US3493489A - Process for the production of jet fuel and middle distillates - Google Patents

Description

Feb. 3,1970

Filed April 17, 1968 G. R. NANICHE PROCESS FOR THE PRODUCTION OF JET FUEL AND MIDDLE DISTILLATES 2 Sheets-Sheet l n a 3 5 i l2 CATALYTIC I CRACMNG HYDRggIZACKING ZONE l\ 9 7 10 Y COKING ZONE \8 F|G.1 74 l 34 7 as 2 so CATALYTIC CRACKING \4 ZONE -2 com ZONE so 7 SI/i a COKING DRUMS 69 6a INVENTOR GEORGES R. NAN/CHE FIG'B 001mm LU 777.

ATTORNEYS United States Patent O 3,493,489 PROCESS FOR THE PRODUCTION OF JET FUEL AND MIDDLE DISTILLATES Georges R. Naniche, Berkeley, Calif., assignor to Chevron Research Company, San Francisco, Calif., a corporation of Delaware Filed Apr. 17, 1968, Ser. No. 722,041 Int. Cl. Cg 37/00, 13/00; C10b 55/00 US. Cl. 208-50 7 Claims ABSTRACT OF THE DISCLOSURE A process for the production of low freeze point jet fuel from a heavy oil feed by catalytically cracking the heavy oil, fractionating the catalytic cracker eflluent, coking a portion of the efiluent, and hydrocracking at least a portion of the coker gas oil and uncoked catalytic cracker eflluent. Preferably the bottoms from catalytic cracker effiuent fractions is coked. A separate second coker feed may be combined with that portion of the fractionated catalytic cracker effiuent to be coked prior to coking.

BACKGROUND OF THE INVENTION This invention relates to processes for the conversion of heavy hydrocarbon fractions to produce jet fuel. More particularly, the invention relates to an improved process wherein a heavy oil is converted by cracking and coking to a high yield of very low freeze point jet fuel.

Today, the petroleum refiner has an increasingly difficult task in meeting the market demand for sufiicient quantities of middle distillate hydrocarbons, principally jet fuels. To meet this demand, the refiner is looking for new processes and ways to realize maximum utilization of old conversion processes. Hydrocracking is one relatively new process to which the refiner has been giving a great deal of attention. This process is able to convert a Wide range of high boiling feeds into gasoline, middle distillates,, and other useful products. At the same time, the older processes, such as catalytic cracking and coking, have been studied to determine ways to increase the conversion to middle distillates and to vary the product distribution of these processes. A number of combinations of catalytic cracking, coking, and hydrocracking have been suggested in the prior art. These combinations have disclosed means by which the feed to the hydrocracker could be tailored by catalytic cracking and coking such that the subsequent hydrocracking would produce a satisfactory yield of the desired products. Generally the original oil that was being converted was a heavy oil, such as crude oil. It was recognized that such an oil fould contain components, such as polynuclear aromatic compounds, which would be harmful to catalysts used in hydrocracking and catalytic cracking. The prior art has disclosed two types of processes aimed at eliminating these harmful products from feeds to cracking units: (1) process combinations wherein these oil fractions are diverted away from the catalytic crackers and hydrocrackers and dumped into products such as heavy heating oils or bunker fuels which then may be sold without further processing, and (2) process combinations wherein oil fractions containing these harmful components are pretreated to convert these compounds to other compounds less harmful to the cracking catalysts.

The feasibility of employing the first of these methods of disposing of the undesirable materials has greatly decreased in recent years, however. The demand for heating oils has decreased because of the increasing use of natural gas for heating in many marketing areas. Similarly, the use of bunker fuel oils has become uneconomical and has decreased in many marketing areas due to the competition from natural gas. In addition, pollution restrictions in many areas have prevented the burning of bunker fuel due to its high sulfur content; and the almost complete dieselization of the railroads and the competitive advantages afforded low-cost foreign bunker fuels have made disposal of the undesirable oil fractions economically unattractive. Thus, attention has largely been directed to processes for the conversion of these fractions to more desirable materials, such as jet fuels.

