Study of the working of a new multicell scrubber applied in the fight against air pollution

The Science o f the Total Environment, 23 (1982) 215--224 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

STUDY OF THE WORKING OF A NEW MULTICELL

SCRUBBER APPLIED

215

IN THE FIGHT AGAINST AIR

POLLUTION L. PERDREAU (*)

(*), S. DJERID

Laboratoire

54042 Nancy Cedex (**) Europoll,

(*), C. BELIN

des Sciences

(**), A. LAURENT

du GEnie Chimique,

(*) and J.C. CHARPENTIER

CNRS-ENSIC,

(*)

l, rue Grandville,

(France)

2 rue Amorteaux,

78730 Saint Arnoult en Yvelines

(France)

ABSTRACT The EUROPOLL multicell for the fight against This contactor

contactor

of which constitutes

that may be use

patterns

called cells

to transfer mass and/or heat between the gas and liquid

Indeed each cell comprises

a convergent

and a divergent

the intersection

the throat and thus leads to a good contact between the phases.

From the hydrodynamic of pressure

is a new gas-liquid

and dust pollution of air.

involves a regular package of elementary

in which it is possible phases.

scrubber

the chemical

point of view,

the scrubber

is characterized

by graphs

losses and liquid holdup and by the domain of the flowrates

to the working

in satisfactory

leading

conditions.

The capacity of the mass transfer of the contactor quid side and gas side mass transfer

coefficients

is presented

in terms of li-

and of gas-liquid

interfacial

area. Finally classical

the performances contactors

of the scrubber are compared with those of the other

employed

use of a diagram plotting

to fight the air pollution.

the mass transfer parameters

This is made with the

versus

the spent energy.

INTRODUCTION The main classical columns,

absorbers

spray columns,

ejector reactors.

and gas-liquid

bubble columns, mechanically

Generally

for the treatment

of operation

are well adapted

is to present

(ref.

reaction.

the new EUROPOLL multicell

its appli-

I). For example,

packed

free of dust par-

Moreover

turbulent solid par-

reactions. The aim of the present paper

scrubber,

then to precise

and its mass transfer performances

limits of its potential

plate

tanks, venturi and as concerns

for the washing of hot gases containing

ticles together with very rapid gas-liquid

mical characteristics

stirred

of the gaseous effluents

ticles during an absorption with a rapid chemical jet reactors

are packed columns,

each one has its own specificity

cation field and has its own flexibility columns are employed

reactors

its hydrodyna-

and finally to define the

use when compared with the other types of absorbers.

0048-9697/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

216 DESCRIPTION

OF THE EQUIPMENT AND EXPERIMENTAL

Description

of the multicell

The multicell

scrubber

- an inferior hopper - a modular

contactor

involves three main parts

for the gas entrance

:

and the recovering

of the washing liquid,

shell for gas washing the length and/or the height of which are variable

by placing side to side and/or by packing

identical

elementary

- a cap for the gas exit and the entrance and distribution A classical

ID

convergent

,

/

section, throat

elementary

the intersection (Fig,

(Fig.

of rectangular of which forms the

]b). The elementary

(80 rmn), the length WIDTH ~

(= 20 degrees)

(= 20 degrees),

ness of the throat

p

(I0 mm),

la. Multicell scrubber

Fig.

the wideness

(200 rr~).

/~

constituted by a vertical

lb. Elementary cell

study is

package of elemen-

tary cells the number of which is comprised between

Experimental

the thick-

(80 mm) and the height

The contactor of the present Fig.

cell inves-

study is defined by

the angles of the convergent and the divergent

5;

la),

cell involves a

and a divergent

tigated in the present

,/

cells

of the liquid.

