Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REDUCTION OF NOX IN LOW CO PARTIAL-BURN
OPERATION USING FULL BURN REGENERATOR ADDITIVES
FIELD OF THE INVENTION
[0001] The present invention relates to the reduction of the concentrations of
nitrogen oxides (N~X) from a fluid catalytic cracking (FCC) regenerator by
operating the regenerator in partial C~ burn mode with a N~X reducing catalyst
system.
BACKGROUND OF THE INVENTION
[OOOZj It has been found that oxides of nitrogen, primarily IVC~ and N~a, are
formed at high temperatures, such as the temperature at which catalyst
utilised in
a hydrocarbon cracking process is regenerated in the presence of carbon
monoxide combustion promoters. ~1s hydrocarbons, such as petroleum
feedstocks, arc cracked, coke is deposited on the catalyst particles. The coke
formation on the particles progressively decreases the activity of the
catalyst
particles. Eventually the activity of the catalyst declines to the point where
the
coke must be burned off the particles. This step, which is normally referred
to as
regeneration, may be done on a batch or a continuous basis by contacting the
catalyst particles with a hot regeneration gas, such as air. Coke is burned
off,
thus restoring catalyst activity and simultaneously heating the catalyst. Flue
gas
formed by burning coke from the catalyst in the regenerator may be treated for
removal of particulates and for conversion of C~ to C~a, after which the flue
gas is normally discharged into the atmosphere.
[0003] Most FCC units now use zeolite-containing catalysts having high
activity and selectivity. These catalysts are believed to work best when the
amount of coke on the catalyst after regeneration is low. There are two types
of
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FCC regenerators commonly used, the high efficiency regenerator and the
bubbling bed type. The high efficiency regenerator mixes recycled regenerated
catalyst with spent catalyst, burns much of the coke from the spent catalyst
in a
fast fluidized bed coke combuster, then discharges the catalyst and flue gas
up a
dilute phase transport riser where some additional coke combustion occurs, and
where most of the C~ is afterburned to C~a. These regenerators are designed
for complete C~ combustion, and usually produce a relatively coke-free
regenerated catalyst, and flue gas with very little C~, and modest amounts of
NIX. The bubbling bed regenerator maintains the catalyst as a bubbling
fluidized bed, to which spent catalyst is added and from which regenerated
catalyst is removed. These regenerators usually require more catalyst
inventory
than the high efficiency regenerator does, because gas/catalyst contacting is
not
as efficient in a bubbling fluidized bed as in a high efficiency regenerator.
Many
bubbling bed regenerators operate in complete C~ combustion mode, i.e.,
wherein the mole ratio of C~a/C~ is at least I0. Definers try to bum C~
completely within the catalyst regenerator to conserve heat and to minimize
air
pollution. It is difficult in a catalyst regenerator to completely burn all
the coke
and convert CO without emitting N~x with the regenerator flue gas.
Increasingly
stringent government regulatory emission standards restrict the amount of N~x
that can be present in a flue gas stream discharged to the atmosphere. In
response to environmental concerns, much effort has been spent on finding ways
to reduce N~,~ emissions.
[0004] There have been several ways suggested to decrease the amount of
NOX emissions from the regenerator including using a catalyst or additive
which
is compatible with the FCC reactor, which suppresses N~X formation or
catalyzes its reduction in a regenerator in complete C~ burn mode. The
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inventors herein, however, have unexpectedly discovered that a N~x reducing
catalyst system typically used in complete C~ combustion regenerators can
provide improved N~X reduction using a regenerator run in partial burn mode at
low C~ concentrations.
