Chemical Engineering: Balances, Limiting Reactants and Duty
What this section tests
Chemical Engineering questions stick to the sophomore toolkit: unit conversion, stoichiometry and limiting reactants, material and energy balances, ideal-gas behaviour, phase equilibrium basics, reaction rate and conversion, and simple unit operations such as distillation and heat exchange.
Core rules
- The mole is the accounting unit. Convert mass to moles before applying any stoichiometric ratio.
- Limiting reactant: divide each feed by its stoichiometric coefficient; the smallest ratio limits. Excess = fed − required.
- Conversion X = moles reacted / moles fed; selectivity = desired product / undesired product; yield = product formed / theoretical maximum.
- Degrees of freedom = unknowns − independent equations. Zero means the balance is solvable.
- Batch balance: accumulation = in − out + generation − consumption; at steady state accumulation is zero.
- Ideal gas: PV = nRT with T in kelvin and R = 8.314 J/(mol·K).
- Q = m·c_p·ΔT for sensible heat, plus latent heat at a phase change.
Worked example
N₂ + 3H₂ → 2NH₃, fed with 10 mol N₂ and 10 mol H₂. Ratios: N₂ 10/1 = 10; H₂ 10/3 = 3.33. Hydrogen limits. N₂ consumed = 10/3 = 3.33 mol, leaving 6.67 mol N₂. NH₃ formed = (2/3) × 10 = 6.67 mol. As conversions: H₂ 100%, N₂ 33.3%.
Traps
Picking the reactant with the smaller absolute mole count instead of the smaller stoichiometric ratio; forgetting to convert °C to K in gas or Arrhenius calculations; treating mass fraction as mole fraction (they differ whenever molecular weights differ); ignoring inert accumulation in recycle loops.