Abstract:
A reactor for converting a feedstream having one or more oxygenated compounds to a product containing olefins is provided. The reactor comprises a fluidized reaction zone defined by a reactor wall and a feedstream inlet located adjacent the reaction zone. The feedstream inlet is operative to feed the reaction zone with said feedstream. A riser extends from said reaction zone and carries a vaporized combination of said feedstream and said catalyst from said reaction zone to a disengaging zone fed by the riser. At least one cooling tube is disposed within the reactor and extends substantially vertically and substantially parallel to the reactor wall. The cooling tube is located adjacent the reactor wall and extends from an upper portion of the reaction zone towards a lower portion of the reaction zone. Also provided is a cooling system for a methanol to olefin reactor. Finally, a method of producing olefins from a feedstream having an oxygenate is provided. The method includes cooling the reaction which produces olefins from the oxygenate feedstream.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to processes and systems for use in cooling reactions within fluidized bed reactors. More particularly, the disclosure relates to hydrocarbon conversion processes utilizing a fluidized bed reaction zone and the reaction which results in the conversion of methanol to olefins. 
       BACKGROUND 
       [0002]    Processes and systems for converting feedstreams containing oxygenates to a product containing olefins are known. One example of such a process, using a fluidized bed reaction, is shown in U.S. Pat. No. 6,166,282. U.S. Pat. No. 6,166,282, which is incorporated herein by reference, shows an oxygenate conversion process and fast-fluidized bed reactor with an upper disengaging zone and a lower reaction zone. The reaction zone has a dense phase zone and a transition zone which extends upwardly from the dense phase zone into the disengaging zone. The feedstock, in the presence of an optional diluent is passed into the dense phase zone, which contains a non-zeolitic catalyst to effect at least a partial conversion of the feedstream to light olefins. The feedstream and catalyst are then passed into the transition zone to achieve essentially complete conversion. A portion of the catalyst is withdrawn from above the transition zone in the disengaging zone, at least partially regenerated through an external regenerator, and returned to a point above the dense phase zone, while catalyst is continuously circulated from the disengaging zone to the lower reaction zone. Additionally, the disengaged catalyst that is not regenerated is collected and recirculated into the dense phase zone via external recirculation standpipes. In this process, the temperature of the reaction is controlled by removing heat from the catalyst via external heat exchangers within the standpipes or adjacent the regenerators. 
         [0003]    U.S. Pat. No. 4,071,573 teaches a process for prolonging the life of a catalyst by disposing of exothermic reaction heat. Owen discloses a vertical reactor vessel through which a feedstream and fluidizable catalyst particles are passed upwardly in a reaction. In one embodiment, the reaction temperature is controlled by applying a quench fluid to the reaction via distributor grids spaced vertically through the reactor. In another embodiment, the catalyst is cooled prior to injection into the reaction zone. In a final embodiment, there are a number of heat exchange tubes spaced vertically about the reactor. The heat exchange tubes form grids that span the reactor&#39;s cross section. Feedstream is passed through the heat exchange tubes, thereby removing heat from the reaction and preheating the feedstream. In addition, the catalyst used in the reaction is cooled prior to injection into the reactor. 
         [0004]    In view of the need to control the temperature of reactions such as those described above, it could be beneficial to provide an efficient and cost effective way to cool such reactions. 
       SUMMARY 
       [0005]    I provide a reactor for converting a feedstream having an oxygenate to a product containing olefins. The reactor comprises a fluidized reaction zone defined by a reactor wall and a feedstream inlet located adjacent the reaction zone. The feedstream inlet is operative to feed the reaction zone with said feedstream. A riser extends from said reaction zone and carries a vaporized combination of said feedstream and said catalyst from said reaction zone to a disengaging zone fed by the riser. At least one cooling tube is disposed within the reactor and extends substantially vertically and substantially parallel to the reactor wall. The cooling tube is located adjacent the reactor wall and extends from an upper portion of the reaction zone towards a lower portion of the reaction zone. I also provide a cooling system for use in said reactor and a method of producing olefins using a reactor and cooling system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic diagram of a fluidized reactor. 
           [0007]      FIG. 2  is a side elevational view of the interior wall of a reactor. 
