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Magnetic field control of gas-liquid mass transfer in ferrofluids
Journal Pre-proofs Research articles Magnetic field control of gas-liquid mass transfer in ferrofluids Mikhail M. Maiorov, Dmitry Zablotsky, E. Blums, A. Lickrastina PII: DOI: Reference:
S0304-8853(19)32782-9 https://doi.org/10.1016/j.jmmm.2019.165958 MAGMA 165958
To appear in:
Journal of Magnetism and Magnetic Materials
Accepted Date:
6 October 2019
Please cite this article as: M.M. Maiorov, D. Zablotsky, E. Blums, A. Lickrastina, Magnetic field control of gasliquid mass transfer in ferrofluids, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/ 10.1016/j.jmmm.2019.165958
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier B.V. All rights reserved.
Graphical Abstract Magnetic eld control of gas-liquid mass transfer in ferro uids Mikhail M. Maiorov,Dmitry Zablotsky,E. Blums,A. Lickrastina
1
Highlights Magnetic eld control of gas-liquid mass transfer in ferro uids Mikhail M. Maiorov,Dmitry Zablotsky,E. Blums,A. Lickrastina Gradient magnetic eld can be used to eectively control the gas-liquid contact volume Foaming can be completely suppressed by the magnetic forces mediated by magnetic nanoparticles Mass transfer intensity is not aected by the magnetic eld
Magnetic eld control of gas-liquid mass transfer in ferro uids ,<
a
Mikhail M. Maiorov
a University
,
Dmitry Zablotsky
,<
a
,
E. Blums
a
and
A. Lickrastina
a
of Latvia, Jelgavas 3, LV-1004 Riga, Latvia
ARTICLE INFO
ABSTRACT
: ferro uid magnetic eld foaming mass transfer
Gas-liquid mass transfer plays a key role in a broad range of industrial processes. The magnetic eld control over the morphology of the gas-liquid interface and solute transport is an attractive feature if it can be realized e ciently. However, the magnetic properties of typical liquids and gases are rather weak. The experimental investigation is carried out to evaluate the eect of the magnetic eld, which is mediated by magnetic nanoparticles, on the gas-liquid mass exchange during the sparging run through a hydrocarbon ferro uid. The results indicate that the gradient eld is especially eective at controlling the gas-liquid contact volume: the foaming of the liquid during the sparging of gas has been completely suppressed by the magnetic forces without any reduction of the mass exchange e ciency.
Keywords
1. Introduction
Gasâliquid mass transfer is of great interest in chemical engineering. Gas-liquid contactors employing the dispersion of gas bubbles to facilitate the transfer of solutes are widely applied in bioprocessing (e.g. oxygenation of aerobic cultures), ue gas scrubbing, wastewater treatment or to conduct a broad range of industrial chemical reactions (e.g. oxidation, chlorination, hydrogenation etc.) [1]. The enhancement of mass transfer in multiphase contactors can be accomplished by increasing the interfacial area between gas and liquid phases and residence times. Hence, small bubble systems aord the desirable improvements to mass ux, but at the same time the use of small bubbles stimulates foaming, which is undesirable and has a potential to upset downstream processing [2]. Magnetic nanoparticles dispersed in a colloidally stable solution, so called ferro uid, aord unprecedented control of the uidity and mass transfer of carrier liquid [3]. It is known that the geometry of the ferro uid gas-liquid interface is sensitive to the applied magnetic eld, sometimes producing unusual shapes [3, 4], whereas the gradient magnetic eld strongly enhances the buoyancy of nonmagnetic shapes immersed within the ferro uid [3]. Hence, there is a real opportunity to aect the state of the gas-liquid interface by electromagnetic eld and ensure the desirable regime of the mass transfer operations as it is determined by the surface area and residence time [5]. Here we report the investigation of the mass exchange kinetics and foaming during the bubbling of gas through a volume of ferro uid and the eect of a strong gradient electromagnetic eld mediated by the dispersed colloidal magnetic nanoparticles as a potential method to introduce magnetic eld control in gas puri cation, transport and processing. Subject of the present work is experimental examination of the in uence of a magnetic eld on the removing of volatile components from the higher-boiling solvent (sparging). < Corresponding
author
[email protected] (M.M. Maiorov); [email protected] (D.
