US2808368A - Predicting bed defluidization in fluid cokers - Google Patents

Description

Oct. 1, 1957 J. F. MOSER, JR ,808,368 PREDICTING BED DEFLUIDIZATION m FLUID COKERS Filed April'4', 1955 HEATED 7 x souos L 5 ll 3 J \JQRESIDUAL on. 44 T STEAM I3 STEAM 4 2 COOL souos John F. Mdser, Jr. Inventor By TX Attorney tered in operating fluid coliers. ;portanceto be ahle to predict when bogging conditions are being approached in order that suitable adjustments United States Patent Ofiice oils in" afliiidi'zed' solids system. It relatesgpart icularly to a method for controlling the coliingreaction whereby maximum 'feed" rates I may be enjoyed while hogging or defluidization of the fluid coking be d i's revented:

In essence, this invention provides =a-method andapparatus for determining -'incipient 'bog-ging conditions in ar-coking zonewhereby proper adjustmentcanbe' made of the operating conditions to .c'ircumventbogging' "and consequent shutdown of the equipment. =ofi determining the onset'of bogging" comprises operating a relatively small'eoking or testunit integrally withthe .maincoking unit. A portion of the fluihized-hed inthe main 'colier along with adhering 1 iquid r'esiduei's" transfenedtothe test coker. feed is injected into' the *testcoker such that the test This method A- small amount a of additional whet-is operating a'tan actual higher WZ/hrz/w. than thc main coker. Thus so l'ong as this test coker or"b'oggi-ng 'c'okcr is operable; it is indicative that bogging condit'ions d'o not exist in themain coken. v

fluid cokingLpr-oc'ess normally consists of a fluid 1 coking vessel and an external heating vessel, e'. g., a flilid bed hurnert A- fluid bed of solids, preferably coke particles-produced by the process; is maintained 'in 'the coking zone by a fluidizing gas, e. g., steam; 'Th'e'tenrperatnreof the bed ismaintained at about 950 F! by "circulating solids: to" the heating vessel and "back.

The

heavy oil to be converted is injected into the bed and =upon= contaet with= the hot solids 'undergoes pyrolysis,

evolving lighter hydrocarbon vapor-s and depositingresi due or coke on the solids.

Themixih'g-actiou of the fiui'd bed normally results in substantially isothermal conditions in the Bed and thorough and rapid distribution of the injectedi lie'avy oil. Product vapors; after having entrained solids removed; are-removed from the coking vessel' and sent to a "scrubher'or'fractionator for coolih g and separation;

Witli-the' customary" feeds to the process, the viscous, near'solid components, e. 'g.', asphalt'enes, of theteedprcid i1ce a-ta'cky filin on theparticles'which; until it iscoked jdry, tends tocause the fluid-particlesto adhere to' each otherand to form 'agglomerates. If the'feed Iateis'toO high,'or i f' the coking reaction is too slow; sufficient age glomer'ation' of the particles will occur to cause coinplete-deflui'd'ization or hogging of thefiuidbed. Bogging "of the bed'isone of the most serious problems encoun- Thus; it is of vital inrin operating conditions can 'be'mad" 'I'thasbe'en thepracticeto operate a cokingprocesswell on the safe side" of conditions that cause bogging, i. e.,

.a'tlow feedlra'tes; high temperatures, high solidcirculation .rates,z etc. In this way, all of the feed is promptly evaporated and cracked and thetacky residue isspread oven.a.large=amount of-the solidssovthatno substantial agglomeration takesvplacer obviously operating wellton this invention clear.

2,808,368 Patented Oct. 1 1957 ;his =invention provifle'sa meaustor detecting the'enset .of boggin'g -of the fluid bed wherehy" operations at mastito operate a a small test 'co'k'er integrally with the main coker; usihgrsol ids; withdrawn from} the main cokin he'd. This: test c'oker operated; a slightly, higher feed "rate suchithat the festcokei Will he the one fi'rs't to bog ifcondition's co ducive toiboggiug arebeing ap roached inthe :main coking bed;

The preseuttiuvehtionresults in the aditant'age thafthe test .coher i'operat'es' on the: same coke presentin themai-n .cohirr rhediaho with thesam-e': feed stockb'ei gives} therefore, attire-picture of: the actual conditions attheimoment' i'n' 't'he coker; of .th'e -test coker' cannot be ohta'ined by operating: on solids from another'sou'rce, e. .,-from: the burner. The change 1 in liquid or residue eonte' t oi the --s'61id's"-'supplied to: the test coker" from 'the mfiimCOkBri's' reflectiveofa E uivalent: operation changeor upset' in the main coker, and is"- used invention 'tWg'ive ansacou'rate picture of eonditions in-the main c'olieri In contrast to previously roposed devicesfor predieting bogging; the -instrument of: this 'nv'ention does-not have to be calibrated. to perform satisfactorily; With schemes reviously proposed, the main coking Bed-had .torbe a'ctually subj'e'cted to conditions: tohog ihg' i11 order to calibrate the predicting deviee} This is highlyuudesirable; I

