Liquid–liquid extraction intensification with magnetite nanofluid single drops under oscillating magnetic field

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JIEC-2055; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Liquid–liquid extraction intensification with magnetite nanofluid single drops under oscillating magnetic field Javad Saien *, Hamid Bamdadi, Shabnam Daliri Department of Applied Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran

A R T I C L E I N F O

Article history: Received 27 March 2014 Received in revised form 10 May 2014 Accepted 18 May 2014 Available online xxx Keywords: Mass transfer Nanofluid Magnetic field Liquid–liquid extraction Single drops

A B S T R A C T

The influence of an oscillating magnetic field was investigated in mass transfer from magnetite nanofluid single drops in liquid–liquid extraction process. The chemical system of toluene–acetic acid–water was used, while hydrophobic magnetite nanoparticles were present in the organic dispersed phase. The magnetic field intensity was within the range of 0.36–1.45 T, accompanied with different nanofluid concentrations within 0.001–0.005 wt%. Compared with no magnetic field, an average enhancement of about 65% and a maximum of 121% (for the smallest used drops) were achieved. This essential matter can be attributed to the induced nanoparticles micro-convection inside the drops. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Engineered suspensions of nanoparticles in liquids, known recently as nanofluids, have attracted a great attention due to their high potential for heat and mass transfer enhancement. These fluids contain nanoparticles smaller than 100 nm, belonging to a new type of composite materials [1]. In order to access lower operating costs, better performance, higher energy efficiency, and rapid advances in nanotechnology, wide researches on different applications of nanofluids in heat and mass transfer operations have been conducted [1–3]. In this regard, some research studies have focused on the thermal conductivity of ‘‘ferrofluids’’ which consist of suspension of ferromagnetic particles in nonmagnetic carrier fluids [4]. The nanoparticles of iron oxide have been extensively used as selected materials for ferrofluids [5]. Magnetite (Fe3O4) is a main iron oxide that fall under the category of super paramagnetic iron oxide behavior [6]. These materials can be magnetized when exposed to an external applied magnetic field and exhibit no hysteresis in their magnetization curve in the absence of the field [5]. Another advantage is their ease of separation after utilization. Effect of magnetic field orientation and its direction on the thermal conductivity enhancement have been confirmed in some accomplished works with magnetic nanofluids [4,7,8]. The enhancement of thermal conductivity in the presence of magnetic field could be attributed to the microscopic structure changes (i.e., * Corresponding author. Tel.: +98 811 8282807; fax: +98 811 8257407. E-mail addresses: [email protected], [email protected] (J. Saien).

aggregate/chainlike structures) occurring in magnetic nanofluids due to an external magnetic field parallel to temperature gradient [9]. Researches on application of nanofluids to improve mass transfer are still developing. The interests include different fields of dye diffusion in based nanofluids [7,10,11], mass transfer in gas– liquid operations [8,12,13] and liquid–liquid extraction [14,15]. In most reported works, mass transfer enhancement has been attributed to the micro-convection caused by Brownian motion [14]. In a work reported by Suresh and Bhalerao [16], in the field of gas–liquid process, it has been shown that 40–50% increase in mass transfer rate can be achieved by employing ferrofluids and causing the system to oscillate using an external magnetic field. Up to now, no investigation has been performed about the application of magnetic field in liquid–liquid extraction process. In this work, the influence of applying an external oscillating electromagnetic field on the mass transfer from organic drops, containing modified magnetite nanoparticles, is experimentally studied. For this aim, single drops are conducted in an extraction column equipped with two oscillating magnetic field generators. Under these conditions, the change in acetic acid solute concentration is followed consistently. Experimental Materials The chemical system of toluene–acetic acid–water [14,17] was used. The main specification of this system is its high interfacial

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Synthesis of mono-layer-coated magnetite nanoparticles

