Drying Operations - Rajshahi University Web Pagedept.ru.ac.bd/achem/images/Khairul-all-docs/3rd_year/Drying.pdf · ii. Vacuum tray b) Continuous dryers i. Tunnel dryer 2. Granular

Drying Operations

Md. Khairul Islam Lecturer

Department of Applied Chemistry and Chemical Engineering.University of Rajshahi

Drying fundamentals

• Definition of drying: Removal of moisture from a substance. Drying usually refers the removal of relatively small amount of water from solid or nearly solid materials.

• Evaporation: Removal of relatively large amount of water from solutions.

• Difference between drying and evaporation:• (i) In drying process major emphasis on solid products.• (ii) Drying involves the removal of water at

temperature below boiling point, whereas evaporation means the removal of water by boiling the solution.

• (iii) In evaporation the water is removed from the material as practically pure water vapor, mixed with other gases only because of unavoidable leaks. In drying water is usually removed by circulating air or some other gas over the materials.

8/15/2015 2Md. Khairul Islam,Lecturer, ACCE,RU

Objectives of drying

• To make the materials easily handled.• To reduce carrying cost.• To avoid the damage of the materials.• To preserve for long time.• To control the quality of a product.


Theory of drying


Theory of drying


Hysteresis or drying loop


Explanation of drying rate curve


Calculation of drying time


Calculation of drying time


Mechanism of moisture movement within the solid

1. Liquid diffusion: Diffusion of liquid moisture may result because ofconcentration gradients between the depths of the solid, where theconcentration is high, and the surface where it is low.

2. Capillary movement: Unbound moisture in granular and porous solids suchas clays, sands, paint pigments, and the like, move through the capillariesand interstices of the solids by a mechanism involving surface tension.The capillaries extend from small reservoirs of moisture in the solid to thedrying surface. As drying proceeds, at first the moisture moves by thecapillary to the surface sufficiently rapidly to maintain uniformly wettedsurface and the rate drying is constant. The water is replaced by airentering the solid through relatively few opening and cracks. The surfacemoisture is eventually drawn to spaces between the granules of thesurface, the wetted area at the surface decreases, and the unsaturatedsurface drying period follows.


Mechanism of moisture movement within the solid

The subsurface reservoirs eventually dry up, the liquid surface recedes intothe capillaries, evaporation occurs below the surface in a zone or planewhich gradually recedes deeper into the solid, and a second falling rateperiod results. During this period, diffusion of vapor within the solid willoccur from the place vaporization to the surface.

3. Especially if heat is supplied to one surface of a solid while drying proceedsfrom another, the moisture may evaporate beneath the surface and diffuseoutward as a vapor. Moisture particles in granular solids, which have beenisolated from the main portion of the moisture flowing through capillaries,may also be evaporated below the surface.

4. Pressure: Owing to shrinkage of outside layers of a solid on drying,moisture may be squeezed to the surface.

Classification of dryers• Dryers are classified on the basis of the form in whch the materials is

handled through the drying process-1. Materials in sheet or masses carried through on conveyers or trays

a) Batch dryersi. Atmospheric compartmentii. Vacuum tray

b) Continuous dryersi. Tunnel dryer

2. Granular or loose matarialsa) Rotary dryers

i. Standard rotary ii. Roto louvre

b) Turbo dryersc) Conveyor dryersd) Filter dryer combination


Classification of dryers3. Material in continuous sheets

a) Cylinder dryersb) Festoon dryers

4. Pastes and sludges od caking crystalsa) Agitator dryer

i. Atmosphericii. Vacuum

5. Materials in solutiona) Drum dryers

i. Atmosphericii. Vacuum

b) Spray dryers6. Special methods

a) Infrared raditionb) Dielectric heatingc) Vaporization from ice


Description of some important dryers

1. Compartment dryer:



2. Tunnel dryer



3. Rotary dryer:



4. Drum dryer



5. Spray dryer:


Dryer control


Thanks all the patience


Documents

Drying Operations - Rajshahi University Web Pagedept.ru.ac.bd/achem/images/Khairul-all-docs/3rd_year/Drying.pdf · ii. Vacuum tray b) Continuous dryers i. Tunnel dryer 2. Granular