Method for removing coke from particulate catalyst

Coke is removed from particulate catalyst by passing coke-containing catalyst downwardly through a regenerator without backmixing, in a countercurrent flow with respect to a regeneration gas having a limited free-oxygen content, so that nitrogen oxides are reacted to form free nitrogen in a substantially oxygen-free atmosphere in an intermediate level of the regenerator, and carbon monoxide formed in the intermediate level is burned in contact with coke-free catalyst with additional oxygen introduced into an upper level of the regenerator.

BACKGROUND OF THE INVENTION 
This invention concerns the art of catalyst regeneration. More 
specifically, the present invention concerns a method for burning 
nitrogen-containing coke off coke-containing particulate catalyst while 
avoiding contamination of flue gas formed in burning the coke. 
Catalytic cracking systems employ catalyst in a moving bed or a fluidized 
bed. Catalytic cracking is carried out in the absence of externally 
supplied molecular hydrogen, in contrast to hydrocracking, in which 
molecular hydrogen is added during the cracking step. In catalytic 
cracking, an inventory of particulate catalyst is continuously cycled 
between a cracking reactor and a catalyst regenerator. In a fluidized 
catalytic cracking (FCC) system, hydrocarbon feed is contacted with 
catalyst particles in a hydrocarbon cracking zone, or reactor, at a 
temperature of about 425.degree. C.-600.degree. C., usually 460.degree. 
C.-560.degree. C. The reactions of hydrocarbons at the elevated operating 
temperature result in deposition of carbonaceous coke on the catalyst 
particles. The resulting fluid products are separated from the 
coke-deactivated, spent catalyst and are withdrawn from the reactor. The 
coked catalyst particles are stripped of volatiles, usually by means of 
steam, and passed to the catalyst regeneration zone. In the catalyst 
regenerator, the spent catalyst is contacted with a predetermined amount 
of molecular oxygen. A desired portion of the coke is burned off the 
catalyst, restoring catalyst activity and simultaneously heating the 
catalyst to, e.g., 540.degree. C.-815.degree. C., usually 590.degree. 
C.-730.degree. C. Flue gas formed by combustion of coke in the catalyst 
regenerator may be treated for removal of particulates and for conversion 
of carbon monoxide, after which the flue gas is normally discharged into 
the atmosphere. 
Most FCC units now use zeolite-containing catalyst having high activity and 
selectivity. Zeolite-type catalyst have a particularly high activity and 
selectivity when the concentration of coke on the catalyst after 
regeneration is relatively low, so that it is generally desirable to 
regenerate zeolite-containing catalysts to as low a residual carbon level 
as is possible. It is also normally desirable to burn carbon monoxide as 
completely as possible within the catalyst regeneration system to conserve 
heat. Heat conservation is especially important when the concentration of 
coke on the spent catalyst is relatively low as a result of high catalyst 
selectivity. Among the ways suggested to decrease the amount of carbon on 
regenerated catalyst and to burn carbon monoxide in a manner which 
provides process heat, is carrying out carbon monoxide combustion in a 
dense-phase catalyst bed in the catalyst regenerator using an active, 
combustion-promoting metal. Metals have been used either as an integral 
component of the cracking catalyst particles or as a component of a 
discrete particulate additive, in which the active metal is associated 
with a support other than the catalyst particles. 
