Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PRODUCING POLYPROPYLENE
FROM C_z OLEFINS SELECTIVELY PRODUCED
IN A FLUID CATALYTIC CRACKING PROCESS
FIELD OF THE INVENTION
The present invention relates to a process for producing polypropylene
from C3 olefins selectively produced from a catalytically cracked or thermally
cracked naphtha stream.
BACKGROUND OF THE INVENTION
The need for low emissions fuels has created an increased demand for
light olefins used in alkylation, oligomerization, MTBE and ETBE synthesis
processes. In addition, a low-cost supply of light olefins, particularly
propylene,
continues to be in demand to serve as feedstock for polyolefin, particularly
polypropylene.
Fixed bed processes for light paraffin dehydrogenation have recently
attracted renewed interest for increasing olefin production. However, these
types of processes typically require relatively large capital investments and
high
operating costs. It is therefore advantageous to increase olefin yield using
processes, which require relatively small capital investment. It would be
particularly advantageous to increase olefin yield in catalytic cracking
processes
so that the olefins could be further processed into polymers such as
polypropylene.
A problem inherent in producing olefins products using FCC units is that
the process depends on a specific catalyst balance to maximize production of
light olefins while also achieving high conversion of the 650°F +
0340°C) feed
components. In addition, even if a specific catalyst balance can be maintained
to
maximize overall olefin production, olefin selectivity is generally low
because of
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undesirable side reactions, such as extensive cracking, isomerization,
aromatization and hydrogen transfer reactions. Light saturated gases produced
from undesirable side reactions result in increased costs to recover the
desirable
light olefins. Therefore, it is desirable to maximize olefin production in a
process that allows a high degree of control over the selectivity of C3 and C4
olefins.
SUMMARY OF THE INVENTION
One embodiment of the present invention is a process for producing
polypropylene comprising the steps of (a) feeding a naphtha stream comprising
less than about 40 wt.% paraffins and between about 15 to 70 wt.% olefins to a
process unit comprising a reaction zone, a stripping zone, a catalyst
regeneration
zone, and a fractionation zone; (b) contacting the naphtha stream with a
fluidized
bed of catalyst in the reaction zone to form a cracked product, the catalyst
comprising a zeolite having an average pore diameter of less than about 0.7 nm
and wherein the reaction zone is operated at a temperature from about
500° to
650°C, a hydrocarbon partial pressure of 10 to 40 psia (about 70- about
280
kPa), a hydrocarbon residence time of 1 to 10 seconds, and a catalyst to feed
weight ratio between about 4 and about 10, thereby producing a reaction
product
wherein no more than about 20 wt. % of paraffins are converted to olefins and
wherein propylene comprises at least about 90 mol.% of the total C3 products;
(c) passing the catalyst through said stripping zone; (d) passing the stripped
catalyst from the stripping zone to the catalyst regeneration zone where the
catalyst is regenerated in the presence of an oxygen-containing gas; (e)
recycling
the regenerated catalyst to the reaction zone; (f) fractionating the cracked
product to produce a C3 fraction, a C4 fraction rich in olefins, and
optionally a CS
fraction rich in olefins; (g) passing at least a portion of the C4 fraction to
the
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reaction zone or the stripping zone, or both; and, (h) separating propylene
from
the C3 fraction and polymerizing the propylene to form polypropylene.
In another embodiment of the present invention the catalyst is a ZSM-5
type catalyst.
In an embodiment of the present invention a CS fraction rich in olefins is
also recycled.
In another embodiment of the present invention the feedstock contains
about 5 to 35 wt. % paraffins, and from about 20 to 70 wt. % olefins.
In another embodiment of the present invention the reaction zone is
operated at a temperature from about 525°C to about 600°C.
