Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 1 63981
--1--
01 THREE-STAGE CATALYST REGENERATION
. . . _ _ .
BACRGROUND OF THE INVENTION
... ... _ _
This invention concerns the art of catalyst
05 regeneration. More specifically, the present invention
concerns a method for burning nitrogen-containing coke off
coke-containing particulate catalyst while avoiding nitro-
gen oxides 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 molecu-
lar 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
2 temperature of about 425C-600C, usually 460C-560C.
The reactions of hydrocarbons at the elevated operating
temperature result in deposition of coke on the catalyst
particles. The resulting fluid products are separated
from the coke-deactivated, spent catalyst and are with-
drawn 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 prede-
termined 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.,
540C-815C, usually 590C-730C. Flue gas formed by
combustion of coke in the catalyst regenerator may be
- treated for removal of particulates and for conversion of
A ~ 7 ~ 9
. ~
- 1 1 6398 1
01 carbon monoxide, after which the flue gas is normall~ dis-
charged into the atmosphere.
Most FCC units now use zeolite~containing
catalyst having high activity and selectivity. Zeolite-
05 type catalyst have a particularly high activity andselectivity when the concentration of coke on the catalyst
after regeneration is relatively lowl so that it is
generally desirable to burn off as much coke as possible
in regenerating zeolite-containing catalysts. It is also
o~ten desirable to burn as much as possible of the carbon
monoxide formed by coke burning within the catalyst regen-
eration system to conserve heat energy. Conservation o~
heat is especially important when the concentration o
coke on the spent cracking catalyst is relatively low as a
result of high catalyst selectivity. Among the ways sug-
gested 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 fluidized catalyst bed in the
catalyst regenerator using an active, carbon monoxide com-
bustion 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. Patent 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 car-
bon dioxide and to prevent afterburning. In U.S. Patent
3,808,121, it is proposed to introduce into a catalyst
regenerator relatively large sized particles containing a
carbon monoxide combustion promoting metal. The circu-
lating particulate solids inventory, comprised o~
Al~10120730
1 3 6398 ~
01 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
05 metals such as cobalt, copper, nickel, manganese, copper-
chromite, etc., impregnated on an inorganic oxide such asalumina, are disclosed. Belgium Patent Publication No.
8~0,181 suggests using cracking catalyst particles
containing 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 hydro-
carbon feed. Inclusion of the promoting metal in thecracking system is said by the publication to decrease
product selectiqity in the cracking step by substantially
increasing coke and hydrogen formation. Catalyst parti-
cles containing the promoter metal can ke used alone or
can be cir~ulated in physical mixture with catalyst
particles free of the combustion promoting metal. U.S.
Patents 4,072,600 and 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 opera-
tions using metal-promoted complete carbon monoxide com-
bustion-type regeneration has been the generation of
undesirable nitrogen oxides (NOX) in the ~lue gas formed
by burning coke. The present invention is directed, in
part, toward providing a catalyst regeneration system
which accomplishes a high degree of coke remova~ and
complete carbon monoxide combustion within a catalyst
regeneration system, while substantially decreasing the
~ .1 63g8.~ .
01 concentration of nitrogen oxide present in flue gas formed
by burning the coke.
Representative of catalyst regeneration patent
literature previously published are the following patents:
05 U.S. Patent No. 3,909,392 describes a scheme for enhancing
carbon monoxide combustion by thermal means. Catalyst
is used to provide a dilute-phase heat sink for the
increased heat production. British Patent Publication No.
2 r 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 regen-
eration being carried out in the second stage with a
separate regeneration gas. U.S. Patent no. 3,767,566
describes a two-stage regeneration scheme in which partial
regeneration ~akes place in an entrained catalyst bed, and
secondary, more complete regeneration takes place in a
dense fluidized catalyst bed. A somewhat similar regen-
eration operation is described in U.S. Patent 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.5. Patent 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 accom-
plishing essentially complete removal of coke from the
catalyst.
