US2645569A - Jet combustion fuel - Google Patents

Description

Patented July 14, 1953 UNITED STATES PATENT OFFICE JE1 COMBUSTION FUEL Jacl; M. God Qy,.Wennah, N. J., assignor to S0- cony-Vacuum Oil Company, Incorporated, a corporation of New York 7 No Drawing. Application April 28, 1949,

Serial No. 90,270

2 Claims, (01. 4463) tube give rise to a reaction effect which drives the tube in a direction. opposite to that of the emission of the gases. The most complicated forms presently proposed consist of they same propulsion or jet tube, plus a compressor to supply air for combustion, plus a gas turbine which extracts enough energy from the departing gases to drive the compressor. In present commercial terms, the compressor and turbine are assembled axiallyupon a common shaft, spaced far enough apart to permit a number of combustion cham bers to be arranged about the shaft between the compressor and turbine with an exhaust tube extending rearwardly from the turbine. In essence, the term jet combustion, as now commonly applied, and; as used in this specification, refers to. a method of combustion wherein fuel, is continuously introduced into and continuously. burned in a confined space for the purpose of deriving power directly from the hot products of combustion.

In practice, the range of conditions over which a jet combustion device may operate may become quite limited for ill-chosen or ill-adapted fuels. Even though a combustible mixture be present, the flame will be blown out if the rate of fuel feed is too far increased. Yet, high rates of fuel feed are necessary to obtain high heat release, which is high power delivery. Fuels with low blow-out levels can furnish only limited power and limited flexibility under conditions of operation. Consequently, the flame stability of a fuel is of major importance.

It has been found that flame stability of a fuel is correlated with a property of the fuel which is readily reproducible on a laboratory basis, using apparatus which is simple when compared to a commercial combustion tube, and with con siderably less procedural difficulty. This correlated property is the rate of flame propagation as measured in a Bunsen-type burner, using the method of Smith and Pickering [J. Res. Natl. Bur. Stds. l7, '7 (1936)]. In this procedure, the rate of flame propagation is measured at a number of fuel-air ratios by photographing the flame and measuring the slope of the. flame, cone. De sirable levels of commercial operab ility may be found in fuels having rates of flame propagation,

of the order of 1.4 feet per second and higher.

Benzol, an excellent fuel, has a rate of flame,

propagation of about 1.6 feet per second, at a fuel mixture temperature of 230. R, a pressure. of 1 atmosphereQand an air, flow rate, of 3,1 5 pounds/hour.

The present invention is predicated upon, the

discovery that small amounts of a material, ob;

tained by vacuum distillation ofthiophene tar, boiling at -125 C. at 2 mm. absolute pressure, disclosed in U. S. Patent No. 2,450,659, when added to a fuel, will materially improve. the com} bustion stability thereof.

As shown in U. S. Patent No. 2,450,659, and in copending application for Letters Patent, Serial No. 721,453, filed on Janua'rylO, 1947 ,now U. S. Patent No. 2,515,927, thiophene tar and thiophene are prepared. by separately preheat ing sulfur and one or more normal aliphatichydrocarbons selected from the group consisting of normal butane, normal butenes, and butadienes,

' to temperatures such that combining sulfur and the hydrocarbon material will give a mixture hav: ing a temperature in excess of about 450 0., mix: ing the preheated sulfur and the preheated hydrocarbon mixture, maintaining the temperature. of the mixture at a temperature in excess of about 450 C. for a period of time of at least 0.01 secondand reducing the temperature of the mixture to less than 450 C. Along with thiophene tar. and thiophene, hydrogen sulfide and small amounts of carbon disulfide are also formed in the process.

