Chemical Process Principles and Thermodynamics
The Chemical Process Principles and Thermodynamics domain checks whether you can close a balance around a real plant - with recycle, purge and inerts - and then justify the temperature, pressure and phase behaviour of every stream. Questions are usually short numerical items, so method marks are earned by stating the balance envelope and the reference state you chose.
Balances: choose the envelope first
- Write total mass (or mole) and component balances; for reactive systems add extent of reaction ξ and use ξ to link all species.
- The tie component (absent from reaction) gives the fastest handle on flows.
- Recycle raises overall conversion at fixed per-pass conversion; only a purge or a separation step removes inerts and by-products, and the purge flow fixes the steady-state inert concentration.
Energy and equilibrium
- ΔH_rxn at 298 K from standard heats of formation, corrected to reactor temperature by Kirchhoff: ΔH(T2) = ΔH(T1) + ∫ΔCp dT.
- Adiabatic temperature rise: ΔT_ad = (−ΔH_rxn)/(Σ n_i Cp_i), valid while no phase change occurs.
- ΔG° = −RT ln K; van't Hoff gives d(ln K)/dT = ΔH°/(RT²). Exothermic reactions lose equilibrium conversion as temperature rises, which is why reactors follow an optimum temperature path.
- Vapour–liquid equilibrium: Raoult's law for ideal systems, modified Raoult (γ_i P_i_sat) for non-ideal, K-values and bubble/dew calculations for flash drums.
Worked example - purge sizing
Fresh feed is 100 kmol/h containing 1 kmol/h inert. The separator off-gas leaving the recycle loop is 200 kmol/h and the catalyst tolerates 10 mol% inert. Steady-state inert balance: 1 = (purge fraction) × 200 × 0.10, so the purge must be 5% of the off-gas. The penalty appears in the reactant balance: the purge also carries 0.05 × 180 = 9 kmol/h of reactant, which must be made up in fresh feed. If the question asks for make-up, the answer is 9 kmol/h reactant plus 1 kmol/h inert.
Traps
- Omitting the purge from the inert balance, which implies infinite accumulation.
- Mixing mass and mole fractions in K-value calculations.
- Using 298 K thermodynamic data for a 700 K reactor without correction.
- Sign errors in ΔH for exothermic reactions, and assuming ideal gas where the compressibility factor is far from unity.