Claims
- 1. A fluid catalytic cracking process for cracking hydrocarbons comprising the steps of:
- (a) cofeeding active hot solid zeolite cracking catalyst and crackable hydrocarbon feed to a cracking zone;
- (b) cracking said feed to hydrocarbon products while depositing coke on said catalyst to evolve spent catalyst;
- (c) disengaging said spent catalyst from said hydrocarbon products;
- (d) flowing said spent catalyst to a regeneration zone;
- (e) passing an oxygen-containing gas upwardly through said regeneration zone at sufficient velocity to fluidize said catalyst contained within said regeneration zone;
- (f) retaining said catalyst in said regeneration zone at elevated temperature for a time sufficient to effect exothermic oxidative regeneration of said catalyst by burning said coke deposited thereon, thereby heating and reactivating said catalyst;
- (g) providing a catalyst stripping zone comprising three superimposed stages, said stages comprising an upper mixing stage, a central dehydrogenation/stripping stage, and a lower steam stripping stage;
- (h) mixing spent catalyst of step (c) with regenerated catalyst of step (f) in said upper mixing stage of said catalyst stripping zone;
- (i) retaining said mixture of step (h) within said upper mixing stage at elevated temperature for a period of time sufficient to effect desorption of cracked products from said spent catalyst;
- (j) flowing said catalyst mixture of step (i) downwardly to said central dehydrogenation/stripping stage;
- (k) introducing a stream containing C.sub.2 -C.sub.4 alkanes to a lower section of said central dehydrogenation/stripping stage and flowing said stream containing C.sub.2 -C.sub.4 alkanes upwardly in countercurrent contact with said catalyst mixture at superficial velocity adequate to maintain said catalyst in a state of sub-transport fluidization, and to strip cracked products from said spent catalyst while providing sufficient contact time between said catalyst mixture and said C.sub.2 -C.sub.4 alkane-containing stream to cool said catalyst mixture by endothermically dehydrogenating at least a portion of alkanes present in said central dehydrogenation/stripping stage to evolve a dehydrogenated product stream;
- (l) flowing said cooled catalyst mixture of step (k) from said central dehydrogenation/stripping stage downwardly to said lower steam stripping stage and flowing said dehydrogenated product stream upwardly to said upper mixing stage;
- (m) introducing steam to a bottom portion of said lower steam stripping stage and flowing said steam upwardly at superficial gas velocity sufficient to fluidize said catalyst mixture and to countercurrently steam strip said downwardly flowing catalyst mixture to remove hydrocarbons from said catalyst mixture;
- (n) withdrawing steam and stripped hydrocarbons from an upper portion of said lower steam stripping stage to prevent substantial flow of steam and stripped hydrocarbons upward from said lower steam stripping stage to said central dehydrogenation/stripping stage and further to minimize steam deactivation of said zeolite cracking catalyst within said catalyst cooling/stripping zone.
- 2. The process of claim 1 wherein the weight ratio of regenerated catalyst to spent catalyst charged to said upper mixing stage is from about 0.5:1 to about 4:1.
- 3. The process of claim 1 further comprising controlling the flowrates of regenerated catalyst to spent catalyst charged to said upper mixing stage to maintain the temperature of said upper mixing stage from about 950.degree. F. to about 1400.degree. F.
- 4. The process of claim 1 wherein said alkane-containing stream of step (k) comprises at least 50% by weight of C.sub.4 -alkanes.
- 5. The process of claim 4 wherein said alkane-containing stream of step (k) comprises at least 70% by weight of C.sub.4 -alkanes.
- 6. The process of claim 1 wherein the process conditions of said dehydrogenation/stripping stage include weight hourly space velocity based on catalyst of from about 0.01 to about 5.0 hr.sup.-1 and temperature of from about 1000.degree. F. to about 1200.degree. F..
