EP0542713B1 - Method and device for mixing fluids - Google Patents

Abstract

The present invention relates to a method and a device for mixing fluids, in which method the mixing of fluids takes place in a mainly cylindrical mixing vessel by means of a revolving mixing element disposed therein. The invention is characterised by the observation that the most effective manner of maintaining the homogeneity of the settling fluid consists in keeping the flow direction in the vessel as vertical as possible. This is ensured by the agitator shaft having attached thereto paddles/blades (26) by means of which the flow in the vessel is kept columnar at the central section as far as the mixing element (32), which deflects the axial flow via a radial flow into a rising annular flow. <IMAGE>

Description

The present invention relates to a method for mixing fluids in a cylindrical mixing container, in which a mixing mechanism is arranged which rotates essentially around the container axis and with which the fluid is set in motion, and a device for mixing fluids. Fluids are understood to be liquid media.

Several such processes and apparatuses are known to date and are commonly used in the paper, paint and food industries to mix different fluids with one another or to mix solids with fluids. The apparatus and processes are generally to be developed in such a way that the highest possible mixing effectiveness with the lowest possible output is achieved.

A generally used method for increasing the mixing effectiveness consists in causing vortex formation in the fluid streams by means of flow guide surfaces. The most common type of flow guide surface is a band-shaped metal plate, which is arranged on the inner wall of the mixing container at a distance therefrom in the direction of the mixer shaft. There are usually two or more flow control surfaces. The plane of the flow guide plate is located on a plane formed by the radius and vertical line of the mixing container, as a result of which it causes turbulence in the fluid when the fluid flow impinges on the plate in the circumferential direction. Such a flow guide surface causes strong turbulence, but has the disadvantage of a high power requirement. However, if less mixing is desired in general storage, no vortex formation is required, but the rising and falling currents that occur in the container keep the contents homogeneous.

DE-A-21 25 698 shows a mixing mechanism in which relatively large guide flaps are mounted on the wall of the mixing container and extend into the interior of the container in such a way that the fluid flow is inclined upwards due to their adjustment or can be directed down. The inclined position of the flow control surfaces reduces the power requirement while maintaining the mixing effectiveness, which means that the mixer, according to the publication mentioned, is more economical to operate than the standard version mentioned above.

A solution is known from FI-C-77 384 in which the flow guide surface is still arranged in a band and at a distance from the wall of the mixing container, but is wound around its axis. This solution has the advantage that the fluid flows impinging on the flow guide surface are deflected in different directions depending on the point at which they hit the flow guide surface. As a result, the flow guide surface causes a large number of flows in different directions, which promote mixing. Although the mixing effectiveness of the apparatus described is of a high level, the energy consumption of the apparatus is remarkably high because the flow guide surfaces represent a very high flow resistance.

• Bei einem relativ hohen Behälter, d.h. wenn die Höhe mehr als ein Anderthalbfaches gegenüber dem Behälterdurchmesser beträgt, reicht ein Mischorgan nicht aus, sondern man ist gezwungen, mehrere Organe einzusetzen. Dabei weist das Strömungsbild anderer als des untersten Organs auch eine bedeutende radiale Strömungskomponente auf, wobei die Strömungen im äußersten Zylinder gestört werden.
• Man ist gezwungen, die untersten Mischorgane möglichst dicht am Mischbehälterboden anzuordnen, wobei der Abstand rund 0.1 - 0.2 * Behälterdurchmesser ist, damit im Behälter auch kleine Mengen des betreffenden Fluids gelagert werden können. Bei den typischen untersten Mischorganen handelt es sich um Turbine, Propeller und Blatt, von denen
• die Turbine typischerweise ein symmetrisches Bild bildet, wobei die unterhalb des Mischorgans entstehenden Strömungen für das Endergebnis der betreffenden Mischung nutzlos sind, und
• Propeller und Blätter wiederum eine wesentliche axiale Komponente erzeugen, die gegen den Boden gedrückt nur eine örtliche Turbulenz bewirkt, die die obengenannte Makroströmung nicht begünstigt.
• Darüber hinaus versetzen die umlaufenden Mischorgane das einzulagernde Gut in eine Drehbewegung, die beim Anstreben der obengenannten Vertikalbewegung nutzlos, d.h. Energieverschwendung ist.

The following problems can be identified with conventional mixer solutions:

• In the case of a relatively high container, ie if the height is more than one and a half times the diameter of the container, one mixing element is not sufficient, but one is forced to use several elements. The flow pattern other than the bottom organ also has an important radial flow component, the flows in the outermost cylinder being disturbed.
• One is forced to arrange the bottom mixing elements as close as possible to the bottom of the mixing tank, the distance being around 0.1-0.2 * tank diameter, so that even small quantities of the fluid in question can be stored in the tank. Both Typical lowest mixing elements are turbines, propellers and blades, of which
• the turbine typically forms a symmetrical image, the flows arising below the mixing element being useless for the end result of the mixture in question, and
• Propellers and blades in turn produce an essential axial component that, when pressed against the ground, only creates local turbulence that does not favor the above-mentioned macro flow.
• In addition, the rotating mixing elements set the goods to be stored in a rotational movement which is useless when striving for the above-mentioned vertical movement, ie is wasted energy.

