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Assignment Presentation Topic: Fluidization

Presenting to you by the Dashing Personalities Mr. Sarfaraz Jalbani {14 CH 48}. and Mr. Rahat Inayat Ali {14 CH 23}.

What Is Fluidization …….???

The Operation by which Fine Solids are Transformed into Fluid-Like State, through Contact with the Gas or Liquid.

Fluidization refers to those Gas-Solids and Liquid-Solids system in which the solid phase is subjected to behave more or less like a Fluid by the upwelling current of gas or liquid stream moving through the bed of solid particles.

Fluidized bed combustion and Catalytic cracking of heavy crude-oil fractions of petroleum are the two good examples of Fluidization.

Some Size Reduction Processes used in Fluidization Process

Attrition

Compression

Impact

Shear

Types When Fluidizing Sand with Water, the

particles move further apart and their motion becomes more vigorous as the velocity is increased, but the bed density at a given velocity is same in all sections of the bed. This is called Particulate Fluidization and is characterized by a large but uniform expansion of the bed at high velocities.

And if ….. Beds of solids fluidized with Air usually

exhibit what is  called Aggregative or Bubbling Fluidization. At superficial velocities much greater than Vmf most of the gas passes through the bed as bubbles or voids which are almost free of solids, and only a small fraction of the gas flows in the channels between the particles.

How Fluidization ..? PROCESS

When a gas flow is introduced through the bottom of a bed of solid particles, it will move upwards through the bed via the empty spaces between the particles. At low gas velocities, aerodynamic drag on each particle is also low, and thus the bed remains in a fixed state. Increasing the velocity, the aerodynamic drag forces will begin to counteract the gravitational forces, causing the bed to expand in volume as the particles move away from each other.

Further increasing the velocity, it will reach a critical value at which the upward drag forces will exactly equal the downward gravitational forces, causing the particles to become suspended within the fluid. At this critical value, the bed is said to be fluidized and will exhibit fluidic behavior. By further increasing gas velocity, the bulk density of the bed will continue to decrease, and its fluidization becomes more violent, until the particles no longer form a bed and are "conveyed" upwards by the gas flow.

When fluidized, a bed of solid particles will behave as a fluid, like a liquid or gas. Like water in a bucket: the bed will conform to the volume of the chamber, its surface remaining perpendicular to gravity; objects with a lower density than the bed density will float on its surface, bobbing up and down if pushed downwards, while objects with a higher density sink to the bottom of the bed. The fluidic behavior allows the particles to be transported like a fluid, channeled through pipes, not requiring mechanical transport (e.g. conveyor belt).

Process Diagram

Fluidization starts at a point when the bed pressure drop exactly balances the net downward forces (gravity minus buoyancy forces) on the bed packing, so

Dp/L = (1-e)(rs - r)g à 1 à 2

Substituting for  Dp/L from Ergun's equation

In most industrial applications involving fluidized beds, the particle diameter is small, and VS also small. In these cases, the second term of the above equation is negligible compared to the first, so that

à 3

For a given bed the above equation can be used for both the unexpanded and the expanded state.

Why Fluidization ..? Application of Fluidization

Fluidized solids can be easily transferred between reactors.

The intense mixing within a fluidized bed means that its temperature is uniform.

There is excellent heat transfer between a fluidized bed and heat exchangers immersed in the bed.

To avoid particles to flow in air.

Example A simplified every-day-life example of a

gas-solid fluidized bed would be a hot-air popcorn popper. The popcorn kernels, all being fairly uniform in size and shape, are suspended in the hot air rising from the bottom chamber. Because of the intense mixing of the particles, akin to that of a boiling liquid, this allows for a uniform temperature of the kernels throughout the chamber, minimizing the amount of burnt popcorn. After popping, the now larger popcorn particles encounter increased aerodynamic drag which pushes them out of the chamber and into a bowl.

Fluidized bed, Used for Fluidization

A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding vessel) is placed under appropriate conditions to cause a solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids, such as the ability to free-flow under gravity, or to be pumped using fluid type technologies. The resulting phenomenon is called fluidization.

When trying to describe the operation of a fluidized bed, one main definition is the

minimum fluidization velocity. This parameter is defined as "the superficial fluid velocityat which the upward drag force exertedby the fluid is equal to the apparentweight of the particles in the bed”.

Another common characteristic of fluidized beds is the bed expansion. When incipient fluidization is achieved, the fluid flowing upwards pushes the particles up and the separation distance between particles increases. ThisIncreases the void volume within the bed of particles and the bed is considered expanded

Uses of Fluidized bed

Fluidized beds are used for several purposes, such as fluidized bed reactors (types of chemical reactors), fluid catalytic cracking, fluidized bed combustion, heat or mass transfer or interface modification, such as applying a coating onto solid items.

Advantages n Disadvantages of Fluidized bed

Compact, simple in construction and easily scalable.

Thermal efficiency Continuous operation Ease of process control due to stable conditions

Non-uniform flow patterns (difficult to predict). Agglomeration of particle decrease effective

surface area. Possible breakage of fine particles. Bed walls erosion

Thank You ……… Special Thanks to: Ma’am Khadija Qureshi