Zablotsky)
ORCID(s):
First Author et al.: Preprint submitted to Elsevier
2. Experimental 2.1. Ferro uid 2.1.1. Synthesis
The ferro uid employied for this study was produced by a method described previously [6]. Brie y, magnetic nanoparticles were synthesized by coprecipitation of ferric-ferrous salts with ammonium hydroxide. The synthetic route yields predominantly magnetite nanoparticles coated with an adsorbed layer of surfactant (oleic acid), which are then suspended in tetradecane. 2.1.2. Characterization
The synthesized colloidal solution was studied by vibrating sample magnetometry (vibrating sample magnetometer Lake Shore Cryotronics, Inc., model 7404VSM) - a noninvasive and non-destructive investigative technique for probing the susceptibility, magnetization or hysteresis of magnetic materials that can provide information on the nanoscale [7]. Brie y, a small sample ( 40 mg) of liquid material is magnetized within a region of uniform magnetic- ux density with variable strength (-10 kOeâ¦+10 kOe) and subjected to vibrational excitation at low frequency, and the magnetic moment (magnetization curve) of the sample is deduced. The method of determining the size distribution of magnetic nanoparticles from the magnetization curve [7, 8] is based on representing the corrected curve .H / in the form of a functional, whose kernel is the Langevin function L .x/ = coth x * 1 . In CGS system: 0 1 2 mH .H / = I f .m/ L dm (1) Ê 1 k T x
m
s
m
B
where f .m/ is the volume density of the distribution function of magnetic moments m of the particles and I is the spontaneous magnetization of the particle material. The particle sizes of the samples were determined by dynamic light scattering (DLS) (Zetasizer nano S90 Malvern Instruments), the particles were imaged by transmission electron microscopy (JEOL Transmission Electron Microscope with 100 kV electron acceleration). s
Page 1 of 4
Short Title of the Article
A
15 10 10
M, emu g-1
5
5
0
0
-5
-5
-10
-10 -1
-15 -10
B
0.3
-5
0 H, kOe
0
5
1
boil
10
C
b
2
0.2
boil
Ni
5 a
0.1
0 1
2
5 10 20 di, nm
50
IR
D 3
1
B
4 air Figure 1: Details of the gas-liquid mass exchange experiment
in ferro uids: A, left - TEM snapshot of the magnetic nanoparticles employed for magnetic eld control, showing good dispersity and roughly spherical shape. Scale bar is 100 nm. The formation of small aggregates is the consequence of the drying of the sample. A, right - measured magnetization curve of the ferro uid sample. The solid line is the simulated reconstruction based on magnetic granulometry showing a good agreement with experimental data. The inset shows a detailed view of the marked region. B - size distribution of (a) the magnetic core reconstructed from the magnetic granulometry analysis of the magnetization curve and (b) hydrodynamic size of the nanoparticles obtained by DLS analysis of the ferro uid sample. C - schematic view of the mass exchange testing apparatus: 1 - dehumidi er, 2 - peristaltic pump driving the air ow through the device, 3 - ferro uid mass exchange cell (test-tube), 5 - gas cell of the IR spectrometer, 4 - permanent magnet assembly providing the gradient eld for magnetic eld control. D - distribution of magnetic eld induction in the middle cross-section above the permanent magnet assembly (4 in C) scanned by a Hall eect sensor. The dimensions of the permanent magnets are 20 mm 15 mm. The dashed line marks the position of the test-tube containing the ferro uid sample.
2.2. Apparatus and measuring procedure The experimental apparatus for testing the eect of the magnetic eld on the ferro uid gas-liquid mass exchange is shown schematically in Fig. 1C. A hermetically sealed glass test-tube with a diameter 12 mm contains a 0.2 ml sample of synthesized ferro uid. A thin copper capillary with a diFirst Author et al.: Preprint submitted to Elsevier
ameter 1 mm pierces the top seal and passes down to the bottom of the test tube. The capillary is in turn connected to a peristaltic pump, which blows dehumidi ed air continuously through the capillary into the volume of ferro uid at a constant ow rate 0.23 ml/s. For testing the kinetics of the gas-liquid mass exchange a 0.01 ml aliquot of hexane is added to the ferro uid sample. The tetradecane is a high-boiling point (T ù 252 * 254ý C ) alkane hydrocarbon and therefore remains predominantly in the liquid phase. In turn, hexane is a low-boiling point (T ù 68ý C ) alkane, which evaporates rapidly at room temperature. The typical duration of a single experiment is approx. 2 hours, during which the added amount of hexane is fully vaporized. The air ow driven by the peristaltic pump travels upwards passing through the volume of the ferro uid and exits the glass test-tube through a second outlet. Then the exhaust is directed into a gas cell of the infra-red (FT-IR) spectrometer (Agilent Varian 640-IR). The IR absorption spectra of the e uent are collected every 2 minutes throughout the duration of the experiment, to determine the concentration of the volatile component in the air passing through the ferro uid during the experiment: the IR spectrum of the e uent contains characteristic C-H bands belonging to hexane. Their intensity depend on the experiment duration or quantity of the air passing through. The relative concentration of the hexane marker in the e uent is calculated from the spectral area of the C-H stretching band (approx. 2850-3000 cm-1 ), which is the most intense.
2.3. Magnetic system For testing the eect of the applied magnetic eld on the gas-liquid mass exchange the experiment is carried out in a similar manner using a magnetic system attached to the bottom of the test-tube. The magnetic eld con guration employed for this study is formed by two laterally conjoined SmCo permanent magnets (remanence B0 approx. 0.6 T) with dimensions 20 mm 15 mm (length L = 50 mm) and their vertical magnetization pointing in opposite directions, as shown in Fig. 1C and D. The magnetic assembly formed in this way produces the highest possible gradient eld along the axis of the test-tube, which is placed directly above the center of the assembly.
