Published: September 19, 2011

r 2011 American Chemical Society 12203 dx.doi.org/10.1021/ie200452q | Ind. Eng. Chem. Res. 2011, 50, 12203–12207

ARTICLE

pubs.acs.org/IECR

Effect of Electrolytes in Aqueous Solution on Bubble Size inGas�Liquid Bubble ColumnsJos�e E. Botello-�Alvarez,*,† Sergio A. Baz-Rodríguez,† Ra�ul Gonz�alez-García,‡ Alejandro Estrada-Baltazar,†

Jos�e A. Padilla-Medina,† Guillermo Gonz�alez-Alatorre,† and Jos�e L. Navarrete-Bola~nos†

†Departamento de Ingeniería Química � Ingeniería Bioquímica � Ingeniería Electr�onica, Instituto Tecnol�ogico de Celaya,Av. Tecnol�ogico y García Cubas s/n, Celaya, Gto 38010, M�exico‡Facultad de Ciencias Químicas, Universidad Aut�onoma de San Luis Potosí, Av. Dr. M. Nava No. 6, San Luis Potosí, SLP 78210, M�exico

ABSTRACT: The effects of three inorganic electrolytes (CaCl2, MgCl2, and NaCl) in aqueous solutions on the bubble size in agas�liquid bubble column under a homogeneous bubbling regime were investigated. The concentration of electrolytes and thesuperficial gas velocity at the entrance of the column were varied. The average bubble diameter in the main section of the equipmentwas estimated using digital image analysis. From an analysis of variance, it was found that the electrolyte concentration is the mainreason for bubble size variation. Moreover, it was found that the coalescent and noncoalescent behaviors in the bubble column areseparated by the transition concentrations of coalescence. The ratio between the electrolyte concentrations and the transitionconcentration of coalescence (reduced concentration) was used to modify an empirical correlation that describes the experimentalbehavior of the Sauter mean diameter within an absolute percentage deviation of 10.47%.

1. INTRODUCTION

Interfacial mass transfer in gas�liquid systems has manyimportant applications in chemical and biochemical indus-tries. When considering the design and operation of bubblecolumns, bubble size is an important parameter because itdefines the available area for mass transfer, the gas hold up,and other hydrodynamic variables. The amount, size, shape,and motion conditions for the bubbles depend on variablessuch as the superficial gas velocity at the entrance, diffuserdesign, frequency of bubble coalescence and break up, andphysicochemical properties of the liquid phase. In this sense,the presence of solutes can be important because they cansignificantly affect the liquid properties and the bubble coales-cence behavior; thus, the interfacial area availability can beaffected too. In particular, the electrolytes are present in manybiotechnological processes that are carried out using bubblecolumns, and their effects on bubble size are a relevant matterto research.

It has been reported in the literature that the averagebubble size in bubble columns is smaller in electrolyte aqu-eous solutions than in pure water.1�3 It is possible that theincrement of the liquid surface tension due to the presence ofinorganic electrolytes causes the reduction in the bubblevolume when they are generated in the diffuser.4,5 Moreover,an important effect of electrolytes is the inhibition of bubblecoalescence.1�3 This allows the maintenance of bubbles assingle elements during the approach or collision with otherbubbles.

There are several attempts to explain why the presence ofelectrolytes causes the inhibition of bubble coalescence. Someauthors claim that coalescence occurs because the liquid isdrained from the space between two approaching bubbles untila minimum film thickness is reached; then, the film ruptures andthe bubble coalesces. If the formation of ordered structures

among ions and water molecules in aqueous electrolyte solutionsprevents the critical liquid draining, coalescence is inhibited.6,7

Other authors have proposed that the inhibition of bubblecoalescence above a minimum film thickness can be explainedin terms of the change of surface tension with concentration.2

The amount of dissolved air that is affected by the electrolyteconcentration also has been suggested as an important factor inthe inhibition of bubble coalescence.8,9 In this context, a transi-tion concentration of coalescence has been defined as theconcentration of electrolyte in aqueous solution from whichthe air bubbles reduce their coalescence above 50%, with respectto that observed in pure water.7

The effects of the presence of electrolytes in aqueoussolutions on the average size of air bubbles in a bubble columnwere studied. For this purpose, the average bubble size wasrelated with the transition concentration of coalescence. Therelevance of the reduced concentration (ratio between theelectrolyte concentration and the transition concentration ofcoalescence) as a reference for assessing the average bubblesize was investigated.

