Fluid mixing apparatus and method
The present invention relates to an apparatus and method for mixing distinct fluids to obtain an essentially homogeneous mixture. 
     German Offenlegungsschrift No. 25 29 987, and corresponding U.S. Pat. No. 4,140,299 describe a mixing device which comprises a cylindrical body disposed in a cooperating casing, with the body having a plurality of helical grooves on the external surface. A large amount of a first liquid is adapted to be fed axially into the casing, and a small quantity of a second liquid is adapted to be fed through an inlet which is radially disposed with respect to the cylindrical body. Where it is desired that the second liquid be highly dispersed to facilitate its mixture with the first liquid, a plurality of concentric annular bodies are provided, together with a plurality of radial inlets associated with each of the annular bodies. This construction results in a very complicated design, and the device must be disassembled into several components to permit cleaning, which is a time consuming and tedious operation. Further, flow conditions in the mixing chamber are not fully controllable, since the liquid delivery lines run through the mixing chamber. 
     It is accordingly an object of the present invention to provide a fluid mixing apparatus and method which effectively avoids the above noted disadvantages. 
     It is a further object of the present invention to provide a fluid mixing apparatus and method which is able to achieve controlled, reproduceable flow conditions, which provides good mixing results, and wherein the apparatus may be readily cleaned. 
     These and other objects and advantages of the present invention are achieved in the embodiment illustrated herein by the provision of a mixing apparatus which includes a housing, a sleeve fixedly mounted in the housing and having an inner cylindrical surface which defines a mixing chamber therewithin, and a mixing head rotatably mounted in the sleeve, with the mixing head having an outer cylindrical surface which is in close proximity to the inner surface of the sleeve. The mixing head further includes at least one helical groove formed in the outer surface thereof. Also, the apparatus includes first duct means including a plurality of openings extending radially through the sleeve for conveying a first fluid radially into the mixing chamber, and additional duct means for conveying an additional fluid into the mixing chamber. Upon rotation of the mixing head, the first fluid which radially enters the mixing chamber is adapted to be sheared by contact with the grooved exterior surface of the rotating mixing head, to form a fine dispersion which is mixed with the additional fluid. 
     In the preferred embodiment, the mixing apparatus further includes a second mixing chamber positioned downstream of the first mixing chamber, and the additional duct means includes a plurality of additional openings extending radially through the sleeve for conveying the additional fluid radially into the mixing chamber in a manner equivalent to that of the first fluid. 
     The openings in the sleeve may take the form of a plurality of distinct, radial holes, which are arranged in axially extending sets of equally spaced apart openings. Alternatively, the sleeve may be composed of a porous sintered metallic material, with the pores of the material defining the openings. In this latter embodiment, the fluids which are fed through the sleeve are suppplied to the mixing head in a highly dispersed, partly pre-mixed condition, since the fluids are divided into myriad streams and tend to mix with one another during passage through the wall of the sleeve. The average size of the pores in the porous metallic material typically ranges between 5 and 200 microns. 
     The width of the space formed between the sleeve and the mixing head typically ranges between about 0.2 and 5 mm. A more narrow space provides an increased shearing effect, whereas a wider space provides an increased remixing effect as further described below. The shearing effect is achieved by reason of the fact that the streams of the fluids emerging from the sleeve are sheared between the sleeve and the mixing head, and with the mixing head rotating at high speeds, a fine dispersion and thorough mixing of the stream may be achieved. For this reason, the mixing apparatus of the present invention permits the addition of fluid components to a variety of materials which can be sheared, such as polymer melts, greases, waxes, pastes, as well as other liquids and gases. 
     The re-mixing effect may be achieved where the mixing head conveys in the conveying direction a substantially greater amount of the fluid than can be discharged through the exit of the mixing apparatus. On the other hand, the re-mixing effect can also be achieved if the mixing head is rotated so as to convey the fluid in opposition to the direction of flow, as described in the above noted German and U.S. patents. 
     It is also preferred that each of the first and additional duct means for the fluids includes a plurality of axially extending channels formed in the housing and communicating with the associated openings at the outer surface of the sleeve. Thus for example, where two components are to be mixed, the axial channels may be arranged such that each channel is connected to one of the component feed lines, and so that the components, after being dispersed within the sleeve, are equally distributed over the circumference of the mixing head. Alternatively, the channels formed in the housing may extend in a circumferential direction. 
     The mixing apparatus according to the present invention is particularly adapted to permit a very small quantity of one liquid to be added to a main liquid stream, for example, at a ratio of 100 to 1. In such case, the main liquid stream is not conveyed radially through openings of the sleeve, but rather may be conveyed axially into one end of the mixing chamber defined by the sleeve. 
    
    
     Some of the objects having been stated, other objects and advantages of the invention will appear as the description proceeds, when taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a sectional elevation view taken through a mixing apparatus which embodies the features of the present invention; 
     FIG. 2 is a sectional view taken substantially along the line A--A of FIG. 1; 
     FIG. 3 is a sectional view taken substantially along the line B--B of FIG. 1; 
     FIG. 4 is an exploded perspective view of the mixing head, sleeve, and housing of the mixing apparatus shown in FIG. 1; 
     FIG. 5 is a sectional elevation view of another embodiment of a mixing apparatus according to the present invention. 
