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INDUSTRIAL A4NDENGINEERING CHEMISTRY
theoretical and near-theoretical mixtures containing small concentrations of diluent nitrogen. For mixtures containing concentrations of diluent nitrogen equal to or greater than that of air, the critical initial temperature a t constant initial pressure is higher than a t constant initial density. Similsr relationships are found in gasoline engines and bombs, and may help explain the effects of supercharging upon fuel combixtion characteristics.
Literature Cited (1) Aubert, M., and Duchene, R., Compt. rend., 191, 123-5 (1930). (2) Brown, G. G., Leslie, E. H., and Hunn. J. V., ISD. ESG.CHEV, 17, 397 (1925).
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(3) Brown, G. G., and Watkins. G. B., Ibid., 19, 280 (1927). (4) Campbell, C., Littler, W. B., and Whitworth, C., J. Chem. Sac., 135, 339-48 (1932). ( 5 ) Dixon, C., Trans. Roy. Sac. (London), 184, A97 (1893). (6) Dumanois, P., Ann. combustibles Ziquides, 9, 143 (1934). (7) Dumanois, P., and Mondain-Monl-al, P., Compt. rend., 189, 761-3 (1929). (8) Eastman, E. D., Bur. Mines, Tech. Paper 445 (1929). (9) Egerton, A, and Gates, S. F., Proc. Roy. Sac. (London), A114, 137-51 (1927). (10) Penning, R. W., Phil. Trans., 225, A331 (1926). (11) Lafitte, P., Compt. rend., 186, 951-3 (1928). (12) Lichty, L. C., Trans. Am. SOC.Mech. Engrs., 51, 37-44 (1929).
RECEIVEDMay 11, 1936.
LIQUID PROPANE Use in Dewaxing, Deasphalting, and Refining Heavy Oils
ROBERT E. WILSON Pan American Petroleum and Transport Company, New York, N. Y.
P. C. KEITH, JR. ROBABLP the most remarkable development in the history of refining heavy oils The M, W. Kellogg Company, for the production of lubricants has been New York, N . Y . the recent development of the use of liquid propane, as a solR. E. HAYLETT vent (or, more accurately, as antisolvent) for the removal of asphalt, wax, and other undesirable constituents from lubriUnion Oil Company, Los Angeles, Calif. cating fractions. The purpose of this paper is to discuss both the theoretical and practical aspects of these new processes and to describe briefly the commercial installations which have alrender the oil marketable. In this paper the term “naphready been made in the United States for carrying them out, thenic” is used inclusively to denote all nonparaffinic conwith ernphasis on certain novel engineering features. stituents other than the asphaltic materials. Thus it may inIn a previous paper (6) the authors pointed out that the clude compounds of olefinic or aromatic characteristics, and important properties of lubricating oils, particularly for use as in any case refers to those undesirable constituents present motor oils, are lorn- carbon-forming tendencies, low pour test, in all lubricating cuts which have a relatively low stability, high viscosity index (i. e., low rate of change of viscositywith low viscosity index, and low hydrogen-carbon ratio. temperature), and high resistance to oxidation and sludging. Reasons were given -why these properties were particularly important from the standpoint of the performance of motor This paper necessarily touches only the more important oils in modern internal c o m b u Y t i o n developments which have occurred in propane refining engines. during the past few years. I t indicates that propane is a cheap, available, safe, and versatile solvent that can Undesirable Constituents in be used advantageously in every step of lubricating oil Lubricating Fractions manufacture. A t low temperatures its properties are To obtain these properties, five types such that wax can be quickly and completely removed; at of constituents present in ordinary lubrihigh temperatures, due to rapid changes in its physical cating oil fractions must be eliminated. properties, it may be used to precipitate various undesirable These five undesirable constituents are: ( a ) paraffin wax, which must be removed constituents, since it tends to eliminate all those comt o obtain a low pour point; (b) asphalt, pounds which the refiner wishes to remove from his raw the removal of which is necessary for lubricating stock. several reasons, including instability and Propane refining makes readily available as by-products excessive carbon-forming tendencies; ( c ) a whole series of high-melting-point waxes and petrolatums the heavy ends of the lubricating oils, which also have high carbon-forming of extremely high quality and new types of asphalt of untendencies; ( d ) the “naphthenic” comusually desirable emulsification properties, as well as excelpounds which are, in general, responsible lent ductility penetration relations. Unquestionably these for low viscosity indices and low resietproducts will make themselves felt commercially in the ance to oxidation; and ( e ) color bodies near future. which must he remowd, primarily to
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INDUSTRIAL AXD ENGINEERING CHEMISTRY
Removal by Selective Solvents Although there are numerous ways of removing these impurities, the oil industry within the past few years has focused its attention upon the use of selective solvents. Selective solvents quickly made progress in the field of separating the naphthenic from the paraffinic constituents of lubricating oils which otherwise required the use of large quantities of sulfuric acid with the attendant acid sludge nuisance. Some solvents were found wLich were able to remove both naphthenic and asphaltic constituents, but, since the resulting extract was a mixture of asphalt and naphthenic hydrocarbons, it required further refining and separation to make marketable by-products and hence was not very promising. However, after the oil refiner had learned of the efficiency and cheapness of selective solvents, he was anxious to find solvents (or antisolvents) which would remove not only one or two impurities, but all of the undesirable constituents of lubricating oils, preferably one at a time in order that the by-products might be recovered in useful form. In an effort to discover such a universal solvent, chemists have searched Bielstein and other sources without finding any single chemical whose solvent properties are such that it will selectively remove all these impurities, although several of the improved solvents for the removal of naphthenic compounds have been studied as a result of this search.
