January, 1927
INDUSTRIAL AND ENGINEERING CHEMISTRY
125
Determination of Oil in Paraffin Wax‘ Direct Refractometer Method By S. H. Diggs and C. C. Buchler STANDARD OIL COMPANY (INDIANA),CASPER,WYO.
HE method described in this paper for the determination of oil in paraffin wax by means of the refractometer has several advantages over the present press method:
T
(1) It is capable of much greater accuracy for small percentages of oil. (2) It gives consistent results. (3) Less time is required t o make a determination.
It has the added advantage over any other method thus far proposed that it is readily adaptable to routine testing. The shortcomings of the present press method for the determination of oil in paraffin wax have already been fully discussed by Wilson and Wilkins,2 and are so generally recognized as to need only passing mention. The various attempts to modify the procedure of the press method, in order to obtain more consistent results, have met with little success. Methods which are based upon the selective solubility of some solvent for either the wax or oil, usually the latter, such as Holde’sY or its modification^,^ are unsatisfactory for samples containing small percentages of oil. Obviously, this is due to the fact that as yet there is available no solvent which is completely selective for only one of the components. Moreover, the necessarily complicated and time-consuming procedure of all such methods makes them unsuitable for routine testing, where speed as well as accuracy is essential. Not until Wilson and Wilkins proposed the “so1ventAndex of refraction method,”2 in which for the first time the use of the refractometer was applied to the determination of oil in paraffin wax, was a really accurate method available. However, in this case, too, the procedure is too complicated for routine work. It therefore still seemed highly desirable to attempt to work out a method involving the use of the refractometer, which would meet the requirements of speed and accuracy demanded by routine testing. The simplest and most rapid procedure would be merely to determine the index of refraction of a sample of %ax a t some definite temperature above its melting point and then, by reference to a series of previously established curves of index of refraction versus per cent oil, to read directly the per cent oil from the curve of the corresponding melting point. Since the basis of such a method depends upon the constancy of the indices of refraction of the two components, namely, the “wax-free” oil and the “oil-free” wax, it was first necessary to establish the existence or non-existence of this constancy. “Wax-Free” Oil Component A series of samples of pressed distillate, which was collected over a considerable period of time, showed conclusively that the index of refraction varied directly with the cold test of the sample. Therefore, the pressed distillate of the lowest cold test obtainable should most nearly represent the oil component of paraffin wax, since such a pressed distillate should have the lowest wax content of any available. Such an oil had a cold test of -22” C. (-7” F.) and an index of refraction of 1.4762 a t 60” C. (140” F.). Received August 28, 1926. Presented before the Division of Petroleum Chemistry at the 72nd Meeting of the American Chemical Society, Philadelphia, Pa., September 5 to 11, 1926. 2 THIS JOURNAL, 16, 9 (1924). 8 Holde-Mueller, “Examination of Hydrocarbon Oils,” p. 37, 1915. Bur. Mines, Tech. Paper S68, 17 (1825). 1
“Oil-Free” Wax Component The next step was to obtain the “oil-free” wax component from a number of samples representing waxes of various melting points, in order to test the constancy of the index of refraction of this component for a given melting point. The difficulty involved in securing the “oil-free” wax component by any usual procedure is obvious. If crystallization from some solvent was used to separate the wax and oil, it was certain that, during the repeated crystallizations which would be necessary to obtain a complete separation, some of the wax would be lost on account of its solubility. However, there were available certain facts which indicated that Attapulgus filter clay selectively adsorbed the oil component from a mixture of paraffin wax and oil. If this selective adsorption by filter clay was quantitative for small percentages of oil, the “oil-free” wax component could be obtained very readily and simply. T a b l e I-Constancy
of I n d e x of Refraction after Filtration t h r o u g h Clay
h‘o.
SAMPLE
OF
FILTRATIONS
54’ C.m. p. refined wax
1
2
Same as above to which oil was added
nx 1.4361 1.4361 1.4361
0
0
1.4370 1.4364 1.4361 1.4361
1 2 3
Various synthetic mixtures of different refined waxes with oil were made, and each sample was filtered through Attapulgus clay until the index of refraction remained constant. For the same refined wax, the final value of the index of refraction was invariably the same, regardless of the number of filtrations to which the sample was subjected. Typical results obtained with one sample of refined wax are arranged in Table I. T a b l e 11-Variation
of Index of Refraction w i t h M e l t i n g P o i n t
MELTING POINT
FILTRATION CONSTANT VALUE
$AFTER
F.
c.
