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 cyclone which has a curved outer wall and imparts a second swirling angular direction to the mixture. The second angular direction is counter to the first angular direction. The apparatus and method assures that a greater proportion of the mixture entering the cyclone is incorporated into the vortex to further enhance separation between the solids and gases.

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. Nos. 4,701,307 B1 and 4,792,437 B1, the contents of which are hereby incorporated by reference. 
     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. Nos. 4,397,738 B1 and 4,482,451 B1 disclose an arrangement for initial quick centripetal separation that tangentially discharges a mixture of gases and solid particles from a central reaction conduit 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 a traditional cyclone vessel. 
     Another method of obtaining this initial quick separation on discharge from the reaction conduit is disclosed in U.S. Pat. No. 5,584,985 B1. 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. The swirl motion of the mixture continues while it rises up the gas recovery conduit. At the end of the gas recovery conduit, the mixture is drawn into cyclones to effect further separation of the particulate catalyst from the gases. This arrangement is known as the UOP Vortex Separation System (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. Nos. 4,670,410 B1 and 2,535,140 B1. 
     U.S. Pat. No. 4,956,091 B1 discloses a separator comprising a swirl chamber that imparts a swirl motion to a mixture of gases and solids in an angular direction. The mixture then enters a swirl tube through swirl veins which intensify the swirl motion of the mixture in the same angular direction to effect separation between the solids and gases. This same principle has been followed in vortex separation systems that are used in conjunction with cyclones. The angular direction of the swirl motion induced by the VSS SM  has the same angular direction as the swirl motion induced by the cyclones. It was, perhaps, thought that consistency between the swirl motion in the VSS SM  and the cyclones will operate to intensify the swirl motion in the cyclone and thereby effect greater separation. 
     U.S. Pat Nos. 5,370,844 B1 and 4,364,905 B1 disclose cyclone inlets that radially extend from and communicate with a central gas recovery conduit. Radially extending cyclone inlets are the most common. However, cyclone inlets of which the long, straight sidewall is disposed just inwardly of a tangent to a central gas recovery conduit have been licensed for commercial use. 
     Another concern involved in arranging cyclones in a vessel is to provide clearance between cyclones to permit adequate access for installation and for maintenance purposes. Clearance between cyclones becomes a greater consideration when more cyclones are installed in a vessel. 
     Accordingly, it is an object of the present invention to improve the efficiency of separating particulate solids from vapors in an FCC unit. It is a further object of the present invention to further improve such efficiency of separation in an FCC unit that utilizes a VSS SM  with one or more cyclones. An additional object of the present invention is to assure adequate clearance between cyclones in a containing vessel. 
     BRIEF SUMMARY OF THE INVENTION 
     It has now been discovered that by orienting the angular direction of a swirl motion from a VSS SM  to counter the angular direction of a swirl motion in a downstream cyclone enhances the separation efficiency. When the swirl motions in the VSS SM  and the cyclones are oriented to agree, the mixture entering into the cyclone is less likely to first encounter the interior surface of the outer wall, which imparts the swirl motion to the mixture. Instead, the mixture is more likely to encounter a center of the cyclone where the inlet to the vapor outlet conduit is located. Consequently, some of the mixture entering the cyclone is permitted to exit the cyclone before a swirl motion is imparted to the mixture by the outer wall. Under these circumstances, some of the mixture exits the cyclone with minimal further separation of solid particles from the gaseous vapors. It has been discovered that by orienting the angular direction of the swirl motion from the VSS SM  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, thereby imparting to the mixture a swirl motion before the mixture can encounter the center of the cyclone. Accordingly, greater separation efficiency results. To achieve counter-direction of swirl motions, the curvature of the VSS SM  swirl arm is oriented so that the opening at the end of the swirl arm angularly faces toward the wall of the inlet to the cyclone that is contiguous with the curved wall of the cyclone. 
     Accordingly, in one embodiment, the present invention relates to a process for the fluidized catalytic cracking of a hydrocarbon feedstock comprising passing a hydrocarbon feedstock and solid catalyst particles into a reaction conduit to produce a mixture of solid catalyst particles and gaseous fluids. The mixture of the catalyst particles and gaseous fluids is induced to swirl in a first angular direction to decrease the catalyst particle concentration and increase the gaseous fluids concentration in the mixture. The mixture is transported to at least one cyclone wherein it is induced to swirl in a second angular direction that is counter to the first angular direction to further decrease the catalyst particle concentration and further increase the gaseous fluids concentration in the mixture. 
