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Effects of organic solvent and ionic strength on continuous demulsification using an alternating electric field
Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 228–233
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Effects of organic solvent and ionic strength on continuous demulsification using an alternating electric field Akinori Muto ∗ , Yuichi Hiraguchi, Koichiro Kinugawa, Tomoyuki Matsumoto, Yuki Mizoguchi, Hayato Tokumoto Department of Chemical Engineering, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• We • • • •
investigated demulsification using an alternating electric field. A square waveform alternating electric field was more effective. A greater relative permittivity promotes the demulsification. Demulsification rates changed with ionic strength of the aqueous phase. Demulsification using an electric field was effective for W/O emulsions.
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 6 June 2016 Accepted 21 June 2016 Available online 22 June 2016 Keywords: Alternating electric field Relative permittivity Organic solvent Electrolyte Ionic strength W/O emulsion
a b s t r a c t We herein report the effects of the following variables on demulsification using an alternating electric field: (1) type and volume fraction of an organic solvent, and (2) type and ionic strength of an electrolyte in water. Ethylbenzene, toluene, o-xylene, m-xylene, p-xylene, cyclohexane, and n-hexane were employed as organic solvents. The effect on demulsification of either KCl or CaCl2 addition to the aqueous phase was also examined. We observed that a square waveform alternating electric field was more effective than a sine waveform. In addition, the demulsification rate varied significantly upon altering the volume fraction of the organic solvent and the ionic strength of the electrolyte. Demulsification using an alternating electric field was effective for W/O type emulsions. Demulsification was promoted using an organic solvent with greater relative permittivity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the worldwide production of emulsions for use as industrial chemicals [1], cosmetic additives [2], food additives [3], and medicinal materials [4] has become widespread. However, emulsions are often generated as unwanted byproducts from the treatment of oil and aqueous solutions. Emulsions often form upon mixing of the organic and aqueous phases, thus rendering
∗ Corresponding author. E-mail address: [email protected] (A. Muto). http://dx.doi.org/10.1016/j.colsurfa.2016.06.037 0927-7757/© 2016 Elsevier B.V. All rights reserved.
separation of these phases difficult and time consuming, often requiring bulky equipment. Consequently, the development of a rapid, convenient, and continuous demulsification method would be of enormous benefit to many chemical processes. To date, several demulsification methods have been developed, such as heating [5,6], centrifugation [7,8], chemical treatment [9–11], and electric field treatment [12–20]. Electrical separation in particular is often favored for the following reasons: (i) No additional material is added, thus eliminating the requirement for further preor post-treatment; (ii) the demulsification process can be carried out at 20–25 ◦ C; and (iii) the demulsification device is structurally and operationally simple. However, this process also has several
A. Muto et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 228–233
disadvantages, such as the requirement for a high electric voltage and the possible occurrence of electrochemical reactions when the electrode is in direct contact with the emulsion. If these issues could be overcome, the electric field method could potentially yield good demulsification performances. We previously investigated the demulsification of a water-in-toluene emulsion by the application of an alternating current (AC) electric field [21]. A fluorocarbon tube of 1 mm diameter was used for the emulsion flow. The waterin-toluene emulsion was smoothly separated into clear water and toluene phases. Thus, we chose to investigate the effects of the following variables on demulsification using an alternating electric field: (1) Type of organic solvent in the emulsion; (2) volume fraction of the organic solvent; and (3) type and ionic strength of the electrolyte dissolved in water. Finally, a suitable emulsion structure for the demulsification using an alternating electric field is examined.
