MINERAL OIL DETERIORATION J. C. BALSBAUGH, A. G. ASSAF, AND W. W. PENDLETON Massachusetts Institute of Technology, Cambridge, Mass
F
OR several years the deterioration of mineral oils in the presence or absence of either copper or paper (or both) has been studied in these laboratories. The results (as well as new tests and equipment, 1 , 2, 4, 9,10) have been reported (6, 6). This work has been continued with an aim toward interpreting the mechanism involved in the deterioration of these oils asd the roles played by the various types of hydrocarbons in the oil with respect to their electrical and chemical stabilities. The procedure used in these deterioration studies was described by Balsbaugh, Howell, and Assaf (6),and detailed sketches of the apparatus used were given by Balsbaugh and Assaf (9). Briefly, the apparatus consists of an all-glass deterioration cell with an internal glass pump, two threeterminal platinum measuring cells for making electrical measurements on the oil and oil-impregnated paper, and an automatic gas feed apparatus for maintaining constant pressure. The samples are deteriorated at 85" C. in the presence of copper spirals (&mil wire) and Whatman No. 41-H ashless filter paper, at a pressure of 760 mm. Electrical measurements are taken from time to time as the deterioration warrants.
Electrical measurements of dielectric constant and conductance are made on the oil and in certain cases on an oilimpregnated paper sample during the deterioration. I n a dissipative circuit the admittance, y, may be expressed in terms of an equivalent parallel conductance, 9, and susceptance, b, as
+ j b = jwCoe = wCb(d' + j e ' )
(1)
where E = e' - je' C. = geometric vacuum capacitance of electrode arrangement, farads w = 27lj
cycles per second 6 =- jfrequency, wc
C = capacitance in farads of the electrode arrangement, including the dielectric
The loss angle, 6, of the dielectric may be expressed in terms of the dissipation factor, tan 6, as
where e'
= e" =
dielectric constant loss factor
(3)
When the dielectric constant of mineral oils is approximately 2 at a frequency of 60 cycles per second, d'f is slightly larger than the 60-cycle power factor expressed in per cent. Thus in the later discussion values of e'f may be readily interpreted in terms of the usual 60-cycle power factors. The specific conductance g,, in mho-cm., can be obtained from values of e'(f as follows: gs = 0.5566' tan 6f10-12= 0.556(10-1~)~"f
(4)
A study has been made of (a)the effect of
Electrical Measurements
y =g
From a practical point of view the electrical loss in a dielectric is expressed in terms of the power factor, cos 8. Since cos 8 is equal to sin 6 in the lower practical range of power factors, cos 8 may be assumed to be practically equal to tan 6. Since the 60-cycle power factor, cos e,, is of practical importance, this value may be readily obtained from values of e'y, and is
temperature on the oxidation characteristics of an oil sample with a viscosity of 100 Saybolt Universal seconds at 100" F., ( b )the continuous oxidation of a series of related samples having viscosities comparable to those of cable saturants, ( c ) the effect of a limited amount of oxygen on a water-white aromatic-free oil, and ( d ) the oxidation and electrical characteristics of cetane and cisDecalin. The results show that up to 95" C. the mechanism of deterioration was approximately the same. The series of related oils displayed varying electrical and chemical stabilities with varying aromaticity. A limited amount of oxygen produced higher electrical losses than those obtained with an unlimited supply; and the hydrocarbons exhibited widely divergent chemical and electrical properties.
Since the conductance g is proportional to the dielectric loss, the loss factor E" is proportional to the loss per cycle and ef'j+is proportional to the conductance or total loss. I n the case of mineral oils, many electrical measurements over the audioDeterioration of an Oil at Different Temperatures frequency range and in the temperature range 25" to 115" C. have indicated that at various stages of oxidation or deterioraPRHPARATION AND PHYSICAL PROPERTIES OF SAMPU. The study of the effect of temperature on the mechanism of detion d'f is practically a constant. This indicates that the loss terioration by oxygen was carried out on Shell sample 100-3. is principally due to the motion of ions. I n the mineral oil This sample is one of a series of oils prepared by the Shell deterioration studies discussed later, the electrical measurePetroleum Corporation from a plant cut of a special West ments on the oil are expressed in terms of d'f. 1321
INDUSTRIAL AND ENGINEERING CHEMISTRY
1322
OXIDATION
TMME
HOURS
IN
FIQURE1. OXIDATIONOF SHELL100-3 PRESSUREI
AT
760 MM. OXYQEN
Figure 1 shows a plot of absorbed oxygen against time for the various temperatures. The runs appear to fall into two separate groups, the 75" and 85" C. tests belonging to one group and the 105" and 115" tests to another. At the higher temperature (105" and 115") it was not possible to obtain consistent check runs, as shown by the two tests a t 115". This varying behavior may be due to the effect of the oil oxidation products upon the paper. Upon inspection a t the end of the tests the paper from the 75" and 85" C. tests appeared merely to be impregnated with the oxidized oil. The 95 O paper was partially charred or blackened, particularly in the places where it had come in direct contact with the copper, while the paper from the 105" and 115" tests was black and brittle. Previous experiments in this laboratory showed that the action of heat alone upon cellulose (in an atmosphere of oxygen) is insufficient to produce this result, even with a temperature as high as 145" C. However, when mineral oil is oxidized, strong organic acids such as acetic are produced, which a t the higher temperatures would attack the paper. KINETICS OF OXIDATIONDATA. In analyzing these oxidation data, use was made of the following formula which was shown in previous work (7) to best represent the kinetics of oil oxidation:
va Texw crude of low pour point, and has a viscosity of approximately 100 Saybolt Universal seconds a t 100" F. Four charges of this plant cut were solvent-extracted to varying degrees, yielding four samples (Shell 100-1, 100-2, 100-3, and 1004) of decreasing aromaticities; Shell 1 0 0 4 was completely freed of aromatics by treatment with fuming sulfuric acid. All of the samples (including the original plant cut, Shell 100) received a finishing treatment of sulfuric acid followed by clay contacting. The complete series therefore consists of five samples varying in aromaticities from highly aromatic (Shell 100) to aromatic-free (Shell 100-4). Shell 100-3 was selected for study because previous work (6) showed it to be a desirable sample from the standpoint of electrical and chemical characteristics. Shell 100-4 was tested with limited amounts of oxygen, as described later. Data on physical properties are given in Table I. These samples are all free of acidity and are stored under nitrogen.
Vol.'33, No. 10
= bt
where V = volume of oxygen absorbed, co./kg. of oil t = time of oxidation, hours a, b = parameters
PROPERTIES OF SHELL 100 SERIES TABLEI. PHYSICAL Sample No. Gravity "A,. P.!. Saybolt'Unw. viscosity (100' F J , SPP.
F I ~ &point (open cup), O F. Fire
100 23.1
100-1 28.1
100-2 30.4
100-3 31.9
110
102
97
94
345
345
350
350
350
390,
390 la 119
390
890
390
Color Il/tf Soeoific disnersion 142 Gercaptan' disulfide S. yo
