US2721168A - Seed coke production in fluid coking systems using oxidation to increase friability - Google Patents

Description

Oct. 18, 1955 c. N. KIMBERLIN, JR, ET AL 2,721,168

SEED COKE PRODUCTION IN FLUID COKING SYSTEMS USING OXIDATION To INCREASE FRIABILITY Filed Oct. 14, 1954 2 Sheets-Sheet l 3 CONVERSION l T PRODUCTS 28 29 Q. FLUE GAS i 1/ I MWWWVBURNER RESIDUAL 6 OIL so ihi-UEL AIR STEAM IO T :1 l4 1 5 2 FLUE j GAS ELUTRIATOFM. L L 9 fi wg umzER i; dag- 3! -5 3 AIR NE COKE 25 PRODUCT ATTRITING GAS Figurel Charles N. Kimberlin, Jr. Ralph B. Mason Inventors Robert W Krebs Oct. 18, 1955 c. N, KIMBERLIN, JR, ET L 2,721,168

sEED COKE PRODUCTION IN FLUID comm; SYSTEMS USING OXIDATION TO INCREASE FRIABILITY Filed Oct. 14, 1954 2 Sheets-Sheet 2 CONVERSION FLUE 3 PRODUCTS GAS FLUID BURNER COKER RESIDUAL COKE PRODUCT Charles N.Kimber|in, Jr. Ralph B. Mason Inventors Robert W. Krebs By CM Attorney United States Patent SEED COKE PRODUCTION IN FLUID COKING SYSTEMS USING OXIDATION TO INCREASE FRIABILITY Charles N. Kimberlin, Jr., Baton Rouge, Ralph B. Mason, Denham Springs, and Robert W. Krebs, Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware Application October 14, 1954, Serial No. 462,206

18 Claims. 01. 202-28 This invention relates generally to an improved hydrocarbon oil fluid coking system wherein heavy oils, containing constituents nonvaporizable without cracking, are subjected to pyrolysis by contact with a fluidized bed of high temperature particulate solids. This invention is concerned with the benefaction of the petroleum coke produced by the fluid coking of oils, and is also concerned with an improved method of producing seed coke in a fluid coking process by subjecting a portion of the coke in the process to controlled oxidation to increase its friability, and reducing in size the coke so treated.

There has recently been developed a coking process for pyrolytically upgrading petroleum oils which utilizes the fluidized solids technique. In this coking process, as is known by the art, a mass of fluidized, high temperature solids, usually coke formed by the process, is used to supply heat for converting oils. A charging stock to be upgraded is sprayed into the fluid bed and upon contact with the high temperature solids undergoes pyrolysis, evolving lighter hydrocarbon vapors and depositing carbonaceous residue, i. e., coke, on the solids. Usually, in order to maintain the coking temperature, a portion of the solids is continuously circulated to an external heating zone and back.

In past processes, the reheating of the coke to supply the process heat requirements is usually carried out by partially combusting the coke in a combustion zone. Thus the coke is oxidized in small increments with intermediate deposition of fresh coke. In each passage of the coke particles through the burning zone about 0.1 to 1.0% of their weight is removed by burning and in subsequent passage through the coking reaction zone their weight is augmented by about 1 to 5% by the deposition of fresh coke.

Because of the deposition of coke on the contact particles, the particles increase in size and steps must be taken to counteract this growth. In fluidized solids systems, it is highly desirable, if not necessary, to maintain an optimum mass, size, and size distribution of fluidized solids. If the solids are too fine, they are readily lost from the system as by entrainment in the fluidizing gases, and if they are too large, fiuidization becomes difficult, particularly within the range of fluidizing gas velocities used, and the circulation of the solids is hindered. It is preferred to operate with particles in the size range of about 40 to 800 microns, although the size range of the particles may vary somewhat beyond these limits, e. g., from to 1000'microns.

