Chapter 6-Agitated Liquid

8/17/2019 Chapter 6-Agitated Liquid

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CHAPTER 6-AGITATED LIQUIDS

Introduction and Definition

Purpose of Agitation & Mixing

Agitated EquipmentTypes of Impeller

Flow Pattern in Agitated Vessel

Standard turbine design


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AGITATION & MIXING OF LIQUID

DEFINITIONS

• Agitation: It refers to the induced motion of a “homogenous” material in a

specified way. (eg: in a circulatory pattern in some container)

• Mixing:

It is the random distribution, into and through one another, of

two or more initially separate phases

PURPOSES OF AGITATION / MIXING

• Suspending solid particles

• Blending miscible liquids• Dispersing a gas through the liquid

• Dispersing a second liquid to form an emulsion or suspension

• Promoting heat transfer


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AGITATION AND MIXING


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INTRODUCTION TO MIXING

Mixing is one of themost common operationscarried

out in the chemical, processing and allied industries.

The term "mixing" is applied to the processes used to

reduce the degree of non-uniformity, or gradient of a

property in a system such asconcentration,viscosity,

temperature and so on.

Mixing is achieved by moving material from one region to

another. It may be of interest simply as a means of

achieving a desired degree of homogeneity but it may alsobe usedto promote heat and mass transfer, often

where a system is undergoing a chemical reaction.


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TYPE OF MIXING

Single-phase liquid mixing

Mixing of immiscible liquids

Gas-liquid mixing

Liquid-solids mixing

Gas-liquid-solids mixing

Solids-solids mixing


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INTRODUCTION TO MIXING

1. Single-phase liquid mixing:

Two or moremiscible liquidsmust be mixed to give a

product of a desired specification.

This isthe simplest type of mixing as it involves neither

heat nor mass transfer, nor indeed a chemical reaction.

Example:

1.The use of mechanical agitation to enhance the rates of heat

and mass transfer between the wall of a vessel, or a coil, andthe liquid (brine solution= HCl+H2O).

2.In the blending of petroleum products of different viscosities.


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MIXING

2. Mixing of immiscible liquids:

Whentwo immiscible liquids are stirred together, one phase

becomes dispersed as tiny droplets in the second liquid whichforms a continuous phase.

Example:Liquid-liquid extraction, a process using successive

mixing and settling stages.

The liquids are brought into contact with a solvent that willselectively dissolve one of the components present in the mixture.

Vigorous agitation causes one phase to disperse in the other and,

if the droplet size is small, a high interfacial area is created for

interphase mass transfer.

When the agitation is stopped, phase separation takes place, but

care must be taken to ensure that the droplets are not so small

that a diffuse layer appears in the region of the interface; this can

remain in a semi-stable state over a long period of time and

prevent effective separation from occurring.


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MIXING

3. Gas-liquid mixing: Numerous processing operations involving chemical reactions,

such as aerobic fermentation, wastewater treatment,

oxidation of hydrocarbons, and so on, require good contacting

between a gas and a liquid.

The purpose of mixing here isto produce a high interfacialarea by dispersing the gas phase in the form of bubbles into

the liquid.

Generally, gas-liquid mixtures or dispersions are unstable and

separate rapidly if agitation is stopped.


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MIXING

4. Liquid-solids mixing:

Mechanical agitation may be used to suspend particles in a liquidin orderto promote mass transfer or a chemical reaction.

The liquids involved in such applications are usually of low

viscosity, and the particles will settle out when agitation ceases.

5. Gas-liquid-solids mixing: In some applications such ascatalytic hydrogenation of

vegetable oils, slurry reactors, froth flotation, evaporative

crystallization, and so on, the success and efficiency of the

process is directly influenced by theextent of mixing between

the three phases.


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MIXING

6. Solids-solids mixing: Mixing together of particulate solids, sometimes referred to as

blending, is a very complex process in that it is very dependent,

not only on the character of the particles — density, size, size

distribution, shape and surface properties.

Mixing of sand, cement and aggregate to form concreteand of the ingredients in gunpowder preparation are examples of

the mixing of solids.

Other industrial sectors employing solids mixing include food,

drugs, and the glass industries.