    
    
     Referring more particularly to the drawings, FIG. 1 illustrates a mixing apparatus according to the present invention, and which comprises a grooved mixing head 1 which is fixed on the rotor shaft 2, with the head being fixedly connected to the shaft by means of a key 16, note FIG. 2. The grooves extend helically on the outer cylindrical surface of the mixing head, at an angle of between about 10 and 80 degrees with respect to the axial direction. In the illustrated embodiment, the grooves are disposed at an angle of about 45 degrees. The shaft 2 is rotatably mounted in the mixer housing, which comprises the interconnected components 3 and 4, and cover 5. 
     As best seen in FIG. 4, the housing component 4 includes a generally cylindrical internal cavity. A first group of circumferentially spaced, axially extending channels 6 are formed in the wall of the cavity, and a second group of like channels 7 are formed in the wall, with the channels 6 and 7 being alternately arranged about the entire circumference of the cavity. The feed ducts for the components X and Y run through the housing component 4, and the housing cover 5, with the channels 6 being connected through an annular groove 8 to the line 10 feeding the component X, and the channels 7 being connected through the annular groove 9 to the line 11 feeding the component Y. 
     A sleeve 12 is fixedly mounted in the cavity of the housing by a press fit or the like, and the sleeve has an inner cylindrical surface which defines a mixing chamber therewithin. The mixing head 1 is coaxially disposed within the inner surface of the sleeve 12, and the outer cylindrical surface of the head is in close proximity to the inner surface of the sleeve, with the space between the inner surface of the sleeve 12 and the outer surface of the head being indicated at S. Also, the housing components 3 and 4 and the cover 5 are fixedly connected to each other by threaded bolts 13, and nuts 14. 
     The shaft 2 is sealingly fitted in the housing cover 5, and the housing component 4 is followed by a generally conventional &#34;pin mixer&#34; which serves as a second mixing chamber and which is positioned in the housing component 3. The pin mixer is thus disposed coaxially downstream of the mixing chamber defined by the sleeve 12, and the pin mixer is operatively connected to such first mixing chamber through holes 15. The structural components of the pin mixer are generally known in the art, and therefore are not described herein in more detail. 
     FIG. 4 is an exploded view of the mixing head 1, the sleeve 12, and the housing component 4, and illustrates the embodiment wherein the sleeve 12 is provided with a plurality of distinct, radial openings 17. More particularly, the openings 17 are arranged in axially extending sets of equally spaced apart openings, with each set being in communication with one of the channels 6 or 7. This view further illustrates the lines 10 and 11, and the manner in which the components move through the annular grooves 8 and 9 to the axial channels 6 and 7 respectively. The components X and Y are supplied to the mixing head 1 via the distinct radial openings 17 in the sleeve 12. In this regard, it will be understood that the size of the openings 17 may vary from set to set, with the size being determined by the viscosity of the particular component and the desired flow rate. 
     A specific example of the operation of the mixing apparatus will now be described. In the described example, the sleeve 12 comprises a porous metallic material, and the component X comprises a polypropylene having a viscosity of 7000 Pas, a temperature of 235° C., and a flow rate of 12 g/min., and the component Y is a fatty amine with a viscosity of 0.03 Pas, a temperature of 180° C. and a flow rate of 4 g/min. The components are fed to the apparatus by suitable gear pumps or the like. The polypropylene is supplied to the porous sleeve 12 via the feed line 10, the annular groove 8, and the axial channels 6, whereas the fatty amine reaches the sleeve via feed line 11, annular groove 9, and axial channels 7. Upon entering the porous sleeve, the individual streams of the components are divided into even smaller individual streams, and during passage through the wall of the sleeve, these streams come into contact with each other, are partly mixed, and then again divide. Thus the individual streams emerging from the porous metal sleeve are differently composed, and for this reason, the liquid flowing into the mixing space between the head 1 and the sleeve 12 is referred to below as the &#34;medium&#34;. The surface of the grooved mixing head is provided with helical grooves arranged at an angle of 45 degrees, and is spaced a distance of about 500 microns from the bore of the sleeve. The head is rotated in such a manner that the medium emerging from the sleeve 12 is sheared immediately upon entering the mixing chamber, and at a point immediately adjacent the wall of the sleeve. The sheared medium is conveyed in the direction of the holes 15 by the force of the transverse shearing operation at a velocity which is greater than that of the medium passing through the holes. Thus the medium in the mixing chamber exerts a force on the streams entering the chamber which is in a direction opposite the direction of the force applied during the transverse shearing step. For this reason, part of the medium is forced to flow back into the sleeve 12. The advantage of this shearing effect and the repeated mixing of the medium flowing against the conveying direction, is that the medium is mixed very thoroughly. This mixing effect is further enhanced by the mixing which takes place in the second mixing chamber in the housing component 3, which is downstream of the first mixing chamber. As will be apparent, this second mixing chamber may take a variety of different forms. 
     The medium which leaves the illustrated pin mixer had a viscosity of 40 Pas and a temperature of 200° C. The mixture was essentially homogeneous, and had the properties of a melt. Even where the amount of polypropylene contained in the mixture was reduced to 20 percent, this mixture will be essentially homogenous, and still have the properties of a melt. 
     The embodiment of FIG. 5 generally corresponds to the embodiment of FIG. 1, with the exception that a main component Z may be axially fed into the mixing chamber between the sleeve 12 and the mixing head 1 via a channel 20 in the cover 5, and an annular channel 21 in the housing component 4. The other components X and Y are radially fed into the space via openings provided in the sleeve 12. This embodiment permits the feeding of a main stream of a liquid, for example a polymer melt, with small quantities of one or more liquid components which are to be added to the main liquid stream. 
     In the drawings and specification, there has been set forth preferred embodiments of the invention, and although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.