Naphtha cracking and hydroprocessing process for low...

Details Naphtha cracking and hydroprocessing process for low... Naphtha cracking and hydroprocessing process for low...

1999-11-10

2001-11-13

Preisch, Nadine (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Plural serial stages of chemical conversion

C208S135000, C208S120010, C208S072000, C208S068000, C208S097000, C208S217000, C208S213000, C208S215000, C208S21600R, C585S648000, C585S649000, C585S650000, C585S651000, C585S653000

Reexamination Certificate

active

06315890

ABSTRACT:

BACKGROUND OF THE DISCLOSURE
1. Field of the Invention
The present invention relates to a process for hydroprocessing a catalytically cracked or thermally cracked naphtha stream. More particularly, the invention relates to a process for cracking an olefinic naphtha using a zeolite catalyst to form a cracked product having a diminished total olefin concentration, and then hydroprocessing at least a portion of the cracked product in a manner that reduces the sulfur concentration while substantially retaining the olefin content in order to maintain octane.
2. Background of the Invention
The need for low emissions, high octane fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of C
2
to C
4
olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production. In parallel with this need, increasingly stringent regulations require motor fuels having a diminished concentration of sulfur and, to a lesser extent, olefins boiling in the gasoline boiling range (C
4
and above).
It is well known that conventional fluid catalytic cracking (“FCC”) processes can be adapted to increase product C
2
to C
4
olefin concentration. Some of the adaptations include dual risers, combinations of cracking and metathesis, and the use of zeolite catalysts. Hydroprocessing cracked naphtha formed in such processes typically results in a product having a diminished concentration of olefinic species and non-hydrocarbyl species such as sulfur-containing species, and an augmented concentration of saturated species. Relatively severe hydroprocessing conditions are generally required to substantially remove sulfur-containing species, particularly in the presence of olefinic species having more than four carbon atoms, and such severe hydroprocessing conditions are known to result in a substantial octane reduction in the hydroprocessed product.
There remains a need, therefore, for new processes for forming C
2
to C
4
olefins together with naphtha having a diminished concentration of sulfur-containing species, while maintaining a sufficient amount of C
4
and larger olefins in the naphtha, preferably C
5
and C
6
olefins, to provide a relatively high octane.
SUMMARY OF THE INVENTION
In one embodiment, the invention is a process for forming a hydroprocessed product comprising:
(a) reacting an olefinic naphtha in the presence of a molecular sieve catalyst under catalytic cracking conditions in order to form a product, and then
(b reacting at least a portion of the product under hydroprocessing conditions in the presence of a hydroprocessing catalyst in order to form the hydroprocessed product.
In another embodiment, the invention is a hydroprocessed product formed according to such a process.
In a preferred embodiment, the olefinic naphtha is reacted in a process unit comprised of a reaction zone, a stripping zone, and a catalyst regeneration zone. In the reaction zone, the naphtha stream is contacted under catalytic conversion conditions a catalytically effective amount of molecular sieve catalyst having an average pore diameter of less than about 0.7 nm, preferably zeolite, and more preferably ZSM-5 catalyst, that is preferably in the form of a fluidized bed. The reaction zone is operated at a temperature from about 500° to 650° C., a hydrocarbon partial pressure of 10 to 40 psia, a hydrocarbon residence time of 1 to 10 seconds, and a catalyst to feed weight ratio of about 2 to 10. Preferably, less than about 20 wt. % of paraffins are converted to olefins.
Preferably, at least a portion of the product from the catalytic cracking unit is conducted to a hydroprocessing unit. Preferably, the hydroprocessing reactor is operated at a temperature from about 250° C. to about 375° C., a hydrogen partial pressure of 50 to 500 psig, and a liquid hourly space velocity of 2-10. The hydrogen treat rate is about 500 to 3000 scf/bbl and the preferred hydroprocessing catalyst is comprised of an alumina support with Co and Mo added to it.
Preferably, the olefinic naphtha feedstock contains about 10 to 30 wt. % paraffins, and from about 20 to 70 wt. % olefins.
DETAILED DESCRIPTION OF THE INVENTION
The invention is based on the discovery that catalytically cracking an olefinic naphtha under appropriate conditions results in the formation of light (i.e., C
2-C
4
) olefins and a cracked naphtha. The invention is also based on the discovery that a portion of such a cracked naphtha may be separated and then hydrotreated under appropriate conditions to yield a product having a diminished sulfur concentration while maintaining or at least not substantially reducing its octane rating. Moreover, it has been discovered that cracking an olefinic naphtha under appropriate conditions results in an overall reduction in olefinic species' concentration and an increased concentration of desirable light (i.e., C
2
to C
4
) olefins in the cracked product. While not wishing to be bound by any theory, it is believed that diminishing the overall olefin concentration permits hydroprocessing of the cracked naphtha fraction under more selective conditions.
Accordingly, the invention is related to a two step process wherein the first step comprises cracking an olefinic naphtha resulting in a cracked product having a diminished total concentration of olefinic species. When the olefinic naphtha feed is obtained from processes such as catalytic cracking, steam cracking, or coking, then the first step may be referred to as re-cracking. The second step comprises hydroprocessing at least a portion of the cracked product to provide a hydroprocessed cracked product having a reduced concentration of contaminants such as non-hydrocarbyl species but without a substantial octane reduction.
Naphtha feeds include olefinic naphthas having hydrocarbyl species boiling in the naphtha range. More specifically, the olefinic naphthas contain 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., preferably from about 65° F. to about 300° F., and more preferably from 65° F. to about 1 50F. The naphtha may be a thermally cracked or a catalytically cracked naphtha. Such naphthas may be derived from any appropriate source, for example, they can be derived from the fluid catalytic cracking (FCC) of gas oils and resids or from delayed or fluid coking of resids. Preferably, the naphtha streams are derived from the fluid catalytic cracking of gas oils and resids. Such naphthas are typically rich in olefins, diolefins, and mixtures thereof, and relatively lean in paraffins.
In one embodiment, the cracking process of the present invention may be performed in one or more process units comprised of a reaction zone, a stripping zone, a catalyst regeneration zone, and a fractionation zone. The naphtha feedstream is conducted 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 500° C. to 600° C. The cracking reaction deposits carbonaceous hydrocarbons, or coke, on the catalyst, thereby deactivating the catalyst. The cracked products may be separated from the coked catalyst and a portion of the cracked products may be conducted 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 in order to retain 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 contain

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