           [0008]      FIG. 3  is a top down cross sectional view of a reactor. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    My processes and systems are discussed in the context of using a representative reactor which employs a fast-fluidized bed reactor. Although the specifics of a fast-fluidized bed reactor are given for purposes of illustrating the significance of temperature control in a reactor, those skilled in the art will recognize the applicability of my processes and systems to reactions occurring in similar reactors. Below, an overview of a typical reactor and reaction process is given, along with detailed illustrations of a preferred a reactor employing a cooling system. 
         [0010]    The illustrated fast-fluidized bed reactor comprises an upper disengaging zone and a lower reaction zone. The lower reaction zone comprises a dense phase zone which operates within a superficial velocity range less than about 1 meter per second (3 feet per second). By the term “superficial velocity”, it is meant the velocity of the gas as it flows through the vessel. The superficial velocity is typically determined by dividing the volumetric flow rate of the gas by the cross-sectional area of the vessel. 
         [0011]    A transition phase zone is disposed above the dense phase zone and extends from the lower reaction zone into the upper disengaging zone. The transition phase zone includes a reducing cone which reduces the flow path diameter from the diameter of the dense phase zone to the diameter of the riser. The superficial velocity within the transition zone ranges between about 1 meter per second (3 feet per second) and about 6 meters per second (13 feet per second). A feedstream of feedstock at effective conditions is introduced into the lower reaction zone wherein it contacts a partially coked catalyst to selectively produce light olefins. As the unreacted feedstock and reaction products pass through the dense phase zone, they are carried into the transition zone with partially coked catalyst particles having a reduced number of active catalyst sites. As the mixture of unreacted feedstock, fluidized catalyst particles, and reaction products enters the transition zone, the reaction continues to essentially complete conversion (about 99 mole percent) of the oxygenate feedstock. At least one catalyst recirculation standpipe is provided to transfer or return a portion of the catalyst mixture from the upper catalyst bed to the dense phase zone. 
         [0012]    Preferably, a catalyst cooling system is provided to cool the catalyst and feedstream as they react. The cooling system, as described in further detail below, comprises a plurality of cooling tubes disposed along the reactor wall within the reactor. The cooling tubes are fed with boiler feedwater which is heated to produce steam that is preferably used elsewhere in the reactor complex. The use of a cooling system employing cooling tubes within the reactor system eliminates the need for costly and complex flow through catalyst coolers. 
         [0013]    Reaction conditions can be determined by those skilled in the art and preferably comprise a temperature of from about 200 degrees to 600 degrees Centigrade, more preferably from about 300 degrees to 500 degrees Centigrade, and a pressure of from about 1 to 200 psia, more preferably from about 20 to 100 psia. Typical processes for producing light olefins are described in U.S. Pat. Nos. 4,499,327 and 4,873,390 cited above and hereby incorporated by reference. 
         [0014]    Preferably, the reaction zone comprises at least one catalyst recirculation standpipe to facilitate return of catalyst to the dense phase zone. A portion of the catalyst from the dense phase zone is withdrawn, optionally stripped in a conventional manner, and passed to a regeneration zone. In the regeneration zone, the coked catalyst is at least partially regenerated to produce a regenerated catalyst. The regenerated catalyst is returned to the reaction zone at a location above the dense phase zone. More particularly, the regenerated catalyst may be returned to the reaction zone at a location above the dense bed such as at a point in the riser or transition zone, or at a point in the disengaging zone such as to the upper catalyst bed. It is believed that by returning the regenerated catalyst to a location above the dense phase zone, contact between the freshly regenerated catalyst and the oxygenate feedstock is minimized, thereby improving selectivity to ethylene and the overall production of coke is reduced. The regenerated catalyst is lifted to the reaction zone with a portion of the net product stream. Preferably, the portion of the net product stream used to lift the regenerated catalyst comprises butene which was fractionated from the net product stream in a fractionation zone producing an ethylene stream, a propylene stream and a butylene stream. 
         [0015]    Referring now to  FIG. 1 , a fast-fluidized bed reactor  10  is illustrated in schematic form. The reactor  10  comprises a disengaging zone  62  and a lower reaction zone having a dense phase zone  44  and a transition phase zone  46 , defined by a reactor wall  45 . The reactor wall may be constructed of stainless steal, with a hardened liner for corrosion protection. 
