U.S. patent application number 14/494704 was filed with the patent office on 2016-03-24 for fcc process with an integrated secondary reactor for increased light olefin yields.
The applicant listed for this patent is UOP LLC. Invention is credited to Zhihao Fei, Hosoo Lim, Charles P. Luebke, Paolo Palmas, Lisa M. Wolschlag.
Application Number | 20160083660 14/494704 |
Document ID | / |
Family ID | 55525175 |
Filed Date | 2016-03-24 |
United States Patent
Application |
20160083660 |
Kind Code |
A1 |
Fei; Zhihao ; et
al. |
March 24, 2016 |
FCC PROCESS WITH AN INTEGRATED SECONDARY REACTOR FOR INCREASED
LIGHT OLEFIN YIELDS
Abstract
A process for increasing the yields of propylene is presented.
The process is an FCC process for producing light olefins, and
utilizes a smaller secondary reactor that uses the same catalyst,
or a different catalyst as in the FCC reactor. The FCC effluent is
separated, and C4 and C5 olefins are recovered. The C4 and C5
olefins are passed to the secondary reactor for cracking to
generate increased light olefin yields.
Inventors: |
Fei; Zhihao; (Naperville,
IL) ; Luebke; Charles P.; (Mount Prospect, IL)
; Lim; Hosoo; (Mount Prospect, IL) ; Palmas;
Paolo; (Des Plaines, IL) ; Wolschlag; Lisa M.;
(Aurora, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UOP LLC |
Des Plaines |
IL |
US |
|
|
Family ID: |
55525175 |
Appl. No.: |
14/494704 |
Filed: |
September 24, 2014 |
Current U.S.
Class: |
585/653 |
Current CPC
Class: |
C10G 70/041 20130101;
C10G 21/27 20130101; C10G 51/026 20130101; C10G 11/05 20130101;
C10G 11/18 20130101; C10G 55/06 20130101; C10G 2400/20
20130101 |
International
Class: |
C10G 55/06 20060101
C10G055/06 |
Claims
1. A process for improving light olefin yields, comprising: passing
a hydrocarbon stream to an FCC reactor to generate an FCC effluent
stream comprising light olefins; passing the FCC effluent stream to
a product recovery unit to generate a first stream comprising light
components, a second stream comprising C4 and C5 hydrocarbons, and
a third stream comprising C6+ compounds; passing the second stream
to an extractive distillation unit to generate a fourth stream
comprising C4 and C5 olefins, and a fifth stream comprising
paraffins; passing the fourth stream to a secondary reactor to
generate a sixth stream comprising light olefins; and passing the
sixth stream to the light olefins separation unit.
2. The process of claim 1 wherein the secondary reactor is a
bubbling bed reactor, a slow fluidized bed reactor or a fast
fluidized bed reactor with, or without, partial regeneration.
3. The process of claim 1 further comprising passing a catalyst to
the secondary reactor, thereby generating a catalyst effluent
stream.
4. The process of claim 3 further comprising passing the catalyst
effluent stream to the FCC reactor.
5. The process of claim 4 wherein the FCC reactor comprises a riser
section, a catalyst separation section and a stripper section, and
wherein the catalyst effluent stream is passed to the stripper
section.
6. The process of claim 1 wherein the catalyst comprises a cracking
catalyst selected from the group consisting of Y-zeolite, ZSM-5,
ST-5, ZSM-11, ZSM-22, beta, erionite, ZSM-34, SAPO-11, non-zeolitic
amorphous silica-alumina, faujasite, chabazite, modernite, and
mixtures thereof.
7. The process of claim 6 wherein the catalyst comprises ZSM-5.
8. The process of claim 1 further comprising: passing a regenerated
catalyst stream to the FCC reactor, to generate an intermediate
stream of catalyst and reactants; passing the intermediate stream
to a reactor separation stage to generate the FCC effluent stream
and an intermediate catalyst stream; passing the intermediate
catalyst stream to a stripping section to generate a stripped
catalyst stream; and passing the stripped catalyst stream to a
regenerator to generate the regenerated catalyst stream.
