During the constant-rate drying period of a solid material, what behavior is observed ?
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Answer: C. Surface evaporation of unbound moisture occurs
Explanation
During the constant-rate drying period, moisture moves to the surface to replenish evaporated unbound water. Surface evaporation of unbound moisture keeps the evaporation rate steady throughout this initial phase. External gas conditions control the rate of drying while unbound moisture is present on the solid. Unbound moisture has a vapor pressure equal to free liquid at the same operating temperature. Internal capillary forces transport liquid to the surface as fast as evaporation takes place. The temperature of the wet solid remains constant at the wet-bulb temperature of the air.
Drying operations in chemical industries are designed to maximize the duration of this constant-rate period. Removing surface moisture quickly prevents surface crusting and case hardening of the dried material. Food processing uses this mechanism to dry outer surfaces before internal diffusion limits take over. Option A is incorrect because high humidity decreases the drying rate during all drying periods. Option B is incorrect because increasing air temperature actually increases the rate of drying. Option D is incorrect because surface evaporation of unbound moisture correctly defines this phase.
- Subject
- Engineering
- Category
- Chemical Engineering
- Topic
- —
- Difficulty
- medium
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