BE/B Eng Bachelor of Engineering Bachelor of Chemical Engineering 4 — Questions and Answers
Question 1: In a plug flow reactor (PFR), how does concentration vary along the reactor length?
- It remains constant throughout
- It changes continuously from inlet to outlet (Correct answer)
- It equals the inlet concentration at all points
- It only changes at the reactor exit
Correct answer: It changes continuously from inlet to outlet
In a PFR, reactant concentration decreases (and product concentration increases) continuously along the axial direction with no back-mixing.
Question 2: What is the significance of the Damköhler number (Da) in reactor design?
- Ratio of reaction rate to convective mass transport rate (Correct answer)
- Ratio of heat generation to heat removal
- Ratio of pressure drop to flow velocity
- Ratio of turbulent to molecular diffusivity
Correct answer: Ratio of reaction rate to convective mass transport rate
The Damköhler number compares the characteristic reaction rate to the convective flow rate, indicating whether reaction or transport limits conversion.
Question 3: Fouling in heat exchangers primarily results in which operational consequence?
- Increased heat transfer coefficient
- Reduced pressure drop across tubes
- Increased thermal resistance reducing heat transfer (Correct answer)
- Lower fluid viscosity on the shell side
Correct answer: Increased thermal resistance reducing heat transfer
Fouling deposits add an additional thermal resistance layer on heat transfer surfaces, reducing overall heat transfer efficiency.
Question 4: Which thermodynamic property is minimized at equilibrium for a closed system at constant temperature and pressure?
- Enthalpy
- Entropy
- Gibbs free energy (Correct answer)
- Helmholtz free energy
Correct answer: Gibbs free energy
At constant T and P, a closed system reaches equilibrium when Gibbs free energy (G) is at its minimum value.
Question 5: In gas absorption with a dilute system, the operating line on a y-x diagram is:
- Curved due to large concentration changes
- Approximately straight because molar flows are nearly constant (Correct answer)
- Horizontal because the gas composition is fixed
- Vertical because liquid loading does not change
Correct answer: Approximately straight because molar flows are nearly constant
For dilute systems, the gas and liquid molar flow rates are approximately constant, making the operating line nearly linear.
Question 6: Which property of a fluid determines whether it obeys Newton's law of viscosity?
- Compressibility
- Thermal conductivity
- Linearity between shear stress and shear rate (Newtonian behavior) (Correct answer)
- Surface tension
Correct answer: Linearity between shear stress and shear rate (Newtonian behavior)
A Newtonian fluid exhibits a linear relationship between shear stress and shear rate, with the proportionality constant being the dynamic viscosity.
Question 7: The Van't Hoff equation relates equilibrium constant K to temperature. If a reaction is exothermic, how does K change as temperature increases?
- K increases because the reaction speeds up
- K decreases because the equilibrium shifts toward reactants (Correct answer)
- K remains unchanged because ΔG° is temperature-independent
- K increases then decreases at the boiling point
Correct answer: K decreases because the equilibrium shifts toward reactants
For an exothermic reaction, increasing temperature shifts equilibrium toward reactants (Le Chatelier's principle), decreasing K.
BE/B Eng Bachelor of Engineering Bachelor of Chemical Engineering 4
In a plug flow reactor (PFR), how does concentration vary along the reactor length?