Agitation and MixingAgitation and Mixing

Mixing in process and chemical industries:

Chemicals

Food, juice, oils, and candy

Pharmaceuticals

Paper

Polymers

Minerals

Agitation and Mixing in Industries

Inhomogeneity in concentration, phase, temperature

aims at reduction of inhomogeneity

Minerals

Environmental

Paints and coatings

Synthetic rubbers and resins

Sealants and adhesives

Catalysts

Acids

Biofuels and Ethanol

Increase mass and/or heat transfer, reaction rate, or

product properties

Chemical industry : up to $10 Billion lost because of poor mixing

Pharmaceutical industry:

Low yield : $100 Million

Poor scale-up $500 Million

Agitation and Mixing Importance

Lost opportunity from poor mixing very large number

Agitation and Mixing Problems

Single-Phase Determine blending time for miscible liquids to obtain uniform mixture

Reduce concentration gradientsMiscible fluids may have different physical properties Chemical reaction may be present

Two-phase

Liquid-LiquidPower required to form 0.01 mm droplet of oil in waterPower required to form 0.01 mm droplet of oil in watergenerate surface area for mass transfer/reactionstable dispersion (emulsion) may be final product

Gas- Liquid Determine rate of mass transfer that can be achieved by sparging a gas into liquid

small bubbles-dynamics high surface area high surface area to give high mass transfer rates and high reaction rates in liquid phase to form a stable dispersion (foam) as a final product

Solid-Liquid Determine minimum impeller speed that will suspend all particles in tank

Dissolving/precipitation /Crystallization Catalyst particles High solids loading pastes

Three-phases Determine reaction rates within liquid in which catalyst particles are solids and one of reactants is gas that dissolves into a liquid to react with a second reactantone of reactants is gas that dissolves into a liquid to react with a second reactant given power, particle and fluid properties, particle diameter, tank geometry

Agitation and Mixing

Agitation refers to the induced motion of a material in a specified way, usually in a circulatory pattern inside some sort of container

Mixing is the random distribution, into and through one another, of two or more initially separate phases

term mixing is applied to a variety of operations, differing widely in the degree of homogeneity of the "mixed" materialhomogeneity of the "mixed" material

A single homogeneous material, such as a tankful of cold water, can be agitated, but it cannot be mixed until some other material is added with it.

1. Suspending solid particles

2. Blending miscible liquids, e.g., methyl alcohol and water

3. Dispersing a gas through the liquid in the form of small bubbles

4. Dispersing a second liquid, immiscible with the first, to form an emulsion or

Liquid-Liquid Mixing

4. Dispersing a second liquid, immiscible with the first, to form an emulsion or

suspension of fine drops

5. Promoting heat transfer between the liquid and a coil or jacket

Reasons for processing particle-liquid systems in mixing equipment:

(a) to promote chemical reactions between particulate solids and liquids

(b) to obtain relatively uniform concentrations of particulate solids in liquids

(c) to promote particle dissolution or crystal growth

(d) to obtain a uniform particle concentration in an effluent stream when a tank is emptied

Particle-Liquid Mixing

Examples for particle-liquid mixing

(a) coal-water slurries,

(b) suspensions of ion-exchange resins

(c) paper pulp slurries

(d) polymer dispersions from polymerization reactions

(e) sugar crystal slurries

(f) paint pigment, clay, or starch slurries

Agitation Equipment

Impellers Classes

Axial-flow impellers

Radial-flow impellers

Impeller-Types ( 95% of the liquid agitation problems are handled by first 3 types)

Propellers

Impellers Classes and Types

Propellers

Paddles

Turbines

Other types

Propellers Axial-flow, high-speed impeller for liquids of low viscosity Speed

small propellers: full motor speed 1150 to 1750 r/min large propellers: 400 to 800 r/min

flow currents leaving the impeller continue through the liquid in a given direction until deflected by the floor or wall of the vessel

propeller blades vigorously cut or shear the liquid persistence of the flow currents, propeller agitators are effective in very large vessels revolving propeller traces out a helix in the fluid, and if there were no slip between liquid

and propeller, one full revolution would move the liquid longitudinally a fixed distance and propeller, one full revolution would move the liquid longitudinally a fixed distance depending on the angle of inclination of the propeller blades

