Removal of aniline from aqueous solution in a mixed flow reactor using emulsion liquid membrane

Journal of Membrane Science 226 (2003) 185–201

Removal of aniline from aqueous solution in a mixedflow reactor using emulsion liquid membrane

S. Datta, P.K. Bhattacharya, N. Verma∗Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

Received 13 February 2003; received in revised form 1 August 2003; accepted 4 September 2003

Abstract

The emulsion liquid membrane (ELM) technique was employed in a mixed flow reactor (MFR) for the removal of anilinefrom dilute aqueous solution. The extent of removal of aniline was experimentally determined under various operatingconditions such as, feed to emulsion ratio (F:E), internal reagent concentration (Ch), stirrer speed (n), residence time in thereactor (τ), and feed concentration (C0). The mass transfer within the emulsion globule was modeled by incorporating thediffusion of solute within the reacted (spent) emulsion phase, and allowing for the gradual shrinkage of the unreacted zone ofthe internal reagent. The reaction between the solute and internal reagent at the moving boundary (interface) of the reactedand unreacted zones was assumed to be first order reversible. The removal of aniline was found to increase with increase inτ, Ch andn, and with decrease in theF:E ratio andC0. The maximum removal of aniline obtained in this study was 98.53%.The data and the model results were found to be in good agreement in most cases. The discrepancy between the model resultsand the data observed at highern andτ were attributed due to the rupture of emulsion globules and non-ideal flow conditionsexisting within the reactor, respectively.© 2003 Elsevier B.V. All rights reserved.

Keywords:Wastewater; Aniline; Emulsion liquid membrane (ELM); Separation; Mixed flow reactor (MFR); Mathematical modeling

1. Introduction

A common problem in most of the industries is thedisposal of large volume of wastewater containing or-ganic solutes, which may be toxic. Their presence,therefore, requires treatment prior to the disposal fromthe points of view of safety and environment. Anilineis one of the harmful and toxic organics, which arecommonly used in a number of industrial applications,for example, in the production of methyldiphenyldiiso-cyanate, rubber accelerators, dyes, pigments and also,herbicides[1].

∗ Corresponding author. Tel.:+91-512-597629;fax: +91-512-590104.E-mail addresses:[email protected] (P.K. Bhattacharya),[email protected] (N. Verma).

Several attempts have been made either to treatsuch types of effluents for facilitating easy disposal,or to recover the chemicals and recycle the processwater. Traditional method of purification such as, dis-tillation, liquid extraction and absorption are still inuse; however, industries are looking for competingalternative technologies which may overcome someof the inherent disadvantages of the traditional pro-cesses[2]. Further, these processes suffer from thedisadvantages that they are not necessarily efficientin removing organic solutes at parts per million levelsfrom dilute solutions. Attempts are, therefore, towardsdeveloping membrane-based separation processes tohandle wastewater containing organic solutes[3–5].The separation process based on the emulsion liquidmembrane (ELM) technique is one such process.

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2003.09.003

186 S. Datta et al. / Journal of Membrane Science 226 (2003) 185–201

In 1968, the ELM technology was introduced as aseparation technique by Li[2] for the separation ofhydrocarbons. Since then ELM has demonstrated con-siderable potential in a wide variety of applications.These include the fractionation of hydrocarbons, therecovery and enrichment of heavy metal ions, and theremoval of trace organic contaminants from wastew-ater[6–8].

ELM is made by forming an emulsion of two immis-cible phases (internal phase and membrane phase) andthen dispersing it in a third phase (continuous phase).In general, the encapsulated phase (internal phase) andthe continuous phases are miscible, but the membranephase must not be miscible with either of them. Tomaintain the integrity of the emulsion during the sep-aration process the membrane phase usually containscertain surfactants and other additives (as stabilizingagents) in a base material (a solvent for all other in-gredients). In general, a large number of globules ofemulsion are desired to produce a large membrane sur-face area for rapid mass transfer from the continuousphase to the internal phase.

Several mathematical models have been proposedto explain the mass transfer within an emulsion glob-ule and to predict the extent of removal of solute fromthe continuous phase. Cahn and Li[9] were the firstresearchers to propose a simple model for phenol per-meation through ELM. It was assumed that the reagentwithin the internal droplet consumed the solute instan-taneously and irreversibly. Kopp et al.[10] adopted amore complex but a realistic approach to mathemat-ically model such behavior. They assumed that con-ceptually there was an interface boundary inside theELM, which advanced towards the globule center asthe internal reagent was consumed. This model wasthe basis of the advancing front model applied in ELMby Ho et al.[6,11]. Ho et al.[11] formulated the prob-lem of mass transfer within an emulsion globule basedon the advancing front approach that included boththe spherical geometry and depletion of solute in thebulk phase. Stroeve and Varanasi[12] extended Ho’sapproach to include the external and internal masstransfer resistances to the globule surface. Teramotoet al. [13] carried out batch experiments with aminesas solute and HCl as internal reagent. They removedthe restriction of reaction irreversibility and introducedthe concept of reversible reaction between solutes andinternal reagent. Bunge and Noble[7] presented a

