Unit Operations and Separation Processes
Unit Operations and Separation Processes tests your ability to select, size and troubleshoot separation equipment - distillation and absorption columns, heat exchangers, pumps, dryers, filters and membrane units - and to justify the choice on energy and capital grounds.
Distillation and absorption
- McCabe–Thiele: operating lines from material balance and reflux ratio R; pinch at the q-line intersection; R_min from the pinch touching the equilibrium curve; typical design R = 1.1–1.5 R_min.
- Fenske for minimum stages, Gilliland for actual stages, Kirkbride for feed stage. Relative volatility falls as pressure rises, so vacuum is used for heat-sensitive or close-boiling systems.
- Capacity limits: flooding (Souders–Brown u_max = K√((ρL − ρV)/ρV)), weeping, entrainment and pressure drop. Packed columns use HETP and F-factor; trays use weir loading and downcomer residence time.
- Absorption: Kremser equations; lean-oil rate and temperature set recovery; stripping is the reverse operation.
Heat transfer and hydraulics
- LMTD with correction factor F for multipass; check shell-side and tube-side film coefficients plus fouling.
- Pump NPSH: NPSHa = (p0 − pv)/(ρg) + z − hf, and it must exceed NPSHr with margin.
Worked example - NPSH margin
Water at 30 °C (pv = 4.25 kPa, ρ = 996 kg/m³) is drawn from an open tank 2 m below the pump with 1.2 m of suction friction loss. NPSHa = (101.3 − 4.25) × 1000/(996 × 9.81) = 9.9 m; subtract 2 m static lift and 1.2 m friction → 6.7 m. If NPSHr is 5.5 m the margin is 1.2 m - acceptable for clean water but marginal for a volatile hydrocarbon, where the fix is a larger suction line or a lower pump elevation.
Traps
- Using total-reflux stages as the design case.
- Forgetting that NPSHa is expressed in metres of the pumped liquid, not of water.
- Applying LMTD without the F correction to a multipass exchanger.
- Quoting relative volatility at the wrong pressure or composition.