US3374168A - Coking process and apparatus - Google Patents

Description

March 19, 1968 w. J. METRAILER I 3,374,168

COKING PROCESS AND APPARATUS Filed June 29, 1966 William Joseph Metruiler INVENTOR United States Patent 3,374,168 COKING PROCESS AND APPARATUS William Joseph Metrailer, Baton Rouge, La., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed June 29, 1966, Ser. No. 561,519 Claims. (Cl. 208-127) ABSTRACT OF THE DISCLOSURE An upflow high temperature coking process and apparatus are described in which fluid coke is produced by pyrolysis of hydrocarbons. The fluid coke is entrained in liberated hydrogen and conveyed to a transfer line burner, heated therein and returned to the fluid coke bed to provide the endothermic heat of cracking for the coking reaction.

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 temperatures, e.g., 850 to 1200 F., and at high temperatures, e.g., 1800" to 2800 F. A typical fluid coking process and apparatus is described in U.S. Patent No. 2,881,- 130. Therein is described a fluid coking unit which consists basically of a reactor vessel for coking and a heaterburner vessel for supplying heat for the coking 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 850 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 hydrocarbons as by-products. Uniform mixing in the fiuid bed results in virtually isothermal conditions and effects instantaneous distribution of the feedstock.

The heat for carrying out the endothermic cracking reaction is usually generated in an auxiliary heater or burner vessel. A fluidized stream of coke is withdrawn horizontally or downwardly from the reactor and transferred as a dense phase fluid solids system to a riser or verticallyaligned conduit.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. Sufficient combustion is carried out to bring the solids up to a temperature sufficient to maintain the system in heat balance. Alternatively, part of the coke solids can be burned with oxygen or oxygencontaining gas to provide heat.

Generally the solids in the burner are maintained at temperatures of about 200 to 400 F. above the coker reactor temperature, depending upon the solids circulation rate, and returned to the reactor bed by means of a standpipe and riser system. The net coke made, above that needed to maintain the level necessary for circulation to provide heat, is withdrawn. A portion of the product coke is ground and is returned to the reactor and used as seed coke to maintain the desired average coke particle size in the reactor for optimum fluidization.

Certain problems and inefliciencies are inherent in the coking operation, particularly when it is carried out at temperatures above about 1600 to 1800 F. First, high ice temperature coking produces large quantities of gaseous products, namely hydrogen, which leave the reactor at very high temperatures and carry away a great deal of sensible heat. This greatly lowers the thermal efficiency of the process.

Moreover, the prior art coking processes require a great deal of special equipment for transferring solids and handling gases. Several transfer line risers, down-comers, 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 makes the coking operation particularly 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 1.90+ g./cc. with a particle size range of about 5 to 5000 microns and averaging generally about 20 to 600 microns. Such solids are difficult to fluidize in that they tend to form fluid beds which are extremely sensitive to slugging and conducive to the production of large gas bubbles.

The prior coking techniques also require the addition of large quantities of extraneous heating gas as fuel in the transfer line burner or heater. Such fuel gas must be compressed to process pressures which requires the investment and maintenance of a high volume, high pressure gas compressor system. Moreover, the processes are dependent upon the availability of inexpensive fuel gases. Absent such gases, it is necessary to burn quantities of the product coke itself in the transfer line burner by adding oxygen thereto. This, of course, is uneconomical and wasteful of valuable product.

Still other facilities are required to provide the numerous bleed gas injection points and metering equipment throughout the various risers and transfer lines.

Furthermore, When hydrocarbon fuels such as methane are used in the transfer line burner, large quantities of carbon dioxide are formed. Carbon dioxide is highly oxidizing to coke at high temperatures, reacting according to the equation:

This gasification of carbon to carbon monoxide results, of course, in the loss of large quantities of valuable coke product.

