Gas-liquid Flows in Cylindrical Venturi Scrubbers

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The Chemrcal Engzneertng Journal, 49 (1992) 55-64

55

Gas liquid flows in cylindrical venturi scrubbers: boundary layer

separation in the ditiser section

B J Azzopardl

Departmeni of Chemacal Engzneerzng, Ur uverszty of Nottmgham, Unaverszty Park, Nottsngham NG7 2RD (UK)

(Recewed August 20, 1991, III

rewsed form November 22,

1991)

Abstract

The

mportance of growth and separation of the gas boundary layer II\ the chfkser section of venhm

scrubbers has been exanuned In particular, expenments m three small ventuns which tiered

only In

their Muser angle were used From these and other data It was estabhshed that a model whch mcorporated

calculation of boundary layer growth and separation gave good pre ctlons of pressure loss over a \ylde

range of comhtlons Calculations ~th the model show that under certam conchuons use of a smaller

Muser angle can appreciably reduce the pressure loss

1 Introduction

The venturi nozzle has a long and titmguahed

lustory m metenng smgle phase flow The name

denves from the 18th century Italian physlclst Glov-

anm Battlsta Venturi who studled the flow of fhnds

through conical reducmg sections and through ex-

pandmg tubes, for the purpose of reducmg tur-

bulence and losses caused by such velocky changes,

and devlsed the venturi meter m whch volumetnc

flow rates can be deduced by measurmg the pressure

drop across a conical reducmg sectlon This ge-

ometry was refined by Herschel

[

1

J

mto what is

still the standard ASME (Amencan Society of Me-

chanical Engmeers) design

Use of ventuns III the cleanmg of gases dates

rom early m the 20th century A patent for their

apphcatlon to the scrubbmg of dust or chenucals

from gases was lirst taken out m 1925, ~th the

irst mdustnal example bemg reported 20 years

ater Two mam types are used In the first, or

Pearce-Anthony, type water is sprayed m through

nozzles mounted m the throat, ensunng good throat

coverage but at the expense of havmg to mamtam

he spray system. The second type 1s known as the

etted approach; here water 1s mtroduced on the

Just before the start of the convergent section

It 1s then atormzed mto fine drops by the gas stream

itself as it passes through the ventun throat In

oth cases the velocity of the drops 1s uutlally low

compared mth that of the gas, which may reach

150 m s-

’ U-I

some designs This high relatlveveloclty

ensures particle collection at high efficiency down

to submlcron sues As the drops accelerate, the

local collectlon efficiency decreases down to the

pomt where the drops attam the gas velocity when

no further mertlal collection occurs With gas de-

celeratlon m the dlvergmg sectlon of the ventun,

the high mertla drops can have a higher velocity

than the gas, allowmg some secondary particle

collectlon to occur. The venturi ISmevltably followed

by an entramrnent separator such as a cyclone,

wluch then removes the dust-laden drops from the

gas stream

The acceleration of gas and dust IS achieved at

the expense of gas-side pressure drop and hence

pumpmg power. Classically, a venturi was used for

this scrubbmg duty smce it was held to @ve the

maxunum gas velocity for a given pressure loss and

hence the maxunum theoretical collection efficiency

It has been noted that the clearung efficiency of

tlus type of scrubber depends on the amount of

energy used and hence the pressure drop across

the umt This may range anywhere from 100 to

1500 mm w.g (0 98-14.7 kPa) The mdth of t&

range 1ssuch that, for converuence, ventun scrubbers

are frequently classtied as Hugh, medn_un and low

energy scrubbers, ~th pressure drops of 500 and

250 mm w g. (4.9 and 2.42 kPa) berg taken as

the arbitrary &-ion pomts High energy scrubbers

can achieve efficlencles m excess of 98 on 1 pm

particles

0300-9467/ 32/$5

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Setnrau and coworkers [?, 31 have showl~ that

for man) types of venturt scrubbet the collectton

effictency ts related prtmarlly to the pressure drop

across the unit Thts j,roposttlon bras been examlncd

by Allen and van Santen [4 J. who h,tve pomted out

the unportance of correct water dtstt tbutton for the

contactmg power concept to ha\e vdltdtty These

studres show that whtle the modellmg of the pat-tlcle

collectton effictency IS useful, the fortiiiilntioti of

reallsttc models for pressure drojl poses the mote

tmpot-tnrit challenge for clestgn pitwo5es

Most appltcattons of ventut~ scrubbers are at or

about atmosphertc pressure Howe\ er. recent e\l-

dence shows that they ate also betng considered

for higher pressure appltcattons such as combmed

cycle power plant and coal g‘astficatton plant [s]