Nomenclature C d E EO¨ H kB K od mP M NPG Re t T ut

v We

solute concentration (wt%) drop diameter (mm) extraction fraction () ¨ tvO ¨ s dimensionless number (g Drd2 =g ) () EO dimensionless group defined by Grace et al. () Boltzman constant (1.3806488  1023 J K1) overall mass transfer coefficient (mm s1) particle mass (kg) Morton dimensionless number ( A g m4c Dr=r2c g 3 ) () inverse of Morton dimensionless number () drop Reynolds number (rut d=m) () drops contact time (s) temperature (K) terminal velocity (m s1) Brownian motion velocity (m s1) drop Weber number (rc u2t d=g )

Greek symbols g interfacial tension (mN m1) m viscosity (Pa s) r density (kg m3) D difference Subscripts and superscripts continuous phase, critical size c dispersed phase d final value f initial value i organic o overall dispersed phase value od terminal t w water equilibrium *

Preparation of nanofluids

tension. Toluene and acetic acid were purchased from Merck with purities of more than 99.9%. De-ionized water of high quality (electrical conductivity of 0.06 mS cm1) was used as the continuous phase. The analysis of collected samples containing acetic acid solute was carried out through simple titration method using O.1 M NaOH titrant (Merck) solutions. The materials used for preparation of nanoparticles including ferric chloride anhydrous (FeCl3), ferrous chloride tetra-hydrate (FeCl24H2O), ammonium hydroxide (30% NH3), and oleic acid (C18H34O2), ultra-pure (used for surface coating of particles); all were purchased from Merck company, and were used to prepare Fe3O4 nanoparticles. The physical properties of the liquid–liquid extraction chemical system at 20 8C are given in Table 1. It has to mention that the interfacial tension (measured with drop weight method [14]) did not vary markedly while the nanoparticles were present within the concentration range used in this work. Table 1 Physical properties of the used chemical system at 20 8C [14]. Property 3

Density, r (kg m ) Viscosity, m (mPa s)

Magnetite nanoparticles were prepared by the chemical coprecipitation method, reported by Ramirez and Landfester [18]. According to this method, FeCl24H2O (12.0 g) and FeCl3 (14.6 g) were dissolved in 50 mL de-ionized water and then 40 mL ammonium hydroxide were added rapidly. Since the surface of produced oxide nanoparticles is hydrophilic and incompatible with non-polar organic liquids, surface modification of magnetite particles was performed by functionalizing of them with a hydrophobic ligand. Oleic acid is often used to modify the surface of magnetite particles [19]. Therefore, after coprecipitation of magnetite particles, 3.0 g oleic acid was added and the suspension was heated to 70 8C for 30 min. In order to evaporate water and excess of ammonium, the temperature was increased to 110 8C. The black lump-like residuum was cooled to room temperature and then washed several times with distilled water and ethanol to remove the excess oleic acid. The modified product was obtained after perfect drying in an oven. Using scanning electron microscopy (SEM, WEGA II TESCAN), the surface morphology of nanoparticles was determined. In this regard, a 0.01 wt% particle suspension was placed on a coated gold grid and dried in air before SEM observations. X-ray diffraction (XRD, ITAL-Structures-APD 2000, Cu Ka1 X-ray source) was also used to determine the crystal structure of the magnetic particles as used by Wei et al. and Liu et al. [20,21]. To identify the bonding structure of functional group (oleic acid) on the nanoparticle surface, Fourier transform infrared (FT-IR) spectrometry (PerkinElmer Spectrum 65) was used. For this purpose, nanoparticles, both before and after modification, were dried and pelletized with KBr powder for the FT-IR study. Furthermore, the stability investigation of nanofluids (toluene + nanoparticle) was done via spectrometry analysis with a UV–visible spectrophotometer (JASCO V630). By monitoring the change in sample UV absorbance with time, the beam absorbance at maximum wavelengths of 300 nm was obtained for magnetite nanofluids.

