The present invention relates to a device for in-vessel treatment of material, particularly for dispersion or homogenisation of liquids, or for suspension of solids in liquids, of the kind comprising a treatment element that is rotationally mounted in the vessel.
In mixing liquids in a vessel, a rotary stirrer usually is used, for example a stirrer having radially outwardly directed vanes. A stirrer of this kind is adapted to transfer energy to the liquid in two ways. Firstly, the liquid is set in motion as the vanes are drawn through liquid, whereby a laminar flow is created. Secondly, the liquid is affected by shearing forces at the vane edges, which causes turbulence.
In some situations, the mixing process is dependent on the presence of extremely strong shearing forces. This is true in the dispersion or homogenisation of liquids that do not spontaneously form a solution (for example oil in water) and in suspending powdered solids in a liquid (such as flour in water). The considerable shearing forces are required to break up for example drops of oil or lumps of flour into atomised particles through xe2x80x9cwhippingxe2x80x9d. When conventional stirrers are used, a very large moment of force is required to deliver shearing forces of this magnitude.
U.S. Pat. No. A 5,205,647 suggests a solution to the above problem. The mixing apparatus described therein has a cylindrical casing, in which is attached a sleeve formed with oblong slots, and a cylinder rotationally mounted in the casing. The cylinder is formed With through-botes extending in the cylinder material in parallel with the cylinder axis, and apertures extending between the bores and the exterior of the cylinder. The mixing apparatus operates by introducing liquids through two inlets while the cylinder is rotating. The liquids are introduced into the bores and thereafter are passed through the apertures to the exterior of the cylinder and thereafter through the oblong slots in the casing to finally exit through an outlet in the casing. On their route, the liquids are exposed to shearing forces.
In accordance with another prior-art apparatus two concentric cylinders are arranged to rotate relative to one another, for example inside a tank. The cylinders are formed with through-outlets and are disposed sufficiently closed to one another to ensure that a scissors-like force is produced, when they rotate relative to one another. Liquid subjected to this scissors-like force is affected by considerable shearing forces.
Several disadvantages are connected with the prior-art technique described above. The apparatuses comprise several components, which are movable relative to one another and between which the spacing by necessity must be extremely narrow if the large shearing forces are to be produced. The manufacturing tolerances as well as the assembly and mounting tolerances with respect to the discrete components therefore are extremely small.
Should some components happen to come into contact with one another during the rotation, there is a risk that particles may separate from the contacting components and pollute the liquid to be mixed therewith. In case of heavy contact, there is also a risk that the apparatus may be seriously damaged.
Considering the large number of components that must be produced, mounted and made to co-operate with a high degree of precision in order to produce the desired effect, the apparatuses become expensive to manufacture and to maintain.
In addition, the narrow spaces formed between the various components are difficult to clean. Particles and viscous liquid may get trapped in these narrow spaces and form obstructions, which impairs the functional ability of the apparatus
One object of the present invention is to provide a device for in-vessel treatment, which is capable of efficiently mixing, dispersing and/or homogenising liquids under the conditions outlined above without requiring a large moment of force. By xe2x80x9cliquidxe2x80x9d as used herein should be understood all fluid substances (media?) as also liquids/fluids containing solid particles.
A second object of the invention is to provide an easy-to-clean apparatus for in-vessel treatment.
A third object of the invention is to provide an apparatus for in-vessel treatment that does not require a large number of components that are movable relative to one another.
A fourth object of the invention is to provide an apparatus for in-vessel treatment that is inexpensive and simple to manufacture.
These and other objects are achieved according to the invention by means of an apparatus of the kind define in the introduction hereto and which is characterised in that the treatment element essentially is of tubular configuration and is formed with a plurality of cuts formed with sharp edges which are drawn through the material in the vessel as the treatment element rotates, in order to transfer shearing forces to said material.
Upon rotation of the treatment element in liquid, the sharp edges thereon generate a resistance force, as they move through the liquid. Because of the sharpness of the edges, the motion of the latter affects the liquid by means of shearing forces, and the moment of force required to rotate the treatment element consequently is transferred to a very high extent to the liquid in the form of shearing forces.
The treatment element preferably is cylindrical and is mounted for rotation about its centre axis. Resistance against the rotary motion then is generated almost exclusively from the sharp edges, since the rest of the treatment element is configured as a rotationally symmetrical element.
In addition, the treatment element can be configured as a multi-piece element. For example, it may be designed in the form of several cylinder sectors, or in the form of several concentric cylinders. In addition one or both ends of the treatment element may be formed with inwardly directed flanges. Alternatively, the treatment element may be formed with a barrel-shaped contour configuration, presenting smaller radii at its ends than in its in-between parts. Owing to this configuration, it becomes more difficult for liquid that is forced against the jacket of the treatment element to flow axially along the jacket of the treatment element and across the edge of the latter, and in consequence thereof the liquid is instead forced to pass through the cuts.
In addition, the cuts may be formed with shovel means the mouths of which are orientated in the direction of rotation of the treatment element, which mouths are formed with sharp edges. Preferably, the shovel means are formed on the inner face of the treatment element. In the course of rotation of the element, the shovel means urge liquid to pass from the inner face of the treatment element, through the cuts to the external face of the treatment element. In addition, the shovel means contribute to setting the liquid in a rotary motion, whereby the centripetal force will convey liquid radially outwards, towards and through the cuts formed in the jacket of the treatment element. Altogether, a pumping action is produced, which makes liquid flow past the sharp edges.
Preferably, the treatment element is fitted on a stirrer or mixer having several radially outwardly directed vanes. Preferably, the treatment element is mounted on the tips of the vanes, whereby the treatment element will form a cylindrically shaped enclosure around the stirrer/mixer. The vanes generate a rotary motion of the liquid in consequence whereof the centripetal force will transport liquid and particles radially outwards, away from the hub of the stirrer/mixer. The flow through the cuts, and thus the flow past the sharp edges, therefore will be larger.