Abstract:
In a fluid catalytic cracking (FCC) unit, a regenerator or a stripper includes a standpipe for circulating catalyst from one vessel to another, the standpipe having an inlet design which reduces gas entrainment during catalyst transport by partial de-fluidization in the standpipe inlet region. The standpipe inlet design could include multiple inlet openings through the top of the standpipe or from the side wall by means of slots, or both, and a horizontal disk surrounding the standpipe below the slots for blocking the upward flow of bubbles, the combination thereby forming a dense fluidization zone above the disk and surrounding the inlet, including the slots. Additionally, the disk may include a downwardly-projecting lip or edge forming an inverted void space around the standpipe and the downwardly-projecting edge may further include vent holes around its circumference which allow bubbles trapped under the disk to be vented outside the standpipe inlet region. Above and below the disk and surrounding the standpipe, gas injection rings may be used to prevent the dense fluidization zone above the disk from complete de-fluidization, thus assisting the catalyst to remain fluidized and flow smoothly into the standpipe either through the slots or at the very top of the open standpipe, or both.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a standpipe inlet design for enhancing particle circulation and reducing gas entrainment, the design being suitable for applications in fluid catalytic cracking (FCC) units and other processes, such as fluid cokers, flexicokers, and fluidized bed combustors which circulate large quantities of particulate solids between different vessels connected with standpipes and risers. 
     2. Description of the Related Art 
     In a typical Fluid Catalytic Cracking (FCC) process consisting of a regenerator, a riser reactor and a stripper, such as that shown in U.S. Pat. No. 5,562,818 to Hedrick which is incorporated herein by reference, finely divided regenerated catalyst leaves a regenerator and contacts with a hydrocarbon feedstock in a lower portion of a reactor riser. Hydrocarbon feedstock and steam enter the riser through feed nozzles. The mixture of feed, steam and regenerated catalyst, which has a temperature of from about 200° C. to about 700° C., passes up through the riser reactor, converting the feed into lighter products while a coke layer deposits on catalyst surface. The hydrocarbon vapors and catalyst from the top of the riser are then passed through cyclones to separate spent catalyst from the hydrocarbon vapor product stream. The spent catalyst enters the stripper where steam is introduced to remove hydrocarbon products from the catalyst. The spent catalyst containing coke then passes through a stripper standpipe to enter the regenerator where, in the presence of air and at a temperature of from about 620° C. to about 760° C., combustion of the coke layer produces regenerated catalyst and flue gas. The flue gas is separated from entrained catalyst in the upper region of the regenerator by cyclones and the regenerated catalyst is returned to the regenerator fluidized bed. The regenerated catalyst is then drawn from the regenerator fluidized bed through the regenerator standpipe and, in repetition of the previously mentioned cycle, contacts the feedstock in the reaction zone. 
     Catalyst circulation is critical to overall performance and reliability of FCC units. The main drive for catalyst circulation comes from stable and adequate pressure build-up in the standpipe. One critical element of the standpipe design is the inlet design because it determines the inlet condition of the catalyst which, in turn, affects the entire standpipe operation. 
     The prior art of standpipe inlet design, for both stripper standpipe and regenerator standpipe, is a conical hopper such as that shown in the open literature in Handbook of Petroleum Refining Process, second edition by R. A. Meyers, which is incorporated herein by reference. The key concept of the inlet hopper design of the prior art is that when catalyst particles are drawn from a fluidized bed into a standpipe, bubbles are also drawn together with the catalyst. The inlet hopper provides residence time for the bubbles to coalesce and grow into large bubbles before entering the standpipe. Since large bubbles have a higher riser velocity, they have a better chance to escape back into the fluidized bed, thus reducing gas entrainment into the standpipe. 
     However, the design concept of the prior art standpipe inlet has several disadvantages. If the inlet hopper is too small, many bubbles drawn into the inlet hopper do not have enough time to grow but flow directly into the standpipe, leading to high gas entrainment. If, on the other hand, when the inlet hopper is large enough to allow small bubbles to grow, large bubbles could form and hang stationary inside the hopper for a period of time as the bubbles try to rise against the downward catalyst flow. These large hanging bubbles can temporarily restrict catalyst flow into the standpipe. When the bubbles finally grow large enough to escape into the fluidized bed, the release of the large bubbles creates a sudden surge of catalyst into the standpipe, leading to a sudden pressure swing in the standpipe. The sequence of growing and releasing of large bubbles leads to a very undesirable condition of unstable standpipe operation. The fundamental flaw of the prior art design is that, while the objective of the standpipe inlet design is supposed to reduce gas entrainment into the standpipe, the design in fact encourages many bubbles to be drawn in. This is inherently very inefficient. Furthermore, the prior art of the inlet hopper design is a bulky structure such that in many FCC units there is not enough room to place it. A common compromise is to use either a straight pipe or an asymmetric hopper for the standpipe inlet which further exacerbates the problems described above. 
