Patent Publication Number: US-2004042942-A1

Title: Continually stirred reactor system

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention is directed to a continually stirred reactor system that is useful in promoting the intermixing of a solid granular catalyst and a liquid phase. The invention also includes a method for using such a reactor system.  
       [0003] 2. Background Art  
       [0004] Various techniques have been deployed for the mixing of reagents in chemical processing. Techniques that include for example circulation of a liquid chemical reagent within an enclosed vessel are sometimes used. However, such approaches may be unable to circulate reagents adequately using commercially reasonable electrical power expenditures. Consequently, mixing at acceptable flow rates often leads to sub-optimal results, even when sufficient electrical power is used to energize a mixing system.  
       [0005] Catalysts are thought to be essential in virtually all industrial chemical reactions, especially in petroleum refining and synthetic organic chemical manufacturing. Since the activity of a solid catalyst is often centered on a small fraction of its surface, the number of active points can be increased by adding promoters which increase the surface area in one way or another, e.g. by increasing porosity. Such approaches, may also yield results that leave something to be desired. Illustrative of the prior art is U.S. Pat. No. 5,972,661 which issued to Kubera et al. on Oct. 26, 1999. That reference discloses a mass transfer system for mixing bulk liquid and gas in an upright tank. The &#39;661 patent is incorporated herein by reference. It discloses a “draught tube” reactor, a simplified schematic of which is illustrated by FIG. 1 of that reference. The reactant slurry, including a solid catalyst, enters the central draught tube and is directed along its axis by impeller(s). A series of vertically oriented baffles positioned between the impellers prevents a swirling flow which might cause solid catalyst segregation. Upon reaching the end of the draught tube, the slurry flows in a counter-current direction through an annulus between the draught tube and the reactor wall. Product is continuously removed, and separated from an entrained catalyst, and worked up to remove solvent, byproducts, etc. The reactor has exhibited acceptable mass transfer rates. But catalyst separation issues have discouraged widespread commercial use.  
       [0006] It is also known that catalyst activity may be decreased by substances that tend to clog and weaken the catalyst surface. It would of course be desirable to avoid such adverse consequences.  
       SUMMARY OF THE INVENTION  
       [0007] Against this background, it would be desirable to make available a reactor system that offers the favorable mass transfer characteristics of a draught tube reactor with less catalyst attrition.  
       [0008] It is a further object of the present invention to provide an improved impeller arrangement which promotes circulation of liquid chemical reagent within a cylindrical bore while offering efficiency in the power acquired to produce a required flow within the reactor system.  
       [0009] Accordingly, this invention pertains to a reactor system for use in chemical processing that meets these, and other objects. The system includes a reactor with a longitudinal cylindrical bore having a shaped bearing carrier. A basket assembly is received in the bore so that the basket is secured in relation thereto. The basket assembly has inner and outer concentric cylindrical baskets that are mounted in the bore of the reactor. Between the inner and outer basket there is defined an annular space which is adapted to accommodate and confine a granular catalyst therewithin.  
       [0010] A rotatable shaft assembly extends axially within the bore inside the inner basket. The shaft assembly has an upper end and a lower end. A central section extends between the upper and lower ends. The upper and lower ends each have a pump for directing flow of a chemical reagent toward the central section. Disposed within the central section are longitudinally oriented, radially extending blades that, when the shaft assembly rotates, urge flow of the reagent outwardly through the annular space and therefore the granular catalyst that is entrapped between the baskets.  