Another disadvantage of prior art coking processes is that extensive clean-up of effluent process gases is required before the gases can be recovered or released into the atmosphere. This is because the gas carries entrained soot as well as various partially combusted hydrocarbons. The clean-up problem is doubly complicate-d because of the necessity for maintaining two effluent streams of gas, i.e. reducing gases from the reactor or coker and oxidized fuel gases from the transfer line burner or heater. It is often also necessary to put additional equipment, such as taking advantage of the entrainment of coke in the lib- .detrainment battles, in the reactor to reduce the amount of small coke particles entrained in the exiting gas.

Part of these problems can be overcome or avoided by erated hydrogen and other reactor efiluent gases to transport the coke to the heater or burner, which in such cases is located above the reactor. This makes use of the phenomenon that a fluid bed of coke is actually composed of two more or less distinct phases. Most of the coke is present as a dense phase, with a small fraction existing as a dilute or disperse phase in the gas phase above the dense phase bed of coke. The dense phase is characterized as being much like a liquid, having a visible level or upper surface. The disperse 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 vigorous boiling or bubbling action of the gas-fluid system. In the upper portion of the disperse phase the particles are just sufficiently large as not to be entrained and carried out of the reactor by the rising gases, i.e., the free fall velocity is only slightly greater than the ascending gas velocity.

By carefully controlling the gas velocity, the amount of coke, and the particle size thereof in the reactor, the upper portion of the disperse phase can be moved up to the reactor outlet. Thus, by increasing the gas velocity in the outlet well above the free fall velocity of the particles, the particles can be easily entrained in the gases. The highvelocity gases along with the entrained solids can then be passed directly to a burner or heater and heated, e.g., by combusting oxygen with the hydrogen or other combustible gases present in the reactor eflluent.

Because of the coarse nature of the fluid coke product and the difliculty involved in fluidizing it, the design of an up-flow reactor-burner system is critical.

In such up-flow 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 circulating rate increases. 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 of a narrow particle size distribution, e.g., 60 to 80 microns and having a relatively low particle density, i.e., about 0.8 to 1.0 g./cc., are used in an up-flow 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.

However, with dense coarse solids of Wide particle size distribution, the geometry of the upper portion of the reactor is extremely important. At superficial gas velocities preferred in the dense bed for high temperature fluid coking, i.e., 0.5 to 3 ft./sec., the up-flow gas velocity is too slow to provide suflicient 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 because under such conditions the bed surges and slugs, this causing extreme pressure fluctuations in the entire system due to gas bypassing and the formation of large gas bubbles in the reactor. This, in turn, thrusts large quantities of coke in slugs into the reactor outlet along with uncracked hydrocarbon feed, which can result in coking and plugging of the reactor outlet line.

Thus, it was heretofore considered impractical, if not impossible, to carry out a high temperature coking process in up-flow apparatus, wherein a burner or heater located above the coker or reactor vessel can employ the combustible reactor 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.

The alleviation of these and other problems is achieved in accordance with the present invention, which contemplates a process and apparatus for high temperature fluid coking wherein hydrocarbon feed is cracked at a temperature ranging from about 1800 to 2800 F. in a reactor containing a fluid bed of coke to coke and hydrogen, a high velocity upflowing gas is supplied to a zone of the dense phase of the fluid bed to convey coke and hydrogen to a burner or heating zone wherein hydrogen is burned with oxygen to heat the coke, and the hot coke is returned to the fluid bed to provide heat to carry out the cracking reaction.

