Dryering

1) Fluid Bed Dryer

Fluid bed dryers are mainly used for the drying of wet particles and granular materials that can be fluidized. They are used in processing of products such as inorganic chemicals, foodstuff, pharmaceuticals, pigments, fertilizers, detergents etc. Main advantages are;

 

- High heat and mass transfer rates

- Good mixing of solid particles

- Easy material transport

- Extended residence time

Hot and cold process air is supplied to the bed through a perforated distributor plate and flows through the bed of solid particulates. At a certain gas velocity, the bed starts to fluidize. This is known as minimum fluidization and the corresponding gas velocity is called minimum fluidization velocity. The gas velocity is such that only very fine particles are carried over to the dust collection system.   

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Fig. 1. Fluid Bed Dryer   

2) Spray Dryer

Spray drying technology is used to convert either solutions or free-flowing slurries into powder particles. They are used in processing of products such as inorganic chemicals, foodstuff, pharmaceuticals, pigments, fertilizers, detergents etc. This process turns a fluid feed into small droplets prior to drying. It is called Atomization. Two techniques are used for atomization. One is centrifugal atomization and the other one is pressure atomization. Both have advantages and disadvantages. Pressure nozzle atomizer uses high pressure pump to feed the liquid to the dryer. Pressure energy is converted into kinetic energy. It produces a powder with a high bulk density and narrow particle size distribution. In centrifugal atomization, spray is created by passing the feed through a rotating disk.

 

Main advantages of spray drying are;

 

- Product properties are more effectively controlled

- Relatively uniform, spherical particles are produced

- Corrosion problems are minimized

- Simple and continuous operation

- Heat sensitive products can be dried

Main disadvantages of spray drying are;

- Relatively low thermal efficiency, because exhaust air stream contains heat

- Relatively high investment costs  

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Fig. 2. Spray Dryer  

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Fig. 3. Atomizer