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INDEX 1. 2.
Introduction Plate contractors 2.1. Definition of tray areas 2.2. Plate types 2.2.1. Bubble cap plates 2.2.2. Valve plates 2.2.3. Sieve plate 2.2.4. Selection of tray type 2.3. Effect of vapor flow conditions on tray design 2.3.1. looding consideration 2.3.2. Sieve tray weeping 2.3.3. !i"uid entrain#ent 2.4. $ray %ydraulic para#eters 2.&. 'olu#n si(ing appro)i#ation 2.*. Provisional plate design 2.*.1. 'olu#n dia#eter 2.*.2. +ole dia#eter, %ole pitc% and plate t%ic-ness 2.*.3. eir %eig%t and weir lengt% 2.*.4. 'al#ing (ones 3. Stepwise design tray procedure References
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1. Introduction $%e typical gas/li"uid contacting operations include distillation, absorption, stripping, leac%ing and %u#idification. Distillation and absorption are two #ost widely used #ass transfer processes in c%e#ical industries. Design of plate colu#n for absorption and distillation involves #any co##on steps of calculation suc% as deter#ination of nu#ber of t%eoretical plates, colu#n dia#eter, plate %ydraulic design, etc.0n absorption process, a soluble co#ponent is absorbed in a li"uid called solvent fro# a gaseous #i)ture. $%e gas and li"uid strea#s leaving t%e tray are in e"uilibriu# under t%e ideal condition. $%e separation in distillation is based on t%e relative volatility of t%e co#ponents. dditional vapor p%ase is generated by t%e vapori(ation of #ore volatile co#ponents called stripping and by condensation of relatively less volatile co#ponentscalled absorption adds to t%e li"uid p%ase. Selection of column type Plate or Pac!ed Pac-ed towers colu#ns are also used as
t%e contacting devices for gas absorption, li"uid/li"uid e)traction and distillation. $%e gaseous #i)ture is allowed to contact continuously wit% t%e li"uid counter/currently in a pac-ed colu#n. $%e li"uid flows downward over t%e pac-ing surface, and t%e gaseous #i)ture flows upward t%roug% t%e space in t%e pac-ing. $%e perfor#ance of t%e colu#n strongly depends on t%e arrange#ent of t%e pac-ings to provide good li"uid and gas contact t%roug%out t%e pac-ed bed. $%e solute gas is absorbed by t%e fres% solvent li"uid entering at t%e top of t%e tower w%ere t%e lean gas leaves syste#. $%e li"uid enric%ed wit% absorbed solute gas, leaves t%e colu#n botto# t%roug% t%e e)it port. 0n a
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plate tower, t%e li"uid and gas are contacted in stage/wise #anner on t%e trays w%ile gas/ li"uid contact is continuous in a pac-ed colu#n. $%ere are always so#e uncertainly to #aintain good li"uid distribution in a pac-ed tower. or t%is reason, it is difficult to accurately esti#ate t%e pac-ed tower efficiency. $%e course content is li#ited to design of plate colu#n only and so#e typical criterions for t%e selection of colu#n type are discussed below.
Plate towers e)%ibit larger pressure drops and li"uid %oldup at %ig%er gas flow rate. %ile, pac-ed towers are not appropriate for very low li"uid flow rates. Pac-ed colu#n is t%e preferred c%oice t%an a plate colu#n to %andle to)ic and fla##able li"uids due to lower li"uid %oldup to -eep t%e unit as s#all as possible for t%e sa-e of safety. Plate colu#ns are nor#ally suitable for fouling li"uids or laden wit% solids. $%ey are easier to clean and could %andle substantial te#perature variation during operation. Pac-ed towers are #ore suitable for foa#ing an d corrosive services. 0t is easier to #a-e t%e provision for t%e installation of internal cooling coils or wit%drawal of side strea#s fro# a plate colu#n.
2. Plate contractors Plate contractors5 towers are vertical cylindrical colu#ns in w%ic% a vertical stac- of trays or plates are installed across t%e colu#n %eig%t as s%own in igure 2.1. $%e li"uid enters at t%e top of t%e colu#n and flows across t%e tray and t%en t%roug% a downco#er cross/ flow #ode to t%e ne)t tray below. $%e gas5vapor fro# t%e lower tray flows in t%e upward direction t%roug% t%e opening5%oles in t%e tray to for# a gas/li"uid dispersion. 0n t%is way, t%e #ass transfer between t%e p%ases gas5vapor/li"uid ta-es place across t%e tray and t%roug% t%e colu#n in a stage/wise #anner.
