Note: Descriptions are shown in the official language in which they were submitted.
F-2928 (2929,2930) ~ 35~il 8
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FCC CATALYST STRIPPING
METHOD AND PPPARATUS
The present invention relates to methods and apparatus for
the separation of a catalyst and hydrocarbon materials in a
fluidized catalytic cracking (FCC) unit and is a divisional
of application Serial No. 485,554, filed June 27, 1985.
Fluid catalytic cracking, has undergone significant
development improvements due to advances in catalyst technology.
With the advent of zeolite cracking catalysts, new areas of
operating technology have been encountered, requiring refinements in
processing techniques to take advantage of the high catalyst
activity, selectivity and operating sensitivity.
The catalyst usually employed in an FCC installation is
preferably a high activity crystalline zeolite catalyst of a
fluidizable particle size. The catalyst is transferred in suspended
or dispersed phase condition with a hydrocarbon feed generally
upwardly through one or more riser reactors (FCC cracking zones),
providing a hydrocarbon residence time of 0.5 to lO seconds, and
usually less than 8 seconds. High temperature riser cracking,
occurring at temperatures of at least 538C (1000F) or higher and
at 0.5 to 4 seconds hydrocarbon residence time in contact with the
catalyst in the riser, is desirable.
Rapid separation of catalyst from cracked hydrocarbons
discharged from the riser reactor is desirable. During cracking,
carbonaceous deposits or coke accumulate on the catalyst particles
and the particles entrain hydrocarbon vapors upon removal from the
riser reactor. The entrained hydrocarbons are removed from the
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catalyst by a separator, which could be a mechanical means, and/or
stripping gas in a separate catalyst stripping zone. Hydrocarbon
conversion products separated and materials stripped from the
catalyst are combined and passed to product fractionation. Stripped
catalyst containing coke, is then passed to a catalyst regenerator.
Cyclones are typically used for efficient separation of
catalyst particles from the gas phase. Cyclones often permit an
undesirable extended residence time of the product vapor within a
large reactor vessel. This extended residence time reduces the
~0 desired product yield by as much as 4% through non-selective
cracking. Recent developments in this art have been concerned with
the rapid separation of catalyst from cracked products.
Fig. 1 in the present application corresponds to a
simplified illustration of Fig. 2 from Anderson et al, U. S. Patent
No. 4,043,899, where similar reference numbers have been utilized to
illustrate similar structures in the two figures. Anderson et al
discloses a method for rapid separation of a product suspension,
comprising the cracked product phase and catalyst (entering riser
reactor 24), by discharging the entire suspension directly from the
riser into a cyclone separator 4. The cyclone is modified to
include a separate cyclonic stripping of the catalyst separated from
the hydrocarbons vapors in an auxiliary stripper. The cyclone
separator is modified to include an additional downwardly extending
section comprising a lower cyclone stage ll. Catalyst separated in
the upper stage of the cyclone slides along a downwardly sloping
helical baffle 12 to the lower cyclone, where stripping steam is
introduced to further separate entrained hydrocarbon products from
the catalyst recovered from the upper cyclone. The steam and
stripped hydrocarbons are passed from the lower cyclone through a
concentric pipe 8, where they are combined with the hydrocarbon
vapors separated in the upper cyclone.
The separated, stripped catalyst is collected and passes
from the cyclone separator 4 by conventional means through a dipleg
22 into a catalyst bed 60 in the bottom of reactor vessel 26 and out
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catalyst exit 44. This lower portion of vessel 26 also acts as a
catalyst stripping section, comprising baffles 32, 34, and 36, with
steam being supplied to the catalyst bed thereunder. Vaporous
material separated in cyclone 4 can also be discharged into cyclone
52 and subsequently passed by way of conduit 54 into chamber 46 and
withdrawn therefrom by conduit 48 for eventual fractionation.
A substantial amount of catalyst stripping occurs in
catalyst bed 60. The stripped hydrocarbon material still contacts
with additional catalyst particles as it is carried upward through
the catalyst bed and into the entrance of cyclone 52 and from there
to chamber 46 and eventual fractionation. This increased
hydrocarbon material/catalyst contact contributes to uncontrolled
and undesired cracking of the hydrocarbon materials.
At each stripper, in Fig. l, stage, represented by baffles
32, 34 and 36, the hydrocarbons stripped from catalyst in the lower
portion of vessel 26 undego further catalyst contact while making
their way to the surface of the catalyst bed. The catalyst bed acts
as a lower seal to the dipleg 22, to prevent the flow of
hydrocarbon-laden gas through dipleg 22 into the catalyst bed. The
dipleg must be extended deep within the catalyst bed in order to
provide a sufficient seal. This depth requirement, plus the
desirability o~ multistage stripping (to ensure that a high
percentage of hydrocarbon material is removed from the catalyst)
requires a substantial volume of catalyst in bed 60, which increases
the uncontrolled contact of hydrocarbons with catalyst.