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MIXING

Miscellaneous mixing applications:

Mixing equipment may be designed not only to achieve a

predetermined level of homogeneity, but also toimprove heat

transfer.

For example, the rotational speed of an impeller in a mixing

vessel is selected so as to achieve a required rate of heattransfer, and the agitation may then be more than sufficient

for the mixing duty.

Excessive or overmixing should be avoided as it is not only

wasteful of energy but may be detrimental to product quality.

It is therefore important to appreciate that overmixing mayoften be undesirable because it may result in both excessive

energy consumption and impaired product quality.


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MIXING

In mixing, there are two types of problems to be considered

— how to design and select mixing equipment for a givenduty, andhow to assesswhether a mixer is suitable for a

particular application. In both cases, the following aspects of

the mixing process should be understood:

(i) Mechanisms of mixing.(ii) Scale-up or similarity criteria,

(iii) Power consumption,

(iv) Flow patterns.

(v) Rate of mixing and mixing time.

(vi) The range of mixing equipment available and its

selection.


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A BASIC STIRRED TANK DESIGN

Amount of energy required for achieving a needed amount of

agitation or quality of mixing are based on;

Size of vesel

Dimensions and arrangement of impellers, baffles and other

internals factors.

The internal arrangements depend on the objectives of theoperation: whether it is to maintain homogeneity of a reacting

mixture or to keep a solid suspended or a gas dispersed or to

enhance heat or mass transfer.

A basic range of design factors, however, can be defined to coverthe majority of cases, for example as in Figure7.5.


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THE VESSEL:

A dished bottom requires less power than a flat one. When a

single impeller is to be used, a liquid level equal to the diameter isoptimum (DT=H), with the impeller located at the center for an

all-liquid system.

BAFFLES:

Baffles are needed to preventvortexing and rotation of the

liquid mass as a whole. A baffle width one-ten the tank

diameter,WB = DT /10; a lengthextending from one half the

impeller diameter,D/2,from the tangent line at the bottom to the

liquid level.

IMPELLER TYPES:

Typically, impeller is placed from the bottom vessel withZ A= D. A

basic classification is into those that circulate the liquidaxially

and those that achieve primarilyradial circulation.



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TYPES OF IMPELLERS

A rotating impeller in a fluid imparts flow and shear to it, the

shear resulting from the flow of one portion of the fluid past

another. The flows are in the axial or radial directions so that impellers

are classified conveniently according to which of these flows is

dominant.

Those generate currents parallel with the axis of the impeller

shaft are calledaxial-flow impellerand those that generatecurrents in a radial or tangential direction are calledradial

flow impeller.

The three main types of impeller (low to moderate viscosity) are :

1. Propellers,

2. Turbines,

3. High – efficiency impeller.

For high viscosity= Helical impellers and anchor agitators


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1. Propeller:

A propeller is an axial-flow, high speed impeller for liquids of low

viscosity. The direction of rotation is usually chosen to force the liquid

downward, and the flow currents leaving the impeller continue

until deflected the floor of the vessel.

Because of the persistence of the flow currents, propeller agitators

are effective in very large vessels.

For deep tank- two or more maybe mounted on the same shaft.

T Type: Standard 3-blade marine

propeller with square pitch (commonin used).

-four blade, toothed/other designed.


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2.Turbines.

3 types;

Type 1: The turbine with flat vertical blades extending to the

shaft is suited to the vast majority of mixing duties up to 100,000CP or so at high pumping capacity. The currents it generates

travel outward to the vessel wall and then flow either upward or

downward. Such impellers are sometimes called paddles.

Type 2/3: Create zones of high shear rate.Good in dispersing gas in a liquid (gas is

forced at high shear rate to flow

radially to the blade tips)


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3.High- efficiency impeller.

. Variations of the pitched-blade turbine have been developed to

provide more uniform axial flow in addition to radial flow forbetter mixing, as well as to reduce the power required for a given

flow rate.

.These impeller are widely used to mix low or moderate viscosity

liquids, but they are not recommended for very viscous liquids or

for dispersing gases.

. Eg: A310 fluid foil impeller


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Highly viscous liquids impeller.