Because none of the prior art devices function sufficiently effectively under all circumstances, particularly in the treatment of settling fluid, a method and apparatus of a new type for mixing fluids have been developed. The invention is therefore based on the object to provide a method and a device for mixing fluids which meet the requirements in terms of energy efficiency and mixing effectiveness better than the devices according to the prior art and which are also completely insensitive to the amount of Container or the fluid level in the container.

The method according to the invention is characterized in that the fluid in the upper part of the container is circulated by the application of an axial force, so that a first part of the flow is a columnar flow around the container axis, mainly in the longitudinal direction of the container, and a second part of the flow is a opposite upwardly moving, surrounding the column or annular flow forms and that the fluid in the lower part of the container is conveyed by radial force in the direction of the container wall.

The device according to the invention for mixing fluids from mixing elements introduced concentrically on a rotating shaft, which shaft is introduced concentrically to the container axis in an essentially cylindrical mixing container, is in turn characterized in that a mixing element arranged in the upper part of the container has a shovel or a blade The tip of which has a guide flap that is essentially parallel to the container axis, that the substantially parallel guide flap points towards the bottom of the container, and that a further mixing element is provided in the vicinity of the bottom of the mixing container, which is designed as a shovel or blade Point is aligned in a plane mainly parallel to the container axis.

The longitudinal axis of the blade or of the blade of the mixing element advantageously forms an angle different from 90 ° with the axis of the container.

The guide flap is advantageously designed such that it comes to rest on the back of the blade or blade. The guide flap advantageously comes to rest mainly on an imaginary surface, concentric with the mixing container, at the tip of the blade or blade or on a surface which mainly touches it.

It is also a preferred embodiment of the device according to the invention that flow guide surfaces are attached to the container wall.

From DE-C-921 927 a mixer solution is already known, with a guide flap parallel to the circumference or even a whole cylindrical one at the blade tip of the mixing element is designed flat ring, the purpose of the guide flap is only to prevent running vibrations and to eliminate the turbulence at the tip. The solution mentioned cannot aim to produce a cylindrical flow, because the mentioned guide flap at the tip of the blade is expressly used at the lowest level of the mixing element, ie close to the bottom of the container, where it would under no circumstances be necessary to keep the flow cylindrical, but already should be redirected to the radial.

• Fig. 1 eine Lösung gemäß dem Stand der Technik und das dadurch hervorgerufene Strömungsfeld,
• Fig. 2 eine zweite Lösung gemäß dein Stand der Technik und das dadurch hervorgerufene Strömungsfeld,
• Fig. 3 ein Mischwerk gemäß einer Ausführungsform der Erfindung und das dadurch hervorgerufene Strömungsfeld,
• Fig. 4a und 4b Schaufel- bzw. Blattkonstruktionen zweier vorteilhafter Ausführungsformen der Erfindung detailliert,
• Fig. 5 eine dritte Schaufel-/Blattlösung der erfindungsgemäßen Vorrichtung, und
• Fig. 6a und 6b eine an der Wandung des Mischbehälters einzusetzende Strömungsleitfläche.

The method and the device according to the invention are explained below by a comparison with solutions according to the prior art and with reference to the accompanying drawings. It shows

• 1 shows a solution according to the prior art and the flow field caused thereby,
• 2 shows a second solution according to the prior art and the flow field caused thereby,
• 3 shows a mixer according to an embodiment of the invention and the flow field caused thereby,
• 4a and 4b blade or blade constructions of two advantageous embodiments of the invention in detail,
• Fig. 5 shows a third blade / blade solution of the device according to the invention, and
• 6a and 6b a flow guide surface to be used on the wall of the mixing container.

1 shows a solution according to the prior art and the flow field generated thereby. In a mixing container 10 there is a mixing mechanism 12, which is composed of a shaft 14 and an upper 16 and lower mixing element 18 attached to it. It is essential that the lower mixing element 18 is structurally different from the upper element 16. Depending on the height of the mixing container, one or more upper organs can thus be present. The upper mixing element 16 consists of shafts, at the ends of which in one blades are attached to the shaft plane inclined direction, which should shift the flow in an axial flow direction. When generating the axial movement component mentioned, however, the blades also expose the fluid to a movement component in the circumferential direction, which directs the flow more or less obliquely towards the container wall. The lower mixing element 18 in turn consists of blades arranged in planes parallel to the shaft, which move the fluid in a mainly radial direction of flow. In addition, said blades produce a vigorous rotational movement in the fluid.