- 7. The process of claim 6 further comprising controlling the flowrate of said C.sub.2 -C.sub.4 alkane-containing stream to provide upward superficial gas velocity within said dehydrogenation/stripping stage of from about 0.3 to about 5 ft/sec.
- 8. The process of claim 7 further comprising controlling the process conditions within said dehydrogenation/stripping stage to provide a turbulent subtransport fluidization regime.
- 9. The process of claim 1 further comprising controlling the rate of steam introduction to said lower steam stripping stage to provide steam superficial velocity within said lower steam stripping zone of less than about 0.2 ft/sec.
- 10. A fluid catalytic cracking process comprising the steps of:
- (a) admixing hot zeolite cracking catalyst with a crackable hydrocarbon feed in the lower section of a reactor riser;
- (b) flowing said admixture of step (a) upwardly through the length of said reactor riser to contact said crackable feed with said zeolite cracking catalyst for a period of time sufficient to effect conversion of crackable hydrocarbons to cracked products while deactivating said cracking catalyst by depositing coke thereon;
- (c) disengaging said deactivated catalyst from said cracked products;
- (d) flowing a first portion of said deactivated catalyst to a regeneration zone under regeneration conditions including pressure from about 20 psig to about 50 psig and temperature from about 1200 to about 1500.degree. F. while injecting sufficient oxygen-containing regeneration gas into said regeneration zone to maintain a dense fluid bed of cracking catalyst in said regeneration zone and to oxidatively regenerate said cracking catalyst;
- (e) providing a catalyst cooling/stripping zone external both to said regeneration zone and to said reactor riser, said catalyst cooling/stripping zone comprising an upper mixing stage superimposed over a central dehydrogenation/stripping stage superimposed over a lower steam stripping stage;
- (f) flowing a second portion of said deactivate catalyst to said upper mixing stage of said catalyst cooling/stripping zone;
- (g) withdrawing a controlled stream of regenerated cracking catalyst from said regeneration zone and introducing said withdrawn regenerated catalyst into said upper mixing stage to admix said withdrawn regenerated catalyst with said second portion of said deactivated catalyst;
- (h) controlling the relative flowrates of said regenerated cracking catalyst and said deactivated catalyst to maintain temperature within said upper mixing stage from about 1050 to about 1250.degree. F.;
- (i) flowing said catalyst mixture downwardly from said upper mixing stage to said dehydrogenation/stripping stage at a rate relative to the combined charged rates of spent and regenerated catalyst to said upper mixing stage such that the residence time of said catalyst mixture within said upper mixing stage is sufficient for the desorption of at least 10% by weight of hydrocarbons sorbed onto said deactivated catalyst while concomitantly conveying sensible heat to said dehydrogenation/stripping stage sufficient to provide the endothermic heat of reaction of rehydrogenation of alkanes in said dehydrogenation/stripping stage;
- (j) introducing a feedstream containing said alkanes into said dehydrogenation/stripping stage to maintain said catalyst mixture in a state of fluidization within said dehydrogenation/stripping stage, said state of fluidization existing in a sub-transport regime operating at a weight hourly space velocity (WHSV) of said alkanes up to about 5 hr..sup.-1 while maintaining said dehydrogenation/stripping stage at a temperature sufficient to convert at least 50% by weight of said alkanes, and concurrently to cool said catalyst mixture while stripping cracked products from said catalyst mixture;
- (k) transporting said cooled catalyst mixture directly from said dehydrogenation/stripping stage, said catalyst mixture now at a temperature from about 1100 to about 1350.degree. F., to said lower steam stripping stage;
- (l) countercurrently stripping said cooled catalyst mixture by flowing steam upwardly in contact with generally downwardly flowing catalyst to remove entrained hydrocarbons from said catalyst mixture; and
- (m) withdrawing steam and stripped hydrocarbons from said lower steam stripping stage to avoid substantial flow of steam upward to said dehydrogenation/stripping and mixing stages and further to minimize steam deactivation of said zeolite cracking catalyst within said catalyst cooling/stripping zone.