I and 8.

apparatus

A schematic

description

of the experimental

The gas and the liquid phases with an initial trickling the gas phase.

apparatus

flow counter-currently

is presented

in figure 2.

inside the absorber either

flow of the liquid phase or with an initial bubble

The liquid is distributed

flow of

at the top of the package by a distributor

having the shape of a cross and containing many holes. The gas is injected at the bottom of the apparatus by a fritted distributor. The range of the liquid and gas flowrates L and G are respectively 4×]0 -5 m3.s-I

and 0 to 0.01 m 3.s-I. The scrubber

prised between The pressure trickling

apparatus

0.8×10 -5 to

at range L/G com-

0.8 and 4 m 3 of liquid per 1000 m 3 of gas to be washed. loss in the equipment

flow operations

(also sometimes

is thus operating

is measured with a U-tube water manometer

and with two piezometric

called at flooded flow conditions).

is measured by weighting

in the equipment

lysis of the gas solute concentration the mass transfer

study.

at bubble

flow operation

The amount of liquid held in the

the liquid that leaves the apparatus

shut of the flows of the phases at the entrance both phases before the entrance

manometers

at

after a rapid

and at the exit. Samples gathered

in

and after the exit allow for the ana-

and of the liquid reactant

concentration

during

217

®

®

®

® ®

I Fig. 2. Experimental apparatus. I. Liquid storage tank 6. 2. Centrifugal pump 7. 3. Liquid rotameter 8. 4. Multicell scrubber 9. 5. Gas-liquid demister I0.

Bottom of the scrubber Liquid rotameter Air compressor Air rotameter Gas solute rotameter

II. 12. 13. 14. 15.

Washing column Gas sampling (in) Gas sampling (out) Liquid sampling (in) Liquid sampling (out)

HYDRODYNAMIC S Operating

zones

I02G (Nm3.s-I)

When the equipment •

\

REGIME o B~BLINGRE~E

•

T

~

is initially working at

trickle flow (trickling and Djerid

regime),

Boldo

(ref. 3) have described

(ref. 2)

the different

flow regimes visually observed and have shown the existence between /

of a discontinuity

for the operation

the zones corresponding

to a trickle flow

or a surging flow of the liquid on the walls of

(17

the cells and the zone corresponding

m~ 05

0.25

operation with a good mixture of both phases.

SURGING FLOW

last zone is limited between

an TRICKLINGFLOW

curves and the "flooding"

BUBBLINGFLOW

plot.

AIR_WATERSYSTEM n.5 IOsL (m3.s-9

When the equipment

curves

in a L versus C

is initially working at bub-

ble flow (bubbling regime), not observed

Thi~

the "picking-up"

Perdreau

the discontinuous

non and led to the conclusion Fig. 3. Operating zones of the multicell scrubber.

to a correct

(ref. 4) did

picking-up

phenome-

that the operating

zone was only limited by the flooding

curve.

218

The operating

zones of the scrubber working

bubbling regime are presented corresponding

to a correct operation

ting zone at bubbling operating

regimes,

contactors Pressure

regime.

either at trickling regime or at

in figure 3. It has to be underlined at trickling

Besides,

regime

and bubbling

concerning

the

5-6).

of the pressure

flow regimes

and liquid flowrates an approximatively At trickling

drop per unit length AP/Z measured

pressure

It should be noted that the flooding occurs for drop as in the case of packed columns

for a given liquid flowrate

the pressure

slowly for the low gas flowrates

(before the picking-up

the picking-up

increases

point and finally

gas flowrate

On the contrary,

rate first very quickly,

regime,

(ref.

7).

drop first increases then present a jump at

quickly up to the flooding point. For a is high.

the pressure drop decreases with the gas flow-

then more slowly up to the flooding.

is all the more weak that the liquid flowrate

-2&P/Z(mbar.m ')

point),

is all the more high that the liquid flowrate

at bubbling

are presented

the location of the flooding zone for the same gas

at both regimes.

constant

regime,

for trickling

in function of the gas and liquid flowrates

in figure 4a. This plot confirms

ll

in the opera-

drop

The variations

constant

is included

the flooding curve, which is common for both

may be predicted by a more general correlation

of tubular type (ref.

that the zone

Also the pressure drop

is high.

~(m

AH/n (m w,ter/cell)

,100 AIR.WATER SYSTEM

020'

60•m 0

AIR_WATER SYSTEM 0.15'

•40

\%---.... )

REGIME

m 3. s -~

F~CKLb'~C;I~I~_ING

0

x

+

0.833

•

o

2.5

•

"

3.333

•

a

0.10"

n=5

"

105 L

II

n. 5

~ +

•20 (b) ~

o

•

0

025

Fig. 4a.-4b.

