US 2874095 A
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Feb. 17, 1959 w.w. BOISTURE EIAL APPARATUS AND PROCESS FOR PREPARATION OF SEED COKE FOR FLUID BED comm;
OF HYDROCARBONS 4 Sheets-Sheet 1 INVENTORS Filed Sept. 5, 1956 w; hm
7i ATTORNEY tFFICIENCY- HF. HR. REG/TON SEED COKE 1959 w. w. BOISTURE ET AL 2,874,095
APPARATUS AND PROCESS FOR PREPARATION OF SEED COKE FOR FLUID BED COKING OF HYDROCARBONS Filed Sept. '5, 1956 4 sheets-sheet 2 FLUE GAS 1 VAPOR (OR WATER GAS) COKER PROD. 225 v 69 I75 Li -2m 1 3. P, 1 n K BURNER 5 (GASIFIER) Ill 22| M! m 195 4 I I 497 |3| fl6l' 12543 5 153 n7 PRoou 223 93 COKE our 12? I 115 k l5l I I83 A ACCELTUBE 4" LONG o ACCELTUBE 8" LONG D ACCELTUBE 24" LONG iNVENTORS LINDSAY I. GRIFFIN, JR.
g Y an WATTORNEY Feb. 17, 1959 APPARATUS AND W W. BOISTURE ET AL PROCESS FOR PREPARATION OF Filed Sept. .5, 1956 RATIO OF 0 TO 00 F HYDROCARBONS 4 Sheets-Sheet 3 I M'=.2o
PARTICLE SIZE DISTRIBUTION I IN COKING SYSTEM NO ELUTRIATION /Mc=l0 1 I MC=4 I I 0 I0 3o 4oso so I00 F IGURE-6 Worth W. Boisfure Byron V. Molsfedf Inventors Richard F Stringer Lindsay I. GrIffin,Jr.
Mm Attorney Feb. 17, 1959 Filed Sept. 5, 1956 RATIO OF TO 00 W. W. BOISTURE ET AL APPARATUS AND PROCESS FOR PREPARATION OF SEED COKE FOR FLUID BED COKING OF HYDROCARBONS 4 Sheets-Sheet' 4 PARTICLE SIZE DISTRIBUTION I coNsTANT souos INVENTORY A MAINTAINED BY WITHDRAWING LARGEST PARTICLES AS PRODUCTS 2.2 -Mc=IO Mc:4 L6 I I.4 -Mc=2 0 I0 20 .40 6O so FIGURE 7 Worth W. Boisfure Byron V. Molsfed'r Inventors Richard F. Stringer LIndsay I. GrIffIn, Jr.
By%,,; L. mAfiorney United States Patent APPARATUS AND PRGCESS FOR'PREPARATION OF 8EED COKE FOR FLUID BED COKING 0F HYDROCARBONS Worth William Boisture, ByronVictor Molstedt, Richard Franklin Stringer, and Lindsay Ira Grifiin, Jr.,' Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware Application September '5, 1956, Serial No. 608,162
Claims. (Cl. 202--25) The present invention relates to an apparatus and process for preparation of seed coke for fluid, bed coking of hydrocarbons such as residual oils andthe like. More particularly, the invention relates both to an apparatus and a process for providing the requisite, small particlesof coke to serve as nuclei for deposition of coke 'formedin the process. ,One object of the, invention is to replace larger coke particles which are Withdrawn from the system as product coke with smaller particles so as to keep the total number of particles in the system approximately constant. The invention includes means for breaking up large coke particles so that the growth in average particle size which occurs by coke deposition is compensated for by the withdrawal of large coke particles and replacement with anequivalent number of small coke. particles.v The invention relates further to an eificient and eiiective tially returning small particles to the coking zone to serve as seed coke. This application is a c0ntinuati0n-inmethod of breaking up the large particles andpreferen i In the conversionof hydrocarbon oils,,especially petroleum .crudeoils, it has been the practice for many years '.to distill Off the more volatile fractions and to subject the heavy residues to thermal cracking to obtain as much motor fuel'and other low .boiling fractions as possible.
In all these prior processes considerable quantities of heavy, viscous, residual oil .or pitch has been produced.
While .this pitch .is usually of relatively low value there :has normally been a.-fair marketfor it, but this has declined in recent years. Henceit is desirable to convert the heavy residua to more valuable products.