SI7MMARY OF THE INVENTION
[0005) An embodiment of the present invention provides a process for the
catalytic cracking of a nitrogen-containing heavy hydrocarbonaceous feed to
lighter products with reduced N~X emissions, which process comprises:
a) cracking said feed by contacting said feed with a catalyst system in a
fluidized catalytic cracking (FCC) reaction zone operating at catalytic
cracking
conditions to produce a mixture of cracked products and spent cracking
catalyst
having nitrogen compounds and coke deposited thereon, wherein said catalyst
system comprises (i) at least one solid acid component, (ii) at least one
metal-
containing component comprised of one or more elements from Groups 1 and 3,
and one or more elements from Groups 4-15 of the Periodic 'Table of the
Elements; and at least one of oxygen and sulfur, wherein the elements from
Groups 1 and 3, Groups 4 -15 and the at least one of oxygen and sulfur are
chemically bound both within and between the groups and (iii) at least one
support, filler; or binder;
b) separating said cracked products from said spent cracking catalyst to
produce a cracked product vapor phase stream, which is charged to a
fractionation zone, and spent catalyst having nitrogen compounds and carbon
deposited thereon, which spent catalyst is charged to a stripping zone;
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c) stripping said spent catalyst of volatile compounds in said stripping
zone to produce a stripped spent catalyst having coke and nitrogen compounds
deposited thereon;
d) regenerating said stripped, spent catalyst with an oxygen-containing
gas in a regeneration zone operated at partial C~ combustion conditions
effective for producing a regenerated catalyst and a flue gas stream
containing
from 0.5 to 4 vol. % C~ and greater than 90 ppm by volume, N~, wherein the
content of N~x in said flue gas stream is reduced; and
e) conducting said regenerated catalyst from the regeneration zone to the
reaction zone.
j0~~6] Another embodiment of the present invention provides a process for
the catalytic cracking of a nitrogen-containing heavy hydrocarbonaceous feed
to
lighter products with reduced N~% emissions, which process comprises:
a) cracking said feed by contacting said feed with a catalyst system in a
fluidized catalytic cracking (FCC) reaction zone operating at catalytic
cracking
conditions to produce a mixture of cracked products and spent cracking
catalyst
having nitrogen compounds and coke deposited thereon, wherein said catalyst
system comprises (i) at least one solid acid component, (ii) at least one
metal-
containing component comprised of one or more elements from Groups 1 and 3,
and one or more elements from Groups 4-15 of the Periodic Table of the
Elements; and at least one of oxygen and sulfur, wherein the elements from
Groups 1 and 3, Groups 4 - 15 and the at least one of oxygen and sulfur arc
chemically bound both within and between the groups and (iii) at least one
support, filler, or binder;
b) separating said cracked products from said spent cracking catalyst to
produce a cracked product vapor phase stream, which is charged to a
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fractionation zone, and spent catalyst having nitrogen compounds and carbon
deposited thereon, which spent catalyst is charged to a stripping zone;
c) stripping said spent catalyst of volatile compounds in said stripping
zone to produce a stripped spent catalyst having coke and nitrogen compounds
deposited thereon;
d) regenerating said stripped, spent catalyst with an oxygen-containing
gas in a regeneration zone operated at partial CO combustion conditions
effective for producing a regenerated catalyst and a flue gas stream
containing
from 0.5 to 1 vol. % CO and greater than 263 ppm by volume, NO, wherein the
content of NOx in said flue gas stream is reduced; and
e) conducting said regenerated catalyst from the regeneration zone to the
reaction zone.
I5E'T~1ILEI5 PESO TIN F TI3E P SENT I ENTI N
[0007] l~.s used herein, the reference to NO,~, or nitrogen oxides) refers to
the
various oxides of nitrogen that may be present in process streams such as, for
example, the off gas of the regenerator of a FCC unit. Thus, the terms refer
to
all of the various oxides of nitrogen including nitric oxide (NO), nitrogen
dioxide (N02), nitrous oxide (Na0), etc. and mixtures thereof. Of the nitrogen
oxides present in the regenerator off gas, NO typically makes up the majority
of
all NOX present. NO will usually represent 90% in the regenerator off gas.
Therefore, the presently claimed process is especially concerned with the
reduction and control of NO.
