Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1993-05-13
1994-03-22
Bennet, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 34, 34 77, F26B 300
Patent
active
052953106
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for drying a particulate material, such as wood fibres for making fibreboards, in which method the material is transported through a two-stage drying plant by means of drying gas, e.g. drying air, the material being supplied, in a first stage, into a first drying conduit where it is dried in a first drying gas flow transporting the material to a first cyclone in which it is separated from the drying gas which is discharged from the first cyclone, and the material separated in the first cyclone being supplied, in a second stage, into a second drying conduit where it is dried in a second drying gas flow transporting the material to a second cyclone in which it is separated from the drying gas which is discharged from the second cyclone.
In a prior art method of effecting a two-stage drying of this type, fresh air is blown in as drying air in the first stage, whereupon this air is separated in the first cyclone and emitted from the first stage. The used drying gas is emitted into the atmosphere but is previously used to preheat the fresh air which is blown in in the first stage. In this connection, the used drying air is passed through a heat exchanger. The heat exchanger is combined with a scrubber for washing the used drying air. In this prior art method, fresh air is blown in as drying air also in the second stage, whereupon this air is separated in the second cyclone and emitted into the atmosphere.
When this prior art method is used to dry glue-coated wood fibres for making fibreboards, the used drying air emitted into the atmosphere contains, despite the washing described above, fibre dust, formaldehyde and hydrocarbons.
The object of the present invention is to provide a method for drying a particulate material, such as wood fibres, in which method the emission of pollutants into the atmosphere is reduced, simultaneously as the drying gas is utilised in an effective manner.
According to the present invention, this object is achieved by a method which is of the type mentioned by way of introduction and characterised in that the major part of the drying gas discharged from the first cyclone is conveyed to a condenser so as to be subsequently recirculated in the first stage, that the remainder of the drying gas discharged from the first cyclone is emitted from the plant, that an amount of the drying gas discharged from the second cyclone, which corresponds to said remainder of the drying gas discharged from the first cyclone, is conveyed to the condenser to be circulated in the first stage, that the remainder of the drying gas discharged from the second cyclone is recirculated in the second stage, and that an amount of fresh drying gas, e.g. fresh air, which corresponds to said remainder of the drying gas discharged from the first cyclone, is supplied to the second stage.
About 80% of the drying air discharged from the first cyclone is preferably conveyed to the condenser.
The second drying gas flow preferably is about 30% of the first drying gas flow.
The first drying flow preferably has a temperature of 150.degree. C.-180.degree. C., while the second drying gas flow preferably has a temperature of 110.degree. C.-135.degree. C.
BRIEF DESCRIPTION OF DRAWING
The invention will now be described in detail with reference to the accompanying drawing which schematically illustrates a plant for carrying out the method according to the present invention.
The plant shown in the drawing and intended for drying wood fibres for making fibreboards, has two drying stages, viz. a first stage 1 and a second stage 2.
In the first stage, use is made of a first fan 3 which blows drying air, which is heated to a temperature of about 160.degree. C. in a first heating device 4, through a first drying conduit 5 opening into a first cyclone 6. Glue-coated wood fibres which are to be dried are supplied through a first supply conduit 7 into the first drying conduit 5 and are transported by the drying air to the first cyclone 6. The wood fibres are dried while being transported. They are
REFERENCES:
patent: 3866333 (1975-02-01), Sarukahanian et al.
patent: 4043049 (1977-08-01), Hedstrom
patent: 4495710 (1985-01-01), Ottoson
ABB Flakt AB
Bennet Henry A.
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