model, which incorporated reaction equilibrium (re-versible reaction). In this model there were no separatereacted and unreacted regions; solutes diffusing intoemulsion globules either reacted with internal reagentor distributed itself between two phases. Baird et al.[14] have carried out batch experiments with aminesusing HCl as internal reagent and showed that the re-versibility of acid–amine reaction along with aminesolubility in membrane phase affected the extractionrates. In the aforementioned study the reversible re-action model satisfactorily predicted the experimentaldata using the measured parameters (distribution co-efficients, partition coefficients and globule size). Inone of the few studies on flow reactor, Rautenbach andMachhammer[15] studied the applicability of ELMprocess in a stirred counter-current flow column bycarrying out experiments to separate NH3 from diluteaqueous solution. A few studies have also been donespecifically to study the phenomenon of the globulerupture resulting into the leakage of solute[16–18]. Inall these studies, the instability of the membrane phasewas cited as one of the most serious obstacles in thesuccessful application of liquid membranes to indus-trial separations. It was also pointed out that the rate ofleakage by membrane rupture was usually a functionof the surfactant concentration and agitation speed.

From the literature survey it is quite evident that asignificant number of researches have been done onthe application of ELM for the removal of organic so-lutes from aqueous solution in a batch reactor. How-ever, such studies are few in a flow reactor. In eithercase, the mass transfer within emulsion globules isdescribed by considering either advancing front or re-versible reaction models. In this work, experimentaland theoretical study has been carried out to removeaniline from aqueous solution using ELM techniquein a mixed flow reactor (MFR), which has not beendone earlier. Mass transfer within emulsion globule isdescribed by considering shrinkage of the unreactedinternal reagent zone and the reversible reaction be-tween solute (aniline) and internal reagent.

2. Theoretical analysis

The theoretical analysis takes into account solutetransfer from bulk continuous phase to the emulsionglobule interface, diffusion within the globule, and

S. Datta et al. / Journal of Membrane Science 226 (2003) 185–201 187

chemical reaction in the unreacted receptor (internalreagent) phase. The analysis also takes into account thegradual shrinkage of unreacted zone within the glob-ule during the separation process. The solute, solublein the membrane phase, permeates from the continu-ous to the receptor phase as a consequence of concen-tration gradients. In the receptor phase the solute mayreact reversibly or irreversibly with internal reagent,and form product, which is insoluble in the membranephase and thus incapable of diffusing back. However,in the case of reversible reaction, some free solute isalways present within the emulsion globule at equi-librium concentration, which is capable of diffusingback.

Mass transport within an emulsion globule is mod-eled by incorporating the radial advancement of thereacted core front during the separation process. In theglobule, the internal phase is evenly distributed overthe membrane phase in the form of very fine droplets.The solute, after permeating through the membranephase, comes in contact with the receptor phase. Thereit reacts with the internal reagent, reversibly and at-tains equilibrium immediately represented by the fol-lowing chemical reaction:

C6H5NH2 + [H+] + [Cl−] ⇔ C6H5NH3Cl (1)

Following attainment of equilibrium, the reaction cen-ters become actually inactive. The centers are practi-cally saturated reaction centers. The saturation of thereaction centers proceeds concentrically into the emul-sion globule.

The various assumptions made in the theoreticalanalysis in developing the mathematical model forcharacterizing the separation of solute in ELM processcarried out in a MFR may be summarized as follows:

1. The membrane and bulk phases as well as themembrane and internal phases are completelyimmiscible.

2. It is assumed that local equilibrium holds be-tween the membrane and droplet phases and theconcentration field within the globules is de-scribed in terms of average local concentration,i.e. instead of composite nature of the emulsion,the globule is treated as a continuum. In otherwords, the theoretical analysis neglects anyconcentration gradient in the reagent dropletsformed within the emulsion globules.

3. In view of the strong presence of the surfactantin the membrane phase, no internal circulationoccurs within the globules.

4. The chemical reaction between aniline and hy-drogen ion is first order reversible.

5. All globules are of uniform size, the effectivemean diameter of which is represented by Sautermean diameter.

6. The coalescence and redispersion of the emulsionglobules are negligible.

7. Leakage of membrane phase due to globulebreakage is neglected.

8. The resistance to mass transfer in the bulk feedphase due to the surfactant monolayer at the outerphase boundary is negligible.

The development of the model to predict the ex-tent of separation of solutes by ELM in a MFR isbased on three governing equations for solute balance:(1) in the continuous mixed flow reactor, (2) withinthe globule, and (3) in the unreacted internal reagentzone.