[OOOgJ The present invention provides a FCC process for cracking a nitrogen-
containing heavy hydrocarbonaceous feed to lighter products with reduced NOX
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emissions. The heavy hydrocarbonaceous feed passes to a FCC process unit
which contains at least one reaction zone, a stripping zone, a catalyst
regeneration zone, and a product fractionation zone. The feed contacts a NOX
reducing catalyst system in a reaction zone at 425°C-600°C,
preferably 460°C-
560°C. The hydrocarbons crack, and deposit coke, sulfur, and nitrogen
compounds on the catalyst. The cracked products are separated from the coked
or spent catalyst. The coked catalyst is stripped of volatiles, usually with
steam,
in the stripping zone. The stripping is preferably performed under low-
severity
conditions to minimize thermal cracking. The stripped catalyst is then passed
to
the regeneration zone where it is regenerated by burning coke on the catalyst
in
the presence of an oxygen containing gas, preferably air or oxygen-enriched
air.
This regeneration step restores catalyst activity and simultaneously heats the
catalyst to a temperature from 1202°F (650°C) to 1382°F
(750~C). The
environment in the regenerator reduces the amount of coke nitrogen that is
converted to reduced nitrogen compounds, such as ammonia, and increases the
amount of nitrogen oxides that are produced in the regeneration .one from coke
nitrogen. The total amount of oxygen in the regeneration zone is limited so
that
the regenerator is operated in partial combustion mode.
[0009] Any conventional FCC feed can be used in the present invention.
Such feeds typically include heavy hydrocarbonaceous feeds boiling in the
range
of 430°F to 1050°F (220°C-565°C), such as gas
oils, heavy hydrocarbon oils
comprising materials boiling above 1050°F (565°C); heavy and
reduced
petroleum crude oil; petroleum atmospheric distillation bottoms; petroleum
vacuum distillation bottoms; pitch, asphalt, bitumen, other heavy hydrocarbon
residues; tar sand oils; shale oil; liquid products derived from coal
liquefaction
processes; and mixtures thereof. Such feeds typically contain an undesirable
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amount of nitrogen compounds that are converted to nitrogen oxides in the
regenerator. The FCC feed may also comprise recycled hydrocarbons, such as
light or heavy cycle oils. Preferred feeds in this process are vacuum gas oils
boiling in the range above 650°F (343°C).
[0010] The NOx reducing catalyst system utilized in the present invention
comprises (i) at least one solid acid component, (ii) at least one metal-
containing
component comprised of one or more elements from Groups 1 and 3, and one or
more elements from Groups 4-15 of the Periodic Table of the Elements; and at
least one of oxygen and sulfur, wherein the elements from Groups 1 and 3,
Groups 4 - 15 and at least one of oxygen and sulfur are chemically bound both
within and between the groups and (iii) at least one support, filler or
binder.
[0011] The solid acid component is preferably a conventional FCC catalyst
including catalysts containing large-pore zeolites such as USY or PeEY.
Additional zeolites, which can be employed in accordance with this invention
include both natural and synthetic zeolites. The large-pore zeolites include
gmelinite, chabazite, dachiardite, clinoptilolite, faujasite, heulandite,
analcite,
Ievynite, erionite, sodalite, cancrinite, nepheline, lazurite, scolecite,
natrolite,
offretite, mesolite, mordenite, brewsterite, and ferrierite. Included among
the
synthetic zeolites are zeolites X, Y, A, L. ZK-4, ZK-5, B, E, F, H, J, M, C~,
T, W,
Z, alpha and beta, omega, and USY zeolites. The more preferred large-pore
zeolites are the faujasites, particularly zeolite Y, USY, and REY.