It has been found in the operation of this process that the relative proportions of sulfur. and hydrocarbon material in the charge may be varied over wide limits. Too much sulfur, however, results in poor efficiency in sulfur utilization per pass and favors the complete sulfurization of hydrocarbon material to carbon disulfide. Yet, too low a proportion of sulfur lowers the conversion per pass and the ultimate yield by increasing the overall thermal degradation of hydrocarbon material. Generally speaking, best results are obtained using a weight ratio of sulfurto a hydrocarbon material varying between about 0.5 and about 4.0, although when butenes and butadienes constitute the bulk of the hydrocar bon charge. the lower limit of the weight ratio may be lower than 0.5. It should be observed, however, that for economical operation of the process, it is preferred not to use a hydrocarbon charge consisting predominantly of butadienes because of their tendency to polymerize under the conditions of the process.

The selectivity of the reaction involved in the process for the preparation of thiophene tars and thiophene depends primarily upon two variables; namely, the reaction temperature at which the normal aliphatic hydrocarbon or hydrocarbons are contacted with sulfur, and the reaction time or the time durin which contact between the reactants is maintained at the reaction temperature.

The limits of operating temperature are fixed by the kinetics of the desired reaction and the kinetics of possible side reactions. It has been found in this connection, that the reaction temperature may vary between about 450 C. and about 760 C. and, preferably, between about 540 C. and about 650 C. when butane is the predominant hydrocarbon reactant in the charge, and between about d80 C. and about 500 C. when butenes and butadienes are the predominant hydrocarbon reactants in the charge. Below the lower limit of the temperature range (about 450 C.), the reaction is so slow as to require a large throughput of sulfur and a higher ratio of hydrocarbon recycle for a fixed amount of end product, thereby detracting from the economics of the operation. Above the upper limit of the temperature range, the secondary reaction of degradation of hydrocarbon material in the charge takes precedence, thereby decreasing the yield of desired product. In addition to this, high temperatures favor the formation of carbon disulfide. It must be noted, also, that at these high temperatures corrosion problems are at a maximum, corrosion increasing perceptibly with increasing temperature.

It has also been found, in connection with this process, that the optimum reaction time depends upon the temperature employed. In general, other variables remaining constant, the lower the temperature, the longer the reaction time. The reaction or contact time and the reaction temperature are somewhat fixed, one in relation to the other, by the degree of degradation of the hydrocarbon material in the charge and by the effect of formation'of undesirable products which may be tolerated. Thus, too long a contact time at high temperature results in severe cracking of the hydrocarbon material in the charge. The reaction proceeds with extreme speed, the only apparent limitation being the rapidity with which heat can be supplied to the reaction mixture. The reaction is highly endothermic, requiring by experimental measure approximately 28,000 calories per gram molecular weight of thiophene produced from normal butane. The lower limit of the range of reaction time is fixed, therefore, by the engineering problem of heat transfer and by mechanical limitations such as allowable pressure drop across the reactor. Relatively long reaction times at temperatures in the neighborhood of the lower limit of the temperature range result in insufficient reaction. Accordingly, it has been found that for best results the time of reaction is fixed by the reaction temperature.

In view of the foregoing, the criteria to be used in determining optimum operating temperatures within the range of 450 C. to 760 C. depend on the degree of conversion desired commensurate with operating costs, such as heat input and equipment cost, bearing in mind that within limits, the shorter the reaction time, and accordingly the higher the temperature, the larger the amount of end product which can be realized from a unit of given size per day.

While the relationship between the temperature of reaction and reaction time is not peculiar to the present process, it has been found that thiophene tar and thiophene may be produced by reacting sulfur and the aforesaid l-carbon hydrocarbons at a temperature between about 450 C. and about 760 C. for a period of time selected to minimize the yields of secondary reaction products such as carbon disulfide, coke-like materials and the like at the selected temperature. Under such condtions, when operating continuously with a reactor coil of suitable size and at a practical charge rate, it has been found that the lowest practical limit of the time of reaction is of the order of 0.01 second at about 760 C. The upper practical limit of the reaction time, other variables remaining constant, will correspond to the lower limit of the reaction temperature and may be of the order of several seconds.