.It.isa tovbe 'appreciated that theldegroe ofi-safety factor desired to anticipate bogging can be r'eadilyadj'usted by varying. the incremental-feed rate to the-testor hogging 'coker. Alternatively, theitest: coker- 'c'an be-o erated at a slightly lower temperature to -achieve a safety factor with less incremental feed. It is obvious that the causing of 'boggin'gtiui the verysihalllcoke'r isznot serious as its operability. can be 'readily restored.-

When-:the test cokerindicates that conditions of hogging: arebei'n'g'i approached in the main: coking bed; then suitableiadjustment' can be lila'de 'ili the operating conditionstto avoidthis'sditii'culty. por example, the feed rate to the cokercan be: decreased}- theeokingzteniperature may be increased, anil fior the-"solids'circula'tio''n rate may heincreased;

Reference to the attached drawingwili serve-tomake The drawing illustrates-a referred modification of this invention andis presented for'purposes orillustrationonly;

Illustrated by the drawing? is a fluid: coking vessel 1 for pyrolytica y converting heavy oils; A .fluid 'bed of solids, e: g1 coke'o'f 40 to800 microns' ih size; having an upper level L is maintained in the vesselby admitting a fiuidizing gas, e. g., steam, to thebase ofth'evessel by line 2 in amounts sufiicient to obtain superficial fluidizing gas velocities in the coker in the range of 0.5 to 4 ft./sec. Coke at a temperature to 300 F. above thetcoking temperature is admitted to. the coker byilihe'3 in amounts sufiicient to maintain a eokin'gte'rnpera'ture in the rang'e'of 900 to 1200 F. The'l'owerportion of thecoker serves as'a stripping zone to removeoccludedhydrocarbons from the coke. Coke is-withdrawn from this strippingzone by line 4 and is circulatedto an external heatingrzone to he reheated.

The :feed; e. g., a residual oilh is' injected into the coker via line 5. Vaporous conversion products are removed overhead by line 6 as product after having entrained solids removed by cyclone system 7.

To anticipate the onset of bogging, a small external fluidizing vessel is supplied continuously with a small .amount of coke withdrawn from themain coke bed via line 9. This coke has a small amount of yet unconverted tacky material adhering to it. A fluidizing gas, e. g., steam, is admittedat thebase of the test coker by line 10 to fluidize the solids therein and to maintain essentially the same gas velocity as that in the main coker. Conversion products and spent fluidizing gas are withdrawn from the top of the test coker by line 11 and may be vented or, more conveniently, be returned to the upper portion of the coker 1 via line 11. Additional amounts of steam may be supplied to line 11 to prevent coking in this line and to aid in regulating the pressure in the test coker.

If it is required, he test coker may be heated as by steam coils 12 or electrical heaters to maintain the coking temperature. With the exception of the feed rate, the conditions in the test coker are maintained substantially equivalent to the conditions in the main coking bed. In general, the capacity of fluid solids of this test coker need notbe greater than about 1 cu. ft. to give an accurate picture of the conditions in the main coking bed, although in some cases, somewhat larger volumes may be desirable.

In some applications, the test coker can be run at a lower temperature, preferably 20 to 50 F. below the coking temperature, to give the desired safety factor. There will be, therefore, more of the tacky compounds on the solids in the test coker because of the lower cracking rate. Thus, bogging conditions will be more closely approached in the test coker.

Solids are overflowed from vessel 8 through line 13 to maintain a constant inventory of solids therein. These solids may conveniently be returned to the coking vessel. Other means of withdrawing solids from the test coker will occur to those skilled in the art.

A small amount of the same residual oil feed introduced into the main coking bed, including any recycle streams, is introduced into the test coker by line 14. -This feed rate may, for example, be in the range of 0.01 to 0.5 1b./hr./lb. of solids contained in the test coker (w./hr./w.), depending upon the degree of safety factor desired.

By introducing fresh feed into the test coker, the test coker is made to operate at an actually higher w./hr./w. than the main coking bed. Thus, it is more amenable to becoming defluidized or bogged. If the fluid bed in the test coker does bog, then it is indicative that bogging conditions are being approached in the main coking bed.

In some applications, the solids bed in the test coker can be mechanically agitated or stirred as by a vaned impeller or paddle to maintain proper motion of the particles without exceeding fluidization gas limits.

A determination that the bed of the test coker has become defluidized can be made in several ways. If the gases in line 11 start to surge or if the coke ceases to flow through line 13, then bogging has or is about to occur. Alternatively, the bed could be probed with a steel rod to see if compaction or agglomeration is occurring. A rapid decrease in the temperature of the small coking bed is also indicative of bogging.