2.1. Solute balance in the flow reactor

Assuming the flow conditions within the contactorsimilar to that in an ideal MFR, the concentration ofaniline at any location within the contactor was as-sumed to be the same as that in the outlet. Therefore,the difference between the inlet and outlet concentra-tions of the solute was related with the residence time,τ within the contactor under steady-state and describedas follows:

τ = V

V0= C0 − Cf

(−rA)(1)

where (−rA) is the rate of the removal of solutefrom the bulk phase, expressed in kmol/m3 s. Understeady-state condition (−rA) may also be related withthe total solute flux to the emulsion globule throughits specific surface area as:

(−rA) = N × J × 4πR2

V(2)

where N is the total number of emulsion globulespresent in the system and is equal to 3(1− ε)V/4πR3.Substituting the expression forN in the above equationand rearranging the resulting equation, an expression


for the solute flux through single emulsion globule wasobtained in terms of the residence time as:

J = (C0 − Cf )R

3(1 − ε)τ(3)

2.2. Solute balance within a globule

The solute flux in the external concentration bound-ary layer of the emulsion globule may be written interms of the continuous phase mass transfer coeffi-cient,k3 and concentration of aniline in the bulk feedphase,C3 (C3 is same asCf , the concentration of ani-line in the MFR):

J = −dNA

dt× 1

4πR2= k3(C3 − C3m) (4)

Within an emulsion globule, we assume that thequasi-steady-stateassumption is valid[19]. As aconsequence of this assumption, the rates of solutediffusion across the outer boundary of the emulsionglobule, across the boundary of the shrinking core,and at any position in the advancing front, were as-sumed to be equal (referFig. 1). Further, the rate ofchange of moles of solute (aniline) due to the chemical

Fig. 1. Schematic of concentration profile of the solute at differentposition within and outside an emulsion globule.

reaction within the globule may also be equated withthe rate of diffusion to the reaction surface. Therefore,the following mathematical relation may be assumedto hold good at any time during separation.

−dNA

dt= 4πR2J |R = 4πζ2 J |ζ= 4πr2J |r = constant (5)

The solute flux for a single globule,J may also bewritten in terms of concentration gradient from Fick’sfirst law of diffusion, as

J = −DedC

dr

IntegratingEq. (5)with the following boundary con-ditions: atr = ζ, C = Ce (aniline equilibrium concen-tration in the depleted internal reagent droplets) and,r = R, C = Cm (aniline concentration in the emul-sion globule at the outer boundary), an expression forthe concentration of solute in the emulsion globule atthe outer boundary can be obtained as:

Cm = Ce + (dNA/dt)(1/ζ − 1/R)

4πDe(6)

From Eqs. (3)–(5), by proper mathematical manipu-lation, an expression for the solute flux for a singleemulsion globule (J) is obtained in terms ofCf as:

J = ϕ(Cf − Ce)

(ϕ/k3) + R2/De(1/ζ − 1/R)(7)

whereϕ is the partition coefficient and is defined as:

ϕ = Cm

C3m

∣∣∣∣r=R

(8)

2.3. Solute balance in unreacted internal reagentzone

The rate of change of moles of solute (aniline) due tothe chemical reaction between aniline and HCl (whichis stoichiometrically in 1:1 molar ratio) can be equatedwith the decrease in the volume of the unreacted coreas:

dNA

dt= −Ch × d

dt

(4

3πζ3

)(9)

Finally, substituting dNA/dt from Eq. (4) into Eq. (9)and integrating the resulting equation with the


condition,t = 0, ζ = R, an expression forJ at timeτcan be shown to be obtained as:

J = Ch

3R2τ(R3 − ζ3) (10)

Eqs. (3), (7) and (10)are three governing equationsrequired to determine the separation of solutes fromthe continuous phase to the receptor phase.

2.4. Solution technique

Mathematical simplifications were carried out tosolve for Cf (concentration of aniline in the MFR)from the following equations obtained fromEqs. (3),(7) and (10), in terms of the known variablesC0, Ch,R, ε, andζ:

Cf = C0 − Ch

R3(1 − ε)(R3 − ζ3) (11)

Except ζ, all other terms in the above equation areknown.ζ may be determined by solving the followingquadric equation, which is obtained fromEqs. (3), (7)and (10):[Ch

(R

De− ϕ

k3

)− 3τϕCh

R(1 − ε)

]ζ4

+[−R2Ch

De

]ζ3 +

[ChR

3(

ϕ

k3− R

De

)

−3R2τ{ϕC0 − ϕCh(1 − ε) − Ce}]ζ

+[ChR

5

De

]= 0 (12)

Once the value ofζ for a givenτ was determined, thetheoretical value ofCf was obtained fromEq. (11).This value was compared to the corresponding exper-imental data to validate the mathematical model forthe prediction of separation of solute (aniline) fromaqueous solution using ELM technique in an MFR.

It may be pointed out that the operating variables,such asC0, Ch, τ andε were known from the exper-imental conditions.ϕ was obtained from a separatebatch experiment. The continuous phase mass trans-fer coefficientk3, the effective diffusivityDe, and theglobule radiusR for different experimental conditionswere calculated from the appropriate correlations andare reported inAppendix A. Thus, it is fair to say that

there are no adjustable parameters in the developedmodel.