Separate preheating of the hydrocarbon reactant and sulfur and quenching of the reaction mixture are necessary for achieving the somewhat close control of the reaction time at a given reaction temperature. This is very important in producing the specific reaction products, thiophene and thiophene tar. It is suspected that a number of reactions occur upon contacting the hydrocarbon reactant with sulfur. In this connection, the following should be noted: cracking of the hydrocarbon reactant destroying the 4-carbon atom chain structure (said 4-carbon atom chain structure being a prerequisite for the formation of thiophene), formation of thiophene tars high in sulfur and formation of carbon disulfide. These reactions compete with one another. It has been found that the rates of the formation of lighter hydrocarbons and of the formation of carbon disulfide are somewhat slower than those required for the formation of thiophene and thiophene tar. Accordingly, proper control of the reaction time at a given reaction temperature, achieved by separate preheating, mixing, heating at a given temperature for a corresponding period of time, and quenching is necessary to produce high yields of thiophene and thiophene tar with limited yields of carbon disulfide, coke-like materials, and fixed gases, due to limited decomposition of the hydrocarbon product. The rate of the reaction producing thiophene tar is fairly close to that required for the formation of thiophene, and the yields of thiophene tar and or" thiophene are approximately the same. Upon standing, the thiophene tar separates from the other products and a separation can be made by decantation or other suitable separating means.

In this process the reaction is effected preferably at atmospheric pressure or under suflicient ressure to cause the flow of the reactants through the reactor and auxiliary system under the desired reaction conditions. Tests have shown that the conversion per pass and ultimate yield of thiophene decreases with increasing pressure. However, even at appreciable pressures, thiophene and thiophene tar are, never theless, produced in substantial amounts.

Vacuum distillation of the above described thiophene tars is a destructive distillation process in which the charge, probably disulfides, polysulfides, etc., is decomposed during the heating process into distillable liquids and hydrogen sulfide. Vacuum distillation of the original tar and subsequent vacuum fractionation of the distillate so obtained yields two distinct fractions, a lower boiling material and a higher boiling material constituting the compound used in this invention. There is nothing critical in the vacuums employed during these distillations.

During the course of the aforesaid vacuum distillation, hydrogen sulfide is evolved, giving rise to frothing and bumping of the tar. These undesirable conditions have been overcome, however, by resorting to any one of several modifications: Smoother operation is realized by eating a capillary tube in the distillation vessel so that its lower end is located below the surface of the boiling tar and directing a stream of inert gas, such as carbon dioxide, nitrogen, or the like, through the tube and thus through the boiling tar. Another expedient involves first evacuating the distillation vessel at room temperature to degas the tar therein and thereafter slowly increasing the temperature of the tar. Hydrogen sulfide evolved from the tar during the distillation is readily removed by scrubbing the evolved gases by passing through towers filled with acid-absorbing media, such as soda lime, sodium hydroxide pellets, etc. This absorption of hydrogen sulfide protects the mechanical moving parts of the pump used to obtain the desired vacuum and hence is highly desirable. However, if a steam ejector system is used to obtain vacuum, the preliminary absorbing step may be omitted, since in this case, the hydrogen sulfide will be exhausted to the atmosphere.

It has been found that maximum distillation efiiciency can be attained by keeping the pressure below 10 millimeters and preferably below 2 millimeters of mercury. If the pressure is permitted to rise to the order of 10 millimeters of mercury, the temperature must necessarily be increased for distillation to occur at a reasonable rate and ultimately the rate of decomposition with evolution of hydrogen sulfide becomes too rapid to maintain an appreciable vacuum. When the temperature of the initial distillation rises to the neighborhood of 250 C., the tar has a tendency to polymerize and coke. Accordingly, the temperature of this distillation should be maintained between about 150 C. and about 250 C. and, preferably, between about 175 C. about 190 C. to attain a maximum yield of red, oily distillate. Under the above specified conditions of temperature and pressure, approximately -50 per cent of the initial charge of thiophene tar is distillable.

