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.
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