DUAL RISER CATALYTIC CRACKING PROCESS FOR MAKING MIDDLE DISTILLATE AND LOWER OLEFINS

Abstract

A fluidized catalytic cracking process and system that provide for the processing of hydrocarbon feedstocks to selectively produce a middle distillate boiling range product and lower olefins. The inventive process uses two riser reactors each having associated therewith a separator/stripper for separating the cracked product and cracking catalyst received from the respective riser reactor and a single regenerator for regenerating coked or spent cracking catalyst received from the separator/strippers. The two riser reactors, two separator/strippers and regenerator are operatively integrated to provide a process system for carrying out the process of the invention.

Description

This application claims the benefit of U.S. Provisional Application No. 61/503,209 filed Jun. 30, 2011, the entire disclosure of which is hereby incorporated by reference.

This invention relates to method and apparatus for the manufacture of a middle distillate product and lower olefins from a hydrocarbon feedstock by the use of a dual riser catalytic system and process.

The fluidized catalytic cracking (FCC) of heavy hydrocarbons to produce lower boiling hydrocarbon products, such as gasoline, has been around since the 1940's. Typically, an FCC unit or process system includes a single riser reactor, a catalyst separator and stripper, and a regenerator. An FCC feedstock is introduced into the riser reactor in which it is contacted with hot FCC catalyst from the regenerator. The mixture of FCC feedstock and FCC catalyst passes through the riser reactor and into the catalyst separator wherein the cracked product is separated from the FCC catalyst. The separated cracked product passes from the catalyst separator to a downstream separation system and the separated catalyst passes to the regenerator where the coke deposited on the FCC catalyst during the cracking reaction is burned off the catalyst to provide a regenerated catalyst. The resulting regenerated catalyst is used as the aforementioned hot FCC catalyst and is mixed with the FCC feedstock that is introduced into the riser reactor.

Many processes and systems are designed so as to provide for a high conversion of the FCC feedstock to yield products having boiling temperatures in the gasoline boiling range. But, some prior art processes provide for the preferential conversion of a hydrocarbon feedstock to a middle distillate product and lower olefins. One such process is disclosed in the US Patent Publication No. US 2006/0231461 of Mo et al. The process taught by Mo et al. includes the use of a riser reactor in combination with a dense bed reactor to process a gas oil feedstock and a gasoline feedstock in a way to preferentially make middle distillate and lower olefins. The disclosed process includes regenerating a spent cracking catalyst and using the resulting regenerated catalyst in the dense bed reactor. Used regenerated catalyst is passed from the dense bed reactor and introduced into the riser reactor wherein it is used in combination with regenerated catalyst in the fluidized catalytic cracking of the gas oil feedstock.

Other publications disclose the use of a combination of riser reactors or a combination of a dense fluid bed reactor with a riser reactor to provide for recracking of a gasoline product from the cracking of gas oil. U.S. Pat. No. 3,928,172 to Davis, Jr. et al. discloses a number of alternative fluid catalyst systems and processes that involve the recracking of cracked gasoline over a zeolite-containing catalyst. It is asserted by Davis that the zeolite catalyst is able to effect a degree of octane improvement that was previously not possible with amorphous silica-alumina catalysts.

One process and system disclosed by Davis uses a dense bed reactor with a single riser reactor arranged in a catalyst flow sequence such that the dense bed is placed between the regenerator and riser. Davis further discloses a hydrocarbon upgrading process that includes a first cracking zone in which gas oil is cracked and a second cracking zone in which gasoline is cracked. The second cracking zone may include a dense bed. In another disclosure of Davis, gasoline is cracked within a dense bed reaction zone in which freshly regenerated catalyst is introduced. Catalyst from the dense bed reaction zone is then used for gas oil cracking in a riser cracking zone.

In the article published by the Chinese Journal of Chemical Engineering, 16(3) 394-400 (2008), entitled “Alternative Processing Technology for Converting Vegetable Oils and Animal Fats to Clean Fuels and Light Olefins,” the authors Tian et al. disclose a catalytic cracking process that utilizes two risers which share a common disengager and regenerator. Fresh feedstock is introduced into the first stage riser, and a recycle stream of gasoline or heavy oil, or both, is introduced into the second stage riser. There is no disclosure, however, of the use of multiple catalyst separators or strippers nor is there any disclosure of the selective separation or stripping of catalyst taken from each of the two risers and the separate or selective recycle thereof.