3. Experimental

Experiments were carried out to study the removalof aniline from aqueous solution using the ELM tech-nique in a MFR under several operating conditionssuch as feed to emulsion ratio, rpm, residence time, in-ternal reagent concentration and initial concentrationof aniline.

3.1. Materials

Isopar-LS, manufactured by Exxon India Ltd. wasused as isoparaffinic solvent and Span-80 (s. d. finechemical Ltd., India) was used as surfactant. Concen-trated HCl (Qualigens Ltd., India) and aniline (s. d.fine chemical Ltd., India) were diluted with distilledwater to get the solutions of required strength.

3.2. Apparatus

Fig. 2 is the schematic of the experimental set-upconsisting mainly of two units:homogeniser and set-tler. A 250 ml glass vessel with a provision of an outletin the lower portion along with a stirring arrangementwas used as a homogenizer. The emulsion preparedin the homogenizer was continuously transferred to acontactor with the help of a peristaltic pump (PP-10,Miclin Corp., India).

A cylindrical glass vessel with two inlets and pro-vision of an outlet in the lower portion along with astirring arrangement was used as a contactor. In thecontactor, emulsion phase was dispersed in continu-ous phase with the help of stirrer (impeller) and ACmotor (1047.2 × 10−1 rad/s (1000 rpm), Remi). Thespeed of the stirrer was measured with an electrodetype tachometer (DT2006 5D rpm counter). From thecontactor, the mixture was directly transferred to afunnel containing Whatman 42 filter paper, throughcock valve. Here, the emulsion with extract and raf-finate was separated, and the raffinate was collectedin a conical flask kept below the funnel. The treatedaniline solution (i.e. raffinate) was analyzed for itsconcentration by a UV-Vis spectrophotometer (ModelUV-1601, Shimadzu Corporation, Japan).


Fig. 2. The experimental set-up to study the removal of aniline using ELM in a MFR.

In the present experimental study, the solution con-taining emulsion and extract phases was discarded.No effort was made to recover the concentrated ani-line in the extract phase and reuse the membranephase. It may, however, be pointed out that, in anindustrial plant the emulsion and the extract phaseare required to be broken (in a coalescence unit) byvarious means including mechanical abrasion, to re-lease the permeated component (aniline in this case)and recycle of the components of the membranephase to re-emulsify the membrane in the homoge-nizer. Else, the handling and disposal of the concen-trated effluent may be environmental and economicalconcern.

3.3. Experimental variables

The experiments were performed under the vary-ing conditions of residence time (10–60 min), feed toemulsion ratio (1:12, 1:15, and 1:20), stirrer speed(209.44×10−1 to 523.6×10−1 rad/s (200–500 rpm)),

hydrogen ion concentration (0.1–1.0 M) and initialconcentration of the feed (0.001–0.002 M).

3.4. Experimental procedure

The membrane phase was prepared by mixing50 ml of organic solvent and 2% (by volume) of thesurfactant in the homogenizer under constant stirringcondition (1047.2× 10−1 rad/s (1000 rpm)) for 2 min.Emulsion phase was then prepared by adding internalreagent (HCl solution) in the membrane phase andemulsifying the resulting mixture. A stirring speed of2618× 10−1 rad/s (2500 rpm) was used for 5 min.

The emulsion phase was transferred into the contac-tor by the peristaltic pump at a constant pre-calibratedflow rate, E. The aniline phase was simultaneouslyintroduced through a valve at a pre-calibrated flowrate,F, to maintain a fixedF:E ratio. Separation wasachieved in the contactor by dispersing the emulsionphase in the continuous phase (aniline solution) withthe help of a stirrer set at a fixed speed. A constant and


uniform mixing in the contactor created a conditionsimilar to that in MFR. For a fixedF:E ratio, the flowrate of the outlet stream was adjusted identical to thetotal flow rates of the inlet streams to maintain the re-quired residence time (τ) and the level of liquid in thecontactor. In order to achieve steady-state concentra-tion in the contactor, sufficient time was allowed be-fore the samples were collected for the analysis fromthe settler.

From the contactor, the mixture was transferred tothe funnel where emulsion phase and treated continu-ous phase (raffinate) were separated by Whatman 42filter paper. The lower raffinate phase was taken outin a conical flask. To make sure that the concentra-tion has reached the steady-state value in the contac-tor, several samples were collected at different timeintervals. The collected samples were analyzed for theconcentration of aniline by UV-Vis spectrophotometerat a wavelength of 280 nm. The steady-state concen-tration value of aniline was taken as the concentrationof aniline in raffinate, i.e. the final concentration ofaniline.

