Reaction Engineering and Reactor Design
Reaction Engineering and Reactor Design tests whether you can choose a reactor type, size it, and predict conversion, selectivity and thermal behaviour from kinetic data - for homogeneous, catalytic and multiphase systems.
Design equations and selection
- Batch: t = N_A0 ∫ dX/(−r_A V).
- CSTR: V = F_A0 X/(−r_A), evaluated at outlet conditions; PFR: V = F_A0 ∫ dX/(−r_A).
- For positive-order kinetics the CSTR needs a larger volume than the PFR; for autocatalytic or inhibited kinetics the CSTR can win.
- CSTRs in series approach PFR behaviour; Levenspiel plots give volume as an area.
- Selectivity: series reactions A→B→C need short residence time and low per-pass conversion; parallel reactions need the concentration that favours the desired path.
Kinetics and catalysis
- Arrhenius k = A exp(−E/RT); obtain activation energy over a temperature range, never from a single point.
- Pore diffusion: Thiele modulus φ and effectiveness factor η; strong diffusion limitation gives η ≈ 1/φ and an apparent activation energy near half the intrinsic value.
- Deactivation by coke, sintering or poisoning; regeneration cycles enter production cost.
Thermal behaviour
- Heat removal must match heat generation; an adiabatic CSTR can show three steady states, with ignition and extinction set by the heat-removal line and the heat-generation S-curve.
- Runaway occurs when heat removal is exceeded; use feed dilution, cooled recycle or a semi-batch feed policy.
Worked example - CSTR versus PFR
First-order reaction, 90% conversion, isothermal. PFR: τ = −ln(1 − X)/k = 2.30/k. CSTR: τ = X/(k(1 − X)) = 9.0/k. The CSTR is about 3.9 times larger. At 99% conversion, PFR is 4.6/k and CSTR is 99/k - a ratio of 21.5, which is why high-conversion duties use PFRs or CSTR trains.
Traps
- Using inlet concentration in the CSTR rate expression.
- Ignoring volume change on reaction (ε) for gas-phase systems.
- Treating a diffusion-limited apparent activation energy as intrinsic.
- Designing at the maximum adiabatic temperature without checking the runaway criterion.