High velocity upflowing gas is supplied to a zone of the dense phase of the reactor fluid bed to pick up coke and transport it to the reactor outlet. Preferably, the high velocity zone will be separated from the rest of the dense phase of the bed as, for example, by a conduit or draft tube extending from below the surface of the dense phase up to within about 12 to 24 inches of the reactor outlet. The ratio of conduit to bed diameter should be no greater than 1:6, and preferably Will range from about 1:10 to about 1:30, to achieve smoothest operation and avoid slugging. The pick-up gas added to the draft tube in combination with the coker by-produot gases increases the overall gas velocity within the draft tube and accelerates the particles therein upwardly into the reactor outlet from which they are conveyed to a transfer line heater or burner. In the burner the evolved hydrogen from the cracked hydrocarbons, along with the pick-up gas and/ or other combustible gases, is burned with oxygen, air, or other oxygen-containing gases. Preferably, the oxygen-containing gas is preheated. The pick-up gas can also be preheated and can be either an inert gas such as nitrogen or it can be additional hydrogen or even hydrocarbon fuel gases, or mixtures thereof. By utilizing hot recycle hydrogen as pick-up gas, a large excess of hydrogen over oxygen can be maintained in the burner to minimize gasification of the circulated coke. The amount of pick-up gas required will be an amount sufflcient to provide, in combination with the reactor gases, a superficial gas velocity in the high velocity zone, e.g., the draft tube, ranging from about 5 to 50 ft./sec., preferably from about 15 to 30 ft./ sec. The precise gas velocity desired will depend, inter alia, upon the exact solids circulation rate desired and the particle size distribution of the solids conveyed. The combustion temperature of the hydrogen or other combustibles and oxygen-containing gas in the transfer line heater or burner is controlled primarily by the recycle rate of circulated coke, the preheat temperature of the oxygen-containing gas, and the ratio of oxygen to hydrogen or other combustibles to be burned. The burning of part of the combustible gases in the heater provides sensible heat to the circulated coke for carrying out the cracking reaction in the dense phase cracking zone of reactor fluid bed. The combustion of the oxygen with the combustible gases, e.g., hydrogen, from the cracking reaction can be carried out in a precombustion zone or in the transfer line burner itself. Depending upon the ratio of hydrogen to carbon in the hydrocarbon feed that is injected into the hot, fluid bed of coke, the amount of hydrogen by-product available for combustion in the transfer line burner will be determined. Generally with lighter hydrocarbons there will be a substantial excess of hydrogen over that needed to maintain the system in heat balance. With heavy hydrocarbons having relatively low ratios of hydrogen to carbon, almost all of the hydrogen evolved will be used to carry out the combustion in the transfer line burner. Generally, the combustion of the hydrogen by-product or other combustible gases to heat the circulated coke products is carried out with a deficient amount of air, i.e., with excess fuel. A sufficient amount of oxygen-containing gas is added to maintain an average temperature in the transfer line burner of about 2200" to 3000 F., and preferably about 2400 to 2600 F.

This temperature will usually be about 200 to 400 F. higher than the average temperature in the fluid coke bed.

The gas velocity in the transfer line burner is maintained at about 20 to ft./sec., and preferably 30 to 60 ft./sec., to obtain a coke residence time in the burner of about 0.1 to 1.0 second, and preferably 0.3 to 0.6 second. The coke circulation rate is very important because the coke serves as a quench to the hot flame of the combustion gases to reduce their average temperature to those temperatures which conventional refractory materials can withstand over a long period of time. The circulation rate is about 15 to 40, and preferably 20 to 30, times the coke production rate in the reactor. The circulation rate will, of course, depend on the amount of combustibles burned and the total hydrocarbon feed rate to the reactor.

The hydrocarbon feed to the high temperature fluid coke process can be any gaseous, liquid or heavy residual hydrocarbon. Vacuum residuum as well as residue which are solid at ambient temperatures can also be used.

The superficialline-ar gas velocity through the fluidized coke bed generally ranges from about 0.3 to 5.0 ft./sec., and preferably about 0.5 to 3.0 ft./.sec. This gas velocity is controlled by the amount of feed used and the terriperature at which the reaction is carried out. If desire-d, an inert gas can be added to provide additional fluidizing gas to the fluid bed.