[0012] The catalysts useful in the present invention can also be a medium-
pore zeolite or a large-pore and medium-pore zeolite mixture. Medium-pore size
zeolites that can be used in the practice of the present invention are
described in
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"Atlas of Zeolite Structure Types", eds. W. H. Meier and I~. H. ~lson,
Butterworth-Heineman, Third Edition, 1992, which is hereby incorporated by
reference. The medium-pore size zeolites generally have a an average pore
diameter less than 0.7 nm, typically from 0.5 to 0.7 nm and includes for
example, MFI, MFS, MEL, MTW, EU~, MTT, HEU, FER, and T~N structure
type zeolites (IUPAC Commission of Zeolite Nomenclature). Non-limiting
examples of such medium-pore size zeolites, include ZSM-5, ZSM-12, ZSM-22,
ZSM-23, ZSM-34, GSM-35, ZSM-3~, GSM-4S, GSM-50, silicalite, and silicalite
2. The most preferred is ZSM-5, which is described in U.S. Pat. Nos. 3,702,86
and 3,770,614. ZSM-11 is described in U.S. Pat. No. 3,709,979; GSM-12 in U.S.
Pat. No. 3,32,449; GSM-21 and GSM-3~ in U.S. Pat. No. 3,94~,75~; ZSM-23 in
U.S. Pat. No. 4,076,42; and ZSM-35 in U.S. Pat. No. 4,016,245. AlI of the
above patents are incorporated herein by reference. ether suitable medium-pore
size zeolites include the silicoaluminophosphates (SAP~), such. as SAP~-11,
SAPO-34, SAP~-4I, and SAP~-42, which are described in U.S. Pat. No.
4,440,71; chromosilicates; gallium silicates; iron silicates; aluminum
phosphates (ALP~), such as ALP~-11 described in U.S. Pat. No. 4,310,440;
titanium aluminosilicates (TAS~), such as TASf?-4S described in EP-A No.
229,295; boron silicates, described in U.S. Pat. No. 4,254,297; titanium
aluminophosphates (TAP~), such as TAP~-11 described in U.S. Pat. No.
4,500,651; and iron aluminosilicates. In one embodiment of the present
invention the SiJAI ratio of said zeolites is greater than 40.
[0013] The medium-pore size zeolites can include "crystalline admixtures"
which are thought to be the result of faults occurring within the crystal or
crystalline area during the synthesis of the zeolites. Examples of crystalline
admixtures of ZSM-5 and ZSM-11 are disclosed in U.S. Pat. No. 4,229,424
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which is incorporated herein by reference. The crystalline admixtures are
themselves medium-pore size zeolites and are not to be confused with physical
admixtures of zeolites in which distinct crystals of crystallites of different
zeolites are physically present in the same catalyst composite or hydrothermal
reaction mixtures.
[0014] The large-pore and medium-pore catalysts of the present invention
will be present in an inorganic oxide matrix component binder that binds the
catalyst components together so that the catalyst product is hard enough to
survive inter-particle and reactor wall collisions. The inorganic oxide matrix
can
be made from an inorganic oxide sol or gel which is dried to "glue" the
catalyst
components together: Preferably, the inorganic oxide matrix is not
catalytically
active and will be comprised of oxides of silicon and aluminum. It is also
preferred that separate alumina phases be incorporated into the inorganic
oxide
matrix. Species of aluminum oxyhydroxides-y-alumina, boehmite, diaspore, and
transitional aluminas such as ~,-alumina, [3-alumina, y-alumina, ~-alumina, a-
alumina, x-alumina, and p-alumina can be employed. Preferably, the alumina
species is an aluminum trihydroxide such as gibbsite, bayerite, nordstrandite,
or
doyelite. The matrix material may also contain phosphorous or aluminum
phosphate. It is within the scope of this invention that the large-pore
catalysts
and medium-pore catalysts be present in the same or different catalyst
particles,
in the aforesaid inorganic oxide matrix.
[0015] Supported acid materials are either crystalline or amorphous materials,
which may or may not themselves be acidic, modified to increase the acid sites
on the surface. Non-limiting, illustrative examples are H2S~4, H3P04, I-I3B~3,
CH2(CO~H)2, mounted on silica, quartz, sand, alumina or diatomaceous earth.,
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as well as heteropoly acids mounted on silica, quartz, sand, alumina or
diatomaceous earth. Non-limiting, illustrative examples of crystalline
supported
acid materials are acid-treated molecular sieves, sulfated zirconia,
tungstated
zirconia, phosphated zirconia and phosphated nobia.
