Abstract:
A process for distributing a deflecting media into an axial center of a riser to push catalyst outwardly toward the feed injectors ensures better contacting between hydrocarbon feed and catalyst.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for mixing hydrocarbon feed and catalyst. A field of the invention may be the field of fluid catalytic cracking (FCC). 
     FCC is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke are deposited on the catalyst to provide coked or carbonized catalyst. This carbonized catalyst is often referred to as spent catalyst. However, this term may be misconstrued because the carbonized catalyst still has significant catalytic activity. Vaporous products are separated from carbonized catalyst in a reactor vessel. Carbonized catalyst may be subjected to stripping over an inert gas such as steam to strip entrained hydrocarbonaceous gases from the carbonized catalyst. A high temperature regeneration with oxygen within a regeneration zone operation burns coke from the carbonized catalyst which may have been stripped. 
     FCC units are being designed increasingly larger because refiners are trying to capitalize on economies of scale. As the reactor riser of FCC units are designed with correspondingly increasing diameter, the distance between the wall mounted feed injectors and the axial center of the riser increases. As FCC reactor risers become larger, care must be taken to ensure hydrocarbon feed and catalyst are adequately contacted. Inadequate contact between catalyst and hydrocarbon feed can result in substantially higher dry gas and coke formation and reduced conversion of hydrocarbon feed, all undesirable performance attributes. 
     Improved apparatuses and processes are sought for the contacting of catalyst and hydrocarbon feed in larger FCC units. 
     SUMMARY OF THE INVENTION 
     We have found that in larger FCC units, hydrocarbon feed from the feed injectors does not penetrate through the flowing catalyst to the center of the riser. Consequently, a high density core of catalyst can develop in the riser which is not impacted by injected feed. The high density core can be very stable and exist while ascending through a significant height of the riser resulting in lack of conversion and poorer selectivity to desirable products. 
     An embodiment of our process for contacting catalyst with a hydrocarbon feed comprises distributing a lift gas to a riser to lift the catalyst upwardly in a reactor riser. A deflecting media is distributed into an axial center of the riser to deflect catalyst away from a center of the riser. Hydrocarbon feed is injected into the riser and hydrocarbon feed is contacted with catalyst in the reactor riser to crack the hydrocarbon feed to produce lighter gaseous hydrocarbons. 
     An embodiment of our apparatus for contacting catalyst with a hydrocarbon feed comprises a riser in which the hydrocarbon feed is contacted with catalyst particles to catalytically crack hydrocarbons in the hydrocarbon feed to produce a gaseous product of lighter hydrocarbons and carbonized catalyst. A lift gas distributor distributes lift gas to the riser. A deflecting media distributor distributes deflecting media to the riser and the deflecting media distributor has a nozzle aligned with the axial center of the riser. A feed injector injects hydrocarbon feed into the riser. The feed injector is above at least one of the lift gas distributor and the deflecting media distributor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, elevational view of an FCC unit incorporating the present invention. 
         FIG. 2  is a perspective view of a lower partial section of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the present invention is for contacting catalyst with a hydrocarbon feed. The present invention may be useful in any solids-gas contacting equipment. However, ready usefulness is found in an FCC unit. 
       FIG. 1  shows an FCC unit  8  that includes a reactor vessel  20  and a regenerator vessel  50 . A regenerator catalyst conduit  12  transfers regenerated catalyst from the regenerator vessel  50  at a rate regulated by a control valve  14  to a reactor riser  10  through a regenerated catalyst inlet  15 . The regenerated catalyst conduit  12  intersects the reactor riser  10  at a regenerated catalyst conduit intersection  90 , which is the highest point at which the regenerated catalyst conduit intersects the riser  10 . A lift gas distributor  16  distributes lift gas to the riser  10 . The lift gas is typically steam, but other light hydrocarbons or hydrogen may be used. The lift gas urges a stream of catalyst upwardly through the riser  10  at a relatively high density traveling at least at 3 meters/second (10 feet/second). 
