ZERO COLD TEST HAROLD
J. TORMEY AND JOSEPH B. ENNIs, JE.,ST.BONAVENTURE COLLEOE, ST.BONA-
VENTURE, NEWYORK The increasing use of the automobile in winter months, especially in regions where winter temperatures are very low, has resulted in a demand for lubricating oils possessing special properties adapting them to use in such regions. The petroleum refiners, in increasing numbers and with a large measure of success, have met this demand by putting on the market lubricating oils whose characteristics are adapted to winter driving. The following may be considered to be the most important desirable characteristics of lubricating oils. The first four properties listed are of particular importance with respect to oils intended for winter use. Desirable Characteristics of Lubricating Oils
1. 2. 3. 4.
5.
6. 7. 8. 9. 10. 11. 12. 13.
Low cold test. Should not solidify under conditions of use. Should not greatly change viscosity under conditions of use. Should have good viscosity at all engine temperatures. a. Should remain fluid in coldest weather. b. Should have some degree of viscosity up to 250°F. High flash and tire tests. a. Oil is thus indicated to be unlikely to take fire and also indicated not to be a blend of light with heavy oils to regulate viscosity. Low carbon residue. Low acid number. Low sulfur content. Low Maumeu* number. No corrosive action on metal surfaces. Minimum amount of gum formation. Slow rate of evaporation. High mechanical efficiency.
It will perhaps be of interest to the motor-car driver, who must select the lubricating oil he is to use from the large number of brands whose merits are presented to him in such technical terms as "cold test," "viscosity," etc., to know the exact meaning of these terms, their significance, and something about the methods used to produce lubricating oils having those desirable characteristics for winter driving which were previously listed. Cold Test The cold test of a lubricating oil is that temperature at which the oil will no longer flow. This temperature is determined in the following way. 2202
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A sample of the oil to be tested is placed in the test cylinder which is equipped with a cork &d a special thermometer. The cylinder is then immersed in a cooling medium and the temperature of the oil sample thus gradually reduced. At each cold test thermometer reading which is a multiple of 5"F., the cold test jar shall be removed from the jacket carefully and shall be tilted just su3iciently to ascertain whether the oil around the thermometer remains liquid. As long as the oil around the thermometer flows when the jar is tilted slightly, the cold test jar shall be replaced in the jacket. The complete operation of removal and replacement shall require not more than three seconds. As soon as the oil around the thermometer does not flow when the jar is tilted slightly, the cold test jar shall be held in a horizontal position for exactly five seconds, and observed carefully. If the oil around the thermometer shows any movement under these conditions, the cold test jar shall be immediately replaced in the jacket and the same procedure shall be repeated at the next temperature reading 5'F. lower. As soon as a temperature is reached a t which the oil around the thermometer shows no movement when the cold test jaris held in a horizontal position for exactly five seconds, the test shall be stopped and this temperature reported as the cold test. Viscosity The resistance experienced by one portion of a liquid in moving over another portion is called viscosity. The term is also defined as "resistance to flow" and internal friction. The extent to which an oil substitutes its own smaller internal friction for the friction of metal surfaces is largely dependent upon its ability to maintain an easily deformable film of oil on these metal surfaces. In general, the power to adhere to metals increases with the viscosity of the oil. The viscosity that an oil should have is dependent on the intended use of the oil. Low pressure and high speeds require the use of a very mobile oil. The use of a highly viscous oil in such a case would mean a large power loss. High pressures and lower velocities require the use of a more viscous oil. The use of a mobile oil in such cases would mean a waste of lubricant. Since the viscosity decreases with increasing temperatures it is common practice to determine viscosities at several temperatures. The viscosity is reported as the number of seconds time required for a standard sample to flow from a small orifice at a definite temperature. It is to be noted that an oil should not be selected merely on the basis of a low cold test at the expense of desirable viscosity characteristics. Poorly refined oils may have low cold tests but will show an excessive drop in viscosity with increasing motor temperatures, which means poor lubrication at higher motor temperatures.
