Note: Descriptions are shown in the official language in which they were submitted.
1~2~92~
~AVY OIL CATALY~IC CRAC~ING
This invention is concerned with a fluidized catalytic
cracking process wherein coked deactivated catalyst is subject to
high temperature stripping to control the carbon ~evel on spent
catalyst. r~ore particularly, the concept employs a high temperature
stripper to control the carbon level on the spent catalyst, followed
by catalyst cooling to control the temperature of the catalyst to
regeneration.
The field of catalytic cracking has undergone progressive
development since 194n. The trend of development of the fluid
catalytic cracking process has been to all riser cracking, use of
zeolite-containing catalysts and heat balanced operation.
. Other major ~rends in fluid catalytic cracking processing
have been modifications to the process to permit it to accommodate a
wider range of feedstocks, in particular, feedstocks that contain
more metals and sulfur than had previously been permitted in the
feed to a fluid catalytic cracking unit.
Along with the development of process modifications and
catalysts, which could accommodate these heavier, dirtier feeds,
there has been a growing concern about the amount of sulfur
contained in the feed that ends up as Sx in the regenerator flue
gas. ~igher sulfur levels in the feed, combined with a more
complete regeneration of the catalyst in the fluid cataly-tic
cracking regenerator tends to increase the amount of SO contained
in the regenerator flue gas. Some attempts have been made to
minimize the amount of Sx discharged to the atmosphere through
the flue gas hy providing agents to react with the Sx in the flue
gas. These agents pass along with the regenerated catalyst back to
the fluid catalytic cracking reactor, and then the reducing
atmosphere releases the sulfur compounds as H2S. Suitable agents
for this purpose have been described in U. S. Patent ~os. 4,071,436
and 3,834~031. Use of a cerium oxide agent for this purpose is
shown in U. S. Patent ~To. 4,001,375.
.
.
132~9~
F-373~ --2~-
Unfortunately, the conditions in most fluid catalytic
cracking regenerators are not the best for Sx adsorption. The
high temperatures encountered in modern fluid catalytic crackin~
regenerators (up to 870C (1600F)) tend to discourage Sx
adsorption. Gne approach to overcome the problem of Sx in flue
gas is to pass catalyst from a fluid catalytic cracking reactor to a
long residence time steam stripper. After the long residence time
steam stripping, the catalyst passes to the regenerator, as
disclosed by U. S. Patent ~To. 4,4~1,1Q3 to Krambeck et al. ~lowever,
this process preferably steam strips spent catalyst at 5noo~50OC
(~32 to lQ22F), which is not sufficient to remove some undesirable
sulfur- or hydrogen-containing components. Furthermore, catalyst
passing from a fluid catalytic cracking stripper to a fluid
catalytic cracking regenerator contains hydrogen-containing
components, such as coke, adhering thereto. This causes
hydrothermal degradation when the hydrogen reacts with oxygen in ~he
regenerator to form water.
U. S Patent ~!o. 4,336,160 to ~ean et al atten~pts to reduce
hydrothermal degradation by staged regeneration. ~owever, the flue
gas from both stages of regeneration contains ~x which is
; difficult to clean.
Another need of the prior art is to provide improved means
for controlling fluid catalytic cracking regeneration temperature.
Improved regenerator temperature control is desirable, because
regenerator -temperatures above 871C (1600F) can deactivate fluid
cracking catalyst. Typically, the temperature is controlled by
adjusting the CO/CC2 ratio produced in the regenerator. This
control works on the principle that production of C0 produces less
heat than production of C02. I~owever, in some cases, this control
is insufficient.
It would be desirable to separate hydrogen from catalyst to
eliminate hydrothermal degradation. It would be further
advantageous to remove sulfur~containing compounds prior to
~32~2~
F-3734 ~
regeneration to prevent Sx from passing into the regenerator flue
gas. Also, it would be advantageous to hetter control regenerator
temperature.
U. S. Paten~ ~o. ~,353,812 to Lomas et al discloses cooling
catalyst from a regenerator by passing it through the shell side of
a heat-exchanger with a cooling medium through the tube side. The
cooled catalyst is recycled to the regeneration zone. This process
is disadvantageous, in that it does not control the temperature of
catalyst from the reactor to the regenerator.