Various ways of employing carbon monoxide combustion-promoting metals in 
cracking systems have been suggested. In U.S. Pat. No. 2,647,860, it is 
proposed to add 0.1-1 weight percent chromic oxide to a cracking catalyst 
to promote combustion of carbon monoxide to carbon dioxide and to prevent 
afterburning. In U.S. Pat. No. 3,808,121, it is proposed to introduce 
relatively large-sized particles containing a carbon monoxide 
combustion-promoting metal into a cracking catalyst regenerator. The 
circulating particulate solids inventory, comprised of relatively 
small-sized catalyst particles, is cycled between the cracking reactor and 
the catalyst regenerator, while the combustion-promoting particles remain 
in the regenerator because of their size. Oxidation-promoting metals such 
as cobalt, copper, nickel, manganese, copper-chromite, etc., impregnated 
on an inorganic oxide such as alumina, are disclosed. Belgian Patent 
Publication No. 820,181 suggests using catalyst particles in platinum, 
palladium, iridium, rhodium, osmium, ruthenium or rhenium to promote 
carbon monoxide oxidation in a catalyst regenerator. An amount of the 
metal between a trace and 100 parts per million is to be added to the 
catalyst particle, either during catalyst manufacture or during the 
cracking operation, as by addition of a compound of the 
combustion-promoting metal to the hydrocarbon feed. Addition of the 
promoting metal to the cracking system is said by the publication to 
decrease product selectivity in the cracking step by substantially 
increasing coke and hydrogen formation. Catalyst particles containing the 
promoter metal can be used alone or can be circulated in physical mixture 
with catalyst particles free of the combustion-promoting metal. U.S. Pat. 
No. 4,072,600 and No. 4,093,535 disclose the use of combustion-promoting 
metals in cracking catalysts in concentrations of 0.01 to 50 ppm, based on 
total catalyst inventory. 
One problem encountered in some cracking operations using metal-promoted, 
complete carbon monoxide combustion-type regeneration has been the 
generation of undesirable nitrogen oxides (NO.sub.x) in the flue gas 
formed by burning coke. The present invention is directed, in part, toward 
providing a catalyst regeneration system, which accomplishes complete coke 
removal and complete carbon monoxide combustion within a catalyst 
regeneration system, while substantially decreasing the concentration of 
nitrogen oxide present in flue gas formed by burning coke. 
Representative of catalyst regeneration patent literature previously 
published are the following patents: U.S. Pat. No. 3,909,392 describes a 
scheme for enhancing carbon monoxide combustion by thermal means. Catalyst 
is used a heat sink for the increased heat production. British Patent 
Publication No. 2,001,545 describes a two-stage system for a regenerating 
catalyst, with partial catalyst regeneration being carried out in the 
first stage and further more complete regeneration being carried out in 
the second stage with a separate regeneration gas. U.S. Pat. No. 3,767,566 
describes a two-stage regeneration scheme in which partial regeneration 
takes place in an entrained catalyst bed, and secondary, more complete 
regeneration takes place in a dense fluidized catalyst bed. A somewhat 
similar regeneration operation is described in U.S. Pat. No. 3,902,990, 
which discusses the use of several stages of regeneration, with dilute- 
and dense-phase beds of catalysts being employed, and with the use of 
plural streams of regeneration gas. U.S. Pat. No. 3,926,843 describes a 
plural-stage regeneration scheme in which dilute-phase and dense-phase 
coke burning are performed. British Patent Publication No. 1,499,682 
discloses use of a combustion-promoting metal for enhancing carbon 
monoxide combustion. None of the above cited patents provides a method for 
forming a flue gas having low concentrations of both carbon monoxide and 
nitrogen oxides, while accomplishing essentially complete removal of coke 
from the catalyst. 
SUMMARY OF THE INVENTION 
I have found that nitrogen-containing coke can be burned off a 
coke-containing a particulate catalyst and a flue gas free from both 
carbon monoxide and NO.sub.x can be formed in burning the coke, by passing 
deactivated catalyst downwardly, without substantial backmixing, through a 
regenerator in countercurrent flow to an upwardly flowing, 
oxygen-containing regeneration gas. Nitrogen oxides formed in a lower, 
complete combustion section of the regenerator are reacted in a 
substantially oxygen-free atmosphere in an intermediate section of the 
regenerator to form elemental nitrogen, and carbon monoxide formed in the 
regeneration gas in the intermediate section is burned in an upper section 
of the regenerator in the presence of coke-free catalyst, using additional 
free oxygen.

