Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE INVE:NI'ION
This invention is concerned with the control of
nitrogen oxides, especially nitric oxicle, in the ~lue gas
l.eaving the regenerator of an FCC u.nit.
BACKGROUND OF THE INVENTION
Sul~ur oxides formed in the reKenerator of a f'luid~ ed
catalyti.c crackin~ unit during the combustion of t;he coke
deposited on the catalyst particles may be controlled by includ-
ing in the ci:rcula.ting 1.nventory o:~ the unit a sulfllr sorbent,
such as reactive alumina, capable of sorbing the sulf'ur oxides
in the regenerator and releasing them as hydrogen sulfide in the
presence of hydrocarbons in the cra.cking vessel. See United
States Patent No. 4,071~436. The efficiency of such a sulfur
oxides control system is enhanced by the presence of a sulfur
dioxide oxidation promoter, such as platinum. These sulfur
dioxide oxidation promoters aid the formation of sul~ur trioxide
in the regenerator which is more readi]y sorbed by the sulf'ur
oxide sorbent than sulfur dioxide. However, a disadvantage
of this method of controlling sulfur oxides is that the promoter
has been observed to increase the amount of nitrogen oxides
present in the regenerator flue gas. Since the oxides of
nitrogen are themselves noxious gases, it is desirable to
control the amount of nitrogen oxides, especially nitric oxide
(NO), in the flue gas.
United States Patent No. 3,900,55LI teaches a method for
selectively reducing nitric oxi.de by mixing an effluent stream
containing nitric oxide with ammonia and oxygen. The resulting
mixture is subjected to a high temperature, preferably in the
presence of a reducing material such as hydrogen. Various other
processes have been described for reducing nitrogen oxides using
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ammonia, but most of these methods utilize a catalyst.
SIJMMA~Y OF T~IE INVENTION
The present invention is direcked to a process for
controlling the oxides of nitrogen, especially nitric oxide,
in a flue gas containi.ng said oxides :from the regenerator o~ a
fluid catalytic cracking unit in wh:ich the sulfur oxides :~ormed
in the regenerator are controlled by means o:t a circulating
particulate sulfur sorbent used in combination with a sulfur
dioxide oxidation promoter, said process comprising:
(a) mixing the flue gas at a tempe:rature between about
500~ a~d about 1600F with a sufficient amount of ammonia or
an ammonia generating compound to provide at least a stoichio-
metric amount of ammonia relative to the degree of nitrogen
oxide control desired; and
(b) passing the mixture of flue gas and ammonia through a
combustion zone having a temperature in the range of from about
1200 F to about 2000 F for a time sufficient to significantly
lower the amount of nitrogen oxides present in the effluent
from the combustion zone relative to that present in the flue
gasO
As used herein, the term "nitrogen oxide" or "nitrogen
oxides" refers to the various oxides of nitrogen which may be
present in the flue gas leaving the regenerator. Thus, the
term refers to nitri.c oxide (N0), nitrogen dioxide (N02),
nitrogen peroxide (N204)~ nitrogen pentoxide (N205), and mixtures
thereofO The process herein described is especially concerned
with the reduction and control of nitric oxide, since nitric
oxide typically comprises greater than 90% of the nitrogen oxides
in regenerator flue gas. Since the other oxides of nitrogen
mentioned above interconvert rapidly at the temperature of the
process~ reduction of the other oxides will also take place.
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In addition, as used herein, the term "flue gas"
will refer to the gases leaving the regenerator of the fluid
catalytic cracking unit. This is to distinguish the
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1ue gases containing the nitrogen oxides formed in the
regenerator from the combustion gases leaving the combus-
oS tion zone where the nitrogen oxide control is effected.These latter combustion gases will be referred to as
"combustion effluent" or "effluent from the combustion
~one" to avoid confusion.
_RIEF_ DESCRIPTION O HE RAWING
/ The figure is a flow diagram which illustrates
one method for carrying out the present invention.
