 23.5 30.2 36.9 43.7 50.5 57.3 64.2 71.1 78.2
Frat- Mean distribution of carbon, tion wt. % No. Total Aro- NaDh- Par-’ feed rings matic th&e affin 35.7 36.9 34.2 33.5 34.1 32.1 34.4 34.1 33.8 32.6 Residue 29.6
1 2 3 4 5 6 7 8 9
Frat-
tion
Tables IV to VI1 contain ring-type analyses, calculated by the n-d-M method, of the light lubricating oil molecular distillates and high-vacuum distillates, and the heavy lubricating oil molecular distillates and high-vacuum distillates, respectively. Properties of heavy lubricating oil reconstituted from its molecular distillates and residue are compared with the properties of the original feedstock in Table VIII, while Table IX records the results of n-d-M analyses of these materials. Figure 15 gives a comparison of the carbon number distribution (C,H2,-6 series) of the original and reconstituted heavy lubricating oil. Simulated distillation curves derived from feedstock carbon number distribution data are shown in Figure 16. DISCUSSION
Separating Power of Centrifugal Molecular Still. The CVC 5-inch centrifugal molecular still is a single stage unit in which the distilland is
No.
feed
n-d-M Ring-Type Analysis for Light Lubricating Oil High-Vocuum Distillation Fractions
Mean distribution of carbon, wt. % Total Aro- Naph- Parrings matic thene affin
35.7 1 33.0 2 33.2 3 33.5 4 33.0 5 33 0 6 33.6 7 34.7 8 33.8 9 34.2 Residue 31.6
6.1 3.6 5.2 4.8 5.1 5.4 5.5 5.2 7.1 8.6 7.1
29.6 29.4 28.0 28.7 27.9 27.6 28.1 29.5 26.7 25.6 24.5
64.3 67.0 -. _ _ 66.8 66.5 67.0 67.0 66.4 65.3 66.2 65.8 68.4
6.1 5.2 4.9 4.7 5.1 6.2 5.7 6.3 6.4 7.4 7.9
...
Table VI. Table V.
n-d-M Ring-Type Analysis for Light Lubricating Oil Molecular Distillation Fractions
Av. No. of rings per moleculeTotal Aro- Naphrings matic thene
~.
1.82 1.56 1.62 1.70 1.71 1.77 1.87 2.00 2.02 2.06 2.34
0.27 0.15 0.23 0.22 0.24 0.25 0.26 0.26 0.36 0.44 0.51
1.55 1.40 1.39 1.48 1.48 1.51 1.60 1.74 1.65 1.62 1.82
29.6 31.7 29.3 28.8 29.0 25.9 28.7 27.8 27.4 25.2 21.7
64.3 63.1 65.8 66.5 65.9 67.9 65.6 65.9 66.2 67.4 70.4
1.82 1.55 1.60 1.62 1.73 1.73 1.87 1.93 2.03 2.06 2.16
0.27 0.20 0.21 0.21 0.23 0.28 0.27 0.31 0.33 0.40 0.51
1.55 1.35 1.39 1.41 1.50 1.45 1.60 1.62 1.70 1.66 1.65
heated only once (per pass) and the entire distillate formed is removed from the still without further evaporation to improve the separation. It may be anticipated, therefore, that the distillation efficiency of the still will be equivalent to that of approximately one theoretical molecular plate. This has been confirmed by plate tests employing the binary mixture di-n-butyl phthalate-di-n-butyl azelate. Using a test mixture of Octoil (di-2-ethylhexyl phthalate) and Octoil-S (di-2-ethylhexyl sebacate) , Biehler, Hickman, and Perry (1) compared the performance of the 5-inch centrifugal molecular still with that of a falling-film still a t different throughputs. The separating power of the centrifugal molecular still was substantially higher than that of the falling-film still and it remained constant a t one theoretical molecular plate over a wide range of throughput. The degree of separation that can be achieved by single-plate molecular distillation of multicomponent feedstocks is illustrated by the yield,/propcrty curves for the light and heavy lubricating oil fractions shown in Figures 4 to 10. The properties change progressively with yield, and the curves for the heavy lubricating oil especially are of con-