The latter method of preparing hydrocracking feedstc cks free of the undesirable components, which involves coking and catalytic cracking and eliminates the necessity of dumping the undesirable products, has been the subject of a number of patents. The particular individual process disclosed in each of these patents, however, has involved one of two basic alternative procedures. In the first, which may be termed the series combination, the fed oil is coked to remove undesirable components and produce coker distillate gas oil suitable for use as a catalytic cracker feed. The catalytic cracker in turn is used to convert the coker distillate to hydrocracker feed, and the hydrocracker is used to produce the final desired products. Such a process is disclosed in US. Patents 3,008,895 and 3,072,560. The other common procedure, which may be termed the parallel combination, requires a separation of the initial oil feed, usually by distillation, into a catalytic cracker feed stream, commonly a gas oil, and a heavier stream, commonly a residual oil, which contains the undesirable components and which is fed to the coker. In this procedure, the coker distillate is fed directly to the hydrocracker, as is the catalytic cracker product. The coker distillate and the catalytic cracker product may be, and in practice are, combined and fed to the hydrocracker as a single stream. In this procedure, it will be noted that the catalytic cracker and coker feeds are entirely separated and only the products are combined. An example of this procedure will be found in U.S. Patent 3,245,900. Modifications of these two procedures, which utilize the same basic principlesi.e., separation or pretreatment of catalytic cracker feed and hydrocracker feed to remove or segregate undesirable componentswill be found in US. Patents 3,019,180 and 3,071,535. In each of these processes, however, a certain proportion of material which would be suitable for use as catalytic cracker or hydrocracker feed must be sacrificed to coke because these processes do not have the capability of separating out those materials which, while boiling in the coker feed boiling range, are not harmful to catalysts and which could be effectively converted to valuable products,

Because of the complex nature of a jet engine and the myriad of conditions under which such an engine may be used, jet fuels are required to meet a number of different specifications, which may include smoke point, aniline point, and luminometer number. Due to the low temperature encountered by jet engine-powered aircraft in arctic climates or at high altitudes, a most important specification is that for freeze point. A jet fuel must have a low freeze point so that it will remain liquid and flow freely without external heating even at very low temperatures. It is apparent that the lower the freeze point of a particular jet fuel, the more suitable it will be for operation under these conditions of extreme cold.

Consequently, it would be desirable to have a process wherein a high yield of low freeze point jet fuel could be produced While maintaining cracking catalysts in good condition and minimizing the production of secondary products such as coke. It would further be desirable if in such a process undesirable components and heavy oils for which there is a reduced market demand could be 3 converted into valuable jet fuels and other middle distillates.

SUMMARY OF THE INVENTION It has now been discovered that a high yield of jet fuel having a low freeze point can be obtained from a heavy oil feed by a process wherein said heavy oil feed is catalytically cracked in a catalytic cracking zone and the effluent of the zone fractionated into a plurality of products including at least a hydrocracker feed fraction and a coker feed fraction. The coker feed fraction has a higher average boiling point than does the hydrocracker feed fraction. At least a portion of the coker feed fraction is passed to a coking zone and converted by coking into a plurality of products, including at least coke and coker gas oil. At least a portion of the coker gas oil and at least a portion of the hydrocracker feed stream are combined, and the combined materials are hydrocracked to produce a plurality of products including low freeze point jet fuel. Feed streams of heavy oils from the same or other sources may be added, if desired, to the heavy oil feed, to the catalytic cracker, and/or to the material being converted in the coking zone. Preferably, the coker feed fraction is the bottoms from the catalytic cracker efllulent fractionation.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates schematically the basic steps of this process. It also illustrates a preferred embodiment of the process wherein a second feed enters the coking zone. This second feed may be, if desired, a heavier portion of the oil from which a lighter portion has been selected as the catalytic cracker feed;

FIGURE 2 illustrates schematically one method of blending the catalytic cracker product and the coker distillate into hydrocracker feed. This scheme is an alternative to that illustrated in FIGURE 1 wherein the coker distillate passes directly to the hydrocracker;

FIGURE 3 illustrates schematically alternative arrangements of the coking drums, fractionator, and feed pipes; and

FIGURE 4 illustrates one preferred embodiment of the process of this invention wherein the catalytic cracker fractionator bottoms comprises the coker feed fraction and further illustrates schematically how the process of this invention may be integrated into an over-all refinery arrangement.

