Direct heat dryer

Direct dryer "AKDRY"

General information

The sludge produced from the primary or secondary water or waste water treatment plants is extremely voluminous with a high content of water. The sludge is often mechanically pressed in order to minimize the water content and reduce the overall volume. The cake produced from the mechanical pressing in the belt filter presses has an average content of dry solids between 15-30%. The high concentration of moisture in the sludge increases the volume and weight while making the transportation and storage difficult.

In order to control and diminish the volume, the moisture content has to be evaporated, therefore, a thermal technology is applied to the cake. ATHANASIOS KOUTSOUKOS S.A. offers the direct drier “AKDRY” for the dehydration and drying of sludge.

Process description

The “AKDRY” is an industrial rotary direct dryer which is capable of drying a wide range of materials since it is able to process materials of various size and composition. The “AKDRY” is employed in order to reduce or minimize the moisture content of the material it is handling by bringing it into direct contact with a heated gas stream. The dryer is made up of a large, rotating cylindrical tube which is supported by several steel beams. In addition, a burner which is installed in the same side as the feed hole, produces a heated gas stream which flows from the feed side to the discharge side (known as co-current flow). Alternatively the burner can be installed on the same side as the discharge hole (known as counter current flow) and the heated gas stream flows in the opposite direction compared to the co- current flow.

The dryer is slightly sloped so that the discharge end is lower than the material feed end, in order to convey the material through the dryer under gravity. The material to be dried enters the dryer from the feed hole and as the dryer rotates, the material is lifted up by a series of internal fins which line the inner wall of the dryer. When the material reaches the top part of the dryer it falls to the bottom part of the dryer under the action of gravity.  The latter action is performed several times until a grain of the material reaches the discharge hole and finally it is dried as it has passed several times through the heated gas stream. The final product is in the form of dry pellets with a maximum water content of 10%.

The produced dry pellets are packed in water proof big-bags in order to facilitate lifting and transportation. The exhausted waste air is free from contamination and a barometric condenser is used to remove the water vapour.

Parts of the system:

1. A feed hopper
2. Screw conveyor for feeding the moisturized cake
3. The dryer including the combustor, the rotary engine and the rotating gear.
4. The insulating wrapper
5. The product hopper
6. The screw conveyor attached to the hopper for feeding the big bag
7. The filling system with the big bag and the frame-scaffold
8. The barometric spray condenser.
9. Electrical control and safety devices.

Screw conveyor

The screw conveyor is Archimedes type. The revolution is given by axial gear motor. The shaft is supported by two cushions in both ends and a transmission chain is used between the shaft gear motor and the screw. The screw type thickness-pitch- is selected according to the feeding sludge. Screw feeders are set up to be flood fed, so that regardless of the amount of material in a hopper, above the inlet, the output rate will remain constant. It is this trait that makes screw feeders a critical part of a system, as they set the rate of material flow for the rest of the equipment downstream. Depending on the product being conveyed and the inlet size, different types of flights are used in screw feeders. Some options include tapered flights, variable pitch flights and cone mounted flights.

Burner

Natural gas, biomass, liquid gas, and diesel oil can be used as fuel for the burner. The innovative design of the combustion head in the “AKDRY” allows combustion with very low polluting emissions. A mobile coupling flange is used to control the penetration of the head into the combustion chamber.

Rotary dryer

The dryer is a rotary slightly sloped metallic cylinder. To support the cylinder two patterns of roll trunnion ball assemblies are used. The cylinder is driven via chain and a shift gear. The drive system is simple and robust.
The cylinder is equipped with blades that lift up the sludge as the former rotates. The mechanical lifting of the material allows rotary dryers to be used to dry materials ranging from fine filter cake to coarse minerals. It also helps in breaking up lumps thus, promoting a more uniform dried material.

The insulating wrapper

The wrapper is constructed from high quality mineral fibres which are enclosed in a stainless steel case. The final structure has high conductivity, excellent thermal dissipation and stability at high temperatures. All parts can be easily uninstalled, therefore, maintenance can be performed quickly and easily.

The filling system

The filling system is a Big Bag type where feed can be performed via gravity, screw or belt. Weighing is achieved via a four load cell platform weight scale. Typical capacity ranges from 250kg to 1500kg. Fully automated big bag filling equipment with pallet magazines, roller conveyors and automatic bag releases are also available.

The barometric condenser

The exhaust fumes are treated in a direct contact barometric spray condenser which is relatively simple in operation and inexpensive. The direct contact condenser has several advantages over shell and tube condensers, for instance it offers a greater efficiency due to a greater available surface area for heat transfer produced by the water droplets.
Initially, cool water droplets of a specific diameter are sprayed into the steam of the exhaust fumes. The fumes are forced to cool due to the difference in the temperature between the water droplets and the air and the water which is enclosed in the exhaust fumes, condenses.  The latent heat which is released during the condensation of the water enclosed in the exhaust fumes, can be utilized by a heat exchanger, thus, converting it to energy which can be used for other usages. The condensed water as well as the water droplets are collected and can be used as the spray.