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"igure 2.1. Sc#ematic diagram of a plate contractor $%1& page 1'().
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2.1.
Definition of tray areas
$%e definition of tray areas and its no#enclature illustrated in igures 2.2 6.3 are followed t%roug%out t%e design procedure. *otal tower cross+section area $ ) $%e e#pty tower inside cross/sectional area
wit%out trays or downspouts. Net area $ ) $also called free area) $%e total tower crosssectional area #inus t%e
area at t%e top of t%e downco#er . $%e net area sy#boli(es t%e s#allest area available for vapor flow in t%e inter/tray spacing. ,u--ling area or actie area $
) $%e total tower cross/sectional area #inus su# of
t%e downco#er top area( and downco#er seal area ()and any ot%er nonperforated areas on t%e tray. $%e bubbling area represents t%e area available for vapor flow 7ust above t%e tray floor. /ole area $ ) $%e total area of t%e perforations on t%e tray. $%e %ole area is t%e
s#allest area available for vapor5gas passage.
"igure 2.2.Sc#ematic of a tray operating in t#e frot# regime $%2& page 10+2).
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"igure 2.3. *ypical cross+flow plate $siee) $%3& page '').
2.2. Plate types 8as and li"uid flow across t%e tray can eit%er be by cross/flow or counter/flow #anner igure 2.4. $%e cross/flow plates are #ost widely practiced and t%e t%ree #ain types of cross flow plates are9 bubble cap, valve and sieve trays wit% downco#er.
"igure 2.0. lassification of plate types -ased on flow mode+ side iew s#own $a) ross+flow plate4 $-) ounterflow plate.
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2.2.1. ,u--le cap plates n en%anced gas/li"uid contact can be ac%ieved %aving bubble caps on t%e tray at very low li"uid flow rates. bubble cap consists of a riser also called c%i#ney fi)ed to t%e tray t%roug% a %ole and a cap is #ounted over t%e riser igure 2.&. $%e gas flows up t%roug% t%e riser, directed downward by t%e cap t%roug% t%e annular space between riser and cap. inally, t%e gas is dispersed into t%e li"uid. nu#ber of slots in t%e lower part of t%e cap %elp in gas bubble dispersion. :n/slotted types of cap designs are also co##on in application. Bubble caps are especially suitable for %ig%er turndown ratio. $urndown ratio is t%e ratio of #a)i#u# operating vapor rate to t%e #ini#u# allowable vapor rate, below w%ic% weeping starts.
"igure 2.'. ,u--le caps $%1& page 155).
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2.2.2. 6ale plates Valve trays or floating cap plate are t%e #odified design of sieve trays w%ere relatively large plate perforations are covered by #ovable caps5valves igure 2.*. Valves cover #ay be round or rectangular. $%e very co##on %ole dia#eter is 4; ## but upto 1&; ## are also used. $%e valve lifts up as t%e vapor flow rate increases and t%e valve sits over t%e perforation at lower flow rate, t%us stops t%e li"uid fro# weeping. Valve trays provide good vapor/li"uid contact at low flow rates %ig% turndown ratio.
"igure 2.5. 6ale tray $%0& page 10+2').
2.2.3. Siee plate $%e sieve tray also -nown as perforated plate is a flat perforated #etal s%eet igure 2.<. $%e %ole dia#eter fro# 1.& to 2& ## are very co##only used. $%e sieve tray layout is a typical s"uare or e"uilateral triangular pitc% %oles. $%e gas5vapor flows upward t%roug% t%e perforation and disperses into t%e flowing li"uid over t%e plate. $%ere is no li"uid seal in case of trays wit%out downco#er and t%e li"uid weeps called weeping t%roug% t%e %oles at low flow rates, reducing t%e efficiency of plate. or t%is reason, sieve tray %as t%e lowest turndown ratio. Sieve tray construction is si#ple and relatively c%eap.
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"igure 2.. Siee tray $%0& page 10+2').
2.2.0. Selection of tray type $%e co#parative perfor#ances of t%ree co##on types of trays are su##ari(ed in $able 2.1. $%e capacity, efficiency, pressure drop and entrain#ent of sieve and valve trays are al#ost sa#e. Bubble cap trays %ave lower capacity and efficiency and but %ig%er pressure drop and entrain#ent co#pared to valve and sieve trays. $%e turndown ratio co#es in t%e order of9 bubble cap=valve=sieve. +owever, valve trays %ave t%e best turndown ratio in case of refinery applications. Sieve trays are t%e least e)pensive and suitable for al#ost all applications. Valve trays can be considered w%ere %ig%er turndown ratio is needed. Bubble cap trays s%ould be used at very low li"uid flow rate w%ic% is not ac%ievable using sieve trays. *a-le 2.1 omparison of t#ree types of cross+flow trays $%'& page 255).