2. Materials and methods 2.1. Reagents All reagents were special grade and were purchased from Wako Pure Chemical Industries Ltd. A water/organic solvent emulsion was prepared in the presence of surfactant using a homogenizer (IKA, Germany, T18BS1). Ethylbenzene, toluene, o-xylene, m-xylene, p-xylene, cyclohexane, and n-hexane were employed as organic solvents. Sorbitan monooleate (Span® 80, reagent grade) was used as a surfactant, and was added to the organic solvent (4 vol%) prior to emulsion preparation. Potassium chloride and
229
calcium chloride were used as additives to the aqueous phase. Distilled and deionized water was employed in all experiments. 2.2. Experimental setup The experimental setup used in this study is outlined in Fig. 1. The apparatus comprises five devices, namely a plunger pump, a demulsification device, a measuring cylinder, a voltage supply, and a function generator. Two copper plates (100 mm × 100 mm × 5 mm) were used as electrodes and were connected to a voltage supply (HEOP-3B10, Japan, Matsusada Precision Inc.) and a wave generator (WF1974, Japan, NF Corporation) to control the waveform and the frequency of the alternating electric field. A fluorocarbon tube (Code No. 1024-01, Japan, ARAM Corporation; internal diameter 1 mm, external diameter 3 mm, length 760 mm), through which the emulsion flowed, was inserted between the two electrodes responsible for application of the electric field. The emulsion was fed into the fluorocarbon tube using a dual plunger pump (YMCK-11-13-P, Japan, YMC Co. Ltd.). The flow rate, and therefore the retention time of the device, was selected to correspond to the duration of the applied electric field. The output was collected in a measuring cylinder, thus allowing the volumes of the oil phase, emulsion phase, and aqueous phase to be measured. 2.3. Experimental procedure The surfactant was dissolved in the organic solvent, and calculated amounts of the organic solvent and water (total volume = 100 mL) were stirred at 24000 rpm for 1 min using a homogenizer to yield the stable emulsion. From preliminary
Fig. 1. Schematic representation of the demulsification device.
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Organic solvent
Pure water
Addition of Span® 80, 4 vol%
Addition of Electrolyte; KCl or CaCl2
Homogenization (24,000 rpm, 1 min)
Emulsion Fig. 2. Outline of the emulsion preparation method.
experiments, we found that a reaction time of 185 s was sufficient to demulsify a test mixture. Consequently, the emulsion was fed into the demulsification device at constant flow rates of 20, 60, 100, 150, and 200 mL/h using a plunger pump. The preliminary experiment confirmed that the plunger pump did not affect demulsification. All emulsions demulsified to approximately ∼5 vol% after 24 h, with no significant changes being observed over 14 days. Demulsification experiments were carried out within 2 h of preparation to prevent spontaneous demulsification taking place. The volumes of the liquid output and the oil phase generated by demulsification were directly measured in the measuring cylinder. The demulsification experiment was conducted using a 1000 kV/m electric field oscillating as a square wave at a frequency of 100 Hz. All demulsification experiments were conducted under these conditions unless otherwise specified. The demulsification rate was defined according to Equation (1): =
Voil V0, oil + V0, water
(1)
where Voil is the volume of the separated organic phase, and V0,oil and V0,water are the original volumes of the organic solvent and water, respectively, prior to mixing. The water content of the oil phase was analyzed by Karl Fischer titrations (KF-21 moisture meter, Japan, Mitsubishi Chemical Analytec). 2.4. Determination of emulsion type The prepared W/O or O/W emulsion was then examined using the dilution method [22]. A drop of each emulsion was added either to water or to the organic solvent used for preparation of the emulsion. The emulsion drop was observed to determine whether it remained as a droplet or whether it was dispersed in either water or the organic phase (Fig. 2). 3. Results and discussion 3.1. Effect of alternating frequency on demulsification efficiency for the hexane-based emulsion An emulsion containing 50 vol% hexane was prepared according to the procedure outlined above. Hexane was selected as it is a typical aliphatic organic solvent used widely in the chemical industry. The emulsion was fed to the demulsification device at a constant flow rate and was applied to two types of alternating electric field, i.e., a square waveform or a sine waveform, for 62 s. The effect of the alternating electric field of the square and sine waveforms on the demulsification rate are shown in Fig. 3, where 0 Hz corresponds to a direct electric field. Upon increasing the frequency of the square waveform to 2 Hz, the demulsification rate
Fig. 3. Demulsification of a hexane/water emulsion (50 vol% hexane, electric field for 62 s).