3.5. Experimental determination of thermodynamicand physical properties

3.5.1. Partition coefficientTo determine the partition coefficient, an approach

specified by Ho et al.[6] was adopted. Batch ex-periments were carried out using organic solventwithout surfactant and aniline solution of known ini-tial concentration in a mechanical shaker operated at25◦C and 209.44 × 10−1 rad/s (200 rpm) for 24 h.Final concentration of aniline was analyzed by UVspectrophotometer. Partition coefficient was then de-termined from correlation described inAppendix A.The measurements of the partitioning coefficientwere also carried out under different volume phaseratios. Under the experimental conditions used in thisstudy, the maximum variation in the values of theportioning coefficient was observed to be less than1–2%.

3.5.2. Viscosity of pure membrane phaseThe viscosity of pure membrane phase was deter-

mined by comparing with a solvent whose viscositywas known (referAppendix A). The experiments werecarried out in Ostwald’s viscometer[20].

3.5.3. Densities of the membrane phase, emulsionphase and the reaction mixtures

The various densities were measured by comparingthe weight of identical volume of the solutions withthat of water in a specific gravity bottle. By knowingthe density of water at that temperature, the densitiesof other solutions were calculated.

4. Results and discussions

4.1. Effect of feed to emulsion ratio (F:E) on thepercent removal of aniline

Fig. 3describes the experimental data on the percentremoval of aniline and the model results under iden-tical experimental conditions. In all the figures drawnin this paper, the data are shown with the error bars(the error estimated in the reported data correspondedto the maximum variation found in the experimentalmeasurement of the solution concentration by the UVspectrophotometer). As observed fromFig. 3, the re-moval of aniline decreases with increase inF:E ra-tio; the rest of the operating conditions includingτ,n, C0 and Ch remaining constant. For example, theremoval decreases from 78 to 73% as theF:E ratiois increased from 12:1 to 15:1. The trend in the ex-perimental data is explained as follows: with increasein F:E ratio, the number of emulsion globules avail-able for aniline per unit volume of the reaction mix-ture decreases. Thus, the interfacial surface area formass transfer decreases, thereby decreasing the rate ofmass transfer (molar flux) from feed to the emulsionglobule. Consequently, the removal of aniline also de-creases. As shown inFig. 3, there is a reasonably goodagreement between the experimental and model pre-dicted results (within the experimental error), particu-larly for the lower values of residence time. However,at higher values of residence time, an over-predictionof the model in comparison with the experimental datais observed. This discrepancy was attributed due to thenon-ideal conditions prevailing in the reactor at higherτ. The phenomenon of non-ideality has been taken upand discussed later in this section with the help of aseparate study on residence time distribution (RTD).The increase in the removal of aniline observed withdecrease inF:E ratio can also be explained with theaid of Eq. (11), which can be rearranged in terms of


Fig. 3. Effect of theF:E ratio on the removal of aniline (stirrer speed= 209.44× 10−1 rad/s (200 rpm),Ch = 0.1N andC0 = 0.002 M).

ε andζ/R as follows:

C0 − Cf

C0= Ch

C0R3(1 − ε)(R3 − ζ3)

= Ch

C0(1 − ε)

[1 −

(ζ

R

)3]

(13)

With decrease in theF:E ratio, the fraction of emul-sion in the reaction mixture (1−ε) increases and there-fore, the corresponding removal also increases. Pointto be noted is that, with decrease inF:E, the Sautermean radius,R increases as per the correlation givenin Appendix A. Consequently, the value of the term[1 − (ζ/R)3] decreases. However, the effect of incre-ment inR is suppressed by that of decrement in voidfraction asF:E ratio decreases. The net effect is over-all increase in the extent of removal of aniline.

4.2. Effect of residence time (τ) on the percentremoval

The experimental data obtained under the oper-ating conditions forFig. 3 were replotted inFig. 4as a function ofF:E ratios to describe the effectsof residence time on the removal efficiency.Fig. 4shows that the removal increases with increase inτ

at a fixedF:E ratio. However, at higherτ the increaselevels off asymptotically. With increase inτ, the timeof contact and hence, the total mass transfer betweenthe emulsion and feed phases increases, though thenet flux (mol/m2 s) remains the same under identicalstirring condition and feed and reagent concentra-tions. Consequently, the removal of aniline increases.The ‘leveling-off’ trend observed at higherτ can beexplained by considering the physical phenomenon


Fig. 4. Effect of residence time on the removal of aniline (stirrer speed= 209.44× 10−1 rad/s (200 rpm),Ch = 0.1N andC0 = 0.002 M).

that, at higherτ the characteristic time of diffusion(R−ζ)2/De of the solute molecules within the reactedzone of the emulsion globules increases and the entireprocess gradually becomes diffusion controlled. Asa consequence, the rate of solute transfer from thefeed phase to the globule becomes independent of theresidence time in the contactor.

The above inference about the effect ofτ on theremoval of aniline can be further elucidated with thehelp of the model predicted values for the unreactedcore radius (ζ). From the model calculations it wasobserved thatζ decreased with increase inτ for a fixedF:E ratio. With increase inτ, the total mass transferbetween the feed and emulsion phase increases. Con-sequently, the reacted reagent front advances andthe unreacted internal reagent core shrinks, thereby

decreasingζ. From Eq. (13), it is evident that asζ decreases the removal efficiency increases. Thecalculations revealed that that thoughR decreasedmarginally from 0.8824 to 0.8411 mm, the correspond-ing decrease inζ was significant (0.8127–0.7478 mm)with increase in theF:E ratio from 12:1 to 20:1 due tothe dependence of the removal on the third power ofζ.