n-d-M Ring-Type Analysis for Heavy Lubricating Oil Molecular Distillation Fractions
F ~ Mean ~ ~distribution of carbon, tion wt. % No, Total Aro- NaphParfeed rings matic thene affin 33.8 35.0 32.3 30.6 31.1 30.3 29.6 29.4 29.7 29.6 28.8 28.7 Residue 31.6 1 2 3 4 5 6 7 8 9 10 11
Av. No. of rings per molecule Total Aro- NaDhrings matic th&e
6.1 5.7 6.0 6.6 6.4
6.7 7.0 7.7 8.4 8.7 9.7 10.0 7.9
27.7 29.3 26.3 24.0 24.7 23.6 22.6 21.7 21.3 20.9 19.1 18.7 23.7
66.2 65.0 67.7 69.4 68.9 69.7 70.4 70.6 70.3 70.4 71.2 71.3 68.4
Av. No. of rings per molecule_-Total Aro- Naphrings matic thehe 3.51 2.60 2.70 2.67 2.75
2.83 2.96 3.10 3.23 3.63 3.56 4.04 5.08
0.51 0.34 0.41 0.47 0.47 0.52 0.58 0.68 0.77 0.88 1.01 1.19 1.04
VOL. 36, NO. 6, M A Y 1964
3.00 2.26 2.29 2.20 2.28 2.31 2.38 2.42 2.46 2.75 2.55 2.85 4.04
1049
Table VII.
n-d-M Ring-Type Analysis for Heavy Lubricating Oil High-Vacuum Distillation Fractions
F ~ Mean ~ ~distribution of carbon, tion wt. % No. Total Aro- NaphParfeed rings matic thene affin 1 2 3 4 5 6 7 8 9 10 11 12
Residue
33.8 34.3 34.0 33.2 32.9 31.9 32.2 30.5 29.5 29.6 29.9 29.0 30.7
6.1 3.8 3.3 4.0 4.1 4.7 5.6 6.9 6.8 7.2 8.1 10.1 9.4
. ..
27.7 30.5 30.7 29.2 28.8 27.2 26.6 23.6 22.7 22.4 21.8 18.9 21.3
...
Av. No. of rings per molecule Total Aro- Naphrings matic thene
66.2 65.7 66.0 66.8 67.1 68.1 67.8 69.5 70.5 70.4 70.1 71.0 69.3
...
...
3.51 2.74 2.84 2.89 2.97 3.02 3.10 2.97 3.13 3.28 3.21 3.01 3.08
0.51 0.25 0.22 0.28 0.30 0.36 0.40 0.56 0.61 0.67 0 74 0.90 0.80
...
...
3.00 2.49 2.62 2.61 2.67 2.66 2.70 2.41 2.52 2.61 2.47 2.11 2.28
Table VIII. Comparison of Properties of Original Heavy Lubricating Oil with Those of Material Reconstituted from Molecular Distillation Fractions
Density at 20' C., gr ams/ml. Refractive index, nnm Kinematic viscosity at 100" F., cs. Kinematic viscosity at 140' F., cs. Kinematic viscosity at 210' F., cs. Viscosity index Sulfur content, nit. % Molecular wt.
..
lecular distillation fractions shown in Figure 8 are slightly lower than the values for the feedstocks (up to 12 VI units for light lubricating oil fractions, and up to 4 VI units for heavy lubricating oil fractions).
I
I
I 101
60
20
D I I T I L L A T L "IELD
Figure 2.
%
Wb
Distillation curves
siderable interest because they are the first data on yield/produc t quality relationships known t o have been published on lubricating oil distillates in this high molecular weight range. Particular attention should be paid to the kinematic viscosities of the fractions presented in Figures 6 and 7 . The final distillate had values of 3800 CS. at 100" F. and 78 cs. a t 210' F., whereas the maximum viscosities reported for the high-vacuum distillates are only 540 cs. and 33 cs., respectively. Another interesting observation is that the viscosity indices of the first few mo-
Table IX.
LO
Figure 3.
