Abstract:
An apparatus and method that more effectively reduces fluid streaking in a motionless mixer includes a series of baffles, including a flow inversion baffle. The flow inversion baffle acts to redirect fluid from the center of the fluid flow stream to the periphery of the fluid flow stream while at the same time redirecting fluid flow from the periphery of the fluid flow stream to the center. The transposition of fluid flows eliminates the “channeling” effect and reduces streaking in the extruded mixture.

Description:
BACKGROUND 
     A number of motionless mixer types exist, such as Multiflux, helical and others. These mixer types, for the most part, implement the same general principle to mix fluids together. In these mixers, fluids are mixed together by dividing and recombining the fluids in an overlapping manner. This action is achieved by forcing the fluid over a series of baffles of alternating twisted geometry. Such division and recombination causes the layers of the fluids being mixed to thin and eventually diffuse past one another. This mixing process has proven to be very effective, especially with high viscosity fluids. These mixers are typically constructed of a series of alternating baffles, of varying geometries, usually consisting of right-hand and left-hand elements disposed in a conduit to perform the continuous division and recombination. Such mixer types, while effective in mixing together most of the fluid being mixed, have a tendency to leave streaks of unmixed material in the extruded mixture. These streaks result from channels of fluid forming along the interior surfaces of the conduit that pass through the mixer essentially unmixed. 
     It has been found that motionless mixers utilizing baffles that have greater degrees of twist are more effective at reducing this streaking phenomenon (e.g., a mixer using 180° baffles will have less streaking than a mixer that employs only 90° baffles). Using baffles with greater degrees of twist, however, raises its own issues. As the degree of baffle twist increases, the length of the mixer required to maintain flow rates equivalent to mixers with baffles of lesser twist also increases. Such an increase in mixer length is unacceptable in most motionless mixer applications. Users of motionless mixers, typically, need to be close to the work piece they are applying the mixture to. The longer a mixer becomes the less manageable it is to work with. In addition, longer mixers will generally have a higher retained volume, entrapping more fluid when the mixer is disposed. 
     There have been attempts made to maintain adequate mixer length while trying to remedy the streaking problem. Much of this effort has focused on using a combination of mixing baffles of varying degrees of twist (e.g., mixing 90° elements with 180° or 270° elements). In such designs, the bulk of the mixing is done in the baffles of lesser twist (i.e., the 90° elements), which reduces the overall length of the mixer, and then, in the baffles of greater twist (i.e., 180° or 270° elements), the fluid moving along the outer periphery of the mixing elements (i.e., along the interior surface of the conduit) is forced into the center of the mixing elements, but only temporarily. In these arrangements, such fluid is eventually worked back to the outer periphery. Examples of such approaches are described in U.S. Pat. No. 3,239,197 to Tollar and U.S. Pat. No. 5,851,067 to Fleischli et al. While such approaches tend to reduce the width of the streak, the streak is not eliminated. Accordingly, there is a need for a motionless mixer that more effectively reduces streaking while maintaining acceptable mixer length and flow rate. There is also a need to do this in a cost effective manner. 
     SUMMARY 
     According to one aspect of the present invention, a motionless mixer includes a conduit and a flow inversion baffle disposed in the conduit in which the flow inversion baffle has a center to perimeter flow portion, a perimeter to center flow portion and a perimeter flow diverter. Fluids introduced into and flowing within the conduit are mixed together by moving the fluids flowing in the center of the fluid flow to the perimeter of flow and by also moving the fluids from the perimeter of the fluid flow to the center of flow. The mixer may also have a plurality of baffle elements. At least one baffle element may be a right-handed baffle element and at least one other baffle element may be a left-handed baffle element. The baffle elements may be integral with one another, and a sidewall may be formed integral with the baffle elements. The baffle elements may be formed by injection molding. 