05

Q75

f lo'G

0

Q25

Q50

(175

Variations of the pressure drop (4a) and of the liquid holdup sus gas and liquid flowrates.

1 (4b) ver-

219 Liquid holdup The variations of the total liquid holdup reported to the total empty volume of the scrubber in function of the gas and liquid flowrates are reported in figure 4b. The general trend observed in this graph is identical to that concerning the pressure drop. At trickling regime, the evolution of the liquid holdup also presents a discontinuity at the level of the picking-up point. The order of magnitude of the liquid holdup is the same at that in a packed column. At bubbling regime, the liquid holdup is greatly superior as compared with the values obtained at trickling regime. Generally the liquid holdup decreases when the gas flowrate increases and the influence of the liquid flowrate is all the more marked that this liquid flowrate is high and is located near the flooding of the scrubber. MASS TRANSFER The chemical techniques for the experimental determination of the mass transfer coefficients kLS (liquid film) and kGS (gas film) and of the interfacial area S (gasliquid) for a gas-liquid reactor by absorption with chemical reaction have been reported in the literature (ref. |). A slow irreversible reaction is used for the measurement of kLS , a fast pseudo m-th order reaction for S and an instantaneous surface reaction for the determination of kGS. The kinetics of each reaction employed has been previously studied in a laboratory model of known interfacial area and contact time (cylindrical wetted wall column). These techniques have been applied in the present equipment. Liquid phase coefficient The liquid mass transfer coefficient kLS was determined by the absorption of dilute carbon dioxyde (2 %) in air into aqueous potassium carbonate and bicarbonate buffer solutions

(0.6 M K2CO 3 + 0.2 M HKC03) in the conditions of a slow reaction totally

controlled by the mass transfer resistance in the liquid phase. The variations of the kLS values in function of the gas flowrates for different given liquid flowrates are presented in figure 5. At trickling regime, kLS is simultaneously influenced both by L and G. At bubbling regime, kLS increases with the gas flowrate and is independent of the liquid flowrate if the accuracy of the measurements

(within + ]0 %) is taken into account. It has to be noted that the values

determined with the bubbling regime are greatly superior. Gas phase coefficient The gas mass transfer coefficient kGS was determined by absorption of dilute sulphur dioxyde

(2 %) in air into aqueous solution of sodium hydroxyde

(IN) in the con-

ditions of an instantaneous surface and irreversible reaction totally controlled by the resistance in the gas phase.

220

103koS(kmol.s-LI~-') I0' WLS(m3.SJ)

ll~i~l o I " I " I bo'u~.~-')lo~31 ~ 13.3331 o/

~o

~/yD

°lol

BUBBLING o lOSL(m3.d) 0833 25 3.333

TRICKUNG

•

•

•

J

I

/

n=3

f

/

I

,,"

-"

i_,,,, '..,.-" I ivilrt

0.5

T~Ki_l,~l

•

I v l •

I

~o~u,,.,-,ilof~ol~63s12~s2sl 102 G (NmS.s -1)

o.~s

o'.s

o.~,s

i

Fig. 5. Liquid-side mass transfer the multicell scrubber

o:~s

--

in

102G(Nms.s-9 075 1' -'

ds

Fig. 6. Gas-side mass transfer in the multicell scrubber

S (m2) 1.2'

The variation

IBUBBLI~I ° ] ~ ] D [

of the kGS values

for different

constant

are reported

liquid flowrates

figure 6. At trickling

10 ,

/I

,,

oi..---o-i'

+

!/

a

independent

of the liquid flowrate.