Use for liquid with viscosities more than 20 Pa.s

-diameter helix approximately to inner diameter of tank

Provide good agitation near the floor of the tank;. No vertical motion

. Promotes good heat transfer to/from the vessel.

a) Double-flight helical-ribbon impeller b) Anchor impeller


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MIXING EQUIPMENT

The wide range of mixing equipment available

commercially reflects the enormous variety of mixing duties

encounteredin the processing industries.

It is reasonable to expect therefore thatno single item of

mixing equipmentwill be able to carry out such a range of

duties effectively.

This hasled to the development of a number of distinct

types of mixer over the years.

Thechoice of a mixertype and its design is therefore

primarily governed by experience. In the following

sections, the main mechanical features of commonly used

types of equipment together with their range of applications

are described qualitatively.


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MECHANICAL AGITATION

This is perhaps the most commonly used method of mixing

liquids, and essentially there are three elements in such devices.

Vessels

These are often vertically mounted cylindrical tanks, up to10 m

in diameter, which typically are filled to adepth equal to

about one diameter, although in somegas-liquid contacting

systems tall vessels are used and the liquid depth is up to aboutthree tank diameters; multiple impellers fitted on a single

shaft are then frequently used.

The base of the tanks may beflat, dished, or conical, or

specially contoured, depending upon factors such as ease of

emptying, or the need to suspend solids, etc., and so on.

For the batch mixing ofviscous pastes and doughs using

ribbon impellers and Z-blade mixers, the tanks may be

mountedhorizontally.


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Baffles

To prevent gross vortexing, which is detrimental to mixing,

particularly in low viscosity systems, baffles are often fitted to the

walls of the vessel.

These take the form of thin strips aboutone-ten of the tank

diameter in width, and typicallyfour equi-spaced bafflesmay

be used.

In some cases, thebaffles are mounted flush with the wall,

although occasionally a small clearance is left between the wall

and the baffle to facilitate fluid motion in the wall region.

Baffles are, however, generallynot required for high viscosity

liquids because the viscous shear is then sufficiently great to

damp out the rotary motion. Sometimes, the problem ofvortexing

is circumvented by mounting impellers off-centre.


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Impellers

Figure 7.20 shows some of the impellers which are frequently used.

Propellers, turbines, paddles, anchors, helical ribbons and

screwsare usually mounted ona central vertical shaftin acylindrical tank, and they are selected for a particular duty

largely on the basis of liquid viscosity.

By and large, it is necessary to move from a propeller to a turbine

and then, in order, to a paddle, to an anchor and then to a helical

ribbon and finally to a screw as theviscosity of the fluids to bemixed increases. In so doing thespeed of agitation or

rotation decreases.

Propellers, turbines and paddlesare generally used with

relatively low viscosity systems and operate at high

rotational speeds. A typical velocity for the tip of the blades of aturbine is of the

order of3 m/s, with apropeller being a little fasterand the

paddle a little slower.


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These are classed as remote-clearance impellers, having

diameters in the range (0.13-0.67) x (tank diameter).

Typical design take asD=0.5DT. Furthermore, minor

variations within each type are possible. For instance,

Figure 7.20b shows a six-flat bladed Rushton turbine,

whereaspossible variations are shown in Figure 7.21.

a six-flat bladed Rushton turbine

Hence it is possible to have retreating-blade turbines,angled-blade turbines, four- to twenty-bladed turbines, and

so on. For dispersion of gases in liquid, turbines are usually

employed.


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Propellers are frequently of the three-bladed marine type

and are used for in-tank blending operations with low

viscosity liquids, and may be arranged as angled side-entryunits, as shown in Figure 7.22.

For large vessels, and when the liquid depth is large

compared with the tank diameter, it is a common practice

to mount more than one impeller on the same shaft. With

this arrangement the unsupported length of the propeller

shaft should not exceed about 2 m.


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In the case of large vessels, there is some advantage to be

gained by using side- or bottom-entry impellers to avoid the

large length of unsupported shaft, though a good gland ormechanical seal is needed for such installations or

alternatively, a foot bearing is employed.

Despite a considerable amount of practical experience, foot

bearings can be troublesome owing to the difficulties oflubrication, especially when handling corrosive liquids.


