Effect of Pressure on Viscosity of Oils and Chlorinated
Diphenyls R. E. DOW, M. R. FENSKE,
n.E.MORGAN T h e Pennsylvania State College, AND
State College, Pa.
T
HE viscosity of several oils as a function
of pressure and temperature was recently investigated by one of the writers ( 3 ) . This DaDer reDort8 further studies on the viscosit; df lubricating oils a t high hydrostatic FIGURE 1. pressures, particularly on the effect of adding Paratone to a sample of California oil, and an investigation of the viscosity a t high prcmures of two chlorinated diohenvls which are of interest in chemical engineering. 'i'wo hf the oils used in these tests were from Pennsylvania and California crudes, and were blended to have the same viscosity of approximately 0.4 poise a t 130'F. (54.4' C.). Tire other oil was a special blend of the California oil already mentioned and 10.1 per cent by weight of Paratone, a complex hydrocarbon liquid that is often added to oils to ebanee their temnerature-viscositv coefficients or viscositv
tains about 48 per cent and 1254 about 54 per ce& by weight. Table I gives the norrrlal viscosity of the five samples a t two temperatures, as well as other physical data. For most of the liquids two independent values of the viscosity a t atmospheric preasure were obtained, one by the experimental method of this paper and the other by the capillary tube or modified Ostwald type of viscometer.
Method of Viscosity Measurement The viscosities a t any pressure were computed from data .obtained with n rolling-ball viscometer, and other conventional high-pressure apparatus. The rolling-ball viscometer was used previously (3,6)to obtain the viscosity of oils a t high pressures. Figure 1 shows the viscometer and pressure apparatus employed in this investigation. When the viscometer was tilted through a s m l l , constant angle about its horizontal axis, a 0.25-inch ball bearing rolled down an axial hole from .one contact to the other; the contacts were located a t opposite .ends of the path. The time of roll can be recorded by any of several simple methods. In these experiments s 0.1-second stop watch and a calibrated electric clock were used satisfactorily. Tho viscometer was surrounded by a constant-ternperature bath of water which was controlled by a vacuuni tube thermostat so that the recorded temperatures were sub-
jected to variations of temperature of only about *0.02" C. 1*0.04"F.). ' Since t h i viscosities computed from the roll tinies were relative, the viscometer was calibrated by observing roll times at atmospheric pressure with liquids of !mom viscosity. As developed by Ikrsey (6),this involves plotting a function ST agnirmt a function U/S. T i s the roll time in seconds, U the kinematic viscosity in Stokes, and S is a function of the densityequal to(po/p - l)"*wherep,is thedensityof thesteel ball and p the density of the oil in grams per cc. Figure 2 shows a typical calibration curve when the angle of tilt i p about 10". Writing the relation,
the equation becomes
ST
= 118
(z)
for long roll times. Solving Equation 2 for the viscosity in poises
(a)
(p = pU)
Thus the coniputation of viscosity is done in one of two ways: If the roll time is short (region where the calihration curve 1) was solved bv deoarts from beine linear). ,. U (Eauation . . referring directly to the curve; if the roll t i n e is long (region where ST varies linearly with U / S ) . Emation 3 was used to give directly. The values of p were olkained by interpolating the I)ressure-volume-temperature data of Dow (4) : since the change of volume of oils with pressure is not. B sirong function of composition, it was sufficiently accurate to use the P-V-T data of Pennsylvania oil interpolated for 130' and 210.2"F. (54.4"and 99" C.). The volume, or density, of the Aroclors as a function of pressure is not known. For the purposes of this paper the P-V-T dat.a of Bridgman (2) for cblorolOi8
SEPTEMBER, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLEI. PHYSICAL DATAAT ATMOSPHERIC PRESSURE Mineral Oils -ViscosityDensity 130° F. 210.2' F. Viscosity at looo F. Liquid (54.4" C.) (99.0"C.) Index (37.8" C.) -CentipoiseGram/cc. Pa. oil 41 (40 a 8 (9.0)a 102 0.863 Calif. oil 42 421" 16 0.912 Calif. oil $- Paratone 42 (43)a 1; 100 0.911 Arochlors (Chlorinated Diphenyls) -ViscosityDensity Viscosity at 86' F. Specific Liquid 86" F. 167' F. Index (30° C.) Heat No. (30' C.) (750 C.) Cal./g./o C. 7-CentipoisesQrom/cc. 0.95 1248 129 (133)" 8.7 (8.8;" - 624 1.445 1.49 1254 2740 (2700)O 24 (24) -1989 1.540 0 Viscosity determined by capillary instrument.
