US2735806A

US2735806A - Method of scouring equipment in a fluid coking process

Info

Publication number: US2735806A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

1956 B. v. MOLSTEDT ET.AL ,806

METHOD OF SCOURING EQUIPMENT IN A FLUID COKING PROCESS Filed Aug. 25, 1954 4 Sheets-Sheet 1 m 0 MD EA wxOU n 2339mm- PUDQOmE All IN V EN TORS BYRON w. MOLSTEDT JOHN F MOSER. JR

W M a m ATTORNEYS Feb. 21, 1956 Filed Aug. 23, 1954 FIG. ]I

B. v. MOLSTEDT ET AL 2,735,806

METHOD OF SCOURING EQUIPMENT IN A FLUID COKING PROCESS 4 Sheets-Sheet 2 5 l0 INCREASE IN A P, REACTOR THROUGH CYCLONE OUTLET, INCHES H O IDAY 8 8 O m m 2 ENTRAINMENT, LBS/BBL OF FEED BYRON w. MOLSTEDT JOHN F. MOSER, JR. INVENTORS 441, a; 4,725., ATTORNEYS O O N Feb. 21, 1956 v, MQLSTEDT ET AL 2,735,806

METHOD OF SCOURING EQUIPMENT IN A FLUID comma PROCESS Filed Aug. 23, 1954 4 Sheets-Sheet. 5

I00 FEED RATE, BBL/DY./$Q. FT.

ENTRAINMENT LBS/BBL FEED 2 3 SUPERFICIAL GAS VELOCITY, FI/SEC.

BYRON w. MOLSTEDT JOHN F. MOSER,JR. INVENTORS )afiJnC/M 4/ 51 ATTORNEY-Y 1956 B. v. MOLSTEDT ET AL 2,735,805

METHOD OF SCOURING EQUIPMENT IN A FLUID COKING PROCESS Filed Aug. 25, 1954 4 Sheets-Sheet 4 FIG. 11

MULTIPLIER 5 IO l5 OUTAGE FEET BYRON w. MOLSTEDT JOHN F. MOSER, JR. INVENTORS M WW4 QMTTORNEYY United States Patent METHOD OF SCOURING EQUIPMENT IN A FLUID COKING PROCESS Byron V. Molstedt and John Frederick Moser, Jr., Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware Application August 23, 1954, Serial No. 451,382

Claims. (Cl. 202-38) This invention relates to a process for converting hydrocarbons. More particularly, it pertains to coking of heavy residual oils by the fluidized solids technique. Specifically, this invention is concerned with an improved hydrocarbon oil fluid coking process wherein a coking charge stock is contacted at a coking temperature with a body of coke particles maintained in a fluidized state in a coking zone.

The hydrocarbon oil which forms the coking charge stock of the present process is preferably a low value highboiling residuum of about to 20 API gravity, about 5 to 50 wt. percent Conradson carbon, and boiling above about 900 to 1200 F. Broadly, however, any hydrocarbon oil may be treated in the present process, including shale oils, tars, asphalts, oils derived from coals, synthetic oils, recycled heavy ends from the coker effluent, whole crudes, heavy distillate and residual fractions therefrom, or mixtures thereof.

The prior art is familiar with a fluid coking process wherein an oil is pyrolytically upgraded by contact at a coking temperature with particulate solids maintained in a fluidized state in a coking vessel. Upon contact with the solids, the oil undergoes pyrolysis, evolving lighter hydrocarbons and depositing carbonaceous residue on the solid particles causing them to grow in size. Theneeessary heat for the pyrolysis is supplied by circulating a stream of the fluidized solids through an external heating zone, e. g., a combustion zone, and back to the coking vessel. This fluid coking process is more fully presented by co-pending application entitled, Fluid Coking of Heavy Hydrocarbon and Apparatus Therefor, S. N. 375,088, filed August 19, 19.53, by Pfeiffer et al.

Because more coke is produced by the coking process than'is required to be burnt to supply heat, the heatcarrying solids will continue to grow in size because of the carbon deposition and a portion of the solids must be withdrawn to maintain the total mass or weight inventory of the particles substantially constant. It is customary in commercial processes to withdraw some of the coke from the system, comminute the coke in some manner to form seed coke or growth nuclei and to return the seed coke to the process to maintain the particle size and particle size distribution relatively constant. This size reduction of solids may be accomplished, for example, by jet attrition grinding. The net coke product of the process may be classified as by elutriation such that only relatively coarse material is withdrawn whereby the coke of seed size in the process is conserved.

