How to Design Overhead Condensing Systems

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T

Th

he

e

state

of

in

in pipelin

pipelines,

es,

a

an

nd

d tthe

he

physical

arrangement

of

equiprnent

around

around th

th e

e

distillation tower, establish

th

th e

e

design

parameters

fo

for

r Ineet

Ineeting

ing

hydraulic

a

an

nd

d

piping

conditions

o

of

f th

the

e

system.

Robert Kern,

Robert Kern, HoffmannHoffmann La La _Roche_{Roche Inc.*}_Inc.*

., .,

Th

Th ee required distance,required distance, H,H, betweebetween n fractionatfractionator or inletinlet an

an dd exchanger centerlineexchanger centerline can becan be calculatedcalculated fromfrom Eg.Eg. (1) (1) as:as: (3) (3) (1) (1) (2) (2) Th

Th ee vaporvapor columncolumn c ac an n b eb e neglectedneglected byby assumingassuming

P2

=

00 inin Eq. Eq. (1(1)) anandd (3(3). ). All All pressurepressuress are inare in psi;psi; densities,

densities, P,P, inin Ib/ft Ib/ft 33_;; _an_and_{d dimensions,}_{dimensions, H,}_{H, in}_in _ft._ft. As

AsEq. (3) shows, for aEq. (3) shows, for a minimumminimum of of elevation differelevation differ ence between

ence between thethe totoppof of the columnthe column anan dd ththee exchangerexchanger centerlin

centerline, e, thethe pipingpipinganan dd componentscomponents resistances mustresistances must also

also bebe minimal.minimal. must

must be equalbe equal toto oror greatergreater thanthan thethe sumsum of of (1)(1)

th

thee pipe-pipe-systesystem m resiresistanstance,ce, betweebetween n refrefereerencence drop,

drop, anan dd (3)

(3) required pressure difference acrossrequired pressure difference across ththeecontrol valve,control valve, in

in arar

of

of ththee terminatingterminating in in FF

III

I

vapor vapor liquid liquid liquid liquid

o

ThThee statestate of of an

an dd thermalthermal conditions in conditions in tillation

tillation colucolu circuits. circuits. The The or or Fluid Fluid ca ca p. p. 107.)107.) In

In this article, wethis article, we examineexamine ththee hydraulics forhydraulics for the following:

the following:

1.

1. Condensers with gravity-flowCondensers with gravity-flow returnreturn lines.lines.

2.

2. _Conden_Condensers_{sers with}_{with pumped-refl}_{pumped-reflux}_{ux li}_lines_nes..

3.

3. Two-stage condensation.Two-stage condensation. Within

Within thethese se grogroups, ups, hydraulichydraulic-design-design anan dd plpmgplpmg design conditions vary, depending

design conditions vary, depending onon ththee statestate of of fluidfluid in

in ththee lineslines anan dd the the physicphysicalal arrangementarrangement of of ththee instalinstal lation.

lation. Vacuum

Vacuum technology has itstechnology has its own own systems,equipmesystems,equipmentnt an

an dd terminology.terminology. PipingPiping designdesign of of vacuum-condensingvacuum-condensing systems

systems areare outside the Euler-Bernoulli-Darcy theoriesoutside the Euler-Bernoulli-Darcy theories an

an dd areare nono tt includedincluded inin this article.this article.

"For

"For biographybiography of of author,author, Eng., Eng., Aug.Aug. 1975. p. 1975. p. 113.113.

II

,,

129 129

CHEMICAL ENGINEERING SEPTEMBER CHEMICAL ENGINEERING SEPTEMBER15.15. 19751975

fluids

tip tip

points A

points A anan dd B; (a) exchanger pressureB; (a) exchanger pressure

Gravity-flow

Horizontal condensers-A

Horizontal condensers-Acondensercondenser rangements

rangementsisislocated abovelocated above ththeelevellevel point

point of of the condenser's outlet line, as shownthe condenser's outlet line, as shown an

andd FF1212.. For theFor the horizontal condenserhorizontal condenser inin FF lIb,lIb, enters

enters ththee exchangerexchanger atat the top,the top, anan dd subcooledsubcooled leaves

leaves atat thth ee bottom.bottom. TheThe looped-outlet pipe looped-outlet pipe ensurensureses aa permanent liquidpermanent liquid levellevel in thein the condenser.condenser. ThisThis