Th1.5 paper considers Important phenomena whtch

occur tn venturt scrubbers and, after a brtef rexlen

of methods avallable for pt edtctmg performance,

parttcularly pressute loss, desct tbc\ exj,et Iments to

examtne one spectfic phenomenon Data from these

and other espetunents ate used to test j)redtct.ton

methods, the most successful of which 15 used to

pro\lde suggesttons to tmprove destgn Thus study

has been confined to cyltndrtcal \rentut IS

2. Important phenomena

The different methods used for the tntroductton

of the lrqutd can affect the performance of ~enturt

scrubbers by ,altermg tmportant parameters such as

the stzes of drops created Ho\\ \ er, there are certatn

featutes whtch are common to most ienturt scrub-

bers

The fit-st ttem IS the presence of a Itqulcl 6Jtn on

the ventut-t walls Thts occurs for all Ilqutcl feed

methods In the wetted approach case the ltqutd IS

obvtously mtttally all travelltng as a film Stgntfkant

atomtzatton occurs tn the throat regton, though not

aU of the Itqutd IS atomtzed Some IS always left

as a f&l on the walk In addttton, some of the

Itqutd atotntzed tn the throat region can re-depostt,

augmenttng the tin flow rate Azzoparclt and Govan

[Cl reported spectal vtsuahzatton expertmen& whtch

showed that the atomtzatton taktng place tn the

throat of the venturt was aery stmtkar to that seen

dunng annular gas-ltqutd flow tn berttcal tubes,

where part of the ltqutd travels a5 a film on the

channel walls and there 1.5constant aTotntzatton and

re-deposttton of the ltqutd

For Pearce-Anthony-type scrubbers, where the

ltqutd IS sprayed tnto the gas flow tn the venturt,

the ltqutd wtll mtttally all be tn the form of drops

However, some of the Ilqutd soon clepostts to form

a wall film There can be te-atotntzatton at the tluoat

tf the hqutd was tntrocluced upstream of the throat

The occurrence of the hqutd film has two unportant

consequences Ftrstly, smce the fihn has a much

loiver surface area j)eI irnlt \ olunie than the droj)s,

that j>‘trt of the llqutcl can be neglected Iti the ga+

clenntng jjrocess SecondIS, the ltqutd film has wak es

on

tts mterface These jlresent a rough surface to

the gas flow and 50 the frtctlonal pressure drop

wtll probably be htgher than what tttlght be expected

for the patttcttktt wall toughness

Azzopat dt ancl Grimm [ l suggested that there

mtght be a?tomtzatton (or enttatnment) at the start

of the thtoat o\el and above that which would occur

for the gas shear stress that the film was expet t-

encmg They postulated that the llqutcl floivtng on

the comergent sectton wall had a radially Inward

component of veloctb that contuiited uir~arcls mto

the gas at the throat u~et In contrast, Letth t’t al

7 leasonecl that the 11qu1clon the wall m the Letitutt

throat had an a\~al component of xcloctty that

conttnued donnnntds tnto the gd?r, thus ntomtzatlon

i\ns Ithel3. at the throat outlet

Another unportant aspect of the dynamtcs of the

flop. pat-t~cukul~ m the dlvrrgenc sectlon of the

ventitn, 1s related to gas core deceleratton In suigle-

phase apphcattons the boundary layer IS expected

to gtoo\v ut the dtffuser owmg to the efiecrs of the

adverse pressure gracltent The gas wtll not dece-

lerate as qutcldy as mtght be expected from a one-

dunenstonal ,attalS s~s Boundary lager effects hs\re

also been obse~ecl tn venturts wtth two-phase flo\\

A consequence of the higher than expectecl gas

veloctttes IS that dtops wtjl not decelerate a5 raptdly

as eypectecj. leadtng to an unclet-reco\‘ery of the

pressure drop

A furthet effect associated wtth the presence of

the boundary layer ut adverse pressure gradients

IS the occurrettce of separatton At separatton the

shear stt ess at the wall goes from posrttve to negattve

and so a “rek\mg” and hence change m the filnl

wtN be expected m the absence of shear A further

consequence of boundary layer separatton IS that

pressure recovery ceaSes

3. Prediction methods

A number of workers have publtshed emptrtcal

equattotts for pressure loss across venturts RIP-

perger and Dau [S] potnt out that they can all be

wrttten m the form



B J A zopa r dz / Gas lzquzd ~70~s zn zmz tu t z scr ubbe rs 57

U,, IS the gas velocity m the throat, pc 1s

nsity and 5 IS an empmcal factor, usually

function of the gas-hquld ratlo

e =h, +h2L

(2)

