Note: Descriptions are shown in the official language in which they were submitted.
1136564
BACKGRO~ OF 1~ INVENTION
Field of the Invention
The present invention relates to Fluidized
Catalytic Cracki-ng. More particularly, it relates to an
improved Fluidized Catalytic Cracking Reactor Vessel. In
particular, it relates to a Fluidized Catalytic Cracking
reactor vessel wherein diplegs from cyclones, for separating
catalyst from hydrocarbon vapors, extend into a stripping
vessel located below the reactor vessel.
In Fluidized Catalytic Cracking of hydrocarbons,
one common arrangement of the Fluidized Catalytic Cracking
Unit comprises an upwardly directed transport reactor riser
conduit discharging into a vertical reactor vessel. Cyclone
separation means are arranged in the upper portion of the
reactor vessel with catalyst diplegs extending into the
lower portion of the reactor vessel. A slide valve, or
other valve means, in the bottom of the reactor vessel
provides communication between the reactor vessel bottom and
a stripper vessel vertically disposed below the reactor
vessel. Vent pipes provide communication for stripping
vapor to flow from the top of the stripper vessel into the
upper portion of the reactor vessel.
I~ operation, a mixture of hot catalyst and
hydrocarbon vapors flow upwardly through the transport
reactor riser under cracking conditions at a velocity such
that the catalyst is transported, along with the hydrocarbon
vapors, into the reactor vessel. The catalyst and
hydrocarbon vapors discharge from the upper end of the
transport reactor riser into the reactor vessel, forming a
dense phase bed of catalyst surmounted by a dilute phase of
catalyst suspended in hydrocarbon vapors. The dense phase
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catalyst bed is generally maintained in a fluidized
condition by injection of primary stripping gas, such as
steam, into the lower portion of the fluidized bed.
From the dilute phase, catalyst and hydrocarbon
vapors flow into cyclone separators, generally comprising
two or three stages, wherein hydrocarbon vapors are
separated from the catalyst. The hydrocarbon vapors flow
from the outlet of the last stage of cyclone separators into
a plenum located in the top of the reactor vessel. From the
plenum, the hydrocarbon vapors, free of catalyst, flow
through an overhead line to a fractionation tower.
Catalyst, separated from hydrocarbon vapors in the
cyclone separators, flow downward from the cyclone
separators through diplegs, and is discharged below the
level of the fluidized dense phase catalyst bed. Although
the bottom dipleg outlets are equipped with dipleg trickel
valves to control the flow of catalyst therefrom, it is
desirable to maintain the level of the fluidized dense phase
catalyst bed at least several feet above the dipleg outlets,
for ensuring hydraulic seals.
Catalyst, in the fluidized dense phase bed, flows
downwardly through the slide valve in the reactor bottom
into the upper portion of a stripper vessel. In the
stripper vessel, the catalyst is contacted with stripping
vapor, generally steam, injected into the lower portion of
the stripper vessel, for vaporizing any volatile
hydrocarbons which may be occluded within the catalyst.
Stripping vapors and volatized hydrocarbons flow from the
top of the stripper vessel through stripper vent lines into
the dilute phase contained in the upper portion of the
reactor vessel. The stripper vent lines extend well above
the upper surface of the fluidized dense phase catalyst bed.
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Stripped catal~st flows via a spent catalyst line,
from the bottom of the stripper vessel into a regeneration
zone.
In such Fluidized Catalytic Cracking Processes,
the reactor vessel must have substantial height to
accommodate the cyclone separators, their diplegs, the
plenum and the depth of fluidized dense phase bed of
catalyst. A substantial portion of this reactor vessel
height is re~uired to provide adequate length for the
cyclone diplegs, which length is re~uired to allow
accumulation of catalyst in the diplegs and thus prevent
under-flow of hydrocarbon vapors from the cyclone separators
back to the lower portion of the reactor vessel. Extension
of the diplegs into the dense phase catalyst bed provides a
hydraulic seal which ensures that a level of catalyst is
maintained in the dipleg.
In processes for cracking hydrocarbons with more
active modern catalysts, such as ion exchanged
alumino-silicate molecular sieves, it has been found
advantageous to contact the hydrocarbons with hot catalyst
under dilute phase conditions for a relatively short time in
a transport reactor riser and avoid contact of hydrocarbon
vapors with spent catalyst in the fluidized dense phase
catalyst bed. Yields and octane of naphtha produced are
increased. In such processes, the only dense bed of
catalyst required in the reactor vessel is one sufficient to
seal the reactor slide valve inlet such that hydrocarbon
vapors cannot flow from the reactor vessel into the stripper
and such that stripping vapors cannot flow through the slide
valve into the bottom of the xeactor vessel.