FIELD OF THE INVENTION
This is a continuation of copending application Ser. No. 509,455, filed on Apr. 16, 1990 now abandoned.
This invention relates to fluid catalytic cracking and more particularly to stripping cracked hydrocarbons from spent cracking catalyst and cooling the catalytic cracking process regenerator. More particularly, the invention relates to an improved three-stage process for stripping entrained hydrocarbons from spent cracking catalyst prior to regeneration while concurrently cooling catalyst withdrawn from the regenerator by endothermically dehydrogenating an alkane-containing feedstream.
The fluid catalytic cracking (FCC) process has become well-established in the petroleum refining industry for converting higher boiling petroleum fractions into lower boiling products, especially gasoline.
In the fluid catalytic process, a finely divided solid cracking catalyst is used to promote the cracking reactions which take place in the feed. The catalyst is used in a very finely divided form, typically with a particle size range of 20-300 microns, with an average of about 60-75 microns, in which it can be handled like a fluid (hence the designation FCC) and in this form it is circulated in a closed cycle between a cracking zone and a separate regeneration zone. In the cracking zone, hot catalyst is brought into contact with the feed so as to effect the desired cracking reactions after which the catalyst is separated from the cracking products which are removed from the cracking reactor to the associated fractionation equipment for separation and further processing. During the cracking reaction, coke is deposited on the catalyst. This deposit of coke masks the active sites and temporarily deactivates the catalyst. Such temporarily deactivated catalyst is commonly called spent catalyst. The catalyst must then be regenerated before it can be reused. Fortunately, the coke deposit can be made to serve a useful purpose. Cracking is an endothermic reaction. Although, in principle, heat could be supplied by raising the temperature of the hydrocarbon feed prior to contact with the catalyst, this would thermally crack the feed so that very little control could be effected over the product distribution. Additionally, the coke formed would deposit on furnace tubes and other equipment used for heating and conveying the feed to the cracker, causing operational problems. For this reason, it is generally preferred to supply the heat to the cracking reaction by means of the catalyst. The feed may, however, be preheated to a certain degree in order to maintain an appropriate heat balance in the cycle.
Heat for the catalytic cracking process is supplied by the regeneration step in which the spent catalyst is subjected to oxidatively regenerated to remove the coke. This coke-burning step is strongly exothermic and raises the regenerated catalyst temperature such the sensible heat imparted to the catalyst during regeneration is sufficient to supply the endothermic heat of reaction for the cracking step.
The regeneration takes place in a separate regenerator vessel. Catalyst is maintained in a fluidized bed in a lower section of the regenerator vessel and an oxygen-containing gas, usually air, flows through a distribution grid which is designed to provide efficient mixing of air with the spent, coked catalyst. During the regeneration step, the coke on the spent catalyst is oxidized and the heat from the oxidation is transferred to the catalyst to raise its temperature to the requisite level for continuing the cracking reactions. The hot, freshly-regenerated catalyst is then returned to the cracking zone for contact with further feed together with any recycle. Thus, the catalyst circulates continuously in a closed cycle between the cracking zone and the regenerating zone with heat for the endothermic cracking reactions being supplied in the regenerator by oxidative removal of the coke deposits which are laid down during the cracking portion of the cycle. In order to maintain the desired level of catalyst activity and selectivity, a portion of the circulating inventory of catalyst may be withdrawn intermittently or continuously with fresh, make-up catalyst being added to compensate for the withdrawn catalyst and the catalyst losses which occur through attrition and loss of catalyst from the system.
A further description of the catalytic cracking process and the role of regeneration may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts", Venuto and Habib, Marcel Dekker, N.Y., 1978. Reference is particularly made to pages 16-18, describing the operation of the regenerator and the flue gas circuit.
