# Mechanical Operations Questions and Answers – Fluidization

This set of Mechanical Operations Multiple Choice Questions & Answers (MCQs) focuses on “Fluidization”.

1. How do solids behave like fluids in fluidization?
a) Solids behave like fluids by blowing liquid downwards through the reactor
b) Solids behave like fluids by blowing gas upwards through the reactor
c) Solids behave like fluids by blowing gas or liquid downwards through the reactor
d) Solids behave like fluids by blowing gas or liquid upwards through the reactor

Explanation: When fluid is introduced from the bottom of the bed, it flows upwards through the spaces present in the bed. When the velocity of flow increases to a value, such that the drag force becomes equal to the weight of the particles of the bed, the particles become suspended in the fluid. At this point the bed is said to be fluidized.

2. When does slugging occur in fluidization?
a) When height of the bed is greater than diameter of the bed
b) When height of the bed is shorter than diameter of the bed
c) When height of the bed is equal to the diameter of the bed
d) Slugging does not depend on the ratio of height of bed and diameter of bed

Explanation: The superficial velocity, at which slug appears on the bed is defined as slugging. When the ratio of height of the bed to the diameter of the bed is large, slugging occurs. In other terms, slugging is the phenomenon when the bubbles almost become as big as the diameter of the bed.

3. Arrange the following stages of fluidization in the increasing order of gas flow.

i. Bubbling regime
ii. Turbulent regime
iii. Slugging regime
iv. Fast fluidization 

a) i, ii, iii, iv
b) i, iii, ii, iv
c) I, iv, iii, ii
d) I, iv, ii, iii

Explanation: The gas in a fluidized system flow from the blower to the drying chamber, and passes through fixed bed regime bubbling regime, slugging regime, turbulent regime and fast fluidization regime. This transition is outlined by the minimum fluidization velocity.

4. Which of the following parts of a fluidized system separates the product from air?
a) Filter bags
b) Air handler
c) High shear mixer
d) Distributor plate

Explanation: Filter bags separate the product from air, before the air is allowed to escape into the atmosphere from the outlets. They are manufactured according to the type of product being separated. Materials usually used to build these bags are nylon fabric and antistatic polymer.
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5. Calculate the porosity of the bed when the initial bed height is 24.5 cm, the final bed height is 34.5 cm and initial porosity is 0.2036.
a) 0.634
b) 0.534
c) 0.434
d) 0.334

Explanation: Given,
Initial bed height, L0 = 24.5 cm
Final bed height, L = 34.5 cm
Initial porosity ε0 = 0.2036
To find: Final porosity ε
Calculation:
Final porosity ε = 1 – [($$\frac {L_0}{L}$$) (1 – ε0)
ε = 1 – [($$\frac {24.5}{34.5}$$) (1- 0.2036 )
ε = 0.434

6. Which of the following is true about Wurster fluid system’s coating process?
a) Inner partition has a diameter that is approximately 30% of the coating chamber’s diameter
b) Air distribution plate has holes drilled of equal sizes all over the plate
c) Spray nozzle is located at the center of the plate
d) Wurster fluid bed was designed in 1850

Explanation: The inner partition’s diameter is about 50% or half the diameter of the coating chamber. The air distribution plate is present at the bottom of the system and has holes of greater diameter towards the center than on the circumference. Wurster had designed this process in the year 1950.

7. Which of the following is false about the Huttlin – Kugel coater?
a) It does not consist of central processor
b) Spray nozzles cannot be located at the bottom
c) The product container is spherical
d) Fluidized air passes through the center of product container

Explanation: Spray nozzles in Huttlin – Kugel coater can also be present at the bottom. This allows the fluidized air to create a balloon effect. This effect keeps the product to be coated, away from the spray nozzles. Multiple spray nozzles may be employed in order to ensure uniform coating.

8. Which of the following happens when the gas velocity increases in fluidization?
a) Bulk density of the increases and fluidization become less aggressive
b) Bulk density of the decreases and fluidization become less aggressive
c) Bulk density of the increases and fluidization become more aggressive
d) Bulk density of the decreases and fluidization become more aggressive

Explanation: When the velocity of the gas fed from the bottom of the column increases, the bed starts losing particles as they get suspended in the fluid. The fluidization also becomes more violent with an increase of the velocity, until all the particles get suspended in the fluid.

9. Calculate the initial porosity of the bed given that the void volume is 0.8901 and the total volume is 1.1.
a) 0.9791
b) 0.2099
c) 1.2358
d) 0.8092

Explanation: Given
Void volume = 0.8901
Total volume = 1.1
Initial porosity ε0 = Void volume/ Total volume
ε0 = 0.8901/1.1
ε0 = 0.8092

10. Water was used as the fluid in a fluidized bed. The length of the fluidization column was 1000mm. The manometer readings were 26 cm and 24 cm. Calculate the pressure difference per length of column. Acceleration due to gravity is used as 9.8 m/s2.
a) 198 kg/m2s2
b) 196 kg/m2s2
c) 200 kg/m2s2
d) 194 kg/m2s2

Explanation: Given
Since water is used, density of fluid ρ = 1000 kg/m3
Length of the fluidization column L = 1 m
Manometer readings h0 = 0.24 m
h1 = 0.26 m
To find: $$\frac {ΔP}{L}$$
Difference in manometer readings, h = h1 – h0
h = 0.26 – 0.24 = 0.02 m
Pressure difference ΔP = ρgh
Pressure difference ΔP = 1000 kg/m3 × 9.8 m/s2 × 0.02 m
ΔP = 196 kg/ms2
ΔPL = 1961 m kg/ms2
ΔPL = 196 kg/m2s2

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