*ray type apacity
Efficiency
Pressure drop +ig%
Entrainment
*urndown ratio
ost
?3 ti#es t%an sieve tray
E)cellent
1;;/2;; @ #ore t%an sieve tray 2;/&;@ #ore t%an sieve tray '%eapest of all types
,u--le cap
>ediu# %ig%
>ediu# %ig%
6ale
+ig% to very %ig%
+ig%
>ediu# to %ig%
>ediu#
4 to 1;.1
Siee
+ig%
+ig%
>ediu#
>ediu#
2.1
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2.3. Effect of apor flow conditions on tray design 2.3.1. "looding consideration E)cessive li"uid buildup inside t%e colu#n leads to colu#n flooding condition. $%e nature of flooding depends on t%e colu#n operating pressure and t%e li"uid to vapor flow ratio. 0t #ay be downco#er bac-up, spray entrain#ent or frot% entrain#ent type floodings.+ig%er tray pressure drop due to e)cessive vapor flow rates %olds up t%e li"uid in t%e downco#er, increases t%e li"uid level on t%e plate and leads to downco#er flooding situation. $%e colu#n flooding conditions sets t%e upper li#it of vapor velocity for steady operation. 8as velocity t%roug% t%e net area at flooding conditioncan be esti#ated using airAs correlation %0&4 page 10+259
#5sC
vapor density, -g5#3 li"uid density, -g5#3 li"uid surface tension, #5# dyn5c# capacity para#eter #5s can be calculated%0& page 10+2 in ter#s of plate spacing and flow para#eter 2.2
=li"uid flow rate, -g5s =vpor flow rate, -g5s $%e design gas velocities is generally F;/F&@ of for non/foa#ing li"uids and <&@ or less for foa#ing li"uids sub7ect to acceptable entrain#ent and plate pressure drop.
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2.3.2. Siee tray weeping eeping occurs at low vapor5gas flow rates. $%e upward vapor flow t%roug% t%e plate perforationsprevents t%e li"uid fro# lea-ing t%roug% t%e tray perforation. t low vapor flow rates, li"uid start to lea-5rain t%roug% t%e perforation called weeping. %en none of t%e li"uid reac%es t%e downco#er at e)tre#e weeping condition at very low vapor flow rate, it is called du#ping. $%e weeping tendency increases wit% increasing fractional %ole area and li"uid flow rates. $%e vapor velocity at t%e weep point w%ere li"uid lea-age t%roug% %oles starts is t%e #ini#u# value for stable operation. or a c%osen %ole area,t%e #ini#u# operating vapor flow velocity ,) at #ini#u# flow rate for stable operation s%ould be above weep point vapor velocity. $%e #ini#u# vapor velocity min at t%e weep point %3& page '5( 9 2.3 %ere, ! %ole dia#eter, ##, vapor density, -g5#3 #a)i#u# value of vapor density "# constant "$ of weep/point correlation depends on t%e dept% of clear li"uid weir crest G weir %eig%t on t%e plate %3& page '1 . eir crest !%& can be deter#ined using t%e rancisA weir correlation %3& page '1 9 ##C
'weir lengt%, # 'li"uid flow rate over t%e crest, -g5s li"uid density, -g5#3 ctual operating #ini#u# vapor velocity9 $o avoid weeping9 , min
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2.3.3. 7i8uid entrainment Entrain#ent is t%e p%eno#ena in w%ic% li"uid droplets are carried by vapor5gas to t%e tray above. $%erefore, t%e less volatile li"uid co#ponents fro# botto# tray are #i)ed wit% li"uid %aving relatively #ore volatile #aterials on t%e over%ead tray. 0t counteracts t%e desired #ass transfer operation and t%e plate efficiency decreases.
increases wit% vapor velocity.$%e fractional entrain#ent
Entrain#ent
can
predicted using airAs correlation in ter#s of t%e flow para#eter
and
actual flooding velocity %0& page 10+2 .