       [0011] The reactor system promotes flow and mixing between the chemical reagent and the catalyst, thereby enhancing the kinetics of chemical reaction, control, and the reproducibility thereof. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a side elevation view of a basket assembly according to the present invention before installation of the assembly within a reactor;  
     [0013]FIG. 2 is a cross-sectional, longitudinal view of the basket assembly after installation within the reactor, but before installation of a top cover;  
     [0014]FIG. 3 is a side elevational view, exploded, that illustrates the rotatable shaft assembly that is also depicted in FIG. 2;  
     [0015]FIG. 4 is also a side elevational view of the rotatable shaft assembly depicted in FIG. 3, illustrating a coupling thereof to members at the top and bottom ends thereof;  
     [0016]FIG. 5 resembles the view of FIG. 1, illustrating the positioning of the basket without the shaft assembly within the longitudinal cylindrical bore of the reactor system;  
     [0017]FIG. 6 is a top view of an upper end cap provided with apertures that permit the bore to be filled and emptied;  
     [0018]FIG. 7 is a top elevation view of the top side of the upper end cap;  
     [0019]FIG. 8 is an underside view thereof;  
     [0020]FIG. 9 is a top view of the lower screen plate; and  
     [0021]FIG. 10 is an underside view of the lower screen plate.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
     [0022] Turning first to FIGS.  1 - 2 , there is depicted a basket assembly (FIG. 1) for use in a reactor system (FIG. 2) that is deployed in chemical processing where, for example, it is desirable to promote the kinetics of mixing between a liquid chemical reagent and a catalyst. The reactor system  10  includes a reactor  12  that has a longitudinal cylindrical bore  14  with a bearing carrier  16 . The bearing carrier  16  is attached to three outer baffles  15  (FIGS. 1, 6). The longitudinal cylindrical bore  14  includes a bottom portion which curves smoothly into a center (FIG. 2). A basket assembly  18  (FIGS.  1 - 2 ) is received in the bore so that the basket assembly is secured in relation thereto. The basket assembly  18  is fixed inside the reactor bore  14  by friction between the three outer baffles  15  and the reactor wall.  
     [0023] As depicted, the basket assembly  18  include an inner, generally cylindrical basket  20  (FIG. 2), surrounding which is an outer, generally cylindrical basket  22  that is co-axial with its inner counterpart. Between the inner  20  and outer  22  baskets, there is defined an annular space  24  which is adapted to accommodate a granular catalyst  26  therewithin.  
     [0024] Additional detail of the shaft assembly is depicted in FIGS.  3 - 4 . The rotatable shaft assembly  28  extends axially within the bore  14 . The shaft assembly  28  includes an upper end  30 , a lower end  32 , and a central section  34  that extends therebetween. Disposed at each of the upper and lower ends  30 ,  32  of the rotatable shaft assembly is a pump or impeller  36 ,  38  which directs the flow of a chemical reagent  40 . Under the influence of the pumps  36 ,  38 , the chemical reagent  40  is propelled generally axially toward the central section  34 . Extending radially outwardly from the central section  34  are blades  42  for urging flow of the reagent outwardly through the annular space  24  between the baskets  20 ,  22 .  
     [0025] When the shaft assembly  28  is rotated in relation to the basket assembly  18 , the reactor system  10  promotes the rate of chemical reaction between the catalyst and the chemical reagent without the catalyst itself being consumed or undergoing a chemical change. Although a granular catalyst is disclosed therein, it will be appreciated by those skilled in the art that suitable catalysts may be inorganic, organic, or complex organic groups and metal halides. Alternatively, the catalysts that may be confined within the annular space  24  may be in solid, rather then granular form. Thus, the reactor system  10  promotes the kinetics of mixing and therefore effectiveness of the granular catalyst with the chemical reagent for a given electrical power consumption. The inventor has found that the disclosed reactor system also enhances the control that can be asserted by the operator over the chemical reaction and enable the results to readily be reproduced.  
     [0026] In one embodiment, the cylindrical baskets are formed from a screen with an average mesh size α. The granular catalyst is prepared such that it has a smallest grain size β. To contain the granular catalyst within the annular space  24  between the basket  20 ,  22 , β exceeds α. One illustrative mesh size is 0.013 wire gage, 30×30 mesh stainless steel. The mesh size may be selected depending upon the particle of grain size of the granular catalyst.  
