A wet solid of 100 kg containing 30 wt% moisture is to be dried to 2 wt% moisture in a tray dryer. The critical moisture content is 10 wt% and the equilibrium moisture content is 1 wt%. The drying rate during the constant rate period is 10 kg/(h·m²). The drying curve in the falling rate period is linear. If the drying area is 5 m², the time required for drying is ____ h (rounded off to 1 decimal place). Note: all the moisture values given are on dry-solid basis.

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Accepted Answer

The correct answer is 0.5 to 0.9. GATE 2025 official key: 0.5 to 0.9 h. Dry solids L_s = 100/1.30 ≈ 76.9 kg. Constant rate period: t_c = L_s(X₀−X_c)/(A·R_c) ≈ 0.31 h. Falling rate period: t_f = [L_s(X_c−X*)/(A·R_c)]·ln[(X_c−X*)/(X₂−X*)] ≈ 0.30 h. Total ≈ 0.61 h.