KR20250025446A - Process for cracking into light olefins - Google Patents

Abstract

A process for the catalytic production of olefins comprises contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream. The first cracked product stream is separated in a main column. An overhead stream from the main column is separated into a second hydrocarbon stream. The second hydrocarbon stream is contacted with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cooling catalyst. A third hydrocarbon stream is obtained from the overhead stream and/or the second cracked product stream. The third hydrocarbon stream is contacted with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cooling catalyst.

Description

Process for cracking into light olefins

Statement of Priority

This application claims priority to U.S. Provisional Patent Application Serial No. 63/391,756, filed July 24, 2022, which is incorporated herein by reference in its entirety.

Technical field

This field relates to the reaction of feedstock with a fluid catalyst. In particular, this field relates to the FCC process for producing light olefins using multiple reactors.

Catalytic cracking can produce a variety of products from larger hydrocarbons. Typically, a feed of heavy hydrocarbons, such as vacuum gas oil, is provided to a catalytic cracking reactor, such as a fluidized catalytic cracking (FCC) reactor. A variety of products, including gasoline products and/or light products, such as propylene and/or ethylene, can be produced from such a system.

In such a system, either a single reactor or a dual reactor may be utilized. Using a dual reactor system may incur additional capital costs, but allows one of the reactors to be operated under controlled conditions to maximize products such as light olefins including propylene and/or ethylene.

It may often be advantageous to maximize the yield of a product from one of the reactors. Additionally, there may be a desire to maximize the production of a product from one reactor that can be recycled back to another reactor to produce a desired product, such as propylene.

Additionally, some dual reactor systems utilize a mixture of catalysts, such as larger pore catalysts and smaller pore catalysts. In some instances, the proportion of smaller pore catalysts limits the production of the desired light olefins. Ultimately, it would typically be advantageous to separate the catalysts to control their reactions.

Therefore, there may be a need to provide a reactor system for catalytic cracking which can maximize operating conditions for maximizing propylene production.

A process for the catalytic production of olefins comprises contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream. The first cracked product stream is separated in a main column. An overhead stream from the main column is separated into a second hydrocarbon stream. The second hydrocarbon stream is contacted with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cooling catalyst. A third hydrocarbon stream is obtained from the overhead stream and/or from the second cracked product stream. The third hydrocarbon stream is contacted with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cooling catalyst.

Additional details and embodiments of the present invention will become apparent from the following detailed description of the present invention.

The drawings are cross-sectional elevation views of the processes and devices of the present disclosure.
definition
The term "downstream connection" means that at least a portion of the fluid flowing into the subject of the downstream connection can be operatively flowed from the object in fluid communication therewith.
The term "upstream communication" means that at least a portion of the fluid flowing from the subject of the upstream communication can operatively flow to an object in fluid communication therewith.
The term "direct connection" means that fluid flow from the upstream component enters the downstream component without passing through any other intervening vessel.
The term "indirect communication" means that fluid flow from an upstream component enters a downstream component after passing through an intervening vessel.
The term "bypass" means that an object is removed from downstream communication with the bypassing subject, at least to the extent of bypassing.
As used herein, the term "dominant" or "dominant" means greater than 50%, suitably greater than 75%, preferably greater than 90%.
The term "column" means a distillation column or columns for separating one or more components of different volatilities. Unless otherwise specified, each column includes a condenser overhead of the column for condensing and refluxing a portion of the overhead stream back to the top of the column, and a reboiler at the bottom of the column for vaporizing and re-returning a portion of the bottoms stream to the bottom of the column. The feed to the column may be preheated. The overhead pressure is the pressure of the overhead vapor at the vapor outlet of the column. The bottoms temperature is the liquid bottoms outlet temperature. The overhead line and the bottoms line refer to the net line from any reflux or reboil column downstream to the column. A stripper column may omit the reboiler at the bottom of the column, and instead provide the heating requirement and separation drive from a fluidized inert medium such as steam. Typically, a stripping column feeds a top tray and takes the main product from the bottom.
As used herein, the term "separator" means a vessel having an inlet and at least an overhead vapor outlet and a bottom liquid outlet, and possibly also having an aqueous stream outlet from a boot. A flash drum is a type of separator that can be in downstream communication with a separator capable of operating at higher pressures.
As used herein, the term "boiling point temperature" means the atmospheric equivalent boiling point (AEBP) calculated from the observed boiling temperature and distillation pressure using the equation provided in ASTM D1160, Appendix A7, entitled "Practice for Conversion of Observed Vapor Temperatures to Atmospheric Equivalent Temperatures."
As used herein, the term "true boiling point" (TBP) means a test method for determining the boiling point of a material equivalent to ASTM D-2892 for the production of liquefied gases, distillate fractions, and residues of standardized quality from which analytical data can be obtained, and for the determination of the yield of said fractions, both by mass and volume, using 15 theoretical plates in a column with a reflux ratio of 5:1, and a graph of temperature versus mass % distilled.
As used herein, "pitch" means a hydrocarbon material boiling in excess of 524°C (975°F) AEBP as determined by any standard gas chromatography simulated distillation method, such as ASTM D2887, D6352 or D7169 (all of which are used by the petroleum industry).
As used herein, the terms “T5” or “T95” refer to the temperature at which 5 mass percent or 95 mass percent of the sample boils, as applicable, using ASTM D-86 or TBP.
As used herein, the term “initial boiling point (IBP)” means the temperature at which a sample begins to boil, as defined in ASTM D-7169, ASTM D-86 or TBP, as applicable.
As used herein, the term “end point” (EP) means the temperature at which a sample is completely boiled off using ASTM D-7169, ASTM D-86 or TBP, as applicable.
As used herein, "vacuum gas oil" means a hydrocarbon material having an IBP of at least 232° C. (450° F.), a T5 of from 288° C. (550° F.) to 392° C. (700° F.), typically not greater than 343° C. (650° F.), a T95 of from 510° C. (950° F.) to 570° C. (1058° F.), and/or an EP of not greater than 626° C. (1158° F.) as determined by any standard gas chromatography simulated distillation method, such as ASTM D2887, D6352 or D7169 (all of which are used by the petroleum industry).
As used herein, "atmospheric residue" means a hydrocarbon material obtained from the bottom of an atmospheric crude distillation column having an IBP of at least 232° C. (450° F.), a T5 of from 288° C. (550° F.) to 392° C. (700° F.), typically less than 343° C. (650° F.), and a T95 of from 510° C. (950° F.) to 700° C. (1292° F.).
As used herein, “vacuum residue” means a boiling hydrocarbon material having an IBP of at least 500°C (932°F).

