# Mass Transfer Questions and Answers – Multicomponent Diffusion

This set of Mass Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Multicomponent Diffusion”.

1. A mixture undergoing multicomponent diffusion is analyzed through the Maxwell-Stefan approach. With regard to this approach which among the following statements are incorrect? The difference in the partial pressure of the Component A over a small distance ∆z (AB)is proportional to
a) The molar concentration at the two points
b) Length of the diffusion path
c) Velocity at the point A
d) Difference in the velocities at point A and B

Explanation: Factual.

2. If all the components except 1 of the multicomponent diffusing mixture are inert, then what will be the expression for Diffusivity of component 1 in the mixture?
a) D1m=(N1-Y1∑Nj)/(∑(YjN1-Y1Nj)/Dij)
b) D1m=(Y1∑Nj)/(∑(YjN1-Y1Nj)/Dij)
c) D1m=N1/(∑[(YjN1-Y1Nj)/Dij])
d) D1m=(1-Y1)/(∑[Yj/Dij])

Explanation: For an inert component Nj=0.

Paragraph for question numbers (3-5)

A mixture of 60% hydrogen and 40% oxygen is blowing over a pan of water at 1atm pressure. The bulk gas temperature is 30°c and the water temperature is 20°C. The vapor pressure of water at this temperature is 17.53mm Hg. The concentration of moisture in the bulk gas is negligible and the diffusivity values are DO2-H2O=0.357cm2/s,DH2-H2O=0.972cm2/s at 34°C and DH2-O2=0.891cm2/s at 43°C.

3. Which of the following statements are correct?
a) Water vapor diffuses through non-diffusing hydrogen and oxygen
b) Hydrogen and Oxygen is diffusing through water vapor
c) Hydrogen is diffusing through water vapor and oxygen
d) None of Hydrogen, Oxygen and water vapor is diffusing

Explanation: Water vapor has a source (pan) as well as a sink (bulk gas) whereas hydrogen and oxygen have no sink.
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4. What will be the values of the diffusivities (in cm2/s) at the mean gas-film temperature?
a) 0.923,0.267
b) 0.483,1.351
c) 0.294,0.368
d) 1.381,0.364

Explanation: Use the relation D∝T1.75
Mean gas-film temperature= (30+20)/2=25°C.

5. What is the value of the flux (kmol/m3.s) of water vapor through a stagnant film of estimated thickness 1.5mm on the water surface?
a) 1.36*10-5
b) 1.96*10-5
c) 1.96*10-1
d) 1.96*10-7

Explanation: Calculate D1m from the equation, D1m=(1-Y1)/(∑〖Yj/Dij〗)
D1m=0.465cm2/s
Use the equation, N1=(D1mC/l)*ln⁡〖(1-Yil)/(1-Y10)〗, where Yil=0(dry air) and Y10=0.023 (17.53/760).

6. For a steady state equimolar counter diffusion of A and B. The N flux of A is ________
a) – N flux of A
b) N flux of A
c) – N flux of B
d) N flux of B

Explanation: For equimolar counter diffusion N Flux of A is opposite to N flux of B.

7. The value of a ratio of N flux of A to the total N flux for a steady state equimolar counter diffusion is
a) 0
b) Infinity
c) 1
d) None of the mentioned

Explanation: As the total flux becomes zero, the value of the ratio becomes infinity.

8. In sulphuric acid plants dry air is used for burning of sulphur. Air is dried in contact with concentrated Sulphuric acid. Moisture diffuses through a layer of air, reaches the surface and gets absorbed in it. Here air is said to be non-diffusing were as moisture is diffusing. Which of the following statements are correct?
a) Moisture has a source and a sink whereas dry air has a source but no sink
b) Moisture as well as dry air have a sink
c) Moisture has a source but no sink whereas dry air has a source as well as sink
d) Moisture as well as dry sir have no sink

Explanation: A component is said to be diffusing if it has both sources as well as a sink.

Sanfoundry Global Education & Learning Series – Mass Transfer.

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