The temperature of the fluid bed of coke in the reactor is maintained from about 1800 to 2800 F., generally at about 1900 to 2500 F., and preferably about 2000 to 2200 F. Control of the temperature is achieved by the amount of recycle of hot fluid coke from the transfer line heater or burner. The coking reactor can be operated at pressures up to 250 p.s.i.g. or higher, although generally pressures will range from about 25 to 150 p.s.i.g., and preferably from about 50 to 100 p.s.i.g.

A preferred embodiment of the invention will be discussed in relation to the attached schematic diagram or drawing. A hydrocarbon fuel boiling in the range of about 700 to 1400 F. is introduced through line 13 into fluidized coke bed 2 of reactor 1. The particle size of the coke in bed 2 ranges from about 20 to about 600 microns, averaging about 200 microns. Hydrocarbon feed is introduced at a sufficient rate that the cracked hydrogen gas eflluent has an average superficial linear gas velocity through the fluidized coke bed of about 2.5 ft./sec. The fluidized coke bed is maintained at a temperature of about 2100 F. by circulation of hot fluid coke particles through transfer line heater 15. Reactor pressure is about 100 p.s.1. The hydrocarbon feed injected into the bed is cracked essentially to hydrogen and carbon, the carbon depositing onthe hot coke particles causing them to gradually grow in size. The evolved hydrogen from the cracking reactor serves as the fluidizing gas and passes up through the surface 4 of the bed into the dilute or disperse phase 5 and entrains a small or minor amount of solid coke particles. The major up-flow of solids, however, is provided for by draft tube 17 in which the up-flowing gas rate is controlled by the introduction of pick-up gas through line 18. The draft tube diameter is only that of the upper surface of the fluid bed. While a cylindrical draft tube is generally employed, other conventional shapes can, of course, be used provided the equivalent diameter is within the described range as compared to the equivalent diameter of the fluid bed. By equivalent diameter is meant the diameter of a cylinder having the same cross-sectional area as the particular conduit in question. Also, multiple draft tubes of satisfactory area may be used. The pick-up gas is preheated hydrogen at reactor temperature. The total superficial gas velocity upwards through the draft tube is about 25 ft./sec.

The hydrogen and coke particles (along with a small amount of uncracked hydrocarbons) flow upwardly into reactor outlet line 7 and then are mixed with preheated air at a temperature of about 1200 F., introduced through line 9 and burned in transfer line burner 15. The gas velocity in the transfer line heater or burner is maintained at about 40 ft./sec., which results in an average residence time of the coke of about 0.5 second. The

combustion temperature of the hydrogen and preheated air is about 35 00 F.; however, this temperature is rapidly quenched by the circulating coke and the average temperature in the dilute phase 20 of the transfer line burner is about 2400 F. The coke is circulated through the burner at a rate of about 25 times the coke production rate in the reactor, i.e., the coke, on the average, makes about 25 cycles through the burner before being withdrawn from the system. The burner temperature is controlled as desired by controlling the oxygen addition rate.

Hot combustion gases and entrained circulating coke pass from the burner 15 by line 16 to cyclone 8, wherein the heated coke is separated from the combustion gases. The combustion gases are then vented through line 19 while the coke particles descend through standpipe 10 to the dense phase of the reactor fluid bed 2. The combustion gases exiting through line 19 can be treated to remove excess hydrogen, which can be used as lift gas by recycling it back to line 18, or alternately, the hydrogen can be kept as a product.

The hot coke particles descending through standpipe 10 are at a temperature of about 300 F. above the temperature of the fluidized coke particles in dense phase 2 and provide the necessary heat for the cracking reaction as sensible heat.

Product coke is withdrawn through line 14 and about 25% of the product is ground so that the average particle size is about microns and reintroduced through line 21 as seed coke to the fluid bed.

The coke product produced in the above described manner has unique physical properties which permit it to be used directly in the formation of carbon electrodes without further thermal treatment; that is, the coke does not have to be calcined in a separate step prior to being used to make coke electrodes. The coke can be used both for Soderberg and prebaked electrodes. Suitable electrode formulations can be prepared by grinding a portion of the coke product to form a fine coke, mixing it with coarse coke and a suitable binder, forming electrodes under pressure and baking the electrodes at temperatures above about 1800" F. to coke the binder material. In the case of Soderberg electrodes, the mixture is formed into a suitable paste and fed to the Soderberg process without prebaking.