level

level isis controlled thrcontrolled throuough gh thethe refluxreflux branchbranch anan dd through

through the takeoff line to storage.the takeoff line to storage. Th

Thee static-head pressure difference,static-head pressure difference, betweenbetweenthth ee

vert

vertical ical overhead overhead linlinee anandd ththee condenser'scondenser's outlet lineoutlet line for the arrangements

for the arrangements inin FF

II

can becan be written as:written as: fluid in the pipelines,

fluid in the pipelines, anan dd ththeehydraulichydraulic condensing syste

condensing systemsms of of disdis mns, are

mns, are the the rereveversrsee of of thosethose inin reboilerreboiler inlet line to condensers can carry superheated inlet line to condensers can carry superheated saturated

saturatedvapor, or dispersed vapor-liquid mixtures.vapor, or dispersed vapor-liquid mixtures. is

is coolcooled ed in in the exchangerthe exchanger,, anan dd partialpartial oror fullfull condensation

condensation taktakes es plaplace.ce. TheThe condenscondenser's outlet er's outlet lineline n

n havehave stratified andstratified and dispersed two-phase flow, satudispersed two-phase flow, satu rated

rated liquid,liquid, oror subcoolsubcoolcd cd liquidliquid.. InIn addition, thaddition, thee ing fluid

ing fluid cacann bebe a mixture of two substances.a mixture of two substances. Thus,Thus, thisthis type

typeof of condensacondensa tion offtion offerers s a wide ranga wide rang ee of of classificationclassification from a thermodynamic standpoint

from a thermodynamic standpoint [1].[1]. In

In contrcontrast, ast, saturatsaturated ed liquliquid id normalnormally ly flflowows s inin thethe downcomer

downcomer of of reboilcrs.reboilcrs. ThThee liquidliquid isis vaporized whilevaporized while passing

passing throughthrough the the exchanexchanger.ger. TheThe reboireboiler'ler's s outletoutlet line carries a dispersed vapor-liquid

line carries a dispersed vapor-liquid mixturemixture havinghaving aa vapor

vapor content of content of 3030 toto

of

of this series for more details,this series for more details, Chern.Chern. Eng.,Eng., Aug.Aug. 4, 4, 1971975,5, tipe;tipe;

tipcv: tipcv:

reflll.x

"gravity-flow "gravity-flow tiPs, tiPs, flow" flow" 90%

90% of of total total flflowow. . (See(See PartPart 99 \vill

\vill

see

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1

)

Vertical reflux condense

vapors on or of

Generally, in

piping is low-about _a

psi/lOO ft. Inletan d outlet ment

resistancean dshould no tbe (Using three

unusual.)

In

the sheIl. This

A baffle (or baffles) in the plane through the

two inletan dtwo

an d reduce entrance an d In this case, th e inlet an d metrical.

Th e subcooled liquid for can be drained or

product stream F /la

cooler before storage.

Vapor 130 CHEMICAL ENGINEERING

I

!-i ,'2

condensers wit gravity-flow

th sheilside tuheside exchanger

e ; : : , ' Q

f " = ~ * ~ J " < : ~ i " . ~ ) < F " , " , ~ ~ = ~ ~ : ! . ~ . - c ; : :

ToC O O ~ ~ i

Point

Drain

condensing systems, the unitloss in the tenth or a hundredth of 1 resistances to process equip usually take a considerable portionof th e pipeline ignoredin the calculations. decimal places in the calculations is _not

horizontal condensers, condensation takes place in gives lower resistance than th e tubeside. exchangeris_{in the horizontal}

exchanger's centerline. If _necessary,

nozzles can halv.e the total flow, exit resistances considerably. outlet piping should be sym the arrangementin _F_/1_b

pumped directly to storage. Th e

is usually directed through a Horizontal condensers with gravity-flow reflux

outlet

fo

a.Saturatedliquid {sheiisid condensation}

Vapor

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Seal loops prevent a flow of vapor in gravity-flow condensing systems

where PI _{is usually}_{vapor density,}_and P2_{is vapor-liquid}

Fo r th e dimensions given in F /5, the static-head difference will be:

(9) (8)

leg (Zz dimension in F /3), th e piping design should be such as to prevent siphoning that can empty the seal.

