US2399805A - Production of aviation gasoline - Google Patents

Description

May 7, 1946.

P. E. HURLEY 'ET AL PRODUCTION OF AVIATION GASOLINE Filed Feb. 7, 1944 PEEP; 538 3% NHE UUCOU NEN O NUHSCLDE CEQOU 550 u rcnm .m 5 sw z Paul El Hun-l2 mixture.

Patented May 7, 1946 PRODUCTION OF AVIATION GASOLINE Paul E. Hurley and John B. Dunlap, Deer Park, Tex., assignors to Shell Development Company,

San Francisco, Calif., a corporation of Dela- Ware Application February 7, 1944, Serial No. 521,454 (01. 196-49) 13 Claims.

This invention relates'to thamanufacture of aviation gasoline by cracking of hydrocarbon distillates and especially to the art of cracking low octane naphtha to produce aviation gasoline having improved properties and high anti-knock value, as well as to increase the production of aromatics, particularly toluene and xylenes.

It is well known that naphthas used in the blending of high grade motor fuels and particularly aviation gasoline must have ahigh antiknock value, must be free of gum-formin components and must maintain their anti-knock properties, under severe engine conditions, i. e. high temperature, high compression and rich For these reasons naphthas, rich in aromatic hydrocarbons and substantially free from olefins. are preferred. Aromatics are known to have excellent anti-knock properties which do not decrease under severe operating conditions and have no tendency to form gums or to deteriorate otherwise during storage. Olefins, on the contrary, frequently oxidize and polymerize during storage, forming gums, and thus render a motor fuel less desirable for airplane engines.

It is well known that, by cracking, i. e. by thermal treatment, resulting in carbon-carbon scission and dehydrogenation or isomerization or polymerization or a combination of these reactions, it is possible to increase the octane number of hydrocarbons whose boiling range is not substantially changed by the cracking or reforming operation, and also to increase their content of aromatics. Thermal means noncatalytic or "in the absence of a catalyst as understood in the conventional sense, in accordance with the accepted usage in the prior art as exemplified by the definition found on page 37 of The Chemistry of Petroleum Derivatives by Carlton Ellis, published in 1934. Such thermal operations when conducted by conventional methods cause, however, at the same time the formation of large amounts of olefins. This drawback has heretofore precluded the use of cracked products, particularly of higher boiling cracked naphthas in aviation gasolines, despite their relatively high octane number and aromaticity.

It is also known that for any given hydrocarbon feed the improvement in octane number of products whose boiling range is not substantially changed increases as the severity of cracking conditions is raised. This may be efiected by increasing the temperature or, the residence time of the hydrocarbons in the cracking furnace or both of these factors. v However, coking of the furnace places a strict practical'limit upon the improvement which may thus be obtained in that there is a certain severity of cracking above which coking becomes so rapid that it precludes commercial operation. This critical value may, to some extent, depend on the design of the furnace in which cracking is conducted, but for a given furnace it is primarily a function of the nature of the feed. Since a furnace requires shutting-down and cleaning as soon as deposition of coke begins to plug it, it is obviousthat only processes in which coking is relatively slow are economically practicaL' 7 It has been proposed that more severe cracking conditions can be maintained without objectionable coking by introducing into the cracking zone, together with the naphtha, gaseous hydrocarbons such as those having one to five carbon atoms in the molecule. Such processes are described, for example, in U. S. Patents 2,135,014 to Ostergaard or 2,220,699 to Angell. The introduction of these gases raises very appreciably the cost of heating, fractionation, compressing and pumping. Moreover, such light hydrocarbons and particularly the light olefins contained therein, or' which are formed by dehydrogenation of the parafllns, tend to form unsaturated polymers which find their way into the cracked products, particularly in the naphtha fractions, thereby increasing their unsaturation. p

It is an object of the present invention to provide a cracking process capable of producing a naphtha having a high aromatic content and unusually low olefin content. It is another object to produce naphtha having essential antiknock and storage properties. Still another object is to provide an improved method for making aviation blending stock and aviation gasoline. A further object is to produce naphtha from which pure aromatics, particularly toluene, may be obtained. Still further objects will be found in the improvements hereinafter disclosed.

