Chemical Engineering PE Exam Flashcards
6 cards from real NCEES practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Chemical Engineering PE Exam flashcards as text
On the NCEES PE Chemical exam, what is the 'relative volatility' (α) between two components in a distillation system?
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.
In the NCEES PE Chemical exam, what does 'minimum reflux ratio' represent in distillation column design?
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.
For the NCEES PE Chemical exam, what is the purpose of a 'relief valve' on a pressure vessel per ASME codes?
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.
On the NCEES PE Chemical exam, what is 'flooding' in a packed or tray distillation column?
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.
In the NCEES PE Chemical exam, which equation is used to calculate the theoretical minimum work (energy) for separating a mixture?
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.
For the NCEES PE Chemical exam, what does 'pseudo-steady-state approximation' assume about reaction intermediates?
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.
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