DETAILED DESCRIPTION OF THE INVENTION The process of this invention can best be understood by reference to the drawings. The basic steps of the proc ess are illustrated schematically in FIGURE 1. A heavy oil feed enters catalytic cracking zone 1 through line 2 and undergoes a moderate amount of cracking. A plurality of products, which may include gasolines and light ends, is passed through line 3 to catalytic cracker fractionation zone 4 and therein separated into several different fractions. A cycle oil fraction which contains materials boiling in the range of from about 400 to about 900 F. is withdrawn from catalytic cracker fractionation zone 4 through line 5 and passed to hydrocracking zone 6. Generally the whole cycle oil fraction will constitute the hydrocracker feed and will be passed to zone 6, but if desired a small portion may be removed through line 13 for other processing. A fraction containing primarily heavier materials which may boil in the range above 850 F. is withdrawn from fractionation zone 4 through line 7 and passed to coking zone 8. This material may, if desired, be combined with additional hydrocarbons which enter coking zone 8 through line 9. In coking zone 8, at least a portion of this heavier fraction is converted to coker products. Unconverted materials may be withdrawn from zone 8 through line 14. The products of coking zone 8 include coke and, usually, light materials boiling below 400 F. The principal product, however, is a gas oil fraction boiling in the range of approximately 400 850 F. This gas oil fraction is withdrawn from coking zone 8 through line 10 and passed to hydrocracking zone 6 wherein it is combined with the cycle oil fraction which entered hydrocracking zone 6 through line 5. The gas oil fraction may be treated in the manner described above for the cycle oil fraction; i.e., generally the entire material will be passed to hydrocracking zone 6, but, if desired, a small portion may be removed through line 15 for other processing. The combined materials are passed over hydrocracking catalysts in the presence of hydrogen which enters zone 6 through line 11 and are converted into valuable jet fuels having low freeze points and other products which are withdrawn from hydrocracking zone 6 through line 12. Hydrocracking zone 6, which will be described in more detail below, may be either a singlestage or two-stage hydrocracker. If desired, other types of feed may be added to hydrocracking zone '6 to be reacted in combination with the coker gas oil and the catalytic cracker cycle oil.

The unexpected advantages of this process over other processes are illustrated in the table below. In this table are compared the properties of 320-500 F. hydrocracked product-s derived from different types of feeds. It will be seen that the jet fuel product obtained by the process of this invention, wherein a portion of the heavier catalytically cracked material is coked prior to hydrocracking in the presence of uncoked catalytically cracked material, is far superior in freeze point characteristics to the jet fuel products obtained from other sources.

The above table shows that the process of this invention produces the highest yield of jet fuel product and that this product has a freeze point which is much lower than that which is characteristic of jet fuels produced by other methods.

Typical detailed embodiments of the coking zone and catalytic cracking zone of this invention are illustrated schematically in FIGURES 2 and 3. In FIGURE 1, it was shown that the coker gas oii may be passed directly to the hydro-cracking zone or may be combined with the catalytic cracker cycle oil just prior to entering the hydrocraking zone. An alternative to this procedure is illustrated in FIGURE 2. The feed to catalytic cracking zone 1 again enters through line 2. After moderate cracking, the efiluent is discharged through line 30 and is eventually passed into catalytic cracker fractionation zone 4. The bottoms fraction is withdrawn from zone 4 through line 7 and passed to coking zone 8. In coking zone 8, as previously described, coke is produced and withdrawn through line 31; and a liquid fraction is also produced and withdrawn through line 32. Additional feed may, if desired, be added through line 9. The liquid fraction which contains the coker gas oil, as well as any light materials, is passed through line 32 to be combined with the catalytic cracker effluent in line 30 and passed to catalytic cracker fractionation zone 4. In zone 4, a continuous separation produces a plurality of products including the previously mentioned bottoms fraction and a hydrocracker feed fraction which is withdrawn from zone 4 through line 33. This hydrocracker feed fraction contains the coker gas oil and catalytic cracker cycle oil which are passed to hydrocracking zone 6 as illustrated in FIGURE 1. In addition, lighter fractions, such as light ends and gasoline, may be produced and withdrawn from zone 4 through lines 34 and 35.