The inventors found that the twice-cracked naphtha or the once-cracked or twice-cracked C4 hydrocarbons can still be cracked to produce more propylene. However, the conditions for cracking the twice-cracked naphtha or C4 hydrocarbons should be adjusted to maximize the propylene production. The inventors propose a third riser to provide favorable reaction conditions for maximizing the yield of propylene.

Now, referring to the drawings where like numbers represent like components, the process and apparatus generally include an FCC unit section (6) and a product recovery section (8). The FCC unit section (6) includes a first FCC reactor (10) including a first reactor unit (12) and a catalyst regenerator (14). Process conditions within the first FCC reactor (10) can include a cracking reaction temperature at the reactor outlet of from 400° C. to 600° C., preferably from 538° C. to 593° C., and a catalyst regeneration temperature of from 500° C. to 900° C. Both cracking and regeneration occur at an absolute pressure of from 100 kPa (14 psia) to 650 kPa (94 psia), preferably from 140 kPa (20 psia) to 450 kPa (65 psia).

Figure 1 illustrates a first FCC reactor vessel (12) in which a first hydrocarbon feedstock in line (15) is contacted via a distributor (16) with a first stream of fluid catalyst entering from a regeneration catalyst standpipe (18) and a recycle catalyst standpipe (19). The first hydrocarbon feedstock may comprise vacuum gas oil, atmospheric resid, deasphalted oil, vacuum resid or any other stream processed in a conventional FCC unit.

The catalyst may be a single catalyst or a mixture of different catalysts. Typically, the catalyst comprises two components or catalysts, a first component or catalyst, and a second component or catalyst. Such catalyst mixtures are disclosed, for example, in U.S. Pat. No. 7,312,370 B2. Typically, the first component may comprise any of the known catalysts used in the art of FCC, such as an active amorphous clay-type catalyst and/or a highly active crystalline molecular sieve. Zeolites may be used as the molecular sieve in the FCC process. Preferably, the first component comprises a large pore zeolite, such as a Y-type zeolite, an activated alumina material, a binder material comprising silica or alumina, and an inert filler such as kaolin.

Typically, a zeolite-like molecular sieve suitable for the first component has a large average pore size. A molecular sieve having a large pore size typically has pores having an effective diameter defined by a 10-membered ring, typically greater than 0.7 nm. The pore size index of the large pores can be greater than 31. Suitable large pore zeolite components can include synthetic zeolites, such as X and Y zeolites, mordenites and faujasites. Some of the first component, such as the zeolite, can have any suitable amount of a rare earth metal or rare earth metal oxide.

The second component may comprise a medium or small pore zeolite catalyst, such as an MFI zeolite, as exemplified by at least one of ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48 and other similar materials. Other suitable medium or small pore zeolites include ferrierite and erionite. Preferably, the second component is a medium or small pore zeolite dispersed in a matrix comprising a binder material such as silica or alumina and an inert filler material such as kaolin. The second component may also comprise some other active material, such as a beta zeolite. Such compositions may have a crystalline zeolite content of from 10 to 50 wt % or more and a matrix material content of from 50 to 90 wt %. A component containing 40 wt % crystalline zeolite material is preferred, although those having higher crystalline zeolite contents may be used. Typically, medium and smaller pore zeolites are characterized by having an effective pore opening diameter of less than 0.7 nm, a ring size of less than 10, and a pore size index of less than 31.

The total catalyst mixture within the first FCC reactor (12) can contain from 1 to 25 wt % of a second component, i.e., a medium to small pore crystalline zeolite, with greater than or equal to 1.75 wt % of the second component being preferred. The first component can comprise the remainder of the catalyst composition. In some preferred embodiments, the relative proportions of the first and second components within the mixture can remain substantially unchanged throughout the first FCC reactor (12). A high concentration of medium or small pore zeolite as the second component of the catalyst mixture can improve selectivity to light olefins. In one exemplary embodiment, the second component can be a ZSM-5 zeolite, and the mixture can comprise from 4 to 10 wt % of the ZSM-5 zeolite, excluding any other components such as a binder and/or filler.

Preferably, at least one of the first and/or second catalysts is an MFI zeolite having a silicon to aluminum ratio greater than 15, preferably greater than 75. In one exemplary embodiment, the silicon to aluminum ratio can be from 15:1 to 35:1.

Contacting can occur in a narrow first riser (20) extending upwardly into the bottom of the first reactor vessel (22). Contact of the first hydrocarbon feedstock and the first stream of fluid catalyst is fluidized by a gas, such as steam, from a fluidized distributor (24). In one embodiment, heat from the catalyst vaporizes the first hydrocarbon feedstock, which is then cracked into a first stream of lighter molecular weight cracked product in the presence of the first catalyst stream, both of which are passed up the riser (20) into the reactor vessel (22) to provide a first mixture of catalyst and product gases.

The pressure within the first riser (20) may be from 200 kPa (29 psia) to 450 kPa (65 psia), but may be lower. A first hydrocarbon feedstock having a steam rate of from 3 to 7 wt % is added to the first riser (20). Unavoidable side reactions occur in the first riser (20) leaving coke deposits on the catalyst, reducing catalytic activity and providing a spent catalyst stream. The first cracked product stream within the first mixture of catalyst and product gases is then separated from the spent catalyst stream using a cyclone separator which may include one or two stages of cyclones (62) within the reactor vessel (22). The gaseous first cracked product stream exits the reactor vessel (22) through the first product outlet (31) in line (32) for transport to the downstream product recovery section (8).

The spent catalyst or coked catalyst requires regeneration for further use. After being separated from the first cracked product stream by the separation device (54) within the first separation chamber (56), the spent catalyst stream falls into the stripping section (34) where steam is injected through the distributor (35) to purge any residual hydrocarbon vapors. After the stripping operation, the stripped coked catalyst is conveyed to the catalyst regenerator (14) through the spent catalyst standpipe (36). Another portion of the stripped coked catalyst can be recycled to the riser (20) by the recycle catalyst standpipe (19) without undergoing regeneration.

