Abstract:
A method and apparatus are disclosed to reduce the amount of coke and unstripped hydrocarbon flowing to the regenerator in an FCC unit. The catalyst stripper section is heated by indirect heat exchange with hot regenerator flue gas.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for the separation of an entrained gas from a fluidized finely divided solid catalyst in a fluidized catalytic cracking unit (FCC). More particularly, it relates to an improved method and apparatus for separating catalyst from a catalytically cracked product in a catalyst stripper zone to minimize or substantially eliminate flow of valuable cracked product to the regenerator. 
     The field of fluid catalytic cracking has undergone significant improvements relating both to catalyst technology and to mechanical process unit design. These advances have enabled refiners to process heavier feedstocks as well as to increase the total yields of gasoline and distillate. However, the significant potential for process improvement resulting from eliminating or substantially reducing flow of cracked products to the regenerator has not been fully realized. 
     By way of background, the hydrocarbon conversion catalyst usually employed in an FCC unit is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed Phase condition generally upwardly through one or more riser conversion zones (FCC cracking zones) providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 1000° F. or higher and at 0.5 to 4 seconds hydrocarbon resistance time in contact with the catalyst in the riser, are desirable for some oPerations before initiating separation of vapor phase hydrocarbon product materials from the catalyst. Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. It is also highly desirable to strip hydrocarbon product materials from the catalyst before the catalyst enters a regeneration zone. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are removed from the catalyst in a separate catalyst stripping zone. Hydrocarbon conversion products separated from the catalyst and stripped materials are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, referred to as coke, is then passed to a catalyst regeneration operation. 
     Coke deposited on deactivated FCC catalyst together with entrained product which is carried over to the regenerator with the deactivated catalyst is referred to by those skilled in the art as &#34;total delta carbon.&#34; For a given FCC unit design, at a fixed catalyst circulation rate, an increase in total delta carbon is accompanied by higher regenerator temperatures. Consequently, one method of limiting FCC regenerator temperature is to reduce total delta carbon by reducing carryover of cracked hydrocarbon product to the regenerator 
     Methods and systems for separating catalyst particles from a gas suspension phase containing catalyst particles and hydrocarbon vapors, particularly the seParation of high activity crystalline zeolite cracking catalysts, have been the subject of recent advances in the art. 
     Anderson et al U.S. Pat. No. 4,043,899 discloses a method for rapid separation of a product suspension comprising fluidized catalyst particles and the vapor phase hydrocarbon product mixture, by discharging the entire suspension directly from the riser conversion zone into a cyclone separation zone. The cyclone is modified to include a separate cyclonic stripping of the catalyst separated from the hydrocarbon vapors. In the method of Anderson et al, the cyclone separator is modified to include an additional downwardly extending section comprising a lower cyclone stage. In this arrangement, catalyst separated from the gasiform material in the upper stage, slides along a downwardly sloping baffle to the lower cyclone where stripping steam is introduced to further separate entrained hydrocarbon products from the catalyst recovered from the upper cyclone. The steamed and stripped hydrocarbons are passed from the lower cyclone through a concentric pipe where they are combined with the hydrocarbon vapors separated in the upper cyclone. The separated and stripped catalyst is collected and passes from the cyclone separator by conventional means through a dipleg. 
     Myers et al U.S. Pat. No. 4,070,159 provides a separation means whereby the bulk of catalyst solids is discharged directly into a settling chamber without passing through a cyclone separator. In this apparatus, the discharge end of the riser conversion zone is in open communication with the disengaging chamber such that the catalyst discharges from the riser in a vertical direction into the disengaging chamber which is otherwise essentially closed to the flow of gases. The cyclone separation system is in open communication with the riser conversion zone by means of a port located upstream from, but not near, the discharge end of the riser conversion zone. A deflector cone mounted directly above the terminus of the riser causes the catalyst to be directed in a downward path so as to prevent the catalyst from abrading the upper end of the disengaging vessel. The cyclone separator is of the usual configuration employed in a catalytic cracking unit to separate entrained catalyst particles from the cracked hydrocarbon products so that the catalyst passes through the dipleg of the cyclone to the body of the catalyst in the lower section of the disengaging vessel, and the vapor phase is directed from this vessel to a conventional fractionation unit. There is essentially no net flow of gases within the disengaging vessel beyond that resulting from a moderate amount of steam introduced to strip the catalyst residing in the bottom of the disengaging vessel. 
