Drying Operations. Md. Khairul Islam Lecturer Department of Applied Chemistry and Chemical Engineering. University of Rajshahi

Transcription

1 Drying Operations Md. Khairul Islam Lecturer Department of Applied Chemistry and Chemical Engineering. University of Rajshahi

2 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. 2

3 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. 3

4 Theory of drying 8/15/2015 Md. Khairul Islam,Lecturer, ACCE,RU 4

5 Theory of drying 5

6 Hysteresis or drying loop 6

7 Explanation of drying rate curve 7

8 Calculation of drying time 8

9 Calculation of drying time 9

10 Mechanism of moisture movement within the solid 1. Liquid diffusion: Diffusion of liquid moisture may result because of concentration gradients between the depths of the solid, where the concentration is high, and the surface where it is low. 2. Capillary movement: Unbound moisture in granular and porous solids such as clays, sands, paint pigments, and the like, move through the capillaries and interstices of the solids by a mechanism involving surface tension. The capillaries extend from small reservoirs of moisture in the solid to the drying surface. As drying proceeds, at first the moisture moves by the capillary to the surface sufficiently rapidly to maintain uniformly wetted surface and the rate drying is constant. The water is replaced by air entering the solid through relatively few opening and cracks. The surface moisture is eventually drawn to spaces between the granules of the surface, the wetted area at the surface decreases, and the unsaturated surface drying period follows. 10

11 Mechanism of moisture movement within the solid The subsurface reservoirs eventually dry up, the liquid surface recedes into the capillaries, evaporation occurs below the surface in a zone or plane which gradually recedes deeper into the solid, and a second falling rate period results. During this period, diffusion of vapor within the solid will occur from the place vaporization to the surface. 3. Especially if heat is supplied to one surface of a solid while drying proceeds from another, the moisture may evaporate beneath the surface and diffuse outward as a vapor. Moisture particles in granular solids, which have been isolated 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.

12 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 dryers i. Atmospheric compartment ii. Vacuum tray b) Continuous dryers i. Tunnel dryer 2. Granular or loose matarials a) Rotary dryers i. Standard rotary ii. Roto louvre b) Turbo dryers c) Conveyor dryers d) Filter dryer combination 12

13 Classification of dryers 3. Material in continuous sheets a) Cylinder dryers b) Festoon dryers 4. Pastes and sludges od caking crystals a) Agitator dryer i. Atmospheric ii. Vacuum 5. Materials in solution a) Drum dryers i. Atmospheric ii. Vacuum b) Spray dryers 6. Special methods a) Infrared radition b) Dielectric heating c) Vaporization from ice 13

14 Description of some important dryers 1. Compartment dryer: 8/15/2015 Md. Khairul Islam,Lecturer, ACCE,RU 14

15 Description of some important dryers 2. Tunnel dryer 15

16 3. Rotary dryer: Description of some important dryers 16

17 Description of some important dryers 4. Drum dryer 17

18 Description of some important dryers 5. Spray dryer: 18

19 Dryer control 8/15/2015 Md. Khairul Islam,Lecturer, ACCE,RU 19

20 Thanks all the patience 20