9. The process of claim 1 wherein the hydrocarbon stream is a VGO
stream.
10. The process of claim 1 wherein the extractive distillation unit
comprises a selective olefin absorption process utilizing a solvent
to generate the fourth stream comprising olefins and the fifth
stream comprising paraffins.
11. The process of claim 10 wherein the solvent is selected from
the group consisting of NMP (n-methyl-2-pyrrolidone), DMF
(dimethylformamide), THF (tetrahydrofuran), ACN (acetonitrile), and
mixtures thereof.
12. A process for improving light olefin yields, comprising:
passing a hydrocarbon stream to a cracking reactor, wherein the
reactor includes a cracking catalyst, to generate a cracking
effluent stream comprising light olefins; passing the cracking
effluent stream to a separation unit to generate a first stream
comprising C3 and lighter compounds, a second stream comprising C4
and C5 hydrocarbons, and a third stream comprising C6+ compounds;
passing the second stream to an extractive distillation unit to
generate a fourth stream comprising C4 and C5 olefins, and a fifth
stream comprising paraffins; and passing the fourth stream to a
secondary reactor, wherein the secondary reactor includes a
cracking catalyst, to generate a sixth stream comprising light
olefins.
13. The process of claim 12 wherein the cracking reactor is a
fluidized catalytic cracking reactor comprising a riser
reactor.
14. The process of claim 12 further comprising: passing a
regenerated catalyst stream to the cracking reactor.
15. The process of claim 12 further comprising passing the sixth
stream to the light olefins separation unit.
16. The process of claim 12 further comprising passing a cracking
catalyst to the secondary reactor to generate a secondary catalyst
effluent stream.
17. The process of claim 16 further comprising passing the catalyst
effluent stream to the cracking reactor, thereby generating a
cracking reactor catalyst effluent stream.
18. The process of claim 12 further comprising: passing a
regenerated catalyst stream to the cracking reactor thereby
generating a spent catalyst effluent stream; and passing the spent
catalyst effluent stream to a regenerator thereby generating the
regenerated catalyst stream.
19. The process of claim 18 further comprising: passing a fresh
catalyst stream to the secondary reactor thereby generating a spent
secondary catalyst stream; and passing the spent secondary catalyst
stream to the cracking reactor.
20. The process of claim 12 wherein the secondary reactor can
comprise a bubbling bed reactor, a slow fluidized bed reactor, or a
fast fluidized bed reactor, and wherein the secondary reactor
utilizes the same catalyst as in the FCC reactor.
Description
FIELD OF THE INVENTION
[0001] The field of this invention relates to hydrocarbon cracking
processes, and in particular the production of light olefins from
cracking a heavy hydrocarbon feedstock
BACKGROUND
[0002] The production of light olefins, ethylene and propylene, are
used in the production of polyethylene and polypropylene. These are
among the most commonly manufactured plastics today. Other uses for
ethylene and propylene include the production of other chemicals.
Examples include vinyl monomer, vinyl chloride, ethylene oxide,
ethylbenzene, cumene, and alcohols. This list is by no means
exhaustive, but is representative of the versatility of ethylene
and propylene. The production of ethylene and propylene is chiefly
performed through the cracking of heavier hydrocarbons. The
cracking process includes stream cracking and catalytic cracking of
hydrocarbon feedstocks, such as naphtha, gas oils, and other
hydrocarbon streams, as well as other sources of carbonaceous
materials, such as recycled plastics and organic materials.
[0003] A light olefins plant involves a very complex combination of
reaction and gas recovery systems. Feedstock is charged to a
thermal cracking zone in the presence of steam at effective
conditions to produce a pyrolysis reactor effluent gas mixture. The
mixture is then stabilized and separated into purified components
through a sequence of cryogenic and conventional fractionation
steps. Ethylene and propylene yields from steam cracking and other
processes may be improved using known methods for the metathesis or
disproportionation of C4 and heavier olefins, in combination with a
cracking step in the presence of a zeolitic catalyst, as described,
for example, in U.S. Pat. No. 5,026,935 and U.S. Pat. No.