Referring to the drawing, there is shown a regeneration vessel 1. Spent, 
coke-containing catalyst is introduced into an intermediate level of the 
vessel through a conduit 3 at a rate adjustable by means of a valve 5. A 
regeneration gas stream containing free oxygen is introduced into the 
vessel through a conduit 7 and a distributor 9. Spent catalyst entering 
the vessel flows generally downwardly, countercurrently to the 
regeneration gas, which is conducted upwardly through the vessel. Catalyst 
is retained above a gas distribution grid 11 at the lower end of the 
vessel. Substantially coke-free catalyst is removed from the regeneration 
vessel above the grid 11 through a conduit 13 and passed into the surge 
vessel 15. A minor portion of the coke-free catalyst in the vessel 15 is 
entrained in a stream of a gas such as steam, introduced through a conduit 
17. Coke-free catalyst is passed upwardly in the entraining gas into an 
upper section of the vessel 1 through a conduit 19. Most of the coke-free 
catalyst is removed from the regeneration system, and returned to 
catalytic service or other desired use, by way of a conduit 21. 
Essentially plug-type downward flow of the catalyst undergoing 
regeneration is enhanced in the vessel 1, and gross backmixing of the 
downwardly moving catalyst is substantially restricted, by including in 
the regeneration vessel internals such as the perforated plates 23, 25, 27 
and 29. Regeneration gas in the lower section of the vessel adjacent the 
grid 11 and the plates 23 has a high free-oxygen concentration. The high 
temperatures generated in this portion of the vessel by burning coke and 
carbon monoxide, preferably in the presence of a metallic combustion 
promoter, in the highly oxidizing atmosphere result in the formation of 
nitrogen oxides in the regeneration gas. At higher levels in the vessel 1, 
in an intermediate section of the bed of catalyst, generally adjacent the 
plates 25, essentially all the free oxygen in the regeneration gas has 
been consumed in burning coke and carbon monoxide. The regeneration gas in 
the intermediate section provides a substantially oxygen-free atmosphere, 
typically including substantial concentrations of carbon monoxide and 
carbon dioxide and substantially no free oxygen. The spent catalyst and 
partly regenerated catalyst in contact with the regeneration gas in this 
intermediate section contain substantial carbon concentrations. Nitrogen 
oxides in the regeneration gas react in the oxygen-free atmosphere to form 
free nitrogen (molecular nitrogen). In an upper section of the vessel 
generally above the plate 27, additional free oxygen is introduced into 
the regeneration gas stream, as, for example, through a conduit 31 and a 
distributor 33. Carbon monoxide present in the regeneration gas is burned 
with the added free oxygen in contact with substantially coke-free 
catalyst in the upper section of the regeneration vessel. The coke-free 
catalyst advantageously supplies a heat sink for heat energy evolved by 
combustion of carbon monoxide with the additional free oxygen. The 
resulting carbon monoxide-free and nitrogen oxides-free flue gas is passed 
into a cyclone separator 35, and any entrained catalyst is separated from 
the flue gas and returned to the bed of coke-free catalyst. The top of the 
coke-free catalyst bed is indicated by a line at 37. The pollutants-free 
flue gas is withdrawn from the top of the vessel through a conduit 39. In 
order to simplify the explanation, various conventional elements of the 
regeneration scheme described above are not shown in the attached drawing 
or described. The operation and disposal of these elements, such as 
control means, valve and pump means, and the like, will be clear to those 
skilled in the art. 
DETAILED DESCRIPTION OF THE INVENTION 
As used herein, the term "oxidizing atmosphere" means an atmosphere 
containing at least 1.0 volume percent molecular oxygen and less than 0.1 
volume percent carbon monoxide. 
As used herein, the term "substantially oxygen-free atmosphere" means an 
atmosphere containing less than 0.5 volume percent free (molecular) 
oxygen. 
As used herein, the term "substantially coke-free catalyst" means catalyst 
which contain less than 0.2 weight percent carbon. 