Because of this growth in the size of the particles, portions of the coke must be withdrawn from the system to maintain the total mass or weight inventory of the solids substantially constant, and small size particles, i. e., growth nuclei, or seed coke must be supplied to the system to keep the numerical inventory of particles substantially constant. Preferably this withdrawal and addition are done continuously. Experience indicates that a desirable seed coke size is within the range of 50 to 200 microns. Below this range, particles agglomerate readily and/ or are blown Fates-lied Got. 18, 1955 from the system and above this range the amount of material that must be comminuted to maintain a size balance in the coking system becomes prohibitive.

It has been found that the amount of coke produced by a fluid coking system is related to the Conradson carbon of the feed. For feed stocks ranging from about 10 to 40 wt. per cent Conradson carbon, e. g. 24 wt. per cent, about 11 to 45 Wt. per cent, e. g., 26 wt. per cent, based on feed, of coke will be produced and about 15 to 45 wt. per cent, e. g., 25 wt. per cent, of this will be burnt to supply heat, the remainder being withdrawn as product. Of the coke in the system, about 0.01 to 0.1 lb./lb. of feed, e. g., 0.07 lb./ 1b., has to be reduced in size in some manner to form seed coke in order to maintain the size and size distribution of the coke particles in the system substantially constant.

As has been appreciated by the art, the coke produced in fluid coking systems is very hard and resistant to grinding. Further, because of its hardness, sulfur content, density, etc., it is not of a quality suitable for use in most carbon consuming processes. Consequently, there has been a desideratum for a method of benefacting fluid coke.

In light of the above, it is an object of the present invention to provide the art with a simple and inexpensive process for comminuting coke in a fluid coking system. Broadly, however, the process of this invention will find application to the general benefaction of fluid petroleum coke other than to improve its grindability. It is another object of this invention to produce a net coke product from a hydrocarbon oil fluid coking process having improved properties. These properties include those that make the coke more easily ground, make the coke more active toward chemical or surface reactions, make the coke more amenable to hydrogen desulfurization, and make the coke of a higher density.

The nature of this invention and other objects and advantages will become more clear from the ensuing description during which the attached drawings, forming a part of this specification, are described in detail. Figure I of the drawings illustrates a preferred embodiment of this invention, adapted to achieve its objects. Figure II illustrates a modification of the invention wherein a somewhat simplified arrangement of equipment is used.

It has now been found that the characteristics of particulate fluid petroleum coke, particularly its friability, may be greatly improved by controlled oxidation of the coke. More specifically, it has been found that oxidizing the coke with a free oxygen-containing gasiform medium to remove 3 to 50%, preferably 5 to 20%, of its weight causes the remaining coke to be very friable besides improving other properties. This oxidation treatment-may be carried out on an amount of coke suflicient to'meet seed coke requirements of the coking process, or an amount sufi'icient to supply the net product coke, but is, preferably, used to concurrently provide seed coke and product coke of improved properties. Although this oxidation treatment may be carried out at a temperature in the range of 600 to 1500 F. it is preferred to maintain the oxidation reaction at a temperature below 1000 F. I

' The lower the temperature at which the oxidation is carried out, the greater the improvement in the properties of the coke for a given percentage of coke removed by oxidation. However, the lower the temperature the greater is the time requirement for the oxidation treatment. Therefore, in selecting the temperature at which to conduct the oxidation, the advantages in improved product quality for the lower temperatures, within the operable range, must be balanced against the disadvantage of the longer time required for the oxidation.

At a temperature in the range of 600 to 1000 F.,

the time of treatment may vary between 10 mins. to 20 hrs., depending upon the temperature within this range, the nature of the coke, and the oxygen concentration of the gas.

The extent of the oxidation of the coke in accordance with this invention is to be compared to a conventional fluid coking process wherein only about 0.1 to 1.0% of the coke is burnt per pass. Such a situation obtains where about 3-5 wt. per cent coke on feed is burnt in the burner to supply heat to the process and a coke/ oil circulation rate of from 5 to 30 is employed. The burning of at least 3 wt. per cent of a coke particle without intermediate deposition of fresh coke has been found necessary to enhance its properties, particularly its friability.

