Effect of Liquid Properties on the Performance of Bubble Column Reactors

Chemical Engineering Science 60 (2005) 1465–1475

www.elsevier.com/locate/ces

Effect of liquid properties on the performance of bubble column reactorswith fine pore spargers

A.A. Mouza, G.K. Dalakoglou, S.V. Paras∗

Department of Chemical Engineering, Aristotle University of Thessaloniki Univ.Box 455, GR 54124 Thessaloniki, Greece

Received 4 November 2003; received in revised form 20 September 2004; accepted 7 October 2004

Abstract

This work is a study of the effect of liquid properties on the performance of bubble column reactors with fine pore spargers. Variousliquids covering a range of surface tension and viscosity values are employed, while the gas phase is atmospheric air. A fast video techniqueis used for visual observations and, combined with image processing, is used for gas holdup and bubble size measurements. New data onaverage gas holdup values, bubble size distributions and Sauter diameters are presented and are consistent with existing physical modelson coalescence/breakage. A correlation based on dimensionless groups for the prediction of gas holdup in thehomogeneousregime isproposed and found to be in good agreement with available data.� 2004 Elsevier Ltd. All rights reserved.

Keywords:Bubble column; Porous sparger; Gas holdup; Bubble size; Coalescence; Liquid properties; Flow regimes

1. Introduction

Bubble columns are widely used in industrial gas–liquidoperations (e.g. gas/liquid reactions, agitation by gas injec-tion, fermentations, etc.) in chemical and biochemical pro-cess industries, due to their simple construction, low oper-ating cost and high-energy efficiency. In all these processesgas holdup and bubble size are important design parameters,since they define the gas–liquid interfacial area available formass transfer. In turn, bubble size distribution and gas holdupin gas–liquid dispersions depend largely on column geome-try, operating conditions, physico-chemical properties of thetwo phases and type of gas sparger (Camarasa et al., 1999).The design of bubble columns has primarily been carried outby means of empirical or semi-empirical correlations basedmainly on experimental data. Since the multiphase flow isin general complex in structure, the design and scale up ofsuch type of equipment is still a difficult task and subject toerrors (Deckwer and Schumpe, 1993).

∗ Corresponding author. Tel.: +302310996174; fax: +302310996209.E-mail address:[email protected](S.V. Paras).

0009-2509/$ - see front matter� 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2004.10.013

Despite the extensive and long lasting study of bubblecolumn performance many basic questions about the effectof important operational parameters remain unanswered.For example, although bubble column characteristics havebeen studied extensively in the past few decades, there is stillconsiderable uncertainty concerning the prevailing mecha-nisms of bubble formation, as well as the most appropriatecorrelations for practical applications. Break-up and coales-cence of fluid objects play a crucial role in a broad spectrumof multiphase flow processes, such as the evolution of thebubble size distribution in stirred tanks and bubble columns(Delhaye and McLaughlin, 2003). Shah et al. (1982)andParasu Veera and Joshi (1999)have reviewed and summa-rized the work done in this area. It is generally acceptedthat, depending on the gas flow rate, two main flow regimescan be readily observed in bubble columns, i.e., thehomoge-neousbubbly flow regime encountered at low gas velocitiesand characterized by a narrow bubble size distribution andradially uniform gas holdup; and theheterogeneous(churn-turbulent flow) regime observed at higher gas velocities andcharacterized by the appearance of large bubbles, formed bycoalescence of the small bubbles and bearing a higher risevelocity hence leading to relatively lower gas holdup values

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1466 A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465–1475

(Joshi et al., 2002; Camarasa et al., 1999; Zahradniket al., 1997). The two regimes differ from one another intheir hydrodynamic and transport characteristics. Depend-ing on the type of the gas distributor and the properties ofthe liquid phase, both regimes can be obtained in the sameequipment by varying the gas input flow rate.Various types of gas spargers, the most common of which

are perforated plate, membrane and fine porous plate, arein use. Among the above sparger types, the perforated platerequires a minimum gas velocity in order to produce a uni-form bubble distribution over the whole sparger area. Onthe other hand, bubble columns equipped with porous sparg-ers offer a greater gas/liquid contact area for efficient massand heat transfer, because the bubbles created by this typeof gas distributor are numerous and far smaller. Finally,the membrane, even though it is able to maintain a homo-geneous flow up to greater flow rates, generates a greaterpressure drop (Hebrard et al., 1996). As a result, the useof a fine porous plate as a gas sparger seems to be ad-vantageous over the other types of multiple injection pointdistributors.To the authors’ best knowledge, limited information is

