Abstract:
An apparatus and process is disclosed for the separation of solids from gases in a mixture which is most particularly applicable to an FCC apparatus. The mixture of solids and gases are passed through a conduit and exit through a swirl arm that imparts a swirl motion having a first annular direction to centripetally separate the heavier solids from the lighter gases. The mixture then enters a gas recovery conduit in which at least one plate radially extending from an inner wall impedes rotational motion of the mixture. The mixture enters cyclones at the other end of the gas recovery conduit without substantial swirling motion.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to an apparatus and a process for the separation of solid particles from gases. More specifically, this invention relates to the separation of particulate catalyst materials from gaseous materials in an FCC process. 
     DESCRIPTION OF THE PRIOR ART 
     Cyclonic methods for the separation of solids from gases are well known and commonly used. A particularly well known application of such methods is in the hydrocarbon processing industry where particulate catalysts contact gaseous reactants to effect chemical conversion of the gas stream components or physical changes in the particles undergoing contact with the gas stream. 
     The FCC process presents a familiar example of a process that uses gas streams to contact a finely divided stream of catalyst particles and effects contact between the gas and the particles. The FCC processes, as well as separation devices used therein are fully described in U.S. Pat. No. 4,701,307 and U.S. Pat. No. 4,792,437. 
     Efficient separation of particulate catalyst from product vapors is very important in an FCC process. Particulate catalyst that is not effectively separated from product vapors in the FCC unit must be separated downstream either by filtration methods or additional separation devices that multiplicate separation devices utilized in the FCC unit. Additionally, catalyst that is not recovered from the FCC process represent a two-fold loss. The catalyst must be replaced, representing a material cost, and catalyst lost may cause erosion to downstream equipment. Severe erosion may cause equipment failure and subsequent lost production time. Accordingly, methods of efficiently separating particulate catalyst materials from gaseous fluids in an FCC process are of great utility. 
     In the FCC process, gaseous fluids are separated from particulate catalyst solids as they are discharged from a reaction conduit. The most common method of separating particulate solids from a gas stream uses centripetal separation. Centripetal separators are well known and operate by imparting a tangential velocity to gases containing entrained solid particles that forces the heavier solids particles outwardly away from the lighter gases for upward withdrawal of gases and downward collection of solids. 
     U.S. Pat. No. 4,397,738 and U.S. Pat. No. 4,482,451 disclose an arrangement for initial quick centripetal separation that tangentially discharges a mixture of gases and solid particles from a central reaction conduit or riser into a containment vessel. The containment vessel has a relatively large diameter and generally provides a first separation of solids from gases. In these arrangements, the initial stage of separation is typically followed by a second more compete separation of solids from gases in cyclones. 
     Another method of obtaining this initial quick separation on discharge from the reaction conduit is disclosed in U.S. Pat. No. 5,584,985. This patent discloses the contacting of feed and catalyst particles in a riser conduit. The exit from the riser conduit comprises an arcuate, tubular swirl arm which imparts a swirling, helical motion to the gases and particulate catalyst as they are discharged from the riser conduit into a separation vessel. The swirling, helical motion of the materials in the separation vessel effect an initial separation of the particulate catalyst from the gases. A gas recovery conduit communicates the separation vessel with cyclones in a reactor vessel. The mixture of gases and entrained catalyst is drawn up the gas recovery conduit and fed into cyclones to effect further separation of the particulate catalyst from the gases. This arrangement is known as UOP&#39;s VSS SM . 
     Cyclones for separating particulate material from gaseous materials are well known to those skilled in the art of FCC processing. Cyclones usually comprise an inlet that is tangential to the outside of a cylindrical vessel that forms an outer wall of the cyclone. In the operation of an FCC cyclone, the entry and the inner surface of the outer wall cooperate to create a spiral flow path of the gaseous materials and catalyst that establishes a vortex in the cyclone. The centripetal acceleration associated with an exterior of the vortex causes catalyst particles to migrate towards the outside of the barrel while the gaseous materials enter an interior of the vortex for eventual discharge through an upper outlet. The heavier catalyst particles accumulate on the side wall of the cyclone barrel and eventually drop to the bottom of the cyclone and out via an outlet and a dipleg conduit for recycle through the FCC apparatus. Cyclone arrangements and modifications thereto are generally disclosed in U.S. Pat. No. 4,670,410 and U.S. Pat. No. 2,535,140. 
     We have found that the swirling of the mixture of gases and entrained catalyst exiting the swirl arms of the riser continues into the gas recovery conduit to a significant degree. The swirling of the mixture continues into the duct that communicates the gas recovery conduit with the cyclones. U.S. Pat. No. 6,841,133 recognized that by orienting the angular direction of the swirl motion of the mixture leaving the swirl arms of the riser to be counter to the angular direction of the swirl motion in the cyclones, the mixture entering the cyclone is more likely to first encounter the outer wall which generates the swirling motion in the cyclone. Hence, greater separation efficiency was achieved. 
     Accordingly, it is an object of the present invention to improve the efficiency of separating particulate solids from vapors in an FCC unit and the durability of the equipment used for such separation. 
