Abstract:
Disclosed is a reactor or sparging vessel suitable for use for reaction mixtures containing vapor and/or gas bubbles. It comprises a mechanical agitation means suitable to redistribute flow radially and at least one perforated plate suitable to provide resistance to axial flow. In desirable embodiments it may include at least one electrically or hydraulically-stimulated rotatable shaft upon which at least one blade impeller is mounted, and at least one perforated plate oriented such that it provides resistance to axial flow. The perforated plate may have channels therethrough whose cross-sectional dimension is smaller than the projected average diameter of the vapor and/or gas bubbles. The reactor or sparging vessel offers improved flow dynamics, including reduced back mixing and narrowed residence time distribution. A method of employing the reactor for a reaction mixture containing vapor and/or gas bubbles is also disclosed. An impeller blade having two curvatures is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to reactions wherein vapor and/or gas bubbles are produced and rising in a fluid. More particularly, the invention relates to reactions wherein autorefrigeration or sparging is employed. 
     2. Background of the Art 
     A number of known reactions may be characterized as including a vapor phase. Some of these reactions involve introduction of a gas through a means such as a sparger. These gases may include, for example, air, oxygen or nitrogen. Other reactions are those categorized as autorefrigeration reactions. In these reactions a solvent-diluent medium is vaporized from a liquid phase in order to remove the exothermic head of reaction and maintain a substantially constant temperature in the medium remaining in solution. In these cases the vaporized solvent-diluent is usually recovered via, for example, a condenser, and the recovered solvent-diluent is returned to the reactor. This solvent-diluent may be a single material such as toluene; a constant boiling azeotrope such as benzenecyclohexane, or a reactant in the system. Temperature control in these cases thus requires a vapor space in the reaction vessel into which the vaporized solvent-diluent medium expands. Some of the solution medium is frequently entrained in the vapor phase and must then be removed separately because it may foul the solvent-diluent vapor removal means. 
     When either of the above-described types of reactions is carried out in conventional autorefrigeration or sparging vessels, the rising vapor and/or gas bubbles induce significant axial, or back, mixing. Such mixing tends to broaden the residence time distribution, and may also undesirably affect the product being produced. The residence time within these types of vessels is therefore generally comparable to that of continuous stirred tank reactors (CSTRs). 
     In view of the above, it would be desirable to have an autorefrigerated reactor or sparging vessel wherein back mixing is reduced and, therefore, residence time distribution may be narrowed. 
     SUMMARY OF THE INVENTION 
     A method for reducing the back mixing of a reaction mixture including vapor and/or gas bubbles has now been found. It includes carrying out the desired reaction in a reactor or sparging vessel comprising at least one mechanical agitation means suitable to redistribute flow radially and at least one perforated plate oriented to resist axial flow. The combination of the specified mechanical agitation means and the perforated plate serves to narrow bubble size distribution and reduce back mixing. 
     A reactor or sparging vessel comprising at least one mechanical agitation means suitable to redistribute flow radially, and at least one perforated plate oriented to resist axial flow, is also disclosed. 
     Finally, the invention includes a reactor or sparging vessel suitable for use for reaction mixtures containing vapor and/or gas bubbles. This reactor or sparging vessel comprises a downward-directed flow inlet and flow outlet, at least one electrically or hydraulically-stimulated rotatable shaft upon which at least one blade impeller is mounted, and at least one perforated plate statically or movably disposed within the reactor. The perforated plate has channels whose cross-sectional dimension is smaller than the projected average diameter of the vapor and/or gas bubbles. In this reactor or sparging vessel, the electrically or hydraulically-stimulated rotatable shaft is suitable to rotate the blade impeller within the reaction mixture such that flow is redistributed radially, and the perforated plate is suitable to provide resistance to axial flow. The result is that back mixing may be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-section of a reactor having disposed therein an impeller assembly and, affixed to the reactor wall, perforated plates. 
         FIG. 2  is a graph showing the overall narrowing of residence time distribution attained by the inventive reactor/vessel as compared with a reactor/vessel without perforated plates. 
         FIG. 3A  is a transparent graphical illustration of a perspective view of a 4 bladed impeller having a blade pitch angle of 0°. 
