Chemical and Process Engineering: Balances, Reactors, Separation and Process Safety
Chemical and Process Engineering in IESL Part III tests material and energy balances, reactor sizing, separation operations, heat transfer and fluid transport in process plants, together with process control and safety analysis.
Core rules and concepts
- Balances are written on a consistent basis, usually molar for reaction and mass for separations; check degrees of freedom before solving.
- Define conversion X, selectivity and yield precisely. For a first-order reaction in a CSTR of volume V with volumetric flow v0: V = v0·X/[k(1 − X)]. A PFR needs V = (v0/k)·ln[1/(1 − X)], always smaller for positive-order kinetics.
- Heat exchangers: Q = U·A·ΔT_lm, with a correction factor F for multipass and cross-flow arrangements, and fouling resistance added as a series resistance.
- Distillation: McCabe–Thiele stepping between the operating lines and the equilibrium curve; minimum reflux links to the pinch point, and the actual reflux ratio is typically 1.2–1.5 times the minimum.
- Safety: HAZOP for deviation analysis, relief valve sizing from the credible worst-case relief scenario, and LOPA/HAZOP follow-up for risk reduction.
Worked example
A first-order reaction with k = 0.5 s⁻¹ must achieve 90% conversion at v0 = 0.01 m³/s. In a CSTR, V = v0·X/[k(1 − X)] = 0.01 × 0.90/(0.5 × 0.10) = 0.009/0.05 = 0.18 m³. In a PFR, V = (0.01/0.5)·ln(1/0.1) = 0.02 × 2.303 = 0.046 m³. The comparison shows why back-mixing costs volume when conversion is high.
Traps that cost marks
Solving balances in mixed mass and molar units; forgetting the inert or solvent stream; using arithmetic mean temperature difference rather than LMTD; neglecting the F correction; confusing conversion with yield when side reactions occur; and selecting a relief scenario that is not the governing one.