Drying and gas or vapor contact with solids – Apparatus – With fluid current conveying of treated material
Patent
1996-03-29
1997-04-01
Sollecito, John M.
Drying and gas or vapor contact with solids
Apparatus
With fluid current conveying of treated material
34168, F26B 1700
Patent
active
056154933
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a spray drying device comprising a drying chamber with at least one fluid spraying means and means for introducing drying air into the chamber as a flow around the spraying means, said drying chamber having a conically downwards tapering section and at the bottom a perforated plate and means for producing an upwardly directed air flow through the perforations of the plate for maintaining a fluidized particle layer on top of the plate.
For spray drying of many different kinds of products a particular interest has for the last decade been taken in spray drying devices of the above type, i.e. apparatuses, in which initially a drying of the sprayed drops of liquid takes place in a drying chamber like in the more conventional spray drying plants, following which the particles thus formed, while still having a considerable moisture content, are received in a fluidized bed which is maintained in the bottom part of the apparatus. In this fluidized bed an after-drying of the moist particles takes place and possibly an agglomeration thereof. Moreover, the too fine particles are blown off to a certain degree and transferred to the spraying chamber for agglomeration in such a way that a product is recovered from the fluidized bed with better particle size distribution than obtainable with a conventional spray drying device without a fluidized bed.
To this may be added a large number of additional advantages which all after the circumstances may be just as important or more important than the particle size distribution. Examples of these other advantages may be a particularly good energy economy, a very compact construction relative to the capacity and a gentle handling of the material to be dried, which is particularly important in connection with heat-sensitive materials.
A more detailed description of this type of spray drying apparatuses is disclosed in published Danish patent application no. 2670/82 and the corresponding patents EP 97484 and U.S. Pat. No. 4,490,403 as well as in French patent no. 1 526 478.
An essential function of the fluidized bed at the bottom of spray drying devices of the type in question is to catch the relatively moist particles from the actual spray drying process, and for that purpose a bigger height of the fluidized layer is required than what is optimal for the dust-removing effect, as the big height of the layer prevents an effective blowing off of fine particles from the lower parts of the layer. To this may be added that on account of the fact that the whole surface of the layer is constantly receiving moist particles from the spray drying zone, the layer consists of particles which have had a widely differing staying time in the layer and which are therefore inhomogeneous in respect of moisture content, and the product removed from the fluidized bed will therefore usually have a higher moisture content than what is desirable in the end product.
It is therefore usual to supplement the known spray drying apparatuses of the type described with an additional classification and/or after-drying device, for instance a vibrating or stationary fluidized bed or a vertical wind classification plant.
If the product is to receive an after-treatment which comprises the spraying on of liquid and a subsequent drying, for instance in view of coating or surface modification, it is a prerequisite according to prior art to use a special after-treatment device therefor, in which a spraying on of liquid takes place in a fluidized bed or for instance in a drum.
Such after-treatment devices with accompanying plants for collecting the fine particles carried along by the outgoing air, such as cyclones, electro filters, filter bags or washing towers, are space demanding, complicated to operate and cost demanding.
The requirements to the quality of the spray dried powders in respect of freedom from dust, agglomeration degree and particle size distribution are constantly increasing. The increased attention to allergic phenomena and work environmental conditions as a whole, both during p
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Gravini Steve
Niro Holding A/S/
Sollecito John M.
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