Example A fluid coker contains 300 tons of coke of a particle size in a range of 40 to 800 microns, 250 microns median particle size. The bed is maintained at a temperature of 950 F. and steam is admitted to the base of the vessel in amounts sufficient to maintain an average superficial fluidizing gas velocity of 1.5 ft./sec. The pressure above the fluid bed is 6 p. s. i. A heavy oil having an API gravity of 42, a Conradson carbon content of 24 wt. percent, and an initial boiling point of 950 F. is introduced into the fluidized bed at a feed rate of 0.5" w./hr./w. Under these conditions, the coker will bog at a feed rate of about 0.65 w./hr./w. All of the oil is converted under these conditions with recycle operation to products boiling below 1000 F. excluding coke. Twenty pounds of coke are maintained in a fluidized condition in a small test coker having an internal diameter of 6 inches and a bed height of 36 inches. Solids are withdrawn from the main coking bed at a rate of lbs/hr. and are supplied to the test coker through a /2 inch line. Additional feed is introduced into the test coker at a rate of about 0.4 w./hr./w. This amounts to about 3 l0- of the feed charged. to the main coking bed. Under these conditions the test coker is operating at a total eflective feed rate of 0.6 w./hr./w., 20% higher than the main coker.

Modifications of this invention will occur to those skilled in the art. For example, other schemes for controlling the coke flow to the test coker can obviously be used. Having described this 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 fluid coking process wherein an oil is pyrolytically converted by contact with a fluidized bed of solids maintained at a coking temperature in a coking zone to vaporous conversion products and residue which is deposited on said solids, a method of determining the onset of bogging of said fluidized bed, which comprises withdrawing a portion of said fluidized bed, introducing said portion into a relatively small coking zone, fluidizing the solids in said small coking zone at a gas velocity substantially equal to that in said fluidized bed, maintaining said small coking zone substantially at said coking temperature, flowing solids from said small coking zone at a rate sufficient to maintain a substantially constant solids holdup therein, removing conversion products overhead from said small coking zone, introducing a small amount of said oil into said small coking zone, and decreasing the liquid content of the solids of said fluidized bed when the solids in said small coking zone begins to bog.

2. The process in claim 1 wherein said decrease in liquid content is achieved by increasing said coking temperature.

3. The process of claim 1 wherein said decrease in liquid content is achieved by decreasing the feed rate of said oil to said coking zone.

4. The process of claim 1 wherein said conversion products are transferred to the upper portion of said coking zone and the solids removed from said small coking zone are returned to said fluidized bed.

5. A process for coking a heavy oil which comprises contacting a heavy oil at a coking temperature with a bed of fluidized solids in a coking zone whereby said heavy oil is converted to vapors and residue which is deposited on .said solids, maintaining in a second relatively small coking zone a bed of fluidized solids sub-' stantially at the condition of said coking zone, flowing solids coked with adhering liquid residue from said coking zone to said smaller coking zone, removing solids from said second coking zone, introducing a small amount of said heavy oil into said second coking zone, whereby said second zone operates at a higher w./hr./w. than said coking zone, and decreasing the concentration of liquid on the solids in said coking zone when said second coking zone indicates bogging.

6. A method of predicting bogging in a hydrocarbon oil fluid coking process wherein an oil is converted to relatively lighter hydrocarbon vapors in a coking zone by contact at a coking temperature with a bed of fluidized solids, which comprises operating a relatively small fluid bed test coker integrally with said coking zone using solids withdrawn from said coking zone along with adhering liquid residue, and introducing a small amount 5 6 of said oil into said test coker whereby said test coker References Cited in the file of this patent operates at a higher effective feed rate and is more prone to bed bogging than said bed of fluidized solids. UNITED STATES PATENTS 7. The invention of claim 6 wherein said relatively 2,690,990 Adams et al. Oct. 5, 1954 small coker is operated at a temperature 20 to 50 F. 5 2,709,676 Krebs May 31, 1955 lower than said coking temperature. 2,735,802 Jahnig Feb. 21, 1956

Claims (1)

1. IN A FLUID COKING PROCESS WHEREING AN OIL IS PYROLYTICALLY CONVERTED BY CONTACT WITH A FLUIDIZED BED OF SOLIDS MAINTAINED AT A COKING TEMPERATURE IN A COKING ZONE TO VAPOROUS CONVSION PRODUCTS AND RESIDUE WHICH IS DEPOSITED ON SAID SOLIDS, METHOD OF DETERMINING THE ONSET OF BOGGING OF SAID FLUIDIZED BED, WHICH COMPRISES WITHDRAWING A PORTIONOF SAID FLUIDIZED BED, INTRODUCTING SAID PORTION INTO A RELATIVELY SMALL COKING ZONE, FLUIDIZING THE SOLIDS IN SAID SMALL COKING ZONE AT A GAS VELOCITY SUBSTANTIALLY EQUAL TO THAT IN SAIDFLUIDIZED BED, MAINING TEMPERATURE, FLOWING SOLIDS FROM SAID SMALL COKING ZONE AT A RATE SUFFICIENT TO MAINTAIN A SUBSTANTIALLY CONSTANT SOLIDS HOLDUP THEREIN, REMOVING CONVERSION PRODUCTS OVERHEAD FROM SAID SMALL COKING ZONE, INTRODDUCING A SMALL AMOUNT OF SAID OIL INTO SAID SMALL COKING ZONE, AND DECREASING THE LIQUID CONTENT OF THE SOLIDS OF SAID FLUIDIZED BED WHICH THE SOLIDS IN SAID COKING ZONE BEING TO BOG.

Images