fluidize bed.docx

7/28/2019 fluidize bed.docx

1/8


2/8

Objective

To determine the relationship between pressure drop and superficial velocity for apacked bed.

To calculate the minimum fluidisation velocity.

TheoryThe upward flow of a fluid through a bed of particles give to a fixed bed (packed bed) at lowflow rates but if the velocity is sufficiently great, the particles will be freely supported in thefluid to give rise to a fluidised bed.

The forces acting on the particles in the bed are its own weight, buoyancy force and the dragforce. At the start of fluidisation the (weight-buoyancy) force is equal to drag force.

For a fixed bed, the Erguns equation is applicable. It is as follows:

[ ( ) ] Where:

For laminar or stream line flow,

( ) ( ) () ( ) For turbulent flow,

( ) ( ) () ( ) Thus a plot of vs v on a log paper should be linear with a gradient of 1 for laminar flow and 2for turbulent flow, and provides a method to determine the flow type. Erguns equation could

also be applied at the start of fluidisation when applied for spherical particles at minimumfluidisation, equation get modified as follows:

( ) ( ) Where:


3/8


4/8

Calculation

Superficial velocity (v),

Where,

Actual bed pressure drop ( ),

Voidage () Density of particles (),

Orifice diameter (cm) 19

Area of the bed (cm 2) 132.25Weight of 100 particles (g) 0.35Volume of 100 particles (ml) 0.4Bed height (cm) 3

Density of fluid ( f ) (kgm-3) 1.2

Viscosity of fluid ( f ) (kgm-1s-1) 0.000016

Coefficient of discharge (C d) 0.62

g (ms -2) 9.81

density ( water ) (kgm-3) 1000

density ( s) (kgm-3) 875

voidage ( ) 0.998991808


5/8

P a c k e d B

e d

P ( O r i f i c e ) ( c m )

P ( E m p t y b e d ) ( c m )

P ( T o t a l ) ( c m )

P ( T o t a l ) ( c m

) B

e d H e i g h t ( c m )

v ( m s -1 )

b e d ( c m

)

P b e d ( P

a )

l o g ( v )

l o g ( P )

