Reaction Engineering and Catalysis
What is examined
Reaction engineering questions test whether you can translate a rate law into a reactor volume, separate intrinsic kinetics from transport limitations, and justify catalyst selection and deactivation strategy. Expect CSTR and PFR sizing, conversion-selectivity trade-offs, Arrhenius and energy balances, and Thiele-modulus reasoning.
Core rules
- Design equations: CSTR V = F_A0 X / (-r_A); PFR V = F_A0 integral of dX / (-r_A). For constant-density first-order liquid in a CSTR, V = v X / [k(1 - X)]; for a PFR, V = (v/k) ln[1/(1 - X)].
- For multiple reactions, selectivity depends on concentration and temperature. A CSTR operates at the low outlet concentration; a PFR follows the concentration profile. Neither is universally better.
- Catalyst effectiveness factor eta is actual rate divided by rate without internal diffusion resistance. eta falls as Thiele modulus phi rises. At strong pore diffusion, apparent activation energy is about half the intrinsic value.
- Catalyst deactivation changes the time-on-stream design. Cost includes regeneration, pressure drop, and lost production, not only catalyst price.
- Heat management: adiabatic temperature rise is set by heat of reaction and heat capacity. Runaway risk rises when heat release is steep and removal is limited.
Worked example
A first-order liquid reaction has k = 0.10 min^-1, volumetric flow v = 10 L/min, and target conversion X = 0.90. PFR: V = (10 / 0.10) ln(1 / 0.10) = 100 x 2.303 = 230 L. CSTR: V = 10 x 0.90 / [0.10 x 0.10] = 900 L. The CSTR is about four times larger because it runs at the outlet concentration of 10% reactant. If the kinetics were zero-order, the volumes would be similar; if the reaction were autocatalytic, the CSTR could be advantageous. Reactor choice is therefore a kinetic question, not a preference.
Traps
- Using conversion in a rate expression without converting to concentration for a gas-phase reaction with changing moles.
- Quoting catalyst activity without defining the basis: per mass, per surface area, or per active site.
- Ignoring external mass transfer. At high temperature the apparent rate may plateau, mimicking a lower activation energy.
- Designing to equilibrium conversion when the reaction is reversible and the approach to equilibrium is the real driver.
State units for rate constants and check the reaction order by changing initial concentration, not by fitting a straight line to a single run.