US3409542A - Coking process and apparatus - Google Patents

Description

Nov. 5, 1968 B. v. MOLSTEDT 3,409,542

COKING PROCESS AND APPARATUS Filed Dec. 21, 1966 -BURNER 15-- 7 +FLUE GAS ""CYCLONE 8-v- REACTOR W GAS GAS FLUIDIZING GAS B. Inventor United States Patent 3,409,542 COKING PROCESS AND APPARATUS Byron V. Molstedt, Baton Rouge, La., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Dec. 21, 1966, Ser. No. 603,490 8 Claims. (Cl. 208127) ABSTRACT OF THE DISCLOSURE A process and apparatus are described for coking hydrocarbons wherein heat is supplied to the process by upflow transfer of coke particles with liberated gases through a transfer line heater and the heated particles are then returned to the coking zone. The top of the reactor is tapered in a trumpet shape. The disclosed invention is especially useful in a high temperature coking process.

This invention relates to the fluid coking of hydrocarbons, especially petroleum hydrocarbons. More particularly, it relates to a process and apparatus for making high temperature fluid coke wherein the endothermic heat for the process is supplied to the coke by burning combustible gases from the coking process itself.

It is known to prepare coke in a fluidized bed at low temperature, e.g., 850 to 1200 F., and at high tempera tures, e.g., 1800" to 2800 F. A typical fluid coking process and apparatus is described in US. Patent No. 2,881,- 130. A high temperature fluid coking unit consists basically of a reactor vessel for conducting the coking reactions and an auxiliary heater-burner vessel for supplying heat for the reactions. The reactor vessel or coker is generally a vertical, elongated vessel with a flat or elliptical top and contains a dense, turbulent, fluidized bed of hot coke particles maintained at a temperature in a range of about 1800 to 2800 F. Feedstock, i.e., hydrocarbon, is injected into the coker and cracked essentially to hydrogen and coke, usually with the formation of a small amount of soot and uncracked, or partially cracked, hydrocarbons as by-products. Uniform mixing in the fluid bed results in virtually isothermal conditions and effects instantaneous distribution of the feedstock.

The heat for carrying out the endothermic coking or cracking reaction is usually generated in an auxiliary heater or burner vessel. A fluid stream of coke is withdrawn horizontally or downwardly from the fluidized bed of the reactor via a downcomer and transported as a dense phase, fluid solids system to a riser or vertically aligned conduit. In the riser the coke solids are conveyed upwardly by injection of large quantities of carrier or lift gas, which converts the dense phase of coke to a disperse or dust phase. The disperse phase is mixed with oxygen or oxygen-containing gases, such as air, passed through a transfer line heater and heated by partial combustion of the coke solids therein or by combustion of an extraneous fuel gas added thereto. Such fuel gas may be added at the bottom of the riser and serve also as the carrier or lift gas. Sufficient combustion is carried out to maintain the coker-burner system in heat balance, i.e., by generating enough heat to balance heat losses resulting from the endothermic cracking reaction, the sensible heat in the exiting solids and gaseous coker products, and the heat losses through the walls of the process equipment.

Certain problems and inetficiencies are inherent in coking operations carried out at high temperatures. Large quantities of gaseous products, mainly hydrogen, leave the reactor at high temperatures and carry away a great deal of sensible heat. This greatly lowers the thermal efficiency of the process.

The prior art coking processes require a great deal of special apparatus for transferring solids and handling 3,409,542 Patented Nov. 5, 1968 gases. Several transfer line risers, downcomers, and cyclones are generally required. Separate gas handling facilities are required for the reactor and the transfer line heater. Thus, the gas leaving the reactor passes through a cyclone or other solids-gas separating means, and similar equipment is required to handle the exhaust gases from the burner. This added equipment is not only extremely expensive as an initial investment, but it also requires extensive maintenance. The complexity of the apparatus has made the coking operating sensitive to plugging and coking of various lines. It also presents problems due to the failure of refractory material at the high temperatures involved in the numerous lines. In any system where there is a great deal of solids circulation and transfer, the unit downtime generally goes up substantially as the number of elements in the apparatus. This is especially true in a very high temperature process such as high temperature coking.

