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& 2010 Elsevier Ltd. All rights reserved.

contac., absgenatia numr, 1992availaer de

transport characteristics which greatly affect bubble column

rgersark,

ts oftentset al.,

recent work, Ruzicka et al. (2008) investigated the effect of salt

Contents lists available at ScienceDirect

w.e

Chemical Engine

Chemical Engineering Science 65 (2010) 58725880destabilizes the homogeneous regime and decreases the gasURL: http://philon.cheng.auth.gr/mouza (A.A. Mouza).additives (acting as surfactants) on bubble coalescence and on thetransition to the heterogeneous regime. They reported that a lowsurfactant concentration stabilizes the homogeneous regimeand increases the gas holdup, whereas a high concentration

0009-2509/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ces.2010.08.011

n Corresponding author. Tel.: +30 2310 994161.

E-mail address: mouza@cheng.auth.gr (A.A. Mouza).gas ow rates, respectively. There are considerable differencesbetween the two regimes concerning the hydrodynamics and the

1999). Most of the published studies are focused on the effect ofn-alcohols (e.g., Veera et al., 2001; Zahradnik et al., 1999). In aand semi-empirical correlations that are based on experimentaldata and are not of general validity.

It is known (Deckwer, 1992) that, depending on the gas owrate, two main ow regimes are observed in bubble columns, i.e.,the homogeneous bubbly ow and the heterogeneous (churn-turbulent ow) regime encountered for relatively low and high

studies are focused on single nozzle or multiple nozzle spa(e.g., Akita and Yoshida, 1973; Hikita et al., 1980; Hughm1967; Kumar et al., 1976).

It is widely accepted that the addition of small amounadditives hinders coalescence between the bubbles and exthe homogeneous regime to higher gas ow rates (Camarasabubble size which in turn are affected by the operating conditions,the physico-chemical properties of the two phases, the gassparger type and the column geometry (Camarasa et al., 1999;Mouza et al., 2005). The design and scale up of bubble columnreactors have been primarily carried out by means of empirical

require more numerous and smaller bubbles (Hebrard et al.,1996; Kazakis et al., 2007). Although there is an extensiveliterature on bubble column performance, limited information isavailable on the behavior of ne pore plate spargers (e.g., Kazakiset al., 2008, 2007; Mouza et al., 2005), since most of the listedChemical processes

Gas holdup

Porous sparger

1. Introduction

Bubble columns are gasliquidwide range of applications (e.gtreatment, fermentation, blood oxyliquefaction) because they providein design and in operation (Deckwetioned processes the interfacial areathe most important design paramettors encountered in aorption, waste wateron, bio-reactions, coalber of advantages both). In all the aforemen-ble for mass transfer isned by gas holdup and

operation. For instance, the homogeneous regime offers a largercontact area (numerous and small bubbles) and provides a lowshear rate environment and thus is most desirable for practicalapplications involving sensitive materials (e.g., bioreactors, bloodoxygenators) (Dhanasekharan et al., 2005; Jones et al., 2002).

Various types of gas spargers are used for the injection of thegas phase, the most common of which are perforated plate,membrane and ne pore plate. Among the above sparger types,porous plate is considered advantageous for applications thatMultiphase owEffect of organic surfactant additives onpseudo-homogeneous regime in bubblewith ne pore sparger

A.D. Anastasiou, N.A. Kazakis, A.A. Mouza n, S.V. Pa

Department of Chemical Engineering, Aristotle University of Thessaloniki, Univ. Box 45

a r t i c l e i n f o

Article history:

Received 6 May 2010

Received in revised form

22 July 2010

Accepted 4 August 2010Available online 11 August 2010

Keywords:

Bubble columns

a b s t r a c t

New experimental data co

were acquired. The effect o

been studied. Three differe

aqueous solutions of eac

concentration and type

dimensionless numbers (i.

