Viscosity Characteristics of
Lubricating Oils Saturated with JUDSON S. SWE4RINGEN AND EDWIN D. REDDING The University of Texas, Austin, Texas
A study of the effects of blending to improve the characteristics of lubricating oils for natural gas compressors a t pressures up to 3500 pounds per square inch is presented. A description of the high-pressure capillary flow viscometer used in this study is given.
Battery of Compressors
NDER the sponsorship of the American Petroleum Institute, Sage, Lacey, Mendenhall, and Sherborne (5-9) have made comprehensive studies of the effect of several hydrocarbon gases a t high pressures on the viscosity of some light crystal oils. Their work, which included a study of the composition of the liquid phase under pressure, was directed toward an understanding of oil flow in natural reservoirs and its bearing on production problems and also toward a study of absorber design. The results of their work showed a great decrease in the viscosity of oils saturated with natural gases a t high pressures. Hocott and Buckley (4) reported similar results on East Texas underground samples. Recent developments in high-pressure compression of natural gas in the gas cycling process for the production of condensate, in numerous process applications, and in highpressure gas pipe lines justifies further study of this effect. In these cases of natural gas compression the high-pressure gas is in contact with the lubricant applied to the compression piston, and accordingly the result is a solution of gas in the injected lubricant. I n view of the increasing operations involving natural gases a t high pressures, a study of the effect of gases a t high pressures on the viscosity,of lubricating oils should provide information useful in selecting the most
suitable lubricant and in improving the characteristics of oils for such compressor use.
U
Viscometer Deaign I n studying viscosities under purely mechanical pressure, Thomas (11) devised an absolute viscometer consisting of two concentric cylinders immersed in the liquid. The motor-driven outer cylinder caused an angular displacement of the inner cylinder against a torsion spring. Sage (6) employed a falling ball type of viscometer in his studies of oils saturated with natural gases at high pressures. Variations of the falling ball type of apparatus were used also by Exline and EnDean ( 3 ) and by Hocott and Buckley (4). It was decided to construct a capillary tube type of apparatus mounted inside a bomb because of its adaptability to contacting the oil and gas. As the capillary flow method of determining viscosity requires a knowledge of the density of the liquid to eonvert viscosity to absolute units, a means for determining the density of the liquid under pressure was necessary. The apparatus of Sage (6) was equipped with B solenoid-operated density balance. A pressure pycnometer was used by Hocott and Buckley (4). In this work a spring-
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Natural Gases at High Pressures I
actuated tensiometer type balance located inside the bomb was used. Figure 1 is a sectional view of the apparatus. Its essential parts are the capillary assembly, A , the windows, B, and the density balance, C. I n operation the apparatus was partly filled with the oil in question and the bomb charged with gas at a pressure just above that required. The bomb was then rotated in a thermostat about supports E to saturate the oil with the gas and the final pressure was then measured and recorded. I n the determination of the viscosity of the resulting solution, the viscometer capillary was purged and filled by the following means. On rotation of the bomb, accumulator F fills with the liquid, whence viscometer reservoir G is filled. The excess oil drains from F through a small hole in its bottom. The stopcock at the bottom of the capillary has a large bore in comparison with the capillary but sufficiently small that surface tension prevents gas bubbles from entering a t the bottom. I n operation the viscometer was thoroughly purged and filled. The time required after the stopcock was opened for the oil to reach a mark observed through windows B was recorded as a measure of the viscosity. The relation of this time of efflux to viscosity is presented under "Discussion of Procedure". Density balance C is merely a submerged aluminum float mounted on an arm which extends from a tortion wire. The position of the float is determined by the electrical contact in the bottom of the bomb. The tortion necessary to be applied to the wire to open the electrical contact varies linearly with the density of the liquid. The temperature was thermostatically controlled to within 0.2"F. as indicated by a calibrated thermometer.