Also because of the coarse material of the fluidized coke in a high temperature coking process, the design of the system is critical. The circulated coke solids are very dense, i.e., about 190+ g./cc. with a particle size range of about 5 to 5000 microns, more generally about 50 to 500 microns, with median particle sizes of about to 250 microns. Such solids are diflicult to fluidize in that they tend to form fluid beds which are extremely sensitive to slugging and the formation of large gas bubbles.

The prior coking techniques also require the compression and addition of large quantities of extraneous heating gas as fuel in the transfer line burner or heater.

Still other gas facilities are required to provide the numerous bleed gas injection points and metering equipment necessary to the operation of the various risers, downcomers and transfer lines.

Another disadvantage of prior art coking processes is that extensive cleanup to remove entrained soot from eflluent process gases is required before the gases can be recovered or released into the atmosphere. The cleanup problem is doubly complicated because of the necessity for maintaining two efiiuent streams of gas, i.e., reducing gases from the reactor and flue gases from the transfer line burner.

One method considered for overcoming or avoiding part of these prior art problems is to take advantage of the entrainment of coke in the liberated hydrogen and other reactor effluent gases to transport the coke to a heater or burner located above the reactor. This makes use of the phenomenoon that a fluid bed of coke is actually composed of two more or less distinct phases, namely, a dense phase and a dilute phase. Most of the coke is present as a dense phase with a smaller fraction existing as a dilute phase above the dense phase bed of coke. The dense phase is characterized as a pseudo liquid, having a visible level or upper surface. The dilute phase, on the other hand, is much like a gas phase and consists partly of coke thrown up above the dense phase surface by the boiling or bubbling action of the gas-fluid system.

The lower portion of the dilute phase contains larger particles continuously falling back into the dense phase. Normally, the solids concentration decreases with height in the dilute phase and, if the height is sufficient, the upper portion of the dilute phase will contain essentially only particles so small as to be entrained in the upflowing gases, i.e., having free fall velocities less than the ascending gas velocity.

By decreasing the dilute phase height, the entrainment rate can be increased until coke solids are being entrained out of the reactor at the same rate at which coke is being produced, thus eliminating the need for standpipe withdrawal systems.

In such upflow reactors the fluid solids circulation rate through the burner is the same as the entrainment rate out of the reactor. It is controlled as a function of the reactor outage, i.e., the distance from the dense phase surface to the top of the reactor. As the outage decreases, the entrainment and circulation rate increases for any given gas rate. The outage adjusts automatically, therefore, to that required for the imposed circulation rate for the system.

When systems containing fluid beds of relatively small particles, averaging 40 to 100 microns with a size distribution in the range of from about to 250 microns and having a relatively low particle density, i.e., about 0.8 to 1.5 g./cc., are used in the conventional upflow entrainment type circulation system, little difficulty is encountered. In such systems, the fluidization, entrainment and outage effects are generally smooth in performance and the geometric design of the reactor vessel is not particularly critical. Satisfactory operation can sometimes even be achieved with solids outside the foregoing ranges. For example, more dense particles can be used if they are very small and conversely larger particles can be used if their density is very low.

However, with solids which are both dense and coarse, such as fluid coke, the geometry of the upper portion of the reactor is extremely important. At superficial gas velocities preferred for high temperature fluid coking, i.e., 0.3 to 2.0 ft./sec., the upflow gas velocity is too slow to provide sufiicient entrainment of solids for good solids circulation unless extremely low outages are employed, i.e., 0.5 to 2 feet. These low outages are found to be impractical or inoperable in the conventional system because beds of coarse, dense solids surge and slug under such conditions, which leads to extreme pressure fluctuations. This behavior is due to massive impacts of the bed on the top of the vessel coupled with intermittent choking of the reactor outlet with dense slugs of solids.

Thus, it was heretofore considered impractical, if not impossible, to carry out a high temperature coking process in upflow apparatus wherein a burner or heater located above the coker or reactor vessel can employ the combustible reactor effluent gases, both as a fuel to supply heat to the circulating coke in the burner and as a solids transport medium for carrying or conveying coke particles from the reactor to the heater.

