Patent Publication Number: US-6334705-B1

Title: Fluid mixing impellers with shear generating venturi

Description:
FIELD OF THE INVENTION 
     The present invention relates to mixing apparatus and particularly to a mixing impeller which produces axial flow and also is effective in dispersing one fluid (either a liquid or gas) into another fluid as the fluids are mixed by the impeller. The mixing apparatus is especially suitable for use in sparging systems for mass transfer (the transfer of the mass of one fluid phase into another) and in stirred reactors or agitators where the other fluid phase is generated during the reaction, as for example in liquid-liquid dispersion processes. 
     The invention provides a mixing impeller which affords efficient axial flow in the direction of the axis about which the impeller rotates, while providing a shear field which reduces the size of the media being dispersed without a significant or practical impact upon the flow efficiency of the impeller. 
     BACKGROUND OF THE INVENTION 
     Mixing impellers, which have been used for introduction of air or another fluid, operate effectively when the gas or other fluid is dispersed in the form of fine droplets. Such fine droplets make mass transfer or dissolving processes more efficient, for example, as measured by the mass transfer coefficient of the mixing system, kLa. The introduction of the gas or other fluid is called sparging. Axial flow impellers, particularly those with large blades, have been effectively used in sparging and provide high efficiency by benefiting from the efficient axial flow now produced by the impeller. One such axial flow sparging system, and mixing impellers used therein, are described in Weetman et al, U.S. Pat. No. 5,046,245 issued Sep. 10, 1991. It is desirable, therefore, to use axial flow impellers, and particularly those with even more efficient pumping as a obtained by narrow blades, such as described in Weetman U.S. Pat. No. 4,468,130 issued Aug. 28, 1984, which are even more efficient in terms of flow than the impellers described in U.S. Pat. No.5,046,245. Nevertheless, the efficiency of sparging is adversely affected by the size of the bubbles of the phase (liquid of gas or gas) being dispersed into the flow produced by the mixing impellers. It has been proposed to provide turbulence adjacent the impeller by various elements which disrupt the flow (See for example, Cooke 4,662,823, May 5, 1987 and Kozma et al, 5,312,567, May 17, 1994 and 5,431,860, Jul. 11, 1995). However, the turbulizing elements block the flow and sacrifice the inherent efficiency of the impeller. 
     SUMMARY OF THE INVENTION 
     It is a feature of the present invention to provide a shear field which effectively breaks down the size of the bubbles or droplets of the phase to be dispersed and disperses the phase in the form of much finer bubbles or droplets than would otherwise be the case. The invention provides structure carried by the impellers which creates the shear field. Such structure is to be distinguished from expedients to reduced turbulence and discontinuities in the flow about an around an impeller or to stabilize the impeller; for example, slotted or multiple blades or fins or other projections attached to the impeller blades. See in this connection the following U.S. Pat. Nos.: Zeides, 4,636,143, Jan. 13, 1987, Tomohiro et al, 4,893,990, Jan. 16, 1990, Mita 5,226,783, Jul. 13, 1993, Kato et al, 5, 277,550, Jan. 11, 1194, Weiss et al, 5,595,475, Jan. 21, 1997, Miura, 5,326,168, Jul. 5, 1994, Connolly et al, 5,525,269, Jun. 11, 1996, and also the proplets which control reverse flow around the tips in axial flow impellers such as described in the above referenced Weetman U.S. Pat. No. 4,468,130 and see also in Weetman U.S. Pat. No. 4,802,771, Feb. 7, 1989. 
     Accordingly, it is the principal object of the present invention to provide improved axial flow impeller apparatus which introduces shear for breaking up bubbles of a fluid phase while preserving axial flow efficiency of the impeller. 
     It is a still further object of the present invention to provide improved axial flow impellers which are especially suitable for use in sparging systems. 
     It is a still further object of the present invention to provide an improved axial flow impeller which shears droplets or bubbles into finer droplets or bubbles and disperses them into an axial flow stream produced by the impeller. 
