Abstract:
Process for increasing mixing in a fluidized bed. A slide, which may be in the form of a tube or trough, transports particles from an upper zone downward to a lower zone at a different horizontal position, thereby changing the horizontal position of the particle and creating lateral mixing in the fluidized bed. Increased mixing may improve efficiency for an apparatus using a fluidized bed. For example, increased lateral mixing in a regenerator may increase temperature and oxygen mixing and reduce stagnation to improve efficiency. A slide may be relatively unobtrusive, inexpensive, and simple for a retrofit or design modification and may improve combustion efficiency at high rates by enhancing the lateral blending of spent and regenerated catalyst.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Division of copending application Ser. No. 11/614,862 filed Dec. 21, 2006, the contents of which are hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to apparatus and processes using fluidized beds. More specifically, this invention relates to increasing the lateral mixing of particles in fluidized beds. 
     DESCRIPTION OF THE PRIOR ART 
     Fluidized beds are used in many industrial applications. One use in particular is in the regenerator of a petroleum refining process. 
     Fluid catalytic cracking (FCC), as well as Resid FCC (RFCC), is a catalytic conversion process for cracking heavy hydrocarbons into lighter hydrocarbons by bringing the heavy hydrocarbons into contact with a catalyst composed of finely divided particulate material. Most FCC units use zeolite-containing catalyst having high activity and selectivity. 
     The basic components of the FCC reactor section include a riser, a reactor, a catalyst stripper, and a regenerator. In the riser, a feed distributor inputs the hydrocarbon feed which contacts the catalyst and is cracked into a product stream containing lighter hydrocarbons. Catalyst and hydrocarbon feed are transported upwardly in the riser by the expansion of the lift gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, upon contact with the hot catalyst. Steam or an inert gas may be used to accelerate catalyst in a first section of the riser prior to or during introduction of the feed. Coke accumulates on the catalyst particles as a result of the cracking reaction and the catalyst is then referred to as spent catalyst. The reactor disengages spent catalyst from product vapors. The catalyst stripper removes absorbed hydrocarbon from the surface of the catalyst. The regenerator removes the coke from the catalyst and recycles the regenerated catalyst into the riser. 
     The spent catalyst particles are regenerated before catalytically cracking more hydrocarbons. Regeneration occurs by oxidation of the carbonaceous deposits to carbon oxides and water. The spent catalyst is introduced into a fluidized bed at the base of the regenerator, and oxygen-containing combustion air is passed upwardly through the bed. After regeneration, the regenerated catalyst is returned to the riser. 
     Oxides of nitrogen (NO x ) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Regulated NO x  emissions generally include nitric oxide (NO) and nitrogen dioxide (NO 2 ), but the FCC process can also produce N 2 O. In an FCC regenerator, NO x  is produced almost entirely by oxidation of nitrogen compounds originating in the FCC feedstock and accumulating in the coked catalyst. At FCC regenerator operating conditions, there is negligible NO x  production associated with oxidation of N 2  from the combustion air. Production of NO x  is undesirable because it reacts with volatile organic chemicals and sunlight to form ozone. 
     The two most common types of FCC regenerators in use today are a combustor-style regenerator and a bubbling bed regenerator. Bubbling bed and combustor-style regenerators may utilize a CO combustion promoter comprising platinum for accelerating the combustion of coke and CO to CO 2 . The CO promoter decreases CO emissions but increases NO x  emissions in the regenerator flue gas. 
     The combustor-style regenerator has a lower vessel called a combustor that burns nearly all the coke to CO 2  with little or no CO promoter and with low excess oxygen. The combustor is a highly backmixed fast fluidized bed. A portion of the hot regenerated catalyst from the upper regenerator is recirculated to the lower combustor to heat the incoming spent catalyst and to control the combustor density and temperature for optimum coke combustion rate. As the catalyst and flue gas mixture enters the upper, narrower section of the combustor, the velocity is further increased and the two-phase mixture exits through symmetrical downturned disengager arms into an upper regenerator. The upper regenerator separates the catalyst from the flue gas with the disengager arms followed by cyclones and return it to the catalyst bed which supplies hot regenerated catalyst to both the riser reactor and lower combustor. 