     A plurality of feed injectors  18  inject feed across the flowing stream of catalyst particles to distribute hydrocarbon feed to the riser  10 . The feed injectors  18  may be circumferentially located on a frustum  19  of the riser. Upon contacting the hydrocarbon feed with catalyst in the reactor riser  10  the heavier hydrocarbon feed cracks to produce lighter gaseous hydrocarbon product while coke is deposited on the catalyst particles to produce carbonized catalyst. The riser has an aspect ratio suitably of at least 10. 
     A conventional FCC feedstock and higher boiling hydrocarbon feedstock are suitable feeds. The most common of such conventional feedstocks is a “vacuum gas oil” (VGO), which is typically a hydrocarbon material having a boiling range of from 343° to 552° C. (650 to 1025° F.) prepared by vacuum fractionation of atmospheric residue. Such a fraction is generally low in coke precursors and heavy metal contamination which can serve to contaminate catalyst. Heavy hydrocarbon feedstocks to which this invention may be applied include heavy bottoms from crude oil, heavy bitumen crude oil, shale oil, tar sand extract, deasphalted residue, products from coal liquefaction, atmospheric and vacuum reduced crudes. Heavy feedstocks for this invention also include mixtures of the above hydrocarbons and the foregoing list is not comprehensive. It is also contemplated that lighter recycle or previously cracked feeds such as naphtha may be a suitable feedstock. 
     The reactor vessel  20  is in downstream communication with the riser  10 . As used herein, the term “communication” means that material flow is operatively permitted between enumerated components. The term “downstream communication” means that at least a portion of material flowing to the component in downstream communication may operatively flow from the component with which it communicates. The term “upstream communication” means that at least a portion of the material flowing from the component in upstream communication may operatively flow to the component with which it communicates. In the reactor vessel, the carbonized catalyst and the gaseous product are separated. The resulting mixture of gaseous product hydrocarbons and carbonized catalyst continues upwardly through the riser  10  into the reactor vessel  20  in which the carbonized catalyst and gaseous product are separated. A pair of disengaging arms  22  may tangentially and horizontally discharge the mixture of gas and catalyst from a top of the riser  10  through one or more outlet ports  24  (only one is shown) into a disengaging vessel  26  that effects partial separation of gases from the catalyst. A transport conduit  28  carries the hydrocarbon vapors, including stripped hydrocarbons, stripping media and entrained catalyst to one or more cyclones  30  in the reactor vessel  20  which separates carbonized catalyst from the hydrocarbon gaseous stream. The disengaging vessel  26  is partially disposed in the reactor vessel  20  and can be considered part of the reactor vessel  20 . A collection plenum  34  in the reactor vessel  20  gathers the separated hydrocarbon gaseous streams from the cyclones  30  for passage to an outlet nozzle  36  and eventually into a fractionation recovery zone (not shown). Diplegs  38  discharge catalyst from the cyclones  30  into a lower bed  29  in the reactor vessel  20 . The catalyst with adsorbed or entrained hydrocarbons may eventually pass from the lower bed  29  into an optional stripping section  40  across ports  42  defined in a wall of the disengaging vessel  26 . Catalyst separated in the disengaging vessel  26  may pass directly into the optional stripping section  40  via a bed  41 . A fluidizing conduit  45  delivers inert fluidizing gas, typically steam, to the stripping section  40  through a fluidizing distributor  46 . The stripping section  40  contains baffles  43 ,  44  or other equipment to promote contacting between a stripping gas and the catalyst. The stripped carbonized catalyst leaves the stripping section  40  of the disengaging vessel  26  of the reactor vessel  20  with a lower concentration of entrained or adsorbed hydrocarbons than it had when it entered or if it had not been subjected to stripping. Carbonized catalyst leaves the disengaging vessel  26  of the reactor vessel  20  through a spent catalyst conduit  48  and passes into the regenerator