[0016] The solid acid component is present with at least one metal-containing
component comprised of one or more elements from Groups 1 and 3, and one or
more elements from Groups 4-15 of the Periodic Table of the Elements. The
remaining component of the catalyst system in accordance with the invention
can be at least one of sulfur and oxygen. Oxygen is preferred. The elements)
from Groups 1 and 3 can be any metal or combination of metals selected from
lithium, sodium, potassium, rubidium, cesium, francium, scandium, yttrium,
lanthanum, cerium, praseodymium, neodymium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and
lutetium. Preferably, the elements from Groups 1 and 3 can be any metal or
combination of metals selected from lanthanum a.nd cerium. The elements)
from Groups 4-15 can be any element or a mixture of elements from Groups 4-
1 S of the Periodic Table of the Elements. Preferably, the elements) from
Groups 4-15 is (are) at least one of titanium, vanadium, chromium, manganese,
iron, cobalt, nickel, copper, zinc, boron, aluminum, phosphorous, gallium,
germanium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium,
silver, indium, tin, antimony, hafnium, tungsten, rhenium, iridium, platinum,
gold, lead and bismuth. More preferably, the elements) from Groups 4-15 is
(are) at least one of copper, palladium, and silver.
[0017] The catalyst system of the present invention can be prepared by
physically mixing or chemically reacting with the metal-containing component
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and, optionally, combined with the binder to form catalyst particles or act as
a
filler to moderate the catalyst activity. The weight ratio of solid acid
component
to the total weight of the metal-containing component can be from 1000:1 to
1:1000. Preferably, this ratio is 500:1 to 1:500. Most preferably, this ratio
is
100:1 to 1:100. The weight ratio of the metal-containing component to the
matrix component can be 100:1 to 1:100.
[001] The metal-containing or NOX reducing component of the catalyst
system may be used as a separate additive particle or as an integral part of a
FCC
catalyst particle. The NO~ reducing component may contain minor amounts of
other materials, which preferably do not adversely affect the N~~ reducing
function in a significant way. More preferably, however, the NO~ reducing
component consists essentially of items (ii)-(iii) mentioned above. '~6Then
the
NC~~ reducing component is used as a separate additive particle, it is added
to the
inventory of circulating FCC catalyst particles in a NO,~ reducing effective
amount. ~n effective amount is an amount, which is effective for reducing the
N~,~ content in the regenerator flue gas below that which would be present in
the
absence of the additive. The NOX reducing component may be combined with at
least one support, filler, or binder to form particles suitable for use in a
FCC
process. If the particle NOx reduction component is an integral part of a FCC
catalyst particle, it preferably contains from 0.001 to 10 percent by weight,
and
more preferably from 0.01 to S percent by weight, and most preferably from
0.05
to 1 percent by weight of the inventory of circulating particulate solids.
[0019) While the NO,~ reducing catalyst system of the present invention is
active for reducing the NOX content of FCC regenerator flue gas, it has a
negligible effect on the hydrocarbon cracking reactions which occur during the
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FCC process. Furthermore, the NOX reducing catalyst system is compatible with
CO oxidation promoters, which may be part of the inventory of circulating
particulate solids. Additionally, the NOX reducing catalyst system of the
present
invention may be used with additives that provide SOX reduction. These SOX
additives may also provide NOX reduction. The SOX additives are preferred to
be
various forms of alumina, rare-earth oxides, alkaline earth oxides, and
spinals.
More preferably, the SOX additive is a magnesium-aluminum spinal. SOX
additives are available from several catalyst suppliers, such as Grace-
Davison's
MESON or SUFERI?ESOX, or Intercat's SOXGETTER or Super-SOXGETTER.
