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THE DISPOSAL OF PETROLEUM FOOTS OIL
By L. G. Marsh1 and L. D. Wyant2
The manufacture of lubricating oils from wax-bearing crude petroleums involves the removal and recovery of the paraffin wax contained in the lubricating distillates. During this process several intermediate products are formed, among them the mixture of waxes and oils known to the refiners as "foots oil." Because of its complex character, foots oil is little understood, and therefore methods of handling it in refineries are often ineffi
The Bureau of Mines is studying the manufacture of lubricating oils, and during the work has investigated the redistillation of foots oil. This report gives in brief the results of the investigation, and recommendations based upon it.
no. 2734 withakewn
Manufacture of foots oil.
The processes involved in the manufacture of foots oil are complex but may be described briefly as follows3:
Paraffin wax is removed from lubricating distillate by chilling and cold pressing, the products of this operation being "pressed distillate" and "slack wax. 11 Pressed distillate (sometimes referred to as "wax-free oil") is used as a base material for the manufacture of lubricating oils. Slack wax is a mixture of waxes and oils, containing usually 40 to 60 per cent of commercial paraffin wax.
The process used in refining slack wax is called "sweating" and involves the following steps: The slack wax is melted and while liquid is run into the pans of the "sweating oven." Each pan contains a substantial screen raised slightly above the bottom and is filled with water deep enough to cover the screen. The melted slack wax thus floats above the screen until it
1 - Associate chemical engineer, Bureau of Mines, Department of Commerce.
Chemical engineer, Bureau of Mines.
3 Much information on these processes is given by Smith, A. D., Handbook of the Petroleum. Industry, chapter by David T. Day and others, Vol. II, pp. 372-5.
has cooled to a solid cake. The water is then withdrawn from the pan and the cake rests on the screen itself. The wax cake is then very slowly heated, and as the temperature rises the oil gradually drains away. The first fractions draining from the cake are oil, containing only a small percentage of commercial paraffin wax. As the temperature of the wax cake is increased, however, more of the commercial wax is melted out, so that the percentage of wax in the oil increases. In commercial practice it is customary to divide the material sweated from the wax cakes into two fractions, "foots oil" and "intermediate wax.!! The foots oil fraction is usually about half the volume of the slack wax charge. It usually contains 55 to 85 per cent of oil, the rest being wax.
Typical wax sweating products.
Finished wax Finished wax Finished 785 92.64 7 (1) Melting points in all tables were determined by the A.S.T.M. Petrolatum Melting Point Method. American Society for Testing Materials, Tentative method of test for melting point of petrolatum: Proceedings, Vol. 22, pt. 2, pp. 766-767.
(1)Melting Point of Wax Content 115.6
The second cut separated by sweating is called "intermediate wax," and is very similar in composition to the original slack wax charged to the sweating pans. It contains all the material drained from the sweating oven from the time the foots oil cut is finished until the wax remaining in the pans is sufficiently free of oil to be considered as "finished" paraffin wax.
Table 1 shows the relative characteristics of several samples of foots oil, intermediate wax, and finished wax.
In refinery practice intermediate wax is usually put back with the new slack wax coming into the sweating plant, and thus recycled through the sweating operations. In refineries where the volume of intermediate wax is large it is often processed through the sweating operations separately, as it yields a special wax with a melting point lower than that of the regular or first-run paraffin. It is very easy, therefore, to dispose of intermediate wax as it works in well with the regular routine of plant operations.
Disposal of foots oil
Methods of recycling foots oil, however, vary widely in different refineries.. Until recently refiners usually mixed the foots oil with part of the wax distillate from the crude and then distillod the mixture without using steam.
Dry or cracking distillation
This dry distillation is referred to in refinery parlance as a "cracking" distillation, but this use of the term should not be confused with its use as applied to pressure distillation for producing gasoline from gas oil or heavier oils. The temperature is not so high and the distillate is very much like the material charged to the still, apparently being only slightly "cracked." This "cracking" of the less volatile part of the lubricating distillate seems to be essential in refinery practice today, although application of improved methods .of distillation may make it unnecessary in the future. For some reason, not understood at present, a large part of the lubricating distillate made by steam distillation directly from wax-bearing crude oil is not in condition for chilling and pressing. "Cracking" changes the character of the unpressable distillate so that the wax can be removed by the usual methods.
Experimental study of foots oil
However, recycling foots oil through the stills not only is unnecessary, but it lowers the wax content of the stock. This is shown by the following series of tests.
Five typical samples of foots oil were obtained from five representative refineries. The wax and oil contents4, the melting point of the wax, and the viscosity (in Saybolt Seconds at 100°F.) of the wax-free oil were determined on each sample as shown in Table 2.
Analyses. of foots oil.
A sample of each foots oil was then vacuum distilled (40 mm. pressure) in the U. S. Bureau of Mines standard Hempel equipment5. The distillation range was noted and cloud points determined on each 10 per cent cut. (See Table 3.) A second sample was distilled dry at atmospheric pressure in a 1000 cc. copper distillation flask equipped with a one-inch, chain-filled Hempel column, 13 inches high. 13 inches high. A third sample was distilled dry in the
Wyant, L. D., and Marsh, L. G., Paraffin wax testing wax, and of analyzing oil-wax mixtures, Bureau of Mines.
Dean, E. W., Hill, H. H., Smith, N. A. C., and Jacobs, W. A., The ánalytical distillation of petroleum and its products, Bull. 207, Bureau of Mines.
properties, methods of
same copper flask. This distillation was run very slowly at atmospheric pressure so that as much cracking of the foots oil as possible would be obtained The data collected from these runs are shown in Table 3.
It will be noted that the recovery is less in the two dry distillations than in the vacuum distillation. This is probably due to the formation of fixed gases which were not condensed. The cloud points indicate that some cracking took place in the dry distillations.
After determining the cloud points, the first five cuts were mixed and tests were made on the combined 50 per cent fraction. The last five cuts were likewise mixed together and tested. The results are given in Table 4.
It is apparent that the viscosity of the oil in the material recovered from the vacuum distillation was almost the same as the viscosity of the oil in original "foots oil" (as shown in Table 2), the slight difference probably being due to the slight amount of heavy residue left in the distillation flask. However, the viscosity of the oils from the two dry distillations was materially lower than the viscosity of either of the original oils. Furthermore, dry distillation decreases the wax content, particularly in the second half of the distillate. In general, also, the slower the distillation the greater are the harmful effects.
In other words, redistillation of foots oil causes cracking, and cracking lowers the viscosity of the oil content and reduces the percentage of commercial wax in the foots oil. Dry distillation of foots oil is, therefore, not recommended. Redistillation with steam would probably not injure the product, but would not increase the wax yield.
The data shown in Table 4 suggest that half of the foots oil could be distilled off, and the residue without further distillation could be run as "slack wax" direct to the sweaters. Accordingly, Samples 5 and 6 were distiller, the distillation being stopped when 50 per cent of the samples had come over. The distillates and residues showed practically the same wax contents as in Table 4. The residues, however, from dry distillation of both slack waxes were dark in color and bad in odor, and the separated paraffin was also dark. The results of the vacuum distillation, however, indicate that distillation in a vacuum or with steam might be successful.
If steam distillation were used to redistill foots oil, the last half of the distillate might be used as slack wax, either to be sweated separately or mixed with the new slack wax coming into the sweating plant. The first half of the distillate might be mixed directly with the pressable distillate and recycled through the pressing and sweating operations. The commercial practicability of this procedure has not, however, been demonstrated.