vessel  50  at a rate regulated by a slide valve  51 . The spent catalyst conduit  48  is in downstream communication with the outlet port  24  of the riser  10 . Optionally a first portion of carbonized catalyst leaves the disengaging vessel  26  through the spent catalyst conduit  48  while a second portion of the carbonized catalyst that has been coked in reactor riser  10  leaves the disengaging vessel  26  of the reactor vessel  20  and is passed through a carbonized catalyst conduit  52  back to the riser  10  at a rate regulated by a control valve  53 . The optional carbonized catalyst conduit  52  is in downstream communication with the reactor vessel  20  and intersects the riser  10  at a carbonized catalyst conduit intersection  94  and defines a carbonized catalyst inlet  97  to the riser  10 . The carbonized catalyst intersection  94  is the highest point at which the carbonized catalyst conduit  52  intersects the riser  10 . The carbonized catalyst conduit intersection  94  is above the lift gas distributor  16  so the lift gas therefrom can lift the catalyst upwardly in the riser  10  to the feed injectors  18 . The carbonized catalyst conduit  52  is in downstream communication with the outlet port  24  of the riser  10  and in upstream communication with the carbonized catalyst inlet  97  to the riser  10 . 
     The riser  10  of the FCC process is maintained at high temperature conditions which generally include a temperature above about 425° C. (797° F.). In an embodiment, the reaction zone is maintained at cracking conditions which include a temperature of from about 480° to about 621° C. (896° to 1150° F.) at the riser outlet port  24  and a pressure of from about 69 to about 517 kPa (ga) (10 to 75 psig) but typically less than about 275 kPa (ga) (40 psig). The catalyst-to-oil ratio, based on the weight of catalyst and feed hydrocarbons entering the bottom of the riser, may range up to 30:1 but is typically between about 4:1 and about 10:1 and may range between 7:1 and 25:1. Hydrogen is not normally added to the riser, although hydrogen addition is known in the art. Steam may be passed into the riser  10  and reactor vessel  20  equivalent to about 2-35 wt-% of feed. Typically, however, the steam rate will be between about 2 and about 7 wt-% for maximum gasoline production and about 10 to about 15 wt-% for maximum light olefin production. The average residence time of catalyst in the riser may be less than about 5 seconds. The type of catalyst employed in the process may be chosen from a variety of commercially available catalysts. A catalyst comprising a zeolitic material such as Y Zeolite is preferred, but the older style amorphous catalysts can be used if desired. Additionally, shape-selective additives such as ZSM-5 may be included in the catalyst composition to increase light olefin production. 
     FCC units have been designed in progressively larger sizes over the past few years because refiners are trying to capitalize more on economies of scale. As the FCC reactor risers have also been progressively designed with increased diameters, the distance between the wall mounted feed injectors and the axial center of the riser has been increasing. Recent gamma scan tomography data from a larger commercial FCC unit has shown that the feed and steam injection from feed injectors circumferentially mounted around the wall of a riser only penetrates the interior of the riser by about 0.6 meters (2 feet). As such, we have found that risers with diameters larger than 1.2 meters (4 feet) can develop a high density core of catalyst in the axial center of the riser. The high density core can be very stable and exist for a significant portion of the overall riser. This results in several performance deficiencies. The formation of a vapor annulus results in hot catalyst coring in the center of the riser and increased particle slip and back-mixing at the walls. The penalties are substantially higher dry gas and coke formation, and reduced conversion of hydrocarbon feed. 
     In the present invention, a deflecting media distributor  100  distributes deflecting media to the riser  10  where a central axial core is expected to develop to deflect catalyst away from the center of the riser and into contact with the hydrocarbon feed. The deflecting media distributor is separate from the lift gas distributor  16  and feed injectors  18 . 
