Aug., 1922
723
T H E JOURNAL OF IiYD U S T R I A L A N D ENGINEERING CHEMISTRY
the change of viscosity with the temperature does not follow any known theoretical laws (except in the case of certain liquids of definite chemical composition) and that mathematically exact calculations cannot be made. Thus a simple method of estimation or interpolation is of value, even though the results may not be very accurate. An examination of empirical formulas leads to the adoption of the following method. 1-Using oils all from the class or crude under consideration, make a logarithmic diagram, A (similar to Fig. 9); where absolute viscosity is plotted against the temperature in F. 2-With data from Diagram A, discarding curved graphs, and using only those which, when prolonged, would approximately meet at a point, construct a second diagram, B (similar to Fig. ll), where, for each oil, the slope of the logarithmic graph is plotted against the logarithm of the absolute viscosity a t 100" F. Average the points on Diagram B by a straight line, and find the slope and the point of intersection of this line with the axis of abscissas. 3--Calculate, by Equations 16 and 17, the point of intersection of the logarithmic graphs on Diagram A. This diagram can then be used t o estimate change of viscosity with the temperature, for oils of medium viscosity, of the crude in question. A new diagram would have to be made only when a new crude was considered.
The temperature of average viscosity was obtained from Fig. 12 by planimeter and by Equation 20 in O F . , with n = 4.26 for the Topila oil having an absolute viscosity of 0.96 a t 100" E'. (37.8" C.) and with n = 4.88 for the Panuco oil, with viscosity of 1.30 a t the same temperature. The results are given in Table V. TAELEV-TEMPERATURE O F AVERAGEVSSCOSITY -BY
PLANIMETER--
Average Temperature of Viscosity Average Viscosity Poises OF. C. Topila.. 3.8 122.9 50.5 Panuco 8.5 118.4 48.0
OIL
. .. ..
-------BY
OF T W O
MEXICAN OILS
EQUATION---
19 20 Temperature of Temperature of Average Viscosity Average Viscosity O F . "C. O F . 'C. 122 50 121.1 49.5 122 50 120.5 49.2
Thus for the particular oils and range of temperature in question, there is little choice in regard to accuracy between Equations 19 and 20, either of which is accurate enough for practical purposes, and far more accurate than any method of estimating the temperature of the ground, and the consequent value of tz. While Equation 20 might prove to be of more general application, Equation 19 is far simpler to use in calculations.
&To estimate the viscosity of an oil at, say, 122°F. (50°C.), locate the point on Diagram A (for the crude jn question) with abscissa equal t o 2 000, the logarithm of the standard temperature of 100" F., and with ordinate equal to the logarithm 6f the absolute viscosity a t that temperature. This point, together with the point of intersection of the logarithmic graphs, previously located, determines the graph of the particular oil in question.
Notes on Determination of Absolute Viscosity of Petroleum Oils'
%As the logarithm of 122 is 2.086, the logarithm of the desired absolute viscosity a t 122" F. may be read from the graph just determined, at the point where the abscissa has that value.
The absolute viscosity of a number of samples of petroleum, animal, and vegetable oils has been examined. Ail of these samples showed changes in viscosity when the material was allowed to stand in glass viscosimeters for 24 hrs. With two samples that were retested ajter standing for a number of weeks, the sign and magnitude of the change was practically the same as that observed after 24 hrs.' standing in the viscosimeter. Apparently, samples of oil intended as viscosity standards should be checked from time to time until more is known regarding the nature and extent of the change on standing.
THE TEMPERATURE OF AVERAGEVISCOSITY I n pumping through pipes, the oil is given an initial temperature, t l , in order that it may not be too viscous when the final temperature, t z , is reached. In order to estimate the loss of pressure, the temperature of average viscosity, tu, must be calculated, for which ManceroZ6gives the equation, due to the Standard Oil Company of California: ta =
tl
"3
+
113
tr
(18)
which is equivalent to tu =
tl
-
-k 3 tl
and may be used with either the Fahrenheit or Centigrade scale. If it is assumed that Equation 8 is correct, it may be shown that 27
By W. H. Fulweiler and C. W. Jordan UNITED
GASIMPROVEMENT
CO.,
319 ARCHST., PHILADELPHIA, P A .
T
H E authors have recently had occasion to make a careful determination of the absolute viscosity of several samples of petroleum oil and have found that in some cases there is considerable change in viscosity on standing. It was further noticed that the viscosities showed marked changes when the oil was allowed to remain in the glass viscosimeter for periods of from 24 to 72 hrs., these changes being of the order of 0.5 per cent when the precision of the observations was of the order of 0.1 per cent. These results may be of interest, especially in connection with the use of samples of oil as viscosity standards over long periods. APPARATUS
Mancero gives two curves, for Panuco and Topila oils, "prepared by La Carona Oil Co.," with absolute viscosity plotted against temperature in "C., from 30" to 90" C. (86' to 194' Fa), Both curves show a point of reverse curve a t about 75" C. (167" F.), which is not so plainly visible in the lower part of Fig. 12, reproduced from Mancero's data. The upper part of Fig. 12 shows logarithmic graphs of these same oils, which are straight except a t the highest temperatures. The irregularity a t high temperatures is probably duo to some inaccuracy in viscosimetry.
The apparatus was of the Bingham and Green type. Two viscosimeters were used on each sample. In the No. 1 instrument the capillary was 24 cm. in length and approximately 0.416 mm. in diameter,Z in the No. 2 instrument the capillary was 10 cm. in length and approximately 0.918 mm. in diameter.2 The thermostat had a capacity of 60 gal. The circulation was obtained by means of an external pump, and the temperature control was such that temperatures constant to within f0.005' were readily maintained. The thermometer had
26 Juan Mancero, OiE Weekly, 17 (1920), 84; translated from the Spanish by John F. Dodge. 27 The writer is indebted t o Dr. H. I,. Dryden of the Bureau of Standards for Equation 20.
1 Presented before the Section of Petroleum Chemistry a t the 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 to 7, 1922. 2 Calculated from the "C" constant of the instruments.
