column internals for distillation and absorption
TRANSCRIPT
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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