Heat Transfer Equipment Agitated Vessel

8/10/2019 Heat Transfer Equipment Agitated Vessel

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Agitated vessel

ERT 216 HEAT & MASS TRANSFER


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1. Introduction

2. Mixing Terminology3. Important Heat Transfer

Considerations4. Heat Transfer in Agitated

Vessels5. Heat Transfer Surfaces and

Effective Areas6. Jackets and Other Applied

Devices


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7.Internal Pipe Coils8.External Auxiliary Devices9.Process-Side Heat-Transfer

Correlations10.Service-Side Heat-TransferCorrelations


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Many processing operations depend

for their success on the effectiveagitation and mixing of fluids.

Agitation and mixing are not

synonymous.Agitation:The induced motion of a material in

a specified way, usually in acirculatory pattern inside somesort of container.


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Mixing:The random distribution into and

through one another, of two ormore initially separate phases.

Agitate water

Add

Agitation & Mixing


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Agitation and mixing:

LiquidsDispersion of liquids and gasses into

other liquids

Suspension of solids in liquids.Purposes of agitation:

1.Suspending solid particles.

2.Blending miscible liquids, (e.g.methyl alcohol & water)

3.Dispersing a gas through the liquid

in the form of small bubbles.


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4. Dispersing a second liquid, immiscible with

the first, to form an emulsion or suspensionof fine drops.5. Promoting heat transfer between the liquid

and a coil or jacket.

Agitated vessels: Liquids are often agitated in tank or vessel,

usually cylindrical in form with a vertical axis.

The top of the vessel may be open to the airor closed. The proportions of the tank vary widely,

depending on the nature of the agitation

problem.


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The tank

bottom isrounded, notflat, to

eliminate sharpcorners orregions into

which fluidcurrents wouldnot penetrate.

Fig 1: Typicalagitation process

vessel.


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The liquid depth is approximately

equal to the tank diameter.An impeller is mounted on an

overhung shaft (a shaft supported

from above).The shaft is driven by a motor,

sometimes directly connected to the

shaft but more often connected to itthrough a speed-reducing gearbox.

Accessories: inlet & outlet lines,

coils, jackets, thermometer etc.


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The impeller causes the liquid to

circulate through the vessel andeventually return to the impeller.

Baffles are included to reduce

tangential motion.


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Heat transfer is an importantconsideration when the fluid motionin the vessel is in the laminar flow

regime.It influences the design andoperation of agitated process vessels

such as reactors, evaporators, andcrystallizers.



Heating and cooling of fluids in these

vessels are necessary to:Remove the heat of reactionProvide uniform temperature in a

vessel.Provide accurate temperature control

in a given process.

Agitation improves heat transfer by itseffect on the process-side (inside theprocess vessel) heat-transfer resistance(the controlling resistance).



The design challenge is to select and

designan agitation system to minimizethe process-side heat-transferresistance while meeting other mixingrequirements.

Proximity and nonproximity impellers arethe two major designs used in mixingapplications.

Proximity relates to distance from thevessel wall.

Fig. 2 (a) shows a nonproximity impeller

typically used for turbulent conditions.


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Its blades are not close to the vessel wall.Close proximity agitators like anchors

and helical ribbons, illustrated in Fig. 2 (b,c), are typically used for high-viscosityapplications. Fig 2:

Types ofmixingimpellers

for heat-transferapplications


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An agitated vessel may be operated in

either a batch, continuous or semi-batchmode.In continuous operations, the typical heat-

transfer requirement is to maintain a setprocess temperature by either adding orremoving heat, depending on the chemicalreaction involved.

In batch operations, the heat-transferprocess can have a number of differentfunctions at different stages of the

operation.


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Examples of the functions include:

i. Establishment of initial reactiontemperatureii. Maintenance of a set temperatureiii.Cooling of a product to a final desired

temperature

The heat-transfer coefficient on both

the process (agitated) and service(jacket) side may change dramaticallyduring the course of processing, usually asa result of physical property or chemicalchanges.


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Heat transfer seldom dictates equipmentdesign.

Process mixing requirements dictatedesign for the majority of agitated tank

systems.Heat transfer is then a necessary

adjunct, and the design objective is to

accommodate a suitable means ofmatching the heat-transfer requirementsto other process requirements.


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Surface area for process heat transferis made available by means ofjackets,coils, baffles, and plates.

When these fail to adequately meet

process requirements, pumps and externalheat exchangers are commonly used.

Under certain conditions, condensers can

be designed to remove process heatthrough the refluxing of a solvent orreactant.


