Note: Descriptions are shown in the official language in which they were submitted.
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ANCILLARY CRACKING OF PARAFFINIC NAPHTHA
IN CONJUCTION WITH FCC UNIT OPERATIONS
FIELD OF THE INVENTION
This invention relates to increasing the production of lighter
hydrocarbons, such as ethylene, propylene and the butylenes, and gasoline in
conjunction with the operation of a fluidized catalytic cracking process.
BACKGROUND OF TH F. INVENTION
Propylene is second in importance only to ethylene as a petrochemical
raw material building block. Propylene has traditionally been obtained as a by-
product from steam cracking to produce ethylene and from refinery fluidized
catalytic cracking processes to produce gasoline. The projected growth in
demand for propylene has started to exceed that of ethylene so that existing
processes cannot satisfy the future propylene demand. To meet the expected
market demand for propylene, a catalytic paraffinic naphtha cracking process
can be utilized.
The catalytic cracking of olefinic naphthas is well known and currently
practiced in all types of FCC units processing a variety of feedstocks.
Recycled
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cracked naphtha, olefinic naphthas from the FCC, from visbreakers or cokers
are easily converted to propylene in the FCC reactor riser with the base
feedstock. The gasoline produced from recycling is high in octane and
aromatics. None of the prior art FCC processes can be used to crack light
straight run ("LSR") naphtha efficiently without significant modifications.
Fluidized catalytic cracking, or FCC, is a well-known and widely
practiced process for converting heavy hydrocarbons, gasoils and residues into
lighter hydrocarbon fractions. In general terms, the process for the cracking
of
hydrocarbon feedstocks relies on contact with heated fluidized catalytic
particles in a reaction zone maintained at appropriate temperatures and
pressures. When the heavier feed. contacts the hot catalyst and is cracked to
lighter products, carbonaceous deposits, commonly referred to as coke, form on
the catalyst and deactivate it. The deactivated, or spent, catalyst is
separated
from the cracked products, stripped of removable hydrocarbons and passed to a
regeneration vessel where the coke is burned from the catalyst in the presence
of
air to produce a substantially regenerated catalyst. The combustion products
are
removed from the vessel as flue gas. The heated regenerated catalyst is then
recycled to the reaction zone in the FCC unit. A general description of the
FCC
process is provided in USP 5,372,704
Various methods and apparatus have been proposed for increasing or
enhancing the output of particular product streams from the FCC unit. In some
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cases, ancillary reactors and other treatment vessels have been provided to
treat
a particular fraction or reaction product stream. In some instsrices, multiple
reactors are provided, each with a different feed, in order to derive a
particularly
desired product stream.
For example, USP 4,090,949 describes a method for upgrading poor
quality olefin.ic gasoline using two reactor risers, one of which receives a
typical
gas feedstream, while the second is used to crack a feedstream consisting
primarily of low quality olefmic gasoline. The reactor temperature is from 450
F to 900 F and the catalyst-to-olefinic gasoline ratio is in the range of
from 1 to
40, with a residence time in the range of from 1 to 30 seconds.
A process employing multiple cracking zones in either a fluidized bed or
in parallel riser reactors is disclosed in USP 3,856,659. In one aspect of
this
integrated process, paraffinic naphtha feed from the crude unit is mixed with
recycled cracked naphtha (olefinic) and fed to one of the riser reactors.
Typical
operating temperatures range from 900 F to 1,3000 F with a catalyst/oil ratio
of
from 3 to 20 and a residence time of from 1 to 10 seconds.
A process using separate multiple fluidized reactors with upward
direction in elongated transfer lines, or risers, is disclosed in USP
4,297,203.
Cracked naphtha feedstock from the first riser reactor is recovered and
recycled
to the second riser along with another hydrocarbon feedstrearn. Reactor
temperatures are somewhat lower than in the prior example.
