INDUSTRIAL A N D ENGINEERING CHEMISTRY
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gradually tapered, experiments show that m does not exceed 0.5. However, since it does vary, the safest procedure is so to desi the viscometers that the term B/t is negligible compared to term Ct and the above equation then becomes
tE
KV = Ct If fluids of widely different viscosities leave different quantities of liquid on the walls of the efflux bulb for routine viscometers (3)
and the master viscometer shown as Figure 1, then C will not be a constant, since this drainage will change V. The master viscometer shown aa Figure 2 is free of drainage error, since the liquid is entering clean dry bulbs. On all oils investigated in the two master-type instruments it was found that C is a true constant. This means that the rate of drainage is inversely proportional to the viscosity. Thus, a 3Wcentistoke oil will drain a t only one third the rate of a 100-centistoke oil but the total drainage time will be three times as great for the 3Wcentistoke oil and so in each case the degree of drainage is the same.
The equations and methods for calculating the various corrections in basic calibration work have been presented in detail (3), and are not repeated here. The magnitude of the corrections for the master viscometer shown as Figure 1 is: (a) kinetic energy correction 0.04% for water a t 20" C., less for more viscous fluids; ( b ) surface tension correction when using water and hydrocarbons in the same instrument, 0.09%; (c) change of viscometer con-
Vol. 16, No. 11
stant with temperature, 0.03% per 30" e. change in temperature. For the master viscometer shown aa Figure 2 the same figures apply, except (c) is 0.1% per 30' C:since the charge is greater. Results are reproducible in these instruments to within 0.1% and consequently relative viscosities can be measured to that degree of precision. Absolute viscosities depend upon the error inherent in the value of 1.007 centistokes taken for water a t 20" C. This is probably in the order of *0.5%. These master viscometers are an improvement over those described earlier (6,6). The capillary diameter required for a given constant C can be calculated from the equations above. LITERATURE CITED (1) Am. SOC. Testing Materials,
"Standards on Petroleum Produotts and Lubricants". Designation D445-39T. (2) Bingham, E. C., and Geddes, J. A., Physics, 4, 203 (1933). (3) Cannon, M. R., and Fenske, M. R., IND. ENO. CHEM.,ANAL. ED.,10, 297 (1938). (4) Ibid., 13, 299 (1941). (5) Cannon, M. R., and Fenske, M. R., Oil and Bas J.,3 3 , 5 2 (1935). (6) Ibid., 34, 45 (1936). (7) Spooner, L. W., and Serex, P., Physics, 6, 162 (1935). (8) Ubbelohde, L., IND.ENG.CHEM.,ANAL.ED.,9, 85 (1937). (9) Zeitfuchs, E. H., Nail. Petroleum News, 29, 68 (1937).
Gum Content of Distillate Diesel Fuels L. W. DICKEY AND R O Y HENRY Standard Oil Company of California, Richmond, Calif. gum content of distillate Diesel fuels is of less significance in their evaluation than is this property of gasolines; there are, however, occasions when it is necessary to compare two Diesel fuels with respect to their tendency to form gum. Furthermore, the advent of increasing quantities of cracked Diesel fuels, complicated by the catalytic action of various metals with which such fuels may come in contact, makes it pertinent to establish a method by which the gum content, preformed and potential, may be determined. The following conditions for the evaporation of the fuel were established as satisfactory, after testing out numerous variations.
the end phase of the evaporation heat the h s k for approximately 15 minutes. Disconnect the flask as previously described, clean the outside thoroughly, and weigh.
APPARATUS
The procedure described constitutes a purely empirical test, as are all similar methods for measuring gum in petroleum products. It is therefore necessary that all details of the test be standardized and followed if results of useful precision are to be obtained. The following variables were investigated in arriving at the selected test conditions. DEGREEOF VACUUM.A vacuum of 50 to 55 cm. (20 to 22 inches) of mercury waa found adequate to volatilize 25 ml. of fuel within an hour, varying the temperature to suit the stock and keeping this to a reasonable maximum. With this degree of vacuum there is little danger of collapsing the flask, and it is easier to avoid trouble from leaks. EVAPOUTION TIME. Polymerization under the conditions existing in the test is a function of both time and temperature. The total period during which the sample is heated is set at about one hour. This period was arbitrarily selected, and further experience with the test, using a wider range of fuels, may show that the heating period can be reduced or that a longer period would be more satisfactory. AMOUNT OF SAMPLE. The amount of sample selected, 25 ml., is sufficient to give a weighable residue with the Diesel fuels available to the authors, and it behaved satisfactorily in the apparatus selected. The residue from smaller samples must be multiplied by a larger factor, and small errors, obviously, would be proportionally magnified.