Effect of
on >urp%ree plate efficiency can be esti#ated using 'olburn e"uation %0&
page 10+2(9
<.*
* =>urp%ree vapor efficiency +'orrected >urp%ree vapor efficiency for li"uid entrain#ent
2.0. *ray #ydraulic parameters *otal plate pressure drop
ll gas pressure drops !- are e)pressed as %eads of t%e clear li"uid and !-is given by9 - =! .
!%& .!% .!/
2.< %ere, ! =dry plate pressure drop, ## !%& =%eig%t of li"uid over weir weir crest, ## !% =weir %eig%t, ## !/residual %ead, ## Dry plate pressure drop $ 0)1 Dry plate pressure drop occurs due to friction wit%in dry s%ort %oles.!can be calculated using following e)pression derivedfor flow t%roug% orifices %3& page '' .
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>a)i#u# vapor velocity9 2.H $%e orifice coefficient, 2 can be deter#ined in ter#s of
and
%3& page
'5. Residual gas pressure #ead $ 3)9
$%e residual pressure drop results #ainly fro# t%e surface tension as t%e gas releases fro# a perforation. $%e following si#ple e"uation can be used to esti#ate !/wit% reasonable accuracy %3& page '' .
H.1; Downcomer -ac!up $ 4) and downcomer residence time
$%e li"uid level and frot% in t%e downco#er s%ould be well below t%e top of t%e outlet weir on t%e tray above to avoid flooding %3& page '5 . =
!%& .!% .!- .!& 2.11
+ead loss in downco#er9 2.12
% Downco#er li"uid flow rate, -g5s =S#aller of clearance area under t%e downco#er apron ) and downco#er area ) $%e average density of aerated li"uid in t%e dow#nco#er can be assu#ed as
of t%e
clear li"uid density.$%erefore, %alf of t%e su# of t%e plate spacing and weir %eig%t s%ould be greater t%an t%edownco#er bac-up. 2.13 Downco#er residence ti#e -/- s%ould be sufficient for t%e disengage#ent of li"uid and
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vapor in t%e downco#er to #ini#i(e entrained vapor. $%e value of -/-=3 s is suggested. Downco#er residence ti#eis given by %3& page ' 9
2.14 !& =56789 6i:;i< 85>
;?,##
2.'. olumn si9ing appro:imation $%e colu#n si(ing is a trial anderror calculationprocedure,starting wit% a tentative tray layout. $%e calculation is t%en revised until anacceptable design is obtained sub7ect to satisfyingt%etray pressure drop, weeping, flooding and li"uid entrain#ent li#its. $%e colu#n si(ing is carried at t%e tray w%ere t%e anticipated colu#n loading is t%e %ig%est and lowest for eac% section. +owever, t%e vapor flow rates %ave t%e %ig%est i#pact on tower dia#eter. or an e)a#ple, t%e si(ing calculation is perfor#ed o n t%e top tray for t%e above feed section and on t%e botto# tray for below feed section, for a single feed distillation colu#n wit% one top and one botto# product. $%e tray spacing deter#ines t%e colu#n %eig%t. !ower tray spacing is desirable to #ini#i(e construction cost by c%ec-ing against t%e colu#n perfor#ance criteria. $%e suggested tray spacing - wit% colu#n dia#eter is appended below %1& page 152. $%e detailed colu#n si(ing calculations are discussed in t%e solved e)a#ple. *ower diameter4 m 1 or less 1/3 3/4
*ray spacing4 mm &;; 1&; ## is #ini#u# *;; <&;
4/F
H;;
2.5. Proisional plate design 2.5.1. olumn diameter $%e colu#n dia#eter is deter#ined fro# t%e flooding correlation for a c%osen plate spacing. $%e superficial vapor5gas velocity at flooding t%roug% t%e net area relates to li"uid and vapor densities according to airAs correlation refer to section2.3.1 . is an e#pirical constant, depends on tray spacing and can be esti#ated against t%e flow para#eter @A) based on #ass flow rate of li"uid and vapor %3& page '54 %0& page 10+2.
$ypically, t%e design velocity t%roug% t%e net area is about F; to F&@ of for non/ foa#ing li"uids and <&@ or less for foa#ing li"uid depending on allowable plate
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pressure drop and entrain#ent. 0t is a co##on practice to %ave unifor# tower dia#eter in all sections of t%e colu#n even t%oug% t%e vapor5gas and li"uid loadings are e)pected to be different to #ini#i(e t%e cost of construction. $%e unifor#ity in tower dia#eter#ay re"uire selecting different plate spacing in different sections of t%e tower.