     It is also known to transfer thermal energy from the regenerator to the reactor. Gross U.S. Pat. Nos. 4,356,082 and 4,411,773 teach a fluid catalytic cracking (FCC) process and apparatus wherein the heat balance between the reactor and the regenerator of the FCC operation is partially uncoupled by transferring at least a portion of thermal energy from the reactor vessel riser to the regenerator vessel. The transfer of thermal energy results in a higher regenerating temperature. The thermal energy is recirculated to the upstream section of the reactor riser through a regenerated catalyst having higher temperature. As a result, the outlet of the reactor vessel is maintained at a substantially constant temperature (about 1000° F.) and the rate of conversion of the oil feed and the octane number of gasoline produced in the process are increased. 
     Krug U.S. Pat. No. 4,574,044 discloses a method for increasing the overal efficiency of an FCC process by decreasing the amount of valuable product burner in the regenerator. Separation of catalyst from hydrocarbon product is enhanced by first stripping the hydrocarbon product from the catalyst and then conditioning the catalyst in the presence of steam at elevated temperatures for a period of about 1/2 to 30 minutes. The benefits of this system include a reduction in coke make. 
     Owen et al U.S. Pat. No. 4,689,206 teaches an apparatus for fluid catalytic cracking (FCC),of a hydrocarbon feed in an open or closed system, which includes a multi-stage stripper system, which comprises a means for spinning a gasiform mixture of catalyst and cracked hydrocarbons emitting from a riser, a first means for stripping the spun gasiform mixture, and a means for deflecting the gasiform mixture to separate catalyst from the cracked hydrocarbons. 
     Commonly-assigned U.S. Patent No. Application Ser. No. 903,365 filed Sept. 3, 1986, of Herbst et al discloses a technique for improving the efficiency of a catalyst stripper section by injecting an inert gas and heating the stripper section by carrying out an exothermic reaction within the stripper. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a catalyst stripping process and apparatus which significantly reduces unstripped hydrocarbon flowing to the regenerator. 
     It is a further object of the present invention to improve the yield of valuable product by reducing or eliminating flow of cracked product to the regenerator. 
     It is a further object of the present invention to reduce the emission of SO m  and SO m  produced by the combustion of cracked product in the regenerator. 
     It is a further object of this invention to reduce the partial pressure of water in the regenerator to reduce the degree of steaming deactivation of the catalyst, thereby increasing catalyst life and reducing fresh catalyst makeup requirements. 
     Briefly, the present invention improves stripping efficiency in an FCC catalyst stripper by heating the stripper section with hot flue gas. More particularly, the method of the present invention achieves the above and other objects by the steps of mixing a hydrocarbon feed with a regenerated catalyst in the lower section of a reactor riser; passing the mixture through the length of the reactor riser under conversion conditions whereby the  hydrocarbon is catalytically cracked and the catalyst is deactivated; separating the cracked product and the deactivated catalyst into a first stream rich in hydrocarbon product and a second stream rich in deactivated catalyst; charging the -second stream to a stripping zone; withdrawing deactivated catalyst- from the stripping zone; regenerating the withdrawn deactivated catalyst in a regeneration zone whereby a hot flue gas is generated; transferring at least a portion of the thermal energy of the hot flue gas to the stripping zone whereby the hot flue gas is cooled and the stripping zone is heated. 
     The method may also include transferring thermal energy from hot flue gas to the stripping zone by maintaining conduit means within the stripping zone and passing hot flue gas through the conduit means at a flow rate such that the stripping zone is heated to a temperature sufficient to enhance separation of catalyst and hydrocarbon product. 