A
10 DIST,LL*TL " . € : : % X I
,
80
300 $00
Normal boiling point curves
Estimated from Figure 1
Original heavy lubricating oil
Reconstituted heavy lubricating oil, No. 1
Reconstituted heavy lubricating oil, No. 2
0.9051 1.4973
0.9043 1.4977
0,9049 1,4976
566.7
571.9
562.1
156.3
157.8
155,5
33.0 96 1.56 694
33.3 96 1.54 720
33.0 96 1.54 720
spread of 25, from 19 to 44 with a peak at 28. The heavy lubricating oil molecular distillates have a wider composition range, as shown in Figure 13. Here the average carbon number spread is 25, while the heavy lubricating oil itself has a carbon number spread of 29, from 32 to 60 with a peak at 44. Figures 11 and 13 also reveal that, as expected in single-plate distillation, there is considerable overlap in composition of adjacent fractions that amounts t o 13 carbon numbers for the light lubricating oil distillates, and 24 carbon numbers for the heavy lubricating oil fractions. Comparison of Molecular and HighVacuum Distillates. Comparison of the properties of the molecular and high-vacuum distillates obtained by distillation of lubricating oils is interesting because, according to theory a t least, slight differences between the two are to be expected. These differences should arise because, in ordinary ebullient distillation, separation of the different components depends on differences in their partial pressures, ,\-hereas in molecular distillation, separation is a function of the ratio of the
The alkylbenzenes carbon number distribution for the light lubricating oil molecular distillates recorded in Figure 11 shows t,hat the fractions have an average carbon number spread of 14. The feedstock itself has a carbon number
n-d-M Ring-Type Analysis for Original Heavy Lubricating Oil and Material Reconstituted from Molecular Distillation Fractions Av. No. of rings Mean distribution of carbon, wt. % per molecule AroSaphTotal ParTotal Aro- Naphmatic thene rings afEn rinrrs matic thene -
I
I S
I
Original heavy lubricating oil Reconstituted heavy lubricating oil, KO. 1 Reconstituted heavy lubricating oil, No. 2
1050
33.8
6.1
27.7
66.2
3.51
0.51
3.00
32.3
7.5
24.8
67.7
3.47
0.65
2.82
32.9
6.9
26.0
G7.1
3.55
0.60
2.95
ANALYTICAL CHEMISTRY
I
I
I
Figure 5. lates
Refractive index of distil-
partial pressure of each component to the square root of its molecular weight. According to Hickrnan (2), the difference between the two kinds of distillation is small and its importance is apt to be exaggerated. This opinion has been confirmed by Jones (d), who reported that there were no significant differences between the properties of distillate fractions obtained by molecular and high-vacuum distillation of crude oil residues. Similarly, only slight differences should be apparent between the properties of the molecular and the high-vacuum distillates of lubricating oil, because the basis of comparison is single-plate distillation. Actually, comparison is made more difficult because the high-vacuum still, being an unstirred pot still, is unlikely to have an efficiency as high as one theoretical plate. This is confirmed by the slightly wider spread in carbon number of the high-v,acuum distillates compared to that of the molecular distillates shown in Figures 11 to 14. Thus, the average carbon number spread for the light lubricating oil high vacuum distillates is 15, compared with 14 for the molecular distillates. To assess the diff ererices in separation achieved by molecular and high-vacuum distillation ideally, the two stills should not only have the flame distillation 0
"
Gh " I C Y Y U D I S I I L L A T I O H
-
0 D I I T I L L I T L "ILL0
Figure 6.
.(.
0
lo
,oo
WL
Kinematicvixosity at 100" F.
