Mass Transfer Questions and Answers – Design of Absorption Tower (NTU,HTU,HETP Concepts)

This set of Mass Transfer test focuses on “Design of Absortion Tower (NTU, HTU, HETP Concepts)”.

1. For the Absorber design, the plotting with mole ratio helps to find the
a) Slope of operating line
b) Slope of equilibrium curve
c) Minimum number of trays
d) Maximum number of trays

Explanation: The plot with mole fraction will give an operating line as a curve; it will be very difficult to find the slope of the operating line. Therefore the use of mole ratio plot will give us a liner operating the line.

2. The equation helps us to find the number of trays in an absorber theoretically is _________
a) Frenske’s Equation
b) Rayleigh’s Equation
c) Kremser’s Equation
d) None of the mentioned

Explanation: Kremser brown derived an equation for determining the number of stages.

3. Absorption factor increases equipment cost decreases
a) True
b) False

Explanation: If absorption factor increases, the number of trays decreases. So the cost decreases.

4. Find the Absorption factor from the given figure,

a) 0.77
b) 1.18
c) 1.5
d) 1

Explanation: Absorption factor= slope of operating line /slope of equilibrium line.

5. Packed towers are _____________
a) Absorption tower
b) Stripper
c) Absorber or Stripper
d) All of the mentioned

Explanation: Packing is just to increase the contact time. Packed towers are used for both absorption and stripping process.

6. Packing height in the packed tower = NTU x __________
a) HTU
b) HETP
c) NTU
d) HETP + HTU

Explanation: HETP- Height equivalent to theoretical plates. It is a packing value, to determine the packing height.

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7. Will the Height equivalent theoretical plates changes with flow rates?
a) True
b) False

Explanation: Flow rates affect the absorption rates.

8.Find the number of trays

a) 0
b) 1
c) 2
d) 1.5

Explanation: It is clearly shown in the figure, two trays between the equilibrium line and the operating line.

9. Find the absorption tower height if
Number of gas phase transfer units 4.09
Height of gas phase transfer units 1.5 meters
a) 2.72 meters
b) 0.36 meters
c) 6.135 meters
d) None of the mentioned

Explanation: Height of the absorption tower = HTUx NTU
= 4.09 x 1.5
= 6.135 meters.
10. Find the HTU if gas rate is 0.07 kmol/sq.m s and the Fg* a ( F type mass transfer co-efficient) is 0.06 kmol/ cu.m s.
a) 1.167
b) 0.85
c) 0.042
d) None of the mentioned
Explanation: HTU= Gas rate/ Fg*a = 0.07/0.06 = 1.167.

11. Kremser brown equation will be invalid for gas phase transfer units if operating and equilibrium line has same slope.
a) True
b) False

Explanation: When the operating line and equilibrium line having the same slope then absorption factor becomes 1. So kremser Brown equation for both gas and liquid phase transfer units becomes invalid.