Dispersed phase with Fe3O4

Continuous phase

873.05–876.69 9 (0.01) 0.589–0.597 (2  103)

998.17 (0.01) 1.003 (2  103)

Interfacial tension of pure toluene/water system, g: 39.5 (0.1) mN m1.

Magnetite nanofluid was prepared by dispersing weighed quantities of produced hydrophobic magnetite nanoparticles in toluene containing acetic acid. The mixture was sonicated in ultrasound bath (BSA-SK2-4.0L) for several minutes to break up any potential clusters of nanoparticles and to obtain uniform suspension. Magnetite nanoparticle concentrations of 0.001, 0.002, 0.003, 0.004 and 0.005 (104) wt% were prepared. Setup and operating procedure A Pyrex glass column (11.4 cm diameter and 51 cm height) was used as the contactor. The set-up (Fig. 1) operation to produce drops and mass transfer along column (without magnetic field) has been described in our previous works [14,22,23]. Different drop size was generated using a variety of glass nozzles, which were located at the bottom of this column. The toluene-phase nanofluid in a glass syringe was conducted to the column by an adjustable syringe pump (JMS SP-500, Japan) and through a rigid tube connected to the glass nozzle while the column was filled with aqueous phase. The oscillating magnetic field was established, just at the middle of the drops path along the column, by means of two similar coils (10 cm length of each) placed next to the column and in horizontal rectangular positions relative to each other. A power supply generating alternative current with adjusted frequency of 41 Hz (constant) and voltage within the range of 10–40 V was used for the aim of different magnetic fields generation. The provided

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Fig. 1. Details of experimental set up.

magnetic field intensity was determined from Faraday’s Law, based on measured electric potential with a secondary coil at the central position of the column where drops pass along and expose the magnetic field. The corresponding magnetic field intensities were 0.36, 0.73, 1.09 and 1.45 (0.03) T. The intensity was about 2 T just at the vicinity of the coils.

To find the most suitable place for the coils, positions at 15, 25 and 35 cm heights were tried, for typical generated drops from nozzle 1, with nanoparticle concentration of 0.004 wt% and magnetic field intensity of 0.73 T. The obtained overall mass transfer coefficients were 118, 135 and 128 (1.3) mm s1, respectively; indicating the midpoint position of 25 cm as the best choice.

Fig. 2. Schematic block diagram of steps of single drop experiments.

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4

100

(311)

intensity (a.u.)

80

60

40

(440) (220)

20

(511) (400)

(422)

0 10

20

30

40

2

50

60

70

80

(°)

Fig. 4. XRD patterns of modified magnetite nanoparticles. Fig. 3. SEM image of modified magnetite nanoparticles.

Drop formation, sampling, hydrodynamic and mass transfer measuring and the procedure of experiments were as described in our previous works [14,22,23]. A schematic block diagram for the steps of experimental procedure is shown by Fig. 2. Mass transfer direction was from dispersed phase to continuous phase. All experiments were conducted at the ambient temperature of 20  2 8C.

Results and discussion Characterization of nanoparticles and nanofluids The SEM image of modified magnetite nanoparticles is shown in Fig. 3 where particles are appeared uniform spherical shape with an average size of 17 nm. To identify the crystalline structure of the products Furthermore, the XRD pattern of modified Fe3O4 nanoparticles, was also prepared and presented in Fig. 4. The pattern agrees with those reported by Jingjing et al. [24] and Wei et al. [20]. The calculated value of particle diameter from the XRD spectrum, based on