of distillates
efficiency, but the still pressures should also be the same, and, in addition, the distillate fractions should be of identical length. With regard to still pressure, the operating pressure in the molecular still decreased from about 3 microns Hg a t the beginning of each run to approximately 1 micron Hg a t the end. The still pressure in the high-vacuum still during the light lubricating oil distillation decreased from about 15 microns Hg a t the beginning to approximately 2 microns Hg a t the end; in the heavy lubricating oil distillation, the pressure decreased from about 20 microns Hg at the beginning to 15 microns Hg midway through, and then increased gradually to 40 microns Hg toward the end, presumably because of slight cracking. With a high-vacuum still, it is a simple matter to collect fractions of equal volume. HoweVer, with the molecular still, the quantity of distillate formed per pass is purely a function of temperature and it is virtually impossible to control the distillation temperature within the limits required to ensure fractions of the same size. Thus, no more than a qua!itative comparison of the properties of highvacuum and molecular distillates is possible in the present work. In addition, because of the complexity of the feedstocks, any differences between the two sets of distillates will be exhibited only by the first and, to a lesser degree, the last fractions. The principal effect of operating an ordinary still at reduced pressure is to reduce the boiling points of all components in accordance with the operating pressure employed. The effect of operating a t reduced pressure, however, is not the same for all types of hydrocarbons. The polynuclear aromatic hydrocarbons become relatively lower-boiling and the n-paraffins become relatively higher-boiling than naphthenic hydrocarbons that are of the same boiling point a t higher pressures. For a given vapor pressure and a given temperature, the n-paraffins have the highest molecular weights of all the hydrocarbon types; the polynuclear aromatic hydrocarbons, the smallest molecular weights; and the naphthenic hydrocarbons assume intermediate values. It follows, therefore, that, beterm which enters cause of the -',1 the molecular distillation separation expression, a more aromatic and a less paraffinic distillate should be produced by molecular distillation than by highvacuum distillation. However, for the reasons stated above, any differences between the two sets of fractions caused by differences between the two distillation processes are likely to show only in the early, and to some extent, in the late fractions.
I
0
Figure 7.
I
40 e.2 DlSTlLLATE "IELD%*L
20
I 80
100
Kinematicviscosityat 2 10" F.
of distillates
Comparison of the properties of the molecular and high-vacuum distillates shows that the differences between the two are small. For the light lubricating oil distillates, there is some evidence to show that the density, refractive index, sulfur content, and molecular weights of the first molecular distillation fract,ion are slightly higher than the corresponding values for the first high-vacuum distillation fraction, and that the kinematic viscosity and viscosity index are slightly lower (Figures 4 to 10). These differences suggest that the molecular distillate is more aromatic and less paraffinic than the high-vacuum distillate, and support the view put forward above. The properties of the next three or four fractions, however, generally show the opposite effectvie., the densities, refractive indices, sulfur contents, molecular weights, and kinematic viscosities of the molecular distillation fractions are lower than the values for the high-vacuum distillation fractions. The properties of the remaining fractions show close agreement with each other, although the kinematic viscosities of the molecular distillates in
VOL. 36, NO. 6, MAY 1964
1051
DISTILLATE WELD %*L
Figure 9. lates
Sulfur content of
distil-
this range are undoubtedly higher than those of the high-vacuum distillates. Differences observed in the properties of those distillates which follow immediately after the first fraction are probably due to better fractionation in the molecular still than in the highvacuum still, rather than to any differences between molecular and highvacuum distillation. Similarly, the differences revealed by n-d-M analyses shown in Tables IV and V are slight; the greatest differences between the two sets of distillates are exhibited by the first fractions. The results indicate that the percentage aromatic carbon (CAYo)and the percentage naphthenic carbon (CNyo) are greater in the molecular distillate than in the high-vacuum distillate, whereas the percentage p a r a f i i c carbon (C,%) is less. There appears to be a slightly greater number of aromatic rings per molecule (EA) in the first molecular distillate fraction compared with the first high-vacuum distillate fraction, but there is no signiiicant difference in the number of naphthene rings per molecule (EN). With regard to the carbon number
I100 40
PO
800
DISTILLAT< YIELD % w t
Figure 10. lates
1052
Molecular weight of distil-
ANALYTICAL CHEMISTRY
distribution data for the light lubricating oil fractions presented in Figures 11 and 12,the greatest differences between the molecular and high-vacuum distillates are again shown by the first and last fractions, although, as before, the differences are only small. One difference between the two sets of fractions which has already been noted is that the average carbon number spread for the high-vacuum distillates is 15, compared to 14 for the molecular distillates. This indicates that there is better fractionation in the molecular still than in the high-vacuum still, presumably because there is inadequate distilland surface renewal in the latter. In addition, the first molecular distillates have a lower mean carbon number than the corresponding high-vacuum distillates, whereas the reverse is true of the last fractions. This again provides evidence of better fractionation in the molecular still than in the high-vacuum still.