THE
Erlenmeyer flask, capacity 50 ml. Condenser, Liebig type, water-cooled, with ada ter, preferably sealed on. Filtering flask, capacity 1 liter. 8il bath. Source of inert gas (natural, artificial, nitrogen, carbon dioxide). Vacuum pump. 9
PROCEDURE
Transfer 25 ml. of the Diesel fuel to a tared 50-ml. Erlenmeyer flask. Connect the flask to the condenser and to the source of inert gas by means of a cork stopper fitted with two glass tubes, the ends of which are flush with the bottom of the cork. Attach the filtering h s k to the condenser adapter by means of a rubber stopper, and connect the side arm of the flask to a source of vacuum. Apply a vacuum of 50 to 55 cm. (20 to 22 inches) of mercury to the assembly, and pass gas into the system a t a rate of approximately 250 ml. per minute. Immerse the flask in an oil bath heated to that temperature a t which the sample will be evaporated practically to dryness in 45 * 5 minutes. (This temperature will generally be approximately 150" F. below the 90% point of, the, A.S.T.M. D158 distillation.) At the end of the specified time increase the bath temperature 50" F., and increase the rate of gas flow to approximately 500 ml. per minute. Maintain these conditions for 10 to 15 minutes; then remove the oil bath, shut off the vacuum, and increase the flow of gas until the pressure in the system is approximately atmospheric. Disconnect the tared Erlenmeyer flask, add 25 ml. of a mixture of equal parts of carbon tetrachloride and acetone to the flask, and evaporate to dryness on the steam bath. Connect the flask to the condenser as before, again apply the vacuum, and with the gas flow and temperature as previously established for
Mg of gum per 100 ml. = mg. gain in weight
x
4
Following the procedure outlined samples of Diesel fuels and various distillate fractions of similar boiling range were tested, primarily to establish the repeatability of the method over as wide a range as waa possible with the stocks available. The data are presented in Table I. VARIABLES INVESTIGATED
November, 1944
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Solution of the residue in acetone-carbon tetrachloride, and re-evaporation to dryComness, eliminated this source of error and mercia1 yielded repeatable results. On the oils DieCracked Naphtha used a single such treatment was suffisel Rerun Still Base. Gsa oil Commercial Diesel, cient; however, it may be desirable to Bottoms -Gas OilunTreated Gas Oil Untreated Treated A B C treated D E A B C (Cracked) repeat this, giving a check on the apGravity, OA.P.1. 29.2 28.9 3 3 . 1 3 1 . 4 3 1 . 8 3 8 . 3 3 4 . 3 2 9 . 1 3 8 . 8 3 9 . 1 41.2 28.4 proach to “constant” weight. Distillation, NONVOLATILE RESIDUE OTHER THAN A.S.T.M. D158 GUM. Diesel fuels may contain small 398 390 456 614 390 378 483 436 560 430 442 442 438 650 432 427 503 538 436 522 530 480 596 480 amounts of lubricating oil or other non510 660 504 492 611 579 648 510 653 640 573 573 volatile oil-soluble substances, either 560 565 548 654 616 692 719 564 715 617 693 End point 617 Gum. mg. added intentionally or present as the re12 8 8 52 8 18 40 60 30 70 82 27 per 100 9 9 48 41 14 sult of contamination. The residue ob6 22 70 58 26 29 86 ml. Evaporation tained by the procedure described will temperature, ’ F. obviously include such material; hence, 500 350 350 450 450 400 350 450 500 450 400 400 Start the entire l‘gum” content may not be 550 400 400 400 500 450 500 400 550 500 450 450 Final deleterious. No satisfactory method of distinguishing between gum-i.e., the Table II. Results of Tests product of the oxidation and/or polymerization of hydrocarbons Residue Sample Mg./100 ml. normally present in the Diesel fuel boiling range-and other Fuel Sample nonvolatile material has been found. 8 Without oxidation Table
I. Gum in Petroleum Products
$!
