Spray Dryer

Spray Dryer

Spray dryer (pneumatic system):

Principle: Spray dryers are used for drying only the liquid materials such as solutions, suspensions, slurries and thin pastes. In this method, the liquid is dispersed as fine droplets into a moving stream of hot air where they are evaporated rapidly before reaching the wall of the drying chamber. The product dries into a fine powder, which is collected into a collection system. There are many types of spray dryers designed to suit the materials to be dried.

Operation: The materials to be dried are fed to the atomizer by gravity or by use of a suitable pump. The rate of feed is adjusted in such a way that each droplet of the liquid is dried completely before it comes in contact with the walls of the drying chamber and yet the dried powder is not overheated in this process. The inlet air temperature is kept constant. Too high a temperature may result in improper drying. Similarly, excessive feed rate will lower the outer temperature due to which the material will be collected on the walls of the drying chamber.

The disc of the atomizer is driven by an air turbine and spins at 35000 rpm. Air is introduced by the help of fan and filtered with inlet air filter. Hot drying air is supplied by the blowing of air over a heat exchange to a maximum temperature of 350^oC.

The sprayed droplets from atomizer come into contact with the hot air. The droplets rapidly evaporate as they are carried by currents of hot air in the drying chamber.

The moist air in the drying chamber is formed due to evaporation of liquid droplet is separated out from the drying chamber through a passage. The dried powder is separated from the exhaust air in cyclone separator or by an airlock and colleted into container. The flow rate of air is controlled by a value.

 

Advantages:

1. The droplets are small; giving a large surface area for heat and mass transfer so that evaporation is very rapid i.e. the drying time for a droplet is only a few seconds.
2. It can be used for drying the heat sensitive or oxidized materials without degradation.
3. It can be designed for drying under sterile conditions.
4. The dried powder will have a uniform particle size and shape.
5. Because of good flow properties, the spray dried powders can be easily compressed in the form of tablets.
6. It is useful in the coating and encapsulation of both solids and liquids.
7. Labor cost is low since the material is dried in a single operation with no handling.

Disadvantages:

1. Solid materials cannot be dried.
2. The equipment is very costly and bulky.
3. Since the equipment is bulky, it takes much time for cleaning.
4. There is lot of wastage of heat.

Application:

1. Spray dryers are used for drying of liquid materials like emulsion, suspension, solution, slurries, thin pastes, etc.
2. Spray drying can be used to dry materials that are sensitive to heat and or oxidation without degrading them, even when high temperature air is employed.
3. The liquid feed is dispersed into droplets, which are dried in seconds because of their high surface area and intimate contact with the drying gas.
4. The product is kept cool by vaporization of the enveloping liquid and the dried product is kept from the overheating by rapid removal from the drying zone.
5. The improvement in flow and reduction of air entrapment make the spray dried material suit ale for use in the manufacture of tablets and capsules.

It is extremely useful in the coating and encapsulation of both solids and liquids.