     According to another aspect of the present invention, a motionless mixer includes a conduit, at least one flow inversion baffle disposed in the conduit and a plurality of alternating mixing baffles disposed in the conduit. The flow inversion baffle has a center to perimeter flow portion, a perimeter to center flow portion and a perimeter flow diverter. In this aspect of the invention, the center to perimeter flow portion has a chamber wall that defines a center to perimeter flow chamber having an entry and an exit, and the perimeter to center flow portion has a chamber wall that defines a perimeter to center flow chamber having an entry and an exit. The center to perimeter flow portion, the perimeter to center flow portion and the perimeter flow diverter may be integral with one another. The perimeter flow diverter may surround the center to perimeter flow portion and define the entry to the perimeter to center flow chamber. Further, the chamber wall of the perimeter to center flow portion may define an angled baffle adjacent the flow chamber exit. In one aspect of the invention, the alternating baffle elements are right-handed and left handed baffle elements. The alternating right-handed and left-handed baffle elements may have a 90° twist. The conduit of the mixer may be circular, and the flow inversion baffle and the alternating baffle elements may be rounded. The baffle elements may be integral with one another, and a sidewall may be formed integral with the baffle elements. The baffle elements may be formed by injection molding. 
     According to another aspect of the invention, a method of reducing fluid streaking in a motionless mixer includes providing a conduit having an inlet and an outlet, a flow inversion baffle and a plurality of alternating mixing baffles disposed in the conduit. The flow inversion baffle has a center to perimeter flow portion, a perimeter to center flow portion and a perimeter flow diverter. The method farther includes introducing a plurality of fluids to be mixed to the conduit inlet, forcing the fluids through baffles in the conduit and extruding a mixed fluid composition from the conduit outlet. In another aspect of the invention, a method of making a flow inversion baffle includes providing a set of forming tools that define the structure for a flow inversion baffle having a center to perimeter flow portion, a perimeter to center flow portion and a perimeter flow diverter, setting the forming tools to form a flow inversion baffle mold and injecting plastic resin into the flow inversion baffle mold to form a flow inversion baffle. According to another aspect of the invention, a method of making a baffle assembly includes providing a set of forming tools that define the structure for a flow inversion baffle having a center to perimeter flow portion, a perimeter to center flow portion and a perimeter flow diverter and that further defines a plurality of alternating mixing baffles, setting the forming tools to form a mold for a baffle assembly having a flow inversion baffle and plurality of alternating mixing baffles and injecting plastic resin into the baffle assembly mold to form a baffle assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 depicts an embodiment of a motionless mixer of the present invention with a portion of the mixer sidewall removed; 
     FIG. 2 is an isometric view of interconnected baffles; 
     FIG. 3 is an isometric view of a 90° right-handed baffle employed in the mixer of FIG. 2; 
     FIG. 4 is an isometric view of a 90° left-handed baffle employed in the mixer of FIG. 2; 
     FIG. 5A is an isometric view of an embodiment of a flow inversion baffle of the present invention; 
     FIG. 5B is a top view of an embodiment of a flow inversion baffle of the present invention; 
     FIG. 5C is a cross-section along the line  5 C— 5 C of FIG. 5B; 
     FIG. 5D is a top view of injection molding tooling for forming an embodiment of a flow inversion baffle of the present invention; 
     FIG. 5E is a cross-section along the line  5 E— 5 E of FIG. 5D along with the cross-section of a formed flow inversion baffle; 
     FIG. 5F is a cross-section along the line  5 F— 5 F of FIG. 5D along with the cross-section of a formed flow inversion baffle; 
     FIG. 6 illustrates the fluid mixing process in an embodiment of the invention; 
     FIG. 7A is an isometric view of another embodiment of a flow inversion baffle of the present invention; 
     FIG. 7B is a top view of the embodiment of a flow inversion baffle depicted in FIG. 7A; 
     FIG. 7C is a cross-section along the line  7 C— 7 C of FIG. 7B; 
     FIG. 8A is an isometric view of another embodiment of a flow inversion baffle of the present invention; 
     FIG. 8B is a top view of the embodiment of a flow inversion baffle depicted in FIG. 8A; 
     FIG. 8C is a cross-section along the line  8 C— 8 C of FIG. 8B; 
     FIG. 9A is an isometric view of another embodiment of interconnected baffles; 
     FIG. 9B is an isometric view of a 90° right-handed baffle of the embodiment depicted in FIG. 9A; and 
     FIG. 9C is an isometric view of a 90° left-handed baffle of the embodiment depicted in FIG.  9 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an embodiment of a mixer  10  of the present invention includes a conduit  12  defining an interior wall  13 , an inlet  14  and an outlet  16 . The mixer  10  further includes a series of alternating left-handed baffles  18 , right-handed baffles  20  and one or more flow inversion baffles  21 . The mixer  10  depicted in FIG. 1 is a twenty-four stage mixer having twenty-four baffle elements  18 ,  20 ,  21 . There are eleven right-handed baffles  18 , eleven left-handed baffles  20  and two flow inversion baffles  21 . The baffles  18 ,  20 ,  21  are disposed within the conduit  12  along a central, longitudinal axis X along which inserted materials flow in a flow direction F. The left-handed and right-handed baffles  18 ,  20  are mirror images of one another. The baffles  18 ,  20  are provided with two forward, angled surfaces  22  and two rear, angled surfaces  23  (FIGS.  2 - 4 ). The front angled surfaces  22  and rear angled surfaces  23  are connected by two planar webs  24 ,  27  that intersect one another. In a preferred embodiment, all of the baffles (i.e., left-handed  18 , right-handed  20  and flow inversion  21 ) are formed together as an integral string and are further integral with a pair of opposing sidewalls  15  to form a baffle assembly  26 . The sidewalls  15  provide support and rigidity to the baffle assembly  26  during insertion of the assembly  26  into the conduit  12  and during operation of the mixer  10 . 