DETAILED DESCRIPTION OF THE INVE_TION
In the typical fluid catalytic cracking unit
(FCC unit), a particulate catalyst is cycled between a
cracking zone in which the hydrocarbon feedstrea~ is
cracked and a regeneration ~one in which the coke depos
i.ted on the catalyst is burned off in the presence of
oxygen. Since the coke generally contains some sulfur
compounds (the amount usually depends on the sulEur con-
tent of the hydrocarbon feed), during regenerationsignificant amounts of sulfur dioxide and sulfur trioxide
are formed. As already noted, these oxides of sulfur may
be controlled by including in the circulating inventory of
the FCC unit, i.e., the inventory of particles cycled
between the cracking zone and the regeneration zone, a
sulfur sorbent such as reactive aluminaO Since sulfur
- trioxide is more readily sorbed by the sulfur sorbent than
sulfur dioxide, a sulfur dioxide oxidation promoter~
usually a noble metal such as platinum, i5 present in the
regenerator. The oxidation promoter is preferably part of
the circulating inventory of the FCC unit. Usually, the
promoter is present on a particulate support separate from
the catalyst particles and the sulfur sorbent.
Although relatively free of sulfur compounds,
the flue gases leaving the regenerator will usually
contain significant amounts of nitrogen oxides chiefly in
the form of nitric oxide. According to the present inven-
tion, the flue gas is mixed with ammonia or a compound
which will yield a~nonia under the conditions of opera-
tion. The temperature of the flue gas/ammonia mixture
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usually will not exceed about 1600F and is generally con-
siderably cooler, i.e., less than 1000F. In the absence
05 of a reducing material such as hydrogen or a catalyst,
very little of the nitrogen oxides will be reduced at
these temperatures.
The Elue yas/ammonia mixture is passed throuyh a
zone of active combustion haviny a temperature falling
10 within the range of from about 1200F to about 2000F.
The residence time of the flue gas/ammonia mixture in the
flame should be of sufficient duration to allow for the
reduction of the nitrogen oxide to nitrogen. Generally,
the reduction will be completed in a second or two,
depending on the temperature of the flame and the manner
of passing the mixture through the flame.
The amount of a~nonia present in the mixture is
not critical so long as at least a stoichiometric amount
is present relative to the amount of nitrogen oxide to be
controlled. However, the efficiency of the process is
improved by increasing the amount of ammonia above this
minimum level. Therefore, the process may be operated
with the ammonia at several times the stoichiometric
amount. In the case of nitric oxide, a stoichiometric
amount is one mole of ammonia to one mole of nitric oxide.
Free oxygen is believed to be required for the
reduction to take place~ However, since oxygen is neces-
sary for the combustion as well, it is not necessary to
add the oxygen separately to the ammonia/flue gas mixture.
However, to insure that sufficient oxygen is present, the
combustion should be carried out in an oxygen-rich mode
rather than in an oxygen lean mode.
The selection of fuels for the combustion zone
is not criticalO Generally, refinery fuel gas is readily
available at the site at which FCC units are located and
provides a convenient fuel Such fuel gases are usually
composed of principally methane and hydrogen with lesser
amounts (generally about 10~ or less) of other lower
hydrocarbons~
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The exact reactlon which occurs in the combus-
tion zone is not well understood, but it is believed the
05 reduction that takes place yie]ds nitrogen and water as
products. Such reductions oE nitrogen oxldes have been
known to occur in the presence of catalysts and under
other conditions. Althouyh it is assumed the same basic
reactions are occurring, the present invention is not
limited by any particular chemical reaction or mechanism.
In order to further clarify the invention, one
preferred way of carrying out the process is illustrated
in the figure. Coked catalyst is carried from the
cracking vessel (not shown) of the FCC unit to ~he cata-
lS lyst regenerator 2 via transfer conduit 4. The spentcatalyst is regenerated in a fluidized bed 6 by burning
the coke off the catalyst in the presence of air intro-
duced at the bottom of the regenerator 2 by means of air
conduit 8. In addition to the particles of cracking
catalyst, a separate particulate sulfur sorbent and sepa-
rate particles of a supported sulfur dioxide oxidation
promoter are present in the circulating inventory of the
FCC unit. Therefore, sulfur dioxide formed in the
regenerator is oxidized to sulfur trioxide and sorbed by
the sulfur sorbent before leaving the fluidized bed 6.