Scherrer’s equation, is about 10 nm. Different average particle sizes have been reported in literature using these methods [25,26]. The oleic acid mono-layer coating was confirmed by the FT-IR spectra. The spectrum of modified Fe3O4 nanoparticles is compared with bare ones in Fig. 5. Three appeared bands at 586, 1637, and 3400 cm1 are corresponding to the bare Fe3O4 nanoparticles. The band at 586 cm1 appeared for the vibration of the Fe–O bond in the crystalline lattice of Fe3O4. The appeared bands at 1637 and 3400 cm1 in the spectrum (a) correspond to the hydroxyl groups which are attached to the particle surfaces during preparation of Fe3O4 nanoparticles by chemical co-precipitation in aqueous medium [19]. Compared with the spectrum of bare Fe3O4 structure, five new bands are appeared in the modified Fe3O4 particles spectrum. The band at 1408.9 cm1 is corresponding to the CH3 of oleic acid and those at 1437 and 1521 cm1 are attributed to the asymmetric and symmetric stretch vibration of COO group, and the bands at 2850 and 2920 cm1 are corresponding to the stretching modes of CH2 group in oleic acid, respectively [19]. The supramagnetic properties of Fe3O4 nanoparticles, generated with similar procedure used in this work, both in the bare state and with sodium oleate have been previously reported [27].

Fig. 5. FT-IR spectra of bare (a) and its modified (b) magnetite nanoparticles.

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JIEC-2055; No. of Pages 8 J. Saien et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 2 The range of generated drops size by the used nozzles.

100

99

A/A 0 (%)

5

98

97

96

95 0

20

40

60

80

100

120

time (min) Fig. 6. Relative absorbance of toluene nanofluid at l = 300 nm versus time. Nanoparticle concentration of 0.005 wt%.

Because of a linear relationship between the UV–vis absorbency intensity and the used concentration of nanoparticles; absorbency analysis was used to evaluate the stability of toluene nanofluids [28]. Accordingly, the maximum used nanoparticle concentration of 0.005 wt% was considered for measurement at wavelength of 300 nm which is a characteristic optical property of nanoparticles. As indicated in Fig. 6, the relative absorbance, A/A0 (A0 is the absorbance just after preparation of the nanofluid), decreases only about 5% after 110 min. It is while; run times for each series of experiments did not exceed 25 min after preparation of the nanofluids. This matter indicates a sufficient stability of the nanofluids. A similar procedure and result have been reported for SiO2 nanoparticles in kerosene, by Bahmanyar et al. [15].

The size of generated drops was within the range of 2.88–4.08 (0.01) mm (Table 2). The size of drops did not vary much with concentration of nanoparticles. It is due to the nearly constant interfacial tension and other physical properties of the phases given in Table 1. There was also no sensible change in drops velocity due to applying magnetic field along a short section of the column.

d (mm) (0.01)

1 2 3

2.88–2.92 3.34–3.37 4.00–4.08

Based on a number of existing criteria [29], the generated drops in this work lie within the conditions of circulating, since the value of the dimensionless group H defined by Grace et al. [30], 2 H ¼ ð4=3ÞEo¨ M0:149 ðmc =mw Þ0:14 , (Eo¨tvos number, Eo¨ ¼ g Drd =g , and Morton number, M ¼ g mc 4 Dr=rc 2 g 3 ) is within the range of 10.85–28.31 (2 < H < 59.3). Also the range of the drops Weber number (We ¼ d ut 2 rc =g ) is within 0.36–0.72 (We < 3.58) [31] and the range of the ratio Re=N0:15 PG , where NPG is the inverse of Morton number, is within 5.02–8.29 (less than 20) [29]. In these formulas, ut is terminal velocity, Dr ¼ rc  rd and indices c, d and w stand for continuous, dispersed and water cases. For circulating drops, the terminal velocity increases with drop size. Mass transfer also has influence in this regard. Henschke and Pfennig [32] illustrated the influence of mass transfer on the terminal velocity. Accordingly, as a more plausible reason, mass transfer induces turbulence and leads to a stochastic and irregular movement of the interface, which causes the drops to move rather slower. Mass transfer investigations Considering mass transfer during the measured contact time (t) for a drop size (d), the overall dispersed phase mass transfer coefficient is obtained from equation: K od ¼ 

Drops hydrodynamic investigations

Nozzle no.