The apparatus and process of the present invention results in substantial savings in fuel costs, investment costs and operating costs. In accordance with the present invention, minimum inventment in apparatus such as transfer lines, risers, downcomers, cyclones and compressors, etc., is attained, resulting in substantial investment savings as well as eliminating many of the elements susceptible to equipment failure in processes of this type. Moreover, this process provides a self-contained unit which produces product coke and its own fuel, utilizing the heat content of such fuel to maximize or optimize process thermal efficiency. The process can be completely independent of the availability of extraneous hydrocarbon fuels. Provisions can also be made for recovering the excess amount of hydrogen produced by condensing out the combustion water and removing other impurities. Coke of especially high purity is obtained because of the elimination of extraneous impurities that might otherwise be introduced by burning an extraneous fuel in the transfer line heater.

What is claimed is:

1. A high temperature fluid coking process comprising injecting a hydrocarbon feed into a dense phase fluidized bed of coke particles in a ooker react-or "to crack the feed into essentially coke and hydrogen, introducing a high velocity upflowing gas to a zone of said bed, conveying and circulating a portion of the coke upflow from the me actor as a disperse phase in a gas comprising said upflowing gas and said hydrogen to a transfer line heater, mixing said hydrogen with an oxygen-containing gas, burning said hydrogen to provide heat to said circulating 7 coke, returning the thus heated coke to said fluidized bed to provide, as sensible heat of the coke, endothermic heat for cracking said hydrocarbon, and recovering coke product.

2. The process of claim 1 wherein the superficial gas velocity in said zone of high velocity upfiowing gas ranges from about to about 50 ft./ sec.

3. The process of claim 2 wherein said superficial gas velocity ranges from about 15 to about 30 ft./sec.

4. The process of claim 1 wherein said high velocity upflowing gas is introduced into a Zone whose boundaries are defined by a conduit immersed in said dense phase fluid bed.

5. The process of. claim 4 wherein said conduit extends to within about 12 to 24 inches of the outlet of said reactor.

6. The process of claim 1 wherein said circulating portion of coke is circulated through said transfer line heater at a rate ranging from about 15 to about 40 times the rate at which coke is produced by said cracking of hydrocarbon feed.

7. The process of claim 6 wherein the hydrocarbon feed rate is suflicient to provide an amount of hydrogen from said cracking to provide a superficial gas velocity through said ifillid bed ranging from about 0.5 to about 3 ft./sec.

8. In a reactor and apparatus suitable for carrying out the high temperature coking of a hydrocarbon feed by contact with a bed of fluid coke contained within said reactor, and wherein is provided means for introducing hydrocarbon feed into said bed, the improvement comprising a conduit extending through the surface of said bed of fluid coke,

means for introducing a high velocity gas into the lower portion of said conduit below the surface of said bed,

an outlet line from the upper portion of said reactor directly above said conduit and in direct and open communication with a transfer line heater,

means for introducing free oxygen-containing gas into said heater,

and an outlet line from said heater in communication with said reactor for returning coke solids thereto.

9. The reactor and apparatus of claim 8 wherein the ratio of the diameter of said conduit to the diameter of said reactor at the upper surface of said bed of coke ranges from about 1:5 to about 1:30.

10. The reactor and apparatus of claim 8 wherein said transfer line heater is above said reactor.

References Cited UNITED STATES PATENTS 2,863,821 12/1958 Dunlop et al 208127 2,916,438 12/1959 Jahnig et al. 208-127 3,090,746 5/1963 Markert et a1. 208127 3,260,664 7/1966 Metrailer et al, 208127 HERBERT LEVINE, Primary Examiner.

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