US3367860A - High density jet fuel and process for making same - Google Patents

Description

United States Patent 3,367,860 HIGH DENSITY JET FUEL AND PROCESS FOR MAKING SAME Robert L. Barnes, 4822 Sanbert St, Placentia, Calif. 92670, and Robert L. Dinsmore, 5328 Coralite St Long Beach, Calif. 90808 No Drawing. Continuation-impart of application Ser. No. 324,881, Nov. 19, 1963. This application Oct. 13, 1966, Ser. No. 588,237

2 Claims. (Cl. 208-15) ABSTRACT OF THE DISCLOSURE A high density supersonic jet fuel having a freeze point of less than 65 F. and a heat of combustion of at least 127,500 B.t.u./gal. and 18,600 B.t.u./lb. and process for making such fuel. The fuel is produced by blending 2 parts of a stove oil out from a naphthenic petroleum stock having less than 20 percent paraffins and from 55 and 65 percent naphthenes and the remainder aromatics with 1 part of a stove oil cut from an intermediate petroleum stock having about 30 percent paraffins, 45 to 55 percent naphthenes and the remainder aromatics. The blend contains about 15 to 35 percent parafiinic hydrocarbons, about 15 to 20 percent aromatics and the balance naphthenes. The blend is hydrogenated to from 80-95 percent completion, in the presence of a hydrogenation, hydrodenitrogenation and hydrodesulfurization catalyst such as a Group VIII-Group VLB metal combi nation under the conditions set forth. The blend is then distilled to separate a 370 to 520 F. heart-cut therefrom. This heart-cut is then hydrogenated to about 98 percent completion in the presence of a Group VIII catalyst. The resultant fuel has a ratio of mononapththenes to polynaphthenes of greater than 1:1.

This is a continuation-in-part of application Ser. No. 324,881 filed Nov. 19, 1963 and now abandoned.

The present invention relates to a high energy fuel for supersonic aircraft and more particularly relates to a high fuel density naphthenic type jet fuel.

High Mach number supersonic bombers and their commercial transport counterparts are designed to utilize thin wing sections to minimize drag. This reduction in wing section tends to limit aircraft volume available for fuel storage and, consequently, will require a fuel having a higher fuel density in terms of B.t.u.s/gal. However, this trend in engine design also requires a fuel having a high heat of combustion (B.t.u.s/lb.) because the aircraft will be weight limited as well as volume limited. Ideally, the best combination would be high density (B.t.u.s/gal.) and high heat of combustion (B.t.u.s/lb.). These two properties are difficult to find in the same fuel. Consequently, the best fuel will have to be a compromise between these two combustion properties.

Aliphatic hydrocarbon fuels have been employed for aircraft engines, however, their fuel density is insufiicient for high Mach number supersonic aircraft. Fuels containing hydrocarbons which are unsaturated with respect to hydrogen have also been used, however, such fuels do not satisfy the thermal stability requirements for supersonic aircraft. It has recently been proposed to pro duce a jet fuel of high thermal stability, high heat content and low smoke point and low freezing point by extracting an aromatic cut from a reformate boiling in the 400 to 500 F. range and subjecting this fraction to hydrogenation. This method not only involves the additional step of reforming, but also requires a more severe hydrogenation to effect complete saturation of the highly aromatic reformate stock. In addition, the reformate fraction is normally so low in parafiinicity that the heat of combustion of the fuel is lower than required to satisfy the requirements for supersonic aircraft fuels. It has also been found that jet fuels obtained by hydrogena-ting aromatic reformates tend to have shorter side chains on the ring(s) than comparable straight run naphthenic stocks. Hence, the composition and the characteristics of jet fuels derived from aromatic reformates are substantially different from the naphthenic type jet fuels envisioned by our present invention. Reforming of the stock produces a high aromatic content requiring a high amount of hydrogen for saturation so that it may be used in jet fuels. By selecting proper crudes or straight run stocks, blending them and treating the blend in accordance with this invention, it has been found that high quality fuels have been obtained. Since the initial aromatic content is lower than that of a reformate stock, considerable savings in time and money can be attained by using the blend of this invention rather than reforming. For example, a typical reformed stock comprises about 80 percent aromatics, whereas the blend contemplated for use to produce the jet fuel of this invention comprises only about 20 percent aromatics. Thus by comparison, in addition to the extra reforming step, the reformates require, typically, four times as much hydrogen as the straight run blends of this invention for saturation.