2. EXPERIMENTAL PROCEDURE

Experiments were carried out in a Pyrex glass bubble columnwith an internal diameter of 0.095 m and 1.2 m height. Aschematic diagram is shown in Figure 1. A porous glass platewith a nominal porous diameter of 160�250 μm was used as adiffuser. Temperature was maintained at 30 �C inside the bubblecolumn using a recirculating bath (PolyScience, 9100 series). Sixsuperficial air velocities at the bottom of the bubble column were

Received: March 7, 2011Accepted: September 19, 2011Revised: September 15, 2011

12204 dx.doi.org/10.1021/ie200452q |Ind. Eng. Chem. Res. 2011, 50, 12203–12207

Industrial & Engineering Chemistry Research ARTICLE

evaluated: 0.0005, 0.0045, 0.0085, 0.0125, 0.0160, and 0.0196m/s.These velocities were measured and controlled with a rotameter(112-02-GL, Aalborg Instruments and Controls, Inc.). Table 1shows the molarities of NaCl, CaCl2, and MgCl2 in aqueoussolutions, which were used as liquid phase; the transition con-centrations of coalescence are indicated too.2 Deionized water wasused to prepare the solutions.

Geometric measures of the bubbles were determined usingdigital image analysis. A 0.15 m height glass cube was placedconcentrically on the column around the location where theimage acquisition was carried out (0.40 m above the diffuser).The cube was filled with water in order to avoid the opticaldistortion of the images due to the column curvature. Thenegligibility of the optical distortion was verified by looking atrigid spheres of known dimensions. Image acquisition wascarried out using a video camera (Sony Handycam DVC-VTR730), and it was zoomed manually for a better focusing ofthe bubbles. A couple of incandescent lamps (60W)were used toenhance the clarity of the bubbles. These lamps were placed atthe lateral sides of the cube, with respect to the camera location,in order to highlight the bubble contours; clear and dark back-grounds were placed outside of the cube (see Figure 1). At thewall, inside the column, a transparent band with a graduateddistance scale in millimeters was placed to establish a pixel/distance relationship.

Video sequences were captured at an average rate of 25 frames/sand then sent to a personal computer. A script of the ImageAcquisition Toolbox of Matlab 7.0 (The MathWorks, Inc.) wasused to monitor the capture. The resolution of video frames was240� 320 pixels, and the average of the pixel/distance ratio was100 pixels/cm. Images were extracted and processed from thevideo sequences by using a script performed in the Image ProcessingToolbox of Matlab 7.0. By enhancing the contrast among thedark and clear zones of the images, the contours of the bubblescould be traced. At least 150 bubbles were isolated from the imagesfor each experimental condition (electrolyte�composition�superficial gas velocity). The bubble shape was assumed to beoblate ellipsoidal. The geometric properties of each bubble wasmeasured, and its diameter associated with an equivalent spherewas calculated using the following equation:

dbi ¼ffiffiffiffiffiffiffiffiffiffiffiffiffidH2dL3

pð1Þ

where dH and dL are the major and minor axis of each bubble,respectively. In order to obtain a representative diameter for eachexperimental condition, the Sauter mean diameter was calculated

Figure 1. Experimental setup.

Figure 2. Effects of superficial gas velocity on the average bubblediameter in NaCl aqueous solutions: (a) concentration below thetransition concentration of coalescence (0.05 M) and (b) concentrationabove the transition concentration of coalescence (0.37 M).

Table 1. Electrolyte Concentrations in Aqueous Solution

electrolyte

concentration in

aqueous solution (M)

transition concentration

of coalescence (Ctc, M)

NaCl 0.05, 0.13, 0.21, 0.29, 0.37 0.175

CaCl2 0.02, 0.04, 0.06, 0.08, 0.10 0.055

MgCl2 0.02, 0.04, 0.05, 0.08, 0.11, 0.14 0.055

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Industrial & Engineering Chemistry Research ARTICLE

as follows:

d32 ¼∑inidbi

3

∑inidbi

2 ð2Þ

where ni is the number of bubbles with a dbi diameter.