     It is a still further object of the present invention to provide improved axial flow impellers which can be used in a tank in which one or more such impellers may be mounted for rotation on a shaft and where fluid is released into the tank in the form of fine bubbles or droplets which are mixed and dispersed throughout the liquid in the tank by the impeller or impellers. 
     Briefly described a mixing impeller provided in accordance with the invention is effective for dispersing a first fluid, that may differ in density or viscosity from a second fluid, into the second fluid while mixing the fluids with each other. The impeller has a plurality of blades oriented with respect to an axis about which the impeller rotates to provide axial flow of the fluids. The blades each have a structure providing a Venturi which is rotatable with the blades defining flow path having a width in a direction radially of the blades which is larger nearer the leading edge of the blades than the trailing edge thereof. The flow path thus converges in a direction from the leading edge to the trailing edge and may be wedge shaped. The Venturi creates a shear field which shears the second fluid into fine bubbles or droplets and disperses it into the first fluid during the mixing of the fluids by the impeller, thereby enhancing sparging and mass transfer processes which are facilitated by the mixing impeller. 
     The foregoing and other objects, features and advantages of the invention, as well as presently preferred embodiments thereof will become more apparent from a reading of the following description in connection with the accompanying drawings, brief descriptions of which follow. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of an axial flow mixing impeller having proplets at the end thereof which induce shear and assist in mass transfer in liquid to liquid and gas to liquid sparging systems in which the impeller may be used; 
     FIG. 2 is an end view of one of the impeller blades having the proplets attached thereto and showing the shape outer proplet in full lines and the tip of the blade and the shape of the inner proplet in dash lines; 
     FIGS. 2A and 2B are an end and a front elevational view of one of the blades near the tip thereof showing an arrangement of the proplets which extend both above and below the upper and lower sides of the impeller, which are the convex, suction side thereof and the concave, pressure side thereof; 
     FIG. 3 is a plan view of the tip region of a blade of the impeller showing in FIG. 1 which illustrates how the proplets form a wedge shaped flow path, open at the leading edge of the blade and forming a scoop for the medium of the tank into the flow path, the flow path also has a section which diverges and opens at the trailing edge of the blade. 
     FIG. 4 is a front elevation of the tip region of the blade shown in FIG. 3; 
     FIG. 5 is a view similar to FIG. 2 showing a side elevation of the tip region of the blade illustrated in FIGS. 3 and 4; 
     FIGS. 6,  7 ,  8  and  9  are diagrammatic views similar to FIG. 3, and showing the action of the proplets in breaking up bubbles or droplets to facilitate the dispersion of a gas or liquid into the liquid in the tank which is being mixed or agitated and pumped by the impeller; 
     FIGS. 10,  11  and  12  are views similar to FIGS. 3,  4  and  5  showing proplets in accordance with another embodiment of the invention where the diverging section of the flow path shown in FIGS. 3-5 is not used and where the leading edges of the proplet are in alignment with the leading edge of the blade; 
     FIGS. 13,  14 , and  15  are views similar to FIGS. 3,  4  and  5  showing another embodiment of the invention where the proplets are formed from a single piece of material and provide a covered section in the vicinity of the trailing edge of the blade; 
     FIGS. 16,  17  and  18  are views of another embodiment of the inventions and are similar to FIGS. 3,  4  and  5 , and showing the trailing edges of at least one of the proplets is serrated (scalloped) so as to provide an additional shear inducing and dispersing region; 
     FIGS. 19,  20  and  21  are also views similar to FIGS. 3,  4  and  5  where the outer one of the proplets is disposed radially outward of the tip of the blade and is connected to the other proplet; 
     FIG. 22 is a plan view of an impeller in accordance with another embodiment of the invention having a pair of closely adjacent blades which form a wedge shape flow path, wider at the leading edge than at the trailing edge of the blades; 
     FIG. 22A shows a blade of an impeller such as shown in FIG. 22 with a serrated, and particularly a scalloped, trailing edge to enhance the dispersion of the bubbles which are sheared by the wedge shaped Venturi structures formed by the blade and by an auxiliary blade or blade segment such as shown in FIGS. 23 and 23A; 
     FIG. 23 is a sectional view along the line  23 - 23  in FIG. 22; and 
     FIG. 23A is a fragmentary perspective view illustrating a pair of Venturi structures provided by blade segments near the tip and extending radially inwardly along the trailing edge of a blade of the impeller shown in FIG.  22 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2 there is shown a mixing impeller  10  having three blades  12 ,  14  and  16  which are welded to a hub  19 . The hub is mounted on an impeller shaft (not shown) alone or with other impellers. The shaft may be rotated about its axis so that leading edges  18  of the blades first intercept the fluid being mixed. The fluid is pumped by the impeller in an axial direction, which is downward as viewed in FIG. 1, that is in a direction along the axis of rotation of the impeller  10 . The blades have trailing edges  20  and may have camber and twist and be disposed so as to provide an angle of attack to obtain efficient axially flow from the impeller. The blade design may be in accordance with the above-referenced U.S. Weetman U.S. Pat. No. 4,468,130, but the invention may use other and different axial flow impeller designs such for example as shown in the other above-referenced Weetman and Weetman et al. Patents. The blades have tips  30  at the radially outward ends thereof. 