Among various. processes which are usedfor coking,
. thefiuid solids process, recentlydeveloped, appears to be among the most efiicient and-it promises to be most successful. In this process, as is Well known, a mass of ,.fluidized preheated solids, such as sand, metal particles, .clay, alumina, beads, and especially coke particles formed in the process, is used to supply the necessary heatfor coking or conversion ofthe residuum. :A'major proportion of the residuum normally is converted to gas oiland other fractions of higher volatility than the feed. 'A ,minor proportion of the feed is converted to coke. Various solids such as those mentioned above can be used to supply the heat of conversion. Since coke is a product ,of the process, hOWVQI,"lI is usually preferred. :The .present invention pertains particularly to means and.
methods for controlling the particle size of the circulating solids, especially those consisting entirely orlargely of coke, to maintain a continuously operable processn-It will be understood that even when solids other than coke best be kept imbalance-by; continuously keeping:.a}rela-' 2,874,095 Patented Feb. 17, 1959 "ice conversion to coke layers on the initial or nuclei particlesin the coking zone.
Moreover, some particles adhere to each other and unite to reduce the total number of discrete particles. It is found necessary to withdraw from the system the large coke particles and to replace them with an equivalent number of small particles, to
keep both the total number and the size distribution in equilibrium. The small particles which serve asnuclei for new coke deposition may be considered seed coke, and this term will apply hereinafter even when some of the particles, initially at' least, are' composed of other materials than coke, as indicated above.
In the catalytic cracking of hydrocarbons where fluidized masses of finely divided solids are used as catalysts, the total number of particles in a given system can vary widely. The catalyst particles, on the average, may be much finer than the coke particles which are used in the fluidized solids coking process. Moreover, the catalyst is not withdrawn from the system as a product but the coke is merely burned therefrom and the catalyst is reused, as long as it retains its activity. Hence there is no problem of removing a solid product and keeping the number of particles in the solids inventory substantially constant. The particle ,size distribution may change gradually until corrected by periodic addition. of fresh solids. In coking there is normally no such periodic addition. I
(The production of the seed coke or other particulate While the degree of breaking up solid particles in the system. may be varied somewhat the ratio ofaverage diameter of the broken or seek coke particles to the averageparticles in the-system'is preferably notlless than about /s .nor morethan about 55 It appears, more over, ,that. particles of about /5 average diameter are deemed to be about as small as "canbe handledefiiciently fOIPlII'PCSBSlDf the invention. Particles of fines are rapidly lost inoverhead flue .gases, for example. Attrition to smaller sizes also consumes power withoutcompensating improvement in the process. .On the other hand, seed coke particles only slightly reduced diameter, e. g., to A of the average particle diameter in the coking system, are about as large ascan be-tolerated economically, a
rapidturnover or circulation of cokebeingrequired when sizereduction is only. minor.
It will be seen, therefore, that the invention involves the :broad aspects of maintenance of a balanced system with a relatively constant number and size distribution of particles as well as arelatively constantly circulatingmass :alsoa narrower aspect of method and apparatus for of heat carrying and coke-accepting particles even though providing within the. systemthe small nuclei or seed par- :ticles to replace the large ones that are withdrawn.
. are used to start the coking operation they-become coated withcoketo such a degree that particles withdrawn from the coking system are composed of coke to a large, extent.
The invention will be more fully understood by reference to the attached drawings which, show alternative -.methods vfortheproduction of the small orseed coke particles and various aspects'thereof.
apparatus or vessel which maybe a' cokingreactor represeated diagrammatically, in which coke ,or other. solids within the system are converted to seed particles in the vessel;
Fig. 2 shows another seed forming or impact system where the necessary small particles are provided by impelling larger particles forcibly against an impact target element in an elutriation zone from which the small and large particles, respectively, may separately be withdrawn;
Fig. 3 shows still another system wherein small or seed coke particles are produced by the action of opposed jets entraining larger particles and striking them against each other in a relatively disperse section of a conveyor or transfer line in a coking system;
Fig. 4 shows a diagrammatical elevation of the major V essential elements of a coking system including a modified form of the present invention;
Fig. 5 is a graphical showing of the effect of particle velocity and apparatus characteristics on the efliciency of seed coke production;
Fig. 6 is a graph showing particle size distribution in a coking system where there is substantially no segregation of larger particles for coke product;
Fig. 7 is a graph showing particle size distribution where a constant inventory of solids is maintained by withdrawing only the coarse particles as product coke.