     A bubbling bed regenerator carries out the coke combustion in a dense fluidized bed of catalyst. Fluidizing combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Only catalyst entrained in the gas exits the reactor with the vapor. Cyclones above the dense bed separate the catalyst entrained in the gas and return it to the catalyst bed. The superficial velocity of the fluidizing combustion air is typically less than 1.2 m/s (4 ft/s) and the density of the dense bed is typically greater than 480 kg/m 3  (30 lb/ft 3 ) depending on the characteristics of the catalyst. The mixture of catalyst and vapor is heterogeneous with pervasive vapor bypassing of catalyst. The temperature will increase in a typical bubbling bed regenerator by about 17° C. (about 30° F.) or more from the dense bed to the cyclone outlet due to combustion of CO in the dilute phase. The flue gas leaving the bed may have about 2 mol-% CO. This CO may require about 1 mol-% oxygen for combustion. Assuming the flue gas has 2 mol-% excess oxygen, there will likely be 3 mol-% oxygen at the surface of the bed and higher amounts below the surface. Excess oxygen is not desirable for low NO x  operation. 
     Refiners often use CO promoter (equivalent to 0.5 to 3 ppm Pt inventory) to control afterburn at the low excess O 2  required to control NO x  at low levels. While low excess O 2  reduces NO x , the simultaneous use of Pt CO promoter often needed for afterburn control can more than offset the advantage of low excess O 2 . 
     Bubbling bed regenerators have a fluidized bed. Fluidized beds generally mix well vertically, up and down, but not laterally, or horizontally. Rising bubbles draw catalyst up with tem in their wakes and the catalyst constitutes about one third of total bubble volume. This is the principle solids mixing mechanism in fluidized beds. In a bubbling bed, also known as a dense catalyst bed, combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed. These bubbles rise with little horizontal displacement. 
     The superficial velocity of the combustion gas is typically less than 1.2 m/s (4.2 ft/s) and the density of the dense bed is typically greater than 640 kg/m 3  (40 lb/ft 3 ) depending on the characteristics of the catalyst. The mixture of catalyst and combustion gas is heterogeneous with pervasive gas bypassing of catalyst. 
     The dilute transport flow regime is typically used in FCC riser reactors. In transport flow, the difference in the velocity of the gas and the catalyst is relatively low with little catalyst back mixing or hold up. The catalyst in the reaction zone maintains flow at a low density and very dilute phase conditions. The superficial gas velocity in transport flow is typically greater than 2.1 m/s (7.0 ft/s), and the density of the catalyst is typically no more than 48 kg/m 3  (3 lb/ft 3 ). The density in a transport zone in a regenerator may approach 80 kg/m 3  (5 lb/ft 3 ). In transport mode, the catalyst-combustion gas mixture is homogeneous without gas voids or bubbles forming in the catalyst phase. 
     Intermediate of dense, bubbling beds and dilute transport flow regimes are turbulent beds and fast fluidized regimes. In a turbulent bed, the mixture of catalyst and combustion gas is not homogeneous. The turbulent bed is a dense catalyst bed with elongated voids of combustion gas forming within the catalyst phase and a less discernible surface. Entrained catalyst leaves the bed with the combustion gas, and the catalyst density is not quite proportional to its elevation within the reactor. The superficial combustion gas velocity is between about 1.1 and about 2.1 m/s (3.5 and 7 ft/s), and the density is typically between about 320 and about 640 kg/m 3  (20 and 40 lb/ft 3 ) in a turbulent bed. 
     Fast fluidization defines a condition of fluidized solid particles lying between the turbulent bed of particles and complete particle transport mode. A fast fluidized condition is characterized by a fluidizing gas velocity higher than that of a dense phase turbulent bed, resulting in a lower catalyst density and vigorous solid/gas contacting. In a fast fluidized zone, there is a net transport of catalyst caused by the upward flow of fluidizing gas. The catalyst density in the fast fluidized condition is much more sensitive to particle loading than in the complete particle transport mode. From the fast fluidized mode, further increases in fluidized gas velocity will raise the rate of upward particle transport, and will sharply reduce the average catalyst density until, at sufficient gas velocity, the particles are moving principally in the complete catalyst transport mode. Thus, there is a continuum in the progression from a fluidized particle bed through fast fluidization and to the pure transport mode. The superficial combustion gas velocity for a fast fluidized flow regime is typically between about 1.5 and about 3.1 m/s (5 and 10 ft/s) and the density is typically between about 48 and about 320 kg/m 3  (3 and 20 lb/ft 3 ). 