     The deflecting media distributor  100  is best shown in  FIG. 2  which is a close up perspective view of the lower end of the riser  10 . The deflecting media distributor  100  comprises a pipe having a horizontal segment  102  that extends into the riser  10  and a vertical segment  104  that extends vertically coincident with the axial center of the riser  10  shown by centerline “A” of the riser  10 . An elbow  103  may communicate the horizontal segment  102  and the vertical segment  104 . The deflecting media distributor terminates at a nozzle  106  on the top of the vertical segment  104 . The nozzle  106  is aligned with the axial center on centerline A. An atomizing device  108  such as an internal swirl vane is depicted in phantom in  FIG. 2  inside an enlarged portion  110  of the vertical segment  104  for shearing the deflecting media and atomizing it before it exits through nozzle  106 . The nozzle  106  may be a cone with an open upper base directed to spray deflecting media upwardly into the axial core of catalyst. In an embodiment, the upper base of the cone of the nozzle  106  may be closed with openings therein. The nozzle  106  may have other suitable configurations. Split couplings (not shown) with tapered retaining rings may be used to secure together assembled components of deflecting media distributor  100 . A support brace  112  such as a pipe secured such as by welding to the deflecting media distributor  100  may be supported by a shelf  114  secured to the wall on the side of the riser  10  opposite to an inlet  116  to the deflecting media distributor  100  to stabilize the deflecting media distributor  100  in the riser  10 . The support brace  112  may be secured such as by welding to the shelf  114 . The deflecting media distributor  100  will be subjected to severe erosion from up flowing catalyst. Hence, the deflecting media distributor  100 , the support brace  112  and shelf  114  should be made of a durable material such as stellite and/or coated with a refractory like the rest of the interior wall of the riser  10 . 
     The deflecting media may be hydrogen, dry gas, light petroleum gas (LPG), naphtha or other hydrocarbon. Steam may be used as the deflecting media. When the deflecting media enters the riser and contacts the hot catalyst it will expand. Liquid deflecting media will vaporize to a greater volume. Hydrocarbonaceous deflecting media may crack to smaller hydrocarbons thereby increasing its moles and its volume. The expanding deflecting media provides a motive force to deflect the hot catalyst from the axial core closer to the feed injectors for improved contact between the hydrocarbon feed and catalyst. 
     It is also contemplated that hydrocarbons be fed to the riser  10  as a hydrocarbon feed through deflecting media distributor  100 . Hydrocarbon feed be may be light hydrocarbons recycled from previously cracked products from the riser  10  recovered in the fractionation recovery zone downstream of outlet  36 . Naphtha and LPG may be recycled to the riser  10  to increase the yield of light olefins. In such a case, a lighter deflecting media may be mixed with the light hydrocarbon feed to act as an atomizing media. The hydrocarbon feed and the lighter atomizing media all act as deflecting media. The atomizing media may be mixed with the hydrocarbon feed within or outside of the deflecting media distributor  100 . In this case, the lighter atomizing media should be gaseous even if the hydrocarbon feed is liquid or partially liquid to achieve atomization of the hydrocarbon feed. Consequently, a light hydrocarbon such as dry gas is superior to steam as an atomizing media when light hydrocarbons are the feed to the deflecting media distributor  100  because light hydrocarbon atomizing media will be less likely to condense at the lower temperature of the light hydrocarbon feed relative to the higher temperature typical of heavier hydrocarbon feed injected into the riser  10  through feed injectors  18 . Dry gas used as a deflecting media and an atomizing media may be obtained from lighter gaseous hydrocarbons previously cracked in riser  10 , recovered in fractionation recovery zone downstream of outlet  36  and recycled to deflecting media distributor  100 . 
     The feed injectors  18  are suitably above one or both of the lift gas distributor  16  and the deflecting media distributor  100 . The lift gas distributor  16  lifts catalyst entering from catalyst inlets  15  and  97  below the feed injectors  18  up to the feed injectors  18 . The deflecting media distributor is suitably above the regenerated catalyst conduit intersection  90  and/or the carbonized catalyst conduit intersection  94  which in an aspect are between the lift gas distributor  16  and the deflecting media distributor  100 . The present invention is most advantageous for risers having a diameter of at least 1.2 meters (4 feet) at the level of the hydrocarbon feed injector because the hydrocarbon feed may be injected from injectors  18  to a point short of the center of the riser shown by centerline A. 
