Gas Dispersed Liquid Dispersed

Sparged Vessels

Mechanically Agitated Vessels

Tray Towers

Venturi Scrubbers

Wetted Wall Towers

Spray Towers & Spray Chambers

Packed Towers

Done by a rotating device- impellers

Used where multistage, countercurrent effect is not required

Produces high turbulent intensities

Increases mass & heat transfer rates

Prevent settling

Typical vessels- vertical circular cylinders

Rectangular tanks also used

Impeller consists of a hollow shaft and a flat blade turbine.

Promotes rapid reactions between gases, liquids, and solids in high-pressure reaction vessels.

Turbine creates suction, drawing gas in at the top of the shaft. Gas travels through the shaft and exits as bubbles that are thoroughly dispersed by the turbine blades..

Different Types Of Mixers and Impellers

Agitation and Mixing

To suspend solids

To blend miscible liquids

To make homogenous solid mixtures and pastes

To disperse gas through liquid

To make emulsions from immiscible liquids

To promote heat transfer

Types Of Mixers:

No moving parts, continuation of pipe

Divides stream until molecular diffusion prevails

Used for gases, low viscosity liquids

Static Mixers

Movement of agitators/mixers/homogenizers responsible for mixing

Can be used for solids, liquids and gases

Additional power required running motors and other accessories

Dynamic Mixers

Static mixer: Static: consist of fins, obstructions, or channels that are mounted in pipes.

Planetary mixer

Ribbon Mixer: has a balanced shaft which carries helical ribbons (continuous or interrupted) and is suitable for blending two or more

powders with varying densities

Twin paddle mixer: Have horizontal rotating shafts with fixed arms and paddle-shaped feet

Screw mixer: Screw: use a rotating screw that moves around the periphery of a conical hopper.

Magnetic mixer: Uses magnetic coupling to convert magnetic energy into rotational. Used in handling toxic, highly corrosive or high purity materials.

Tumbling mixer: Vessel rotates on a horizontal shaft. Generally used for solids.

Basic Mixer or agitated vessel

Motor

Gear arrangement

Baffle

Impeller shaft

Vessel

Impeller

Drain valve

Vessel and Turbine Design

Type and Location of Impeller

Proportions of vessel

Number and Proportions of Baffle

Typical Proportions

Da:Dt = 1:3, H:Dt = 1

J:Dt = 1:12, E:Dt = 1:3

W:Da = 1:5, L:Da = 1:4

Types of Impellers

Radial flow Axial Flow

Generate currents in the radial or tangential direction.

Blades are parallel to the axis of the mixer shaft.

Generate currents parallel to the axis of the shaft

Blades make an angle of less than 90° with the mixer shaft.

Turbine impellerPropeller impeller

For small scale application

Axial flow

Used for liquids of low viscosity

Speed of operation: 400 to 1750 rpm

Low efficiency

Maximum diameter: 18 inch

Power required is lesser

For large scale application

Axial or radial flow depending on design

Can be used for liquids of high viscosity

Speed of operation: 20 to 150 rpm

High efficiency

Maximum diameter: 5 m

Power required is more

Flow pattern generated by impellers

Propeller impeller Turbine impeller

Power For Mixing

Power for mixing: P = Npρn3Da5

Np= Power Number (unitless)Da= Impeller diameter

Laminar Flow; NRe < 10Np = KL/NReP = KL n2Da

3µ

Turbulent Flow; NRe >= 10,000Np = KT

P = KTρn3Da5

KL and KT values given in Table 9.2 of Mc Cabe and Smith

Marine Propeller:

High speed

Low viscosity

High liquid circulating capacity.

Different Types Of Impellers

Hydrofoil Left hand (LH) propeller

In majority of industrial mixing applications

A Clockwise rotation (motor end), creates a downward flow to produce optimal tank turnover.

One piece Hydrofoil Impeller

Flat blade turbine

Offers very high torque,

blending at the expense of efficiency, or when high shear is required.

Ex: liquid-liquid emulsions, high intensity solids scrubbing.  

                       

Curved blade turbine

Most flow-efficient,

Radial flow impeller used to break up plug flow in flow-velocity sensitive applications, such as in a multi-stage aluminum digester.