available in the literature regarding the behavior offineporousplate spargers.Hebrard et al. (1996), who studied theinfluence of gas sparger characteristics on the hydrodynamicbehavior of bubble columns, concluded that bubble size andholdup values depend primarily on the physico-chemicalproperties of the liquid phase and the type of gas distrib-utor. On the other hand,Parthasarathy and Ahmed (1996),who conducted their experiments with non-coalescing liq-uids, stated that there is confusion regarding the averagesize value and size distribution of bubbles generated by finepore spargers.Zahradnik et al. (1997)investigated the ef-fect of various parameters (i.e., column geometry, distributortype, liquid properties) on the gas–liquid flow regime sta-bility and gas holdup in bubble column reactors equipped,among others, with porous spargers.Camarasa et al. (1999),who examined the influence of the liquid properties andof the gas sparging method on hydrodynamics and bub-ble characteristics in a bubble column, used water-alcoholsolutions to simulate the behavior of non-coalescing or-ganic liquids and compared their data with those concern-ing standard air–water systems.Kaji et al. (2001)studiedexperimentally the behavior of bubble formation using var-ious porous spargers with pore diameter 5–400�m and in-vestigated the effect of surface tension on the gas holdupdistribution.The purpose of this work is to study the effect of liquid

properties on bubble size distribution in a bubble columnequipped with two different fine porous spargers. Variousliquids covering a range of surface tension and viscosityvalues are employed, while atmospheric air is used as the gasphase for all experiments. Experimental data on average gasholdup, bubble size distribution and mean Sauter diameter,obtained from image analysis of fast video recordings, are

reported. A correlation is also proposed for the prediction ofaverage gas holdup.

2. Experimental set-up and procedures

The experimental set-up (Fig. 1) consists of a vertical rect-angular Plexiglas� column 1.5m height, having a squarecross-section (side length 10 cm). The column is equippedwith appropriate rotameters for gas phase flow measure-ment and control. The rectangular geometry was preferredover the cylindrical one, because it facilitates both the directflow visualization and the use of optical measuring meth-ods by minimizing optical distortion. For the injection anduniform distribution of the gas phase, agas sparger, i.e.,a round metal porous disk, 2.0 cm in diameter, is installedat the center of the bottom plate. In the present experi-ments, two 316L SS porous disks (made by Mott Corp.)with nominal pore size of 20 and 40�m were alternativelyused as gas spargers. However, in order to check the effectof the usage time on sparger performance various porousdisks were employed during the period of the experiments.Several liquids, whose physical properties are presented inTable 1, were employed as liquid phase, whereas the gasphase was atmospheric air for all runs. All the experimentswere conducted at ambient pressure and temperature condi-tions. Each experimental run started by first filling the col-umn with the appropriate liquid phase up to 80 cm above thesparger. All the experiments were performed with no liquidthroughput, while the gas phase was injected and distributedinto the liquid phase by passing through the porous disksparger.A high-speed digital video camera (Redlake MotionScope

PCI � 1000S) is used both for direct flow visualization andfor bubble size and gas holdup measurements. The camerais fixed on a stand very close to the area of observationin such a way that the test section is located between thecamera and an appropriate lighting system placed behind adiffuser to evenly distribute the light. Although the imag-ing system used was capable of recording up to 1000 fullframes per second, a speed of 500 fps is considered a suit-able recording rate for the present experiments. The shutterrate employed was of 1/10000. It must be pointed out thatthe optical system used offers a very narrow depth of field(few mm).The recorded imageswere also used to extractquantitative

information on bubble size distribution and gas holdup val-ues. Using proper lighting, the gas–liquid interface aroundthe bubble circumference can be clearly outlined on the pic-tures. The calibration of the measuring system, needed toensure the accurate measurement of the bubbles, is accom-plished by measuring a microscale placed at the focusingplane. Subsequent image processing (e.g. noise reduction,brightness improvement, contrast enhancement, shadow anddouble images removal) results to a sharp bubble–liquid in-terface. A detailed description of the technique can be also

A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465–1475 1467

Fig. 1. Experimental set-up.