[0020] In a regenerator ran under typical partial combustion mode conditions,
CO concentrations are generally greater than 4.0 vol.% with very little NO
being
present, generally less than SO ppm by volume NO. The majority of nitrogen
containing species exiting the regenerator at such high CO concentrations are
in
the form of I~CN and NI-i3. This is not the optimum use for a NOX reducing
catalyst system. The CO concentration of the present invention, however, is
roan
in partial combustion mode with preferably 0.5 to 4.0 vol.°/~ CO, more
preferably from 0.75 to 3.0 vol.% CO, and most preferably from 1.00 to 2.0
vol.% CO. Additionally, NO concentration is preferably greater than 90 ppm by
volume, more preferably greater than 150 ppm by volume, and most preferably
greater than 200 ppm by volume. As CO concentration is reduced, more NO is
produced, causing more NO reduction with CO, and thereby reducing NOX
emissions. The presence of a circulating NOX reducing catalyst system
increases
the rate of NO reduction by CO, thereby causing additional reduction in NOX
emissions.
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[0021] The hot regenerated catalyst is recycled to the FCC reaction zone
where it contacts inj acted FCC feed.
[0022] The reactor operation will usually be conventional all riser cracking
FCC, such as disclosed in LT.S. Pat. No. 4,421,636 incorporated herein by
reference. Typical riser cracking reaction conditions include catalyst/oil
ratios of
0.5:1 to 15:1, preferably 3:1 to ~:1, and a catalyst contact time of 0.1-50
seconds,
preferably 0.5 to 10 seconds, most preferably 0.75 to S seconds, and riser top
temperatures of 900°F to 1100°F, preferably 950°F to
1050°F.
[002] It is preferred to have good mixing of feed with catalyst in the base of
the riser reactor, using conventional techniques such as adding large amounts
of
atomizing steam, use of multiple nozzles, use of atomizing nozzles and similar
technology. The Atomax nozzle, available from the 1~I. W. Kellogg C~, is
preferred. Details of a suitable nozzle are disclosed in LT.S. Pat. Nos.
5,29,976
and 5,306,41, which ire incorporated herein by reference. It is preferred, but
not essential, to havc a riser catalyst acceleration zone in the base of the
riser. It
is also preferred, but not essential, for the riser reactor to discharge into
a closed
cyclone system for rapid separation of cracked products from spent catalyst. A
closed cyclone system is disclosed in LT.S. Pat. No. 4,502,947 to Haddad et
al.,
which is incorporated herein by reference. It is also preferred but not
essential,
to rapidly strip the catalyst as it exits the riser upstream of the catalyst
stripper.
Stripper cyclones disclosed in LT.S. Pat. No. 4,173,527, Schatz and Heffley,
incorporated herein by reference, may be used. It is preferred, but not
essential,
to use a hot catalyst stripper. Hot strippers heat spent catalyst by adding
hot,
regenerated catalyst to spent catalyst.. A hot stripper is shown in IJ.S. Fat.
No.
3,821,103, ~wen et al, incorporated herein by reference. After hot stripping,
a
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catalyst cooler may cool heated catalyst before it is sent to the regenerator.
A
preferred hot stripper and catalyst cooler is shown in U.S. Pat. No.
4,820,404,
~wen, incorporated herein by reference. Conventional FCC steam stripping
conditions can be used, with the spent catalyst having essentially the same
temperature as the riser outlet, and with 0.5 to 5% stripping gas, preferably
steam, added to strip spent catalyst. The FCC reactor and stripper conditions,
per
se, can be conventional.
[0024] Two types of FCC regenerators can be used in the process of the
present invention, the high efficiency regenerator and the bubbling bed type.