SUMMARY OF THE INVENTION
I have found that nitrogen-containing coke can
be burned oEf a coke-containing particulate catalyst to
provide a low level of residual carbon on the catalyst,
6398 1
01 and a flue gas low in both carbon monoxide and NOX can be
formed in burning the coke, by the process of (a) burning
substantially all coke off a first portion of the coke-
containing catalyst with a regeneration gas comprising
05 free oxygen in a first regeneration zone, and burning
substantially all carbon monoxide formed in the first
zone, sufficient free oxygen ~ing introduced into the
first zone to provide at least 1 volume percent residual
free oxygen in the regeneration gas after burning the coke
and carbon monoxide, whereby nitrogen oxides are formed in
the first zone; (b) passing the regeneration gas from the
first ~one into a second zone, forming carbon monoxide and
carbon dioxide and generating a substantially oxygen-free
; atmosphere in the second zone by burning coke off a second
lS portion o the coke-containing catalyst a~d burning carbon
: monoxide with substantially all the residual free oxygen,
and decreasing the amount of nitrogen oxides in the regen-
eration gas by reacting at least a portion of the nitrogen
oxides in the oxygen-free atmosphere to form free nitro-
gen; and (c)-passing the regeneration gas from the second
zone into a third regeneration zone, burning substantially
: all carbon monoxide generat~d in the second regeneration
zone with additional free oxygen in contact with substan-
tially coke-free catalyst in the third zone.
In another embodiment, the present invention
concerns a method for burning nitrogen-containing coke off
a coke-containing particulate catalyst, which comprises;
(a) introducing a major portion of the coke-containing
catalyst into a first fluidized bed comprising substan-
tially coke-free catalyst in a lower zone in a vertically
extending regeneration vessel; (b) passing a regeneration
gas comprising free oxygen upwardly through the lower
zone, burning substantially all coke off the major portion
of coke-containing catalyst i.n the first bed, and burning
substantially all carbon monoxide formed in the first bed
~ 121~l3~
3 ~ 639~ 1
01 within the lower zone, sufficient free oxygen being intro-
duced into the lower zone to provide at least 1 volume
percent residual free oxygen in the regeneration gas at
the upper end of the lower 20ne, whereby nitrogen oxides
05 are generated in the lower zone; (c) introducing a minor
portion of the coke-containing catalyst into a second
fluidized bed of cataIyst in a vertically intermediate
zone in the regeneration vessel, passing nitrogen oxides-
containin~ regeneration gas from the lower zone upwardly
through the intermediate zone, forming carbon monoxide and
carbon dioxide and generating a substantially oxygen-free
atmosphere in the second bed by reacting substantially all
the residual free oxygen with coke and carbon monoxide in
the second bed, and decreasing the amount of nitrogen
oxides in the regeneration gas by reacting at least a
portion of the nitrogen oxides in the intermediate zone to
form free nitrogen; and (d) passing carbon monoxide-
containing regeneration gas from the intermediate zone
upwardly through a third fluidized bed comprising
substantially coke-free catalyst in an upper zone in the
regeneration vessel, and burning substantially all carbon
monoxide introduced into the upper zone with additional
free oxygen in contact with the third bed.
DESCRIPTION OF THE DRAWING
The attached drawing is a schematic representa-
tion of one preferred embodiment o the present invention9
Referring to the drawing, there is shown a ver-
tically extending regeneration vessel 1. Catal~st having
nitrogen-containing coke deposited thereon is introduced
into the system through a conduit 3 and is divided into
two portions, which pass into two conduits 5 and 7. The
relative amounts of spent catalyst passing into these two
conduits is controlled by adjustment of valves 9 and 11.
A major portion of the coke-containing catalyst flows
through the conduit 7 into a dense-phase fluidized bed 13
~ Zur~4
1 J 6398 1
01 of substantially coke-free catalyst maintained in a first,
lower regeneration zone or section 15 in the vessel 1. A
stream of free oxygen-containing regeneration gas is
introduced into the vessel through a conduit 17 and a gas
05 distributor 19. The regeneration gas stream passes
upwardly through a distribution grid 21 and through the
fluidized bed 13, the upper end of which is indicated by a
line at 23. Substantially all the coke introduced into
the lower zone 15 with the major portion of coke-
containing catalyst is burned within the bed 13. Substan-
tially all the carbon monoxide formed in the zone 15 isalso burned. Suficient excess free oxygen is introduced
into the zone 15 in:the regeneration gas to provide at
least 1 volume percent residual free oxygen in the
regeneration gas at the upper end of the zone 15 after
combustion of coke and carbon monoxide. Nitrogen oxides
are formed in the regeneration gas by burning the
nitrogen-containing coke in the oxidizing atmosphere
present in the zone 15. The regeneration gas stream,
containing residual free oxygen and nitrogen oxides,
passes out of the lower zone 15 through a gas distribution
grid 25. The minor portion of coke-containing catalyst
flows through the conduit 5 into a dense-phase fluidized
bed 27 of partially regenerated catalyst in a second,
vertically intermediate regeneration zone or section 29 in
the vessel 1. The top of the dense-phase bed 27 is
maintained at a level indicated by a line at 31.
