A process and apparatus for drying and homogenisation of material containing liquids

Abstract

 A process for drying and homogenisation of materials containing liquids.
While continuously mixing simultaneously vibration energy is radiated by one or more vibration generators (9) . towards the free surface (10) of the product mass in the mixing/drying container.
Thereby substantially only the liquid, which is a polar material, is heated and vaporized so that a quick drying of the product is obtained without significant damage thereto.

Description

[0001] The invention relates to a process for drying and homogenisation of materials containing liquid, which are subjected to a mixing operation by means of one or more mixing means in a closed drying/mixing container through heat supplied to the mixture and suction of the obtained vapour from the container.

[0002] Said process is known and is used for all kinds of materials, e.g. in the food products, pharmaceutical and chemical industries. The heat supply is effected in that the mixing container is constructed with a double wall, a heating fluid, e.g. steam, being carried through the chamber between both walls thereof. This known process has several disadvantages.

[0003] The heat exchange is a two-dimensional heat exchange, whereby a temperature gradient is present in the product mass so that temperature fluctuations will occur, which are often disadvantageous for the product. Some products dissociate above a predetermined temperature, other become sticky, whereby they adhere to the wall of the mixing/drying container and/or to the mixing elements. Due to this adhesicn to the wall a strong increase of the heat resistance of the wall occurs due to the insulating properties of the product, and thereby an increase of the drying time of the product.

[0004] A long drying time is necessary due to the fact that also the product absorbs heat, during which time the mixing operation continues. Some products are damaged by the continuous rubbing effect or they loose their granular shape or stick together in that the liquid is not removed quickly enough.

[0005] Therefore, in order to prevent the disadvantage of the long drying time and the product damage in the mixing/drying container, for many materials pre-drying was practised of the material prior to the final drying, namely on a porous conveyor belt, if necessary with passage therethrough of hot air, or in a centrifuge. The draw back of the above mentioned method in that the conveyor belt and/or the centrifuge claim much extra space and additional cost of investment. Also there is the additional draw-back that after the pre-drying has taken place the material has to be transported to a so called final dryer.

[0006] The invention aims at removing the said disadvantages. This is achieved according to the invention in that the heat supply is effected in that by one or more generators, positioned on the outer wall of the mixing/drying container, high frequency vibration energy is supplied to a free surface of the mass in the mixing/drying container with continuous movement of the product mass in the mixing/drying container, whereby subsequently all liquid present in the mass is heated and vaporized..

[0007] An extra advantage is that predrying and final drying may be carried out in the same apparatus and also that, due to the fact that the product mass is continuously moved, it is possible for more products to obtain uniform, homogeneous quantities (batches).

[0008] The high frequency vibration energy is only absorbed by molecules having a dipole. The molecules of about 95% of the solvents used in the industry have a dipole, otherwise stated they are polar substances. The solvent therefore absorbs the energy and is heated and vaporized thereby and is removed from the mixing/drying container by suction, whereas the product is not heated by the high frequency energy. Therefore much less heat has to be supplied per unitj of volume so that the drying time is considerably shortened. The product mass therefore, during drying is for a much shorter period exposed to the mechanical influence of the mixing process and therefore is not damaged..

[0009] The high frequency energy is irradiated towards the free surface of the mass to be mixed, said surface constituting the boundary between the space taken by said mass in the mixing/drying container and the remaining space from which the solvent vapour is exhausted.

[0010] The vibration energy imparted to the material and therefore the generated heat is proportional to the frequency and to the dielectrical constant. Since a polar material has a high dielectrical constant, contrary to a little polar material the product, substantially only the solvent is heated.

[0011] The vibration energy has a predetermined penetration depth so that only the solvent in the boundary layer is vaporized. Due to the continuous mixing in the container, the boundary layer is constantly changed and after some time the solvent in the total mass has been heated and vaporized.

[0012] A correct mixing, minimal damage and short drying times are obtained if the mixing/drying is carried out in a conical mixing container including a mixing screw which is simultaneously rotatable around its own axis and around the container axis.

[0013] The invention also includes a mixer for carrying out the process according to claim 1 or 2, in which mixer one or more high frequency generators are secured to the outer wall of the container.