132
56
1 1 3
1.4361 1.4359 1.4353
125
52
1
1.4351
TO
The constancy of the final value of the index of refraction of a mixture of wax and oil, regardless of the amount of filtration through filter clay, proves conclusively that filter clay not only quantitatively adsorbs the oil component from such a mixture, but that the wax component does not suffer any fractionation. Thus an “oil-free” wax may be defined as a
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INDUSTRIAL AND ENGINEERING CHEMISTRY
wax whose index of refraction does not change upon filtration through filter clay. It would seem that this definition is not purely arbitrary, since, in general, the indices of refraction of “oil-free” waxes vary with the melting point-the higher melting points corresponding to a higher index of refraction (Table 11). The assumption that the variation in melting point is due to the presence of varying amounts of oil would necessitate a higher index of refraction for a lower melting point, since the oil component has a higher refractive index than the wax. A further examination of the data in Table 11 shows that the indices of refraction of [‘oil-free” waxes of a given melting point are not sufficiently constant to form the basis of a method for the determination of oil in paraffin wax directly from the index of refraction. This inconstancy is undoubtedly due to the fact that paraffin wax is a composite of hydrocarbons of different melting points in varying amounts, the relative quantities depending upon refining conditions. T h e Method
As the work progressed, it became evident that this variation of the indices of refraction might be overcome by filtering each individual sample, in order to obtain the [‘oil-free” wax component, the index of refraction of which would establish the zero per cent oil point of a curve of index of refraction versus per cent oil. Thus the zero per cent oil point of an index of refraction-per cent oil curve is not a fixed point, but it must be determined for each individual sample of wax. On the other hand, since neither of the refining operations of pressing nor sweating has a fractionating effect upon the oil component, the assumption is made that this component is the same in all samples of wax. Such will be the case as long as the wax distillate is of fairly uniform quality. Therefore the 100 per cent oil point is a fixed point for a given wax distillate. Since, however, the zero per cent oil point is a variable point, no single index of refractionper cent oil curve is in itself sufficient unless the following formula is used: Let n, = index of refraction of “oil-free’’ wax no = index of refraction of “wax-free’’ oil On a n experimentally established curve n, = 1.4342 no = 1.4762 Then no-n, = 0.0420 = change in index of refraction corresponding to a change of oil content from 0 t o 100 per cent. Now let n = index of refraction of any mixture of wax and oil, in which the index of refraction of the “oil-free’’ wax component = n’w,and the index of refraction of t h e “wax-free’’ oil component = no. Then n’,yn, = n‘,-l.4342 = correction to be applied for 0 per cent oil. Let a = correction to be applied (usually subtracted) for any given sample,
n -n (ntW - n,) no- nur 1.4762-n (n’, - I . 4342) Substituting, a = 0.0420 Let n, = corrected index of refraction t o be read from the
Then a =
established curve, Thenn, = n - a = n-
1.4762-n (n’u- 1 .4342) 0.0420
However, since such calculations would not only tend to complicate the method, but would also be a probable source of error, the same results have been accomplished in a purely mechanical manner ,by means of a specially designed chart.5 The Chart
The relationship between the index of refraction and the per cent oil was established by making up by weight mixtures from 0 to 100 per cent of an ‘[oil-free” wax and a “wax-free” pressed distillate, and determining the indices of refraction at 60” C. (140”F.). (Table 111) 6
Designed in conjunction with G. D. Graves of this laboratory.