The process of this invention comprises the following essential steps:

(1) Thermally cracking a straight run naphtha having a selected boiling range.

(2) Fractionating the resulting cracking prod-'- ucts to obtain a selected second naphtha fraction.

(3)! Recracking said second fraction and a portion of a third fraction produced in the process. (4) Separating and fractionating the resulting recracked product to produce said third fraction.

Recycling a portion of said third fraction to said recracking step (3).

In addition to these essential steps, the third fraction may be further fractionated and treated to improve its aviation blending characteristics.

The success of the present invention, and particularly of the recracking depends on the proper selection of the straight run and both the cracked naphtha fractions. It is only when all three of these fractions are properly chosen and cracked under the proper conditions that the full benefits of this invention are obtained.

The nature of the invention will be better understood from the following detailed description, taken together with the drawing forming a part of this specification. This drawing presents a simplified flow diagram of a preferred embodiment of the invention.

For simplicity the drawing does not show pumps, heat exchangers, valves, by-passes, vents, reboilers, condensers, and other auxiliaries, the proper placement of which will at once be evident to those skilled in the art.

The straight run hydrocarbon naphtha which forms the starting material of the present process may originate from any crude, though naphthenic oils are preferred to paraffinic ones. This starting material, or first naphtha fraction preferably should be substantially free of C4 and lighter hydrocarbons, and preferably free of C5 hydrocarbons. That is, it should have an A. S. T. M. initial boiling point of not below about 100 F. Its end boiling point by A. S. T. M. distillation should be below about 500 F. and preferably below 450 F. The reason for these limita- 1 tions lies in the fact that most of the hydrocarbons boiling outside these limits tend, upon cracking, to produce relatively large amounts of olefins boiling within the boiling range of the cracked naphtha selected for re-cracking. If this latter cracking stock is very rich in olefins, then the product of recracking, i. e. aviation blending stock, will also be relatively rich in olefins. That this must be avoided has been pointed out before.

In addition to the above boiling range 1imitations the straight run naphtha selected for the first cracking should preferably have a U. 0. P. characterization factor below 11.6 and preferably below 11.4; that is, it should be naphthenic and/0r aromatic in nature. The U. 0. P. characterization factor is a measure of paraffinicity of an oil. A highly paraflinic oil has a factor of about 12.5 and a highly aromatic and naphthenic oil has a lower factor approaching as a minimum. (See Characterization of Petroleum Fractions by Watson, Nelson and Murphy of U. 0. P., Industrial and Engineering Chemistry, volume 27, No. 12, page 1460.)

In the first step of this process the selected straight run naphtha fraction is introduced into cracking zone I through valved line 2 under conditions as severe as is practical without excessive coking. Such conditions have been found to correspond to cracking temperatures between about 1000 F. and 1040 F. and outlet pressures between about 500 and 1000 lbs. per square inch. The pressure drop across the cracking coil is in general in the order of 200 to 400 lbs. per square inch. If desired, the time of cracking may be extended and controlled with the aid of a reaction chamber not shown.

The cracking operation is thu conducted in the psuedo liquid phase as opposed to vapor phase cracking in which lower pressures are maintained, and larger proportions of olefins are formed. Actually the naphtha is above its critical temperature and pressure, so that the terms liquid and vapor do not have the conventional meaning.

In the second step, the resulting cracked products from zone I are withdrawn through lines 3 and 4, and are quenched and fractionally distilled in the distillation column 5 to separate therefrom those products boiling above about 450 F., and preferably above 400 R, which are withdrawn from the bottom of the column 5 through line 6. The remaining lighter portion of the products are withdrawn from the top of the column through line 1 and introduced into another fractional distillation column or depentanizer 8 wherein the products boiling below C5 and preferably below C6 are withdrawn through vapor line 9 at the top of column 8. The remaining fraction, which has a boiling range between about 100 and 400 F. to 450 F. will be referred to hereinafter as the second naphtha fraction, and is withdrawn from the bottom of column 8 through line Ill.