Table 1Liquid phase properties at 20◦C

Index Liquid phase Viscosity, Density, Surface�L (mPas) �L (kg/m3) tension,

�L (mN/m)

w Water 1.0 998 72b1 n-butanol 0.6% w/w 0.9 994 60b2 n-butanol 1.5% w/w 0.9 991 48g1 Glycerin 33.3% w/w 3.5 1081 70g2 Glycerin 50.0% w/w 8.2 1126 68g3 Glycerin 66.7% w/w 22.5 1173 67

found in a recent paper byColella et al. (1999). Image anal-ysis, using appropriate software (SigmaScan Pro�), allowsbubble size prediction. The bubbles were approximated byellipses whose major and minor axes were computed by thesoftware. The equivalent diameter of a sphere with the samevolume as the ellipsoid was computed (Colella et al., 1999;Polli et al., 2002). Approximately 800 bubbles were mea-sured in each experimental run, a number considered tobe adequate for statistical calculations (Tse et al., 2003;Hebrard et al., 1996). The advantage of the method is thatit permits both in situ and non-intrusive measurements. Theuncertainty of the measurements has been estimated to beless than 10%.The average gas holdup is estimated by the bed expansion.

The liquid level is measured on two different pictures takenprior to gas inflow and after gas is injected and steady stateis established. The difference in liquid level, measured bysuperposition of the two pictures, gives a measure of theaverage gas holdup. The uncertainty of the measurements isestimated to be less than 15%.

3. Results

3.1. Visual observations

As already mentioned, depending on the gas flow rate, thetwo flow regimes observed in bubble columns are theho-mogeneousbubbly flow regime encountered at low gas ve-locities and theheterogeneous(churn-turbulent flow) regimeobserved at higher gas velocities. The photos inFig. 2 givea visual image of the flow patterns observed in a bubble col-umn according to the gas flow rate when the liquid phase iseither water or 50% glycerin solution. At the beginning thebubbles are gathered at the core of the flow, but after the first20 cm they are spread uniformly covering the whole columnarea. It is also worth noticing that after the first 40 cm of thecolumn height the gas phase distribution does not seem tochange significantly.For the lower gas velocities applied the homogeneous flow

regime is encountered, where relatively small gas bubblesare formed and almost uniformly distributed throughout thewhole column area (Figs. 2a and b). The bubbles have asymmetric ellipsoid shape and rise almost vertically withthe same speed and without coalescence drifting an amountof liquid to the top of the column. As it is also describedby Ruzicka et al. (2001), the amount of liquid carried up bythe bubbles hinders the uprising bubbles on its way down,resulting in an increase of gas holdup.By increasing the gas flow rate the bubbles begin to grow

in size and large bubbles appear to coexist with the smallerones. The uprising bubbles begin to exhibit also a reciproca-tive movement which retards their upward movement en-hancing coalescence. The above observations correspond toan intermediate transition regime. By further increasing the


Fig. 2. Flow patterns in homogeneous (QG = 0.8× 10−5m3/s) and heterogeneous (QG = 7.7× 10−5m3/s) regimes for water–air (a, c) and glycerin50.0%—air (b, d) systems (dp = 40�m).

gas flow rates (Figs. 2c and d) the heterogeneous flow regimeis encountered where big gas masses, presumably formedby coalescence, begin to rise resulting in a kind of churnflow pattern. In this regime velocities display pronouncedradial profiles resulting to strong circulations and enhancingbubble rise speed. This results to a further decrease of gasholdup value. A detailed and comprehensive description ofthe flow regimes in bubble columns is also given byRuzickaet al. (2001).The flow patterns observed for some of the liquids used

(i.e., water, 50% glycerin and 1.5% butanol) are presentedin Fig. 3. It is worth noticing that the bubble shape and con-centration obtained by the glycerin solutions is practicallythe same with that of water (Figs. 3a and b). The bubbles ob-served in the low surface tension non-coalescing butanol so-lutions (Fig. 3c) differ significantly from those encounteredin the coalescence promoting media (i.e., water and glycerinsolutions). The former are spherical, considerably smaller insize and hence, for a given gas flow rate, far more numerousthan those of water (Papatzika, 2002). These bubbles forma kind of plume that quickly covers the whole column areaproviding an interfacial area much greater than that obtainedwith water. A radial circulation is also observed, which be-comes more pronounced by increasing gas flow rate, result-ing in a decrease of holdup value (transition regime).

