US 2849384 A
Description (OCR text may contain errors)
Aug. 26, 1958 A. VOORHIES, JR., ETAL 2,849,334
. FLUID COKING PROCESS Filed June so, 1954 PRODUCT RESIDUAL OIL " Alexis Voorhies Jr.
Edward W. S. Nicholson 7 Inventors By C m/Aflorney United States Patent @fiice 2,849,384 Patented Aug. 26, 1958 FLUID COKIWG PROCESS Alexis Voorhies, In, and Edward W. S. Nicholson, Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware Application June 30, 1954, Serial No. 440,366
2 Claims. (Cl. 196-55) The present invention relates to the art of coking of heavy petroleum oils, in contact with a bed of fluidized solids at a coking temperature in a coking zone. More particularly, this invention is concerned with a specific improvement of a fluidized solids hydrocarbon oil coking vessel, whereby product distributions and quality are improved.
Recently it has been proposed to coke hydrocarbon oils such as petroleum residua by injecting them into a coking vessel containing a fluidized bed of high temperature finely divided solids, e. g., coke, sand, spent catalyst and the like. In the coking vessel, the oil under goes pyrolysis in the fluidized bed, evolving lighter hydrocarbons and depositing carbonaceous residue on the solid particles. The necessary I heat for the hyrolysis is supplied by circulating a stream of the fluidized solids through an external heater, generally a combustion zone, and back to the coking vessel. The solids are partially combusted in the heater to supply heat to the process. This fluid coking process is more fully depicted in copending application entitled, Fluid Coking of Heavy Hydrocarbons and Apparatus Therefore, Serial No. 375,088, filed August 19, 1953, by Pfeiffer.
One problem associated with this type of coking is that of obtaining a uniform dispersion of the heavy oil feed on the particulate solids in the bed. Because the feed stock to such a coking process is highly viscous, considerable agglomeration of the particles in the bed takes place unless particular care is taken in injecting the feed into the bed. If this agglomeration continues, a point is soon reached where the fluidized bed ceases to be fluid and bogs down. This may result in a complete shut-down of the apparatus, requiring that it be opened up and cleaned out 'before operations can be resumed. In commercial applications of this process, this results in considerable economic loss.
It is an object of the present invention to obtain better feed dispersion on or among the solid particles which comprise the fluid bed, thereby permitting the high operating rates and making the process considerably more eflicient.
Also, in the fluid coking of heavy residual oils, it is important to minimize the time during which the cracked vapor products are held at the cracking temperature. It is most advantageous to rapidly remove and cool the vapors formed by the pyrolysis. Not to do so, permits degradation of the reaction products as a result of polymerization or further cracking and this leads to poor yields and quality of the conversion products. It is equally important, however, that the solid particles on which the nonvaporized liquid fractions of the feed are deposited should be held in the reaction zone for a long enough time to permit all cracking possible to occur, and in order that the solids be sufficiently dried before conveyance to the heater to minimize loss of potential hydrocarbon products by combustion.
In a conventional fluidized solids coking reactor, adequate holding time for the solids is provided. However, at the gas velocities that can be used in such a reactor, vapor holding time is too great and undue thermal degradation of the products occurs. It is a further object of the present invention to present the art with an improved coking reactor design that provides for a minimum gas holding time in order to substantially inhibit thermal degradation of the product vapors.
According to the present invention, it has been found that these objects may be met through the use of a draft tube of conical shape within the fluidized solids bed, and by specially configuring the coking reactor. By insertion of such a draft tube, it is possible to create within the tube an upwardly diverging stream of solids. By injection of the coking feed stock into the lower portion of this draft tube, the cracked product vapors supply most, if not all, of the necessary conveying or fluidizing gas. The design of the draft tube is such that a concentric annular path is created within the fluid bed, wherein the solids issuing from the upper portion of the draft tube move downwardly to the base of the coking reactor.
This conically shaped draft tube makes possible the maintenance of high velocities in the bottom of the draft tube where the fresh feed is introduced without requiring large quantities of an aerating gas, e. g., steam. Further, the reactor is designed to provide 2 to 4 times as much holding time for the solids in the annular part of the reactor as in the central draft tube. As a result of this arrangement, the time of contact of the vaporous reaction products with the solids is materially reduced as compared to the contact time in conventionally designed fluid coking reactors.
Because much less steam is required in the central draft tube, a greater percentage of a given quantity of steam can be used in the annular section for stripping reaction products from the downwardly moving solids. This stripping action is greatly enhanced in the upper part of the annulus as a result of this design because the crosssectional area of the annulus is smallest at that part of the vessel. Thus, a high stripping velocity and more efficient stripping is accomplished in the zone where the greatest amount of reaction products are being released from the solids emerging from the draft tube. This minimizes thermal degradation of the reaction products in the annular soaking section.
