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Note

Experimental investigation of Marangoni effect in 1-hexanol/water system

Zhihui Wang, Ping Lu, Guangji Zhang, Yumei Yong, Chao Yang

n

, Zai-Sha Mao

National Key Laboratory of Biochemical Engineering, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 21 July 2010 Received in revised form 21 March 2011 Accepted 24 March 2011 Available online 31 March 2011 Keywords: Marangoni effect Mass transfer Surfactant Drop Interface Experiment

a b s t r a c t

In this work, the interfacial phenomena of a single hanging drop have been observed and captured by a Schlieren optical system. The extraction fractions at different hanging times were determined. For the system without surfactant, the Marangoni effect induced by interphase mass transfer of a solute displays regular convection patterns. The addition of surfactants changed the mode of interfacial instability signiﬁcantly but in different ways: SDS enhanced the mass transfer and Triton X-100 reduced the extraction fraction.

1. Introduction

The Marangoni effect, induced by interfacial tension gradient generated in the process of heat or mass transfer, exists in many industrial gas–liquid and liquid–liquid processes. As numerous reports manifested, the Marangoni effect takes on diversified forms, such as cellular convection, drop pulsation, localized eruption and kicking. And in the referenced investigations, it was realized that such interfacial hydrodynamic conditions chan-ged by the Marangoni effect would in turn affect mass transfer efficiency significantly (Mao and Chen, 2004; Mao et al., 2008;

Wang et al., 2008;Wegener et al., 2007).

Surfactants are commonly present in practical extraction systems. Due to the complexity of the interfacial phenomena, the effect of surfactant on Marangoni effect and mass transfer has not been well understood. However, plenty of researches have suggested that surfactants have noticeable inﬂuence on interfacial instability and mass transfer efﬁciency (Li and Mao, 2001;Li et al. 2003;Liang and Slater, 1990)

Most of the researches indicated that the presence of surfactants was disadvantageous in mass transfer processes, either due to the hydrodynamic effect (Beitel and Heideger, 1971; Lee et al. 1998;

Arendt and Eggers, 2007) or the formation of an interfacial barrier to the effective interphase mass transfer (Huang and Kintner,1969;

Chen and Lee, 2000). However, having studied ﬁve binary systems

with six surfactants,Agble and Mendes-Tatsis (2000)found that the ionic surfactant can introduce or intensify interfacial convection and hence increase mass transfer rate, while nonionic surfactants contributed the adverse effect. For ternary systems, although most investigations suggested at first the addition of surfactants created adverse effect on mass transfer efficiency, with further addition of surfactants the mass transfer coefficient would increase slightly, then it decreased to a minimum value rapidly (Chen and Lee, 2000). However, for the cases described above, the mass transfer rate with surfactants is still lower than that in the pure system. In the present experiments, we found that surfactants can increase the mass transfer rate even higher than that in the pure system under certain conditions. In this paper, the effects of surfactants on interfacial instability and interphase mass transfer were investigated.

2. Material and Methods 2.1. Material

The liquid–liquid extraction system in our experiments is 1-hexanol (drop phase, Sinopharm Chemical Reagent Co., AR, Z98.5%)/water (continuous phase, deionized water). The solute is acetic acid (Beijing Chemical Reagent Co., AR, Z99.5%), and surfactants (Farco Chemical Supplies, Hongkong, Z99%) added into the system are ionic surfactant sodium dodecyl sulphate (SDS) and nonionic surfactant Triton X-100. The properties of the 1-hexanol/ acetic acid/water system and surfactants are listed inTables 1and2, Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/ces

Chemical Engineering Science

n

Corresponding author. Tel.: þ86 10 62554558; fax: þ86 10 82544928. E-mail address: [email protected] (C. Yang).

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respectively. To avoid the additional mass transfer of solvent, both 1-hexanol and water were pre-saturated with each other before all experiments conducted (Table. 2).

