US3527693A - Process for making jet fuel - Google Patents

Description

United States Patent Int. Cl. C07C 23/02; C10] 1/04 US. Cl. 208-57 4 Claims ABSTRACT OF THE DISCLOSURE A process for making jet fuel by contacting straight run stove oil (kerosene) first with a hydrodesulfurization catalyst and then with a saturative hydrogenation catalyst is disclosed. The straight run stock is first contacted with a catalyst conventionally of a Group VIII metal in combination with a Group VI-B metal such as cobalt molybdenum, nickel-molybdenum, cobalt-tungsten, and molybdenum-tungsten supported on alumina or silica alumina at 250 to 800 F., 300 to 2,000 p.s.i.g., a WHSV of 0.2 to 6.0 and a hydrogenation rate of 1500 to 8000 s.c.f./bbl. The second stage hydrogenation is performed with a Group VIII hydrogenation catalyst such as plat inum at 200 to 850 F., 100 to 2000 p.s.i.g., a weight hourly space velocity of 1.5 to 6.0 and a hydrogen flow rate of 1500 to 6000 s.c.f./bbl.

CROSS-REFERENCE TO RELATED APPLICATIONS This is a division of application Ser. No. 658,066 now abandoned, filed Aug. 3, 1967, which is a continuationin-part of application Ser. No. 588,237, filed Oct. 13, 1966, now Pat. No. 3,367,860, which is a continuationin-part of application Ser. No. 324,881, filed Nov. 19, 1963, now abandoned.

BACKGROUND OF THE INVENTION This invention is directed to a high thermal stability, low vapor pressure fuel for jet engines in Mach 3 to 3.5 aircraft and to the process for producing such fuels and operating such engines. These engines require fuel having a high luminometer number, a high heat of combustion, a low freeze point, a low viscosity at low temperatures and a moderately high flash point. In addition to these burning properties, the fuel must be of high thermal stability at temperatures of about 700 F. The required burning properties and thermal stability have been found to be generally unattainable with straight run stocks.

The heat of combustion may be expressed both as B.t.u./ gal. and B.t.u./lb. For simplicity in nomenclature, the heat of combustion expressed in B.t.u./gal. will be referred to herein as fuel density All percentages of fuel components given herein are intended to refer to percent by volume unless specified otherwise.

Many of the fuels developed thus far for use in Mach 3 to 3.5 aircraft have been unsatisfactory in that the luminometer number has been undesirably low. The luminometer number is indicative of the tendency of the fuel to smoke during combustion in the engine. High luminometer number fuels burn cleaner than those of lower luminometer number and, consequently, provide a more desirable jet fuel. Preferably, the luminometer number of Mach 3 to 3.5 jet aircraft fuel should be or greater.

The luminometer number of a jet fuel can be increased generally by increasing its parafiinicity. Increased parafiinicity, however, also normally causes a rise in the fuel freeze point and a decrease in the fuel density due to the comparatively high hydrogen to carbon ratio of the parafiins. It has now been discovered that the removal of normal paraihns from the parafiin content of the fuel significantly reduces the detrimental effect of the parafiins on fuel freeze point without otherwise reducing the beneficial effects of high parafiinicity. The branched or isoparaffins still beneficially increase the fuel luminometer number but the freeze point remains low and surprisingly only a mild reduction in fuel density has been observed. Also, contrary to some teachings of the prior art, it has been found that jet fuels of this invention having high isoparaflin content also have good stability at high temperatures.

SUMMARY OF THE INVENTION Low vapor pressure, high thermal stability jet fuel compositions contain from 30 to 60 percent isoparaflins, up to about 6.5 percent n-parafiins, from 20 to 50 percent monocyclic naphthenes and from 10 to 25 percent polycyclic naphthenes. The fuels are hydrogenated by the process of this invention to completion and contain substantially no aromatics. These fuel compositions possess excellent burning characteristics and are thermally stable at temperatures on the order of 700 F. They have a heat of combustion of greater than 18,750 B.t.u./lb., a fuel density of greater than 124,000 B.t.u./gal., and a luminometer number of greater than 75. The fuel freeze points of these fuels are about -75 F. or lower. These jet fuels are formed by blending select, completely hydrogenated stocks in the proper proportions to obtain the ranges of components and properties set forth.

In particular, blending components which have shown exceptionally beneficial characteristics are hydrogenated stove oils from high normal paraffin content crudes which have been treated with a molecular sieve or in some other manner to remove n-paraffins.

One object of this invention is to produce a fuel for jet engines of Mach 3 to 3.5 aircraft.

An additional and principal object of this invention is to provide a novel method for producing high quality jet fuels.

