Drying of slurries on a rotating drum: In a single-drum or twin-drum dryer, which phenomenon most commonly controls the overall drying rate for thin films?

Introduction / Context:
Drum dryers spread a thin layer of slurry on a hot rotating cylinder. As the drum turns, heat conducts from the metal surface through the liquid film, causing surface evaporation and producing a dried sheet that is scraped off. Understanding the controlling resistance helps set steam pressure, drum speed, and film thickness.

Given Data / Assumptions:

• Thin films with short residence times (seconds).
• Well-wetted metal surface, negligible internal diffusion limitations for many foods/chemicals.
• Evaporation limited by energy supplied through the drum wall.

Concept / Approach:
For thin films typical of drum drying, the rate is frequently governed by heat transfer (supply of latent heat) rather than slow diffusion through a thick solid. The overall resistance lies in the metal/condensate/liquid-film path, not deep mass diffusion like in thick cakes. Hence steam pressure, condensate removal, and film contact dominate performance.

Step-by-Step Solution:
Recognize that the wet film is thin and exposed to high temperature at the surface.Heat must be transmitted quickly from condensing steam → drum wall → liquid film.Drying rate aligns with available heat flux; thus heat transfer controls.Select the option highlighting heat transfer control.

Verification / Alternative check:
Plant tuning shows increases in steam pressure or improved condensate drainage raise throughput markedly, consistent with heat-transfer control; changing air-side conditions has smaller effects.

Why Other Options Are Wrong:
Internal diffusion (a) is minor for very thin films and short paths.

Both/neither (c/d) are overly broad and do not reflect the dominant mechanism in typical drum dryers.

Common Pitfalls:

• Allowing condensate pooling inside the drum, which reduces heat-transfer coefficient.
• Letting films get too thick, inadvertently introducing diffusion limits.

Final Answer:
Heat transfer to the wet film (evaporation controlled by heat supply)