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Substantially all the reactor vessel height must
be maintained, however, to accommodate the required dipleg
lengths, and the required depth of fluidized dense phase
catalyst bed to hydraulically seal the diplegs.
1136564
SUMMARY OF THE INVENTION
Now according to the method of the present invention we have dis-
covered an improved configuration for the apparatus in a Fluidized Catalytic
Cracking reaction zone and stripping zone which substantially reduces the
required regenerator vessel height.
More particularly, according to the present invention, there is
provided in a Fluidized Catalytic Cracking Unit comprising a vertical,
cylindrical reactor vessel; an upwardly directed riser transport reactor
discharging into said reactor vessel; a stripping vessel vertically disposed
below said reactor vessel, cyclone separator means in the upper portion of
said reactor vessel for separating catalyst from hydrocarbon vapors, hydro-
carbon vapor outlet means for transferring hydrocarbon vapors from said
cyclone separator means to processing facilities outside said reactor
vessel, spent catalyst transfer means for transferring catalyst from the
bottom of said reactor vessel into the upper portion of said stripping
vessel, and stripper vent lines communicating from the top of said stripping
vessel and the upper portion of said reactor vessel; the improvement which
comprises: a) cyclone separator catalyst diplegs extending from the bottom
of said cyclone separator means within said reactor vessel into said strip-
ping vessel; and b) means for maintaining a column of catalyst within said
diplegs to prevent blow by of either hydrocarbon vapors from said cyclone
separator means or stripping vapors from said stripping vessel through said
diplegs.
The advantages of this improved configuration includes a reduction
in reactor vessel height as well as associated structural members and piping.
Such reduction in height results in substantial reduction in cost of a
Fluidized Catalytic Cracking Unit. AdditionallyJ the depth of dense phase
catalyst bed in the reactor vessel may be reduced to the minimum required
to properly seal the reactor vessel slide valve, thus substantially elimin-
ating overcracking hydrocarbons in the dense phase bed, in the situation
where the modern cracking catalysts are employed.
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BRIEF DESCRIPTION OF THE DRAWING
The drawing is a schematic representation of a Fluidized Catalytic
Cracking reaction zone and stripping zone embodying the improvements of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
In order to better describe the improvement of the present invention,
attention is now drawn to the drawing. The drawing is a schematic represen-
tation of the reaction and stripping zones of a Fluidized Catalytic Cracking
Unit which embodies the improvements of the present invention. The drawing
is in only such detail as required to clearly demonstrate the present in-
vention, and many elements commonly found in Fluidized Catalytic Cracking
Units, such as piping, pumps, instrumentation, etc., but which are unneces-
sary for a complete description of the present
-5a-
1136564
invention, have been omitted for the sake of clarity. The
drawing is of on~ embodiment of the present invention only,
and is not intended as a limitation upon the invention which
is set-out in the appended claims.
S In the drawing, a riser transport reactor 1
extends upwardly through the side wall of a reactor vessel
2, and riser 1 terminates in a downwardly directed discharge
head 3. Hot, regenerated catalyst and vaporized hydrocarbon
charge flow upward in riser 1 at cracking conditions
including a hydrocarbon vapor velocity in the range of about
20 to 60 ft/sec., sufficient to transport the catalyst along
with the flowing hydrocarbon vapor. The catalyst and
hydrocarbon vapor exit riser 1 via discharge head 3 into
reactor vessel 2 wherein the catalyst and hydrocarbon vapors
separate forming a dense phase catalyst bed, having an upper
surface 4, in the lower portion of reactor vessel 2 and a
dilute phase of catalyst suspended in hydrocarbon vapor in
the upper portion of reactor vessel 2. In the middle area or
reactor vessel 2, extending from about 7 to 10 feet above
discharge head 3 to the dense phase bed upper surface 4, the
catalyst and hydrocarbon vapors form a transition zone
wherein catalyst and hydrocarbon vapors are undergoing the
separation process. The exact height of the transition zone
varies with the superficial vapor velocity of vapors flowing
upward in reactor vessel 2, wherein increased superficial
vapor velocities, commonly in the range of 1 to 6 ft/sec,
resulting in increased height of the transition zone.
rn the drawing, a steam ring 5, supplied with
steam from a steam header 6 is located in the lower portion
of reactor vessel 2. Steam from header 6 and ring 5 flows
into the lower portion of reactor vessel 2, thereby
1136564
maintaining the dense phase bed of catalyst as a fluidized
bed, and stripping a portion of any vaporizable occluded
hydrocarbons from the catalyst. Preferably, with modern
Fluidized Cracking Catalysts, the height of the fluidized
dense phase catalyst bed is maintained at the minimum
required for stable flow of spent catalyst from the bottom
of reactor vessel 2, as is herein below described.
