Process Design, Simulation and Scale-Up
Scope
The assessment tests whether you can turn a process concept into a defensible design basis: mass and energy balances, PFD and P&ID hierarchy, thermodynamic model selection, convergence strategy, and scale-up logic. It is not a software test; it is a judgement test.
Core rules
- Fix the design basis first: feed rate and composition, product specification, operating pressure and temperature, utility conditions, ambient design, turndown, and design margin. Every later number inherits these assumptions.
- A PFD shows process streams and major equipment. A P&ID adds instruments, control loops, isolation, relief, and line sizes. Do not blur the two.
- Choose a property package for the chemistry: ideal for light gases at low pressure; cubic equations of state for hydrocarbons; activity models for polar or non-ideal liquids; electrolyte models for acids and bases.
- Recycles require convergence. Tear streams, apply damping, and set the convergence criterion on the right variable. A converged but non-physical result is still wrong.
- Scale-up must preserve the controlling dimensionless group: Reynolds number, power number, tip speed, residence time, or heat-transfer coefficient. Geometric similarity alone does not preserve mixing or heat transfer.
Worked example
A stirred tank is scaled from D1 = 1.0 m to D2 = 2.0 m with geometric similarity. The 1 m tank runs at N1 = 100 rpm, and the criterion is constant power per unit volume. Since P is proportional to N^3 D^5 and V to D^3, P/V is proportional to N^3 D^2. Setting (P/V)1 = (P/V)2 gives N2 = N1 (D1/D2)^(2/3) = 100 x (0.5)^0.667 = 63 rpm. Constant tip speed would give 50 rpm, and constant Reynolds number would give a different answer again. Each criterion preserves a different phenomenon, so the engineer must state which one governs dispersion, heat transfer, or shear.
Traps
- Reporting a simulation result without a balance check or a hand calculation.
- Selecting a property package because it converges rather than because it represents the chemistry.
- Double-counting design margin in both the basis and the equipment specification.
- Scaling up mixing by geometry only and losing heat-transfer capacity or creating dead zones.
Document the model version, property package, assumptions, and convergence tolerance. A design that cannot be audited is not a design.