Abstract:
An apparatus for the recovery of fine particulate material of the type that uses a tube sheet arrangement and a multiplicity of centrifugal separation devices provides improved separation by extending a skirt to confine a particle outlet stream and raise its superficial velocity. The separation arrangement partially confines a downwardly directed flow of particles and gas from the separator thereby raising the effective superficial velocity of the particle stream without increasing the portion of the gas steam that exits with the recovered particles. As a result the device provides a greater recovery of particles without increasing the quantity of gas that accompanies the separated fine particles without the need for complete confinement of the particles that exit the separator.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to separators that use a plurality of tube-type separators to remove fine particulate matter such as catalyst particles or dust from a fluid such as hydrocarbon vapors or air. This invention relates more particularly to centrifugal separators that remove particulate material from high temperature fluid streams. 
     BACKGROUND OF THE INVENTION 
     Separators of the type that arrange a plurality of multiple tube-type separators in a parallel flow arrangement for removal of particular material from fluid streams are well-known. U.S. Pat. No. 2,941,621 and 2,986,278 and 3,061,994 generally disclose such separators and the individual tubular centrifugal separators contained therein. These arrangements house the inlets to the separators between upper and lower tube sheets that also retain upper and lower portions respectively of the tubular separation elements. Specific design criteria for such arrangements are well known. Other variations of the individual centrifugal separators and the separator layout itself can be found in U.S. Pat. No. 3,443,368 and 5,690,709. 
     These separators are commonly used to separate fine, particulate material from high temperature streams associated with the use of fine, particulate catalyst. One such process is the fluidized catalytic cracking process. The fluidized catalytic cracking of hydrocarbons is the principal process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. The FCC process is carried out by contacting the hydrocarbon feed with a catalyst made up of a finely divided or particulate solid material. In the FCC process large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. During the cracking reaction, coke will be deposited on the catalyst. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally passed to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. 
     The flue gas generated by combustion of coke from the FCC catalyst produces a high temperature gas stream that, after passage through cyclone, still retains a substantial amount of fine catalyst particles that must be removed prior to further treatment or energy recovery from the flue gas. The multiple tubular separator arrangements are particularly suited to the removal of this very fine particulate material. 
     Application of multiple tubular separators for removal of dust and particulate material from such high temperature fluid streams has required further consideration and special arrangements. FIGS.  1  through 3 of U.S. Pat. No. 4,863,500 and of U.S. Pat. No. 3,541,766 each show a separator arrangement that is suitable for the usual high temperature operations of processes such as the FCC process. The three different arrangements of U.S. Pat. No. 3,541,768 and 3,415,042 each suspend multiple centrifugal separators from an inner housing to accommodate different expansion rates between the internals of the separator and an outer housing. Internal insulation normally covers the inside of the outer housing to reduce its wall temperature thereby increasing its strength for pressure containment, but reducing its expansion relative to the internals. 
     FIGS. 1 and 3 of both of the above patents show a completely closed inner housing contained within an outer housing vessel. In these two arrangements the inner housing retains the separation elements in a closely conformed housing that introduces a number of design complexities to the overall arrangement. Complexities associated with these arrangements involve the use of expansion elements, additional inlets or outlets, and purge requirements for retaining all of the separate centrifugal separators in a completely enclosed housing. In some cases the complexity of the separators can impose additional pressure drop from the total system. 
     FIG. 2 of the &#39;766 patent and of the &#39;042 patent shows a less confined separator arrangement. The generally simplified arrangement of the separator shown in FIG. 2 has lead to its adoption in different forms in most commercial applications. 
     To provide flexibility for the radial growth of the tube sheets supporting the tubular separators, the more open separator arrangement results in a significant space between the wall of the containment vessel and the closest outlet of the tubular separators. Up until this time, the containment of the gas stream below the outlets of the tubular separators in this type of arrangement has received little attention. This is particularly true in the arrangement of the more open-type separator as shown in the above mentioned FIG.  3 . 