Such a process is now practical and highly advantageous when practiced in accordance with the present invention.

This invention contemplates a process whereby hydrocarbon is heated and cracked essentially to hydrogen and coke in a coking zone containing a dense phase bed of fluid coke particles having a bed height-to-diameter ratio ranging from about 0.5 to 3, preferably 0.5 to 1.5, and a dilute phase zone of fluid coke particles above and contiguous to the dense phase bed. The liberated hydrogen is then passed upwardly at progressively increasing superficial gas velocities through the coking zone to entrain coke particles in the dilute phase zone and convey the hydrogen and entrained coke particles as a disperse phase to a heating or burning zone. In the heating zone an oxygen-containing gas is added to the disperse phase to combust at least a portion of the hydrogen, thereby heating the coke particles. Heated coke is then separated from the disperse phase and returned to the coking zone to provide heat to carry out the endothermic cracking reaction therein.

The progressive increase in superficial gas velocity can be achieved by passing the liberated hydrogen through a specially tapered zone until adequate transport velocities are reached. It is important to increase the gas velocity smoothly, but rapidly, to avoid surging and slugging of solids in the coking zone outlet.

Maximum benefits are achieved when the superficial gas velocity within the dense phase fluid bed of coke ranges from about 0.3 to about 2.0 ft./sec., preferably about 0.3 to 1.0 ft./sec. and increases to velocities in the range of 25 to 100 ft./sec. at the top of the tapered zone.

At such conditions it is desirable to maintain the upper surface of the dense phase fluid bed less than about 5 feet, preferably less than 3 feet below the tapered zone. More preferably, the surface of the dense phase bed will be within the lower part of the tapered zone where superficial gas velocities range from about 1 to about 5 ft./ sec. Smoothest operation is generally achieved when the superficial gas velocity at the surface of the bed is about 2 to 3 ft./sec.

In a preferred embodiment of this invention, a portion of the fluid coke particles is withdrawn from the dense phase fluid bed of the reactor, and conveyed as a dense or disperse phase to the heating zone through a separate riser, by-passing the tapered zone. The flow rate by weight of the coke particles through the tapered zone is about 0.2 to 10 times, and preferably 0.5 to 5 times, the flow rate of coke particles by-passing the tapered zone. At these conditions, optimum entrainment of solids by the liberated hydrogen to provide the smoothest and most efficient operation of the tapered zone is achieved and any potential coke deposition in the top of the reactor or in the outlet is eliminated. Preferably, the total coke rate through the heating zone ranges from about 20 to times the rate at which coke is formed by the cracking of hydrocarbon feed in the cracking zone.

Suitable apparatus according to this invention requires a reactor with a smoothly. tapered top. The top should be generally trumpet shaped, designed according to the following equation:

where H=the height in the tapered top above the reactor proper,

D =diameter of the tapered top at any height H,

D =diameter of the reactor proper,

K=shape factor.

The value of the shape factor, K, ranges from about 1.5 to 2.5 and preferably about 1.85 to 2.15.

Reactor tops conforming to the foregoing equation will provide gas velocity gradients which result in the desired entrainment of fluid solids without slugging when used in combination with a main reactor body of sufficient size to provide an overall bed height-to-diameter ratio of about 0.5 to 3, preferably 0.5 to 1.5. The main reactor body or reactor proper is that portion of the reactor below the tapered top adapted to contain the main body of fluidized solids. In the case of non-circular reactors, the diameters D and D defined above are intended to include pseudo-diameters, i.e., diameters of circles having the same cross-section as the reactor or tapered top in question.

The prefer-red apparatus includes in combination with the tapered top reactor a by-pass riser adapted to transport coke particles at a rate of about 0.1 to 5 times, preferably 0.2 to 2 times the rate at which coke is passed through the tapered top.

As a practical matter of construction, the smoothly tapered trumpet shape of the reactor top can be approximated by a series of at least 2, and preferably 3 or more, converging cones.