The amorphous cracking catalysts which were initially used in the FCC process were characteristically low activity catalysts which gave a relatively low hydrocarbon conversion with a relatively low carbon lay-down on the catalyst. Because the carbon provides the heat for the regeneration process, the carbon lay-down is a measure of the heat which can be produced during the regeneration and, consequently, of the regeneration temperature. Thus, the use of amorphous catalysts implied the use of relatively low regeneration temperatures.
The development of synthetic zeolite cracking catalysts, especially the zeolite cracking catalysts represented mainly by the synthetic faujasite zeolite Y, typically in the form of rare earth exchanged zeolite Y (REY) or ultrastable Y (USY) represented a considerable advance in the technology of the FCC process, but it was accompanied by its own problems. In contrast to the older, amorphous cracking catalysts which they rapidly supplanted, the zeolite catalysts were characterized as relatively high conversion catalysts which produced a relatively high carbon lay-down on the catalyst. The relatively higher carbon lay-down resulted in higher regenerator temperatures and higher burning rates both for the carbon on the catalyst and for the carbon monoxide produced during the combustion process. With the production of greater heat in the regenerator, the catalyst circulation rate was reduced since the process as a whole needs to remain in a heat balanced condition and this was desirable since it enabled the catalyst make-up rate to be reduced, a valuable economic factor.
The zeolite cracking catalysts are, in general terms, more sensitive to residual carbon than the amorphous catalysts, particularly with respect to selectivity. This sensitivity, coupled with the fact that operation under high temperature regeneration conditions was desirable for other reasons, as indicated above, provided an incentive for higher regenerator temperatures and lower residual carbon levels on the regenerated catalyst.
However, it has been found that the most desirable conversion selectivity may be obtained, and more difficult feeds may be processed, by introducing the regenerated catalyst to the fresh feed at a temperature below that determined by the metallurgical limitations of the regenerator. In other words, while the regenerated catalyst upper temperature limit is determined by the maximum safe operating temperature in the catalyst regenerator, the optimum catalyst temperature for the conversion process is generally accepted to be a lower temperature.
Viewing the regenerator as a controlled combustion chamber, it can be seen that the operating temperature may be lowered by decreasing net heat input or by withdrawing heat from the vessel. Heat input sources for the regenerator vessel include sensible heat is from the spent catalyst, as well as heat generated within in the vessel from the combustion of coke and entrained hydrocarbon products. Thus operating temperature could effectively be lowered in the regenerator vessel by decreasing spent catalyst flow, decreasing fuel flow, or by cooling the regenerator by direct or indirect heat exchange. However, the rate of catalyst circulation is substantially set by the cracking feedstock flowrate to the riser reactor and the desired conversion, thus practically limiting regenerator temperature control methods to reducing fuel flow and cooling via heat exchange.
The coke deposited on spent cracking catalyst together with entrained product carried over to the regenerator with the spent catalyst is referred to by those skilled in the art as "total delta carbon". For a given FCC unit design, at a fixed catalyst circulation rate, an increase in total delta carbon is accompanied by higher regenerator temperatures. Consequently, one method of limiting FCC regenerator temperature is to reduce total delta carbon by reducing carryover of cracked hydrocarbon product to the regenerator.
Recent research efforts in the field of FCC technology have contributed new processes and devices for efficiently separating catalyst particles from a fluidized suspension phase, as exemplified by the following references.
U.S. Pat. No. 4,070,159 to Myers et al. provides means for separating cracking catalyst from cracked product in which the bulk of catalyst solids is discharged directly into a settling chamber without passing through a cyclone separator. However, the Myers et al. process strips hydrocarbon product from the spent catalyst using hot flue gas; See column 10, lines 12-18. Cracked products could be burned in contact with oxygen-containing flue gas at elevated temperature, thus decreasing the net product yield from the cracking process.