Here li, and k2 can be constants or fimctlons of

vanables studied Although these correlations

re sunple, they are only valid mthm the ranges

f the variables from which they were produced

Early models for pressure drop usually assumed

hat the gas deceleration m the dfluser exactly

equalled the gas acceleration m the convergmg

section Thus the total pressure loss IS caused by

acceleration and deceleration of drops and by wall

fnctlon Calvert [9J mtegrated the equations over

the throat of the venturi, assunung that drops achieve

the gas velocity by the end of the throat This

procedure produced an explicit equation, though

it contamed an emplrlcal variable Behle and Beek-

mans [ lo] and Yung et

l

[ 111 both allowed that

drops dtd not achieve the gas velocity by the end

of the throat However, Yung et l [ 111 ignored

the fnctlonal pressure drop, assurrung that it was

balanced by the pressure recovery of gas III the

dtiuser A further group of workers

[

12-16

I

have

mtegrated the equations over the entu-e venturi,

taking mto account convergence and divergence of

the channel as necessary Some of these workers

have considered gas cleanmg, others pressure drops,

while some have dealt wqth both Boll [ 12 J takes

mto account frlctlon, however, he allows for the

effect of the liquid solely by aaustmg the gas density

to account for the drops Other workers have pro-

duced vanatlons on Boll’s analysis In particular,

Placek and Peters [ 1 T] and Bayvel [ 1 S] both allow

for the occurrence of a dlstrlbutlon of drop sizes

Boll [ 121 shows good predictions of pressure drop

across the venturi Recently, Vlswanathan

et l

[ 181

have allowed for the fact that some of the hquld

travels as a film on the channel walls They mcor-

porated this effect through wall frlctlon and by

allowmg that not all the hquld flowed as drops

which had to be accelerated However, they had to

provide the fraction of llqud travellmg m the film

as an mput parameter

A more thorough analysis was presented by AZ-

zopardl and Govan [6] In ths they allowed for the

fact that liquid was entramed (or atonuzed) from

the liquid film and that drops re-deposlted on the

film along the entire length of the venturi They

used equations smular to those of Boll but calculated

the film flow rate from a mass balance on the hquld

6lrn, specifying the rates of deposition and entram-

ment from equations denved from annular flow m

tubes Tlus model gives good descrlptlons of the

film flow rates and gas cleanmg m ventuns but gave

poor predlctlons of the pressure loss across the

venturi,, particularly m the diffuser where the model

predicted too much pressure recovery The model

gives predictions which agree well wth experunental

data up to the venturi throat In contrast, the pre-

dictions of Boll [ 121 can give reasonable predlctlons

of the overall pressure change, though the values

throughout the venturi are not well predicted

Recently, Azzopardlet al. [ 191 extended the earlier

work of Azzopardl and Govan [6 ] by mcludmg growth

of the boundary layer UI the dfluser Atomlzatlon

of liquid from the wall film and re-deposltlon of

drops back on the lihn, together \\qth drop accel-

eration and deceleration, were calculated as m the

earlier model The thickness of the boundary layer

and hence the gas velocity m the centre of the

channel were computed usmg the momentum m-

tegral equation, the effect of the liquid phase trav-

ellmg as drops was mcluded through extra mo-

mentum terms m the core of the diffuser The llquld

travellmg as a film on the walls was considered to

present the gas flow \?ntha rough wall

This extended

model gave predlctlons which agreed well wth data

for cases with and lnthout hquld

4.