Subsequent vacuum fractionation of the red, oily distillate is ordinarily carried out at pressures below 10 millimeters of mercury and, preferably, at 4 millimeters of mercury or below. Such re-distillation yields two distinct fractions, a low-boiling fraction (4045 C. at 2 millimeters) constituting 60-85 per cent of the initial distillate and a high-boiling fraction (120-125 C. at 2 millimeters) constituting approximately 1540 per cent of the initial distillate.

The following example will serve as an illustration:

EXAlWPLE A mixture containing 30 per cent by volume of 1,3-butadiene and 70 per cent by volume of normal butane was charged into a preheater at a rate of grams per minute and heated to a temperature of 590 C. Sulfur was charged to a separate preheater at a rate of 28 grams per minute and heated to a temperature of 590 C.

constructed of 27 per cent chromium stainless steel, maintained at a temperature of 650 C. The reaction product was quenched with a water spray passed through a small Cottrell precipitator to remove tar mist and scrubbed through a hot counter-current caustic tower. Liquid product was condensed and'separated in a water cooler and ice trap and the residual gas was metered. Of the hydrocarbon material charged, 49 per cent was converted to a liquid product and tar. Fractionation of a portion of the liquid product after removal of C'i-hydrocarbons and lighter constituents showed the following composition:

Per cent Carbon disulfide 9.0 Thiophene 80.5 Residue (mostly thiophene) 10.5

One hundred parts by Weight of the thiophene tar were vacuum distilled in a distillation vessel immersed in a heating bath. l/Vhile warming the tar, a stream of nitrogen was bubbled through the heavy liquid until the tar was degassed. A scrubbing tower for removal of hydrogen sulfide and a Dry Ice-acetone condenser for removal of light liquids were connected in series before the vacuum pump. The bath. temperature was allowed to rise slowly and then was maintained at 175-200 C. A pressure of 1 millimeter of mercury was initially obtained but this gradually rose upon prolonged heating of the tar until a maximum pressure of 10 millimeters of mercury was reached. The product consisted of 226 parts by weight of a red, oily distillate boi ing between C. and 175 C. The yield of said distillate, based on the weight of tar, was 45.3 per cent;

The red oil was then vacuum-fractionated at a pressure of 2 millimeters of mercury, whereupon the following fractions were obtained:

- Wei ht 13011111 Fraction 7 Point Range gggga g atzmm" Original Tar) Material boiling within the range of fraction 2 was shown to have the following properties:

Molecular Weight i 157 Carbon 34. 46

Hydrogen 2. 60 ulfur G2. 0

Molecular formula O-(HiSl Refractive index, 2 0 C 1. 70

opecific gravity 25 C. 1.446

Color Deep Red sired, although, in general, no outstanding re- The two streams were sent through a mixing sults seem to occur. conceivably and within the scope of the present invention, the jet fuel additives contemplated herein may be marketd or procured as concentrates, vi z., jet combustion fuls containing upwards of 10% and up to 49% by weight of the additive. These concentrates are subsequently added to a jet combustion fuel in such proportions as to produce the effective concentration of additive in the fuel desired, i. e., a suificient amount to improve the jet combustion properties of the jet combustion fuel.

The jet combustion fuels of the present invention may contain other materials or additives for improving other characteristics thereof. Carbon deposition-reducing additives, gum inhibitors, and starting aids are mentioned by way of non-limiting examples of other additives which may be present in the jet combustion fuels of the present invention.

The addition of the thiophene tar fraction (fraction 2 as hereinbefore indicated) may be made to fuels of a fairly wide variety of boiling ranges, specific gravity, etc. Suitable base fuels for use in accordance with this invention include .1

those having the character of light gasolines up to those having the character of gas oils: Synthetic fuels, such as those manufactured by the Fischer-Tropsch process, can be used, as can be fuels derived from coal or wood distillation. It is also contemplated to add these combustion improving additives to liquid alcohols or combinations of alcohols with other base fuels. The preferred fuel is, however, a hydrocarbon distillate fuel boiling within the range of about 100 F. (37.8 C.) to about 600 F. (315.6 C.).