A more specific primary object of this invention is to provide a method for producing high quality jet fuels by the two step hydrogenation of straight run stove oil (kerosene) stocks.

These and other objects of this invention will become more apparent from the following discussion and the appended claims.

Briefly, the fuels of this invention are formed by totally hydrogenating a stove oil out from selected straight run crudes and blending these hydrogenated stove oil cuts.

Second, the fuels can be produced by hydrogenating a stove oil cut from a straight run stock to completion in two stages and removing n-parafiins, as by means of a molecular sieve or by extraction with a urea complex.

Thirdly, these fuels can be produced by hydrogenating the stove oil cut from a straight run stock to completion, extracting the normal paraffins from the hydrogenated stock, as through the use of molecular sieves, and blending the resultant product.

Suitable blending agents have been found to be a mixture of hydrogenated stove oils from naphthenic and intermediate crudcs such as a stove oil mixture containing from to 40 percent parafiins of which 99 percent are isoparaffins, from 38 to 48 percent monocyclic naphthenes from 22 to 28 percent dicyclic naphthenes and up to 2 percent tricyclic naphthenes.

It has been found that the straight run kerosene (stove oil) is best hydrogenated in two stages. The first hydrogenation or hydrofinishing is necessary to remove sulfur and nitrogen from the stock and may be conducted at a temperature from 250 to 850 F., a pressure from 300 to 2,000 p.s.i.g., a liquid hourly space velocity of from 0.2 to 6.0, and a hydrogenation rate of from 1,500 to 8,000 s.c.f./bbl. The preferred first stage conditions are a temperature from 600 to 800 F., a pressure from 450 to 850 p.s.i.g., a liquid hourly space velocity from 0.5 to 2.0 and a hydrogenation rate of 2,500 to 3,500 s.c.f./bbl.

The hydrogenation catalyst for the first stage must also be a hydrodenitrogenation and hydrodesulfurization catalyst. Therefore, it must be a catalyst which is not fouled by sulfur and nitrogen such as a supported metal combination of a Group VIII metal and Group VI-B metal oxides and sulfides. Typical catalysts of this variety are cobalt-molybdenum oxide, nickel-molybdenum oxide, cobalt-molybdenum sulfide, nickel-molybdenum sulfide, cobalt-tungsten sulfide, nickel-tungsten sulfide and molybdenum-tungsten sulfide on a conventional support mate rial such as alumina or silica-alumina.

The second stage hydrogenation conditions are a temperature from 200 to 850 F., a pressure of from 100 to 2,000 p.s.i.g., a liquid hourly space velocity of 1.5 to 6.0, and a hydrogenation rate of 1,500 to 6,000 s.c.f./bbl. The preferred conditions are temperatures from 400 to 750 F., pressures from 500 to 1,000 p.s.i.g., a liquid hourly space velocity from 0.5 to 4.0, a hydrogen flow rate of 3,500 to 4,500 s.c.f./bbl. Second stage hydrogenation should substantially saturate the feed stock. Preferably, the feed stock should be saturated to at least 98 percent completion to form the component of the jet fuels of this invention. Platinum group metal catalysts and preferably platinum have been found to produce the best results in the second stage.

Low boiling cracked and partially cracked products of the initial hydrogenation step may be removed by a convenient separation process such as distillation of a 370 to 520 F. heart cut prior to second stage hydrogenation. Separation may also be accomplished by other means such as hydrogen stripping.

It has been found that a highly isoparaffinic blending material can be provided by selecting a straight run stove oil (kerosene) which, after hydrogenation, is relatively high in paraffins and has a high heat of combustion and fuel density and by removing substantially all n-parafiins from the oil. Stove oil from Four Corners crude has been found to be the only straight run stock which satisfies these requirements. Typically, the stove oil fraction, boiling in the range of from 290 to 520 F., of this crude when hydrogenated contains from 22 to 30 percent isoparafiins, from 15 to 20 percent n-paraflins, from to 41 percent monocyclic naphthenes and from 15 to 20 percent polycyclic naphthenes when hydrogenated in the two-stage hydrogenation process already discussed.

The hydrogenated Four Corners stove oil typically has a heat of combustion of 18,771 B.t.u./lb., a fuel density of 124,658 B.t.u./gal., a luminometer number of 84 and a freeze point of F. After removal of the n-paraffins from the oil, the heat of combustion, fuel density and luminometer number remained Substantially constant while the freeze point decreased to below F. providing a full scope of excellent properties for a jet fuel.

This invention may be further understood from a consideration of the foregoing discussion in conjunction with the following specific examples of the high Mach jet fuel prepared in accordance with our invention. The examples are not intended to limit the scope of this invention beyond that of the appended claims.