Figure 1 1. Carbon number distribution (C,H2,-a series) for molecular distillates from light lubricating oil
As shown in Figures 4 to 10, there is generally good agreement between the properties of the very first fractions of the heavy lubricating oil distillates. Such small differences between the molecular and high-vacuum distillates as are apparent from the properties of the fractions which follow immediately after the first, and which correspond to a cumulative distillate yield of about 40% weight on charge, suggest that the efficiency of separation is higher in the centrifugal molecular still than in the high-vacuum still. The properties of the remaining fractions, however, show an increasing divergence as distillation proceeds. That thermal cracking is responsible for this is evident from Figure 10, which shows that the molecular weight of the high-vacuum distillates reaches a maximum of 754 a t 57% weight distillate yield and that subsequent fractions have a somewhat lower value. The molecular weight of the molecular distillates, on the other hand, increases progressively with each suecessive fraction up to a value greater than 950.
Figure 12. Carbon number distribution (C,,H2"-e series) for high-vacuum distillates from light lubricating oil
The results of n-d-M analysis of the heavy lubricating oil fractions, presented in Tables VI and VII, show the same general trend as the light lubricating oil fractions-namely, that the greatest differences between the two sets of distillates are exhibited by the first fractions. There is a greater CA% in the molecular distillate than in the highvacuum distillate, a smaller C,%, and the same CP%. There appears to be slightly more Ra in the first molecular distillate fraction than in the first highvacuum distillate fraction, but there are fewer RN. Carbon number distribution data for the heavy lubricating oil fractions, given in Figures 13 and 14, reveal that there is little difference between the spread of carbon number in the molecular and in the high-vacuum distillates, although the first molecular distillation fraction appears to have a somewhat lower peak carbon number than the corresponding high-vacuum distillate (37 compared to 39). Higher molecular distillation fractions, however, have somewhat higher peak carbon numbers than the highvacuum distillation fractions. This can be attributed partly to better fractionation in the molecular still and partly to the incidence of thermal cracking in the high-vacuum still.
_-
_'9 _ _ _ ~ _ _ _ ~--Jeo0 0 D 5
LLl-i
D
%ut
Figure 13. Carbon number distribution (C,H2,-s series) for molecular distillates from heavy lubricating oil
Figure 14. Carbon number distribution (CnH2n--6 series) for high-vacuum distillates from heavy hbricating oil
I n comparison with results of the light lubricating oil (distillations, the work on heavy lubricating oil has shown that, in general, there are smaller differences between the properties of the first molecular and high-vacuum distillates prepared from tht: latter feedstock than from the former. This is not surprising in view of the greater complexity of the heavy lubricating oil and, also, of the lower vapor pressures of the hydrocarbons present in the heavy lubricating oil. Thermal Hazard in Centrifugal Molecular Still. Reference has already been made to the experimental evidence which points to the occurrence of thermal cracking in the highvacuum still. However, i t was of interest to establish the extent, if any, of thermal hazard in the centrifugal molecular still. Comparison of the density, refractive index, kinematic viscosity, sulfur content, and molecular weight of the original and reconstituted heavy lubricating oils, shown in Table VIII, reveals that there are no significant differences in properties betneen them. Similar conclusions are reached both from consideration of the n-d-1\13. ring-type analyses of these materials presented in Table I X and of their carbon number distributions shown in Figure 15. There can be no doubt, therefore, that therms1 decomposition did not occur in the centrifugal molecular still, a t least to temperatures of 300" C. Distillation Temperature in Centrifugal Molecular Still. The outstanding feature of the 5-inch centrifugal molecular still is its ability to distill materials a t much low'3r temperatures than those obtaining in other vacuum stills, including other types of molecular stills. The temperature measured in the still is that of the liquid leaving the evaporator. A vapor temperature would be extremely difficult to measure accurately, and indeed, if conditions in the still are truly representa-
tive of molecular distillation, it would have no real significance because under these conditions there is no boiling point. Molecular distillation occurs whenever there is a difference in temperature between evaporator and condenser. Increasing this difference and the absolute distillation temperature increases the rate of evaporation, but there is no abrupt transition point that corresponds to a phase change as in conventional distillation. It might be considered that measurement of liquid temperature in this way would have no practical value because of the number of variables which influence this reading-e.g., flow rate, viscosity and heat capacity of the distilland, and difference in temperature between the evaporator and condenser (4). Also, it has been reported (4) that the temperature of the oil film varies with the radius of the rotor and reaches a maximum a t a point usually slightly inward from the periphery of the rotor, not always a t the edge or in