U.S. Pat. No. 4,574,044 to Krug discloses a method for increasing the overall efficiency of an FCC process by decreasing the amount of valuable product burned in the regenerator. Separation of catalyst from hydrocarbon product is enhanced by first stripping the hydrocarbon product from the catalyst and then conditioning the catalyst in the presence of steam at elevated temperatures for a period of about 1/2 to 30 minutes. The benefits of this system include a reduction in coke make.
The FCC process converts petroleum feedstocks in the gas oil boiling range to lighter products such as gasoline. While a wide variety of catalysts may be used in the catalytic cracking process, most preferred is a zeolite cracking catalyst which exhibits loss of catalytic activity when coked. Thus a substantial quantity of coke must be removed from the catalyst when it is regenerated. As a result, regenerators are designed to be "hot-operated" and under pressure, that is, operated at a pressure in the range from about 25 psig to 40 psig, and as high a temperature as is practical from a materials standpoint. The temperature within a regenerator typically ranges from about 538.degree. C. to about 815.degree. C. (1000-1500.degree. F.).
Examples of FCC regenerators with catcoolers are disclosed in U.S. Pat. Nos. 2,377,935; 2,386,491; 2,662,050; 2,492,948; and 4,374,750, inter alia. These catcoolers remove heat by indirect heat exchange, typically a shell and tube exchanger. None removes heat by direct heat exchange, for example, by continuously diluting hot regenerated catalyst with cold catalyst, or by blowing cold air through the hot catalyst; more particularly, none removes heat by functioning as a reactor which supplies heat to an endothermic reaction.
U.S. Pat. No. 4,422,925 discloses the step-wise introduction of ethane, propane, butane, recycle naphtha, naphtha feed, raffinate naphtha, and fractionator bottoms recycle in the riser reactors of a FCC unit. In the riser reactors, the lower alkanes are contacted, in a transport zone, with hot regenerated catalyst which would dehydrogenate the alkanes, progressively decreasing the temperature of the suspension of catalyst and hydrocarbons as they progress upwards through the risers. The mixture of catalyst and reaction products is then contacted with a hydrocarbon feedstock suitable for catalytic cracking, such as virgin naphtha, virgin gas oil, light cycle gas oil, or heavy gas oil. (see col 2, lines 29-33). This contrasts with the present process which concurrently strips entrained hydrocarbons from spent catalyst while cooling hot regenerated catalyst.
The concept of cooling hot regenerated catalyst by using an endothermic reaction, specifically the catalytic dehydrogenation of butane, is taught in U.S. Pat. No. 2,397,352 to Hemminger. While the teachings of the Hemminger patent are wholly unrelated to operation of a FCC unit, regeneration of the catalyst was required before it was returned to the dehydrogenation reactor. The Hemminger patent teaches the use of a catalyst (chromic oxide supported on alumina or magnesia) heating chamber for supplying heat to the dehydrogenation reaction, and to preheat, at least in part, the butane to raise its temperature to reaction temperatures.
U.S. Pat. No. 4,840,928 to Harandi and Owen teaches a fluid catalytic cracking process in which catalyst withdrawn from the regenerator is cooled by direct contact with an alkane-rich stream in an external catalyst cooler. However, the Harandi and Owen process differs from the process of the present invention in that the Harandi and Owen process teaches a single stage dehydrogenation reactor in which only the hot regenerated cracking catalyst contacts the alkane-rich stream. In contrast, the present invention provides a three-stage catalyst cooling and steam stripping process which efficiently strips hydrocarbons from spent cracking catalyst while cooling the regenerated catalyst in controlled stages to minimize steam deactivation of the catalyst.
The process of the present invention improves the operational flexibility of a fluid catalytic cracking process by a three-stage process for stripping spent catalyst and for cooling the regenerator. The process of the present invention includes three sequential stages, and their order is critical to the effectiveness of the process. The process is suitably conducted in a partitioned vessel physically separate from the catalytic cracking unit regenerator and reactor vessels.