The prior art also includes fluid catalytic cracking
~r processes which utilize dense or dilute phase regenerated fluid
catalyst heat removal zones or heat-exchangers that are remote from,
and external to, the regenerator vessel to cool hot regenerated
catalyst for return to the regenerator. Fxamples of such processes
are found in U. S. Patent 1~70s. 2,970,117 to Harper; 2,873,175 to
Owens; 2,~62,798 to ~cKinney; 2,5~6,7~ to Watson et al, 2,515,156
to Jahnig et al; 2,~92,9~8 to ~erger; and 2,506,123 to ~'atson. The
processes disclosed in these patents have the disadvantage that the
regenerator operating temperature is affected with the temperature
Z0 of catalyst from the stripper to the regenerator.
Accordinglyg the present invention comprises a fluid
catalytic cracking process and apparatus which employs a high
temperature stripper, followed by cooling of the stripped catalyst
to control a regenerator inlet temperature.
The present invention provides a process for controlling
~;~ the fluid catalytic cracking of a feedstock containing hydrocarbons,
comprising the steps of:
passing a mixture comprising catalyst and the feedstock
through a riser conversion zone under fluid catalytic cracking
conditions to crack the feedstock;
passing the mixture, having a riser exit temperature, from
the riser into a fluid catalytic cracking reactor vessel;
separating a portion of catalyst from the mixture, with the
remainder of the mixture forming a reactor vessel gaseous stream;
F-373~ -74-._ 132~92~
heating the separated catalyst portion by combining the
separated catalyst portion with a portion of regenerated catalyst
from a fluid catalytic cracking regenerator vessel to form combined
catalyst;
stripping the combined catalyst, by contact with a
stripping gas stream, at a stripping temperature between 55C
(lQ0F) above the riser exit temperature and 816C (1500F), the
regenerated catalyst portion having a temperature between 55C
(lQ0F) above the stripping temperature and ~71C (16Q0F~ prior -to
heating the separated catalyst;
.. cooling the stripped catalyst, prior to passing it into the
regenerator vessel, to a temperature sufficient to cause the
regenerator vessel to be maintained at a temperature between 55C
~ (100F~ above the stripping temperature and ~71~ (16nQF); and
-~ 15 regenerating the cooled catalyst stream in the fluid
catalytic cracking regenerator vessel by contact with an
oxygen-containing stream at fluid catalytic cracking regeneration
conditions.
The riser exit temperature is defined as the temperature of
the catalyst-hydrocarbon mixture exiting from the riser. The riser
-~ exit temperature may be at any suitable temperature. ~owever, a
riser exit temperature of 482-593C (900 to 1100F) is preferred,
and 538~566C (1000 to 1050F) is ~ost preferredO
~lore particularly the present invention provides a process
for controlling the fluid catalytic cracking of a feedstock
containing hydrocarbons and sulfur-containin~ compounds, comprising
the steps of:
passing a ~ixture comprising catalyst and the feedstock
through a riser conversion zone at fluid catalytic cracking
conditions to crack the feedstock;
passing the mixture, having a riser exit temperature
between 53~~566C (1000 and 105QF), from the riser conversion
zone to a closed cyclone system located within a fluid catalytic
cracking reactor vessel;
~32~
F-3734 --5--
separating a portion of catalyst from the mixture in the
closed cyclone system, with the remainder of the mixture forming a
reactor vessel gaseous stream;heating the separated catalyst portion
by combining the separated catalyst portion in the reactor vessel,
with a portion of regenerated catalyst from a fluid catalytic
cracking regenerator vessel to form combined catalyst;
stripping the combined catalyst, by contact with a
stripping gas stream in the reactor vessel, under stripping
conditions comprising a stripping temperature between 83C (150F)
above the riser exit temperature and 760C (1400F) and a residence
.. ti~e of a gaseous stream from 0.5 to 5 seconds, the regenerated
catalyst portion having a temperature between 83C (lSnF) above the
stripping temperature and 871C (1600F~ prior to heating the
~ separated catalyst, wherein the separated catalyst portion comprises
; lS sulfur-containing compounds and hydrocarbons derived from thefeedstock, the stripping conditions are sufficient to separate 45 to
; 55% of the sulfur-containing compo~mds and 70 to 80~ of hydrogen
~, from the hydrocarbons in the separated catalyst portion of the
combined catalyst to produce the gaseous stream, and the gaseous
stream comprises stripping gas and molecular hydrogen, hydrocarbons
and the sulfur-containing hydrocarbons separated from the separated
catalyst;
cooling the stripped catalyst strea~ to between 2~-~3C
: (50 and 150F) below the s~ripping temperature by indirect
heat-exchange with a heat-exchange medium in a heat~exchanger
located outside the reactor vessel, causing the regenerator vessel
to be maintained at a temperature between 83C (150F) above the
~:~ stripping temperature and 871C (1600~), thereby maintaining said
regenerator vessel temperature independently of the stripping step
temperature; and
regenerating the cooled catalyst stream in the fluid
catalytic cracking regenerator vessel, by contact with an
oxygen~containing stream under fluid catalytic cracking regeneration
conditions.