     BRIEF SUMMARY OF THE INVENTION 
     We have found that the swirling of the mixture of gases and entrained catalyst exiting the swirl arms of the riser continues into the gas recovery conduit to a significant degree. The swirling of the mixture continues into the duct that communicates the gas recovery conduit with the cyclones. Consequently, the mixture swirls into the upstream wall of the duct at high velocity, presenting a potential erosion problem. The present invention is the provision of one or more plates in the gas recovery conduit that block and impede swirling motion in the gas recovery conduit. Accordingly, the mixture of gas and catalyst enter into the ducts of cyclones without or with minimal swirling. The walls of the ducts are not significantly impacted thereby mitigating erosion. Additionally, separation efficiency is maintained irrespective of the relational orientation of the riser swirl arms and the cyclones. 
     Additional details and embodiments of the invention will become apparent from the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an FCC unit. 
         FIG. 2  is a perspective view of the swirl impeding device of the present invention. 
         FIG. 3  is a cross-section of  FIG. 1  taken along segment A-A. 
         FIG. 4  is computational flow dynamics model showing velocity vectors of a swirling gas-solid mixture entering cyclones. 
         FIG. 5  is computational flow dynamics model showing velocity vectors of a gas-solid mixture impeded from swirling entering cyclones. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is the schematic illustration of an FCC unit that will serve as a basis for illustrating several embodiments. The FCC unit includes a separation arrangement in a reactor vessel  10 . A conduit in the form of a reactor riser  12  extends upwardly through a lower portion of the reactor vessel  10  in a typical FCC arrangement. The central conduit or reactor riser  12  preferably has a vertical orientation within the reactor vessel  10  and may extend upwardly through the bottom of the reactor vessel or downwardly from the top of the reactor vessel. Reactor riser  12  terminates in a separation vessel  11  at swirl exits in the form of swirl arms  14 . Each swirl arm  14  is a curved tube that has an axis of curvature that is parallel to the reactor riser  12 . The swirl arm  14  also has one end connected to the reactor riser  12  and another open end comprising a discharge opening  16 . Swirl arms  14  discharge a mixture of gaseous fluids comprising cracked vaporous product and solid catalyst particles through the discharge opening  16 . Tangential discharge of gases and catalyst from the discharge opening  16  produces a swirling helical motion about the interior of separation vessel  11 . Centripetal acceleration associated with the helical motion forces the heavier catalyst particles to the outer portions of separation vessel  11 . Catalyst particles from discharge openings  16  collect in the bottom of separation vessel  11  to form a dense catalyst bed  17 . The gases, having a lower density than the solid catalyst particles, more easily change direction and begin an upward spiral with the gases ultimately traveling into a gas recovery conduit  18  through an inlet  20 . The gases that enter a lower end of a gas recovery conduit  18  through inlet  20  will usually contain a light loading of catalyst particles. Inlet  20  recovers gases from the discharge openings  16  as well as stripping gases from a stripping section  27  which is hereinafter described. The loading of catalyst particles in the gases entering gas recovery conduit  18  are usually less than 16 kg/m 3  (1 lb/ft 3 ) and typically less than 2 kg/m 3  (0.1 lb/ft 3 ). The swirl motion imparted by the swirl arm  14  continues in the same angular direction up through the gas recovery conduit  18 . A swirl impeding device  8  comprising at least one vane or plate  13  is attached such as by welding to an inner wall of the gas recovery conduit  18  to impede swirling of the mixture of vapor product and entrained catalyst. The plate  13  may extends radially from the inner wall of the gas recovery conduit and stops short of center of the conduit  18 . Alternatively, the plate may chordally or diametrically extend all the way across the gas recovery conduit  18 . Preferably, a group of plates  13  attached to the inner wall of the gas recovery conduit  18  all extend radially to converge at a center of the gas recovery conduit. The plates may radially extend to just short of center or may extend diametrically all the way across the gas recovery conduit  18 . A group of chordally arranged plates  13  is also contemplated. To prevent swirling from reforming, a first stage of plates  13  may be used in conjunction with a second stage B of plates  15  such that the stages A &amp; B and the plates  13  &amp;  15 , respectively, therein are vertically spaced in the gas recovery conduit  18 . Preferably, two stages A &amp; B of four radial plates  13  and  15  oriented at 90° with respect to adjacent plates in the stage comprise the swirl impeding device  8 . 