     It is a constant goal in the use of the multiple tube separators to improve separation efficiency. Separators will usually operate with a separation efficiency of over 60%—with some providers of such separators claiming efficiencies up to 90%. The separation efficiency is defined as the difference between the particles entering the separator and the particles leaving through the fluid outlet of the separator divided by the rate of particles entering the separator. The efficiency related to selected particle size ranges and the overall efficiency are both important for evaluating the operation of a separator with the efficiency in particular particle size ranges being particularly important in many applications. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Surprisingly, it has now been discovered that the separation efficiency of a separator vessel that suspends multiple, tubular separators from a tube sheet for open discharge into a containment vessel can have improved overall recovery of particulate material, and more importantly, this improvement may be attained over particular particle size ranges. This invention confines the discharged particles and fluid from the outlets of the multiple, tubular separators as the mixture passes to a lower outlet. This type of containment improves the recovery of particles without diverting a greater fraction of the total gas flow from the main gas outlet to the particle outlet of the separation vessel. In addition, the improved containment does not restrict the free downward or radial expansion of the suspended tube sheet arrangement. Recovering additional particulate material without increasing the flow of fluid to the particle outlet improves the separator operation by minimizing the amount of fluid that exits with particles and which requires special recovery or recycle. This separation efficiency is attained without the added complexity of the more closed systems that inherently confine the discharge particles and fluids from the outlets of the tubular separators. 
     Reducing the flow area for the particle and fluid mixtures that exit the tubular separators increases the superficial velocity of the combined mixture stream as it passes to the outlet of the separator vessel. Until this discovery, it was not appreciated that the increase in diameter of the containment vessel, necessary for thermal expansion of the tube sheet and normal vessel maintenance, abruptly increased the area for the mixture thereby interfering with the separation efficiency. Extending a confining skirt downwardly, below the outlets of the tubular separators, raises the superficial velocity of the combined stream by 36% or more in a typical separator arrangement. Raising the superficial velocity in the zone below the particle outlets of the separators is particularly effective in increasing the removal of particles having a size greater than two microns. Confinement of the particle mixture below the separator outlets will usually maintain a superficial velocity of the mixture of at least 0.15 ft/sec and more preferably of at least 0.2 ft/sec. 
     The apparatus of this invention is particularly useful to modify the operation and arrangement of existing separator units. Modification of existing separators requires minimal retrofitting and can usually be accomplished through modifications made in the field. 
     Accordingly, in a broad apparatus embodiment, this invention is an apparatus for separation of fine particulate material from a fluid stream. The apparatus comprises a vessel having a vessel inlet and a fluid outlet, both an upper and a lower chamber closure partially defining a separation chamber, a plurality of centrifugal separators, and a skirt bordering the perimeter of multiple discharge openings of the separators. The vessel defines a fluid outlet, a bottom closure, and a vessel inlet which receives an input stream. The vessel inlet receives an input stream which contains particulate material and the fluid outlet discharges a principally fluid stream of particulate material having a reduced concentration relative to the input stream. The bottom closure defines a bottom outlet stream for a particulate stream which contains solids which were removed from the input stream. The bottom closure also defines at least the bottom of a particulate collection chamber. Within the separation vessel, an upper and a lower chamber closure partially defines a separation chamber which communicates with the vessel inlet and the fluid outlet. A plurality of centrifugal separators is located at least partially in the separation chamber. Each of the centrifugal separators defines a separator inlet, a separator outlet, and a solids discharge opening. The separator inlet communicates with the vessel inlet to receive the input stream. The separator outlet communicates with the fluid outlet to discharge a principally fluid stream having a reduced concentration of particulate material relative to the input stream. And the solids discharge opening downwardly discharges particulate material into the particle collection chamber. The skirt extends below the discharge openings that it surrounds and defines a bottom outlet which is in direct communication with the solids outlet through the particle collection chamber. 
     In a more limited embodiment, this invention is an apparatus for separating particulate material from a fluid stream that comprises a vessel which defines both a vessel inlet and a fluid outlet and which has a tapered bottom closure, a separator chamber, a plurality of centrifugal separators, and a skirt bordering the perimeter of the particle outlets for the separators. The vessel inlet receives an input stream which contains particulate material. The fluid outlet discharges a principally fluid stream having a reduced concentration of particulate material relative to the input stream. The tapered bottom closure defines a solids outlet for a stream which contains an increased concentration of solids relative to the input stream. The separation chamber is located within the vessel, is defined at least in part by an upper tube sheet and a lower tube sheet, and has a chamber inlet which is in isolated communication with the vessel inlet. The centrifugal separators are located at least partially within the separation chamber and spaced apart horizontally as an array of separators. Each centrifugal separator has an upper end fixed to the upper tube sheet, a lower end fixed to the lower tube sheet, a separator inlet positioned within the separation chamber for communication with the chamber inlet, a separator outlet extending above the upper tube sheet to communicate with the fluid outlet, and a particle outlet extending below the lower tube sheet for communicating particles downwardly to the particle outlet. The skirt which borders the perimeter of particle outlets in the array of separators extends below every particle outlet by a distance of at least 6″ and defines a skirt opening at the bottom of the skirt in direct communication with the tapered bottom closure and the solids outlet. 
    
