Pharmaceutical Engineering
Drying Rate Calculator
Finds the drying rate per unit area from the moisture removed, the drying time and the exposed area, and names the drying period.
Drying rate N
Enter both moisture contents, the drying time, the dry mass and the drying area.
Drying parameters
Moisture contents as mass ratios (kg water per kg dry solid), plus the batch and dryer geometry.
Moisture content at the start of the run.
Target moisture content at the end.
Total time in the dryer.
Bone-dry mass of solid, not the wet batch weight.
Total surface exposed to the drying air.
Typical moisture contents
Starting points when a measured value is not to hand.
| Material | X₁ | X₂ | Example |
|---|---|---|---|
| Food grains | 0.20 | 0.14 | Wheat, rice |
| Ceramics | 0.25 | 0.02 | Clay, tiles |
| Pharmaceuticals | 0.15 | 0.05 | Granules, powders |
| Wood | 0.40 | 0.12 | Lumber |
| Textiles | 0.30 | 0.08 | Fabric |
N = [M × (X₁ − X₂)] / (t × A)
M is the bone-dry mass of solid (kg), X₁ and X₂ the initial and final moisture contents as mass ratios (kg water per kg dry solid), t the drying time (h) and A the exposed area (m²).
M × (X₁ − X₂) is the water removed in kg; dividing by t gives the average rate in kg/h; and dividing again by A gives the rate per unit area, which is what lets two dryers of different sizes be compared. The rate is shown to three decimals, the two intermediate figures to two.
References
- Aulton ME, Taylor KMG, editors. Aulton's Pharmaceutics: The Design and Manufacture of Medicines. 6th ed. Edinburgh: Elsevier; 2022.
Common questions
No — as a ratio. 25% moisture on a dry basis is 0.25 kg water per kg dry solid. Entering 25 will give a rate a hundred times too high.
Dry basis. M is the bone-dry mass of solid and X is kg water per kg of that dry solid, so a material can exceed 1.0 kg/kg. Wet-basis figures must be converted first: X = w / (1 − w).
This is the average over the whole run. Drying starts in the constant-rate period, where the surface is fully wet and the rate is highest, then falls once internal diffusion controls it. The average always sits below the initial rate.
It is a rough classification based only on the final moisture content: above 0.5 kg/kg the run is still in the constant-rate period, 0.1–0.5 the first falling-rate period, below 0.1 the second. A real determination needs the critical moisture content from a drying curve.
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