Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2000-03-02
2002-05-14
Preisch, Nadine (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S648000, C585S651000, C585S654000, C585S324000, C585S329000, C208S118000, C208S120010, C208S122000, C208S071000, C208S067000
Reexamination Certificate
active
06388152
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing polypropylene from C
3
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.+(~340° 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 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 C
3
and C
4
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 from about 10 to 30 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 C
3
fraction, a C
4
fraction rich in olefins, and optionally a C
5
fraction rich in olefins; (g) passing the C
4
fraction to the reaction zone or the stripping zone, or both; and, (h) separating propylene from the C
3
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 C
5
fraction rich in olefins is also recycled.
In another embodiment of the present invention the feedstock contains about 10 to 30 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 C
2
, C
3
, and C
4
olefin yields are those streams boiling in the naphtha range containing from about 5 wt. % to about 35 wt. %, preferably from about 10 wt. % to about 30 wt. %, and more preferably from about 10 to 25 wt. % paraffins, and from about 15 wt. %, preferably from about 20 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 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 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 C
3
fraction, a C
4
fraction, and optionally a C
5
fraction. The C
4
fraction and the C
5
fraction will typically be rich in olefins. One or both of these fractions can be recycled to the reactor. They can be recycled to either the 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 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 C
2
to C
4
olefins (particularly propylene) selectivity with relatively high conversion of C
5
+ 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 comprisi
Asplin John E.
Chen Tan-Jen
Ladwig Paul K.
Stuntz Gordon F.
Cromwell Michael A.
ExxonMobil Chemical Patents Inc.
Hughes Gerrard J.
Preisch Nadine
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