Mineral oils: processes and products – Chemical conversion of hydrocarbons – Plural serial stages of chemical conversion
Reexamination Certificate
2001-03-16
2003-05-27
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Plural serial stages of chemical conversion
C208S074000, C208S072000
Reexamination Certificate
active
06569315
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for converting cycle oils produced in catalytic cracking reactions into olefinic naphthas. More particularly, the invention relates to an out-board process for converting a catalytically cracked cycle oil such as heavy cycle oil (“HCO” or “HCCO”), light cycle oil (“LCO” or “LCCO”), and mixtures thereof into olefins and naphthas using a zeolite catalyst.
BACKGROUND OF THE INVENTION
Cycle oils such as LCCO produced in fluidized catalytic cracking (“FCC”) reactions contain two-ring aromatic species such as naphthalene. The need for blendstocks for forming low emissions fuels has created an increased demand for FCC products that contain a diminished concentration of multi-ring aromatics. There is also an increased demand for FCC products containing light olefins that may be separated for use in alkylation, oligomerization, polymerization, and MTBE and ETBE synthesis processes. There is a particular need for low emissions, high octane FCC products having an increased concentration of C
2
to C
4
olefins and a reduced concentration of multi-ring aromatics and olefins of higher molecular weight.
Hydrotreating a cycle oil and re-cracking hydrotreated cycle oil results in conversion of the cycle oil to a motor gasoline blend-stock. The hydrotreated cycle oil may be recycled to the FCC unit from which it was derived, or it may be may be re-cracked in an additional catalytic cracking unit.
Hydrotreating cycle oil such as LCCO partially saturates bicyclic hydrocarbons such as naphthalene to produce tetralin. Hydrotreatment and subsequent LCCO re-cracking may occur in the primary reactor vessel. Hydrotreated LCCO may also be injected into the FCC feed riser at a point downstream of feed injection to provide for feed quenching. Unfortunately, such re-cracking of hydrotreated LCCO results in undesirable hydrogen transfer reactions that convert species such as tetralin into aromatics such as naphthalene.
There remains a need, therefore, for new processes for forming naphthenic blendstocks from hydrotreated cycle oils such as LCCO.
SUMMARY OF THE INVENTION
In one embodiment, the invention is a method for catalytically cracking a cycle oil comprising:
(a) catalytically cracking a primary feed in a first FCC riser reactor under catalytic cracking conditions in the presence of a first catalytic cracking catalyst in order to form a cracked product;
(b) separating at least a cycle oil from the cracked product;
(c) injecting the cycle oil into a second FCC riser reactor; and
(d) cracking the cycle oil under cycle oil catalytic cracking conditions in the presence of a second catalytic cracking catalyst.
DETAILED DESCRIPTION OF THE INVENTION
The invention is based on the discovery that re-cracking a cycle oil such as LCCO in a second riser reactor results in beneficial conversion of the cycle oil into naphtha and light olefins such as propylene. It is believed that injecting the cycle oil into the second reactor suppresses undesirable hydrogen transfer reactions that would otherwise occur if the cycle oil were re-cracked in the first FCC reactor. Re-cracking in a second reactor under cycle oil cracking conditions (i.e. conditions that exclude gas oils and residual oils from the reaction zone) substantially eliminates hydrogen transfer reactions between hydrogen donors present in the cycle oil and hydrogen acceptors present in the gas oil or residual oil. Preferably, the cycle oil is hydroprocessed before re-cracking because hydrogen transfer reactions are further suppressed when the cyclic and multi-cyclic species present in the cycle oil are at least partially saturated. More preferably, the cycle oil is hydroprocessed to form a significant amount of decahydronaphthalene in the hydroprocessed product because the related naphthalene and tetrahydronaphthalene species are not as readily re-cracked into light olefins. Optionally, the hydrotreated LCCO is combined with naphtha before re-cracking. The naphtha may be, for example, one or more of a thermally or catalytically cracked naphtha obtained from an FCC unit, coker, or steam cracker.
Preferred hydrocarbonaceous feeds (i.e. the primary feed) for the catalytic cracking process described herein include naphtha, hydrocarbonaceous oils boiling in the range of about 430° F. (220° C.) to about 1050° F. (565° C.), such as gas oil; heavy hydrocarbonaceous oils comprising materials boiling above 1050° F. (565° C.); heavy and reduced petroleum crude oil; petroleum atmospheric distillation bottoms; petroleum vacuum distillation bottoms; pitch, asphalt, bitumen, other heavy hydrocarbon residues; tar sand oils; shale oil; liquid products derived from coal and natural gas, and mixtures thereof.
Cycle oil formation, in accord with this invention, may occur in one or more conventional FCC process units under conventional FCC conditions in the presence of conventional FCC catalyst. Each unit comprises a riser reactor having a reaction zone, a stripping zone, a catalyst regeneration zone, and at least one fractionation zone. The primary feed is conducted to the riser reactor where it is injected into the reaction zone wherein the primary feed contacts a flowing 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 500° C. to about 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 fractionator separates at least a cycle oil fraction, preferably an LCCO fraction, from the cracked products.
The coked catalyst flows through the stripping zone where volatiles are stripped from the catalyst particles with a stripping material such as steam. The stripping may be preformed under low severity conditions in order to retain adsorbed hydrocarbons for heat balance. The stripped catalyst is then conducted 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, e.g., 650° C. to 750° C. The hot catalyst is then recycled to the riser reactor at a point near or just upstream of the second reaction zone. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
The primary feed may be cracked under conventional FCC conditions in the presence of a first fluidized catalytic cracking catalyst. Preferred process conditions in the reaction zone include temperatures from about 500° C. to about 650° C., preferably from about 525° C. to 600° C.; hydrocarbon partial pressures from about 10 to 40 psia, preferably from about 20 to 35 psia; and a catalyst to primary feed (wt/wt) ratio from about 3 to 12, preferably from about 4 to 10; where catalyst weight is total weight of the catalyst composite. Though not required, it is also preferred that steam be concurrently introduced with the primary feed into the reaction zone, with the steam comprising up to about 10 wt. %, and preferably ranging from about 2 wt. % to about 3 wt. % of the primary feed. Also, it is preferred that the primary feed's residence time in the reaction zone be less than about 10 seconds, for example from about 1 to 10 seconds.
Any conventional FCC catalyst may be used for primary feed cracking. Such catalysts are set forth, for example, in U.S. Pat. No. 5,318,694, incorporated by reference herein.
In one embodiment, at least a cycle oil fraction is separated from the cracked products of the primary FCC unit, and at least a portion thereof is hydroprocessed. Cycle oil hydroprocessing may occur in one or more hydroprocessing reactors under hydroprocessing conditions in the presence of a hydroprocessing catalyst. Cycle oil hydroprocessing is
Stuntz Gordon F.
Swan, III George A.
Winter William E.
ExxonMobil Research and Engineering Company
Griffin Walter D.
Hughes Gerard J.
Singleton Wilson Erika
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