419
T H E JOURNAL OF I N D U S T R I A L AAiD En'GINEERIATG CHEMISTRY
Vol. 14, No. 5
Effect of Paraffin W a x on Viscosity of Petroleum Oils'" By E. W. Dean3 and M. B. Cooked BUREAUOF MINES, PETROLEUM DIVISION, CHEMICAL
Experiments have been performed to defermine the effect upon viscosity of changing the parafin wax content of several petroleum oils. I t was found that, as a general rule, the viscosity of any mixture is intermediate between the viscosity of the parafin and that of the wax-free oil. That this rule is approximate rather than absolute is proved by the fact that mixtures of parafin and an oil of practically equal oiscosity were slightly less oiscous than either of the constituents. Parafin wax, when dissolved in oil, behaves lite an oil of low viscosity at temperatures considerablg below its melting point. Commercial parafin wax is a complex mixture which can be easily resoloed into fractions of differing physical properties.
need for information on the relation between paraffin wax content and viscosity of petroleum oils was recently encountered. The lack of published figures and unsatisfactory character of the information supplied to the Bureau by individuals associated with the petroleum refining industry led to the belief that this subject had probably never been given the detailed study which it merits and therefore experiments, the results of which are recounted herein, were undertaken. Acknowledgment is made of the assistance rendered in experimental determinations by L. E. Jackson, formerly petroleum chemist, and F. W. Lane, organic chemist, of the Pittsburgh petroleum laboratory of the Bureau. A preliminary report5 covered the results of Mr. Jackson's work. SCOPEOB I~WESTIGATION The work involved measurements of the viscosities of several commercial petroleum oils before and after adding predetermined percentages of commercial paraffin, and similar tests upon two distillates specially prepared in the laboratory from crude oil, before and aft8erremoving successive portions of crystalline wax. The commercial oils used were water-white kerosene, transformer oil, spindle oil, medium automobile oil, and Libwty aero oil. All the oils and the wax used were, according to the best available information, derived from Pennsylvania crude petroleum. The laboratoiy distillates were also derived from Pennsylvania crude oil and were very similar in general character to crude '%ax distillates'' as prepared in oil refineries. The paraffin wax was a well-known brand, sold largely for household use. Changes in wax content varied from a fraction of 1 per cent up to 10 per cent. For experiments involving the removal of wax from the laboratory distillates the percentages were necessarily odd figures. For the other tests, the quantities of paraffin added were 1,5, and 10 per cent, respectively. Viscosity determinations were made on each original oil and on each oil-paraffin mixture. Measurements were made on each sample a t several temperatures ranging between 100" and 210" F. Results are reported in terms of kinematic viscosity for the entire series of tests, Saybolt Universal figures also being given for all products except the kerosene. The specific gravities of most of the mix-
A
January 11, 1922. Published by permission of the Director, U. S. Bureau of Mines. 3 Petroleum Chemist, Pittsburgh Experiment Station, Bureau of Mines. 4 Assistant Petroleum Chemist, Pittsburgh Experiment Station, Bureau of Mines. 5 E. W. Dean and I,. E. Jackson, "The Effect of Crystalline Paraffin Wax upon the Viscosity of Lubricating Oil, "Bur- Mines, ReOls of Investiaalions 3149, May 1921. 1 Received 2
SECTION,
PITTSBURGH, PENNSYLVANIA
tures are also reported, which makes it possible to calculate absolute viscosities if such figures are desired. To obtain supplementary informatlion, the several products studied were subjected to conventional physical tests. Experiments were also performed to demonstrate that commercial paraffin wax is a mixture of constituents which vary appreciably in physical properties. EXPERIMENTAL DESCRIPTION O F PRODUCTS STUDIED-The tW0 laboratory distillates were prepared by distilling a 5-gal. charge of Pennsylvania crude petroleum in an electrically heated iron still equipped with a coil for supplying 'ropeii" steam. The primary fractions obtained by means of this "steam" distillation were redistilled in a vacuum of 40 mm., and final products obtained that came over between temperature limits of 250' to 275' C. (482" t o 527' Fa),and 275" to 300" C. (527" to 572" F.). TABLE I-PHYSICAL PROPERTIES OF PRODUCTS USGDIN EXPERIMENTS Specific Flash Fire Saybolt Universal Gravity Point1 Point1 -Viscosity at-NATURE OF PRODUCT atf3OOF. ' BC. O F . O F . 100'F. 13OOF. 210'F. Water-white kerosene 0.795 4 6 . 1 , , . .. Laboratory distillates 0.835 , ,, 6 3 . 8 43: 4 34: 3 Laboratory distillates 0.847 69.2 50.8 36.3 Transformer oil 0.852 34:3 340 39s 72.7 51.7 36.6 Spindle oil 0.883 28.6 415 480 212.7 111.0 4 6 . 8 Medium auto oil 0.883 28.6 420 470 283.0 141.0 52.0 Liberty aero oil 0.888 27.7 475 530 874.0 383.0 89.1 Paraffin wax4 .... . . . .... 52.0 37.0 1 Cleveland open cup. 2 Distillation limits at mm vacuum 250" to 275O C. (482O to 527OF.). 8 Distillation limits at %mm: vacuum' 275"to 300° C. (527O to 572OF.); 4 Melting point of paraffin wax, 126.'0° F.; specific gravity at 130 F., 0,7830.