4.3. Effect of internal reagent concentration (Ch) onthe percent removal

To determine the effects of internal reagent concen-tration (Ch) on the removal of aniline, the experimentswere conducted over various values ofCh (0.1, 0.5and 1.0 M) forτ ranging from 10 to 60 min, whilekeeping the remaining experimental variables such as


Fig. 5. Effect ofCh on the removal of aniline (stirrer speed= 209.44× 10−1 rad/s (200 rpm),F:E = 12:1 andC0 = 0.0015 M).

n, F:E and C0 constant. InFig. 5, the experimentalas well as the corresponding model results for theremoval of aniline were plotted as a function ofτ,with Ch as a parameter. As observed fromFig. 5, theremoval of aniline increases with increase inCh for afixed value ofτ. However, at higherCh the increasegradually levels off. For example, at a constantτ of30 min, the removal of aniline increased from 81 to88% asCh was increased from 0.1 to 0.5 M. How-ever, the increase was from 88 to 90% only asCh wasfurther doubled from 0.5 to 1.0 M. This trend in theexperimental observation forCh at a constantτ canbe explained as follows: with increase inCh, the con-centration of hydrogen ions (H+) within the emulsionglobules increases to react with the aniline molecules.Consequently, the efficiency of removal increases.The “leveling-off”, observed at higherCh, may bedue to the fact that the reaction between aniline andH+ ions approaches pseudo-first-order reaction type

Table 1Effect of Ch on ξ for varying τ

τ (min) Ch (mol/m3) τ × 104 (m) Ch (R3 − ζ3) ×1012 (mol)

10 100 8.425 891500 8.737 1006

1000 8.780 1022

15 100 8.385 975500 8.731 1075

1000 8.777 1092

30 100 8.325 1101500 8.722 1178

1000 8.773 1184

45 100 8.298 1157500 8.719 1208

1000 8.772 1212

60 100 8.282 1190500 8.717 1231

1000 8.771 1235


with respect to the aniline concentration, making it in-dependent of H+ ion concentration. The increase inremoval withCh is consistent withEq. (13). For a con-stant rpm andF:E ratio,Rand (1−ε) remain constant.Hence, as seen fromEq. (12), the extent of removal islinearly dependent onCh. It may be pointed out herethat with increase inCh, ζ also increases resulting incounter decrease in the removal of aniline. However,the net result is increase in the separation efficiency.The calculated numerical values ofζ and the valuescorresponding to the right hand side ofEq. (13)arereported inTable 1 for the purpose of clarity. The‘leveling-off’ at higherCh is also evident fromTable 1.As observed fromFig. 5, there is a reasonably goodagreement between the experimental and model pre-dicted results within the experimental error.

Fig. 6. Effect of stirrer speed on the removal of aniline for varyingF:E (residence time= 30 min,Ch = 0.1N andC0 = 0.001 M).

4.4. Effect of stirrer speed (n) on removal of aniline

The effects of stirrer speed on the removal of ani-line were studied by carrying out two different sets ofexperiments for varying stirrer speeds (209.44×10−1,314.16×10−1, 418.88×10−1 and 523.6×10−1 rad/s(200, 300, 400 and 500 rpm)):

(1) TheF:E ratio was varied over a range (12:1, 15:1and 20:1), whereasτ, Ch, andC0 were kept con-stant at 30 min, 0.1 and 0.001 M, respectively.

(2) Ch was varied as 0.1, 0.5 and 1.0 M, whereasτ,F:E andC0 were kept constant at 30 min, 15:1 and0.0015 M, respectively.

Fig. 6 describes the effect of stirrer speed on theextent of removal of aniline for varyingF:E ratio. The


analysis of the data obtained under the condition of theexperimental set (1) reveals the following two obser-vations: (a) for a constant speed, the removal increasesappreciably with increase inF:E ratio, and (b) for aconstantF:E ratio, the removal increases significantlywith increase in speed at lower rpm. However, increaseis marginal at higher rpm. The first observation can beexplained in the similar manner as done in the explana-tion for the data onFig. 3. The second observation canbe attributed due to the increase in continuous phasemass transfer coefficient (k3) and the decrease in glob-ule size with increase in stirrer speed, thereby increas-ing the interfacial mass transfer surface area per unitvolume of the reaction mixture. Referring toEq. (A.2)in Appendix A, the globule size (the Sauter mean di-ameter) has an inverse dependence on the stirrer speed(d ∝ n−1.4). Thus, the other operating conditions

Fig. 7. Effect of stirrer speed on the removal of aniline for varyingCh.