2.2. Methods

All experiments were conducted in a rectangular quartz glass extraction cell at 2572 1C. As shown inFig. 1, a drop with almost the maximum volume but still attached on a needle was formed at the tip of the stainless needle (ID¼1.19 mm) in 2 s by a computer controlled precision syringe pump (Model 22, Harvard Apparatus, USA), i.e., after hanging for a speciﬁed time interval, the pump was run again to eject the hanging drop and form a new one. To observe and capture the interfacial phenomena, a Schlieren optical system with a video CCD Camera (Watec-902B) was set up, which was equipped with a microscope. For mass transfer measurement, a funnel was installed at the top of the extraction cell to collect about 20 drops to measure the sample concentration by GC (Shimadzu, Japan, 2014C). To get the ﬁguration and calculate the interfacial tension of a drop, the picture was analyzed by software FTA 32 (First Ten Angstroms, FTA 32 Video 2.0). In mass transfer measurements, the extraction fraction Ef was calculated as

fol-lows, where Cd,fis the mean concentration of the collected drops

just formed without hanging on the needle and Cd,tis the mean

concentration of the drops hanging at time t: Ef,t¼

Cd,fCd,t Cd,f

ð1Þ

3. Results and discussion 3.1. Previous model equations

A variety of equations can be found in the literature applied for the calculation of mass transfer into or out of droplets.Arendt and Eggers (2007) found that the internally circulating model pro-posed byKronig and Brink (1950)

Ef ,circ¼1 3 8 X1 n ¼ 1 B2 nexp

l

n 64Ddt d2 ð2Þ agreed well with their experiment results on pendant drops in case of Reynolds number smaller than one. The values for Bn and

l

n

were given byElzinga and Banchero (1959)as a function of Bi ¼kcd

Dd

ð3Þ where kcis the mass transfer coefﬁcient in the continuous phase,

Dd is the diffusion coefﬁcient in the drop phase and d is the

volume-equivalent spherical diameter of the drop. In this work. The volume-equivalent spherical diameters of the drops in differ-ent aqueous solutions are in the range 2.5–2.8 mm due to the interfacial tension changing with surfactants concentration.

We ﬁnd the model with eigenvalues selected at the following Biot numbers:

Bi ¼

57:37 CA,d¼3 g=L 180 CA,d¼15 g=L

1 CA,d¼20 g=L, or CA,d¼3 g=L with CSDS¼0:5g=L 8

> < >

: ð4Þ

ﬁts well with our experimental results, which demonstrates the resistance in the continuous phase is reduced due to the Marangoni convection intensiﬁed along with the increasing initial solute concentration.

To manifest the effect of Marangoni convection on mass transfer by comparing our experimental results with the predic-tion of the stagnant model at corresponding time instants, the enhancement factor F is calculated by

F ¼Ef ,t Ef ,S

ð5Þ The internally stagnant droplet model (for drop without Marangoni effect) given byGr ¨ober (1925)

Ef,S¼16 X1 n ¼ 1 Cnexp

c

2n 4Ddt d2 ð6Þ is used to estimate Ef,S. For long contact time, taking only the term

of n¼1 (

c

1¼3.142 and C1¼0.101) into account the equation can

be reduced to a constant Sherwood number kdﬃ 2

p

2 3 Dd d or Shd¼ kdd Dd ﬃ6:58 ð7Þ

in which kdis the mass transfer coefﬁcient in the dispersed phase.

In this model, the liquid inside the drop is assumed to be completely motionless, and the mass transfer takes place by

Table 1

Physical properties of the extraction system components at 25 1C. Material r(kg/m3

) m(mPa s) D (109

m2

/s)

Water 997.3 0.89 1.25

1-hexanol 813.6 4.44 E0.39 (Wilke and Chang, 1955) Acetic acid 1049.3 0.1229

Table 2

Critical micelle concentration (CMC) and molecular mass (M) of surfactants.

Surfactant CMC (kg/m3

) M (g/mol)

SDS 2.36 288.3

Triton X-100 0.15 646.85

Fig. 1. Sketch of experimental setup. 1—light, 2—convergent lenses, 3—slit, 4—fully reﬂecting mirror, 5—concave fully reﬂecting mirror, 6—extraction cell, 7—CCD camera, 8—syringe pump, 9—stainless steel needle, 10—glass funnel, 11—sample bottle and 12—computer.

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diffusion alone. The eigenvalues

c

n and Cn were also reported by

Elzinga and Banchero (1959). We calculated the prediction values with eigenvalues in case of Bi¼kcd/Dd¼shd(kc/kd)ﬃshd(Dc/Dd)ﬃ 21.