increased from ∼5% to ∼50%, while the demulsification ratio of the sine waveform (at 2 Hz) increased to only ∼7%. However, for both waveforms, the demulsification rate increased with increasing frequency. At frequencies >8 Hz, the demulsification rate of the square waveform remained relatively constant at ∼80%. Indeed, the square waveform was more effective than the sine waveform, suggesting that a drastic change in the magnitude and direction of the electric field is important in the demulsification process. Upon reversing the electric field suddenly, vibration of the dispersed phase took place, resulting in collisions within the dispersed phase, promoting coalescence. 3.2. Effect of organic solvent volumetric fraction on demulsification The emulsion structure was examined to determine the reason for the dramatic variation in demulsification rate upon changing the volume fraction of the organic solvent. Fig. 4 shows the results of the demulsification experiments carried out using the p-xylene and toluene emulsions. Photographic representations of the dilution method are also shown. High demulsification rate emulsions represent the W/O type emulsion, while the O/W type emulsion exhibited no demulsification. Comparable results were obtained for other organic solvents, suggesting that electric field demulsification is effective for W/O type emulsions. The effect of the solvent volume fraction on demulsification was then examined. Fig. 4 shows the effect of the volume fraction when toluene or p-xylene was employed as the solvent. Demulsification did not occur at volume fractions <20 vol%. However, upon increasing the p-xylene volume fraction to 20 vol% or the toluene volume fraction to 30 vol%, the demulsification rate increased sharply to 80%. To clarify the reason for this remarkable increase in demulsification rate, the structure of the emulsion was examined using the diffusion method, where drops of each emulsion were added either to water or to the appropriate solvent (see corresponding photographic images in Fig. 4). The emulsions that were demulsified by the alternating field method dispersed well in water, but did not disperse in the organic solvent. This indicates that these emulsions are O/W (oil-in-water) type. In general, the opposite was observed for the emulsions that exhibited good demulsification. For example, Eow et al. [14] reported that “For drops with a permittivity greater than that of the suspension medium, as in the case of water drops in oil, they move toward the place of greatest field intensity. This does not require charged particles and it depends on the force felt by all polar materials when in a non-uniform electric field.” Our
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Fig. 4. Effect of emulsion structure on the relationship between demulsification and volume content of the organic solvent.
results therefore implied that these emulsions were W/O (waterin-oil) emulsions, indicating that demulsification by an electric field is optimal for W/O emulsions. Reversal of the dipole moment of the dispersed phase by an alternating current promotes coalescence, i.e., demulsification and fusion of the dispersed droplets. 3.3. Effect of organic solvent on demulsification efficiency The effect of the type of organic solvent on demulsification was then examined. Experiments were carried out using a range of aliphatic and aromatic organic solvents with varying relative permittivities, as shown in Table 1. The dispersion phase diameter of all emulsions was 5–20 m. Fig. 5 shows the effect of the oil phase volume fraction on demulsification using various solvents. In all organic solvents, the demulsification rate varied significantly with a change in the oil phase volume fraction, although the effect differed slightly between the different types of solvent. This suggests that the demulsification rate is affected by the structure and physical properties of the emulsion or the physical properties of the organic solvent. As shown in Fig. 5, the demulsification rate rose rapidly upon increasing the oil phase volume fraction of the emulsion. The initial and final demulsification rates (EL and EH ) shown in the figure represent the oil phase volume fractions VL and VH of the emulsion. In practice, EL is close to zero, and so the critical volume fraction (VC ) of the oil phase upon demulsification can be defined by Equation (2): VC = VL + (VH − VL )/2