     In another embodiment, the present invention relates to an apparatus for the fluidized catalytic cracking of a hydrocarbon feedstock. The apparatus comprises a reaction conduit for contacting a hydrocarbon feedstock and solid catalyst particles to produce a mixture of solid catalyst particles and gaseous fluids. The reaction conduit has a swirl exit configured to induce the solid catalyst particles and gaseous fluids to swirl in a first angular direction. A cyclone in communication with the swirl exit has a swirl inducing outer wall that induces the solid catalyst particles and gaseous fluids to swirl in a second angular direction that is counter to the first angular direction. 
     In a further embodiment, the present invention relates to an apparatus for the fluidized catalytic cracking of a hydrocarbon feedstock. The apparatus comprises a reaction conduit for contacting a hydrocarbon feedstock and solid catalyst particles to produce a mixture of solid catalyst particles and gaseous fluids. The reaction conduit has a curved tubular swirl arm connective with the reaction conduit and an open exit end. A cyclone in communication with the open exit end of the swirl arm has a curved outer wall, wherein the swirl arm curves in an angular orientation counter to the angular orientation in which the outer wall of the cyclone curves. 
     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 cross-section of  FIG. 1  taken along segment A—A. 
         FIG. 3  is a cross-section of  FIG. 1  taken along segment B—B. 
         FIG. 4  is a partial view of  FIG. 2  showing the flow path of particulate material when the swirl motions are the same. 
         FIG. 5  is an alternative cross-section of  FIG. 1  taken along segment B—B. 
         FIG. 6  is a partial view of  FIG. 2  showing the flow path of particulate material when the swirl motions are countered. 
         FIG. 7  is a further alternative cross-section of segment A—A in FIG.  1 . 
     
    
    
     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. Two alternative cross-sections are taken from segment A—A of  FIG. 1  which are  FIGS. 2 and 7 . Moreover, two alternative cross-sections are taken from segment B—B which are  FIGS. 4 and 6 . 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 a swirl exit in the form of a swirl arm  14 . The swirl arm  14  is a curved tube that has an axis of curvature that is parallel to the reactor riser  12 . (See FIG.  4 ). The swirl arm  14  also has one end connected to the reactor riser  12  and another open end comprising a discharge opening  16 . Swirl arm  14  discharges a mixture of gaseous fluids comprising cracked 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 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 . Gas recovery conduit  18  passes the separated gases into cyclones  22  that effect a further removal of catalyst particulate material from the gases in the gas recovery conduit  18 . 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 . 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 the cyclones  22  in more detail by a cross-sectional view taken along segment A—A in FIG.  1 . Each cyclone  22  comprises a radial cyclone inlet  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 radial cyclone inlet  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  34   a  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  36   a  to curved outer wall  38 . The radial cyclone inlet  30  to the cyclones  22  radially exits from the gas recovery conduit  18 . Radial exit from the gas recovery conduit  18  to the cyclone  22  is generally characterized in that a mid-line “C” laterally bisecting radial cyclone inlet  30  where it exits gas recovery conduit  18  would substantially intersect the cross-sectional center of the gas recovery conduit  18 . In operation, a mixture of gases and particulate material exits gas recovery conduit  18  into the radial cyclone inlet  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. 
     The orientation of curvature of swirl arms  114  is shown in FIG.  3 .  FIG. 3  is a cross-sectional view of  FIG. 1  taken at segment B—B. A mixture containing particulate material and gaseous fluids ascending through reactor riser  12  will exit the reactor riser  12  through swirl arms  114  out discharge opening  16  swirling in a clockwise angular direction. As the mixture exits the separation vessel  11  and transports through gas recovery conduit  18 , the mixture will retain the same swirl motion in a clockwise angular direction. 