2.5.2. /ole diameter4#ole pitc#and plate t#ic!ness $%e plate %ole dia#eters ! fro# 3 to 12 ## are co##only used. $%e bigger si(es are susceptible to weeping. $%e %oles #ay be drilled or punc%ed and t%e plate is fabricated fro# stainless steel and ot%er alloys t%an carbon steel. $%e centre to centre distance between two ad7acent %oles is called %ole pitc% BC). Perforations can be arranged in s"uare or e"uilateral triangular arrays wit% respect to t%e vapor5gas flow direction. $%e nor#al range of BC is fro# 2.& to & ti#es of ! %1& page 15 . or triangular pitc%9 2.1& Plate t%ic-ness --) typically varies fro# ;.2 to 1.2 ti#es of t%e %ole dia#eter and s%ould be verified by c%ec-ing t%e allowable plate pressure drop %3& page '5 .
2.5.3. ;eir #eig#tand weir lengt# $%e dept% of li"uid on t%e tray is #aintained by installing a vertical flat plate, called weir. +ig%er weir %eig%t !% increases t%e plate efficiency. But it increases plate pressure drop, entrain#ent rate and weeping tendency. eir %eig%ts fro# 4; to H; ## are co##on in applications for t%e colu#ns operating above t%e at#osp%eric pressure. or vacuu# operation, !%* to 12 ## are reco##ended. $%e weir lengt% % deter#ines t%e downco#er area. weir lengt% of *; to F;@ of tower dia#eter is nor#ally used wit% seg#ental downco#ers. $%e dependency of % on downco#er area is calculated against t%e percentage value of
%3& page '2.
2.5.0. alming 9ones $wo blan- areas called cal#ing (one, are provided between t%e inlet downco#er or inlet weir and t%e perforation area, and also between t%e outlet weir and perforation area. 0nlet
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cal#ing (one %elps in reducing e)cessive weeping in t%is area because of %ig% vertical velocity of t%e entering li"uid in t%e downward direction. Iutlet cal#ing (one allows disengage#ent of vapor before t%e li"uid enters t%e downco#er area. cal#ing (one between &; to 1;;## is suggested.
3. Stepwise design tray procedure 0terative tray design approac% %3& page '55 is listed below. $%e design is perfor#ed separately bot% above feed plate top section and below feed plate botto# section for single feed two product distillation colu#n. Step <19 Deter#ine t%e nu#ber of t%eoretical plate and vapor and li"uid flow/rates separately bot% in top and botto# sections. Step <29 Ibtain t%e p%ysical properties of t%e syste# Step <39 Select a trial plate spacing Step <09 Esti#ate t%e colu#n dia#eter based on flooding considerations Step <'9 Decide t%e li"uid flow arrange#ent reverse, single/pass, or #ultiple/pass. guideline is provided in igure 11.2F %3& page '5 . Step <59 >a-e a provisional tray layout including downco#er area, active area, perforated area, %ole area and si(e, weir %eig%t, weir lengt% Step <9 '%ec- t%e weeping rate, if not satisfactory go bac- to step J* and reselect tray layout Step <9 '%ec- t%e plate pressure drop, if too %ig% return to step J* Step <(9 '%ec- downco#er bac-/up, if too %ig% go bac- to step J* or J3 Step <1=9 Decide plate layout including cal#ing (ones and unperforated areas and c%ec- %ole pitc%, if unsatisfactory return to step J* Step <119 Kecalculate t%e percentage of flooding based upon selected tower dia#eter Step <129 '%ec- for entrain#ent, if too %ig% t%en return to step J4 Step <139 Ipti#i(e design9 repeat steps J3 to JH to find s#allest dia#eter and plate spacing acceptable to get t%e lowest cost for t%e specified application Step <109 inali(e design9 draw up t%e plate specification and s-etc% t%e layout
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References %1&. %2&. %3&. %0&. %'&.
Kobert E. $reybal, >ass $ransfer Iperations, >c8raw/+ill, 0nc., 3rd ed. 1HF1. PerryAs '%e#ical EngineersA +andboo-, >c8raw/+ill, 0nc., Ft% ed. 1HH<. K. L. Sinnott, 'oulson 6 Kic%ardsonAs '%e#ical Engineering9 '%e#ical Engineering Design vol. *, Butterwort%/+eine#ann, 3rd ed. 1HHH. PerryAs '%e#ical EngineersA +andboo-, >c8raw/+ill 'o#panies, c8raw/+ill, 0nc., 1st ed. 1HH2.
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