5,026,936. The cracking of olefins in hydrocarbon feedstocks
comprising C4 mixtures from refineries and steam cracking units is
described in U.S. Pat. No. 6,858,133; U.S. Pat. No. 7,087,155; and
U.S. Pat. No. 7,375,257.
[0004] Currently, the majority of light olefins production is from
steam cracking and fluid catalytic cracking (FCC). However, the
demand for light olefins is growing and other means of increasing
the amount of light olefins have been sought. Other means include
paraffin dehydrogenation, which represents an alternative route to
light olefins and is described in U.S. Pat. No. 3,978,150 and
elsewhere. More recently, the desire for alternative, non-petroleum
based feeds for light olefin production has led to the use of
oxygenates such as alcohols and, more particularly, methanol,
ethanol, and higher alcohols or their derivatives. Methanol, in
particular, is useful in a methanol-to-olefin (MTO) conversion
process described, for example, in U.S. Pat. No. 5,914,433. The
yield of light olefins from such a process may be improved using
olefin cracking to convert some or all of the C4+ product of MTO in
an olefin cracking reactor, as described in U.S. Pat. No.
7,268,265. Other processes for the generation of light olefins
involve high severity catalytic cracking of naphtha and other
hydrocarbon fractions. A catalytic naphtha cracking process of
commercial importance is described in U.S. Pat. No. 6,867,341.
[0005] Another process for enhancing propylene yield is disclosed
in U.S. Pat. No. 4,980,053, where a deep catalytic cracking process
is disclosed. The process requires 5-10 seconds of contact time,
and uses a mixture of Y-type zeolite and a pentasil,
shape-selective zeolite. However, the process reports relatively
high yields of dry gas.
[0006] Other patents disclose short catalyst contact times, but do
not recognize significant light olefin yields, such as in U.S. Pat.
No. 5,965,012 which discloses an FCC process. The process has a
catalyst recycle arrangement with a very short contact time of the
feed with the catalyst. However, further cracking takes place in a
contacting conduit where regenerated and carbonized catalyst
contacts the feed, and not in the riser. Another FCC process is
disclosed in U.S. Pat. No. 6,010,618 where there is a very short
catalyst and feed contact time in the riser, and the cracked
product is quickly removed below the outlet of the riser. Other
patents, such as U.S. Pat. No. 5,296,131 disclose very short FCC
catalyst contact times, but these processes are operated to improve
gasoline production rather than production of light olefins.
[0007] Other patents, U.S. Pat. Nos. 4,787,967, 4,871,446, and
4,990,314, disclose the use of two component catalysts used in FCC
processes. The two component catalyst systems use a large-pore
catalyst for cracking large hydrocarbon molecules and a small-pore
catalyst for cracking smaller hydrocarbon molecules.
[0008] To enhance propylene yields, shape selective additives are
used in conjunction with conventional FCC catalysts containing
Y-zeolites. The additives all have essentially the same selectivity
characteristics. The problem with current catalysts is that
selectivity is limited, and the amount of propylene produced is
only a function of the amount of additive used in the catalyst
mixture. The propylene yield reaches a maximum at a crystalline
shape selective zeolite content in the catalyst blend of
approximately 10-12%.
[0009] To overcome this, the FCC operation severity (temperature,
catalyst/oil ratio, etc.) is increased to increase light olefin
yield, but at the cost of increased undesirable yields of coke, dry
gas, or methane and ethane, as well as C4 and C5 olefins. The final
olefin yields are limited by the equilibrium distribution even at
high severity.
[0010] Despite the variety of methods for generating light olefins
industrially, the demand for ethylene and propylene is still
increasing faster than new processes can provide. Moreover, further
demand growth for light olefins is expected. A need therefore
exists for new methods that can economically increase light olefin
yields from existing sources of both straight-run and processed
hydrocarbon streams.
SUMMARY
[0011] There is an increase in demand for light olefins, and in
particular propylene. The present invention provides for a process
to increase the yields of light olefins from a hydrocarbon
feedstock.
