NCEES Chemical Engineering PE Exam 2

Question 1

On the NCEES PE Chemical exam, what is the 'relative volatility' (α) between two components in a distillation system?

Accepted Answer

The ratio of the K-values (y/x equilibrium ratios) of the two components, measuring ease of separation

Answer

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?

Accepted Answer

The reflux ratio at which the column requires infinite theoretical stages to achieve the desired separation

Answer

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?

Accepted Answer

To protect the vessel from overpressure by automatically opening when pressure exceeds the set point

Answer

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?

Accepted Answer

A hydraulic instability where liquid cannot descend due to high vapor velocity, causing loss of separation efficiency

Answer

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?

Accepted Answer

Gibbs free energy of mixing (ΔGmix)

Answer

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.

NCEES National Council of Examiners for Engineering and Surveying Practice Practice Test

NCEES National Council of Examiners for Engineering and Surveying Practice Practice Test

NCEES Chemical Engineering PE Exam 2