This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Fluidised Beds – Mechanism of Heat Transfer”.
1. The substance used in fluidised bed is _______
a) Same as Packed Bed
b) Powdered Substance
c) Large balls
d) Finely divided Solid material
Explanation: The pellets used in fluidised bed are finely graded solids, such as a tube bundle immersed in a bed of sand or coal particles.
2. At fluidisation, the upward drag force is _________ the weight of the particles.
a) Equal to
b) Slightly higher than
c) Less than
d) Negligible to
Explanation: When the upward drag force is equal to the weight of the solids, they become free but not free enough to have random movements inside the fluid, so for fluidisation, upward drag force must be little higher.
3. The pressure drop in fluidised bed on fluidisation __________ with increasing flow rate.
a) Remains same
b) Linearly increases
c) Linearly decreases
d) Remains zero
Explanation: After fluidisation, the particles behave as complete fluids which theoretically offer no resistance to the flow and hence the pressure drop remains same.
4. At fluidisation, the temperature of the bed is _______
a) Constant but non-uniform
b) Same for fluid and particles
c) Non uniform
d) Increasing from the bottom
Explanation: At fluidisation we observe high grade of mixing as the pellets are then behaving as fluids, hence the temperature of the total bed reaches a constant final value.
5. The thermal conductivity of the bed on complete fluidisation is________
c) In the range 0-100
d) In the range 10-120
Explanation: At fluidisation we observe high grade of mixing as the pellets are then behaving as fluids, hence the temperature of the total bed reaches a constant final value. The resistance of the bed becomes zero and hence conductivity is infinity.
6. Coal combustion cannot be performed with fluidised bed as it would damage the walls.
Explanation: Coal combustion is one of the important uses of fluidised bed as it ensures complete combustion because of the fluidisation that take place in these setups, here the finely divided particles are bituminous coal.
7. The fluidised bed reactor is a ______ reactor.
a) Single phase
b) Two phase
c) Three phase
d) Gas phase
Explanation: The fluidised bed reactor is either a Gas-Solid or Liquid-Solid phase reactor, as the particles involved also participate in the reactions.
8. What does the following diagram represent?
a) Cracking and reforming of hydrocarbons (oil)
b) Carbonization and gasification of coal
c) Fischer -Tropsch synthesis
d) Limestone calcination
Explanation: Limestone has the chemical formula CaCO3. In the calcinations process, CaCO3 pellets are passed through a stream of very hot gases and gets converted to CaO and CO2. Cao remains as pellets and CO2 leaves with the exit gas.
9. The substance used as bed in fluidised bed reactors is also known as _________
a) Particulate Solid material
b) Distributed Solid material
c) Packed Solid material
d) Segregated Solid material
Explanation: The pellets used in fluidised bed are finely graded solids, such as a tube bundle immersed in a bed of sand or coal particles. These are tiny dust like particles to ensure maximum surface area for effective reaction; hence these particles are called Particulate Solid material.
10. At minimum fluidisation flow rate, the gas flow velocity is also known as __________ of the pellets.
a) Superficial velocity
b) Terminal velocity
c) Average velocity
d) Transport velocity
Explanation: The Gas flow velocity at the fluidisation state Is very commonly known as the terminal velocity, the velocity of free fall of a solid same as the velocity of surrounding when the body is a rest in air.
11. At minimum fluidisation with gas flow, which one of the following is correct?
a) All the particles are in fluidisation with random movement
b) The particles at the top of the bed are floating in random movement
c) The particles are stable and not moving
d) Half or partial quantity of the solids are in random movement
Explanation: When fluidisation is reached in gas-solid fluidised bed, the terminal velocity for smaller and lesser density particles is reached earlier which makes them free in the tube. Hence we usually observe fluidisation at the point when half or partial quantity of the solids are in random movement.
12. Fluidised beds are always used in a vertical arrangement.
Explanation: Fluidised bed can be used in vertically only for an of operation because the fluidisation of the particulate matter is not achievable in horizontal setups.
13. Which one of the following is not a functioning model of fluidised bed?
a) Fixed bed
b) Stationary fluidized bed
c) Fluidized bed with particle feeding at the bottom and overflow at the free surface of the bed
d) Packed with fluid sliding on top
Explanation: The operation given as “Packed with fluid sliding on top” is not possible as a setup for fluidised bed because in this case the particles would get compressed and not in random free movement.
14. In Fluidised bed the particle size are___________
a) Usually Equal
b) Necessarily equal
c) Range of varying sizes
Explanation: The particle size in fluidised bed is usually kept equal so that the fluidisations of all the particles are achieved at a particular flow velocity which is dependent on the pellet diameter.
15. In dense phase flow, the bed is ______
a) Moving as a block
b) Totally fluidised
c) Fixed bed
d) Stationary fluidized bed
Explanation: In dense phase flow, the liquid used is highly viscous and hence complete mixing is not achieved as the fluid fails to enter the voids. Hence the bed moves as a block.
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