Fluidization

What Is Fluidization?

• An operation through which fine solids are transformed from a static solid-like state to a dynamic fluid like state through contact with either a gas or a liquid.

• This process occurs when a fluid (liquid or gas) is passed up through the granular material.

Fundamentals:

• When a fluid is passed downwards through a bed of fine particles at a low flow rate:– the fluid merely percolates through the void

spaces between the stationary particles.

• This is fixed bed.• The pressure drop across the bed is directly

proportional to flow rate.

• When a fluid is passed upwards through a bed of fine particles at a low flow rate:– pressure drop is same as for downwards.

• With increase in velocity:

– A point is reached when the upward drag force exerted by the fluid on the particles is equal to the apparent weight of particles in the bed

– At this point the particles are lifted by the fluid, the separation of the particles increases and the bed becomes fluidized.

• The force balance across the fluidized bed dictates that the fluid pressure loss across the bed of particles is equal to the apparent weight of the particles per unit area of the bed

• Low Velocity• Fluid does not impart enough drag to overcome

gravity and particles do not move. » Fixed Bed.

• High Velocity• At high enough velocities fluid drag plus buoyancy

overcomes the gravity force and the bed expands.» Fluidized Bed.

• Superficial velocity: gas flow rate divided by total column surface area.

Response to Superficial Velocities

• Plot of fluid pressure loss across the bed vs superficial fluid velocity through the bed:

OA is the packed bed region∆p proportional to V0.

Bed height remains the same.Solid particles do not move relative to one another

BC is the fluidized bed region∆p across the bed remains constantBed height increases with increasing flow

At point A actual fluidization starts

Types of fluidization • Fluidization can be broadly classified into

particulate fluidization or bubbling fluidization.

• Particulate fluidization occurs in liquids.• As the velocity of the liquid is increased past the

minimum fluidization velocity, the bed expands uniformly, and uniform conditions prevail in the liquid solid mixture.

• In contrast, bubbling fluidization occurs in gas-fluidized beds.

• Here, when the bed is fluidized, large pockets of gas, free of particles, are seen to rise through the bed.

Advantages of Fluidization

1. Smooth liquid like flow of particles allows contionus automatically controlled operations.

2. Rapid mixing of solids leads to isothermal conditions throughout the reactor.

3. Suiatable for high exothermic reactions.

4. H.T and M.T rates between gas & particles are high

5. H.T. Rates between bed and immersed object is high-requires less H.T.area.

Disadvantages of Fluidization

1. For catalytic reactions: movement of porous catalyst particles continously capture and release the gas reactant

2. So back mixing : reduces the yield and performance

3. For non catalytic reactions at high temp

4. Erosion of pipes & vessels from abrasion of particles

Minimum Fluidization Velocity

• Superficial fluid velocity at which the packed bed becomes a fluidized bed is known as the minimum fluidization velocity, Umf

• Sometimes referred to as the velocity at incipient fluidization

• Umf increases with particle size and particle density and is affected by fluid properties

• At the point of incipient fluidization drag force exerted by the fluid on the particles is equal to the net weight of particles in the bed.

• For the whole particle bed:

• Drag force = product of (pressure drop& cross sectional area)

• Net weight = product of (bed volume, density, fraction of bed occupied by particles , acceleration due to gravity)

• Kozney-Carman equation works well for fine particles.

• But for larger particles, minimum fluidization velocity is high & Kozney-Carman equation is inadequate & predicts far too low pressure drop.

• Ergun equation is more accurate.

AHgPA fp 1 HgP fp 1

• Umf increases with particle size and particle density and is affected by fluid properties

• To derive expression for Umf, equate expression for pressure loss in a fluidized bed with pressure loss across a packed bed

• Applying the Ergun equation,

• Writing Ergun’s equation for minimum fluidization conditions & for spherical particles

3

2

322

2 175.11150

d

u

d

u

H

P f

1

3

2

32

2 175.11150

mf

mffmf

mf

mfmf

mf d

u

d

u

H

P

• Substituting pressure drop from equation 1

• Multiply b.s by

3

2

32

75.11150

mf

mff

mf

mfmffp d

u

d

ug

2

3

df

3

2

3

32

222

32

3

Re75.1Re1150

75.11150

mf

mf

mf

mfmfa

mf

mff

mf

mffmfffp

G

ududdg

2

3

dg

G ffpa

Galileo number also known as Archimedes

number

And Remf is the Reynolds number

at incipient fluidization mff

mf

udRe

• In order to obtain a value of Umf, we need to know the voidage of the bed at incipient fluidization,

• e = emf

• A typical value of emf is 0.4

• It can be written in the form

• This can be rearranged to give

2Re3.27Re1406 mfmfaG

0ReRe 2 amfmf G

22Re

5.02

amf

G

Void Fraction at Min. Fluidization

• depends on the shape of the particles. For spherical particles it is usually 0.4 – 0.45.mf

Minimum Fluidization

What if emf (and maybe Fs) is unknown?• Wen and Yu found for many systems

Thus a reasonable estimate of minimum velocity can be obtained from

14

13 mfs

7.330408.07.335.02 GaRemf

• Bed Length at Minimum Fluidization:• Once we obtain the minimum void fraction

pmf

Bedmfb S

ML

1,

Minimum fluidization velocity as a function of terminal falling velocity

• Richardson(1971) summaries the method for predicting Minimum fluidization velocity as a function of terminal falling velocity of a particle.