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Other Properties The flash point of an oil is the lowest temperature of the heated oil a t which the vapors arising from it will ignite with a flash of short duration when a flame is brought near the surface of the heated sample. The burning point of an oil is the lowest temperature of the heated oil at which the vapor arising from it will ignite and continue to burn with a steady flame when a flame is brought near the surface of the heated sample. The carbon residue is the percentage of carbon remaining as residue when a 10-gram sample of the oil is burned as described in the Conradson Carbon Residue Test. The acid number of a lubricating oil is the number of milligrams of potassium hydroxide necessary to neutralize one gram of the oil. In many cases high acidity of a lubricating oil wiU cause corrosion of journals and bearings; consequently, first-class lubricants should be relatively free of acid. The sulfur content of an oil is the percentage of total sulfur present. Since a considerable proportion of this sulfur may be present in the form of corrosive acids, or substances easily converted to corrosive acids, a good lubricant should have a low sulfur content. The Maumend number is the number of degrees Centigrade rise in temperature occurring when 10 cc. of concentrated sulfuric acid is added to 50 grams of oil. The significance of this number lies in the fact that the Maumend number of an oil compounded with animal or vegetable oil will be higher than for the pure mineral oil. The extent to which an oil substitutes its own smaller internal friction for the friction between the surfaces to be lubricated is a measure of its mechanical efficiency. The mechanical efficiency depends largely on viscosity and, to a lesser extent, on various other properties and is consequently determined largely by these properties. De-waxing The problem of producing lubricating oils having desirable characteristics for winter driving has been found to be largely one of removing from the oil in the refining process those substances which form solids or semi-solids a t low temperatures, notably the paraffin wax which remains in solutiou in the oil a t moderate temperatures, but which crystallizes out a t lower temperatures. The removal of a certain amount of the paraffin wax content of lubricating oils has for some time been a common refinery practice. A more complete removal of the wax has become necessary to produce oils suited to winter driving and this has been accomplished by improvements in the process, notably by the introduction of the centrifuge and operation of the process at lower temperatures.
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A description of a de-waxing process capable of producing lubricating oils suited to winter driving in the coldest regions follows. The process employs Sharples centrifuges and operates at temperatures lower than those generally used until recently. The filtered lubricating oil stock and the naphtha with which it is to be
mixed are pumped from their respective storage tanks to a blending tank where they are mixed in the ratio of 60% of naphtha to 40% of stock. If the oil were not diluted with naphtha it would become so viscous on cooling that it could not be readily pumped. This mixture is heated to 100°F.,and then is cooled in the chilling tanks to temperatures ranging
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from minus 10°F. to minus lS°F., during a period of 48 hours. These tanks are provided with mechanical stirrers by means of which the contents are agitated from time to time to insure complete solution of the oil in the naphtha. The chilling of the oil in these tanks is accomplished by circulation of brine.
Better operation on the refrigerating machine and better control on the individual chilling tanks can be obtained by the use of two brine circuits. The first circulates from the brine tank to the brine cooler and then at about minus 20°F. to each of the tank circuits. Here cold brine is taken into each tank circuit as required and the balance returns to the brine tank.
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The second circuit operates in parallel, brine being circulated through the coils of each chillmg tank independently. The temperature control is obtained by admission of cold brine from the main circuit as required. This admission displaces an equal amount of brine in the secondary circuit which overflows into the brine tank and hence into the main brine circuit. After being chilled to temperatures ranging from minus 10°F. to minus lS°F. in the brine tanks, the solution of the oil in naphtha is pumped through a series of chillers maintained a t increasingly lower temperatures by means of direct ammonia expansion. The temperature of the solution as it leaves this series of chiiers ranges from minus 3S°F. to as low as minus 50°F. Chillmg of the solution to this temperature range requires chilling medium temperatures of from minus 50 to minus 60°F. and these chilling medium temperatures are best obtained by ammonia expansion. The solution now is pumped to a constant level lank where, by means of float control, a constant head is maintained on the centrifuges to which the stock flows from the constant level tank. The Sharples centrifuges used to remove the wax operate continuously, the chilled solution being fed in a t the bottom of the centrifuge and the wax and oil from which it has been separated discharging separately. The actual separation of wax from oil is accomplished by means oi centrifugal force. As the solution is fed into the rapidly revolving bowl of the machine (17,000 r. p. m.) the amorphous wax, being heavier than the oil solution, is thrown to the outside where it forms a layer. Water a t a temperature of 145'F. is jetted into the top of the bowl at the point of wax discharge and is camed in part into the bowl forming a thii layer of liquid next to the bowl shell. This water keeps the wax from becoming solid and makes possible the continuous upward flow and subsequent discharge of this mixture at the top of the bowl. Next to the wax layer but nearer the center of the bowl is formed a layer of bright wax-free oil. Separation of the two layers at the top of the bowl is accomplished by a ~ u l adischarge r rings which can be adjusted to obtain desired thickness of layers of wax, oil, and water. The mixing of warm water (145'F.) with the discharging wax at the top of the bowl keeps the discharging wax in a free-flowing condition. From the centrifuge this mixture of wax and water flows to a tank where the wax and water are separated by gravity, the wax flowing to one compartment from which it is pumped to storage and the water to another compartment where it is kept warm and re-used in the centrifuge. The Sharples centrifuges will de-wax from 55 to 65 barrels of the oilnaphtha solution in a 24-hour period, producing from 900 to 1150 gallons of de-waxed oil, depending on the desired specifications. Power required is 1kw.hour per unit and refrigeration 6.5 tons per 24 hours per unit.