     Standpipe inlet geometry not only affects catalyst circulation, the entrained gas can also have a negative impact on the performance of a stripper of a FCC unit. It is common practice that the stripper includes special trays, such as shown in the invention by Johnson et al in international patent PCT/US95/09335 which is incorporated herein by reference. The special trays in the main vessel enhance the efficiency of hydrocarbon vapor stripping by steam. The spent catalyst is then transported to the regenerator through a stripper standpipe with a hopper inlet as shown in the prior art. The hopper inlet for the stripper standpipe has been shown to be rather ineffective in reducing gas entrainment. The study of Nougier et al in the Second FCC Forum (May 15-17, 1996, The Woodlands, Texas) shows that, even after intensive stripping in the main vessel, the vapor leaving the stripper still contains 20 to 25% by mole (or about 40% by weight) of hydrocarbon products. Gas entrainment from the stripper standpipe into the regenerator has two negative impacts in addition to the impact on catalyst circulation discussed above. First, the entrained gas from the stripper to the regenerator represents a loss in hydrocarbon products which could have been recovered as products. Second, the entrained hydrocarbon has to be burned in the regenerator which consumes limited air available in the regenerator and generates additional heat that has to be removed. Thus, it is essential to reduce gas entrainment into the stripper standpipe. 
     One objective of the instant invention is to reduce gas entrainment into standpipes by a standpipe inlet design. This will lead to increases in overall pressure build-up in the standpipe and catalyst circulation rate as well as improving standpipe stability. The reduction in gas entrainment will also reduce hydrocarbon entrainment from the stripper to the regenerator of a FCC unit, as discussed above. 
     SUMMARY OF THE INVENTION 
     In a fluid catalytic cracking (FCC) unit, a regenerator or a stripper includes a standpipe for circulating catalyst from one vessel to another, the standpipe having an inlet design which reduces gas entrainment during catalyst transport by partial de-fluidization in the standpipe inlet region. The standpipe inlet design could include multiple inlet openings, e.g., through the top of the standpipe or from the side wall by means of slots, or both, and a horizontal disk surrounding the standpipe below the slots for blocking the upward flow of bubbles, the combination thereby forming a dense fluidization zone above the disk and surrounding the inlet, including the slots. Additionally, the disk may include a downward-projecting lip or edge forming an inverted void space around the standpipe and the downward-projecting edge may further include vent holes around its circumference which allow bubbles trapped under the disk to be vented outside the standpipe inlet region. Above the disk and surrounding the standpipe, gas injection rings may also be used to prevent the dense fluidization zone above the disk from complete de-fluidization, thus assisting the catalyst to remain fluidized and flow smoothly into the standpipe, either through the slots or at the very top of the open standpipe, or both. The disk itself may also include vent holes for preventing complete de-fluidization. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of the lower portion of a regenerator of a FCC unit including a regenerator standpipe. 
     FIG. 2 is an enlarged sectional view of a portion of FIG. 1 of the regenerator standpipe inlet. 
     FIG. 3 is an alternative embodiment of FIG. 2 of the regenerator standpipe inlet. 
     FIG. 4 is another embodiment of the regenerator standpipe inlet when catalyst is drawn from a space near the bottom wall of the regenerator vessel of a FCC unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The main drive for catalyst circulation in FCC units comes from stable and adequate pressure build-up in the standpipe. One critical element of the standpipe design is the inlet design because it determines the inlet condition of the catalyst which, in turn, affects the entire standpipe operation. It is essential to reduce gas entrainment by a properly designed standpipe inlet. 
     The key concept of the instant invention of the standpipe inlet design is totally different from the inlet hopper design of the prior art, which has many disadvantages as discussed previously. The design concept of the instant invention relies on partial defluidization, rather than bubble coalescence and growth inside the hopper, to reduce gas entrainment which is discussed in detail below. 
