NCEES Chemical Engineering PE Exam 1 ā Questions and Answers
Question 1: On the NCEES PE Chemical exam, which equation describes the relationship between mole fractions in vapor-liquid equilibrium for an ideal system (Raoult's law)?
- yi Ć P = xi Ć Pi_sat (Correct answer)
- yi = xi Ć Ki where Ki = Hi / P
- yi / xi = exp(A/T)
- P_total = Ī£(xi Ć Hi)
Correct answer: yi Ć P = xi Ć Pi_sat
Raoult's law states that for ideal mixtures, the partial pressure of component i equals its mole fraction in the liquid (xi) times its pure-component vapor pressure (Pi_sat): yi Ć P = xi Ć Pi_sat.
Question 2: In the NCEES PE Chemical exam, what does the 'LMTD' (log mean temperature difference) method calculate in heat exchanger design?
- The maximum operating temperature of the heat exchanger
- The effective driving force for heat transfer when temperature difference varies along the exchanger length (Correct answer)
- The log-scale pressure drop across the tube bundle
- The ratio of hot to cold fluid flow rates
Correct answer: The effective driving force for heat transfer when temperature difference varies along the exchanger length
LMTD is the logarithmic average of the terminal temperature differences in a heat exchanger, providing the correct mean driving force for calculating required heat transfer area: Q = U Ć A Ć LMTD.
Question 3: For the NCEES PE Chemical exam, what is the 'residence time distribution (RTD)' concept used for in reactor design?
- To determine the startup time for a batch reactor
- To characterize mixing behavior and deviation from ideal reactor models in real reactors (Correct answer)
- To calculate the recycle ratio in distillation columns
- To measure heat removal from exothermic reactors
Correct answer: To characterize mixing behavior and deviation from ideal reactor models in real reactors
RTD analysis (using tracer experiments) characterizes how long different fluid elements spend in a reactor, revealing mixing patterns and deviations from ideal CSTR or PFR behavior.
Question 4: On the NCEES PE Chemical exam, which dimensionless number characterizes the ratio of convective heat transfer to conductive heat transfer in a fluid?
- Reynolds number
- Nusselt number (Correct answer)
- Prandtl number
- Grashof number
Correct answer: Nusselt number
The Nusselt number (Nu = hL/k) is the dimensionless convective heat transfer coefficient, representing the enhancement of heat transfer by convection relative to conduction through the same fluid layer.
Question 5: In the NCEES PE Chemical exam, what is the 'Fanning friction factor' used to calculate in pipe flow?
- The drag force on particles settling in a fluid
- The pressure drop due to friction in a pipe per unit length (Correct answer)
- The heat transfer enhancement in turbulent flow
- The pump efficiency at off-design conditions
Correct answer: The pressure drop due to friction in a pipe per unit length
The Fanning friction factor f (or Darcy-Weisbach friction factor) is used in the Darcy-Weisbach equation to calculate frictional pressure drop in pipe flow: ĪP = f(L/D)(ĻVĀ²/2).
Question 6: For the NCEES PE Chemical exam, what does 'adiabatic flame temperature' represent in combustion engineering?
- The minimum temperature for ignition of the fuel-air mixture
- The maximum theoretical temperature achieved when fuel burns completely with no heat loss to surroundings (Correct answer)
- The temperature of the exhaust gas after heat recovery
- The autoignition temperature listed in safety data sheets
Correct answer: The maximum theoretical temperature achieved when fuel burns completely with no heat loss to surroundings
Adiabatic flame temperature is the theoretical upper limit of combustion temperature calculated from energy balance assuming all heat of combustion heats the products with zero heat loss.
NCEES Chemical Engineering PE Exam 1
On the NCEES PE Chemical exam, which equation describes the relationship between mole fractions in vapor-liquid equilibrium for an ideal system (Raoult's law)?