In the drawing, catalyst collector 7 comprising an
open cylinder having a slotted upper wall 15 and a solid
lower wall 16 extends through the bottom of reactor vessel 1
into a vertical cylindrical stripper vessel 8. The solid
lower wall 15 of catalyst collector 7 is of a height, in the
range of about 2 to 4 feet, sufficient to provide a static
head of catalyst which will overcome the pressure
differential between stripper vessel 8 and reactor vessel 2,
as will be herein below described. Stripper vessel 8 is
vertically disposed beneath reactor vessel 2 and
communicates at the bottom with a spent catalyst transfer
line 12. A slide valve 9 provides communication from the
bottom of catalyst collector 7 and the upper portion of
stripper vessel 8. Stripper vent lines 13 and 14 provides
communication between the top of stripping vessel 8 and the
dilute catalyst phase maintained in the upper portion of
reactor vessel 2. A stripper steam ring 10, supplied from a
steam header 11, is located in the lower portion of stripper
vessel 8. Spent, co~e contaminated cracking catalyst,
containing some occluded vaporizable hydrocarbons, flows
downwardly from the fluidized dense phase catalyst bed in
reactor vessel 2, through catalyst collector 1 and slide
valve 9 into the upper portion of stripping vessel 8. Slide
valve 9 is adjusted to control the flow of spent catalyst
113~56~
from reactor vessel 2 to essentially the same rate as
catalyst enters reactor vessel 2 via riser 1, such that the
upper level 4, of the fluidized dense phase catalyst bed is
maintained at a desired elevation. In stripper vessel 8,
steam from steam ring 10 contacts spent catalyst, vaporizing
hydrocarbon therefrom. ~tripped, coke contaminated catalyst
exits the bottom of stripper vessel 8 via spent catalyst
transfer line 12 to a regeneration zone, not shown.
Stripping steam and hydrocarbon vapor exit the top of
stripper vent lines 13 and 14 and flow into the dilute
catalyst phase maintained in the upper portion of reactor
vessel 2. Two stripper vent lines 13 and 14 are shown in the
drawing. This number of vent lines is, however, not
limiting and the actual number of vent lines will equal the
number of cyclone diplegs, as described herein below. The
free cross-sectional area of stripper vent lines 13 and 14
is calculated, considering their height and the flow rate of
stripping steam into stripper vessel 8, to provide about 1
to 4 psi pressure drop such that stripping vessel 8 will
operate at a higher pressure than the upper portion of
reactor vessel 2. The height of the lower portion 16 of
catalyst accumulator 7, as hereinabove described is selecte~
such that the static head of catalyst therein will overcome
the pressure differential between stripping vessel 8 and
reactor vessel 2, such that spent catalyst will flow from
the bottom of reactor vessel 2 into the top of stripping
vessel 8.
In the Drawing, a primary cyclone separator 17,
having a primary cyclone inlet 18, in communication with a
primary cyclone vapor conduit 19 and a primary cyclone
catalyst dipleg 20, is located in the upper portion of
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reactor vessel 2 at an elevation such that primary cyclone
inlet 18 will be within the dilute catalyst phase above the
transition zone heretofore described. Primary cyclone vapor
conduit 19 communicates between the upper portion of primary
cyclone separator 17 and the inlet of a secondary cyclone
separator 21. From the upper portion of secondary cyclone
separator 21, a secondary cyclone vapor outlet 22
communicates with a plenum 23 located in the top of reactor
vessel 2. A hydrocarbon vapor line 24 communicates with
plenum 24 through the top of reactor vessel 2. The bottom of
secondary cyclone 21 is in communication with the upper end
of a secondary cyclone catalyst dipleg 25.
Dilute phase, comprising catalyst suspended in
hydrocarbon vapor, and having a density in the range of
about 0.1-3 pounds per cubic foot, enters primary cyclone
separator 17 via inlet 18, wherein catalyst is separated
from hydrocarbon vapor. Separated catalyst flows downward
from primary cyclone 17 into primary cyclone dipleg 20 and
hydrocarbon vapor, containing a small amount of catalyst
flows upward into primary cyclone vapor conduit 19.
Hydrocarbon vapor from primary cyclone vapor conduit 19
flows into secondary cyclone separator 21, wherein
subst~ntially all remaining catalyst is separated from
hydrocarbon vapor. Separated catalyst flows downward from
secondary cyclone 21 into secondary cyclone catalyst dipleg
25. Hydrocarbon vapor flows upward from secondary cyclone
21 through secondary cyclone vapor outlet 22 into plenum 23.