         [0016]    A feedstream is passed via line  50  to the feedstream inlet  14  in the presence of a diluent. The feedstream preferably comprises at least one oxygenate feedstock selected from the group consisting of methanol, ethanol, dimethyl ether, and the like. The feedstock and diluent admixture passes through a feed distributor  34  and enters the dense phase zone  44 . The feed distributor  34  may consist of a uniformly flat sieve plate which permits the vapor phase feed admixture to pass through while retaining a catalyst above the sieve plate. Generally, the feed distributor  34  is supported by a ring having an overall diameter smaller than the outside diameter of the generally circular feed distributor  34 . The ring may be supported by a cylinder with perforations or vents extending therethrough to prevent accumulation of catalyst against its side. The cylinder may be typically welded to the ring at right angles to the sieve plate to form a feed distributor assembly and the feed distributor assembly is rigidly disposed on the base of the lower reaction zone above the feed inlet  14 . The ring serves to support the catalyst bed and to reduce vibrations in the feed distributor  34 . Alternatively, a spider type distributor may be used. 
         [0017]    The catalyst in the dense phase zone  44  and the transition phase zone  46  may comprise a non-zeolitic small pore catalyst such as SAPO-34, SAPO-17, and mixtures thereof. As the feedstock enters the dense phase zone  44 , the feedstock contacts the non-zeolitic small pore catalyst and reacts at effective conditions to produce a reaction product stream. The reaction product stream typically comprise light olefins, including ethylene, propylene, and butylene. In the course of the reaction, a carbonaceous deposit is produced on the catalyst, reducing the activity of the catalyst. The reaction product stream and a catalyst mixture comprising active catalyst and some deactivated catalyst are conveyed into the transition phase zone  46  in an intermediate portion of the reaction zone. 
         [0018]    The reaction of the feedstock with the catalyst is exothermic, producing excess heat in the reactor  10 . As with many reactions, it is important to keep the reactor  10  at a controlled, generally uniform temperature throughout the reaction process. Referring now to  FIGS. 1 and 2 , removal from the reactor  10  is facilitated by a cooling system  64 . The cooling system  64  shown here comprises a plurality of cooling tubes  66  located within the reactor  10 . The cooling tubes  66  have a generally “U” shaped configuration, with first tubes  68  and second tubes  70  connected to each other by a generally semicircular bottom portion  72 . 
         [0019]    Upper portions of the first and second tubes  68 ,  70  are fluidly connected to inlet tubes  74  and outlet tubes  76 , respectively. The inlet and outlet tubes  74 ,  76  respectively supply and evacuate cooling medium to and from the cooling tubes  66 . The inlet tubes are fed by a feedwater manifold  78 , which supplies cooing medium to the first tube  68  via the inlet tube  74 . Cooling medium passes through the first tube  68 , the bottom portion  72  and the second tube  70 . Upon exiting the second tube  70 , the cooling medium flows through the outlet tube  76 , which fluidly connects the second tube with an outlet manifold  82 . Flow through the cooling tubes  66  may be controlled by a valve  80 , which is positioned on the inlet tube  74 . Locating valves  80  on inlet or outlet tubes  74 ,  76  allow flow control for individual cooling tubes  66 . The valve  80  may be positioned anywhere along the flow path of the cooling medium to control flow of the cooling medium. By way of an alternative example, location of at least one valve  80  upstream from the individual tubes, such as along the feed manifold  78  allows an operator to control flow through the cooling tubes  66  with a single valve. 
         [0020]    The feedwater manifold  78  and outlet manifold  82  may be segmented (e.g., in quadrants) so that, in case of a perforation or a leak, the faulty section can be isolated. Mixing in the fluidized bed is usually so vigorous that the temperature distribution remains uniform provided that the remaining cooling tube sections are sized to remove the excess heat. 
         [0021]    Referring now to  FIG. 3 , The cooling tubes  66  are vertically positioned along the reactor wall  45  and, in the structure shown, spaced evenly around the perimeter of the reactor  10 . Because of the mixing characteristics of the catalyst and feedstream within the fluidized bed reactor, the perimeter location of the cooling tubes  66  facilitates even heat distribution throughout the reactor  10 . Heat is transferred as the catalyst and feedstream mixture comes into proximity with the cooling tubes and is then transferred to the cooling medium. 