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A method for upgrading or cracking virgin naphtha is disclosed in USP
4,830,728 using a single catalyst of the Y zeolite type, or a combination of
Y zeolite ZSM-5. In this process, ethylene is mixed with the virgin naphtha in
the riser reactor. This process mentions upgrading straight run naphtha, which
is mixed with recycled cracked material and injected into a separate upflow
riser
reactor. It is apparently the objective of this process to upgrade naphtha in
two
riser reaction zones with gasoil and/or resid catalytically cracked in the
first
riser and ethylene and catalytically cracked naphtha recycle and/or other
naphtha(s) catalytically cracked in the second riser and in a dense fluidized
1.0 reactor.
A method is described in USP 5,372,704 that employs spent catalyst in a
re-cracking reactor for limited conversion of FCC naphtha or other thenually
produced olefmic naphtha to light products with an increase in the product
naphtha octane rating. The operating conditions for this process are
relatively
mild with temperatures in the 800 F to 1,1000 F range and a residence time of
from 1 to 100 seconds.
A review of the disclosures of the patents discussed above, as well as
other prior art sources, has failed to identify a process in which a virgin
paraffinic naphtha feedstream is cracked in conjunction with an FCC unit to
produce primarily light olefins consisting of ethylene, propylene and
butylenes,
and gasoline.
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It is therefore an object of the present invention to provide a process in
which a virgin paraffinic naphtha feedstream withdrawn as a fraction from an
external source, such as a crude atmospheric distillation column, toppers, as
a
by-product stream from a hydrotreater or hydrocracker units, or other high
paraffinic naphtha stream from an extraction process, is further cracked to
provide a light reaction product stream.
It is a further object of the invention to provide such a process that can be
run efficiently utilizing the same catalyst employed in the FCC unit.
Yet another object of the invention is to provide a novel process for
efficiently cracking a paraffinic naphtha feedstock to produce a lighter
hydrocarbon product stream consisting of ethylene, propylene, butylenes and
gasoline, which reaction product stream can either be recovered separately and
further fractionated to recover the individual components or combined with an
effluent stream from the FCC unit for further fractionation.
The term "paraffinic naphtha feed" shall be understood to include any
hydrocarbon charge stock boiling in the range of pentane (C5) hydrocarbons up
to about 450 F that contains 40% to 80% by weight of paraffmic components
with very little olefin components. The remaining components will be
naphthenes, aromatics and olefins in descending order of composition. This
paraffmic naphtha feed usually comes from crude or other atmospheric
fractionation columns, but can also be derived from other processes which
produce paraffinic-containing hydrocarbons. For example, hydrotreater
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processes known in the refining and petrochemical art will produce paraffinic
hydrocarbons from olefmic and aromatic type feed streams that can be used in
the practice of the invention. The term "paraffinic naphtha" will also be
understood to include light straight run (LSR) naphtha, or virgin naphtha,
such
as that obtained from a crude distillation unit, and will also include high
paraffinic naphtha feedstreams resulting from extraction processes, all of
which
are known in the art.
SUMMARY OF THE INVENTION
The above objects and further advantages are achieved by the
improved process and apparatus of the invention in which a downward flow
fluidized catalyst reactor is added as an ancillary reactor to the existing
FCC
process unit operation. The ancillary downflow reactor system utilizes the
same
hot regenerated catalyst as is used in the FCC unit. The regenerated catalyst
and
a virgin paraffinic naphtha feedstream derived from a source that is
independent
of the FCC unit are introduced and thoroughly mixed in an upper portion of the
downflow reactor that is above the reaction zone.
The mixture passes through the reaction zone with a residence time of OA
seconds to 5 seconds, and preferably in a range of 0.2 seconds to 2 seconds,
where the reaction zone operating temperature is from 900 F to 1,200 F. The
ratio of catalyst-to-naphtha, also referred to as the catalyst-to-oil ratio,
in the
reaction zone is in the range of from 10 percent to 80 percent by weight, with
a
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preferred operating range of from 20 percent to 50 percent by weight. The
determination of the catalyst-to-oil ratio is an indication of operating
severity
and its determination is well known to the art.