F-373~ 2~2~
In its apparatus respects, the present invention provides
an apparatus for controlling the fluid catalytic cracking of a
feedstock comprising hydrocarbons, comprising:
means defining a riser conversion zone through which a
mixture comprising catalyst and the feedstock passes at fluid
catalytic cracking conditions to crack the feedstock;
: a fluid catalytic cracking reactor vessel;
means for passing the mixture from the riser into the fluid
catalytic cracking reactor vessel, the mixture having a riser exit
temperature as it passes into the reactor vessel;
.. means for separating a portion of catalyst from the
mixture, with the remainder of the mixture forming a reactor vessel
: gaseous strea~;
means for heating the separated catalyst portion,
comprising means for combinir-g the separated catalyst portion with a
portion of regenerated catalyst to form combined catalyst;
: means for stripping the combined catalyst by contact with a
stripping gas stream to form a stripped catalyst stream;
a fluid catalytic cracking regenerator vessel for producing
the portion of regenerated catalyst; and
~: a heat~exchanger for cooling the stripped catalyst stream,
the heat~exchan~er being located outside the reactor vessel, the
fluid catalytic cracking regenerator vessel thereby regenerating the
cooled catalyst stream hy contact with an oxygen-containing stream
at fluid catalytic c-racking regenerator conditions.
: In its more particular apparatus aspects, the present
invention provides an apparatus for controlling the fluid catalytic
cracking of a feedstock comprising hydrocarbons and
sulfur~containing compounds, comprising:
means defining a riser conversion zone through which a
mixture comprising catalyst and the feedstock passes at fluid
catalytic cracking conditions to cracX the feedstock;
a fluid catalytic cracking reactor vessel,
~3~a~2~
F~3734 --7--
means for passing the mixture from the riser conversion
zone to a closed cyclone system located within the fluid catalytic
cracking reactor vessel, the mixture having a riser exit temperature
hetween 538-566C (1000 and 1050F) as it passes from the riser to
the closed cyclone system, the closed cyclone system including means
: for separating a portion of catalyst from the mixture and forming a
reactor vessel gaseous stream from the remainder of the mixture;
means for heating the separated portion of catalyst,
comprising means for combining a portion of regenerated catalyst
with the separated catalyst portion to form a comhined catalyst ;.n
.. the reactor vessel;
~ means for stripping the combined catalyst by contact with
i~; stripping gas in the reactor vessel, thereby maintaining the~: ~ combined catalyst in the means for stripping at a stripping
~ 15 temperature between 83C (150F) above the temperature of the
~ mixture exiting the riser and 760C (1400F) and a residence time of
~` gas in the means for stripping from n.5 to 5 seconds, the separated
catalyst portion comprising hydrocarbons and sulfur-containing
. compounds derived from the feedstock, the means for stripping
thereby separating 45 to 55% of the sul~ur-containing compounds and
70 to 80% of hydrogen from the hydrocarbons in the separated
catalyst portion;
`~ a stripped catalyst effluent conduit, attached to the
reactor vessel for passing the stripped catalyst strea~ therethrough;
a fluid catalytic cracking regenerator vessel ~or producing
the portion of regenerated catalyst at a temperature between 83C
(150F) above the stripping te~perature and 871C (1600F); and
an indirect heat~exchanger attached to the reactor effluent
conduit, whereby the indirect heat~exchanger is sufficiently sized
~or cooling the stripped catalyst stream to a te~perature between
28~83C (50 and 150F) below the stripping temperature, thereby
causing the catalyst in the regenerator vessel to be maintained at a
temperature between 83C (150F) above the stripping temperature and
~2~
F~373~
871C (1600F), causing the temperature of the catalyst in the
regenerator vessel to be maintained independently of the stripping
temperature, the regenerator vessel regenerating the cooled catalyst
stream by contacting it with an oxygen~containing stream under fluid
ca~alytic cracking regeneratlon conditions.
