Unit Operations and Separation Processes
This domain tests selection and design of distillation, absorption, extraction, drying, filtration, and membrane units. The exam emphasizes equilibrium stages, operating lines, minimum reflux, mass-transfer coefficients, and the difference between theoretical and actual stages.
Core rules
- For staged separations, couple an equilibrium relation with an operating line. In binary distillation, the rectifying line passes through (xD, xD) and the q-line intersection; the stripping line passes through (xB, xB).
- Minimum reflux corresponds to a pinch; total reflux gives minimum stages. Actual stages are theoretical stages divided by stage efficiency.
- Absorption and stripping can be sized with the Kremser equations or NTU/HTU methods when equilibrium and operating lines are approximately linear.
- Separation choice depends on relative volatility, selectivity, energy use, product purity, and fouling. Distillation is usually favored for large-scale, volatile mixtures; membranes and adsorption suit dilute or thermally sensitive systems.
Worked example
A binary distillation column processes F = 100 kmol/h with zF = 0.40 benzene, producing xD = 0.95 and xB = 0.05. Overall benzene balance gives D = F(zF - xB)/(xD - xB) = 100(0.35)/(0.90) = 38.9 kmol/h, so B = 61.1 kmol/h. The reasoning: the overall and component balances fix both product rates before any stage calculation; reflux and stage count then follow from the operating lines and equilibrium curve.
Traps
Assuming constant molal overflow when enthalpy effects are large, forgetting the q-line for a subcooled feed, confusing minimum reflux with total reflux, and sizing a column from equilibrium stages without applying efficiency.