It previously has been thought that jet fuels for best performance should have low paraffinic and monocyclic naphthene content as exemplified by US. Patent 3,126,- 330 issued Mar. 24, 1964 to W. I. Zimmerschied et al. Accordingly, multiple distillation and extraction processes have been employed to reduce the paraflin content of the fuels and to increase the proportion of polycyclic naphthenes to monocyclic naphthenes in the fuel. The jet fuels produced by these additional treating steps normally have less than 6 percent paraflins and less than percent monocyclic naphthenes with the balance polycyclic naphthenes. These fuels have been found to have a heat of combustion on the order of 18,500 B.t.u/lb.

Normally, in evaluating supersonic jet fuels, basically comprising paraffins, monocyclic naphthenes and polycyclic naphthenes, it has been thought that the preferred fuels should contain a high percentage of polycyclic naphthenes compared to monocyclic naphthenes and that the parafi'in content be maintained at a low level. It has been expected that reverse proportioning of the naphthene content and high paraffin contents would decrease the fuel density below acceptable levels. We have discovered that by properly proportioning the monocyclic naphthenes to the polycyclic naphthenes and by using a larger amount of parafilns the fuel density can be maintained within safe limits and at the same time the heat of combustion can be increased. This is particularly important since it is exceedingly difiicult to increase the heat of combustion by even as little as B.t.u./lb. once a level of about 18,300 to 18,400 B.t.u./lb. has been attained. In particular, it has been found that the jet fuels of this invention have a minimum heat of combustion of at least 18,600 B.t.u./lb. Additionally, the minimum fuel density for the fuels of this invention has been foundto be 127,500

B.t.LL/gal.

Accordingly, it is an object of the present mventron to provide a jet fuel having a high thermal stability, a high fuel density and a high heat of combustion.

It is also an object of the present invention to provide a high fuel density supersonic jet fuel having a high heat of combustion, high thermal stability, low viscosity and a low freeze point.

Another object of this invention is to provide a jet fuel produced from a blend of non-reformed crude oils and having a fuel density of greater than 127,500 B.t.u./ gal., heat of combustion greater than 18,600 B.t.u./lb. and a gOOd thermal stability at temperatures above 700 F.

Yet another object of this invention is to produce a thermally stable supersonic jet fuel from a blend of straight run stocks in a plurality of hydrogenation steps.

Another object of the present invention is to provide an improved process for producing a hydrogenated jet fuel of the naphthenie type.

Other objects and a more complete understanding of our present invention may be had by reference to the following specification and the appended claims.

We have discovered a fuel composition which contains the proper balance of constituents for a supersonic jet fuel having high fuel density and a heat of combustion greater than 18,600 B.t.u./lb., a viscosity maximum of centistokes at F., a freeze point below 65 F. and a thermal decomposition temperature above 700 F., the constituents of these fuels which are believed to be critical to obtain the above properties are approximately 15 to percent paraffinic hydrocarbons with the remainder consisting essentially of mono and polycyclic naphthenes wherein the ratio of monoto polycyclic naphthenes is at least 1:1 and preferably about 2:1. This composition having the above properties which are necessary for a supersonic jet fuel is obtained by treating a hydrocarbon mixture derived from one of several sources according to the process of our present invention. The hydrocarbon mixture may be obtained from a stove oil fraction of a straight run stock or a blend of stove oils from straight run stocks and of certain treated refinery stocks such as an isocracked deasphalted gas oil. It is contemplated that various materials could be blended which would yield the composition desired for processing to obtain the supersonic jet fuel of our present invention.

The naphthenic stock is treated according to the process of our present invention by first hydrogenating with a desulfurizing catalyst to remove both sulfur and nitrogen and to partially hydrogenate aromatics in the oil; second- 1y, distilling the hydrogenated material to obtain a product boiling between about 370 to 520 F., and, finally, hydrogenating to completion or about 98 percent saturation with a milder hydrogenation catalyst to stabilize the jet fuel. It has been found that when the fuel is hydrogenated to completion in a single step and then distilled, the heating required for distillation is so high due to the severe onestage hydrogenation that it causes cracking which tends to form gum precursors, and thus adversely affect the thermal stability of the hydrotreated fuel. Additionally, hydrogenation in one stage is impractical because sulfur and nitrogen poison the hydrogenation catalysts.