Referring in detail to the drawings, a reactor vessel 11 which may be of any suitable shape and size is shown diagrammatically in Fig. 1. An attrition or grinding system is shown inside this vessel. Coke particles or equivalent fluidizable heat carrying solids are brought into vessel 11 by any suitable means such as a line 13 leading from a heater or burner, not shown, where the particles are raised to a suitable coking temperature, for example, between 900 and 1250" F. more or less. The particles may be brought in through line 13 either in fluidized state or in a disperse state by means of an aerating or lifting gas or vapor such as steam, as is well known in the art. It will be understood that the attrition mechanism may be placed in or connected to a transfer line, a reheater or burner, or
I face 17. In the present case advantage is taken of the steam or other gas used for fiuidizing the solids, to break up or cause attrition of the solid particles and to thereby furnish the seed coke or related solids referred to above. As shown herein, jets of steam are introduced through inlets 19 and 21 respectively, and are directed through venturi orifices or nozzles 23 and 25 towards each other. Where a target of solid material is used, they may be directed against the opposite sides of a steel plate or the like, 27. Theconfiguration and arrangement of the venturi members is such that the solid particles of coke, or equivalent, surrounding the members 23 and 25 are drawn into and through the venturis by the high velocity jets of steam or gas and are impacted with considerable force against each other and/or the plate 27. The spacing between the members 23 and 25 and the plate 27 respectively is such that the jet of steam plus particles flows upwardly and downwardly through the openings, as indicated above, with the result that these spaces'are relatively clear and impact against the plate is relatively unimpeded. As the solids become spent by cooling and/or becoming coated with new coke layers they are withdrawn through a line 29. From here they may be returned to the burner. Oil feed may be introduced through suitable nozzles 31 as is well known in the art. As will be shown below, the
ating tubes or nozzles have important effects on the effi-= ciency of the grinding or breaking up of the particles.
It will be understood that particles, e. g. of coke, are continuously withdrawn from the vessel through line 29 and continuously returned thereto through line 13.
The volatile fractions produced by coking are withdrawn from the reactor vessel through a suitable gassolids separator 35, the separated solid particles, i. e. of coke, being returned to the bed 15 through the conventional solids return line 37. The vaporous and/ or gaseous products are taken overhead to a suitable recovery system through outlet line 39.
In the system of Fig. 2, a stream of solid particles having a larger average size than is desired within a coking reactor, may be fed through an inlet 51 into a conduit 53. A stream of fluid such as steam or gas is passed through conduit 53 at sufliciently high velocity to cause fracturing of the particles entrained therein when they strike each other or any hard impact surface. This velocity may be from about 100 to 1000 feet per second, the range of 200 to 600 usually being preferable. The solids entering through line 51 are picked up in the high velocity gas stream or jet coming through line 53, and carried forcibly against an impact target 55. The impact target, as shown in Fig. 2, is formed with a concave impact surface. With the proper configuration the particles striking this surface rebound convergently in such a manner so as to maintain a cushion of particles on the concave face of the plate 55. With this arrangement most of the impact force is spent on the particles rather than on the impact plate so that the latter is notrapidly worn away by abrasion. However, in some cases a certain amount of wear is not objectionable, since this member can readily be replaced and impact against a flat hard surface produces more efiicient grinding in terms of steam consumption. Where wear is not objectionable a flat plate or targetsuch as the plate 27 in Fig. 1 may be used. Conversely, a plate or target of double concave type could replace the plate 27 in Fig. 1 if desired.
The impact plate 55 is located in a relatively enlarged conduit section 57. This conduit, however, is not of sufficiently large cross-section to permit dense phase fluidization of the solids therein. Preferably, the particles are entrained in an upwardly flowing gas stream, i. e., in disperse phase, though this is not always necessary. The conduit 57 connects with an enlarged elutriating section 59. The latter is of sufficient cross-sectional area that the gas stream is inadequate to carry the heavier or larger particles. All the particles, fine and coarse, emerge from conduit 57 under the impulse of the fluid stream through line 53 but the heavier particles drop into an annular area 60 outside conduit 57 and inside the elutriator 59. A grid member 61 is placed across a lower part of the annular area 60 of elutriator 59 to separate the heavy particles from a chamber 63 within which a fluidizing gas may be introduced through a line 65.
