Assuming psychrometric ratio to be unity, milk of 50% total solids is spray dried to powder of 5% moisture content on dry basis. Dry bulb and wet bulb temperatures of the inlet air to the spray dryer are 200 °C and 50 °C respectively. Latent heat of vaporization at the wet bulb temperature is 2393 kJ kg⁻¹. Assuming no sensible heating of powder the outlet air temperature is 80 °C. If inlet air absolute humidity was 0.015 kg H₂O kg dry air⁻¹, then kg of dry air required per kg feed is
Options:
(A) 4.7
(B) 5.9
(C) 7.4
(D) 9.5
Official Answer Key:
D
Step-by-Step Solution & Explanation
Water to be removed per kg feed: feed is 50% TS (50% moisture, wet basis); product retains the same 0.5 kg solids but at 5% moisture (dry basis) ⟹ water in product = 0.5×0.05 = 0.025 kg. Water removed = 0.5−0.025 = 0.475 kg/kg feed. Energy balance (sensible heat drop of drying air supplies the latent heat of evaporation): ṁ_dryair = (water removed × λ) / [c_p,air(T_in−T_out)] = (0.475×2393)/(1.005×120) = 1136.7/120.6 = ≈9.5 kg dry air/kg feed.
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