In rotary dryer design and operation, what typical fraction of the drum volume is occupied by the solid hold-up (material actually resident inside the rotating shell during steady operation)?

Introduction / Context:
Rotary dryers are widely used in chemical and process industries to reduce moisture in granular, crystalline, or lumpy solids. A key operating parameter is the solid hold-up, meaning the fraction of the dryer shell volume that is actually occupied by solids at any moment under steady operation. Correctly estimating hold-up helps size the dryer, set residence time, and predict gas–solid contacting efficiency.

Given Data / Assumptions:

• Cocurrent or countercurrent direct-fired rotary dryer handling free-flowing solids.
• Flight (lifter) arrangement is typical for industrial units.
• Operation is steady with normal fill levels—not start-up or emergency conditions.

Concept / Approach:
Inside a rotary dryer, solids are lifted and showered through the hot gas stream. Only a relatively small fraction of the cylinder cross-section is filled because excessive fill would suppress cascading and curtain formation, hurt heat/mass transfer, and increase power draw. Industry practice and pilot data typically show hold-up values in the low percentage range of total shell volume.

Step-by-Step Solution:

Verification / Alternative check:
Mass balance on solids (throughput = hold-up / residence time) combined with measured residence times in test units yields hold-up values generally in this 0.05–0.15 range for most granular products. Higher values are exceptional and can indicate suboptimal lifter design or sluggish materials.

Why Other Options Are Wrong:

• 0.25 to 0.50: Too high; would impair falling curtain formation and cause excessive power consumption.
• 0.50 to 0.80 and 0.80 to 0.90: Unrealistically high for rotary dryers; these fills are more like rotating kilns with bed flow, not efficient dryers.

Common Pitfalls:
Confusing mass hold-up with volumetric hold-up; ignoring the effect of flight design and rotational speed; assuming sticky or highly cohesive solids behave like free-flowing materials.

Final Answer:
0.05 to 0.15