One of the objects of this invention is to provide method and apparatus for the preferential conversion of a hydrocarbon feedstock to a middle distillate product and lower olefins.

Accordingly, provided is a dual riser cracking process for making middle distillate and lower olefins, wherein said process comprises: catalytically cracking a first hydrocarbon feedstock within a first riser reactor zone by contacting under first catalytic cracking conditions within said first riser reactor zone said first hydrocarbon feedstock with a combination of a clean spent catalyst and a first portion of a regenerated cracking catalyst to yield a first riser reactor product comprising a first cracked product and a coked spent catalyst; catalytically cracking a second hydrocarbon feedstock within a second riser reactor zone by contacting under second catalytic cracking conditions within said second riser reactor zone said second hydrocarbon feedstock with a second portion of said regenerated cracking catalyst to yield a second riser reactor product comprising a second cracked product and said clean spent catalyst; passing said first riser reactor product to a first separator/stripper providing means for separating said first riser reactor product into a separated first cracked product and a separated coked spent catalyst; passing said second riser reactor product to a second separator/stripper providing means for separating said second riser reactor product into a separated cracked second cracked product and a separated clean spent catalyst; using at least a portion of said separated clean spent catalyst as said clean spent catalyst of said combination; and passing said separated coked spent catalyst and a remaining portion of said separated clean spent catalyst to a regenerator that defines a regeneration zone and provides means for regenerating said separated coked spent catalyst and said remaining portion of said separated clean spent catalyst to yield said regenerated cracking catalyst.

FIG. 1 is a process flow schematic illustrating certain aspects of one embodiment of the inventive process.

The invention includes process and apparatus that provide for the processing of hydrocarbon feedstocks to selectively or preferentially produce a middle distillate boiling range product and lower olefins. The inventive process uses two riser reactors each having associated therewith a separator/stripper for separating the cracked product and cracking catalyst received from the respective riser reactor and a single regenerator for regenerating coked or spent cracking catalyst received from the separator/strippers. The two riser reactors, two separator/strippers and regenerator are operatively integrated to provide a process system for carrying out the process of the invention.

In the inventive process, a first hydrocarbon feedstock is introduced into the bottom of a first riser reactor zone that is defined by a first riser reactor. Hot cracking catalyst (e.g., the first portion of the regenerated cracking catalyst, as defined below, and, optionally, at least a portion of, or, alternatively, a remaining portion of, the separated clean spent catalyst, as they are defined below) is also introduced into the first riser reactor zone, wherein it is mixed and contacted with the first hydrocarbon feedstock under suitable first catalytic cracking conditions to provide for catalytically cracking the first hydrocarbon feedstock.

A second hydrocarbon feedstock is introduced into the bottom of the second riser reactor zone that is defined by a second riser reactor. Regenerated catalyst (e.g., the second portion of regenerated cracking catalyst, as defined below) is also introduced into the second riser reactor zone, wherein it is mixed and contacted with the second hydrocarbon feedstock under suitable second catalytic cracking conditions to provide for catalytically cracking the second hydrocarbon feedstock.

The fresh catalytic cracking catalyst used in the inventive process and circulated within the process system can be any suitable cracking catalyst known in the art to have cracking activity under the catalytic cracking conditions contemplated by the invention. Preferred catalytic cracking catalysts for use in the inventive process include fluidizable cracking catalysts comprised of a molecular sieve having cracking activity dispersed in a porous, inorganic refractory oxide matrix or binder.

The term “molecular sieve” as used herein refers to any material capable of separating atoms or molecules based on their respective dimensions. Molecular sieves suitable for use as a component of the cracking catalyst include pillared clays, delaminated clays, and crystalline aluminosilicates. Normally, it is preferred to use a cracking catalyst that contains a crystalline aluminosilicate. Examples of such aluminosilicates include Y zeolites, ultrastable Y zeolites, X zeolites, zeolite beta, zeolite L, offretite, mordenite, faujasite, and zeolite omega. The preferred crystalline aluminosilicates for use in the cracking catalyst are X and Y zeolites with Y zeolites being the most preferred.