The invention will be better understood by reference to the attached drawing, which shows an embodiment wherein the upflow of coke particles in the reactor is provided by gradual tapering of the top of the vessel into three successive cones, which causes the gas velocity to progressively increase as it proceeds upwardly in the vessel, thereby conveying entrained coke particles from the reactor into the transfer line heater, from which heated coke particles are returned to the reactor via a cyclone separator. To supplement the solids conveyed to the trans fer line heater through the tapered reactor top, a dense phase riser is provided in which the solids transfer is controlled by the rate of aerating the transfer line riser. The tapered top is designed to approximate a trumpetshape in which the diameter, D at any height H above the main reactor body, whose diameter is D is determined in accordance with the equation described hereinabove.

Referring specifically to the drawing, hydrocarbon feed is introduced into the reactor by line 21 and is cracked to coke and hydrogen, the hydrogen comprising the fluidizing gas which maintains fluidization of dense phase coke bed 2. Any convenient feed can be used, depending on the cracking temperature employed. Suitable feeds, for example, are low molecular weight gases such as methane, heavy atmospheric or vacuum residua and intermediate naphthas, gas oils and the like. The upflow of coke particles and hydrogen product gas proceeds through dilute phase zone 5, through a tapered zone 6, into line 7, through which the disperse phase of coke and hydrogen is conveyed into transfer line heater 15. Preheated combustion air is introduced through line 9 into the transfer line heater and combusted with hot hydrogen therein. The resulting gases as well as the heated solids, are conveyed downwardly through line 16 into cyclone 8 wherein the heated solids are separated from the combustion gases. Off-gases are vented through line 11. The solids, at temperatures generally about 200-400 F. above the reactor bed temperature, are conveyed downwardly through downcomer 10 and are injected into reactor bed 2 by providing aerating gas through line 19. Additional coke particles are withdrawn from dense bed 2 and conveyed via riser 12 by means of aerating gas injected through line 20. Coke particles circulation is effected by controlling the aerating gas in line 10 and line 12 as well as the bed level in the trumpet-shaped tapered bed reactor. Aerating gases can also be injected into bed 2 via line 13 to optimize fluidization. Product coke is withdrawn through line 14 and about 20 to 40 wt. percent is ground to provide seed particles, which are returned to the fluid bed (by means not shown) to control the average particle size within a fluidizable range.

The invention will be better understood by the following example:

Residuum feed is preheated to 500 F. and injected into a coker reactor having a height-to-diameter ratio of 1.5. A reactor vessel is used having three successive tapered sections at the top with the included angles being 60, 30 and 15 for the lower, middle and upper cones respectively. The tapered sections provide for increasing the superficial gas velocity from about 1.0 ft./sec. in the dense phase fluid bed to about 75 ft./sec. at the outlet of the upper cone. This increase in gas velocity provides the lifting means for conveying coke solids upflow as a disperse phase from the coking reactor to a transfer line heater. The level of the dense phase fluid bed is maintained about 1 to 2 feet up into the lower cone section where the superficial gas velocity is about 2 to 3 ft./sec. This causes the gas to transport solids smoothly out of the tapered zone at the rate of about 05-075 pounds of solids per cubic foot of gas. Additional coke solids are conveyed to the transfer line heater through a dense phase riser. The amount of these solids is controlled at about of the rate at which coke circulates through the tapered zone of the reactor by controlling the rate at which aerating gas is introduced to the dense 'phase riser.

Product coke is withdrawn from this apparatus at a rate sufficient to maintain the reactor bed level. About /3 of the product coke is ground to a particle size of minus 300 mesh and recycled as seed coke. This maintains the average particle size of minus 300 mesh particles in the fluid coke bed between 20 and 30 wt. percent. The coke in the fluid bed has an average particle size of about 200 microns and an average particle density of about 1.9 g'./cc.

Hydrogen from the cracking reaction is evolved at a sufficient rate to provide a superficial linear gas velocity in the fluid bed of 1.0 ft./sec. based on the full reactor diameter below the tapered top. Coke is withdrawn from the reactor and circulated to the transfer line heater at a total rate through the dense phase riser and the tapered reactor top of times the production rate of product coke. The hot evolved hydrogen gas from the reactor is combusted in the burner with air preheated to a temperature of about 1000 F., to heat the circulating coke particles to a temperature of about 2300 F. The heated coke particles are separated from the combustion gases in a cyclone separator and returned via a standpipe to the dense phase bed of coke particles in the reactor, to maintain the temperature therein at about 1200 F.