It has also been found that by conducting this oxidation operation in a fluidized solids system, the internal attrition within the fluidized bed of the friable particles created by the oxidation will account for a substantial production of finer particles. However, in a preferred embodiment of this invention, the oxidized coke is subjected to a mechanical size reduction to produce further amounts of finer particles of seed coke size. Preferably this mechanical size reduction is attained by jet attrition within the fluid oxidation bed.

An alternative mode of operation of this invention is to classify, as by elutriation, the coke withdrawn and subjected to the oxidation in order that only relatively coarse coke is subjected to this size reduction process. In this way, needless treatment of coke particles already in the desired size range .is avoided. In comparison with the above, another alternative which may be used is to classifythe material from the grinder to segregate a relatively fine fraction and a coarse fraction, the fine fraction being returned to the coking process as seed coke and the coarse fraction being recycled to the size reduction system or being removed as net coke product of the process. Recycling of the coarse fraction is desirable in that it has been found that coke, once subjected to grinding, requires less power to be further reduced in size.

In brief compass, this invention proposes an improved petroleum oil fluid coking process which comprises contacting a heavy petroleum oil containing constituents nonvaporizable without cracking in a coking zone with fluidized particulate coke maintained at a coking temperature to secure relatively lighter hydrocarbon vapors and carbonaceous residue which is deposited on the par ticulate coke causing it to accrete in size, withdrawing the vapors from the coking zone as product, withdrawing coke from the process as net coke product whereby the inventory of coke is maintained substantially constant, withdrawing another portion of the coke and fluidizing it with a free oxygen-containing gasiform medium, e. g., air, in a combustion zone to burn 3-50 wt, per cent, preferably 5-20 wt. per cent, of the coke at a temperature in the range of 600 to 1500" F., preferably 600 to 1000 F., whereby the coke becomes friable and a substantial amount of fine particles are produced by attrition, and returning the coke so oxidized to said process as seed coke. In a preferred operation, the oxidized coke is mechanically comminuted to produce further amounts of fine particles. In a subsidiary mode of operation, conducted separately or in conjunction with the above, the net coke product of the process is also withdrawn from the oxidation zone as a fluid coke having substantially improved properties. In

tions. The data were collected by subjecting fluid coke to attrition by a jet of air in a standardized attrition test conducted in a Roller analyzer, described hereinafter. The fluid coke employed had a median particle size of about 235 microns and was produced by coking Hawkins residuum at 1000 F. in a semi-commercial fluid coking plant of 100 bbls./day capacity. The Hawkins residuum feed was obtained by the vacuum distillation of crude oil from the Hawkins field. The residuum had a gravity of 43 API, aConradson car: bon of 26 wt. percent, an initial'boiling point of 882 F. (atms; press. equivalent), and a 10% point of 1010 F. In the Roller attrition test, the particle size analyzer manufactured by the American Instrument Company is employed. In this test, 15 cc. of fluid coke are subjected to the action of a jet of air at a rate of 21 liters per minute and a jet velocity of about 450 F./S. fora period of 5 hours. Fines of a particle size of 10 microns or less produced by the jetting action are elutriated from the bulk of the sample, are passed through a settling and classifying zone, and are collected in a filter. The amount of fines collected is weighed at hourly intervals and is reported as percent of the charge attritted per hour. a

In comparison to the above, the marked advantage of an oxidation treatment of the fluid coke, in accordance with the teaching of this invention, is shown-by'the folanother alternative operation, augmenting the above, the

lowing Table II. by oxidizing samples of fluid coke at several oxidation conditions and then subjecting the oxidized samples to the Roller attrition test as described above. The coke employed had a median particle diameter of about 235 microns and was produced by coking Hawkins residuum at 1000 F. as described above. The oxidation was carried out by treating a fluidized bed of the fluid coke with air at a superficial velocity of 0.2 ft./sec. at the temperatures and for the times indicated in the table. In Table II, the first column shows the oxidation conditions, i. e., the temperature and the time of treatment with air. The second column shows the weight percent yield of oxidized coke obtained from the oxidation treatment. The third column shows the weight percent of fines produced in the coke by the oxidation treatment itself, i. e., without the coke being subjected to any extraneous grinding or attrition. Column four shows the hourly average rate of attrition of the oxidized coke'in the Roller attrition test.