The efficient operation of this process is dependent upon the introduction
of a single feedstream consisting of a virgin paraffinic naphtha feed. The
relatively low residence times and higher catalyst-to-oil ratios of 20 to 50
percent by weight are specific to the paraffinic naphtha feedstream. The
introduction of other hydrocarbons into the feedstream of the secondary
downflow reactor will adversely affect the yields of the desired lighter
hydrocarbon reaction products.
The downflow reactor provides several advantages including the relative
ease of separating the desired end products from other components.
The improved process of the invention can be utilized with prior art
FCC units, whether they employ riser cracking in an upward or downward flow
reaction scheme, or bed cracking to catslytically convert naphthas to the
desired
lighter hydrocarbons.
Any existing FCC catalyst can be employed in the practice of the
improved process of the invention. Typical FCC catalysts with or without
catalyst additives are suitable for use in this process improvement.
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BRIEF DESCRIPTION OF '111E DRAWINGS
The apparatus and method of the invention will be described in further
detail below and with reference to the attached drawings in which:
FIG. 1 is a simplified schematic illustration of a typical FCC apparatus
and process of the prior art; and
FIG. 2 is a simplified schematic illustration of an embodiment of the
apparatus and process of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As indicated above, the method and apparatus of the present invention
can be employed with any number of FCC process units known to the prior art.
With reference to Fig. 1, a typical prior art FCC process is schematically
illustrated. The reactor vessel (10) receives the hydrocarbon, or oil,
feedstock
(12) that is admitted into the lower end of reactor riser (14) where it is
mixed
with fresh and/or regenerated catalyst that is transferred by a conduit (22). -
For
the purpose of this simplified schematic illustration and description, the
numerous valves, temperature sensors, electronic controllers and the like that
are customarily employed and well known to those of ordinary skill in the art
are not included.
In this continuous process, the mixture of catalyst and FCC reactor
feedstream proceed upward through the riser into a reaction zone in which the
temperature, pressure and residence time are controlled again within ranges
that
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are conventional and related to the operating characteristics of the one or
more
catalysts used M. the process, the configuration of the apparatus, the type
and
characteristics of the feedstock and a variety of other parameters that are
well
known to those of ordinary skill in the art and which form no part of the
present
invention. The reaction product is withdrawn through conduit (16) for recovery
and/or further processing in. the refinery.
The spent catalyst from the FCC unit is withdrawn via transfer line (18)
for delivery to the lower portion of regeneration vessel (20), most
conveniently
located in relatively close proximity to FCC unit (10). The spent catalyst
entering through transfer line (18) is contacted by at least a stream of air
admitted through conduit (24) for controlled combustion of any accumulated
coke. The flue gases are removed from the regenerator via conduit (26), and
the
temperature of the regenerated catalyst is raised to provide heat for the
endothermic cracking reaction in the reactor vessel (10).
The method of the present invention will now be described with reference
to Fig. 2. It will be understood that the reactor (10) and regeneration vessel
(20)
include components common to those described in connection with Fig. 1 and
their description and functioning will not be repeated. The novel apparatus
component and method of operation depicted in Fig. 2 relates to the downflow
reactor (30) which hot receives regenerated catalyst via transfer line (28)
that is
introduced into an upper portion of the vessel (30). Feedline (32) introduces
a
paraffinic naphtha feedstrearn from a source other than the FCC unit for
mixing
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with the incoming regenerated catalyst from regenerator (20). The mixture of
naphtha and catalyst passes into a reaction zone (33) that is maintRined at a
temperature that ranges from 900 F to 1,200 F. The ratio of the catalyst-to-
naphtha is in the range from 20 percent to 50 percent by weight. The residence
time of the mixture in the reaction zone is from about 0.2 seconds to about 2
seconds.
