DRYER USED IN PROCESS INDUSTRY

Drying of solid means the removal of relatively small amount of water or other liquid from the solid material to reduce the content of residual liquid to an acceptably low value. Drying is usually the final step in the series of the operations and the product from the dryer is often ready for the final packaging. Drying is a relative term and means merely that there is reduction in liquid contents from an initial value to some acceptable final value. In the majority of processing industries, drying is carried out for one or more of the following reasons:
 To reduce the cost of transport.
To make the material more suitable for handling for example soap, powders, dye stuffs and

fertilizers.
To provide the definite properties such as maintaining the free flowing nature of salt.

To remove the moisture which may otherwise lead to corrosion, e.g., the drying of coal gas o 

benzene prior to chlorination.

CLASSIFICATION OF DRYERS

There is no simple way of classifying drying equipment, dryers may be

Continuous

 Batch wise.

Agitate the solids.

Essentially un agitated.

 Some dryers can handle almost any kind of material.

 Others are severely limited in the type of feed they can accept.

  PRINCIPLES OF DRYING

Because of wide variety of materials that are dried in commercial equipment and many types of equipment that I used there is no single theory of drying that covers all the material and the dryer types. Variation in shape and size of stock, in moisture equilibria, in the mechanism of flow of moisture through the solid and in the method of providing the heat required for vaporization - all prevent unified treatment.

Dryers are seldom design by the users but are bought from the companies that specialized in the engineering and fabrication of drying equipment. Today, dryers are in operation in most manufacturing industries including chemicals, pharmaceutical, process and food. Products that are dried range from organic pigments to proteins, as well as minerals to dairy products. Because of the spectrum of duties required, there is a great variety of dryers available. The correct choice depends on the properties of the feed material and the desired characteristics of the final products.

DRYING EQUIPMENT

Dryers are grouped under two categories

 first group includes

• rigid or granular solids
• semi solid pastes

2. the second group includes

• slurry
• liquid feeds

 Dryers for Solids and Paste

§  tray dryers

§  screw conveyor dryers

§  tower dryers

§  rotary dryers

§  screen conveyor dryers

§  fluid bed dryers

§  flash dryers

Dryers for Solutions and Slurries

§  spray dryers

§  thin film dryers

§  drum dryers
SELECTION OF DRYER

Selection of the dryer for our requirement is depend on the following factors

§  nature of the feed

§  drying time

§  sensitivity of the material

§  temperature

§  cost

Nature of Feed

Our feed which is to be dried is of slurry nature. Moisture content is 85 % so it is in the solution form. 

Drying Time

We select our drying equipment on the basis of drying time as we require short drying time so we select that dryer whose drying time is in the range of 3 - 6sec in co current operation.

Sensitivity of Material

Selection criteria for dryer are mainly depend on the sensitivity of the material. As our feed is a heat sensitive material because it is a food product so we select that dryer that is mainly used for food product due to its sensitivity.

Temperature

As the food product is heat sensitive material so temperature is selected by which food product will never damage.

Cost

There are two methods for the removal of water contents

§  evaporation

§  drying

But in the case of evaporation cost is too high so we select dryer for the removal of water contents.

Conclusion

We have discussed five parameters we are important in the selection of dryer. When we are considering the above five points the most suitable dryer is spray dryer. It is mostly used is food industries.

SPRAY DRYERS

A spray dryer, as the name implies, is a device for drying, utilizing a spray. A spray dryer mixes a heated gas with an atomized (sprayed) liquid stream within a vessel (drying chamber) to accomplish evaporation and produce a free flowing dry powder with a controlled average particle size. The unit operation of spray drying includes the following key components:

§  A method for atomizing a solution or slurry

§  An air/gas heater, or a source of hot air such as a waste flue gas

§  A gas/spray mixing chamber with adequate residence time and droplet trajectory distance for achieving the heat and mass transfer

§  A means for recovering the solids from the gas stream

§  A fan to induce the required air/gas through the spray drying system.

The Spray Drying Process

The basic spray drying process consists of three processing steps:

§  Atomization of the feed slurry into tiny droplets. This is accomplished using rotary or nozzle atomizers.

§  Mixing of the atomized spray with the hot drying gas, typically air, and drying of the individual droplets into solid particles. This is carried out inside the drying chamber, and it is important that the particles are dry (at least on the surface) before they contact the internal surfaces of the drying chamber.

§  Recovery of the dried product. This can be done partly from the base of the drying chamber and partly from separation equipment for the spent drying air.