In
bubbling bed regenerators, much of the regeneration gas, usually air, passes
through the bed in the form of bubbles. These pass through the bed, but
contact
it poorly. These units operate with large amounts of catalyst. The bubbling
bed
regenerators are not very efficient at burning coke so a large catalyst
inventory
and long residence time in the regenerator are needed to produce clean burned
catalyst. The carbon levels on regenerated catalyst can be conventional,
typically
less than 0.3 wt ~/o coke, preferably less than 0.15 wt ~/o coke, and most
preferably even less. By coke we mean not only carbon, but also minor amounts
of hydrogen associated with the coke, and perhaps even very minor amounts of
unstripped heavy hydrocarbons which remain on catalyst. Expressed as wt
carbon, the numbers are essentially the same, but 5 to 10% less.
[0025] There should be enough CO present in the flue gas so that the FCC
regenerator can be reliably controlled using control techniques associated
with
partial C~ combustion.
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[0026] The process or the present invention can also be used with high
efficiency regenerators (HER), with a fast fluidized bed coke combustor,
dilute
phase transport riser, and second bed to collect regenerated catalyst. Tt will
be
necessary to operate these in partial C~ burn mode to make CC specifications.
[0027] HERs inherently make excellent use of regeneration air. Most operate
with 1 or 2 mole % ~2 or more in the flue gas when in complete C~ burn mode.
When in partial C~ burn mode most operate with little excess oxygen, usually
in
the ppm range, preferably less than 1/lOth %. For HER's, significant
reductions
in the amount of air added may be necessary to produce a flue gas with the
correct C~/~2 ratio. Reducing or eliminating C~ combustion promoter may be
necessary to generate a flue gas with twice as much C~ as oxygen.
[002] Although most regenerators are controlled primarily by adjusting the
amount of regeneration air added, other equivalent control schemes are
available
which keep the air constant and change some other condition. Constant air
rate,
with changes in feed rate changing the coke yield, is an acceptable way to
modify regenerator operation. Constant air, with variable feed preheat, or
variable regenerator air preheat, are also acceptable. Finally, catalyst
coolers can
be used to remove heat from a unit. If a unit is not generating enough coke to
stay in heat balance, torch oil, or some other fuel may be burned in the
regenerator.
EXAMPLE 1:
[0029] The present invention was tested and developed using a commercial
FCC process unit. In the experiment, CO concentration in the flue gas at the
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outlet of the FCC unit regenerator was varied from above approximately 0.5
vol.% to approximately 5.5 vol.%. Over that range of CO concentration, NO
concentration varied from a maximum of 266 ppm by volume down to a
minimum of approximately 31 ppm by volume (Table 1 ). The accepted reaction
mechanism for minimizing NOX emissions from the FCC unit regenerator
requires operation in a regime where significant concentrations of both NO and
CO exist.
[0030] In partial-burn operations, at very high CO concentrations there is
little NO available (nitrogen species are primarily NH3 and HCN) for reduction
with CO. I~Ioreover, relatively high NOX emissions are found. As CO
concentration was reduced, more N~ was produced, causing more NO reduction
with CO, and thereby reducing NO,~ emissions. below 1°/~ CO, the NO
concentration asymptoted to a constant, maximum value, and NOX emissions
were at the lowest value. See Table 1 and Figure 1.
[0031] 'The region of very low CO concentration simulates CO and NO
concentrations that may be found in a full CO burn operation at low excess Oa.
For such full-bum units, NO~ reducing catalyst systems have proven to be
effective. In partial-burn units, where the CO concentration is typically
above 4
vol.%, these NOX reducing catalyst systems are relatively ineffective. This
invention recognizes that at very low CO operation, the NOx reducing catalyst
system is much more effective than in typical partial-burn operation because
the
key species (NO and CO) both exist in significant concentrations.
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TABLE 1
N~ concentrations
are at a
maximum at
low C~
concentrations.
C~ o1.~/~ N~ v m
0.50 266
0.75 266
1.00 263
1.25 257
1.50 248
I .75 23 7
2.00 223
2.25 209
2.50 193
2.75 176
3.00 158
3.25 141
3.50 123
3.75 106
4.00 90
4.25 75
4.50 62
4.75 50
5.00 41
5.25 35
5.50 32
5.75 31