3. Results and Discussion
The TEM snapshots (Fig. 1A, left) indicate that the synthesized nanoparticles have mostly spherical shape and are rather monodisperse. The measured magnetization curve for a produced ferro uid sample (Fig. 1A, right) shows a characteristic superparamagnetic response with no hysteresis. The reconstruction using the method of magnetic granulometry quite accurately reproduces the experimental curve as evidenced by the solid line in Fig. 1A, right. The calculated distribution of the sizes of the magnetic core of the nanoparticles is narrow and varies from approx. 5 to 13 nm. The dynamic light scattering (DLS) yields the hydrodynamic size of the particles, which varies from approx. 7 to 20 nm. The Page 2 of 4
Short Title of the Article
c/c0
T, %
dierence is ascribed to the thickness of the surfactant coat- A 100 ing (approx. 2 nm) and the magnetically "dead" layer di90 rectly on the surface of the inorganic core [7]. The ferro uid is stable and of good quality and therefore suitable for the 80 experiment. 70 The con guration of the magnetic eld produced by the 60 magnetic assembly was scanned within its central cross-section using a Hall eect sensor and shown in Fig. 1D. Due to the 50 4000 symmetry of the magnetic system the induction vector above the center of the assembly, i.e. along the centerline of the test-tube, is parallel to the surface of the magnets. The soluB 0 tion of the Laplace problem for the magnetic eld induced by 1 a an in nite periodic system of permanent magnets with opposite orientations of magnetization suggests the exponenb tial law to attenuate the amplitude of the main harmonic of the horizontal magnetic eld induction B .z/ along the axis of the test-tube with distance z from the surface of the magnets: 0 0 B .z/ = B0 exp * z (2) a
where a is the width of the magnet pole. For the non-ideal nite assembly composed of just 2 magnets the correlation between the measured maximum value of the horizontal component of the magnetic eld induction and the distance to the surface of the magnets is also near-exponential. The coe cient in the exponent varies from approx. 0.15 for z ~ a, which is close to the theoretical value ( _a ù 0:16), to approx. 0.09 (at z = 20 mm). The example of the IR spectral analysis of the e uent is shown in Fig. 2A collected at dierent times during the experiment. The two readily identi able absorption bands at approx. 1350-1500 cm-1 and 2850-3000 cm-1 belong respectively to the C-H bending and the more energetic stretching vibrations of linear alkanes [9]. The intensity of the hexane bands show an increase at the beginning of the experiment that is followed by a decay of the spectral lines down to complete disappearance. While technically these bands are generic to alkanes, such as hexane or tetradecane, it is clear that here they are predominantly produced by the volatile hexane marker, whereas the high-boiling tetradecane is present in the e uent in trace amounts and is not detected by IR analysis. Hence, the change of intensity of these bands is associated with the evaporation of the hexane characterizing the rate of the mass exchange at the gas-liquid interface in the ferro uid during the sparging of air. The small absorption peak in the vicinity of 2341 cm-1 is ascribed to the presence of atmospheric CO2 in the e uent [10]. The application of the magnetic eld has no qualitative eect on the collected spectra and does not cause the appearance of additional absorption bands. Fig. 2B shows the evolution of the volatile hexane marker concentration in the exhaust of the apparatus during the massexchange experiment. The initial rapid increase of the hexane concentration is associated with the gradual lling of the gas probe by the e uent after the beginning of the experiment, whereas the subsequent slow decay is determined by the mass transfer operations within the ferro uid. The First Author et al.: Preprint submitted to Elsevier
C-H bending
a C-H stretching
b 1
3500
3000
3050 3000 2950 2900 2850 2800
2500
2000
2 1500
1000
500
1/λ, cm-1 t, min 30
60
C a
24 min
500 Vair, ml
b
48 min
1000
no field
Figure 2: Results of the mass-exchange testing in ferro uid: A
- characteristic IR transmittance spectra of the e uent measured at (a) 24 min and (b) 48 min after the beginning of the experiment. C-H bending (1) and stretching (2) vibrations of alkanes were identi ed, inset shows a detailed view of spectral band (1) used to measure hexane marker concentration in the e uent. B - kinetics of the volatile solute (hexane) concentration in the e uent measured by IR spectrometer as a function of the volume of transported gas (a) without the magnetic eld and (b) in gradient eld of the permanent magnet assembly. Markers shows the points where the spectra in A were collected; the black line is an exponential t to the concentration decay region. C - morphology of the gas-liquid interface in the ferro uid test-tube during the mass exchange experiment (a) without the magnetic eld and (b) in gradient eld showing a complete suppression of foaming by nanoparticle mediated magnetic eect.