Pitch of a propeller: ratio of this distance to the propeller diameter A propeller with a pitch=1 is said to have square pitch Standard three-bladed marine propellers with square pitch - most common four-bladed, toothed, and other designs - employed for special purposes

rarely exceed 18 in diameter regardless of the size of the vessel Propeller arrangements in deep tanks:

two or more propellers may be mounted on the same shaft, usually directing the liquid in the same direction

two propellers work in opposite directions, or in "push-pull," to create a zone of especially high turbulence between them

Paddles

for simple problems- an effective agitator consists of a flat paddle turning on a vertical shaft

Two-bladed and four-bladed paddles are common

Sometimes the blades are pitched; more often they are vertical

turn at slow to moderate speeds in the centre of a vessel

push the liquid radially and tangentially with almost no vertical motion at the impeller

unless the blades are pitched

currents they generate travel outward to the vessel wall and then either upward or

downward

deep tanks several paddles are mounted one above the other on the same shaft deep tanks several paddles are mounted one above the other on the same shaft

Anchor agitator

In some designs blades conform to the shape of a dished or hemispherical vessel so

that they scrape the surface or pass over it with close clearance

useful for preventing deposits on a heat-transfer surface, as in a jacketed process

vessel, but they are poor mixers

They nearly always operate in conjunction with a higher speed paddle or other

agitator, usually turning in the opposite direction

Industrial paddle agitators turn at speeds between 20 and 150 r/min.

Total length of a paddle impeller is typically 50 to 80 percent of the inside diameterof the vessel

The width of the blade is one-sixth to one-tenth its length

At very slow speeds a paddle gives mild agitation in an unbaffled vessel; at higherspeeds baffles become necessary. Otherwise the liquid is swirled around the vessel at high speed but with little mixing

Paddles contd...

high speed but with little mixing

Helical ribbon :-for viscous fluids 25 Pa. S to 25000 Pa.s

Anchor impeller :-For good agitation at the floor of tank- NO vertical motion- Less effective than helical ribbon- promotes heat transfer

Impellers for high-viscosity liquids (a) Double-flight helical-ribbon (b) Anchor Impeller

Most of them resemble multi-bladed paddle agitators with short blades, turningat high speeds on a shaft mounted centrally in the vessel

The blades may be straight or curved, pitched or vertical

The diameter of the impeller is smaller than with paddles, ranging from 30 to 50 percent of the diameter of the vessel

Turbines are effective over a very wide range of viscosities

Turbines

In low-viscosity liquids turbines generate strong currents that persist throughout the vessel, seeking out and destroying stagnant pockets

Near the impeller is a zone of rapid currents, high turbulence, and intense shear

The principal currents are radial and tangential

The tangential components induce vortexing and swirling, which must be stoppedby baffles or by a diffuser ring if the impeller is to be most effective

Airfoil

Generally most efficient because it produces the maximum pumping with the lowest shear

Pitch Blade

Ideal for viscous mixtures and for applications requiring a combination of pumping and

shearing

Radial Blade Marine-Type PropellersRadial Blade

Ideal for applications where shear is the primary requirement, or where agitation close

to the bottom of the tank is desired

Marine-Type Propellers

for low-viscosity, high-speed direct drive mixers

Impellers for moderate viscosity liquids (a) Three-blade marine propeller(b) Simple straight-blade turbine, (c) Disk turbine, (d) concave-blade, (e) pitched-blade turbine

Standard Turbine Design

Baffles: usually 4 in numberImpeller blades: 4 16 (generally 6-8)

Measurements of turbine

Da/Dt =1/3; H/Dt=1; J/Dt =1/12 E/Dt=1/3; W/Da=1/5; L/Da=1/4

Large number of choices to make as to type, location of impeller, proportion of vessel, number and proportions of the baffles etc.

