Abstract:
Fluid catalytic cracking units having risers with improved hydrodynamics through the use of baffles are described. The baffles break up the high concentration of catalyst in the slower moving outer annulus and redistribute it into the faster moving, more dilute center of the riser flow.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to fluid catalytic cracking units, and more particularly to fluid catalytic cracking units having risers with improved hydrodynamics through the use of baffles. 
     In a fluid catalytic cracking (FCC) unit such as illustrated in  FIG. 1 , hydrocarbons are contacted in a reaction zone with a catalyst composed of finely divided particulate material. Inert diluent, such as steam, enters the riser and is mixed with catalyst. The hydrocarbon feed and an inert diluent, such as steam, are introduced to the riser  10  by a hydrocarbon feed distributor  5  which atomizes the hydrocarbon feed as it enters the riser  10 . The hydrocarbon feed and inert diluent fluidize the catalyst and transport it in the riser  10 . The catalyst promotes the cracking reaction. As the cracking reaction proceeds, a substantial amount of highly carbonaceous material, referred to as coke, is deposited on the catalyst. The coke-containing catalyst is separated from the hydrocarbon product in a separation zone  20  and removed from the reactor through conduit  30 , while the hydrocarbon product exits through the top of the reactor. The coke is burned from the catalyst by contact with an oxygen-containing stream that serves as a fluidization medium in a high temperature regeneration zone  25 . Coke-containing catalyst is replaced by essentially coke-free catalyst from the regeneration zone  25  through conduit  35 . In some FCC units, there is a conduit  40  in which a portion of the catalyst is recycled without going through the regeneration zone  25 . 
     FCC risers have traditionally suffered from vapor-catalyst slip caused by the inherent non-uniformities of upward moving particle-containing flows. These non-uniformities manifest themselves primarily as core-annular structures: the core of the flow is dilute and moves upward at a higher velocity, while there is a high concentration of catalyst near the wall which forms a dense, slow-moving annulus. The annulus can actually move downward in some cases. This annular flow results in decreased conversion in the riser because the faster moving dilute core under-converts the feed and the slower moving and/or downward moving annulus over-cracks the primary FCC products, leading to increased dry gas production. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a riser reactor. In one embodiment, the riser reactor includes a vertical riser having a hydrocarbon feed inlet; and a row of baffles located more than 6 m above the hydrocarbon feed inlet, a front face of the baffle facing the center of the riser, a lower end of the baffle attached to a wall of the riser and the baffle inclined inward from the wall at an angle of about 90° or less. 
     In another embodiment, the riser reactor includes a vertical riser having a hydrocarbon feed inlet; and a row of baffles located more than 6 m above the hydrocarbon feed inlet, a front face of the baffle facing the center of the riser, a lower end of the baffle attached to a wall of the riser, and the baffle inclined inward from the wall at an angle of about 90° or less. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of one embodiment of an FCC unit. 
         FIG. 2A  is a cross-section of one embodiment of a riser pipe with internal baffles. 
         FIG. 2B  is an illustration of one embodiment of a riser pipe with internal baffles. 
         FIG. 3  is a sectional view along A-A of  FIG. 2A  of one embodiment of a baffle. 
         FIG. 4  is a sectional view along A-A of  FIG. 2A  of another embodiment of a baffle. 
         FIG. 5  is a sectional view along A-A of  FIG. 2A  of another embodiment of a baffle. 
         FIGS. 6A-C  are illustrations of one embodiment of two subsets of a row of baffles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The use of baffles in the mixing zone of the riser alters the flow profile so that it approaches true plug flow, alleviating the problems associated with the core-annulus structure. The baffles break up the outer annulus and redistribute the catalyst into the center of the riser flow. This results in higher conversion in the riser and less overcracking of the products. 
     The attachment of baffles to the riser wall in the mixing zone above the hydrocarbon feed inlet has been shown to make the catalyst holdup distribution in the riser more uniform using Computational Flow Dynamics (CFD) computer simulation. The baffles also improve the flow profile in the riser by slowing down the upward core flow, which results in less short-circuiting. In addition, the baffles minimize the downward flow of the annulus. 