DETAILED DESCRIPTION OF THE INVENTION
Feedstreams that are suitable for producing the relatively high CZ, C3, and
C4 olefin yields are those streams boiling in the naphtha range containing
less
than about 40 wt.%, preferably from about S wt. % to about 35 wt. %, more
preferably from about 10 wt. % to about 30 wt. %, and most preferably from
about 10 to 25 wt. % paraffins, and from about 15 wt. %, preferably from about
wt. % to about 70 wt. % olefins. The feed may also contain naphthenes and
aromatics. Naphtha boiling range streams are typically those having a boiling
range from about 65°F to about 430°F (about 18°C to about
225°C), preferably
20 from about 65°F to about 300°F (about 18°C to about
150°C).
The naphtha can be a thermally cracked or a catalytically-cracked
naphtha. The naphtha streams can be derived from the fluid catalytic cracking
(FCC) of gas oils and resids, or they can be derived from delayed or fluid
coking
of resids. Preferably, the naphtha streams used in the practice of the present
invention derive from the fluid catalytic cracking of gas oils and resids. FCC
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naphthas are typically rich in olefins and/or diolefins and relatively lean in
paraffins. It is within the scope of the instant invention to feed or co-feed
other
olefinic streams that are not catalytically- or thermally-cracked naphthas,
such as
an MTBE raffinate, into said reaction zone with the primary feed. It is
believed
that this will increase the yield of propylene.
The process of the present invention is performed in a process unit
comprising a reaction zone, a stripping zone, a catalyst regeneration zone,
and a
fractionation zone. The naphtha feed is fed into the reaction zone where it
contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and
cracks the feed at a temperature from about 500°C to about
650°C, preferably
from about 525°C to about 600°C. The cracking reaction deposits
coke on the
catalyst, thereby deactivating the catalyst. The cracked products are
separated
from the coked catalyst and sent to a fractionator. The coked catalyst passes
through the stripping zone where a stripping medium, such as steam, strips
volatiles from the catalyst particles. The stripping can be preformed under
low-
severity conditions to retain a greater fraction of adsorbed hydrocarbons for
heat
balance. The stripped catalyst is then passed to the regeneration zone where
it is
regenerated by burning coke on the catalyst in the presence of an oxygen
containing gas, preferably air. Decoking restores catalyst activity and
simultaneously heats the catalyst to a temperature from about 650°C to
about
750°C. The hot regenerated catalyst is then recycled to the reaction
zone to react
with fresh naphtha feed. Flue gas formed by burning coke in the regenerator
may be treated for removal of particulates and for conversion of carbon
monoxide. The cracked products from the reaction zone are sent to a
fractionation zone where various products are recovered, particularly a C3
fraction, a C4 fraction, and optionally a CS fraction. The C4 fraction and the
CS
fraction will typically be rich in olefins. At least a portion of one or both
of
these fractions can be recycled to the reactor. They can be recycled to either
the
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main section of the reactor, or a riser section, or a stripping section. It is
preferred that they be recycled to the upper part of the stripping section, or
stripping zone. Recycling at least a portion of one or both of these fractions
will
convert at least a portion of these olefins to propylene.
While attempts have been made to increase light olefins yields in the FCC
process unit itself, the present invention uses its own distinct process unit,
as
previously described, which receives naphtha from a suitable source in the
refinery. The reaction zone is operated at process conditions that will
maximize
C2 to C4 olefins (particularly propylene) selectivity with relatively high
conversion of CS+ olefins. Suitable catalysts used with the present invention
contain a crystalline zeolite having an average pore diameter less than about
0.7
nanometers (nm), said crystalline zeolite comprising from about 10 wt. % to
about 50 wt. % of the total fluidized catalyst composition. It is preferred
that the
crystalline zeolite be selected from the family of medium-pore size (< 0.7 nm)
crystalline aluminosilicates, otherwise referred to as zeolites. Of particular
interest are the medium-pore zeolites with a silica to alumina molar ratio of
less
than about 75:1, preferably less than about 50:1, and more preferably less
than
about 40:1, although some embodiments may incorporate a silica to alumina
ratio greater than 40:1. The pore diameter, also referred to as effective pore
diameter, is measured using standard adsorption techniques and
hydrocarbonaceous compounds of known minimum kinetic diameters. See
Breck, Zeolite Molecular Sieves, 1974 and Anderson et al., J. Catalysis 58,
114
( 1979), both of which are incorporated herein by reference.
