Chemical Process Principles and Thermodynamics
This domain tests whether you can set up material and energy balances, choose thermodynamic models, and apply phase and reaction equilibria to process equipment. Expect questions on reference states, fugacity, activity coefficients, equations of state, and the sign conventions that make or break a balance.
Core rules
- Start with a clearly bounded control volume and a basis (1 h, 1 kmol, 1 kg). Write total and component balances; do not count an inert as reacting.
- Energy balances use enthalpies relative to a declared reference state. For ideal mixtures, H = Σ xi Hi; for nonideal systems add heat of mixing.
- Phase equilibrium requires equal fugacity of each component in every phase. Use Raoult's law only for near-ideal, low-pressure systems; otherwise use an EOS or activity-coefficient model.
- The Gibbs phase rule F = C - P + 2 gives intensive degrees of freedom for equilibrium, not for a whole flowsheet.
Worked example
An equimolar benzene-toluene liquid at 373 K is flashed at low pressure. At 373 K, take P_sat(benzene)=180 kPa and P_sat(toluene)=74 kPa. Ideal bubble pressure is P = 0.5(180)+0.5(74)=127 kPa. Benzene vapor mole fraction y = 0.5(180)/127 = 0.709. The reasoning is: Raoult's law gives each component's partial pressure from its liquid mole fraction and pure vapor pressure; total pressure is the sum; vapor composition is the partial-pressure ratio.
Traps
Mixing gauge and absolute pressure, using a different enthalpy reference for each stream, applying Raoult's law to a strongly nonideal mixture, and forgetting that an adiabatic flash also requires an energy balance.