According to the figure, the upper mixing element is not able to give the fluid a columnar flow direction, apart from a relatively small diameter area in the very center of the container, but the fluid flow moving obliquely downward mixes with the at the edges of the container upward flow and creates local turbulence areas between the mixing elements, the energy being lost without contributing to the mixing of the entire fluid volume. The lower mixing element, on the other hand, produces an effective radial flow, but also generates useless turbulence below which the fluid does not necessarily mix with the fluid higher in the container.

A solution is shown in FIG. 2, the lower mixing element according to the solution of FIG. 1 being replaced by a mixing element 16 ′ identical to the upper mixing element. With this solution, a local turbulence area is created halfway between the mixing elements, which wastes energy unnecessarily and makes it difficult to keep the fluid homogeneous. Depending on the orientation of the blades of the mixing element, the element 16 'can exert an unnecessarily strong flow component against the bottom of the container.

As can be seen from the conventional solutions outlined above and the flow fields generated thereby, the most difficult problem when striving for optimal mixing is in the non-axial flow generated by the mixing elements in the upper and middle part of the container, which slows the upward and downward flow along the container walls and the mixing prevents.

When considering the devices according to the prior art and testing new design solutions of different types, it has been found that the most effective way of keeping the fluid homogeneous is in no way to maintain a large number of local turbulence centralizations, but rather the direction of flow in the container as possible parallel to the container axis, generally perpendicular and sinking in the middle of the container. Experiments have shown that local turbulence uses far more energy than would be expected after its mixing effect. Thus, the clearest version is to maintain effective mixing in a cylindrical container in a vertical, heally cylindrical column that moves downward at the core of the flow and upward along the edges.

In Fig. 3 a solution according to an embodiment of our invention is shown, the mixer consisting of a shaft 20 and mixing elements 22 and 32 attached to the shaft, the element 22 being of a type such that it is everywhere except in the immediate vicinity of the tank bottom can be used for what an organ 32 has been developed. The mixing element 22 is a modification of the mixing element shown in FIGS. 1 and 2 in such a way that the element consists entirely of either a blade starting at the shaft 20 or a shaft 24 and a blade 26 attached to the end thereof and inclined with respect to the plane of rotation. The sheet 26 is provided at least at the tip with a guide flap prevent the formation of a radial component and should direct the flow down as effectively as possible as a columnar plug. Said additional guide flap 28 is located at the tip of sheet 26 such that the direction of flow of the fluid sliding radially along the sheet 26 is deflected axially by the action of guide flap 28 to form a boundary layer of the columnar downward flow. With the exception of the lowest mixing element, a plug-shaped flow parallel to the container axis should be generated on each mixing element level. If required, several of the mentioned guide flaps can also be arranged in connection with each blade or each blade, whereby they also make the blade construction more stable.

As can be seen from FIG. 3, the flow field generated by such a mixing element 22 is as close as possible to the optimal column, wherein a turbulent area can only be determined in the boundary layer of the opposite flows.

4a shows the optimal shape of the blade 26 of the mixing element 22 in more detail. According to the figure, a guide flap is arranged at the tip or the tip is bent into a guide flap 28 which extends from the blade in the direction of flow of the fluid. The orientation of the guide flap 28 is essentially the same as the direction of movement of the flowing fluid column, the flow deflected by the guide flap 28 smoothly merging into the fluid flow. In Fig. 4b, on the other hand, a solution according to a second embodiment is shown, the blade 26 'of the mixing element being arranged obliquely such that a component of the force field generated when it rotates is directed obliquely towards the container axis, the direction of the resultant being from the above Force and the centrifugal force of the axial direction is closer than in the situation of Fig. 4a. At the tip of the blade 26 'there is a baffle 28' which is still substantially parallel to the axis. The guide flap 28, 28 'comes to lie on an imaginary surface concentric with the container at the tip of the blade or blade or on a surface which touches it. It is of course also possible that the orientation of the guide flap 28 or 28 'is intentionally deflected from the directions mentioned, for example in order to impart the desired direction of movement to the fluid impinging on the blade. It is essential for this upper blade / blade that one wants to prevent through its / its construction that the radial components of the fluid flow disturb the ring flow flowing in the opposite direction. The blades / blades can be produced by folding from a steel or other corresponding metal plate in such a way that the blade / blade consists of a number of planar surfaces. However, if it is considered appropriate in some cases to manufacture the blade or blade from plastic, for example, the blade can be cast directly into the final shape.