[0012] A first embodiment of the invention is a process for
improving light olefin yields, comprising passing a hydrocarbon
stream to an FCC reactor to generate an FCC effluent stream
comprising light olefins; passing the FCC effluent stream to a
product recovery unit to generate a first stream comprising light
components, a second stream comprising C4 and C5 hydrocarbons, and
a third stream comprising C6+ compounds; passing the second stream
to an extractive distillation unit to generate a fourth stream
comprising C4 and C5 olefins, and a fifth stream comprising
paraffins; passing the fourth stream to a secondary reactor to
generate a sixth stream comprising light olefins; and passing the
sixth stream to the light olefins separation unit. An embodiment of
the invention is one, any or all of prior embodiments in this
paragraph up through the first embodiment in this paragraph wherein
the secondary reactor is a bubbling bed reactor, a slow fluidized
bed reactor or a fast fluidized bed reactor with partial
regeneration. An embodiment of the invention is one, any or all of
prior embodiments in this paragraph up through the first embodiment
in this paragraph further comprising passing a catalyst to the
secondary reactor, thereby generating a catalyst effluent stream.
An embodiment of the invention is one, any or all of prior
embodiments in this paragraph up through the first embodiment in
this paragraph further comprising passing the catalyst effluent
stream to the FCC reactor. An embodiment of the invention is one,
any or all of prior embodiments in this paragraph up through the
first embodiment in this paragraph wherein the FCC reactor
comprises a riser section, a catalyst separation section and a
stripper section, and wherein the catalyst effluent stream is
passed to the stripper section. An embodiment of the invention is
one, any or all of prior embodiments in this paragraph up through
the first embodiment in this paragraph wherein the catalyst
comprises a cracking catalyst selected from the group consisting of
Y-zeolite, ZSM-5, ST-5, ZSM-11, ZSM-22, beta, erionite, ZSM-34,
SAPO-11, non-zeolitic amorphous silica-alumina, faujasite,
chabazite, modernite, and mixtures thereof. An embodiment of the
invention is one, any or all of prior embodiments in this paragraph
up through the first embodiment in this paragraph wherein the
catalyst comprises ZSM-5. An embodiment of the invention is one,
any or all of prior embodiments in this paragraph up through the
first embodiment in this paragraph further comprising passing a
regenerated catalyst stream to the FCC reactor, to generate an
intermediate stream of catalyst and reactants; passing the
intermediate stream to a reactor separation stage to generate the
FCC effluent stream and an intermediate catalyst stream; passing
the intermediate catalyst stream to a stripping section to generate
a stripped catalyst stream; and passing the stripped catalyst
stream to a regenerator to generate the regenerated catalyst
stream. An embodiment of the invention is one, any or all of prior
embodiments in this paragraph up through the first embodiment in
this paragraph wherein the hydrocarbon stream is a VGO stream. An
embodiment of the invention is one, any or all of prior embodiments
in this paragraph up through the first embodiment in this paragraph
wherein the extractive distillation unit comprises a selective
olefin absorption process utilizing a solvent to generate the
fourth stream comprising olefins and the fifth stream comprising
paraffins. An embodiment of the invention is one, any or all of
prior embodiments in this paragraph up through the first embodiment
in this paragraph wherein the solvent is selected from the group
consisting of NMP (n-methyl-2-pyrrolidone), DMF
(dimethylformamide), THF (tetrahydrofuran), ACN (acetonitrile), and
mixtures thereof.