     Turning back to  FIG. 1 , the regenerator vessel  50  is in downstream communication with the reactor vessel  20 . In the regenerator vessel  50 , coke is combusted from the carbonized catalyst delivered to the regenerator vessel  50  by contact with an oxygen-containing gas such as air to provide regenerated catalyst. The regenerator vessel  50  may be a combustor type of regenerator, which may use hybrid turbulent bed-fast fluidized conditions in a high-efficiency regenerator vessel  50  for completely regenerating carbonized catalyst. However, other regenerator vessels and other flow conditions may be suitable for the present invention. The spent catalyst conduit  48  feeds carbonized catalyst to a first or lower chamber  54  defined by outer wall  56  through a spent catalyst inlet chute  62 . The carbonized catalyst from the reactor vessel  20  usually contains carbon in an amount of from 0.2 to 2 wt-%, which is present in the form of coke. Although coke is primarily composed of carbon, it may contain from 3 to 12 wt-% hydrogen as well as sulfur and other materials. An oxygen-containing combustion gas, typically air, enters the lower chamber  54  of the regenerator vessel  50  through a conduit  64  and is distributed by a distributor  66 . As the combustion gas enters the lower chamber  54 , it contacts carbonized catalyst entering from chute  62  and lifts the catalyst at a superficial velocity of combustion gas in the lower chamber  54  of perhaps at least 1.1 m/s (3.5 ft/s) under fast fluidized flow conditions. In an embodiment, the lower chamber  54  may have a catalyst density of from 48 to 320 kg/m 3  (3 to 20 lb/ft 3 ) and a superficial gas velocity of 1.1 to 2.2 m/s (3.5 to 7 ft/s). The oxygen in the combustion gas contacts the carbonized catalyst and combusts carbonaceous deposits from the catalyst to at least partially regenerate the catalyst and generate flue gas and regenerated catalyst. 
     In an embodiment, to accelerate combustion of the coke in the lower chamber  54 , hot regenerated catalyst from a dense catalyst bed  59  in an upper or second chamber  70  may be recirculated into the lower chamber  54  via an external recycle catalyst conduit  67  regulated by a control valve  69 . Hot regenerated catalyst enters the lower chamber  54  through an inlet chute  63 . Recirculation of regenerated catalyst, by mixing hot catalyst from the dense catalyst bed  59  with relatively cooler carbonized catalyst from the spent catalyst conduit  48  entering the lower chamber  54 , raises the overall temperature of the catalyst and gas mixture in the lower chamber  54 . 
     The mixture of catalyst and combustion gas in the lower chamber  54  ascend through a frustoconical transition section  57  to the transport, riser section  60  of the lower chamber  54 . The riser section  60  defines a tube which is preferably cylindrical and extends preferably upwardly from the lower chamber  54 . The mixture of catalyst and gas travels at a higher superficial gas velocity than in the lower chamber  54 . The increased gas velocity is due to the reduced cross-sectional area of the riser section  60  relative to the cross-sectional area of the lower chamber  54  below the transition section  57 . Hence, the superficial gas velocity may usually exceed about 2.2 m/s (7 ft/s). The riser section  60  may have a lower catalyst density of less than about 80 kg/m 3  (5 lb/ft 3 ). 