Typical embodiments of the coking zone are illustrated in FIGURE 3. The coker feed fraction, preferably the bottoms fraction from zone 4, illustrated in FIGURES 1 and 2, enters the coking zone through line 7. In one embodiment, the coker feed fraction continues on through line 60 to line 61 where it may, if desired, be combined with a fresh feed which enters through line 9. The coker feed fraction is passed through line 61 into one or more coking drums 62. In the coking drums, coke and a liquid product are produced. A particularly preferred embodiment requires the use of two or more coking drums. In this embodiment, at least one of the coking drums is out of service at any one time so that the deposited coke may be removed from it through line 63. The remaining coking drums are on-stream and are being used to convert the coker feed fraction to liquid product and coke which deposits in the coking drums. Generally a schedule is maintained such that each drum may be shut down and decoked in regular rotation. The liquid product is removed from the on-stream coking drums through line 64 and passed into coker fractionator 65 wherein it is separated into a plurality of products, which may include light ends and gasoline, removed through lines 66 and 67. Coker gas oil is also produced and is removed from coker fractionator 65 through line 68 and many be passed directly to hydrocracking zone 6 as illustrated in FIGURE 1 or passed back to catalytic cracker fractionation zone 4 as illustrated in FIGURE 2. if desired, a recycle stream of heavy material may be taken as the bottoms of coker fractionator 65 and passed through line 69 back to the coking drums. A small portion of this recycle stream may be withdrawn through line 71 for further processing or to prevent the buildup of contaminants in the recycled materials. As a preferred alternative to this embodiment, catalytic cracker fractionator bottoms from zone 4 may enter through line 7 and then pass through line 70 to line 64 and fractionator 65 wherein some of the light materials and gas oil will be separated and may be withdrawn through lines 66, 67 or 68 as desired. A substantial portion of the material, however, will pass through line 69 to the coking drum wherein it will be converted to coke and liquid products as heretofore described. Because some of the components in the material from zone 4 are separated by coker fractionator 65 prior to coking, a coking zone of lesser capacity is required than in the embodiment previously described.

Catalytic cracking zone 1, as used in the process of this invention, is a conventional catalytic cracker. It is preferably of the well-known fluid bed type or the gas lift or bucket elevator moving bed type wherein a cracking catalyst, such as silica-alumina powder or pellet, or zeolite, is continuously circulated between a reaction zone and a regeneration zone under reaction zone conditions of 8001,150 F. and regeneration zone conditions of 8501,400 F., at a pressure of about 0-100 p.s.i.g. The manner of controlling the catalytic cracking process, primarily by means of the catalyst circulation rate and feed preheat temperature to maintain a balance between the heat released by burning coke from the catalyst in regeneration and the heat absorbed by the cracking reaction, is well known.

Coking zone 8 may comprise either one of two different types of coking steps; namely, delayed coking or fluid coking. Delayed coking uses a pipestill heater operating with a maximum of 900950 F. heater outlet temperature in order to avoid coke formation in the heater tubes and transfer lines. There are usually at least two coke drums connected to a coking heater, some drums being on-stream while the others are being cleaned and prepared for use. The coke drums operate at a lower temperature, usually in the range of 800-900 F. because of flashing and endothermic heat of reaction, and the drums provide long residence time favoring the formation of coke and lighter products. The drums usually operate at 40-50 p.s.i.g. The superficial vapor velocities in the drum are of the order of 0.2 foot per second; and, with 6 a drum height of approximately 40 feet, residence time in the drum is of the order of 200 seconds, and total residence time, if the heater tubes and transfer lines are included, will be somewhat higher.

Fluid coking differs fundamentally from delayed coking in that contact times are much shorter and the reaction temperatures higher because the necessary feed preheat and heat of reaction are provided by the circulation of hot coke particles from a burner which uses the coke product itself as a fuel. With reactor velocities in the range of 2 feet per second, the residence times are in the order of 15-20 seconds at substantially isothermal reaction temperatures, normally 9501,000 F. Operating pressures in the fluid coker reactor are ususally about 10 p.s.i.g. and theoretically, at least, the lower pressures, compared to delayed coking, favor the cracking reaction which is accompanied by a very large increase in the volume of vapor products. As a result of the higher temperatures and shorter residence time in the fluid coker reactor, gas oil fractions in the feed tend to flash off rather than crack. The fluid coker recycle stream is generally a 1,000 F.+ material. With the fluid coker reactor normally operating at about 950 F., the heavy gas oil recycle will be converted at this temperature predominantly in the vapor phase and will be severely degraded to gas because of the relatively high vapor cracking intensity and because of the high recycle rate. Fluid coker gas oil usually will have an end point of about 9501,000 F. compared to 800-950 F. for the delayed coking process.

