Static mixer

Abstract: A static mixer for mixing of different media comprises mixing elements arranged within a tube, at which the mixing elements are twisted to divide and divert the media. The mixer is constructed of units each comprising at least one mixing element (1) arranged within a tube part (4). The mixing element is firmly connected to the tube part along its whole length of extension. The mixing elements seen in a cross-section has wing shape with the largest cross-section in the front part (2) of the element seen in the direction of flow of the media, while the rear part (3) seen in the flow direction has a thinner cross-section.

Description: The present invention relates to a static mixer for continuous mixing of different media. Such mixers are commercially available and are used for mixing liquid/liquid or gas/liquid. The geometrical design of the mixers may vary but the main principle for the mixing is mainly the same. The mixer comprises stationary (unmovable) mixing elements and the media which are to be mixed are pumped to and through the mixer. Static mixers were from the beginning developed mainly for laminar flow and in such a flow mixing takes place because the mixing elements divide and divert the steams of different media a number of times in order to obtain a mixing between the media. The necessary energy for the mixing is taken from the pumping energy. The number of mixing elements that are needed in order to obtain a desired degree of mixing depends to some extent on the mixing ability of the media. A higher number of elements are needed for a more difficult mixing duty. Early examples of static mixers are described for example in US 3 051 452 and US 3 206 170. In the first mentioned patent there is described a static mixer which divides a flow through a tube into two concentric flows which are forced to flow radially inwards and outwards while the flows are divided into different partial streams. US 3 206 170 describes mixing of two flows which are brought to flow through a number of elements with rectangular channels which are widened and divided into new channels which are widened and so on. A static mixer with a low pressure drop is described in US 3 286 992. This mixer comprises twisted mixing elements arranged within a tube. The mixing elements consist of discs which are twisted 60-210°. The discs extend to the walls of the tube and divide the same into two separate channels. Each twisted disc is connected to the next disc. Static mixers with similar twisted elements are described in for example GB 1 386 955 and GB 2 127 316. DE-A-3 539 426 describes a static mixer in which several axially adjacent twisted mixing elements are disposed within a tube, each of the mixing elements being formed from an essentially rectangular plate of constant thickness having holes therein or notches to define holes in the tube. US-A-4 099 268 discloses a static mixer in which a tube houses a series of mixing elements which are not twisted. The elements are flat on one side and have a shallow V-form on the other side with the front edge being rounded, so that different flow rates are obtained either side of the elements. The static mixers described above work well for clean liquids but for liquid products comprising particles problems may arise as a consequence of the fact that the twisted mixing elements have sharp edges which may create blocking and problems with cleaning. This applies even to a greater extent if the product consists of food products containing fruit or vegetables which may be damaged by too sharp edges. The present invention presents a solution to the problem of mixing different kinds of food products. According to the invention there is provided a static mixer comprising a tube and twisted mixing elements mounted in the tube for dividing and diverting media flowing through the tube, whereby the mixer comprises at least two units each including a tube part with at least one mixing element secured therein, said mixing element spans the diameter of the tube part over substantially the entire length of the element, and is connected firmly to the tube part over substantially the entire length of the element, the mixing element has a rounded leading edge, and the thickness of said mixing element is greatest adjacent said leading edge and reduces gradually away from the leading edge. With such a design of the mixing elements there is reduced risk of particles in a liquid product passing the mixer being damaged in the same. Due to the fact that the mixer is constructed with soft edges cleaning will be safer. The mixer may also easily be adapted to each given mixing duty in that the number of units and consequently the mixing elements may vary. Since the mixing elements are permanently joined to the tube part along the length of the elements, the flow ways for the products are smooth and easily cleaned. As has been mentioned above this is of value especially for products comprising fibres. A static mixer with the described "flow friendly" mixing elements may, even if including many mixing elements, have a relatively low pressure drop. Usually the mixing in the proposed mixer takes place under laminar flow as the mixer is especially suitable to highly viscous products. Of course the mixing may also take place under turbulent flow if it is suitable for the product. The units are with advantage designed such that the mixing elements are arranged at a distance from the end of the tube parts, in such a way that the free space is created between the mixing elements when the