3.2. Pure system

As the photographs inFig. 2manifested, the solute Marangoni effect took on the mode of regular roll cells. The roll cells developed to larger scales as the solute transferred in acceleration through the interface and then decayed as the process of mass transfer proceeded to the end. Extraction fractions agree well with that predicted by Kronig and Brink’s model in the initial mass transfer period, but higher than experimental values when time longer than 40 s (CA,d¼3 and 15 g/L) or 50 s (CA,d¼20 g/L),

which demonstrates well the transient property of Marangoni effect. With higher solute concentration, the Marangoni convec-tion became stronger and lasted longer. Consistent with the development trend of interfacial convection, the extraction frac-tion increased with the increase of initial concentrafrac-tion as shown inFig. 3. Compared to the stagnant model, the largest enhance-ment factor F is 4.33 for the initial solute concentration in drop phase at 20 g/L and 2.77 for CA,d¼3 g/L.

3.3. Effect of surfactant

In the case of the initial concentration of solute acetic acid CA,d¼3 g/L, the effect of surfactants on interphase mass transfer

and Marangoni effect was investigated. As described byAgble and Mendes-Tatsis (2001), ionic surfactants can create Marangoni effect while nonionic surfactants cannot. As shown inFig. 4, the ionic surfactant can produce larger interfacial tension gradient over a wide concentration range, while the system involving a nonionic surfactant shows a very large initial gradient which rapidly ﬂattens off. Hence, the former can offer a sustainable change in interfacial tension to initiate or promote the Marangoni effect, while the latter is unlikely to provide sufﬁcient driving force.

For this system with surfactant, as shown inFig. 5, both ionic and nonionic surfactants dampened solute interfacial convection at low surfactant concentration, which matches the variation of extraction fraction shown in Figs. 6 and 7a. However, SDS

introduced new instability after 25 s while no interfacial instabil-ity observed for Triton X-100. The presence of SDS at high concentration induced the intensiﬁed interfacial instability accompanied with drop oscillation, and the extraction fraction increased with the increase of surfactant concentration as shown in Fig. 7b. The enhancement factor of extraction introduced by SDS at the concentration of 0.5 g/L is 3.37–6.3, larger than that provided by the solute at the initial concentration of 20 g/L in the pure system at the corresponding time. Meanwhile, increasing the concentration of Triton X-100 (CT) to CT¼0.5 g/L (higher than

Fig. 2. Effect of solute (acetic acid) concentration on interfacial phenomena of a single 1-hexanol drop hanging in water.

Fig. 3. Inﬂuence of solute concentration on extraction fraction.

Fig. 4. Equilibrium interfacial tension of 1-hexanol/water system versus surfac-tant concentration in the continuous aqueous phase.

Fig. 5. Effect of surfactant on interfacial instability of a 1-hexanol drop hanging in aqueous phase with interphase mass transfer (CA,d¼3 g/L).

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CMC), another mode of interfacial instability (which may be due to the Rayleigh effect caused by the density difference between the interfacial ﬁeld and the bulk phase) at the late period (around

45 s) of mass transfer was triggered, which looked like smoke rising up along the interface but no drop oscillation was observed as shown inFig. 5. Nevertheless, as shown inFig. 6, the extraction fraction is lower than the prediction from the stagnant model, because the introduced instability is too weak to counteract the increased interfacial barrier (the surfactant with long chain tends to produce larger interfacial barrier).

4. Conclusion

For the ternary system consisting of 1-hexanol/water with the solute of acetic acid, the Marangoni convection was intensiﬁed with the increase of initial solute concentration and ionic surfactant concentration. Marangoni convection introduced by the presence of SDS can also enhance mass transfer rate effec-tively. Based on the different effects of ionic surfactant SDS and nonionic surfactant Triton X-100 on interfacial instability and mass transfer rate, using surfactant to modify interfacial phenom-ena is a feasible way to manipulate the interphase mass transfer process.

Acknowledgements

Financial support from the National Science Fund for Distin-guished Young Scholars (21025627), the National Natural Science Foundation of China (nos. 20990224, 20976177), 973 Program (2010CB630904 and 2007CB613507), Beijing Natural Science Foundation (2112038) and the Jiangsu Province Projects (BY2009133 and BE2008086) is gratefully acknowledged. The ﬁrst author thanks Dr. Mirco Wegener, Prof. Anja R. Paschedag and Prof. Kai Sundmacher for the guidance and valuable advices on experimental work in Germany.

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