     The reason that FCC catalyst can be maintained at the fluidization state in the regenerator or the stripper is by a continuous supply of upflowing, fluidizing gas. Thus, as soon as the supply of the fluidizing gas is cut off, the fluidized catalyst starts to settle, or defluidize, immediately. In the initial stage of this de-fluidization process, bubbles escape very quickly from the fluidized bed, as shown by Khoe et al in Powder Technology Vol. 66 (1991) which is incorporated herein by reference. After the depletion of all bubbles, FCC catalyst can still be maintained at a dense fluidization state for a certain period of time before becoming completely de-fluidized, as also shown by Khoe et al. In Khoe et al&#39;s experiments, the de-fluidization process was triggered by shutting off fluidization gas supply, leading to de-fluidization of the entire fluidized bed. However, one could trigger a local de-fluidization process within a fluidized bed by strategically blocking off the upflowing, fluidizing gas in a selective area. The instant invention of standpipe inlet design utilizes this special characteristic of FCC catalyst by partial de-fluidization in a strategic area to eliminate the bubbles and by allowing only densely fluidized catalyst to flow into the standpipe. 
     Referring now to FIG. 1 which is a sectional view of the lower portion of a typical regenerator  20  of a FCC unit with a regenerator standpipe  10  which includes an inlet portion  60  to draw in regenerated catalyst according to the instant invention. Catalyst flow is shown by the arrows in the Figures. Spent catalyst is transported from a stripper (not shown) through a typical spent catalyst transport duct  70  and enters the regenerator  20  where coke deposition on catalyst is burned off by air which is supplied by main air grid  30 . The air from grid  30  and the resulting combustion gas rise through the regenerator, thus keeping the catalyst fluidized in fluidized bed  40 . The combustion gas and entrained regenerated catalyst are separated in the upper part of the regenerator by cyclones (not shown). The combustion gas exits from the upper part of the regenerator and the regenerated catalyst, separated by cyclones (not shown), is returned to the fluidized bed  40 . Typical density of the fluidized bed  40  in regenerator  20  is in the range of 20 to 40 lb/ft 3 , with the presence of many rising gas bubbles. The density of the fluidized bed  40  is controlled mainly by the air flow from air grid  30  where higher fluidizing air flow leads to more gas bubbles and lower density of fluidized bed  40 . The fluidized bed  40  is maintained at a certain level  50  by a slide valve (not shown), or other means, located at the bottom of the regenerator standpipe  10  to control the rate of regenerated catalyst being drawn into regenerator standpipe  10 . The top of the regenerator standpipe  10 , including a standpipe inlet  60  according to the instant invention, is shown as enclosed by the dotted circle, is completely submerged in the fluidized bed  40  inside regenerator  20 . Although the standpipe  10  is shown in FIG. 1 to be vertical and protruding into regenerator  20  from the bottom, the instant invention of the standpipe inlet  60  can be applied to other configurations where the standpipe  10  might protrude into regenerator  20  through the side wall, instead of the bottom, and it might be inclined, instead of vertical. 
     Referring now to FIG. 2 for the details of standpipe inlet  60  in FIG. 1, the regenerator standpipe  10  is typically a cylindrical duct with a diameter in the range of about 1 to about 5 feet. Regenerated catalyst is drawn into the standpipe  10  through one of two types of openings, or both, according to the instant invention. The first is the top opening  11  of the standpipe and the second is a plurality of openings  12  cut through the walls of the upper portion of the standpipe  10 . Although slots are shown in FIG. 2 for openings  12 , other forms, such as circular holes, could also be used. Below the openings  11  and  12  is a horizontal disk  13  surrounding the standpipe  10 . In the following discussion, the element  13  will be referred to as a “disk”, which is the most logical form for a cylindrically-shaped vessel. It will be appreciated, however, that the element  13  may simply be a plate of any desired shape. Since the entire standpipe inlet  60  is submerged in the fluidized bed  40  where catalyst is fluidized by the continuous upflow of fluidizing gas from air grid  30  (see FIG.  1 ), disk  13  strategically blocks off the supply of the fluidizing gas coming from below and triggers the local de-fluidization process in the region directly above disk  13 . As fully fluidized regenerated catalyst together with gas bubbles are being drawn toward the standpipe openings  11  and  12 , the fluidizing gas is blocked off by disk  13  (except as described below) and bubbles migrating toward standpipe openings  11  and  12  run out of the continuous supply of fluidizing gas very quickly. This creates a dense fluidized zone  14 , shown enclosed by the dashed line in FIG. 2, with almost no bubble presence in the near proximity to the standpipe openings  11  and  12 . This allows catalyst to partially de-fluidize by eliminating gas bubbles before entering standpipe  10 , but not to the extent of complete de-fluidization