• terminal falling velocity ut in terms of Galileo number.

• For Stoke’s law region:

• Multiply b.s by

18

2 PP

t

gDu

df

2

3

18

fPftf gddu

In terms of Galileo Number

tt GaGa

Re1818

Re

Stokes law is valid only for Re<0.2 which becomes Ga<3.6

• For Newton’s law region:

• Squaring &multiplying b.s.by

• Now if voidage at minimum fluidization velocity is known,

• Then for given value of Galileo number , the ratio of

• Can be calculated from Ergun equation.

2

22

df

f

fPPt

gDu

75.1

Ga

dgdut

fpfft 222

32

2

222

75.1Re75.1

2Re33.0 tGa For Galileo number greater than 10^5

mf

mf

t

mf

t

u

u

Re

Re

• For transition region:

• Which is valid in the range 3.6<Ga<10^5

687.0Re15.01Re18 ttGa

Fluidized bed: Operation• Porosity increases

• Bed height increases

• Fluidization can be sustained until terminal velocity is reached

• If the bed has a variety of particles (usually same material, but different sizes)

– calculate the terminal velocity for the smallest particle

• Range of operability = R

• Minimum fluidization velocity = incipient velocity (min range)

• Maximum fluidization velocity = terminal velocity (max range)

• Other parameters may limit the actual range further

– e.g. Column may not withstand the pressure, may not be tall enough etc

• R = Vt/VOMTheoretically R can range from 8.4 to 74

The Geldart Classification of Particles

• Group C:cohesive, or very fine powders• Normal fluidization is difficult.Rise as a plug of solids.

– Examples: Face powder, flour, and starch.

• Group A:Aeratable,Small particle size or density less than 1.4 g/cc.

• Smooth fluidization at low gas velocity, small bubbles at high velocity.– Major example is the FCC catalyst.

• Group B:sand like particles (40 µm<dp<500µm)• Or density1.4< <4 g/cc.• Fluidize with vigorous bubbling & bubbles grow

large.– Majority of gas-solid reactions occur in this regime.

• Group D: Spoutable, large and/or dense particles– Deep bed difficult to fluidize: spouting or shallow beds.

– Examples include drying grains, peas, roasting coffee beans, gasifying coals and roasting of metal ores.

For any solids of known density & particle size: graph shows the type of fluidization to be observed.

P

Industrial Applications of Fluidized Bed

Coal Gasification• Coal gasification is the process of producing syngas–a

mixture of  – carbon monoxide(CO), 

– hydrogen (H2), 

– Carbon dioxide (CO2) and

– water vapour (H2O)–from coal and water.

– but can include other gaseous constituents; the composition of which can vary depending upon the conditions in the gasifier and the type of feedstock.

3C (i.e., coal) + O2 + H2O → H2 + 3CO

• It can be converted into transportation fuels such as gasoline and diesel through the Fischer-Tropsch process.

–  

• Temp:9800C

• Gasification reactors are classified by type of reaction bed (fixed, entrained, or fluidized), the operating pressure (pressurized or atmospheric).

• The gasifying medium, mixture of steam and air or oxygen is supplied in two stages.

• The first stage supply is adequate to maintain the fluidised bed at the desired temperature.

• While the second stage, supplied above FBD, serves to convert entrained unreacted char particles and hydrocarbons into useful products.

• The process was developed in 1926 by Rheinbraun AG (now RWE)  in Germany using lignite coal.

• Currently, the HTW gasifier can operate about 800 to 900°C. The temperature is controlled to ensure that it does not exceed the ash softening point. 

• Operating pressure can be as high as: 25 to 30 bar.

• In December 2010 ThyssenKrupp Uhde acquired the HTWtechnology from RWE and have a number of projects under development, including biomass-to-methanol projects in Sweden and India.

Fluid Catalytic Cracking

• It is one of the most important conversion processes used in petroleum refineries.

• It is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline, olefinicgases, and other products.

• The feedstock to an FCC is heavy gas oil or vacuum gas oil (HVGO).

• The FCC process vaporizes and breaks the long-chain molecules of the high-boiling hydrocarbon liquids into much shorter molecules by contacting the feedstock, at high temperature and moderate pressure, with a fluidized powdered catalyst.