Example I A stove oil blend comprised a mixture of 2 parts of a stove oil from a naphthenic type crude, i.e., having from 55 to 65 percent naphthenes, less than 20 percent paraffins and the balance aromatics and 1 part of a stove oil from an intermediate type crude, i.e., stove oil fraction having from 45 to 55 percent naphthenes, about 30 percent paraflins and the remainder aromatics. The stove oil blend was hydrogenated in two steps the first of which was carried out at 750 p.s.i.g., 750 F., 1.0 liquid hourly space velocity, and 3,000 s.c.f./bbl. in the presence of a cobalt-molybdenum oxide catalyst on alumina. The second stage was at 600 p.s.i.g., 500 F., 1.0 liquid hourly space velocity, and 4,000 s.c.f./bbl. in the presence of a platinum catalyst. The final fuel contained:

Percent by volume n-Paraflins 0.8 Isoparaflins 52.1 Monocyclic naphthenes 29.5 Dicyclic naphthenes 17.2 Tricyclic naphthenes 0.4

This fuel was tested in the laboratory and found to have the following properties:

The fuel had good thermal stability at 700 F.

Example H A stove oil cut from a Four Corners crude was hydrogenated to completion under the two stage hydrogenation conditions of Example I and then passed through a Linde 5A molecular sieve to remove n-parafiins. The resultant stove oil was 30.3 percent by volume isoparafiins, 48.3 percent by volume monocyclic naphthenes, and 21.4 percent by volume tricyclic naphthenes.

This Four Corners stove oil had the following properties after hydrogenation and prior to removal of the nparafiins.

Gravity, API 45.9 Heat of combustion, B.t.u./lb. 18,771 Fuel density, B.t.u./gal. 124,658 Freeze point, F. 42 Viscosity, CS at 30 F.

Luminometer number 84 After removal of n-parafiins, the fuel had the following properties:

Gravity, API 46.0 Heat of combustion, B.t.u./lb. 18,740 Fuel density, B.t.u./gal. 124,375 Freeze point, F. Below -80 Viscosity, CS at 30 F. 10.94 Luminometer number 82.0 Flash point 166 After removal of the n-parafiins the fuel had an excellent viscosity and freeze point. Four Corners Crude of the composition given has been found to be the only crude which could be hydrogenated and sieved and then directly used as a fuel without blending.

EXAMPLE III A mixture, including a hydrogenated Mid-east stove oil, was prepared. The stove oil was hydrogenated to completion in two stages under the following conditions:

First Stage: nickel-molybdenum oxide on alumina catalyst, 750 p.s.i.g., 750 F., 1.0 liquid hourly space velocity, and 3,000 s.c.f./bbl.

Second Stage: platinum catalyst, 600 p.s.i.g., 500 F., 1.0

liquid hourly space velocity, and 3,000 s.c.f./bb1.

The fuel blend, which included percent hydrogenated alkyl trimers, was tested in the laboratory and found to possess the following properties:

Gravity, API 43.5 Heat of combustion, B.t.u./lb. 18,750 Fuel density, B.t.u./gal. 126,225

Freeze point, F. 80 Viscosity, CS at 30 F. 16.7 Luminometer number 80 Diltillation, F.:

IBP 388 10% 403 50% 427 90% 479 EP 514 Flash point 164 Although the stove oils of many crude oils were examined and one was found which could be hydrogenated and used directly as a high Mach aircraft jet fuel, it is theoretically possible that such a stove oil might be available. Accordingly, with such a stove oil it would be possible to produce high Mach aircraft jet fuels of the disclosed composition using only the two stage hydrogenation described.

Many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof and therefore only such limitations should be applied as are indicated in the appended claims.

We claim:

1. A process for forming a fuel for jet engines of Mach 3 to 3.5 aircraft comprising the steps of hydrogenating a composition selected from the group consisting of a straight run stove oil from a Four Corners type crude oil and a blend of naphthenic and intermediate straight run stove oils to produce a stock consisting of from 22 to 30 percent isoparafiins, from to percent n-paraffins, from to 41 percent monocyclic naphthenes, and from 15 to 20 percent polycyclic naphthenes, said hydrogenation being conducted in a first stage at a temperature of from 250 to 850 F., a pressure of from 300 to 2,000 p.s.i.g., a liquid hourly space velocity of from 0.2 to 6.0, a hydrogenation rate of from 1,500 to 8,000 s.c.f./ bbl. in the presence of a hydrodesulfurization catalyst; and hydrogenating the resultant product in a second stage substantially to completion at a temperature from 200 to 850 F., a pressure from 100 to 2,000 p.s.i.g., a liquid 6 hourly space velocity of from 1.5 to 6.0 and a hydrogen rate of from 1,500 to 6,000 s.c.f./bbl. in the presence of a platinum group metal catalyst; and passing said stove oil fraction through a molecular sieve for extracting nparaflins therefrom to provide a fuel having a heat of combustion of at least 18,750 B.t.u./lb., a fuel density of at least 124,000 B.t.u./gal., a luminometer number of at least 75 and a freeze point of at least --60 F.