Catalysts that are best adapted for regeneration by the method of this 
invention are those in the form of particulate solids. Preferably, 
catalyst to be regenerated is sized appropriately for catalytic use in an 
entrained bed or fluidized bed system. With reference to catalytic 
conversion systems presently in commercial use, the invention is 
especially advantageous for regeneration of FCC catalysts; however, the 
invention is not limited to FCC catalyst regeneration, and can be used in 
burning coke off any coke-containing particulate catalyst. 
Regeneration according to the invention can be carried out in any 
vertically extended vessel or chamber which is capable of containing the 
regeneration gas and catalyst particles at the temperatures and pressures 
employed in the operation. A variety of suitable vessels will be apparent 
to those skilled in the art from the description herein. Preferably, the 
vessel employed is equipped with a type of internals structure which 
prevents gases and catalyst particles from by-passing each other, 
substantially restricts backmixing of catalyst particles in fluidized bed 
operation, and provides catalyst flow downwardly in the vessel that is 
essentially plug-type flow in fluidized bed operation. Such internals may 
be in the form of fixed internals such as perforated plates, baffles, 
rods, or the like, or can be a packing material. When the catalyst is 
regenerated in a moving bed operation, as opposed to a fluidized bed, 
backmixing of catalyst is not found to be a problem, so that internals are 
not usually useful. 
The regeneration gas or gas mixture employed must have an appropriate 
free-oxygen (molecular oxygen) content. Normally, air is quite suitable 
for use in supplying free oxygen but use of air is not essential. For 
example, pure oxygen of oxyge-enriched air can also be used, if desired. 
Conventional gases used in commercial FCC operations, such as free 
nitrogen (molecular nitrogen), carbon dioxide, steam, and the like, are 
suitable for use as fluidizing and entrainment gases. 
In general, regeneration conditions employed in the process include a 
combination of temperature and pressure sufficient to permit coke 
combustion, carbon monoxide combustion and nitrogen oxides reaction to 
take place in the manner discussed below. Temperatures of 540.degree. 
C.-815.degree. C. are normally quite suitable. Temperatures of 590.degree. 
C.-730.degree. C. are preferred. The flow of regeneration and entrainment 
gases and catalyst particles are preferably maintained at rates which 
provide a fluidized bed of catalyst in the regeneration zone, although a 
moving bed of catalyst can also be used, if desired. Fluid bed operation 
can be accomplished in a conventional manner by maintaining a superficial 
regeneration gas velocity appropriate to the size and density of catalyst 
particles undergoing regeneration and by maintaining catalyst introduction 
and withdrawal rates at proper levels. Downward movement of fluidizied 
catalyst in the vessel can be accomplished by simply removing catalyst 
from the bottom of the bed. The operation pressure is usually not 
particularly critical. Pressures of 1-20 atmospheres (absolute) are 
generally quite suitable. Pressures of 1-5 atmospheres are preferred. 
The use of a carbon monoxide combustion-promoting metal to aid in burning 
carbon monoxide in the regeneration gas is preferred in carrying out the 
invention. Metals and compounds of metals previously suggested for use as 
carbon monoxide combustion promoters, such as many of the transition 
metals, can be used. Preferred for use in promoting carbon monoxide 
combustion in the present system are metals or compounds of metals 
selected from platinum, palladium, iridium, rhodium, ruthenium, osmium, 
manganese, copper, and chromium. A combustion-promoting metal is used in a 
concentration sufficient to enhance the rate of carbon monoxide burning to 
the degree desired. In commercial FCC operations, the use of platinum in 
various forms as a carbon monoxide combustion-promoting metal is well 
known. A combustion-promoting metal may be included as a component of all 
or a major or minor fraction of the catalyst particles or may be included 
as a component of discrete, substantially catalytically-inert particles 
which are mixed with the catalyst inventory in essentially a physical 
mixture with the catalyst particles. A preferred metal for use in discrete 
CO-combustion promoter particles is platinum. 