     A combustor-style regenerator is a type of regenerator that completely regenerates catalyst in a lower, first combustion chamber under fast fluidized flow conditions with a relatively small amount of excess oxygen. A riser carries regenerated catalyst and spent combustion gas to a separation chamber wherein significant combustion occurs. Regenerated catalyst in the separation chamber is recycled to the lower combustion phase to heat the spent catalyst about to undergo combustion. The regenerated catalyst recycling provides heat to accelerate the combustion of the lower phase of catalyst. Combustor-style regenerators are advantageous because of their efficient oxygen requirements. 
     As greater demands are placed on FCC units, combustor vessels are being required to handle greater catalyst throughput. Greater quantities of combustion gas are added to the combustor vessels to combust greater quantities of catalyst. As combustion gas flow rates are increased, so does the flow rate of catalyst between the combustion and separation chamber increase. Hence, unless the combustion chamber of a combustor vessel is enlarged, the residence time of catalyst in the lower zone will diminish, thereby decreasing the thoroughness of the combustion that must be achieved before the catalyst enters the separation chamber. 
     An enlarged first chamber diameter increases the diameter of the fluidized bed and therefore the distance between the spent catalyst, at a cooler temperature, input and recycled catalyst, at a hotter temperature, is increased. Areas of temperature difference and generally stagnant zones of the high oxygen concentrations and may result and combustion efficiency may decrease. In the first chamber vertical mixing may occur, but there is usually little horizontal, or lateral, mixing. There exists a need for better lateral mixing in fluidized beds. 
     SUMMARY OF THE INVENTION 
     Apparatus and process for increasing mixing in a fluidized bed. A slide, which may be in the form of a tube or trough, transports particles from an upper zone downward to a lower zone at a different horizontal position, thereby changing the horizontal position of the particle and creating lateral mixing in the fluidized bed. Increased mixing may improve efficiency for an apparatus using a fluidized bed. 
     For example, in a regenerator areas of temperature and oxygen level differences, as well as general stagnation may occur. Recycle and recirculation standpipe inlet and outlet positions in may further exasperate these differences in temperature and oxygen concentration. Increasing lateral mixing in a regenerator may increase temperature and oxygen mixing and reduce stagnation to improve efficiency. A slide may be relatively unobtrusive, inexpensive, and simple for a retrofit or design modification and may improve combustion efficiency at high rates by enhancing the lateral blending of spent and regenerated catalyst. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational diagram showing an FCC unit with a bubbling bed style regenerator with a slide. 
         FIG. 2  is a cross section view from line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a cross section view of a regenerator with a plurality of slides. 
         FIG. 4  is a cross section view of a regenerator with an arrangement of slides. 
         FIG. 5  is an elevational diagram showing a combustor-style regenerator with a slide. 
         FIG. 6  is a cross section view from line  6 - 6  of  FIG. 5 . 
         FIG. 7  is an elevational diagram showing a combustor-style regenerator with an alternative embodiment of a slide. 
         FIG. 8  is a cross section view from line  8 - 8  of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     The FCC process may use an FCC unit  10 , as shown in  FIG. 1 . Feedstock enters a riser  12  through a feed distributor  14 . Feedstock may be mixed with steam in the feed distributor  14  before entering. Lift gases, which may include inert gases or steam, enters through a steam sparger  16  in the lower portion of the riser  12  and creates a fluidized medium with the catalyst. Feedstock contacts the catalyst to produce cracked hydrocarbon products and spent catalyst. The hydrocarbon products are separated from the spent catalyst in the reactor  18 . 
     The blended catalyst and reacted feed vapors enter the reactor  18  and are separated into a cracked product vapor stream and a collection of catalyst particles covered with substantial quantities of coke and generally referred to as spent catalyst or coked catalyst. Various arrangements of separators to quickly separate coked catalyst from the product stream may be utilized. In particular, a swirl arm arrangement  20 , provided at the end of the riser  12 , may further enhance initial catalyst and cracked hydrocarbon separation by imparting a tangential velocity to the exiting catalyst and cracked product vapor stream mixture. The swirl arm arrangement  20  is located in an upper portion of a separation chamber  24 , and a stripping zone  26  is situated in the lower portion. Catalyst separated by the swirl arm arrangement  20  drops down into the stripping zone  26 . 