     Gas recovery conduit  18  passes the separated gases from an upper end into cyclones  22  that effect a further removal of catalyst particulate material from the gases in the gas recovery conduit  18 . The swirl impeding device  8  is suitably installed at the lower end of the gas recovery conduit  18 , e.g., in the lower half and preferably in the lower quarter, so the straight flow pattern will have distance to develop before reaching the cyclones  22 . Cyclones  22  create a swirl motion inside the cyclones to establish a vortex that separates solids from gases. A product gas stream, relatively free of catalyst particles, exits the cyclones  22  through vapor outlets  24  and outlet pipes  49 . The product stream then exits the reactor vessel  10  through outlet  25  for further processing. Catalyst solids recovered by cyclones  22  exit the bottom of the cyclone through hoppers  19  and diplegs  23  and pass to a lower portion of the reactor vessel  10  where it forms a dense catalyst bed  28  outside the separation vessel  11 . Catalyst solids in dense catalyst bed  28  enter a stripping section  27  through windows  26 . Catalyst solids pass downwardly through the stripping section  27 . A stripping fluid, typically steam, enters a lower portion of stripping section  27  through at least one distributor  29 . Counter-current contact of the catalyst with the stripping fluid through a series of stripping baffles  21  displaces product gases from the catalyst as it continues downwardly through the separation vessel  11 . Stripped catalyst from stripping section  27  passes through a conduit  31  to a catalyst regenerator  37  that regenerates the catalyst by high temperature contact with an oxygen-containing gas by oxidizing coke deposits from the surface of the catalyst. Following regeneration, catalyst particles enter the bottom of reactor riser  12  through a conduit  33  where a fluidizing gas from a distributor  35  pneumatically conveys the catalyst particles upwardly through the riser  12 . As the mixture of catalyst and conveying gas continues up the riser  12 , nozzle  40  injects feed into the catalyst, the contact of which vaporizes the feed to provide additional gases that exit through discharge openings  16  in the manner previously described. 
       FIG. 2  illustrates a segment of the gas recovery conduit  18  containing a preferred configuration of the swirl impeding device  8  in more detail by a perspective view. The first lower stage A of plates  13  are oriented at 90° from adjacent plates  13  in the stage. Plates  13  radially converge from an inner wall of the gas recovery conduit  18  toward the center  40  of the gas recovery conduit  18 , each stopping short of the center  40 . The second stage B of plates  15  are oriented at 90° from adjacent plates  15  in the stage. Plates  15  radially converge from an inner wall of the gas recovery conduit  18  toward the center  40  of the gas recovery conduit  18 , each stopping short of the center  40 . The plates  13 ,  15  are preferably made of steel of 0.25 to 1 inch (0.64 to 2.54 cm) thick. The plates  13 ,  15  may have a width of ⅙ to ⅖ the diameter of the gas recovery conduit  18  and a height that is 0.5 to 2 times the width. Other dimensions may be suitable. The inner wall of the gas recovery conduit  18  is lined with refractory to resist abrasion. Hence, the plates  13 ,  15  will also be preferably lined with refractory. 
       FIG. 3  shows a cross section of segment A-A of  FIG. 1 .  FIG. 3  illustrates the plates  13 ,  15  in respective stages A, B of the swirl impeding device  8  are oriented out of phase by 45°. Other orientations are contemplated. Each cyclone  22  may comprise a radial cyclone duct  30  and a barrel chamber  32 . A vapor outlet  24  disposed in the center of the barrel chamber  32  provides for the exit of product gases along with only fine amounts of particulate material from the cyclone  22 . Hopper  19  provides for the discharge of particulate material from the cyclone  22  into the dense catalyst bed  28  as described with respect to  FIG. 1 . The cyclone duct  30  is defined by a long, straight sidewall  34  and a short, straight sidewall  36 . The long, straight sidewall  34  has a continuous, gradual transition to and, preferably, is tangential with a curved outer wall  38  which defines the barrel chamber  32  of the cyclone  22 . The short, straight sidewall  36  has an abrupt, acute transition to curved outer wall  38 . The cyclone duct  30  to the cyclones  22  radially exits from the gas recovery conduit  18  but other duct configurations are contemplated. In operation, a mixture of gases and particulate material exits gas recovery conduit  18  into the radial cyclone duct  30  of cyclone  22 . The long, straight sidewall  34  and the curved outer wall  38  cooperate to provide a continuous surface which imparts a swirl motion to the mixture entering the cyclone  22  to generate the vortex which separates the particulate material from the gases. If the mixture entering the ducts  30  is swirling, catalyst will impact the downstream sidewall  34 ,  36 , depending on the direction of swirl, potentially causing erosion of the sidewall. However, the swirl impeding device in the gas recovery conduit  18  mitigates or eliminates swirling and the resulting erosion. Moreover, if the rotational swirl motion induced by the upstream swirl arms  14  from the riser is counter-clockwise in  FIG. 3 , the mixture could tend toward the middle of the cyclone  22  and exit the vapor outlet  24  without encountering the outer sidewall  34  which induce swirling. Impeding swirling before entry into the ducts  30  ensures higher separation efficiency. The swirl impeding device  8  ensures that the mixture enters the cyclone in a straight-in fashion to allow the cyclones  22  to impart swirling motion as designed. The swirl impeding device  8  is compatible with all types of cyclone configurations. 
     EXAMPLE 
     Computational flow dynamics (CFD) modeling was conducted to study velocity vector gradient at the duct into a cyclone fed by a gas recovery conduit in upstream communication with a swirl exit from a riser with and without the using a swirl impeding device of the present invention. Scales in  FIGS. 4 and 5  indicate ft/sec.  FIG. 4  shows the velocity vectors concentrating on downstream sidewalls of cyclone ducts when fed by a swirling mixture from a gas recovery conduit.  FIG. 5  shows the velocity vectors evenly entering cyclone ducts without bias when swirling is impeded in the gas recovery conduit according to the present invention.