the column and the sparge

surfactants, non-ionic surf

journal homepage: wwas holdup in theolumns equipped

s

R 54124 Thessaloniki, Greece

rning the gas holdup in bubble columns equipped with porous sparger

rfactant additives on gas holdup in the pseudo-homogeneous regime has

ommercial surfactants (Triton X-100s, SDSs, CTABs) were used and four

one were employed, in order to study the effect of the surfactant

, non-ionic, anionic, cationic). A general correlation, which includes

roude, Archimedes and Bond) as well as the geometric characteristics of

an predict the gas holdup in various systems (i.e., pure substances, ionic

nts) with reasonable accuracy.

lsevier.com/locate/ces

ering Science

holdup. This behavior is ascribed to phenomena occurring onbubble surface, although the complete mechanism has not beenclaried yet. To the authors best knowledge no work is availablein the literature concerning the effect of the addition of organicsurfactants on the operation of bubble columns equipped withne pore spargers.

Organic surfactant molecules have both a polar hydrophilichead and a non-polar hydrophobic tail and their addition in waterreduces the surface tension of the system. Depending on whetheran ion is present on the hydrophilic part of the molecule,surfactants are divided into ionic and non-ionic. The ionicsurfactants, in turn, are classied into anionic, cationic andzwitterionic, depending on the charge of the hydrophilic part.Above a critical concentration, which depends on the specicsurfactant and is known as critical micelle concentration (CMC), thesurfactant molecules tend to assemble into aggregates (micelles)while the surface tension remains almost constant (Myers, 1999).

increases almost linearly with increasing gas velocity, but no

all surfactants, from the four concentrations studied, two wereunder the CMC, one was at the CMC and one was much higherthan the CMC.

All the experiments are conducted at ambient pressure andtemperature conditions. The liquid phase viscosity is measuredwith a KPGs Cannon-Fenske (Schott) viscometer, while thesurface tension is measured using the pendant drop method(KSVs CAM 200). Each experimental run starts by rst lling thecolumn with the appropriate liquid phase to at least 40 cm abovethe sparger, to ensure that the gas holdup measurements areindependent of the liquid height (Ruzicka et al., 2001). All theexperiments are performed with no liquid throughput, while thegas phase is injected and distributed into the liquid phase bypassing through the porous disk sparger.

The average gas holdup is estimated by measuring the bedexpansion, i.e., the liquid level difference prior to gas inow andafter gas is injected and steady state is established, and it isrecorded by means of a digital video camera (Redlake MotionScopePCIs1000S). The camera is xed on a stand very close to the areaof observation in such a way that the test section is located

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 58725880 5873uniformity exists in the radial bubble distribution near thesparger region (Kazakis et al., 2007). The scope of the presentwork is to study the effect of organic surfactant addition on thebubble column performance and to formulate a general validitycorrelation for the prediction of gas holdup at the pseudo-homogeneous regime based on both new and previously acquireddata (Kazakis et al., 2007; Mouza et al., 2005). The newexperiments will be conducted using various aqueous solutionsof three commercial surfactants and for concentrations bothunder and over the CMC.

2. Experimental set-up and procedure

The experimental set-up (Fig. 1) consists of a verticalcylindrical Plexiglass column of 9.0 cm i.d. and 1.5 m height.The column is equipped with appropriate rotameters for gasphase ow measurement and control. For the injection anduniform distribution of the gas phase, a ne pore gas sparger, i.e., around 316 L SS porous disk, 4.48 cm in diameter (Mott Corp.) withIn previous works conducted in this Laboratory (Kazakis et al.,2007; Mouza et al., 2005) the inuence of the liquid propertiesand sparger characteristics (i.e., diameter, pore size) on gasholdup in bubble columns equipped with ne pore sparger wasstudied. New correlations for the prediction of gas holdup at thepseudo-homogeneous regime based on dimensionless groups wereproposed. The term pseudo-homogeneous is used to describe theow regime encountered at relatively low gas ow rates, in whichdiscrete bubbles are generated from the sparger and gas holdupFig. 1. Experimental set-up.nominal pore size of 40 mm, is installed at the centre of thecolumn bottom.

