Chemical Engineering Fundamentals and Applied Thermodynamics
Scope of the domain
The assessment tests whether you can close mass and energy balances, choose a thermodynamic model, and calculate phase equilibrium without hiding behind software defaults. Expect bubble and dew points, flash calculations, activity-coefficient behaviour, compressibility, refrigeration duties, and steam-cycle energy balances.
Core rules
- Use absolute pressure and absolute temperature in every equation of state. Gauge pressure is a recurring trap.
- Vapour-liquid equilibrium is y_i P = x_i gamma_i P_i_sat for a modified Raoult model. Ideal Raoult is the gamma_i = 1 case.
- Fugacity is the rigorous currency: the fugacity of each component must be equal in the phases present. Activity coefficients correct liquid non-ideality; an equation of state corrects vapour non-ideality.
- State the energy-balance sign convention once and keep it. Heat into the system positive and work done by the system positive is common, but any consistent choice is acceptable if declared.
- A flash calculation needs the Rachford-Rice equation for vapour fraction; first check that the feed lies between bubble and dew conditions.
Worked example
A liquid mixture of benzene (1) and toluene (2) at 100 C has x1 = 0.40. At 100 C, P1_sat = 180 kPa and P2_sat = 74 kPa. Assuming ideal liquid and vapour, find the bubble pressure and vapour composition. P = x1 P1_sat + x2 P2_sat = 0.40(180) + 0.60(74) = 72.0 + 44.4 = 116.4 kPa. y1 = x1 P1_sat / P = 72.0 / 116.4 = 0.619. The bubble pressure lies between the pure-component vapour pressures, and y1 exceeds x1 because benzene is more volatile. If the liquid were non-ideal, insert gamma_i; a positive deviation raises P and can create an azeotrope. The calculation is a consistency check on any simulator result.
Traps that cost marks
- Using degrees Celsius in the ideal-gas law or in an Antoine correlation without checking its units.
- Reporting an activity coefficient without its standard state. Henry's law is not the same reference as Raoult's law.
- Forgetting that a compressor energy balance includes shaft work, heat loss, and interstage cooling.
- Treating a non-condensable gas in a condenser as negligible. It controls partial pressure and can blanketing the heat-transfer surface.
Keep calculations dimensional. A correct number with wrong units usually loses full marks because the examiner cannot see whether the concept was understood.