Process Equipment and Plant Design
In a rotary drier, the average retention time of solid is(where Z = length of the drier, metres ρ = apparent solid density, kg/m³ L = flow rate of dry solids, kg/see. m² drier cross-section H = hold up of solid.)

H.ρ/L
L/Z.H.ρ
Z.H/L.ρ
Z.H.ρ/L

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Process Equipment and Plant Design
Overall tray efficiency of a distillation column is the ratio of the number of

None of these
Real trays required to the number of ideal trays required
Ideal trays required to the number of real trays required
Overall gas transfer units to the number of ideal trays required

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