Drying and gas or vapor contact with solids – Process – By centrifugal force
Patent
1998-07-15
2000-07-11
Gravini, Stephen
Drying and gas or vapor contact with solids
Process
By centrifugal force
34326, 34333, 34363, 34371, 34378, 34579, 34135, 34138, 34169, 34 86, F26B 508
Patent
active
060854405
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and an apparatus for producing a powder by spin flash drying.
The invention specifically relates to a process and an apparatus for drying of a material in the form of a paste or a filter cake, with a vertical, cylindrical dryer chamber having a rotating coaxially placed stirrer, with a variable speed drive screw feeder and with apertures for supply of hot drying gas and for removal of the spent drying gas and removal of the dried material.
Products preferably dried according to the invention are fruit and beet pulps, distillers residues, pesticides, pigments, dyes, ceramics, active coal, sludge and zeolites.
It is well known, e.g. from EP 0 141 403 to dry materials in the form of a paste or filter cake to obtain a powder.
In this apparatus the produced powder particles' size are all under a given cut size and the residual drying gas is furthermore emitted to the environment.
In U.S. Pat. No. 5,291,668 is described a spin flash type of system for drying of sludge in water vapour. The system uses a louver separator for production of fine particles; the system requires steam recompression for energy utilization.
It is the object of the present invention to provide a process and an apparatus of the spin flash dryer type, which is particular suitable for drying of pastes and filter cakes into a powder with a desired mean particle size and a narrow particle size distribution. Low consumption of energy and little or no emission to the environment should secondarily be obtained.
By the novel means according to the invention presented in claim 1 for the process and claim 6 for the apparatus, particles are separated by a classifier in the chamber in such a way that oversize fraction is withheld in the chamber and the desired fraction is removed. A possible undesired undersize fraction is strained off and recycled externally back to the dryer chamber.
The cut sizes of the classifier is adjustable.
The spent drying gas is recycled to the air distributor of the dryer chamber and is reheated by indirect or by direct heat. With indirect reheat, the composition of the drying gas will be water vapour; with direct reheat it will be air with a high moisture content. In either, the energy in the spent drying gas may be made use of in a heat exchanger and there is no or only little emission to the environment.
By this process it is possible to obtain a powder with a desired mean particle size and a narrow particle size distribution by spin flash drying, with a low consumption of energy and with little or no emission to the environment.
During operation of the spin flash dryer the drying gas will create a high velocity, whirling fluidized bed of drying particles which moves up through the chamber during the drying process.
Heavy, still wet lumps are forced towards the chamber walls. Disintegration, attrition and drying cause particles to become smaller and lighter and, as a consequence, a balanced fluid bed is created in which smaller particles move towards the axis of the dryer chamber.
By selecting operating conditions, a state of equilibrium is obtained in which the feed rate of moist material is in balance with the corresponding drying capacity (a principle, known to a person skilled in the art of drying) and with the discharge rate of the dried product. It is a surprising effect of the spin flash dryer, that the particles remain in the drying zone until they obtain the desired particle size.
By the invention it is possible, within certain limits, to remove particles under a given particle size (which equals the desired maximum) by selecting the diameter of the orifice of the first classifier means in the upper part of the dryer chamber, above which drying gas with entrained particles is removed from the dryer.
Undesired undersize particles are removed above the second classifier means having a second orifice with an annular opening, and recycled to the lower part of the dryer chamber where they will agglomerate with wet product and form larger particles. Produced powders, leaving
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APV Anhydro AS
Gravini Stephen
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