The data in Table II were collected.

It can be seen from a comparison 'of the above two tables that oxidation of fluid coke greatly increases its friability. Further, it is evident from Table 11 that the lower the temperature of the oxidation, within the operable range, the greater the improvement in the friability of the coke for a given yield loss. Thus, although the friability of the coke may be improved by burning away at least 3%, preferably more than 5%, of its weight 5 at temperatures as high as 1500 F., the preferred temperatures for the oxidation are within the range of 600 to 1000" F. and even within this more restricted range the greatest improvement in quality for a given loss in coke yield is obtained at the lower temperatures within the range. However, the lessened eflect of oxidation at the higher temperatures within the operable range, may be compensated for by accepting a greater loss in yield of the oxidized coke, and, because of the slower rate of oxidation at the lower temperatures, the greatest improvement in coke quality for a given time period of oxidation is obtained at the higher temperatures of oxidation. Therefore, in selecting the temperature, within the operable range, at which to conduct the oxidation under a given set of circumstances, it is desirable to weigh the greater time requirement at a lower temperature against the greater yield loss at a higher temperature for a given improvement in quality.

The improved grinding characteristics of coke oxidized in accordance with this invention can be further illustrated by means of ball milling tests. In this test, a portion of fluid coke produced by coking Hawkins residuum was oxidized with air in a fluid bed at a temper-- ature of 650 F. for a period of 13 hours. The superficial velocity of the air during the oxidation was 0.2 foot per second. The yield of oxidized coke'was 93 wt. percent based on the original coke charge. For the ball milling test 12 pounds of oxidized coke and 60 pounds of 1 inch diameter steel balls were charged to a 3 gallon, 10.5 inch I. D. ball mill. The ball mill was rotated at a rate of 36 R. P. M. for two hours. Another 12 pound portion of nonoxidized coke was subjected to ball milling under exactly the same conditions. Table III shows screen analyses of the original unoxidized coke, of the unoxidized coke after ball milling, and of the oxidized coke after ball milling. The screen analyses are reported as the cumulative Weight percent retained by screens of the indicated sieve size. It can be seen from Table III that the preoxidation of the coke increased the rate of production of fines below 74 microns size during ball milling by a factor of five and the rate of production of fines below 43 microns size was increased by a factor of thirty-four.

Table 1H Sieve Size Cumulative Percent Retained After Ball Milling Opening,

Mesh Number Microns Unoxidjzed Oxidized I To show in another way the advantage of the process of this invention, with a fluid coke produced by fluid coking of a residual oil at 950 F. having an average size of about 350 microns (by screen analysis), about horsepower hours/ton are required with an efficient ball mill to produce seed coke of a size that passes a 200 mesh (74 micron) screen. By oxidizing the same coke, at a temperature of 650 F. such that 7 wt. per cent of the coke is consumed, only about 15 horsepower hours/ton of original coke is required to produce the same amount and quality of seed coke. 75

With particular reference to the attached drawing, a preferred coking process incorporating the teachings of this invention will be described.

A heavy, low value oil is injected by line 1 into a coking vessel 2. Charging stocks customarily used in fluid coking operations include petroleum tars, asphalts, vacuum and atmospheric residua and similar high-boiling, low value heavy oils. Broadly, however, charging stocks suitable for use in the present process include shale oils, synthetic oils, asphalts, tars, coal tars, extracts, cycle stocks, whole crudes, distillates and residual fractions therefrom, etc. or mixtures thereof.

There is contained in the coking vessel 2 a fluid bed of particulate coke maintained at a coking temperature in the range of 850 to 1500 F., preferably 900 to 1000 F. when gas oils suitable for catalytic cracking are desired as the major product. The oil, upon contact with the high temperature coke particles, undergoes pyrolysis evolving substantial proportions of relatively lighter hydrocarbon vapors and depositing carbonaceous residue on the coke particles. The vapors along with the fluidizing gas, after having the entrained solids removed, are taken off overhead via line 3. The heavy high boiling ends of these vapors may be condensed or recycled to the coker and the remainder can be processed by conventional means, e. g., fractionation, to recover the various product fractions desired. Steam or other substantially inert gas is admitted to the base of the coker by line 4 as fluidizing gas. As is known by the art, the rate of fiuidizing gas addition is controlled such that the superficial vapor velocity is within the range of about 0.2 to 5 ft./sec., suflicient to cause the coke particles in the coker to form a dense ebullient mass having a well defined upper level. This fluidizing gas first serves to strip the coke particles of adhering hydrocarbons in the lower portion of the coker and then flows upwardly, fiuidizing the main bed.