Serious problems have been encountered in the development of this type of coking. One problem in particular is-the building up of coke deposits on the confines of the vapor space above the fluidized bed. These deposits cause the pressure drop through the coker and overhead lines to increase to such an extent as to require the coker to be shut down periodically and cleaned out.

As the vapors leaving'the coking bed are at or near their dew'or condensation point, they will readily condense. This condensation is aided by endothermic polybe greatly inhibited or substantially eliminated by maintaining in the vapors withdrawn overhead entrained solids from the fluid coking bed in amounts above a certain critical minimum, specifically, above 400 lbs./bbl. of coking charge stock. Entrained solids in amounts above this critical level help to uphold the temperature of the vapors, scour attendant surfaces thereby removing carbon deposits and provide surfaces upon which condensing vapors are absorbed.

It has not been appreciated heretofore that entrained solids from the coking bed, if present above a critical level in the coker product vapors, prevent coke deposition and fouling in the overhead system of the coking reactor. The present invention resides in the discovery that by proper control of the operating conditions of a coking process, partly by control of the particle size, particle size distribution and fluidization gas velocities, solid particle entrainment rates from the fluid bed can be controlled, and by controlling the entrainment rate and hold.- ing it above a certain critical minimum, coke deposition is substantially eliminated.

The nature and objects of the present invention will more clearly appear during the following description of the drawings attached to and forming a part of this specification. In the drawings, Figure I schematically presents a preferred hydrocarbon oil fluid coking process adapted to achieve the objects of this invention. Figures II, III and IV are graphical presentations of data illustrating this invention and its advantages.

Referring to Figure I, the major items of equipment shown are a coking vessel 1 and a combustion vessel or burner 2 used to supply heat to the process. The fluid coker 1 contains a fluidized bed of high temperature solids having an upper level L. Preferably, the solids used in the coking process are finely divided coke particles produced by the process. Other solids such as sand, spent catalyst, pumice, etc. may, however, be used. In this particular coking vessel design, the lower portion 1-A of the vessel serves as a stripping zone. The intermediate portion, 1-3, is conical in shape so as to minimize the consumption of fluidizing gas by permitting conversion products generated in the lower portion of the coker to serve as fluidizing gas in the upper portions. The upper portion l-C is necked down so as to increase the velocity of the vapors withdrawn overhead thereby decreasing the secondary vapor phase cracking of the products. As will later appear, the extent of reduction in the cross-sectional area of the coker at this point is an important design consideration, as the velocities of the vapors affect the solid entrainment rate.

The oil to be upgraded, such as a vacuum residua, is injected into the vessel at a plurality of points via line 3. The feed rate is preferably maintained at a rate between 25 to bbL/day/ft. of reactor cross-section area at the upper level of the bed. The oil undergoes pyrolysis at a temperature in the range of 850 to 1660 F, preferably 950 to 1050 F. When gas oils for catalytic cracking are desired, the coker is operated at a temperature in the range of 950 to 1050 F. When lighter products are desired, e. g., naphthas and heating oils, the operating temperature is about 1050 to 1200" F. and when chemicals and chemical intermediates are desired,

the temperature is l200 to 1600 F., preferably l300 to l450 F. Steam is admitted to the base of the vessel as by line 4. This steam serves first to strip the coke particles before the coke is circulated to the burner and then passes upwardly through the vessel fluidizing the solids therein. The reaction products are taken oif overhead by line 5 after having entrained solids removed by cyclone 6 and may be further processed, as desired.

In order to supply heat to the process, solids are circulated from the base of the coking vessel by line 7 to a burner vessel 2. Here the particles are fluidized by an oxidizing gas, e. g., air, supplied by line 8. The resulting combustion heats the particles to a temperature 100 to 300 F. higher than the coking temperature. After having entrained solids removed, flue gases are removed overhead from the burner vessel by line 9 and may be vented to the atmosphere. Heated solids are transferred to the fluid coker by line 10. Other means may, of course, be used to reheat the coke particles including gravitating bed burners, transfer line burners, shot heating systems, and other direct and indirect heating means. Line 11 removes from the coking vessel the net coke product and agglomerates produced by the process.

