Industrial Dryers

Spray drying is defined as the transformation of feed from liquid / fluid state into a dried form by spraying the feed into hot drying medium. It is a one step continuous processing operation of these industrial dryers.

The feed may be in the form of liquid or solution, suspension or paste. The dried product conforms to powder consisting of single particles or agglomerates, all depending upon the physical & Chemical properties of the feed and the design of the dryer and its operation.

Intensive research and development has undergone during last few decades, so that modern equipment can meet the requirement to produce a powder with tailor made specification.

When a gas is passed through a permeable support on which material rests which is able to flow, the packing of the bed of material will becomes less dense i.e. the bed will start to expand when a certain velocity is reached. At still greater velocities the particles in the bed will start to expand when a certain velocity is reached. At still greater velocities the particles in the bed will begin to be in turbulent motion. Than all particles of the product are thoroughly mixed in the turbulent layer and they dry therefore at uniform rate.

A further increase in the stream of gas finally leads to a velocity at which the particles float, which occurs when the upward force equals the force of gravity acting on the particles. Beyond this velocity it is impossible to maintain a fluidized bed is therefore maintained at air velocities, which lie between those at which the bed starts to expand tha those at which the particles float.

Particles of a moist product can be put into turbulent motion using only air provided moisture or thermoplasticity does not stick there surfaces together. In such a case fluidized bed drying is carried out in a relatively simple installation. If the particles stick together initially, using mechanical stirrers to break up the particle agglomerates can produce stable fluidized beds. Another possibility of forced mixing, which at the same time, also provides forced transport. The fluid bed dryer is based on a vibrating conveyor which can be up to 10 m long and up to 1 wide. Eccentric motors produce the vibration of the drier, which may be supported on helical or leaf springs. The direction of the vibration can be changed by rotating the motor and the amplitude by changing its eccentricity to ensure a uniform distribution of air over the whole area of the air permeable base plate.

The resistance offered by the plate to the airflow should be in the range of 1000 to 2000 pa (corresponding to a column of water (100-200mm high) depending on the type of product and the depth of the bed. The perforation occupy usually between 1.5 and 5% of the area of the plate, and the depth of the bed, which can be adjusted by means of an overflow weir, is usually between 50 and 300 mm.

The vibrating fluidized bed has the following advantages: forced conveyance in a given direction, fluidization even at low air velocities and a residence time of the product which can be at least partially controlled.

Flash dryers are used to dry efficiently those materials that can be transported pneumatically in large volume. During this conveying operation, small solid particles can be dried by pre heating the carrier air or gas to such an extent that it act as a drying medium. These types of industrial dryers always operate in a co-current manner, and the drying takes place in a very short period of time. Flash dryers are probably the most suitable forms of atmospheric pressure continuous direct dryers. These industrial dryers are most suitable for drying wet materials with a moisture content is in the form of recoverable solvents.

Ring dryers are especially suitable for drying of adhesive products that dry in falling rate periods and tend to agglomerate. The product is pumped to a special extruder discharging directly into the path of high speed rotating disintegrator beaters. The disintegrator is continuously swept with hot air borne recycled material flowing through the dryer for a second pass. Most water is removed in and immediately above the disintegrator. The final drying takes place in ring shaped duct. This reduce the natural stickiness of the product and prevents material build up within the dryer