     Referring to FIGS. 2-4, a portion of an embodiment of a baffle assembly  26  including right and left-handed baffles  18 ,  20  is depicted. Referring to FIG. 3, the right-handed baffle  20  is provided with a first, generally planar web  24  that has opposing sides  24   a  and  24   b  and a second, generally planar web  27  having opposing sides  27   a  and  27   b . The webs  24 ,  27  extend generally parallel to the flow direction and intersect one another. The right-handed baffle  20  is also provided with a first, forward surface  22  wherein the surface  22  is perpendicular to one side of the web  24   a  and at an angle to a plane perpendicular to the material flow. A second, forward surface is shown in FIG. 3 wherein the surface  22  is perpendicular to the side of the web  24   b  at an angle to a plane that is perpendicular to the material flow. FIG. 3 also shows a first, rear surface  23  wherein the surface is perpendicular to one side of the web  27   b  and at an angle to a plane that is perpendicular to the material flow. The right-handed baffle  20  also has a second, rear surface  23 . The second, rear surface  23  is perpendicular to the side of the web  27   a  and at an angle to a plane that is perpendicular to the material flow. In addition, one of the webs  24 ,  27  extends past the rear angled surfaces  23  to form a rear fin  25  that extends in the flow direction. 
     FIG. 4 is a detailed view of a baffle designated as a left-handed baffle  18 . The left-handed baffle  18  is formed as a mirror image of the right-handed baffle  20  shown in FIG.  3 . Embodiments of the invention may be formed from baffle elements employing geometries differing from those described above. 
     Referring to FIGS. 5A,  5 B and  5 C, an embodiment of a flow inversion baffle  21  of the present invention is depicted. The flow inversion baffle  21  includes a center to perimeter flow portion  30  and a perimeter to center flow portion  32 . In the embodiment depicted, the center to perimeter flow portion  30  is integral with the perimeter to center flow portion  32 . The flow inversion baffle  21  includes a perimeter flow diverter  34  that surrounds the center to perimeter flow portion  30  and defines an entry  36  to a perimeter to center flow chamber  48 . The perimeter flow diverter  34 , in this embodiment, is integral with the mixer sidewalls  15  and, when inserted in the conduit  12 , also contacts the conduit wall  13 . As described in detail below, the perimeter flow diverter  34  acts to direct all fluid from along the periphery of the baffle assembly  26  into the perimeter to center flow chamber entry  36 . The center to perimeter portion  30  includes a chamber wall  38  which defines a center to perimeter flow chamber  40  having an entry  42  and an exit  44 . The perimeter flow diverter  34  surrounds and is integral with the chamber wall  38 . The perimeter to center flow portion  32  also includes a chamber wall  46  which defines the perimeter to center flow chamber  48 . The perimeter to center flow chamber  48 , in addition to the entry  36 , has an exit  52 . The perimeter to center flow portion  32  may further include an angled baffle  54  to aid in the flow inversion process. The dimensions of the flow inversion baffle  21 , and in particular the dimensions of the center to perimeter flow chamber  40  and the perimeter to center flow chamber  48 , may obviously be varied to accommodate the application of use and/or the production or molding of the baffle. In a preferred embodiment, the flow inversion baffle  21  is made by an injection molding process. Referring to FIGS. 5D-F, exemplary injection molding tooling for an embodiment of a flow inversion baffle  21  is depicted. The molding tooling for this embodiment includes a first tool plate  56  and a second tool plate  58 . The tool plates  56 ,  58  define the structure for the flow inversion baffle  21  to be formed. FIGS. 5E and 5F illustrate the cross-section of a flow inversion baffle  21  formed using tooling plates  56 ,  58 . The flow inversion baffle  21  of the depicted embodiment is designed such that the chamber walls  38 ,  46  have an open top and bottom, respectively. This design accommodates the injection molding process. By having an open top chamber wall  38  and an open bottom chamber wall  46 , the tool plates  56 ,  58  can be brought together and aligned in a relatively simple fashion to form a flow inversion baffle mold. With the mold formed, the creation of the flow inversion baffle  21  is a relatively simple process known in the art of injecting the plastic resin into the mold and allowing it to cool and form. It should be understood that the entire baffle assembly  26 , including left and right-handed baffles  18 ,  20  and flow inversion baffles  21 , could be injection molded together at one time. 