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Power Number, Np:

Np is also a measure of the relative dragof the impeller.Streamline curved blades, like hydrofoils

and retreatcurve impellers, have lessdrag than flat blades; consequently, theirpower numbers are lower than those forflat-blade impellers.

The calculation of power from impellerdiameter, speed, and liquid density isgiven by:


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Flow Number, Nq:

The magnitude of the flow number is ameasure of an impellers ability toproduce flow.

The larger the flow number, the greateris the flow.The total impeller flow consists of the

direct discharge flow plus entrainedflow.Most reported flow numbers include

both flows.


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Mixing time, M:M, is the time it takes to mix initially

segregated materials to a specifieddegree of uniformity.

For example, it takes 60% longer to mixto 99% uniformity than to mix to only95%.

Total flow (direct+entrained),

m3

/s (ft3

/s)

Speed, s-1Impeller diameter, m (ft)


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For certain reactions, it is important to

have mixing times shorter than reactiontimes.The Damkohler number, Da, is the ratio

of mixing to reaction times.

Impeller Reynolds number, Re, NRe, andvessel Reynolds number, Re:The impeller and vessel Re are the ratios

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Molecular diffusivity

Kolmogoroffslength scale

Initial or localconcentration of B


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They are indicators of flow conditions:turbulent, laminar, or transitional.

They are used to correlate otherquantities such as the power number, &the inside heat-transfer coefficients.

Fig A1:Reynolds

number vs.

powernumber forsix turbineimpellers.


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Fig A1:A log-log plot of the power number,

Np, vs. the Reynolds number forseveral impellers in a fully baffledvessel.

A few important features are notedhere:a. In the laminar region (1 Re

10), the power number decreaseslinearly with increasing Reynolds

number.


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b. In the transition region (10 Re 10,000), the power numberdecreases more gradually, and forsome impellers, it then begins toincrease, while for others, itcontinues to decrease withincreasing Reynolds number.

c. In the fully turbulent region (Re 10,000), power numbers areconstant, but design dependent.


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The impeller Reynolds number is definedfor stirred vessels and given:

The power of agitation:

1000

513

3 mDsN

mkgN

)kW(PP

NDRe

2


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EXAMPLE: Determine the power needed to agitate a

fluid using a Rushton impeller, given thespecific gravity of the liquid is 1.0, thetank diameter is 3.0 m, the height of

liquid is 3.0 m, the impeller diameter is1.0 m, the speed is 1.0 s1, and the liquidviscosity is 1.0 cP.


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Solution: Reynolds number:

Flow is fully turbulent.The power number for a Rushton impeller,

the top curve in Figure A.1, NP = 5.0The density = 1000 kg/m3,

1000

0101100005 53 ...

P =5.0 kW

0010

1000101 2

.

.Re

=106


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Micro-Mixing:

It is the smallest scale of mixing. In terms of dimensions, it is at or belowthe Kolmogoroff microscale that canbe calculated mixing is the scaleinvolved with kinetically controlledchemical reactions.

Effective micromixing usually requires

high-energy input.


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Fig A2:Turbulence energy spectrum, with the

Kolmogoroff scale(T) as the length scale.

Kolmogoroffscale:


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Meso-Mixing: It is used to describe the intermediate

scale of mixing between micro-mixing andmacro-mixing.

More specifically, it is the turbulent

exchange between turbulent impellerflow and the surrounding fluid.Macro-Mixing: It is distributive mixing caused by large-

scale flows. It is analogous to convective mixing. The rapid blending in a stirred vessel is

due to macro-mixing.


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The following are some of the key issues

to consider for designing a new system orto troubleshoot an existing one:

[1] Process characteristics:

Is the process continuous, semi-batch, orbatch? Is an exothermic reaction involved? Is the heat of reaction known? What is

the magnitude of heat release? Is there a wall temperature limitation?

(reactivity, purity, fouling)


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Are internal surface area devices

acceptable?Is temperature control important?Can desirable heat-transfer rates

be maintained by controlling thereaction?

Is corrosivity a problem?Are gases evolved from processing?

If so, can gas release rates becontrolled?


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[2] Batch operations:

What is the minimum level that will bemaintained and its level relative to theagitator?

Is heat-transfer surface area available atall stages of processing?How do the physical properties change

during the course of processing?


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[3] Fouling:

Can any undesirable reactions occur in thevapor space of the vessel that maydeposit on the upper surface?

