Drying and gas or vapor contact with solids – Process – With nondrying treating of material
Patent
1992-02-08
1994-12-20
Gromada, Denise L.
Drying and gas or vapor contact with solids
Process
With nondrying treating of material
34385, 34420, 34449, 34 60, 34182, 34183, F26B 700
Patent
active
053736478
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method of wholly or partly dehydrating sludge by evaporation which originates from e.g. sewage, said sludge being transported on a travelling grate from an inlet to a discharge opening in a drying chamber with addition of heat.
To protect nature and to prevent environmental damage very strict requirements are made today with respect to the purification of the sewage which is discharged to a recipient from municipal or industrial purification systems. An essential part of the required purification consists in passing the sewage through relatively large settling basins or tanks, in which the sewage then stays so long and under so quiet conditions that its content of sewage settles, or as is more common today, is subjected to mechanical treatment during which the sludge is dehydrated to a suitable extent by means of e.g. centrifugation.
In some cases, the sludge, which generally mainly consists of organic materials, may be spread directly over agricultural areas and be of use here as a fertilizer for the grown plants, but this will frequently not be possible since the sludge may have a content of substances which are harmful to animals and humans, and it is consequently not acceptable that these are included in the food chain via the crops in question.
Accordingly, methods have been developed for drying the sludge and making it useful as fuel or compost. For the latter purpose, additional heat treatment frequently takes place in connection with the drying to free the sludge of its content of harmful substances to the necessary extent.
It is preferred in many cases to initiate drying of the sludge directly in the wet and tacky state in which the sludge is conveyed to the drying system. In such a known method the sludge is thus thrown from e.g. a rotating wheel in countercurrent to a hot stream of air in a more or less finely divided form, so that the sludge rapidly dries to a powder and/or granulate. This method is extremely efficient, but on the other hand uneconomic in operation since it necessarily requires large amounts of air which must first be heated and is then discharged to the atmosphere, where the air per se, without a preceding expensive and cumbersome separation and filtration process, may constitute a source of environmental pollution since it very easily entrains pulverized sludge from the drying process.
WO 88/02284 discloses a method of drying sludge in a rotary kiln. In this case, an extruder is inserted in front of the rotary kiln, said extruder kneading the sludge with a separate binder and producing pellets from the kneaded mixture which are supplied to the rotary kiln and burnt to a solid, almost ceramic-like form which is suitable for disposal. However, the method cannot be used for producing pellets which can be burnt or composted.
Further, DE Offenlegungsschrift 3 531 748 discloses a method where the sludge is dried on a conveyor belt in a traverse kiln. In order that the sludge can be treated in this manner, the wet sludge is mixed in a suitable proportion with already dried sludge to form a sufficiently loose and friable material which can be distributed on the grate without sticking to it. During drying the sludge shrinks greatly already from the beginning and will hereby form a granulate with a dense and compact structure, which impedes the biological processes which are subsequently to take place during composting. The granulate cannot be used as a fuel either, since the end product has a much to high water content for this purpose. To this should be added that the system is relatively uneconomic in operation, since a considerably portion of the already dried sludge must constantly be recirculated and mixed with new and wet sludge.
Moreover, WO 90/09967 discloses a device for dehydration of sewage sludge, which is subjected to relatively high thermal impacts during transport through a plurality of augers. During this transport, the sludge is constantly rotated about, while relative movement takes place between the sludge and the augers, whereby the materia
REFERENCES:
patent: 2439384 (1948-04-01), Fetzer
patent: 4244287 (1981-01-01), Maffet
patent: 4756092 (1988-07-01), Anderson
patent: 4962592 (1990-10-01), Orizand
Bernes Steen
Jensen John R.
FLS Industries A/S
Gromada Denise L.
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