units are assembled together in a mixer. Such a design diminishes the risk of particles becoming caught. The two flows which are to be mixed with each other may meet in a common pipe either immediately prior to the inlet to the mixer or at some distance from the same. It must not be a disadvantage to locate the mixing point at some distance from the inlet to the mixer in such a way that some tube bends are passed by the two not yet mixed flows. Each mixing element may be twisted 75-135° but a suitable twisting is 80-100°. To little twist does not give a sufficient diversion of the flows which are to be mixed. Too much twist gives a poorer utilisation of the available space. The mixing elements are with advantage arranged such that every second mixing element is twisted clockwise and the other element is twisted counter clockwise. The desired diversion of the partial flows, is obtained when the products being mixed meet the front part of the mixing element with rounded leading edge. The mixing element is with advantage designed such that it has a mainly semi-circular profile at its leading edge. In this way there is obtained a soft diversion of the product mixture which is pumped through the tube. A gentle handling of the product mixture is also obtained if the mixing elements in cross-sectional profile have a continuously narrowing shape. The number of tube parts including mixing elements may be adjusted to the mixing duty but is suitably 5-15 mixing elements, preferably 10-12 mixing elements. A highly viscous product demands a higher number of mixing elements. By constructing the mixer with a desired number of elements it is, for example, possible to obtain an incomplete mixing if, for example, one wants to be able to see one of the products within the other. In order to fulfil such a duty only a limited number of elements are needed. The tube parts are with advantage provided with guiding means in order to ensure correct assembly. These guiding means may consist of notches and projections within end flanges. Of course some kind of guide pins may also be used. A mixer according to the invention may also be designed with double tube walls where the inner tube wall is perforated, which makes it possible to inject a heating medium such as steam, directly into the products which are being mixed. A mixer of the kind described above may of course be used for many mixing duties but as has been mentioned above it is especially suitable for different kinds of food products comprising particles, such as long fibres, spheres, cubes, rods and so on. These fibres may consist of fruit flesh or vegetable parts from orange, pineapple, mango, asparagus or tomato. The spherical particles may consist of peas, berries or small meat balls. The cubes may consist of cut carrots, potatoes or meat, and the rods may be wax beans, asparagus parts, pasta or noodles. The mixer according to the invention is described further with reference to the attached drawings, in which Fig 1 shows a mixing element according to the invention prior to the twisting, Fig 2 shows a mixing element after one of its parts has been twisted 90° in relation to the front part, Fig 3 shows a unit of a static mixer according to the invention seen in four different sections, Fig 4 shows how these units are connected to form a part of a static mixer according to the invention, Fig 5 shows how the mixer may be provided with a surrounding jacket in order to function as a steam injection heater simultaneously with the mixing. A mixing element 1 for a mixing unit of a static mixer according to the invention has, as may be seen in Fig 1, been designed with a streamlined symmetrical shape referred to hereinafter as a "wing" shape, with the largest cross-section at the leading or front edge portion 2. Suitably this front portion 2 is designed such that it has a radius of at least 5 mm, preferably 7-10 mm if it is intended to be inserted in a tube with a diameter of 60 mm. For other tube dimensions the front edge curvature is suitably increased or decreased in relation to the larger or smaller tube size. As may be seen in Fig 2 the rear part 3 of the wing is twisted for example 90°, in relation to the front part. In the embodiments shown in the drawing the twist has consistently been 90°, but this is mainly due to convenience for the drawing, and within the scope of the invention the twisting may be 80-100°. Also a larger or somewhat smaller twisting of 75-135° may be used if it is considered suitable. The static mixer according to the invention is built up of units and an embodiment of such a mixing unit is shown in Fig 3, which also shows four different sections through such a unit. A twisted mixing element may be arranged, as is seen in section IV-IV, within a tube part 4. The tube 4 is at both its ends provided with flanges 5, 5' and guiding means which ensure the correct assembly of the units. The mixing element 1 is arranged somewhat displaced in relation to the front edge of the tube part in order to create a free space between axially adjacent mixing elements, when the tube parts are connected to a mixer. As is seen in the embodiment shown in Fig 3, the mixing element is firmly connected to the tube part along its entire length. The thinner rear part 3 of the mixing element is twisted 90° in relation to the front part which is seen in the section II-II and III-III. The flow of media which enters into the tube part 4 is divided into two partial streams. Also the rear part of the mixing element ends inside the flanges of the tube part. Such an arrangement