       FIG. 2A  shows one embodiment of a riser  100  having a row of baffles  115  extending inward from the wall  110 . The front face  140  of the baffles is facing the center of the riser. As shown, the baffles  115  are equally spaced around the circumference of the riser  100  and cover substantially the whole circumference of the riser. 
     In one embodiment, the baffles are positioned symmetrically around the circumference of the riser. In another embodiment, the baffles are arranged non-symmetrically. 
     In some embodiments, the baffles can cover less of the circumference, if desired. For example, typically at least about 30% of the circumference is covered with baffles, or at least about 40%, or at least about 50%, or at least about 60%, or at least about 70%, or at least about 80%, or at least about 90%, or at least about 95%. 
     The baffles extend inward from the wall a distance d up to about 25% of the radius R of the riser, typically in the range of about 15% to about 25%. The baffles desirably extend over about ⅛ of the cross-sectional area of the riser  110 . 
     The baffles are typically in the range of about 0.15 to about 0.30 m in length. The length depends in part on the radius of the riser and the angle from the wall. 
     The angle of the baffle (about 90° from vertical or less) coupled with the ceramic lining ensure the erosion resistance of the attachment. 
     The riser desirably has at least two rows of baffles along its length so that the core-annulus structure does not return to its original state as it flows up the riser. However, if there are too many rows of baffles, the catalyst-laden vapors flowing upward will simply bypass the baffles altogether, effectively reducing the diameter of the riser. 
       FIG. 2B  illustrates a riser pipe  10  with three rows of internal baffles  115 A,  115 B, and  115 C. The riser pipe  10  has a lift zone  50  and a reaction zone  55 . The regenerated catalyst enters the lift zone  50  through conduit  35 , and the recycled catalyst (if present) enters through conduit  40 . The hydrocarbon feed enters through the feed distributer  5  which separates the lift zone  50  from the reaction zone  55 . Three rows of baffles  115 A,  115 B, and  115 C are located in the riser pipe  10 . As an example, the lift zone  50  could be about 10 m, and the reaction zone about 20 m. The first row of baffles  115 A could be about 6 m above the feed distributor  5 , with a second row of baffles  115 B about 5 m above the first, and a third row  115 C about 5 m above the second row. 
     There are typically up to three rows of baffles for a riser that is about 30 m high. The first row of baffles is located more than about 6 m above the highest feed inlet in the riser (steam, hydrocarbon, catalyst, etc.), typically in the range of about 6 m to about 6.5 m above the feed inlet(s). Additional rows can be positioned at evenly spaced intervals, e.g., about 5 m apart. The separation between the rows will vary depending on the length of the riser, the number of rows of baffles, and whether any of the rows are divided into subsets as discussed below. Generally, the rows will be in the range of about 5 m to about 10 m apart. In one embodiment, the baffles are arranged in the same position around the circumference for all of the rows. In another embodiment, the baffles in one row are offset from the baffles in the previous row. 
     In one embodiment, each row has the same number of baffles. In another embodiment, there can be a different number of baffles in at least two rows. 
     The bottom of the baffle is attached to the wall of the riser, for example, by welding. The baffles are inclined inward from vertical at an angle b of up to about 90°. In one embodiment, the baffles are inclined from the vertical at an angle of about 90°. In another embodiment, the baffles are inclined in a range of about 10° to about 45°. 
       FIG. 3  shows a one embodiment of a baffle  115 . The baffle  115  has a support plate  120 . The support plate  120  has a ceramic liner  125  on the upper end and the front face  140  (the side facing the upward flow). The baffle is typically welded to the riser  110  forming an angle b of about 90°. The baffle  115  can be supported by a support  130 , if desired. The support  130  can be metal plate welded to the wall  110  and the support plate  120 , for example. 