Unique Properties of Propane as a Solvent Unfortunately, during most of this search for solvents, the refmer neglected to look a t his own raw materials. In every refinery and in every crude field, millions of tons of propane gas are available, which, by simple compression and liquefaction, can be converted into a solvent with the unique property (under proper conditions of temperature and pressure) of tending to separate every one of the undesirable constituents. Further, assuming recovery facilities are available, propane is the cheapest liquid per gallon available in the refinery with the exception of water; it is nontoxic, noncorrosive, and extremely stable, and can be used econoinically both as a diluent and a self-refrigerant in the complete removal of wax. Kot only does it throw out of solution under proper conditions each of the impurities mentioned, but for the removal of the three constituents wax, asphalt, and heavy ends, it is in general more efficient than any other known solvent. It has the further advantage of removing each of these three types of impurities separately from the other two, which greatly facili-
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tates by-product utilization. For the removal of naphthenic: constituent,s and color bodies it is very helpful but muitt generally be used with other refining agents such as selective solvents, sulfuric acid, or clay, in order to obtain the best results. As a result of these unique properties propane is today being commercially employed in numerous large installations for the separation of all five of the undesirable constituents of lubricating oils. That this development should have taken place in the short period of three years is a remarkable tiibute both to the versatility of propane and the progressiveness of the petroleum refiner. If we consider the diverse chemical and physical characteristics of the five undesirable constituents listed, it would seem almost inconceivable to anyone familiar with the theory of fractional solution that any one solvent could possibly throw all of these different compounds out of solution. As a matter of fact, propane cannot be expected to, and does not, separate them all at the same temperature and pressure. It owes its versatility as a precipitant to the fact that its properties change rapidly over the particular temperature range between -44" and +215" F. Over this range it possesses the properties of a series of solvents, any one of which can be obtained by raising or lowering the temperature or changing the pressure or combining those two operations. This rapid change in properties makes it possible to throw out the various impurities separately.