Interfacial

area

The interfacial

area S was determined by

the absorption with the rapid irreversible

"1

fast pseudo first order reaction of dilute

/'

carbon dioxyde

OA 10SL('%O06801t633 2~51

o~o

o~s

(2 %) in air into aqueous

solution of sodium hydroxyde

102G(Nm 3 . s -~) o~s

regime,

linear function of the gas flowrate and is

n.5

TR~Lm "/" o

in

kGS increa-

in the studied zone, kGS is practically

"i,,',,,:./ I

regime,

ses with both L and G. At bubbling

'

0.6 #

in func-

tion of the gas flowrates

The variations

i~

of the S values

tion of L and G are reported Fig. 7. InterfacJal scrubber

(0.2N). in func-

in figure 7.

area in the multicell It is observed

that S increases with both

L and G in the trickling and bubbling flows. However at bubbling

regime,

liquid flowrate when L is higher

the interfacial

area becomes

independent

of the

than 2.5×I0 -5 m 3os -!. The area obtained with this

regime is also bigger than with the trickling

regime.

221 COMPARISON OF THE PERFORMANCES To tempt an exhaustive pes of gas-liquid rature results, necessary

confrontation

absorbers

of the various

scrubber

SCRUBBER WITH OTHER ABSORBERS

between

the performances

is very often a challenge

for this comparison.

the EUROPOLL

OF THE MULTICELL

conditions

of the different

ty-

due to the scatter of the lite-

of the tests and/or of the lack of the data

We are going to try to present now the performances

in a diagram with the plot - mass transfer

spent energy - in both trickling

and flooding

parameter versus

regimes and then to confront

formances with those of other contactors when the literature

of

these per-

data are available.

kLS parameter

k.{~L ~4./3 L"~-~L~2)

Perdreau determined powers

10-;

®/

in the EUROPOLL

the values of the scrubber necessary

to obtain the values of the liquid side mass transfer kLS reported

in figure 5.

These are comprised between

I to 2 watts

for each cell in the trickling regime and between

1.5 and 4 watts

the bubbling

regime

for each cell in

(ref. 4). It was also

shown that for a constant

dissipated

power

in each cell, the kLS value at bubbling

N

gime is thrice as high as at trickling

reregi-

me together with a liquid holdup ratio of ten. This leads to suggest the potential of the EUROPOLL

10_10_,6

10°

101

gas-liquid

10"

chemical

2 ,,,-',~ 7[P(~I L p~9~)

absorptions

employed

for oxidation

can be seen that the EUROPOLL stirred vessel and the stirred

processes

such as those of effluents or in aero-

bic fermentations. Figure 8 presents form the volumetric

in a dimensionless mass transfer

coeffi-

cient kLa as a function of the specific power requirement of the EUROPOLL

tional reactors

followed by a'slow

in the treatment

with slow kinetics

use

in the cases of

reaction processes

encountered

Fig. 8. Dimensionless form of the volumetric liquid mass transfer coefficient for various contactors I. Stirred loop reactor 2. Stirred vessel (coalescing) 3. Stirred vessel (hollow stirrer) 4. Injector 5. Stirred vessel (non coalescing) 6. Multicell scrubber

scrubber

(ref. 8). Comparing

scrubber achieves loop reactor.

and gives the location

scrubber among the convenregions

I, 5 and 6, it

same mass transfer

rates than the

222 kGS parameter The comparison of the k~S values

NG

EUROPOLL

9 8

BUBBLING

scrubber

in the

shows that the operation

at the bubbling regime results

in gas phase

efficiencies

99.6 % and

(a.3)

7

comprised

between

99.97 % while the operation ling regime results

at the trick-

in gas phase efficien-

cies comprised between 83 and 93 %. The Number of Transfer phase is plotted against

DUALFLOWTRAY

Units

in the gas

the energy dissipa-

ted per unit volume of treated gas in figure 9. It is seen that the performances the absorber

of

strictly vary with the dissipa-

ted energy for the studied gas-liquid and whatever

~ I

I

I

0.1

I

I I i i I

05

I

I

I

I

1

the flow regime. For a compari-

son with other absorbers,

(kJ. m-3)

regroups

---

5

the same diagram

the data obtained with the same

gas-liquid

system in a plate column invol-

ving 4 dual flow trays

Fig. 9. Comparison between the attainable number of gas mass transfer units for dual flow tray, packed column and multicell scrubber,

and distance between in a packed column

(diameter

the trays

(height

: 0.5 m

: 0.4 m) and

: I m, packings

are I inch and 2 inches polypropylen rings),

(ref. 9). This confrontation

for the treatment

at trickling

system

shows the potential

Pall

use of the EUROPOLL scrubber

flow regime of the gaseous effluents when an absorp-

tion with a rapid and/or instantaneous

reaction

is required.