Used where high wall velocity is required, ex in heat transfer applications.

Disc Turbines

For gas dispersion applications.

Primarily used for very high intensity mixing Power number of 4.75, the highest of any common impeller.                     

Pitched blade turbine

Produces axial flow

Imparts varying degrees of shear.

By varying blade width and pitch angle, these impellers can be configured to optimize process performance.

Concave blade CD-6 Impeller

For gas dispersion applications

Has a unique blade design that handles higher gas rates for improved process efficiency.

Characterized by curved pipe sections mounted to a disc.

Results in a lower power number (ungassed = 3.20) and increased efficiency.    

Helical Impellers

Used for highly viscous liquids, µ> 20 Pa.S

Diameter of helix close to that of tank

Can handle viscosity up to 25,000 Pa.S

Anchor Impeller

To provide good agitation near floor of the tank

Creates no vertical motion, hence less effective than helical ribbon

Promotes heat transfer to or from wall

Paddles

Versatile form of turbines- contains as many forward agitators as the backward.

This pattern allows better and speedier mixing.

Paddle agitator can be used for small batch precision mixing as it can take as little as the 20% of the maximum capacity.

Suitable for both wet and dry ingredients, it is recommended for fertilizers, chemicals and minerals.

2 blade paddle 4 blade paddle 6 blade paddle

Commercially used paddles

Saw toothed disperser

Np= 0.45 Nq= 0.32

Lightnin A315

(Axial Flow, down pumper)

Np= 0.75 Nq= 0.73

Holmes and Narver Pumper Mixer

Curved bladed pumper

Lightnin A 6100

Advanced fiber reinforced composite

Highly corrosion resistant plastic matrix

Np=0.23, Nq=0.59

VORTEX FORMATION & PREVENTION

WHAT IS VORTEX????

A vortex is a spinning, often turbulent, flow (or any spiral motion) with closed streamlines.

It is a flow involving rotation about an axis.

The shape of media or mass rotating rapidly around a center forms a vortex.

vortex formation

Vortex created by the passage of an aircraft wing, revealed by colored smoke

FLOW PATTERNS IN THE AGITATED VESSELS

The factors in which the type of flow in an agitated vessel depend are:

Type of impeller.Characteristic of fluid.Size and proportion of vessel baffles n

agitator.

How vortex is formed???

In an open vessel with a gas-liquid surface where an axially located impeller operating at low speeds, the liquid surface is level and the liquid circulates about the axis.

As the impeller speed is increased to produce the turbulent conditions, power required to turn the impeller increases and a vortex begins to form around the shaft.

Air is drawn into the liquid, the impeller operates partly in the air.

Power required drops.

At higher speeds vortex eventually reaches the impeller

Vortex formation n circulation patterns in an agitated vessel.

Vortex at very high impeller speed.

Why do we need to prevent the formation of vortex????

Gas is drawn into the liquid phase so the reaction doesn’t occur in one phase. so, the Conversion achieved is minimum.

In vortexing, the surface of the liquid takes roughly U-shape n efficient mixing no longer takes place.

When suction from river is done with the help of pump, due to vortex formation on river surface air will enter in the pump, n the efficiency of pump will decrease.

Prevention of vortex

Operation in the laminar range for the impeller (Re=10 to 20).

we should keep low speed of impeller but this is impractically slow for mass transfer purposes.

Off-center location of the impeller.locating the impeller on a shaft entering the

vessel at an angle to the vessel axis

Installation of BAFFLES.

Now…..what are baffles???Baffles are flat vertical strips arranged radially

at 90 deg. intervals around the tank wall, extending for the full liquid depth.

It is common practice to use four baffles

Specifications of the baffles:Width of the baffle should be one-tenth to

one-twelfth of the tank diameter.Baffle height should be at least twice the

diameter of the impeller.Baffles should be set out from the wall with

the gap of about one-fifth of baffle width between the baffle n vessel to minimize the accumulation of solids on or behind them.

Advantages…… -supports for helical heating or cooling coils. -to eliminate stagnant pockets in which

solids can accumulate. -reduces swirl n increases vertical liquid

currents.Disadvantages… -baffles increases the power requirement.