2. A process for forming a high Mach jet engine aircraft fuel comprising:

hydrogenating a refinery tetramer stream consisting essentially of by volume up to 30 percent C olefins, from 50 to 70 percent C olefins, from 3 to 45 percent C olefins and up to 5 percent C olefins, at a temperature of from 250 to 850 F., a pressure of from 300 to 2,000 p.s.i.g., a liquid hourly space velocity of from 0.2 to 6.0 and a hydrogen rate of from 1,500 to 8,500 s.c.f./bbl. in the presence of a platinum group metal catalyst; providing a hydrogenated stove oil selected from the group consisting of a straight run stove oil from a Four Corners type crude oil and a blend of naphthenic and intermediate straight run stove oils consisting essentially of from 30 to 45 percent by volurrie isoparaffins, from 35 to 50 percent by volume monocyclic naphthenes, from 15 to 25 percent by volume polycyclic naphthenes and substantially no n-parafiins; blending a sufiicient quantity of said tetramer with said stove oil to form a jet engine fuel having 20 to 50 percent by volume tetramer and the balance stove oil, said fuel having a heat of combustion of at least 18,750 B.t.u./lb., a fuel density of at least 124,000 B.t.u./gal., a luminometer number of at least 75, a freeze point of 75" F. or lower and a viscosity of less than 15 centistokes; wherein said hydrogenated stove oil is prepared by hydrogenating at a temperature from 250 to 850 F., a pressure from 300 to 2,000 p.s.i.g., a liquid hourly space velocity from 0.5 to 6.0, and a hydrogen rate of from 1,500 to 8,000 s.c.f./bbl. in the presence of a hydrodesulfurizing catalyst; separating a 370 to 520 F. heart cut from said hydrogenated product; and hydrogenating said heart cut at a temperature of from 200 to 850 F., a pressure of from to 2,000 p.s.i.g., a liquid hourly space velocity of from 1.5 to 6.0, and a hydrogen rate of from 1,500 to 6,000 s.c.f./bbl. in the presence of a platinum group metal catalyst.

3. A process for forming a Mach 3 to 3.5 aircraft jet engine fuel comprising the steps of:

hydrogenating a composition selected from the group consisting of a straight run stove oil from a Four Corners type crude oil and a blend of naphthenic and intermediate straight run stove oils having at least 35 percent by volume paraffins at a temperature from 600 to 800 F., a pressure from 450 to 850 p.s.i.g., a liquid hourly space velocity of from 0.5 to 2.0 and a hydrogen rate of from 2,500 to 3,500 s.c.f./bbl. in the presence of a hydrodesulfurization catalyst in a first stage;

hydrogenating the resultant product in a second stage at a temperature of from 400 to 750 F., a pressure from 500 to 1,000 p.s.i.g., a liquid hourly space velocity of from 0.5 to 4.0 and a hydrogen rate of 3,500 to 4,500 s.c.f./bbl. in the presence of a Group VIII hydrogenation catalyst; and

passing said stove oil through a molecular sieve having a pore size of about 5 angstroms to remove n-paraffins from said stove oil and form a jet fuel having from 30 to 60 percent by volume isoparafiins, from 20 to 50 percent by volume monocyclic naphthenes, from 10 to 25 percent by volume polycyclic naphthenes and substantially no n-parafiins.

8 4. A process as defined in claim 3 wherein said jet fuel 3,242,066 3/1'966 Myers 20815 consists of about 30 percent by volume isoparaflins, about 3,369,998 2/1968 Bercik et a1. 208210 49 percent by volume monocyclic naphthenes and about 3,367,860 2/ 1968 Barnes et a1. 208144 21 percent by volume polycyclic naphthenes. FOREIGN PATENTS References Cited 5 836,104 6/1960 Great Britain. UNITED STATES PATENTS 870,431 7/1961 Great Britain.

714,530 7/1965 Canada. 3,125,503 3/1964 Kerr et a1. 20815 3,126,330 3/1964 Zimmerschied et a1. 20815 HE E 3,155,740 11/1964 Schneider 20815 10 RBERT L VINE Pnmary Exammer 3,185,739 5/1965 Gray et al. 20815 3,175,970 3/1965 Bercik et a1. 208212 3,231,489 1/1966 Mahar 20815 ,9 ,1 4