This second naphtha fraction is characterized by a relatively high content of monocyclic aromatics and naphthenes, as opposed to hydrocarbons boiling above about 450 F., which contain a relatively large amount of polycyclic aromatics and naphthenes. These latter compounds should not be included in the selective fraction, because upon cracking under severe conditions, they tend to promote coke formation and lower the quality of the final product. Moreover, as pointed out before, relatively high boiling hydrocarbons tend to form large amounts of olefins upon cracking, which boil within gasoline boiling range. This second naphtha fraction must have an octane number of at least 74 or 75, and could be used with advantage as a motor fuel, but not as aviation fuel.

By further treating this particular fraction, as described below, its octane number can be greatly improved, its aromatic content increased, and its parafiin, naphthene and particularly olefin content decreased so that it can be used for aviation fuel.

The gases and light hydrocarbons withdrawn from the top of column 3 through line 9 may be separated and employed in other processes, for example, the C5 and lighter hydrocarbons may be alkylated to produce a blending stock for gasolines, and any C6 or higher hydrocarbons which may have been removed with the C5 hydrocarbons may be employed as a blending stock for motor gasolines if desired, after the toluene therein, if any, has been removed. The C4 hydrocarbons may be converted into octylenes, cracked to produce butadiene, or alkylated to produce aviation blending stocks. The C3 hydrocarbons may be chemically treated to produce acetone, and the C1 and C2 hydrocarbons may be employed as fuel.

In the third step the second naphtha fraction is blended with recycle naphtha which is a portion of a third naphtha fraction from line H, joining with line ID to form line I 2, before entering the second cracking zone I3. The amount of this third naphtha fraction employed in the mixed feed to zone 13 ranges between 10 and 50% by volume and preferably between and by volume of said feed.

The recracking operation in zone l3 should be conducted under the most severe conditions which can be tolerated without excessive coking. These conditions may be more severe than those in the first cracking zone I because many components of the original fuel, which tend to decom pose readily, and cause coking, have been cracked in the first cracking step and eliminated in the fractional distillation columns and 8. The conditions correspond, for example, to temperatures ranging between about 1030 and 1100 F., preferably about 1060 F., and pressures ranging between about 250 and 750 lbs. per square inch. The resulting recracked products are withdrawn through lines M and i5, quenched and fractionated as described in the second step.

The severity of the cracking may be defined either by the cracking conditions, or by the changes brought about by these cracking conditions in the properties of the hydrocarbon, and particularly in its boiling range, The latter method is particularly applicable when the feed stock has a relatively narrow boiling range, as is the case in the present process, because in such a case, portions whose boiling ranges are changed are most likely to boil outside the boiling limits of the feed. When defined according to this method, the cracking conditions which, for this process, are such that a cracked fraction having the same boiling range as the naphtha feed amounts to about 60% to 90% of the cracked products in each cracking operation, and preferably about 80% in the first operation and about 70% in the second operation.

Another easily measured change efiected by cracking is the change in octane rating. It is, in general, convenient to measure the octane number of a debutanized gasoline having an end point of 400 F. For the purpose of this invention, it is preferred that such a gasoline separated from the first cracking condition have an octane number of 74 to '78 by the C. F. R. Motor method, and one separated from the products of the second cracking step have an octane number at least 5 points higher, e. g. of 80 to 85.

Still another method for measuring the severity of cracking is by means of specific dispersion which is a measure of the percentage of aromatics in the resulting product. (See "Industrial and Engineering Chemistry, volume 29, No. 3, March, 1937, pages 319-325.) For example, a fraction which has a specific dispersion of about 145 contains at least about 50% aromatics. Thus the selected third naphtha fraction obtained from the products of zone l2 should have a specific dis- 5 persion of at least about 143 and preferably at least about 145.

It is essential that the cracking and recracking operation of this process be carried out as separate steps. If it were attempted to maintain cracking conditions sufiiciently severe to produce the desired result in one operation without intermediate separation of light and heavy components, heavy and tarry components produced in the initial stages would cause rapid coking in the subsequent stages. Moreover, the light unsaturated components formed at the beginning would be present in a large concentration, would polymerize to form olefins boiling within the naphtha range, and thus produce finally a more unsaturated naphtha, which is not desired.

