Patent Publication Number: US-8110092-B2

Title: Process for recovering power from FCC product

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of application Ser. No. 11/771,136 filed Jun. 29, 2007, now U.S. Pat. No. 7,799,209, the contents of which are hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention is power recovery from a fluid catalytic cracking (FCC) unit. FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. 
     In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator. The heat of combustion in the regenerator typically produces flue gas at elevated temperatures of 677° to 788° C. (1250° to 1450° F.) which is an appealing focus of power recovery. 
     FCC gaseous products exiting the reactor section typically have a temperature ranging between 482° and 649° C. (900° to 1200° F.). The product stream could be an attractive source power recovery but is instead introduced directly into a main fractionation column meaning that no unit operations are interposed on the line between the FCC product outlet and the inlet to the main column. Product cuts from the main column are heat exchanged in a cooler with other streams and pumped back typically into the main column at a tray higher than the pumparound supply tray to cool the contents of the main column. Medium and high pressure steam is typically generated by the heat exchange from the main column pump-arounds. Low pressure steam is typically generated at 241 to 448 kPa (gauge) (35 to 65 psig). Medium pressure steam is typically generated at 1035 kPa (gauge) (150 psig) and high pressure steam is typically generated at approximately 4137 kPa (gauge) (600 psig). For example, a stream from the main column bottom may be circulated through heat exchangers to impart process heating or steam generation. The cooled main column bottoms stream is typically returned above the main column flash feed zone to quench the vapors entering the main column from the FCC reactor. The FCC reactor vapors are cooled from 482° to 649° C. (900° to 1200° F.) to temperatures of approximately 371° C. (700° F.) in the main column flash zone. In this way, the FCC reactor effluent vapors are quenched. 
     However, steam at greater than these pressures can be used to generate incremental power recovery. Very high pressure (VHP) steam is typically generated at 6200 to 11030 kPa (gauge) (900 to 1600 psig). The FCC reactor effluent vapors are at sufficient temperature to generate steam at the pressure levels required to generate this VHP steam. 
     SUMMARY OF THE INVENTION 
     We have discovered a process for recovering power from FCC product gas directly upon exiting the FCC reactor section. The FCC product gas is heat exchanged with a heat exchange media such as water to produce steam. The steam is then routed to a generator to recover power. Additionally, it may be preferable to circulate cycle oil from an FCC product recovery section to enter the heat exchanger with the FCC product gases. Any coke precursors accumulating on the heat exchanger equipment would be washed away by the cycle oil. Advantageously, the process can enable the FCC unit to be more energy efficient. 
     Additional features and advantages of the invention will be apparent from the description of the invention, FIGURE and claims provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The FIGURE is a schematic drawing of an FCC unit, a power recovery section and an FCC product recovery section. 
     
    
    
     DETAILED DESCRIPTION 
     Now turning to the FIGURE, wherein like numerals designate like components, the FIGURE illustrates an FCC system  100  that generally includes an FCC unit section  10 , a power recovery section  60  and a product recovery section  90 . The FCC unit section  10  includes a reactor  12  and a catalyst regenerator  14 . Process variables typically include a cracking reaction temperature of 400° to 600° C. (752° to 1112° F.) and a catalyst regeneration temperature of 500° to 900° C. (932° to 1652° F.). Both the cracking and regeneration typically occur at an absolute pressure below 507 kPa (74 psia). The FIGURE shows a typical FCC process unit of the prior art, where a heavy hydrocarbon feed or raw oil stream in a line  16  is contacted with a newly regenerated cracking catalyst entering from a regenerated catalyst standpipe  18 . This contacting may occur in a narrow riser  20 , extending upwardly to the bottom of a reactor vessel  22 . The contacting of feed and catalyst is fluidized by gas from a fluidizing line  24 . Heat from the catalyst vaporizes the oil, and the oil is thereafter cracked to lighter molecular weight hydrocarbons in the presence of the catalyst as both are transferred up the riser  20  into the reactor vessel  22 . The cracked light hydrocarbon products are thereafter separated from the cracking catalyst using cyclonic separators which may include a rough cut separator  26  and one or two stages cyclones  28  in the reactor vessel  22 . Product gases exit the reactor vessel  10  through an outlet  31  to line  32  to subsequent product recovery section  90 . Inevitable side reactions occur in the riser  20  leaving coke deposits on the catalyst that lower catalyst activity. The spent or coked catalyst requires regeneration for further use. Coked catalyst, after separation from the gaseous product hydrocarbon, falls into a stripping section  34  where steam is injected through a nozzle to purge any residual hydrocarbon vapor. After the stripping operation, the coked catalyst is fed to the catalyst regeneration vessel  14  through a spent catalyst standpipe  36 . 