Figure 1 illustrates a regenerator (14), known as a combustor. However, other types of regenerators are also suitable. In the catalyst regenerator (14), a stream of oxygen-containing gas, such as air, is introduced through an air distributor (38) and contacts the coked catalyst. The coke is combusted from the coked catalyst in a combustion chamber (80) to provide regenerated catalyst and flue gas. The catalyst regeneration process imparts a significant amount of heat to the catalyst, providing energy to offset the endothermic cracking reactions occurring in the first riser (20). The catalyst and air flow together upwardly in the combustion chamber (80) of the regenerator (14) and, after regeneration, are initially separated by discharge through a separator (40) and enter a separation chamber (86). Additional recovery of the regenerated catalyst and flue gas exiting the separator (40) is accomplished using first and second stage separator cyclones (44, 46), respectively, within the separation chamber (86) of the catalyst regenerator (14). The catalyst separated from the flue gas is distributed through the deep leg from the cyclones (44, 46), while the flue gas, which is relatively lighter among the catalysts, sequentially exits the cyclones (44, 46) and exits the regenerator vessel (14) through the flue gas outlet (47) in the flue gas line (48). The regenerated catalyst is transported back to the riser (20) through the regenerated catalyst standpipe (18). As a result of the coke combustion, the flue gas vapor exiting from the top of the catalyst regenerator (14) contains CO, CO2, N2 and H2O along with lesser amounts of other species.

The product recovery section (8) is in downstream communication with the product outlet (31). In the product recovery section (8), the first cracked product stream in line (32) is directed to the lower section of the FCC main fractionation column (92). The main column (92) is in downstream communication with the first product outlet (31). Several fractions of the FCC product can be separated and taken from the main column including the heavy slurry oil from the bottom in line (93), the heavy cycle oil stream in line (94), the light cycle oil in line (95) taken from outlet (95a), and the heavy naphtha stream in line (96) taken from outlet (96a). Any or all of lines (93-96) can be cooled and pumped back to the main column (92), typically at a higher elevation to cool the main column. Gasoline and gaseous light hydrocarbons are removed from the main column (92) in the main overhead line (97) and condensed prior to entering the main column receiver (99). The main column receiver (99) is in downstream communication with the product outlet (31), and the main column (92) is in upstream communication with the main column receiver (99). A second hydrocarbon stream and possibly a third hydrocarbon stream are taken from the main overhead line (97).

The aqueous stream is removed from the boot in the main column receiver (99). Additionally, a condensed light naphtha stream is removed in line (101), while an overhead stream is removed in line (102). The overhead stream in line (102) contains highly olefinic gaseous light hydrocarbons. The streams in lines (101 and 102) can enter the vapor recovery section (120) of the product recovery section (8).

The vapor recovery section (120) is shown as an absorption-based system, but any vapor recovery system, including a cold box system, may be used. To obtain sufficient separation of the light gas components, the gaseous stream in line (102) is compressed in compressor (104). More than one compressor stage may be used, but typically a dual stage compression is utilized. The compressed light hydrocarbon stream in line (106) is joined by the streams in lines (107, 108, and 422), cooled, and conveyed to a high pressure receiver (110). The aqueous stream from the receiver (110) may be sent to a main column receiver (99). The gaseous hydrocarbon stream in line (112) is sent to a primary absorber (114) where it is contacted with the destabilized gasoline from the main column receiver (99) in line (101) to effect separation between the C3+ and C2- hydrocarbons. The primary absorber (114) is in downstream communication with the main column receiver (99). The liquid C3+ hydrocarbon stream in line (107) is returned to line (106) before cooling. The primary off-gas stream in line (116) from the primary absorber (114) can be directed to a secondary absorber (118) where a circulating stream of light cycle oil in line (121) diverted from line (95) absorbs most of the remaining C5+ and some C3-C4 hydrocarbons in the primary off-gas stream. The secondary absorber (118) is in downstream communication with the primary absorber (114). Light cycle oil from the bottom of the secondary absorber in line (119), which is richer in C3+ hydrocarbons, is returned to the main column (92) via a pump-around to line (95). The overhead from the secondary absorber (118), which comprises dry gas of C2- hydrocarbons, mainly hydrogen sulfide, ammonia, carbon oxides and hydrogen, is removed in the secondary off-gas stream in line (122).

Liquid from the high pressure receiver (110) in line (124) is sent to a stripper (126). Most of the C2- hydrocarbons are removed overhead from the stripper (126) and returned to line (106) via overhead line (108). A liquid bottoms stream from the stripper (126) is sent in bottoms line (128) to a first debutanizer column (130). The first debutanizer column (130) provides an overhead stream in line (132) comprising a C3-C4 hydrocarbon stream from the first debutanizer column. The bottoms stream in line (134) may comprise a first debutanized naphtha stream.

In a first embodiment, the first recycle light cracked naphtha stream may be taken in line (137) from the first debutanized naphtha stream in line (134) through a control valve thereupon, while the remainder of the first debutanized naphtha stream in line (141) may be further processed into gasoline or other products through a control valve thereupon. In this embodiment, it is envisioned that the naphtha splitter column (136) may be located upstream from the product recovery section (8).

In an alternative embodiment, the first debutanized naphtha stream in line (134) may be fed to a naphtha splitter column (136) in line (142) via a control valve thereon. In this alternative embodiment, the naphtha splitter column is located downstream from the product recovery section (8) as shown in the drawing, and the control valves on lines (137 and 141) would be closed. The naphtha splitter column separates the debutanized naphtha stream into a first split light naphtha stream in overhead line (138) comprising C5-C7 hydrocarbons and a heavy naphtha stream in bottoms line (140). The alternative first recycle light cracked naphtha stream may be taken in line (139) via a control valve thereon, while the remainder of the first split light naphtha stream in overhead line (138) may be further processed into gasoline or other products.

The C3-C4 hydrocarbon stream taken in line (132) can be separated into a C3 hydrocarbon stream in overhead line (146) and a C4 hydrocarbon stream in bottom line (148) in a C3-C4 splitter column (144). The first recycle C4 hydrocarbon stream can be taken in line (149) through a control valve thereon, while the remaining C4 hydrocarbon stream can be further processed into other products in line (147). The C3 hydrocarbon stream in overhead line (146) can be further processed for propylene recovery.

