OIL OXIDATION The Reaction Which I s Apparently Retarded by the Products REVIOUS work has shown that oxygen absorption experiments provide a simple means of studying the kinetics of oil oxidation in several cases: the autocatalytic oxidation of oils (f), and the oxidation of oils whose rate of oxidation is unaffected by the nonvolatile reaction products (2). In the Latter case the catalytic effect of metals on the oxidation of the oil was also determined by the oxygen absorption procedure. This investigation presents results on a third type, namely, the oxidation of oils whose rate of oxygen absorption continuously decreases. These three types of oxidation reactions classify most of the oils available. Oxidation Experiments The apparatus and experimental procedures previously described (1) were used in the present study. A detailed study was made of a typical oil whose rate of oxygen absorp-
RALPH W. DORNTE,C. VAUGHAN FERGUSON, AND CARYL P.HASKINS General Electric Company, Schenectady, N. Y.
tion decreases continuously throughout the oxidation. In this way some of the numerous factors involved in the oxidation kinetics were evaluated. The catalytic effects of several metals on the oxidation of this one oil were studied, but only the oxygen absorption data on a few other oils of this group were included. The effect of metals was studied by adding 1-inch (2.5-cm.) lengths of 10-mil copper wire and 7-mil iron t o the absorption cell. For the experiments with tin and lead the granular metal was used (1-2 mm. particles). An active catalyst was not desired in these experiments but rather the effect of the massive metals which are usually in contact with lubricating oils. For this reason the metals were cleaned with solvents and with dilute acid only. Two types of experiments were used to show the catalytic effects of metals: The first involved a constant weight (or surface) ratio of metal to oil a t various temperatures, and the second involved various metal-oil ratios a t constant temperature. The effect of the oxygen pressure was also investigated. The properties of the oils investigated are as follows: Oil designation SP. gr. a t 15.5' C. Flash point C. Fire point. C. Savbblt universal viscosity seo.: 100' F. 138' C.) 210' F. (100' C . ) Acid No. Pour test C Color (N.' P. d.)
E
A 0.878 222 260
B 0.893 144 160
C 0.916 165 194
D 0.881 194 216
0.893 201 227
315 53 0 -15 6
76 35 0 -45 2
151 42 0.01
156 43 0.02 0 1
184 44 0.02 0 2
-10
1
F 0.887 255 290 691
71 0.01 -20 6
These oils were selected to represent the probable variations in the oxidation characteristics of oils whose rate of oxygen absorption continuously decreases as the reaction progresses.
1
4
The rate of oxygen absorption in a circulatory system is applied to the study of lubricating oils whose rate of oxygen absorption continuously decreases. This decrease is probably due to a decreasing concentration of reacting components of the oil. I t is simpler in this case to treat the oxygen absorption data by an empirical equation which indicates a retardation of the reaction by the oxidation products. The rate of reaction varies linearly with the partial pressure of the oxygen. The principal oxidation products are water, carbon dioxide, and acids; no peroxides are found and only small amounts of carbon monoxide can be de-
6
FIGURE1. TOTAL OXYQENABSORPTIONCURVES I342
NOVEMBER,1936
INDUSTRIAL AND EXGINEERING CHEiMISTRY
The essential feature of the oxidation of oil A a t various temperatures a t a constant oxygen pressure is a gradually decreasing rate of oxygen absorption. This decrease may be regarded as either a retardation due t o nonvolatile productsthe volatile oxidation products are continuously removed- or a decrease in the concentration of the reacting components of the oil. The data for run 333 a t 175' C. with a constant oxygen pressure of 76 cm. are given below where the total volume of oxygen absorbed is calculated to standard conditions (0' C. and 76 cm.) for 100 grams of oil:
1343
TABLE I. RESULTS OF ABSORPTION EXPERIMENTS Tpp.,
Run No.