remaining the same, the globule sizes were calculatedto be within the range of 2.396–8.643 mm correspond-ing to that of 523.6×10−1 to 209.44×10−1 rad/s (500–200 rpm) of the stirrer speed. As a result, the masstransfer of solute (aniline) from the bulk feed phase tothe globule increases. The “leveling-off” at higher rpmmay also be due to the possible rupture of globules,resulting from high shear force. This may have trans-ferred back some solute from emulsion to continuousphase. As observed fromFig. 6, the experimental andthe simulation results match reasonably well at lowerrpm. The over-prediction of the model for removal ef-ficiency in comparison to the experimental values athigher rpm is, therefore, attributed to the leakage ofthe internal phase at higher rpm (as pointed out ear-lier in text). Such a physical phenomenon (rupture ofglobules) has also been reported elsewhere[16–18].


The effects ofnon the separation of aniline for vary-ing Ch are graphically represented inFig. 7. The figureshows the identical effect ofnon the separation as exp-lained for the data onFig. 6. The data and the simula-tion results again match reasonably well, except athigher rpm. The discrepancy observed between theexperimental and model predicted values at higher rpmis due to the identical reason as explained for the dataon Fig. 6.

4.5. Effect of feed concentration (C0) on removal ofaniline

Finally, to determine the effects of feed concentra-tion on the exit concentration of aniline, the experi-ments were carried out for varyingC0 (0.002, 0.0015and 0.001 M) over a range ofτ (10–60 min), keeping

Fig. 8. Effect ofC0 on the removal of aniline (F:E = 15:1, stirrer speed= 209.44× 10−1 rad/s (200 rpm) andCh = 0.1N).

F:E, n andCh constant. InFig. 8, the experimental dataalong with the model predicted results for the removalefficiency are presented for varyingC0. As observedfrom the figure, the removal decreases with increasein C0 for all the values ofτ due to obvious reasons.The linear nature of the plot can be explained math-ematically fromEq. (13), which shows an inverselyproportional relationship betweenC0 and the percentremoval. The experimental and model predicted val-ues for the extent of removal are once again in goodagreement.

4.6. Residence time distribution

In order to establish the mixing characteristics (e.g.uniform mixing, dead-volume, etc.), a separate ex-perimental study was carried out to determine the


Fig. 9. RTD plots for MFR used in separation experiments (stirrer speed= 209.44× 10−1 rad/s (200 rpm)).

residence time distribution of the fluid within the re-actor (contactor). This was necessary to explain forthe over-prediction of the model, particularly at higherresidence time, as observed fromFigs. 3–5. The RTDexperiments were carried out as per the standard pro-cedure[21]. A tracer (K2Cr2O7 solution) was injectedas a pulse input (instantaneous addition) into the reac-tor. The operating conditions (F:E, n, etc.) were keptidentical as selected for the previous separation exper-iments, except this time there was no internal [H+]reagent (i.e. no reaction allowed in the contactor). Theexperimental RTD function,E(t), was determined andplotted for low (30 min) to high (60 min) residencetimes corresponding to the volumes of 127 and 255 cc,respectively. These RTD data were compared to the-oretical values for an ideal MFR, as shown by solid

dotted lines inFig. 9. From the figure, it may be ob-served that the experimental and theoretical values arein good agreement for the residence times of 30 and45 min. The marginal deviation observed during thelatter stages of the experiment may be due to the exis-tence of some dead or stagnant volume in the reactor,especially at the bottom of the reactor towards the cor-ners. As a consequence, any tracer elements presentin these volumes mix with the bulk phase only bydiffusion, relatively at a slower rate. This effect typ-ically becomes pronounced towards the latter part ofthe RTD experiment as also observed inFig. 9. Thisexplains the under-prediction of the experimental datain the figure by the theoretical model of an ideal MFR.The identical behavior is observed also in the caseof the RTD data obtained at higher residence time of


60 min. However, under these conditions the deviationof the theoretical values from the corresponding RTDdata is observed to be prevalent throughout the exper-iment, including during the beginning stages of theexperiment. This is attributed due to the existence ofnon-ideal conditions in the reactor in addition to thatof dead-volume, at a relatively larger reaction volume.As prevalent in most of the stirred tank reactors, thetangential flow patterns (circulatory path) responsiblefor non-mixing become more important in comparisonwith the radial and longitudinal currents, if the vol-ume of the reaction mixture (depth of the fluid in thereactor) becomes larger vis-à-vis the size and numberof impeller blade[21]. These effects are only qualita-tively discussed in this paper for the sake of brevity.

5. Conclusions

Aniline was shown to be effectively removed fromaqueous solution using ELM technique. The maxi-mum removal of aniline was observed to be 98.53%under the following conditions:F:E = 12:1, Ch =0.1 M, n = 418.88×10−1 rad/s (400 rpm),τ = 45 minandC0 = 0.001 M. The removal of aniline was foundto increase with increase in the internal reagent con-centration and stirrer speed, and with decrease inF:Eratio andC0. Beyond certain internal reagent concen-tration and stirrer speed, the removal efficiency leveledoff. A residence time of 45 min was found to be favor-able for the maximum removal of aniline in the con-tactor. A mathematical model developed in this workexplained the data reasonably well within the experi-mental error.