     Structures, which create shear fields in the vicinity of the tips  30  of the blades where the velocity of the blades is highest, are provided by pairs of proplets  32  and  34 . The proplets  32  and  34  are tilted toward each other to define a flow path which converges in a direction toward the trailing edges of the blades. The width of the flow path, that is the separation of the proplets in the radial direction is larger at the entrance of the flow path that is near the leading edge  18  is larger than near the trailing edge. Preferably the entrance width is approximately three percent (3%) of the impeller diameter (twice the radius of the individual blades between the axis of rotation and the tip  30 ) and the width is approximately one percent (1%) of the impeller diameter at the exit of the flow path, which is near the trailing edge of the flow path. The width of the path is dictated by the nature (examples being density, viscosity and surface tension) of the fluid which is to be dispersed by the impeller and may be determined experimentally by selecting different flow path dimensions, that is different sizes of the wedge formed between the proplets  32  and  34  by running the system with the media, that is the liquid and fluid to be dispersed into and mixed with the liquid, for a period of a few hours and measuring the droplet or bubble size which is produced by the impeller. The wedge size, and entry and exit opening widths, providing finest droplets or bubbles is then determined to be optimum. 
     In the embodiment shown in FIGS. 1 and 2, the outer proplet has a front section  38  which tapers rearwardly and is spaced forwardly of the leading edge of the inner proplet  34 . This structure forms a scoop which collects the medium to be dispersed, which may be moving radially of the impeller, and directs that fluid into the wedge shaped flow path. 
     The structure providing the wedge shaped flow path provides a Venturi which restricts the flow thereby shearing the fluid to be dispersed into fine droplets or bubbles which are released at the exit end of the Venturi and dispersed in a cloud due to the different pressure downstream of the Venturi then in the constricted part thereof. 
     To facilitate the manufacture of the proplet structure the outer one of the proplets may be aligned with the tip  30  while the inner proplet  34  is tilted toward the outer proplet. The proplets may be attached to the blades by welding or other suitable attachment means. 
     Referring to FIGS. 2A and 2B there is shown a proplet structure using two proplets  32  and  34  which form the wedge shaped flow passage as shown in FIGS. 1 and 2. The proplets extend both above the suction  22  of the blades  12  and below the pressure sides  24  thereof. The blades sides may be flat or concave and convex as shown in FIG.  2 A. The proplets and the flow passageway may be on the suction side or the pressure side or both, depending upon the nature of the fluid medium being dispersed. A series of proplet panes (wedges) may also be spaces along the radius of the blades. The proplets are most advantageously disposed where the blade velocity is the greatest (at the tip). Lower density fluid, such as gases, usually collects on the suction side of the blade. 
     Referring to FIGS. 3,  4  and  5  there is shown an arrangement of proplets  40  and  42 , the outer one of which is disposed along the tip  30  of the blade and follows the tip while the inner proplet  42  converges inwardly toward the outer proplet  40  to a neck  44  and then flares outwardly in a section of the proplet structure  46  which extends beyond the trailing edge  20  of the blade. This flared section  46  directs and aids in dispersing droplets or bubbles which are sheared in the Venturi flow passage defined by the proplets  40  and  42 . 