0 2
k
C. 155 155 165 165 165 175 175
0.53 X 106 1.37 x 105 1.85 X 106 4.84 X 106 11.70 X 10s 23.8 X 106 16.7 X 106 48,200 log k = 29.705 4.58 T
337 348 335 347 344 333 334
Pressure, Cm. Hg 76 76 76 76 76 76 78
~
Effect of Oxygen Pressure OP
02a --
02a
Time 100 Grams Time 100 Grams Time 100 Grams Hours Cc. Hours Cc. HOUTS Cc. 1629 2.25 2191 285 1.25 0.25 2.50 2324 0.50 665 1.50 1827 1960 3.00 2574 0.75 1065 1.75 2.00 2056 1.00 1445 3.25 2681 At normal temperature and pressure.
Time 100 Grams Hours Cc. 3.50 2802 3.75 2894 4.00 2947 4.50 3006
These results may be represented by the equation V2 = kt =
total vol. of oxygen absorbed, cc. at K T. P./100
t
= =
time, hours rate constant obtained by plotting V *against t
grams of oil
This equation is satisfactory for all oxygen absorption data except in the presence of copper where a linear relation between V and t is obtained: V
k't
(2)
The experimental results of the absorption experiments with oil A are given in Table I which lists the run number, the temperature, the value of k or k', and other experimental conditions. The Arrhenius equation, log k = a
-- &/4.58T
338 339 340
175 175 175
355 354 349 345
125 135 145 155
353 349 350 351 357 379 356 358
145 145 145 145 135 135 145 145
(1)
where '5 k
Gas Pressure, Cm. H g 0 2 Nt 15.2 60.8 41.3 34.5 49.1 26.9
01Q
(3)
which best represents the temperature variation of k for the several groups of experiments is also recorded. The products of oxidation of oil A a t 175" C. and 76 cm. of oxygen are summarized in Table 11. Peroxides could not be detected a t any time during the reaction. Only small amounts of carbon monoxide, hydroxy compoundsJ and saponifiable matter were found. The unsaturation increased slowly during the oxidation of oil A. The rates of oxygen absorption of the five other commercial lubricating oils of various viscosities and types of refining
tected. The net heats of activation for oils of this type vary between 33,000 and 73,000 calories. The catalytic effects of copper, iron, tin, and lead are studied by the oxygen absorption procedure. The catalytic effect of these metals depends upon the oxygen pressure and the ratio of metal surface to oil. The rate of oxygen absorption in the presence of copper is constant, which indicates a change in the type of the oxidation reaction. The net heats of activation for the catalyzed oxidation of the oil are determined for a fixed metal-oil ratio. Only copper and lead have a marked catalytic effect on the oxidation of the oil.
3.82 x 105 6.52 x 105 16.0 X 106
Effect of Copper G. Cu/100 G. Gas Pressure, Cm. k' Oil 0 2 NZ 228 38.4 76 .. 606 35.6 76 .. 1543 36.8 76 .. 2880 39.2 78 . .. log k'
359 361 367 360
145 155 155 165
-
-
19.583
864 1543 1717 2425 260 432 1267 1233
*zi
18.8 36.8 51.1 101,o 40.6 40.2 40.1 42.4
Effect of Iron G. Fe/100 G. k Oil 3.62 X 106 7.2 15.9 X 106 7.3 11.2 x 106 7.5 50.3 X 106 6.4 48,200 log k 3 30.724 4.58 T 6.5 X 105 4.7 44.1 X 10' 19.2 57.5 X 106 41.3 4.62 X 106 6.4 10.1 x 10s 6.7 6.15 X 106 6.7
..
76 76 76 76 15.2 41.0 15.2 41.0
.. .. 60:s 15.0 60.8 15.0
76 76 76 76
......
76 76 76 15.2 15.2 35
..
..