       FIG. 4  shows how particulate material  50  radially exiting the gas recovery conduit  18  enters the cyclone  22 . Only one cyclone is shown in  FIG. 4  for purposes of simplicity. A swirl motion of clockwise angular direction “D” of the mixture containing particulate material  50  in gas recovery conduit  18  is generated by swirl arms  14  having the orientation of curvature shown in FIG.  3 . The orientation of curvature of swirl arm  14  is the angular direction it defines from inlet to outlet. The cyclone  22  has an orientation of curvature defined by the angular direction taken by a continuous, gradual transition  34   a  between the long, straight sidewall  34  and the curved outer wall  38 . The orientation of curvature is determined commensurately with the direction of flow. The orientation of curvature of the cyclone  22  will impart a swirl motion of clockwise angular direction “E” to the mixture containing particulate material  50 . When the swirl arms  114  have the same orientation of curvature as the orientation of curvature of the cyclones  22 , they impart to the mixture containing particulate material  50  the same swirl motions of clockwise angular direction “D” in the gas recovery conduit  18  and “E” in the cyclone  22 , as in the prior art. Consequently, the particulate material  50  entering the cyclone has a tendency to approach the vapor outlet  24  instead of following the interior surface of the curved outer wall  38  to generate the swirl motion desired. Consequently, it is believed that some of the particulate material  50  ends up going out the vapor outlet  24  before it is incorporated into a vortex which serves to separate the particulate material  50  from the gases. Accordingly, separation efficiency of the gas from the particulate material  50  is diminished. 
       FIG. 5  is a cross-section of  FIG. 1  taken along segment B—B which shows the orientation of curvature of the swirl arms  14  which is counter to the orientation of curvature of the cyclone  22  according to an embodiment of the present invention. The swirl arms  14  in  FIG. 5  have an orientation of curvature opposite to that of the swirl arms  114  in FIG.  3 . The swirl arms  14  are differentiated from the swirl arms  114  in  FIG. 3  by subtracting 100 from the reference numeral. Other elements common to both  FIGS. 3 and 5  will be designated with the same reference numeral. The discharge openings  16  in  FIG. 5  face oppositely to discharge openings  16  in FIG.  3 . Consequently the orientation of curvature of the swirl arms  14  is counter to the orientation of curvature of the cyclone  22 .  FIG. 5  shows four swirl arms  14 . More or less swirl arms can be used. 
       FIG. 6  demonstrates the interaction between the counter swirling angular directions in the gas recovery conduit  18  and the cyclone  22 .  FIG. 6  shows the gas recovery conduit  18  and just one cyclone  22  for purposes of simplicity. The mixture exiting discharge openings  16  in the swirl arms  14  in  FIG. 5  will swirl in a counterclockwise angular direction “F”. The mixture will continue to swirl in a counterclockwise motion as the mixture ascends the gas recovery conduit  18 . However, the swirl motion in the cyclones  22  shown in  FIG. 2  will be in a clockwise angular direction “E”. As the mixture containing particulate material  50  enters the radial cyclone inlet  30  of the cyclone  22 , the angular momentum of the mixture is carried toward the long, straight sidewall  34 , which is contiguous and has a continuous, gradual transition  34   a  with the curved outer wall  38 , instead of toward the center of the barrel chamber  32 . The long, straight sidewall  34  and curved outer wall  38  are consequently able to impart a swirl motion of clockwise angular direction “E” to more of the mixture, thereby incorporating more of the mixture in the vortex that separates the particulate material  50  from the gases. The heavier particulate material  50  swirls at the curved outer wall  38  of the cyclone  22  where it eventually falls down to the hopper  19  to enter dipleg  23  and eventually join the dense catalyst bed  28 . That the swirl arms  14  swirl the mixture in a counter-clockwise angular direction and the cyclones swirl the mixture in a clockwise angular direction is not a limiting factor, but the counter relationship between the angular directions of swirl motion from the swirl arms  14  and the cyclones  22  is the point of importance. 