decay of the solute concentration is near-exponential indicating that a single rate coe cient mass transfer × e* air (k = 7:2 10*3 ml-1 ) at the gas-liquid interface is in eect after the complete lling of the probe. The condition of ferro uid cell during the mass exchange experiment with and without the magnetic eld is shown in Fig. 2C. Visual inspection indicates a surprising eect of the gradient eld on the morphology of the gas-liquid interface. The total volume of the gas-liquid system, where the solute transfer takes place, is decreased by a factor of approx. 3-4 concurrently with a negligible detrimental eect on the intensity of the mass exchange, as indicated by Fig. 2B. The foaming of the liquid is completely suppressed by the action of the eld mediated by suspended magnetic nanoparticles. The impact of the magnetic eld on the bubble formation can be assessed by considering the modi cation to the buoyant force - the hydrostatic equation for the ferro uid placed in a gradient magnetic eld has an added contribution due to kV
Page 3 of 4
Short Title of the Article
the Kelvin's force [3]:
(P = g + 0 M ð(H ð
(3)
The density of the ferro uid is approx. 1070 kg/m3 . In the following it can be assumed that the magnetic eld B created within the test-tube by the permanent magnet assembly (Fig. 1D) varies predominantly with distance z from the surface of the magnets and is relatively uniform in the horizontal direction due to the con guration of the magnets and the curved bottom of the test-tube. According to Eq. 3 the augmentation of the buoyant force can be interpreted in terms of the eective gravitational acceleration g . Taking into account the variation of ferro uid magnetization (Fig. 1A, right) as a function of the magnitude of the applied eld and the measured eld distribution within the test-tube, a simple estimate shows that g varies from approx. 85g to 20g as the distance from the surface of the magnets z grows from 1 mm to 10 mm, which constitutes approx. 1 to 2 orders of magnitude increase of the buoyant force due to the gradient magnetic eld. While there is no reliable general correlation for the diameter of a rising gas bubble, which is aected by various experimental factors and operating parameters, some studies suggest that the maximum size of a stable bubble decreases as a function of the lifting force d × g *0 4 [1, 5, 11, 12], i.e. 3 to 6 times, due to the gradient magnetic eld, which seems reasonable (cf. Fig. 2C). Assuming that no deformation or compression of the bubbles takes place, the total surface area of the gas-liquid interface according to a simple estimate based on the conservation of the introduced gas volume should increase by approx. the same factor - 3 to 6 times. The increased contact area makes a signi cant impact on the mass exchange process, however, the enhanced buoyancy should also reduce the time the bubbles spend in the ferro uid and participate in the mass exchange. If the bubble travel path is too short to reach the terminal velocity 1 2 *1 u = 18 g d (ferro uid dynamic viscosity ù 2 mPa s), the residence time should scale as t × g *0 5 . The increase of the contact area and the decrease of the residence time can theoretically compensate, hence the gradient magnetic eld does not have an in uence on the intensity of the mass exchange. One of the eventual reasons may also be the achievement of equilibrium solute concentration in the air ow in both cases. Nevertheless, the impact of the gradient magnetic eld on the mode of the mass transfer process in the two-phase system within the mass exchange cell is very signi cant as the foaming is completely supressed and gasliquid contact volume is many times smaller when the magnet is set without appreciable reduction of the mass transfer intensity. ef f
ef f
is especially eective at controlling the gas-liquid contact volume: the foaming of the liquid during the sparging of gas has been completely suppressed by the magnetic forces without any reduction of the mass exchange e ciency. The magnetic eld control of the gas-liquid interface is an attractive feature for a broad range of industrial applications if it can be realized. While many gases and liquids posses magnetic properties, e.g. oxygen in air is slightly paramagnetic, whereas nitrogen is slightly diamagnetic, they are typically very weak. In turn, the present work demonstrates that the magnetic eld eect mediated by colloidal magnetic nanoparticles dispersed in su cient quantity in a liquid of interest allows for eective control of the morphology of the gas-liquid interface and the mode of mass transfer operations. Ferro uids can be prepared based on a broad range of solvents - both hydrocarbon and aqueous. Due to these properties the utilization of the ferro uids is a promising technology in the mass exchange techniques. For instance, it may be eective for puri cation of air and other gases from impurities (even radioactive ones), gas enrichment and processing, as well as gas transport inside ferro uid [13], gas recovery for analysis and other purposes. The superparamagnetic properties of ferro uids aord a possibility to eectively control the listed processes by the magnetic eld.