         FIG. 3B  is a transparent graphical illustration of a side view of a 4 bladed impeller having a blade pitch angle of 0°. 
         FIG. 3C  is a transparent graphical illustration of a top view of a 4 bladed impeller having a blade pitch angle of 0°. 
         FIG. 4A  is a transparent graphical illustration of a perspective view of a 4 bladed impeller having a blade pitch angle of 30°. 
         FIG. 4B  is a transparent graphical illustration of a side view of a 4 bladed impeller having a blade pitch angle of 30°. 
         FIG. 4C  is a transparent graphical illustration of a top view of a 4 bladed impeller having a blade pitch angle of 30°. 
         FIG. 5A  is a schematic illustration of a blade having a blade pitch angle of 0°. 
         FIG. 5B  is a schematic illustration of a blade having a blade pitch angle of 30°. 
         FIG. 6  is a schematic illustration of an impeller having the blades swept 30°. 
         FIG. 7  is a schematic illustration of an impeller shaft and blade connectors. 
         FIG. 8  is a side view of the illustration of  FIG. 7 . 
         FIG. 9  Is a schematic illustration of an impeller blade and connector fitting. 
         FIG. 10  is a schematic illustration of an impeller blade having a curvature. 
         FIG. 11  is an illustration of the end on curvature of an impeller blade. 
         FIG. 12  is an illustration of the side curvature of an impeller blade. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The invention provides a reactor/vessel design that may be generally characterized as offering reduced residence time distribution by addressing, and reducing, the effect of the back-mixing that results from a combination of axial flow and the formation, or introduction, and rising of vapor and/or gas bubbles. As used herein, the term “reactor/vessel” refers to a reactor, a sparging vessel, or both, according to the context within which it is used. Such bubbles may be artificially produced, by operation at a temperature whereby a given component of the reaction nears or reaches the boiling point for that component, such as in autorefrigerated reactors; or they may be the result of the reaction itself, such as when oxygen, carbon dioxide, hydrogen or another gas are formed; or they may be introduced via a sparging means, which may be used to incorporate a gas reactant, to modify viscosity, or for any other reason. 
     The back-mixing greatly reduces the efficiency of flow in a given reaction system. The inventive reactor design offers the advantage of facilitating what is termed herein as “quasi plug flow”. This term refers to flow that, because of the geometry of the vessel and actual flow rate, does not meet precise definitions of plug flow, but which may approximate the desirable mixing level and inherently low back mixing features of plug flow. This is accomplished in the invention through a combination of agitation of the reactants, inducing radial flow and mixing, along with resistance to axial flow of vapor and/or gas bubbles and some portion of their entraining liquid. 
     The agitation of the reactants may be accomplished by any means known to those of ordinary skill in the art. In general such means may constitute mechanical flow facilitators, such as the use of blade impellers or other “stirring” means which increase the mixing of the reactants and, importantly, promote radial flow, i.e., flow toward the perimeter of the reactor. These means may also serve to induce shear, which may concomitantly reduce viscosity and thereby also may enhance the reaction and decrease the bubble size, which may therefore increase the residence time. Where blade impellers are selected they are typically deployed on one or more rotating shafts. Desirably, in one embodiment, the electrically or hydraulically-stimulated rotation may occur at a rate ranging from about 10 to about 100 rpm (0.2 to 2 ft/min (6 to 61 cm/min), more desirably from about 15 to about 60 rpm (0.3 to 1.2 ft/min (9 to 36 cm/min). Routine engineering design and modeling may be used to determine size and blade configurations as well as the operating speed that may be useful to optimize flow. Such blade impeller may be employed singly or multiply, depending upon the size and geometry of both blade impeller(s) and reactor/vessel, and may conveniently be paired. They are typically manufactured from metals or alloys, such as stainless steel, but may also be prepared from polymers or composite material. Optimal fabrication materials will depend upon the selected reactants, reaction products, and conditions under which the reaction will be carried out. 
     A key in reducing the back-mixing that may be induced or enhanced by the rising vapor and/or gas bubbles is the use of perforated plates. As used herein, the term “perforated plate” refers to a solid three-dimensional construct as to which two dimensions are at least several times, and desirably many times, the third dimension, and having segregated channels (perforations) axial to that third dimension that represent a significant volume of the construct as a whole. In some embodiments this means that the height and length of a plate, which may be, for example, a rectangular or approximately rectangular structure, or the diameter of a substantially circular plate, is much greater than the thickness of the plate, and that the channels (perforations) desirably represent at least about 5 percent, more desirably at least about 10 percent, and most desirably at least about 15 percent, of the total surface area of the plate. 