In the fourth step, the cracked product from line I5 is introduced into the fractional distillation column l6 wherein the products boiling above about 400 F. are removed through bottom line H and the remaining portion is Withdrawn through top line 18 into the depentanizer l9, similar to column 8. wherein the C5 and lighter hydrocarbons are withdrawn through vapor line 20 to produce the third naphtha fraction-which is withdrawn through bottom line 2|, into surge tank 22.

This third naphtha fraction has an olefin content much lower and an aromatic content much higher than that of the corresponding fraction obtained from the products of the first cracking zone I as well as a product from the second cracking zone l3 which had not been cracked with any recycle from line II. It is worth noting that if successive narrow boiling fractions are separated from said third naphtha fraction and the olefin content of these narrow fractions is plotted against their boiling range, a very distinct temperature will be found at which the olefin content decreases abruptly. Below about 240 F. the olefin content and octane number are both high. Above about 270 F. the olefin content is low while the octane number is high. In other words, there is a definite break in the curve in which the octane number is plotted against oiefins at various boilingtemperatures. This break occurs between 240 and 270 F.

The lighter hydrocarbons withdrawn through line 20 may be joined with those from the top of column 8 in line 9 and separated for use in the same manner as those described from line 9 above.

In the fifth step, a portion of the third naphtha fraction is withdrawn from tank 22 through line 23 and valved line H for recycle through the recracking zone l3. This recycle of a portion of the third naphtha fraction materially increases the aviation blending characteristics of said third naphtha fraction, as will be seen in the examples described later, As indicated bef0re,recyc1e ratics, i. e. ratio of recycle to the second naphtha should be between about 1:9 and 1:1 and preferably between about 1:4 and 1:2.

An alternative flow for the recycle of cracked products through zone l3 may comprise passing a portion of the recracked products through line i l leaving the'recracking zone I 3, through valved line 32 to join with line l, entering distillation column 5, and fractionating the recycle in columns 5 and 8 together with the second naphtha fraction from zone 1.

A further alternative flow which is not disclosed might be to remove hot naphtha from the topitray of column 5 and charge it through a furnace not shown and then return it to column 5 through line 4; then recrack the product from column 8 in furnace 13 without any recycle through line H.

The cracked residues obtained from the bottom of columns 5 and 16, respectively, through lines 6 and H, are suitable for blending into Bunker fuels, the residue from the bottom of column [6 having a particularly high aromatic content and yielding very stable black fuel oils.

The remaining portion of the third naphtha fraction in tank 22 not recycled to recracking zone l3, may be passed through valved line 24 into distillation column 25 wherein the Ca and possibly some C7 hydrocarbons are removed and then through bottom line 26 into distillation column 21, from which is removed a toluene concentrate through line 28, which is passed to a toluene extraction plant not shown, for the production of nitration grade toluene according to the process disclosed in Dunn et a1. Patent No. 2,288,126. The remaining portion of the fraction boiling above the toluene concentrate is Withdrawn from the bottom of column 21 through line 29, and then may be treated in zone 30 to reduce its content of gum forming components,

improve its color, odor, stability, etc., as well as increase its octane number.

This fraction from the bottom of the column 21 normally has a boiling range between about 250 F. and 400 F. and preferably 335 F., and normally contains between about 45 and 60% aromatics. Or if desired, this remaining portion may be passed directly from tank 22 to treater 30 without going through distillation column 2-5 or 21 or both.

The finishing treatments in treater 30 may include, for example, passing the fraction over adsorption agents such as clay, bauxite, fullers earth, diatomaceous earth, silica gel, etc., for example, at elevated temperatures below incipient cracking; contacting with sulfuric acid, phosphoric acid of suitable concentrations to Produce a sludge containing the undesirable constituents which can be separated; selective hydrogenation the presence of a suitable hydrogenation catalyst, such as finely divided nickel, chromium oxide, molybdenum or tungsten sulfide or a combination of these, etc.; treatment with catalyst of the clay type under conditions which do not cause substantial cracking; destruction or removal of harmful sulfur compound by doctor treatment, extraction with alkaline solution in the presence of a solutizer for mercaptans, or oxidation in the presence of copper catalysts, etc.