Although a variety of catalysts can be utilized in the process, it will be
understood that the same catalyst used in the main FCC unit is also employed
in
the catalytic cracking of the paraffinic naphtha feedstream in downfiow
reactor
(30). In the practice of the invention it is preferred that zeolite catalysts
of the Y
type be used alone or in combination with ZSM-5 catalysts. As will be
understood by those of ordinary skill in this art, catalysts additives can
also be
used with either of these systems. The choice of the catalyst(s) system does
not
form a part of the present invention.
With continuing reference to Fig. 2, the light reaction product stream is
recovered via line (34). In accordance with the method of the invention, the
light hydrocarbon reaction product stream containing ethylene, propylene,
butylenes, gasoline, and any other by-products from the cracking reactions, is
withdrawn and can be either recovered separately in a segregated recovery
section (not shown) or combined with the reaction product stream from the FCC
unit for further fractionation and eventual recovery.
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Stripping steam is admitted through line (36) to drive off any removable
hydrocarbons from the spent catalyst. These gases are discharged from the
downflow reactor (30) and introduced into the upper portion of the stripper
vessel (37) where these combined gases, or vapors, pass through cyclone
separators (39) and out of the stripper vessel via line (34) for product
recovery
in accordance with methods known to the art.
The spent catalyst from the downflow reactor (30) is discharged through
transfer line (40) and admitted to the lower end of the diptube, or lift
riser, (29)
which extends from the lower portion of the modified catalyst regenerator
(20).
In this embodiment, air is introduced below the spent catalyst transfer line
(40)
at the end of diptube or lift riser (29) via pressurized air line (25). A more
detailed description of the functioning of the secondary downflow reactor (30)
is provided below.
The configuration and selection of materials for the downflow reactor
(30), as well as the specific operating characteristics and parameters will be
dependent upon the specific qualities and flow rate of the paraffinic naphtha
feed introduced at feedline (32), which in turn will be dependent upon the
source of the feedstock. More detailed operating conditions are also set forth
below.
It is to be understood that the present invention broadly comprehends a
method of producing a product stream consisting primarily of the light olefins
ethylene, propylene and butylenes, and of gasoline in conjunction with the
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processing of a petroleum feedstock in a fluidized catalytic cracking (FCC)
unit
containing a catalyst of specified composition, the FCC and associated
downflow reactor catalyst feed being regenerated from spent catalyst, and the
method including the steps of:
a. providing a separate paraffinic naphtha feedstresm and
directing it into an upper portion of a downflow reactor that
is proximate the FCC unit;
b. introducing regenerated catalyst of the same type used in the
FCC unit into the downflow reactor for mixing with the
paraffinic naphtha feedstream in a ratio of catalyst-to-
feedstream in the range from 10 percent to 80 percent by
weight;
c. passing the catalyst and paraffinic naphtha mixture through a
reaction zone in the downstream reactor that is maintained at
a temperature that ranges from 900 F to 1,200 F for a
residence time of from 0.1 seconds to 5 seconds to crack the
naphtha;
d: separating the reaction products stream containing light
olefins and gasoline from spent catalyst;
e. recovering the reaction product stream; and
f. passing the spent catalyst from the downflow reactor to a
separate regeneration vessel that also contains spent catalyst
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from the FCC unit for regeneration and recycling to the FCC
unit and the downflow reactor.
With continuing reference to Fig. 2, the hot regenerated catalyst at
approximately 1250 F to 1500 F is transferred from the regenerator vessel
(20) of the FCC process by conventional means, e.g., through a downwardly
directed conduit or pipe (28), commonly referred to as a transfer line or
standpipe, to a withdrawal well or hopper (31) at the top of the downflow
reactor above the reaction zone (33) where the hot catalyst flow is allowed to
stabilize in order to be uniformly directed into the mix zone or feed
injection
portion of the reaction zone (33). A pressure stabilization line (38) connects
the
top of the withdrawal well (31) to the existing regenerator (20).