Thus, as particles emerge from the top of the section 57 their velocity is immediately reduced and heavier particles fall back on to or toward the grid 61. The fluid, e. g., steam or hydrocarbon gas, introduced through line 65, makes a fluidized bed of them. The velocity of the aerating gas in the elutriator 59, is suflicient to elutriate therefrom any finer particles produced by impact against the plate 55. Most of the fines are carried overhead directly from conduit 57. The fine particles pass overhead directly from conduit 57. The fine particles pass overhead out of theelutriator 59 through an outlet 69. From here they may be returned to the coking system. The coarser fluidized coke particles in bed 60 may be withdrawn from the system through an outlet line 71 controlled by a valve 74. The coarse particles in bed 60 are often partially broken and it is usually advantageous to recycle at least part of them to the grinder or attritor 53, 55. This may be done by connecting line 75, or a branch thereof, to line 51 as will be obvious.
Still another arrangement is shown in Fig. 3 wherein .solids. are fed by lines to two opposed jets 81 from whence theyare d'riverf into-airenlarged -section ssa The" particles in the respective-jetsambroken by mutual im' pa'ct'a't high-velocity. The" pa'rticles, atter impact,- -are' swept around a'nd upwardly asshOWnb'ythe arroWs, un-' der the-impelling effect of a fluidstream and guidance of the housing 84, 85; Here they arepreferably in relatively'disperse phase. they flow upwardly, still in disperse'phase, through a conduit 87 into a disperse zonein the upper part ofan elutriating vessel 89. The heavier particles'fall by gravity onto a grid-90 supported inthe annular'space between inlet conduit 87 and the outer walls of vessel 89. A'fluidizing gas is fed into the space below the grid by a "line'91. It passes through the grid to form a fluid-bed 92 of-the' coarse solids which disentrain by gravity from the upfiowing stream emerging from conduit 87-. The upper leverof this bed is indicated at 93. A draw-oif line 94 equipped with valve 95 is provided and the coarse solids may be recycled to attritor jets 81, or withdrawn as coke product, or taken me burner.
Referring now to Fig. -4,'there'is shown a fluid bed coking system, including a burner or'heater; with an attritor elutriat'or fitted into'the transfer lines. The coking vessel 101 is of the conical or tapered type. Feed, suitably pre-- heated; is introduced through manifold feed lines 103, 105 and a plurality of nozzles'107, 109 into a-bed of preheated, heat-carrying 1 fluidized solids, specifically coke particles'of a size between about 4O and 500 microns average particle diameter. These particlesfinorrnally are' preheated'to a temperature aboVe'lOOO" F., up to .1200" F. or more, in a burner vessel 11!; The hot particles tlowl from'the burne'r'through a 'standpip'ezlll ja relatively. sharp U-bend 115, and a steep riser 117, under control of valves 110; 121. The spent coke particles from reactor vessel 101 flowthrough a coarsenscreenin'g device 123 in a 'stripping sectionor :zone 125," down into a standpipe 127, a relativelysharp U-bend '129, a 'steep riser 131 witha control or cut-off valve 133, into an'inleti135'to a deflector 13Tin the burner 'vessel' 111; v
Stripping gas, such assteam is introduced'into'the stripping zone 125' thi'ougha line 141. Large lumps or agglomerated particles of coke, and other product coke, if desired, may be removed from the reactor vessel'throughoutlet 143under control of valve 145;
As coke withdrawn from the reactor flows downwardly, it may be aerated or partly aerated and impelledthrough the angle bend (sharp U-bend120) bya jet of steam or the like introduced through-a line 147. Other aerating lines 149, 151, 153, 155 and 157 are provided for lifting the" coke' into ne'burnr' vesslL' Air may be fed into some-orfall ofthese'to start combustion as the coke is flowing into the burner. The burner vessel preferably has a combustion chamber or starter at the bottom, indicated at 159, to which air and/or fuel Imaybe fed through line 161 for initially heating up 'theburner. Air and/or fuel may continue to be fedto the burner. from this source if desired.