Sulfur oxides present in the regeneration gas as a result of burning 
sulfur-containing coke, may advantageously be removed from the gas by 
using a solid reactant, or acceptor, as a component of the particulate 
solids in the regeneration zone. Sulfur oxides in the regeneration gas can 
be reacted with or adsorbed on the reactant or acceptor to form 
sulfur-containing solids in the regenerator. In this way, the sulfur 
oxides content of the flue gas leaving the regenerator may be 
substantially reduced. A preferred solid reactant for use in this manner 
is alumina. Alumina reacts with sulfur oxides to form a sulfur-containing 
solid. The alumina used should have a surface area of at least 50 square 
meters per gram. Alpha alumina is not suitable. Alumina may be included as 
a component of the catalyst particles or may be included in discrete 
particles which are present in the regenerator in physical mixture with 
the catalyst particles. If discrete alumina-containing particles are mixed 
with the catalyst, a sufficient amount of alumina is preferably mixed with 
the catalyst to provide a substantial removal of sulfur oxides from the 
regeneration gas. Usually, good results can be achieved if 0.1 to 25 
weight percent alumina is added. If alumina is present as a component of 
all or part of the catalyst particles themselves, the catalyst particles 
are preferably selected to include at least 50 weight percent alumina in 
the catalyst, on a zeolite-free basis, and particularly preferably, at 
least 60 weight percent. 
It will be apparent to those skilled in the art that the amount of coke 
contained in spent catalyst, as well as the amount of nitrogen and sulfur 
impurities in the coke, will vary widely depending on such factors as the 
composition and boiling range of the hydrocarbon feed being converted 
using the catalyst, the composition of the catalyst itself, the type of 
catalytic reaction system in which the catalyst is used (e.g., moving bed, 
fluid bed, entrained bed), etc. The benefits of burning coke according to 
the invention can be obtained for catalysts which contain an amount of 
coke varying in a broad range, and also for coke with a broad range of 
nitrogen content. 
In accordance with the invention, spent catalyst is introduced into an 
intermediate level of a vertically extending regeneration zone. The vessel 
or chamber used to provide the regeneration zone must be of sufficient 
vertical height to allow for maintaining the three sections and to allow 
for a solids retention time sufficient to accomplish essentially complete 
combustion of coke in the catalyst reaching the lower end of the 
regeneration zone. Spent catalyst is introduced into the regeneration 
vessel far enough from the bottom of the regeneration zone to permit 
essentially all the coke to be burned off the catalyst particles as the 
catalyst passes downwardly from the spent catalyst inlet to the lower end 
of the regeneration zone. Spent catalyst must therefore be introduced into 
the regeneration vessel sufficiently far from the top of the regeneration 
zone to provide for a bed of coke-free catalyst in an upper section of the 
regeneration zone. Preferably, the portion of the bed of the catalyst 
below the spent catalyst inlet constitutes from 60 to 95% of the total 
catalyst bed volume in the regenerator, particularly preferably, from 80 
to 90% of the total bed volume. The height of the upper section of the 
regeneration zone containing the bed of regenerated catalyst must be 
sufficient to permit essentially complete combustion of carbon monoxide in 
the regeneration gas stream in contact with the coke-free catalyst. 
A regeneration gas is introduced into the bottom of the regeneration zone. 
According to the invention, the amount of free oxygen (molecular oxygen) 
originally introduced in the regeneration gas is (1) sufficient to react 
stoichiometrically with substantially all the coke carbon introduced into 
the regeneration zone in the spent catalyst to form carbon monoxide, and 
(2) restricted to an amount less than needed to react stoichiometrically 
with substantially all the coke carbon introduced into the regeneration 
zone in the spent catalyst to form carbon dioxide. When the amount of free 
oxygen introduced into the lower end of the regenerator in the 
regeneration gas is maintained within the proper range, the composition of 
the regeneration gas changes from a highly oxidizing atmosphere with a 
high oxygen concentration and low carbon monoxide concentration in contact 
with essentially coke-free catalyst in the lower section of the 
regeneration zone to a substantially oxygen-free atmosphere, generally 
having a relatively high carbon monoxide concentration, in contact with 
spent catalyst and partially regenerated catalyst in an intermediate 
section of the regenerator. 