     Referring to FIG. 6, the mixing characteristics of a right-handed baffle  20  and a flow inversion baffle  21  of the embodiment of the mixer  10  described above are depicted. Two fluids  60   a ,  60   b  are introduced into the mixer  10  for mixing. (The fluid  60   b  has been spot marked along the outer edge to track the mixing of the fluids where channeling typically occurs.) As the two fluids  60   a ,  60   b  intersect the leading edge of the right-handed baffle  20 , at point  62 , the fluid flow is divided in half. As the divided fluid continues to flow through the right-handed baffle  20 , the material is shifted laterally by the sub-surfaces of the right-handed baffle  20  at point  64 . At point  66 , as the fluid leaves the trailing edge of the right-handed baffle element  20 , the now mixed fluids stretch to occupy the open space in the baffle assembly  26  within the conduit  12 . From the right-handed baffle  20 , the mixed fluid continues to flow into the flow inversion baffle  21 . As indicated at point  70 A, the mixed fluid moving in the interior of the stream is captured by the wall  38  and directed into the center to perimeter flow chamber  40  through the entry  42 . The mixed fluid outside of the wall  38  makes contact with the perimeter flow diverter  34 . As indicated at points  70 B and  70 C, as the fluid continues to flow through the mixer  10 , the fluid in contact with the perimeter flow diverter  34  moves up the perimeter flow diverter  34 , and the fluid captured within the center to perimeter flow chamber  40  exits the center to perimeter flow chamber  40  and expands outward towards the perimeter of the baffle assembly  26  and conduit  12 . As indicated at points  70 C and  70 D, as the fluid continues to flow through the mixer  10 , the fluid in contact with the perimeter flow diverter  34  is directed into the perimeter to center flow chamber  48 . As point  70 D indicates, the fluid captured in the perimeter to center flow chamber  48  flows through the chamber  48 . At the same time, the fluid that exited the center to perimeter flow chamber  40  and expanded is forced by the chamber wall  46  and the sidewall  15  upward around the chamber wall  46 . At point  70 E, the fluid from the perimeter to center flow chamber  48  exits the chamber  48  into the center of the fluid mixing stream ( 72 ) surrounded by the fluid that exited the center to perimeter flow chamber  40  ( 72 ). As the marking spot indicates, the spot is now mixed within the interior of the fluid flow, and the streaking caused by the “channeling” effect is eliminated. 
     Referring to FIGS. 7A-7C and  8 A- 8 C, other embodiments of flow inversion baffles  21  are depicted. In the embodiment in FIGS. 7A-7C, the center to perimeter flow chamber entry  42  and the perimeter to center flow chamber exit  52  are positioned in line with the flow direction F. In the embodiment in FIGS. 8A-C, the flow inversion baffle  21  is rounded to fit in a round or circular conduit  12 . FIGS. 9A-9C depict another embodiment of interconnected baffles in which the baffle elements are rounded. The baffle arrangement depicted in FIGS. 9A-9C could be combined with the flow inversion baffle  21  depicted in FIGS. 8A-C to form a baffle assembly  26  for use in a round or circular conduit  12 . 
     While the invention has been discussed in terms of preferred and specific embodiments, it should be appreciated by those of skill in the art that the invention is not so limited. The embodiments are explained herein by way of example, and there are numerous modifications, variations and other embodiments that may be employed that would still be within the scope of the present invention. The baffles, for instance, could employ a helical design as opposed to the embodiments described herein.