Stripped coke is continuously removed from the coker by line 5, and a portion of it is transferred to a combustion vessel, e. g., a fluid bed burner 6, wherein it is reheated to a temperature of to 300 F. above the coking temperature. Usually no more than about 0.1 to 1.0 wt. per cent, e. g., 0.3 wt. per cent, of this coke per pass will be consumed in the burner. The reheated coke is circulated by line 7 to the coker. As will later appear, a major portion or all of the contents of line 5 may be subjected to other processing, i. e., classification, before being transferred to the heater.

Air or other oxidizing gas is injected into the burner vessel 6 by line 27. This air serves to fluidize coke contained in the vessel and supports a partial combustion of the coke, thereby reheating the coke. The gaseous combustion products, after having the entrained solids removed, are taken off overhead by line 28 and vented to the atmosphere. if desired, heat exchange means 29 may be used to recover heat from the flue gases as by heat exchange with various feed streams to the process or to generate steam. In applications where the value of the coke product of the process is greater than other extraneous fuel sources, other fuels may be preferentially combusted in the burner to supply heat to the coke particles. Thus'a heavy oil or light tail gases can be injected into the vessel 6 via line30 to be burnt.

In accordance with this invention, a portion, or all, of the stripped coke in line 5 is transferred by line 8 to an oxidation vessel 9. This portion of coke may be cooled as by quenching with water to the oxidation temperature. In the oxidation vessel, it is subjected to oxidation at the temperaturein the range of 600 to 1000" F., sufficient to consume 3 to 50 wt. percent of the coke. In some cases this oxidation Zone can conveniently be integrated into the lower portion of burner 6. A free oxygen-containing gas is admitted to vessel 9 by line 12 and fluidizes the solids therein. Preferably this gas comprises air. In some applications, however, the oxygen V to the process.

7 concentration of the air may be decreased as by dilution with flue gases, steam, etc., or oxygen-enriched gases such as air-oxygen and oxygen-steam mixtures may be used. This oxidation, as before explained, increases the friability of thecoke, aside from other properties, and causes attrition of the coke. The oxidized coke may then be transferred by lines 13 and 14 to the coking vessel. The excess heat generated by the oxidation may be suitably removed as by steam generating coil or other cooling means inserted within the fluidized bed.

Flue gases are removed overhead from vessel 9 after having entrained solids removed, by line 10. The heat content of the flue gases may be suitably extracted, as

by heat exchange in heat exchanger 11 with feed streams Because of the low temperatures of oxidation that may be used in the oxidizer, all of the oxygen in the air may not be consumed. In some applicationsof this invention, it is advantageous to transfer the contents of line 10 without cooling, to the burner vessel to serve as fluidizing and oxidizing gas therein. This re-use of the partially spent air saves compressor investment, and has the further advantage that the sensible heat of the gas is utilized in burner 6.

For the processing sequence above described, wherein only a sufficient amount of coke is oxidized to supply the necessary seed coke to the process without the use of extraneous grinding facilities, about 1 to 10%, e. g., 3% of the circulating coke stream will be subjected to oxidation at a temperature in the range of 600 to 1000 F., e. g., 800 F. to consume 5 to wt. per cent of the material in the oxidation zone, e. g., 7 wt. per cent,

to produce seed coke of a size in the range of 50 to 200 microns for the coking process. In this way, the size andsize distribution of the heat-carrying particulate coke in the coking process is maintained.

In a preferred embodiment of this invention, more coke than is necessary for seed coke production is transferred through line 8 to vessel 9 and the net coke product of the process is removed by line 16. By so treating the net coke product, it has improved properties.

