Column Internals for Distillation and Absorption

8/10/2019 Column Internals for Distillation and Absorption

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ColumnInternals

for

Distillation

andAbsorption


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StagedColumn

Internals

Tray ahorizontalplatewhichsupportsthevaporliquidmixtureandservesasanequilibriumstage.

Downcomer anopeninginthetraywhichallowstheliquidtoflowdownthecolumnandprovidessufficient

volume

and

residence

time

for

the

liquid

to

be

freed

of

entrainedvaporbeforereachingthebottomandenteringthenextplate.

Weir a

vertical

plate

or

dam

at

the

downcomer to

provideagivenvaporliquidmixturedepthonthetray.


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TrayTypes

SieveorPerforated simplyatraywithvapor

holes

BubbleCap acapplacedoverthetraysvapor

holes

Valve a

valve

placed

over

the

trays

vapor

holes


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fBinaryMixture

It is the simplest type of tray,

which consists of a metal plate

with 315 mm holes, spaced in a

regular pattern.

SieveTray


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DistillationofBinaryMixture

Detailsofacontacting onasieve

trayA 2.1mdiameter perforated (sieve) tray. The down

comer to the next tray is located on the left.

SieveTray(contd.)


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DistillationofBinaryMixture

A bubble cap tray has

bubble caps that consists of

a fixed cap, 3 6 inches indiameter, mounted over and

above a riser of 2 3 inches

in diameter. The cap has

rectangular or triangularslots cut around its side cap.

The vapor flows up through

the tray opening into the

riser, turns around andpasses.

Bubblecap

Bubblecaptray


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Lecture19 7

Vapor/LiquidFlowPathsinaBubbleCapColumn


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Valve

Tray

Theopeningsinvalvetrays

are

covered

with

liftablecaps.The riserofthevalve

capissupportedbythe

momentumoftheup

flowingvapor.

At

high

velocities,theriserisfully

open,whileatlowervapor

velocitiestheriseris

partiallyor

completely

lowered.


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Vapor

flow

path

Valvetray Bubblecap Sievetray


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Tray

Comparison

Theturndownratioistheratioofthemaximumvaporflowrate

(flooding)tominimumvaporflowrate(excessiveweeping).


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What

to

Avoid

in

the

Column

Floodingoccursathighvaporflow

rates

excessive

entrainment

of

liquidovercomesthedowncomercapacityandthecolumnfloods orlargeliquidflowrates.

Weepingoccursatlowvaporflow

rates liquid

flows

or

pulses

back

throughthetrayvaporopenings.

Foamingoccurswhenthecomponentsformastablefoam

efficiencyof

the

column

drops

and

thecolumnmayflood.


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OperatingRanges

Vaporvs.

Liquid

Flow

Rates

LimitsofStableOperation


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Efficiency

vs Vapor

flow

rate

Thevaporflowrateaffectsthecolumnoperatingparametersincluding

entrainment,flooding,weepingandtheflowregime;thus,inmanysystems,

theoverallefficiencyisastrongfunctionofvaporflowrate


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CalculationofcolumndiameterFairs Method

The column diameter is important to prevent excessive entrainment, low column efficiency, and

potential flooding. Fairs method estimates the vapor velocity at flooding conditions, uses a rule-

of-thumb to determine the operating velocity, and then calculates the column diameter.

Flooding Vapor Velocity, uflood:

V

VL

flood

Ku ft/s Eq. (12-7)

where the s are the densities of the liquid, L, and vapor, v, and the K factor is

2.0

sb

20CK

Eq. (12-8)

where is the surface tension and Csbis the capacity factor, which is a function of the flow

parameter, FLV:

5.0

L

LV

V

LF

V Eq. (12-9)

where L and V are the mass flow rates of liquid and vapor, respectively.

One also needs to assume the tray spacing in inches.

Calculating FLVfrom Eq. (12-9), one can determine Csbfrom the Fair correlation, Figure 12-16, p.

387, Wankat.

Once Csbis obtained, the K factor and then the flooding velocity, uflood, can be calculated from Eqs.

(12-8) and (12-7), respectively.

(1)

(2)

(3)

(3)

(2) (1)


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Correlation

for

capacity

factor


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AdditionalFactors


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Column

(Tray)

Diameter Theminimumcolumndiameterfortraycolumnsistypically0.75m;otherwise,packedcolumnsareused.

Themaximumdiameterofthecolumncanbequitelarge upto5m althoughitmaybedecidedto

operate2or

more

separate

columns

in

place

of

an

otherwise

large

diameter

single

column.

Asthecolumndiameterdecreases,thevaporvelocityincreasesforagivenvaporflowrate.

Theminimumcolumndiameterisbaseduponthemaximumvaporvelocitythatcausesexcessiveentrainmentandflooding.

Themaximum

column

diameter

is

based

upon

maintaining

ahigh

enough

velocity

to

prevent

excess

weeping.

Theoperatingvaporvelocity,andhenceactualcolumndiameter,isspecifiedasafractionofthefloodingvaporvelocity typically0.65to0.90.

Thefinal

consideration

is

column

cost

a

larger

diameter

column

is

more

expensive

than

asmaller

diametercolumn,althougheconomiesofscaleenterintothecost.


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Other

Factors

Thetotalareaofthetrayholeopening Baseduponvaporflowpertray

Sizedto

prevent

weeping,

minimize

pressure

drop,

and

reduce

entrainment

at

agiven

vapor

velocity.

Thelayoutofthetrayholes: Differentpatternsavailable

Layoutchosentoensureanevenandwellmixedflowofvaporandliquidacrossthetraysothattherearenodryspotsandbypassingofvaporonthetraythatwouldreduceefficiency.

Theliquiddepthonthetrays,hence,theweirheight: Typicallyrangefrom12to75cm

Baseduponvaporandliquidflowpertray

Sizedtopreventdryspots,increaseliquidvaporcontacttime,andtopreventasprayregimethatreducesefficiency.


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Efficiency

of

Trays


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Packedcolumns


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Introduction

usedextensivelyforabsorption,strippingand

distillation. usedwheneverlowholduporlowpressure

dropisdesirable

liquidandvapordistributionareimportant

considerations

filledwith

random

or

structured

packings


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Types

of

packings

eitherrandomorstructured

random

packings:

small

hollow

structures

with

largesurfaceareaperunitvolume;dumpedrandomlyinthetower

structured

packings:

layers

of

elements

fabricatedfromexpandedmetal,wovenwirescreenorsheetmetal,andarestackedintothe

tower

very

carefully

high

mass

transfer

efficiency choiceofpackingisbasedonpressuredropand

masstransferefficiency


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Randompackings


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Knittedstructure

packing

Gauzepacking Tcipac structuredpacking

StructuredPackings


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Support

plates primarypurposeofapackingsupportistoretainabed

ofpackingwithoutexcessiverestrictiontogasand

liquidflow.

alsoservestodistributebothstreams.

unlesscarefullydesigned,thesupportplatecanalso

cause

premature

column

flooding. designofthesupportplatesignificantlyaffectscolumn

pressuredropandstableoperatingrange.

Twobasictypesofsupportplatesmaybeutilized:

Countercurrent

Separateflowpassagesforliquidandgas


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Packingsupports(countercurrent) Supportplate,captype(gasinjectiontype)

Beamtypegasinjectionsupportplateforlargecolumns

TypesofSupportPlates


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Troughtypeliquiddistributor

Perforated

pipe

distributor

LiquidDistributors

Column Internals for Distillation and Absorption

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