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 crosssection H = hold up of solid.) ?

• A. Z.H.ρ/L
• B. L/Z.H.ρ
• C. H.ρ/L
• D. Z.H/ρL