Question

Drying of solids involves simultaneous transfer of

• A. Heat only
• B. Mass only
• C. Momentum only
• D. Heat and mass

Hint:

Logic Tip: Think about a hairdryer. You need the hot coils to provide Heat (to boil the water out of your hair), and you need the blowing air to carry away the Mass (the water vapor) so your hair actually gets dry!

Answer 1

The Correct Option is D

Concept:
Industrial drying is a complex unit operation where a liquid (typically water) is separated from a wet solid by evaporating it into a surrounding unsaturated gas phase (typically air). This process fundamentally requires two distinct transport phenomena to occur at the same time.

Step 1: To convert liquid water residing inside the solid into water vapor, the latent heat of vaporization must be continuously supplied to the system.

Step 2: This required thermal energy must be transferred from the hot drying medium (the air) to the surface and interior of the wet solid. This is Heat Transfer.

Step 3: Once the water absorbs the heat and vaporizes, the moisture must physically move from the interior of the solid to the surface, and then diffuse into the bulk air stream.

Step 4: The physical movement of this moisture (a substance) from a region of high concentration (inside the solid) to low concentration (the dry air) is Mass Transfer.

Step 5: Because evaporation cannot occur without thermal energy, and the removal of moisture is inherently the transport of a substance, the drying of solids necessarily involves the simultaneous transfer of both heat and mass.