Partially regenerated catalyst in the bed 27 passes
through an overflow well 33 into the lower section 15 at a
rate sufficient to maintain the desired level in the bed
27. The residual free oxygen contained in the regenera-
tion gas entering- the intermediate zone is completely
consumed in.the intermediate zone by burning coke and
carbon monoxide in the bed 27, resulting in a substan-
tially oxygen-free atmosphere~ Combustion of coke forms
Al'1O 120 735
~ 3 639~ 1
--8--
~1 carbon dioxide and carbon rnonoxide. Nitrogen oxides
contained in the regeneration gas are reacted to form free
nitrogen in the oxygen~free atmosphere in contact with the
bed 27, so that the amount of nitrogen oxid~s in the
05 regeneration gas is substantially decreased. Additional
free oxygen-containing gas is introduced into the carbon
monoxide-containing regeneration gas stream in the top of
the intermediate section 29 above the dense-phase bed 27
by means of a conduit 35 and a distributor 37. The free
oxygen-enriched regeneration gas passes out of the
intermediate section 29 through a distribution grid 39.
Substantially coke-free catalyst is removed from the bed
13 in the lower zone 15 through a conduit 41 at a rate
controlled by a valve 43 and passed into a surge vessel
~5. Part of the coke-free catalyst in the vessel 45 is
entrained upwardly with steam introduced through a conduit
47. The steam and entrained catalyst are passed through a
riser conduit 49 into a dense-phase fluidi~ed bed 51 of
co~e-free catalyst maintained in a third, upper regenera-
tion zone or section 53 in the vessel 1. The top of the
dense-phase bed 51 is maintained at a level indicated by a
line at 55. Heated catalyst in the bed 51 is passed
through an overflow well 57 into the lower section 15.
Coke-free catalyst is introduced into and removed from the
bed 51 at a rate sufficient to maintain the bed 51 at a
desired temperature. The regeneration gas passes from~the
distribution grid 39 upwardly through the dense-phase bed
51. Carbon monoxide in the regeneration gas is substan-
tially completely burned with the added free oxygen in
contact with the coke-free catalyst in the bed 51, so that
the catalyst in the bed 51 absorbs essentially all the
heat of combustion. The resulting carbon monoxide-free
flue gas stream passes upwardly out of the bed 51 and into
a cyclone separator 59. Plural cyclones or cyclone stages
can, of course, be used. Any catalyst entrained in the
' i ~ i .~
i~
~ 1 6398:1
01 flue gas stream is separated in the cyclone and returned
to the bed 51, and the regeneration gas (flue gas) is
removed from the vessel 1 through a conduit 61. To
simplify the foregoing description, various necessary,
05 conventional elements of the embodiment depicted are not
shown or described. Such elements, e.g., control meansl
pumping and compressing means, and the like, and their use
and disposition in the embodiment depicted will be
apparent to those skilled in the art.
DETAILED_DESCRIPTION OF THE INVENTION
As used herein, the term "oxidizing atmosphere"
means an atmosphere containing at least 0.5 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" refers to catalyst which contains an
average of less than 0.2 weight percent carbon.
As used herein, the term '~dense~phase fluidized
bedl' means a fluidi~ed bed of particulate solids having a
density of 8 ~o 15 pounds per cubic foot, depending on the
particle density and the gas velocity.
Catalysts that are best adapted for treatment
according to this invention are those in the form of
particulate solids. Preferably, the catalyst is sized
appropriately for catalytic use in an entrained bed or
fluidized bed operation. With reference to catalytic
conversion systems presently in commercial use, the
present invention is especially advantageous for burning
coke off spent FCC catalysts; however, use of the present
regeneration system is not limited to FCC catalysts and
can be used for treating any coke-containing particulate
catalyst which can be beneficiated by coke burnoff.