Each of the above affects circulation rate of liquid, velocity patterns and power consumed

Variation of pitched blade turbine to provide more uniform axial flow and better mixing-To reduce power required

HE-3 : 3 slanted blade with crimped at tip to decrease blade angle at tip

A310 Airfoil shaped blades

High Efficiency Impellers

High-efficiency impellers (a) HE-3 impeller, (b) A310 fluid-foil impeller

A310 Airfoil shaped blades

-Taper narrower at tip than the base- widely used to mix low or moderate viscosity liquids- NOT recommended for very viscous liquids or dispersing gases

Key Factors

1. Tank type and volume

determines the amount of fluid your tank can hold-up

will determine the size and position of the fluid mixer and its mounting

2. Viscosity

thickness or internal friction of the fluid

will determine the impeller and horse power configurations

3. Specific gravity

density of the solid if present, any

will determine the type of pumping action that is required to adequately mix

4. the process

Motor with suitable horse power and energy source

Gear drive optimized for torque capacity

Mounting fitted according to your tank needs

Engineering the Mixer/Agitator

Mounting fitted according to your tank needs

Impeller sized to maximize efficiency

Higher Horsepower does NOT guarantee better mixing!

Torque Pumping and Mixing

Given a standard amount of input force, a longer lever - a greater amount of torque

Rotating force, or torque, is transferred to the fluid to create motion within the

application

Both pumping and mixing are often rated in m3/s

Torque Pumping - Mixing

Both pumping and mixing are often rated in m /s

Each impeller type has a Pumping Number associated with it, and the pumping capacity

of the mixer can be predicted through this value

Pumping numbers are empirical and proprietary (and so some mixer suppliers tend to

overstate the pumping values of their mixers)

Torque per Equivalent Volume is a standard and simple calculation of Mixer

Torque/Tank Size best measure to compare mixer performance

Torque per equivalent volume is what you are investing for when purchasing a mixer !

To achieve a medium level of mixing of a water-like substance in a 12 Ht x 12 Diameter cylindrical tank, we can compare the following mixer configurations

Torque Pumping - Mixing

Flow Patterns

factors affecting flow patterns

impeller type

characteristics of the fluid

size and proportions of the tank, baffles, and agitator

velocity of the fluid at any point

three components - overall flow pattern in the tank depends on the variations in

these three velocity components from point to pointthese three velocity components from point to point

first velocity component - radial and acts in a direction perpendicular to the

shaft of the impeller

The second component - longitudinal and acts in a direction parallel

with the shaft

The third component - tangential, or rotational, and acts in a

direction tangent to a circular path around the shaft

Flow Patterns contd...

Addresses solid suspension and stratification

Flow Patterns contd...

Viscosity: 0 50,000 cpsThe pitch blade impeller is the most versatile impeller and was the standard until the development of the airfoil. Theyre useful in blending two or more liquids and are effective in low bottom clearance with less liquid submergence

In the usual case of a vertical shaft, the radial and tangential components are in a horizontal plane, and the longitudinal component is vertical

The radial and longitudinal components are useful and provide the flow necessary for the mixing action

When the shaft is vertical and centrally located in the tank, the tangential component is generally disadvantageous

The tangential flow follows a circular path around the shaft and creates a vortex in the

Flow Patterns contd...

The tangential flow follows a circular path around the shaft and creates a vortex in the liquid

Swirling flow pattern with a radial-flow turbine in an un-baffled vessel

Exactly the same flow pattern would be observed with a pitched-blade turbine or apropeller

The swirling perpetuates stratification at the various levels without accomplishing longitudinal flow between levels

If solid particles are present, circulatory currents tend to throw the particles to the outside by centrifugal force, from where they move downward and to the center of the tank at the bottom. Instead of mixing, its reverse, concentration, occurs.

Since, in circulatory flow, the liquid flows with the direction of motion of the impeller

Flow Patterns contd...

Since, in circulatory flow, the liquid flows with the direction of motion of the impeller blades, the relative velocity between the blades and the liquid is reduced, and the power that can be absorbed by the liquid is limited

In an un-baffled vessel circulatory flow is induced by all types of impellers, whether axial flow or radial flow

If the swirling is strong, the flow pattern in the tank is virtually the same regardless of the design of the impeller

At high impeller speeds the vortex may be so deep that it reaches the impeller leads to drawing of gas above the liquid- undesirable

Power Requirements

Power requirement is a function of

1. Geometric details such as Diameter, thickness, width etc. of an impeller

2. Type of impeller

3. Geometric details of vessel such as diameter, number of baffles and their dimensions3. Geometric details of vessel such as diameter, number of baffles and their dimensions

4. Rotational speed

5. Fluid properties