Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
Mass diffusivity of water vapour in m² s⁻¹ during the falling rate phase of drying is
Options:
(A) 7.01×10⁻⁴
(B) 5.07×10⁻⁴
(C) 3.71×10⁻⁵
(D) 1.07×10⁻⁵
Official Answer Key:
D
Step-by-Step Solution & Explanation
The falling-rate period is normally treated as internal (Fickian) diffusion-controlled, with an effective diffusivity extracted from the unaccomplished-moisture-ratio decay against elapsed time in the falling-rate period. The passage as transcribed does not give the *end time* of the falling-rate period (only the critical-point time is stated), which is needed to fit the exponential decay and solve for D uniquely — so I could not independently reproduce a value to the same confidence as the other questions here. Typical effective moisture diffusivities for vegetable/tuber tissue during air drying fall in the 10⁻¹⁰–10⁻⁹ m²/s range for *liquid* diffusion, but the magnitude of the printed options (10⁻⁵–10⁻³ m²/s) is consistent instead with vapour-phase diffusivity in a porous solid, for which (D) 1.07×10⁻⁵ m²/s is the physically closest of the four to typical water-vapour-in-air/porous-media diffusivities. This answer should be treated as not independently verified and rechecked against the full original passage if a time value for the falling-rate period is available.
Section 7 — Dairy and Food Engineering
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