Process Design, Simulation and Optimization
This domain covers flowsheet synthesis, steady-state simulation, degrees of freedom, recycle convergence, heat integration, and optimization. The exam tests whether you can specify a flowsheet correctly and judge whether a converged simulator answer is physically meaningful.
Core rules
- Count degrees of freedom before simulation: equipment specifications must equal independent variables. Over-specification causes conflicts; under-specification leaves an unsolvable flowsheet.
- Sequential-modular simulators solve units in order and iterate recycles; equation-oriented simulators solve all equations simultaneously. Convergence is not proof of correctness.
- Pinch analysis identifies minimum utility targets from composite curves. The pinch separates regions that should not exchange heat across it.
- Optimization requires an objective, decision variables, constraints, and a feasible starting point. Do not optimize a local variable while ignoring the plant-wide constraint.
Worked example
A water stream of 2 kg/s is heated 20 K. With Cp = 4.18 kJ/kg·K, Q = mCpΔT = 2(4.18)(20) = 167.2 kW. For countercurrent exchange with LMTD = 15 K and U = 500 W/m²·K, area A = Q/(UΔTlm) = 167,200/(500×15) = 22.3 m². The reasoning: the energy balance fixes the duty; the design equation then converts duty, driving force, and U into area. Check units: W/(W/m²·K × K) = m².
Traps
Over-specifying a simulator, accepting recycle convergence without a mass balance, ignoring the utility pinch, and optimizing yield while violating product specification or relief capacity.