Claims
- 1. In a fluidized catalytic cracking process wherein a fluidizable catalyst cracking catalyst and a hydrocarbon feed are charged to a reactor riser at catalytic riser cracking conditions to form catalytically cracked vapor product and spent catalyst which are discharged into a reactor vessel having a volume via a riser reactor outlet equipped with a separation means to produce a catalyst lean phase comprising a majority of said cracked product, and a catalyst rich phase comprising a majority of said spent catalyst, said catalyst rich phase is discharged into a dense bed of catalyst maintained below said riser outlet and said catalyst lean phase is discharged into said vessel for a time, and at a temperature, which cause unselective thermal cracking of the cracked product in the reactor volume before product is withdrawn from said vessel via a vessel outlet, the improvement comprising addition, after riser cracking is completed, and after separation of cracked products from catalyst, of a quenching stream into said vessel above said dense bed of catalyst, via a quench stream addition point which allows the quench stream to contact at least a majority of the volume of the vessel above said dense bed.
- 2. Process of claim 1 wherein said quenching stream is selected from the group consisting of water, steam and hydrocarbons.
- 3. Process of claim 1 wherein said quenching stream is steam.
- 4. Process of claim 3 wherein the amount of steam quench added to said vessel, expressed as weight percent of said hydrocarbon feed, is 0.1 to 10 wt %.
- 5. Process of claim 4 wherein 0.5 to 5 wt % steam is added as quench.
- 6. Process of claim 3 wherein said separation means comprises at least one cyclone separator connected to the reactor riser outlet.
- 7. Process of claim 3 wherein said separation means comprises a downwardly discharging reactor riser outlet, positioned about 3 to 20 meters above said dense bed of catalyst.
- 8. Process of claim 7 wherein the amount and temperature of steam quench addition to said reactor are sufficient to reduce the thermal cracking in said reactor volume, as measured by equivalent reaction time at 800.degree. F., by at least 4%.
- 9. Process of claim 1 wherein said separation means comprises a primary cyclone attached to said riser outlet, and wherein a secondary cyclone is provided at an elevation above said dense bed and connective with said vessel outlet, and said quench is added to said vessel at an elevation intermediate said dense bed and said secondary cyclone.
- 10. Process of claim 8 wherein thermal cracking is reduced by at least 50%.
- 11. A fluidized catalytic cracking process wherein a stream of hot regenerated catalyst contacts a hydrocarbon feed comprising gas oils, vacuum gas oils, topped crudes, tar sands, shale oil, or asphaltic fractions in a conventional reactor riser, and is cracked in said riser to form cracked hydrocarbon vapor products of reduced molecular weight and spent catalyst; said vapor products and spent catalyst are discharged into a vessel having a volume through a cyclone separator which produces a catalyst-rich phase which is discharged into a dense bed of spent catalyst maintained in a lower portion of said reactor vessel and a vapor phase which is separately discharged into said vessel above said dense bed, steam quench is added, after riser cracking is completed, and after separation of cracked products from catalyst, to said cracked product vapor phase via a quench steam addition point which allows quenching of at least a majority of the volume in the vessel and quenched vapor products are removed from said vessel as a product of the process.
- 12. Process of claim 11 wherein the amount of steam quench added to said vessel, expressed as weight percent of said hydrocarbon feed, is 0.1 to 10 wt %.
- 13. Process of claim 11 wherein 0.5 to 5 wt % steam is added as quench.
- 14. Process of claim 11 wherein quench steam is added in an amount equal to at least 1 wt % of the hydrocarbon feed.
- 15. Process of claim 11 wherein the amount and temperature of steam quench addition to said reactor are sufficient to reduce the thermal cracking, as measured by equivalent reaction time at 800.degree. F., by at least 4%.
- 16. Process of claim 11 wherein thermal cracking is reduced by at least 50%.
- 17. A fluidized catalytic cracking process comprising a conventional reactor riser wherein hot regenerated catalyst contacts a hydrocarbon feed comprising gas oils, vacuum gas oils, topped crudes, tar sands, shale oil, or asphaltic fractions and is cracked in said riser to form catalytically cracked hydrocarbon products of reduced molecular weight and spent catalyst; said cracked products and spent catalyst are discharged into a vessel through a primary cyclone separator which produces a catalyst phase containing a minor amount of said cracked product which is discharged into a dense bed of catalyst maintained in a lower portion of said vessel and a cracked product vapor phase discharged via a primary cyclone vapor outlet and wherein a quench stream is added, after riser cracking is completed, and after separation of cracked products from catalyst, to said vessel 0.15 to 5 meters above said dense bed and wherein there is a volume in said vessel above said dense bed and the quench stream contacts at least a majority of the volume of the vessel above said dense bed and at least about 90% of said vapor phase passes from said primary cyclone vapor outlet into an inlet of a secondary cyclone within one second and said minor amount of cracked product passes from said dense bed through said vessel to said secondary cyclone inlet in a period of time in excess of one second and additional separation, of entrained spent catalyst from cracked product and said quench stream, occurs in said secondary cyclone to produce a cracked product and quenched stream vapor.