       FIG. 4  shows another embodiment of the baffle  115 . In this embodiment, the baffle  115  forms an angle b of between about 10° to about 45° from the side of the riser  110 . The support plate  120  is covered on the front face  140  and the upper end by ceramic  125 . 
     A number of factors can be considered in determining the appropriate angle for the baffles in a particular riser. One consideration is mixing, with larger angles producing greater mixing. Another factor is the amount of erosion, which is greater for larger angles. Still another factor is the pressure drop generated by the baffles, which is greater for baffles having larger angles than for those with smaller angles. In addition, the effect of thermal differential growth should be evaluated. When the angle b is about 90°, the wall and the baffle might expand at different rates, which could potentially lead to cracking. With smaller angles, such as about 10° to about 45°, the relatively long inclined support plate provides a longer path for heat transfer. This minimizes the thermal differential growth of the baffle, especially under transient conditions, such as start-up or shut-down. 
       FIG. 5  shows another embodiment of the baffle  115 . A ceramic sleeve  135  covers the front and back faces  140 ,  145  of the support plate  120 . The ceramic sleeve  135  is attached to the support plate. Erosion resistance is improved because both sides of the support plate  120  are covered with ceramic. 
     In some embodiments, a row of baffles (or more than one) can be divided into one or more subsets, with each subset being positioned at a different vertical level in the riser, as illustrated in  FIGS. 6A-C . As shown in  FIG. 6A , subset A is positioned at level A, while subset B is positioned at level B. The baffles  115 A in subset A can be angularly offset from the baffles  115 B in subset B, as shown in  FIGS. 6B-C . As shown, the baffles in subset A are at 90° intervals around the riser. The baffles in subset B are also at 90° intervals, but they are offset 45° from the baffles of subset A. This may help to promote mixing in some embodiments. 
     Although  FIG. 6  shows two subsets with four baffles in each subset and a 45° offset from one level to the next, those of skill in the art will understand that more than two subsets can be used, there can be the same or different numbers of baffles in each subset, and other offset angles can be used as desired. 
     In one embodiment, the baffles in the subsets can form a stair step arrangement on the riser wall. 
     In one embodiment, the baffles in the subsets are arranged symmetrically around the riser wall, and in other embodiments, the baffles are arranged non-symmetrically. 
     The baffles in a subset will generally be within about 1 to 2 m of each other. 
     The baffles are made of a material having sufficient erosion- and temperature-resistance to withstand the riser conditions. Suitable materials include metal plates, such as stainless steel plates, covered with ceramic on at least the front face facing the upward flow to prevent erosion. The back side away from the flow can be covered with abrasion-resistant refractory. Alternatively, both sides can be covered with ceramic. 
     The baffles can be made of fusion-cast ceramic tiles with embedded metal, such as Corguard® made by St. Gobain. If an extended metal piece is used during manufacture, the baffles can be welded to the riser wall as shown in  FIG. 3 , for example. The welding area can then be re-coated with standard FCC riser refractory. 
     Another method of making the baffles involves welding metal pieces, (e.g., trapezoid-shaped metal pieces) to the riser wall as shown in  FIG. 1 . Prefabricated ceramic sleeves can then be attached to the welded metal pieces. The ceramic sleeves can be further secured by creating a lip, for example, by bending or welding, on the metal element. Alternatively, they can be additionally secured using a low-expansion cementing compound between the sleeve and the metal element. This method is not limited to the use of Corguard® tiles. 
     The attachment of the baffles to the riser can take place in situ, if desired. The refractory material inside the riser can be removed manually in the area where the baffles are being installed. The metal pieces would then be welded to the riser wall. The ceramic liner would be attached to the metal piece. The affected areas could then be re-coated with refractory. 
     It is to be understood that the features of any of the embodiments discussed above may be recombined with any other of the embodiments or features disclosed herein. While particular features and embodiments of a process and reactor system has been shown and described, other variations of the invention will be obvious to those of ordinary skill in the art. All embodiments considered to be part of this invention are defined by the claims that follow.