,~, (,, . ~, f :. ~
l,
I ~ 63981
--10--
01 Regeneration according to the invention can be
carried out in plural vessels or chambers or in a single,
vertically extended vessel or chamber, suitably divided
into three zones. The regenerator must, of course, be
05 capable of containing the regeneration gases a~d catalyst
particles at the temperatures and pressures employed in
the operation. Suitable vessels will be readily apparent
to those skilled in the art from the description herein.
When a single, vertically elongated vessel is employed, it
can suitably be divided into three regeneration zones or
sections in any convenient manner, as by the use, for
example, of gas distribution grids, screens, or the like,
sized to allow the regeneration gas streams to flow
upwardly through the vessel, while preventing substantial
flow of catalyst particles between the regeneration zones,
except for the controlled catalyst flow described below.
The regeneration gas employed must have an
appropriate free oxygen (molecular oxygen) content. Norm-
ally, air is quite suitable for use in supplying free oxy-
gen, but use of air is not essential. For example, pureoxygen or oxygen-enriched air can also be used 9 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
include a combination of temperature and pressure suffi-
cient to permit the specified degree of coke combustion,
carbon monoxide combustion and nitrogen oxides reaction to
take place, in the manner discussed below. Temperatures
of 540C to 815C are normally quite suitable. Tempera-
tures of 590C to 730C are preferred. The rates of flow
of regeneration gases, entrainment gases and catalyst
particles are preferably maintained at levels which pro-
vide a dense-phase fluidized bed of ca~alyst in each of
~ X()7
i ~ B39~ :~
.
01 three regeneration ~ones, although moving beds or
entrained beds of catalyst can also be used~ if desired,
with appropriate, obvious mechanical differences from a
fluid-bed operation. The preferred fluid-bed operation
05 can be carried out in a conventional manner by maintaining
upward superficial gas velocities appropriate to the size
and density of catalyst particles undergoing regeneration
and by maintaining proper catalyst introduction and with-
drawal rates. The operating 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 strongly 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 metals for use in promoting carbon monoxide
combustion in the present system include platinum, palla-
dium, iridium, rhodium, ruthenium, osmium, manganese,
copper, and chromium. The metal is used in a concentra-
tion sufficient to enhance the rate of carbon monoxide
burning to the desired degree. Usually, sufficient carbon
monoxide combustion promoter is included to provide for
compLete combustion of carbon monoxide within a dense-
phase fluidized bed. In commercial FCC operations, the
use of platinum in various forms as a carbon monoxide
combustion-promoting metal is known. A carbon monoxide
3Q combustion-promoting metal may be included as a component
of all, or a major or minor fraction, of the catalyst
particles in the system or may be included as a component
of discrete, substantially catalytically inert particles
which are mixed with the catalyst inventory and are circu-
lated in a physical mixture with the catalyst par~icles.
a~ J7
63~1
-12-
01 A preferred metal for use in discrete promoter particles
is platinum.
Sulfur oxides contamination of the flue gas, as
a result of burning sulfur-containing coke off the cata-
05 lyst, may advantageously be avoided by using a solid reac-
tant, or acceptor, as a component of the particulate
solids subjected to regeneration. Sulfur oxides in the
flue gas react with, or adsorb on, the reactant or
acceptor to form sulfur-containing solids in the regen-
erator. This is particularly the case in the oxygen-rich
atmosphere provided in the first and third regeneration
zones, as discussed below. In this way, the sulfur oxides
content of the flue gas leaving the regenerator may be
substantially reduced. A preferred solid reactant is
alumina, which, in its active form, reacts with sulfur
oxides to form a sulfur-containing solid. The active
alumina used for reaction with sulfur oxides has a surface
area of at least 50 square meters per gram. Alpha alumina
is not suitable. Alumina may suitably be included as a
component of all or part of the catalyst particles, and
may be included in discrete, substantially catalytically
inactive particles physically admixed 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 substan-
tial incremental removal of sulfur oxides from the regen-
eration gas. Usually~ good results can be achieved if 0.1
to 25 weight percent alumina is mixed with the catalyst.