3. RESULTS AND DISCUSSION

In Figure 2, the cumulative frequencies of the bubble equiva-lent diameters formed in aqueous solutions of NaCl at twodifferent molarities are plotted. Each data series in the figure depictsa constant superficial gas velocity. The lowest composition (0.05M) depicts coalescing conditions, whereas the highest composi-tion (0.37 M) is the opposite. The equivalent bubble diameterincreases as the superficial gas velocity increases, and it decreasesas the electrolyte concentration increases. It was found that thesize distribution dispersion is lower at high concentration values.

At the lowest superficial gas velocity, bubble coalescence doesnot occur even if medium properties are propitious; therefore,the bubble diameter does not change, and it is independent of thesalt content in the solution. This is shown in Figure 3a, where

bubble size distribution has almost the same behavior at differentNaCl molarities. This happens when the superficial gas velocity islow because the bubbles tend to behave as isolated bodies; whenthe volumetric gas fraction is low, the possibility of collisionsdecreases, and then the coalescence falls.10 On the other hand, athigher superficial gas velocities, a clearly different behavior exists.Figure 3b shows that at concentrations lower than 0.21 M, thereare coalescent conditions, whereas at equal or higher concentra-tions, this does not occur significantly. The hydrodynamicconditions allow the bubble interactions due to spatial proximityto be conducted. Thus, the effects of the increase in electrolyteconcentration on the coalescence behavior can be observed.It was found that the transition concentration (0.175 M for NaClaqueous solutions) separates the coalescence occurrence in thebubble column. While the concentration is lower than transitionconcentration, the bubble coalescence occurrence decreases asthe content of electrolyte increases. Once the transition con-centration is reached, the occurrence of coalescence is no longersignificant, and the bubble size is essentially the same for higherconcentrations.

The experimental data of the equivalent bubble diameter foreach of the cases studied have a log-normal distribution, giventhat their distribution frequency is not symmetrical. A slanttoward the right occurred because no bubble size could equalor be less than zero. This is in agreement with the results of Lageand Esp�osito11 for bubbling a homogeneous regime in bubblecolumns. From each distribution, the Sautermean diameter (d32)was calculated as a representative parameter. Using a Box�Coxtest, it was determined that the Sauter mean diameter datafollows a normal distribution function. Using an analysis ofvariance, it was found that the superficial gas velocity at theentrance, the concentration, and the chemical nature of the salthave significant effects on the Sauter mean diameter. However,the salt concentration is the most sensitive variable and thelargest source of variation, as shown in Table 2. Salt composi-tion gradually generates noncoalescence behavior.6 From thisresult, it is advisible to suggest that a good correlative variablecan be a “reduced concentration”, Cr: the ratio between theconcentration of the salt (Ci) and the transition concentrationof coalescence (Ctc).

Cr ¼ Ci

Ctcð3Þ

Figure 3. Effects of concentration in aqueous solution on the averagebubble diameter in NaCl aqueous solutions at (a) Usg = 0.0005 m/s and(b) Usg = 0.0196 m/s.

Table 2. Analysis of Variance

source

sum of

squares

degrees of

freedom

mean

square F-value P-value

model 1.600 49 0.033 78.61 <0.0001

A superficial

gas velocity

0.190 5 0.038 90.76 <0.0001

B electrolyte

concentration

1.020 4 0.260 615.14 <0.0001

C electrolyte

species

0.047 2 0.024 56.75 <0.0001

AB 0.210 20 0.010 25.07 <0.0001

AC 0.017 10 1.653 � 10�3 3.9 <0.0001

BC 0.094 8 0.012 28.44 <0.0001

residual 0.015 37 0.0004

total 1.61 86

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Industrial & Engineering Chemistry Research ARTICLE

This variable was used previously by Ribeiro and Mewes.12

They found that gas hold up and flow regime in bubble columnscan be related to the ratio of the electrolyte concentration and thetransition concentration of coalescence in the aqueous electro-lyte solution. Figure 4 shows bubble Sauter diameter behaviorfor the three different salts against the reduced concentration,Cr, at superficial gas velocity Usg = 0.0125 m/s. The datatrend allows the assumption that the coalescence may follow abehavior similar to the principle of corresponding states ofsingle fluids. Once the transition concentration is reached (Cr > 1),the Sauter mean diameter no longer varies significantly. Asmentioned above, this can be explained by the almost totalinhibition of bubble coalescence. This leads to the conclusionthat the experimental data of transition concentration ob-tained from experiments addressed to force the contact ofbubble pairs7 are good references to assess the average bubblesize in bubble columns, where there are interactions betweenmultiple bubbles.

