Process Calculations and Stoichiometry
Core concepts
Stoichiometry is the numerical backbone of the exam. You must convert freely between mass, moles, volume and composition (mass fraction, mole fraction, ppm), handle basis selection, and solve material balances with and without chemical reaction. Understand limiting reactant, excess, conversion, selectivity and yield. Combine material and energy balances using heats of formation, combustion, reaction and sensible/latent heat. Master recycle, bypass and purge streams, and the concept of overall vs once-through conversion. Know gas laws (ideal and real), humidity and psychrometric charts, and combustion calculations (theoretical air, excess air, Orsat analysis). Crystallisation, drying and evaporation balances also appear.
Traps
Always fix a basis (e.g., 100 kg feed or 1 mol product) before writing equations. Do not confuse mole fraction with mass fraction in gas problems. In combustion, excess air is relative to theoretical oxygen demand, not total air. Recycle requires an overall balance plus a balance around the mixing point. Watch for inert accumulation requiring a purge.
Worked rule of thumb
For combustion of a fuel with excess air, theoretical O2 from stoichiometry, actual O2 = (1 + excess)·theoretical, and N2 follows from air composition (79/21). Dry flue gas analysis excludes water; convert carefully. A quick check: if a stream is 21% O2 by mole in air, 1 mol O2 brings 3.76 mol N2.
Exam strategy
Draw and label the flowsheet, choose a convenient basis, then write independent balances equal to the number of unknowns.