FIGURE1. PRESSURE VISCOMETER
Discussion of Procedure I n the viscosity determinations the times of efflux varied from 120 to 5000 seconds. At this highest rate the velocity component accounts for about 0.3 per cent of the head. Considering the buoyancy effect of the dense gas phase and including a term for the pressure drop consumed by the velocity head in turbulence losses as being proportional to density/(time of efflux)2, the viscosity relation to time of efflux reduces to the form: u = kl at - ka/t where IC,, k, = constants density of gas phase a = l density of liquid phase These two constants were evaluated from data on a standard oil and on water. The Reynolds number for each and the dimensions of the viscometer were such that the velocity distribution across the capillary diameter at its outlet end approached a steady state (IO). The density of the gas phase was calculated by the method of Brown and Holcomb (1). The following oil samples were tested: 1. A nationally advertised high-grade automotive lubricant rated as S. A. E. 30. Vacuum distillation showed this oil to contain light material. The boiling range of the first 13 per cent of
this oil was 780" to 870" F. The boiling point of the remaining 87 per cent rose steadily to about 950" F. 2. A blend of the following two oils t o essentially meet S. A. E. 30 specifications: (a) 165-second bright stock (at 210' F.) prepared from Ranger crude by vacuum distillation and acid treatment; the vacuum distillation curve for this stock was almost straight. ( b ) ZOO-second neutral oil (at 100' E".) prepared from Ranger crude by vacuum distillation and acid treatment. 3. A blend of the bright stock described under sample 2 with 18 per cent of well-refined No. 1 kerosene to meet S. A. E. 30 specifications approximately. 4. A sample of the pure bright stock used in the preparation of samples 2 and 3. 5. Refined castor oil. Other specificationsof these samples are given in Table I. The composition of the natural gas used in the measurements was as follows, in mole per cent: Methane Ethane Propane Isobutane n-Butane Total
90.66 5.84 2.57
0.53 0.40
i00.00 Comparison of Results The results are presented in graphical form in Figure-2. The properties of the castor oil were uniquely different from those of the oils of petroleum origin. It became visibly 1497
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
TABLE I.
4. 5.
Oil Sample Commercial S. A. E. 30 Bright stock and 200-sec. neutral Bright stock and kerosene Brieht stock Recned castor oil
Density a t 60’ F. 0,881 0,890 OI900
0.912 0.963
Vol. 34, No. 12
SPECIFICATIONS O F O I L S A M P L E S
Ssybolt Universal See. 100‘ F.
519 440 507 2975 1370
0
210” F. 67 57 67 165 105
’i’iscosity Index 104 76 108 94 94
Ratio, Abs. Viscosity a t Atm. Pressure t o T h a t a t 3000 Lb. Gage c
85’ F.
11.6 10.3 8.7 .
I
120’ F. 5.1 5.3
154’ F.
186’ F.
8.7
3.5 4.7 2.8 4.1
2.4
2.0
2.4 3.9
4.1
2.8
...
1.8
opaque a t pressures above the break in the curves, the phenomenon evidently being due to the form a t i o n of a n additional phase. Its quantity could be roughly estimated from Einstein’s relation (9) of viscosity of a dispersion to the viscosity of the disperse phase and the volume of the dispersed particles if suitable assumptions as t o the viscosity of the continuous phase were made. Table I compares these oils by the ratios of the viscosities when saturated with the natural gas at atmospheric pressure and at 3000 p o u n d s per s q u a r e i n c h gage. These ratios indicate the fall in the viscosity of the lubricant with subjection to working conditions; hence in 1000 zoo0 3000 , general, the smaller PRESSURE LBS. PER sa IN GAGE the ratio, the more suitable the lubricant. The data on samples 1, 2, and 3 consistently indicate that the addition of a light oil t o a heavy oil to produce one of intermediate viscosity substantially reduces the drop in viscosity with subjection to natural gas under pressure. Also, it is noted that this desirable reduction in viscosity drop varies with the light oil component, being greater with the lighter component.
Literature Cited (1) Brown and Holcomb, Petroleum Engr., 11, No. 5 (1940).
FIGURE2. VISCOSITYCHARACTERISTICS OF BLENDS
(2) Einstein, Ann. Physilc, 19, 289 (1906); 34, 591 (1911). (3) Exline and EnDean, “Drilling and Production Practice”, p. 659, Am. Petroleum Inst., 1939. (4) Hocott and Buckley, Petroleum Tech., 3, No. 3 (1940). CHEM.,ANAL.ED.,5 , 261 (1933). (5) Sage, IND.ENQ. (6) Sage and Lacey, IND. EEG.CHEM.,32, 587 (1940). (7) Sage and Lacey, Oil W e e k l y , 77, No. 10, 29 (1936). (8) Sage, Mendenhall, and Lacey, Ibid., 80, No. 13, 30 (1936 (9) Sage, Sherborne, and Lacey, Ibid., 80,No. 12, 36 (1933). (10) Schiller, 8. angew. Math. Mech., 2, 96 (1922). (11) Thomas, IND. EXQ.CHEM.,31, 1267 (1939).