U.S. Pat. No. 3,130,007, the disclosure of which is hereby incorporated by reference in its entirety, describes Y-type zeolites having an overall silica-to-alumina mole ratio between about 3.0 and about 6.0, with a typical Y zeolite having an overall silica-to-alumina mole ratio of about 5.0. It is also known that Y-type zeolites can be produced, normally by dealumination, having an overall silica-to-alumina mole ratio above about 6.0. Thus, for purposes of this invention, a Y zeolite is one having the characteristic crystal structure of a Y zeolite, as indicated by the essential X-ray powder diffraction pattern of Y zeolite, and an overall silica-to-alumina mole ratio above 3.0, and includes Y-type zeolites having an overall silica-to-alumina mole ratio above about 6.0.

The stability and/or acidity of a zeolite used as a component of the cracking catalyst may be increased by exchanging the zeolite with hydrogen ions, ammonium ions, polyvalent metal cations, such as rare earth-containing cations, magnesium cations or calcium cations, or a combination of hydrogen ions, ammonium ions and polyvalent metal cations, thereby lowering the sodium content until it is less than about 0.8 weight percent, preferably less than about 0.5 weight percent and most preferably less than about 0.3 weight percent, calculated as Na₂O. Methods of carrying out the ion exchange are well known in the art.

The zeolite or other molecular sieve component of the cracking catalyst is combined with a porous, inorganic refractory oxide matrix or binder to form a finished catalyst prior to use. The refractory oxide component in the finished catalyst may be silica-alumina, silica, alumina, natural or synthetic clays, pillared or delaminated clays, mixtures of one or more of these components and the like. Preferably, the inorganic refractory oxide matrix will comprise a mixture of silica-alumina and a clay such as kaolin, hectorite, sepiolite and attapulgite.

A preferred finished catalyst will typically contain between about 5 weight percent to about 40 weight percent zeolite or other molecular sieve and greater than about 20 weight percent inorganic, refractory oxide. In general, the finished catalyst may contain between about 10 to about 35 weight percent zeolite or other molecular sieve, between about 10 to about 30 weight percent inorganic, refractory oxide, and between about 30 to about 70 weight percent clay.

The crystalline aluminosilicate or other molecular sieve component of the cracking catalyst may be combined with the porous, inorganic refractory oxide component or a precursor thereof by any suitable technique known in the art including mixing, mulling, blending or homogenization. Examples of precursors that may be used include alumina, alumina sols, silica sols, zirconia, alumina hydrogels, polyoxycations of aluminum and zirconium, and peptized alumina.

In a preferred method of preparing the cracking catalyst, the zeolite is combined with an alumino-silicate gel or sol or other inorganic, refractory oxide component, and the resultant mixture is spray dried to produce finished catalyst particles normally ranging in diameter between about 40 and about 80 microns. If desired, however, the zeolite or other molecular sieve may be mulled or otherwise mixed with the refractory oxide component or precursor thereof, extruded and then ground into the desired particle size range. Normally, the finished catalyst will have an average bulk density between about 0.30 and about 0.90 gram per cubic centimeter and a pore volume between about 0.10 and about 0.90 cubic centimeter per gram.

The first hydrocarbon feedstock may be any suitable hydrocarbon feedstock that is chargeable to a fluidized catalytic cracking unit or that will result in providing a particularly desired product mix. In one preferred embodiment of the inventive process, the first hydrocarbon feedstock is a gas oil. Hydrocarbon mixtures boiling in the range of from 345° C. (653° F.) to 760° C. (1400° F.) can suitably be used as the first hydrocarbon feedstock of the invention. Examples of the types of refinery feed streams that can make suitable gas oil feedstocks include vacuum gas oils, coker gas oils, straight-run residues, thermally cracked oils and other hydrocarbon streams.

The catalytic cracking conditions can be defined by such parameters as the average residence time of the hydrocarbons in a particular riser reactor, the catalyst-to-oil ratio, and the riser reactor temperature.

The first catalytic cracking conditions at which the first riser reactor zone is operated can include an average residence time of the hydrocarbons (e.g., first hydrocarbon feedstock) in the first riser reactor zone that is generally in the range of upwardly to about 5 to 10 seconds, but, usually, it is in the range of from 0.1 to 5 seconds. The weight ratio of catalyst (e.g., the first portion of regenerated cracking catalyst, as defined below, and, optionally, at least a portion of, or, alternatively, a remaining portion of, the separated clean spent catalyst, as defined below) to first hydrocarbon feedstock (i.e., the catalyst/oil ratio) introduced into the first reactor zone generally can be in the range of from about 2 to about 100 and even as high as 150. More typically, the catalyst-to-oil ratio can be in the range of from 5 to 100. The temperature in the first riser reactor zone generally can be in the range of from about 400° C. (752° F.) to about 600° C. (1112° F.). More typically, the first riser reactor temperature can be in the range of from 450° C. (842° F.) to 550° C. (1022° F.).