where catalyst can no longer flow. To prevent complete de-fluidization in the dense fluidization zone  14  above the disk  13 , a small gas flow can be supplied, either by vent holes  13   c  in the disk  13  or through a gas injection ring  15  located above the disk  13 . Although a gas injection ring  15  is shown in FIG. 2, other means, such as a gas injection grid, can also be used to achieve the same objective of preventing complete de-fluidization in the dense fluidization zone  14  above the disk  13 . The disk  13  may include a downwardly-projecting side or lip  13   a  which circles the disk  13 , preferably at its circumference. The void below disk  13  surrounded by lip  13   a  allows the disk to capture fluidizing gas coming from below. To continuously vent off the fluidizing gas, lip  13   a  may further include a plurality of vent holes  13   b  which allows fluidizing gas to be vented off outside the dense fluidization zone  14 . Alternatively, a vent tube  16  may be used to discharge fluidizing gas from below the disk  13  to a location above the dense fluidization zone  14 . Although a horizontal disk  13  is proposed as one means to achieve local de-fluidization in the dense fluidization zone  14  in FIG. 2, other means can be applied to achieve the same objective. One such alternative is shown in FIG.  3 . 
     Referring now to FIG. 3, regenerated catalyst is again drawn into the standpipe  10 ′ through the top opening  11 ′, or a plurality of openings  12 ′, or both. Instead of using a horizontal disk  13  as in FIG. 2, FIG. 3 shows that below the openings  11 ′ and  12 ′ is a conical disk  13 ′ surrounding the standpipe  10 ′. The function of the conical disk  13 ′ is to strategically block off the supply of the fluidizing gas coming from below and to trigger the local de-fluidization process in the region directly above disk  13 ′. This creates a dense fluidized zone  14 ′, shown enclosed by the dashed line in FIG.  3 . To prevent complete defluidization in the dense fluidization zone  14 ′, a small gas flow can be supplied, either by vent holes  13   c ′ in the disk  13 ′ or through a gas injection ring  15 ′ located above the disk  13 ′. Although a gas injection ring  15 ′ is shown in FIG. 3, other means, such as a gas injection grid, can also be used to achieve the same objective of preventing complete de-fluidization in the dense fluidization zone  14 ′ above the disk  13 ′. The void below conical disk  13 ′ allows the disk to capture fluidizing gas coming from below. To continuously vent off the accumulation of the fluidizing gas, disk  13 ′ may further include a plurality of vent holes with extension pipes  13   b ′ which allows fluidizing gas to be vented off outside the dense fluidization zone  14 ′. Alternatively, a vent tube  16 ′ may be used to discharge fluidizing gas from below the disk  13 ′ to a location above the dense fluidization zone  14 ′. One advantage of the conical disk  13 ′ over the horizontal disk  13  in FIG. 2 is that catalyst is less likely to become stagnant when gas flow from the gas injection ring  15 ′ is turned off. 
     FIG. 4 shows another embodiment of a regenerator standpipe inlet using a design similar to the concept of FIG. 1 except when the FCC process prefers to draw regenerated catalyst from a region very close to the bottom of regenerator  120 . Spent catalyst is transported from a stripper (not shown) through a spent catalyst transport duct  170  and enters the regenerator  120 . The regenerated catalyst is separated from flue gas in the upper part of the regenerator by cyclones (not shown). The flue gas exits from the upper part of the regenerator and the regenerated catalyst separated by cyclones (not shown) is returned to the lower part of the regenerator  120  to form the fluidized bed  140  by the continuous upflow of fluidizing air and combustion gas from air grid  130 . The fluidized bed  140  is maintained at a level  150  by a slide valve (not shown), or other means, located at the bottom of the regenerator standpipe  110  to control the rate of regenerated catalyst being drawn into the regenerator standpipe  110 . The regenerator standpipe  110  still has one of two types of inlet openings, or both, to draw catalyst from the fluidized bed  140  of the regenerator. The first opening is the top opening  111  of the standpipe  110  and the second is a plurality of openings  112  cut through the walls of the upper portion of the standpipe  110  just above the bottom vessel wall  113  of regenerator  120 . Although the standpipe  110  is shown in FIG. 4 to be vertical, the instant invention of the standpipe inlet can also be applied to other configurations where the standpipe  110  might be inclined. The function of the bottom wall  113  in FIG. 4 is similar to that of the disk  13  in FIG. 2, i.e., to induce local defluidization and to create a dense fluidization zone (as in zone  14  of FIG. 2) with almost no bubbles present in the near proximity to the standpipe openings  111  and  112 . To prevent complete de-fluidization near the vessel wall, a small gas flow can be supplied through a gas injection ring  115 . Although a gas injection ring  115  is shown in FIG. 4, other means, such as a gas injection grid  130 , can also be used to achieve the same objective of preventing complete de-fluidization in the dense fluidization zone above the vessel wall  113 . 