One or both of a first recycle light cracked naphtha stream taken in line (137) from a first debutanized naphtha stream in line (134) from a first debutanized naphtha stream in line (134) from a main column (92) in downstream communication with the main column (92) containing olefinic C5-C7 hydrocarbons or an alternative first recycle light cracked naphtha stream taken from an overhead line (138) of a naphtha splitter column (136) in downstream communication with the main column (92) in line (139) containing olefinic C5-C7 hydrocarbons and a first recycle C4 stream comprising olefinic C4 hydrocarbons taken from a bottom line (148) of a C3-C4 splitter column also in downstream communication with the main column (92) in line (149) are fed as a second hydrocarbon stream to a second FCC in a second charging line (150). The second hydrocarbon stream can be preheated to a temperature of 221° C. (400° F.) to 704° C. (1300° F.) and charged to the second FCC reactor (202).

The drawing depicts a second FCC reactor (200) comprising a second reactor unit (202) and a catalyst heater (238). The second reactor unit (202) comprises a second riser (212) wherein a second hydrocarbon stream in line (150) charged through a distributor (213) or more near the base of the second riser (212) contacts a second stream of fluid catalyst in the second riser (212). The second hydrocarbon stream can comprise at least 20 wt % olefins, suitably at least 60 wt % olefins, preferably at least 70 wt % olefins. The second hydrocarbon stream can comprise at least 1 wt % paraffins, suitably at least 15 wt % paraffins, preferably at least 25 wt % paraffins. The second hydrocarbon stream can comprise once cracked C4 to C7 hydrocarbons.

A second riser (212) extends upwardly through the second reactor vessel (210) within the second FCC reactor (202). A second stream of fluid catalyst may be fluidized with steam distributed from a distributor (218) at the bottom of the second riser (212). A second hydrocarbon stream contacts the second stream of fluid catalyst in the second riser (212). The second stream of fluid catalyst may be provided by a mixture of a first stream of hot catalyst from the first hot catalyst pipe (220) and a first stream of recycle catalyst from the first recycle catalyst pipe (222). The second hydrocarbon stream is vaporized and converted or cracked into a second cracked product stream comprising ethylene and propylene in a higher concentration than in the second hydrocarbon stream. By molar expansion, the second hydrocarbon stream and the second cracked product stream rapidly ascend the second riser (212) entraining a second stream of fluid catalyst as a second mixture of catalyst and product gases.

The second stream of fluid catalyst can comprise less than 20 wt %, preferably less than 5 wt %, of the first component and at least 20 wt % of the second component. In a preferred embodiment, the second stream of fluid catalyst can comprise at least 20 wt % of the ZSM-5 zeolite and less than 20 wt %, preferably less than 5 wt % of the Y-zeolite. In another preferred embodiment, the second stream of fluid catalyst can comprise primarily the second component, and in a further embodiment can contain only the second component, preferably the ZSM-5 zeolite, as the catalyst.

In one embodiment, the second stream of the fluid catalyst can contain from 0.005 wt % to 1.2 wt % coke. The presence of coke at these concentrations helps to passivate the acid sites and prevent the production of dry gas. Dry gas can be defined as H ₂ , H ₂ S, carbon oxides and C ₁ -C ₂ hydrocarbons. Dry gas represents a yield loss of valuable products and increases operating and equipment costs due to the processing of larger gas flow rates. The presence of coke in the riser in amounts from 0.005 wt % to 1.2 wt % sufficiently passivates the acid sites, which ultimately limits the production of dry gas.

The coke concentration in the second riser is increased by circulating the non-regenerated catalyst to the riser and promoting coke formation in the riser. If the FCC reaction produces coke in the catalyst, recycling the first stream of catalyst in the first recycle catalyst pipe (222) to the second riser (212) will increase the coke in the riser because it bypasses regeneration. Increasing the recycle rate of the first recycle catalyst relative to the recycle rate of the first stream of hot catalyst from the first hot catalyst pipe (220) will increase the coke concentration in the second riser. Some types of catalyst, such as ZSM5, produce little coke in an FCC reactor. If coke formation in the second riser is not sufficient, other methods can be used to increase the coke concentration in the second riser, which are described below.

Process conditions in the second riser (212) will be more stringent than in the first riser (20) because the second hydrocarbon stream was previously cracked in the first riser (20) and is therefore less crackable than the first hydrocarbon stream. The second riser (212) may operate at one or more of the following conditions relative to the first riser (20): a higher outlet temperature, a lower hydrocarbon partial pressure, or a different catalyst density. The hydrocarbon partial pressure is reduced by reducing the total pressure within the second riser (212) independent of the pressure within the first FCC reactor (10) and possibly by adjusting the steam rate to the second riser (212).

Conditions in the second riser (212) can include a cracking reaction temperature of from 400° C. to 650° C., preferably from 565° C. to 635° C. at the reactor outlet. Cracking occurs at an absolute pressure of from 100 kPa (14 psia) to 506 kPa (74 psia), preferably from 138 kPa (20 psia) to 310 kPa (45 psia). A second hydrocarbon stream at a steam flow rate of from 5 to 25 wt % is added to the second riser (20). However, the steam velocity in the second riser (212) can be as low as 2 to 25 wt % or can be eliminated. Control valves on the first high temperature catalyst pipe (220) and the first recycle catalyst pipe (222) can be used to adjust the catalyst density within the second riser (212), and thus to allow control of the space velocity therein. Additionally, increasing the flow rate of the first recirculation catalyst into the second riser (212) within the first recirculation catalyst pipe (222) can increase the catalyst density within the second riser without affecting the heat input to the second riser that supplied the first stream of high temperature catalyst within the first high temperature catalyst pipe (220) supplied to the second riser (212).

The second riser (212) terminates at the upper end of a second separation chamber (211) located within the second reactor vessel (210) in a curved duct (214) or a plurality thereof. The curved duct (214) can centrifugally discharge a second mixture of product gas and catalyst into the second separation chamber (211). The centrifugal discharge causes the first mixture to be discharged from the inside to the outside. The centrifugal discharge of the gas and catalyst creates a swirling spiral pattern within the second separation chamber (211), thereby separating the second mixture of catalyst and product gas into a second decomposed product stream and a first stream of cooled catalyst within the second separation chamber (211).