Experimental arrangement

A series of experunents were carned out to test

the effect of flow separation on the pressure changes

through venturls These tests were particularly se-

vere, mvolvmg a large ch,ange m cross-sectional

area and a range of hquld loadmgs extendmg to

values much higher than those usually employed

for scrubber purposes The experunentswere carned

out on the flow loop shown schematically m ng

1 Filtered air was drawn from a constant-pressure

receiver and metered usmg an on6ce plate It was

mtroduced mto a 0 98 m length of copper tube

(0 032 m mner diameter) whch acted as a calmmg

section A short length of

alurmn~um

honeycomb

was mserted at the entrance of this tube to act as

a flow straightener Water was pumped from a supply

tank, metered by calibrated rotameters, and mtro-

duced mto the test section mediately upstream

of the venturi through a porous wall section, z e

the venturi was operated as a wetted approach type.

Downstream of the ventun the two-phase flow passed

mto a large separator vessel The m was released

to the atmosphere and the water returned to the

supply tank

The three ventuns used m this senes of tests

were made up of mterchangeable sections machmed

out of acrylic resm blocks The contra&on (34”)



Displacement

Pump

P

4

Water Inlet

II

enturi Test Sectlon

Ftg 1 Schenlatlc dqranl of flow loop

and throat sectlon (0 01 m mner dmmeter) were

commo11

to all expenments, whde dtffuser sections

\nth 5”,

10” and 15” mcluded angle were used

These are xefened to as venturls A, B and C re-

spectn ely Pressure tappmgs were pro\lded along

the length of the \renturls

Full details of the locations

can be found UI the report by Azzopardi

et cd [2 ]

Tests uere carried out wM1 the venturi mounted

both horizontally and vertically (downflow)

The pressuxe profiles along the tentun were de-

termmed by connectmg each pressure tappmg III

tunl

to a cahbrated dlfferentlal pressure cell Each

tappmg HIS hnked to a separator pot by a naITo\

horizontal tube The dtierentlal pressure cell was

connectecl to the top of the separator pot \r1a a

Scamvahe which was drnen by a computer This

automatically s\\ltched from one tapping to another

after a certam tune delay, which wti long enough

to ensure that each subsequent pressure readmg

was not affected by the pre\lous one Any hqurd

entermg the sep‘arator pot could be dramed through

a valve at the bottom This arrangement was em-

ployed for each tappmg to ensure that the mea-

surement hnes to the pressure cell were always full

of gas This elunmated any uncertamty from the

measurements that could occur owmg to the pres-

ence of small hquld plugs m the lmes The reference

pressure was measured at the gas mlet,Just upstream

of the water mlet pomt The data-gathermg system

IS described m more detad by Dlckmson [21]

5. Evidence of boundary layer separation

Data from the expenments described m the pre-

VLOUSsection have been ex‘anuned for ekldence of

separation of the boundary layer and Its conse-

quences

The mformatlon available IS m the form

of pressure-axl‘al distance plots For convemence

the data are shown as mlet pressure

ITIUILIS

local

pressure Figure 2 1s <an example which tiustrates

G ,s ll@wratc

L 5 ntur1

kg/s

k

L

~10063

<A

*

x

DX

sxXX

D

Xx x

D

b a

,Ol?F,

0

+

,I OlB9

+

0

0 n 2 5 ?

D

X

Prr.F,urr

;

ts<*r

D

b

b

b

b

b

’ DDD~~Db

I+g 2 Effect of gas flow rate on pressure profiles for Lenhms

A and C (vertwal downflow), hqlud flow rate 0 032 kg SK’



B J Azzoparda / Gas-lzquzd f lows m ventun scrubbers

59

a number of unportant pomts The pressure dlf-

ference mcreases as the flulds accelerate mto the

hroat The maxunum value of this pressure dlf-

ference shows a dependence on the square of the

gas flow rate In the dsuser section there LS some

recovery of pressure, though this 1s much greater

for venturi A than for C In fact the data from C

show the almost constant-pressure profile charac-

erlstlc of boundary layer separation This 1s more

clearly seen m I;‘lg 3, where the data are plotted

m terms of cross-sectional area

For design purposes the most unportant aspect

of these pressure profiles ls the pressure loss across

the venturi,, smce this controls the pumpmg power

requu-ed to pass the duty gas through the scrubber

Data gathered from ventun C are shown m fig. 4,

plotted as number of gas throat velocity heads

agamst liquid loadmg Tnterestmgly, the data from

different gas flow rates do not he on a common

curve This ti be considered m more detail later

more unportant feature IS that the data show a

change m slope This might be related to separation

of the boundary layer, smce when separation occurs,

the pressure recovery 1s expected to fall off The

conditions correspondmg to the change m slope

have been plotted on a graph of liquid flow rate

agamst gas flow rate m Fig. 5 Also shown are the

equivalent mformatlon for venturi A together w h

the condltlons under which separation was first

observed as a local thlckemng of the hquld film

This local thlckenmg of the film has been observed

durmg the measurements described above In ad-

dition, slgru6cant changes UI the structure of the

film mterface were seen by Blrchenough et al [22]