     The FIGURE depicts a regenerator vessel  14  known as a combustor. However, other types of regenerator vessels are suitable. In the catalyst regenerator vessel  14 , a stream of oxygen-containing gas, such as air, in line  30  is introduced through an air distributor  38  to contact the coked catalyst, burn coke deposited thereon, and provide regenerated catalyst and flue gas. A main air blower  50  is driven by a driver  51  to deliver oxygen into the regenerator  14 . The driver  52  may be, for example, a motor, a steam turbine driver, or some other device for power input. The catalyst regeneration process adds a substantial amount of heat to the catalyst, providing energy to offset the endothermic cracking reactions occurring in the reactor conduit  16 . Catalyst and air flow upward together along a combustor riser  40  located within the catalyst regenerator vessel  14  and, after regeneration, are initially separated by discharge through a disengager  42 . Finer separation of the regenerated catalyst and flue gas exiting the disengager  42  is achieved using first and second stage separator cyclones  44 ,  46 , respectively within the catalyst regenerator vessel  14 . Catalyst separated from flue gas dispenses through diplegs from cyclones  44 ,  46  while flue gas relatively lighter in catalyst sequentially exits cyclones  44 ,  46  and exits the regenerator vessel  14  through line  48 . Regenerated catalyst is recycled back to the reactor riser  12  through the regenerated catalyst standpipe  18 . As a result of the coke burning, the catalyst transferred to the reactor riser  20  is very hot supplying the heat of reaction to the cracking reaction. 
     The product gas leaving the FCC reactor section  12  in line  32  through outlet  31  is very hot, at over 482° C. (900° F.), and carrying much energy. The present invention proposes a power recovery section  50  to recover power from the hot product gas. A first heat exchanger  52  is in downstream communication with the outlet  31  of the reactor  12 . Line  32  delivers the product gas stream to a hydrocarbon side  52   a  of a first heat exchanger  52  to indirectly heat exchange the gaseous product hydrocarbon stream with a preferably vaporous heat exchange media delivered in line  54  to a heat exchange media side  52   b . The indirect heat exchange provides superheated heat exchange media in line  56  and provides a hot product hydrocarbon stream in line  58 . The stream in line  58  is cooler than the stream in line  32 ; whereas, the stream in line  56  is hotter than the stream in line  54 . The heat exchange media is preferably steam but other media may be suitable. Steam in line  56  is superheated above its saturated vapor temperature based on the delivery pressure from vessel  80 . An expander  60  is in downstream communication with the heat exchange media side  52   b . The superheated heat exchange media is directed through a control valve to the expander  60  in which it turns a shaft  62  coupled through an optional gear box  64  to electrical generator  66  to generate electrical power. A condenser  70  is in downstream communication with the expander  60 . The heat exchange media exhausted from the expander in line  68  may be further condensed in the condenser  70  thereby further reducing the volume of the heat exchange media. In this way, the heat exchange media exhausted from the expander is exhausted to near vacuum pressure to increase the power production in generator  66 . The condenser  70  is preferably a heat exchanger which indirectly exchanges heat with a second heat exchange media provided by line  71 . Condensed heat exchange media exits condenser  70  in line  73 . The product gas stream in line  32  preferably encounters first heat exchanger  52  directly, without encountering any unit operation before entering the first heat exchanger  52 . At least one heat exchanger  52 ,  72  or  86  is on a line communicating the reactor with the main fractionation column 
     The hot product hydrocarbon stream in line  58  can still be used to heat up heat exchange media. Line  58  delivers a hot product hydrocarbon stream to a hydrocarbon side  72   a  of a second heat exchanger  72  which indirectly heat exchanges the hot product hydrocarbon stream in line  58  against preheated heat exchange media from line  74  in a heat exchange media side  72   b . The hydrocarbon side  72   a  is in downstream communication with the hydrocarbon side  52   a of the first heat exchanger  52 . Intermediately heated heat exchange media exits from the second heat exchanger  72  in line  76 . A warm product hydrocarbon stream leaves second heat exchanger  72  in line  78 . The stream in line  78  is cooler than the stream in line  58 ; whereas, the stream in line  76  is hotter than the stream in line  74 . A heat exchange media drum  80  is in downstream communication with the heat exchange media side  72   b . Line  76  delivers intermediately heated heat exchange media to heat exchange media drum  80 . A vaporous overhead stream from heat exchange media drum  80  provides vaporous heat exchange media in line  54 , which is preferably steam. The heat exchange media side  72   b  of the second heat exchanger is in downstream communication with a liquid blowdown outlet line  82  from the heat exchange media drum  80  via lines  82  and  74 . The liquid blowdown stream in line  82  provides a portion of preheated heat exchange media in line  74  and a purge in line  83 . A third heat exchanger  86  has a hydrocarbon side  86   a  and a heat exchange media side  86   b . The hydrocarbon side  86   