Provided that the hydrodynamic conditions are favorable for ahomogeneous bubbling regime that ensures that the bubblesinteract, the effects of electrolyte concentration on bubble size

must be taken into account. Even if the physicochemical proper-ties do not vary significantly with a moderate change in theelectrolyte concentration, the bubble coalescence behavior does.This is relevant because the interfacial area availability, related tothe average bubble size, is crucial for the mass transfer achieve-ment in many biotechnological processes, in which electrolytesare present as source of mineral nutrients.13

Previously, Pohorecki et al14 correlated the Sauter diameterusing the Froude number (Fr):

Fr ¼ Usg2

d32g¼ k

FLUsg4

σg

!αμLUsg

σ

� �β

ð4Þ

where k,α, and β are adjustment parameters;Usg is gas superficialvelocity; g is gravity; FL and μL are the liquid density and viscosity,respectively, and they were measured by Baz-Rodríguez;13 and σ isthe liquid surface tension and was obtained from the literature.9 It isreasonable to propose that the adjustment parameters should be afunction of the reduced concentration. These parameters wereestimated using the subroutine GREG:15

k ¼ 0:138 þ 3:345Cr � 0:712Cr2 ð5Þ

α ¼ 0:412 þ 0:059Cr � 0:013Cr2 ð6Þ

The β parameter is almost constant and equal to 0.1. Figure 5shows a comparison between the experimental results and theprediction of the proposed correlation. The correlation agreeswithin an absolute average percentage deviation of 10.42, 9.47,and 11.53% for CaCl2, MgCl2, and NaCl aqueous solutions,respectively. Data used to fit eqs 4�6 are encompassed withina homogeneous bubbling regime (0.0005 m/se Usge 0.0196m/s; 0.034e εge 0.1443, where εg is the volumetric gas fraction),and 0 e Cr e 2.545. This is a first attempt to obtain a generalcorrelation for the Sauter mean diameter using the critical con-centration of coalescence in a bubble column.

4. CONCLUSION

In this work, it was found that the coalescence and the averagediameters of bubbles in electrolyte aqueous solutions in bubblecolumns are influenced mainly by the solute concentrations andthe hydrodynamic conditions. The experimental data of transi-tion concentrations of coalescence2 obtained from experimentsaddressed to force the contact of bubble pairs are good referencesto assess the average bubble size in bubble columns. The transitionconcentrations marked the point where the bubble coalescencewas drastically inhibited. A new correlation was developed that isa function of the reduced concentration (the ratio between theelectrolyte concentration and the transition concentration ofcoalescence); the estimated data of this work were used to fit theproposed equation. The new correlation is based upon theFroude number and predicts the Sauter mean diameter withinan absolute average percentage deviation of 10.47%.

’AUTHOR INFORMATION

Corresponding Author*E-mail: joseb@itc.mx. Tel.: +52 (461)6117575 ext. 415.

’ACKNOWLEDGMENT

The authors respectfully acknowledge the Consejo Nacionalde Ciencia y Tecnología (National Council for Science and

Figure 4. Reduced concentration (Cr) against Sauter mean diameter forUsg = 0.0125 m/s.

Figure 5. Experimental vs calculated data of reduced concentration(Cr) for three inorganic electrolytes.

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Industrial & Engineering Chemistry Research ARTICLE

Technology) (CONACyT), M�exico, for their financial supportthrough a Graduate Scholarship (SABR). They thank Dr. GustavoIglesias for some interesting remarks.

’NOMENCLATURECi = electrolyte concentration in aqueous solution (M)Cr = reduced concentrationCtc = transition concentration of coalescence (M)d32 = Sauter mean diameter (m)dbi = bubble diameter associated with an equivalent sphere (m)dH = major axes of the bubble (m)dL = minor axes of the bubble (m)Fr = Froude number based on Sauter mean diameter and

superficial gas velocityg = acceleration of gravity (m/s2)k = fitting paramenter in eq 4Usg = superficial air velocity at the entrance of the bubble column

(m/s)

’GREEK LETTERSα = fitting paramenter in eq 4β = fitting paramenter in eq 4εg = volumetric gas fractionμL = viscosity of liquid (Pa 3 s)FL = density of liquid (kg/m3)σ = surface tension of liquid (N/m)

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