There is still a need to reduce total contact time between
hydrocarbon materials and catalysts to reduce, to the extent
possible, non-selective cracking.
Accordingly, the parent invention provides a fluidized
catalytic cracking process wherein a hydrocarbon feed contacts a
conventional FCC catalyst in a riser reactor to produce a riser
effluent comprising spent FCC catalyst with entrained cracked
hydrocarbons and a vapor phase cracked hydrocarbon stream
characterized by separating in a separator the riser effluent into a
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gaseous effluent and separated catalyst, immediately and
non-cyclonically stripping, within a stripper, entrained and
absorbed hydrocarbons from catalyst particles in the separated
catalyst as the catalyst particles exit the separator, and passing
the stripped hydrocarbons to a stripper exitO
In another emoodiment, the parent invention prov`ides an
improved method of sealing the catalyst exit of a catalyst stripper
comprising the steps of passing the stripped catalyst seal pot
comprising at least one drain sized to permit between lO and 90% of
stripped catalyst to flow through the drain.
In another embodiment, the parent invention provides a
fluidized catalyst cracking process wherein a hydrocarbon feed
~ contacts a conventional FCC catalyst in a riser reactor to produce a
riser effluent comprising spent FCC catalyst with entrained cracked
hydrocarbons and a vapor phase cracked hydrocarbon stream
characterized by separating in a separator the riser effluent into a
gaseous effluent and separated catalyst, passing separated catalyst
through a stripper vessel having an entrance, a catalyst exit and a
stripped hydrocarbon exit, causing separated catalyst to follow a
circuitous path from the entrance through the stripper vessel to the
catalyst exit, injecting a stripping gas so that it contacts with
only a portion of the separated catalyst that is above the stripping
gas injecting location and located between the location and stripped
hydrocarbon exit, and passing stripper gas and hydrocarbons stripped
from separated catalyst directly to the stripped hydrocarbon exit.
The present divisional invention provides a
method of fluid catalytic cracking of a hydrocarbon feed comprising
the steps of passing a mixture, as a suspension, of the hydrocarbon
feed and a catalyst through a riser conversion zone contained within
a reactor vessel and cracking the hydrocarbon feed in the riser
conversion zone, passing the mixture through a deflection zone in
which the mixture is physically deflected From an exit of the
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riser conversion zone towards an exit of the deflection zone, and
passing the deflected mixture from the deflection zone exit to a
separation zone.
In related embodiments, the present invention provides
apparatus to accomplish the above processes.
Fig. l is a diagrammatic sketch of the riser reactor,
including catalyst stripping zone, illustrated in Fig. 2 of U.S.
Patent No. 3,043,899 to Anderson et al;
Fig. 2a is a top view of a riser conversion zone
illustrating the connection to two cyclone separators;
Fig. 2b is a side view of the subject matter in fig. 2a;
Fig. 3 is a side view partially in section of one
embodiment of a short contact time catalyst stripper;
Fig. 4 is a side view partially in section of a further
embodiment of a short contact time catalyst stripper;
Fig. 5 is a side view partially in section of a four-stage
countercurrent stripper which can be directly connected with the
exhaust barrel of the cyclone separator illustrated in Figs. 2a and
2b;
Fig. 6 is a side view partially in section of one
embodiment of a catalyst seal pot;
Fig. 7 is a side view partially in section of a further
embodiment of a catalyst seal pot;
Fig. 8 is a side view partially in section of a short
contact time catalyst stripper located adjacent the exhaust barrel
- of a cyclone separator;
Fig. 9 is a side view partially in section of a modified
cyclone separator exhaust barrel and its interconnection with a
short contact time stripper;
Fig. lO is a side view partially in section in which
baffles deflect catalyst particles into the inlet of a cyclone
separator adjacent a two-stage short contact time stripper whose
lower end is sealed by a catalyst seal pot.
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The present invention is directed to a catalyst particle
deflector which reduces uncontrolled cracking, as illustrated in
Figs. 2a, 2b and 10.
Because each of these methods and apparatus can be utilized
separately or in various combinations, a discussion of each and
their interaction follows.
The present invention is also directed to a stripper as a
method and apparatus for reducing uncontrolled cracking, and various
embodiments are shown in Figs. 3, 4 and 8-10.
The present invention is also directed to a method and
apparatus for reducing uncontrolled cracking by utilizing a catalyst
seal pot, which is illustrated in Figs. 6, 7 and 10.