The present invention strips catalyst at a temperature
higher than the riser exit temperature to separate hydrogen, as
molecular hydrogen or hydrocarbons from the coke which adheres to
catalyst, to eliminate hydrothermal degradation, which typically
occurs when hydrogen reacts with oxygen in a flllid catalytic
. cracking regenerator to form water. The high temperature stripper
(hot stripper~ also removes sulfur from coked catalyst as hydrogen
sulfide and ~ercaptans, which are easy to scrub. In contrast,
removing sulfur from coked catalyst in a regenerator produces SOx,
which passes into tlle regenerator flue gas and is more difficult to
scrub. Furthermore, the high te~.perature stripper removes
additional valuable hydrocarbon products to prevent burning -these
hydrocarbons in the regenerator. ~n additional advantage of the
high temperature stripper is that it quickly separates hydrocarbons
from catalyst. If catalyst contacts hydrocarbons for too long a
time at a temperature greater than or equal to 538C (1000F), then
diolefins are produced which are undesirable for do~nstream
processing 9 such as alkylation. However, the present invention
allows a precisely controlled, short colltact time at 53~C (1000F)
or greater to produce premium, unleaded gasoline with high
selecti.Yity .
The heat~exchanger (catalyst cooler) controls regenerator
temperature. This allows the hot stripper to run at a desired
temperature to control sulfur and hydrogen without interfering with
a desired regenerator temperature. It is desired to run the
regenerator at least 55C (100F) hotter than the hot stripper.
However, the regenerator temperature should be kept below 871C
(1600F) to prevent deactivation of the catalys-t.
F~3734 9 ~ ~ ~2~2~
The drawing is a schematic representation of a high
\ temperature stripper and catalyst cooler of the present invention.
The figure illustrates a fluid catalytic cracking system of
the present invention. In the figure, a hydrocarbon feed passes
from a hydrocarbon feeder l to the lower end of a riser conversion
zone 4. Regenerated catalyst from a standpipe 102, having a control
valve 104, is combined with the hydrocarbon feed in the riser ~,
such that a hydrocarbon~catalyst mixture rises in an ascending
dispersed stream and passes through a riser effluent conduit 6 into
a first reactor cyclone ~. The riser exit temperature, ~efined as
.. the temperature at which the mixture passes from the riser 4 to
conduit 6, ranges between 482 and 593C (900~ and 1100F~, and
~; preferably between 538 and 566C (1000 and 1050F). The riser
exit temperature is controlled by monitoring and adjusting the rates
and temperatures of hydrocarbons and regenerated catalyst into the
riser 4. ~iser effluent conduit 6 is attached at one end to the
riser 4 and at its other end to the cyclone 8.