The regenerated catalyst, sulfur sorbent plus sorbed
sulfur trioxide, and sulfur dioxide oxidation promoter are
returned to the cracking vessel via transfer conduit 10.
In the presence of hydrocarbons in the cracking vessel,
the sorbed sulfur compounds are released as hydrogen
~ulfide.
Referring back to the regenerator 2, nitrogen
oxides, especially nitric o~ide, formed in the oxidizing
environment of the regenerator pass out of the fluidized
bed 6 and leave the regenerator with the flue gas via
conduit 12. In FCC units as described herein, nitrogen
oxides are generally present in the fIue gas in amounts of
about ~00 to 1300 ppm. From the regenerator, the flue gas
is carried via conduit 12 to a flue gas cooler 14 where
heat from the hot flue gas is recovered~ The te~perature
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of the flue gas leaving the flue gas cooler vla conduit 16
is typically between about 500F and 700F. Ammonia is
05 injected into the cooled flue gas by means of conduit 18,
and the resulting ammonia/flue gas mixture i5 carried by
conduit 20 to an electrostatic precipitator 22. In the
electrostatic precipitator, particulate materials are
removed from the ammonia/flue gas mixture and collected at
the bottom o~ the precipitator. This arrangement is par-
ticularly advantageous since the presence of the ammonia
in the flue gas is known to enhance the removal of partic-
ulates by the electrostatic precipitator. A further
advantage of carrying out the process in this manner is
that it has been found that the silica-alumina catalyst
fines trapped by the precipitator may serve as a catalyst
for the reductio~ of nitroyen oxides in the presence of
ammonia. Thus, some control (about 5% to 15%) has been
observed to take place in the precipitator itself before
the mixture of ammonia and flue gas passes to the combus-
tion zone~ In carrying out this phase of the process at
least a stoichiometric amount of ammonia is mixed with the
flue gas, i~e., sufficient ammonia to reduce all of the
nitrogen oxides present. Preferably, excess ammonia is
~s used often at two or three times the stoichiometric
amounts.
The ammonia/flue gas mixture is carried via
conduit 24 to a boiler 26 where the mixture is passèd
through a flame 28. The flame is fueled by refinery fuel
gas entering the boiler by means of conduit 30~ Air for
combustion is supplied by conduit 32. A portion of this
air may be pre-mixed with the ammonia/flue gas mixture.
The nitrogen oxides are reduced to nitrogen gas during the
passage of the flue gas/ammonia mixture through boiler
flame 28. In the combustion effluent leaving the boiler
26 via conduit 34, reductions in the amount of nitrogen
oxides have been observed in the order of about 75% using
a stoichiometric amount of ammonia. The combustion
effluent is sent to a stack 36 where it is released to the
atmosphere. Despite the excess ammonia present in the
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flue gas/ammonia mixture, very little ammonia, i.e., less
than 10 ppm, has been observed leaving the stack with tlle
05 combustion effluent. Therefore, the use of the excess
ammonia does not result in a serious em~ssion problem when
the combustion effluent leaves the stack.
The above description i9 directed to one pre-
ferred means for carrying out the present invention.
Those skilled in the art will recognize that other means
which are equally effective could be devised for carrying
Ollt the spirit of this invention.
It is also possible to carry out this process as
a two-step or multi~step combustion process in which addi-
lS tional ammonia is mixed with the partially cooled combus-
tion effluent from the combustion zone. This effluent/
ammonia mixture may be passed to a second combustion zone
where the ammonia reacts to reduce any remaining nitrogen
oxides. ThiS may be advantageous where the first combus-
tion zone is fueled by a high nitrogen fuel (some fueloils and fuel gases contain substantial amounts of
nitrogen). In this embodiment, ammonia is mixed with the
combustion efluent from one or more combustion æones
fired with a high nitrogen fuel oil. The combustion
effluent from these primary combustion zones may contain
significant amounts of nitrogen oxide. The effluent/
ammonia mixture from each of the primary combustion zones
would be sent to a secondary combustion æone fueled by a
low nitrogen fuel The final stack gas vented to the
atmosphere after secondary treatment would contain signi-
ficantly less nitrogen oxide.
In another embodiment of the invention, the
combustion effluent is mixed with additional ammonia and
recycled to the combustion zone~