         [0022]    As shown in  FIGS. 1 and 2 , the cooling tubes are positioned within the reactor  10  so that the lower portion of each cooling tube  66  is disposed within the dense phase zone of the reactor  10 . Above the dense phase zone  44 , the upper portion of the cooling tubes  66  also extend through the transition phase zone  46 . The tubes  66  may also have mounts  84  which secure the tubes to the reactor wall  45  or to each other (not shown). The mounts  84  serve to hold the cooling tubes  66  in place and reduce vibration of the cooling tubes  66  within the reactor  10 . 
         [0023]    The cooling tubes  66  are preferably constructed of hardened steel to reduce the risk of corrosion which may occur within the reactor. Alternatively, the cooling tubes  66  may be constructed of any other material having the properties to withstand conditions occurring within the reactor  10 . Although the structure shows cooling tubes  66  having a smooth surface, it may desirable to construct cooling tubes having a ribbed or finned surface or having a series of ridges located on the outer surface of the tube. Such surface features may serve to increase the surface area of the outer surface, thereby increasing heat transfer. 
         [0024]    Heat is typically removed from the reactor  10 , via the cooling tubes  66 , to produce steam which can be used elsewhere in the complex. By way of example, steam produced within the cooling tubes may be used as a scrubber in connection with regenerating the catalyst. 
         [0025]    As the reaction proceeds, the activity of the catalyst in the reaction zone gradually is reduced by the buildup of coke on the catalyst. As the reaction product and the catalyst mixture continue moving upwardly through the lower reaction zone into a riser section  26 , the cross-sectional area of the flow path through the fast-fluidized bed reactor is reduced from the cross-sectional area of the dense phase zone  44  by a reducing means  25 , or cone section, to the cross-sectional area of the riser section  26 . In the fast-fluidized bed reaction system, the superficial velocity through the transition phase zone  46  varies between about 5 cm/s and 1 meter per second. The riser section  26  has a smaller diameter and a smaller cross-sectional area than the dense phase zone  44  which increases the superficial velocity through the riser relative to the dense phase zone  44 . Because the superficial velocities in the riser section  26  are higher for the same feed rate, the cross-sectional area of the overall reactor zone can be decreased by about a factor of 2 to 3 times compared to the cross-sectional area of a bubbling bed reactor. In addition, the fast-fluidized bed reaction zone provides more precise control of the feedstock and catalyst rates without the need for external catalyst addition or removal. As a result, the fast-fluidized bed reaction system provides significantly decreased catalyst inventories over a bubbling bed reactor. 
         [0026]    The reaction product stream and catalyst mixture continue to be conveyed through the riser section. The riser section discharges the reaction product stream and catalyst mixture through a separation zone consisting of distributor arms  24 , or discharge opening, and a separation vessel  22 . The discharge opening  24  tangentially discharges the reaction product stream and catalyst mixture to create a centrifugal acceleration of the catalyst and gas within the separation vessel  22  that provides an initial stage cyclonic separation. The catalyst mixture falls to the bottom of the disengaging zone  62  which defines a particle outlet for discharging fluidized catalyst particles and the vapor portion of the reaction product stream passes upwardly through a gas recovery outlet for withdrawing gaseous fluids from the separation vessel  22 . Other configurations of separation zones may be suitable. The vapor, comprising entrained catalyst, continues upwards to a dilute phase separator typically in the form of a series of one to three conventional cyclone separation stages shown in the drawing as  20  and  21 . Cyclone separation stage  20  represents a primary cyclone separation wherein a primary cyclone vapor stream is passed to a secondary cyclone separation stage  21  and the secondary vapors from the secondary cyclone separation stage  21  are conveyed via conduit  17  to a plenum chamber  16 . Optionally, external cyclones bay be provided in addition to the cyclones shown within the reactor  10 . 