A partial cracking of the oil is always experienced in the hydrodesulfurizing and dehydronitrogenizing stage. The light materials resulting from this cracking must be separated as by distillation to upgrade the fuel. In addition to removing the lighter materials along with the sulfur and nitrogen compounds by distilling a heart cut from the oil, these materials may be removed by stripping, i.e. by passing the oil through a countercurrent stream of hydrogen. In this case, the oil may be passed through a hydrogen stripper intermediate the two hydrogenation steps in the place of the distillation.

The severity of the initial hydrogenation step is dependent in part on the initial sulfur and nitrogen content. Generally, when the initial content of these materials is low, less cracked light materials are formed. In this instance, the separation step intermediate the two hydrogenation steps may be adjusted accordingly. For example, if only mild cracking has been experienced so that the initial boiling fraction of the oil is about 340 F., the lighter materials can be separated by hydrogen stripping more conveniently than by fractionating the heart cut. Alternatively, any conventional means may be employed for separating the cracked lighter boiling oil fractions produced in the first hydrogenation stage from the oil.

The hydrogenation. catalyst used in the initial hydrogenation step serves as a hydrodenitrogenation and a hydrodcsulfurization catalyst and also catalyzes the hydrogenation of aromatics. This type of catalyst should be of the type which is not subject to fouling by sulfur and is conventionally a Group VIII metal in combination with a Group VIB metal. Typical examples of such combinations are cobalt-molybdenum sulfide, nickel-molybdenum sulfide, cobalt-tungsten sulfide and molybdenum-tungsten sulfide, supported on a conventional support such as alumina or silica-alumina. The first stage hydrogenation may be conducted to at least percent saturation and preferably to 95 percent saturation at 600 to 800 F., 600 to 5000 p.s.i.g., with a weight hourly space velocity of 0.2 to 6.0 at a hydrogenation rate of 1500 to 8000 s.c.f./ barrel. The second stage hydrogenation following the distillation process may be performed with a Group VIII hydrogenation catalyst such as the platinum or nickel catalysts supported on alumina, kieselghur or silica at 400 to 650 F., 200 to 1000 p.s.i.g. at a weight hourly space velocity (WHSV) of .5 to 10 at a hydrogenation rate of 1500 to 6000 sci/barrel. The preferred conditions are: temperatures from 400 to 650 F.; pressures from 200 to 800 p.s.i.g. and WHSV from 2 to 8. Platinum catalysts have been found to produce the best results and are preferred in the second hydrogenation step.

Various straight run stocks were examined to determine their suitability for treatment to produce a supersonic jet fuel having the desired properties. The stocks having parafiinic contents exceeding about 35 percentdid not meet the freeze point requirements for supersonic fuel. Stocks from intermediate crudes that have too high a freeze point may be satisfactory if the n-paraflins are removed by use of urea, molecular sieves or bacteria. Other stocks having parafiin content lower than about 10 percent had satisfactory freeze points, however, their heat of combusion (B.t.u.s/lb.) was low. It has been found that the monocyclic naphthenes should be present in a greater amount than the polycyclic naphthenes in order to increase the heat of combustion (B.t.u.s/lb.). As pointed out, reformate stocks normally have in excess of 50 percent aromatics and, consequently, require a more severe hydrogenation than straight run naphthenic type stocks which have 25 percent or less aromatics. A fuel meeting the requirements for a supersonic jet fuel in accordance with this invention may be made by treating a mixture of 55 to 75 percent naphthenic type crude (one having less than 20 percent parafiins and between 55 to 65 percent naphthenes with the remainder aromatics) and 25 to 45 percent intermediate type crude (a crude having about 30 percent parafiins with 45 to 55 percent naphthenes and the remainder aromatics) according to the process of the present invention.