     The action of the Venturi provided by the wedge shaped flow passage between the proplets  40  and  42  is shown in FIGS. 6 through 9. The arrow  50  indicates the direction of rotation of the blade  12 . Large bubbles or droplets are collected at the entry end of the wedge shaped flow passage and are squeezed into elongated shape in the front of the passage. At the neck or at the exit end where there is no flared section  46 , the shear field breaks the elongated bubbles into small or fine bubbles or droplets  54  which disperse at the exit of the flow passage, as illustrated by the droplet leaving the exit end in FIG.  9  and the droplets which have already left the exit end. Thus, the large droplet or bubble  52  is broken into a plurality of fine bubbles or droplets  54 . 
     Referring to FIGS. 10,  11  and  12  there is shown a pair of proplets  56  and  58  which are similar in size and shape and which extend between the leading edge  18  and extend past the trailing edge  20  thereby providing a longer flow passage than in the other above illustrated embodiments and locating the highest shear region past the blade  12 . 
     FIGS. 13,  14  and  15  illustrate a pair of proplets  60  and  62  which may be formed of three or of one piece of plate material and bent into a generally U shape having a cover  64  at the trailing end thereof. This covered trailing end may extend beyond the trailing end  20  of the blade. The cover encloses the high shear filed region of the wedge shaped Venturi. 
     FIGS. 16,  17  and  18  show a proplet structure similar to that shown in FIGS. 3,  4  and  5 . The proplets  66  and  68  are serrated along their trailing edges. The serrations provide a scalloped trailing edge  70  in the design illustrated in FIGS. 16 through 18. The serrations or scallops enhance the dispersion of the droplets as they leave the Venturi passageway. 
     In the previously described embodiments, the proplets are inboard of the blade with the outer proplet at the tip and the inner proplet spaced radially inwardly thereof. In FIGS. 19,  20  and  21  the inner proplet  72  is disposed along the tip of the blade and forms an angle with an outer proplet  74 . The proplets are held together as by a pin or pins  76 . This arrangement places the shear field outwardly beyond the tip of the blades and benefits from the increased impeller velocity at this outboard location. 
     Referring to FIGS. 22 and 23 there is shown an impeller  80  having three blades  82 ,  84  and  86  on a hub  88 . The hub is attached to a shaft  90 , so that the impeller rotates with the shaft. The connection of the blades at the hub end thereof to the hub may be by means of a weld  92 . 
     In order to create a shear field, a blade  94  is disposed below the blade  84 . Alternatively, the blade  94  may be above the blade  84 . The blades  84  and  94  have different blade angles; that is, the angle which the chord of the blades makes with a horizontal plane blade perpendicular to the axis of rotation of the impeller  80 . The blades  84  and  94  form a generally wedge shaped passageway which provides the Venturi. The Venturi generates the shear field which breaks up bubbles and controls bubble and droplet size as was explained in connection with FIGS. 6 through 9. The blades, at least in a portion of the trailing edges  96  thereof have a serrated or scalloped edge  98 , as shown in FIG. 22A, to facilitate dispersion of the bubbles or droplets as they leave the wedge shaped Venturi formed between the blades  84  and  94 . The blades may be held together by pins  100  or other means for minimal obstruction with the flow path between the blades. 
     Referring to FIG. 23A there is the tip end  104  shown a blade  102 . This blade rotates so that one edge  106  is the leading edge of the blade, while the edge  108  is the trailing edge of the blade. Over a portion of the blade extending from the tip end  104  radially inward along the trailing edge  108  are a pair of plate segments which have curvatures similar to those of the blade  102 . Each of these segments defines a separate Venturi with the upper and lower surfaces of the blade. The Venturi establishes a high shear at the trailing end thereof for bubble and droplet shearing and dispersion so as to provide a fine stream of bublets or droplets which are mixed by the impeller. 
     From the foregoing description it will be apparent that there has been provided improved mixing impellers especially suitable for sparging applications. Variations and modifications in the herein described mixing impellers will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.