3.2. Gas holdup

In this section the measured gas holdup values are given.The flow regimes can be distinguished by plotting the aver-age gas holdup (�G) versus the gas flow rate (QG). Fig. 4

shows the dependence of gas holdup on corresponding gassuperficial velocity for the two spargers used. A typical flowregime map (Ruzicka et al., 2001) is also included for com-parison. The gas superficial velocity is defined as

UGS = QG

A, (1)

whereQG is the gas flow rate andA the column cross sectionarea. A first observation is that for the two spargers testedthe gas holdup is not significantly affected by the spargerpore size, a remark that is in agreement with previous ob-servations (e.g.Kaji et al., 2001).The first part of the curve corresponds to the homogeneous

regime, where the gas holdup increases with the gas veloc-ity. A transition regime follows where a slight decrease ingas holdup is observed. Finally, at the heterogeneous regimethe gas holdup continues to increase but with a lower slopethan the homogeneous regime. As it is also pointed out byRuzicka et al. (2003), if the bubbles could travel unaffectedat their terminal velocity, the gas holdup would increase lin-early with the gas flow rate. In the homogeneous regime,as the gas holdup increases the hindrance progressively re-duces the bubble velocity leading to a further increase of thegas holdup. The opposite holds true for the heterogeneousregime, where the bubble velocity increases in the centralcore of the column resulting in a decrease of the gas holdupvalue with gas flow rate.In Fig. 5 the data are plotted in terms of gas holdup ver-

sus gas phase superficial velocity for air–water, air–butanoland air–glycerin systems. As it is expected, gas holdup in-creases with gas flow rate. A slight increase in gas holdup


Fig. 3. Characteristic photos of the homogeneous regime for various liquids (a) water, (b) glycerin 50.0% and (c) butanol 1.5%(QG = 1.5× 10−5m3/s, dp = 40�m).

0.0 0.2 0.4 0.6 0.8 1.0

20

40

0.0

1.0

2.0

3.0

4.0

UGS, cm/s

ε G, %

dp, µm

Fig. 4. Effect of pore diameter on gas holdup for the water–air system.

values is also observed when the lower surface tension bu-tanol solutions are used as liquid phase, whereas air–waterand air–glycerin solutions have almost identical gas holdupvalues. It is evident that the effect of viscosity is negligible.The present experimental data are compared with corre-

lations included in a comprehensive review paper byShahet al. (1982)and in general they are found to deviate con-siderably from the proposed empirical models. For instance,the prediction by the correlations ofAkita and Yoshida(1973) andHikita et al. (1980)exhibit for air–water sys-tems a 20% deviation from the present data and those of

0

1

2

3

4

5

0.0 0.2 0.4 0.6 0.8 1.0

w

b1

b2

g1

g2

g3

UGS, cm/s

ε G, %

dp=40µm

Fig. 5. Effect of type of liquid on gas holdup (dp = 40�m).

Camarasa et al. (1999)for porous spargers, whereas forglycerin (Fig. 6) and butanol solutions the correspondingdeviations exceed 40%. This can be attributed to the fact thatthe aforementioned correlations are based on data obtainedwith single and multi-nozzle spargers (Shah et al., 1982).

3.3. Bubble size distributions

The number frequency of the bubble size was calculatedat five distances above the sparger surface (i.e., 3, 10, 20, 30


0.0

1.0

2.0

3.0

0.0

1.0

2.0

3.0

0 0.1 0.2 0.3 0.4

exp data

present work Eq.8

Akita & Yoshida (1973)

Hikita et al. (1980)

exp data

present work Eq.8

Akita & Yoshida (1973)

Hikita et al. (1980)

ε G, %

ε G, %

UGS, cm/s

0 0.1 0.2 0.3 0.4

UGS, cm/s

µL=3.5 mPa.s

µL=22.5 mPa.s

(a)

(b)

Fig. 6. Comparison of the present experimental data with values calculatedby proposed Eq. (8) and the models of Hikita et al. (1980) and Akitaand Yoshida (1973) for two glycerin solutions: (a) 33.3% and (b) 66.7%.Error bars correspond to±10 deviation.