     The present invention achieves the above and other objects in an apparatus for separating entrained hydrocarbon vapors from a fluidized catalyst bed comprising a longitudinally extensive cylindrical reactor shell having inlet and outlet ports; a cylindrical riser conduit extending longitudinally through said reactor shell; a plurality of frustoconical members attached to the inner surface of the reactor shell; a plurality of frustoconical members attached to the outer surface of the riser conduit; and conduit means extending through the reactor shell for providing indirect heat exchange between a hot gas flowing through the conduit means and the gaseous stream containing solid catalyst flowing around the outer surface of the conduit means. 
     The apparatus may further comprise a multiple-tube heat exchanger positioned in the annular space between the outside surface of the riser conduit and the inside surface of the reactor shell. 
     The apparatus may further comprise flow control means adapted to control the hot gas flow rate through the tubes to maintain a desired temperature in the catalyst stripper. 
     The present invention reduces coke loading on deactivated catalyst by reducing the amount of valuable product carried over to the regenerator. This lowers regenerator temperature for a given catalyst circulation rate. Cooler regenerated catalyst permits operation at an increased catalyst to oil ratio and consequently increases conversion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram showing the major components of an FCC unit wherein regenerator flue gas provides thermal energy to heat the catalyst stripper section. 
     FIG. 2 is a simplified schematic diagram showing an FCC unit reactor riser and spent catalyst stripper including the novel catalyst stripper design of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a hydrocarbon oil feed such as gas oil or higher boiling material is introduced through a conduit 2 to the bottom or upstream section of a riser reactor 70. Hot regenerated catalyst is also introduced to the bottom section of the riser by a standpipe 6 equipped with a flow control valve 8. A vapor liquid suspension is formed in the lower bottom section of the riser 70 at an elevated temperature at about 525° C. to 650° C. (980° F. to 1200° F.) and is usually at least 540° C. (1000° F.), depending on the degree of hydrocarbon conversion desired and on the composition of the feed. The suspension is formed in the bottom section of the riser and is passed upwardly through the riser under selected temperature and residence time conditions. Residence of the hydrocarbon charge stock in the riser is usually between 0.1 and 15 seconds, typically 0.5 to 4 seconds before the suspension passes through suitable separating means, such as a series of cyclones 11 rapidly effecting separation of catalyst particles from vapor hydrocarbon conversion products. Thus, in the apparatus shown in FIG. 1, the suspension is discharged from the riser 70 into one or more cyclonic separators attached to the end of the riser and represented by a separator means 11. Catalyst articles separated in the cyclone 11 pass countercurrently in contact with stripping gas introduced by conduit 16 to a lower portion of the cyclone. Thus, the contacted and separated catalyst is withdrawn by a dipleg 14 for discharge into a bed of catalyst in the lower section of the reactor. 
     The end of the riser 70 with attached separation means 11 as shown in FIG. 1 is housed in the larger vessel 17 designated herein as a receiving and catalyst collecting vessel. The lower portion of the vessel 17 has generally a smaller diameter than the upper portion thereof and comprises a catalyst stripping section 73 to which a suitable stripping gas, such as steam is introduced, e.g. by a conduit 75. The stripping section is provided with a plurality of frustoconical baffles 74A, 74B and 74C (only three are designated) over which the downflowing catalyst gasses countercurrently to upflowing stripping gas. Hot flue gas is charged to the stripping section 73 through conduit means 80 near the bottom of the stripping section 73. Once inside the stripping section 73, the conduit means may comprise a single tube 76 threaded helically between the baffles, or the conduit means may comprises a plurality of vertical or horizontal tubes (not shown). Control valve 90 maintains flue gas flow to control the stripper section temperature to optimize separation of catalyst Particles and cracked product. Flue gas enters the stripper, section at between about 650° C. and 760°  C. (1200° F. to 1400° F.) and leaves the stripping section at a temperature between about 590° C. and 710° C. (1100° F. to 1300° C. The cooled flue gas flowing through conduit 81 may then be routed to a heat recovery section, e.g. steam generation, (not shown). 
     A cyclone 24 is provided in the upper portion of the vessel 16 for recovering stripped hydrocarbon products and stripping gas from entrained catalyst particles. As is well known in the art, there may also be provided a second sequential stage (not shown) of catalyst separation for product vapors discharged from the separator 11 by a conduit 26. 