5 hours in bomb, no catalyst 5 hours in bomb, iron and brass catalyst.
Fuel Sample 2 Without oxidation 5 hours in bomb, no catalyst 5 hours in bomb, iron and brasa catalyst
12 8 16 16 20 88 92 90
124 128 124 212 216 216 1464 1452 1460
Av. 9
Av. 19 Av. 90 Av. 125 Av. 215 Av. 1458
RATEOF GAS FLOW.I n order to keep oxidation and polymerization at a minimum the oil vapors should be removed speedily from the flask. A current of inert gas does this satisfactorily. Natural gas was used in this work because it was available and cheap, but nitrogen or carbon dioxide would be equally satisfactory. Artificial gas varies considerably in composition and purity; however, there seems no reason why it would not serve the purpose as well as natural gas. The rate of gas flow does not appear to be critical. At 250 ml. per minute the velocity is just sufficient to sweep out the oil vapors. A higher rate of gas flow accelerates the evaporation, but oil was carried over mechanically and there was some cooling of the flask, both factors causing erratic results. However, it was found that the gas flow could be increased to advantage when the evaporation was practically complete. TEMPERATURE OF EVAPORATION. It is obvious from the distillation range of oils covered by the general term “distillate Diesel fuels” that it is not possible or a t least not desirable to specify a single bath temperature for the test, if a limiting evaporation period is also specified. From the standpoint of polymerization a uniform temperature for all fuels is desirable; however, this would force completion of the evaporation of certain samples at an undesirably rapid rate, whereas the evaporation of other fuels would be unduly prolonged. The authors’ work indicated that the temperature of the 90% point of the A.S.T.M. D158 distillation minus 150” F. correlated fairly well with the desired evaporation period, at least sufficiently to give a good lead to the operator testing an unknown stock. RE-EVAPORATIONOF RESIDUE. The residue obtained by simply evaporating to an apparently oil-free condition contained varying amounts of oil held by a surface layer of nonvolatile gum.
ACCELERATED TEST
Having established a reasonably repeatable method for determining the “gum” in a sample by evaporation, various means of establishing an accelerated test, by which the rate of gum formation in stored Diesel fuel might be predicted, became available. The dominant factors operating upon an unstable fuel are heat, pressure, oxygen, and contact with catalytically active metals, all of which speed up the reactions which form gum. The possibilities in apparatus and range of conditions are endless, and the method which correlates most closely with service and storage conditions cannot be established until considerably more is known on the subject. For the authors’ purpose it seemed suitable to subject the fuel to the same conditions as are set up by the Government for aviation gasoline-Le., a bomb under 45 kg. (100 pounds) oxygen pressure, a t 212’ F., using a catalyst. Iron and brass were the catalysts selected, and in the tests reported below approximately 100 sq. cm. (15 square inches) surface, of both metals in each test bottle, were used; a 5-hour test was judged to be suitable; and a 200-ml. sample in a 235-m1. (&ounce) sample bottle was taken. Obviously, in a standardized test the composition of the metal rods, as well as their surface area, should be specified. Table I1 shows the results of tests made under these conditions, employing the previously developed evaporation method for measuring the gum formed. Continuous pressure charts were made during the heating of these samples. Sample 1 showed practically no pressure drop; sample 2 without catalyst showed an early pressure drop, but the curve did not continue downward appreciably; the same sample, with catalyst, showed a similar early drop, which continued throughout the 5 hours. The indications of the pressure-drop curves are in line with the gum tests of the samples. SUMMARY
The evaporation procedure selected is sharply repeatable even with such a high “gum” residue as 1400 mg. per 100 m1.-Le., using successive portions of the same oxidized sample-and the authors believe that it gives a useful measure of nonvolatile gummy material dissolved in the oil when tested. They have no data as to the amount which can be tolerated in actual service. The Army induction bomb and test with iron and brass catalyst may be used to obtain an accelerated test. The authors have no data on the correlation of this accelerated test with service.
