Subtopic: Drying and milling of cereals, pulses and oilseeds
Question Statement:
A rotary dryer is used to dry 1200 kg h⁻¹of paddy containing 30% moisture (wet basis) to give a product containing 15% moisture (wet basis). Alternately, a portion of the dry product may be recycled and mixed with the fresh feed such that the mixed feed enters the dryer with moisture content of 20% (wet basis). The moisture evaporation rate without recycle and the paddy recycle rate in kg h⁻¹ respectively in the dryer are
Options:
(A) 211.76 and 2400
(B) 211.76 and 600
(C) 256.5 and 2400
(D) 256.5 and 600
Official Answer Key:
A
Step-by-Step Solution & Explanation
Without recycle: Dry matter (constant) = 1200×(1−0.30) = 840 kg/h.
Product mass (at 15% moisture) = 840/(1−0.15) = 988.24 kg/h.
Evaporation rate = 1200−988.24 = 211.76 kg/h.
Recycle rate: Let R = recycle rate (kg/h of finished 15%-moisture product mixed back with fresh feed). Mass balance on the mixed feed (target 20% moisture):
Dry matter in mix = 840 + 0.85R; total mix mass = 1200+R
(840+0.85R)/(1200+R) = 0.80 ⇒ 840+0.85R = 960+0.80R ⇒ 0.05R = 120 ⇒ R = 2400 kg/h. Answer: (A) 211.76 and 2400.
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