A first stream of cooled catalyst is collected in the dense catalyst bed (228). A second stream of product gas passes upward through a second gas recovery conduit (226), is further separated from the catalyst in a cyclone (232), and is discharged from the second reactor vessel (210) through an outlet (230) in a product line (231).

The drawing depicts a third FCC reactor unit (302) wherein a third hydrocarbon stream in line (315) distributed through a distributor (313) or more near the base of the third riser (312) contacts a third stream of fluid catalyst in the third riser. In one embodiment, the third FCC reactor unit (302) is integrated with the second FCC reactor unit (202) into the second FCC reactor (200). However, the third FCC reactor unit (302) may exist standalone in its own FCC reactor from the second FCC reactor unit (202). The third hydrocarbon stream may comprise at least 20 wt % olefins, typically at least 30 wt % olefins, suitably at least 50 wt % olefins and preferably at least 60 wt % olefins. The third hydrocarbon stream can comprise at least 25 wt% paraffins, and preferably at least 35 wt% paraffins. The second hydrocarbon stream is typically more olefinic and/or more crackable than the third hydrocarbon stream. The third hydrocarbon stream can comprise a twice-cracked light cracked naphtha stream and/or a twice-cracked C4 hydrocarbon stream and/or a once-cracked C4 hydrocarbon stream. The third hydrocarbon stream can be preheated to a temperature of from 221° C. (400° F.) to 704° C. (1300° F.) and charged to the third FCC reactor (302).

A third stream of catalyst can be fluidized with steam distributed from a distributor (358) at the bottom of the third riser (312). The third stream of catalyst can have the same catalyst composition as the second stream of catalyst. The first stream of catalyst has a different catalyst composition than the catalyst compositions of the second stream of catalyst and the third stream of catalyst. The third hydrocarbon stream contacts the third stream of fluid catalyst within the third riser (312). The third stream of fluid catalyst can be provided by a mixture of the second stream of hot catalyst from the second hot catalyst pipe (362) and the second stream of recycle catalyst from the second recycle catalyst pipe (264). The third hydrocarbon feedstock is converted or cracked into a third cracked product stream comprising hydrocarbons of lower molecular weight than the third hydrocarbon stream. By molar expansion, the third hydrocarbon stream and the third cracked product stream rapidly ascend the third riser (312) entraining the third stream of fluid catalyst as a third mixture of catalyst and product gases.

Process conditions in the third riser (312) can be more stringent than in the second riser (212) and the first riser (20). The third riser (312) can operate at one or more of the following conditions relative to the second riser (212): a higher outlet temperature, a lower hydrocarbon partial pressure, and a different catalyst density. The hydrocarbon partial pressure can be reduced by reducing the total pressure within the third riser (312) independent of the pressure within the first FCC reactor (10) and possibly by adjusting the steam velocity to the third riser (312).

Conditions in the third riser (312) can include a cracking reaction temperature of from 400° C. to 705° C., preferably from 565° C. to 675° C. at the reactor outlet. Cracking occurs at an absolute pressure of from 100 kPa (14 psia) to 506 kPa (74 psia), preferably from 138 kPa (20 psia) to 310 kPa (45 psia). Steam at a third hydrocarbon stream rate of from 25 to 50 wt % is added to the third riser (312). However, the steam rate in the third riser (312) can be as low as 2 to 50 wt % or can be absent. Control valves on the second high temperature catalyst pipe (362) and the second recycle catalyst pipe (264) can be used to adjust the catalyst density within the third riser (312), and thus to allow control of the space velocity therein. Additionally, increasing the flow rate of the second recycle spent catalyst stream into the third riser (312) within the second recycle catalyst pipe (264) can increase the catalyst density within the third riser (312) without affecting the heat input to the third riser.

The coke concentration in the third riser (312) is increased by circulating the non-regenerated catalyst to the riser and promoting coke formation in the riser. If the FCC reaction produces coke in the catalyst, recycling the second stream of recycle catalyst in the second recycle catalyst pipe (264) to the third riser (312) will increase the coke in the riser because it bypasses regeneration. Increasing the recycle rate of the second recycle catalyst stream relative to the recycle rate of the second stream of hot catalyst from the second hot catalyst pipe (362) will increase the coke concentration in the third riser (312). Some types of catalyst, such as ZSM5, produce little coke in an FCC reactor. If coke formation in the third riser (312) is not sufficient, other methods can be used to increase the coke concentration in the third riser, which are described below.

The third riser (312) of the third FCC reactor (302) may be located external to the second reactor vessel (210), but may share the second reactor vessel with the second riser (212) and the second FCC reactor (202). The third riser (312) includes a discharge opening (349) within a third separation chamber (360). The third separation chamber (360) receives the discharge opening (349) of the third riser (312). The third separation chamber (360) may be within the second reactor vessel (210). In one embodiment, the horizontal transfer line (348) of the third riser (312) terminates at the third separation chamber (360). The discharge opening (349) of the third riser (302) tangentially discharges the third mixture of catalyst and product gases into the third separation chamber (360). In other embodiments, the horizontal transfer line (348) may be exchanged for an alternative connector, such as a T-type connector, or an elbow having a more acute or more obtuse angle. When the third mixture of catalyst and product gases is discharged tangentially from the third riser (312) through the discharge opening (349), a spiral spiral pattern is created within the interior of the third separation chamber (360). When the third mixture of catalyst and product gases is separated into a second stream of cooled catalyst and a third cracked product stream, the second mixture of catalyst and product gases can be separated outwardly and concentrically into the first stream of cooled catalyst and the second cracked product stream. It is important that the second mixture of catalyst and product gases is not mixed with the third mixture of catalyst and product gases until the bulk of the catalyst is removed from the product gas to maximize selectivity to propylene.

In one embodiment, the second stream of cooled catalyst is collected from the dense catalyst bed (228) together with the first stream of cooled catalyst. In a further embodiment, the third stream of cracked product passes upward through the second gas recovery conduit (226) together with the second stream of cracked product, is further separated from the catalyst in a cyclone (232), and is discharged from the second reactor vessel (210) through an outlet (230) in the product line (231) as a second product stream.