)E:]:
,
benzene was used to compute the relative densities under pressure conditions. This liquid more nearly resembles the diphenyls in chemical composition than any other pure liquid that has been tested. I n any case, the change of density is relatively small in comparison to the change of viscosity with pressure and the approximation does not involve a serious error. The densities of the five samples at atmospheric pressure were determined in a conventional way with the use of specific gravity bottles. The apparatus for generating pressure was part of the general equipment of the high-pressure laboratory of the Department of Physics. It consisted of a hand pump that could be operated to pressures of 20,000 pounds per square inch (1400 kg. pes sq. em.) and a n intensifier which permitted about a fourfold increase of pressure. The test liquids were separated from the pump liquid by means of a sylphon chamber which was contained in a separate pressure chamber. This chamber also contained a manganin coil which was used to measure the pressure to a greater degree of a c c u r a c y t h a n would have been possible otherwise. The electrical resista n c e of m a n g a n i n i n creases linearly with pressure, and it has been used for many years by investigators as a reliable gage over a wide range of pressure. The highest pressure used i n t h i s investigation w a s a b o u t 5 5 , 0 0 0 pounds per square inch (3820 kg. per sq. cm.); this pressure is the FIGURE2. CALIBRATION limiting one for most oils a t CURVEOF VISCOMETERAT t e m p e r a t u r e s of 212°F. ATMOSPHERICPRESSURE (100" C.) or lower, for beyond this pressure solidification soon starts. The recorded data for a viscosity determination were: The roll times taken for the two directions of inclination a t a certain angle of tilt and then averaged, the pressure as determined by the resistance of the manganin gage, and the temperature of the bath as read on a mercury thermometer. FPom the roll times and the corresponding densities, the absolute viscosity of the samples was computed in centipoises by the method already given.
Preparation of Samples The California type oil containing Paratone, with the same viscosity a t 130' F. (54.4' C.) as the other two oils with a viscosity index of 100, was obtained as follows: The original California t y e oil was vacuum-fractionated in an efficient column to yiefl three fractions. Fraction 1-A constituted the first 15 per cent by weight obtained, and had a
1079
viscosity of 8.97 centistokes at 130' F. (54.4' C . ) . Fraction 1 was the first 23.7 per cent by weight of the California oil and had a viscosity of 11.49 centistokes at 130' F. Fraction 2 constituted the material distilling off between 23.7 and 37.9 per cent by weight of the original oil, and had a viscosity of 29.97 centistokes at 130' F. By proper blending of these three fractions with Paratone and a quantity of the original California oil, it was possible t o obtain the desired viscosity at 130" F. as well as 100 viscosity index. This Paratone blend having a viscosity of 47.7 centistokes (43 centipoises) at 130' F. and a viscosity index of 100 was composed of the following weight percentages: 43.0 original California oil, 25.2 fraction 1-A, 12.1 fraction 1, 9.6 fraction 2, and 10.1 Paratone. The Paratone had a viscosity of 3616 centistokes at 130' F. and 626.7 centistokes a t 210' F. The original California oil was characteristic of this type, and the viscosity of 42 centipoises at 130' F. was obtained by blending two distillates, one slightly more viscous and the other slightly less viscous than this figure. In addition to the pro erties listed in Table I, it had a flash point of 395" F. (201.7' a fire point of 445" F. (229.4' C.), a color of 3.5 A. S. T. M. and a pour point of -5' F. (-20.6' C.). The Pennsylvania oil was prepared by blending to the desireo viscosity a typical Pennsylvania neutral oil of about 39 centistokes at 100 F. with a tvuical Pennsvlvania bright stock of about 510 centistokes a t loyo" F. In addition to &e propertieb shown in Table I, it had a flash point of 460" F. (237.8" C.), a fire point of 510" F. (265.6"C.), a color of 7.25 A. 5. T. M., and a pour point of +25' F. (-3.9' C.).
8.)
Table I1 is a summary of the viscosity data a t various pressures recorded a t two temperatures for the Pennsylvania, California, and California plus Paratone o h Table I11 contains similar data for the two Aroclors. TABLE11. VISCOSITY-PRESSURE DATA OF OILS Viscosity 130' F. (54.4' C.). 210.2' F. (99.0' e.) Cahf. Calif.