       FIG. 7  depicts a further embodiment of the present invention that provides substantially tangential exit to the cyclones from the gas recovery conduit  18  and in which the swirl motion of counter-clockwise angular direction “F” of the mixture in the gas recovery conduit  18  is counter to the swirl motion of clockwise angular direction “H” induced in the cyclones.  FIG. 7  is taken as an alternative cross-section of  FIG. 1  along segment A—A. The reference numeral for each element in  FIG. 7  related to an inlet that is configured differently from a corresponding element in  FIG. 2  will be designated by adding 200 to the reference numeral in FIG.  2 . Other elements common to both  FIGS. 2 and 7  will retain the same reference numeral. The section at segment B—B of  FIG. 1  that corresponds to the embodiment illustrated in  FIG. 7  is illustrated in FIG.  5 . Swirl arms  14  impart a swirl motion of counter-clockwise angular direction “F” to the mixture containing particulate material  50  discharging from the reactor riser  12 . This counter-clockwise angular direction “F” of swirl motion continues as the mixture travels up gas recovery conduit  18 . The mixture exits the gas recovery conduit  18  through cyclone inlets  230  which are substantially tangential to the gas recovery conduit  18 . The mixture enters each cyclone  22  through a tangential cyclone inlet  230  defined by long, straight sidewall  234  and short, straight sidewall  236 . A line “I” coplanar or co-linear with the short, straight sidewall  236  is substantially tangential to a cross-sectional profile of the gas recovery conduit  18 . The short, straight sidewall  236  may be spaced slightly inwardly of tangent to facilitate its welding to the gas recovery conduit  18 . This arrangement permits installation of more cyclones  22  in the reactor vessel  10  with greater clearance between each of the cyclones  22 . The long, straight sidewall  234  is contiguous and has a continuous, gradual transition  234   a  with a curved outer wall  238  which defines the barrel chamber  232  of the cyclone  22 . The short, straight sidewall  236  has an abrupt, acute transition  236   a  with the curved outer wall  238 . A mixture with a greater concentration of particulate material  50  than that entering the cyclone  22  exits downwardly through hopper  19  while a mixture with a greater concentration of gaseous fluids than that entering the cyclone  22  exits upwardly through vapor outlet  24 . The long, straight sidewall  234  and curved outer wall  238  cooperate to impart a swirl motion to the mixture entering cyclone  22 , thereby establishing a vortex which separates the particulate material  50  from the gases. In this embodiment, the swirl motion of counter-clockwise angular direction “F” imparted by the swirl arms  14  from the reactor riser  12  is counter to a clockwise angular direction “H” of swirl motion imparted by the cyclones  22 . Consequently, the particulate material  50  in the mixture is more likely to first encounter the long, straight sidewall  234  and/or curved outer wall  238  and be subjected to the swirl motion of the vortex than it would be to first encounter the center of the cyclone  22  and be discharged from the cyclone with gases through the vapor outlet  24 . Accordingly, because greater proportions of the mixture are likely to be subject to the swirl motion than tending toward the center of the cyclone, greater efficiency in separation is realized. This arrangement also provides counter angular directions of swirl motion in the gas recovery conduit  18  and the cyclones  22 , which formerly agreed, by modifying the orientation of the cyclones  22  instead of the swirl arms  114 . 
     EXAMPLE 
     Computational flow dynamics (CFD) modeling using a FLUENT program was performed to study separation efficiencies for three sets of conditions. The following was assumed for all three sets of conditions: the minimum catalyst size was 40 micrometers, the gas density was 2.75 kg/m 3 , the gas velocity was 0.02 c.p., the velocity of the mixture exiting each swirl arm was 20.8 m/sec, the pressure was 299 kPa and the temperature was 549° C. 
     The first set of conditions involved a model where radial cyclone inlets  30  to the cyclones  22  were disposed with respect to the gas recovery conduit  18  as shown in FIG.  2  and the swirl arms  114  were disposed as in FIG.  3 . This model focused on the case where the angular direction of the swirl motion imparted by the swirl arms was the same as the angular direction of the swirl motion imparted by the cyclones  22  as shown in FIG.  4 . The CFD modeling indicated that in this model, 21% of the mixture entering the cyclone veered toward the center of the cyclone instead of veering toward the periphery of the cyclone to join the vortex to further separate the gases from the solids, representing a loss in efficiency. 
     A second set of conditions had the same cyclone configuration shown in  FIG. 2  as in the previous model. However, the swirl arms  14  were oriented as shown in  FIG. 5 , so that the angular direction of swirl motion generated by the swirl arms  14  was counter to the angular direction of swirl motion generated by the cyclones  22  as shown in FIG.  6 . Modeling indicated that only 10% of the mixture entering the cyclone veered toward the center of the cyclone where the vapor outlet is disposed without veering toward the vortex for further separation.