and 40 cm) for thehomogeneousregime. InFig. 7the bubblesize distribution curve is presented for the air–water systemand for both sparger types used. The measurements wereperformed at a height of 40 cm above the sparger surfacewhere the flow was found to be developed. It seems that forthe spargers tested the sparger pore size has practically noeffect on the distribution curve and this holds true for all theliquids tested. The above observation agrees with the resultsof Parthasarathy andAhmed (1996), who reported that belowa sparger pore diameter of 50�m there is a negligible changein bubble size.Fig. 8shows bubble size distributions at two locations i.e.,

just above the sparger surface and 40 cm away from it, forwater and for the glycerin solutions. In all cases it is obvi-ous that the form of bubble size distribution curve does not

0

10

20

30

40

50

0 1 2 3 4 5 6

2040

num

ber

freq

uenc

y, %

d, mm

dp, µm

h=40 cm

Fig. 7. Effect of pore diameter on bubble size distribution (water–airsystem,h ≈ 40 cm, QG = 0.8× 10−5m3/s).

practically change with height, an observation that is alsoreported byColella et al. (1999)for air–water systems. Thewater data, both in the vicinity of(h ≈ 3 cm) and awayfrom (h ≈ 40 cm) the sparger area, are well fitted by a log-normal distribution curve (Fig. 8a). Similarly, the two glyc-erin solutions (i.e.,g1 andg2) data obtained close to thesparger follow a log-normal distribution curve (Figs. 8a andb). However, theg1 andg2 solutions data away from thesparger(h ≈ 40 cm) cannot be fitted by such type of curvebecause asecondpeak makes its appearance (presumablydue to coalescence), which becomes more distinct for thehigher viscosity glycerin solution (g3). In these cases thedata are best fitted by the summation of two normal distri-bution functions (Figs. 8a and b). It must be noted that thebubble size distributions of the butanol solutions were notcalculated due to the limitations of the measuring techniqueapplied. In this case the problems arise from bubble over-lapping on the focusing plane due to high bubble concentra-tion. It is believed that the limited available data on bubblesize distribution for low surface tension liquids is due to thedifficulty in performing such measurements. The visual ob-servations reveal a behavior for the butanol solutions simi-lar to that of the low viscosity glycerin solutions. Moreover,Camarasa et al. (1999)report that for non-coalescing alco-hol solutions a narrow distribution of small bubbles is ob-served and that the bubble size within the column is nearlythe same as that of bubbles formed at the porous distributor.For the first two glycerin solutions (Figs. 8a and b) the

unimodalbubble size distribution curve suggests that themajority of the bubbles have the same size. This impliesthat the bubbles, after detaching from the sparger surface,


0

10

20

30

40

50

0 1 2 3 4 5 6

340

340

num

ber

freq

uenc

y, %

d, mm

3.5

h, cm

1.0

µL, mPa.s

0

10

20

30

40

50

0 1 2 3 4 5 6

3403

40

num

ber

freq

uenc

y, %

d, mm

µL, mPa.sh, cm

22.5

8.2

(a)

(b)

Fig. 8. Bubble size distributions in the homogeneous regime(QG = 1.5× 10−5m3/s) for: (a) water, glycerin 33.3% and (b) glycerin50.0%, glycerin 66.7%.

retain their initial size and neither a coalescence nor a break-age process occurs during their course to the column exit.The bimodal distribution observed at the higher viscosityvalue (Fig. 8b) can be attributed to coalescence phenomenathat seem to commence onto the sparger surface. The morepronounced bimodal distribution observed ath ≈ 40 cmimplies that the bubblescontinueto coalesce as they risetowards the liquid surface. This is in agreement with theobservations ofZahradnik et al. (1997), who noticed an un-favorable effect of liquid viscosity that they ascribed to theexistence of drag forces promoting bubble coalescence in

Table 2Mean Sauter diameter(d32) and standard deviation(�) for the 40�mpore sparger ath ≈ 40 cm

Liquid phase QG = 0.8× 10−5m3/s QG = 1.5× 10−5m3/s

d32 (mm) � (mm) d32 (mm) � (mm)

