Chemical Engineering
What the Chemical Engineering domain tests
This domain checks whether you can set up and solve process problems from first principles: material and energy balances, phase and chemical equilibrium, reaction kinetics and reactor sizing, and the unit-operation correlations used in fluid flow, heat transfer, and mass transfer. Most items are numerical, so marks come from choosing a correct basis and carrying units cleanly.
Core rules and concepts
- Write balances in moles unless the problem is explicitly mass-based; mole balances preserve stoichiometry.
- Identify the limiting reactant by dividing each feed by its stoichiometric coefficient; do not compare raw molar amounts.
- Percent excess = (moles supplied − moles required for complete reaction of the limiting reactant) / moles required × 100.
- Conversion = moles reacted / feed; yield = product formed / theoretical product from limiting reactant; selectivity = desired product / undesired product.
- For reactors: CSTR volume V = F_A0 X / (−r_A); PFR volume V = F_A0 ∫ dX / (−r_A). Use the rate law at outlet conditions for a CSTR and integrate from inlet to outlet for a PFR.
- For ideal vapor-liquid equilibrium, Raoult's law gives y_i P = x_i P_i^sat; check that Σ y_i = 1.
Worked example
Reaction: A + 2B → C. Feed is 10 mol/h A and 30 mol/h B. B required to consume all A = 2 × 10 = 20 mol/h. Since 30 > 20, A is limiting and B is in excess by 30 − 20 = 10 mol/h, or 50% excess. If 8 mol/h A reacts, conversion of A = 8/10 = 80%; C formed = 8 mol/h; yield on A = 8/10 = 80%. A common error is to call B limiting because its molar feed is larger.
Traps that cost marks
Using mass fractions in a mole-based stoichiometric balance; forgetting that air is 21% O2 on a molar basis; reporting conversion of the excess reactant as if it were limiting; ignoring purge or recycle when a balance loop is present; mixing heat-transfer sign conventions and forgetting LMTD correction factors for non-countercurrent exchangers.