If alumina-containing catalyst is used, the catalyst
preferably includes (on a zeolite-free basis) less than 50
weight percent silica and more than 25 weiqht percent
alumina.
,~M~lZ07b~0
1 3 ~39~ 1
-13-
.
01 It will be apparent to those skilled in the art
that -the amount of coke in coke-containing catalyst, as
well as the concentration of nitrogen and sulfur impur-
ities in the coke, will vary widely depending on such
05 factors as the composition and boiling range of the hydro-
carbon feed being converted using the catalyst, the
composition of the catalyst, the type of hydrocarbon
conversion system in which the catalyst is used prior to
coke burnoff (e.g., moving bed, fluid bed, entrained bed),
etc. The benefits of regeneration according to the
invention can be ob~ained in burning coke off catalysts
which contain an amount of coke varying in a broad range r
and also for catalysts containing coke having a nitrogen
content which can vary over a broad range.
In accordance with the invention, a first
portion of the coke-containing catalyst is introduced into
a first regeneration zone. A free oxygen-containing
regeneration gas is passed through the catalyst in the
first regeneration zone. The first portion of coke-
containing catalyst generally includes from about 60
percent to about 95 percent of the coke-containing
catalyst particles. Preferably, the first portion
includes form 80 percent to 90 percent of the coke-
containing catalyst. The amount of free oxygen (molecular
oxygen) introduced into the first regeneration zone is at
least sufficient to react stoichiometrically with substan-
tially all the coke carbon contained in the major portion
of coke-containing catalyst and to react with substan-
tially all the carbon monoxide generated in the first
zone, with sufficient residual free oxygen being intro-
duced to provide at least 1 volume percent, and, prefer-
ably, at least 3 volume percent, free residual oxygen in
. the regeneration gas withdrawn from the first zone. The
composition of the regeneration gas changes, during its
passage through the first regeneration zone, from a highly
,, "i~l ' ,1 I~` 1
~ ~ 639~1
-14-
01 oxidizing atmosphere with a high oxygen concentration and
no carbon monoxide when introduced, to a less oxidizing
atmosphere, preferably having a relatively low residual
free oxygen concentration, preferably less than 10 volume
05 percent, particularly preferably less than 5 volume
percent, when the regeneration gas is withdrawn from the
first regeneration zone after combustion of the coke and
carbon monoxide.
Because of the oxidizing atmosphere provided in
the first regeneration zone and the essentially c`omplete
coke and carbon monoxide combustion carried out, combus-
tion of nitrogen-~ontaining compounds in the coke on the
catalyst in the first regeneration zone results in genera-
tion of nitrogen oxides, especially in the presence of a
carbon monoxide combustion-promoting metal, such as
platinum. Accordingly, the regeneration gas is contamina-
ted with nitrogen oxides when it is removed from the first
regeneration zone. Nitrogen oxides in the regeneration
gas are reacted, or reduced, to form free nitrogen
(molecular nitrogen) in a second regeneration zone in a
substantially oxygen-free atmosphere generated by combus-
tion of essentially all the remaining free oxygen with
carbon monoxide and coke on spent and partially regen-
erated catalyst in the second regeneration zone.
In accordance with the invention, a second
portion of the coke-containing catalyst, preferably 10-20
percent of the coke-containing catalyst, is introduced
into a second regeneration zone. The nitrogen oxides-
containing regeneration gas withdrawn from the first
regeneration zone is passed through the catalyst in the
second regeneration æone. Coke-containing catalyst is
introduced into the second regeneration zone in an amount
at least sufficient to react with essentially all of the
residual oxygen in the regeneration gas to form carbon
~ LC ~
~ 3 63~` :~
-15-
01 monoxide and carbon dioxide and to generate a substan-
tially oxygen-free atmosphere in the regeneration gas in
contact with the catalyst bed in the second regeneration
zone. The oxygen-free atmosphere preferably contains at
05 least 0.5 volume percent carbon monoxide and particularly
preferably at least 2.0 volume percent carbon monoxide.