Water and twelve aqueous surfactant solutions, whose surfacetension values are presented in Table 1, were employed as liquidphase, whereas the gas phase was atmospheric air for all runs. Allsurfactant solutions exhibit Newtonian behavior (Lioumbas et al.,2006) and have practically the same viscosity (mL1.0 mPa s) anddensity (rL1000 kg/m3). It must be noted that, due to theinteraction between the surfactant molecules and the varioussalts present in tap water, the surfactants remained practicallyinsoluble in tap water, while white sediment was produced,especially in the case of the ionic surfactants (SDS and CTAB). Forthis reason distilled water was used for the preparation of all thesurfactant solutions.

Fig. 2a illustrates the dependence of surface tension on thesurfactant concentration for Triton X-100. The CMC value can belocated from this curve. In the present study the CMC for eachsurfactant is determined using the method proposed by Zhengand Obbard (2002). According to this method, when the chart isdrawn in semi-log scale, the point at which the slope changesindicates the CMC. The exact CMC value is given by theintersection of the two lines (Fig. 2b). Table 2 displays the CMCvalues for all surfactants used and the corresponding surfacetension values, which agree with those given by the supplyingcompany (SIGMATM). As it can be seen from Tables 1 and 2, for

Table 1Liquid phase properties at 25 1C.

Liquid phase % w/w Index Surface tension,

s (mN/m)

Water w 72.0

Triton X-100 solution 0.002 T1 59.00.006 T2 45.00.020 T3 32.00.100 T4 32.0

CTAB solution 0.010 C1 58.50.021 C2 46.00.037 C3 37.50.100 C4 37.5

SDS solution 0.060 S1 59.00.150 S2 44.00.280 S3 36.00.500 S4 36.0between the camera and an appropriate lighting system placed

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 58725880587450

60

70

80N

/m

micelle formation

Triton X-100behind a diffuser to evenly distribute the light. Details of themethod can be found elsewhere (Kazakis et al., 2007).The transition between the ow regimes was determined visuallyusing a high speed video camera and it was also estimated by thedrift ux analysis (Wallis, 1969).

The liquid level after the gas injection is estimated byaveraging the level of ve consecutive pictures covering a timeextension of about 8 s, for each gas ow rate. The difference inliquid level, measured by superposition of the two picturesusing appropriate software (SigmaScan Pros), gives a measure of

et al., 1999), mainly due to the induced coalescence inhibitionbetween the bubbles. When a surfactant is present, bubble surface

20

30

40

0.00

, m

surfactant concentration, % w/w

CMC

20

30

40

50

60

70

0.001

, m

N/m

surfactant concentration, % w/w

CMC

Triton X-100

0.02 0.04 0.06 0.08 0.10 0.12

0.01 0.1 1

Fig. 2. (a) Surface tension vs surfactant concentration for Triton X-100 and(b) CMC value determination.

Table 2Critical micelle concentration (CMC) and corresponding surface tension for the

surfactants employed.

Surfactant Type CMC (% w/w) Surface tension

at CMC (mN/m)

Triton X-100 Non-ionic 0.018 32.0

CTAB Cationic 0.036 37.5

SDS Anionic 0.260 36.0mobility is restricted by the development of a gradient in itsconcentration, and thus a gradient in surface tension. In this case,bubble surfaces can be considered immobile and the rate of lmthinning is then very small especially at low values of the thickness.The surfactant molecules are adsorbed at the bubble interface and,thus, the bubble coalescence is hindered (Marrucci, 1969).

Fig. 4 illustrates the effect of the surfactant concentration onthe column behavior for the anionic surfactant SDS for the samegas ow rates as in Fig. 3. The cationic surfactant CTAB exhibits asimilar behavior. One can readily observe that, regardless of thegas ow rate and surfactant concentration, the presence of anionic surfactant results in the production of more numerous andsmaller bubbles compared to water (Fig. 3) and leads to theformation of a dense bubble cloud even at low gas ow rates.The formation of these small bubbles can be partially attributed tothe low surface tension of the surfactant solutions. Consequently,and contrary to the case of water, the heterogeneous regime is notreached regardless of the surfactant concentration and even forthe higher gas ow rates tested.

