US2738316A - Fluid coke calcining process employing a dual bed - Google Patents

Description

March I3, 1956 w. J. METRAILER FLUID COKE CALCINING PROCESS EMPLOYING A DUAL BED Filed Jan. 25, 1955 FLUID COKE SUPPLY PREIIEATEO O CONTAININC CA5 APRC cons mo RATE CONTROL C PREIIEATED 0 4 commmc GAS AUXILI ARY FUEL COKE NITNORAIAL STRIPPINC CA8 RATE CONTROL CALCINED FLUID CONE TO STORACE INVENTOR WILLIAM J. NETRAILER BY A 64 ATTORNEY William JosephMetrailer, Baton Rouge, La., assignor to v United States PatentO Esso Research-and Engineering Company, a corpcra- This-inventionrelates to improvements in calcining fluid coke. More particularly it relates to a dual bed operation wherein a fluidized bed of. the coke being treated is superimposed on a moving bed.

There has. recently been developed an improved process known as the fluid coking process for the production of fluid coke and the thermal conversion of heavy hydrocarbon oils to lighter fractions, e. g., see Serial No. 375,088, filed August 10, 1953. For completeness the process is described in further detail below although it should be understood that the fluid coking process itself is no part of this invention.

The fluid coking .unit consists basically of a reaction vessel or coker and a heater or burner. vessel. In a typical operation the heavy oil to be processed isinj'ected into the reaction vessel containing'a dense turbulent fluidized bed of hot inert solid particles, preferably coke particles. A transfer line or staged reactors can be employed. Uniform'temperature exists in the coking bed. Uniform mixing in the bed results in virtually isothermal condi-' tions and effects instantaneous distribution of the feed stock. In the reaction zone the feed stock is partially vaporized and partially cracked. Product vapors are removed from the coking vessel and sent to a fractionator for the recovery of gas and light distillates therefrom. Any heavy bottoms is usually returned to the coking vessel. The coke produced in the process remains in the bed coated on the solid particles. Stripping steam is, injected into the stripper to remove oil from the coke particles prior to the passage of the coke to the burner.

The heat for carrying out the endothermic coking reaction is generated in the burner vessel, usuallybut not necessarily separate. A stream of coke is thus transferred from the reactor to the burner vessel, such as a transfer line or fluid bed burner, employing a standpipe and riser system; air being supplied to the riser for conveying the solids to the burner. Sufficient coke or added carbonaceous matter is burned in the burning vessel; to bring the' solids therein up to a temperaturesuflicient to maintain the system in heat balance. The burner solids are maintained at a higher temperature than the solids in the reactor. About 5% of coke, based on the feed, is burned for this purpose. This may amount to approximately 15% to 30% of the coke made in the process. The net coke production, which represents the coke make less the coke burned, is withdrawn.

Heavy hydrocarbon oil feeds suitable for the coking process include heavy crudes, atmospheric and crude vacuum bottoms, pitch, asphalt, other heavy hydrocarbon petroleum residua or mixtures thereof. Typically, such feeds can have an initial boiling point of about 700 F. or higher, an A. P. I. gravity of about to 20, and. a Conradson carbon residue content of about to 40 wt. percent. (As to Conradson carbon residue see A. S. T. M. Test D--18052.)

, The method of fluid solids circulation described above is well known in the prior art. Solids handling tech 2,738,316 Patented 'lVlar. 13 1 956 2. nique is described broadly in. Packie Patent 2,589,124, issued March 11, 1952.

The fluid coke. product particles have a particle diameter predominantly, i. e.,, about 60 to 90 wt. percent, in the range of 20 to mesh, a sulfur content in, many cases; above 6 wt. percent, and a volatile content of 2 to 10 wt. percent. They have a real density of about 1.4 to 1.7 which is too low for use in the manufacture of carvbon electrodes in making aluminum and other purposes. Increased density and lower sulfur and. volatile content are particularly necessary before the fluid coke is suitable for manufacture into electrodes, one of the major uses of petroleum coke. These can be accomplished. by calcining the coke at high temperatures, e. g., minimum temperatures of 2100 F. or higher. These temperatures and the times required make the calcining operation relatively difiicult and expensive.