. . . . . ..... . .
.....
...
The commercial products were obtained from refining companies operating and marketing in the Pittsburgh district. The physical properties of both the laboratory and commercial products are indicated by figures in Table I. OPER.4TINC DETAILS-The method of separating wax from the laboratory distillates was an adaptation of the commercial process of "filter pressing" wax distillate. The actual procedure involved forcing the cooled oil through a specially designed pressure filter, the separating medium being ordinary filter paper. The quantities separated were determined by weighing the wax scraped from the filter paper after the liquid oil had been driven through. The resultant figures are, of course, only approximate,' but' the accuracy is sufficient for present purposes. Additions of wax to the commercial oils were made on the basis of percentage by weight of paraffin in the mixture. Viscosity determinations were made either with the Saybolt Universal viscosimeter or with Ostwald viscosimeters. The latter instruments were calibrat,ed so that results could be expressed in terms of either kinematic viscosity or Saybolt viscosity. The kinematic viscosity equivalents of the figures obtained by direct use of the Saybolt viscosimeter were calculated by means of the familiar Bureau of Standards equation.6 Results for all oils except the kerosene are reported in terms of both systems of viscosity units. DISCUSSION OF RESULTS Results of the principal series of viscosity tests are shown in Table 11. It will be observed that the viscosity of the original oil has a marked influence on the effect of changes in wax con-. 6
Kinematic viscosity = 0.00220 X Saybolt viscosity
1.80
Saybolt viscosity
.
May, I922
THE JOURNAL OF IhTDUSTRIAL AA'D EiZ'GINEERING CHEMISTRY TABLE
11-CHANGES
DESCRIPTIONOF OIL Water-white kerosene
CAUSED BY VARYING PARAFFIN !&'AX CONTENT O F SEVERAL PETROLEUM OILS Percentage Change Specific Saybolt Universal Viscosity Kinematic Viscosity Paraffin Content Gravity a t --a-ta-t 7 of Oil 100' F. 100' F. 130' F. 210'F. looo F. 130' F. 210' F.
I N vISCOSITY
---
0
+ I
+
+ 250' t o 275' C. laboratory distillate1
275' t o 300' C. laboratory distillate'
Transformer oil
Spindle oil
Medium automobile oil
Liberty aero oil
411
5 10 0 .6 -1.9 -2.5 -3.3 0 -1.5 -3.0 -5,Q -9.0 0 f 1 + 5
-
+ 100 f f
1 5
+ 100
+I!100 + I +
+
5
0,7800 0,7806 0.7810 0.7812
....
.... ....
....
.... .... .... .... ....
0.8400 0.8391 0.8374 0.8353 0.8691 0.8683 0.8fl56 0.8606 0,8703 0.8675 0,8%1 0.8615 0,8744 0.8723 0,8698 0.8650
....
....
.... ....
53.8 (51.4) 54.5 54.7 54.9 69.2 (64.5) 69.1 68.7 71.4 69.3 68.3 67.3 67.5 213 206 188 168 283 267 246 215 874 826 688 561
43.4 43.5 43.7 43.8 43.9 50.8 50.7 50.7 50.3 51.0 51.7 51.5 51.2 60.8
34.3 34.3 34.4 34.3 34.2 36.3 36.1 35.9 35.9 36.3 36.5 35.9 35.1 34.5 46.8 45.0 43.3 43.0 52.0 60.9 50.4
....
....
111
....
'
109 100 91.5' 141 137 128 115 384 389 319 272
0.01930 0.01864 0.01994 0.02166 0.0849 (0.0781) 0.0870 0,0874 0.0580 0.1262 (0.1139) 0.1260 0.1250 0.1320 0.1264 0,1239 0.1213 0.1218 0.4602 0.4445 0.4040 0.3589 0.6162 0.5584 0.5339 0.4646 1.923 1.817 1.514 1.234
48.2 89.1 87.8 78.3 72.5
0.01415 0.01446 0.01532 0.01655 0.0539 0.0543 0.0548 0,0551 0.0555 0.0763 0,0760 0.0760 0.0749 0.0769 0.0789 0.0783 0.0775 0.0760 0.2280 0,2233 0,2020 0.1816 0.2974 0.2883 0.2675 0.2374 0.8448 0.7898 0,6962 0.5918
10 1 Some viscosity measurements for the two laboratory distillates were made a t temperatures slightly different from those listed. are interpolated t o permit tabulation on an equivalent basis.