[0013] A second embodiment of the invention is a process for
improving light olefin yields, comprising passing a hydrocarbon
stream to a cracking reactor, wherein the reactor includes a
cracking catalyst, to generate a cracking effluent stream
comprising light olefins; passing the cracking effluent stream to a
separation unit to generate a first stream comprising C3 and
lighter compounds, a second stream comprising C4 and C5
hydrocarbons, and a third stream comprising C6+ compounds; passing
the second stream to an extractive distillation unit to generate a
fourth stream comprising C4 and C5 olefins, and a fifth stream
comprising paraffins; and passing the fourth stream to a secondary
reactor, wherein the secondary reactor includes a cracking
catalyst, to generate a sixth stream comprising light olefins. An
embodiment of the invention is one, any or all of prior embodiments
in this paragraph up through the second embodiment in this
paragraph wherein the cracking reactor is a fluidized catalytic
cracking reactor comprising a riser reactor. An embodiment of the
invention is one, any or all of prior embodiments in this paragraph
up through the second embodiment in this paragraph further
comprising passing a regenerated catalyst stream to the cracking
reactor. An embodiment of the invention is one, any or all of prior
embodiments in this paragraph up through the second embodiment in
this paragraph further comprising passing the sixth stream to the
light olefins separation unit. An embodiment of the invention is
one, any or all of prior embodiments in this paragraph up through
the second embodiment in this paragraph further comprising passing
a cracking catalyst to the secondary reactor to generate a
secondary catalyst effluent stream. An embodiment of the invention
is one, any or all of prior embodiments in this paragraph up
through the second embodiment in this paragraph further comprising
passing the catalyst effluent stream to the cracking reactor,
thereby generating a cracking reactor catalyst effluent stream. An
embodiment of the invention is one, any or all of prior embodiments
in this paragraph up through the second embodiment in this
paragraph further comprising passing a regenerated catalyst stream
to the cracking reactor thereby generating a spent catalyst
effluent stream; and passing the spent catalyst effluent stream to
a regenerator thereby generating the regenerated catalyst stream.
An embodiment of the invention is one, any or all of prior
embodiments in this paragraph up through the second embodiment in
this paragraph further comprising passing a fresh catalyst stream
to the secondary reactor thereby generating a spent secondary
catalyst stream; and passing the spent secondary catalyst stream to
the cracking reactor stripping zone. An embodiment of the invention
is one, any or all of prior embodiments in this paragraph up
through the second embodiment in this paragraph wherein the
secondary reactor can comprise a bubbling bed reactor, a slow
fluidized bed reactor, or a fast fluidized bed reactor, and wherein
the secondary reactor utilizes the same catalyst, or a different
catalyst, as the FCC reactor.
[0014] Other objects, advantages and applications of the present
invention will become apparent to those skilled in the art from the
following detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 presents the FCC reactor and the secondary reactor
for the process of enhancing light olefin production; and
[0016] FIG. 2 presents the process with the product recovery unit
for the recycle of the butenes and pentenes to the secondary
reactor.
DETAILED DESCRIPTION
[0017] The demand for light olefins, ethylene and propylene,
continues to increase. Methods to increase the production include
trying new catalysts, and other flow processes, but the primary
source of light olefins is the cracking of a hydrocarbon stream
through either steam cracking or fluidized catalytic cracking (FCC)
reactor. The principal hydrocarbon stream is naphtha, but heavier
hydrocarbon streams, such as a vacuum gas oil (VGO) can also be
used. However, the yields are not as great as with naphtha, and
also cracking a heavier hydrocarbon stream also produces heavier by
product streams.
[0018] Other processes for increasing propylene yields can include
operating at higher severity, but these also need substantial
amounts of ZSM-5 additive. Due to equilibrium constraints, the FCC
reactor generates a substantial amount of other olefins, such as
butenes and pentenes. This is in particular true for a typical
Arabian Light VGO feedstock. While the current technology generates
about 18 wt % of propylene, the process also generates about 20 wt
% or more of butenes and pentenes. By recovering and passing the
butenes and pentenes to a separate, but smaller reactor, the yields
of propylene can be increased.
[0019] The present invention allows for the use of a heavier
hydrocarbon stream, and is a new method that adds a smaller
secondary reactor, wherein the catalyst for the reactor flows
through the reactor and then into the FCC reactor. The integration
will increase the propylene yield and is one of the objects of this
invention.