In the calcination a considerable quantity of volatile matter is given off from the coke. This volatile matter, which comprises principally hydrogen and methane, together with heavy hydrocarbons, can cause the finely dividedfluid coke to agglomerate and bridge in a calcining vessel when the calcining is done while the fluid coke is, in the form of a moving bed. This obstructssolids flow and interferes with smooth operation of the calcining unit.

This invention provides an improved process for calcining the fluid coke which overcomes these difliculties.

.The process. comprises calcining the coke. at a temperature in the range of 2000 to 3000 F. and preferably 21300 to 2700- F. utilizing a dual bed. The. fluid coke .is maintained in. theform of a dense turbulent. fluidized bed directly superimposed on a moving bed of the coke.

The holding time in the-fluid bed isv in the range of 0.1 to 0.01 of the total treating time whereby substantially all of the volatile matter is removed from the coke while it is in the form of the fluidized bed. Additional fluid coke charge stock is fed to the fluid bed and calcined" product coke is withdrawnfrom the moving bed.

The total holding time consequently is in the range of 0.2 to 20- hours, and preferably 0.3 to- 12 hours with the distribution as indicated above.

The terms dense turbulent fluidized bed and moving bed are employed in their usual engineering connotations, i. e., see Industrialand Engineering Chemistry, volume 41., page 1249 (June 1949).

This invention will be better understood by reference to the following example, description and the flow dia gram shown in the drawing.

Hot product coke from. the fluid coking process is fed from supply line 1, through a flow control valve 2, to. line 3 and/or 3A. The coke fed through line 3 is picked'up by a preheated; gas containing. oxygen entering'through line 4 and the mixture is. distributed uniformly at thebottom of thefluidized bed 6 in fluidized bed section 9. Coke fed through line 3A is discharged. uniformly. into the: dispersed phase above the fluid bed which hasan upper level 7. If an auxiliary fuel is desired it would be fed through line 5, mixed with the 02 containing gas and distributed uniformly at the bottom of the fluid bed 6. The rate of coke addition is controlled by a slide valve actuated by a pressure differential controller 8 set to maintain a constant level in the fluid bed section. Coke is withdrawn from the calcining vessel through a cooler 15 with the rate of withdrawal being controlled by a slide valve 16. The fluid bed 6 is in direct communication with the moving bed 11 in moving bed section 10, and as coke is withdrawn from the bottom of the moving bed section, additional coke will enter the top of this section, falling by gravity from the fluid bed section. The total holding time is regulated by controlling the rate of coke withdrawal through slide valve 16. The relative holding time in the fluid bed and moving bed sections is regulated by coke level in the fluid bed section.

A stripping gas such as hydrogen, natural gas or nitrogen is introduced into the bottom of the moving bed and/or cooler through lines 13, 14 and/or 14A, to remove any volatile materials formed in the moving bed section. Provisions are also made to introduce a preheated oxygen containing gas through line 19 and an auxiliary hydrocarbon fuel line 18 through line 14 to the bottom of the moving bed section to provide heat only when starting up the unit. In normal operations all heat is supplied in the fluidized bed section. Evolved vapors are taken off through cyclone 17 and line 12 along with other gases. The superficial gas velocity in the fluid bed section is between 0.5 and 8 feet per second and preferably 2 to 5 feet per second. The superficial gas velocity in the moving bed section is between 0.005 and 0.05 feet per second. The cross section area of the fluid bed section 9 can be reduced to as much as one twentieth that of the moving bed section to minimimize the quantity of fluidizing gas necessary.

