US2934489A - Heating of coker cyclone and outlet - Google Patents

Description

- April 26, 1960 ca. P. CANEVARI HEATING OF COKER CYCLONE AND OUTLET Filed April 2, 1957 FlG.-l

Ravi

Inventor Gerard P. Canevori By C Wflorney HEATING F COKER CYCLONE AND OUTLET Gerard P. Canevari, Metuchen, N .J., assignor to Esso Research and Engineering Company, a corporation of Delaware Application April 2, 1957, Serial No; 650,253

1 Claim. (Cl. 208-48) from.

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 US. Patents 2,735,349 and 2,735,806.

The fluid coking process unit consists basically of a reaction vessel and a heater or burner vessel. In a typical operation the heavy oil to be processed is injected into the reaction vessel containing a dense, turbulent, fluidized bed of hot inert solid particles. A staged reactor can be employed. Uniform temperature exists in the coking bed.'. Uniform mixing in the bed results in virtually isothermal conditions and effects instantaneous distribution of the feed stock. In the reaction zone the feed stock is partially vaporized and partially cracked. Effluent 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 or heater vessel, usually separate. the reactor to the burner vessel, such as a fluid bed or transfer line burner, employing a standpipe and riser system; air being supplied to the riser for conveying the solids to the burner. Sufiicient coke or added carbonaceous matter is burned in the burner vessel to bring the solids therein up to a temperature sufficient to maintain the system in heat balance. The burner solids are maintained at a higher temperature than the solids in the reactor. Coke, equivalent to about based on 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 The net coke production, which represents the coke make less the coke burned, is withdrawn.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 vacuum bottoms from crude, 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 0 to 20, and a Conradson carbon residue content of about 2 to 40 wt. percent. (As to Conradson carbon residue see A.S.T.M. Test D-l89-41.)

It is preferred to operate with solids having a particle size ranging between 100 and 1000 microns in diameter with a preferred particle size range between 150 and 400 microns. Preferaby not more than 5% has a par- A stream of coke is thus transferred from a ice ticle size below about microns, since small particles tend to agglomerate or are swept out of the system with the gases. While coke is the preferred particulate solid, other inert particulate solids such as spent catalyst, pumice, sand, kieselguhr, Carborundum, and alumina can be employed.

Serious problems have been encountered in the development of this type of coking process. Typically the product vapors are removed overhead through a cyclone or usually cyclones and confined outlet lines therefrom, located in the upper portion of the reactor. Since these vapors leaving the coking bed are at or near their dew or condensation point, they readily condense. This condensation and consequent coke deposition is particularly serious on the surfaces having a temperature of about 700 to 1000 F. The consequent deposits on the inner surfaces of the cyclone and outlet line therefrom sometimes causes the pressure drop to increase to such an extent as to require the unit to be shut down periodically for cleaning.

One proposed solution has been to inject hot solids into the disperse phase to prevent this deposition and condensation by heating the vapors and by scouring deposited coke from the cyclone inlet. This has in turn necessitated a separate hot solids riser system. The latter has proved to be difficult to operate and has given rise to difficulties. These difliculties have included bridging of the solids across the hot solids lines, with resulting stoppage of flow, and erosion of the hot solids lines when aeration rates were increased to help prevent bridging. The required solids rate to the disperse phase is small compared to the rate of solids circulated from the heating to the coking zone. Thus, the hot solids lines to the disperse phase are much smaller than the lines carrying the major solids circulation, and are, consequently, much harder to keep flowing freely.

This invention provides an improved method of overcoming the coke deposition. The method comprises injecting a controlled small amount of an oxygen-containing gas into the cyclone to combust a portion of the product hydrocarbon vapors. The temperature of the inner surfaces are thereby raised to the extent that coke deposition is prevented. Preferably an additional small amount of oxygen-containing gas is also injected into the confined outlet line leading from the cyclone.

While gases of various oxygen concentration can be employed, air is preferred for reasons of economy. Only a small controlled amount of the gases is employed since localized heating only is desired. No significant amount of heat is introduced into the reactor overhead product stream or is there any significant dilution of the reactor products themselves. Consequently the amount of air employed is from about 0.5 to 2 s.c.f.m. (standard cubic ft./min.) per 1000 cu. ft. reactor effluent. This is total amount and would be divided into the number of injection points.