S parameter The comparison of the dissipated

of the interfacial

(ref. 4). It was observed

that at the trickling

a = 100-200 m2.m -3 for a dissipated at the bubbling

regime,

m2.m -3 for a dissipated Moreover Nagel et al. characteristic flowrate

area in the EUROPOLL equipment

the interfacial

1 and 1.5 kW.m -3 while

the interfacial

to define a criterion

for the confrontation

bers. Using such a plot,

figure

of the EUROPOLL

shows that the EUROPOLL

10 allows

the use of another

interesting

area created per unit volumetric

and the selection

for a comparison

scrubber and other contactors.

scrubber

100 and 200

2 and 3.6 kW.m -3.

10) have proposed

diagram plotting

the order of magnitude was

area was comprised between

power comprised between (ref.

regime,

power comprised between

in function of the energy supplied per unit volumetric

performances

in function

energy per unit volume of scrubber has been made by Perdreau

is only suitable

of gas-liquid

gas

in order

absor-

to be made between

the

This working diagram

for production

transfer areas, but a such diagram is only qualitatively ted chemical system.

gas flowrate,

of average mass

valid for the investiga-

223

$/G (m~. S. m-z')

CONCLUSION

EAT cotuMN

I I

The present study shows that the

B. RPE FLOW C. COUNTERCURRENTPACKEDCOLUMN

EUROPOLL scrubber may be employed at trickling regime for the treatment of gaseous effluents containing eventually particles or dust in suspension and when a gas-liquid absorption accompanied

103

by a rapid or an instantaneous chemical reaction occurs (abatement of S02, H2S, NH3, acidic aerosols...).

I02!~ /~

It seems also

that this equipment could be employed at bubbling regime for the treatment of gas effluents by liquids where a slow chemical reaction happens (absorption of Nitrogene oxides or oxygenation of loa-

(BUBBLI~) E CELLULARSCRUBBER(TRICKLNG)

I

1010! DO _

I

P/G(kJ'm-3)

ded liquids) in trying in future to compare its performances with those of bubble columns.

10

Fig. 10. Comparison of interfacial transfer areas and specific power dissipations for any gas-liquid reactors.

Summarizing due to its flexibility depending its operating regime encountered, this new type of multicell contactor should allow to propose solutions adapted to the problems of the fight against the air pollution.

NOTATIONS a e g G kG

kL L n

NG P S V Z B PL

hL AP

interfacial area per unit volume of reactor thickness of the throat gravitational acceleration volumetric gas flowrate gas mass transfer coefficient liquid mass transfer coefficient volumetric liquid flowrate number of elementary cells number of gas mass transfer units dissipated power total interfacial area total volume of reactor height of column total liquid holdup liquid density dynamic viscosity of liquid gas pressure drop

REFERENCES 1 .

J.C. Charpentier, Mass transfer rates in gas-liquid absorbers and reactors, in Adv. Chem. Engng., Ed. T.B. Drew and T. Vermeulen, Vol. 11, Academic Press,

1981,

1-133.

224

2. 3. 4. 5. 6. 7. 8. 9. 10.

P. Boldo, A. Laurent, C. Belin and J.C° Charpentier, La Houille Blanche, 6/7 (1979) 435. S. Djerid, DEA, Nancy (1980). L. Perdreau, Th~se INPL, ENSIC, Nancy (1981). T. Takahashi, Y. Akagi and K. Fujita, J. Chem. Eng. Japan, ! (1973), 97. T. Takahashi, Y. Akagi, K. Fujita and T. Kishimoto, J. Chem. Eng. Japan, 3 (1974) 223. L. Musil, C. Prost and P. Le Goff, Chem. Ind. Ggnie Chimique, 5 (1968) 674. G. Keitel and U. Onken, Ger. Chem. Eng. 4 (1981) 250. P. Krotzsch and M. Molzahn, Ger. Chem. Eng. 3 (1980) 257. O. Nagel, B. Hegner and H. K~rten, Chem. Ing. Techn. 50 (1978), 934.