     A regenerator standpipe inlet according to FIG. 4 was installed in one of Assignee&#39;s FCC units which originally had a hopper standpipe inlet of the prior art design. The original inlet hopper was removed and four slots measuring 6 inches wide by 40 inches long were created on the standpipe wall. After the installation of the new regenerator standpipe inlet, catalyst circulation rate of the FCC unit was increased by 30%, with an additional 3 psi pressure build-up in the regenerator standpipe. This was a clear indication that the standpipe inlet of the instant invention was very effective in reducing gas entrainment from the regenerator thus allowing the standpipe to run at higher density and to build more pressure for increasing catalyst circulation. Furthermore, the standpipe operation became more stable even at a higher catalyst circulation rate compared to previous operation. 
     From the discussion above, it is demonstrated that the standpipe inlet design of the instant invention has several advantages over the inlet hopper design of the prior art when it is applied to the regenerator standpipe of a FCC unit: 
     More Stable Operation—The inlet design of the instant invention does not rely on the mechanism of the prior art inlet hopper to draw in lots of bubbles, letting them coalescence and grow into large bubbles. Instead, the new inlet design minimizes bubble entrainment by strategically eliminating bubbles around the standpipe inlet region with local de-fluidization. Since the new design does not require the formation and release of large bubbles in the hopper design, which leads to standpipe instability, the design of the instant invention is inherently more stable. 
     More effective in reducing gas entrainment—The concept of the prior art inlet hopper is to draw in lots of bubbles while trying to reduce gas entrainment. This is inherently a very inefficient design. On the other hand, the basic design of the instant invention is to strategically eliminate bubbles by local de-fluidization of the catalyst before it enters the standpipe. Thus, the design of the instant invention is inherently more efficient in reducing gas entrainment into the standpipe. 
     Better Control—The prior art hopper inlet has little control of gas entrainment around the inlet. As the catalyst circulation rate increases, more and more bubbles are drawn into the hopper, leading to higher and higher gas entrainment. The design of the instant invention, on the other hand, maintains complete control of the flow condition near the inlet by eliminating all bubbles, then introducing only a small amount of gas necessary for smooth operation. 
     Simplicity—The design of the instant invention is simpler and more robust than the prior art hopper design. 
     When the standpipe inlet design of the instant invention is applied to the stripper standpipe, it provides several additional advantages over the prior art inlet hopper design for enhancing stripper and regenerator performance of a FCC unit. This is in addition to the benefits already discussed for application in the regenerator standpipe where catalyst circulation and standpipe stability are the main concerns: 
     Higher stripping efficiency—The standpipe inlet design of the instant invention is shown to be more effective in reducing gas entrainment into the standpipe. Since the entrained gas from the stripper standpipe may contain about 40% by weight of hydrocarbon products, the standpipe inlet design of the instant invention effectively increases hydrocarbon products by reducing hydrocarbon loss to the gas entrainment. 
     Lower regenerator loading—Since the stripper standpipe inlet design of the instant invention is more effective in reducing gas entrainment, less hydrocarbon will enter the regenerator. This leads to lower air requirement and less heat to be removed as less hydrocarbon is to be burned in the regenerator. More importantly, many FCC units today are limited by air supply or heat removing capacity in the regenerator. Thus, the instant invention can be used to debottleneck the unit. 
     Although the above discussion focuses on the applications of the instant invention in FCC units, a similar standpipe inlet design can also be applied to improve circulation of particulate solids and reduce gas entrainment in other petrochemical processes, such as fluid cokers and flexicokers, and processes other than petrochemical, such as circulating fluidized bed combustors, where large quantities of particulate solids are circulated between different vessels connected by standpipes and risers.