:
max
t
ef f
ef f
max
:
r
4. Conclusions
ef f
In summary, we have investigated the kinetics of the mass exchange at the gas-liquid interface of a hydrocarbon ferro uid carrying Fe3O4 nanoparticles and the eect of the magnetic eld. The results indicate that the gradient eld First Author et al.: Preprint submitted to Elsevier
References [1] C. Leonard, J.-H. Ferrasse, O. Boutin, S. Lefevre, A. Viand, Bubble column reactors for high pressures and high temperatures operation, Chem. Eng. Res. Des. 100 (2015) 391421. [2] R. Prudhomme, S. Khan (Eds.), Foams: Theory, Measurements and Applications, volume 57 of Surfactant Science, CRC Press, 1995. [3] E. Blums, A. Cebers, M. M. Maiorov, Magnetic Fluids, de Gruyter, New York, 1997. [4] I. Torres-Díaz, C. Rinaldi, Recent progress in ferro uids research: novel applications of magnetically controllable and tunable uids, Soft Matter 10 (2014) 85848602. [5] T.-J. Lin, K. Tsuchiya, L.-S. Fan, Bubble ow characteristics in bubble columns at elevated pressure and temperature, AIChE Journal 44 (1998) 545560. [6] G. Kronkalns, M. Kodols, M. Maiorov, Structure, composition and magnetic properties of ferro uid nanoparticles after separation, Latv. J. Phys. Tech. Sci. 50 (2013) 5661. [7] M. M. Maiorov, D. Zablotsky, E. Blums, A. Krumina, Model colloids to study surface ligand interactions in nanosized Fe3O4, IOP Conf. Ser.: Mater. Sci. Eng. 503 (2019) 012029. [8] M. M. Maiorov, E. Blums, K. Raj, Inverse task for evaluation of particle size distribution of polydisperse magnetic uids, Phys. Procedia 9 (2010) 7477. [9] J. Coates, Interpretation of Infrared Spectra, A Practical Approach, American Cancer Society, 2006. doi:10.1002/9780470027318.a5606. [10] G. Gonzalez-Gaitano, J. R. Isasi, Analysis of the rotational structure of CO2 by FTIR spectroscopy, Chem. Educ. 6 (2001) 362364. [11] V. M. Polunin, P. A. Ryapolov, E. A. Sokolov, Dynamics of the gas bubbles in the magnetic uid in the non-uniform magnetic eld, IOP Conf. Ser.: Mater. Sci. Eng. 581 (2019) 012037. [12] V. Bashtovoi, M. Kovalev, A. Reks, Instabilities of bubbles and droplets ows in magnetic uids, J. Magn. Magn. Mater. 289 (2005) 350352. [13] M. H. K. Darvanjooghi, M. Pahlevaninezhad, A. Abdollahi, S. M. Davoodi, Investigation of the eect of magnetic eld on mass transfer parameters of CO2 absorption using Fe3 O4 -water nano uid, AIChE Journal 63 (2016) 21762186.
Page 4 of 4
S0304-8853(19)32782-9 https://doi.org/10.1016/j.jmmm.2019.165958 MAGMA 165958
To appear in:
Journal of Magnetism and Magnetic Materials
Accepted Date:
6 October 2019
Please cite this article as: M.M. Maiorov, D. Zablotsky, E. Blums, A. Lickrastina, Magnetic field control of gasliquid mass transfer in ferrofluids, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/ 10.1016/j.jmmm.2019.165958
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier B.V. All rights reserved.
Graphical Abstract Magnetic eld control of gas-liquid mass transfer in ferro uids Mikhail M. Maiorov,Dmitry Zablotsky,E. Blums,A. Lickrastina
1
Highlights Magnetic eld control of gas-liquid mass transfer in ferro uids Mikhail M. Maiorov,Dmitry Zablotsky,E. Blums,A. Lickrastina Gradient magnetic eld can be used to eectively control the gas-liquid contact volume Foaming can be completely suppressed by the magnetic forces mediated by magnetic nanoparticles Mass transfer intensity is not aected by the magnetic eld
Magnetic eld control of gas-liquid mass transfer in ferro uids ,<
a
Mikhail M. Maiorov
a University
,
Dmitry Zablotsky
,<
a
,
E. Blums
a
and
A. Lickrastina
a
of Latvia, Jelgavas 3, LV-1004 Riga, Latvia
ARTICLE INFO
ABSTRACT
: ferro uid magnetic eld foaming mass transfer
Gas-liquid mass transfer plays a key role in a broad range of industrial processes. The magnetic eld control over the morphology of the gas-liquid interface and solute transport is an attractive feature if it can be realized e ciently. However, the magnetic properties of typical liquids and gases are rather weak. The experimental investigation is carried out to evaluate the eect of the magnetic eld, which is mediated by magnetic nanoparticles, on the gas-liquid mass exchange during the sparging run through a hydrocarbon ferro uid. The results indicate that the gradient eld is especially eective at controlling the gas-liquid contact volume: the foaming of the liquid during the sparging of gas has been completely suppressed by the magnetic forces without any reduction of the mass exchange e ciency.
Keywords
1. Introduction
Gasâliquid mass transfer is of great interest in chemical engineering. Gas-liquid contactors employing the dispersion of gas bubbles to facilitate the transfer of solutes are widely applied in bioprocessing (e.g. oxygenation of aerobic cultures), ue gas scrubbing, wastewater treatment or to conduct a broad range of industrial chemical reactions (e.g. oxidation, chlorination, hydrogenation etc.) [1]. The enhancement of mass transfer in multiphase contactors can be accomplished by increasing the interfacial area between gas and liquid phases and residence times. Hence, small bubble systems aord the desirable improvements to mass ux, but at the same time the use of small bubbles stimulates foaming, which is undesirable and has a potential to upset downstream processing [2]. Magnetic nanoparticles dispersed in a colloidally stable solution, so called ferro uid, aord unprecedented control of the uidity and mass transfer of carrier liquid [3]. It is known that the geometry of the ferro uid gas-liquid interface is sensitive to the applied magnetic eld, sometimes producing unusual shapes [3, 4], whereas the gradient magnetic eld strongly enhances the buoyancy of nonmagnetic shapes immersed within the ferro uid [3]. Hence, there is a real opportunity to aect the state of the gas-liquid interface by electromagnetic eld and ensure the desirable regime of the mass transfer operations as it is determined by the surface area and residence time [5]. Here we report the investigation of the mass exchange kinetics and foaming during the bubbling of gas through a volume of ferro uid and the eect of a strong gradient electromagnetic eld mediated by the dispersed colloidal magnetic nanoparticles as a potential method to introduce magnetic eld control in gas puri cation, transport and processing. Subject of the present work is experimental examination of the in uence of a magnetic eld on the removing of volatile components from the higher-boiling solvent (sparging). < Corresponding
author
[email protected] (M.M. Maiorov); [email protected] (D.