Since essentially all the free oxygen in the regeneration
gas is consumed in burning coke and carbon monoxide, the
free oxygen concentration of the regeneration gas in the
second regeneration zone is decreased to less than 0.5
volume percent and preferably less than 0.1 volume
percent. The presence of a substantially oxygen-free
atmosphere in the regeneration gas causes the nitrogen
oxides to react, at least partially, to form free nitrogen
(molecular nitrogen~. The rate of introduction of spent
catalyst into the second regeneration zone and the rate of
withdrawal of catalyst are preferably adjusted so that
catalyst removed from the second zone is partially regen-
erated, which may be defined as catalyst having a coke
carbon content of less than the coke carbon content the
original coke-containing catalyst and greater than 0.2
weight percent~ Preferably, catalyst in the second
regeneration zone is maintained as a dense-phase fluidized
bed of coke-containing partially regenerated catalyst
Preferably, catalyst is withdrawn from the second regene-
ration zone after partial coke removal and introduced intothe first regeneration zone in order to provide complete
coke removal.
Additional free oxygen is introduced into the
oxygen-free, typically carbon monoxide-containing, regen-
eration gas after nitrogen oxides have been reacted toform free nitrogen in the substantially oxygen-free atmos-
phere. 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
~I(J~ 743
J
1 36398t
~. .
Ol introduced into the regeneration gas is preferably at
least sufficient to react stoichiometrically with all the
carbon monoxide in the regeneration gas leaving the second
regenertion zone to form carbon dioxide. Particularly
05 preferably, enough free additional oxygen is introduced
into the regeneration gas to provide a residual free
oxygen concentration of at least 3 volume percent in the
regeneration gas (flue gas) after combustion of substan-
tially all the carbon mon.oxide in the regeneration gas in
1~ a third regeneration zone. The regeneration gas is intro-
duced into the third regeneration zone, either before or
after the additional free oxygen is introduced into it.
Combustion o carkon monoxide contained in the
regeneration gas with the added free oxygen releases a
substantial amount of heat into the regeneration gas in
the third regeneration zone. It is advantageous to
recover this heat energy from the regeneration gas prior
to its removal from the regeneration systemO The heat
energy conserved is often useful for carrying out a
related catalytic conversion operation (e.g., FCC con-
version) with the resulting coke-free catalyst.
Typically, regeneration gases have a low heat capacity, so
that combustion of carbon monoxide can heat regeneration
gas to an extremely high temperature if the combustion
takes place where the regeneration gas is not in contact
with a substantial amount of catalyst, with a consequent
possibility of temperature damage to apparatus contacted
by the flue gas, such as cyclones, conduits, etc.
According to the inventlon, heat energy evolved
by carbon monoxide combustion in the third regeneration
zone is conserved by using substantially coke-free
catalyst, preferably supplied from the first regeneration
zone, to provide a heat sinkO Since substantially coke-
free catalyst, such as regenerated catalyst rec~vered from
the first regeneration zone, is used for heat absorption,
~ l44
1 1 ~39~ 1
01 little or no further heat from coke burning is added to
the regeneration gas in the third zone, and little or no
further nitrogen oxides are generated. Consequently, flue
gas withdrawn from the third regeneration zone is low in
05 both nitrogen oxides and carbon monoxide.
Preferably, the amount of substantially coke-
free catalyst maintained in the thlrd regeneration zone is
sufficient to provide a dense-phase fluidized bed of
substantially coke-free catalyst in contact with the
regeneration gas during carbon monoxide combustion larye
enough to allow absorption of essentially all the heat
released by carbon noxide combustion. Particularly
preferably, the ~eat sink provided by the coke-free
catalyst is effective to restrict any temperature rise in
the regeneration gas in the third regeneration zone to
less than 50C above the temperature of the regeneration
gas in the second regeneration zone. The rate of coke-
free catalyst introduction and removal from the third zone
is preferably controlled to restrict any increase in
temperature in the flue gas to less than 50C. The amount
of coke-free catalyst maintained in the third regeneration
zone is sufficiènt to permit combustion of at least a
major portion of the carbon monoxide in the regeneration
gases while the gas is in contact with the bed of coke-
free catalyst. Particularly preferably, the rates ofcoke-free catalyst introduction into and withdrawal from
the third regeneration ~one~ and the amount of coke-free
catalyst maintained in the third regeneration zone, are
sufficient to permit substantially complete combustion of
all carbon monoxide in the regeneration gas while the
regeneration gas is in contact with a dense-phase fluid-
ized bed of coke-free catalyst.