     The regenerator vessel  50  may also include an upper or second chamber  70 . The mixture of catalyst particles and flue gas is discharged from an upper portion of the riser section  60  into the upper chamber  70 . Substantially completely regenerated catalyst may exit the top of the transport, riser section  60 , but arrangements in which partially regenerated catalyst exits from the lower chamber  54  are also contemplated. Discharge is effected through a disengaging device  72  that separates a majority of the regenerated catalyst from the flue gas. In an embodiment, catalyst and gas flowing up the riser section  60  impact a top elliptical cap  65  of the riser section  60  and reverse flow. The catalyst and gas then exit through downwardly directed discharge outlets  73  of disengaging device  72 . The sudden loss of momentum and downward flow reversal cause a majority of the heavier catalyst to fall to the dense catalyst bed  59  and the lighter flue gas and a minor portion of the catalyst still entrained therein to ascend upwardly in the upper chamber  70 . Cyclones  82 ,  84  further separate catalyst from ascending gas and deposit catalyst through diplegs  85 ,  86  into dense catalyst bed  59 . Flue gas exits the cyclones  82 ,  84  and collects in a plenum  88  for passage to an outlet nozzle  89  of regenerator vessel  50  and perhaps into a flue gas or power recovery system (not shown). Catalyst densities in the dense catalyst bed  59  are typically kept within a range of from about 640 to about 960 kg/m 3  (40 to 60 lb/ft 3 ). A fluidizing conduit  74  delivers fluidizing gas, typically air, to the dense catalyst bed  59  through a fluidizing distributor  76 . In a combustor-style regenerator, approximately no more than 2% of the total gas requirements within the process enter the dense catalyst bed  59  through the fluidizing distributor  76 . As such, gas is added not for combustion purposes but only for fluidizing purposes, so the catalyst will fluidly exit through the catalyst conduits  67  and  12 . The fluidizing gas added through the fluidizing distributor  76  may be combustion gas. In the case where partial combustion is effected in the lower chamber  54 , greater amounts of combustion gas will be fed to the upper chamber  70  through fluidizing conduit  74 . 
     From about 10 to 30 wt-% of the catalyst discharged from the lower chamber  54  is present in the gases above the outlets  73  from the riser section  60  and enter the cyclones  82 ,  84 . The regenerator vessel  50  may typically require 14 kg of air per kg of coke removed to obtain complete regeneration. When more catalyst is regenerated, greater amounts of feed may be processed in a conventional reactor riser. The regenerator vessel  50  typically has a temperature of about 594 to about 704° C. (1100 to 1300° F.) in the lower chamber  54  and about 649 to about 760° C. (1200 to 1400° F.) in the upper chamber  70 . Regenerated catalyst from dense catalyst bed  59  is transported through regenerated catalyst conduit  12  from the regenerator vessel  50  back to the reactor riser  10 . The regenerated catalyst travels through the control valve  14  and an inlet  15  provided by the regenerated catalyst conduit  12  into the riser  10  where it again contacts feed as the FCC process continues. The regenerated catalyst conduit intersection  90  is above the lift gas distributor  16  so the lift gas therefrom can lift the catalyst upwardly in the riser  10  to the feed injectors  18 . 
     We have also found when a stream of carbonized catalyst and a stream of regenerated catalyst are both fed into the riser  10 ; they tend not to mix before contacting the hydrocarbon feed. Accordingly, the feed can encounter catalyst at varying temperatures resulting in non-selective cracking to a composition with relatively more undesirable products. To ensure mixing between the carbonized catalyst and the regenerated catalyst, the regenerated catalyst conduit intersection  90  is above the carbonized catalyst conduit intersection  94  and the regenerated catalyst inlet  15  is above the carbonized catalyst inlet  97 . Steam can have a dealuminating effect on the zeolitic catalyst and this dealuminating effect increases proportionally with temperature. By bringing the cooler carbonized catalyst into the riser between the fluidizing gas which is typically steam from nozzle  16  and the regenerated catalyst from regenerated catalyst conduit  12 , the carbonized catalyst has an opportunity to cool the regenerated catalyst before the regenerated catalyst stream encounters the steam. Consequently, the regenerated catalyst encounters the steam only at a reduced temperature at which the dealuminating effect is minimized. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. 
     In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.