~ The first reactor cyclone 8 separates a portion of catalyst
- from the catalyst-hydrocarbon ~ixture and passes this catalyst down
a first reactor cyclone dipleg 12 to a stripping zone 30 located
therebelow. The remaining gas and catalyst pass from the first
reactor cyclone 8 through a gas effluent conduit 10. The conduit lQ
is provided with a connector 24 to allow for thermal expansion. The
catalyst passes through the conduit lQ, then through a second
reactor cyclone inlet conduit 22, and into a second reactor cyclone
14. The second cyclone 14 separates the stream to form a catalyst
stream, which passes through a second reactor cyclone diple~ 18 to
the stripping zone 30 located therebelow, and an overhead stream
.The second cyclone overhead stream, which contains the
remaining gas and catalyst, passes through a second cyclone gaseous
effluent conduit 16 to a reactor overhead port 20. ~ases from the
atmosphere of the reactor vessel 2 may pass through a reactor
overhead conduit 22 into -the reactor overhead port 20. The gases
~ 3~2~
F~3734 ~ 107~
which exit the reactor 2 through the second cyclone gaseous effluent
conduit 16 and the reactor overhead conduit 22 are combined and exit
through the reactor overhead port 20. It will be apparent to those
skilled in the art that although only one series connection of
cyclones 8, 14 is shown in the embodiment, more than one series
connection and/or more or less than two consecutive cyclones in
series connection could be employed.
The mixture of catalyst and hydrocarbons passes through the
first reactor cyclone overhead conduit lQ and the second reactor
n cyclone inlet conduit 22 without entering the reactor vessel 2
,. atmosphere. ~owever~ the connector 24 may provide an annular port
to admit stripping gas from the reactor vessel 2 into the conduit 10
to aid in separating catalyst from hydrocarbons adhering thereto.
' The closed cyclone system and annular port is described more fully
; 15 in U. S. Patent ~o. 4,502,9~7 to lladdad et al.
The separated catalyst from cyclones 8, 14 pass through
respective diplegs 12, 18 and are discharged therefrom after a
suitable pressure is generated within the diplegs hy the buildup of
the catalyst. The catalyst falls from the diplegs into a bed of
catalyst 31 located in the stripping zone 3n. The first dipleg 12
is sealed by being extended into the catalyst bed 31. The second
dipleg 18 is sealed by a trickle valve 19. The separated catalyst
is contacted and combined with hot regenerated catalyst from -the
regenerator 80 in the stripping zone 30. The regenerated catalyst
has a temperature in the range between 55C (100F) above that of
the stripping zone 30 and 871C (1600F) to heat the separated
catalyst in bed 31. The regenerated catalyst passes from the
regenerator 80 to the reactor vessel 2 throllgh a transfer line 106
attached at one end to the regenerator vessel 80 and at another end
to the reactor vessel 2. The transfer line 106 is provided with a
slide valve 108. Combining the separated catalyst ~ith the
regenerated catalyst promotes the stripping at a temperature in the
range between 55C (100~) above the riser exit temperature and
~3209~
F-3734
816C (1500F). Preferably, the catalyst strippin~ zone operates at
a temperature between R3C (150F) above the riser exit temperature
; and 760C (1400F).
The catalyst 31 in the stripping zone 30 is contacted at
high temperature, discussed above,~ith a stripping gas, such as
steam, flowing co~mtercurrently to the direction of flow of the
catalyst. The stripping gas is introduced into the lower portion of
the stripping zone 30 by one or more conduits 34 attached to a
stripping gas header 36. The catalyst residence time in the
stripping zone 30 ranges from 2.5 to 7 minutes. The vapor residence
time in the catalyst stripping zone 30 ranges from 0.5 to 30
seconds, and preferably 0.5 to 5 seconds. The stripping zone 3Q
removes coke, sulfur and hydrogen from the separated catalyst which
has heen combined with the regenerated catalyst. The sulfur is
removed as hydrogen sulfide and mercaptans. The hydrogen is removed
as ~olecular hydrogen, hydrocarbons, and hydrogen sulfide. ~ost
preferably, the stripping zone 30 is ~aintained at temperatures
between ~3C (150F) above the riser exit temperature, which are
sufficient to reduce coke load to the re~enerator by at least ~0%,
remove 70~80~ of the hydrogen as molecular hydrogen, light
hydrocarbons and other hydrogen~containing compounds, and remove 45
to 55% of the sulfur as hydrogen sulfide and mercaptans, as well as
a portion of nitrogen as ammonia and cyanides.
The catalyst stripping zone 30 may also be provided with
trays (baffles) 32. The trays ~ay be disc~ and doughnut-shaped and
may be perforated or unperforated.