Variations in Solvent Properties of Propane Figures 1, 2, and 3 illustrate the remarkable changes in the physical properties of propane. Figures 1 and 2 show the vapor pressure curves of propane and n-butane. Pure propane boils at -44" F. under 1 atmosphwe pressure, exerts a pressure of 126 pounds per square inch a t 70" F., and has a critical pressure of 643 pounds at 212.2" F. Figure 3 reproduces a graph from a recent paper by Sage, Ychaafsma, and Lacey (4) which shows how the specso gravity of propane varies with pressure and gives the typical isotherms for that type of diagram. Figure 3 indicates the remarkable variations in the properties of propane over the ordinary temperature range. The dome-shaped curve represents the liquid line and shows that the specific gravity of liquid propane under its own vapor pressure drops from 0.50 a t 70" F. to 0.25 a t 212" F., just below the critical temperature. Incidentally, its viscosity and surface tension decrease to nearly negligible values as the critical temperature is approached. Over this range propane changes from a typical liquid to a fluid possessing substantially the properties of a gas. As the 220" F. isotherm indicates, gaseous propane at a pressure of 1000 pounds per square inch is much more dense (and as will be indicated later, has higher solvent power) than liquid propane a t 212" I?. and its saturation pressure. Also, as the liquid approaches its critical temperature, it becomes highly compressible. These facts make it fairly clear why propane has such different properties at different temperatures. Between 70" and -44" F., its main uses are in connection with dewaxing, and are due to (a) its extremely low viscosity, even at low temperatures; (6) its ability to give self'-refrigeration by allowing part of the propane to evaporate; and (e) its low solvent power for wax at low temperatures. At temperatures above 70" F. propane finds its use as a preferential precipitant, For example, it has been demonstrated that in the range of 100" to 140" F. asphalt' is only slightly soluble in propane; a t these same temperatures both oil and wax, in general, dissolve completely. In the temperature inIn this paper 1 Unfortunately no good definition of asphalt is possible. asphalt denotes a mixture of black, pitchy material, with a melting point of SOo F. or higher.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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be settled out and drawn off as a liquid because it contains a substantial quantity of dissolved propane. The remaining solution can then be rapidly chilled by the simple expedient of lowering the pressure and allowing a small proportion of the propane to evaporate off until a temperature of -20' to -40' F. is reached; a t this point all wax is thrown out of solution and may be removed by settling or preferably by atration. The propane solution is then brought back to room temperature, and may either be treated with selective solvents or relatively small quantities of sulfuric acid for the removal of color bodies and naphthenic constituents, or be heated, without any additions, in stepwise fashion to around 200" F. Under these conditions a series of cuts of varying composition is separated out; the first is composed of very heavy ends, resinous constituents, and the more heavy naphthenic constituents of the oil, and each cut becomes lighter and more paraffinic until the last material left in solution is a relatively low-boiling, highly paraffinic, light-colored oil.
T€MPEPATURf
Solubility Diagram Figure 4 illustrates more quantitatively the factors affecting the solubility of oil in propane a t various temperatures and pressures. This is a temperature-pressure diagram for a solution of Midcontinent distillate (S. A. E. 50 grade) in propane. The single curve to the left represents the vapor pressure of a solution of oil in propane. Up to 174" F. there is only a single liquid phase present, but a t this temperature a second liquid phase begins to separate, giving a characteristic "phase point" (the minimum temperature a t which phase separation occurs for any specific oil and propane concentration). The amount of this separate oil phase increases as the temperature rises. This second liquid phase can be put back into solution in either one of two ways: (a) The pressure may be raised sufiiciently above the normal vapor pressure to increase the density of the propane to a point such that the oil again goes completely into solution; or (b) the pressure may be lowered until enough propane evaporates into the vapor phase to restore the solution to homogeneity. For this particular solution a t a temperature in the neighborhood of 174"F., the change in pressure required to restore the single-phase condition is small, but, as the temperature is increased, a greater pressure change in either direction is required to accomplish this result.
terval of 100" to 212" F. and a t the vapor pressure of propane, the surprising and novel property of propane is that, instead of behaving like an ordinary liquid and dissolving more of any partially soluble substances as the temperature is raised, it actually dissolves less. In general, we find that the heavier and more naphthenic compounds are thrown out first; but as the physical properties of propane become increasiqgly those of gases, it dissolves less and less oil until no viscous oil remains in the solution a t 212.2" F., the critical temperature of propane. This property makes it possible to recover by simple decantation a large portion of the propane in a propane-oil solution as substantially pure propane without having vaDorized it bv the usual method. However. some bropane remains dissolved in the oil phase at this temperature, and this portion of the propane must be recovered by distillation. The heat economy involved in this procedure is small, but there are numerous conditions under which it is advantageous At temperatures near the critical, increasing the pressure (which increases the density, as indicated in Figure 3) increases the solubility of oil in pro; pane. The dissolving power of propane appears to be roughly proportional to the density of the propane, and even compressed propane gas a t 220" F. 5 and 1000 pounds per square inch pressure disLL solves subrtantially more oil than does liquid propane at 600 pounds and 204" F. Thus the solvent Z f----properties of propane can be strikingly changed by b 020 changes in either temperature or pressure or both; and in the magnitude of the resulting change it is probably unique among liquids. To utilize these varying properties of propane on a stock containing all of the undesirable constituents mentioned, a typical cycle would be t o mix oil with three or four times its volume of liquid 200 400 600 800 1000 1200 propane a t about 120°F., which will throw out of PRESSURE P O U N D S PER S Q U A R E INCH solution practically all of the asphalt; the latter can FIGURE3. SPECIFIC GRAVITY ISOTHERMS FOR PROPAXE