     The channels themselves may present in cross-section any of a variety of configurations, e.g., round, square, rectangular, etc., but their cross-sectional dimension is desirably selected to be less than the average diameter of the vapor and/or gas bubbles in the reaction mixture. For example, where routine engineering, modeling, monitoring and/or observation suggest that the average vapor bubble diameter throughout the reaction mixture is about 1.5 cm, it is desirable that the average cross-sectional dimension of the channels (perforations) be less than that, and more desirably at least about 50 percent less. Those skilled in the art will understand that many parameters may need to be taken into account when modeling is applied, and may include factors such as viscosity, agitation rate, reactor geometry, flow and reaction rates, sparging means and rate, and the like. It will be appreciated that a variety of channel sizes may be incorporated in one plate, or that more than one plate may be used with each having one or more discretely sized channels. 
     The perforated plates may be disposed within the reactor in a variety of orientations. However, they are desirably disposed in such a way as to provide resistance to axial flow. This means that they will desirably be oriented, whether statically or movably, such that axial flow (which, because of the radial flow-inducing effect of the mechanical agitation means, tends to be more predominantly counter-current flow) is directed toward the perforated plate&#39;s larger or largest dimension surface and therefore impliedly through its channels. In other words, the perforated plate&#39;s surface is desirably substantially perpendicular to the direction of such axial flow. In one embodiment this may be promoted by shaft-mounting the perforated plate or plates and rotating it/them at the same speed as the impellers. In fact, in some embodiments the plates may be mounted on the same shafts as the blade impellers. In other embodiments the plate or plates may be disposed in permanent, or static, positions within the reactor or vessel, in orientations where routine engineering design indicates flow will be axial or approximately axial. For example, the plate or plates may be disposed in a substantially cylindrical reactor with their circumferences affixed to the reactor wall. One desirable embodiment is shown in  FIG. 1 , wherein the rotatable shaft, upon which a series of paired blade impellers are mounted, is centrally and axially disposed, and passes through a series of perforated plates which are circumferentially affixed to the reactor wall, with each plate occupying essentially the entire diameter of the reactor. 
     In still other embodiments, the plates may be disposed in locations where gas flow relative to them will not be truly axial or approximately axial; however, this embodiment may result in a greater level of undesirable back mixing and therefore broadened residence time distribution. 
     In one embodiment it may be desirable to artificially “sectionalize” the reactor/vessel into more than one agitation “zone”. This may be accomplished by using a series of mechanical agitation means disposed within the reactor/vessel, as well as a corresponding series of perforated plates. 
     The inventive reactor design is suitable for use in a wide variety of applications. For example, it may be used for reactors for preparing polymers such as polystyrene, high impact polystyrene, styrene-butadiene, polymethylmethacrylate, polyamines, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, combinations thereof, and the like. In one embodiment, it is used for fermentation for the production of bio-polymers. As will be obvious to those skilled in the art, it is thus possible to have one reactant, such as a monomer which is being polymerized to form a homopolymer, or more than one reactant, such as two or more monomers which are being used to prepare a copolymer. It is also possible to include a gas sparger to incorporate a gas, as a reactant or as a process modifier, into a reactor into which at least one liquid reactant is being flowed. The products produced using the inventive reactor or sparging vessel may then be employed in uses ranging from manufacture of, for example, durable goods such as furnishings, toys, housings, automobile parts, films, and the like, to disposable goods such as food containers and packaging in general. 