Stripped catalyst passes through a stripped catalyst
effluent conduit 38 to a ca~alyst cooler 40. The catalyst cooler 40
is a heat~exchanger which cools the stripped catalyst from the
reactor vessel 2 to a temperature sufficient to maintain the
regenerator vessel 80 at a temperature between 55C (100F) above
the temperature of the stripping zone 30 and 871C (1600F).
Preferably, the catalyst cooler ~0 cools the stripped catalyst
~32~92~
F-3734 --12--
stream to a temperature sufficient to control the regenerator vessel
8Q at a temperature to between 83C (150F) above the temperature of
the stripping zone 30 and 871C (1600F). Most preferahlv, the
stripped catalyst stream is cooled between 28 and R3C (50 and
: 5 150F) below the stripping zone temperature, so long as the cooled
catalyst temperature is at least 593C (1100F).
The catalyst cooler 40 is preferably an indirect
: heat~exchanger located outside the reactor vessel 2. A
heat-exchange medium, such as liquid water (boiler feed water),
passes throllgh a conduit 50, provided with a valve 54~ into a set of
~r tubes 48 within the catalyst cooler 40. The catalyst passes through
the shell side 46 of the catalyst cooler 40. The catalyst cooler 40
is attached to an effluent conduit 42 provided with a slide valve
44. The cooled catalyst passes through the conduit 42 into a
regenerator inlet conduit 60.
In the regenerator riser 60, air and cooled catalyst
combine and pass upwardly through an air catalyst disperser 74 into
a fast fluid bed 62. The fast fluid bed 62 is part of the
regenerator vessel 80. In the fast fluid bed 62, combustible
materials, such as coke which adheres to the cooled catalyst, are
burned off the catalyst by contact with lift air. Air passes
through an air supply line 66 ~hrough a control valve 68 and an air
transfer line 68 to the regenerator inlet conduit 60. ~ptionally~
if the temperature of the cooled catalyst from the conduit 42 is
less than 593C (1100F), a portion of hot regenerated catalyst from
the standpipe 102 passes ~hrough a conduit 101, provided with a
control valve 103, into the fast fluid bed 62. The fast fluid bed
62 contains a relatively dense catalyst bed 76. The air fluidizes
the catalyst in bed 76, and subsequently transports the catalyst
continuously as a dilute phase throu~h the regenerator riser R3.
The dilute ph.ase passes upwardly through the riser 83, through a
radial arm 84 attached to the riser 83, and then passes downwardly
to a second relatively dense bed of catalyst ~2 located within the
regenerator vessel. 80.
F-3734 --13-~ 1 3 ~
The major portion of catalyst passes downwardly through the
radial arms 84, while the gases and remaining catalyst pass into the
atmosphere of the regenerator vessel 80. The gases and remaining
catalyst then pass through an inlet conduit 89 and into the first
regenerator cyclone 86. The first cyclone 86 separates a portion of
catalyst and passes it through a first dipleg 90, while remaining
catalyst and gases pass through an overhead conduit 88 into a second
; regenerator cyclone 92. The second cyclone 92 separates a ~ortion
of catalyst and passes the separated portion through a second dipleg
96 having a trickle valve 97, with the remaining gas and catalyst
.. passing through a second overhead conduit 9~ into a regenerator
vessel plenum chamber 9~. A flue gas stream 110 exits from the
regenerator plenum chamber 98 through a regenerator flue gas conduit
100 .
The regenerated catalyst settles -to form the bed ~2, which
is dense compared to the dilute cata]yst passing through the riser
83. The regenerated catalyst 'ned 82 is at a substantially higher
temperature than the stripped catalyst from the stripping zone 30,
due to the coke burning which occurs in the regenerator 80. The
catalyst in bed 82 is at least 55C (100F) hotter than the
temperature of the strippin~ zone 30, preferably at least 83C
(150F) hotter than the temperature of the stripping zone 30. The
regenerator temperature is, at most, 871C (1600F) to prevent
deactivating the catalyst. Coke burning occurs in the regenerator
inlet conduit 60, as well as the fast fluid bed 62 and riser 83.
Optionally, air may also be passed from the air supply line
64 to an air transfer line 70, provided with a control valve 72, to
an air header 78 located in the regenerator 80. The regenerated
catalyst then passes from the relatively dense bed 82 through the
conduit 106 to the stripping zone 30 to provide heated catalyst for
the stripping zone 30.