Coking deposits will normally form in the coking vessel on the surfaces above the fluid bed level L including the surfaces of cyclone 6 and overhead line 5 unless steps are taken to prevent their formation. This invention is directed primarily towards the prevention of coking between the fiuid bed level I. and the inlet, C, to the cyclone, although operation in accordance with the present invention will substantially reduce coking of the equipment beyond inlet C. It is in this area between L and C where the greatest amount of coke deposition occurs in normal coking operations.

In some coking processes, the solids separating means, i. e., cyclone. is located exteriorly of the coking reactor. The present invention is also applicable to such designs.

According to the present invention, the superficial velocity of the vapors, which comprises reaction products and fluidizing gas, through the upper surfaces L of the fiuid bed is regulated to obtain an entrainment above 400 lbs./bbl. of stock charged to the coker. By maintaining this entrainment rate, coking of the equipment is substantially prevented.

The criticality of maintaining a certain entrainment rate will be appreciated by reference to Figure II. Figure II presents data obtained from a fluid coker operating under conventional conditions. The abscissa of Fig ure II indicates the increase in pressure drop from the level. L of the fluid bed through the cyclone outlet due to coking and fouling in a coking vessel, as related to the amount of solids entrained in the vapors withdrawn overhead. As can readily beseen, coking of equipment is virtually nonexistent when the entrainment rate is above 400 lbs./bbl. of feed. When the entrainment rate is less than 400 lbs./bbl. then the increase in pressure drop due to coking increases almost exponentially with decreases in entrainment rate. The figure also shows that there is no practical advantage obtained by maintaining the entrainment rate above 800 lbs. of solids/bbl. of feed.

The entrainment rate of solids from the fluid bed is controlled primarily by control of the superficial fluidized gas velocity. Attention must be paid, however, to the particle size and size distribution of the fluidized coke. For a relatively coarse material, fluidizing gas velocity will. have to be higher to obtain a given entrainment as opposed to the use of a finer material.

The fluidizing gas velocity may be controlled by various I methods. The amount of fluidizing steam or other inert gases used may, of course, be controlled to regulate the superficial gas velocity. Excessive use of fluidizing gas is to be avoided, however, as it results in uneconomical operation. The rate of feed injection into the coking vessel may also be controlled so as to control fluidizing gas velocity. This method of control per se is not too attractive as it may mean in some instances that the coker will have to be operated at capacities less than maximum. A preferred method of controlling the superficial gas velocity is to control the level L of the fluid bed along a tapered portion of the reactor as shown. By increasing the amount of coke hold-up in the reactor, the level L will move upwardly along this tapered portion and consequently the cross-sectional area of the surfaces of the fluid bed will be decreased. Thus the fluidizing gas velocity through this surface will be increased and the entrainment rate will thereby be increased. The extent of this taper is, of course, a design consideration and can be made to provide for a fairly wide range of operating conditions. By proper design, the reactor may be configured to achieve an entrainment rate of over 400 lbs. of coke/bbl. of feed without the necessity of resorting to any special control techniques.

So far as it is known, it is believed that the entrainment from a fluid bed with a given particle size and superficial gas velocity and the amount of solids entrained in withdrawn vapors is substantially independent of the reactor geometry or configuration at the top or above the fluid bed with the exception of reactor outage. Outage is the distance from the surface of the fluid bed to the cyclone outlet, indicated by the dimension 0 on the drawing. As outage is increased, the amount of solids contained in the vapors in the uppermost portions of the reactor will be decreased.

Figure III shows in simplified relations a method of controlling the entrainment rate. Although the coke particle size used in fluid coking may vary as much as 0-1000 microns or more, the preferred particle size is within the range of 40-500 microns, with 200-300 microns being the median particle size. The particle distribution may vary within these ranges. The preferred particle distribution is such that 10 to 20 wt. percent of the coke is smaller than 147 microns, 30 to 60 wt. percent is smaller than 175 microns, 60 to wt. percent is smaller than 246 microns and 0 to 5 wt. percent is no larger than 400 microns. This particle size and distribution are controlled by controlling the rate and size of seed coke additions and coke product withdrawal. The coke particles normally have a true particle density in the range of 90 to lbs./ft. For material this size, the superficial gas velocities in the reactor will lie in the range of 1-5 ft./ sec. and bed densities will be in the range of 30 to 55 lbs./ft.