Medium-pore size zeolites that can be used in the practice of the
present invention are described in "Atlas of Zeolite Structure Types",
eds. W. H. Meier and D. H. Olson, Butterworth-Heineman, Third
Edition, 1992, which is hereby incorporated by reference. The
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medium-pore size zeolites generally have a pore size from about 5~r, to
about 7A and include for example, MFI, MFS, MEL, MTW, EUO, MTT,
HEU, FER, and TON structure type zeolites (IUPAC Commission of
Zeolite Nomenclature). Non-limiting examples of such medium-pore
size zeolites, include ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-34, ZSM-
35, ZSM-38, ZSM-48, ZSM-50, silicalite, and silicalite 2. The most
preferred is ZSM-5, which is described in U.S. Patent Nos. 3,702,886 and
3,770,614. ZSM-11 is described in U.S. Patent No. 3,709,979; ZSM-12 in U.S.
Patent No. 3,832,449; ZSM-21 and ZSM-38 in U.S. Patent No. 3,948,758; ZSM-
23 in U.S. Patent No. 4,076,842; and ZSM-35 in U.S. Patent No. 4,016,245. All
of the above patents are incorporated herein by reference. Other suitable
medium-pore size zeolites include the silicoaluminophosphates (SAPO), such as
SAPO-4 and SAPO-11 which is described in U.S. Patent No. 4,440,871;
chromosilicates; gallium silicates; iron silicates; aluminum phosphates
(ALPO),
such as ALPO-11 described in U.S. Patent No. 4,310,440; titanium
aluminosilicates (TASO), such as TASO-45 described in EP-A No. 229,295;
boron silicates, described in U.S. Patent No. 4,254,297; titanium
aluminophosphates (TAPO), such as TAPO-11 described in U.S. Patent No.
4,500,651; and iron aluminosilicates.
The medium-pore-size zeolites can include "crystalline admixtures"
which are thought to be the result of faults occurring within the crystal or
crystalline area during the synthesis of the zeolites. Examples of crystalline
admixtures of ZSM-5 and ZSM-11 are disclosed in U.S. Patent No. 4,229,424,
which is incorporated herein by reference. The crystalline admixtures are
themselves medium-pore-size zeolites and are not to be confused with physical
admixtures of zeolites in which distinct crystals of crystallites of different
zeolites are physically present in the same catalyst composite or hydrothermal
reaction mixtures.
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The catalysts of the present invention may be held together with an
inorganic oxide matrix material component. The inorganic oxide matrix
component binds the catalyst components together so that the catalyst product
is
hard enough to survive interparticle and reactor wall collisions. The
inorganic
oxide matrix can be made from an inorganic oxide sol or gel which is dried to
"bind" the catalyst components together. Preferably, the inorganic oxide
matrix is
not catalytically active and will be comprised of oxides of silicon and
aluminum. It
is also preferred that separate alumina phases be incorporated into the
inorganic
oxide matrix. Species of aluminum oxyhydroxides-g-alumina, boehmite, diaspore,
and transitional aluminas such as a-alumina, b-alumina, g-alumina, d-alumina,
e-
alumina, k-alumina, and r-alumina can be employed. Preferably, the alumina
species is an aluminum trihydroxide such as gibbsite, bayerite, nordstrandite,
or
doyelite. The matrix material may also contain phosphorous or aluminum
phosphate.