Water 3.1 0.5 3.8 0.9n-butanol 0.6% w/w 1.5 a 1.7 a

n-butanol 1.5% w/w 1.1 a 1.3 a

Glycerin 33.3% w/w 2.2 0.6 2.8 0.8Glycerin 50.0% w/w 2.4 0.8 3.5 1.0Glycerin 66.7% w/w 3.0 1.0 4.8 1.5

aNot available.

the distributor region. Finally, the broad distribution curveof the water, which retains its initial shape through the col-umn, may be attributed to the fact that coalescence occursonto the sparger surface but does not continue during thebubble course through the bulk of the liquid.The mean Sauter bubble diameter (d32), defined as

d32 =∑N

i=1 d3i∑Ni=1 d2i

(2)

is a popular representation of themean bubble size. Its valuesand the standard deviation of the drop size distribution(�)

for all liquids as well as for two gas flow rates are givenin Table 2. Values for the low surface tension solutions areonly a rough estimation, because they are calculated from aseries of small samples (e.g. 30 bubbles) taken from areaswith low bubble concentration and for this reason� valuesfor the butanol solutions are not included inTable 2.As it is expected the mean bubble size increases with in-

creasing gas flow rate but it is worth noticing that the in-crease is more pronounced for the high-viscosity glycerinsolutions (30–60%) than it is for the low viscosity butanolsolutions (∼ 15%). It is evident thatd32 also increases withviscosity. The only exception is water which, despite its rel-atively low viscosity value, exhibits ad32 comparable to thatof the high-viscosity glycerin solutions. It must be noted thatthe calculatedd32 value of the glycerin solutions is muchgreater than the more frequent bubble diameter(<1mm),as it can be seen inFig. 8, a fact that is also reported byTseet al. (2003), who found that during coalescence the forma-tion of large bubbles is accompanied by a great number ofsmall daughter bubbles (100–200�m).

3.4. Homogeneous–heterogeneous regime transition

A common procedure to locate the transition point be-tween the homogeneous and heterogeneous regime is to ap-ply the drift flux analysis, which is based on mass conser-vation equations and relates velocities and concentrations ofthe phases (Wallis, 1969). The model looks at the relativemotion of the two phases and is suggested for flows with flatradial profiles. The basic quantity is the drift flux,j, which


0.000

0.002

0.004

0.006

0.008

0.010

0 1 2 3 4 5

w

b1

b2

g1

g2

g3

drift

flux

, m/s

εG, %

trans

Fig. 9. Regime transition using the drift flux model.

Table 3Transition between homogeneous and heterogeneous regime(dp =40�m)

Liquid phase �trans (%) Utrans (cm/s) js (cm/s)

Water 1.2 0.20 6.4n-butanol 0.6% w/w 2.2 0.23 7.3n-butanol 1.5% w/w 2.2 0.23 7.3Glycerin 33.3% w/w 1.1 0.29 9.2Glycerin 50.0% w/w 1.1 0.28 8.9Glycerin 66.7% w/w 1.2 0.28 8.9

represents the gas flux through a surface moving at the av-erage velocity of the mixture and is given by

j = UGS(1− �G), (3)

whereUGS is the superficial gas velocity and�G the averagegas holdup. If the drift flux is plotted versus the gas holdup,the change in the slope of the curve indicates the transitionfrom the homogeneous to the heterogeneous regime (Shahet al., 1982).Fig. 9 presents the calculatedj values (Eq. (3)) versus

the corresponding gas holdup experimental data for all theliquids tested. The points where the change of the slope oc-curs are located and the corresponding superficial velocities(UGS) are calculated and presented inTable 3. A generalcomment is that for all cases the transition occurs at a liquidsuperficial velocity between 0.2 and 0.3 cm/s. These super-ficial velocity values correspond to gas fluxes through thesparger,js , 6.0 to 9.0 cm/s (Table 3), which are in the sameorder of magnitude with those reported in the literature (e.g.Camarasa et al., 1999; Zahradnik et al., 1997). It has beendetermined that (for the two spargers employed) the transi-tion point does not depend on the pore size. It is also evidentthat an increase in liquid phase viscosity shifts the transi-

tion point to slightly higher velocities. The only exceptionis water whose transition velocity is lower than that of bu-tanol solutions despite its slightly higher viscosity. This be-havior can be attributed to the simultaneous effects of bothrelatively low viscosity and high surface tension. The dif-ference between the calculated transition velocities and theones reported in the literature (e.g.Zahradnik et al., 1997;Camarasa et al., 1999; Ruzicka et al., 2001, 2003) is not un-expected since it has been reported (e.g.Hebrard et al., 1996;Zahradnik et al., 1997) that differences in size and type ofthe distributor shifts the limit of the homogeneous regime.