A mixed stream of cooled catalyst separated from the dense catalyst bed (228) passes downwardly through a stripping section (284). A stripping fluid, typically steam, enters the lower portion of the stripping section (284) through a distributor (234). Countercurrent contact of the stripping fluid and the catalyst through a series of stripping baffles, packing or grates displaces product gases from the catalyst as it continues downwardly through the stripping section (284).

A first stream of stripped catalyst from the stripping section (284) passes through a heater conduit (236) to a catalyst heater (238). In one embodiment, the catalyst heater (238) heats the catalyst by heat exchange with regenerated catalyst from the regenerator. In a second embodiment, the catalyst heater (238) contacts the first stream of stripped catalyst with an oxygen feed gas from line (219) to combust coke from the catalyst. The flue gas is discharged in line (242). The second embodiment is applicable when the catalyst produces sufficient coke in the FCC reaction. The catalyst heater (238) provides a first stream of hot catalyst in a first hot catalyst pipe (220) that supplies to a second riser (212) and a second stream of hot catalyst in a second hot catalyst pipe (362) that supplies to a third riser (312).

A second stream of catalyst stripped from the dense layer (228) passes through the recirculation conduit (240) to provide a first stream of recirculated catalyst within the first recirculation catalyst pipe (222) to the second riser (212) and a second stream of recirculated catalyst within the second recirculation catalyst pipe (264) to the third riser (312). In the first embodiment of the catalyst heater (238), the catalyst within the second FCC reactor (202) and the third FCC reactor (302) may not be coked as much as in the first FCC reactor (12). Thus, in the second and third FCC reactors (202, 302), insufficient coke may be burned to balance the heat demand within the reactors. To supplement heat in the second FCC reactor (202) and the third FCC reactor (302), a portion of the high temperature regenerated catalyst may be conveyed to a catalyst heater (238) within the regenerator heater standpipe (42). In the catalyst heater, a portion of the second stream of fluid catalyst may be heat exchanged with a portion of the high temperature regenerated catalyst prior to contacting the second stream of fluid catalyst with the second hydrocarbon stream. Additionally, a portion of the third stream of fluid catalyst may be heat exchanged with a portion of the high temperature regenerated catalyst prior to contacting the third stream of fluid catalyst with the third hydrocarbon stream. In the catalyst heater (238) of the first embodiment, the high temperature regenerated catalyst may be on one side of an indirect heat exchanger, while the second stream of fluid catalyst and the third stream of fluid catalyst may be on the other side of the indirect heat exchanger. Catalyst from the second FCC reactor (202) and the third FCC reactor (302) may be conveyed to the catalyst heater (238) by heater conduits (236). The cooled regenerated catalyst can be returned to the regenerator (14) from the catalyst heater (238) within the cooled regenerated catalyst conduit (48).

It is envisioned that a heat exchange fluid may also be used in the first embodiment of the catalyst heater (238) to transfer heat to the second and third catalyst streams from the regenerated catalyst instead of direct heat exchange between the catalyst streams. In one embodiment, heat may be exchanged directly or indirectly from flue gas from the flue gas outlet (47) to heat the second and third catalyst streams. In another embodiment, the catalyst heater (238) may have fuel combustion to supply the necessary heat. A portion of the second stream of fluid catalyst is heated in the catalyst heater (238) prior to contacting the second stream of fluid catalyst with the second hydrocarbon stream. A portion of the third stream of fluid catalyst is heated in the catalyst heater (238) prior to contacting the third stream of fluid catalyst with the third hydrocarbon stream. The flue gas stream generated in line (242) out of the catalyst heater (238) may be suitably sent to a flue gas treatment unit. In another embodiment, the fuel combustion in the catalytic heater (238) can be replaced by an electric coil powered by renewable or fossil fuel-based electricity.

To generate more coke on the catalyst, the C4+ hydrocarbon stream can be fed to the catalyst heater (238) of the first embodiment from one or more of the first coking line (221) directly to the catalyst in the catalyst heater (238), or the second coking line (223) as a first stream of hot catalyst in the first hot catalyst pipe (220), the third coking line (225) as a second stream of hot catalyst in the second hot catalyst pipe (362), the fourth coking line (227) as a recirculation catalyst in the first recirculation catalyst pipe (222), and the fifth coking line (229) as a recirculation catalyst in the second recirculation catalyst pipe (264). The C4+ injection into the catalyst pipe will help to optimize the coke level in the catalyst which will maximize the propylene selectivity in the second and third risers. The C4+ hydrocarbon injection should be located downstream of the control valve in the catalyst pipe. The coke distribution within the catalyst will be from 0.005 to 5 wt % coke on the catalyst, preferably from 0.1 to 2 wt % coke on the catalyst in the second riser (212), and from 0.1 to 1 wt % coke on the catalyst in the third riser (312). In one embodiment, the coke on the catalyst can be from 0.005 to 1.2 wt % coke on the catalyst in the second riser (212) and the third riser (312). Additionally, if desired, a hydrocarbon stream can be supplied to the catalyst heater (238) of the second embodiment from the first coking line (221) to increase the heat of combustion and provide more heat input for the FCC reaction in the second reactor (202) and the third reactor (302).

Coked catalyst from the stripping section (284) can be directly and periodically distributed to the catalyst regenerator (14) within the distribution conduit (250). The distributed catalyst can be combined with the catalyst mixture within the catalyst regenerator (14) and can provide additional catalytic activity to the catalyst within the FCC unit section (6).

The second product stream in line (231) from the second riser (212) and the third riser (312) can be passed to a wash column (392). In the wash column (392), the second product stream in line (231) is contacted with a wash stream. In one embodiment, the wash stream can be a fresh charge stream in line (394) that contacts the hot second product stream to achieve direct heat exchange. The direct heat exchange quenches the hot second product stream and absorbs catalyst fines from the second product stream. The quenched charge stream having catalyst fines exits the bottom of the wash column (392) in line (15) and is taken as the first hydrocarbon stream that is charged to the first FCC reactor (12).

In another embodiment, the second product stream in line (231) can be used to preheat the second hydrocarbon stream (150) and/or the third hydrocarbon stream (315) by indirect heat exchange prior to passing to the wash column (392). In another embodiment, the second hydrocarbon stream (150) and/or the third hydrocarbon stream (315) can be preheated by the flue gas stream (48) from the flue gas outlet (47).