Any conventional fluid catalytic cracking catalyst can be
used in the present invention. ~Tse of zeolite catalysts in an
132~2~
F-3734 --14 ~
amorphous base is preferred. Many suitable catalysts are discussed
in U. S. Patent ~To. 3,926,778 to Owen et al.
One example of a process which can be conducte~ in
accordance with the present invention begins with a 343 to 593C
(650 to 1100F) boiling poin-t hydrocarbon feedstock ~hich passes
into a riser conversion zone 4, where it combines with hot
regenerated catalyst at a temperature of about 815C (1500F) from a
catalyst standpipe 102 to form a catalyst~hydrocarbon mixture. The
catalyst~hydrocarbon mixture passes upwardly through the riser
conversion zone 4 and into a riser effluent conduit 6 at a riser
.. exit temperature of about 538C (1000F). The catalyst passes from
the conduit 6 into -the first reactor cyclone 8, where a portion of
catalyst is separated from the mixture and drops through a dipleg 12
to a bed of catalyst 31 contained within a stripping zone 30
therebelow. The stripring zone 30 operates at about 704C
(1300F). The remainder of the ~ixture passes upwardly through the
first overhead conduit lQ into a second reactor cyclone 14. T,he
second cyclone 14 separates a portion of catalyst from the first
cyclone overhead stream and passes the separated catalyst down the
second dipleg 18. The remaining solids and gases pass upwardly as a
second cyclone overhead stream through conduit 16 into the reactor
vessel overhead port 20.
In the stripping zone 30, the catalyst from diplegs 12, 18
combines with catalyst from regenerator 80, which passes through a
conduit 106 and is stripped by contact with steam from a steam
header 36. me regenerated catalyst from the conduit 106 is at a
temperature of about ~15C (1500F) and Frovides heat to maintain
the stripping zone 30 at about 704C (1300F). The stripped
catalyst passes through a conduit 38 into a catalyst cooler 40 at a
temperature of about 704C (1300F). The catalyst cooler 40 cools
the 704C (1300F) catalyst to about 621C (1150F). The cooling
occurs by indirect heat~exchange o~ the hot stripped catalyst with
boiler feed water, which passes through a conduit SO to form steam
which exits through a conduit 52.
F-3734 ~15-- ~32~2~
The cooled catalyst, at a temperature of about 621C
(1150F), combines with lift air from a conduit 66 in a regenerator
inlet conduit 60 to form an aircatalyst mixture. The mixture
passes upwardly through the conduit 60 into fast fluid hed 76. The
catalyst continues upwardly from fast fluid bed 7fi through the
regenerator riser 83 and into a regenerator vessel 80. The catalyst
is then separated from gases by the radial arm 84, as wel] as
cyclones 86 and 92, and passes downwardly through the regenerator to
form a relatively dense bed R2. The coke adhering to the strip~ed
catalyst burns in the conduit 60, the fast fluid bed 62, the riser
.. ~3, and the regenerator vessel ~n. riue to the coke burning, the
catalyst in bed 82 is heated to a temperature of about P,15C
(1500F). Catalyst bed ~2 then supplies catalyst for -the standpipe
' 102, which combines with the hydrocarbon feedstock. ~ed ~2 also
provides catalyst for conduit 106 which passes to the stripping zone
30. Gaseous effluents pass through a first cyclone g6 and second
cyclone 92 and leave the regenerator 80 as a flue gas stream 110
through a flue gas conduit lQ0.
Operating the stripping zone as a high temperature (hot)
stripper, at a temperature between 55C (100F) above a riser exit
temperature and 816C (1500~), has the advantage that it separates
hydrogen, as molecular hydrogen as well as hydrocarbons, Erom
catalyst. ~Iydrogen removal eliminates hydrothermal degradation,
which typically occurs when hydrogen reacts with oxygen in a fluid
catalytic cracking regenerator to form water. The hot stripper also
removes sulfur from coked catalyst as hydrogen sulfide and
mercaptans, which are easy to scrub. ~y removing sulfur from coked
catalyst in the hot stripper, the hot stripper prevents formation of
Sx in ~le regenerator. It is more difficult to remove Sx from
regenerator flue gas than to remove hydrogen sulfide and mercaptans
from a hot stripper effluent. The hot stripper enhances removal of
hydrocarbons from spent catalyst, and thus prevents burning of
valuable hydrocarbons in the regenerator. Furthermore, the hot
F~3734 ~ 32~92~
stripper quickly separates hydrocarbons from catalyst to avoid
overcracking.