[0020] The process of the present invention is shown in FIG. 1, and
includes passing a hydrocarbon stream 8 to an FCC reactor 10 to
generate an FCC effluent stream 12. The FCC effluent stream 12 is
passed to a product recovery unit 100, as shown in FIG. 2, to
generate a first stream 202 comprising light components, a second
stream 204 comprising C4 and C5 hydrocarbons and a third stream 194
comprising C6+ compounds. The second stream 204 is passed to an
extractive distillation unit 210 to generate a fourth stream 212
comprising C4 and C5 olefins, and a fifth stream 214 comprising
paraffins. The fourth stream 212 is passed to a secondary reactor
20 to generate a sixth stream 22 comprising light olefins. The
sixth stream 22 is passed to the product recovery unit 100.
[0021] The secondary reactor 20 can comprise a bubbling bed
reactor, a slow fluidized bed reactor, or a fast fluidized bed
reactor with regeneration. With a fast fluidized bed reactor, the
regeneration of the catalyst can be partial of total.
[0022] The process further includes passing a catalyst stream 32 of
fresh catalyst from the fresh catalyst feed hopper 30 to the
secondary reactor 20. A catalyst effluent stream 24 is generated
during the movement of catalyst through the secondary reactor 20.
The catalyst effluent stream 24 is passed to the FCC reactor 10,
and enters the cycle of catalyst in the FCC system. The catalyst
cycle is well known to those in the FCC arts, and comprises flowing
a regenerated catalyst stream 42 through the FCC reactor 10. The
catalyst is separated from the product stream 12 and a spent
catalyst stream 14 and passed to a regenerator 40.
[0023] The FCC reactor 10 comprises a riser section 52, a catalyst
separation section 54, and a stripper section 56. The spent
catalyst from the separation section is passed to the stripper
section to collect in a moving bed, where a gas is passed through
the moving bed to remove residual hydrocarbons and other adsorbed
materials that reduce the efficiency of the regenerator 40. The
regenerator 40 generates a regenerated catalyst stream 42 and
passed the stream to the FCC reactor. The FCC reactor generates an
intermediate stream leaving the FCC riser section 52. The stream
leaving the riser section 52 enters the separation stage 54 wherein
an intermediate catalyst and an FCC effluent stream are separated.
The intermediate catalyst stream enters the stripping section 56 to
generate a stripped catalyst stream, and the stripped catalyst
stream 14 is passed to the regenerator.
[0024] The FCC reactor uses a catalyst, and the present invention
uses the same catalyst for performing the cracking function.
Suitable cracking catalyst are selected from one or more of
Y-zeolite, ZSM-5, ST-5, ZSM-11, ZSM-22, beta, erionite, ZSM-34,
SAPO-11, non-zeolitic amorphous silica-alumina, faujasite,
chabazite and modernite. A combination catalyst can comprise two or
more zeolites mixed into a common catalyst pellet, or can comprise
a mixture of catalyst pellets of different types of catalytic
materials. A preferred catalyst is ZSM-5.
[0025] The product recovery unit 100, as shown in FIG. 2, includes
passing the FCC reactor effluent stream 12 and the secondary
reactor effluent stream 22 to the main FCC column 110. The main
column 110 generate a heavy hydrocarbon stream 114, and an overhead
stream 112 comprising lighter components, including light olefins.
The heavy hydrocarbon stream 114 is a residue stream from cracking
and normally comprising a light cycle oil (LCO) stream. The
overhead stream 112 is passed to a first separation vessel 120 to
generate a first vapor stream 122 and a first liquid stream 124. A
portion of the first liquid stream 124 is used as reflux for the
main FCC column 110. The first vapor stream 122 is passed to a
compressor 130 to generate a first compressed stream 132. The first
compressed stream 132 is passed to a second separation vessel 140
to generate a second vapor stream 142 and a second liquid stream
144. The second vapor stream 142 is passed to a second compressor
150 to generate a second compressed stream 152. The second liquid
stream 144 and a portion of the first liquid stream 124 are passed
to a naphtha splitter 160. The naphtha splitter 160 generates a
naphtha bottoms stream 164 comprising C7+ hydrocarbons, and a
naphtha overhead stream 162 comprising C7- hydrocarbons. The
naphtha bottoms stream 164 can be passed to other process units in
a refinery.