         [0027]    A net reaction product stream comprising less than about 100 ppm-wt catalyst is withdrawn via line  48  from the reactor outlet  12 . Preferably, the net reaction product stream withdrawn from the fast-fluidized bed reaction zone comprises less than about 70 ppm-wt catalyst. Catalyst separated in the primary cyclone separation stage  20  drops through dip leg  59  into the bottom of the disengaging zone  62 . Catalyst separated from the reaction product in the secondary cyclone separation stage falls through dip leg  60  into the bottom of the disengaging zone  62 . Dip legs  59  and  60  are optionally fitted with flapper valves (not shown) at their base to prevent the back flow of vapors through the cyclone separators. Catalyst accumulated in the bottom of the disengaging zone  62  is allowed to achieve an upper catalyst level and any excess catalyst is passed through at least one external catalyst recirculation standpipe  28  through a recirculation slide valve  32 , and returned to the dense phase zone  44 . Preferably, at least two external catalyst recirculation standpipes are employed to return catalyst from the disengaging zone  62  to the dense phase zone  44 . 
         [0028]    To maintain the conversion and selectivity of the reaction at acceptable levels, a portion of the catalyst mixture is withdrawn as a spent catalyst stream from the upper disengaging zone  62  and passed through a spent catalyst standpipe  42 . In the spent catalyst standpipe  42 , the spent catalyst stream may be stripped with a stripping medium such as steam produced in the cooling tubes  66 , which is introduced in line  37  to produce a stripped catalyst stream  56 . The spent catalyst standpipe  42  typically includes a stripping section that contains grids or baffles to improve contact between the catalyst and the stripping medium. The stripped catalyst stream is conveyed through line  38  and the spent catalyst slide valve  39 . The stripped catalyst stream  56  is passed to a catalyst regeneration zone (not shown). 
         [0029]    In the catalyst regeneration zone, the spent catalyst stream is at least partially regenerated by oxidation to produce a regenerated catalyst stream. Such regeneration is well known to those skilled in the art of fluidized bed reaction systems. A regenerated catalyst stream  52  is returned to the lower reaction zone via a regenerated catalyst standpipe comprising line  40 , regenerated catalyst slide valve  41 , and line  36  to a point above the dense phase zone  44 . The regenerated catalyst return is shown at a point above the dense phase zone. However, the return of the regenerated catalyst to the reaction zone may be provided at any point in the riser or in the upper catalyst bed. Preferably, the dense phase zone is operated to maintain a bed height of between about 2 meters (7 feet) and about 6 meters (20 feet) above the feed distributor  34  and below the intermediate portion of the reaction zone in the dense phase zone. More preferably, the bed height of the dense phase zone comprises between about 2.4 meters (8 feet) and about 4 meters (13 feet). By maintaining this bed height in the dense phase zone  44 , it is believed that feedstock flow variations and “jet penetration” at the feed distributor are minimized to provide a well-mixed reaction zone comprising catalyst having a carbon content of between about 3 and 20 weight percent. It is believed that returning the regenerated catalyst to the point above the dense phase zone  44  improves the selectivity of the overall reaction toward ethylene and propylene. Freshly regenerated catalyst has the potential to crack the oxygenate feedstock to produce unwanted by-products. By contacting the feedstock with a partially regenerated catalyst in the dense phase zone and contacting the reaction products and unreacted material in the transition zone with a catalyst mixture which is a relatively more active catalyst mixture, the combination of spent catalyst with freshly regenerated catalyst, more complete conversion to the desired light olefin products is achieved. 
         [0030]    The operating conditions depend, of course, on a particular conversion process and can be readily determined by those skilled in the art. Typical reaction parameters which control the reaction severity include temperature, space velocity, catalyst activity, and pressure. In general, reaction severity increases with increasing temperature, increasing catalyst activity, and decreasing space velocity. The effect of pressure on the reaction severity depends upon the particular reaction. Although any of the above described variables can be adjusted as necessary to obtain the desired hydrocarbon conversion, it is advantageous to have catalyst activity, directed to providing an effective amount of active catalyst sites within the moving bed reaction zone, to enhance the conversion to desired products while not enhancing the conversion to undesired by-products. 
         [0031]    A variety of modifications to the structures and processes described will be apparent to those skilled in the art from the disclosure provided herein. Thus, the processes and systems may be in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of my disclosure.