Example I A blend of 2 parts of a naphthenic type crude and 1 part of an intermediate type crude was hydrogenated and found to produce an excellent supersonic jet fuel. The properties of the blended as compared with the unblended stocks are shown in Table I. Fuels A and B were unblended stocks. Fuels A, B and C were all hydrogenated first with a denitrogenation and desulfurization nickeltungsten sulfide catalyst to about percent saturation at 650 F., 2800 p.s.i.g., with a weight hourly space velocity (WHSV) of 1.0 and a hydrogen rate of 3000 s.c.f./barrel. The materials were fractionated to remove the 370 to 510 F. heart cut and the heart out material hydrogenated with a nickel catalyst at 500 F., 500 p.s.i.g. at a WHSV of 5.0 and a hydrogen rate of 2240 s.c.f./barrel.

TABLE I Composition, Volume Percent Fuel A Fuel B Fuel Naphthenie Type Crude 100 67 Intermediate Type Crude 100 33 Gravity, API 39.0 41. 30.8 Freeze Point, F below 70 -66 -70 Viscosity, (:5. at 30 F 17.9 12. 1 14. 28 Net Heat of Combustion:

B.t.u.s/lb 18, 615 18, 670 18, 640

B.t.u.s/gal 128, 300 127, 200 127,920 Smoke Point, mm 28 27.0 Luminometer Number 70 62.1 Thermal Decomposition Temp, F l 714 Specific Heat, B.tluJs/lb. F. at 212 0. 554 apor Pressure, mm. Hg:

At 200 F 18 At 400 F 610 Although our present invention has been described with a certain degree of particularity, it is to be understood that our invention is not to be limited to the details set forth but should be given the full scope of the appended claims.

We claim:

1. A hydrogenated high density jet fuel derived from a stove oil fraction boiling in the range of 350 to 520 F. consisting essentially of a hydrogenated product of a blend of about 2 parts straight run stove oil from a naphthenic hydrocarbon stock and about 1 part straight run stove oil from an intermediate hydrocarbon stock; the ratio of monocyclic naphthenes to polycyclic naphthenes in said blend being .at least 1:1 and said blend having from 15 to 35 percent by volume paraflinic hydrocarbons with a major portion of said parafiinic hydrocarbons being isoparafiins, said fuel having a freeze point below -65 F., a viscosity less than 15 centistokes at --30 F., a smoke point of 25 minimum, and a fuel density of at least 127,500 B.t.u.s/ gal. and a minimum heat of combustion of 18,600 B.t.u.s/lb.

2. A hydrogenated high density jet fuel derived from a stove oil fraction boiling in the range of 350 to 520 F. consisting essentially of a blend of 55 to 75 percent by volume of a straight run stove oil stock consisting essentially of from 55 to 65 percent by volume naphthenes, less than 20 percent by volume paraflins and the remainder aromatics, and 25 to percent by volume of a straight run stove oil stock consisting essentially of 45 to percent by volume naphthenes, 30 percent by volume parafiins and the remainder aromatics, said blend having a ratio of monocyclic naphthenes to polycyclic naphthenes of at least 1:1 and said hydrogenated fuel having a freeze point below F., a viscosity less than 15 centistokes at 30 F., a smoke point of 25 minimum, and a fuel density of at least 127,500 B.t.u.s/gal. and a heat of combustion of at least 18,600 B.t.u.s/lb.

References Cited UNITED STATES PATENTS 2,765,617 10/1956 Gluesenkamp et a1. 208l5 3,000,815 9/1961 Haney 208--15 3,006,843 10/1961 Archibald 208-2l7 3,012,961 12/1961 Weisz 20815 3,077,733 2/1963 Axe et a1 208--15 3,126,330 3/ 1964 Zimmershield et al. 20815 3,175,970 3/1965 Bercik et a1. 208144 3,222,274 12/1965 Carl 208--89 3,236,764 2/ 1966 Herder et a1. 2082l0 OTHER REFERENCES Symposium on Jet Fuels, Division of Petroleum Chemistry of the American Chemical Society, New York, N.Y., vol. 5, N0. 4-C, Sept. 11-16, 1960, pp. C-29-31 and C-35-37.

DANIEL E. WYMAN, Primary Examiner. P. E. KONOPKA, Assistant Examiner.