In the drawings like numerals represent like elements
throughout the several views.
In Fig. 8, a short contact time stripper 110 (described in
detail below) is located adjacent the exit barrel 4B of cyclone
separator 4. This construction is such that extensions o~ the exit
barrel walls 4B make up the walls of the stripper vessel 111.
However, the various baffles and steam injection mechanisms could be
located in an extension of the exit barrel itself, if so desired.
Separated catalyst particles exiting the separator 4 will
immediately be processed by the catalyst stripper, reducing to an
absolute minimum additional contact time of hydrocarbon vapor and
materials with separated catalyst particles.
Fig. 8 illustrates a short contact stripper. A
conventional catalyst stripper could also be used in some
circumstances in the separator exit barrel or immediately connected
thereto, and a known four-stage countercurrent stripper 112 which
may be used is illustrated in Fig. 5. Depending on the specifics of
the stripper and the separator designs utilized, conical section
114, or some other mounting means joining the stripper to the
separator exhaust, could be utilized, as illustrated in Fig. 9.
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Fig. 9 illustrates the combination of a conica] diffuser connecting
the exit barrel of cyclone separator 4 with the inlet of stripper
110 .
To ensure that hydrocarbon vapors released in the stripper
travel either out of the cyclone separator gas exhaust (in the case
of a conventional multi-stage stripper) or throùgh the appropriate
stripper exhaust conduit (conduits 130, 132 and 134 in Figs. 3 and 4
for the short contact time stripper), it is necessary that the
bottom of the strippers provide a sufficient resistance to gas flow
in the downwa~d direction. However, the lower portions cannot be
completely sealed, as the catalyst particles gathering in the lower
portion of the stripper must be removed for recycling and reuse in
the reactor.
In the past, the extension of dipleg 22 into the catalyst
bed 60 tFig. 1) served to provide a sufficient pressure differential
to force gas ~low in the desired direction. However, the large
volume of catalyst in the bed and the requirement that the catalyst
stripper level extend to any separator dipleg (as in Fig. 1)
provided additional catalyst through which injected stripping gas
and separated hydrocarbons must pass, thus providing additional
"residence" time and a further level of uncontrolled cracking for
hydrocarbons in contact or entrained with the catalyst particles.
Consequently, in another embodiment of the present invention, the
lower portion of the stripper unit extends into a catalyst seal pot
140, as shown in Figs. 6 and 10. As shown in Figs. 6 and 10,
catalyst level 142 (which may be above, at or below steam injection
points) in the stripper is maintained, such that the gas flow
resistance from the catalyst level 142 through the seal pot and to
the overflowing catalyst 144, is sufficient to prevent substantial
hydrocarbon~laden steam flow therethrough. However, the volume of
- catalyst contained in the seal pot 140 is relatively small, such
that any hydrocarbon-laden steam which is entrained therein does not
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F-2928 (2929, 2930) - 8 -
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have the long "residence" time expected of the normal catalyst
bed/dipleg seals.
Catalyst seal pot 140 has one or more drains 146 sized to
permit a flow o~ catalyst equal to between 10 and ~0% and preferably
30 to 50% of the catalyst flow rate through the catalyst seal pot.
The remainder of the catalyst not flowing through drains 146
overflows the catalyst seal pot at 144, whereupon the overflowing
and draining catalysts are recovered and reused. Catalyst
overflowing and draining from the seal pot is contained within the
bottom of the reactor and passes through a reactor exit, in much the
same manner as shown in Fig. 1 with referenc to exit 44, and the
catalyst is reused.
Fig. 6 illustrates a two drain seal pot, with Fig. 7
showing a single drain seal pot. In Fig. 7, the bottom of the seal
pot has sloping sides 148, which serve to ensurs that no catalyst
remains trapped within the seal pot and that ultimately all catalyst
will drain or overflow therefrom. Although the angle o~ the
sloping sides 148 is not critical, the optimum angle would appear to
be 60.
The present invention also provides a riser deflector which
assists in moving catalyst particles, along with hydrocarbon vapors,
toward the exit of riser 24. In a conventional riser outlet
arrangement~ where the cyclone inlet is located below the top of the
riser conversion zone 24, as shown in Fig. 1, catalyst particles
impact against the closed top o~ the riser and rebound back towards
the riser conversion zone. Thus, the velocity of the rebounding
particles must be reduced by dynamic pressure of the rising
hydrocarbon feed before again moving upwards and ultimately into the
cyclone separator 4. This additional "residence" time (the time
during which the catalyst is in immediate contact with hydrocarbon
~2~SS~8
F-2928 (2929, 2930) - 9 -
.. ...
vapor and hydrocarbon material) causes overcrac~ing and loss of
precise control of the cracked products.