     [0027] Turning now to FIGS.  5 - 10 , there are depicted additional members of the reactor system  10 . They include a lower end cap  44  (FIG. 8) and an upper end cap  46  (FIG. 6) between which the baskets  20 ,  22  extend. Although not depicted, if desired, a loop may be provided on the upper side of the top end cap to facilitate removal of the basket. As shown in FIGS.  6 - 8 , the upper end cap is provided with one or more apertures  48  that enable the annular space  24  to be filled with and emptied by the granular catalyst. In FIG. 7, details of the upper end cap are depicted, including (preferably) a 45° chamfer that surrounds three depicted apertures in order to facilitate catalyst loading. Preferably, the apertures are provided with chamfered peripheries to facilitate the loading of the granular catalyst. After charging the annular space with the granular catalyst, a cover  50  (FIG. 2) closes off the apertures.  
     [0028] FIGS.  5 - 6  illustrate how the basket assembly  18  fits within the bore  14  of the reactor  12 . In practice, the basket assembly  18  is secured in relation to the bore  14  so that the former does not rotate in relation to the latter.  
     [0029] In FIG. 8, an underside view of the upper end cap is depicted which illustrates grooves that are provided to accommodate the baskets. FIGS. 9 and 10 respectively illustrate the lower end cap (top view) including screen grooves that support the base of each basket. FIG. 10 depicts slots that are sized for registry with inner baffle bars that secure the basket assembly in relation to the bore and impede axial flow inside the baskets.  
     [0030] During assembly, the basket assembly is hand fit to the reactor bore by sanding or grinding the outer baffles. The basket assembly, as noted earlier, includes inner and outer baskets  20 , 22 . Each basket  20 , 22  is formed from a mesh or screen which is configured cylindrically. The screen cylinders are overlapped and spot welded at intervals.  
     [0031] Although various capacities of reactor/vessel can be used, in practice a 300 cc reactor vessel and a 500 cc reactor vessel have been used with good effect. The reactor vessels are manufactured by Autoclave Engineers ((http://www.snap-tite.com). The 500 cc assembly is similar to the 300 cc version, but different in certain ways. When small particles are used in the granular catalyst, a fine screen is required to contain them. One consequence is that it is difficult to force reaction fluids through a defined catalyst particle bed and the two fine screen baskets. Accordingly, stir shaft-to-catalyst basket clearances need, to be about 0.040 inches between the upper and lower pumps and the basket to ensure that the pumps work at maximum efficiency with little pumping loss. The three inner baffle bars  17  between the catalyst bed and the stir shaft vertical blades tend to reduce internal liquid swirl and help direct the liquid through the screen and catalyst bed. Such baffles are absent from the 300 cc version because of its small size. The baffles, however, are preferred in the 500 cc version. Preferably, the 300 cc basket is about 4 inches in height, with a bore of about 1½ inches. The 500 cc basket has a depth of about 6 inches with a diameter of about 2 inches.  
     [0032] Thus, the continually stirred reactor system contains catalyst particles and efficiently circulates reaction liquids across the surfaces of and between the catalyst particles. The screen mesh size is chosen to be as large as possible and still contain the smallest catalyst particle from creating the least amount of resistence to liquid flow. The top pump draws the liquid down into the central section, while the lower pump urges liquid up into the central section. Attached to the central shaft between the upper and lower pumps are four vertical blades that re-direct the liquid from a vertical motion to a radial motion. Thus, the liquid is redirected outwardly, through the catalyst bed, to the reactor wall and can then continue circulating in this manner as long as the impeller is running.  
     [0033] In one example the disclosed reactor system is used in the hydrogenation of acetophenone to methyl benzyl alcohol. In one series of experiments the typical hydrogenation catalyst of various particle sizes was exposed under pressures between 200-1500 psig and temperatures between 50-250° C. Stirrer RPMs varied between 200-2000 RPMs.  
     [0034] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation. It is understood that various changes may be made without departing from the spirit and scope of the invention.