The quenched second product overhead stream is taken from the wash overhead line (396) from the top of the wash column (392) after some stages of cooling pump-around to further cool the second product stream in line (396) to the product recovery section (90). The wash column (392) is in downstream communication with the second product outlet (231). In the product recovery section (90), the second product stream in the wash overhead line (396) is directed to the wash column receiving section (399). The wash column receiving section (399) is in downstream communication with the second product outlet (231).

The aqueous stream is removed from the boot within the wash column receiver (399). Additionally, a condensed light naphtha stream is removed in line (401), while an overhead gas stream is removed in the wash receiver overhead line (402). The overhead stream in line (402) contains highly olefinic gaseous light hydrocarbons. The streams in lines (401 and 402) may enter the wash recovery section (420) of the product recovery section (90). In another embodiment, the wash column (392) may be an integral part of the product recovery section (8) by utilizing a downstream depropanizer column (not shown) within the unillustrated and first debutanizer column (130) within the vapor recovery section (120).

The wash recovery section (420) is shown as an absorption-based system, but any vapor recovery system, including a cold box system, may be used. To obtain sufficient separation of the light gas components, the overhead stream in line (402) is compressed in compressor (404). More than one compressor stage may be used, but typically a dual stage compression is utilized. The compressed light hydrocarbon stream in line (406) is passed to a high pressure receiver (410). Aqueous streams may be taken from receivers (399 and 410). The high pressure liquid stream from the bottom of receiver (410) in line (412) is sent to the depropanizer column (420) along with the light naphtha stream in line (401) from the bottom of the wash column receiver (399) in line (414).

The depropanizer column (420) separates the high pressure liquid stream in line (412) and the light naphtha stream in line (401) into a C3- hydrocarbon stream in line (422), which is added to the compressed light hydrocarbon stream in line (106) and processed therein. The depropanized bottoms stream in line (424) can be sent to a debutanizer flash drum (426). In the debutanizer flash drum (426), the depropanized bottoms stream is separated into a second C4 hydrocarbon stream in overhead line (432) and a debutanizer naphtha stream in bottoms line (434). The second C4 hydrocarbon stream can be recycled in line (432) through a control valve thereon to a second recycle stream in line (314).

The debutanizer feed stream in line (434) is fed to a second debutanizer column (440). The second debutanizer column (440) separates the debutanizer feed stream into a C4 product stream in overhead line (442) and a second light cracked naphtha stream in bottoms line (444). The second recycle light cracked naphtha stream can be taken in line (446) via a control valve thereon, while the remainder can be further processed into gasoline or other products. The second recycle light cracked naphtha stream in line (446) can be fed to the second recycle stream in the second recycle line (314).

One or both of the second recycle light cracked naphtha stream taken from the second debutanizer column (440) in line (446) and downstream from the wash column (392) and containing olefinic C5-C7 hydrocarbons and the second recycle C4 stream taken from the debutanizer flash drum (426) in line (432) and also downstream from the wash column (392) can be recycled as a third hydrocarbon stream in the third charge line (315) to the third FCC reactor (302). The third hydrocarbon stream can be preheated to a temperature of from 204° C. (400° F.) to 704° C. (1300° F.) and charged to the third FCC reactor (302).

The second hydrocarbon stream and/or the third hydrocarbon stream may be supplemented with a C4 hydrocarbon stream, a C5 hydrocarbon stream or a light cracked naphtha stream from a refinery or a steam cracking unit. Such streams will typically contain greater than 20 wt% olefins.

In a further embodiment, part or all of the first C4 hydrocarbon stream in line (149) may be taken from line (160) through a control valve thereon and fed to the second recycle line (314). The first C4 hydrocarbon stream fed to the second recycle line (314) may be taken as all or part of the third hydrocarbon stream in the third charge line (315) as a third hydrocarbon stream.

The third FCC reactor (302) can be used to further crack the olefinic feed to produce additional propylene by cracking difficult to crack feeds in the third riser (312) under conditions that are more severe than those in the second riser (212) and are uniquely favorable for propylene production for the third hydrocarbon stream.

Example

Example 1

The hydrocarbon feed was cracked in three separate risers having the compositions given in the table. The C4 hydrocarbons from the first riser effluent were charged to the second riser and consequently the C4 hydrocarbons from the second riser were fed to the third riser. Only propylene was reported in the third riser effluent. The yields are given in Table 1. The riser conditions were consistent with those taught in the detailed description.

[Table 1]

By using three separate risers, it is possible to produce more than 26 wt% propylene as a percentage of the feed.

Example 2

The hydrocarbon feed was cracked in two separate risers having the compositions within the table. All C5-C7 hydrocarbons from the first riser effluent were fed to the second riser. In Case 1, the MFI-type catalyst in the second riser was completely regenerated such that the catalyst contained less than 0.005 wt. % coke. In Case 2, the catalyst in the second riser contained coke with a distribution ranging from 0.005 to 1.2 wt. % over the MFI-type catalyst. The total product yields for the first and second risers are provided in Table 2. The riser conditions are consistent with those taught in the Detailed Description.

[Table 2]

By recycling C5-C7 hydrocarbons from the first riser to the second riser, more light olefins such as ethylene, propylene and butylenes are produced. With ZSM-5 catalyst, propylene selectivity of 40 to 60% is achieved. The partially coked catalyst improves propylene and butylene yields while reducing dry gas and heavy material production.

Specific implementation examples

While the following is described with specific implementation examples, it will be understood that this description is for illustrative purposes only and is not intended to limit the scope of the foregoing description and the appended claims.