m vlsuallzatlon experunents carned out on these

ventuns usmg lllummatlon by a hght sheet from a

pulsed laser The agreement between data from

observation and change m slope mdlcate support

for the suggestlon that the change m slope m fig

4 1s related to the onset of separation It must be

noted that what 1s observed here 1s fully separated

flow Before that occurs there could be what IS

known as transitory “stall”, which nught not be so

vlslble

The effect of dtiuser angle IS seen m Fsg 6,

where the ratio of the pressure recovery for venturls

A and C to that for venturi B are plotted agamst

liquid loadmg As nught be expected, the smaller

the diffuser angle, the greater 1s the pressure re-

covery However, there are cases at lugh loadmg

for which this general rule does not apply These

are probably condltlons under wiuch separation 1s

present Equivalent mformatlon has been published

by Overcamp and Bowen [23 ] However, smce their

data were taken from liquid loadmgs m the range

O-2 1 m-‘, they did not report de\qatlons for 5”

dflusers They did note that for a throat 0 01 m

long the pressure recovery was better for a 10”

dfluser than that v&h a 5” mcluded angle This

result was attnbuted to the drops not havmg been

accelerated totally by the end of the throat

Although the above dlscusslon has been confined

to cyhndncal ventuns, sutular effects have been

recorded from venturls urlth rectangular cross-sec-

tlons Overcamp and Bowen [23] observed that

“ the droplets travelled u-~he centre of the dfluser

The gas flow separated from the walls and there

was little pressure recovery” The boundary layer

flow and m particular this region of reverse flow

were also discussed by Behle and Beekmans [ 10 I

“I

2

ii

1

t

A

c?’

z

k Converging section A

w

++**,

Diffuser

’ ’ * c ,

*

*

--_- _

-

-

enturl A Venlurl C

c

_

t

-

Inlet area / ocal area

Q 3 Dmenslonless pressure plotted agamst raho of mlet to local cross-sectronal area, gas and hqwd flow rates 0 0252 kg

s-’



60

10

Gas flow rate

(kg/s)

05 ---

02

0 5

1 2

53

10 20

Liquid loading (I/m

)

Fig 4 Effect

of l~rju~d londulg on nunther of

\eloclh heads lost tar \entun C

Xl

45

Is

Vermm A

Venturi A Venturi C

eparat~oryObserved

Change of Slope Change of Slope

-1 _~ __-

~~ ~_~~~

j? 003

(II

5

2 0025

L

015

/

001

1

1

1

_I

0

01

0 012 0

014 0 016

0018 0 02

0022

0 024

0 026

Gas flow rate (kg/s)

Fig 5 Condltlons for lnceptlon of separation

They

described this saJlng “The au movmg through

the dfluser Llolently unpacted on this water held

w?thm the stall region and re-atomized a portlon

of It and returned It to the maul au- flow”

The occurrence of boundm layer separation IS

more strmgent m flows with hquld present than m

gas only flows In the latter case the onset of

separation depends on the cllffuser angle and the

ratlo of throat diameter to dfluser length Exam-

matron of pubhshed charts mdlcate that for smgle

low no appreciable separation or ‘stall” IS expected

for dfluser A Dfluser B would be borderlme, whde

would be expected to be m “large transitory

tall” Thus contrasts \\qth the experunental obser-

vatlons, which Imply fully developed separation m

both A and C dt sufficiently large flow rates

6.

Accuracy of prediction methods

Pressure loss data taken m the experurlents de-

scribed aboLe have been used to test the model of

Azzopardl et ul 1191 Figure 7 shams that the

predIctIons are reasonable over the tange of llqulcl

loadtngs usually encountered m venturi scrubbers,

though there IS a tendency for underpredlctlon at

higher loadmgs and at larger gru flow rates However,

It must be noted that this IS a very severe test



B J Axaparda / Gas-lzquzd Jlaws m clentun scrubbers

61

16-

L 04 -

a

0

Gas flow rate (kg/s)

“$@A001zf OO ” 0-p “0;” 00189

02.

