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.)

Z.H/L.ρ
L/Z.H.ρ
H.ρ/L
Z.H.ρ/L

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Process Equipment and Plant Design
Overall distillation column efficiency for most of the refinery columns can be given by (for μ = 0.07 - 1.4 cp and relative volatility < 4) (where η = overall column efficiency, % μ = molal average viscosity of feed at average column temperature and pressure, cp Δp = pressure drop per tray, psi)

Η = 24.6 (Δp/μ)
Η = 1.66 - 0.25 log μ
Η = 17 - 61.1 log μ
Η = 0.25 log μ - 1.67

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