In another embodiment of the invention, all or substantially all, of the stripped coke removed from the coking reactor is transferred by line 17 to a classification zone, e. g., an elutriation vessel 18. This serves to conserveseed coke in the process. Elutriating gas is admitted to the base ofthe elutriator by line 19 in amounts suflicient to attain superficial velocities in the range of 5 to 20 ft./sec., e. g., 8 ft./sec. whereby fine coke is conveyed upwardly through the vessel and removed by line 20. Preferably this elutriating gas comprises steam,

'ever, be transferred directly to the coker via line 21.

The relatively coarse material having a size above about 300 to 600 microns, e. g.,above 500 microns, remaining in the elutriator is then transferred by line 22 to the oxidation vessel 9 for treatment therein.

In a particularly preferred processing sequence, friable oxidized coke is removed from vessel 9 and transferred 'by line 23 to an external grinding means.

As shown, a jet impact grinder or attriter is used and is preferred,

although other grinding means such as ball mills, roller mills, crushers, etc. may be used. The contents of line 23 are picked up by an attriting gas, e. g., steam, supplied by line 24, and accelerated to a maximum solids velocity in the range of 250 to 1000 ft./sec. The high velocity stream is then directed into an attriter drum 25 wherein its strikes a fixed target thereby causing the coke to be fractured and comminuted. The comminuted partil ticles are then transferred by line 14 to the coking vessel, the spent attriting gas being used to convey the solids.

Another alternative is to transfer the comminuted coke from the grinder and/or the burner by lines 31 and/or 32 to an elutriator. Advantageously, this elutriator may be the same one that is used to classify the feed to the oxidizer 9. In this way, only the finer part of the comminuted product will be returned to the coking system and the coarser fraction will be recycled for further treatment. By recycling material that has been partially comminuted, the power requirements necessary to obtain further size reduction are greatly reduced.

Advantageously this jet grinder may be operated to produce fine coke in amounts beyond that'required for seed coke requirements whereby a small particle size net coke product of the coking process can be produced suitable for use as a boiler or furnace fuel. Thus the jet attriter can be operated at higher solids velocities to produce an amount of material smaller in size than about to microns, e. g., 74 microns, which can then be separated and withdrawn as net coke product- 'The I coarser portion of the attriterproduct can then be returned to the process as seed coke.

From the above, it can be seen that this invention basically proposes that the properties of fluid coke, particularly its friability, may be improved by controlled oxidation or combustion of the coke. Because of the increase in friability of the coke during the oxidation in a fluid bed, the coke is attrited or comminuted and, under the proper conditions, a sufiicient amount of seed coke will be created to supply the needs of the coking process. The invention also proposes, however, as a further step, that the oxidized coke may advantageously be ground as by jet attrition'to augment the seed coke produced during the oxidation step. This invention further proposes that this process of oxidation of fluid coke With its consequent improvement in the coke properties may advantageously be used to treat'the net coke produced by the coking process, whereby a fluid coke of improved properties, i. e., improved friability, increased surface, increased density, and improved hydrod esulfurization characteristics are obtained. The present invention also proposes various refinements of the process, namely, classification of the coke charged to the oxidizer whereby only relatively coarse coke is treated, and classification of the oxidized and comminuted coke.

With particular reference to Figure II, an attractive alternative processing scheme and equipment arrangement will be described. The arrangement of processing equipment shown when it is desired to conduct the oxidation at the higher temperatures within the operable range, for example, at temperatures up to l500 F. 'Items of equipment similar to those of Figure I bear the same identification number. i. e., 3% or more, necessary to improve the properties of the coke product is obtained in this arrangement by staging the burning so that coke before withdrawal passes through a second stage of burning. The gas andheat generated in the second stage flow to the first stage,

There is shown a fluid coking vessel 2 and a burner vessel 6. The burner vessel is modified so that it con-' tains'an integral oxidation zone 35. Coke to be reheated is transferred from the coker 6 to the burner by line '15 quirements plus net product coke. The solids interchange between the two is impeded by the use of perforated plates; or baffles 35 or coarse packing, screens, etc. The burner vessel and oxidation zone are, however, in fluid in Figure II may be preferred.