4. Result interpretation

The optimum operating conditions of a bubble columnwould be the ones that enhance mass transfer and this isaccomplished by maximizing the gas/liquid interfacial area,a measure of which is given by

a = 6�Gd32

, (4)

where�G is the gas holdup andd32 the mean Sauter diam-eter. Consequently, the homogeneous bubbly flow regimeencountered at the lower gas flow rates is most desirablefor mass transfer operations, since, by exhibiting a large gasholdup value accompanied by relatively small bubble size,provides a greater interfacial area.As it is already mentioned the mean bubble size depends

on the liquid properties which may either promote or inhibitcoalescence of the primary bubbles formed on the spargersurface. It is generally admitted that coalescence occurs inthree steps, i.e., collision, liquid film drainage and rupture.When two bubbles collide, the liquid film formed by thesmall amount of liquid trapped between them begins to drainuntil it becomes sufficiently thin to be ruptured due to an in-stability mechanism. The above described sequence is lead-ing to a coalesced bubble. It is also believed that the liquidfilm drainage is the rate controlling step while the ruptur-ing step is almost instantaneous (Chaudhari and Hofmann,1994). Bubble coalescence is also a function of the contacttime between two bubbles that depends on the bubble risingvelocity, which in turn is a function of the bubble size andthe turbulence intensity. It is evident that, when a poroussparger is used, the proximity of the pores promotes coales-cence on the sparger surface as soon as the bubbles enterthe column.The viscosity seems to play a dual role. At relatively

low viscosity values, an increase in viscosity hinders filmdrainage during the thinning process and thus inhibits coa-lescence. This remark is also supported by the shape of thedistribution curves of the two lower viscosity glycerin solu-tions tested (Fig. 10) and the photos inFigs. 11a and b fromwhich is evident that the bubbles away from the sparger arearetain their relative monodispersity and coalescence mech-anism plays a minor role. However, a further increase of


0

10

20

30

40

50

0 1 2 3 4 5 6

1.0

3.5

8.2

22.5

num

ber

freq

uenc

y, %

d, mm

h=40 cm

µL, mPa.s

Fig. 10. Effect of liquid viscosity on the form of bubble size distributioncurve (QG = 0.8× 10−5m3/s, h ≈ 40 cm).

liquid viscosity leads to a decrease of turbulence in the liq-uid phase favoring large bubble formation by coalescence(Figs. 10and11c) which leads to an increase of the largerbubble number in expense of the smaller ones. This ex-planation is also supported by other authors (e.g.Ruzickaet al., 2003). Thus at higher viscosity values the wide shapeof the bubble size distribution curve implies that the smallerbubbles coalesce, an assumption also supported by the bi-modal bubble size distribution observed. It is noted thatParthasarathy andAhmed (1996)believe that in a coalescingmedium bubbles continue to coalesce as they rise.The water is an exception to the above trends and the

bubbles formed have larger sizes than that expected by con-sidering only the physical properties, i.e., the mean bubblesize has a higher value than that of the 50% glycerin solutionwhose viscosity value is 8.2mPas (Fig. 10). This peculiarbehavior can be attributed to the simultaneous effects of bothlow viscosity and relatively high surface tension that favor

Fig. 11. Images illustrating the effect of viscosity on bubble size distribution for: (a) glycerin 33.3%, (b) glycerin 50.0% and (c) glycerin 66.7%.(QG = 0.8× 10−5m3/s, dp = 40�m).

coalescence on the sparger surface as the bubbles enter thecolumn.An attempt wasmade to formulate a correlation that would

permit the prediction of gas holdup, a variable that greatlyaffects the bubble column operation. From the visual obser-vations and the careful inspection of the experimental results(from various investigators) it can be concluded that the gasholdup value is the result of the interaction of several pa-rameters, the most important of which are:

• the gas phasesuperficial velocity,• thephysical propertiesof the liquid phase (i.e., surfacetension, viscosity),

• thecolumn cross sectionand• the spargercross section.