I I I I 1

0 1 2 hqufd 4 5 6 7

loading (I/m )3

Rg 6 Effect of

&i&user angle

on pressure recovery

F

30-

25 -

z

i i

9 -

s 15 -

3

f z 10 -

5-

2

4

l_&d loadlni (I/IT?

)

10

Rg

7

Accuracy of predxtlon methods for ventuns A C

Other

predlctlve methods did not perform as well

For example, the correlation of Johnstone and Rob-

erts [241 overpredlcts substantially ms could be

due to the fact that it was derived from larger-scale

umts where most of the hqud was travellmg as

drops In these small-scale experunents It has been

observed that most of the hquld remams as a film

on the ventun walls [S 1. Unhke drops, films wdl

not be accelerated to the gas velocity and will

therefore not produce as much pressure loss Thus

nught explam why the data from dfierent gas flow

rates do not he on one curve m @ 4, z e. the

fraction of hquld entramed depends on the gas

shear The model of Azzopard~ et al [ 191 predicts

the mceptlon of separation under condltlons well

below those plotted m fig 5 However, m the

expenments it 1sprobably fully developed separation

bemg recorded m contrast to the transrtory “stall”

computed.

The results of a second senes of tests of the

accuracy of prediction methods can be seen m ng

8 Here the data were taken on a 2 5 m3 s-l (5300

scfm) pilot venturi operated m the wetted approach

mode [25] The convergence and dfluser angles

on the venturi were 25” and 9” respectively. Most

of the predictive methods considered underpre-

dlcted, some substantially In contrast, the model

of Azzopa.r& et al. [ 19 ], though overpredctmg,

showed the correct vanatlon m axial pressure profile

and trend wth hquld loadmg The model has also

been shown to be accurate XI its calculation of dust

removal, e g_ measured efficiency 99 3 , calculated

efficiency 98 5



67

6

Hesketh 26

Johnstone, RoberIz4

Boll l2

Rlpperger Dau 8

,

Azopardl er al

19

I

1 I

5 06

07

Liquid lo:d:ng (I/IT?;’

1 1 1

Rg 8 Accurac) of predvmon methods for pilot scale scrubber

A further test has been carried out agamst data

flom the experunents of k’ung et al [5 J, n ho studled

the effect of system pressure on the performance

of a Lentun scrubber opelatmg m the Pearce-

Anthony mode The convergence

mcl

dlffuset angles

OII theu- venturi weIe 25” and 14 -I” respectively

With such a large value of the d&user angle, sep-

aratlon IS expected Flgule 9 shows predlctecl pres-

sure profiles

for hv0

cases at 1 and 10 bar re-

spectlvely, note the change m scale m the ordmates

of the graphs Throat \eloc~ and hquld loadtng

mere held constant at 54 8 m

s-

’ and 2 I nm3

respectl\elJ The cumes labelled 1-D ale from the

model of Azzopardl and Govan [6], 1 lle BL refers

to the boundary layer model of Azzop~arcl~ ct 01

[19] Knowledge of the sue of ortices through

which the hquicl was Ir\lected IS requued to predict

the diameters of the drops produced Unfortunately,

Yung

et trl [5J

do not specify this dmlensIon

However, It can be seen that the predlctlons are

not kery sensltlve to this parameter Elgure 9 II-

lustrates the decrease m pressure recovery and

possible separation predlcted by the model and the

good agreement uqth the pressure loss measured

by Yung et (11 [5] figure 10 shows that this

agreement holds over the range of gas velocltles

studled There IS a small but systematic underpred-

lctlon \\hlch IS also seen at the other pressures

studled In contrast, the correlations of Johnstone

and Roberts [24 ] and Hesketh [ 26 1both overpredlct

at 10 bar but underpredlct at atmospheric pressure

Obviously, the model of Azzopardl ct al [ 191,

wth Its allowance for boundary layer growth and

capabthty of handlmg mclplent separation, can

a- -

( --

I

P

BL model

--___-

DO,= 1 mm

I

I&,,=

4mm

1 D model

Fg 0

Effect of utiet pressure on aual \anatlon of pressure

drop (a) 1 bar, (b) 10 bar

produce accurate predIctIons of pressure profiles

and pressure loss

Although the calculations described here have

been car-x-led out for cyhndncal ventuns, the

model

should be eqwally applxable to rectangular ge-

ometnes



B J Azopa r dz / Gas l r quzd f l ows m wn t un sc r ubbe r s

63

100

Johnstone/Robert?4 HeskethZ zzopardl el a BYung et ;I

(Exp) 5

/’