The high percentage of burning,

communication. An oxidizing. medium, e. g., air, is admitted to the base of the oxidation zone by line 36. The air is admitted as high velocity jets, e. g., 600 to 1000 ft./sec. jet velocities whereby the friable coke is attrited. The superficial velocity of the gases in the oxidation zone can be adjusted so that finer coke particles below about 200 microns are conveyed upwardly through the oxidation zone past the perforated baffies and returned to the coking system. In this arrangement, the operating temperature of the oxidation zone is preferably higher than that of the burner. Therefore, heat flows from the secondary oxidation to the burner by means of the air-pluscombustion products stream. The temperature in the secondary burner can be adjusted by controlling the air temperature and/ or its oxygen concentration. It is most advantageous to have all, or nearly all, of the combustion necessary to supply heat to the coking process to occur in the oxidation zone. The net coke product in the process is removed from the oxidation zone by line 37. This coke is relatively coarse because of the elutriation attained in the oxidation zone 35. The coke may be suitably quenched or otherwise cooled to prevent ignition upon contact with the atmosphere before it is sent to storage.

From the above description of Figure II, it can be seen that this arrangement advantageously provides for, in a single vessel, the supplying of heat for a coking process, for improving the properties of the coke product, the elutriation of the coke charged to the oxidation zone, the attrition or grinding of the oxidized coke to provide seed coke, and the elutriation of the comminuted coke whereby only coarse coke is withdrawn from the product. In some applications, however, it may be desired to use this arrangement only to improve the properties of the coke product. In such a case, back flow of the solids from the second to the first zone and jet attrition in the second zone need not be provided for.

Other similar variations and alternative modes of operation will be apparent to those skilled in the art.

Having described the invention, what is sought to be protected by Letters Patent is succinctly set forth in the following claims.

What is claimed is:

1. In a hydrocarbon oil fluid coking process wherein an oil is pyrolytically converted in a coking zone by contact with fluidized particulate coke maintained at a coking temperature to gasiform conversion products and carbonaceous residue which is deposited on said solids, a method of supplying seed coke to said process which comprises withdrawing a portion of said particulate coke from said process and burning 3 to 50 wt. per cent of the coke so withdrawn at a temperature in the range of 600 to 1500 F., said burning being accomplished by fluidizing the coke particles with a free-oxygen containing medium in a combustion zone, whereby the coke becomes friable and a substantial amount of fine particles are produced by attrition, and returning the portion of coke so burnt to said process, said portion so returned containing a sutficient amount of fine particles in the size range of 50 to 200 microns to meet the seed coke requirements of the process.

2. The method defined in claim 1 wherein a relatively coarse portion is preferentially withdrawn and subjected to said burning.

3. The method of claim 1 wherein the coke particles subjected to said burning are classified to segregate at least a fine fraction and a coarse fraction, the fine fraction being returned to said process and the coarse fraction being recycled for further burning and attrition.

4. The method of claim 1 comprising in addition thereto comminuting the coke so burnt and returning the comminuted coke to said process as said seed coke.

5. An improved petroleum oil fluid coking process which comprises contacting a heavy petroleum oil containing constituents nouvaporizable without cracking in a coking zone with fluidized particulate coke maintained at a coking temperature to secure relatively lighter hydrocarbon vapors and carbonaceous residue which is deposited on said particulate coke causing said coke to accrete in size, withdrawing said vapors from said coking zone as product, withdrawing coke from said process as net coke product, withdrawing a portion of said particulate coke, fluidizing said portion with a free oxygen-containing gasiform medium in a combustion zone to burn 5 to 20 wt. per cent of said portion at a temperature in the range of 600 to 1000 F., comminuting the coke so combusted and returning the comminuted coke to said coking zone as seed coke whereby the mass, particle size and particle size distribution of the coke are maintained relatively constant.

6. The process of claim 5 wherein said coking temperature is maintained by circulating said particulate coke from said coking zone to an external heating zone and back.