     The cracked product comprising cracked hydrocarbons including gasoline and light olefins and some catalyst may exit the separation chamber  24  via a gas conduit  28  in communication with cyclones  30 . The cyclones  30  may remove remaining catalyst particles from the product vapor stream to reduce particle concentrations to very low levels. The product vapor stream may enter into a reactor plenum  31  and exit the reactor  18  through a product outlet  32 . Catalyst separated by the cyclones  30  may return to the reactor  18  through reactor diplegs  34  into a dense bed  36  where catalyst passes through chamber openings  38  and enter the stripping zone  26 . The stripping zone  26  removes entrained hydrocarbons between catalyst particles and adsorbed hydrocarbons from the surface of the catalyst by counter-current contact with steam over optional baffles  40 . Steam may enter the stripping zone  26  through a line  42 . A spent catalyst conduit  44  transfers spent catalyst to a regenerator  50 . 
     The regenerator  50  receives the spent catalyst into a vessel  52 , shown as a bubbling bed regenerator vessel in  FIGS. 1-4 , or a combustor, or first chamber, in a combustor-style regenerator shown in  FIGS. 5-8 , through an inlet  54 . Spent catalyst may enter into a fluidized bed  56  in the vessel  52 . The fluidized bed  56  may have a mixing apparatus. 
     A mixing apparatus for a fluidized bed  56  may have multiple embodiments. The mixing apparatus may be a slide  70 . The slide  70  may have a first end  71  in the upper zone  60  and a second end  72  at a different horizontal position in the lower zone  62 . 
     In a bubbling bed regenerator, rising bubbles move catalyst from the lower zone  62  to the upper zone  60 . The first end  71  may receive particles and transport the particles down the slide  70  to be dispensed from the second end  72  into a different horizontal position in the lower zone  62 . Bubbles then may transport catalyst from the new position on in the lower zone  62  to a new position in the upper zone  60 . An emulsion phase flows counter to the draft that is created by the flow into and out of the slide  70  to maintain the overall bed level. 
     In a combustor-style regenerator  50  catalyst mixes well vertically and particles traveling downward from the upper zone  62  may be received by first end  71  and transported laterally to dispense from second end  72 . Fluidizing medium may then force the particle into the upper zone  60  at this new horizontal position. Lateral mixing occurs as a result of the change in horizontal position. 
     The slide  70  may be a tube, a trough, or a channel. The slide  70  may be made of angle iron or channel iron. As shown in  FIGS. 1 and 2 , an accumulator  74  may attach to the first end  71  of the slide  70  to funnel particles into the first end  71 . The slide  70  may be attached to the wall  76  for stability. A tube is preferred because a tube can generate head, or pressure, due to density differences between the fluidized bed  56  and the fluidized materials in the tubes and will drive greater flow rates. Slide  70  may be perforated. The opening at the bottom of a slide  70  may have a vertical edge to decrease upward moving gases and particles from entering. A one-way valve on the bottom opening may be used to decrease the entrance of upward moving particles and gases. Dashed lines with arrowheads in the vessel  52  of the FIGURES represent particles entering the first end  71  of the slide  70  and exiting from the second end  72  at a different horizontal location with the arrowhead indicating the direction of movement. 
     Multiple slides  70  may be positioned in the bed at strategic locations at an angle equal to or greater than the angle of repose of the solid being fluidized. As shown in  FIGS. 3-4 , slides  70  may be arranged in patterns to generate additional mixing in the fluidized bed  56 . The number of slides  70  and the diameter of each slide  70  may depend on the size of the fluidized bed  56  and the amount of mixing to be generated. Length of the slide  70  may be a function of the bed  56  height. A larger and longer slide  70  may be used to generate flow from one general area to another and counter flow or natural circulation to reestablish the level. Thus, the number and dimensions of slide  70  may be adjusted for optimal mixing for the particular fluidized bed  56  diameter, height, inlet-outlet configuration, and rates. 
     In one embodiment, as shown in  FIGS. 7-8 , slide  70  may be attached to the inside of the vessel  52  with the elevated first end  71  and transfer particles near and along the wall  76  to the second end  72  at a different horizontal position. The slope of the slide  70  relative to horizon may be between about 10° and 60°, preferably between about 12° and about 25°. The width of the slide  70  may vary to accommodate different sized vessels  52  and to take into consideration affects on the upward movement of particles in the vessel  52 . Preferably, the width of the slide  70  is equal to between about 1% and about 15% of the diameter of the vessel  52 , even more preferably between about 2% and about 10%. 