5 shows the mixing element 32 to be used at the lowest level of the mixing element (the flow is directed downwards in the middle of the container), which is shaped in such a way that the flows above the mixing element 32 are directed exclusively downwards, but at the level of the Mixing member flows laterally from it towards the wall of the container 10 and the outgoing flows from the central region of the mixing member are directed obliquely downwards and represent only a fraction of the flows above. The mixing element 32 can thus consist of a shaft 34 and a sheet 36 fastened to the end thereof, the orientation of its flat surface / s at the axial end being at least almost radial, then some fluid being conducted downwards and angled back again in such a way that the Blade tip 38 already clearly guides fluid in a radial-peripheral direction. In other words, in the embodiment of the figure, the first, shaft-side plane deviates approximately 30 degrees from the plane drawn over shaft 20 and shaft 34 in the direction downward of the fluid and the other Level about 60 degrees in the same direction. Then the tip of the blade begins to rotate in an increasingly fluid direction towards the container wall. According to one embodiment, the orientation of the blade tip forms an angle of approximately 45 degrees with the shaft, the blade thus causing the fluid to make a vigorous radial movement. The blade end on the shaft side gives the fluid a low axial flow velocity, the central flow being conducted as an annular flow over the bottom of the mixing / storage container and ascending along the wall. The mixing element can of course also be a blade, the tip of which is strongly bent almost in an axial plane, the direction of flow of the fluid flowing into the influence area of the mixing element being largely deflected into the radial direction.

It is also possible to arrange a paddle wheel similar to the impeller of a centrifugal pump without a housing or even with a type of housing in the immediate vicinity of the base part of the container, the said impeller effectively deflecting the central flow first parallel to the base and then to an annular flow along the walls .

In addition, flow control surfaces can be attached to the wall of the mixing container in such a way that at least two inclined flow control surfaces are arranged at the level of the lowermost mixing element. The inclination of these flow control surfaces should be at least approximately 30 to 75 degrees. The flow guide surfaces are intended to redirect the flow circulating along the container wall to a component which raises the flow. Economically speaking, the most advantageous flow guide surface is a straight flow guide surface at an angle of 45 degrees, but the most advantageous functionally is a flow guide surface whose angle becomes steeper as the container bottom rises. The flow guide surface can either consist of a curved flow guide surface or several straight flow guide surfaces be made, the inclination of which should change step by step, for example three flow control surfaces (FIG. 6a) with inclinations of around 30, 60 and 90 degrees. The width of the flow guide surfaces in the solution according to our invention should be approximately 1/8 to 1/15 times the container diameter, and its distance from the wall should be approximately 0.1 times the container diameter. When comparing the flow field achieved with flow guide surfaces according to FIG. 6a with a flow field achieved by conventional vertical, i.e. state of the art flow guide surfaces, it turns out that the flow through flow guide surfaces according to our invention is deflected very gently from a peripheral flow into a vertical ring flow can. As is apparent from the above, a completely new mixing mechanism and mixing method, which overcomes the disadvantages of the prior art, has been developed, of which only a few advantageous embodiments are shown above.

Claims (6)

1. A method of mixing fluids in a cylindrical mixing vessel, in which a mixer rotating substantially around the centre-line of the vessel is arranged, by means of which mixer the fluid is made move, characterized in that the fluid in the upper part of the vessel is made circulate by exposing it to axial forces, so that a first part of the flow forms a pillar-shaped flow which moves principally downward in the longitudinal direction of the vessel around the centre-line of the vessel, and a second part of the flow forms a ring flow or side flow encircling said pillar and moving in the opposite direction upward, and that the fluid in the lower part of the vessel is made move toward the wall of the vessel by exposing it to radial forces.
2. An apparatus for mixing fluids by means of mixing members arranged concentrically on a shaft which is arranged in a substantially cylindrical mixing vessel concentrically with the centre-line of the vessel, characterized in that a mixing device (22) arranged in the upper part of the vessel is provided with a blade or paddle (26, 26'), the end of which is composed of a proplet (28, 28') which is substantially parallel with the centre-line of the vessel, that the substantially parallel proplet (28, 28') points toward the bottom of the vessel, and that there is yet another mixing member (32) arranged in the vicinity of the bottom of the mixing vessel, shaped like a blade or paddle (36) the end (38) of which is directed to a plane principally parallel with the centre-line of the vessel.
3. An apparatus according to claim 2, characterized in that the longitudinal axis of the blade or paddle (26') in the mixing member (22) forms with the centre-line of the vessel (10) an angle deviating from 90°.
4. An apparatus according to claim 2 or 3, characterized in that the proplet (28, 28') is located on the back side of the blade or paddle (26, 26').
5. An apparatus according to claim 2, 3 or 4, characterized in that the proplet (28, 28') is principally located on an imaginary surface concentric with the mixing vessel at the end of the blade or paddle (26, 26'), or on a surface principally tangential with this surface.
6. An apparatus according to any of the claims 2 - 5, characterized in that the proplet surfaces (40, 42, 44) for the flow are attached to the wall of the vessel.

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