Hydrocracking zone 6 may be either a oneor twostage unit, and may include some degree of recycle. Dinitrification by contacting the feed in the presence of hydrogen over a sulfactive hydrogenation catalyst, such as Groups VI and VIII metals or compounds thereof on alumina or other supports, at elevated temperatures and pressures, may be included as part of the hydrocracking, but is not necessary in every case. Hydrocracking zone conditions generally comprise a hydrogen feed rate of 2,000-30,000 s.c.f./bbl. of feed oil, and preferably 2,00O 15,000 s.c.f./bbl.; a liquid hourly space velocity of 0.2- 150, preferably 0.4-3.0; a pressure of at least 1,000 p.s.i.g., preferably 1,0003,000 p.s.i.g.; and a temperature of 400950 F. The preferred initial on-stream temperature is about 500650 F., with a progressive increase to the preferred operating range of 750 950 F. in order to maintain catalyst activity at a controlled evel.

The catalyst employed in the hydrocracking zone is one wherein a material having a hydrogenating-dehydrogenating activity is deposited or otherwise combined with a catalyst support. The cracking component may comprise any one or more nonacidic, weakly acidic or strongly acidic materials such as silica, alumina, bauxite, silicaalumina, silica-magnesia, silica-alumina-zirconia and the like, as well as various zeolites, acid-treated clays, and similar materials. The hydrogenating-dehydrogenating component of the catalyst can be selected from any one or more of the various Groups VI, VII and VIII metals, as well as the oxides and sulfides thereof, alone or together with promoters and stabilizers. Examples of suitable hydrogenating-dehydrogenating components are the oxides and sulfides of molybdenum, tungsten, vanadium, chromium, and the like, as well as metals such as iron, nickel, cobalt and platinum. More than one hydrogenating-dehydrogenating component may be present, and favorable results may be obtained with catalysts containing composites of two or more of the oxides of molybdenum, cobalt, chromium, tungsten, nickel, tin, and zinc, and with mixtures of said oxides with fluorine. The amount of the hydrogenating-dehydrogenating component can be varied within wide limits from about 05-30% based on the weight of the entire catalyst.

FIGURE 4 illustrates schematically a typical refinery operation in which the process of this invention is integrated with conventional refinery processes. It also illustrates schematically one manner in which additional heavy oil feed streams can combine with the basic feed streams of this process. In this example, as asphalt-containing crude oil is passed through line 100 to first distillation zone 101 wherein it is separated into a plurality of streams including light ends which are removed from zone 101 through line 102, straight-run gasoline removed through line 103, and straight-run middle distillate removed through line 104. A straight-run gas oil is removed through line 105. The asphalt-containing residue is passed through line 106 to second distillation zone 107. First distillation zone 101 is customarily an atmospheric distillation column while second distillation zone 107 is a vacuum distillation column. In zone 107, additional gas oil is separated from the residual oil and may be passed through line 108 directly to catalytic cracking zone 1. However, it is preferred to pass the additional gas oil through line 109 to be combined with the gas oil in line 105 to provide gas oil to blending zone 110. Also in zone 107, asphalt is produced and removed through line 111. The principal product of zone 107 is a heavy feed for the coker which is withdrawn through line 112.

Meanwhile, a second crude oil is being passed through line 113 to third distillation zone 114 wherein the oil is separated into a plurality of products, such as light ends, which are removed through line 115, straight-run gasoline removed through line 16, and straight-run middle distillate removed through line 117. Through line 118, straight-run gas oil is passed to blending zone 110 wherein it is combined with the straight-run gas oils from zones 101 and 107. A residual fraction is withdrawn from zone 114 through line 119 and passed to residuum stripping zone 120. In zone 120, the material in line 119 is separated into gas oil which is withdrawn through line 121 and residual oil which is withdrawn through line 122 and combined with the residual oil from zone 107 in line 112 to produce straight-run coker feed which is passed through line 123 to coking zone 8.