The experimental results of this study show that the spargerpore size used does not practically affect the holdup val-ues, a remark that is also supported by other investigators(e.g.Parthasarathy and Ahmed, 1996; Kaji et al., 2001). Itmust be also noted that the use of several porous disks withthe same nominal porosity during the period of the exper-iments proved that the data are quite reproducible and thatthe porous disk usage time does not significantly affect itsperformance.In order to formulate a generalized correlation that would

incorporate the relative effect of all the above factors, di-mensional analysis was performed. The results show that theeffect of gas velocity and column dimensions can be takeninto account by defining aFroudenumber:

Fr = U2GS

dCg, (5)

whereUGS is the gas superficial velocity anddC the col-umn diameter or, in case of non-circular cross-sections, thehydraulic diameter. Similarly, the effect of the liquid phaseproperties can be included in the appropriateArchimedes(Ar) andEötvos(Eo) numbers defined as follows:

Ar = d3C�2Lg

�2L, (6)


0

5

10

15

20

25

103 104 105 106 107

present workKaji et al.(2001)Camarasa etal. (1999)correlation

ε G, %

Fr0.5Ar0.1Eo2.2 (dS/dC)

Fig. 12. Gas holdup correlation for the homogeneous regime.

Eo = d2C�Lg

�L

, (7)

where�L,�L and�L are the liquid density, viscosity andsurface tension respectively. Finally, the ratio (dS/dC) ofsparger to column diameter was also included to account forthe different geometrical configurations of the gas entrance.The attempt to formulate a generalized relation that wouldbe valid for both homogeneous and heterogeneous regimeswas unsuccessful. Finally, a correlation is formulated that isvalid only for thehomogeneousregime:

�G = 0.001

[Fr0.5Ar0.1Eo2.2

(dS

dC

)]2/3(8)

and it is plotted inFig. 12, where the data ofCamarasaet al. (1999)and Kaji et al. (2001)are also included forcomparison. It seems that the proposed correlation is in fairlygood agreement (±10%) with all the available data for thehomogeneous regime.

5. Concluding remarks

In bubble column reactor design the homogeneous flowregime is usually the most desirable, because it enhances theefficiency of the equipment by providing a greater gas–liquidinterfacial area. For this regime new data concerning aver-age gas holdup values, bubble size distributions and Sauterdiameters are given for a number of liquids covering a rangeof surface tension and viscosity values. It was found thatbubble size depends on the gas flow rate and is affected bythe liquid properties and that an increase in gas flow rateincreases bubble collision probability resulting in greater

bubble sizes. An increase in liquidviscosity favors largerbubble formation by decreasing turbulence, a fact that bothpromotes bubble coalescence and hinders breakage. On theother hand, an increase in liquidsurface tensionfavors smallbubble formation by promoting breakage and demoting co-alescence. The bubble size distribution data are generallyunimodal. Only for the relatively high-viscosity liquids asecond peak arises as a result of bubble coalescence, andtherefore the data are best fitted by the summation of twonormal distribution functions. It is found that for the geom-etry studied the superficial velocity that marks the homoge-neous regime limit does not exceed 0.3 cm/s. Finally, a newcorrelation based on dimensionless groups for the predictionof gas holdup in the homogeneous regime is proposed andfound to be in good agreement with available data.It is generally accepted that porous plates hold advantages

over the other types of gas distributors used. It has beenalso proved that bubble size distribution in bubble columnsequipped with fine porous spargers depends mostly on co-alescence/breakage phenomena, which take place either di-rectly onto or in the vicinity of the sparger surface.Delhayeand McLaughlin (2003)stated that both experimental andtheoretical analyses are needed in order to establish rigor-ous criteria for the coalescence and breakage of fluid objectsat the microscopic level. Consequently, future experimentalwork must be focused on the phenomena occurring onto thesparger surface with the intention to gain an insight on thebubble formation mechanisms. Moreover, the knowledge ofthe range of parameters over which a particular regime isencountered and the conditions under which a transition oc-curs would facilitate the design of a bubble column reactor.

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Effect of Liquid Properties on the Performance of Bubble Column Reactors