Abstract:
This invention is directed to a process for fluid catalytic cracking, including, fluidizing a hydrocarbon stream in a riser, cracking the hydrocarbon stream with catalyst in the riser to produce a cracked stream and spent catalyst, separating the cracked stream and the spent catalyst in a primary separator to obtain a cracked stream with a first concentration of spent catalyst, and transporting the cracked stream with the first concentration of spent catalyst through a conduit to a multi-cyclone separator comprising multiple cyclones extending through a tube sheet to obtain a cracked stream with a second concentration of spent catalyst. The invention is also directed to an apparatus for catalytic cracking including a riser, a primary separator, a disengagement vessel surrounding the primary separator to collect the catalyst, a gas conduit having a first end in fluid connection with the disengagement vessel, and a multi-cyclone separator comprising a plurality of cyclones.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention is directed to a method and an apparatus for catalytically cracking heavy hydrocarbons and separating the spent catalyst from the cracked product stream. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Fluid catalytic cracking (FCC) is a catalytic conversion process of heavy hydrocarbons into lighter hydrocarbons accomplished by contacting the heavy hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. Most FCC units now use zeolite-containing catalyst having high activity and selectivity. 
         [0003]    The basic components of the FCC process include a riser, a reactor vessel for disengaging spent catalyst from product vapors, a regenerator and a catalyst stripper. In the riser, the hydrocarbon feed contacts the catalyst and is cracked into a product stream containing lighter hydrocarbons. In the riser, regenerated catalyst and the hydrocarbon feed are transported upward by the expansion of the gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, upon contact with the hot catalyst. Upon contact with the catalyst the hydrocarbon feed is cracked into lower molecular weight products. Coke accumulates on the catalyst particles as a result of the cracking reaction and the catalyst is then referred to as “spent catalyst.” The spent catalyst must be removed from the cracked products to reduce catalyst losses from the system and to avoid contamination of the products. High temperature regeneration burns coke from the spent catalyst. The regenerated catalyst is then returned to the reaction zone. Spent catalyst is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. 
         [0004]    The current state of the art FCC reactor design includes a riser external to the reactor vessel that continues into the reactor vessel and typically terminates in a primary separation device. After leaving the primary separation device the reactor vapors and entrained catalyst enter into a secondary catalyst separation device, which may be cyclones. The reaction vapors leaving the cyclones are further combined typically in a plenum chamber before exiting the reactor and flowing to the main column. The outlet of the internal riser, the primary separation device, the cyclones and the plenum chamber are all contained within a large reactor vessel. The reactor is very large and therefore costly to manufacture and construct. The reactor vessel also adds costs to the FCC operation due to the amount of steam required for catalyst fluidization and dome steam for reactor vessel purging. It is preferable to reduce the amount of utilities necessary to maintain the reactor operation. 
       SUMMARY OF THE INVENTION 
       [0005]    This invention is directed to a process for fluid catalytic cracking, including, fluidizing a hydrocarbon stream in a riser, cracking the hydrocarbon stream with catalyst in the riser to produce a cracked stream and spent catalyst, separating the cracked stream and the spent catalyst in a primary separator to obtain a cracked stream with a first concentration of spent catalyst, and transporting the cracked stream with the first concentration of spent catalyst through a conduit to a multi-cyclone separator comprising multiple cyclones extending through a tube sheet to obtain a cracked stream with a second concentration of spent catalyst. The invention may also include regenerating and recycling the regenerated catalyst to the riser. The invention may also include collecting the spent catalyst in a collection vessel below the third stage separator after the further separating step. The further separating step may include providing differential pressure in the third stage separator. In another aspect of the invention, the spent catalyst may be in a disengagement vessel encircling the primary separator prior to the regenerating step. 
         [0006]    In still another aspect, the invention is directed to an apparatus for catalytic cracking including a riser, a primary separator located proximate an outlet end to substantially separate the catalyst from the cracked stream, a disengagement vessel surrounding the primary separator to collect the catalyst, a gas conduit having a first end in fluid connection with the disengagement vessel, and a multi-cyclone separator comprising a plurality of cyclones extending through a tube sheet and a second end of the gas conduit in fluid connection with the multi-cyclone separator. The collection vessel may be flowably connected to the disengagement vessel at a position below the primary separator. 