The process of the present invention can be carried out with a minimum investment cost for apparatus. It also comprises a simple, economical scheme for carrying out the coking reaction, heating the solids in the transfer line burner, and providing heat to the coking reaction by the sensible heat of the circulated solids. The apparatus eliminates various transfer line risers and downcomers. Most important, no gas-solids separator device, such as a cyclone, is required over the reactor outlet. Moreover, in this process, a self-contained unit is feasible, not only to produce coke, but also to produce its own fuel.

The process can thus be completely independent of the availability of extraneous hydrocarbon fuels required in conventional prior art coking processes. Provisions can also be made for recovering the excess amount of hydrogen produced by condensing out the combustion water and removing other impurities. High purity coke is obtained because of the elimination of extraneous impurities that may be introduced into conventional processes which burn an extraneous fuel in the transfer line heater.

The invention is not to be limited by the preceding example, which is illustrative. Many other variations of the invention will be apparent to those skilled in the art.,

What is claimed is:

1. A process for making coke comprising heating and cracking a hydrocarbon feed essentially to hydrogen and coke in a ocking zone containing a dense phase bed of fluid coke particles which has a height-to-diameter ratio ranging from about 0.5 to 3 and a dilute phase zone of fluid coke particles above and contiguous to said dense phase bed,

passing said hydrogen upwardly at progressively increasing superficial gas velocities through said coking zone to entrain coke particles in said dilute phase zone,

conveying said hydrogen and entrained coke particles as a disperse phase to a heating zone,

adding oxygen-containing gas to said disperse phase to combust at least a portion of the hydrogen to heat the coke therein,

separating heated coke from said disperse phase and returning said coke to said coking zone to provide heat to carry out the endothermic cracking reaction therein.

2. The process of claim 1 wherein said superficial gas velocities are progressively increased by passing said hydrogen upwardly through a tapering zone, the cross-section of which decreases with height approximately as the cross-section of a trumpet-shaped zone defined by the equation:

1 DH DR\/K(H/0.25DR) where:

zone at any 3. The process of claim 2 wherein said superficial gas velocity progressively increases in said tapering zone from a velocity in the range from about 0.3 to 2.0 ft./sec. to a velocity in the range from about 25 to 100 ft./sec.

4. The process of claim 2 wherein the upper surface of said dense phase bed lies less than about 5 feet below said tapering zone.

5. The process of claim 4 wherein said upper surface is within said tapering zone such that the superficial gas velocity at said surface is in the range from about 1 to 5 ft./sec.

6. The process of claim 2 wherein fluid coke particles are withdrawn from said dense phase fluid bed, entrained in a gas, and conveyed as a disperse phase to said heating zone by-passing said tapering zone.

7. The process of claim 6 wherein the ratio of the flow rate by weight of coke particles through said tapering zone to the flow rate of coke particles by-passing said zone ranges from about 0.2 to about 10.

8. An apparatus for coking hydrocarbon feed comprising a coking reactor adapted to contain a bed of fluidized coke particles which has a height-to-diameter ratio in the range from about 0.5 to 3, said reactor having a main reactor body and a reactor top which is generally trumpetshaped and tapered from the diameter of said main reactor body up to a narrow upper zone, said upper zone being in communication with a transfer line heater, said transfer line heater also being in direct communication with said reactor through a dense phase riser provided with an aerating gas means, said riser by-passing the tapered reactor top, and means for introducing oxygencontaining combustion gas to said transfer line heater, said transfer line heater being in communication with a gas-solids separator, and means for removing gaseous combustion products from said separator and for passing coke particles back to said reactor.

References Cited UNITED STATES PATENTS 2,912,315 11/1959 Haney 23288.3

DELBERT E. GANTZ, Primary Examiner.

HERBERT LEVINE, Assistant Examiner.

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