The demulsification rate, E, corresponded to the VC expressed by the EC , which gave the relationship indicated in Fig. 6 when EL = 0. The value of the critical volume fraction VC therefore indicates the volume fractions where the emulsion type is W/O (water-in-oil) or O/W (oil-in-water). The relationship between the relative permittivity of the organic solvent used in the preparation of the emulsion and the critical volume fraction is shown in Fig. 7. When the relative permittivity of the organic solvent increased, the critical volume fraction also increased, indicating that the organic solvent was easily polarized with an increase in relative permittivity. These results suggest that a greater relative permittivity promotes demulsification using an alternating electric field. 3.4. Effect of aqueous phase ionic strength on demulsification Either potassium chloride or calcium chloride were dissolved in the aqueous phase to enhance the dielectric polarization of the dispersed phase, and the effect of the aqueous phase ionic strength on the demulsification is shown in Fig. 8. With an oil phase volume fraction of 16.7 vol%, demulsification was not observed for any of the organic solvents tested in the absence of electrolyte in the
(2)
Table 1 Physical properties of the organic solvents at 293 K [23]. Material
Relative permittivity
o-xylene ethyl benzene toluene m-xylene p-xylene cyclohexane n-hexane
2.562 2.446 2.379 2.359 2.274 2.024 1.887
Fig. 5. Effect of different types of organic solvent and volume contents on the demulsification rate (electric field = square waveform, 100 Hz, 62 s).
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100
(+
Demulsification η [%]
80
Linear approximation
60
Critical volume content of organic phase; Vc
(+
40
20
(/
0 0
20
40
60
Volume content of organic solvent [vol%] Fig. 6. Concept and evaluation of the critical volume content (VC ) of the organic phase.
Fig. 7. Effect of relative permittivity of the organic solvent on the critical volume fraction.
100
Demulsification
[%]
80
m-xylene p-xylene cyclohexane
n-hexane
60
n-hexane cyclohexane
40
m-xylene p-xylene
0.00
0.05
0.10
0.15
aqueous phase. The black legends show the results obtained upon dissolving KCl in the aqueous phase, while the white legends show the results obtained using CaCl2 . In both cases, the demulsification rates varied dramatically with the ionic strength of the aqueous phase. This suggests that dissolution of the electrolyte promotes demulsification due to enhancement of the dielectric polarization of the aqueous phase in the emulsion, and also due to the difference in ionic behavior between CaCl2 and KCl. For example, Ca2+ affects the oil-water interface in the interface structure due to ionic interactions. The critical ionic strength, IC , was then determined for KCl and CaCl2 , and the relationship between the relative dielectric constant of the organic solvent and the IC is shown in Fig. 9. When KCl was dissolved in the aqueous phase, the critical ionic strength increased with an increase in the relative permittivity of the organic solvent. Upon application of an electric field to the emulsion, a localized charge was generated in the dispersed phase. If the ionic strength of the aqueous phase and the relative permittivity of the organic solvent are high, this effect is expected to be more pronounced. However, upon increasing the relative permittivity and the ionic strength, the positive and negative charges at the oilwater interface cancel each other out, and thus demulsification is not promoted. Therefore, as a high organic solvent relative permittivity is required for demulsification, the aqueous phase must have a high ionic strength. When CaCl2 was dissolved in the aqueous phase, the critical ionic strength remained constant at ∼0.04 mol/L for all organic solvents. Indeed, calcium-containing emulsions, with adsorbed surfactants at the oil-water interface have been reported to be stable [24]. However, the addition of calcium chloride is beneficial for all organic solvents in the demulsification using an electric field. Further studies into this phenomenon are therefore required. 4. Conclusions
20 0
Fig. 9. Effect of relative permittivity of the organic solvent on critical ionic strength. ,䊉 = m-xylene, , = p-xylene,䊐,䊏 = n-hexane, ♦, = cyclohexane.
