Drying

Drying removes moisture from solids by evaporation. Learn moisture content (wet and dry basis), equilibrium and critical moisture, constant-rate and falling-rate periods, drying-time calculations, heat requirements, and dryer types (tray, rotary, spray, fluid-bed, freeze).

From pharmaceuticals and milk powder to minerals and timber, drying is one of the most energy-intensive unit operations — often 10–25 % of industrial energy use in some sectors.

Understanding drying rates lets engineers size dryers and avoid damaging heat-sensitive products.

An analogy

Drying washing: at first the surface stays wet and dries at a steady rate (constant rate); later moisture must travel from inside, so drying slows (falling rate).

Moisture measures

Dry-basis moisture X (kg water per kg dry solid) is convenient for calculations because dry solid mass stays constant. Equilibrium moisture X* depends on air humidity; free moisture X − X* can be removed.

Drying-rate curve

Constant-rate period: surface wet, rate set by heat and mass transfer from air (like a wet-bulb evaporator). Critical moisture X_c: surface starts drying. Falling-rate period: internal diffusion or capillary flow controls, rate falls toward zero at X*.

Dryer types

Tray and tunnel dryers (batch), rotary dryers (minerals, fertilisers), fluidised-bed dryers (granules), spray dryers (milk, detergents, from liquid feed), drum dryers, freeze dryers (pharma, coffee — sublimation under vacuum), conveyor and microwave dryers.

Energy and quality

Energy per kg water is typically 3–6 MJ (vs 2.3–2.5 MJ latent heat) because of exhaust losses. Heat recovery, closed-loop superheated-steam drying and heat pumps save energy. Product quality issues: case hardening, shrinkage, thermal degradation, dust explosion risks.

Formulas

• Dry vs wet basis: X = w/(1 − w), w = X/(1 + X)
• Constant-rate time: t_c = M_s (X₁ − X_c) / (A N_c)
• Falling-rate time (linear): t_f = M_s X_c/(A N_c) · ln(X_c/X₂) (X* ≈ 0)
• Evaporation heat: Q ≈ m_water λ + sensible heating

Worked example: Batch drying time

Given: 100 kg dry solid, X 0.5 → 0.1; X_c = 0.2, A = 10 m², N_c = 2 kg/m²·h

Find: Drying time.

• Constant: 100 × (0.5 − 0.2)/(10 × 2) = 1.5 h
• Falling: 100 × 0.2/20 × ln(0.2/0.1) = 0.69 h
• Total ≈ 2.2 h; water removed 40 kg

Answer: ≈ 2.2 h

Worked example: Wet to dry basis

Given: Wet cake 35 % moisture (wet basis)

Find: Dry-basis moisture.

• X = 0.35/0.65 = 0.54 kg/kg

Answer: 0.54 kg water/kg dry solid

Common mistakes

• Wrong: Mixing wet and dry basis. — Right: Convert to dry basis for calculations.
• Wrong: Drying too fast at high temperature. — Right: Case hardening and degradation — control the falling-rate period.

Applications

• Food & dairy: Spray-dried milk, freeze-dried coffee, dehydrated vegetables.
• Pharmaceuticals: Fluid-bed granule drying, lyophilised injectables.
• Minerals & chemicals: Rotary dryers for fertilisers, clays and salts.

Key takeaways

• Use dry-basis moisture X.
• Constant-rate then falling-rate periods.
• t_c = M_s(X₁ − X_c)/(AN_c).
• Drying is energy-intensive — recover heat.

Going further

• Psychrometric design: Inlet/outlet air conditions set dryer airflow and efficiency.
• Diffusion-controlled drying: Fick's second law predicts falling-rate behaviour.
• Dust explosion safety: Dryers handling organic powders need venting and inerting.