The second hydrocarbon feedstock may be any suitable hydrocarbon feedstock that is chargeable to a fluidized catalytic cracking unit or that will provide the particularly desired product mix. In a preferred embodiment of the inventive process, the second hydrocarbon feedstock include hydrocarbon mixtures boiling in the naphtha or gasoline boiling temperature range. Generally, gasoline feedstocks comprise hydrocarbons boiling in the temperature range of from about 32° C. (90° F.) to about 204° C. (400° F.). Examples of refinery streams that may be used as the naphtha or gasoline feedstock of the inventive process include straight run gasoline, straight run naphtha, catalytically cracked gasoline, and coker naphtha.

The second catalytic cracking conditions at which the second riser reactor zone is operated can include an average residence time of the hydrocarbons (e.g., second hydrocarbon feedstock) in the second riser reactor zone generally in the range upwardly to about 20 seconds, but usually the average residence time is in the range of from 0.1 to 10 seconds. The weight ratio of catalyst (e.g., the second portion of regenerated cracking catalyst, as defined below) to second hydrocarbon feedstock (i.e., the catalyst/oil ratio) can generally be in the range of from about 2 to about 100 and even as high as 150. More typically, the catalyst-to-oil ratio can be in the range of from 5 to 100. The temperature in the second riser reactor zone generally can be in the range of from about 482° C. (900° F.) to about 871° C. (1600° F.). More typically, the second riser reactor zone generally can be in the range of from 538° C. (1000° F.) to 732° C. (1350° F.).

The hot cracking catalyst that is introduced into the first riser reactor zone along with the first hydrocarbon feedstock includes a first portion of regenerated cracking catalyst taken from the regenerator. In another embodiment of the inventive process, at least a portion of the separated clean spent catalyst is introduced into the first riser reactor zone along with the first hydrocarbon feedstock and the first portion of the regenerated cracking catalyst. The remaining portion of separated clean spent catalyst, which is the portion that is not introduced into the first riser reactor zone, is passed and introduced into the regenerator. In an alternative embodiment of the invention, at least a portion of the separated clean spent catalyst is, instead, passed and introduced into the regenerator; and, then the remaining portion of the separated clean spent catalyst, which is the portion that is not introduced into the regenerator, is passed and introduced into the first riser reactor zone.

The clean spent catalyst is referred to herein as being “clean” because it is derived from the product of the second riser reactor zone that is defined by a second riser reactor. The second riser reactor zone of the process is operated under suitable second catalytic cracking conditions that are more severe than the reaction conditions under which the first riser reactor zone is operated. The second hydrocarbon feedstock charged to the second riser reactor zone is, preferably, a lighter feedstock than the first hydrocarbon feedstock charged to the first riser reactor zone, thus resulting in less coke yield.

Due to the cracking of a lighter feedstock, the used cracking catalyst yielded from the product of the second riser reactor zone has a lower concentration of coke than the spent or coked spent catalyst yielded from the product of the first riser reactor zone. Thus, the used cracking catalyst from the second riser reactor product is referred to herein as being “clean” for the purpose of distinguishing it from the spent or coked spent catalyst yielded from the first riser reactor product.

Yielded from the first riser reactor zone is a first riser reactor product that comprises a first cracked product and a coked spent catalyst. The first riser reactor product is passed to the first separator/stripper associated with the first riser reactor. The first separator/stripper provides means for separating the first riser reactor product into a separated first cracked product and a separated coked spent catalyst.

The separated coked spent catalyst has a coke content, generally, in the range of from about 0.5 to about 5 weight percent (wt. %), based on the total weight of the catalyst and the carbon. More typically, the coke content on the separated coked spent catalyst is in the range of from or about 0.5 wt. % to or about 1.5 wt. %.

A second riser reactor product is yielded from the second riser reactor zone and comprises a second cracked product and a clean spent catalyst. This second riser reactor product is passed to the second separator/stripper that is associated with the second riser reactor. The second separator/stripper provides means for separating the second riser reactor product into a separated second cracked product and a separated clean spent catalyst.

