NCEES Chemical Engineering PE Exam 2 — Questions and Answers
Question 1: On the NCEES PE Chemical exam, what is the 'relative volatility' (α) between two components in a distillation system?
- The ratio of their boiling points in absolute temperature
- The ratio of the K-values (y/x equilibrium ratios) of the two components, measuring ease of separation (Correct answer)
- The difference in their vapor pressures at the feed tray temperature
- The ratio of top to bottom product flow rates
Correct answer: The ratio of the K-values (y/x equilibrium ratios) of the two components, measuring ease of separation
Relative volatility α₁₂ = K₁/K₂ = (y₁/x₁)/(y₂/x₂) measures the ease of separating two components; α >> 1 means easy separation, α ≈ 1 means very difficult.
Question 2: In the NCEES PE Chemical exam, what does 'minimum reflux ratio' represent in distillation column design?
- The reflux ratio at which the column requires infinite theoretical stages to achieve the desired separation (Correct answer)
- The reflux ratio below which the condenser floods
- The ratio at which reboiler duty equals condenser duty
- The reflux ratio at maximum column throughput
Correct answer: The reflux ratio at which the column requires infinite theoretical stages to achieve the desired separation
At minimum reflux ratio (Rmin), a pinch point exists in the column (often at the feed tray) where concentration profiles flatten, requiring infinite stages — a theoretical lower bound on reflux.
Question 3: For the NCEES PE Chemical exam, what is the purpose of a 'relief valve' on a pressure vessel per ASME codes?
- To control normal operating pressure within ±5%
- To protect the vessel from overpressure by automatically opening when pressure exceeds the set point (Correct answer)
- To vent inert gases during startup
- To maintain vacuum conditions during distillation
Correct answer: To protect the vessel from overpressure by automatically opening when pressure exceeds the set point
Pressure relief valves (PRVs) automatically open to release excess pressure when a vessel exceeds its maximum allowable working pressure (MAWP), preventing catastrophic failure.
Question 4: On the NCEES PE Chemical exam, what is 'flooding' in a packed or tray distillation column?
- Liquid overflow from the reflux drum
- A hydraulic instability where liquid cannot descend due to high vapor velocity, causing loss of separation efficiency (Correct answer)
- Excessive bottoms product removal rate
- The condition when all theoretical stages are utilized
Correct answer: A hydraulic instability where liquid cannot descend due to high vapor velocity, causing loss of separation efficiency
Flooding occurs when vapor velocity is high enough to entrain liquid upward or prevent liquid from flowing down, resulting in a rapid pressure drop increase and collapse of the separation.
Question 5: In the NCEES PE Chemical exam, which equation is used to calculate the theoretical minimum work (energy) for separating a mixture?
- Gibbs free energy of mixing (ΔGmix) (Correct answer)
- The McCabe-Thiele graphical method
- Kremser equation for extraction stages
- Ponchon-Savarit enthalpy-composition method
Correct answer: Gibbs free energy of mixing (ΔGmix)
The minimum reversible work of separation equals the negative of the Gibbs free energy change of mixing (−ΔGmix), representing the thermodynamic lower bound on separation energy.
Question 6: For the NCEES PE Chemical exam, what does 'pseudo-steady-state approximation' assume about reaction intermediates?
- Intermediates are present in high concentrations throughout the reaction
- The rate of change of intermediate concentration is approximately zero, simplifying kinetic rate expressions (Correct answer)
- Intermediates do not participate in the rate-limiting step
- The intermediate concentration equals the reactant concentration
Correct answer: The rate of change of intermediate concentration is approximately zero, simplifying kinetic rate expressions
The pseudo-steady-state (PSS) assumption treats reactive intermediates as if d[I]/dt ≈ 0, allowing algebraic elimination of intermediate concentrations from complex rate expressions.
NCEES Chemical Engineering PE Exam 2
On the NCEES PE Chemical exam, what is the 'relative volatility' (α) between two components in a distillation system?