t8ent. Addition of wax to the kerosene increased its viscosity. The change in the case of the two laboratory distillates is practically negligible. For the other four oils there is a decrease in viscosity, as a result of adding paraffin, Bhe decrease varying with the viscosity of the original oil. For example, the maximum decrease for the transformer oil is of the order of 5 per cent and for the Liberty aero oil is of the order of 35 per cent. The obvious conclusion is that this investigation is simply one phase of the familiar general problem of the viscosity of mixtures. The effect of the paraffin wax seems to be practically identical with what might have been expected i f , instead of the wax, a liquid oil of equal viscosity had been used. Actual measurements (see Table I) showed that at temperatures above its melting point, the viscosity of the paraffin was approximately the same as that of the transformer oil, When mixed with the more fluid kerosene -it caused an increase in viscosity, and when mixed with the 'more viscous oils there was a decrease. The transformer oil and the two laboratory dist'illates might seem to be exceptions to the simple qualitative rule that h.olds for the other products, The wax and transformer oil are pract,ically equal in viscosity, and the mixtures &re less viscous than either of the constituents, This occurrence, though interesting, is not at all surprising, as the viscosities !of oil blends are usually less than the values calculated by "the most satisfactory rule for a d d i t i ~ i t y . ~ There is R more definite explanation for the results obtained. with the two laboratory distillates. Neither of these products showed appreciable changes in viscosity when -various percentages of paraffin were removed. These two oils were considerably different in viscosity, and as .a logical inference it was to be expected that the properties ,of the paraffin removed from one distillate would not be identical with those of the paraffin removed from the other. The separated paraffin, unfortunately, was not retained, and this probability could not be investigated by direct experimentation, It was shown, however, that the wax used in the other tests could be resolved without difficulty into fractions of differing physical properties. To determine this experimentally a 15-g. sample of paraffin was dissolved in 7 . 5 g. of pure benzene by warming to a temperature of approxie
,=,.
H, Herschel, ',Saybolt Technologic Paper 164 (1920).
viscosity of Blends,,, Bur,
0.00842 0.00867 0,00902 0,00958 0.0230 0.0230 0,0234 0.0230 0.0228 0.0303 0.0295 0.0290 0.0290 0.0303 0.0310 0.0288 0.0260 0.0237 0.0642 0.0590 0.0537 0.0527 0.0798 0.0766 0,0762 0.0687 0.1758 0.1727 0.1492 0.1341
The figures given
mately 60" C. The solution was then cooled to 30" C., and the resulting crystals of wax were removed by filtration. The benzene was then removed from both "precipitate" and "filtrate" by evaporating for 3 hrs. on a steam bath. The yield of paraffin from the precipitate was approximately 7 g., while that from the filtrate was 8 g. The properties of the original paraffin and of the two fractions are indicated by the figures in Table 111. TABLE I
I
I
-
T~~~ ~ ~ cOMMERCIAL ~ ~ ~ PARAFFIN ~ ~ W A X Is
A MIXTURE
OF CONSTITUEXTS HAVINGVARYIXGPHYSICAL PROPERTIES
Melting Point NATURE OF MATERIAL 30"
c.
Saybolt Universal Viscosity a t 150' F.
F.
126
44
from a solution of 2 parts
~ ~ r ~ ~ ~ Fraction remaining in ~ 0 1 ~ tion a t 30' C. when 2 parts of parafin were dissolved in 1part. of benzene
~132 ~
119
i
$
46 ,
~
~
42.5
It will be noted that even so simple a method of separation as this shows that commercial paraffin wax is a mixture of constituents of differing physical properties. Such being the case, it is entirely justifiable to assume that the paraffin separated from the laboratory fraction distilling between the limits of 250" to 275" C. is less viscous than that separated from the 275" to 300" C. fraction. It has been impossible to formulate a quantitative rule for the effect of varying the paraffin wax content of oil from the figures in Table 11. In a majority of cases the decreases 'or increases in viscosity are of such size as to be somewhat influenced by normal experimental errors in the determinations; furthermore, there is no satisfactory general rule for calculating the visc0sit.y of blends. The method outlined by Herschel7 is admittedly only an approximat'ion, and is part,icularly unsatisfactory for mixtures in which the relative percentages of constituents are so different. One of t'he most interesting facts brought out by these experiments is that, when dissolved in oil, paraffin wax seems to behave like an oil of low viscosity a t temperatures appreciably below its melting point. Its effect upon viscosity at 100" F., which is 26" F. below its melting point, is similar to its effect a t 130' and 210" F. The temperatures a t which the oil-wax mixtures would begin to deviate from the general-though perhaps indet,erminate-rules covering the viscosity of blends were not determined, but it seems safe to assume that they are related to the so-called cloud points, or point,s at which solid paraffin begins to separate.
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~
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