This invention will be better understood by reference to an example and the flow diagram. The flow diagram is shown in Figure 1 and a more detailed view of the cyclone and outlet line in Figure 2.

Referring now to the drawings, numeral 1 is a coking vessel constructed of suitable materials for operation at 950 F. A bed of coke particles preheated to a suflicient temperature, e.g., 1125 F., to establish the required bed temperature of 950 F. is made up of suitable parti cles of to 400 microns. The bed of solid particles reaches an upper level indicated by the numeral 5. Above is disperse phase 6. The bed is fluidized by means of a gas such as stripping steam entering the vessel at the stripping portion near the bottom thereof via pipe 3. The fluidizing gas plus vapors from the coking reaction pass upwardly through the vessel at a velocity of 1 ft./sec. establishing the solids at the indicated level. The fiuidizing gas serves also to strip the vapors and gases from the coke which flows down through the vessel to the heater, not shown. A stream of solid particles is removed from the coking vessel via line 8 and transferred to the heater.

A reduced crude oil to be converted is introduced into the bed of hot coke particles via line 2, but preferably at a plurality of points in the system. The oil upon contacting the hot particles undergoes decomposition and the vapors resulting therefrom assist in the fiuidization of the solids in the bed and add to its general mobility and turbulent state. The product vapors leave through cyclone separator 7 and confined outlet 4-. As stated previously, several cyclones per stage can be employed with air injected into each one.

Typically the outlet lines are insulated or steam traced at 9 in an effort to maintain a temperature suflicient to prevent coke deposition. Quite often this is ineffective. Coke deposition is known to be particularly severe at points 10 and 11 because the inner surface temperature drops to 10-20 F. below reactor etfiuent temperature depending upon many factors such as insulation, steam tracing, etc. To prevent this from happening air is injected at cyclone inlet 12 through line 13 in an amount of /6 s.c.f.m. per 1000 cu. ft. of reactor efiluent at design conditions, and conveniently also into confined outlet line 4 through lines 14 and 15 in an amount of /3 s.c.f.m. per 1000 cu. ft. of reactor eflluent at design conditions. A small controlled amount of product vapors are thereby combusted providing local heating which raises the temperature at points 10 and 11 well above reactor effiuent temperature. The combustion zone temperature is approximately 3000 F. although this is very localized and dissipated quickly by the efiiuent stream. Coke deposition is thereby completely prevented.

The advantages of this invention will be apparent to those skilled in the art. Coking of the cyclone andoutlet line is prevented in a simple, economical manner with the consequent avoidance of plant shutdowns. No significant amount of heat is introduced into the vapors to degrade them thermally and dilution problems are also avoided.

The conditions usually encountered in a fluid coker for fuels are also listed below for completeness.

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

What is claimed is:

In a process for coking a heavy hydrocarbon oil charge stock by contacting the charge stock at a coking temperature with a dense, turbulent, fluidized bed of inert particulate solids in a coking zone wherein the oil is converted to product vapors and carbonaceous solids are continuously deposited 011 the coke particles, removing efiluent product vapors through a cyclone separating zone and a confined outlet line and wherein coke tends to deposit on the inner surfaces of the cyclone separating zone and said outlet line, the improved method of preventing coke deposition in said cyclone separating zone and said outlet line therefrom which consists in injecting a controlled small amount of only air directly into the inlet of said cyclone separating zone and directly into said confined outlet line therefrom to burn only a small portion of the effiuent stream of product vapors at localized confined regions to quickly obtain localized heating and temperatures higher than that of the effiuent stream of product vapors at the regions of fair injection without introducing any insignificant amount of heat into the effluent stream of product vapors and without any significant dilution of said efiiuent stream of product vapors whereby coke deposition is prevented and the localized heating is quickly dissipated by the effiuent stream of product vapors, the total amount of air injected into said cyclone separating zone inlet and into said confined outlet line being in the range between about 0.5 and 2 s.c.f.m./1000 cubic feet of efiluent stream of product vapors.

References Cited inthe file of this patent UNITED STATES PATENTS 1,987,972 Rhein et al. Jan. 15, 1935 2,485,315 Rexet al. Oct. 18, 1949 2,549,117 Nelson Apr. 17, 1951 2,793,173 Fritz .May 21, 1957

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