The hot coke,--or part orig-may be withdrawn as product from theburner side ofthevessel, if desired,-throngh an outlet 163- under control of a valve 165. lt maybecooled as withdrawn by stripping it with a stream of Water or steam introduced through iine'167; Byoperatingthe burner'atsuitably hightemperature, water gas may, thus be generated, the temperature of product coke and/or coke returning to, the coking reactor thus-being brought down to the desired levels. The combustion and/or-com bustible gas from the burner passes*through a conventional solids separator 169. Entrained solids are returned-t the fluid burnerrbed 171 through solidsreturn line173. The gases, substantially-free of entrained solids,
pass overhead through an outlet line 175 under control of valve 177 to a suitable heat recovery and/or disposal system. Thesolids returningfrom-the burner to'the-reactor are suitably aerated and buoyed .up by-steam -hydro carbon gas; or? other: inert 'gasiform fluid introduced Under the impellinggasstream enough aeraiing ta s lsrg 1-83 -185, a's i's well understood in the. art. Because the solid particles 1 are much} coarser; on the average, than the solids used in older fluid'bed sys tents, e. g.- than catalytic cracking catalysts, they tend to become deaerated quickiy and lose buoyancy. For this reason the risers 117'and 13:1 should be quite steep, at least 50 from-"horizontal and preferably-60 or more;
The cokefrom the reactor'is taken to "a grinder 190, flowing up through line- 191 under the driving force of a stream of gas injected by line 151. The line 151-may" project'througn or partially through the riser'131; or other jet means may be introduced. into 1ine"191,.above or below control valve 193,.so as 'to drive the coke particles upwardly at high velocity, to'lOOO-feet per"second togrind them. The high velocityjet itself is an effec tive grinder but it may be aided by a target plate195 arranged within'an enlarged conduit 197. The arrangement is substantiallyidentical with that of Fig. 2.
The coke particles, broken; partly broken, fine and coarse,are all carried upwardly intoelutriator-yessel 199 wherezthe" fine particles continue to be entrainedand carried overhead by the impelling gas stream throughout let 201; From here they may pass-through valve 203 and line 205 'into'riser'117 and are returned to the coker'. Alternatively, the fine particles, or part of them, may be passed'to'the burner'through line 207 andvalve209, a propelling jet of suitable gas being provided as'indicated at211.
The heavierparticles fallback into an annular area 213 onto a grid 215'throughwhich a fluidizing gassuppliedbyline. 217 is'fiowingupwardly; The fluidized particles," which are co'arser, may be withdrawn through line 219- under control of a valve 221' and recycled to'line 191 by a lifting or propelling fiuid injected by a'line-223. From line filthey mayxberec'ycled to the attritoror grinder 195," which is often desirable because partially crackedparticles usuallyflbreak up more easilythan =uncrackedparticles, orthey maybe fed-back-into riser-131 to go to the burner. If-desired; product coke may be "withdrawn at 225.
The vapor products from the coker pass overhead through a conventional solids separator 221 with solids return line -223"a'nd product 'outlet line 225.
It willbe;understoodthat-the grinding of coke par ticles to reduce their size for reuse isdisclosed ina'patent to Kuhl No. 2,339,932; The-present invention-involves not only an improvedgrinding technique but-also the concept, stated inpart above, of-consta'ntly replacing large coke particles withdrawn from the system-with anequivalent-number'ofsmall or seed coke particles,-=tokeep the coke receiving or deposit surfaces substantially constant in total area and'keeping the total coke mass or inventory in--the coker substantially constant;
The nature of-thecokingprocess'is such thatcokeais, being formed constantly and deposited on the solids which supply the heat for conversion. Theseparticlesgrow in average diameterby :accretion' 1 of coke, layers at a grate that is substantially independent of-particle size.- That is, whether aparticle has a diameter of 100 micronsonlO. microns, growth by 10 microns takes substantially equal-- time,von the average. The-100 micron particle grows to microns while the 10 micron particle grows to 20: v
The ratio of increase in weight is greatly different. The larger'particle increases'about 33-%-.while the smaller one,- increasesabout;700%' Operation-eta large scale pilotplant using-fresh Discocoke of highash 1 content, and analysis of the; graduallyv lowering ash content as particles, grow gave clear-proof; that coke is deposited-as stated abovet- Coke I was de posited i i-direct proportion to surface area, i. e inlayerse of equalcthickness during .agiven time, regardlessteofparticle'size.