Will controlling the wall temperaturesprevent fouling?Will the process foul the surface of the

vessel or any internals?Are solids formed upon cooling or in the

course of reaction? Is the design suitable for cleaning?


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[4] Safety:

Is the heat release due to mixing?Is there a choice of heat-transfermedia?

Can temporary power loss create asudden heat release when power isrestored?


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The basic heat-transfer equation for heat

transfer between two fluids separated bya wall:

The individual resistances comprising UOand how it is affected by impellerselection and surface area.

Heat flow,

kW (Btu/h) Overall heat-transfer coefficient,kW/m2K

(Btu/hft2F)

Area for heattransfer, m2

(ft2)

Temperaturedriving force,

K (F).

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The overall heat-transfer coefficient for ajacketed vessel can be obtained from theindividual resistances:

Heat-transfer filmcoefficient (insidejacket), kW/m2K

(Btu/hft

2

F)

Wallthickness,

m (ft)

Thermalconductivity of

wall W/mK

(Btuft/hft2F)

Referencearea, m2

(ft2)

Areainside

of

jacket

Foulingresistance

(insidesurface of

vessel)

Foulingresistance

(insidejacket)

Overall heat-transfer

coefficient,

W/m2K

(Btu/hft2F)

Heat-transfer filmcoefficient (insidesurface of vessel),

kW/m2K(Btu/h

ft2

F)


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Fig 3: Heat-transfer

resistances.


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In situations where both ajacket and an

internal device are used, the overallcoefficients for each type of surface should becalculated separately, and the two Qs shouldbe added to obtain the overall heat-transfer

capability.


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Jackets, internal helical pipe coils, tube

baffles, and plate coil baffles are usedto provide heat-transfer surface area.

The only surface area effective for

heat transfer is that portion that iswetted by both service and processfluids.

The effective heat-transfer area forsome items may be determined as follows:i.Use the total wetted area for plain or

spirally baffled jackets.


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ii. The area between the half-pipes is not totallyeffective for heat transfer when using half-pipe coil jackets, usually fabricated from 2-, 3-, or 4-in. pipes with typical 3/4-in. spacing.

iii. The total outside wetted area is effective for

internal helical coils:

Number ofcoil turns

per foot ofcoil height

Totalheightof coil

Centerlinediameter of coil

helix, ft


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Jackets form what amounts to a double

wall on the mixing vessel.

Fig 5: Jacket designs

Spiral

Halfpipe

Dimpled

Agitation

nozzle


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Factors to consider when selecting the

type of jacket to use are listed:1. Cost: in terms of cost the designs can be

ranked, from cheapest to mostexpensive, as below:i. simple, no bafflesii.agitation nozzles

iii.spiral baffleiv.dimple jacketv. half-pipe jacket


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2. Heat transfer rate required:

select a spirally baffled or half-pipejacket if high rates are required.3. Pressure: as a rough guide, the pressure rating

of the designs can be taken as:i. jackets, up to 10 barii. dimpled jackets, up to 20 bar

iii. half-pipe, up to 70 bar.[So, half-pipe jackets would be used for

high pressure].


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The plain jacket is the simplest

construction and therefore the lowestinitial cost.It is suitable for condensing heating

fluids such as steam, but results in verypoor performance using sensible heat-transfer fluids.

Large passage areas limit the ability to

create good wall velocities.


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Installation of agitating nozzles is

recommended if sensible liquid heating orcooling is to be used with plain jackets.Nozzles produce liquid jets directing the

inlet jacket fluid in a spiral fashion intothe jacket.This increases the effective velocity

and turbulence level. Vendors have information dealing with

their performance and installation.


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[1] Spiral-baffled jacket:

The spiral-plate baffle consists of a spiralstrip welded edgewise to the shell.This forms a channel that raises the

velocity next to the wall.The largest drawback to this baffle isthat there will inherently be someclearance between the edge of thebaffle and the tank wall, allowing fluidsto bypass the spiral passageway.


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[2] Half-pipe jacket:

It consists of a half-pipe section welded tothe vessel wall.This construction is quite good if highjacket pressures are required, but it isalso an expensive method, because eachcourse requires two long welds along eachedge of the cut pipe.

It is suitable for sensible heating/cooling but not for condensing/vaporizing fluids.


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Heat-transfer predictability is welldefined, since the geometry is known and

no bypassing is possible.[3] Dimpled jacket :It consists of an outer shell having regular

indentations of the shell material.These dimples are intended to promote

turbulence by creating high local

velocities at the dimple.Heat-transfer information concerning

dimple-jacketed vessels are proprietary

to the fabricators (little information)


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[4] Other devices:

There are other special devices such asclamp-on plate coils, weld-on plate coils,etc.They are often used when an unjacketed

vessel needs limited heat-transfercapability.


