250+ TOP MCQs on External Mass Transfer and Answers

Bioprocess Engineering Multiple Choice Questions on “External Mass Transfer”.

1. When the boundary layer is present in the shell mass transfer then?
A. C_As = C_Ab
B. C_As ≠ C_Ab
C. C_As < C_Ab
D. C_As > C_Ab

Answer: D

2. What is the dimension of liquid – phase mass-transfer coefficient “k_S”?
A. LT^-1
B. LT
C. L^-1T^-1
D. L^-1T

Answer: A

3. If Ω << 1, then _________
A. C_AS = C_Ab
B. C_AS < C_Ab
C. C_AS > C_Ab
D. C_AS = C_Ab = 0

Answer: A

4. Removing the boundary layer, C_AS would decrease.
A. True
B. False

Answer: B
Explanation: Removing the boundary layer would increase the value of C_AS, thus establishing a greater driving force for internal mass-transfer and reducing the likelihood of C_A falling to zero inside the particle.

5. Mass transfer resistance leads to an increase in the concentration of reactants at the catalyst surface in bulk fluid.
A. True
B. False

Answer: B
Explanation: In absence of mass transfer there is no difference in concentration of reactant at bulk and near the catalyst surface. However, presence of significant mass transfer resistance results in a decrease in the concentration of reactants at the catalyst surface compared to that in the bulk fluid. Consequently, the observed rate is less than the intrinsic rate evaluated at bulk fluid reactant concentration.

6. Denitrifying bacteria are immobilised in gel beads and used in a stirred reactor for removal of nitrate from groundwater. At a nitrate concentration of 3 g m^-3, the conversion rate is 0.011 g s^-1 m^-3 catalyst. The effective diffusivity of nitrate in the gel is 1.5×10^-9 m² s^-1, the beads are 6 mm in diameter, and the liquid-solid mass-transfer coefficient is 10^-5 m s^-1. K_m for the immobilised bacteria is approximately 25 g m^-3. What is the value of an observable modulus for external mass transfer, Ω?
A. 0.35
B. 0.36
C. 0.37
D. 0.38

7. Refer to Q6 and calculate the value of C_As.
A. 1.9 g m^-3
B. 0.9 g m^-3
C. 2.9 g m^-3
D. 3.9 g m^-3

Answer: A
Explanation: C_As/C_Ab = 1 – Ω = 0.63
C_As = 0.63 C_Ab = 0.63 (3 gm^-3) = 1.9 g m^-3.

8. If the reaction is endothermic, the temperature at the catalyst surface will be less.
A. True
B. False

Answer: A
Explanation: If the reaction is endothermic, the temperature at the catalyst surface will be less than the bulk fluid temperature and the observed rate will be less than that determined at the bulk fluid temperature.

9. Rate at the surface will be higher due to heat transfer limitations.
A. True
B. False

Answer: A
Explanation: If the reaction is endothermic, the temperature at the catalyst surface will be less than the bulk fluid temperature and the observed rate will be less than that determined at the bulk fluid temperature. If the reaction is exothermic, the temperature of catalyst surface will be more than the bulk fluid temperature. Therefore, the observed rate will be higher or lower than corresponding bulk fluid conditions, depending on both heat transfer and mass transfer effects. Rate at the surface will be higher due to heat transfer limitations and lower due to mass transfer limitations. Depending on the relative magnitude, the overall rate will be increased or decreased.

10. If a species appears in the numerator of the rate law, it is probably a product.
A. True
B. False

Answer: B
Explanation: If a species appears in the numerator of the rate law, it is probably a reactant and if a species appears in the denominator of the rate law, it is probably adsorbed in the surface.

11. Molar flux consist of ______________
A. Molar flux = partial flux + diffusion
B. Total flux = bulk motion + diffusion
C. Molar flux = solid motion + flow rate
D. Total flux = partial flux + flow rate

Answer: B
Explanation: Molar flux consists of two parts:
– Bulk motion of the fluid, B_A
– Molecular diffusion flux relative to the bulk motion of the fluid produced by a concentration gradient, JA
– W_A = B_A + J_A (total flux = bulk motion + diffusion).

12. What is the unit of heat transfer coefficient?
A. Watt m² / k
B. Watt m² k
C. Watt / m² k
D. Watt / m²k²

Answer: C
Explanation: Heat flux form a bulk fluid at T_∞ to the surface of a spherical particle of diameter d_p at T_S: Newtons law of cooling:
q = h (T_∞ – T_S)
Where, q (Heat flux) = Watt / m²
h (Heat transfer coefficient) = Watt / m²K.

13. What is the unit of mass transfer coefficient?
A. ms
B. m/s
C. ms²
D. ms^-2

Answer: B
Explanation: Molar flux form a bulk fluid at concentration C_Ab to the surface of a spherical particle of diameter dp at concentration C_As:
W_A = k_C (C_Ab-C_As)
Where, W_A (Molar flux) = moles/m²s
k_C (Mass transfer coefficient) = m/s.

14. The ratio of momentum diffusivity (kinematic viscosity) and mass diffusivity is referred as:
A. Nusselt number
B. Schimdt number
C. Sherwood number
D. Reynolds number

Answer: B
Explanation: Schmidt number (SC. is a dimensionless number defined as the ratio of momentum diffusivity (kinematic viscosity) and mass diffusivity, and is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes.

15. The ratio of inertial forces to viscous forces is called as ________
A. Nusselt number
B. Schimdt number
C. Sherwood number
D. Reynolds number

Answer: D
Explanation: The Reynolds number is the ratio of inertial forces to viscous forces and is a convenient parameter for predicting if a flow condition will be laminar or turbulent. It can be interpreted that when the viscous forces are dominant (slow flow, low R_e) they are sufficient enough to keep all the fluid particles in line, and then the flow is laminar. Even very low R_e indicates viscous creeping motion, where inertia effects are negligible. When the inertial forces dominate over the viscous forces (when the fluid is flowing faster and R_e is larger) then the flow is turbulent.