- 18. Process of claim 17 wherein steam quench is added to said vessel in an amount, expressed as weight percent of said hydrocarbon feed, of 0.1 to 10 wt %.
- 19. Process of claim 17 wherein the amount and temperature of quench stream added are sufficient to reduce the thermal cracking in the reactor vessel, as measured by equivalent reaction time at 800.degree. F., by at least 50%.
- 20. Process of claim 17 wherein said secondary cyclone inlet is substantially radially aligned with, and is over said primary cyclone vapor outlet, and wherein all of the quench stream and cracked product enter said secondary cyclone inlet with the vapor from said primary cyclone, and wherein said quenched hydrocarbon represents less than about 10% of the total amount of hydrocarbon entering said secondary cyclone.
BACKGROUND OF THE INVENTION
1. Cross-Reference to Related Application
This is a continuation of copending application Ser. No. 272,196, filed on Nov. 16, 1988, now abandoned, which is a continuation-in-part of prior copending application Ser. No. 666,533, filed on Oct. 30, 1984 now abandoned.
This invention relates to an improved fluidized catalytic cracking process, and in particular to an all riser reactor FCC unit operating with quench, preferably steam quench in the catalyst/cracked vapor disengaging space downstream of the riser reactor.
The fluidized catalytic cracking, or FCC, process is one of the work horses of modern refineries.
In somewhat oversimplified terms, hot catalyst contacts a relatively heavy oil feed, producing coked catalyst and cracked products. The coked catalyst is regenerated by burning the coke from coked catalyst in a regenerator. The catalyst is heated during the regeneration, because the coke burns. The hot regenerated catalyst is recycled to contact more heavy oil feed.
In 1940's vintage FCC units, the heavy oil feed contacted the catalyst in a relatively short transfer line which mixed the catalyst and oil together, and discharged the catalyst/oil mixture into a dense bed reactor.
Gradually refiners learned that riser cracking (with a very short residence time, typically on the order of under ten seconds) was more beneficial than dense bed cracking (with catalyst/oil residence times on the order of 10 seconds-60 seconds or more).
The desired reactions happened quickly in the catalyst riser. Some additional conversion occurred in the dense bed reactor, but a significant amount of overcracking also occurred in the dense bed and in the reactor vessel.
Modest conversions of feed to fuel oil and gasoline fractions occurred in the riser reactor. Very modest incremental conversion of feed to lighter components was obtained in the dense bed reactor, but there was also a significant amount of cracking of very valuable gasoline and fuel oil components to coke and light gases.
Accordingly refiners have attempted to maximize riser cracking, and minimize dense bed cracking. Generally this has been done by extending the catalyst riser and cutting down on the amount of catalyst inventory in the relatively large vessel into which the riser reactor discharged.
Some FCC units have attempted to practically eliminate dense bed cracking, by causing the riser reactor to discharge into a rough cut cyclone, or to discharge down toward the dense bed without agitating it, whereby substantial separation of cracked products from deactivated catalyst can be quickly obtained. Such an approach is shown in U.S. Pat. No. 3,785,962, which is incorporated by reference.
Because of the number and size of FCC units in modern refiners, with over 5 million barrels per day of FCC capacity in the USA alone, there has been tremendous incentive to improve this process even more.
A profound improvement was the shift to the use of zeolite based catalyst which resulted in a tremendous increase in catalyst activity.
Another development was the CO-afterburning regenerator which resulted in more complete combustion of coke to CO.sub.2, rather than CO, in the FCC regenerator.
U.S. Pat. No. 4,072,600, of which is incorporated by reference, disclosed adding Pt, Pd, etc. to the circulating catalyst inventory to promote afterburning of CO to CO.sub.2 in the FCC regenerator. This led to higher regenerator temperatures, hotter catalyst, and reduced yields of coke.
Although quenching the reactor vessel was ignored, the problem of coke formation in the top of the reactor vessel has not been ignored.
It is well-known in FCC processing that there are four kinds of "coke" which are produced during the course of the FCC reaction. Refiners have long known, but rarely discussed, a fifth kind of coke production which occurs generally downstream of the FCC reactor zone. This fifth type of coke was reported and extentively discussed in REACTOR COKING PROBLEMS IN FLUID CATALYTIC CRACKING UNITS, L. J. McPherson, in a paper presented at the Ketjen Catalyst Symposium, May 27-30, 1984, Amsterdam The Netherlands.
The author reported that coking occurs in dead spaces in FCC reactor vessels, and that the problem of coke formation was most severe when a riser cracking FCC unit was in use. This coke adheres to internal surfaces of the reactor and downstream equipment and causes many problems. Not only does this coke represent a safety hazard (the coke burns when the reactor vessel is opened for inspection) but can cause an unplanned shutdown of the unit when large coke deposits, in the order of several feet in thickness, break off and plug various process lines. This article is incorporated herein by reference.