Figure 15. Heavy lubricating oil carbon number distribution (C,H*,,-s series) before and after molecular distillation
the residue-collecting system. The sharpness of this variation is said to be due to a number of causes, one of the more important being the amount of preheat supplied to the oil in the feed line. Pl'otwithstanding these limitations, the temperature of the oil in the rotor gutter is very near the expected vapor temperature, as shown by distillation of pure compounds (4). Xs Figure 2 shows, during distillation of multicomponent mixtures of wide boiling range such as light and heavy lubricating oils the temperatures measured in the still reflect the usual general rise in nominal boiling point or decrease in vapor pressure as the lighter fractions are distilled off. Moreover, these temperature/yield curves appear to be characteristic of the material being distilled: the wider its boiling range, the steeper the inclination of the curve.
0 b p OER(YED
F R O M MOLECULAR STlLL TEMPERATURES AND PRESSURES
'
100
The light lubricating oil distills over a t temperatures ranging from 90" to 155' C. (90% off), whereas the heavy lubricating oil has a distillation range of 180' to 310' C. (90% off). These temperatures are remarkably low considering the molecular weight range of the fractions (315 to 465 for light lubricating oil fractions; 505 to 945 for heavy lubricating oil fractions). In ordinary or equilibrant distillation, a mixture can be characterized by its boiling point curve, sometimes known as the true boiling point curve, in which distillate yield is plotted against the normal boiling point at 760 mm. Hg. In molecular distillation, on the other hand, ebullition does not occur, and consequently there is no boiling point. Hence, it is not possible to derive true boiling point curves. For this reason, it has been customary in the past to present molecular distillation data in terms of the elimination curve, as proposed by Hickman (S), in which the concentration of a given constituent in a series of fractions is plotted against distillation temperature. These elimination curves have proved to be extremely useful for comparative purposes, but they have no absolute significance as TBP curves have. It would represent a ronsiderable advance if it \+-erepovible to derive TBP data from measured molecular still temperatures and pressures. In 1956, W. C. Jones published a paper (4) in which normal boiling point us. yield curves were presented for a number of different feedstocks. Good agreement waq shown to exist between these curves and those obtained from ebullient distillation measurements. These experiments were carried out with a Centrifugal Molecular Still (CMS) 5, a CMS 14 molecular still, and a vacuum pot still, and satisfactory correlation was obtained between the three units. VOL 36, NO. 6, M A Y 1964
1053
The T B P curves shown in Figure 3 fully confirm these findings. The normal boiling-points referred to have been derived from the observed centrifugal molecular still temperatures and pressures using Maxwell and Bonnell's vapor pressure correlation (6). Superimposed on these curves are boiliig points estimated from the ebulliometrically determined molecular weights of the distillates with the boiling point molecular weight relationship for nalkanes and narrow petroleum fractions given in Figure 1. The estimated boiling points generally lie within 10' to 15" C. of the distillation curves, and agree with Jones' results (4). This raises the question whether conditions in the centrifugal molecular still are equilibrant rather than molecular. Experiments on this problem, to be described in another paper, were performed in which the evaporation rate of di-n-butyl phthalate was measured over a wide range of temperature, pressure, and distilland feed rate. The experimentally determined distillation rates were in close agreement with the values calculated from Langmuir's equation (6) for the theoretical molecular distillation evaporation rates. Moreover, recycling residue from a single-pass distillation to the evaporator a second time, a t the same temperature as the first pass, produces a further quantity of distillate, which shows that the distillation process is molecular and not equilibrant in character. Maxwell and Bonnell's vapor pressure correlation has been verified experimentally at pressures as low as 0.001 micron Hg, in which region molecular distillation conditions and not equilibrant distillation conditions presumably exist, and it is possible that, contrary to accepted theory, still temperatures measured in the centrifugal molecular still a t a pressure of 1 micron Hg can be converted to normal boiling points. Another more likely explanation is that there exists an intermediate range of pressure in which both -equilibrant and molecular distillation can occur, and that this region extends ,down to 1 micron Hg.