         [0007]    In another aspect of the invention, the disengagement vessel has a top above the outlet end and the top and the multi-cyclone separator are connected by a conduit that redirects flow by about 180 degrees. The invention may also include an outflow line for channeling the cracked stream leaving the multi-cyclone separator and a pressure controller on the outflow line creates differential pressure in the separator. The disengagement vessel may also include baffles that encircle the riser below the primary separator. The collection vessel may be flowably connected to the disengagement vessel at a position above the baffles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is cross-sectional elevation view of an FCC apparatus with a riser, a primary separator, and a multi-cyclone separator. 
           [0009]      FIG. 2  is a cross-sectional elevation view of the multi-cyclone separator shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    An FCC reaction may occur without a reactor vessel and the spent catalyst may be separated from the cracked stream first in primary separator  20  and then in multi-cyclone separator  30 . Heavy hydrocarbon feed may be added to riser  10  via feed injection nozzles  15 . The cracking reaction may be mostly completed in riser  10  and produce a cracked stream. The spent catalyst and the cracked products may be separated at primary separator  20  located on top of riser  10 . The separated spent catalyst may travel downwardly through disengagement vessel  25  and into a regenerator  90 . Catalyst may be regenerated in the regenerator  90  by combustion with air. The cracked stream with some entrained spent catalyst may be carried upwardly into a multi-cyclone separator  30  for separating substantially all of the entrained spent catalyst. The cracked stream may then go to a main column (not shown) for initiation of cracked product separation. 
         [0011]    As shown in  FIG. 1 , a hydrocarbon feed stream may be fed to a riser  10  at feed injection nozzles  15  and may be contacted and vaporized by hot regenerated catalyst entering through the nozzles  15  and fluidized by a gas such as steam from a nozzle  17 . The catalyst cracks the hydrocarbon feed stream and a mixture of spent catalyst particles and gaseous cracked hydrocarbons exit discharge openings  23  (only one shown) in swirl arms  22  into a disengagement vessel  25 . Tangential discharge of gases and spent catalyst from the swirl arms  22  produce a swirling helical motion about the interior of the disengagement vessel  25 , causing heavier catalyst particles to fall down a stripping section  26  of the disengagement vessel  25 . The spent catalyst particles may be stripped of entrained cracked vapors over baffles  27  with a stripping medium such as steam entering from at least one stripping nozzle  24 . At least about 90 wt-%, and preferably at least about 95 wt-%, of the spent catalyst may be separated from the cracked stream by a primary separator  20 . The spent catalyst particles disengaged by the primary separator  20  may be the first concentration of spent catalyst separated from the cracked stream. 
         [0012]    Tangential discharge of cracked stream vapor and spent catalyst from the swirl arms  22  may produce a swirling helical motion about the interior of the disengagement vessel  25  causing heavier catalyst particles to fall downwardly through the baffles  27  and a mixture of spent catalyst entrained in vaporous cracked products to travel upwardly into a transport conduit  21  which makes a U-bend into a multi-cyclone separator  30 . 
         [0013]    Continuing with  FIG. 1 , stripped spent catalyst from the stripping section  26  of the disengagement vessel  25  may travel through a spent catalyst pipe  28  regulated by a control valve  29  into the regenerator  90 . The spent catalyst may be distributed into a dense bed  92  by a distributor  94  where high temperatures in the presence of oxygen will combust the coke from the catalyst particles and regenerate, or restore, the activity of the catalyst particles. The entrained regenerated catalyst may be separated from the regeneration gases by cyclones  93  with the catalyst particles falling back towards the dense bed  92 . The regenerated catalyst may be returned to the bottom of the riser  10  by a return conduit  98 . Regeneration flue gas may exit the regenerator  90  by a flue gas outlet  100 . 