In Fig. 4 one can also observe regardless of the surfactantthe average gas holdup. The calibration of the measuring systemis accomplished by measuring a scale placed at the focusing plane.The uncertainty of the measurements is estimated to be lessthan 20%.

It would be interesting to also acquire data on bubble sizedistribution downstream the sparger. However, the measuringtechnique (i.e., high speed photography) used in previous studiesconducted in this Lab (i.e., Kazakis et al., 2007; Mouza et al., 2005)is not suitable for the estimation of bubble size distribution in thepresent study, because, due to the high bubble concentration, thebubbles overlap on the focusing plane.

The experiments are conducted for gas supercial velocities(UGS) up to 0.035 m/s. The gas supercial velocity is given by

UGS Q

A1

where Q is the gas ow rate and A the column cross section. Theupper limit of the gas supercial velocity was imposed by the fact thatabove this ow rate the uncertainty of the holdup is highly increaseddue to the intense foaming on the free surface of the liquid phase.

3. Results and discussion

Fig. 3, which illustrates the typical behavior of common liquid(i.e., water) and the bubble characteristics in a column equippedwith porous sparger, is used as the reference case. The three mainow regimes are presented. At low gas ow rates (pseudo-homogeneous regime) discrete bubbles are produced from theporous sparger and ascend with relatively low velocity andnegligible interactions between them. As the gas ow rate isincreased, more pores are activated and, thus, more numerousand relatively small bubbles are produced. At the transitionregime bubbles commence to coalesce, mainly in the vicinity ofthe porous sparger, leading to the formation of larger bubbles,which coexist with the smaller ones produced by the poroussparger. Finally, at even higher gas ow rates, the heterogeneousregime is reached where bubbles coalesce on the porous spargersurface. Large bubbles of irregular size are formed that ascendwith relatively high velocity at the centre of the column,producing intense turbulence.

It is known that the addition of small amounts of surfactantextents the homogeneous regime to higher gas ow rates (Camarasaconcentration, as the gas ow rate is increased more numerous

IO

rati

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 58725880 5875(Table 1). The gas ow rates employed are the same as in Fig. 4.It is obvious that the type of the surfactant greatly affects thebubble column behavior. In the case of the non-ionic surfactant(i.e., Triton X-100), less numerous and larger bubbles appearcompared to the ionic surfactant solutions (i.e., CTAB and SDS) forall gas ow rates, which are the result of the coalescence betweensmaller bubbles produced by the porous sparger. On the othercolsolhanalmdifsurtheMonusolsurbupro

Figgas(Tr(FigSDrsinctioisgenumn are presented for three types of surfactant and foutions that have almost the same surface tension valuedembubbles are produced that ll the whole column cross section andascend as a swarm. Similarly, for the same gas ow rate, as thesurfactant concentration increases, more numerous and smallerbubbles appear, due to the pronounced bubble coalescence inhibition.

In conclusion, the inhibition of coalescence when a surfactantis present can be attributed to:

the low surface tension of these solutions and the effect of the surfactant molecules on the mobility of thebubble surface.

The effect of surfactant type on bubble column operation isonstrated in Fig. 5, where the characteristics of the bubble

r

PSEUDO-HOMOGENEOUS

TRANSIT

Fig. 3. Snapshots of bubble column oped, in the case of the ionic surfactant solutions, coalescence isost fully inhibited even at relatively high gas ow rates. Thisference in behavior may be attributed to the charge of the ionicfactants, due to which repulsive forces are developed betweenbubbles, similar to the case of salt solutions (Tse et al., 1998).reover, in the anionic SDS solutions bubbles are moremerous and smaller compared to the cationic surfactantutions (CTAB), while it was also observed that the effect offactant concentration on the size and the number of thebbles, as well as on the coalescence possibility, is morenounced in the case of the non-ionic surfactant (Triton X-100).The aforementioned trends (Figs. 4 and 5) are better illustrated ins. 6 and 7. In Fig. 6 the gas holdup is plotted versus the supercialvelocity for various concentrations of the non-ionic surfactant