60 -

0 1

I

I I

30 40

50 60 70

90 90 100

Gas velocrty

n throat (m/s)

mg 10

Accuracy of pretictlon methods for Hugh pressure venturi

7 Implications for design

In the se&Ions above the model of Azzop‘ardl e t

l [ 191 was shown to have a sound physical basis

and to give reasonably accurate predIctIons over a

parameters Here Its use in the op-

muzatton of the design of venturi scrubbers IS

considered The parameter chosen for optunlzatlon

IS the angle of the diffuser section The smaller the

angle, the less hkely it IS that separation might

occur and hence the greater the pressure recovery

achieved The model was used to study two cases

he first was the high pressure case studies by Yung

et l

[

51 The second case considered was at

atmospheric pressure For this the geometry of the

pilot-scale scrubber of van Santen

[

25

1was

selected

hese two particular cases were selected so that

would be a lmk mth experunental data at one

angle In the calculations dtiuser angles between

1 25” and 20” were exammed Other dunenslons as

low rates and physlcal properties were kept

onstant

Figure 11 summarizes the results of the com-

It shows that for the h@h pressure case

(A) there could be a savmg because of the slgnticant

ecrease m pressure loss It must be noted that

here would be an mcrease m the capital cost smce

length of the venturi would mcrease 2 4-fold

(from 0 75 to 1.8 m) while savmg 40 on pressure

loss However, capital cost 1s usually of less un-

ortance m sunple equipment such as venturi scrub-

ers A further consideration 1s the mcrease m

reeboard used should the urut be mounted vertically.

For the atmosphenc pressure case (B) the un-

rovement m pressure loss was small. The shape

of the curve of pressure loss agamst dfiuser angle

-1

2

Rg

11 Effect of dfluser angle on

pressure

loss and

penetration

IS surular

to what has been seen m smgle-phase

flow The muurnum ases from the competmg effects

of gas deceleration, 2-e pressure drop decreasmg

urlth decreasmg diffuser angle, and wall fnctlon, i e.

pressure drop mcreasmg tnth decreasmg Muser

angle and hence mcreasmg channel length It 1s

also logical that the effect should be more pro-

nounced at 10 bar than at atmospheric pressure

In the higher pressure case the proportion of pres-

sure change wluch relates to gas accelera-



tlon-deceleration IS much greater than tn Ihe at-

mosphenc pressure case

It 1s mtetesttng to note that m both cases the

effictency of dust ternoval, plotted ‘as penetratton

(1 -effictency), shows \*ery little dependence on dtf-

fuser angle Thts IS hardly surpnsmg, stnce most

of the scrubbmg IS effected m the throat The smaJ.1

decrease \slth decreasmg angle could be attrtbutecl

to the greater residence ttme m the longer dlffuset

It must be noted that the unpto~~ements tn design

dlscussed here ha\e not been confirmed cxpert-

mentally Howevet, because of the sound physical

basts of the tnodel and tts abtllty to ptedlct the

wade range of data agarnst which tt w(as tested,

these suggested m~pro~~ements mertt further con-

sideration

8. Conclusions

F’rotn the above work the followmg conclustons

can be stated

(1) Boundary layer separatton occuts m the dtf-

fuser sectton of ventut I scrubbers Condltlons undet

whtch this occurs ‘ate probably mote I estrlctmg than

for smgle-phase flow A consequence of boundaQ

layer separation is loss of pressure reco\eq

(2) The model of Azzopardl et cl

[

191, which

mcludes a descnptlon of boundary layet growth and

separation, gnes good predicttons over a wide range

of condlttons

(3) Calculattons show that m certam cases use

of a smallet ‘angle of dtvergence tn the dtffuser

section can result m substantial reductton tn pressure

loss

Acknowledgement

The author would hke to thank Dr A H Govan

(currently wkh BP Exploration) for his help tn some

of the c<alculattons

References

I

7

,3

3

5

G

5

8

9

10

11

12

13

11

15

16

17

18

19

‘0

‘1

27

d3

21

15

If

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Gas-liquid Flows in Cylindrical Venturi Scrubbers

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