         [0014]    The temperature in the riser  10  may be between about 454° C. and about 593° C. (between about 850° F. and about 1100° F.), preferably between about 482° C. and about 566° C. (between about 900° F. and about 1050° F.), and more preferably between about 510° C. and about 538° C. (between about 950° F. and about 1000° F.). The regenerator  90  may regenerate catalyst at between about 593° C. and about 896° C. (between about 1100° F. and about 1500° F.), preferably between about 649° C. and about 760° C. (between about 1200° F. and about 1400° F.), more preferably between about 660° C. and about 732° C. (between about 1220° F. and about 1350° F.). 
         [0015]    After the FCC reaction, the gaseous mixture of gaseous cracked hydrocarbons and entrained spent catalyst particles may leave the disengagement vessel  25  and travel up and down the transport conduit  21  and enter the multi-cyclone separator  30 . The transport conduit  21  may extend vertically upwardly from the disengagement vessel  25  and bend about 90 degrees to extend horizontally and then bend about 90 degrees to extend vertically downwardly to connect to the top of the multi-cyclone separator  30 . The transport conduit  21  may bend about 180 degrees between the disengagement vessel  25  and the multi-cyclone separator  30 . 
         [0016]    As shown in  FIG. 2 , the multi-cyclone separator  30  receives the gaseous mixture via a separator inlet  31 . The multi-cyclone separator  30  may contain numerous individual cyclones  53 . Although only four cyclones  53  are represented in  FIG. 2 , between about 10 and about 200 cyclones  53  may be used depending on the size of the FCC unit. The separator inlet  31  may face an upper tube sheet  56  that retains top ends  54  of the cyclones  53 . The upper tube sheet  56  at least partially defines an inlet chamber  55  and limits communication between the inlet chamber  55  and the rest of the multi-cyclone separator  30 . The gaseous mixture may be distributed via a diffuser  50  to the inlets  51  of the plurality of cyclones  53  containing swirl vanes  52 , which may be structures that restrict the passageway through which incoming gas can flow, thereby accelerating the flowing gas stream. The swirl vanes  52  may also change the direction of the gaseous mixture to provide a helical or spiral formation of gas flow through the length of cyclones  53 . The spinning motion imparted to the gaseous mixture sends the higher-density catalyst toward the wall of the cyclone  53 . The catalyst falls down the cyclones  53  and out of open bottom ends  58  into a solids chamber  57  defined between the upper tube sheet  56  and a lower tube sheet  59 . In one embodiment the bottom ends  58  are closed and the catalyst exits slots formed in the wall of the cyclone  53 . In another embodiment, the solids outlet tube  34  extends from the solids chamber  57  into a collection vessel  35  and transports solids collected on the lower tube sheet  59  into the collection vessel  35 . As shown in  FIGS. 1 and 2 , the bottom of the multi-cyclone separator  30  may be defined by a hemispherical region  32  which is a clean gas area. Essentially all of the catalyst is transferred out of the multi-cyclone separator by the solids outlet tube  34 . 
         [0017]    Continuing with  FIG. 2 , clean gas, flowing down the center of cyclones  53 , passes through open-ended cyclone gas outlet tubes  72  below the lower tube sheet  59  and into a clean gas chamber  73 . The combined clean gas stream, representing the bulk of the gaseous mixture fed to the multi-cyclone separator  30  then exits into a main column line  41 . The lower tube sheet  59  limits communication between the clean gas chamber  73  and the solids chamber  57 . 
         [0018]    Referring back to  FIG. 1 , a differential pressure controller  40  on the main column line  41  regulates differential pressure across the upper tube sheet  56  and the lower tube sheet  59  to regulate flow through the solids outlet tube  34 . Catalyst level is regulated in a collection vessel  35  by use of the slide valve  39  in a spent catalyst return conduit  38 . The differential pressure controller  40  keeps a slightly higher pressure in the multi-cyclone separator  30  than in the main column line  41 . The pressure difference drives the flow of catalyst down the solids outlet tube  34 . A transfer pipe  80  which connects the collection vessel  35  to the main column line  41  acts to equalize the pressure between the collection vessel  35  and the main column line  41 , so that gas and catalyst may flow through cyclones  53  and the spent catalyst may be effectively separated from the cracked stream. 