d lnð1  EÞ 6t

(1)

where E is the extraction fraction, calculated from: E¼

C di  C d f C di  Cd

(2)

in which Cdi, Cdf, and Cd are drop side initial, final and equilibrium solute concentrations, respectively. For dispersed to continuous

300 0.001 wt% (nozzle 3)

250

0.002

Kod ( m.s–1)

0.005 0.001 wt% (nozzle 2)

200

0.002 0.005

150

0.001 wt% (nozzle 1) 0.002

100

0.005

50

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

magnetic field (T) Fig. 7. Variation of overall mass transfer coefficient with magnetic field intensity for different nozzles and nanoparticle concentrations.

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Fig. 8. Nanoparticles in drops without (a) and with applied magnetic field (b).

phase mass transfer direction Cd is zero, since the solute concentration in aqueous phase was zero. Meantime, the continuous phase is considered uniform in the bulk phase and thus has a unique bulk concentration. The slope of equilibrium curve (m), within the used concentration range (acetic acid concentrations less than 2.5 wt% in organic phase), is much low [14], and mass transfer resistance exists mainly in the organic phase [33–35]. It can be justified, considering the Whitman two film theory, where the overall mass transfer resistance is obtained from sum of the local mass transfer resistance in phases, i.e. 1=K od ¼ 1=kd þ m=kc , where kd and kc are dispersed and continuous phase mass transfer coefficients. Since m=kc is negligible, so K od ’ kd . As Fig. 7 presents, the rate of mass transfer increases with magnetic field intensity within the range used, and a mild variation is observed as the intensity tends to about maximum used intensity of 1.45 T. The obtained experimental Kod values are within (33–153), and (33–154  1.3) mm s1. Compared with no magnetic field (i.e. just with nanoparticles) an average enhancement of about 65% is achieved for different magnetic field intensities and for different nanparticle concentrations. Significant enhancement, as great as 121% in overall mass transfer coefficient, is achieved with the magnetic field intensity of 1.45 T, and nanoparticles concentration of 0.005 wt% for the used smallest drops. This observation is presumably due to Brownian motion and consequently micro-convection

intensification because of oscillating magnetic field. Brownian motion is basically the random dynamic mode of particles in liquids where the particles move and collide each other [36], leading to mass and heat transfer enhancement in nanofluids. Two magnetic field sources in this work give different direction intensified movements of nanoparticles and consequently mass transfer improvement. A scheme of the magnetic field impact and the induced nanoparticles motion inside drops is presented in Fig. 8. The variation of overall mass transfer coefficient for two nozzles is shown in Fig. 9. An ascending variation of Kod is first observed for nanoparticle concentrations up to about 0.002 wt%, and then a descending variation is relevant in agreement with observations in our previous work [14]. Increase in nanoparticle concentration, up to a certain value, leads to enhance mass transfer coefficient due to Brownian motions and subsequent micro-convection [37]. Higher nanoparticle concentrations cause a decrease in the particles suspension and possibility of aggregation. As hydrophobic nanoparticles contact each other, the tails of the oleic acids are physically absorbed on the primary layer by forming an interpenetrating layer with the tails of the primary layer and leads aggregation [19] followed by a reduction inffi the global pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Brownian motion velocity according to v ¼ 3kB T=mP , [38], where v is the root-mean-square velocity of a Brownian particle with the mass of mP, kB is the Botzman constant and T is temperature. Although magnetic field assists the function of nanoparticles

320

280

1.45 T (nozzle 5)

240

Kod ( m.s–1)

1.09 0.73

200

0.36 0

160

1.45 T (nozzle 1) 1.09 0.73

120

0.36 0

80

40

0 0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Cnanoparticle (wt%) Fig. 9. Variation of overall mass transfer coefficient with nanoparticle concentrations for two nozzles and different magnetic field intensities.

