FF0 Agitation Mixing -Part I

8/7/2019 FF0 Agitation Mixing -Part I

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CHE C 213

Fluid Flow OperationsLecture Notes/Slides on

(Lecture 1, 2 and 3)Dr. ASHISH M GUJARATHI

Birla Institute of Technology and Science, PilaniEmail: [email protected]

You may give your course related feedback.


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Dr. Ashish M Gujarathi

BITS Pilani ASHISH GUJARATHI, BITS PILANI

A Typical Chemical Plant


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Lecture Plan

Agitation and Mixing

Introduction

Applications

Impellers

Propeller

Turbines High efficiency impellers

Anchor Agitator

Helical and Ribbon type impellers


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Lecture Plan


Turbine Design

Flow Patterns


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Agitation:

Agitation refers to forcing a fluid by mechanicalmeans to flow in a specified way usually in

circulator attern.

Mixing

Mixing refers to random distribution betweenseparate phases

E.g. A tankful of cold water can be agitated, but it cannot bemixed until some other material (i.e., salt or hot water) is

added to it


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Mixing term is applied to a variety of

operations (differing in degree ofhomogeneity)

thoroughly blended and

Sand, gravel, cement, and water tumbled in arotating drum for a long time

Though products are not equally homogeneous ,

In both the cases product is said to be mixed


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Purposes of agitation

Suspending solid particles in liquid

Blending miscible liquids

small bubble

Dispersing second liquid, immiscible with first toform an emulsion or suspension of fine drops

Promoting Heat Transfer between the liquid and aheating coil or jacket

Often one Agitator serves purposes at the same time


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Agitated Vessel (with agitator)Inlet Connection

Top of vessel

(Either OpenOr Sealed with

Gasket in betweenTank and rounded

Rounded Bottom (Not Flat)

Vary widely,depending onthe nature of

agitation problem Impeller causes liquidto circulate through

the vessel and returnto impeller

Baffles used to reducetangential motion


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Agitation Equipments

Agitation Equipments

Gas Sparger, Jet Mixer, Bubble Column

Mixing of gas with liquid

,

Agitation of Liquid with Liquid

Double cone mixer or concrete mixer , ScrewConveyer

Mixing of solids with solid

Colloid Mills, Mixing rolls, Pan mixer or puttychasers for clay mixing

Mixing of solids with liquids


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Selection of Agitator

Impeller Agitator

Axial Flow impeller (a)

Those that generate

currents arallel with

the axis of the

impeller shaft

Radial flow impeller (b)

Those that generate

currents in a radial (a) (b)

or tangential direction


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Selection of Impellers

The selection of impeller depends on viscosity

Low to moderate viscous liquids

Propellers (Viscosity < 3 Pa-s)

Turbines Viscosit < 100 Pa-s

High efficiency impellers

Very Viscous liquids

Anchor Agitator ( 50 < Viscosity < 500 Pa-s)

Helical and Ribbon type impellers (Viscosity > 500 Pa-s)

Each type includes many variations and subtypes

which are not discussed here


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Impellers: Propeller

Propeller (for Viscosity < 3 Pa-s)

Axial flow, high speed impeller for liquids

400-1750 rpm motor speed

, ,used

Three bladed turbine are common with squarepitch

Propeller with a pitch 1.0 is : Square pitch

diameterPropeller

bladepropellerofninclinatioofanglecertainat

revolutiononeinliquidby themoveddistancealLongitudin

Pitch =


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Propeller

The direction of rotation is usually chosen to forcethe liquid downward, and the flow currents

leavin the im eller continue until deflected b

floor of the vessel.

Three Blade marine propeller


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Propeller

Propellers rarely exceeds 18 in. in diameter

In deep tanks two or more propellers may be,

liquid in the same direction

Because of persistence of flow currents

Propellers are effective in very large vessels

Six Blade propeller and Shaft


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Impellers: Turbines

Simple straight blade turbine (Visco. < 100 Pa-s)

Pushes liquid radially and tangentially with almostno vertical motion at the impeller

-

common

In Process vessels they turn at 20 150 rpm

Length of Paddle : 50-80% of vessel diameter

Width of blade: 1/6 1/10 of the length


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Impellers: Turbines

Turbine (continued..)

Anchor Agitator: Blades conform to the shape of dished or hemispherical

vessel, so that they scrap the surface. Useful in

prevailing deposits on a heat transfer surface

Baffles

Necessary to reduce swirl around the vessel


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Impellers for high viscosity liquids

High efficiency impellers

Pitched Blade turbine (Fig. e) Used when good overall

circulation is im ortant because

it provides some axial flow in

addition to radial flow

High efficiency impeller HE3

A 310 fluid foil impeller


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Impellers : Turbines

Impellers for highly viscose liquids

Double flight helical ribbon (a) Used widely in Polymerization reactors


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Turbine Design:Typical DimensionsThe Designer of Agitated vessels has an unusually large number of choicesto make as to Type and Location of Impeller, Proportions of vessels, numberAnd proportions of baffles, Motor Speed, Speed of Agitator, etc.

3

1=

t

a

D

D1=

tD

H

12

1=

tD

J

3

1=

aD

E

5

1=

tD

W

4

1=

aD

L

Side View


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Flow Patterns

The way liquid moves in an agitated vessel

depends onCharacteristics of liquid i.e. viscosity, size and

,

The velocity at any point in the tank dependson three components and the flow pattern in

tank depends upon variations in thesecomponents


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Flow Patterns

Velocity components

Radialand acts in a direction perpendicular to shaftof impeller

shaft

Tangential or rotationaland acts in a directiontangent to a circular path around the shaft

Overall flow pattern in the tank/vessel depends onvariations in these 3 Velocity Components


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Flow Patterns

Velocity components and Vortex Formation

In usual case of vertical shafts, the radial andtangential components are in horizontal plane and

the lon itudinal com onent are in a vertical

Radial and longitudinal components are usefuland provide the flow necessary for mixing action

When shaft is vertical and located at central,

tangential component is disadvantageous

Tangential flow follows a circular path around theshaft and creates a vortex


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Vortex in a reactor

Unbaffled vessel with vortex

At high impeller speeds, Vortex may touchimpeller: Undesirable

Shaft

Top ViewSide View

Cylindrical Vessel

Swirling flow pattern with radial-flow turbine in an unbaffled vessel


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Prevention of Swirling

Circulatory flow or swirling can be prevented

byMounting impeller off-center (in small tanks)

,then tilted in plane perpendicular to the direction

of move

In larger tank, the agitator may be mounted in theside of the tank, with a horizontal plane but an

angle with a radius


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Prevention of Swirling

Fig. Flow Pattern with off-center propeller


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Draft Tubes

Draft Tubes

When direction and velocity of flow to the sectionof the impeller are to be controlled, draft tubes

are used.

This devices are useful when high shear at theimpeller is desired.

Loop Reactor??

hi h j hi


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Draft tubes Draft tubes for propellers are mounted around the

impeller, and those for turbines are mountedimmediately above the impeller

Turbine Draft tube Propeller draft tube

D A hi h M G j thi


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FF0 Agitation Mixing -Part I

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