The separated clean spent catalyst has a coke content that is typically lower than the coke content of the separated coked spent catalyst. Generally, the coke content of the separated clean spent catalyst is in the range of from about 0.1 to about 1 weight percent (wt. %), based on the total weight of the catalyst and the carbon. More typically, the coke content on the separated clean spent catalyst is in the range of from or about 0.1 wt. % to or about 0.6 wt. %.

Each separator/stripper of the inventive fluidized catalytic cracking process system includes a vessel that defines a separation zone and a stripping zone. Within the separation zone there may be one or more cyclones that define one or more cyclone separation zones. The cyclones may be operated in series flow or in parallel flow and they provide means for receiving a riser reactor effluent and for separating spent catalyst and catalytically cracked vaporous hydrocarbons of the riser reactor effluent. The separated vaporous hydrocarbon product exits the cyclones and the separator/stripper apparatus to pass downstream for further processing such as with a main fractionator of the fluidized catalytic cracking unit. The separated spent catalyst passes from the cyclones through diplegs into the stripping zone or section of the separator/stripper apparatus. The separated spent catalyst is stripped of hydrocarbons within the stripping zone, typically, by the use of stripping steam that is introduced into the stripping section of the separator/stripper apparatus. Stripped catalyst is removed from the stripping section of the stripping section of the separator/stripper apparatus by way of a catalyst standpipe conduit.

The first riser reactor product is received into first separation means of the first separator/stripper which defines a first separation zone. First separation means provides for separating the first riser reactor product into a separated first cracked product and a separated coked spent catalyst.

The second riser reactor product is received into second separation means of the second separator/stripper which defines the second separation zone. Second separation means provides for separating the second riser reactor product into a separated second cracked product and a separated clean spent catalyst.

Any suitable means known in the art may be used as either first separation means or second separation means, but, typically, and preferably, such means include cyclone separators that utilize centrifugal flow and gravity to provide for the separation of hydrocarbon gases and catalyst particles. Many of the various types or designs of suitable cyclone separators and their uses are known to those skilled in the art.

The separated first cracked product and separated second cracked product that are respectively yielded from the first separator/stripper and second separator/stripper pass to the downstream where they may further be processed. It is preferred for the separated first cracked product and the separated second cracked product to pass either separately or in combination to one or more fractionators, but, typically, a combined stream is passed to a main fractionator.

The main fractionator defines a fractionation zone and provides means for separating the separated first cracked product or the separated second cracked product, or a combination of both, into one or more product streams including a naphtha product stream. Other product streams may include a lower olefins stream, a cracked gasoline stream, and a cracked gas oil stream.

The first separator/stripper further defines a first stripping zone generally contained within the bottom section of the first separator/stripper. In the operation of the first separator/stripper, the separated coked spent catalyst falls from first separation means into the first stripping zone, wherein it is stripped of hydrocarbons. Any suitable stripping fluid may be used to strip the hydrocarbons from the separated coked spent catalyst, but, the preferred stripping fluid is steam, which may, in general, be introduced into the bottom of the first stripping zone or section of the first separator/stripper. The stripped hydrocarbons will pass to downstream along with the separated first cracked product for further processing and the separated coked spent catalyst passes from the first stripping zone to be introduced into the regeneration zone of the regenerator.

The second separator/stripper further defines a second stripping zone generally contained within the bottom section of the second separator/stripper. In the operation of the second separator/stripper, the separated clean spent catalyst falls from second separation means into the second stripping zone, wherein it is stripped of hydrocarbons. As with the first separator/stripper, any suitable stripping fluid may be used to strip the hydrocarbons from the separated clean spent catalyst with the preferred stripping fluid being steam. The stripped hydrocarbons will pass to downstream along with the separated second cracked product for further processing.

The separated clean spent catalyst passes from the second stripping zone and at least a portion of the separated clean spent catalyst including up to the entire flow of the separated clean spent catalyst from the second separator/stripper is introduced into the regeneration zone of the regenerator. In another embodiment of the invention, the remaining portion of the separated clean spent catalyst that is not introduced into the regenerator is further passed to and introduced in combination or along with a first portion of the regenerated cracking catalyst into the riser reactor zone of the first riser reactor. In still another embodiment of the inventive process, at least a portion of the separated clean spent catalyst, instead, is introduced, in combination or along with a first portion of a regenerated cracking catalyst, into the first riser reactor zone of the first riser reactor wherein the catalyst is contacted with the first hydrocarbon feedstock. A remaining portion of the separated clean spent catalyst, which is that portion of the separated clean spent catalyst that is not passed to and introduced into the first riser reactor zone, is introduced into the regeneration zone of the regenerator.

