Air Dispersion Dryers Drying of wet cakes, pastes and sludges.

When producing powders from aqueous suspension, it is a distinct advantage to first mechanically dewater so as to reduce the plant initialization and operating costs of the final drying stage. During dewatering suspensions are converted to either filter cakes, pastes or sludges.

Drying of such material thereafter is ideal in Air Dispersion Dryers. The selection of a particular dryer depend on the nature of wet deed cake particle size distribution of final powder and the drying nature is usually characterized by Drying Curve as shown. The selection of the most suitable dryer depends upon nature of the wet feed cake.

Liquid to be dried is fed to the atomizer wheel through a liquid distributor. The wheel rotates at a high speed, causing disintegration of the material into fine droplets. The rotary atomizer can handle wide range of products, including slurries, with high viscosities.

Principle

A peristaltic pump delivers liquid from the sample container to the 2 fluid nozzle mounted on the drying chamber. Compressed air from the inbuilt compressor is also supplied to the nozzle, resulting in atomization of the liquid into a fine spray.

Air from the blower is heated by an electrical air heater and blown through the drying chamber. Due to the large surface area, drying is very rapid, resulting in a sharp drop in temperature. The dry powder particles are separated from the air in the cyclone separator and collected in the glass collection bottle. The exhaust air is directed through a flexible SS hose into the atmosphere. A bag filter can be provided for secondary powder collection.

Construction

• The blower, electrical air heater, air compressor and the pump are placed in a stainless steel panel, with free and easy access.
• The air entering the panel is filtered through a fine cloth filter.
• The chamber and cyclone are of borosilicate glass with clamp fillings, designed for easy assembly and removal.
• The peristaltic pump is mounted on the front of the panel with easy accessibility..

Controls

The dryer has ease of operations. The inlet air temperature is precisely controlled by the inlet temperature PID controller. The Panel includes:

• Inlet Temperature controller.
• Outlet Temperature indicator
• On/Off switch and indicator for blower
• On/Off switch and indicator for heater
• On/Off switch and indicator for air compressor.
• On/Off switch and indicator for feed pump
• Pump speed control with LCD display
• Compressed air pressure control with LCD display of air pressure.

Atomized liquid and hot air from the air disperser come in intimate contact in the drying chamber. With the intense direct contact, heat and mass transfer occurs between the two phases. Due to the large surface area created by the fine atomization, drying is very rapid and occurs in a matter of seconds. Drying can be co-current/counter current depending on the particular application. The chamber can be a flat roof design (with rotary/nozzle atomizer), top cone design (nozzle atomizer), tall form design with a bustle construction (for coarser powder), two stage design (with subsequent drying in a fluid bed dryer), with an integrated fluid bed (placed in the bottom cone of the chamber).

The vibrated fluid bed dryer is used for fluidization of materials which are difficult to handle in a static fluid bed dryer. These materials are often characterized by having one or more of The following physical properties:

• Wide particle size distribution
• Low strength of wet or dry particles
• Stickiness or thermo plasticity of the particles
• Pasty properties of the wet feed


In the vibrated fluid bed dryer, the material is easily transported through the dryer by the combined effect of fluidization and vibration. The fluidization velocity can be judiciously selected, to minimize entertainment of the smaller particles, to avoid gas bubbling m the fluid bed thereby minimizing attrition. The residence time can be adjusted, by varying the bed height, through the overflow weir position. A cooling section can also be provided, by partitioning of the plenum chamber.

Typical products handled

• Salt
• Milk products (used as a secondary dyer)
• Chemicals
• Tea