7
+
+
Pa. 41 51 60 64 73
42 57 68 80 99
5000 6000 7000 8000 9000
82 91 100 111 124
124 154 190 232 281
97 110 129 151 180
143 181 249 315 408
340 490 692 960 1320
524 663 830 1030 1260 1560
1830 2510 3400 4540
10 12 14 16 18
X X X X X
(351.5)
f% :3 (562.4) (632.7)
103 103 103 103 103
(0.70 X (0.84X (0.98X (1.12 X (1.26 X
108) 103) 108)
103) 103)
20 X IO3 22 X 103
(1.41 X 103) (1.54X 103)
24 x loa 26 108 28 X 10s 30 X 108
(1'68x loa) 11:s~ 103) 1 96 X 103) 2.11 X 103)
32 34 36 38 40 42 44 46 48 50 52 54
x x x x x
x x x x x x
x
103 103 103 103 103 103
(2.25x (2.39x (2.53x (2.67x (2.81x (2.95x
1133) 103) 103)
103 103 103 103 103 103
(3.09 x (3.23 x (3.37 x (3.52 x (3.66X (3.80x
103) 103) 103) 103) IO3) 103)
103) 103) 103)
1960 2460
.. .. .. .. .. .. .. .. ..
..
Calif.
Paratone Pa. Centipoise8 8 42 9 50 60 10 13 71 15 82
-PressureLb./sp. in. (kg./sq. cm.) 14.2 (0.998) 1000 (70.3 2000 (140.61 3000 (210.9) 4000 (281.2)
.. .. .. .. .. .. ..
.. .. .. ..
..
.. ..
Calif.
Paratone
9 14 17 19 21
10 11 13 15 17
17 19 20 22 24
24 25 27 30 33
19 22 24 28 31
216 303 410 540 739
26 31 36 44 52
37 45 55 70 90
34 41 49 54 61
1008 1396 1970 2769
62 73 87 103 123 145
116 153 202 260 327 408
79 92 113 144 181 230
171 202 242 287 337 393
510 655 846 1080 1400 1790
288 344 405 469 539 613
457 535 627 732 846 973
2270 2890
690 765
.. .. , . .. .,
., , . .. , , , ,
,. , ,
,
.
..
..
.. ..
..
... . .. ... . ..
Oils The change of viscosity of the Pennsylvania sample (viscosity index 102) with pressure was considerably less than for the Califprnia, sample (viscosity index 16). For example, a t 130' F. (54.4' C.) a pressure change of 20,000 pounds per square inch increased the viscosity of the Pennsylvania oil about thirteen times, as against an increase of about fortythree times for the Califoreia oil. At the higher temperature and same pressure interval, the viscosity increased by eight
and thirteen times, respectively. Assuming that the California oil is more complicated in composition from the standpoint of viscosity, probably because of the greater number of ring compounds (naphthenes and aromatic structures), it is not surprising that the effect of pressure is greater for this sample than for the Pennsylvania oil since, in general, liquids of the more paraffinic or chain-type structures have lower pressure coefficients of viscosity than those of the cyclic and benzene-ring types. TABLE111. VISCOSITY-PRESSURE DATAON AROCLORS -86O
Pressure
Lb./sq. in. ( k g . / s q . cm.) 14.2 (0,998) 500 (35.2) (70 3) 1000 1500 (105 5) 2000 (140.6)
Visoosity F. (30' C.1-167' F. (75' C.)No. 1248 No. 1254 Centipoises 129 2,740 8.7 24 3,140 ..26 i53 4020 9.3 5:670 .... itci 8,740 10 26
No. 1248 No. 1254
7
....
2500 3000 3500 4000 5000
If: n,, (281.2)
(351.5)
287 384
6000
(421 8) 492 1) 1562.4) 632.7)
553 833 1270 1950
....
(0.70X 103) (0.84 x 103 (0.98 X loa{ 1.12 x 103) 1.26 X 103)
2940
7000 8000
9000 10 X 108 12 x 1 0 8 14 X 103
22 24 26 28 30 32
x
VOL. 29, NO. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
1080
103 X 10: X 10 X 103 x 103 X 108
(175.8) 223
.. .. .. ..
..
14,300 23,600 40,500
....
I
.
.
,
.... 11 . . I .
12 13
..
..29 ..33 40
....
15 17 19 21
48 59 73 92
....
24 30 39 53 75 111
198 345 642 1290 2580
.... .... ....
....
..... ... ....