     Combustion of coke from the spent catalyst particles raises the temperatures of the catalyst. Flue gas consisting primarily of N 2 , H 2 O, O 2 , CO 2  and traces of NO x , CO, and SO x  passes upwardly from the dense bed into a dilute phase of the regenerator  50 . Typically above the fluidized bed in a bubbling bed regenerator  50 , or in an upper chamber  100  of a combustor-style regenerator  50  may be a regenerator cyclone  80  or other means to remove entrained catalyst particles from the rising flue gas, usually having a regenerator dipleg  82  for releasing catalyst. Gases may enter a plenum  84  before exiting through a vent  86 . Depending on the size and throughput of a regenerator  50 , between about 6 and 60 regenerator diplegs  82  may be utilized. In a combustor-style regenerator catalyst from regenerator dipleg  82  may enter a regenerator dense bed  94 . From this regenerator dense bed  94  in a combustor-style regenerator, or from the vessel  52  in a bubbling bed regenerator, catalyst may pass, regulated by a control valve, through a regenerator standpipe  88 , which attaches to the bottom portion of riser  12 . 
     As shown in  FIG. 5-8 , the upper chamber  100  may receive flue gas and catalyst from the vessel  52  through a disengager  102 . Regenerated catalyst may be recycled into the vessel  52  through a recycle standpipe  104 .  FIG. 6  shows a cross section of the vessel  52  indicating the positions of the spent catalyst conduit  44  and recycle standpipe  104  on opposite sides of the vessel  52 . Bubbling bed regenerators may also have a recycle standpipe  104  and recycle regenerated catalyst to the lower zone  62  of the vessel  52 . 
     The hottest and most completely regenerated catalyst is recirculated to the lower zone of the vessel  52 , in a bubbling bed regenerator, or the lower chamber in a combustor-style regenerator, making the hot spot hotter, while the least completely regenerated catalyst is returned to the riser  12 . Preferably, it would be better to reverse this, returning the most completely regenerated catalyst to the riser  12  and recirculating the less regenerated material to the first chamber  52  for another pass. This may permit more stable operations at lower regenerator temperatures. 
     Analysis of temperature data from a large diameter vessel  52  of a combustor-style regenerator with extensive thermometry indicated the presence of relative hot spots where cooler fresh and hotter regenerated catalyst standpipes enter the vessel  52 . In this combustor-style regenerator the data shows a relatively cool spot of about 640° C. to about 670° C. very near the entry of spent catalyst. The temperature of the cool spot is just above the mid point between the about 740° C. regenerated catalyst temperature and the 530-540° C. spent catalyst. With perfect mixing it could roughly be two thirds of the regenerated catalyst temperature. A hot spot, of about 25-40° C. hotter, exists at the bottom of the vessel  52  at the return of the regenerated catalyst recirculation standpipe  104 . The temperature profiles at higher elevations show that the hot and cool areas propagate vertically through the vessel  52  up to bottom of the upper chamber  100 . As the flue gasses and catalyst rise, the exotherm of combustion and lateral mixing and dispersion reduce the magnitude of the differences hot and cool spot temperatures 5-10° C. 
     Mixing in a regenerator  50  promotes more uniform temperatures and catalyst activity through improved fuel distribution to promote a more efficient reaction between the gases and catalyst. The improved mixing Refiners often use high levels of Pt CO combustion promoter and high levels of excess O 2  to accelerate combustion and reduce afterburning in their FCC unit, especially when operating at high throughputs. These practices may increase NO x  by up to 10-fold from the 10-30 ppm possible when no platinum is used and excess O 2  is controlled below 0.5 v-%. 
     A process for increasing mixing, especially lateral mixing, in a fluidized bed  56  may include one or more of the described apparatus. Increasing lateral mixing in the bed  56  may be accomplished by including a slide  70 . Such a process may include introducing catalyst to a vessel  52  through an inlet  54 . Gas is distributed to the vessel  52  below said inlet. Particles of a fluidized bed  56  may be directed from an upper zone  60  of the vessel  52  to a different horizontal position in a lower zone  62  of the vessel to increase the lateral mixing of the bed  56 . This process may occur in a combustor-style or a bubbling bed regenerator  50 . 
     The examples and figures provided are mostly in reference to embodiments used in FCC and RFCC regenerators; however, the invention should not be limited to only regenerators or to the these processes.