Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2000-03-02
2002-01-15
Preisch, Nadine (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C208S118000, C208S120010, C208S122000, C208S071000, C208S067000, C585S648000, C585S651000, C585S654000, C585S324000, C585S329000
Reexamination Certificate
active
06339180
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 use 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 feed for polyolefins production, particularly polypropylene production.
Fixed bed processes for light paraffin dehydrogenation have recently attracted renewed interest for increasing olefins production. However, these types of processes typically require relatively large capital investments as well as high operating costs. It is therefore advantageous to increase olefins yield using processes, which require relatively small capital investment. It would be particularly advantageous to increase olefins yield in catalytic cracking processes.
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 olefins production, olefins 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 olefins production in a process that allows a high degree of control over the selectivity to C
2
-C
4
olefins that are processed and polymerized to form products such as polypropylene and polyethylene.
SUMMARY OF THE INVENTION
An embodiment of the present invention comprises a process for producing polypropylene comprising the steps of (a) contacting a naphtha feed containing between about 10 and about 30 wt. % paraffins and between about 15 and about 70 wt. % olefins with a catalyst to form a cracked product, the catalyst comprising about 10 to about 50 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nm, the reaction conditions including a temperature from about 500° C. to 650° C., a hydrocarbon partial pressure of 10 to 40 psia (70-280 kPa), a hydrocarbon residence time of 1 to 10 seconds, and a catalyst to feed ratio, by weight, of about 4 to 10, wherein no more than about 20 wt. % of paraffins are converted to olefins and wherein propylene comprises at least 90 mol. % of the total C
3
products; and, (b) separating the propylene from the cracked product and polymerizing the propylene to form polypropylene.
In another preferred embodiment of the present invention the catalyst is a ZSM-5 type catalyst.
In still another preferred embodiment of the present invention the feed contains about 10 to 30 wt. % paraffins, and from about 20 to 70 wt. % olefins.
In yet another preferred 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
Suitable hydrocarbons feeds for producing the relatively high C
2
, C
3
, and C
4
olefins yields are those streams boiling in the naphtha range and 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. (18-225° C.), preferably from about 65° F. to about 300° F. (18-150° C.).
The naphtha feed can be a thermally-cracked or catalytically-cracked naphtha derived from any appropriate source, including fluid catalytic cracking (FCC) of gas oils and resids or delayed- or fluid-coking of resids. Preferably, the naphtha streams used in the present invention derive from the fluid catalytic cracking of gas oils and resids because the product naphthas are typically rich in olefins and/or diolefins and relatively lean in paraffins.
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 650° C., preferably from about 525° C. to 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 is passed through the stripping zone where volatiles are stripped from the catalyst particles with steam. 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 between about 650° C. and about 750° C. The hot 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 and a C
4
fraction.
While attempts have been made to increase light olefins yields in the FCC process unit itself, the practice of 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. Catalysts suitable for use in the practice of the present invention are those which are comprising 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 incorporate silica-to-alumina ratios 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 medium-pore-size zeolites generally have a pore size from about 0.5 nm, to about 0.7 nm 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
Asplin John E.
Henry B. Erik
Ladwig Paul K.
Stuntz Gordon F.
Wachter William A.
Crowell Michael A.
ExxonMobil Chemical Patents Inc.
Hughes Gerrard J.
Preisch Nadine
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