Pneumatic

Pneumatic

Pneumatic Conveyor dryer or ‘Diluent bed dryer’:

Principle: An air-solid contacting system in which the solid is exist in a dilute condition is termed as pneumatic system.

The dryers are called pneumatic because the quantity and velocity of the air is sufficient to leave and convey the solid against the force of gravity.

Mechanism of drying: The materials to be dried are introduced through the hopper into the duct, carrying gas at high velocity and the air can be made fresh by air filter, the fan propels the air to the heater to heat the air. As a result, the particles are carried to the drying chamber where cyclone is produced by high velocity heated gas. So, volume of the materials expanded. Distance between the particles increases and particles exist in single entities. Thus, the particles are highly exposed to the hot gas, so heat and mass transfer occur instantly.

The airlock prevents the panning out of air and drying is performed rapidly and efficiently. If the material is partially dried the product may recycle.

Pneumatic conveyors may be single stage or multi stage.

The single stage is employed for ordinary heating and cooling operations and drying processes requiring evaporations of small quantities of surface moisture. Whereas multi-stage installations are used for difficult drying processes, e.g. drying heat-sensitive products containing large quantities of moisture and drying materials initially containing internal as well as surface moisture.

Application: Pneumatic conveyors are suitable for materials which are granular and free flowing when dispersed in the gas stream. Tablet granules and capsule powders are dried by this process.

Advantages:

1. All materials (except liquid) which are granular and free flowing can be dried.
2. It is suitable for—

-          heat sensitive

-          explosive

-          easily oxidized and

-          flammable materials

Which can’t be exposed to process conditions for extended period.

3. It can be operated satisfactorily in either batch or continuous processes.
4. It can be single stage or multi-stage as it is necessary.
5. Thermolabile substances such as hormones, antibiotics, blood products, etc can be dried efficiently.
6. There is no destruction of materials by heat as low temperature is required accordingly fuel cost is minimized.
7. The machine is simple and yield value is excellent i.e. high output.

Disadvantages:

1. It is not suitable for highly abrasive materials.
2. It may result significant particle size reduction.
3. The machine is costly.

Liquid materials can’t be dried.