The author reported overcoming coke deposition in dead spaces in the reactor vessel by adding minor amounts of purge steam to the dome or top of the reactor.
Although not reported in the article, it is believed that many FCC operators, whether using riser cracking FCC units, or older units including some or all dense bed cracking, add minor amounts of steam purge to the top of the reactor vessel to minimize the buildup of coke in dead spaces, such as the "dome," in the top of reactor vessel, and to minimize somewhat coking in the transfer line intermediate the reactor and the main fractorator.
Addition of purge steam in this way minimizes formation of coke in the dome of the reactor, but does not change conditions in the bulk of the reactor vessel. This is because steam flows up in the vessel. Steam added to the dome does not do anything to the vapors below, because the steam goes up, not down.
The dome represents a small fraction of the volume of a typical reactor vessel, typically only one-fifth to one-tenth of the volume of the vessel is in the dome. Adding purge steam to the dome of a "closed cyclone" FCC can quench thermal reactions in the dome, but unselective thermal reactions would be undiminished in the remaining 80-90% of the vapor space of the vessel. Many refiners add dome steam without quenching the dome significantly. They add hot, dry purge steam to the dome, in the belief that localized cooling would lead to increased coke formation.
We realized that other workers in this area overlooked one problem, or if they recognized the problem, failed to see its solution.
The problem was the thermal cracking that occurred after riser cracking but before the cracked products could be removed from the reactor vessel and subjected to conventional product recovery techniques.
We discovered a way to significantly minimize the unnecessary losses of valuable normally liquid products which occurred due to thermal cracking after riser cracking had been completed, but before the cracked product could be removed from the reactor and subjected to conventional product recovery.
Accordingly the present invention provides a fluidized catalytic cracking process wherein a fluidizable catalytic cracking catalyst and a hydrocarbon feed are charged to a reactor riser at catalytic riser cracking conditions to form catalytically cracked vapor product and spent catalyst which are discharged into a reactor vessel having a volume via a riser reactor outlet equipped with a separation means to produce a catalyst lean phase comprising a majority of said cracked product, and a catalyst rich phase comprising a majority of said spent catalyst, said catalyst rich phase is discharged into a dense bed of catalyst maintained below said riser outlet and said catalyst lean phase is discharged into said vessel for a time, and at a temperature, which cause unselective thermal cracking of the cracked product in the reactor volume before product is withdrawn from said vessel via a vessel outlet, the improvement comprising addition of a quenching stream into said vessel above said dense bed of catalyst, via a quench stream addition point which will allow the quench stream to contact at least a majority of the volume of the vessel above said dense bed.
In another embodiment the present invention provides a fluidized catalytic cracking process wherein a stream of hot regenerated catalyst contacts a relatively heavy hydrocarbon feed in a conventional reactor riser, and is cracked in said riser to form cracked hydrocarbon vapor products of reduced molecular weight and spent catalyst; said vapor products and spent catalyst are discharged into a vessel having a volume through a cyclone separator which produces a catalyst-rich phase which is discharged into a dense bed of spent catalyst maintained in a lower portion of said reactor vessel and a vapor phase with relatively low catalyst content which is separately discharged into said vessel above said dense bed, steam quench is added to said cracked product vapor phase via a quench steam addition point which will allow quenching of at least a majority of the volume in the vessel and quenched vapor products are removed from said vessel as a product of the process.
In another embodiment the present invention provides a fluidized catalytic cracking process comprising a conventional reactor riser wherein hot regenerated catalyst contacts a relatively heavy hydrocarbon feed and is cracked in said riser to form catalytically cracked hydrocarbon products of reduced molecular weight and spent catalyst; said cracked products and spent catalyst are discharged into a vessel through a primary cyclone separator which produces a catalyst phase containing a minor amount of said cracked product which is discharged into a dense bed of catalyst maintained in a lower portion of said vessel and a cracked product vapor phase discharged via a primary cyclone vapor outlet and wherein a quench stream is added to said vessel 0.15 to 5 meters above said dense bed and at least about 90% of said vapor phase passes from said primary cyclone vapor outlet into an inlet of a secondary cyclone within one second and said minor amount of cracked product passes from said dense bed through said vessel to said secondary cyclone inlet in a period of time in excess of one second and additional separation of entrained spent catalyst from cracked product and said quench stream occurs in said secondary cyclone to produce a cracked product and quenched stream vapor.
US Referenced Citations (12)
Continuations (1)
|
Number |
Date |
Country |
| Parent |
272196 |
Nov 1988 |
|
Continuation in Parts (1)
|
Number |
Date |
Country |
| Parent |
666533 |
Oct 1984 |
|
Patent Grant
4978440