When coke -is burned from thepar-ticles; it-is alsorremoved inalayers 'of substantiallytuniform thickness per unit "of time, quite regardless of particle-size; A car'eful gamma study of burning particles of widely'ditferent sizes showed clearly that the weight of coke burned was directly proportional to surface area.
- Hence, both product coke deposition and coke burning, the increase or reduction in diameter, proceeds at a uniform rate independent of particle size. Theoretically, then, if all the coke produced were burned to supply heat and the rate of burning were precisely the same as the rate of deposition, addition of seed coke would not be necessary. The system would stay in equilibrium. In practice, however, only about 20% or less of the coke produced is burned to supply heat requirements, even with the lowest coke-producing oils suitable for coking. Losses of fines from the system are normally considerably larger than any natural attrition and it may be said that in all cases some grinding is essential.
In a typical large scale pilot plant operation, coking approximately 100 barrels per day of heavy residuum, product coke was withdrawn at a rate that varied from 1300 to 2200 pounds per day to keep the inventory constant. For five days fine seed coke (100 to 200 mesh) was added at a rate varying between 100 and 500 pounds per day. Frequent sampling of the circulating solids showed that the circulating coke gradually was becoming finer in particle size.
At the end of the fifth day, about 53% by weight of the coke would pass through a 100 mesh (150 microns) sieve. Thereupon the addition of seed coke was discontinued. Within about 2 days particle size had increased so that only 18% of the coke would pass the 100 mesh sieve. 80 mesh and coarser jumped from 20% to 50 in the same 2 day period. The unit would very shortly have become inoperable if it had continued in operation without seed addition.
Operation of commercial cokers shows clearly, moreover, that sufiicient fine solids must be included to permit some entrainment of fines overhead. They serve as scouring agents to keep the apparatus clean. Not only are they effective scouring agents but they add heat capacity and they provide surface for deposition of coke by the overhead vapors, helping to keep it off the apparatus.
In order to control particle size, additions of small or seed particles constantly or essentially constantly is required. Proportions larger than-about 5% by weight of 200 mesh-material (smaller than about 75- microns) cannot be kept in the system because of entrainment. On the other hand, if there are more than about 20% by weight of particles larger than 48 mesh (about 295 microns) the system operates very roughly, slugging occurs in the transfer lines and bumping in the fluid bed because of erractic fluidity. These limits, therefore, are quite critical. Further, by keeping average particle size in the system below 300 microns, the maximum operable feed rate of the unit can be increased.
-As shown in Figs. 6 and 7,'the particle size distribution required to maintain equilibrium in a stable system depends to a very important extent on the degree of segregation of large particles in the product coke. With no segregation, for example, a much wider distribution is found than with complete segregation of the largest particles as product.
In Fig. 6 the ratio of D to Do is the ratio of diameter of a given size of particle to the diameter of the seed coke. The various curves are based on various grinding ratios. In the top curve, M =20 is the ratio of product coke by weight to the ground seed coke recycled to the reactor. That is, 5% as much coke is ground as is withdrawn as product. If this coke were ground only to a particle size, say, of 100 mesh (about 150 microns) 20% of the coke would have a particle size of over 900 microns (6.2 times 150) which canot be tolerated, as mentioned above. Hence an upper limiting factor on seed particle size when there is no elutriation to'concentrate coarse particles in the product coke, is determined by dividing the 48mesh'coke size (approximately 300'microns) by the This small size seed coke would be lost from the system.
Without elutriation or segregation of coarse particles in the product coke, only the two lower curves, M =4 and M =2 are operable. The limit is indicated approximately by the dotted line L.
With elutriation to segregate coarse particles in product coke,.all of the conditions shown are operable. Here (Fig. 7) the diameter ratio of particles at any point on a curve away from the origin is generally lower than in Fig. 6. Even so, however, constant seed coke replenishment is necessary but its rate can be much lower. In practical operations, about 3% of seed coke based on product coke is the minimum requirement. The particle size ratio limits mentioned earlier in this specification of to ,5 will be seen to lie fairly near the rnidpoints of curves M =20 of Fig. 6 and M =2 of Fig. 7, respectively. They are indicated as x and y.
