Mass Transfer Multiple Choice Questions on “Diffusion through Variable Area”.
1. Consider steady-state equimolar counterdiffusion through the converging-diverging tube. Given: r1=1cm; r2=2cm; NA=10-6 kmol/m2.s at the vessel 1 end of the tube. The rate of transport of B at the vessel 2 end of the tube is
a) 2*10-6 kmol/s, towards vessel 1
b) 2.5*-7 kmol/s, towards vessel 1
c) 5*10-6 kmol/s, towards vessel 2
d) 9*10-7 kmol/s, towards vessel 2
Answer: b
Clarification: None.
2. Two large vessels containing gaseous mixture of A and B at different concentrations but at the same total pressure are connected by a tapered tube of length 15cm and end diameters 1cm and 4 cm. what should be the diameter of a cylindrical tube of the same length that allows the same rate of transport of A?
a) 2.5 cm
b) 2 cm
c) 3cm
d) 3.5 cm
Answer: b
Clarification: None.
3. At which point of the following locations in the tapered tube of the previous problem is the magnitude of the flux maximum?
a) At the larger diameter end
b) At the smaller diameter end
c) Midway in the tube
d) Somewhere in between the midway and the larger diameter end
Answer: b
Clarification: None.
4. Equimolar counterdiffusion of A and B occurs through a spherical film of i.d=ri and thickness δ. The flux of A is NA1 at r=ri. The flux through a flat film of the same thickness is NA2. if NA1 is larger than NA2 by 5%, the film thickness δ is
a) 0.025ri
b) 0.05ri
c) 0.1ri
d) 0.01ri
Answer: b
Clarification: None.
5. A napthelene hemisphere is placed on a glass plate and a napthelene ball of identical diameter is kept suspended in the stagnant air in two large rooms. The respective shapes of the ball are maintained while they sublime and reduce in size. Which of the following statements are true in this connection?
a) They take the same time for complete sublimation
b) The sphere disappears first
c) The hemisphere disappears first
d) Nothing definite can be said
Answer: a
Clarification: None.
6. Sublimation of a long cylinder made of napthelene occurs in a large volume of stagnant air. The time required for its complete sublimation is
a) infinite
b) half the time of sublimation of a sphere of the same mass
c) twice the time of sublimation of a cylinder of the same mass and lengthy/diameter=1
d) half the time of sublimation of a cylinder of the same mass and lengthy/diameter=1
Answer: a
Clarification: None.
7. A tapered tube of length 8cm and end diameters 2cm and 4cm connects two vessels containing mixtures of A and B. if the tapered tube is replaced with a cylindrical tube of diameter 6cm, what will be the required length of the cylindrical tube to maintain the same flow rate as in the previous case?
a) 8cm
b) 16cm
c) 36cm
d) 48cm
Answer: c
Clarification: – (Area)*(flux) =Constant=W
W1 for cylinder= (πr2 DAB(P1-P2))/RTL
W2 for tapered tube = (πDAB*r1*r2(P1-P2))/RTL
Equate W1 and W2.
8. In a binary gas mixture containing A and B, If NA=-2NB, what will be the expression for NA?
a) 2(D*(P1-P2))/RTL
b) (D*(P1-P2))/RTL
c) DP*(2RTZ)-1*ln[(P-P1)/(P-P2)].
d) 2DP*(RTZ)-1*ln[(P-P1)/(P-P2)].