The physical characteristics of a few examples of suitable base fuels are given hereinafter for illustrative purposes:

3. Hydrocarbon distillate fuel Boiling range 320-470 F.

Gravity 365 A. P. I.

Freezing point"- Below -76 F.

Sulfur 0.035 by weight Bromine No 1 Aromatics 12.1 by volume Viscosity 1.48 centistokes, at 100 F.

The following examples are given for the purpose of illustrating the present invention and for indicating the advantages thereof. It must be clearly understood, however, that these examples are non-limiting. It will be appreciated by those skilled in the art that numerous types of jet combustion fuels, other than the standard reference fuel described hereinafter, may be used for the purpose contemplated herein.

In these tests a reference fuel was used which was substantially 2,2,4-trimethylpentane, commonly known as S-reference fuel. The rate of fiame propagation using the standard reference fuel was compared with that of a blend of the S-reference fuel with 0.5% by weight of the additive material hereinbefore disclosed, in accordance with the procedure of Smith and Pickering also hereinbefore disclosed. The fuel mixture was maintained at a temperature of about 230 F., under a pressure of 1 atmosphere, and the air flow rate was controlled at 3.15 pounds/ hour.

Table Rate of flame propagation reference fuel 1.45 ft./sec. 99.5% reference fuel+0.5% C4H4S3 additive material 1.50 ft./sec.

It will be seen that by adding as little as 0.5% of the additive material to the fuel, an increase of approximately 4% in rate of flame propagation can be obtained, which, as indicated hereinbefore, means a substantial improvement in combustion stability.

This application is a continuation-in-part of copending application Serial No. 26,477, filed May 11, 1948, now abandoned, which in turn is a continuation-in-part of application Serial No. 744,024, filed April 25, 1947, now abandoned.

I claim:

1. A liquid fuel capable of being utilized in jet combustion mechanisms, which comprises a hydrocarbon distillate having an initial boiling point of about 40 C. and a final boiling point of about 315 C. and boiling substantially continuously between said boiling points, and between about 0.1 per cent and about 2.0 per cent by weight of a fraction boiling at l20-125 C., at a pressure of 2 mm. of mercury, obtained by the process which comprises separately preheating sulfur and a Gil-hydrocarbon selected from the group consisting of normal butane, normal butenes, and butadienes, to temperatures such that combining said sulfur and said hydrocarbonwill give a reaction mixture having a temperature falling within the range varying between about 450 C. and about 760 C.; mixing the preheated sulfur and the preheated hydrocarbon; reacting said preheated sulfur with said preheated hydrocarbon at a reaction temperature falling within the range Varying between about 450 C. and about 760 C. for a period of time selected to minimize the yields of hydrocarbons containing less than four carbon atoms per molecule and carbon disulfide at said reaction temperature, to yield a mixture containing thiophene tar; immediately reducing the temperature of the mixture containing said thiophene tar to a temperature of less than about 450 C. separating the thicphene tar from said mixture; subjecting said thiophene tar to vacuum distillation at temperatures Varying between about 150 C. and about 250 C. and at a pressure of less than about 10 mm. of mercury, to produce a first distillate; and subjecting said first distillate to a vacuum fractionation at a pressure of less than about 10 mm. of mercury, to produce said fraction boiling at -125 C., at a pressure of 2 mm. of mercury.

