# Separation Processes Questions and Answers – Two Film Theory and Overall Mass transfer Coefficients

This set of Separation Processes Interview Questions and Answers for Experienced people focuses on “Two Film Theory and Overall Mass transfer Coefficients”.

1. What results due to addition of surfactants?
a) Ease of diffusion
b) Interfacial mass transfer resistance
c) Ease of mass transfer
d) A cloud over the liquid interface

Explanation: The surfactants which reduce the surface tension results in Interfacial mass transfer resistance.

2. Which of the following is true about rate of mass transfer in gas?
a) NA = kp(pab-pai)
b) NA = kp(cab-cai)
c) NA = kc(pab-pai)
d) NA = kc(cab-cai)

Explanation: For diffusion in gas, partial pressure of the gas is considered.

3. Which of the following is true about rate of mass transfer in liquid?
a) NA = kp(pab-pai)
b) NA = kp(cab-cai)
c) NA = kc(pab-pai)
d) NA = kc(cab-cai)

Explanation: For mass transfer in liquid, molar concentration is considered.

4. Which of the following is the incorrect form of Henry’s law?
a) pA=HAxA
b) pA = cA/HA
c) Xa=HAxA
d) YA = HAxA

Explanation: All are the correct forms of henry’s law except Xa=HAxA.

5. Which of the following is true about overall mass transfer coefficient KL?
a) KL =HA/kp +1/kc
b) 1/KL = HA/kp +1/kc
c) KL =1/HA kp +1/kc
d) 1/KL = HA/kp +kc

Explanation: Using henry’s law and slope point form in the graph we get 1/KL = HA/kp +1/kc.
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6. What is the correct expression for overall mass transfer coesfficient based on mole fraction driving forces?
a) 1/Kx = 1/KAky + 1/kx
b) Kx = 1/KAky + 1/kx
c) 1/Kx = KA/ky + 1/kx
d) 1/Kx = 1/KAkx + 1/ky

Explanation: 1/Kx = 1/KAky + 1/kx is the correct representation of overall mass transfer coefficient.

7. What will be the overall mass transfer coefficient when the resistance by gas phase is negligible?
a) 1/Kx = 1/KAky + 1/kx
b) Kx = 1/KAky + 1/kx
c) 1/Kx = KA/ky + 1/kx
d) 1/Kx = 1/KAkx + 1/ky

Explanation: When the resistance by gas is negligible 1/KAky>>1/kx.

8. What are the correct SI units for kx, ky?
a) kmol/s-m
b) kmol/s-m2
c) mol/s-m2
d) kmol/s

Explanation: The SI units for kx and ky is kmol/s-m2.

9. Which of the following is true for mass transfer across liquid-liquid interface?
a) 1/Kx(2) = 1/KAkx(1) + 1/ky(2)
b) 1/Kx(2) = 1/KAky(2) + 1/kx(2)
c) 1/Kx(2) = 1/KAkx(1) + 1/kx(2)
d) 1/Kx(2) = 1/ky(1) + 1/kx(2)

Explanation: For mass transfer across liquid-liquid interface 1/Kx(2) = 1/KAkx(1) + 1/kx(2) holds true.

10. What is the SI unit of kp?
a) kmol/s-m2-pa
b) kmol/s-m-kpa
c) kmol/m2-kpa
d) kmol/s-m2-kpa

Explanation: The value of kp involes partial pressure and its SI unit is kmol/s-m2-kpa.

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