Post on 08-Mar-2016
description
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