Preferably the hot stripper is maintained at a temperature
between 83C (150F) above a riser exit temperature and 760C
(14~0F) to reduce coke load to the regenerator by at least ~%, and
strip away 70 to 80% of the hydrogen as molecular hydrogen, light
hydrocarbons and other hydrogen-containing compounds. ~he hot
stripper is also maintained within the desired temperature
conditions to remove 45 to 55~ of the sulfur as hydrogen sulfide and
mercaptans, as well as a portion of nitrogen as ammonia and cyanides.
This concept advances the development of a heavy oil
(residual oil) catalytic cracker and high temperature cracking unit
for conventional gas oils. The process combines the control of
catalyst deactivation with controlled catalyst carbon contamination
level and control of te~perature levels in the stripper and
regenerator.
The hot s-tripper temperature controls the amount of carbon
removed from the catalyst in the hot stripper. Accordingly, the hot
stripper controls the amount of carbon (and hydrogen, sulfur)
remaining on the catalyst to the regenerator. This residual carbon
level controls the te~perature rise between the reactor stripper and
the regenerator. The hot stripper also controls the hydrogen
content of the spent catalyst sent to the regenerator as a function
of residual carbon. Thus, the hot stripper controls the temperature
and amount of hydrothermal deactivation of catalyst in the
regenerator. This concept may be practiced in a ~ulti-sta~e,
multi~temperature stripper or a single stage stripper.
Fmploying a hot stripper, to re~ove carbon on the catalyst,
rather than a regeneration stage reduces air pollution, and allo~s
all of the carbon made in the reaction to be burned to C02, if
desired.
; The stripped catalyst is cooled (as a function of its
carbon level) to a desired re~enerator inlet temperature to control
:~2~
F~3734 ~17~
the degree of regeneration desired, in combination with the other
variables of C0/CO2 ratio desired, the amount oE carbon burn~off
desired, the catalyst recirculation rate from the regenerator to the
hot stripper, and the degree of desulfurization/
denitrification/decarbonization desired in the hot stripper.
Increasing CO/C~2 ratio decreases the heat generated in the
regenerator, and accordingly decreases the regenerator temperature.
~urning the coke, adhering to the catalyst in the regenerator, to C0
removes the coke, as would burning coke to CO2, but kurning to C0
produces less heat than burning to C02. The amount of carbon
(coke) burn-off affects regenerator temperature, because greater
carbon burn-off generates greater heat. The catalyst recirculation
rate from the regenerator to the hot stripper affects regenerator
' temperature, because increasing the amount of hot catalyst rrom the
regenerator to the hot stripper increases hot stripper temperature.
Accordingly, the increased hot stripper temperature removes
increased amounts of coke so less coke need burn in the regenerator;
thus, regenerator temperature can decrease.
The catalyst cooler controls regenerator temperature,
thereby allowing the hot stripper to be run at temperatures between
55C (100F) above a riser exit temperature to 816~C (1500F)~ which
facilitate controlling sulfur and hydrogen, while allowing the
regenerator to be run independently at temperatures at least lQ0F
hotter than the stripper, while preventing regenerator temperatures
greater than 871C (1600F) which deactivate catalyst.
Use of the catalyst cooler on catalyst exiting the stripper
also allows circulation of catalyst to the regenerator riser to
increase catalyst density in the regenerator riser, while
controlling the regenerator temperature. This reduces catalyst
deactivation and provides additional control.
While specific embodiments of the method and apparatus
aspects of the invention have heen shown and describedS it should be
apparent that the many modifications can be made thereto without
departing from the spirit and scope of the invention~ Accordingly,
the invention is not limited by the foregoing description~ but is
only limited by the scope of the claims appended thereto.