For a fluid coking vessel operating under normal coking conditions, e. g., temperature 950 F., pressure 6 p. s. i. g., fluidizing steam 5 wt. percent based on feed, coke circulation rate to burner 15 lbs./lb. of feed etc., a chart in the nature of Figure III may be prepared. Figure III relates feed rate in bbls./day/sq. ft. of cross-sectional area of the reactor to the entrainment rate in lbs./bbl. of feed which is dependent upon the superficial gas velocity and size distribution of the coke in the figure. Wt. percent retained on 80 mesh is used in the chart as being indicative of the size distribution of the coke. Thus for a set of conditions, the fluidizing gas velocity necessary to obtain the minimum entrainment needed to avoid coke deposition can be obtained from Figure III following lines X, Y and Z in the direction indicated.

A commercial fluid coker may be designed to provide sufiicient gas velocity in the top of the reactor to give suflicient entrainment under normal conditions. It is quite possible, however, that a coking unit may be called upon to operate at widely varying feed rates. For example, the unit may process 10,000 B./D. of oil in the winter and yet only be required to handle 4000 B./D. in the summer. At the lower feed rate, the gas velocity in the top of the reactor will be greatly reduced. This would mean that unless special operating controls were invoked, the entrainment rate would become dangerously low, leading to coke deposits in the top of the reactor followed by a shut down of the unit. By use of a plot similar to Figure III, however, proper entrainment can be maintained. As the feed rate is reduced, both the velocity and the particle size of the circulating coke can be varied to hold the entrainment above 400 lbs./bbl. of feed. Velocity can be adjusted by increasing the quantity of fiuidizing gas and by controlling the position of the surface of the fluid bed with respect to upper tapered portion of the reactor, if the reactor be so designed. The particle size and distribution can be varied by adjusting the amount and size of seed coke or growth nuclei added to the system.

For example, with reference to Figure III, a coking reactor may normally handle an 80 B./D./ft. of reactor cross-section, of feed. "The entrainment at this rate may be desired to be about 600 lbs./'bbl. of feed. The lines X, Y and Z on Figure III represent the conditions necessary to maintain the desired entrainment. If the feed rate becomes 40 B./D./ft. then the entrainment will drop below the minimum for safe operation, even though this decrease is partially compensated for by a decrease in particle size, as indicated by lines X, Y and Z". But by a change both in'g'as velocity and in particle size, as indicated by lines X, Y and Z, the entrainment can be brought back to the desired 600 lbs./bbl.

Figure III was based upon a coking vessel having about ll ft. outage. Figure IV illustrates a method that may be used to extrapolate the data of Figure III to a coking vessel having a different outage. The abscissa of Figure 1V indicates the outage in feet and the ordinate yields a multiplier which can be used to adjust the curves in the left field of Figure III upwardly or downwardly.

It should be understood that the Figures III and IV have been used only to illustrate one method of controlling and determining the entrainment rate and this invention is not to be limited thereby. Other methods may well be used. It is important, however, that the entrainment in the vapors withdrawn overhead be above 400 lbs./bbl. of feed.

Solids entrained according to this invention are most beneficial in preventing coke deposits before the cyclone outlet. However, coke deposition in the cyclone and beyond is also inhibited because the high entrainment minimizes temperature drop and, therefore, inhibits condensation of the vapors. The solids also absorb any condensate and polymeric material formed from the vapors. The entrained solids will also provide a scouring action in the cyclone and, due to some coke losses through the cyclone, will scour the lines after the cyclone.

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 conversion process wherein a coking charge stock is contacted with particulate solids maintained at a coking temperature in a coking zone to obtain relatively lighter hydrocarbon vapors, wherein said solids are maintained in said coking zone as a dense fluid ized bed, and wherein said vapors are withdrawn overhead from the upper surface of said bed and thence from said coking zone; the improvement which comprises maintaining an entrainment of said solids in said vapors withdrawn overhead above 400 lbs./bbl. of said charge stock.

2. The process of claim 1 wherein said particulate solids include coke produced in said process having a particle size substantially in the range of 40 to 500 microns.

3. The process of claim 1 wherein said particulate solids have a particle size substantially in the range of 40 to 500 microns and have a particle size distribution substantially in the ranges of 10 to 20 wt. per cent smaller than 147 microns, 30 to 60 wt. per cent smaller than 175 microns, 60 to wt. per cent smaller than 246 microns and 0 to 5 wt. per cent no larger than 400 microns, and wherein the superficial velocity of ascending gases through said upper surface is controlled within the range of 1 to 5 ft./sec. to attain said entrainment.