The specific conditions involved in the treating processes as above mentioned may be readily obtained from the book Chemical Refining of Petroleum, by Kalichevsky and Stagner, published by Reinhold Publishing Corporation, 1942. In this book are descriptions of processes for the treatment of hydrocarbons with sulfuric acid (see Chapter II), with alkaline reagents (see Chapter IV), sweetening processes (see Chapter V), refining by adsorption (see Chapter VI) and refining with a solvent (see Chapter VII).

A particular advantage of the process of this invention is that these treatments ma be relatively light, i. e. muchlighter than treatments which would have to be applied to conventionally cracked naphthas or to the naphtha obtained in the first cracking step of this process, in order to obtain a product having the same color, stability, low degree of unsaturation and low mercaptan content.

Sulfuric acid treatment, which is known ordinarily to reduce octane number of cracked gasoline, does, on the contrary, further raise the octane number of the third naphtha prepared according to the invention. The acid employed in the treating may be fresh acid or may be spent alkylation acid.

The treated naphtha may be redistilled to remove higher boiling constituents formed during the treating operations, If it is redistilled, the bottoms from the distillation, including the polymers formed during the treating steps, may be blended into fuel oils. These bottoms are usually highly aromatic oils.

The finished naphtha suitable for aviation blending stock is withdrawn through line 3!. It is highly aromatic and therefore has very high anti-knock value when tested by the 3C or other methods which emphasize rich mixture or highly supercharged operation. Naphthas of still further improved anti-knock properties, having octane numbers up to 93-94, may be obtained by extraction of this naphtha fraction by means of selective solvents, having greater solvent power for aromatic than for parafiinic hydrocarbons, such as $02, nitrobenzene, furfural, acetone, aniline, phenol, methyl acetate, ethyl cellosolve, etc., and separation of the extract from the solvent.

The treated naphthas from zone 30 normally have an alkylate replacement value between about .3 and .9, however, with treatment this value may be increased, A more positive and definite way of expressing this alkylate replacement value is to state that the naphthas have an index number between 94 and 139, which is equivalent to the performance of fuels having a rich mixture rating in the Coordinating Research Council F-4 engine test carried out in a 30 supercharged C. F. R. engine between that of (1) 98% iso-octane and 2% normal heptane and (2) pure iso-octane containing about 2 cc. of tetra ethyl lead.

It may then be blended with suitable blending agents to produce a finished aviation gasoline. Suitable blending agents include light straight run gasolines, isopentane, cyclopentane, neohexane, 2,3-dimethyl butane, diisopropyl ether, triptane, iso-octane, alkylation gasoline produced by reacting isobutane with C5 and lower olefins, etc. These blending agents give the proper boiling range to the final blend and its octane number may be further improved by the addition of tetraethyl lead, aromatic mono-cyclic amines, etc. Many of these blending agents may be obtained or produced from the byproducts of this process.

It may be noted that the preferred embodiment of this process permits the best utilization of a given petroleum crude oil for the manufacture of aviation fuel in that both the high boiling naphtha components are improved to give a highly aromatic aviation gasoline base, and that Cl and C5 gases obtained both from the crude and in the cracking operation may be utilized to further improve the octane number and add susceptibility of this base and to give the finished aviation gasoline the proper boiling range.