The naphtha feedstock is injected into the mixing zone through feed
injection nozzles (32a) placed in the immediate vicinity of the point of
introduction of the regenerated catalyst into the downflow reactor (30). These
multiple injection nozzles (32a) result in the catalyst and oil mixing
thoroughly
and uniformly. Once the paraffinic naphtha feedstock contacts the hot catalyst
the cracking reactions occur. The reaction vapor of hydrocarbon cracked
products and unreacted naphtha feed and catalyst mixture quickly flows through
the remainder of the downflow reactor and into a rapid separation section (35)
at
the bottom portion of the reactor. The residence time of the mixture in the
reaction zone is controlled in accordance with apparatus and procedures known
to the art.
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If necessary for temperature control, a quench injection (50) is provided
for the naphtha feed, recycle cracked naphtha or other light olefinic
hydrocarbon near the bottom of the reaction zone (33) immediately before the
separator. This quench injection quickly reduces or stops the cracking
reactions
and can be utilized for controlling cracking severity and allows for added
process flexibility.
The reaction temperature, i.e., the outlet temperature of the downflow
reactor, is controlled by opening and closing a catalyst slide valve (not
shown)
that controls the flow of regenerated catalyst from the withdrawal well (31)
and.
into the mix zone. The heat required for the endothermic cracking reaction is
supplied by the regenerated catalyst. By changing the flow rate of the hot
regenerated catalyst, the operating severity or cracking conditions can be
controlled to produce the desired yields of light oleflnic hydrocarbons and
gasoline.
The rapid separator (35) along with the end portion of the downflow
reactor (30) is housed in the upper section of a large vessel referred to as
the
catalyst stripper (37). The rapid separator directs the reaction vapor and
catalyst
directly into the top part the stripper vessel (37).
The reactor vapor stream moves upward from the rapid separator outlet
into the stripper, combines with stripped hydrocarbon product vapors and
stripping gas from the catalyst stripping section of this vessel and passes
through conventional separating means such as cyclones (39), which further
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separate any entrained catalyst particles from the vapors. The catalyst from
the
separator that is captured in the cyclones is directed to the bottom of the
stripper
vessel (37) through a cyclone dipleg for discharge into the bed of catalyst
that
was recovered from the rapid separator in the stripping section.
After the combined vapor passes through the cyclones and out of the
stripper vessel, it is directed through a conduit or pipe commonly referred to
as
a reactor vapor line (34) to a conventional product recovery section known in
the FCC art.
The catalyst from the rapid separator and cyclone diplegs flows to the
lower section of the stripper reactor vessel (37) that includes a catalyst
stripping
section into which a suitable stripping gas, such as steam, is introduced
through
streamline (36). The stripping section is provided with several baffles or
. structured packing (not shown) over which the downwardly flowing catalyst
passes counter-currently to the flowing stripping gas. The upwardly flowing
stripping gas, which is typically steam, is used to "strip" or remove any
additional hydrocarbons that remain in the catalyst pores or between catalyst
particles.
The stripped catalyst is transported by the combustion air stream (25)
through a lift riser (29) that terminates in the existing regenerator (20) in
a
typical FCC process to burn off any coke that is a by-product of the naphtha
cracking process. In the regenerator, the heat produced from the combustion of
the by-product coke produced in the first reaction zone (10 and 14) of a
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FCC process from cracking heavy hydrocarbons and from the naphtha cracking
in zone (33) of the downflow reactor (30) is transferred to the catalyst.
The regenerator vessel (20) can be of any conventional previously known
design and can be used with the enhanced process and downflow reaction zone
of this invention. The placement of the regenerator-to-reactor conduit (28) or
regenerated catalyst transfer line for the regenerator will be such that it
insures a
steady and continuous flow of a substantial quantity of regenerated catalyst
that
is needed to meet the maximum design requirements of the downflow reactor.
The catalyst requirements for the process of the invention can be
determined in conjunction with any catalyst conventionally used in FCC
processes, e.g., zeolites, silica-alumina, carbon monoxide burning promoter
additives, bottoms cracking additives, light olefin-producing additives and
any
other catalyst additives routinely used in the FCC process. The preferred
cracking zeolites in the FCC process are zeolites Y, REY, USY, and RE-USY.