The regenerator of the inventive process or system provides means for regenerating the separated coked spent catalyst and the at least a portion or the remaining portion of the separated clean spent catalyst to yield regenerated cracking catalyst. The regenerator defines a regeneration zone into which the separated coked spent catalyst and the at least a portion, or the remaining portion, of the separated clean spent catalyst are introduced and wherein deposited carbon is burned to provide the regenerated cracking catalyst having a reduced carbon content. The regenerator, typically, is a vertical vessel of any suitable configuration that defines the regeneration zone and wherein the separated coked spent catalyst and the remaining portion, or at least a portion, of the separated clean spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, such as air.

The regeneration temperature within the regeneration zone is, in general, maintained in the range of from about 621° C. (1150° F.) to 760° C. (1400° F.), and more typically, in the range of from 677° C. (1250° F.) to 715° C. (1320° F.).

The pressure within the regeneration zone typically is in the range of from about atmospheric to about 345 kPa (50 psig), and, preferably, from about 34 to 345 kPA (5 to 50 psig).

The residence time of the separated coked spent catalyst and the at least a portion, or the remaining portion, of the separated clean spent catalyst within the regeneration zone is in the range of from about 1 to about 6 minutes, and, typically, from or about 2 to or about 4 minutes.

The coke content on the regenerated cracking catalyst is less than the coke content on the separated coked spent catalyst and the at least a portion, or remaining portion, of the separated clean spent catalyst that are introduced into the regeneration zone of the regenerator. The coke content of the regenerated cracking catalyst will, thus, generally be in the range of from or about 0.01 wt. % to or about 0.5 wt. %. It is preferred for the coke concentration on the regenerated cracking catalyst to be less than 0.1 wt. % and, it will preferably be in the range of from 0.01 wt. % to 0.1 wt. %.

The regenerated cracking catalyst yielded from the regenerator is used as a hot cracking catalyst that is introduced into the first riser reactor zone and the second riser reactor zone for contacting with the respective hydrocarbon feedstocks. Thus, a first portion of the regenerated cracking catalyst is introduced into the first riser reactor zone wherein it is contacted with the first hydrocarbon feedstock, and a second portion of the regenerated cracking catalyst is introduced into the second riser reactor zone wherein it is contacted with the second hydrocarbon feedstock.

Now referring to FIG. 1 which presents a process flow schematic representing certain aspects of the inventive process 10. In process 10, a first hydrocarbon feedstock, which preferably is a gas oil feedstock, is passed by way of conduit 12 and introduced into first riser reactor zone 14 that is defined by first riser reactor 16.

First riser reactor 16 is an elongated conduit that extends vertically. The first hydrocarbon feedstock is introduced into first riser reactor zone 14 at or near the bottom of first riser reactor 16, wherein it is mixed or contacted with hot catalyst. The mixture of first hydrocarbon feedstock and hot catalyst passes through first riser reactor zone 14, which is operated under suitable first catalytic cracking conditions so as to provide a first riser reactor product.

The first riser reactor product, which comprises a first cracked product and a coked spent catalyst, is yielded from first riser reactor zone 14 and passes by way of conduit 18 from outlet 20 of first riser reactor 16 to be introduced into first separator/stripper 22 through inlet 24.

The sources of hot catalyst introduced into first riser reactor zone 14 include a first portion of the regenerated cracking catalyst taken from regenerator 26, and, in an alternative embodiment of the invention, at least a portion, or, in another alternative embodiment of the invention, a remaining portion, of the separated clean spent catalyst taken from second separator/stripper 30. Thus, a first portion of the regenerated cracking catalyst passes from regenerator 26 by way of conduit 32 and is introduced into first riser reactor zone 14. Or, a first portion of the regenerated cracking catalyst in combination with at least a portion of the separated clean spent catalyst is introduced into first riser reactor zone 14. Or, a first portion of the regenerated cracking catalyst in combination with a remaining portion of the separated clean spent catalyst is introduced into first riser reactor zone 14. The at least a portion of the separated clean spent catalyst is introduced into first riser reactor zone 14 by way of conduit 34. The remaining portion of the separated clean spent catalyst, likewise, is introduced into first riser reactor zone 14 by way of conduit 34.