In practice, seed coke is not ground to a uniform size and the factors given above are not precisely applicable to average particle sizes of grinding but the relationships are reasonably approximate, even when the spread of seed particle sizes is rather broad.
Bybalancing the grinding and the selection of large particles to be withdrawn to keep the system in equilibrium maximum efiiciency and continuing stable operation are obtained.
As suggested above, still another feature of this invention relates to the discovery that there is an optimum jet velocity and an optimum jet apparatus design for high efiiciency in grinding or attriting the coke. In order to study these factors, various jet velocities and various lengths of accelerating tubes were tested in impacting the solids against a steel target plate. The arrangement of the test nozzles and target were substantially as in Figs. 1 and 2, that is, the coke was fed into a high velocity gas jet, and the length of the tube from solids inlet to the tube outlet was measured as the length of accelerating tube.
The coke tested had the following particle size dis tribution before the attrition or grinding tests:
Table I Tyler Mesh Min. Particle Wt. Percent Size, Microns on Screen At a gas inlet velocity (at the point where coke was introduced'into the accelerator tube) of feet per second there was only a small amount of attrition. Particles smaller than microns (about 90 mesh) increased by only 2.2% of the total mass. Particles larger than 240 microns (about 60 mesh) decreased by the same amount.
At a gas velocity of 208 feet per second, there was an increase of 9% of the total Weight material of less than 200 microns (about 68 mesh) and a decrease of 9% in coarse material larger than 240 microns. At 246 feet per second there was an increase of 21 weight percent of material smaller than 200 microns average diameter.
The tests showed that there is an increase in all cases of material smaller than 200 microns and a decrease of material'larger than 200 microns. They showed also that-in the gas velocity range of 200 to 250 feet per mangoes:
=second there is no -apparent material difiereirc'e in 1 selec the general range of '50 to 200 microns. At 200 feet per. second the material passing through a 100 meshscreen (about 15.0 microns) increased by 2%. At 250 feet. per second it'increased 14% by weight. I
I Fig. 5 of the drawing shows graphically the relation between jet vvelocity and efliciency of grinding, in terms of horsepower hours required to produce a ton of seed coke (through 100 mesh-i. e., below about 150 microns). At 230 feet'per second the power required, in an 8 inch accelerator tube, toproduce one tonof seed coke was only 48 horsepower hours. .At 160 feet per second power requirements were 115 horsepower hours.
Fig. 5 also shows the effect of the length of accelerator tubes. The 8" tube was much more efiicient than either a 4" tube or a 24" tube in the diameters tested, and the diameter in test apparatus, 2 mm. vs. 4 mm., appeared to make no measurable difference. In larger scale apparatus it appears that the same is approximately true, proportionately. For small diameters the accelerator tube should be between about 4 and 24 inches long, with about 8" being the optimum for efilciency. Apparently the 4" tube is too short to bring particles up to gas velocity, due to slippage. The 24" tube showed too great a pressure drop, due to friction, for efiicient operation of a 4 mm. tube. With larger diameters, optimum lengths are proportionately greater.
Within the range of 0.3 to 1.0 pound of coke per s. f. c. of gas, the solids loading rate appeared to be about optimum and variations were narrow. At a loading rate of less than 0.1 pound per s. c. f., the power requirements increased very greatly. Results are indicated in Table II, using the 8" accelerator tube nozzle.
Similar results were obtained, using a narrow size range of coke particles (48-60 mesh, 249295 microns) for feed. The correlation of gas velocity vs. efiiciency was substantially unchanged. Power requirements were slightly higher than for the widersize range coke of Table I. Note that power requirements leveloff and become relatively constant at 260 to 300 F. P. S. and
By comparison, a ball mill grinder required 97 horsepower hours power input to grind a ton of coke, of about the same size distribution, to pass a 100 mesh screen, roughly or nearly double the power requirements for a steam jet. Aside from operating costs, investment costs for a ball mill are much higher. The impact attritor system is even more efficient operating on relatively large and agglomerated coke particles. With agglomerated coke of 20 to 35 mesh (400 to 800 microns) 96.7% by weight was reduced in particle size in a lightly loaded jet at 250 feet per second. By comparison, a 48 to 60 mesh sample (246 to 295 microns) showed size reduction of only 26.2% in one pass.