Vol. 19, KO. 1
Table Ill-Relationship between Index of Refraction a n d Per c e n t Oil Per cent oil Per cent oil :n ng by weight by weight 0 1.4342 50 1.4543 5 1.4362 60 1.4586 1.4381 10 70 1.4631 1.4421 20 80 1.4674 1.4461 30 90 1.4720 1.4502 40 1.4762 100
This relationship, which is not quite linear on account of the varying densities of the mixtures, is projected to a straight line on tracing cloth. The smallest division on the x-axis equals 0.3 em. and corresponds to 0.2 per cent oil, while the same division on the y-axis corresponds to 0.0002 index of refraction. The chart is approximately 61 by 122 cm. The tracing cloth is mounted on beaver board, and the whole incased in a wooden frame to permit suspension on the ~ 1 1 . The surface is sprayed with a transparent pyroxylin lacquer, so that it may be readily cleaned. A movable brass arm, hinged a t the 100 per cent oil point, passes through the wooden frame a t the lower end, and is held in place by friction. The lower edge of this arm serves as any one of a family of curl-es, all of which pivot a t the 100 per cent oil point, by merely raising or lowering the zero per cent oil end. R e c o m m e n d e d Procedure
For refined and semi-refined waxes, namely, waxes containing less than 2.5 per cent oil, the index of refraction is determined a t 60” C. (140’ F.). Then approximately 50 grams of wax are filtered through about an equal weight of Attapulgus or Floridin filter clay, and the index of refraction of the first few cubic centimeters through is determined. The lower edge of the movable brass arm is made to coincide with the value of the filtered sample a t zero per cent oil, and the per cent oil corresponding t o the index of refraction of the sample is read directly from the chart. For samples containing more than 2.5 per cent oil-namely, crude scale and slack wax, it is necessary only to determine the index of refraction a t 60” C. (140” F.) and, using a previously established value for the “oil-free” wax component, to read directly the per cent oil corresponding to the index of refraction of the sample. D e t e r m i n a t i o n of the “Oil-Free” Wax C o m p o n e n t of C r u d e Scale and Slack Wax
The “oil-free” wax component of crude scale or slack wax may be obtained in either of two ways. The oil content of the sample is reduced to approximately 2.5 per cent by pressing between linens and blotters as in the press method, or the oil may be removed by crystallization from ethylene dichloride, as recommended by Wilson and Wilkins. I n either case, some low-melt wax is lost; however, since the quantity is small, and the wax which is lost has a relatively low index of refraction compared with the whole sample, the error introduced is negligible. The sample of wax thus obtained is filtered through clay, and the index of refraction of the first several cubic centimeters is the value of the “oil-free” wax component. This value is redetermined about once a month. However, experience over a considerable period of time has shown that the index of refraction of the “oil-free” wax component of either crude scale or slack wax varies only slightly, from 0.0002 to a maximum of 0.0004. Therefore, the determination of the per cent oil in such samples is always within a t least 1.0 per cent, which is usually sufficiently accurate for refinery purposes. Results
The method was checked by determining the oil content of a large number of synthetic mixtures of wax and oil, a few of which are given in Table IV.
I-VDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1927
Table IV-Oil C o n t e n t of S y n t h e t i c Oil-Wax Mixtures CAI.CUI,ATED FOUND Per cent Per cent 2.2 2.0 1.5 1.4 1.0 1.0 0.7 0.5 0.5 0.4 0.2 0.3
I n order to check the method further, and a t the same time to secure some data on the agreement obtainable between different operators, a check determination on regular refinery samples was made in the research laboratory without the previous knowledge of the regular wax inspector. A few typical results are given in Table V.
It should be noted that a very satisfactory agreement was obtained under normal operating conditions. Comparison with Other Methods
SOLI-ES.I-~KDESOF REFRACTION YIETHOD-A comparative test between the “solvent-index of refraction method” and the method described in this paper was made on three different samples. Three separate determinations were made on each sample of wax by both methods. Table VI-Comparison
SAMPLE 1 2 3 a
of Solvent-Index a n d Direct Refractometer Methods“
SOLVENT-ITDEX METHOD DIRECTREFRACTOMETER METHOD Per cent Max. Per cent Max. oil variation oil T ariation 0.613 1.046 0.870
0.14 0.23 0.26
0.47 1.00 0.93
The results (Table VI) obtained by the two methods are practically identical. This agreement would seem to indicate that the per cent oil determined is not merely an arbitrary figure but the actual oil content of the wax. PRESS3fETHoD-Table VI1 gives the results of a comparison of the refractometer method and the press method using the procedure suggested by Committee D-2 of the American Society for Testing -l.laterials.6 Table VII-Comparison
of Press Method a n d Direct Refractometer Method
S.4MPl.E
PRESS REFRACTOMETER METHOD METHOD Per cent oil Per cent oil
-
_-
0.0 0.25 0.24 1.00 0.53
54; C . refined 30 C. refined 50’ C . refined 50’ C. semi-refined 50‘ C. semi-refined
Table V-Per c e n t Oil by Different Operators Routine latioratory Research laboratory 0.
