FLUIDISATION; AGITATION AND MIXING

Fluidisation

When fluid moves downward through a solid bed no relative movement between particles takes place

Unless the initial orientation of solids is disturbed

Pressure drop increases by increasing the rate of flow

Fluidisation

When fluid moves upwards the solid bed; pressure drop is same for downward flow for low flow rates

When frictional drag on the particles becomes equal to their weight (actual weight less than the buoyancy force)

the particles get rearranged and bed starts to expand

If velocity is increased further; particles separate from each other and are freely supported in the fluid Fluidised.

Fluidisation

Further increase in the velocity causes the particles to separate still further from one another

In practice, the transition from the fixed to the fluidised bed condition is not uniform mainly due to irregularities in the packing

over a range of velocities, fixed and fluidised bed regions may co-exist.

Fluidisation

with gases, surface-related forces give rise to the formation of conglomerates of particles through which there is a minimal flow

Resultantly, much of the gas may pass through the bed in channels.

Regions of the bed may remain in an unfluidised state even though the overall superficial velocity may be much higher than the minimum fluidising velocity.

Fluidisation for liquid systems

At high fluid velocities, there is usually a fairly sharp distinction between the behaviour of the liquid and gaseous systems.

With a liquid, the bed continues to expand as the velocity is increased and it maintains its uniform character, with the degree of agitation of the particles increasing progressively.

This type of fluidisation is known as particulate fluidisation.

Fluidisation for gaseous systems

With a gas, however, uniform fluidisation is frequently obtained only at low velocities.

At higher velocities two separate phases may form—a continuous phase, or dense or emulsion phase; and a discontinuous phase known as the lean or bubble phase.

The fluidisation is then said to be aggregative.

Fluidisation for gaseous systems

At much higher velocities, the bubbles tend to break down—a feature that leads to a much more chaotic structure.

closely resembles a boiling liquid, with the lean phase corresponding to the vapour and the dense or continuous phase corresponding to the liquid.

The bed is then often referred to as a boiling bed, as opposed to the quiescent bed usually formed at low flowrates.

Fluidisation for gaseous systems

At high flowrates in deep beds, coalescence of the bubbles takes place, and in narrow vessels, slugs of gas occupying the whole cross-section may be produced.

These slugs of gas alternate with slugs of fluidised solids that are carried upwards and subsequently collapse, releasing the solids which fall back.

Froude Number

Froude number (u2mf /gd)

umf is the minimum velocity of flow, calculated over the whole cross-section of the bed, at which fluidisation takes place,

d is the diameter of the particles, and g is the acceleration due to gravity. At values of a Froude group of less than

unity, particulate fluidisation normally occurs at higher values, aggregative fluidisation

takes place

Applications of Fluidisation

Extremely high surface area contact between fluid and solid per unit bed volume

High relative velocities between the fluid and the dispersed solid phase.

High levels of intermixing of the particulate phase.

Frequent particle-particle and particle-wall collisions.

Applications of Fluidisation in industry

1. Fluidized bed reactors (types of chemical reactors

2. Fluid catalytic cracking3. Fluidized bed combustion4. Heat or mass transfer

Fluidisation—explained in another way

Fluidisation: a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state.

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

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, drag on each particle is also low, and thus the bed remains in a fixed state.

Fluidisation—explained in another way

Increasing the velocity, the 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.

Further increase than the superficial velocity

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.

Continuous Fluidisation; slurry and pneumatic transport

When the velocity of the fluid through the solids becomes large enough, all the particles are entrained in the fluid and carried along with it to give continuous fluidisation

Principal applications of continuous fluidisation are in transporting solids from point to point

1. Hydraulic conveying or slurry transport 2. Pneumatic conveying

Hydraulic or slurry transport

Particles smaller than 50micro meter in dia settle very slowly and readily suspended and transported in liquid

Larger particles are harder to suspend esp when dia is 0.25 mm or higher

In that instance, a fairly large liquid velocity is needed to keep the particles from moving at all, esp in horizontal pipes

The critical velocity Vc, below which particles will settle out is typically 1 and 5 m/s

Vc Critical velocity depends on..

Density difference b/n solids and liquid Particle diameter Slurry concentration and Size of pipe

Vc is larger in big pipe than in small pipe

Pressure drop in hydraulically conveyed systems

Pressure drop may be found from equations for a homogeneous liquid

with appropriate allowance for the increased density and viscosity

For settling velocities (when near critical velocity Vc), pressure drop is higher as compared to single phase liquid

As velocity increases (> Vc) pressure drop approaches as that of a single phase liquid

When velocity is 3Vc or greater, pressure drop in the slurry and that in the equivalent single phase liquid are equal.

The velocity in a long slurry pipeline is nearly 1.5-2 times Vc.

Pneumatic Conveying

A pneumatic conveying system transfers powders, granules and other dry bulk materials through an enclosed pipeline

the motive force for this transfer comes from a combination of pressure differential and the flow of a gas, such as air or nitrogen.