Zablotsky)
ORCID(s):
First Author et al.: Preprint submitted to Elsevier
2. Experimental 2.1. Ferro uid 2.1.1. Synthesis
The ferro uid employied for this study was produced by a method described previously [6]. Brie y, magnetic nanoparticles were synthesized by coprecipitation of ferric-ferrous salts with ammonium hydroxide. The synthetic route yields predominantly magnetite nanoparticles coated with an adsorbed layer of surfactant (oleic acid), which are then suspended in tetradecane. 2.1.2. Characterization
The synthesized colloidal solution was studied by vibrating sample magnetometry (vibrating sample magnetometer Lake Shore Cryotronics, Inc., model 7404VSM) - a noninvasive and non-destructive investigative technique for probing the susceptibility, magnetization or hysteresis of magnetic materials that can provide information on the nanoscale [7]. Brie y, a small sample ( 40 mg) of liquid material is magnetized within a region of uniform magnetic- ux density with variable strength (-10 kOeâ¦+10 kOe) and subjected to vibrational excitation at low frequency, and the magnetic moment (magnetization curve) of the sample is deduced. The method of determining the size distribution of magnetic nanoparticles from the magnetization curve [7, 8] is based on representing the corrected curve .H / in the form of a functional, whose kernel is the Langevin function L .x/ = coth x * 1 . In CGS system: 0 1 2 mH .H / = I f .m/ L dm (1) Ê 1 k T x
m
s
m
B
where f .m/ is the volume density of the distribution function of magnetic moments m of the particles and I is the spontaneous magnetization of the particle material. The particle sizes of the samples were determined by dynamic light scattering (DLS) (Zetasizer nano S90 Malvern Instruments), the particles were imaged by transmission electron microscopy (JEOL Transmission Electron Microscope with 100 kV electron acceleration). s
Page 1 of 4
Short Title of the Article
A
15 10 10
M, emu g-1
5
5
0
0
-5
-5
-10
-10 -1
-15 -10
B
0.3
-5
0 H, kOe
0
5
1
boil
10
C
b
2
0.2
boil
Ni
5 a
0.1
0 1
2
5 10 20 di, nm
50
IR
D 3
1
B
4 air Figure 1: Details of the gas-liquid mass exchange experiment
in ferro uids: A, left - TEM snapshot of the magnetic nanoparticles employed for magnetic eld control, showing good dispersity and roughly spherical shape. Scale bar is 100 nm. The formation of small aggregates is the consequence of the drying of the sample. A, right - measured magnetization curve of the ferro uid sample. The solid line is the simulated reconstruction based on magnetic granulometry showing a good agreement with experimental data. The inset shows a detailed view of the marked region. B - size distribution of (a) the magnetic core reconstructed from the magnetic granulometry analysis of the magnetization curve and (b) hydrodynamic size of the nanoparticles obtained by DLS analysis of the ferro uid sample. C - schematic view of the mass exchange testing apparatus: 1 - dehumidi er, 2 - peristaltic pump driving the air ow through the device, 3 - ferro uid mass exchange cell (test-tube), 5 - gas cell of the IR spectrometer, 4 - permanent magnet assembly providing the gradient eld for magnetic eld control. D - distribution of magnetic eld induction in the middle cross-section above the permanent magnet assembly (4 in C) scanned by a Hall eect sensor. The dimensions of the permanent magnets are 20 mm 15 mm. The dashed line marks the position of the test-tube containing the ferro uid sample.
2.2. Apparatus and measuring procedure The experimental apparatus for testing the eect of the magnetic eld on the ferro uid gas-liquid mass exchange is shown schematically in Fig. 1C. A hermetically sealed glass test-tube with a diameter 12 mm contains a 0.2 ml sample of synthesized ferro uid. A thin copper capillary with a diFirst Author et al.: Preprint submitted to Elsevier
ameter 1 mm pierces the top seal and passes down to the bottom of the test tube. The capillary is in turn connected to a peristaltic pump, which blows dehumidi ed air continuously through the capillary into the volume of ferro uid at a constant ow rate 0.23 ml/s. For testing the kinetics of the gas-liquid mass exchange a 0.01 ml aliquot of hexane is added to the ferro uid sample. The tetradecane is a high-boiling point (T ù 252 * 254ý C ) alkane hydrocarbon and therefore remains predominantly in the liquid phase. In turn, hexane is a low-boiling point (T ù 68ý C ) alkane, which evaporates rapidly at room temperature. The typical duration of a single experiment is approx. 2 hours, during which the added amount of hexane is fully vaporized. The air ow driven by the peristaltic pump travels upwards passing through the volume of the ferro uid and exits the glass test-tube through a second outlet. Then the exhaust is directed into a gas cell of the infra-red (FT-IR) spectrometer (Agilent Varian 640-IR). The IR absorption spectra of the e uent are collected every 2 minutes throughout the duration of the experiment, to determine the concentration of the volatile component in the air passing through the ferro uid during the experiment: the IR spectrum of the e uent contains characteristic C-H bands belonging to hexane. Their intensity depend on the experiment duration or quantity of the air passing through. The relative concentration of the hexane marker in the e uent is calculated from the spectral area of the C-H stretching band (approx. 2850-3000 cm-1 ), which is the most intense.