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320

270

0

7

T (0.001 wt%)

0.73 1.45 0

T(0.002 wt%)

0.73 1.45

Kod ( m.s–1)

220

0

T (0.005 wt%)

0.73 1.45 170

120

70

20 2.75

3.00

3.25

3.50

3.75

4.00

4.25

d (mm) Fig. 10. Variation of overall mass transfer coefficient with drop size for different magnetic field and nanoparticle concentrations.

0.9

0.8

0.7

E

0.6

0.5 0.001 wt% (nozzle 3) 0.002 0.005

0.4

0.001 wt% (nozzle 2) 0.002 0.005

0.3

0.001 wt% (nozzle 1) 0.002 0.005

0.2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

magnetic field (T) Fig. 11. Variation of extraction fraction with magnetic field for different nanoparticle concentrations and nozzles.

within all concentration range; however, the influence is more for the small drops with the highest used concentration of 0.005 wt% (an average enhancement of 104%). The variation of mass transfer coefficient with drop size should also be considered. As shown in Fig. 10, the mass transfer coefficient increases with drop size (under constant conditions) due to tendency of drops to higher internal circulation or turbulence as their size increase, and so, the rate of mass transfer increases. In addition, terminal velocity for larger drops is more, and therefore lower contact time provides higher overall mass transfer coefficients according to Eq. (1). Another involved parameter which has to be regarded, is the extraction fraction [Eq. (1)]. As shown in Fig. 11, the extraction

fraction increases with magnetic field. The average relative enhancement is about 34.8%. As expected, small drops are more benefited under specified magnetic field intensities because of lower moving velocity and subsequently longer time exposure to the magnetic field. Conclusions Although mass transfer enhancement in the presence of just nanoparticle is significant [14,15]; using magnetic field, in a short section of extraction column, can intensify this effect due to presumably induced stronger micro-convection. It is practically favorite, using magnetite nanoparticles and then easily separation

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from suspension by means of a simple magnet. The range of magnetic field intensity was within (0.361.45) T, accompanied with magnetite nanoparticle concentration of within (0.001– 0.005) wt%. For a specified nanoparticles concentration, small drops were found to benefit more. An average enhancement of about 65% was achieved in the rate of mass transfer and a maximum of 121% for the smallest drops, with 1.45 T magnetic field and nanoparticle concentration of 0.005 wt%. The extraction fraction plays the most important role in the overall mass transfer enhancement. Acknowledgment The authors wish to acknowledge the university authorities for providing the financial support to carry out this work. References [1] R. Saidur, K. Leong, H. Mohammad, Renew. Sustain. Energy Rev. 15 (2011) 1646. [2] S. Kakac¸, A. Pramuanjaroenkij, Int. J. Heat Mass Transfer 52 (2009) 3187. [3] J. Buongiorno, D.C. Venerus, N. Prabhat, T. McKrell, J. Townsend, R. Christianson, Y.V. Tolmachev, P. Keblinski, L.W. Hu, J.L. Alvarado, J. Appl. Phys. 106 (2009) 094312. [4] R. Ganguly, S. Sen, I.K. Puri, J. Magn. Magn. Mater. 271 (2004) 63. [5] J. Jang, H. Lim, Microchem. J. 94 (2010) 148. [6] H. El Ghandoor, H. Zidan, M.M. Khalil, M. Ismail, Int. J. Electrochem. Sci. 7 (2012) 5734. [7] J. Veilleux, S. Coulombe, J. Appl. Phys. 108 (2010) 104316. [8] J.K. Kim, A. Akisawa, T. Kashiwagi, Y.T. Kang, Int. J. Refrig. 30 (2007) 1086. [9] I. Nkurikiyimfura, Y. Wang, Z. Pan, Exp. Therm. Fluid Sci. 44 (2013) 607. [10] X. Fang, Y. Xuan, Q. Li, Appl. Phys. Lett. 95 (2009) 203108. [11] S. Krishnamurthy, P. Bhattacharya, P. Phelan, R. Prasher, Nano Lett. 6 (2006) 419.

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