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In comparing propellers and turbines, the following features

may be noted:

Propellers:

(a) are self-cleaning in operation,

(b) can be used at a wide range of speeds,

(c) give an excellent shearing effect at high speeds,

(d) do not damage dispersed particles at low speeds,

(e) are reasonably economical in power, provided the pitch is

adjusted according to the speed,

(f) by offset mounting, vortex formation is avoided,

(g) if horizontally mounted, a stuffing box is required in theliquid, and they are not effective in viscous liquids.


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Shrouded turbine

(a) are excellent for providing circulation,

(b) are normally mounted on a vertical shaft with the stuffing boxabove the liquid,

(c) are effective in fluids of high viscosity,

(d) are easily fouled or plugged by solid particles,

(e) are expensive to fabricate,

(f) are restricted to a narrow range of speeds, and

(g) do not damage dispersed particles at economical speeds,

Open impellers

(a) are less easily plugged than the shrouded type,

(b) are less expensive, and

(c) give a less well-controlled flow pattern.


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FLOW PATTERN

The way a liquid moves in an agitated vessel depends on;

a) the type of impeller;

b) the characteristics of the liquid, especially its viscosity;c) the size and proportions of the tank, baffles and impeller.

.The liquid velocity at any point in the tank has three

components, and the overall flow pattern in the tank depends on

the variations in these three velocity components from point to

point.

.The first velocity component - radial and acts in a direction

perpendicular to the shaft of the impeller.

.

The second component- longitudinal and acts in a directionparallel with the shaft.

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

direction tangent to a circular path around the shaft.


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FLOW PATTERN


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ASSIGNMENT 2 – (4 MEMBERS/GROUP)

MIXING OPERATIONS AVAILABLE IN INDUSTRY

Choose 1 type of mixing operation available in industry andyour discussion shall includes;

1. AGITATION AND MIXING PROCESSES BACKGROUND

2. DESIGN OF MIXING TANK - Production Rate

- Tank Dimension (D and H), Agitated System, No of baffles,

3. THE FLOW PATTERNS

4. CALCULATION OF POWER CONSUMPTION

Date of Report Submission: wk 13 (13 Dec 2013)

Date of Presentation: 18 Dec 2013


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MIXING MECHANISMS

If mixing is to be carried out in orderto produce a uniform

mixture, it is necessaryto understand how liquids move

and approach this condition.

In liquid mixing devices, it is necessary that two

requirementsare fulfilled.

1. There must be bulk or convective flow so that there are no

dead (stagnant) zones.

2. There must be a zone of intensive or high-shear mixing in

which the inhomogeneities are broken down.

MIXINGMECHANISMS


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MIXING MECHANISMS

Both these processes are energy-consuming and ultimately

the mechanical energy is dissipated as heat; the proportion of

energy attributable to each varies from one application to

another.

Depending upon the fluid properties, primarily viscosity, theflow in mixing vessels may be laminar or turbulent, with a

substantial transition zone in between the two, and

frequently both flow types will occur simultaneously in

different parts of the vessel.


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Laminar mixing.

Laminar flow is usually associated with high viscosity liquids

(in excess of 10 N s/m2) which may be either Newtonian or

non-Newtonian. In laminar flow, mixing process occurs when the liquid is

sheared between two rotating cylinders. During each

revolution, the thickness of the fluid element is reduced, and

molecular diffusion takes over when the elements are

sufficiently thin. This type of mixing is shown schematically in Figure 7.3 in

which the tracer is pictured as being introduced

perpendicular to the direction of motion.

Finally, mixing can be induced by physically slicing the fluid

into smaller units and re-distributing them. In-line mixersrely primarily on this mechanism, which is shown in Figure

7.4.

Thus, mixing in liquids is achieved by several mechanisms

which gradually reduce the size or scale of the fluid elements

and then redistribute them in the bulk.

LAMINARFLOW


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LAMINAR FLOW

TURBULENTFLOW


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TURBULENT FLOW

Turbulent mixing.

For low viscosity liquids (less than 10 mN s/m2), the bulk flowpattern in mixing vessels with rotating impellers is turbulent.

The inertia imparted to the liquid by the rotating impeller is

sufficient to cause the liquid to circulate throughout the vessel

and return to the impeller.

Mixing is most rapid in the region of the impeller because of

the high shear rates due to the presence of trailing vortices,

generated by the impeller.

Documents

Chapter 6-Agitated Liquid