    
     Other advantages, aspects, embodiments and details of this invention are set forth in the following detailed description of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section elevation of a typical arrangement of a separator as used in the prior art. 
     FIG. 2 is an enlarged cross-section of a typical tubular separation element used in the separator of FIG.  1 . 
     FIG. 3 is a cross-section of a separator as shown in FIG. 1, but modified in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The arrangement of this invention can improve the operation of separators that use an open tube sheet arrangement of multiple tubular separators. This invention can use any type of multiple separator that has a downward discharge of removed particles and fluid that will benefit from increased superficial velocity below the outlet of the tubular separators. Multiple tube separators of this type will usually house anywhere from 48 to 160 individual separation tubes with the number of individual tubes typically ranging from 60 to 120. A typical swirl tube will have a maximum height of about 48 inches and a maximum diameter of from 8 to 10 inches. The typical pressure drop across the individual tubular separators is normally about 1.5 psi and total pressure drop across the entire separation system averages about 2.5 psi. Separators used with this invention will typically remove particles having a size of 25 microns and smaller. The preferred applications of this invention may remove particles in a size range of from 20 to 5 microns and more preferred applications may remove particles in a size range of from 15 to 7 microns. 
     FIG. 1 shows the general arrangement for a typical separator that may be improved by this invention. In a typical arrangement, a vessel  10  has an inlet  12  for receiving a mixture of fluids and particles that the separator  10  divides into a principal fluid stream having a reduced concentration of particles relative to the input stream that exits through a fluid outlet  14  and a stream containing an increased concentration of solids, relative to the input stream, that exits through a solids outlet  16 . A conduit  18  delivers the input stream from nozzle  12  into a separation chamber  20  across a chamber inlet  22 . A screen element  24  covers inlet  22  and blocks the entrance of oversize material into chamber  20  The bottom of conduit  18  has a dished bottom  25  that serves as a well for retaining oversize material. An upper tube sheet  26  and lower tube sheet  28  form the top and bottom respectively of separation chamber  20 . 
     A circular cylinder  30  encloses the outside of chamber  20  and depends from a frustro-conical support  32  that is rigidly attached to separation vessel  10 . Circular cylinder  30  has freedom to expand downwardly and outwardly. An annular space  58  surrounds cylinder  30  to permit free radial growth of the tube sheet and tubular separator internals relative to the wall of vessel  10 . Vertical clearance between a bottom closure  60  of vessel  10  and the bottom of an outlet tube  40  or circular cylinder  30  permits the cylinder and tube sheet assembly to freely expand in a downward direction. 
     The difference in thermal expansion between the cylinder walls and the tube sheet elements usually results from relative cooling of the walls of vessel  10  with respect to the internals of the separator. FIG. 1 shows a layer of insulation  62  that shields the externally exposed elements of vessel  10  from the full internal temperature of the fluid stream passing through the vessel. Such cooling is usually necessary to give the metal defining the vessel adequate strength to withstand the internal pressure that it contains. 
     This invention is especially useful where the processing of high temperature fluids results in the use of an internally insulated vessel. Such thermal design considerations typically become important in the design of a separator that processes a fluid at temperatures of more than about 500° F. 
     Tubular separation elements  34  vertically bridge the space between tube sheets  26  and  28  of chamber  20 . As shown more clearly by reference to FIG. 2, a fluid tube  36  of tubular separator element  34  depends from an attachment plate  33  that is sealed to fluid tube  36  about an inner edge and rests on the top of upper tube sheet  26  with a seal to the tube sheet  26  about its opposite edge. Ring  33  covers an enlarged hole  39  that is used to install fluid tube  36  from a collection chamber  52  located above upper tube sheet  26 . Once installed, an outlet tube  40  surrounds the lower portion  38  of fluid tube  36  and the outer portion of a vane assembly  42  that encircles the portion  38  of fluid tube  36 . Sealed attachment of outlet tube  40  fixes it to lower tube sheet  28  and positions outlet tube  40  relative to the fluid tube  36 . An annular space between outlet tube  40  and fluid tube  36  defines an inlet  44  for tubular separator element  34  to receive its distributed share of the inlet stream comprising the particle-containing fluid. As the input stream passes downwardly through the vane element  42 , centripedal acceleration of the particle containing fluid forces the heavier, particulate material towards the outside of outlet tube  40  and to the outside of annular space  46 . The particles and a small portion of the fluid exit a bottom outlet  48  of outlet tube  40 . 