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[5] Internal devices:

Internal devices comprise coiled pipe (ortubing), baffles of various types that arealso heat-transfer devices, and sometimeseven the agitator.All internals interfere with the flow

patterns within thevessel and likely lead

to the formation ofstagnant regions.


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The use of internal devices needs to be

carefully considered from the standpointof harm to good mixing.If fouling is a problem (known or

suspected), any internals should beavoided.Internal devices, of any type, increasemechanical complexity and maintenance.

It is recommended that internalconnections be welded, not flanged, tominimize maintenance.


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Internal pipe coils consist of one to three

helical (concentric) coils of pipe locatedinside the mixing vessel for sensibleheating/cooling.

The effectiveness of these coils isdirectly related to the flow patternsgenerated by the agitator.

Considerable area can be added in thismanner, and if all the heat-transfercapability must be provided within themixing vessel itself, this is an effective

means of doing so.


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Potential difficulties include cleaning,

mechanical integrity (the coil must besupported), and installation (both weightand access).

The presence of coils and their supportstructures always detracts from mixingperformance.

The recommended geometry for the use

of coils is shown in Fig 6.


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Fig 6:Recommended

geometry forinternal pipe

coils


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An external auxiliary heat-transfer

device is usually one of the following:A standard condenser,A reflux (or knock-back) condenser,A sensible heat exchanger through

which the vessel contents are circulated.

These external auxiliary devices are onlyrequired when the heat-transferrequirements cannot be met by use ofjackets or internal devices.


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Heat-transfer coefficients on theagitated side are determined by the sameprinciples as for any other heat-transferprocess.Correlations have been developed for

each of the major impeller types.These are all basically of the same

form, but differ only in the pre-proportionality constant and values ofthe exponents.


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The general form of the correlating equation:

The process-side heat-transfercoefficient (for flat-blade turbines) forheat transfer to a jacket is based on thework of Brooks (1959):

Constant(geometry

variations)

Prandtl

number

Nusseltnumber

Reynoldsnumber

Viscosity(Process fluid)

Tank

diameter Viscosity(Vessel wall)


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The basic correlations have been

extended to include more details of theimpeller geometry, such as blade width,pitch, and number of blades.

As long as the process is in the turbulentregime, most of these geometricalvariables have little impact on heattransfer, and their use is not

recommended until details of an agitationsystem are selected or in place.


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For agitated vessels:

The values of constant C and the indices a, b andc depend on the type of agitator, the use ofbaffles, and whether the transfer is to the

vessel wall or to coils.

c

w

b

p

a

v

k

C

NDC

k

Dh 2Heat

transfercoefficientto vessel

wall or coil,Wm-2 oC-1

Agitator

diameter, m

Agitator,

speed, rps

Liquid specific heat

capacity, J kg-1 o

C-1

Liquid thermalconductivity,

Wm-1 oC-1

Liquiddensity,kg/m3

Liquidviscosity,

Nm-2s.


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A general equation that can be used :


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Flat blade paddle, baffled or un-baffledvessel, transfer to vessel wall, Re < 4000:

Flat blade disc turbine, baffled or un-

baffled vessel, transfer to vessel wall, Re< 400:

Flat blade disc turbine, baffled vessel,transfer to vessel wall, Re > 400:


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Propeller, 3 blades, transfer to vessel wall,Re > 5000:

Turbine, flat blades, transfer to coil,baffled, Re= 2000 - 700,000:

Paddle, flat blades, transfer to coil,baffled,


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For proximity impellers (e.g. helical ribbon)for Re


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A conservative estimate for the

condensing-steam coefficient used inplain jackets is 5.678 kW/m2K (1000Btu/hft2F).Any organic fluid will have a lower value

due mainly to its lower thermalconductivity.

Bondi (1983) proposes the followingequations for sensible fluid in plain jacketwith no agitating nozzles.


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Turbulent flow conditions (Re>10 000),

The equivalent diameter of the jacket:

The flow area Ax,

Equivalentdiameter

of thejacket

Axis is used to calculate

the velocity (V) in Re.


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In this case, Reynolds number is

defined as:

All properties pertain to thejacketfluid

Laminar flow conditions (Re10 000),

Documents

Heat Transfer Equipment Agitated Vessel