As a specific example of the above described process fluid coke at 1200 F. from the fluid coking process is fed to a calcining vessel at a rate of about 60 tons per day. The calcining vessel proper is about 11 feet in internal diameter and 30 feet tall, not including the attendant cooling section and fine solids recovery equipment. The moving bed section is 20 feet deep and the fluidized bed section about 2 feet deep. This provides a total holding time of about 1 hour in the calcining vessel with slightly less than per cent of the total holding time being in the fluid bed section because of the lower densities in the fluid bed section. Air, preheated to about 1200" F. by indirect heat exchange with the exit gases from the calcining vessel, is distributed through porous refractory tubes uniformly across the bottom of the fluidized bed. The rate of air addition is adjusted to maintain a temperature of 2700" F. in the fluidized bed. This rate should be approximately 500,000 S. C. F./hr. for this size plant but varies somewhat with the burning characteristics of the coke. This gives a superficial velocity of about 5.0 feet per second in the fluidized bed section. The fluid coke enters the calcining vessel through the top of the vessel and is distributed uniformly in the dispersed phase above the fluidized bed. Nitrogen is added at the bottom of the moving bed at a rate of about 2000 S. C. F./hr. to maintain a superficial velocity of about 0.02 foot per second in the moving bed section. The calcining vessel is thermally insulated or otherwise protected from excessive heat loss so that the temperature at the bottom of the moving bed is about 2600 F. The treated coke withdrawn through line is cooled to about 500 F. by indirect heat exchange and withdrawn through a slide valve.

The calciner itself advantageously can be constructed of two materials, i. e., an inner layer of a shock resistant porous refractory such as carbon, graphite or silicon carbide. This can be coated with a relatively thin layer of a non-porous refractory which has only poor resistance to thermal shock, e. g., porcelain, vitrified alumina and stabilized zirconia.

Some of the advantages of this invention are as follows:

Agglomeration and bridging are avoided.

The removal of the volatile matter early in the process means that smaller equipment can be used subsequently, i. e., the moving bed section of the calciner, the solids recovery system and piping.

The evolved volatile matter form a part of the fluidizing gas.

Oxygen containing gases are injected into the small fluidizing section so only this section has to be constructed with expensive oxygen resistant refractories.

Advantage can be taken of the high heat transfer to a fluid bed and at least part of the heat can be supplied by indirect heat transfer in the fluid bed.

The sulfur content of the coke is reduced to below 3 wt. per cent and even below 2 wt. per cent by the process of this invention and the real density raised to a minimum of 1.8.

The conditions usually encountered in a fluid coker for fuels are also listed below to further illustrate how the coke was prepared.

Conditions in fluid coke! reactor It is to be understood that this invention is not limited to the specific examples which have been offered merely as illustrations and that modification may be made without departing from the spirit of the invention.

What is claimed is:

1. In a process for dcsulfurizing and increasing the density of fluid coke particles by calcination at a temperature in the range of 2000 F. to 3000 F. the improvement which comprises the steps of maintaining the fluid coke in the form of a dense, turbulent, fluidized bed directly superimposed on a moving bed of fluid coke, the holding time of the fluid bed being in the range of 0.1 to 0.01 of the total treating time whereby substantially all of the volatile matter on the coke is removed while the coke is in the form of the dense, turbulent, fluidized bed; feeding additional fluid coke charge stock to the dense, turbulent, fluidized bed and withdrawing calcined product coke from the moving bed.

2. The process of claim 1 in which the total treating time is in the range of 0.2 to 20 hours.

3. The process of claim 2 in which the coke particles being calcined are brought to the required temperature by utilization of an oxygen containing gas as the fluidizing gas for the dense, turbulent, fluidized bed.

No references cited.

Claims (1)

1. IN A PROCESS FOR DESULFURIZING AND INCREASING THE DENSITY OF FLUID COKE PARTICLES BY CALCINATION AT A TEMPERATURE IN THE RANGE OF 2000* F. TO 3000* F. THE IMPROVEMENT WHICH COMPRISES THE STEPS OF MAINTAINING THE FLUID COKE IN THE FORM OF A DENSE, TURBULENT, FLUIDIZED BED DIRECTLY SUPERIMPOSED ON A MOVING BED OF FLUID COKE, THE HOLDING TIME OF THE FLUID BED BEING IN THE RANGE OF 0.1 TO 0.01 OF THE TOTAL TREATING TIME WHEREBY SUBSTANTIALLY ALL OF THE VOLATILE MATTER ON THE COKE IS REMOVED WHILE THE COKE IS IN THE FORM OF THE DENSE, TURBULENT, FLUIDIZED BED; FEEDING ADDITIONAL FLUID COKE CHARGE STOCK TO THE DENSE, TURBULENT, FLUIDIZED BED AND WITHDRAWING CALCINED PRODUCT COKE FROM THE MOVING BED.

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