Indirect rotary dryers differ from convective-direct dryers in that heat is transferred to the wet product by conduction and radiation through the cylinder wall or through a series of tubes located circumferentially to the cylinder.
Indirect rotary dryers use as heating medium hot gases coming from a combustion chamber.
They are particularly suited for the drying of dusty products which are not sensible to heat or of products which cannot be exposed to combustion gas.
Indirect Heated Fired Rotary Dryers
Indirect Heated Fired Rotary Dryers are commonly used for carbon black pellets drying.
Carbon black coming from the reactor, which is not part of Idreco technology, is fed to a wet pelletizer where water together with molasses are added to form wet pellets. Molasses are used as a binder for beading / pelletising of carbon black particles, as required to obtain carbon black with desired characteristics.
Then, wet pellets are fed inside the rotary dryer through a chute or a screw feeder.
Carbon black pellets are dried inside a rotary cylinder, where they are gentle moved.
At the feed end of the cylinder a retaining ring and cone followed by spiral flights for rapid advancing of the product are provided.
Spiral flights are followed by lifting flights to agitate, and to revert product to dry properly avoiding to brake the pellets.
The active length of the cylinder is enclosed in a refractory lined annular furnace consisting of a steel casing inside lined with ceramic fiber modules and insulating bricks for the floor.
Annular chamber receives hot gases produced in a combustion chamber which recover as heating medium tail gas coming from carbon black reactor. Tail gas has poor heating value (around 700 kcal/Nm3) and combustion chamber is provided with honeycomb wall appositely studied to assure the complete combustion.
To increase the thermal efficiency, a side stream of the heating gas can enter in the rotary dryer cylinder through a snorkel: gas flows in countercurrent with the pellets and is discharged at the feed head side together with the evaporated water vapour.
The Purge Gas Blower provides to convey the vapors from dryer to the Purge Gas Filter (out of scope of supply).
Dry pellets with low residual moisture (lower than1 %) are discharged from the dryer for final storage.
Eventually a small diameter cooler rotating together with the dryer can be provided to cool down carbon black pellets before storage.
The hot flue gas at the annular furnace exit are conveyed to a Air Preheater, for heating combustion air and reduce tail gas consumption, and finally are discharged to the atmosphere through a stack.
Indirect Flue Gas Tube Dryer
Another type of indirect rotary dryer is the flue gas tube type.
Hot gases generated inside the combustion chamber are conveyed inside concentric rows of tubes installed circumferentially to the cylinder. Along the length of the tubes, intermediate supports are provided.
The wet product is fed externally to the tubes and advance countercurrent to the hot gases.
There is no contact between product and gases. Heat is transferred from hot gases to product through the tubes wall by radiation and conduction.
Flue gases leave the cylinder on the wet product feed side and are partially recycled inside the combustion chamber to reduce fuel consumption and partially discharged to atmosphere with eventual heat recovery.
To conserve heat the cylinder is externally insulated.
Vapor coming from evaporation is extracted from the dryer by means of purge ambient air and successively dedusted inside a fabric filter before its discharge to atmosphere.
Products that can be processed by Indirect Flue Gas Tube Dryers:
- Carbon black
- Soda ash