     Deactivated stripped catalyst is withdrawn from the bottom of the stripping section at an elevated temperature which may vary with individual unit operation but typically ranges between about 560° C. and 600° C. (1050° F. to 1100° .F), by a standpipe 72 equipped with a flow control valve 32. The catalyst is then passed from the standpipe 72 into the bottom portion of a regenerator riser 34. A regeneration gas is introduced into the bottom of riser 34 through a conduit 35. The regeneration gas may comprise air or may optionally comprise preheated air or oxygen supplemented air at about 150° C. to 260° C. (300° F. to 500° .F) and about 270 kPa (25 psig) to 450 kPa (50 psig), typically about 380 kPa (40 psig). The amount of lift gas introduced into the regenerator riser is sufficient for forming a suspension of catalyst in lift gas, which suspension is forced to move upwardly through riser 34 under incipient or partial regenerator conditions and into the bottom portion of an enlarged regenerator vessel 36. Regenerator vessel 36 comprises a bottom closure member 38 shown in the drawing to be conical in shape. Other suitable shapes obvious to those skilled in the art may also be employed, such as rounded dish shapes. 
     The regenerator vessel 36 comprises a smaller diameter cylindrical vessel means 40 in the lower section provided with a cyclindrical bottom containing a cyclindrical opening, whose cross section is at least equal to the cross section of the riser 34. An annular space 49 is formed by the chambers 36 and 40 and serves to recirculate regenerated catalyst to the dense bed. 
     Vessel 40 is provided with a conical head member 46 terminating in a relatively short cylindrical section of sufficient vertical height capped at its upper end by means 47 to accommodate a plurality of radiating arm means 48. The radiating arm means 48 are opened on the bottom side and operate to discharge a concentrated stream of catalyst substantially separated from the combustion product gases generally downward into the space 49. 
     In the upper portion of vessel 36, a plurality of cyclonic separators 54 and 56 is provided for separating combustion flue gas from entrained catalyst particles. The separated flue gas passes into plenum 58 for withdrawal by a conduit 60. A controlled amount of flue gas is routed to the catalyst stripper section 73 through conduit 80 as described above. The balance of the flue gas is sent to a heat recovery section, e.g. steam generation, through conduit 96. 
     The illustrated catalyst regenerator operation is designed to provide regenerated catalyst at an elevated temperature above 450° F. and preferably at 1300° F. to 1500° F. having residual coke on catalyst of less than about 0.15 and typically 0.1 to 0.05 weight percent. However, the process of the present invention can be successfully used with any regenerator coupled to an FCC reactor. Accordingly, the regenerator operation illustrated in the embodiment of FIG. 1 is used as an example of one suitable regenerator and is not to be considered a limitation of the present invention. 
     FIG. 2 details the catalyst stripper section of reactor vessel 17 shown in FIG. 1. The catalyst stripper section 73 comprises a cylindrical longitudinally extensive outer shell 93 having a plurality of frustoconical members 74A and 74B (only two are designated) attached to the inner surface thereof. Riser conduit 70 extends longitudinally through the stripper section and is equipped with a plurality of frustoconical members 74C (only one is designated) attached to its outside surface. A mixture of deactivated catalyst and entrained catalytically cracked product flows downward from a dense bed 95 to the inlet 94 of the catalyst stripper. Steam is introduced to the catalyst stripper near the bottom through conduit 75 and perforated steam distribution ring 71. Steam flows upward around the frustoconical baffles, stripping catalytically cracked product off the deactivated catalyst. The catalyst flows downward through the catalyst stripper and emits through valved standpipe 72. 
     Hot flue gas enters the catalyst stripper through conduit 80. Conduit 80 may join a single tube 76 which winds through the frustoconical baffles 74A, 74B and 74C. Cooled flue gas leaves the catalyst stripper through conduit 81. In an alternate embodiment, not shown, conduit 80 may be joined with a plurality of vertical or horizontal tubes resembling a heat exchanger bank. Cooled flue gas flowing out of the tubes is consolidated and similarly leaves the catalyst stripper through conduit 81 for further processing.