Unit Operations and Equipment Design
What the domain tests
Unit Operations and Equipment Design tests selection, sizing, and specification of the equipment that actually makes the product: reactors, centrifuges, filters, dryers, mills, blenders, chromatographs, and clean-in-place systems. Questions often combine a simple sizing calculation with a cleanability or materials constraint.
Core rules
- Size equipment from a duty, not a guess: mass or energy rate, residence time, settling velocity, filtration flux, or drying rate.
- For dryers, the energy balance must include latent heat, sensible heat of solids and gas, and heat losses. Air flow is set by the allowable humidity and temperature drop.
- For filtration, flux declines with cake resistance; constant-pressure filtration follows the Ruth equation, and scale-up by filter area alone fails if the cake is compressible.
- For centrifugation, sigma theory gives Q = Σ × v_g for dilute systems; the sigma factor depends on geometry and speed. Real feeds with high solids need pilot testing.
- Design for cleaning: avoid dead legs (typically ≤ 1.5D or ≤ 2D depending on the standard), provide drainability, use hygienic connections, and specify surface finish where product contact occurs.
- Chromatography: scale by column diameter at constant bed height and linear velocity, then verify pressure drop and resolution.
Worked example
A fluid-bed dryer must evaporate 120 kg/h of water at 60 °C. Latent heat is about 2,358 kJ/kg, so duty = 120 × 2,358 = 283,000 kJ/h. Air enters at 120 °C and leaves at 60 °C; cp ≈ 1.01 kJ/kg·K. Air flow = 283,000/(1.01 × 60) = 4,670 kg/h. Add a 20% margin for heat losses and non-ideal contact: 5,600 kg/h. If the air were heated only to 90 °C, flow would rise to about 9,340 kg/h, which may exceed fan and filter capacity. The reasoning is to fix the duty, choose the driving ΔT, then check the equipment constraint-not to pick a dryer size first.
Traps
- Using 100 °C latent heat for water evaporated at 60 °C.
- Ignoring the sensible heat to warm the wet solids.
- Scaling a filter by area without accounting for compressible cake resistance.
- Designing a hygienic vessel with threaded fittings or undrainable low points.