The feed to catalytic cracking zone 1 which enters through line 124 comprises the straight-run gas oil from blending zone 110 which passes through lines 125 and 126 as well as the gas oil in line 121 and, if desired, the gas oil in line 108. The effiuent of catalytic cracking zone 1 passes through line 3 to fractionation zone 4 wherein it is fractionated into a stream containing butanes and lighter materials which is removed through line 34, C 400 F. cracked gasoline which is removed through line 35, 450800 F. cycle oil which is removed through line 33, and 850 F fractionator bottoms which is removed through line 7 and passed to coking zone 8. In typical refinery operations, line 7 will contain some sort of intermediate tankage or settling apparatus to allow separation of any catalyst particles which might have been carried out with the fractionator bottoms from the catalytic cracking zone. If desired, a portion of the cycle oil may be removed from line 33 through line 127.

The fractionator bottoms in line 7 and the residual oil in line 123 are combined and coked as heretofore described in coking zone 8. Coke is removed through line 31 while the liquid product is passed through line 64 to fractionator 65. In fractionator 65, the liquid product is separated into a stream containing butanes and lighter gases which is removed through line 66, C -3 80 F. coker gasoline removed through line 67, and 380-850 F. coker gas oil removed through line 68. This gas oil is combined with the gas oil from line 125 and the cycle oil in line 33; and the combined materials are passed through line 128 to hydrocracking zone 6. Hydrogen is introduced into hydrocracking zone 6 through line 11. The

hydrocracked efiluent is withdrawn through line 12 and passed into separation zone 129 wherein it is separated into a gasoline fraction which is withdrawn through line 130, light and heavy naphtha fractions withdrawn respectively through lines 131 and 132, and jet fuel and middle distillate which are withdrawn through line 134.

The above-described example is given for illustrative purposes only. It is apparent that many widely diiferent embodiments of this invention may be made without departing from the scope and spirit thereof; and, theerfore, it is not intended to be limited except as indicated in the appended claims.

I claim:

1. An improved process for the production of jet fuel having a low freeze point for a heavy oil feed which comprises catalytically cracking in a catalytic cracking zone said heavy oil feed, fractionating the cracked oil into a plurality of fractions, including at least a hydrocracker feed fraction and a coker feed fraction, said coker feed fraction having a higher average boiling point than said hydrocracker feed fraction, passing at least a portion of said coker feed to a coking zone, converting said portion in said coking zone to a plurality of products, including at least coker and coker gas oil, combining at least a portion of said coker gas oil with at least a portion of said hydrocracker feed into a combined feed, hydrocracking said combined feed in the presence of hydrogen over a hydrocracking catalyst in a hydrocracking zone, and recovering from said hydrocracking zone a plurality of products, including at least a jet fuel having a low freeze point.

2. The process of claim 1 wherein at least a second heavy oil feed is catalytically cracked simultaneously with said first heavy oil feed.

3. The process of claim 1 wherein at least one heavy oil is combined with said coker feed prior to said conversion of said coker feed in said coking zone.

4. The process of claim 3 wherein said jet fuel has a freeze point of less than -70 F.

5. The process of claim 1 wherein said coker feed fraction comprises the bottoms fraction from the fractionation of the eflluent from said catalytic cracking zone.

6. The process of claim 5 wherein said bottoms fraction is combined with at least a portion of the efliuent from said coking zone and the combined materials are simultaneously fractionated to produce a plurality of streams including a heavy stream which comprises 10-40 volume percent of said bottoms fraction, and then passing said heavy stream to said coking zone.

7. The process of claim 1 wherein at least a portion of said coker feed fraction is passed to a coker fractionator wherein it is combined with at least a portion of the effluent of said coking zone, the combined materials are fractionated in said coker fractionator into a plurality of streams, including at least one coker bottoms stream, and said coker bottoms stream is passed into said coking zone.

References Cited UNITED STATES PATENTS 3,132,087 5/1964 Kelley et al 708-60 3,193,486 7/1965 Payne 208-50 3,245,900 4/1966 Paterson 208-56 DELBERT E. GANTZ, Primary Examiner A. RIMENS, Assistant Examiner US. Cl. X.R. 20856, 60, 111

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