A first embodiment of the present invention is a process for the catalytic production of olefins, comprising: contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream; separating the first cracked product stream in a main column; separating an overhead stream from the main column into a second hydrocarbon stream; contacting the second hydrocarbon stream with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cooling catalyst; and obtaining a third hydrocarbon stream from the overhead stream and/or from the second cracked product stream; and contacting the third hydrocarbon stream with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cooling catalyst. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, wherein the second hydrocarbon stream is more olefinic and/or more crackable than the third hydrocarbon stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, wherein the second hydrocarbon stream is a light cracked naphtha stream, further comprising separating an overhead stream from the main column into a first C4 hydrocarbon stream and taking the first C4 hydrocarbon stream as a third hydrocarbon stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, further comprising washing the second cracked product stream in a wash column and obtaining a third hydrocarbon stream from the wash column. One embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, wherein the second hydrocarbon stream is a first C4 hydrocarbon stream, and the third hydrocarbon stream is a second C4 hydrocarbon stream. One embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, further comprising the steps of compressing an overhead stream from the wash column to provide a compressed overhead stream, depropanizing the compressed overhead stream to provide a depropanized compressed overhead stream, and taking a third hydrocarbon stream from the depropanized compressed overhead stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, further comprising separating the depropanized compressed overhead stream into a second C4 stream and a second light cracked naphtha stream, and taking a third hydrocarbon stream from the second light cracked naphtha stream and/or from the second C4 stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, wherein the second stream of fluid catalyst and the third stream of fluid catalyst comprise from 0.005 to 1.2 wt % coke. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, wherein the third riser operates at a higher outlet temperature and/or a lower hydrocarbon partial pressure and/or a different catalyst density than the second riser. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, further comprising regenerating a stream of spent catalyst by combustion of coke from the spent catalyst to provide a high temperature regenerated catalyst, and heat exchanging a portion of the high temperature regenerated catalyst with a portion of the second stream of fluid catalyst prior to contacting the second stream of fluid catalyst with the second hydrocarbon stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, further comprising regenerating a stream of spent catalyst by combustion of coke from the spent catalyst to provide a high temperature regenerated catalyst, and heat exchanging a portion of the high temperature regenerated catalyst stream with a portion of the third stream of fluid catalyst prior to contacting the third stream of fluid catalyst with the third hydrocarbon stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the first embodiment of this paragraph, wherein the third hydrocarbon stream has greater than 20 wt % olefins. One embodiment of the present invention is one, any or all of the preceding embodiments of this paragraph through the first embodiment of this paragraph, further comprising the step of recycling the C4+ stream to a catalyst pipe that provides catalyst to the second riser and/or the third riser.

A second embodiment of the present invention is a process for the catalytic production of olefins, comprising: contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream; separating the first cracked product stream in a main column; obtaining a second hydrocarbon stream from the main column; contacting the second hydrocarbon stream with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cooling catalyst; and separating the second cracked product stream in a wash column and obtaining a third hydrocarbon stream from the wash column; and contacting the third hydrocarbon stream with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cooling catalyst. An embodiment of the present invention is one, any or all of the preceding embodiments through the second embodiment of this paragraph, further comprising the steps of compressing an overhead stream from the wash column to provide a compressed overhead stream, depropanizing the compressed overhead stream to provide a depropanized compressed overhead stream, and taking a third hydrocarbon stream from the depropanized compressed overhead stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the second embodiment of this paragraph, wherein the third hydrocarbon stream has greater than 20 wt % olefins.

A third embodiment of the present invention is a process for the catalytic production of olefins, comprising: contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream; separating the first cracked product stream in a main column; obtaining a second hydrocarbon stream from an overhead line of the main column system; contacting the second hydrocarbon stream with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cooling catalyst; and obtaining a third hydrocarbon stream from the first cracked product stream or the second cracked product stream; and contacting the third hydrocarbon stream with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cooling catalyst. An embodiment of the present invention is one, any or all of the preceding embodiments through the third embodiment of this paragraph, wherein the second hydrocarbon stream is more olefinic and/or more crackable than the third hydrocarbon stream. An embodiment of the present invention is one, any or all of the preceding embodiments through the third embodiment of this paragraph, wherein the third hydrocarbon stream has greater than 20 wt% olefins. An embodiment of the present invention is one, any or all of the preceding embodiments through the third embodiment of this paragraph, further comprising operating the third riser at a greater stringency than the second riser.

Without further elaboration, it is believed that, using the foregoing description, one skilled in the art can readily ascertain the essential characteristics of the present invention to the fullest extent possible, and thus make various changes and modifications of the present invention and adapt it to various uses and conditions, without departing from the spirit and scope thereof. Accordingly, the above-described preferred specific embodiments are not to be construed as limiting in any way the remainder of the present disclosure, but rather as illustrative only, and it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

In the above, unless otherwise indicated, all temperatures are given in degrees Celsius and all parts and percentages are by weight.

Claims (10)

As a process for the catalytic production of olefins,
A step of contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream;
A step of separating the first decomposed product stream from the main column;
A step of separating an overhead stream from the main column into a second hydrocarbon stream;
contacting said second hydrocarbon stream with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cooled catalyst; and
obtaining a third hydrocarbon stream from said overhead stream and/or from said second cracked product stream; and
A process comprising the step of contacting the third hydrocarbon stream with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cooled catalyst. A process in which the second hydrocarbon stream is more olefinic and/or more crackable than the third hydrocarbon stream. A process in accordance with claim 1, wherein the second hydrocarbon stream is a light cracked naphtha stream, further comprising the step of separating the overhead stream from the main column into a first C4 hydrocarbon stream and taking the first C4 hydrocarbon stream as the third hydrocarbon stream. A process, further comprising the step of washing the second decomposed product stream in a washing column and obtaining the third hydrocarbon stream from the washing column, in the first aspect. A process in which, in claim 4, the second hydrocarbon stream is a first C4 hydrocarbon stream, and the third hydrocarbon stream is a second C4 hydrocarbon stream. A process further comprising the steps of: in claim 1, compressing an overhead stream from the wash column to provide a compressed overhead stream, depropanizing the compressed overhead stream to provide a depropanized compressed overhead stream, and taking the third hydrocarbon stream from the depropanized compressed overhead stream. A process further comprising the step of separating the depropanized compressed overhead stream into a second C4 stream and a second light cracked naphtha stream, and taking the third hydrocarbon stream from the second light cracked naphtha stream and/or from the second C4 stream. A process in which the second stream of the fluid catalyst and the third stream of the fluid catalyst have the same catalyst composition. A process in which, in the first aspect, the third riser operates at a higher outlet temperature and/or a lower hydrocarbon partial pressure and/or a different catalyst density than the second riser. A process according to claim 1, further comprising the step of regenerating a stream of said spent catalyst by combustion of coke from said spent catalyst to provide a high temperature regenerated catalyst, and heat exchanging a portion of said high temperature regenerated catalyst with a portion of said second stream of said fluid catalyst prior to contacting said second stream of said fluid catalyst with the second hydrocarbon stream.