First separator/stripper 22 includes and defines first separation zone 36 and first stripping zone 38. One or more cyclones (not shown) may be included within first separation zone 36 to provide first separation means for separating the first riser reactor product into a separated first cracked product and a separated coked spent catalyst. Provided within first stripping zone 36 are baffles or trays (not shown) that provide for enhanced contact between the falling catalyst and a stripping fluid, such as steam, that is introduced into the first stripping zone by way of conduit 40, so as to assist in the stripping of the hydrocarbons from the falling catalyst. The separated coked spent catalyst passes from first stripping zone 38 by way of conduit 44 to be introduced into regeneration zone 46 of regenerator 26.

Regenerator 26 defines regeneration zone 46 and provides means for regenerating the separated coked spent catalyst by contacting the separated coked spent catalyst with an oxygen-containing gas, such as air, under carbon burning conditions to remove carbon therefrom. The oxygen-containing gas is introduced into regeneration zone 46 through conduit and the combustion gases pass from regeneration zone 46 by way of conduit 50 to downstream for further handling or processing.

Second riser reactor 54 is an elongated conduit that extends vertically. The second hydrocarbon feedstock, which is preferably a naphtha or gasoline feedstock, is introduced by way of conduit 55 into second riser reactor zone 56 at or near the bottom of second riser reactor 54, wherein it is mixed or contacted with a second portion of regenerated cracking catalyst. The second portion of regenerated cracking catalyst passes from regenerator 26 by way of conduit 58 and is introduced into second riser reactor zone 56 by way of conduit The mixture of second hydrocarbon feedstock and the second portion of regenerated cracking catalyst passes through second riser reactor zone 56, which is operated under suitable second catalytic cracking conditions so as to provide a second riser reactor product.

The second riser reactor product, which comprises a second cracked product and a clean spent catalyst, is yielded from second riser reactor zone 56 and passes by way of conduit 60 from outlet 64 of second riser reactor 54 to be introduced into second separator/stripper 30 through inlet 66.

Second separator/stripper 30 provides means for receiving the second riser reactor product by way of conduit 60 and through inlet 66. Second separator/stripper 30 includes and defines second separation zone 68 and second stripping zone 70. One or more cyclones (not shown) may be included within second separation zone 68 to provide second separation means for separating the second riser reactor product into a separated second cracked product and a separated clean spent catalyst. Provided within second stripping zone 70 are baffles or trays (not shown) that provide for enhanced contact between the falling catalyst and a stripping fluid, such as steam, that is introduced into the second stripping zone by way of conduit 72, so as to assist in the stripping of the hydrocarbons from the falling catalyst.

The separated clean spent catalyst, which may be introduced into regeneration zone 46 of regenerator 26, passes from second stripping zone 70 by way of conduit 74. At least a portion of the separated clean spent catalyst up to and including the entire portion or flow of the separated clean spent catalyst can be introduced into regeneration zone 46 by way of conduit 76. Then, the remaining portion of the separated clean spent catalyst that is not introduced into regenerator 26 passes by way of conduit 34 for introduction into first riser reactor zone 14. Or, in another alternative embodiment of the invention, a remaining portion of the separated clean spent catalyst, which is that portion of the separated clean spent catalyst not passed to first riser reactor zone 14 through conduit 34, can be introduced into regeneration zone 46 by way of conduit 76. In this embodiment, at least a portion of the separated clean spent catalyst is passed by way of conduit 34 to be introduced into first riser reactor zone 14.

Regenerator 26 further provides means for regenerating the separated coked spent catalyst and the separated clean spent catalyst that are charged to it. Thus, regenerator 26 defines a regeneration zone 46 and provides means for regenerating separated coked spent catalyst and a remaining portion, or at least a portion, of separated clean spent catalyst to yield regenerated cracking catalyst.

A separated first cracked product passes from first separator/stripper 22 by way of conduit 80. A separated second cracked product passes from second separator/stripper 30 by way of conduit 82. Either the separated first cracked product or separated second cracked product, or a combination of the two streams, may be passed to a main fractionation column or system (not shown). The main fractionation column or system may be any separation system know to those skilled in the art for recovering and separating cracked product streams into various FCC products, such as, for example, cracked gas, cracked gasoline, cracked gas oils and cycle oil that respectively pass from main fractionation column. A main fractionation system may include such systems as absorbers and strippers, fractionators, compressors and separators or any combination of known systems for providing recovery and separation of the cracked products that may make up the separated first cracked product or the separated second cracked product, or both. In a preferred embodiment of the process, a combination of the separated first cracked product and the separated second cracked product passes to a main fractionation column which provides means for their separation into one or more product streams, as indicated above, including the cracked gasoline or a naphtha product stream.