     The majority of the fluid stream changes direction and flows upwardly into an inlet  50  and through fluid tube  36 . Collection chamber  52  collects the particle de-entrained fluid from outlets  54  of fluid tubes  36 . Fluid flows out of chamber  52  via nozzle  14 . Particulate material and the underflow fluid pass downwardly through a particle flow chamber  56  and out of a separator through solids outlet  16 . 
     FIG. 3 shows the improvement of this invention where an extended band of cylinder  30  provides a skirt section  64  bordering the outer perimeter of the discharge openings  48  and extending below the discharge openings that effectively reduces the diameter of particle flow area  56  ′. Providing a standard skirt section  64  cuts off the volume of vessel  10  below the discharge openings  48  and isolates an extended annular volume in an annular space  58 ′. The bottom of skirt extension  64  is spaced away from the bottom enclosure  60  and the insulation  62  by approximately the minimum distance needed for radial and downward expansion of the skirt section  64 . For most arrangements, vertically off-setting the skirt from the lining of the pressure vessel wall by a minimum of 3 inches will provide adequate clearance for thermal expansion during high temperature operation. Any remaining gap between the bottom of extended skirt  64  and the bottom closure  60  will have minimal impact on the superficial velocity since the portion of the bottom enclosure adjacent to the bottom of skirt will have about the same diameter as the skirt bottom. It may also be desired to provide some minimum clearance between the periphery of outlet tube  40  and the inside of skirt  64 . Such clearance will usually be in a range of about 3 inches and aids in establishing a regular vortex pattern below outlet  48 . 
     In regard to the vortex pattern, the vortex-type flow of fluid that is established by vane element  42  typically extends below outlet  48  as the fluid changes direction. It is believed that the added confinement and increased superficial velocity, provided by this invention, restricts the vortex from expansion below the outlet  48  while at the same time pulling particles from the reversing fluid stream thereby improving the separation efficiency. Maintaining an undisturbed vortex pattern immediately below outlet  48  is also believed to be beneficial in improving the separation efficiency and the skirt  64  will typically provide a uniform diameter section that extends at least 6 inches and more preferably at least 12 inches below the lowest outlet  48 . 
     For structural reasons the tube sheets  26  and  28  that support the separators supports will usually have a curved shape. Normally the bottom of the outlet tubes  40  are spaced a uniform distance from where the center-line of the outlet tube  40  intersects the tube sheets. Uniform extension of outlet tubes  40  below their tube sheet intersection point locates their outlets  48  at different elevations. At minimum, skirt  64  extends below the lowermost outlet  48  of the outlet tubes  40 . 
     The annular space  58  provided between the outside of circular cylinder  30  and the inside vessel  10  is routinely made larger than necessary for thermal growth purposes to provide inspection and/or service access to the insulating lining  62 . Similarly the spacing between the bottom of circular cylinder and the bottom closure  60  is often sized to permit visual inspection and/or service of the refractory from the bottom of annular space  58  by a person who enters discharge space  56  through the access nozzle  63 . The practice of this invention need not prevent such access and annular space  58  may be made accessible by providing bolted sections of skirts  64  which are suitable for temporary removal during inspection or service. Where removable sections will provide access, the skirt will preferably have a minimum vertical dimension of at least 18 inches. Alternately, additional access nozzles, as represented by number  63  in the drawings may be provided in the side of vessel  10 . 
     While this description has shown a skirt extension to provide the confinement, the function of the skirt extension may be provided by other means known to those skilled in the art. For example additional refractory or other filling material may reduce the diameter of the separation vessel to approximately that of the outlets  48  and thereby provide the same function of the extended skirt. The diameter of the filler material would only be increased by the minimum amount necessary to provide the necessary radial growth of the lower tube sheet. Such filler material can comprise a builtout portion of insulation  62  or other suitable lining. In this manner the overall arrangement of the separator materials remains relatively simple and the benefits of the invention are achieved without extending the skirt per se. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                   
                   