2

0 . 7

2

2 4 . 0 4

1 . 3

1 2 7 . 5 3

1 . 3 8 0 8 6 7

2 .1 0 5 6 1 2

5

1 . 8

5

3 8 . 0 0

3 .2

3 1 3 . 9 2

1 . 5 7 9 8 3 7

2 . 4 9 6 8 1 9

5 . 7

1 . 9

3 . 5

3 .1

4 0 . 5 8

1 . 6

1 5 6 . 9 6

1 . 6 0 8 2 8 9

2 .1 9 5 7 8 9

6 . 5

2 .1

3 . 9

3 .2

4 3 . 3 3

1 . 8

1 7 6 . 5 8

1 . 6 3 6 8 0 9

2 .2 4 6 9 4 2

7 . 6

2 . 6

4 . 4

3 . 7

4 6 . 8 6

1 . 8

1 7 6 . 5 8

1 . 6 7 0 7 5 9

2 .2 4 6 9 4 2

8 . 8

2 . 9

4 . 8

4

5 0 . 4 2

1 . 9

1 8 6 . 3 9

1 . 7 0 2

5 9 3

2 .2 7 0 4 2 3

9 . 5

3 .2

5 .1

4 . 5

5 2 . 3

9

1 . 9

1 8 6 . 3 9

1 . 7 1 9 2 1 4

2 .2 7 0 4 2 3

1 0 . 7

3 . 5

5 . 4

5 .2

5 5 . 6 0

1 . 9

1 8 6 . 3 9

1 . 7 4 5 0 4 4

2 .2 7 0 4 2 3

1 0

3 .2

5 .1

5

5 3 . 7 5

1 . 9

1 8 6 . 3 9

1 . 7 3 0 3 5 2

2 .2 7 0 4 2 3

8 . 4

2 . 8

4 . 6

4 . 4

4 9 .2 6

1 . 8

1 7 6 . 5 8

1 . 6 9 2

4 9 2

2 .2 4 6 9 4 2

7 . 5

2 . 4

4 .2

3 . 8

4 6 . 5 5

1 . 8

1 7 6 . 5 8

1 . 6 6 7 8 8 3

2 .2 4 6 9 4 2

5 . 8

2

3 . 9

3 . 5

4 0 . 9 3

1 . 9

1 8 6 . 3 9

1 . 6 1 2

0 6 6

2 .2 7 0 4 2 3

3 . 5

1 .2

2 . 6

3

3 1 . 8

0

1 . 4

1 3 7 . 3 4

1 . 5 0 2

3 8 6

2 .1 3 7 7 9 7

2

0 . 8

1 . 7

2 4 . 0 4

0 . 9

8 8 .2 9

1 . 3 8 0 8 6 7

1 . 9 4 5 9 1 2

1 . 4

0 . 5

0 . 9

2 0 .1 1

0 . 4

3 9 .2 4

1 . 3 0 3 4 1 6

1 . 5 9 3 7 2 9

1 .2

0 . 4

0 . 8

1 8 . 6 2

0 . 4

3 9 .2 4

1 .2 6 9 9 4 3

1 . 5 9 3 7 2 9

Decreasing Increasing

F l u i d i z e d B

e d


6/8

Gradient = tan ( ) = 0.97

Log 10 ( )

1

10

100

1000

9.00 90.00

P

( P a

)

v (ms -1)

P vs v

Increasing

Decreasing

Maximum Fluidisation Velocity


7/8

Discussion

1. Cause for the hump in the graph of log ( ) vs log (v)

In the beginning granules are packed inside the bed. When air flow velocity increases passing through those granules, then they start to mobilize after some point. But the pressuredrop needed to mobilize them is greater than the pressure drop needed to pack them fromfluidised bed to packed bed when increasing the air flow velocity. This is the reason to have anovershoot in the increasing side of the graph.

2. Deviation of fluid pressure drop when increasing the fluid flow and when decreasingthe fluid flow

Particles are in a static state in the initial state. When the air flow velocity increases particle has to mobilize from a static state. In the static state particles are packed and reaction

forces are acting on particles due to mass of the particles. This cause the pressure differenceneeded is higher compared to the decreasing air flow.

When the air flow is decreasing the particles are already in a dynamic state wherenegligible reactions occur due to masses. So its easy to reach static state compared to increasingflow velocity instance. This is the reason to follow different routes by fluidised bed when flowrate increases and decreases.

3. Applications of fluidised bed Reactors

A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry outa variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is

passed through a granular solid material (usually a catalyst possibly shaped as tinyspheres) at high enough velocities to suspend the solid and cause it to behave as though itwere a fluid.

o Cracking hydrocarbonso Coal gasificationo Carbonizingo Calcination

Drying operationsThe process air is supplied to the bed through a special perforated distributor plate

and flows through the bed of solids at a velocity sufficient to support the weight of particles in a fluidized state. Bubbles form and collapse within the fluidized bed of material, promoting intense particle movement. In this state, the solids behave like a freeflowing boiling liquid. Very high heat and mass transfer values are obtained as a result of the intimate contact with the solids and the differential velocities between individual

particles and the fluidizing gas.


8/8

Food industryFluidized beds are used to accelerate freezing in some IQF tunnel freezers. IQF

means Individually Quick Frozen, or freezing unpackaged separate pieces. Thesefluidized bed tunnels are typically used on small food products like peas, shrimp or slicedvegetables.

Heat transfer Fluidized bed combustion (FBC) is a combustion technology used in power

plants. Fluidized beds suspend solid fuels on upward-blowing jets of air during thecombustion process. The result is a turbulent mixing of gas and solids. The tumblingaction, much like a bubbling fluid, provides more effective chemical reactions and heattransfer.

Growth of particles Adsorption/desorption

Bio fluidisation

fluidize bed.docx

Documents