a  is in downstream communication with the hydrocarbon side  72   a  of the second heat exchanger  72 . The warm product hydrocarbon stream in line  78  is further heat exchanged in the hydrocarbon side  86 a against fresh heat exchange media from line  84  in the heat exchange media side  86   b  of the third heat exchanger  86 . The heat exchange media side  72   b  of the second heat exchanger  72  is in downstream communication with the heat exchange media side  86   b  of the third heat exchanger  86 . Preheated heat exchange media leaves heat exchanger  86  in line  88  to provide the other portion of preheated heat exchange media in line  74 . A lower heat hydrocarbon stream leaves the third heat exchanger in line  89 . The main fractionation column  92  is in downstream communication with the hydrocarbon side  86   a . The stream in line  89  is cooler than the stream in line  78 ; whereas, the stream in line  88  is hotter than the stream in line  84 . The pressure drop in the product streams  32 ,  58 ,  78  and  89  is minimal so as to avoid elevated pressures in the FCC reactor. These product streams may be processed at about 69 to 483 kPa (10 to 70 psia) and preferably at about 206 to 345 kPa (30 to 50 psia). The pressure of the heating media should be high enough to create high power generation efficiency in expander  60 . The pressure of the heating media streams in lines  84 ,  88 ,  74 ,  82 ,  76 ,  54  and  56  may be about 6177 to about 12659 kPa (896 to about 1836 psia) if the heating media is steam. The first heat exchanger should bring the temperature of the heating media in line  56  above its saturation temperature, which is approximately 279° to 329° C. (535° to 625° F.) for steam at 6180 to 12665 kPa (896 to 1836 psia). The steam temperature in line  56  may be superheated to between about 371° and 482° C. (700° to 900° F.). The first, second and third heat exchangers  52 ,  72  and  86 , respectively, may be a shell and tube heat exchangers with the hydrocarbon on the shell side and the heat exchange media on the tube side, but other heat exchangers and arrangements may be suitable. 
     In the product recovery section  90 , at least a portion of lower heat FCC product stream in line  89 , which is at least a portion of the gaseous product stream from the FCC reactor in line  32 , the hot product stream in line  58 , or the warm product stream in line  78  is directed to a lower section of an FCC main fractionation column  92  through inlet  91 . Inlet  91  is in downstream communication with the first, second and third heat exchangers  52 ,  72 ,  86 , respectively, and the product outlet  31  of the FCC reactor  12 . Several fractions may be separated and taken from the main column including a heavy slurry oil from the bottoms in line  93 , a heavy cycle oil stream in line  94 , a light cycle oil in line  95  and a heavy naphtha stream in line  96 . Any or all of lines  93 - 96  may be cooled and pumped back to the main column  92  to cool the main column typically at a tray location higher than the stream draw tray. However, because sufficient heat is removed from the FCC product stream, the bottoms pump around may be unnecessary. However, it is contemplated that slurry oil in bottoms line  93  may be used to heat the fresh heat exchange media in line  84 . Gasoline and gaseous light hydrocarbons are removed in overhead line  97  from the main column  92  and condensed before further processing. 
     Very heavy oil droplets may not be completely vaporized in the FCC reactor vapors and could form coke in the first, second and third heat exchangers  52 ,  72  and  86 , respectively. Therefore, a cyclic oil such as LCO from line  95  or HCO from line  94  from the main column  92  may be circulated with the gaseous hydrocarbons from line  32  to keep the tubes of the first heat exchanger  52  or subsequent downstream second and third heat exchangers  72  and/or  86 , respectively, wetted on the tube walls. In the FIGURE, first, second and third heat exchangers  52 ,  72  and  86 , respectively, are in downstream communication with a product line  94 . For example, a portion of the HCO stream in line  94  is recycled in line  98  and joins line  32  carrying the gaseous hydrocarbon products before entering the first heat exchanger  52 . Alternatively, both lines  32  and  98  could enter the heat exchanger separately. It is also contemplated that in a shell and tube heat exchanger, the hydrocarbon product would be on the shell side, and the heat exchange media be on the tube side, but vice-versa may be acceptable. Suitably, about 5 to 25 wt-% and preferably, about 10 to 15 wt-% of the hydrocarbon fed to the first heat exchanger  52  should be recycled cycle oil which will be processed with the hydrocarbon products downstream. When the temperature of the hydrocarbon products decrease in the first heat exchanger  52 , the cycle oil will wet on the tube wall. This liquid phase will help wash away heavy cyclic coke precursors and avoid coking on the tube walls. This same washing effect may also occur in the subsequent heat exchangers  72  and  86 . 
     EXAMPLE 
     We determined the steam that could be regenerated from FCC product vapors at a temperature of 513° C. (955° F.) and 229 kPa (33.2 psia) was equivalent to 0.1 kg (0.175 lb) of superheated very high pressure steam per pound of hydrocarbon feed fed to an FCC unit. Hence, 0.52 kW of power may be recovered per pound per hour of feed fed to an FCC unit. This equates up to 20 MW-h of power generated from a 70,000 barrel per day FCC unit. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.