Bentonite Drying Drum - 2D Rotary Dryer Moisture and Temperature Profile Lab

Residence time

Solids move through a sloped rotating drum by cascading; an empirical (Friedman-Marshall) formula gives the mean residence time θ = 0.23 L / (S N^0.9 D) in minutes, with L and D in metres, slope S as a fraction and N in rpm. Faster rotation, a steeper slope or a wider drum shorten it.

Drying and heat balance

Moisture X (kg water per kg dry clay) falls as dX/dt = -k (X - X_e), where k = k_ref exp[(E/R)(1/T_ref - 1/T_s)] rises with the solids temperature (a simple Page-type form with equilibrium moisture X_e = 0.02). The solids warm towards the gas, (c_s + X c_w) dT_s/dt = (c_s + X c_w)(T_g - T_s)/τ - h_fg (-dX/dt), and the gas, flowing in the same direction, gives up exactly the heat the solids gain, G c_g dT_g = -F (c_s + X c_w) dT_s - F h_fg (-dX). The efficiency is the latent heat used divided by the heat the gas brings above ambient.

Honest simplifications: plug flow with no axial mixing, a single lumped heat-transfer time constant, no wall losses, no dust carry-over, constant specific heats, an assumed rate constant (not measured for any particular clay) and no counter-current option. Use it for trends, not for plant design.

• Normal operation: the moisture curve falls steeply near the gas inlet, where the gas is hottest.
• Overloaded feed: the same gas has more water to remove, so the product stays wet and the exhaust cools.
• Hot gas, fast drum: raise the gas temperature but shorten the residence time and watch the solids temperature.
• Drag the rotation speed slider and use the third graph to see how the residence time changes.