iton X-100) (Fig. 6a), as well as the anionic surfactant (SDS). 6b). The behavior of the cationic surfactant (CTAB) is similar toS. The holdup values for water are also included for comparison. At remark is that for the non-ionic Triton solutions gas holdupreases linearly with gas velocity only for the higher concentra-ns employed, whereas for the ionic surfactant solutions this trendobvious for all concentrations, i.e., no transition to the hetero-eous regime occurs. Another observation is that the effect of thesurfactant concentration is more signicant for the non-ionic (TritonX-100) surfactant (Fig. 6a) than for the ionic surfactants (Fig. 6b).Also in the case of the ionic surfactants (Fig. 6b) the gas holdupvalues are always much higher than those of water. It is also worthnoticing that the holdup values provided by the solution T1 (Fig. 6a)are similar to those of water despite its lower surface tension,signifying that the behavior of these solutions cannot be attributedto their low surface tension alone. In this solution, the amount ofTriton X-100 is not large enough to fully prevent bubble coalescence.Thus, large bubbles are also formed (Fig. 5a), leading to holdupvalues comparable to those of water. As the surfactant concentrationincreases the gas holdup values also increase. This trend cannot beattributed to the surface tension effect alone, since gas holdup valueskeep increasing even above the CMC, despite the fact that the surfacetension retains a constant value. It seems that the number ofsurfactant molecules also plays a crucial role to this trend.

Fig. 7 depicts the effect of the surfactant type on the gasholdup values for three solutions (T1, C1 and S1) that have similarsurface tension values (59 mN/m). It is obvious that the holdupvalues measured for the non-ionic Triton X-100 solution are muchlower than the corresponding values of the ionic surfactantsolutions, which are almost the same. As already mentioned, thisdifference may be attributed to the charge of the surfactant

N HETEROGENEOUS

on at the three ow regimes for water.10 cmmolecules in these solutions. In ionic solutions, due to thepresence of the electric charge, coalescence is inhibited to agreater extent compared to the Triton X-100 and, thus, smallerand more numerous bubbles appear as the gas ow rate isincreased. On the other hand, coalescence between the producedbubbles is more pronounced in the T1 solution and, consequently,larger bubbles with relatively high velocity appear at lower gasow rates, leading to lower gas holdup values. In addition, S1solution exhibits slightly higher gas holdup values compared tothe C1, probably due to the smaller and more numerous bubblesproduced in the S1 solution, as shown in Fig. 5.

3.1. Prediction of gas holdup at the homogeneous regime

Two correlations proposed by Akita and Yoshida (1973) andKazakis et al. (2007) for the prediction of gas holdup for systemswith ionic surfactants are tested (Fig. 8). It is obvious that none ofthe correlations can predict the gas holdup value with reasonableaccuracy. It is also worth noticing that the correlation by Kazakiset al. (2007) gives reasonable predictions for the lower gassupercial velocities (i.e., UGSo0.10 m/s).

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 587258805876gasexp(TaThthe

The correlation proposed by Kazakis et al. (2007) refers to theholdup prediction in the homogenous regime, is based oneriments conducted in this lab and by other investigatorsble 3) and concerns columns equipped with porous sparger.is correlation includes the most important parameters affectinggas holdup, i.e.,

the supercial velocity of the gas phase,the physical properties of the liquid phase (i.e., surface tension,viscosity, density),the column cross section andthe porous sparger characteristics (i.e., diameter and pore size)

Increasing gas floFig. 4. Effect of gas ow rate and surfactant concentration on band is based on dimensionless numbers, namely

the Froude number : Fr U2GS

dCg, 2

the Archimedes number : Ar d3Cr2L gm2L

3

and

the Eotvos number : Eo d2CrLgs 4

where dC is the column diameter and rL, mL and s are the liquiddensity, viscosity and surface tension, respectively.

w rate

Incr

easi

ng su

rfacta

nt co

ncen

tratio

n

ubble column operation for SDS: (a) S1, (b) S2 and (c) S3.