2. A liquid fuel capable of being utilized in jet combustion mechanisms, which comprises a hydrocarbon distillate having an initial boiling point of about 40 C. and a final boiling point of about 315 C. and boiling substantially continuously between said boiling points, and between about 0.1 per cent and about 10.0 per cent by weight of a fraction boiling at 120-125 C., at a pressure of 2 mm. of mercury, obtained by the process which comprises separately preheating sulfur and a Ci-hydrocarbon selected from the group consisting of normal butane, normal butenes, and butadienes, to temperatures such that combining said sulfur and said hydrocarbon will give a reaction mixture having a temperature falling within the range varying between about 450 C. and about 760 0.; mixing the preheated sulfur and the preheated hydrocarbon; reacting said preheated sulfur with said preheated hydrocarbon at a reaction temperature falling within the range varying between about 450 C. and about 760 C. for a period of time selected to minimize the yields of hydrocarbons containing less than four carbon atoms per molecule and carbon disulfide at said reaction temperature, to yield a mixture containing thiophene tar; immediately reducing the temperature of the mixture containing said thiophene tar to a temperature of less than about 450 C.; separating the thiophene tar from said mixture; subjecting said thiophene tar to vacuum distillation at temperatures varying between about 150 C. and about 250 C. and at a pressure of less than about 10 mm. of mercury, to produce a, first distillate; and subjecting said first distillate to a vacuum fractionation at a pressure of less than about 10 mm. of mercury, to produce said fraction boiling at 120-125 0., at a pressure of 2 mm. of mercury.

JACK M. GODSEY.

References Cited in the file Of this patent UNITED STATES PATENTS Number

Claims (1)

1. A LIQUID FUEL CAPABLE OF BEING UTILIZED IN JET COMBUSTION MECHANISMS, WHICH COMPRISES A HYDROCARBON DISTILLATE HAVING AN INITIAL BOILING POINT OF ABOUT 40* C. AND A FINAL BOILING POINT OF ABOUT 315* C. AND BOILING SUBSTANTIALLY CONTINUOUSLY BETWEEN SAID BOILING POINTS, AND BETWEEN ABOUT 0.1 PER CENT AND ABOUT 2.0 PER CENT BY WEIGHT OF A FRACTION BOILING AT 12/-125* C., AT A PRESSURE OF 2 MM. OF MERCURY, OBTAINED BY THE PROCESS WHICH COMPRISES SEPARATELY PREHEATING SULFUR AND A C4-HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF NORMAL BUTANE, NORMAL BUTENES, AND BUTADIENES, TO TEMPERATURES SUCH THAT COMBINING SAID SULFUR AND SAID HYDROCARBON WILL GIVE A REACTION MIXTURE HAVING A TEMPERATURE FALLING WITHIN THE RANGE VARYING BETWEEN ABOUT 450* C. AND AOUT 760* C.,; MIXING THE PREHEATED SULFUR AND THE PREHEATED HYDROCARBON; REACTING SAID PREHEATED SULFUR WITH SAID PREHEATED HYDROCRBON AT A REACTION TEMPERATURE FALLING WITHIN THE RANGE VARYING BETWEEN ABOUT 450* C. AND ABOUT 760* C. FOR A PERIOD OF TIME SELECTED TO MINIMIZE THE YIELDS OF HYDROCARBONS CONTAINING LESS THAN FOUR CARBON ATOMS PER MOLECULE AND CARBON DISULFIDE AT SAID REACTION TEMPERATURE, TO YIELD A MIXTURE CONTAINING THIOPHENE TAR; IMMEDIATELY REDUCING THE TEMPERATURE OF THE MIXTURE CONTAINING SAID THIOPHENE TAR TO A TEMPERATURE OF LESS THAN ABOUT 450* C.; SEPARATING THE THIOPHENE TAR FROM SAID MIXTURE; SUBJECTING SAID ATHIOPHENE TAR TO VACUUM DISTILLATION AT TEMPERATURES VARYING BETWEEN ABOUT 150* C. AND ABOUT 250* C. AND AT A PRESSURE OF LESS THAN ABOUT 10 MM. OF MERCURY, TO PRODUCE A FIRST DISTILLATE; AND SUBJECTING SAID FIRST DISTILLATE TO A VACCUM FRACTIONATION AT A PRESSURE OF LESS THAN ABOUT 10 MM. OF MERCURY, TO PRODUCE SAID FRACTION BOILING AT 120-125* C., AT A PRESSURE OF 2 MM. OF MERCURY.