Reaction Engineering and Catalysis
This domain examines reactor sizing, rate-law interpretation, selectivity, and catalytic behavior. Questions often require you to choose between batch, CSTR, PFR, and fluidized-bed models, and to connect laboratory kinetics to plant performance.
Core rules
- Write the rate law with explicit units; first-order k has units of inverse time, second-order k has volume per mole per time.
- Design equations: batch time t = NA0 ∫ dX/(-rA V); CSTR V = FA0 X/(-rA); PFR V = FA0 ∫ dX/(-rA).
- For gas-phase reactions, volumetric flow changes with conversion; do not use a liquid-phase constant-density equation without checking.
- Catalyst particles can be diffusion-limited. The Thiele modulus compares reaction rate to pore diffusion; the effectiveness factor η multiplies the intrinsic rate.
- Selectivity and yield depend on residence time, temperature, and mixing. A higher conversion can reduce selectivity in series reactions.
Worked example
A liquid-phase first-order reaction A → B has k = 0.1 min⁻¹ and Q = 2 m³/min. For 90% conversion, a PFR gives V = (Q/k) ln[1/(1-X)] = (2/0.1) ln(10) = 46.1 m³. A CSTR gives V = QX/[k(1-X)] = 2(0.9)/[0.1(0.1)] = 180 m³. The reasoning: the PFR integrates over decreasing reactant concentration, while the CSTR operates at the low outlet concentration throughout, so it needs much more volume for the same conversion.
Traps
Using the CSTR equation for a PFR, ignoring density change in gas reactions, reporting k without units, and treating a diffusion-limited catalyst as if η = 1.