From plenum 27, hydrocarbon vapor exits reactor vessel 2 via
vapor line 24, and flows to a primary fractionation column,
not shown.
1136S64
In the drawing, only one primary cyclone 17 and
one secondary cyclone 21 have been shown. It is to be
understood however that additional cyclone separators, in
series and in parallel arrangement, may be employed
according to the present invention as required for obtaining
the substantially complete separation of catalyst from
hydrocarbon vapor. Commonly, in Fluidized Catalytic
Cracking Units, two stages of cyclone separators in series
are employed. However, it is known to use three stages in
series. Whether a single cyclone or a plurality of cyclones
in parallel are used at a particular stage of separation,
will depend upon engineering design considerations including
available space, weight, etc.
Primary cyclone catalyst dipleg 20 extends
downward from the bottom of primary cyclone 17 through
stripper vent line 13 into stripping vessel 8, wherein
dipleg 20 terminates in a flapper valve 26. Secondary
cyclone catalyst dipleg 25 extends downward from the bottom
of secondary cyclone 21, through stripper vent line 14 into
stripping vessel ~, wherein dipleg 25 terminates in a
flapper valve 28. For process units wherein more than two
cyclones are employed, each cyclone catalyst dipleg extends
downward through a stripper vent line into stripper vessel
B. Catalyst from primary cyclone 17 and secondary cyclone
21 flows through primary cyclone catalyst diplegs 20 and 25
into stripping vessel 8. Catalyst flow is controlled by
flapper valves 26 and 28. The differential pressure between
stripping vessel 8 and the upper portion of reactor vessel
2, preferably in the range of 1 to 8 psi, provides a back
pressure sufficient to maintain a column of catalyst in
diplegs 20 and 21, thus preventing blow-by of hydrocarbon
--10--
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vapor down diplegs 20 and 25. Flapper valves 26 and 28 are
typical of dipleg flapper valves commonly employed in
Fluidized Catalytic Cracking units and operate such that
they close, preventing ingress of vapor into the diplegs,
until the weight of the column of catalyst accumulated in
the diplegs is sufficient to open the flappers. If desired,
in accord with the present invention, a dense phase bed of
catalyst, having an upper surface 29, may be maintained in
stripping vessel 8 with upper surface 29 being above dipleg
flapper Yalves 26 and 29 for providing additional hydraulic
seal for diplegs 20 and 25.
Thus, in accordance with the improvement of the
present invention, the cyclone diplegs 20 and 25 are
extended into stripper vessel 8. By so doing, reactor
vessel 2 may be shortened substantially. That is, the
portion of the height of reactor vessel 2 previously
required to accommodate discharge of diplegs 20 and 25 and
to accommodate the additional depth of fluidized bed
required to provide a hydraulic seal for diplegs 20 and 25
is no longer required and may be dispensed with. This
height savings is in the range of lO feet or more for a
commercial scale Fluidized Catalytic Cracking Unit, and
represents a very substantial reduction in cost of building
such a unit. The Reactor vessel 2 may be shortened lO feet
or more, as well as supporting structural steel and
associated piping. Considering the size of such units, this
savings is substantial.
In ~he Drawing, diplegs 20 and 25 pass through
stripper vent lines 13 and 14, forming annular openings.
Stripping vapors flow upward from stripping vessel 8 into
the upper portion of reactor vessel 2 through these annular
1136564
openings. As heretofore stated, the size of stripper vent
lines 13 and 14 are based upon the cross sectional area of
the annular openings formed between the inside of vent lines
13 and 14 and the outside walls of diplegs 20 and 25.
As hereinabove described, improved apparatus for
Fluidized Catalytic Cracking of hydrocarbons is disclosed
wherein cyclone diplegs are extended from a vertical,
cylindrical reactor vessel into a stripping vessel
vertically disposed below said reactor vessel. Such
extension of the diplegs allow reduction of the heights of
dense phase bed required to be maintained in reactor vessel
2. Consequently, the heights of reactor vessel 2 may be
shortened by a commensurate amount. This reduction in
reactor vessel 2 height, and also the height of related
piping and structural steel provides a substantial savings
in cost of building such a unit. Additionally, reduction in
the dense phase bed maintained in reactor vessel 2 avoids
overcracking of hydrocarbons which results in increased
yield and octane of naphtha product.
It will be understood that the above description
is illustrative of a preferred embodiment of the present
invention. Additional modifications within the spirit and
scope of the present invention, will occur to those skilled
in the art, and such additional modifications may fairly be
presumed to be within the scope of the invention as defined
in the claims below.