Process conditions include temperatures from about 500°C to about
650°C, preferably from about 500°C to 600°C; hydrocarbon
partial pressures
from about 10 to 40 psia (about 70-about 280 kPa) to about, preferably from
about 20 to 35 psia (about 140- about 245 kPa); and a catalyst to naphtha
(wt/wt)
ratio from about 3 to 12, preferably from about 4 to 10, where catalyst weight
is
total weight of the catalyst composite. Steam may be concurrently introduced
with the naphtha stream into the reaction zone, with the steam comprising up
to
about 50 wt. % of the naphtha feed. Preferably, the naphtha residence time in
the reaction zone is less than about 10 seconds, for example from about 1 to
10
seconds. The reaction conditions will be such that at least about 60 wt. % of
the
C5+ olefins in the naphtha stream are converted to C4- products and less than
about 25 wt. %, preferably less than about 20 wt. % of the paraffins are
converted to C4- products, and that propylene comprises at least about 90
mol.%,
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preferably greater than about 95 mol % of the total C3 reaction products with
the
weight ratio of propylene/total C2- products greater than about 3.5.
Preferably, ethylene comprises at least about 90 mol.% of the C2
products, with the weight ratio of propylene:ethylene being greater than about
4,
and that the "full range" CS+ naphtha product is enhanced in both motor and
research octanes relative to the naphtha feed. It is within the scope of this
invention to pre-coke the catalysts before introducing the feed to further
improve
the selectivity to propylene. It is also within the scope of this invention to
feed
an effective amount of single-ring aromatics to the reaction zone to also
improve
the selectivity of propylene versus ethylene. The aromatics may be from an
external source such as a reforming process unit or they may consist of heavy
naphtha recycle product from the instant process.
The following examples are presented for illustrative purposes only and
are not to be taken as limiting the present invention in any way.
Examples 1-12
The following examples illustrate the criticality of process operating
conditions for
maintaining chemical grade propylene purity with samples of cat naphtha
cracked over
ZCAT-40 (a catalyst that contains ZSM-5) which had been steamed at
1500°F
0815°C) for 16 hrs to simulate commercial equilibrium. Comparison of
Examples 1
and 2 show that increasing Cat/Oil ratio improves propylene yield, but
sacrifices
propylene purity. Comparison of Examples 3 and 4 and 5 and 6 shows reducing
oil
partial pressure greatly improves propylene purity without compromising
propylene
yield. Comparison of Examples 7 and 8 and 9 and 10 shows increasing
temperature
improves both propylene yield and purity. Comparison of Examples 11 and 12
shows
decreasing cat residence time improves propylene yield and purity. Example 13
shows
an example where both high propylene yield and purity are obtained at a
reactor
temperature and cat/oil ratio that can be achieved using a conventional FCC
reactor/regenerator design for the second stage.
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TABLE 1
Feed Temp. Oil Cat Wt.% Wt.% Propylene
Res. Res.
ExampleOlefins.~ at/ Oil Time. Time, C~ ~' Puri
wt% i1 Asia sec sec
1 38.6 566 4.2 36 0.5 4.3 11.4 0.5 95.8%
2 38.6 569 8.4 32 0.6 4.7 12.8 0.8 94.1%
3 22.2 S 10 8.8 18 1.2 8.6 8.2 1.1 88.2%
4 22.2 511 9.3 38 1.2 5.6 6.3 1.9 76.8%
38.6 632 16.6 20 1.7 9.8 16.7 1.0 94.4%
6 38.6 630 16.6 13 1.3 7.5 16.8 0.6 96.6%
7 22.2 571 5.3 27 0.4 0.3 6.0 0.2 96.8%
8 22.2 586 5.1 27 0.3 0.3 7.3 0.2 97.3%
9 22.2 511 9.3 38 1.2 5.6 6.3 1.9 76.8%
22.2 607 9.2 37 1.2 6.0 10.4 2.2 82.5%
11 22.2 576 18.0 32 1.0 9.0 9.6 4.0 70.6%
12 22.2 574 18.3 32 1.0 2.4 10.1 1.9 84.2%
13 38.6 606 8.5 22 1.0 7.4 15.0 0.7 95.5%
5 Table 1 Continued
Ratio of C3- Ratio
of C3
Example tW .%C~-Wt.%C~' ~- tg~- Wt
1 2.35 2.73 4.9 4.2 11.4
2 3.02 3.58 4.2 3.6 12.8
3 2.32 2.53 3.5 3.2 8.2
4 2.16 2.46 2.9 2.6 6.3
5 6.97 9.95 2.4 1.7 16.7
6 6.21 8.71 2.7 1.9 16.8
7 1.03 1.64 5.8 3.7 6.0
8 1.48 2.02 4.9 3.6 7.3
9 2.16 2.46 2.9 2.6 6.3
10 5.21 6.74 2.0 1.5 10.4
11 4.99 6.67 1.9 1.4 9.6
12 4.43 6.27 2.3 1.6 10.1
13 4.45 5.76 3.3 2.6 15.0
C2 = CH4 + CZH4 + CZH6
The above examples (1,2,7 and 8) show that C3-/C2 > 4 and C3-/C2- > 3.5
10 can be achieved by selection of suitable reactor conditions.