As mentioned above, it has been found advantageous to recycle broken coke particles of large size to the accelerating nozzle. Using an 8" accelerating tube of 4 mm. inside diameter with a loading of 0.25 pound per s. c. f. gas at 200 feet per second, 94 horsepower 1'0 hr.'/to'n wasrequired to produce coke asstng'momesfi. When the coke-was recycled at 200 feetper second the power requirement was cut to 46 horsepower hr./ton. The discharged material from the two runs showed the following particle size distribution:
Table III Run A- Run B}- Wt. on Mesh Original Recycle Coke 5. 6 0.9 53. 4 29. 5 71. 4 5o. 5 80.5 til. 2 89. 8 73. 6 93. 6 80. 7 88. 2 99. 2 93. 4 .97. 2
Hence, the recycle of the coarse coke from line to line 51 in Fig. 2, from line 94 to line of Fig. 3, or through line 219 in Fig. 4, appears to be highly advantageous as regards grinding efiiciency.
In elutriating the fine particles from a bed or mass of ground or attrited coke, efiiciency of separation may be increased by packing the bed with Berl saddles, Raschig rings, or the like, as will be understood by those skilled in the art. The beds 60, Fig. 2, 92, Fig. 3 and 213, Fig. 4 may be so packed, if desired, so that the finely divided solids may be more effectively separated from the coarser solids.
Various improvements and modifications, such as the use of cyclones or other size separation devices instead of elutriators, will suggest themselves to those skilled in the art and it is intended, in the claims which follow, to cover such, so far as the state of the art permits.
What is claimed is:
1. In the process of coking heavy hydrocarbon oils by contacting them with a fluidized mass of preheated coke particles, wherein the coke particles are continuously passed through a fluidized coking zone to grow by accretion of coke therein and at least part of said particles are continuously reheated and recycled through coking zone inlet and outlet lines, the improvement which consists in maintaining a substantially constant mass, number and size distribution of coke particles in the total system by constantly withdrawing relatively large coke particles from the system so as to keep the particles of over 300 microns diameter generally below about 20% of the weight of the total coke in the system, and constantly replacing the withdrawn particles by a substantially equal number of smaller particles whereby the accretion of coke'on the particles is compensated for and the coking zone is maintained in fluidized condition.
2. Process according to claim 1 wherein the smaller particles in the system do not exceed about 5% by weight of particles below 75 microns average diameter.
3. The process of coking hydrocarbon oils which comprises impelling particles of coke at a velocity of to 1000 feet per second against solid target material so as to produce small particles of seed coke size, elutriating said small particles from larger particles, passing said small particles to a coking reactor, feeding oil into contact with said small particles to coke said oil and deposit: coke upon said particles, thereby causing said particles...
7 weight of solids in the system are of greater size than.
about 300 microns average diameter.
4. The process of claim 3 wherein not more than;
"11 5% by weight of the solids in said coking system are smaller than 75 microns average diameter.
5. An improved method for pyrolizing heavy hydrocarbon feed in a fluid bed at high feed rates while we serving smooth fiuidization, which comprises, maintaining a mass of inert particles at a reaction temperature, introducing fiuidization gas through said mass so as to form a fluid bed, contacting hydrocarbon feed with said inert solids thereby converting said feed to lighter hydrocarbon products which are removedoverhead, and carbonaceo'us residue which constantly deposits on said solids, withdrawing a portion of said carbonaceous coated solids, attriting at least 3 weight of said withdrawn solids, elutriating said attrited solids and returning thus treated solids to said reaction mass so as to maintain a constant mass, number and size distribution of solids therein, said size distribution being characterized by less than 20 weight percent of said solids having a diameter greater than 300 microns and less than 5 weight percent having a diameter below 75 microns.
References Cited in the file of this patent UNITED STATES PATENTS 1,325,676 McKelvey Dec. 23, 1919 2,339,932 Kuhl Jan. 25, 1944 2,560,807 Lobo July 17, 1951 2,606,144 Lefier Aug. 5, 1952 2,624,696 Schutte Jan. 6, 1953 2,661,324 Leffer Dec. 1, 1953 2,717,866 Doering et al Sept. 13, 1955 2,717,867 Jewell et al Sept. 13, 1955 2,736,690 Mattox Feb. 28, 1956 FOREIGN PATENTS 381,591 Great Britain Oct. 10, 1932