2.3. Magnetic system For testing the eect of the applied magnetic eld on the gas-liquid mass exchange the experiment is carried out in a similar manner using a magnetic system attached to the bottom of the test-tube. The magnetic eld con guration employed for this study is formed by two laterally conjoined SmCo permanent magnets (remanence B0 approx. 0.6 T) with dimensions 20 mm 15 mm (length L = 50 mm) and their vertical magnetization pointing in opposite directions, as shown in Fig. 1C and D. The magnetic assembly formed in this way produces the highest possible gradient eld along the axis of the test-tube, which is placed directly above the center of the assembly.
3. Results and Discussion
The TEM snapshots (Fig. 1A, left) indicate that the synthesized nanoparticles have mostly spherical shape and are rather monodisperse. The measured magnetization curve for a produced ferro uid sample (Fig. 1A, right) shows a characteristic superparamagnetic response with no hysteresis. The reconstruction using the method of magnetic granulometry quite accurately reproduces the experimental curve as evidenced by the solid line in Fig. 1A, right. The calculated distribution of the sizes of the magnetic core of the nanoparticles is narrow and varies from approx. 5 to 13 nm. The dynamic light scattering (DLS) yields the hydrodynamic size of the particles, which varies from approx. 7 to 20 nm. The Page 2 of 4
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c/c0
T, %
dierence is ascribed to the thickness of the surfactant coat- A 100 ing (approx. 2 nm) and the magnetically "dead" layer di90 rectly on the surface of the inorganic core [7]. The ferro uid is stable and of good quality and therefore suitable for the 80 experiment. 70 The con guration of the magnetic eld produced by the 60 magnetic assembly was scanned within its central cross-section using a Hall eect sensor and shown in Fig. 1D. Due to the 50 4000 symmetry of the magnetic system the induction vector above the center of the assembly, i.e. along the centerline of the test-tube, is parallel to the surface of the magnets. The soluB 0 tion of the Laplace problem for the magnetic eld induced by 1 a an in nite periodic system of permanent magnets with opposite orientations of magnetization suggests the exponenb tial law to attenuate the amplitude of the main harmonic of the horizontal magnetic eld induction B .z/ along the axis of the test-tube with distance z from the surface of the magnets: 0 0 B .z/ = B0 exp * z (2) a
where a is the width of the magnet pole. For the non-ideal nite assembly composed of just 2 magnets the correlation between the measured maximum value of the horizontal component of the magnetic eld induction and the distance to the surface of the magnets is also near-exponential. The coe cient in the exponent varies from approx. 0.15 for z ~ a, which is close to the theoretical value ( _a ù 0:16), to approx. 0.09 (at z = 20 mm). The example of the IR spectral analysis of the e uent is shown in Fig. 2A collected at dierent times during the experiment. The two readily identi able absorption bands at approx. 1350-1500 cm-1 and 2850-3000 cm-1 belong respectively to the C-H bending and the more energetic stretching vibrations of linear alkanes [9]. The intensity of the hexane bands show an increase at the beginning of the experiment that is followed by a decay of the spectral lines down to complete disappearance. While technically these bands are generic to alkanes, such as hexane or tetradecane, it is clear that here they are predominantly produced by the volatile hexane marker, whereas the high-boiling tetradecane is present in the e uent in trace amounts and is not detected by IR analysis. Hence, the change of intensity of these bands is associated with the evaporation of the hexane characterizing the rate of the mass exchange at the gas-liquid interface in the ferro uid during the sparging of air. The small absorption peak in the vicinity of 2341 cm-1 is ascribed to the presence of atmospheric CO2 in the e uent [10]. The application of the magnetic eld has no qualitative eect on the collected spectra and does not cause the appearance of additional absorption bands. Fig. 2B shows the evolution of the volatile hexane marker concentration in the exhaust of the apparatus during the massexchange experiment. The initial rapid increase of the hexane concentration is associated with the gradual lling of the gas probe by the e uent after the beginning of the experiment, whereas the subsequent slow decay is determined by the mass transfer operations within the ferro uid. The First Author et al.: Preprint submitted to Elsevier
C-H bending
a C-H stretching
b 1
3500
3000
3050 3000 2950 2900 2850 2800
2500
2000
2 1500
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500
1/λ, cm-1 t, min 30
60
C a
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500 Vair, ml
b
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Figure 2: Results of the mass-exchange testing in ferro uid: A
- characteristic IR transmittance spectra of the e uent measured at (a) 24 min and (b) 48 min after the beginning of the experiment. C-H bending (1) and stretching (2) vibrations of alkanes were identi ed, inset shows a detailed view of spectral band (1) used to measure hexane marker concentration in the e uent. B - kinetics of the volatile solute (hexane) concentration in the e uent measured by IR spectrometer as a function of the volume of transported gas (a) without the magnetic eld and (b) in gradient eld of the permanent magnet assembly. Markers shows the points where the spectra in A were collected; the black line is an exponential t to the concentration decay region. C - morphology of the gas-liquid interface in the ferro uid test-tube during the mass exchange experiment (a) without the magnetic eld and (b) in gradient eld showing a complete suppression of foaming by nanoparticle mediated magnetic eect.