A~0120745
`~ ~ 639~ ~
01 PREFERRED EMBODI~ENT
The invention can best be further understood by
referring again to the specific, preferred embodiment
shown in the attached drawing.
05 In carrying out a preferred embodiment of the
invention, spent zeolite-type FCC catal~st, preferably
containing a substantial amount of alumina capable of
reacting with sulfur trioxide to form a sulfur-containing
solid, is regenerated. A carbon monoxide combustion
promoting metal additive is employed in the system,
preferably in the form of alumina particles containing 0.1
weight percent platinum. The additive particles are
preferably mixed with the catalyst particles in an amount
sufficient to provide burning of carbon monoxide within
the dense-phase catalyst b~d 13. 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, and the
amounts of nitrogen and sulfur compounds in the coke vary
substantially, depending on the specific feed, conversion
conditions and catalyst employed. The mixture of spent
catalyst and combustion-promoting additive is introduced
into a dense-phase fluidized be~ of regenerated, coke-free
catalyst in the lower section 15 of regeneration vessel 1
throuyh the conduit 3 at the rate of about 2700 tons per
hour. Spent catalyst enters the bed 13 at the rate of
about 2000 tons per hour. Oxygen-containing regeneration
gas such a~; air is introduced into the regeneration vessel
through the distributor 19 at a rate sufficient to provide
enough free oxygen to burn substantially all the coke on
the catalyst and any carbon monoxide generated in the bed
13, and to provide with at least 3 volume percent residual
free oxygen after complete coke and carbon monoxide
1 1 6398 1
--19--
01 burning. Preferably, enough free oxygen is introduced
into the bed 13 to supply between about 3 and 5 volume
percent free oxygen in excess of the free oxygen necessary
for stoichiometric combustion to carbon dioxide of
05 substantially all the coke carbon in the catalyst in the
bed 13. Preferably substantially all coke and carbon
monoxide are burned within the dense-phase bed 13. Steam
is added as necessary to maintain the regeneration gas
flow rate and superficial velocity at a proper level to
fluidize the catalyst particles in the bed 13.
Coke-containing spent catalyst is introduced at
the rate of 400 tons per hour, into the dense-phase fluid-
; ized bed 27 of partially regen~rated catalyst. Catalyst
in the bed 27 has a coke content less than that of the
spent catalyst and greater than 0.2 weight percent.Carbon monoxide and carbon dioxide are formed, and essen-
;~ tially all the residual free oxygen in the regeneration
gas is consumed, by combustion of coke and carbon monoxide
in the fluidized bed 27. Nitrogen oxides in the regen-
eration gas are reacted in the resulting oxygen-free
atmosphere to form free nitrogen. The carbon monoxide
~;~ concentration in regeneration gas leaving the top 31 of
the bed 27 is preferably between about l and 15 volume
percent. The temperature of the regeneration gas as it
passes above the top 31 of the bed 27 is preferably in the
range from about 575 to 750C, for example, about 670Co
Additional free oxygen, in a gas such as air or air and
steam, is introduced into the regeneration gas stream, by
means of the distributor 37~ preferably in an amount
3 sufficient to provide enough free oxygen for essentially
complete combustion of all the carbon monoxide in the
regeneration gas and to provide for at least 1 volume
percent residual free oxygen in the flue gas. Partic-
ularly preferably, enough additional free oxygen is intro-
duced into the regeneration gas to provide a residual free
J ;'~ /
I J ~39~ 1
-20-
01 oxygen concentration of at ].east 3 volume percent in flue
gas removed from the regeneration vessel through the
conduit 61. The flue gas stream above the top 55 of the
coke-free catalyst bed 51 in the upper zone 53 of the
S vessel is preferably maintained at a temperature between
575C and 750C, such as, for example, about 660C. The
flue ga.s stream preferably contains less than 500 parts
per million, by volume, of carbon monoxide.
A preferred embodiment of the present invention
having been described, numerous modifications and varia-
tions of the invention within the scope of the invention
will be apparent to those skilled in the art.
~ 120-7