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 58725880 5877The aforementioned correlation has the general form

eG C1 FrC2ArC3EoC4dSdC

C5 dpdS

C6" #C75

where the ratio of sparger to column diameter (dS/dC) and theratio of mean pore diameter to sparger diameter (dp/dS) are alsoincluded to account for the different geometrical congurations of

Increasing gas

Fig. 5. Effect of surfactant type on bubble column operation: (a) T1, (b) C1 athe gas entrance and the porous sparger characteristics, respec-tively. It is worth mentioning that Akita and Yoshida (1973), whostudied the effects of the liquid physical properties on the gasholdup, proposed a gas holdup correlation that incorporatesthe same dimensionless numbers, i.e., Bond (or equally Eotvos),Froude and Galileo (or equally Archimedes), but they do notincorporate the geometrical characteristics of the device (i.e.,column, sparger and pore diameter).

flow rate nd (c) S1; all solutions have similar surface tension value (59 mN/m).

Table 3Column and porous sparger characteristics in studies of other investigators.

Studies Column

Diameter, dC (cm)

Ellenberger and Krishna (1994) 10.0 and 38.0

Camarasa et al. (1999) 10.0

Kaji et al. (2001) 20.0

Krishna et al. (1997) 10.0 and 38.0

Krishna et al. (1999) 5.0

Olmos et al. (2001) 10.0

Vial et al. (2000) 10.0

Zahradnik et al. (1997) 14.0

0

5

10

15

20

0

G,

%

0

5

10

15

20

0.000UGS, m/s

0.005 0.01 0.015 0.02 0.025 0.03 0.035

0.005 0.010 0.015 0.020 0.025 0.030 0.035

G,

%

UGS, m/s

w

S1S2S3 (at CMC)S4

w

T1

T2

T3 (at CMC)T4

Fig. 6. Effect of surfactant concentration on gas holdup for: (a) Triton X-100 and(b) SDS.

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 587258805878Porous sparger

Diameter, dS (cm) Pore size, dp (lm)

10.0 and 38.0 50 and 150200

10.0 1016

2.0 50

10.0 and 38.0 50

5.0 2040

10.0 1016

10.0 1016

14.0 100160 and 160250

20

T1In order to formulate a more accurate correlation for thepresent system the constants C1C7 of the general equation havebeen properly adjusted to give

eG 0:14 Fr1:0Ar0:15Eo1:85dSdC

0:2 dpdS

0:3" #0:376

where eG (%) is the gas holdup. The new modied correlation is infairly good agreement (720%) with the available data for ionicsurfactants (Fig. 9).

The experimental data of the non-ionic Triton X-100 solutionsare not included, due to their different behavior compared to thesolutions of the ionic surfactants (SDS and CTAB), as alreadydiscussed. In these solutions the effect of gas ow rate on gasholdup is smaller (Fig. 7) and consequently the exponent of Froudenumber must be reduced (compared to Eq. (6)). In the case of theTriton X-100 solutions by properly adjusting the constants C1, C2and C7 of Eq. (5) a suitable correlation is formulated

eG 0:0034 Fr0:6Ar0:15Eo1:85dSdC

0:2 dpdS

0:3" #0:527

0

5

10

15

0.000

C1

S1 G

, %

UGS, m/s

0.005 0.010 0.015 0.020 0.025 0.030 0.035

Fig. 7. Effect of surfactant type on gas holdup; all solutions have similar surfacetension value (59 mN/m).

regKa

solshosur

It is also known (Deckwer, 1992) that the interfacial area keeps

4. Concluding remarks

0

5

10

15

20

25

30

100 101 102 103 104 105 106

present work (SDS & CTAB)correlation (Eq. 6)

G,

%

Fr1.0Ar0.15Eo1.85(dS/dC)0.2(dp/dS)-0.3

Fig. 9. Gas holdup correlation for the homogeneous regime compared with datafrom ionic surfactants experiments.