[0014] The invention will be further explained below with reference to the drawing, showing a schematic embodiment of a mixer having a high frequency generator for carrying out the process according to the invention.

[0015] The mixer/dryer as shown has a conical vessel 1 which is suspended through the intermediary of supports and/or a carrier ring (not shown) in a concrete and/or steel structure and provided with a discharge spout 2 and with an upper cover 3 closing the mixing/ drying container. A drive unit 4 is mounted on the cover, said unit comprising two electromotors and transmission means to which inside the container a pivotable arm 5 is connected carrying a mixing screw 6. The drive and transmission means are such that the mixing screw 6 may be rotated due to the rotation of the output shaft 7 of the drive 4 along the interior of the conical wall of the mixing container 1 and also the mixing screw 6 may be rotated around its own axis.

[0016] A product mass 8 is present within the container 1 and comprises material containing liquid. Due to the operation of the mixing screw 6 said product mass may be mixed into a homogeneous mass.

[0017] According to the invention in a certain position on the exterior of the mixing/drying container a generator 9 of high frequency vibration energy is secured. In the embodiment shown a generator 9 is provided on the cover 3 of the mixer. Different locations as well as more than one generator 9 belong to the possibilities.

[0018] If electrical energy is supplied to the generator 9 it is converted thereby into electro-magnetical energy, which is irradiated with high frequency towards the product. Said energy penetrates into the mass along a predetermined penetration depth from the free surface of the product (the boundary layer, between the space occupied in the container by the product mass and the space over it). Thereby the molecules of the solvent, which is a polar material, are induced into strong movements whereby the vibration energy is converted into heat. This heat vaporizes the liquid, which escapes through the boundary layer 10 and is exhausted by suction from the mixing container 1 through a conduit, schematically indicated 11. Said vapour may be condensed in order to be re-usable.

[0019] The vibration energy issued by the generator 9 is proportional to the frequency thereof and to the dielectrical constant of the solvent. Since the dielectrical constant of the product material itself is much lower than that of the solvent, the energy is converted into heat substantially only in the solvent.

[0020] It will be clear that dependent on the temperature sensibility of the product, the power of the vibration generator may be controlled in conformity therewith. If said power is decreased less heat per unity of time is generated. The penetration depth of the high frequency waves into the surface of the mass is larger in accordance with the frequency and the dielectrical constant of the product material or the liquid respectively being smaller. Should the penetration depth be. too large, then at that depth the- vaporized liquid cannot escape quickly enough and yet a local superheating of the product could occur. Therefore the penetration depth must not be too large. It can also be adjusted by changing the power of the vibration generator.

[0021] Although the drawing shows a conical mixor/dryer the process according to the invention may be applied to all different Xinds of dryers in which simultaneously the products are homogenised. The axis of rotation of the mixing screw, moreover, may extend horizontally as well as vertically.

[0022] ^. It is important that the high frequency energy is irradiated during the processing since thereby in a short time all liquid in the mixer/dryer is reached and vaporizes. The vaporization is furthered by a stronger suction vacuum in the suction conduit 11. It may be adjusted dependent on the nature of the product. One could also have the vapour merely escape through the conduit 11, i.e. at atmospheric pressure.

Claims

1. A process for drying and homogenisation of material containing liquids, which are subjected to a mixing operation by means of one or mixing elements in a closed mixing/drying container, through heat supplied to the mixture and exhausting the obtained vapour through suction from the mixing/drying container, characterized in that the heat supply is carried out in that high frequency vibration energy is supplied by one or more generators(9) provided on the exterior wall of the mixing/drying container (1) to a free surface (10) of the mass in the mixing/drying container, with continuous movement of the product mass in the mixing/drying container, whereby subsequently all liquid present in the mass is heated and vaporized.

2. A process according to claim 1, characterized in that the mixing/drying is carried out in a conical vessel (1) having a mixing screw (6) which is simultaneously rotatable around its own axis and around the vessel axis.

3. A mixer for carrying out the process according to claims 1 or 2, characterized in that one or more high frequency generators (9) are secured on the exterior wall of the vessel (1).

Drawing