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ANALYTICAL CHEMISTRY
The remaining explanation is that agreement between the experimental and the estimated distillation curves is purely fortuitous. Up to the present time this seemed to be unlikely because the good agreement which had been observed covered the results of experiments that used a wide range of feedstocks, including crude oil residues of widely differing characteristics and wax distillates, in addition to lubricating oils. However, more recent experimental evidence, to be discussed in a later paper, now seems to suggest that it is merely coincidental that normal boiling point curves can be derived from molecular distillation temperatures and pressures. Whatever the explanation, the present investigation has clearly shown that the 5-inch centrifugal molecular still can be used to provide TBP data extending to temperatures beyond 700' C., and that it is no longer necessary to employ the elimination curve technique with its limitations. Simulation of Distillation Curves by Mass Spectrometry. Molecular weights can be estimated with fair accuracy from carbon number data determined spectrometrically (8). In view of the close agreement between the boiling points derived from the still temperatures and pressures, and those estimated from the ebulliometrically determined molecular weights of the distillate fractions, it seemed possible that distillation curves might be simulated by mass spectrometry. This has actually been confirmed. Agreement between the distillation curves simulated from the spectrometric data and those derived from the molecular still temperatures and pressures is fair, as Figure 16 shows. For light lubricating oil, the simulated boiling points have a mean value about 15" C. higher than the boiling points calculated from the observed still readings. The difference decreases from 20" C. a t the beginning of the distillation to 5" C. a t the end. Agreement between the simulated and experimental curves is good for the heavy lubricating oil up to a distillate yield of 30% weight. Thereafter there is increasing divergence between the two curves, until a t the end of the distillation, the simulated boiling
points are approximately 100" C. below the experimental values. Two factors are responsible for the poor agreement in the high molecular weight region : incomplete volatilization of the higher molecular weight constituents, and decrease in instrument sensitivity with increase in molecular weight. Improved sampling techniques and the provision of calibration data in this range would undoubtedly lead to increased accuracy in carbon number determination. These curves represent the preliminary results of an investigation which is still in progress. It is known that better correlation between the experimental and the simulated distillation curves is obtained by using the carbon number distribution data for the molecular distillation fractions rather than those for the feedstock. Nevertheless, the results are sufficiently promising to suggest that a method of simulating distillation curves up to normal boilingpoint temperatures of 700' C. can be developed based on mass spectrometry. This would be of considerable value as a means of extending beyond approximately 500' C. the range of temperature over which it is possible a t present to simulate distillation curves with gas liquid chromatography. LITERATURE CITED
(1) Biehler, R. M., Hickman, K. C. D., Perry, E. S., ANAL. CHEM. 21, 638 (1949). (2) Hickman, K. C. D., Chem. Rev. 34, 51 (1944). (3) Hickman, IC C. D., Ind. Eng. Chem. 29, 968 (1937).
(4) Jones, W. C., American Vacuum
Society, Third National Symposium on Vacuum Technology, Transactions, Pergamon Press, p. 161, 1956. (5) Langmuir, I., Phys. Rev. 2,329 (1913). (6) Maxwell, J. B., Bonnell, L. S., Znd. Ens. Chem. 49, 1187 (1957). (7) Nes, K. van, Westen, H. A. van, "Aspects of the Constitution of Mineral Oils," Elsevier, Amsterdam, 1951. (8) Thornton, E., West, A. R., 2. Anal. Chem. 170, 348 (1959). RECEIVED for review September 23, 1963. Accepted January 10, 1964. Permission to publish this paper was given by the chairman and directors of The British Petroleum Co., Ltd.