The system’s basic elements include amotive air source (also called an airmover, such as a fan or blower), a feeder or similar material-introduction device, a conveying line, a termination vessel (such as an airmaterial receiver), and a dust collection system.

Pneumatic conveying

While some of the most common materials transferred by pneumatic conveying systems are flour, cement, sand, and plastic pellets,

any of hundreds of chemicals, food products, and minerals can be pneumatically conveyed.

These materials are usually fairly dry; materials in slurry or paste form typically aren’t suited to pneumatic conveying.

Pneumatic conveying

Gas (mostly air) Velocity b/n 15 – 30 m/s in pipes ranging from 50-

400 mm in dia 4 types1. -ve pressure (vacuum) systems, useful in

transferring solids from multiple intake points2. +ve pressure systems, best for one intake point

and one or more points of delivery3. Vacuum-pressure systems, which combine the

advantages of the previous two4. Prefluidised systems, which require less air and

consequently less power, than any other method

How Pneumatic Conveying systems works?

The air pressure in the conveying line is changed by the system’s air mover, which generates pressure or vacuum.

Where the air mover is located in the system determines whether it generates one or the other:

When located at the system’s start, the airmover pushes air through the system and the system operates under pressure.

When located at the system’s end, the air mover pulls air through the system and the system runs under vacuum.

By controlling the pressure or vacuum and airflow inside the system, the system can successfully transfer materials.

Types of Pneumatic conveying systems

Dilute Phase Dense Phase

Dilute Phase

Dense Phase

SOLID CONVEYORS

Solid Conveyers

Gravity Chute Belt Conveyors Screw Conveyors Bucket Elevators Vibrating Conveyors Pneumatic/Hydraulic Conveyors

Gravity Chute

A chute is a vertical or inclined plane, channel, or passage through which objects are moved by means of gravity.

Or through which the solids fall under the action of gravity

Belt Conveyor

where the solids are conveyed horizontally,

or at small angles to the horizontal, on a continuous moving belt

Screw Conveyor

Solids are moved along a pipe or channel by a rotating helical impeller

Bucket Elevators

Particles are carried upwards in buckets attached to a continuously moving vertical belt

Vibrating conveyor

Particles are subjected to an asymmetric vibration and travel in a series of steps over a table.

During the forward stroke of the table the particles are carried forward in contact with it

but the acceleration in the reverse stroke is so high that the table slips under the particles.

With fine powders, vibration of sufficient intensity results in a fluid-like behaviour

Pneumatic and Hydraulic conveying

Discussed before.

AGITATION AND MIXING

Agitation and Mixing

Many operations depend upon effective agitation and mixing of components

Agitation: induced motion of a material Mixing: random distribution of two

initially separate phases A single homogeneous material such as

water in a tank can be agitated but not mixed until another material is added to tank

Agitation and mixing

agitation is a means whereby mixing of phases can be accomplished and by which mass and heat transfer can be enhanced between phases or with external surfaces

Mixing is concerned with all combinations of phases of which the most frequently occurring ones are

1. gases with gases 2. gases into liquids: dispersion.3. gases with granular solids: fluidization, pneumatic conveying;

drying4. liquids into gases: spraying. 5. liquids with liquids: dissolution, emulsification, dispersion6. liquids with granular solids: suspension. 7. pastes with each other and with solids. 8. solids with solids: mixing of powders.

Agitation and Mixing

interaction of gases, liquids, and solids also may take place

Example: hydrogenation of liquids in the presence of a slurried solid catalyst where the gas must be dispersed as bubbles and the solid particles must be kept in suspension

Purpose of Agitation

Suspending solid particles in a liquid Blending miscible liquids e.g. methanol-

water Dispersing a gas through a liquid in the

form of small bubbles Dispersing a second liquid, immiscible

with first to form an emulsion or suspension of fine drops

Promoting heat transfer between liquid and a coil or jacket.

Agitated Vessels

Round bottom to eliminate corners where fluid cannot penetrate

Impeller is mounted on a shaft

Shaft driven by a motor

Flow patterns will be discussed later

Baffles to reduce tangential motion of fluid

Design of an Agitated Vessel

Impellers

2 types1. Generate currents parallel

with the axis of impeller shaft Axial-flow impeller

2. Generate currents in a radial or tangential direction Radial flow impellers

Axial flow impellers impose essentially bulk motion, and are used on homogenization processes

Radial flow impellers impose shear stress to the fluid, and are used to mix immiscible liquids

3 types used based upon low-to-moderate viscosity liquids

1. Propellers 2. Turbines3. High efficiency impellers

Propeller

Axial flow, high speed impeller for liquids of low viscosity

Its rotation forces liquid downward until deflected by the floor vessel

Propeller blades cut or shear the liquid Produces a helix in the fluid

Propeller

One revolution will move the liquid longitudinally a fixed distance

depending upon angle of inclination of impeller blades

Ration of this distance to propeller diameter is called pitch of impeller. (square pitch=1)

2 or more propellers may be used on a single shaft; directing liquid in same direction

Straight blade turbine impeller

Straight blade force liquid radially and tangentially with no vertical movement.