0.20
Ionic strength [mol/L] Fig. 8. Effect of aqueous phase ionic strength on demulsification (16.7 vol% organic phase, solid line = NaCl, dotted line = CaCl2 ).
We herein discussed the results of our studies into the effects of the following factors on demulsification using an alternating electric field: (1) Type of organic solvent present in the emulsion; (2) volume fraction of the organic solvent; and (3) type and ionic strength of the electrolyte dissolved in water. The organic solvents employed were ethylbenzene, toluene, o-xylene, m-xylene, p-xylene, cyclohexane, and n-hexane. In addition, sorbitan monooleate (Span® 80) was employed as a surfactant, and the effects of KCl or CaCl2 addition on the demulsification were examined. The main conclusions that could be drawn from the
A. Muto et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 228–233
study were as follows: (1) A square waveform alternating electric field was more effective than a sine waveform electric field; (2) the demulsification rate varied significantly with changes in volume fraction of the organic solvent and ionic strength; (3) demulsification using an alternating electric field was effective for W/O type emulsions; and (4) demulsification was promoted when the organic solvent exhibited a greater relative permittivity. We therefore expect that our system will be useful for application in the treatment of emulsions in the chemical industry. Studies into demulsification using other types of surfactants and clarification of the mechanism of surfactant demulsification will be carried out in the near future. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 24560943. We would like to thank Editage (www.editage.jp) for their help with English language editing. References [1] H.M. Shewan, J.R. Stokes, Review of techniques to manufacture micro-hydrogel particles for the food industry and their applications, J. Food Eng. 119 (2013) 781–792. [2] K.R. Agnieszka, J. Teofil, Zinc oxide—from synthesis to application: a review, Materials 7 (2014) 2833–2881. [3] B.C. Claire, K. Schroën, Pickering emulsions for food applications: background, trends, and challenges, Annu. Rev. Food Sci. Technol. 6 (2015) 263–297. [4] L.J. Wang, X. Zhang, T. Wang, C. Huang, Z.L. Xiang, A review on phospholipids and their main applications in drug delivery systems, Asian J. Pharm. Sci. 10 (2015) 81–98. [5] C.C. Chan, Y.C. Chen, Demulsification of w/o emulsions by microwave radiation, Sep. Sci. Technol. 37 (2002) 3407–3420. [6] R. Martínez-Palou, R. Cerón-Camacho, B. Chávez, A.A. Vallejo, D. Villanueva-Negrete, J. Castellanos, J. Karamath, J. Reyes, J. Aburto, Demulsification of heavy crude oil-in-water emulsions: a comparative study between microwave and thermal heating, Fuel 113 (2013) 407–414. [7] A. Cambiella, J.M. Benito, C. Pazos, J. Coca, Centrifugal separation efficiency in the treatment of waste emulsified oils, Trans IChemE Part A, Chem. Eng. Res. Des. 84 (2006) 69–76. [8] T. Krebs, C.G.P.H. Schroen, R.M. Boom, Separation kinetics of an oil-in-water emulsion under enhanced gravity, Chem. Eng. Sci. 71 (2012) 118–125.