decay of the solute concentration is near-exponential indicating that a single rate coe cient mass transfer × e* air (k = 7:2 10*3 ml-1 ) at the gas-liquid interface is in eect after the complete lling of the probe. The condition of ferro uid cell during the mass exchange experiment with and without the magnetic eld is shown in Fig. 2C. Visual inspection indicates a surprising eect of the gradient eld on the morphology of the gas-liquid interface. The total volume of the gas-liquid system, where the solute transfer takes place, is decreased by a factor of approx. 3-4 concurrently with a negligible detrimental eect on the intensity of the mass exchange, as indicated by Fig. 2B. The foaming of the liquid is completely suppressed by the action of the eld mediated by suspended magnetic nanoparticles. The impact of the magnetic eld on the bubble formation can be assessed by considering the modi cation to the buoyant force - the hydrostatic equation for the ferro uid placed in a gradient magnetic eld has an added contribution due to kV
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the Kelvin's force [3]:
(P = g + 0 M ð(H ð
(3)
The density of the ferro uid is approx. 1070 kg/m3 . In the following it can be assumed that the magnetic eld B created within the test-tube by the permanent magnet assembly (Fig. 1D) varies predominantly with distance z from the surface of the magnets and is relatively uniform in the horizontal direction due to the con guration of the magnets and the curved bottom of the test-tube. According to Eq. 3 the augmentation of the buoyant force can be interpreted in terms of the eective gravitational acceleration g . Taking into account the variation of ferro uid magnetization (Fig. 1A, right) as a function of the magnitude of the applied eld and the measured eld distribution within the test-tube, a simple estimate shows that g varies from approx. 85g to 20g as the distance from the surface of the magnets z grows from 1 mm to 10 mm, which constitutes approx. 1 to 2 orders of magnitude increase of the buoyant force due to the gradient magnetic eld. While there is no reliable general correlation for the diameter of a rising gas bubble, which is aected by various experimental factors and operating parameters, some studies suggest that the maximum size of a stable bubble decreases as a function of the lifting force d × g *0 4 [1, 5, 11, 12], i.e. 3 to 6 times, due to the gradient magnetic eld, which seems reasonable (cf. Fig. 2C). Assuming that no deformation or compression of the bubbles takes place, the total surface area of the gas-liquid interface according to a simple estimate based on the conservation of the introduced gas volume should increase by approx. the same factor - 3 to 6 times. The increased contact area makes a signi cant impact on the mass exchange process, however, the enhanced buoyancy should also reduce the time the bubbles spend in the ferro uid and participate in the mass exchange. If the bubble travel path is too short to reach the terminal velocity 1 2 *1 u = 18 g d (ferro uid dynamic viscosity ù 2 mPa s), the residence time should scale as t × g *0 5 . The increase of the contact area and the decrease of the residence time can theoretically compensate, hence the gradient magnetic eld does not have an in uence on the intensity of the mass exchange. One of the eventual reasons may also be the achievement of equilibrium solute concentration in the air ow in both cases. Nevertheless, the impact of the gradient magnetic eld on the mode of the mass transfer process in the two-phase system within the mass exchange cell is very signi cant as the foaming is completely supressed and gasliquid contact volume is many times smaller when the magnet is set without appreciable reduction of the mass transfer intensity. ef f
ef f
is especially eective at controlling the gas-liquid contact volume: the foaming of the liquid during the sparging of gas has been completely suppressed by the magnetic forces without any reduction of the mass exchange e ciency. The magnetic eld control of the gas-liquid interface is an attractive feature for a broad range of industrial applications if it can be realized. While many gases and liquids posses magnetic properties, e.g. oxygen in air is slightly paramagnetic, whereas nitrogen is slightly diamagnetic, they are typically very weak. In turn, the present work demonstrates that the magnetic eld eect mediated by colloidal magnetic nanoparticles dispersed in su cient quantity in a liquid of interest allows for eective control of the morphology of the gas-liquid interface and the mode of mass transfer operations. Ferro uids can be prepared based on a broad range of solvents - both hydrocarbon and aqueous. Due to these properties the utilization of the ferro uids is a promising technology in the mass exchange techniques. For instance, it may be eective for puri cation of air and other gases from impurities (even radioactive ones), gas enrichment and processing, as well as gas transport inside ferro uid [13], gas recovery for analysis and other purposes. The superparamagnetic properties of ferro uids aord a possibility to eectively control the listed processes by the magnetic eld.
:
max
t
ef f
ef f
max
:
r
4. Conclusions
ef f
In summary, we have investigated the kinetics of the mass exchange at the gas-liquid interface of a hydrocarbon ferro uid carrying Fe3O4 nanoparticles and the eect of the magnetic eld. The results indicate that the gradient eld First Author et al.: Preprint submitted to Elsevier
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