                 CASE 1 
                 CASE 2 
                 WEIGHT RATIO 
               
               
                 PARTICLE 
                 INLET 
                 OUTLET 
                 OUTLET 
                 OUTLET CASE 1 
               
             
          
           
               
                 SIZE μm 
                 WEIGHT % 
                 LB/HR 
                 WEIGHT % 
                 LB/HR 
                 EFFICIENCY 
                 WEIGHT % 
                 LB/HR 
                 EFFICIENCY 
                 OUTLET CASE 2 
               
               
                   
               
               
                 14.7 and up 
                 36.16 
                 49.43 
                 10.23 
                 5.42 
                 89.0 
                 2.22 
                 .99 
                 98.0 
                 5.5 
               
               
                 12.4 to 14.7 
                 2.33 
                 3.17 
                 0.82 
                 .44 
                 86.4 
                 .20 
                 .09 
                 97.0 
                 4.5 
               
               
                 7.2 to 12.4 
                 7.84 
                 10.71 
                 2.80 
                 1.48 
                 86.2 
                 1.68 
                 .75 
                 93.0 
                 2.0 
               
               
                 4.2 to 7.2 
                 6.5O 
                 8.88 
                 2.14 
                 1.12 
                 87.3 
                 5.00 
                 2.23 
                 75.0 
                 0.5 
               
               
                 2.8 to 4.2 
                 9.50 
                 12.98 
                 9.07 
                 4.81 
                 63.0 
                 11.68 
                 5.20 
                 60.0 
                 0.9 
               
               
                 1.4 to 2.8 
                 13.25 
                 18.12 
                 19.61 
                 10.38 
                 42.7 
                 24.38 
                 10.87 
                 40.0 
                 1.0 
               
               
                 0.9 to 1.4 
                 8.58 
                 11.73 
                 19.28 
                 10.20 
                 13.0 
                 18.39 
                 8.20 
                 30.0 
                 1.2 
               
               
                 0.0 to 0.9 
                 15.84 
                 21.65 
                 36.05 
                 19.07 
                 11.9 
                 36.45 
                 16.25 
                 25.0 
                 1.2 
               
               
                 TOTALS 
                 100.00 
                 136.6 
                 100.0 
                 52.9 
                   
                 100.00 
                 44.5 
               
               
                   
               
             
          
         
       
     
     EXAMPLES 
     EXAMPLE 
     In order to demonstrate the advantages of increasing superficial velocity in accordance with this invention, two separators are designed to process 295,000 lbs/hr of an FCC flue gas having fines loading as shown in Table 1 and broken down by size distribution. The following example is based on engineering design calculations and empirical data from experimental modeling and operating separators. In both cases, the separator has a configuration as generally shown in FIG.  1 . The separator contains 64 swirl tubes and has a vessel diameter of 20′2½″ and a skirt diameter of 15′3″. The flue gas enters the separator at a temperature of 1250° F. and a pressure of 35 psi. The average molecular weight of the flue gas stream is 29.2. The cross-sectional area of the inside of the separator vessel is approximately 321 ft 2 . The inside of the skirt has a cross-sectional area of approximately 183 ft 2 . 
     Case 1 
     The separator operated in case 1 to simulate the relatively lower superficial velocity that would occur when the fluid and particle mixture (also referred to as the underflow) leaving the outlets of the swirl tubes passes into the cross-sectional area of the separator below the swirl tube outlets. Approximately 3% of the entering flue gas stream or 8850 lb/hr exits the separator as the underflow stream that contains the particles removed from the flue gas stream. On a volumetric basis this produces a flow rate of about 31 ft 3 /sec and reduces the superficial velocity to 0.097 ft/sec. in the region below the outlets. The table lists the wt % and lbs/hr of particles remaining in the flue gas stream after it leaves the flue gas outlet of the separator. The overall efficiency for the separation is about 61.3. The table also lists a breakdown by particle size. 
     Case 2 
     To demonstrate the benefit of extending the skirt downwardly below the swirl tube outlets, this case shows the effect of increasing the downward superficial velocity by confining the cross-sectional area to that of the ID of the skirt that surrounds the swirl tube outlets. The reduction in diameter increases the superficial velocity to 0.17 ft/sec. The table shows the resulting decrease in the particle loading in the flue gas as it leaves the separator as a result of the increased superficial velocity. Looking at the entire particle loading, the invention has the advantage of increasing the overall efficiency to 67.4 or by about 10% over the efficiency of case 1. More importantly, when looking at individual efficiencies associated with particle sizes, case 2 provides a more substantial increase in the efficiency of the particle removal for the larger particle sizes. More dramatically, the ratio of the remaining particles in the largest particle size range is relatively high. Removal of particles having sizes of about 15 microns and up is a very significant improvement since these particles have the greatest propensity to damage equipment that is used for the further processing of the flue gas downstream of the separator. Removal of particles in the 12.4 to approximately 15 micron size range is also significant and is also accomplished to a much higher degree in case 2. Particle removal in the size range of from 7.2 to 12.5 microns removal is also important and improves significantly in case 2. The removal of particles in the size range of about 4.2 to 1.4 microns is not as important for most flue gas operations while the removal of particles having sizes of below about 1.4 microns is of little consequence. Accordingly, the operation represented by case 2 shows a very beneficial decrease in particles that escape with the flue gas from the separator and that are in the most beneficial size ranges for removal.