       FIG. 1  illustrates one embodiment of the inventive reactor/vessel. In this drawing the reactor/vessel  12  is fed by reactant inlet  15 . Flow of the reactants is indicated by the arrows. Disposed within the reactor/vessel  12  is a rotatable shaft  18 . Mounted on the rotatable shaft  18  are paired sets of impeller blades  21 ,  24  and  27 , respectively, which are aligned with the vertical axis of the reactor/vessel  12 . When the blades are rotated, this orientation will facilitate radial flow while minimizing axial flow in general. In the embodiment illustrated in this  FIG. 1 , the lengths of the blade impellers are “stepped”, such that the paired blade impellers  21  closest to the inlet  15  have the shortest length, and the paired blade impellers  27  farthest from the inlet have the greatest length. Also disposed within the reactor, and circumferentially affixed thereto, are two approximately circular perforated plates  30  and  33 . These perforated plates  30  include channels (perforations)  36  thereihrough. The rotatable shaft  18  is operated via electrical or hydraulic-stimulation to an appropriate mechanical means (not shown) disposed outside of reactor/vessel  12 . 
     In use the desired reactant or reactants may be flowed, via reactant inlet  15 , into reactor/vessel  12 , where the rotating impeller blades  21 ,  24  and  27  operate to direct flow radially while at the same time the overall flow pressure from the inlet  15  generally establishes current flow toward the outlet  39 . In this “outward and downward” flow path the reactants, and therefore the entrained vapor and/or gas bubbles, pass through the two perforated plates  30  and  33 , and eventually to the outlet  39 . Back mixing is reduced because counter-current flow, induced by the natural tendency of the vapor and/or gas bubbles to rise in what is essentially a counter-current, or back, flow, is axially hindered by the plates  30  and  33 , and those vapor and/or gas bubbles that do rise to reach the plates  30  and  33  become, or already are, reduced in size due to passing through the plate perforations  26 . This size reduction itself tends to reduce associated flow turbulence and therefore also contributes to desirably decreased back mixing. The gas is also redistributed radially by the motion of 24 between 30 and 33, and the bubbles are reduced in size by the energy dissipated by 24. As a consequence, the gas residence time is increased. 
     In one embodiment, the invention includes an impeller that has been designed to provide near plug flow behavior in single phase fluids and to provide efficient gas dispersions in mixed phase fluids. This can be accomplished by reducing downward flow while optimizing the mixing of the phases. The impellers may have more or fewer blades, but desirably have from 3 to 5 blades. In one embodiment, the impellers have four blades. 
     The blades of the impeller may be configured to optimize the mixing of multiple phases.  FIGS. 3A-C  show three views of a four bladed impeller having the blades in a 0.degree. blade pitch configuration. In these figures, the impeller  300  has a shaft,  301 . On the shaft are up to four blade connectors  303   a - d , connecting four blades  302   a - d  to the shaft  301 . In contrast,  FIGS. 4A-C  shows the same impeller having the blades in a 30.degree. blade pitch configuration. These two configurations are further illustrated in  FIGS. 5A and 5B  wherein the pitch angle of the blade  302  is shown to be 0.degree. and 30.degree. respectively. The impeller may have a blade pitch of from 0.degree. to about 60.degree. In one embodiment, the impeller has a blade pitch of from 20.degree. to about 55.degree. In another embodiment, the impeller has a blade pitch of from 25.degree. to about 45.degree. 
     In addition to blade pitch, the impellers can be adjusted to vary the degree the blades are swept relative to the shaft. In  FIG. 6 , an impeller  300  is shown in a top view wherein  301  is the shaft,  302   a - d  are blades, and  303   a - d  are blade connectors. In this configuration, the blades are show swept in 30° relative to the shaft. The impellers may have the blades swept from 0° to 45°. 
       FIG. 7  show one embodiment of attaching the blades of the impeller to the shaft. The blades may be attached using any method known to be useful to those of ordinary skill in the art. In  FIG. 7 , the impeller  400  is shown from a top view with the shaft  401  having attached thereto four connectors  403   a - d  and for hex nuts  402   a - d . A side view is shown in  FIG. 8  wherein the shaft  501  ahs three visible connectors  503   a - d  and three visible hex nuts  502   a - c . In this embodiment, the connector fitting ( FIG. 9 ,  504 ) on an impeller blade ( FIG. 9 ,  301 ) is mated with the connector  503 , the blade pitch adjusted to the desired point, and then the blade  301  is secured in place with the hexnut  502 . 
     In  FIG. 10 , a blade  600  useful with the impeller is shown to have curvature in two perspectives. The blades are curved both when viewed end on  601  and from the side  602 . These curvatures of the blades can be illustrated by example. In  FIG. 11 , the end on curvature of the blade  601  is defined by the radius of curvature at a given angle, alpha ( α ). The blades then have a radius of curvature defined by Table 1: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Impeller 
                 Radius of 
                 Curves 
                   