Current moves outward to vessel wall and then either upward or downward

Also called paddles

Turbines

Disk Turbine like straight blade turbine creates zones of high shear rate

Dispersing a gas in a liquid

Pitched blade turbine is used when good overall circulation is important

High Efficiency Impellers

Designed to produce more uniform axial flow and better mixing

Reduce power required Blades are sometimes

crimped to decrease the blade angle near tip

Used to mix low to moderate viscosity liquids but not for very viscous liquids or dispersing gases.

Impellers for highly viscous liquids

Anchor

Helical ribbon

Helical ribbon impeller Having diameter almost

equal to inside dia of tank Promotes liquid motion all

the way to the tank wall with very viscous liquids

Anchor Impeller Creates no vertical motion Less effective than helical Promotes better heat

transfer May have scrapers to

remove liquid from tank wall

Flow patterns in Agitated Vessels

Depends upon Type of impeller Characteristics of the liquid (esp

viscosity) Size and proportions of the tank, baffles

and impeller

3 velocity components

Liquid velocity at any point has 3 components

1. Radial – perpendicular to shaft of impeller2. Longitudinal – parallel with shaft3. Tangential or rotational – tangent to a

circular path around the shaft. Radial and longitudinal comp are useful in

mixing When shaft is vertical and centrally located;

tangential component is disadvantageous – creates a vortex

Vortex formation and its disadvantages

If solid particles are present, they will be thrown at the outside by centrifugal force; and move downward and to the centre of the tank at bottom

Instead of mixing; (reverse) concentration occurs

Relative velocity b/n blades and liquid is reduced

Hence power that can be absorbed by the liquid is limited

Prevention of Swirling (Vortex)

In small tanks, impeller can be mounted off centre

In large tanks, agitator may be mounted in the side of the tank with an angle with the radius

Installing baffles in large tanks (2,3 or 4)

Flow patterns in agitated vessels– Axial flow impellers

When swirling is stopped; flow patterns depends on the type of impeller

Propeller agitators drive the liquid down to the bottom of the tank, where stream spreads radially in all directions toward the wall and flows upward along the wall and returns to the suction of the propeller from top.

For keeping solid particles in suspensions

Axial flow impellers change their flow pattern from axial flow at low liq viscosity to radial at high viscosity.

Flow patterns in agitated vessels– flat blade turbines

Flat blade turbines give good radial flow Stream moves at the wall and divides One portion flows downward along the

wall and back to the center of the impeller from below

Another portion flows upward toward the surface and back to impeller from above

Flow patterns in agitated vessels

Flow patterns in agitated vessels

In unbaffled tank there are strong tangential flows and vortex formation at moderate stirrer speeds

With baffles, vertical flows are increased and is more rapid mixing of liquid.

2 or more impellers can be mounted on single shaft for a long vertical cylindrical tank

Lowest impeller is usually a radial flow impeller (straight blade turbine); the upper is axial flow

Lowest impeller is about one impeller diameter above the bottom of the tank.

Mixing and Blending

mixing is putting things together. Blending is the combination of things in a

unified smoothed manner mixing is the combining of a number of

dry ingredients which when combined with a liquid is blended to achieve a uniform (usually) processed product

Mixing and Blending

Mixing is more difficult operation than agitation

Criteria depends upon the experimenter Often good mixing is visual

Color change of an acid base indicator Solid-liquid mixtures

Rate of decay of concentration or temperature fluctuations; variation in the analysis of small samples taken from various parts of the mix

Blending of Miscible liquids

Miscible liquids are blended in small process vessels by propellers, turbines or high efficiency impellers

In large storage tank, the agitator may turned on only to blend the stratified layers of liquid that are formed as the tank is being filled

Stratified blending if often very slow

Blending in Process Vessels

Impeller in process vessel produces high velocity stream

Liquid is well mixed in the region close to impeller bcos of intense turbulence

There is probably little mixing in the direction of flow

The fluid completes a circulation loop and returns to the eye of the impeller where vigorous mixing again occurs

Calculations show that complete mixing (99%) is achieved if contents are circulated about 5 times

Blending

Rate of Mixing– Circulation rates

For efficient mixing, volume of fluid circulated by impeller must be great enough to sweep out the entire vessel in a reasonable time

The velocity of the stream leaving the impeller must be enough to carry the currents to the remotest parts of the tank

Circulation rate is not the only factor; turbulence in moving stream is often very imp

Turbulence results from properly directed currents and large velocity gradients in the liquid

Rate of Mixing– Circulation rates

Circulation and turbulence both consume energy

Flow rate and power dissipation increase with stirrer speed

However selection of the type and size of impeller influences the relative values of flow rate and power dissipation

Large impellers moving at medium speed promote flow

Smaller impellers at high speed are used where intense turbulence is required.

Power consumption