7. The process of claim 5 wherein the net coke product is removed from said combustion zone whereby a coke product of improved properties is obtained.

8. The process of claim 5 wherein said portion treated in said combustion zone comprises relatively coarse coke obtained by classifying said particulate coke so withdrawn.

9. The process of claim 8 wherein said relatively coarse coke is obtained by elutriation in a separate classification zone.

10. The process of claim 5 wherein said comminuted coke is produced by jet impact grinding.

11. The process of claim 10 wherein said jet impact grinding is carried out in said combustion zone.

12. The process of claim 5 wherein said comminuted coke is classified to segregate at least a relatively coarse fraction and a relatively fine fraction, the fine fraction being returned to said process as seed coke and the coarse being recycled for further comminution.

13. The process of claim 12 wherein the comminuted coke is elutriated in a separate classification zone to secure said fractions.

14. The process of claim 5 wherein said coking temperature is maintained by circulating said particulate coke to an external fluid bed burning zone wherein 0.1 to 1.0% of said coke is consumed per pass and back, and wherein said combustion zone is in fluid communication with the fluid bed of said burning zone.

15. A system for pyrolytically converting hydrocarbon oils to relatively lighter products which comprise a coking vessel, a burning vessel and an oxidation vessel, means for establishing and maintaining fluidized beds of hot coke particles in said vessels, means for feeding said oil into said coking vessel, means for circulating coke between said burner vessel and coking vessel whereby the coke particles in said coking vessel are maintained at a coking temperature, means for circulating coke from said coking vessel to said oxidizing vessel, coke size reducing means for comminuting oxidized coke from said oxidation vessel and means for returning comminuted coke from said size reducing means to said burner vessel.

16. The system of claim 15 comprises in addition thereto classification means for classifying said coke circulated to said oxidation vessel and classification means for classifying said comminuted coke.

17. A process for the benefaction of particulate fluid coke produced in a hydrocarbon oil fluid coking process which comprises fluidizing said coke in an oxidation zone with a free oxygen-containing gasiform medium and oxidizing said fluid coke without intermediate deposition of fresh coke thereon at a temperature in the range of 600 to 1500" F. for a period of time sufiicient to consume at least 3 wt. per cent of said fluid coke.

18. An improved coking process for the production of particulate petroleum coke of improved properties which comprises contacting a heavy petroleum oil in a coking 7 zone with fluidized particulate coke maintained at a coking temperature to secure relatively lighter hydrocarbon'vapots and carbonaceous residue which is deposited on said particulate coke, withdrawing and fluidizing the coke from said coking zone with a free oxygen-containing gasiforrn medium in an oxidation zone to burn 3 to 50 wt. per cent of the coke at a temperature in the range of 600 to 1500 F., comminuting the oxidized coke from said oxi

Claims (1)

1. IN A HYDROCARBON OIL FLUID COKING PROCESS WHEREIN AN OIL IS PYROLYTICALLY CONVERTED IN A COKING ZONE BY CONTACT WITH FLUIDIZED PARTICULATE COKE MAINTAINED AT A COKING TEMPERATURE TO GASIFORM CONVERSION PRODUCTS AND CARBONACEOUS RESIDUE WHICH IS DEPOSITED ON SAID SOLIDS, A METHOD OF SUPPLYING SEED COKE TO SAID PROCESS WHICH COMPRISES WITHDRAWING A PORTION OF SAID PARTICULATE COKE FROM SAID PROCESS AND BURNING 3 TO 50 WT. PER CENT OF THE COKE SO WITHDRAWN AT A TEMPERATURE IN THE RANGE OF 600* TO 1500* F., SAID BURNING BEING ACCOMPLISHED BY FLUIDIZING THE COKE PARTICLES WITH A FREE-OXYGEN CONTAINING MEDIUM IN A COMBUSTION ZONE, WHEREBY THE COKE BECOMES FRIABLE AND A SUBSTANTIAL AMOUNT OF FINE PARTICLES ARE PRODUCED BY ATTRITION, AND RETURNING THE PORTION OF COKE SO BURNT TO SAID PROCESS, SAID PORTION SO RETURNED CON-

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