Rate of Drying Curve.



  • Drying is defined as the removal of liquid from a product usually with application of heat.

Rate of Drying Curve.

  • Drying process can be divided into three periods

    1. Initial Adjustment Period.

    2. Constant drying rate period.

    3. First falling drying rate period.

    4. Second falling rate period.

  1. Initial Adjustment Period (A-B):

  • Also called the “Heating up” period.

  • In this period the substance gets heat and increases in temperature.

  • Drying has not yet started.

  1. Constant drying rate period (B-C):

    • During this period the temperature of the solid and the rate of drying remain constant.

    • The moisture evaporating from the surface is replaced by water diffusing from the interior of the solid at a rate equal to the rate of evaporation. 

    • The moisture content at the end of constant rate (point C) is referred to as the critical moisture content (CMC). 

    • At CMC, dry spots start appearing and drying rate starts falling.

  2. First falling drying rate period (C-D):

    • This period is also called the period of unsaturated surface drying

    • During this period the rate of evaporation is insufficient to saturate the air in contact with the surface.

    • This is because the surface water is no longer replaced at a rate required to maintain continuous film on the surface. 

    • Between points C and D, the number and area of dry spots continue to increase, and the rate of drying falls steadily. 

    • Point D is referred to as the second critical point, at this point, the film of surface water has completely evaporated.

  3. Second falling rate period (D-E):

    • At the end of the first falling rate period, the rate of drying falls even more rapidly. 

    • The plane of vapourization moves From the surface into the body of the solid. 

    • The evaporation takes place from within the solid and the vapour reaches the surface by molecular diffusion through the material.

    • Point E is referred to as the equilibrium moisture content.

  1. Equilibrium moisture content (EMC):

    • Beyond this point drying is not possible as remaining moisture is bound moisture.

    • Drying rate here becomes zero.

Commonly Asked Questions.

  1. Explain theory of drying with the help of a drying curve.

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