It is a further ancillary feature of this invention to mini- 'mize vapor holding time above the fluid coking bed by use of a very short disengaging height or outage from the pseudo-liquid level of the bed to the reaction products outlet. This is accomplished by reduction in the free vapor passageway above the fluid bed, as by placing fillings in the top of the reactor or by swaging down the upper section.
The invention will be more clearly understood by reference to the drawing attached to and forming a part of this specification. The drawing illustrates a preferred design of a hydrocarbon oil, fluidized solids, coking vessel adapted to achieve the objects of this invention.
This invention is capable of handling any of the feed stocks customarily charged to a fluid coking process such.
as coal tars, shale oils, extracts, catalytic cycle stocks, whole petroleum crudes, distillate and residual fractions therefrom, mixtures thereof, cycle stocks from the coker product fractionation system, etc. 7
Referring now to the attached drawing, the oil to be upgraded, such as a residual oil, is injected via line 3 into a draft tube 1 Within a fluidized bed of high temperature particulate solids contained in a cylindrical coking vessel 2. This oil can be suitably preheated to temperatures below the cracking temperature of the oil, preferably to 500700 F., and an atomizing gas, such as steam, can be used as an aid in introducing the oil into the coking vessel.
The coking vessel contains a fluidized bed of solids having a temperature of about 950 F. when gas oil suitable as a charging stock to catalytic cracking processes is desired as the primary product. Higher temperatures are used, of course, when fuels or chemicals are desired as primary products. Preferably, coke produced by the process is the heat carrying solid used. The coke has, preperably, a size range of about 40-500 microns, although it may vary from 1,000 microns or more.
Steam or other substantially inert gas is admitted to the base of the coking vessel by lines 4 and 5, and serves to strip and fluidize the coke particles therein. A distributing grid 6 passes the steam into the annular stripping space 7 and inhibits the flow of the steam into the draft tube. The oil upon contact with the coke undergoes pyrolysis depositing carbonaceous residue on the coke and evolving considerable quantities of hydrocarbon vapors. These vapors serve to fiuidize and convey upwardly the particles in the draft tube. The upward velocity of the particles, which depends upon the rate of feed injection, is preferably in the range of l to ft./sec., but may vary from 0.1 to ft./sec. The density of the suspension in zone 8 may vary from to 50 lbs./cu. ft., but is maintained preferably in the range of 30 to 45 lbs/cu. ft. As the cracking continues, there will be, of course, additional quantities of vapors produced and for this reason, the draft tube is conical in shape. This prevents excessive velocities in the upper portion of the draft tube and serves to allow a suflicient time for primary cracking products to be produced.
The solids emerging from the draft tube overflow into the annular stripping or soaking zone formed by the draft tube and the vessel walls. The extent of the stripping is greatest in the upper portion of this zone because of the narrow cross-sectional area. This serves to enhance the recovery of the initial cracking products and inhibits the secondary cracking of the vapors as they have only to traverse a short distance of the fluid bed.
As there is a greater volume of fluidized solids in the stripping zone than in the draft tube, the particulate coke will move downwardly at a somewhat lesser velocity, e. g., 0.02 to 6.0 ft./sec. Thus, the residence time of the coke along with adhering liquid hydrocarbons is much longer in the annular stripping zone than in the stripper of a conventional coker. This favors the cracking of the heavier constituents of the oil to an extent greater than that attained in conventional fluid coking operations and eliminates the need of a secondary soaking zone for the coke. By the time the coke particles reach the base of the reactor, they are completely dry and are, therefore, in proper condition for circulation to-the burner or for recirculation through the draft tube. The density of the coke suspension in the stripping zone at the lowest portion can vary from 30 to 55 lbs./ cu. ft., preferably from 40 to 50 lbs./cu. ft.
A portion of the stripped coke is continuously removed by line 9 and circulated to an external heater wherein the temperature is raised to about 1000" to 1800 F., i. e. 100 to 300 F., above the coking temperature. Heated coke is returned to the coking vessel by line 10 and is preferably introduced into the base of the draft tube as depicted in the drawing. This external heater may be one of several types of heaters known by the art such as a fluid bed combustion burner, a transfer line burner, gravitating bed burner, etc. Also, indirect heat exchange means may be used if desired such as heat exchange with flue gases, heat carrying, heavy inert solids, e. g., shot, etc.
The vapors emerging from the fluidized bed are conveyed upwardly to a cyclone system 11 wherein entrained solids are removed and returned to the bed by line 12. The vapors are then taken off overhead by line 13 to a conventional products separating means, e. g., fractionation, not shown.
A preferred feature of this invention is to so configure the upper portion of the reactor so as to minimize vapor holding time and to quickly cool or quench the products to substantially stop the cracking reaction. Thus, filling 14 is inserted in the upper portion of the reactor to take up dead space. This filling may be conveniently designed to act as a baffie, whereby entrained solids in the vapors are deflected downwardly into the coking bed. By these means, the vapors emerging from the bed are made to have a velocity in the range of 3 to 10 ft./sec. and a holding time in the dilute solids suspension phase above the bed of less then 3 seconds.