         [0019]    As shown in  FIGS. 1 and 2 , the bottom of the collection vessel  35  may be defined by the hemispherical region  37 . The shape of hemispherical region  37  may help collect catalyst , so it will not enter the clean gas in main column line  41  though line  80 . 
         [0020]    The underflow may be the portion of the vapor that may be directed to the solids outlet tube  34  at the bottom of multi-cyclone separator  30 . The amount of underflow corresponds to the amount of flow carrying the fines away from the clean cracked stream. The underflow rate may be typically between about 3 vol-% and about 5 vol-% of the total flow rate. In one instance the underflow would carry the catalyst into the collection vessel  35  where the level would be controlled by a slide valve  39  on the conduit  38 . The underflow vapor would then turn back up the vessel  35  and into the transfer pipe  80  to the main column line  41  to the main column (not shown). There may be a critical flow orifice (not shown) on the main column line  41 . The critical flow orifice may be a Venturi-type flow instrument that is naturally restrictive and allows a predetermined flow without the use of a control valve. The conduit  38  preferably returns separated catalyst from multi-cyclone separator  30  back to disengagement vessel  25 . The catalyst then falls down stripping section  26  though baffles  27 . 
         [0021]    After passing through the multi-cyclone separator  30 , at least about 98 wt-%, and in one embodiment at least about 99 wt-%, of entrained spent catalyst may be removed from the cracked stream. The catalyst recovered from the multi-cyclone separator  30  may be a second concentration of catalyst recovered. 
         [0022]    The amount of steam required for an FCC unit without a reactor may be significantly less than in a traditional FCC unit. In a traditional FCC unit, acceleration steam input to the steam distributor  17  at the base of the riser  10 , feed steam to the feed distributors  15  in the riser  10 , stripping steam for stripping spent catalyst in the stripping section  26  prior to regeneration, fluidization steam to direct catalyst from a reactor vessel to a regenerator, and dome purge steam to purge the reactor shell are all necessary steam streams. In the FCC unit disclosed in  FIG. 1 , acceleration, feed and stripping steam are necessary, but there may be no need for fluidization steam and dome purge steam because there is no reactor vessel. Along with the elimination of the fluidization and purge steams, the respective steam control valves may also be eliminated. Less instrumentation may be necessary than in a traditional FCC unit because not as many thermocouples may be needed in an FCC unit without a reactor vessel. The number of thermocouples needed in the new FCC unit may be between about 5 and about 8, and in one embodiment about 6. Furthermore, the catalyst level and density taps and cyclone differential pressure taps may not be needed in an FCC unit without a reactor vessel. Without all the pressure taps, the dry-gas purge points may be decreased by at least about 30%, and in one embodiment by at least about 50%. 
         [0023]    In the new FCC unit design, no dead areas in the unit may accumulate coke deposits and cause maintenance problems. In a traditional FCC unit, there may be dead spaces in the reactor and large expansion joints that get covered with coke in normal operation conditions. In the new FCC unit, about 100% of the riser  10 , the primary separator  20 , the disengagement vessel  25 , the multi-cyclone separator  30 , and the collection vessel  35  may be activated with flowing materials, so no coke deposits can accumulate. 
         [0024]    Furthermore, the catalyst inventory to the new FCC system may be reduced because there may be no longer a reactor so the entire volume of the FCC unit may be reduced. The normal operating volume of catalyst necessary for the reactor cyclones and for the reactor dilute phase may be reduced. In one embodiment, a traditional FCC unit may utilize about 181,437 kg (about 200 tons). In the same embodiment, the FCC unit utilizing this invention may utilize about 154,221 kg (about 170 tons). For a traditional FCC unit, the new design should decrease the total catalyst inventory by between about 18,140 kg (about 20 tons) and about 45,360 kg (about 50 tons), and in one embodiment about 27,200 kg (about 30 tons). The decrease may be between an about 10 and an about 20 wt-% reduction in catalyst inventory, and in one embodiment about 15 wt-% reduction in catalyst inventory. Not only does this catalyst inventory decrease lead to decrease in initial catalyst loading costs, but it has the additional advantage of requiring less additives, such as NOx reduction, SOx reduction and propylene producing additives, to be added to the system to bring base catalyst loading up to design for the individual additives.