helps prevent particle or fibres getting caught. The connections between the inner wall of the tube and the twisted wing are suitably manufactured with a bending of at least 2 preferably 3-5 mm radius in order to prevent objects getting stuck and in order to facilitate cleaning. In order to ensure that the tube parts are arranged in such a way that a unit with a mixing element which is twisted clockwise is followed by a unit with a counter clockwise twist there are notches and projections in the flanges of the tube units. In the tube part 4 there is a small notch 6 in the flange 5' and a larger notch 7 in the flange 5. The tube parts with the other kind of twist has corresponding projections which fit into the notches. By providing the tube parts with notches and projections of different size it is also ensured that the thicker front portion of the mixing element is arrange to be met firstly by the product media. In Fig 4 there is shown how three tube parts are connected to a part of a static mixer according to the invention. The first unit is twisted clockwise, the second counter clockwise and the third clockwise again. With such a connection of the mixing elements there is obtained an optimum use for the available space. As may be seen on the drawing there is a certain distance between the front part of the next mixing element and the rear part of the previous element, and this distance is suitable 3-7 mm. Finally there is shown in Fig 5 a combined static mixer and steam injection heater. By drilling a number of small holes (0,5 - 3 mm) in the tube 8 and surrounding the tube parts with a further tube 9 provided with an inlet conduit 10 for steam there is obtained a good mixing and condensing of steam during simultaneous heating an mixing. Such a combined mixer and stem injection heater is especially suitable for highly viscous or particle- and fibre-rich liquids. A static mixer of the kind described above is especially suitable for mixing two media which each have passed through a separate pasteurisation and which are to be mixed prior to packaging. One of the media products may consist of soup containing particles while the other consists of sauce, i.e. a liquid product. Due to the difficulties of heat treating food products comprising particles in a correct way on tries, where it is possible, to limit the amount which must be subjected to this difficultly calculated treatment and with advantage a liquid phase is separated off prior to the heat treatment. The liquid may then be heat treated on its own and remixing of the products takes place after the heat treatment. Another mixing operation which with advantage may be carried through in the mixer according to the invention consists in mixing with gas in order to get fluffy and foaming food products. A static mixer according to the invention comprises usually 10-12 mixing units. Due to the modular construction it may easily be adapted to different mixing operations, in certain cases incomplete mixing may be desirable, for example, if jam is to be mixed into a yoghurt gel. The mixing units are with advantage manufactured by casting or injection moulding and they can be made of stainless steel or some plastic materials suitable for foods, for example, polysulphone or polyetheretherketone, if the mixer is to be used for food applications.

Claims: A static mixer comprising a tube and twisted mixing elements (1) mounted in the tube for dividing and diverting media flowing through the tube, characterised in that the mixer comprises at least two units each including a tube part (4) with at least one mixing element (1) secured therein, said mixing element spans the diameter of the tube part over substantially the entire length of the element and is connected firmly to the tube part over substantially the entire length of the element, the mixing element has a rounded leading edge, and the thickness of said mixing element (1) is greatest adjacent said leading edge and reduces gradually away from the leading edge. A static mixer according to claim 1, characterised in that the units are so arranged that adjacent mixing elements (1) are separated from each other by a free space when the units are assembled together. A static mixer according to claims 1 or 2, characterised in that each mixing element (1) is twisted 80-100°. A static mixer according to claims 1, 2 or 3 characterised, in that every second mixing element (1) is twisted counter clockwise while every other element is twisted clockwise. A static mixer according to any one of claims 1-4, characterised in that the leading edge of the mixing element has a substantially semicircular profile. A static mixer according to claim 5, characterised in that the mixing element has a continuously narrowing form in cross-sectional profile. A static mixer according to any one of the preceding claims, characterised in that the tube parts are provided with guiding means to ensure correct assembly of the mixing units. A static mixer according to any one of the preceding claims, characterised in that the mixer comprises tube parts with 5-15 mixing elements. A static mixer according to claim 8, characterised in that the mixer comprises 10-12 mixing elements. A static mixer according to any one of the preceding claims, characterised in that the mixer has double tube walls (9, 10), and the inner tube wall (9) is perforated in order to make addition of a heating medium possible.

Inventors: PALM BENGT, PALM, BENGT
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 1993
Main classification: B01F3
IPC codes: B01F5