If dimension ZI is smaller than Z2' an d th e pressure

just before th e seal loop an d _at _{th e terminating point}

after th e seal loop is identical (for example, with greatly reduced flow), liquid ca n be siphoned ou t of th e seal;

an d intermittently, th e condenser will no t operate well. This can be prevented if ZI is designed to be longer

than Zz (see F /3a).

Fo r th e arrangement in F /3b, th e final vertical leg has a larger diameter than th e gravity-flow reflux line. Again, this ca n prevent th e siphoning of liquid from th e seal loop.

Another arrangement (F /3c) ha s a closed vent line. This ca n be openedat reduced condensate flow to keep th e seal loop filled with liquid. With this type of vent ing, th e pressure difference across the vent valve should be zero. Therefore, it is essential to connect th e end points of the vent line to locations where pressures ar e expected to be about equal.

Pumped-reflux arrangements

Typical overhead lines for hydrocarbon distillation columns are shown in F /4. Fluid circulation in the piping is th e result of th e thermosiphon effect in grav ity-reflux condensation. Fo r th e systems shown in F /4,

there is (and most of the time must be ) a pressure difference between th e to p of th e tower an d reflux drum. Th e reflux drum ha s a pump, which returns th e liquid to th e to p of th e tower or sends it to storage.

In _these arrangements (besides the sum of the static heads), actual pressure differences,

Control valves in a low point of Sufficient static vaporization no t receive a Vertical with (F In gravity-flow -provided t o condenser's outlet ing a liquid an d F/2c. If _{th e} 131 jj.197j . - - - . _ . _.._ - - - , \/ent reversed

6.P also enter into th e calculations:

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svstems should be located at

the line an d product stream.

head the valve inlet will prevent

across A product cooler should

liquid-vapor mixture.

condensers-Arrangements for these condensers gravity-flow are shown in F /2. Conden sation ca n take in the shell (F /2a), or tubes /2b). A single-pass vertical condenser is more suitable for liquid subcooling than a horizontal one. Th e seal loop heightca n be adjusted within a greater range than

with horizontal condensers (F/2c). Th e required liquid level in th e exchanger shell is determined by the ex changer's designer.

Th e hydraulic balance for the shown

in F /2 is:

(1/144)(H I PI

+

₍₄₎

HI _H 2P2

+

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where PI _{is th e density of condensate in the reflux line,} P3_{is the vapor density in the overhead line,} _{an d} P2_is

th e average density in the vertical exchanger:

Seal loop _preve_nts flow reversal

condensing systems, a seal loop is

prevent a flow of vapor in the

line.This loop ca n be used for hold in the condenser, as shown in F /la

gravity-flow reflux line terminates in a vertical is th e sum of piping, exchanger, control-valve (if any), resistances:

=

+

(5)

Th e elevation difference, as expressed from Eq. (4), between th e condenser's outlet an d th e reflux inlet noz zle will be:

these r('tun; before the valve. outlets place arrangements i'2) _(1/144)H 3P3

+

6.P (l/PI)(f!;Ii'3 1446.P) re"'ersed level ...here 6.P 6.P 6.Pe' ·and 6.Pcv' 6.P 6.P t:..Pe 6.pcv

(4)

(5)

I

! I

L

PI

P1H 1 = o.

Expressing H 2 from Eq. (12) in feet, we get:

H 2 (13)

In layout design, usually th e reflux drum is elevated first in accordance with the required NPSH (net posi tive suction head) of th e reflux pump. Th e dimensions shown in Detail A of F /5 _{will establish the condenser}

elevation from grade.

Elevated condensers and details of reflux drum

Slug flow

is

_undesirable

Slug flow ca n develop in th e pocketed condenser outlet line shown in F /5, depending on vapor-liquid proportion an d fluid velocity. Slug flow should be

avoided because it ca n cause undesirable pressure surges.

An empirical relation can be used to estimate th e

slug-flow region. If th e velocity (calculate d with two phase density) in the pipeline is smaller than (5Pl/

P _v)1/2, slug flow is possible. Th e type of flow ca n also sure of th e overhead vapor line'can be neglected.

Con-sider: Froth flow

. Liquid and vapor velocities type of flow

133

CHEMICAL SEPTEMBER IS,1975

I

=:: (144/P2)(M D.p D.Pe) \J 3 determine ENfiINEERING

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