-
363 362 364 387 386 385
155 155 155 155 165 165
.. 6018 60.8 41
Effect of Tin
403 407 402 406
145 155 165 175
405 404 408 409 410
155 155 165 165 165
390 389 391
145 155 165
393 395 394 396 397 399 398
155 155 155 165 165 165 165
G. Sn/100 G. Oil 0.49 X 105 6.8 2.64 x 105 6.9 11.8 X 106 6.9 61.8 X 105 7.1 58,500 log k 35.251 4.58 T 2.64 X 106 3.2 6.42 X 10s 13.9 2.93 X 106 6.6 5.95 x 105 7.4 6.76 x 105 7.0
-
..
76 76 76 76
..
..
..
-
Effect of Lead G . Pb/100 G. Oil 122.5 X 105 6.6 237.5 X 106 6.7 731 X 106 6.8 33,000 log k = 24.332 - 4.58 T .~ 145 X 106 3.1 272 X 10s 13.5 48 8 X 106 6.4 123 X 105 6.4 136 X 106 6.7 177 X 106 6.9 217 X 105 6.8
76 76 15.2 37.0 47.0
.
.. ..
76 76 76
..
..
7R ._
76 15.2 15.2 15.2 39.0 46 5
60:8 60.8 60.8 37.0 29.5
TABLE11. PRODUCTS OF OXIDATIONOF OIL A Time, Hr.
-Equivalent HzO
Cc.
0 2
at h-.T.P./100G . Oil-
coz
I
60:s 39.0 29.0
Acid
1344
INDUSTRIAL -4ND ENGINEERING CHEMISTRY
VOL. 28, NO. 11 cording to either of these rate laws, a three- or fourfold decrease in the value of a rate constant is also found after an absorption of 4000 cc. per 100 g r a m s of oil. The ratedetermining step may be either of the following reactions: A+O*---t 2 A +02+
Since the oxygen is in excess and a t constant pressure, it does not enter the rate law expression. The data, however, do not permit the exclusion of either of these cases. In view of this difficulty and the limited oxygen absorption in this work, it seemed more desirable to calculate the rate constants by the simple empirical rate law (Equation l). For certain purposes (dependent upon the application of the oil) it may be desirable to neglect the initial 3000 cc. of oxygen absorption and base the calculation of FIGURE2. TEMPERATURE COEFFICIENT OF REACTION RATES k on the slower reactions which follow. I n the present work all calwere studied. The values of the constants of Equations l and culations are based on the initial oxygen absorption, The 3 are as follows: absorption data for oil A a t various temperatures and an oxygen pressure of 1 atmosphere are shown in Figure 1 when k X 10-5 for Or at 76 Cm-. Oil 135' C. 145' C. 155' C. 165' C . 175' C. 185' C. a Q plotted according to Equation 1. A .. . . .. .. 1.37 4 . 8 4 18.7 .. 2 9 . 7 0 5 48,200 The temperature coefficient of the oxidation of oil A a t an B .. .. 0 ...1 4. 7. 00 .. 15 22 37 01 ..59945 1, .. 1. 1 .. .. 22.250 33,500 C 28.717 47,000 oxygen pressure of 76 cm. is given by the equation: ... . . 2 4 . 4 8 6 41,000 D 0:0066 0.0116 0 . 0 3 6 0 , 1 1 0 E .... 0 . 5 4 2 . 1 7 9 . 5 2 2 1 . 4 43,400 log k =: 29.705 - 48,200/4.58T F .. . . . . . . . . . 0 . 5 0 2 . 5 6 2i.'4 42 17 .. 34 27 22 73,000 The probable error in the net heat of activation, &, is of the Discussion of Results order of 4000 to 5000 calories. This rather large error The empirical treatment of the oxygen absorption data on probably arises in the empirical calculation of k which tends oil A requires some discussion. The experimental oxidation to exaggerate the experimental errors. The experimental data runs were carried to a total oxygen absorption of a t least for this equation are shown in Figure 2. The Arrhenius equa3000 cc. Der 100 grams of oil. However, a single value for k tion is also applicable to the results with metal catalysts. in Equadon 1 is i o t sufficient to represent the absorption curve when the total absorption is e x t e n d e d t o a b o u t 8000 cc. per 100 grams of oil. The value of k decreases twoto fourfold beyond an oxygen a b s o r p t i o n of 4000 cc. but follows the same empirical rate law (Equation 1). This behavior favors the view that the decreasing rate of the reaction is due to a decreasing concentration of the various reacting components of oil. The addition of oxidized oil to unoxidized oil A did not appreciably decrease the reaction rate so that the p r o d u c t s were not causing the retardation. This interpretation receives further support from the fact that the absorption data can also be represented by either a firstor second-order reaction rate HRS. law with respect to the oil components. When the abIN FIGURE3. EFFECT OF OXYQENPRESSWRE FIGURE4. TOTALOXYGEN ABSORPTION PRESENCE OF COPPER sorption data are plotted acON REACTION RATE 7 -
NOVEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
The values of k’ obtained with copper are plotted on a separate scale in Figure 2. The results with metals in Figure 2 are for a fixed ratio of metal surface to oil and for a constant oxygen pressure of 1 atmosphere. The effect of the oxygen pressure on the rate of the oxidation of oil A was obtained by adding nitrogen so that the total gas pressure was 76 cm. of mercury. This procedure gives the net effect of an inert gas and the effect of the oxygen partial pressure. The results of these experiments are shown in Figure 3 where the values of k are plotted against the corresponding partial pressures of the oxygen. The linear relation obtained indicates a first-order reaction with respect to oxygen. The oxidation of oil A in the presence of copper proceeds a t a constant rate as indicated by Equation 2. Some of the results are shown in Figure 4 where the total oxygen absorption is plotted as a function of time. The values of k’ increase rapidly with ratio of copper to oil. The values of the net activation heat for the oxidation catalyzed by copper is only 31,000 calories which is not directly comparable to the value for the uncatalyzed oxidation since different empirical rate laws are used in the two cases. Some of the experimental results in the presence of iron are shown in Figure 5 . The empirical rate law for the oxidation of oil A is unchanged by iron. The values of k are increased about tenfold by iron although the net heat of activation is apparently unchanged. This effect may arise from the large errors involved in the determination of the values of Q or to a change in the reaction caused by the metal. The
7 L
LO
1345
TIME (HRSJ
FIQURE
6. DISTRIBUTION OF OXYGlN
AFTER
ABSORPTION
values of k for constant temperature and oxygen pressure increase with the ratio of the iron surface to oil. This increase with iron is much less than in the case of copper. Tin has no effect on the empirical type of the oxidation reaction of the oil A. Although the values of k are increased two- or threefold by tin, the net activation heat of the reaction is increased about 10,000 calories. This anomaly probably arises from the empirical treatment of the absorption data. The more cumbersome calculation of k for first- or second-order reaction would obviate this difficulty. The values of k are only slightly increased by increasing the ratio of tin $0 oil. Lead has a marked catalytic effect on the oxidation of oil A. This catalytic activity is shown by an increase of several hundred fold in the values of k and a decrease of 15,000 calories in the net heat of activation, which is the normal effect of a catalyst. The values of k show a marked increase with the ratio of lead to oil. Water, carbon dioxide, and acids are the only products found in appreciable amounts as products of the oxidation of oil A. These three products account for 60 to 70 per cent of the total oxygen which has reacted. The water formation follows the empirical rate law (Equation 1) while the rate of formation of carbon dioxide is nearly constant. I n the presence of copper the rates of both water and carbon dioxide are constant and nearly equal. This result might be expected since the rate of oxygen absorption is also constant in this case. No peroxide can be detected a t any time in the oxidation of oil A. The unsaturation showed small increase after oxidation and only small amounts of carbon monoxide could be found.
Literature Cited (1) Dornte, R. W., IND.ENGI. CHEM.,28, 26 (1936). (2) Dornte, R. W., and Ferguson, C.V., Ibid., 28, 863 (1936). RECEIVED July 31, 1936. e
FIQURE
I
1
HRS.
3
4
5. TOTALOXYQEN ABSORPTIONIN PRESENCE OF IRON