0

5

10

15

20

25

30

100 101 102 103 104 105 106 107 108

T1, T2, T3, T4correlation (Eq. 7)

G,

%

Fr0.6Ar0.15Eo1.85(dS/dC)0.2(dp/dS)-0.3

Fig. 10. Gas holdup correlation for the non-ionic surfactant solutions.

0

SDS 0.06%Kazakis et al., (2007)Akita & Yoshida (1973)

0

0.1

0.2

0.3

0.4U

GS,

m/s

eG/(1-eG)40.01 0.02 0.03 0.04

Fig. 8. Comparison of experimental data of SDS with gas holdup predictions fromtwo correlations.

A.D. Anastasiou et al. / Chemical Engineering Science 65 (2010) 58725880 5879The effect of the addition of organic surfactants on thebehavior and the gas holdup in a column equipped with poroussparger has been studied. Both the effect of surfactant type andconcentration has been investigated. The most important conclu-sions are:

The addition of an organic surfactant in a bubble column:J inhibits bubble coalescence,J increases gas holdup andJ extends the homogeneous regime to higher gas ow rates.The gas holdup depends to a great extend on the surfactant type.

Even at concentrations above CMC the gas holdup continues toincrease with the surfactant concentration, while this trend ismore pronounced for the non-ionic surfactant.

The behavior of the surfactant depends largely on the structureof the molecules.

It has been proved that a reasonable prediction of gas holdup canbe made using a correlation with the general form of Eq. (5).Adjusting the constants C1C7 depending on the examined system(i.e., pure liquid, ionic-surfactant system and non-ionic surfactantsystem) a quite accurate prediction (720%) can be made. Also itis important to mention that in the case of Triton moreexperimental data would help for safer conclusions about thevalidity of the correlation. It is would be also interesting to checkthe proposed correlation against data acquired from largediameter columns that are used in conventional industrialapplications. However, and to the authors best knowledge suchdata are not available in the literature.

In conclusion, the addition of an organic surfactant is anincreasing with the gas ow rate until the transition point to theheterogeneous regime is reached, where it gets its maximumvalue. Taking into account that when surfactants are present notransition is observed and that more numerous smaller bubblesare produced, it is obvious that the addition of a small amount of asurfactant enhances the specic interfacial area.of aapputions is always smaller than the predicted one. As alreadywn, the gas holdup values are also always greater for thefactant solutions and consequently it is obvious that the valuewill be greater.roations it is known that the mean Sauter diameter for theseobs

not been tested for surfactant solutions, from the visual

erv

hasbubbles produced by a porous sparger at the homogeneousime might be estimated by the correlation proposed byzakis et al. (2008). Although the aforementioned correlationtheEq. (7) is plotted in Fig. 10 along with the experimental dataacquired using the Triton X-100 solutions. In this case thecorrelation is in good agreement (720%) with the available data.

3.2. Interfacial area at the homogeneous regime

In bubble columns, the most important factor, that affects themass transfer between the two phases, is the available interfacialarea (Deckwer, 1992), a measure of which is the value of theparameter a:

a 6eGd32

8

where a is the specic interfacial area and d32 is the mean Sauterdiameter of the bubbles. The initial mean Sauter diameter (d32) ofpriate action if the aim is to increase the interfacial area and

to shift the upper limit of the homogeneous regime to higher gasow rates.

Nomenclature

a specic interfacial area, m1

A column surface, m2

Ar Archimedes number dened by Eq. (3), dimensionlessC1,y, C7 constants, dimensionless

References

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Effect of organic surfactant additives on gas holdup in the pseudo-homogeneous regime in bubble columns equipped with...IntroductionExperimental set-up and procedureResults and discussionPrediction of gas holdup at the homogeneous regimeInterfacial area at the homogeneous regime

Concluding remarksNomenclatureAcknowledgementReferences