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KREVOX European Environmental Centre
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DDS drum dryer

Drum dryers manufactured by Andritz can be found at 95 locations and are characterized by their heat energy consumption ratio of 0.85 kWh per kg of vaporized water.

Process description

The mixed sludge does not stick to the surface of the transporting equipment. Moreover, transporting sludge with such dry mass content to the dryer results in the outer layer of the granulate not being dried first (with a humid core structure).
From the mixer, the sludge is then passed on to the dryer inlet via a set of feeding screws with variable rotations, from there it is transported to the inside of the device by a stream of hot air from the furnace or heat exchangers.


The dryer, the heart of the entire installation, consists of three coaxial cylinders, rotating slowly with the speed of 10-16 rotations per minute. The sludge is transferred by a stream of hot air first via an internal cylinder, then a central one and finally via an external cylinder. During this process, the sludge granules rotate, which makes it easier for a stable granulate to form. The external cylinder intercepts the heat from two internal cylinders, thanks to which the heat losses inside the dryer are minimal.
The sludge granules passing through all three rotating cylinders are directed via a set of specially shaped deflectors in the direction of the internal drum walls and then fall thanks to the effect of gravity by a stream of hot air. The lighter sludge particles, which are now more dried, are transported by the stream of air faster and remain in the dryer shorter than large particles. Different particles with different sizes retention time ensures identical level of dry mass content at the outlet inside each granule.


After leaving the dryer, the sludge is transported by a stream of air to the separator, where the sludge and air are separated. Then, the sludge is placed in the correct polycyclone, which it leaves via a rotation hole, maintaining correct pressure.
The circulation air leaves the polycyclone with a temperature of 90 –100oC and passes through the fans to the condenser. The steam is being condensed thanks to cold process water injected by the nozzles. The mixture of process water and condensed steam is returned to the treatment plant inlet.


Most of the air (approx. 85%) is being recirculated. The circulation air has a low oxygen content maintained throughout the whole time in order to prevent the creation of an explosive atmosphere. The rest of the air (approx. 15%) is separated and directed to the biofilter with a small amount of air directly from the dryer (th dryer itself has a small undepressure, preventing odors from leaving it) and air from the dehydrated sludge silo as well as air from the sludge transport zone in the separator.
After leaving the polycyclone, the granulate is transferred to a screw conveyor, where it is cooled to the temperature below 70oC and passed on to the vibration screen.


Thanks to a special construction, the transfer of vibrations to the floor is minimized and no dust leaves the screen. On the screen, three types of granulate are being separated, depending on the size of the granules. The most desired material has a size of 0.5-4 mm and such product goes into the silo or the containers.

If the amount of too large and too small granules mixture is not enough for the recirculation purposes, part of the material with the desired size of 0.5-4 mm is also returned via a rotation hole to the storage chamber, to maintain constant sludge level inside. The dust from the filter cleaning the aspirated air is also directed there.


The dried sludge can be used as an alternative fuel in the cement or heat plant industry or as a source of energy for the drying process through combustion.


The entire process is fully automatic. It is controlled by a controller and a computer, which visualizes the entire process.

References

Drum dryer - Valenton, Paris France

Installation type    

3 x DDS

Material

25 - 28% d.m.

Product

> 90% d.m.

Efficiency

3 x 7,000 kg/h H2O

Startup

2005

 Drum dryer - Bran Sands, UK

Installation type    

Phase 1: 3 drying lines

Phase 2: 4 drying lines

Material

dehydrated 28% DS

Product

90 - 93% d.m.

Efficiency

7 x 5000 l/h H2O

Startup

Phase 1: 1998

Phase 2: 2001

 


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