Because of the highly oxidizing atmosphere provided by a high free oxygen 
concentration and low carbon monoxide concentration in the regeneration 
gas in the lower section of the regeneration zone, combustion of 
nitrogen-containing compounds present in the coke burned in the lower 
section tends to form nitrogen oxides, especially in the presence of a 
carbon monoxide combustion-promoting metal. According to the invention, 
these nitrogen oxides are reacted to form free nitrogen (molecular 
nitrogen) in the intermediate section of the regeneration zone in the 
presence of the oxygen-free atmosphere provided by the absence of free 
oxygen. Therefore, regeneration gas leaving the intermediate section of 
the regeneration zone may typically contain a substantial amount of carbon 
monoxide, but is relatively free from nitrogen oxides. Simultaneously, 
catalyst particles reaching the bottom end of the regeneration zone are 
substantially coke-free. 
Above the spent catalyst inlet level, additional free oxygen is added to 
the, oxygen-free, typically carbon monoxide-containing, regeneration gas. 
The additional free oxygen can suitably be added in any free 
oxygen-containing gas, such as pure oxygen, air, or the like. The amount 
of additional free oxygen introduced is preferably at least sufficient to 
react stoichiometrically with all the carbon monoxide present in the 
regeneration gas leaving the intermediate section of the regenerator to 
form carbon dioxide. Particularly preferable, enough additional free 
oxygen is introduced to provide at least 3 volume percent (excess) free 
oxygen in the regeneration gas in addition to the free oxygen required for 
stoichiometric combustion of all the carbon monoxide in the regeneration 
gas. 
Combustion of carbon monoxide in the regeneration gas with added free 
oxygen releases a substantial amount of heat energy into the regeneration 
gas. It is highly desirable to recover this heat energy from the 
regeneration gas prior to its removal from the regenerator. The additional 
heat energy is often useful for carrying out a subsequent catalytic 
conversion operation (e.g., FCC conversion) using the coke-free, 
regenerated catalyst. 
Typically, the regeneration gas has a low heat capacity, so that carbon 
monoxide combustion in the absence of catalyst could heat the flue gas to 
an extremely high temperature, with a consequent possibility of 
temperature damage to equipment contacted by the flue gas, such as 
cyclones, conduits, etc. In order to recover the heat evolved by carbon 
monoxide combustion and provide a heat sink, coke-free catalyst is 
supplied to the upper section, as by conducting a portion of the 
regenerated catalyst from the lower end of the regeneration zone into the 
upper section. Since the regenerated catalyst at the lower end of the 
regenerator is substantially coke-free, essentially no further heat or 
combustion products are added to the regeneration gas in the upper section 
and, in particular, no further nitrogen oxides are formed. Consequently, 
flue gas leaving the regeneration system is free from both nitrogen oxides 
and carbon monoxide. 
Preferably, the substantially coke-free catalyst is passed into the upper 
section of the regeneration zone at a rate sufficient to maintain enough 
coke-free catalyst in the carbon monoxide burning region to absorb 
essentially all the heat released by carbon monoxide combustion. 
Particularly preferably, the heat sink provided by the coke-free catalyst 
is effective to restrict the maximum temperature of the regeneration gas 
in the upper section to less than 27.degree. C. above the maximum 
temperature in the intermediate section of the regeneration zone. The 
height of the bed of essentially coke-free catalyst maintained in the 
upper section of the regeneration zone is sufficient to permit combustion 
of at least a major portion of the carbon monoxide in the regeneration gas 
in contact with the regenerated catalyst. Particularly preferably, the 
amount of regenerated catalyst introduced into the upper section of the 
regeneration zone and the height of the bed of regenerated catalyst 
maintained in the upper section, are sufficient to permit substantially 
complete combustion of all carbon monoxide in the regeneration gas, while 
the gas is in contact with the regenerated catalyst bed. 
