Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By c content reduction – e.g. – cracking – etc.
Reexamination Certificate
2000-05-19
2001-11-06
Preisch, Nadine (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By c content reduction, e.g., cracking, etc.
C585S649000, C585S650000, C585S651000, C585S653000, C208S135000, C208S120010, C208S072000
Reexamination Certificate
active
06313366
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing C
3
olefins 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 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 propylene 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 and aromatics 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° 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.
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.
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. Applicants have found that selectivity to propylene versus propane and propylene versus ethylene can be increased by reducing olefin partial pressures. At low olefin partial pressures, secondary reactions to generate aromatics and disproportionation reactions to other olefins are minimized. The addition of a separate aromatic stream also minimizes hydrogen transfer reactions that convert propylene to propane. To improve selectivity to propylene, an additional stream of aromatics are added to the feedstock to reduce the olefin partial pressure and to retard aromatization of olefins to aromatics, thereby improving selectivity to propylene. The additional stream of aromatics preferably comprises single-ring aromatics in an amount greater than about 50 wt. %, more preferably greater than about 75 wt. %. As used herein, single-ring aromatics includes single-ring aromatic species that may or may not have one or more substituents or functional groups.
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 le
Asplin John E.
Carpency Jay F.
Chen Tan-Jen
Fung Shun C.
Henry Brian Erik
ExxonMobile Chemical Patents Inc.
Preisch Nadine
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