In one embodiment of the inventive process, at least a portion of the cracked gasoline or naphtha product stream passing from the main fractionation system is recycled and utilized as the second hydrocarbon feedstock introduced into second riser reactor zone 34 by way of conduit 44.

Claims

1. A dual riser catalytic cracking process for making middle distillate and lower olefins, wherein said process comprises:

catalytically cracking a first hydrocarbon feedstock within a first riser reactor zone by contacting under first catalytic cracking conditions within said first riser reactor zone said first hydrocarbon feedstock with a combination of a clean spent catalyst and a first portion of a regenerated cracking catalyst to yield a first riser reactor product comprising a first cracked product and a coked spent catalyst;

catalytically cracking a second hydrocarbon feedstock within a second riser reactor zone by contacting under second catalytic cracking conditions within said second riser reactor zone said second hydrocarbon feedstock with a second portion of said regenerated cracking catalyst to yield a second riser reactor product comprising a second cracked product and said clean spent catalyst;

passing said first riser reactor product to a first separator/stripper providing means for separating said first riser reactor product into a separated first cracked product and a separated coked spent catalyst;

passing said second riser reactor product to a second separator/stripper providing means for separating said second riser reactor product into a separated cracked second cracked product and a separated clean spent catalyst;

using at least a portion of said separated clean spent catalyst as said clean spent catalyst of said combination; and

passing said separated coked spent catalyst and a remaining portion of said separated clean spent catalyst to a regenerator that defines a regeneration zone and provides means for regenerating said separated coked spent catalyst and said remaining portion of said separated clean spent catalyst to yield said regenerated cracking catalyst.

2. A process as recited in claim 1, which further comprises: passing said separated first cracked product and said separated second cracked product to a fractionator defining a fractionation zone and providing fractionation means for separating either said separated first cracked product or said separated second cracked product, or both, into one or more product streams including a naphtha product stream.

3. A process as recited in claim 2, which further comprises: using at least a portion of said naphtha product stream as at least a portion of said second hydrocarbon feedstock.

4. A process as recited in claim 3, wherein said first separator/stripper defines a first separation zone that includes first separation means for separating said first riser reactor product into said separated first cracked product and said separated coked spent catalyst, and wherein said first separator/stripper further defines a first stripping zone, and wherein within said first stripping zone said separated coked spent catalyst is stripped of hydrocarbons.

5. A process as recited in claim 4, wherein said second separator/stripper defines a second separation zone that includes second separation means for separating said second riser reactor product into said separated second cracked product and said separated clean spent catalyst, and wherein said second separator/stripper further defines a second stripping zone, and wherein within said second stripping zone said separated clean spent catalyst is stripped of hydrocarbons.

6. A dual riser cracking process for making middle distillate and lower olefins, wherein said process comprises:

catalytically cracking a first hydrocarbon feedstock within a first riser reactor zone by contacting under first catalytic cracking conditions within said first riser reactor zone said first hydrocarbon feedstock with a combination of a clean spent catalyst and a first portion of a regenerated cracking catalyst to yield a first riser reactor product comprising a first cracked product and a coked spent catalyst;

catalytically cracking a second hydrocarbon feedstock within a second riser reactor zone by contacting under second catalytic cracking conditions within said second riser reactor zone said second hydrocarbon feedstock with a second portion of said regenerated cracking catalyst to yield a second riser reactor product comprising a second cracked product and said clean spent catalyst;

passing said first riser reactor product to a first separator/stripper providing means for separating said first riser reactor product into a separated first cracked product and a separated coked spent catalyst;

passing said second riser reactor product to a second separator/stripper providing means for separating said second riser reactor product into a separated cracked second cracked product and a separated clean spent catalyst; and

passing said separated coked spent catalyst to a regenerator and passing said separated clean spent catalyst to said regenerator, wherein said regenerator defines a regeneration zone and provides means for regenerating said separated coked spent catalyst and said separated clean spent catalyst to yield said regenerated cracking catalyst.