Nomenclature

C concentration of aniline in reactedcore (M)

C0 concentration of aniline in the feedphase (M)

Cf final concentration of aniline in the bulkfeed phase (M)

Cm concentration of aniline in the emulsionphase at the outer boundary (M)

C3m concentration of aniline in the continuousphase at the outer surface of emulsionglobules (M)

d Sauter mean diameter of the emulsionglobules (mm)

De effective diffusivity of aniline (m2/s)Di diffusivities of aniline inith phase (m2/s)DR impeller diameter (m)E volumetric flow rate of the emulsion

phase (m3/s)E(t) Residence time distribution (RTD)

function (min−1)F volumetric flow rate of the feed phase

(m3/s)g acceleration due to gravity (m/s2)J solute flux (mol/m2 s)k3 mass transfer coefficient of aniline in

continuous phase (m/s)KC equilibrium constant for the reaction

between aniline and HClMA molecular weight of aniline (kg/kmol)Msol molecular weight of solvent (kg/kmol)Mw molecular weight of water (kg/kmol)n stirrer speed (rpm)N number of emulsion globules in reaction

mixtureP power consumed by the agitator (W)r radial coordinate in the emulsion

globules (m)R Sauter mean radius of the emulsion

globules (mm)t time (s)T temperature (K)V total volume of the reaction mixture

(m3)V0 volumetric flow rate of the feed (m3/s)vA molal volume of aniline (m3/mol)Xe equilibrium conversion of aniline

Greek lettersα volume fraction of the receptor phase

in the emulsion phaseε void, i.e. fraction of continuous phase

in the reaction mixtureζ radius of unreacted core within the

emulsion globule (mm)


η viscosity of pure solvent (kg/m s)θi time of falling for theith liquid in the

Ostwald’s viscometer (s)µi dynamic viscosity ofith species/phase

(kg/m s)ρi density ofith species/phase

(kg/m3)σEM−3 interfacial tension between emulsion

and continuous phase (kg/s2)τ residence time of solute molecules

in the contactor (s)νi kinematic viscosity ofith species/phase

(m2/s)φ association factor for waterϕ partition coefficient between the

membrane and continuous phase

Subscripts1 receptor phase2 membrane phase3 continuous phaseA anilinee emulsion phasef feed phaseh hydrogen ions concentrationm membrane phasemix reaction mixturesol organic solventw water

Dimensionless numbersNRe Reynolds numberNSc Schmidt number

Appendix A

A.1. Viscosity of the emulsion phase[15]

The kinematic viscosity of the emulsion phase wascorrelated with the kinematic viscosity of the puremembrane phase as:

νEM = ν2 exp[5.32(α − 0.1)] (A.1)

The viscosity of pure membrane phase,µ2 was de-termined experimentally with the help of Ostwald’s

viscometer. In this experiment, the viscosity of puremembrane phase was related with that of water as[20]:

µ2 = µwρ2θ2

ρwθw

A.2. Size of the globules[15]

The Sauter mean diameter was calculated from thefollowing correlation:

d = 0.11× DR ×[n2D3

Rρ3

σEM−3

]−0.7

×[νEM

ν3

]0.32

×[

E

E + F

]0.1

(A.2)

whereDR is stirrer diameter= 0.028 m,n the stirrerspeed in rotation/s,ρ3 the density of the continuousphase at 298 K= 997.045 kg/m3, σEM−3 the interfa-cial tension between emulsion and continuous phase=5 mN/m2, νEM the kinematic viscosity of emulsion,m2/s,E the volumetric flow rate of the emulsion phase,m3/s, F the volumetric flow rate of the feed phase(continuous phase), m3/s.

A.3. Mass transfer coefficient in continuous phase[21]

The continuous phase mass transfer coefficientk3in highly agitated systems was calculated from thefollowing correlation:

k3N2/3Sc = 0.13

[(P/V)µ3

ρ23

]0.25

(A.3)

whereNSc is the Schmidt number= De/νC, P/V thepower dissipated by agitator per unit volume of re-action mixture, N/m2 s, µ3 the viscosity of continu-ous phase, kg/m s,ρ3 the density of continuous phase,kg/m3.

A.4. Partition coefficient

From the batch experiments, initial (C0) and finalvalues (Cf ) of the concentration of aniline were ob-tained. From these values the partition coefficient (ϕ′)


was calculated as (C0 −Cf )/Cf . In order to obtain thepartition coefficient of aniline between the exhaustedemulsion mixture and the continuous phase (ϕ), ϕ′ wasmodified as per the procedure adopted by Ho et al.[6] as:

ϕ = Vi + ϕ′Vm

Vi + Vm(A.4)

References

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Removal of aniline from aqueous solution in a mixed flow reactor using emulsion liquid membrane

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