For enhanced naphtha cracking potential, a preferred shaped selective catalyst
additive typically used in the FCC process to produce light olefins and
increase
FCC gasoline octane is ZSM-5 zeolite crystal or other pentasil type catalyst
structure. This ZSM-5 additive is mixed with the cracking catnlyst zeolites
and
matrix structures in conventional FCC catalyst and is preferably used in the
method of the invention to maximize and optimize the paraffinic naphtha
cracking in the downflow reactor.
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A particular advantage of this invention as an. enhancement to an existing
FCC process for typical FCC heavy hydrocarbon feedstocks is the amount on
coke produced from these cracking reactions. In naphtha cracking, the overall
unit operational efficiency is adversely effected by the limited amount of
coke
produced during the cracking reactions. The amount of coke produced is not
sufficient to produce enough heat during catalyst regeneration to allow for
the
paraffinic naphtha cracking reactions to occur in the downflow reactor. By
comparison, the coke produced during the heavy oil or gasoil cracking in the
typical FCC process is more than adequate to provide the required heat to the
downflow reactor. In the method of the invention, this heat is transferred
from
the regenerator to the downflow reactor by the regenerated catalyst by mixing
the spent catalysts from the two sources during the regeneration processing in
vessel (20).
A further advantage of the present invention as an enhancement to
existing FCC processes for co-processing paraffinic naphtha is that the
products
can be recovered in the existing recovery section of the unit. The unconverted
paraffinic naphtha can be recycled with the olefinic naphtha in the FCC
process
to produce additional light olefins from cracking the olefinic naphtha or for
use
as a blending stock in finished gasoline. The process has the advantage of
providing for the separate recovery of the naphthas from each reactor for
further
separate downstream processing, with the alternative of combining the two
streams for partial recycling to the FCC unit or for gasoline blending.
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Examples
A bench scale pilot plant was used to determine the operating conditions
for obtaining desired product yields from cracking a typical paraffinic
naphtha
feedstock. A pilot plant unit was used to represent the cracking conditions in
the downflow reactor.
In the following examples, two catalyst systems are utilized to
demonstrate the potential for cracking paraffinic naphtha to produce light
olefin
yields. One catalyst was a typical low rare earth, low hydrogen transfer USY
zeolite catalyst that is commercially available. The second catalyst system
was
the same commercially available USY zeolite cracking catalyst blended with a
shape selective ZSM-5 zeolite type cracking catalyst additive.
The following Table summarizes the effects of varying the cracking
severity by changing the reactor temperatures in the pilot unit for both
catalyst
systems.
Catalyst USY USY + ZSM-5
Temperature C 510 550 625 510 550 625
F 950 1022 1157 950 1022 1157
Conversion wt. % 29 39 . 44 31 43 52
Product Yields wt%
Total Gas 28 39 44 30 42 51 :
_Liquid 71 60 56 69 57 48
Coke 0.5 0.5 0.5 0.5 0.5 0.5
Gas Composition Wt.%
Ethylene 0.8 2.0 2.9 2.2 4.2 7.1
Propylene 4.3 7.5 13 6.4 10.5 16.8
Butylenes 2.7 - 4.2 8.7 4.0
5.9 8.6
_H2, Cl, C2 0.4 _ 1.8 3.4 3.8 1.3 3.3
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Paraffins
C3-C4 Paraffins 20 23 16 17.2 20.1 15.2
Selectivity
Propylene 15 19 29 21 25 33
C3-C4 Paraffin 71 59 36 56 48 30
As used in the Table, the term "Selectivity" is defined as the ratio of the
amount of a particular Component to the Total Gas, e.g., Propylene/Total Gas.
It will be understood that the embodiments described above are
illustrative of the invention and that various modifications can be made by
those
of ordinary skill in the art that will be within the scope of the invention,
which
is to be determined by the claims that follow.
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