[0026] The second compressed stream 152 is passed to a third
separation vessel 170 to generate a third vapor stream 172 and a
third liquid stream 174. The third liquid stream 174 is passed to a
deethanizer 180 to generate a deethanizer overhead 182 comprising
C2 and lighter gases, and a deethanizer bottoms stream 184
comprising C3 and heavier hydrocarbons. The deethanizer bottoms
stream 184 is passed to a depentanizer 190 to generate a
depentanizer overhead stream 192 comprising C3 to C5 hydrocarbons,
and a depentanizer bottoms stream 194 comprising heavier
hydrocarbons. The depentanizer overhead stream 192 is passed to a
depropanizer 200 to generate a depropanizer overhead stream 202 and
a depropanizer bottoms stream 204 The depropanizer overhead stream
202 comprises C3s and is passed to a C3 splitter to recover
propylene. The depropanizer bottoms stream 204 comprises C4s and
C5s. and is passed to an extractive distillation unit 210. The
extractive distillation unit 210 generates a paraffins stream 214
and an olefin stream 212. The olefin stream 212 comprises C4 and C5
olefins and is passed to the secondary reactor 20.
[0027] The naphtha overhead stream 162 is passed to a light gas
stripper 220. The third vapor stream 172 is passed to the light gas
stripper 220. The depentanizer bottoms stream 194 can be passed to
other process units in a refinery, or a portion, can be passed to
the light gas stripper 220. The bottom stream 222 of the light gas
stripper 220 is recycled to the third separation vessel 170. The
light gas stripper overhead 224 can be passed to a sponge absorber
230 for removing contaminants to generate a lean gas stream
232.
[0028] The extractive distillation unit 210 comprises a selective
olefin absorption process utilizing a solvent to generate the
olefin stream 212, and the paraffin stream 214. Suitable absorbents
for the extractive distillation unit can include one or more
absorbents selected from n-methyl-2-pyrrolidone (NMP),
dimethylformamide (DMF), tetrahydrofuran (THF), and acetonitrile
(ACN).
[0029] The flow configuration of the product recover unit 100 lends
itself to the heat exchange of several streams leaving or entering
the different separation columns. This is well known and not
elaborated further here.
[0030] This novel reactor configuration does not need additional
catalyst for high propylene operation except the fresh makeup ZSM-5
catalyst for the FCC system. The ZSM-5 makeup catalyst is due to
attrition losses in the FCC system during operation. However, this
is a relatively small amount added per day (on the basis of total
catalyst in the system) to maintain a constant level of activity.
The makeup catalyst is first passed through the secondary reactor
before passing into the FCC reactor. Since the spent catalyst will
be regenerated in the FCC regenerator, catalyst regeneration
process for this additional catalytic cracking process is optional.
The separated reactor will allow the reaction condition to be
optimized independently, so ethylene and propylene concentrations
will not be constrained by the FCC riser condition. As a result,
high ethylene and propylene yields of single pass can be achieved
from this reactor.
[0031] Unlike an FCC riser, the catalyst density in this secondary
reactor is much higher, and can be at least 10 times higher. Hence,
the reactor size is much smaller than a second riser for the same
purpose. And, unlike a fixed bed reactor such as the olefin
cracking process, where dual reactors loaded with special catalyst
are needed to maintain a continuous operation during catalyst
regeneration. The secondary bed reactor also allows a catalyst heat
exchanger to work. The reactor, like the FCC reactor, will be
operated at low pressure, 170 to 210 kPa (absolute) and high
temperature 580 C or above. Therefore, total high C3=yield (26+ wt
% on VGO) and C2=yield (10+ wt % on VGO) can be achieved in
integrated system with typical VGO feedstock. Although the
secondary bed reactor is integrated with the FCC unit, the FCC unit
itself is a conventional FCC system. It can be operated with other
modes such as gasoline mode by shutdown down this additional
reactor.
[0032] While the invention has been described with what are
presently considered the preferred embodiments, it is to be
understood that the invention is not limited to the disclosed
embodiments, but it is intended to cover various modifications and
equivalent arrangements included within the scope of the appended
claims.
* * * * *