117
;:(1.82 1: (1.96 (2.11 (2.25
At high pressures the viscosity of the Paratone-blended oil is considerably below that of the original California sample a t both temperatures. Considering a pressure change of 20,000 pounds per square inch again, a t the lower temperature, the forty-three fold increase of the original California sample was reduced to twenty-four fold for the case of the Paratone blend; a t the higher temperature the same pressure interval reduced the thirteen fold increase of viscosity of the California type oil to eightfold for the Paratone blend. The composition of Paratone is unknown but it seems likely that it should be fairly rich in paraffinic or chain-type compounds. The enrichment of the California type oil then, by the addition of these compounds, is sufficient to alter its viscosity characteristics in such a way that the viscosity is not affected as much by pressure. These results are believed to be sufficiently important to warrant a more thorough examination of the viscosity characteristics a t high pressures of oils of various viscosity indices that have been mixed with Paratone, and other similar high-molecular-weight substances in various proportions. 1 ap The temperature coefficient of viscosity, ti, computed between the temperatures of 130" and 210.2'F. (54.4" and 99" C.), increased as the pressure increased in a way that is normal for most liquids. The increase, however, was smaller than for most pure liquids for the same pressure range. For the Pennsylvania type sample, the increase of the temperature coefficient taken between atmospheric pressure and 20,000 pounds per square inch was 10 per cent; for the California sample alone, and the California oil plus Paratone, the increases were 19 and 21 per cent, respectively, for the same range of pressure. It is evident here that the Pennsylvania type oil has a smaller temperature coefficient with pressure.
Aroclors The nearly fifteen fold increase of viscosity a t 86" F. of Aroclor 1254 over a pressure range of 3500 pounds per square inch is particularly interesting since this undoubtedly represents the greatest increase of viscosity that has been measured for any liquid. The greatest coefficient of viscosity of a liquid measured previously over a pressure range of a few thousand pounds per square inch a t this temperature was reported by Bridgman (1) who found a twofold increase of the viscosity of cineole over a pressure range slightly greater than 7000 pounds per square inch. It appears probable that the difference of increase of viscosity of these Aroclors cannot be due solely to their difference of chlorine content. The addition of a chlorine atom to the benzene ring may not increase the viscosity a t high pressures, for it has been shown that the viscosity of chlorobenzene is lower than that of benzene ( I ) , a t a certain elevated pressure, even though a t ordinary pressure chlorobenzene is more viscous than benzene. The kinematic viscosity indices (6) for the Aroclors (Table I) indicate that these liquids differ widely from petroleum oils in their viscosity-temperature relations. For example, a t atmospheric pressure, for a temperature fall of 8 1 ° F . (45'C.) the viscosity of Aroclor 1254 increased by over one hundred fold, and for Aroclor 1248 by about fourteen fold. At a pressure of 3000 pounds per square inch, the increase amounted to eight hundred fold for Aroclor 1254 and twenty fold for 1248. Thus the effect of temperature in changing viscosity increased decidedly with pressure. The Aroclors have been found useful as heat transfer media. If the temperatures a t which they are used are relatively high, which is probably the case, then the effect of pressure (at relatively high temperatures) on the viscosity of these fluids is not very important. Consequently the work of pumping or coefficients of heat transfer, in so far as these are dependent upon viscosity, would not be greatly affected by relatively large increases in pressure a t relatively high temperatures. There would, however, be marked changes a t the lower temperatures. The Reynolds number is an important criterion of the properties of fluids in motion. Estimating the ratio of Reynolds numbers a t high pressure for the condition of constant mass velocity, it is found that for Aroclor 1254 a t 167°F. and a pressure increase of 1000 and 10,000 pounds per square inch the ratio of Reynolds number equals 0.96 and 0.205, respectively. In other words, the Reynolds number for this material flowing a t constant mass velocity a t 167'F., and 10,000 pounds per square inch is only about 20 per cent of the value a t atmospheric pressure.
Acknowledgment Acknowledgment is due to The Pennsylvania Grade Crude Oil Association for their cooperation and financial aid; also to J. A. Pollock and C. E. Fink of the Petroleum Refining Laboratory, who prepared the Paratone blend and made absolute determinations of the viscosity of the liquids a t atmospheric pressure. The pressure measurements of viscosity were made in the High Pressure Laboratory of the Department of Physics.
Literature Cited Bridgman, P. W., Proc. Am. Acad. Arts Sci., 61,57 (1926). (2) Ibic?., 66, 185 (1931). (3) Dow,R. B., S. AppZied Physics, 8,367 (1937). (4) Dow,R.B., J . Wash. Acad. Sci., 24,516 (1934). (5) Hersey, M. D.,and Shore, H., Mech. Eno. 50,221 (1928). (61 Hersh. R. E.,Fisher, E. K., and Fenske, M. R., IND. ENG. CI~EM., 27, 1441 (1935). (1)
RECEIVED May 6, 1937.