               
               
                   
                 diameter 
                 circle: R 
                 length: L 
                 Angle: α 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 6.75″ 
                 1.5″ 
                 2.29″ 
                 45° 
               
               
                   
                 5″   
                 1.125″ 
                 1.73″ 
                 45° 
               
               
                   
                   
               
             
          
         
       
     
     In  FIG. 12 , the side curvature of a blade useful with the impeller is illustrated. In some embodiments, it may be necessary to increase the length of the blades as blade pitch is increased. 
     The impeller can be of any dimension suitable for use with the reactor in which it is to be installed. One of ordinary skill in the art of preparing reactors will be able to size the impeller. 
     EXAMPLES 
     The following hypothetical examples are provided for purposes of illustration. The examples are not intended to limit the invention&#39;s scope and should not be construed as such. 
     Example 1 and Comparative 
     A cylindrical reactor/vessel is constructed of stainless steel having the overall dimensions of 5 ft length×3 ft diameter. Disposed therein is a rotatable shaft having one set of paired blade impellers fabricated from stainless steel and having overall dimensions for each blade of 4 inches (10 cm) and 0.8 ml 0.02 mm) thickness. Disposed therein above the paired blade impellers is a perforated plate, with the rotatable shaft passing through the center of the perforated plate. The perforated plate is made of stainless steel and measures approximately 0.95 m in diameter by 1 cm in thickness, i.e., the plate crosses essentially the entire inner diameter of the reactor vessel. The plate is perforated by channels having a circular cross-section of approximately 3 cm. The blade impeller is located approximately 0.5 m above the bottom of the reactor/vessel. 
     A reaction is carried out in which styrene and a polybutadiene rubber solution are fed into the reactor/vessel. The rotatable shafts are rotated at approximately 20 rpm. The reacted product, which is styrene-butadiene, is flowed out of the outlet. Vapor and/or gas bubbles formed by the reaction are entrained in the reacted product. The average bubble diameter of the product at the outlet is found to be about 2 cm. This is approximately 50 percent less than that of bubbles formed under identical conditions wherein a rotatable shaft and impeller blade are disposed in the reactor/vessel, but wherein no perforated plates are employed. It is also found that the bubble size distribution is narrower in the example reactor/vessel than for the reactor/vessel without the perforated plates. 
     Example 2 and Comparative 
     A hypothetical experiment is carried out to determine residence time distribution for the inventive reactor/vessel as compared with a reactor/vessel without perforated plates. The reactor/vessel is as described in Example 1. Identical reactants are flowed into each reactor/vessel at a feed rate of 0.36 liters per minute, and with a gas flow rate of 20 liter per minute. Three sets of blade impellers, disposed as shown in  FIG. 1 , are rotated at a speed of about 50 rpm. The residence time of the reactants is measured using conductivity measurements respectively at the inlet and the outlet of the reactor/vessel. A negligible volume, compared to the reactor/vessel volume, of an electrically conductive liquid of the same viscosity, density and temperature as the reactants is injected during a negligible time, compared to the overall residence time, at the inlet of the reactor/vessel and is followed at the outlet of the reactor/vessel. The results are as shown in  FIG. 2 . In that figure it is seen that the overall residence time distribution achieved by the inventive reactor/vessel is considerably narrower, as shown by the relatively flat solid line, for the inventive reactor/vessel than for the comparative vessel, as shown by the broken line. The dotted line shows the inlet conductivity against which the residence time distributions may also be compared. 
     The description provided hereinabove is intended to generally describe the features and some useful embodiments of the invention. However, it will be appreciated that modifications may be made to many aspects of the invention without departing from its scope and generalized objects. For example, selection of temperatures, pressures, reactor vessel configurations, feed rates, fabrication materials, impeller configurations, rotation speeds, perforated plate configurations, plate orientations, and the like, not explicitly listed but which do not substantially alter the outcome of the process or the character of the products, will still fall within the scope of the invention.