4. In a hydrocarbon oil conversion process wherein a coking charge stock boiling above about 900 F. is contacted with particulate coke having a true particle density in the range of 90 to lbs/ft. and maintained at a coking temperature in the range of 850 to 1600 F., wherein said coke is maintained in said coking zone as a dense fluidized bed, and wherein conversion products are withdrawn from the upper surface of said bed as a relatively dilute solids-gas suspension; the improvement which comprises maintaining the particle size of said particulate coke substantially in the range of 40 to 500 microns, controlling the particle size distribution of said particulate coke so that 10 to 20 wt. per cent of said particulate coke is smaller than 147 microns, 30 to 60 wt. per cent is smaller than 175 microns and 60 to 90 wt. per cent is smaller than 246 microns, maintaining the density of said fluidized bed in the range of 30 to 55 lbs./ft. introducing said feed stock into said coking zone in an amount in the range of 25 to 150 bbl./day/ft. of reactor cross-sectional area, adjusting the superficial velocity of gases passing upwardly through said upper surface to a velocity in the range of 1 to 5 ft./sec., so as to secure a minimum entrainment of said particulate coke in said vapors withdrawn of at least 400 lbs./bbl. of said charge stock whereby coke deposition on the confines of said solids-gas suspension is substantially eliminated.

5. In a hydrocarbon oil conversion process wherein a coking charge stock is contacted with particulate coke maintained at a coking temperature in a coking zone to obtain relatively lighter hydrocarbon vapors, wherein said coke is maintained in said coking zone as a dense fluidized bed, wherein said vapors are withdrawn overhead from the upper surface of said bed and thence from said coking zone and wherein net coke product is withdrawn from said process and seed coke is added to maintain the weight inventory, particle size and particle size distribution of said particulate coke substantially constant, the improvement which comprises controlling the operating conditions of said process to obtain an entrainment of said solids in said vapors withdrawn overhead in the range of 400 to 800 lbs./bbl. of said charging stock.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

4. IN A HYDROCARBON OIL CONVERSION PROCESS WHEREIN A COKING CHARGE STOCK BOILING ABOVE ABOUT 900* F. IS CONTACTED WITH PARTICULATE COKE HAVING A TRUE PARTICLE DENSITY IN THE RANGE OF 90 TO 110 LBS./FT.3 AND MAINTAINED AT A COKING TEMPERATURE IN THE RANGE OF 850* TO 1600* F., WHEREIN SAID COKEIS MAINTAINED IN SAID COKING ZONE AS A DENSE FLUIDIZED BED, AND WHEREIN CONVERSION PRODUCTS ARE WITHDRAWN FROM THE UPPER SURFACE OF SAID BED AS A RELATIVELY DILUTE SOLIDS-GAS SUSPENSION; THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE PARTICLE SIZE OF SAID PARTICULATE COKE SUBSTANTIALLY IN THE RANGE OF 40 TO 500 MICRONS, COTROLLING THE PARTICLE SIZE DISTRIBUTION OF SAID PARTICULATE COKE SO THAT 10 TO 20 WT. PER CENT OF SAID PARTICULATE COKE IS SMALLER THAN 147 MICRONS, 30 TO 60 WT. PER CENT IS SMALLER THAN 147 MICRONS AND 60 TO 90 WT. PER CENT IS SMALLER THAN 246 MICRONS, MAINTAINING THE DENSITY OF SAID FLUIDIZED BED IN THE RANGE OF 30 TO 55 LBS./FT.3, INTRODUCING SAID FEED STOCK INTO SAID COKING ZONE IN AN AMOUNT IN THE RANGE OF 25 TO 150 BBL./DAY/FT.2 OF REACTOR CROSS-SECTIONAL AREA, ADJUSTING THE SUPERFICIAL VELOCITY OF GASES PASSING UPWARDLY THROUGH SAID UPPER SURFACE TO A VELOCITY IN THE RANGE OF 1 TO 5 FT./SEC., SO AS TO SECURE A MINIMUM ENTRAINMENT OF SAID PARTICULATE COKE IN SAID VAPORS WITHDRAWN OF AT LEAST 400 LBS./BBL. OF SAID CHARGE STOCK WHEREBY COKE DEPOSITION ON THE CONFINES OF SAID SOLIDS-GAS SUSPENSIONIS SUBSTANTIALLY ELIMINATED.