PREFERRED EMBODIMENT 
The invention can best be further understood by referring to the specific, 
preferred embodiment shown in the attached drawing. 
In carrying out a preferred embodiment of the invention, spent zeolite-type 
FCC catalyst containing a discrete alumina phase constituting at least 50 
weight percent of the catalyst (zeolite-free basis) is regenerated. A 
combustion-promoting metal additive is employed in the system in the form 
of alumina particles containing 0.1 weight percent platinum. The additive 
particles are mixed with the catalyst particles in an amount sufficient to 
provide 1 part per million, by weight, of platinum in the mixture of 
catalyst and additive. The spent FCC catalyst to be regenerated typically 
contains about 0.3-2.0 weight percent coke, of which typically 0.01-1 
weight percent is nitrogen and 0.25-5.0 weight percent is sulfur. It will 
be apparent to those skilled in the art that the amount of coke contained 
in typical spent FCC catalyst varies substantially above and below this 
concentration, depending on the specific feed and catalyst employed. The 
spent catalyst and combustion-promoting additive are introduced in the 
regeneration vessel 1 through the conduit 3 at the rate of 2,400 tons per 
hour. The spent catalyst entering the regeneration vessel mixes with 
fluidized previously regenerated catalyst moving generally downwardly from 
above the perforated plate 27. Air is introduced into the regeneration 
vessel through the distributor 9 at a rate sufficient to provide the 
desired amount of free oxygen. Steam is added as necessary to maintain the 
regeneration gas flow rate and superficial velocity at a proper level to 
fluidize the particles in the regeneration vessel. Backmixing of catalyst 
particles in the fluidized bed is restricted by the perforated plates 23, 
25, 27 and 29, so that the catalyst particles tend to move downwardly 
through the regeneration zone in plug-type flow. Sufficient coke is burned 
off the catalyst particles before they reach the distribution grid 11 at 
the bottom end of the regeneration vessel so that catalyst at the lower 
end of the bed contains less than 0.1 weight percent coke. Coke-free, 
regenerated catalyst is withdrawn through the conduit 13. Part of the 
regenerated catalyst is passed through the conduit 19 and introduced into 
the upper section of the regenerator vessel 1 at the rate of 600 tons per 
hour. The remainder of the regenerated catalyst is withdrawn from the 
regeneration system through the conduit 21 for catalytic use at the rate 
2,400 tons per hour. The amount of free oxygen contained in the 
regeneration gas introduced through the distributor 9 is restricted 
sufficiently so that the free oxygen content of the regeneration gas, as 
it passes through the perforated plate 27, is less than 0-1 volume 
percent. The carbon monoxide concentration is about 2 volume percent. The 
maximum temperature of the regeneration gas as it passes through the plate 
27 is about 650.degree. C. Additional free oxygen, in a gas such as air or 
mixed air and steam, is introduced into the regeneration gas stream, by 
means of the distributor 33, at a rate sufficient to provide free oxygen 
for essentially complete combustion of all the carbon monoxide in the 
first regeneration gas to form carbon dioxide and to provide a residual 
free oxygen concentration of at least 3 volume percent in flue gas removed 
from the regeneration vessel through the conduit 39. The temperature of 
the regeneration gas stream above the top of the regenerated catalyst bed 
(flue gas) in the upper section of the vessel as shown at the line 37 in 
the drawing is about 670.degree. C. The flue gas stream leaving the 
regenerated catalyst bed and passing into cyclone 35 contains less than 
0.1 volume percent carbon monoxide and less than 200 parts per million, by 
volume, of nitrogen oxides. 
A preferred embodiment of the present invention having been described, 
various modifications and equivalents of the invention within the scope of 
the invention, as defined in the appended claims, will be apparent to 
those skilled in the art.