In the first stage, spent catalyst withdrawn from the reactor vessel is admixed with hot regenerated catalyst withdrawn from the regenerator vessel. This admixing step raises the spent catalyst temperature to effect desorption of at least a portion of the entrained cracked product.
Next, the mixture of spent and regenerated catalyst flows to a dehydrogenation/stripping zone where an added C.sub.2 -C.sub.4 alkane-rich stream countercurrently strips entrained heavier cracked products from the catalyst mixture. The alkane-rich stream endothermically dehydrogenates in the dehydrogenation/stripping zone thus cooling the catalyst mixture.
Finally, the cooled catalyst mixture flows to a steam stripping stage where it is countercurrently stripped with upwardly flowing steam. The cooled, stripped catalyst then returns to a lower section of the regenerator for further processing.
More specifically, the process encompasses an improved method for stripping and regenerating spent cracking catalyst in a fluidized catalytic cracking process comprising the steps of:
(a) cofeeding active hot solid zeolite cracking catalyst and crackable hydrocarbon feed to a cracking zone;
(b) cracking said feed to hydrocarbon products while depositing coke on said catalyst to evolve spent catalyst;
(c) disengaging said spent catalyst from said hydrocarbon products;
(d) flowing said spent catalyst to a regeneration zone;
(e) passing an oxygen-containing gas upwardly through said regeneration zone at sufficient velocity to fluidize said catalyst contained within said regeneration zone;
(f) retaining said catalyst in said regeneration zone at elevated temperature for a time sufficient to effect exothermic oxidative regeneration of said catalyst by burning said coke deposited thereon, thereby heating and reactivating said catalyst;
(g) providing a catalyst stripping zone comprising three superimposed stages, said stages comprising an upper mixing stage, a central dehydrogenation/stripping stage, and a lower steam stripping stage;
(h) mixing spent catalyst of step (c) with regenerated catalyst of step (f) in said upper mixing stage of said catalyst stripping zone;
(i) retaining said mixture of step (h) within said upper mixing stage at elevated temperature for a period of time sufficient to effect desorption of cracked products from said spent catalyst;
(j) flowing said catalyst mixture of step (i) downwardly to said central dehydrogenation/stripping zone;
(k) introducing a stream containing C.sub.2 -C.sub.4 alkanes to a lower section of said central dehydrogenation/stripping zone and flowing said stream containing C.sub.2 -C.sub.4 alkanes upwardly in countercurrent contact with said catalyst mixture at superficial velocity adequate to maintain said catalyst in a state of sub-transport fluidization, and to strip cracked products from said spent catalyst while providing sufficient contact time between said catalyst mixture and said C.sub.2 -C.sub.4 alkane-containing stream to cool said catalyst mixture by endothermically dehydrogenating at least a portion of alkanes present in said central dehydrogenation/stripping zone to evolve a dehydrogenated product stream;
(l) flowing said cooled catalyst mixture of step (k) from said central dehydrogenation/stripping stage downwardly to said lower steam stripping zone and flowing said dehydrogenated product stream upwardly to said upper mixing stage;
(m) introducing steam to a bottom portion of said lower steam stripping stage and flowing said steam upwardly at superficial gas velocity sufficient to fluidize said catalyst mixture and to countercurrently steam strip said downwardly flowing catalyst mixture to remove hydrocarbons from said catalyst mixture;
(n) withdrawing steam and stripped hydrocarbons from an upper portion of said lower steam stripping stage to prevent substantial flow of steam and stripped hydrocarbons upward from said lower steam stripping stage to said central dehydrogenation/stripping stage.
US Referenced Citations (32)
Non-Patent Literature Citations (1)
| Entry |
| P. B. Venuto et al., Fluid Catalystic Cracking with Zeolite Catalysts, Marcel Dekker, N.Y., 1978. |
Continuations (1)
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Number |
Date |
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| Parent |
509455 |
Apr 1990 |
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Patent Grant
5059305