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Examples 14 - 17
The cracking of olefins and paraffins contained in naphtha streams (e.g.
FCC naphtha, coker naphtha) over small or medium-pore zeolites such as ZSM-
S can produce significant amounts of ethylene and propylene. The selectivity
to
ethylene or propylene and selectivity of propylene to propane varies as a
function of catalyst and process operating conditions. It has been found that
propylene yield can be increased by co-feeding steam along with cat naphtha to
the reactor. The catalyst may be ZSM-5 or other small or medium-pore zeolites.
Table 2 below illustrates the increase in propylene yield when 5 wt. % steam
is
co-fed with an FCC naphtha containing 38.8 wt. % olefins. Although propylene
yield increased, the propylene purity is diminished. Thus, other operating
conditions may need to be adjusted to maintain the targeted propylene
selectivity.
TABLE 2
Steam Temp. Oil Cat Wt% Wt% Propylene
Res. Res.
Example Co-feedC Cat/Oil Time, Time, ProRylenePropanePurity,
Oil psia sec sec
14 No 630 8.7 18 0.8 8.0 11.7 0.3 97.5%
1 S Yes 631 8.8 22 1.2 6.0 13.9 0.6 95.9%
16 No 631 8.7 18 0.8 7. 13.6 0.4 97.1
8
17 Yes 632 8.4 22 1.1 6.1 14.6 0.8 94.8%
Examples 18 - 21
ZCAT-40 was used to crack cat cracker naphtha as described for the
above examples. The coked catalyst was then used to crack a C4 stream
composed of 6 wt.% n-butane, 9 wt.% i-butane, 47 wt.% 1-butene, and 38 wt.%
i-butene in a reactor at the temperatures and space velocities indicated in
the
table below. As can be seen from the results in the table below, a significant
fraction of the feed stream was converted to propylene.
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TABLE 3
WHSV, Hr-1 35 18 12 6
Temperature C 575 575 575 575
Butylene Conversion wt.
Product Yields. wt.%
Ethylene 2.4 4.7 5.9 8.8
Propylene 20.5 27.1 28.8 27.4
Butylenes 39.7 29.0 25.5 19.2
C,-C4 Light Saturates 18.2 19.2 19.8 22.0
CS+ Products 19.3 20.0 20.0 22.6
Light olefins resulting from the preferred process may be used as feeds
for processes such as oligimerization, polymerization, co-polymerization, ter-
polymerization, and related processes (hereinafter "polymerization") to form
macromolecules. Such light olefins may be polymerized both alone and in
combination with other species, in accordance with polymerization methods
known in the art. In some cases it may be desirable to separate, concentrate,
purify, upgrade, or otherwise process the light olefins prior to
polymerization.
Propylene and ethylene are preferred polymerization feeds. Polypropylene and
polyethylene are preferred polymerization products made therefrom.
