Chemical & Process Engineering
Chemical & Process Engineering in IESL Part I tests material and energy balances, reaction stoichiometry, thermodynamics, fluid flow, heat and mass transfer, reactor performance and separation processes such as distillation and absorption.
Core rules and concepts
- Material balance: accumulation = input − output + generation − consumption. Choose a basis (1 kg, 1 kmol or 1 hour) and state it.
- Conversion, selectivity and yield must be defined on the correct limiting reactant. Recycle raises overall conversion; purge controls inert build-up.
- Energy balance: Q = m·cp·ΔT for sensible heat; add latent heat for phase change. For heat exchangers, Q = U·A·ΔTlm.
- Fluid flow: Reynolds number Re = ρVD/μ; Bernoulli with friction losses; pump power = ρgQH/η.
- Distillation: operating lines, reflux ratio and McCabe–Thiele stages; minimum reflux corresponds to a pinch.
Worked example
Water at 2 kg/s is heated from 20 °C to 60 °C with cp = 4.18 kJ/kg·K. Q = 2 × 4.18 × 40 = 334.4 kW. If saturated steam at 120 °C condenses with hfg = 2200 kJ/kg, steam flow = 334.4 / 2200 ≈ 0.152 kg/s. Adding 5% for losses gives about 0.160 kg/s. The reasoning is to calculate the duty from the process stream first, then divide by the latent heat to size the heating medium.
Traps that cost marks
- Confusing gauge and absolute pressure in thermodynamic or steam tables.
- Using mass fractions where mole fractions are required, or vice versa.
- Ignoring heat losses, fouling and the correct LMTD flow arrangement.
- Forgetting to identify the limiting reactant before calculating conversion.