To prevent the problems caused by rebounding particles, a
V-shaped or conical deflector 100 transforms the upward velocity
vectors of catalyst particles contained in the hydrocarbon feed to a
direction towards the inlet of cyclone separator 4. With the
deflector shown in Figs. 2a and 2b, the particle trajectories are as
illustrated by the dotted line arrow in Fig. 2b, which reduces
- "residence" time due to the rebounding of catalyst particles.
In one embodiment of the deflector, the angle ~ of the
deflector surface with respect to the horizontal (for a vertical
riser) is between 60 and 70, although different angles could be
used depending upon the location of the cyclone separator inlet with
respect to the deflector, the diameter of the riser, the distance
from the riser to the separator inlet, etc. Existing risers may be
converted to incorporate deflectors by the addition of baffle-type
deflectors 102 and/or 104, as shown in Fig. ~0. The surface of
deflector 100 need not be planar and a smoothly contoured curve from
the lowest point of the deflector to the upper surface of the
cyclone separator inlet 4 is advantageous in redirecting catalyst
travel and reducing pressure drop between the upper portion of rlser
24 and cyclone separator 4. Baffle-type deflectors 102 and/or 104
may also be curved to direc the catalyst/hydrocarbon vapor flow to
separator 4. While a cyclone separator has been illustrated, the
deflector would be equally useful with other typef of known
separators.
The present invention seeks to minimize uncontrolled
cracking. To do this, a catalyst stripper is provided at the
exhaust barrel of a separator. The stripper is preferably a short
contact time stripper.
- ~x~
F-2928 ( 2929, 2930 ) - 10
i,:,,
Fig. 3 illustrates one embodiment of a short contact time
stripper 110 which is adapted to be located concentrically around
riser 24. A hydrocarbon/catalyst feed ascends vertically through
riser 24, passes through a separator and a catalyst stream with a
minor amount of entrained cracked hydrocarbons enters the upper
portion of stripper 110. Ba~fles 116 and 118 direct the descending
catalyst towards perforated baffles 120 and 122. Steam is provided
at outlets 124 and 126 and travels through only a portion of the
flowing catalyst 128. The "portion" referred to is that catalyst
located between the steam injection level and the intake of the
inverted funnels. Since the steam does not flow through the
catalyst above its associated funnel intake, it does not place the
hydrocarbons entrained therewith in further contact with catalyst.
Although all catalyst is contacted with steam, a given amount of
steam does not contact all catalyst contained thereabove in the
stripper vessel.
The hydrocarbon stripped from the catalyst by the stripping
stream is prevented from further contacting catalyst by baffles 116
and 118 which serve as inverted funnels forcing the steam and
entrained hydrocarbons into concentric pipes 130 and 132 for either
'further separation and~or stripping or fractionation (now shown).
In the vertical arrangement shown in Fig. 3, the baffles 116 and 118
serve to move the catalyst in at least a partially horizontal
direction, with perforated baffles 120 and 122 doing likewise. The
descending catalyst particles 128 follow a circuitous route which
permits a number of steam exposure locations or "stages."
Although stripper 110 in Fig. 3 is mounted concentrically
around riser 2~, the stripper could also be mounted apart from the
riser, such as in the exhaust barrel of a separator, as shown in
Fig. 10. Although two stages of steam injection are shown in Fig.
3, more or less stages could be added, depending on the amount of
stripping desired and the desired level of complexity. There is no
requirement that the stripped hydrocarbon vapors be carried in
separate conduits 130 and 132, as shonw in Fig. 3. As shown in Fig.
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.. ..
4, a single conduit 134 could be used with baffles 116 and 118 and
perforated baffles 120 and 122, as discussed with reference to Fig.
3. Fig. 4 illustrates stripper which includes riser 24
concentrically mounted therein, but the riser does not have to be
concentrically mounted, and the Fig. 4 stripper, like that of Fig.
3, can also be mounted in the exhaust barrel of a separator.
Conduits 130 and 132 in Fig. 3 and 134 in Fig. 4 conduct the
stripped hydrocarbons away from further contact with catalyst to
avoid increased "residence" time.
The present invention can combine stripper in the exhaust
barrel of a cyclone with a short contact timc stripper and~or a
catalyst deflector which reduce the "residence" time during which
hydrocarbons contact catalyst. In a preferred embodiment, the
present inventions are all used in a single ~luid catalytic cracking
process or apparatus, such as shown in Fig. 10, which includes the
catalyst deflectors 102 and 104, the catalyst stripper located in
the barrel of cyclone separator 4, and the two-stage short contact
time stripper, sealed with a low volume catalyst seal pot.