Within the cyclone itself, or upon emerging from the cyclone, the vapors are preferably quenched by a cooling medium supplied by line 15 to a temperature below about 750 F. This cooling medium is preferably cool recycled oil from the product fractionation system, but may also be any other suitable type of coolant such as water, stream, cool particulate solids, cool gases, etc.
The optimum design of the conical draft tube will, of course, depend upon many factors such as reactor size, feed rate, feed stock compositions, etc. For proper performance over a fairly wide range of operating conditions, it is preferred to have the central portion of the bed within the draft tube contain 20 to 50% of the fluidized bed volume. The draft tube is designed as a truncated cone having a total included angle in the range of 2 to 16, preferably 3 to 8. Also, for proper performance, the length of the draft tube should be 50 to of the height of the fluidized coke bed when considering the bed height as being measured from its upper level L to the distributing grid 6.
Table I summarizes the preferred range of pertinent operating conditions for the present invention and presents a specific example. Table II indicates the products obtainable from a coker designed in accordance with the teaching of the present invention when the injected oil has the inspections listed and the coker is operated in accordance with the example of Table I.
Table 1 Range Example Temperature, F 9001, 600 950 Pressure, p. s. i. g. coker outlet 0-400 11 Coke particle size, microns 0-1, 000 40500 Velocity of coke particles, ft./sec.
In draft tube 0.1 to 10 1. 5 In stripping section 0. 02 to 6 1. 0 Steam rate, lbs./lb. coke circulated in annular stripper 0.00015 to 1.0 0.00093 Oil injection rate, lbs/lb. coke circulated in draft tube 0.002 to 0.2 O, 012 Coke circulated to heater, lbs/lb. oil
injected 1.5 to 25 10 Temperature of coke from heater, F 1,000t0 l, 800 1,100 Average vapor residence time in draft tube, secs 1 to 5 2. 4 Average solids residence time in draft tube, secs 6 to 60 20 Gas velocity in draft tube, f./s 1 to 15 4. 3
Table II C3 wt percent 7.9 C 430 F. vol. percent 23 430-l050 F. vol. percent 61.5 Coke, wt. percent 19.2
It is to be noted that in the above example, about 18 wt. percent steam, based on fresh feed, is used. This is to be compared to conventional fluid coking operations wherein about 40 wt. percent steam is required to obtain the same low vapor holding time.
Various modifications of this invention will be readily apparent to those skilled in the art. For example, al-
though the coking reactor has been shown as a straightwall vessel, in certain applications it may be desired to have the coker of conical shape also. Further, although a cyclone has been shown as removing entrained solids from the product vapors, other means could be used such as oil sprays, helical vanes, baffles, etc. Further, this invention is not to be limited to the specific means of heated solids withdrawal and introduction into the vessel. The solids from the heating vessel may be introduced into the stripping zone, into the dilute phase above the fluidized bed, may be commingled with the oil feed prior to its injection into the draft tube, etc., although the arrangement shown is preferred.
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 process for coking a heavy hydrocarbon oil by contacting the oil coking charge stock at a temperature in the range of 900 to 1600 F. with a body of coke particles maintained in the form of a dense, turbulent, fluidized bed in a reaction zone, removing product vapors from the reaction zone, circulating the coke particles through an extraneous heating zone wherein the particles are heated and back to the reaction zone to supply heat thereto, the improvement which comprises the steps of injecting the coke particles from the heating zone upwardly at a velocity in the range of 0.1 to
conversion products; circulating the coke particles from the upper portion of the passageway downwardly at a velocity in the range of 0.02 to 6 ft./sec. through an annular path of increasing cross sectional area defined by said passageway and the confines of the reaction zone to the lower portion of the passageway, the fluidized bed within the path having a density of to lbs/cu. ft. and greater than that in the passageway; introducing an inert stripping gas into the lower portion of said path; passing the stripping gas upwardly through the path, the stripping gas having an increasing velocity as it proceeds upwardly through said path; withdrawing at a velocity in the range of 3 to 10 ft./sec. and rapidly cooling the vaporous conversion products from the upper portion of the bed to a temperature below about 750 F. within 3 seconds to substantially end the conversion reaction by limiting the free vapor passageway above the level of the dense, turbulent, fluidized bed.
2. The method of claim 1 wherein said oil is introduced into said passageway in an amount in the range of 0.002 to 0.2 lb. per lb. of said coke particles circulated to the lower portion of said passageway, and when 0.00015 to 1.0 lb. of said stripping gas is introduced into the lower portion of said path per lb. of said coke particles circulated in said path.
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