The following comparative examples further illustrate :the invention. Two cracked naphthas are compared:

(a) One which is recracked without any recycle,

and

(1)) One which is recracked under substantially identical conditions but with recycle at a recycle ratio of any lie-reformer operating conditions: i Charge rate B/D- (a) Reformed gasoline (second naphtha fraction) 4410 3300 (b) Recycle (recycle rc-rcformed gasoline) (third naphtha fraction) None 1700 Transfer temperature F 1060 1057 Properties of charge:

ASTM octane number 73. 2 74. 0 Initial boiling point F. 153 152 End point 380 382 Properties of rc-reformed gasolm ASTM octane number 78. 2 80.2 Initial boiling point 11. 182 End point F 349 406 Aromatics percent 27.0 0 Acid treated and rerun third naphtha fraction after 1firtsit removing toluene and lighter material by distila on:

Initial D ling point ,.F 267 26G 10% point .F 276 276 00% poiut ."F.. 30% 310 End point F 354 340 Aromatics ..pcrcent 37.5 63.0 Index number l32 From these data it may be seen that the effectiveness of the third naphtha fraction is materially increased by acid treatment, specifically the heavier the acid treatment the lower the bromine number. For comparison, tw identical fractions treated to 5 and 1.5 bromine numbers respectively by 2 and 6 dumps of 10 pounds per barrel of 94% sulfuric acid, produced respectively octane numbers of 83.5 and 84.5%, per cent aromatics of 50 and 53, and peroxide numbers of .04 and .01, which after two Weeks in a closed drum changed to .93 and .08.

We claim as our invention:

1. A process for increasing the aromatic content of straight run naphthas substantially free from C4 and lighter hydrocarbons and boiling below about 500 F., comprising the steps of noncatalytic cracking-said naphtha in the pseudo liquid phase, separating from the resulting cracked product a second naphtha fraction substantially free from C5 and lighter hydrocarbons and boiling below 450 F., non-catalytic recracking in the pseudo liquid phase a mixture consisting of said second fraction and a portion of a third fraction produced in the process, separating from the resulting recracked product said third fraction substantially free from C5 and lighter hydrocarbons and boiling below about 400 F., recycling a portion of said third naphtha fraction to said recracking step said cracking steps being conducted at temperatures above 1000 F.

2. The process of claim 1 wherein the first naphtha fraction is cracked at a temperature between 1000 F. and 1040 F., and a pressure between 500 and 1000 lbs. p. s. i. and said second naphtha fraction is cracked at a temperature between about 1030 F. and 1100 F. and a pressure between about 250 and 750 p. s, i.

3. The process of claim 1 wherein the cracking operations are conducted to yield between 60 and 90% of products boiling in the same temperature range as the feed thereto.

4. The process of claim 1 wherein the first cracking operation is conducted under conditions to yield a gasoline having an octane number above '74 and said second cracking operation is conducted to yield a gasoline having an octane number at least 5 units higher.

5. The process of claim 1 wherein the straight run naphtha to be cracked boils between about 100 F. and 450 F.

6. The process of claim 1 wherein said second naphtha fraction boils below about 400 F.

7. The process of claim 1 wherein the remaining portion of the third naphtha fraction which is not recycled is further treated to improve its stability.

8. Th process of claim 1 wherein the remaining portion of the third naphtha fraction, which is not recycled, is treated with sulfuric acid to reduce its bromine number whereby its octane number is not lowered.

9. The process of claim 1 wherein between about 10% and of said third naphtha fraction is recycled.

10. The process of claim 1 wherein between about 25% and 35% of said third naphtha fraction is recycled.

11. The process of claim 1 wherein said straight run naphtha has a U. 0, P. characterization factor below 11.6.

12. In a process for manufacturing naphthas having improved anti-knock rating and low unsaturation and being suitable for aviation gasoline, the steps comprising non-catalytic cracking a Straight run naphtha fraction substantially free of C5 and lighter hydrocarbons and boiling below about 450 F., separating from the cracked products a second naphtha fraction substantially free of C5 and lighter hydrocarbons boiling below 400 F. and having an octane number above '74, non-catalytic recracking a mixture consisting of said second naphtha fraction and a portion of a third naphtha fraction produced in the process, separating from the recracked products said third naphtha fraction having. an octane number above 80, and recycling a portion thereof for recracking with said second naphtha fraction said cracking steps being conducted at temperatures above 1000 F.

13. The process of claim 12 wherein said third naphtha fraction has an octane number above 85.

PAUL E, HURLEY. JOHN B. DUNLAP.

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