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[9] Y. Wang, L. Zhang, T. Sun, S. Zhao, J. Yu, A study of interfacial dilational properties of two different structure demulsifiers at oil—water interfaces, J. Colloid Interface Sci. 270 (2004) 163–170. [10] D.D.D. Pezron, C. Dalmazzone, C. Noik, D. Clausse, L. Komunjer, Elastic properties of crude oil/water interface in the presence of polymeric emulsion breakers, Colloid Surf. Physicochem. Eng. Aspect 270–271 (2005) 257–262. [11] S.B. Zhang, X.J. Liu, Q.Y. Lu, Z.W. Wang, X. Zhao, Enzymatic demulsification of the oil-rich emulsion obtained by aqueous extraction of peanut seeds, J. Am. Oil Chem. Soc. 90 (2013) 1261–1270. [12] M. Yamaguchi, A. Kobayashi, K. Ohbori, T. Katayama, A method of continuous phase separation of water in oil emulsion by applying D.C. electric field, Kagaku Kogaku Ronbunshu 11 (1985) 729–734. [13] T. Kataoka, T. Nishiki, Experimental study of electrical demulsification in liquid surfactant membranes, Kagaku Kogaku Ronbunshu 12 (1986) 16–22. [14] J.S. Eowa, M. Ghadiri, A.O. Sharif, T.J. Williams, Electrostatic enhancement of coalescence of water droplets in oil: a review of the current understanding, Chem. Eng. J. 84 (2001) 173–192. [15] T. Ichikawa, K. Itoh, S. Yamamoto, M. Sumita, Rapid demulsification of dense oil-in-water emulsion by low external electric field, Colloid Surf. Physicochem. Eng. Aspect 242 (2004) 21–26. [16] W.-T. Kwon, K. Park, S.D. Han, S.M. Yoon, J.Y. Kim, W. Bae, Y.W. Rhee, Investigation of water separation from water-in-oil emulsion using electric field, J. Ind. Eng. Chem. 16 (2010) 684–687. [17] F.L.M.C. Silva, F.W. Tavares, M.J.E.M. Cardoso, Phase transition of water?in?oil emulsions over influence of an external electric field, Colloids Surf. A: Physicochem. Eng. Aspects 326 (2008) 10–17. [18] K. Rayat, F. Feyzi, Estimation of the electric field strength required for breaking the water-in-oil emulsion: a thermodynamic approach considering droplets deformation and the effect of interfacial tension, Fluid Phase Equilib. 316 (2012) 156–163. [19] M. Mohammadi, S. Shahhosseini, M. Bayat, Electrocoalescence of binary water droplets falling in oil: experimental study, Chem. Eng. Res. Des. 92 (2014) 2694–2704. [20] S.H. Mousavi, M. Ghadiri, M. Buckley, Electro-coalescence of water drops in oils under pulsatile electric fields, Chem. Eng. Sci. 120 (2014) 130–142. [21] A. Muto, T. Matsumoto, H. Tokumoto, Continuous flow demulsification of a water-in-toluene emulsion by an alternating electric field, Sep. Purif. Technol. 156 (2015) 175–182. [22] M. Al-Yaari, I.A. Hussein, A. Al-Sarkhi, M. Abbad, F. Chang, Effect of water salinity on surfactant-stabilized water—oil emulsions flow characteristics, Exp. Therm. Fluid Sci. 64 (2015) 54–61. [23] W.M. Haynes, in: The CRC Handbook of Chemistry & Physics, 96th ed., CRC Press, New York, 2015, pp. 6-186–6-207. [24] A.L. Márqueza, A. Medranoc, L.A. Panizzolo, J.R. Wagnerb, Effect of calcium salts and surfactant concentration on the stability of water-in-oil (w/o) emulsions prepared with polyglycerol polyricinoleate, J. Colloid Interface Sci. 341 (2010) 101–108.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Effects of organic solvent and ionic strength on continuous demulsification using an alternating electric field Akinori Muto ∗ , Yuichi Hiraguchi, Koichiro Kinugawa, Tomoyuki Matsumoto, Yuki Mizoguchi, Hayato Tokumoto Department of Chemical Engineering, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• We • • • •
investigated demulsification using an alternating electric field. A square waveform alternating electric field was more effective. A greater relative permittivity promotes the demulsification. Demulsification rates changed with ionic strength of the aqueous phase. Demulsification using an electric field was effective for W/O emulsions.
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 6 June 2016 Accepted 21 June 2016 Available online 22 June 2016 Keywords: Alternating electric field Relative permittivity Organic solvent Electrolyte Ionic strength W/O emulsion
a b s t r a c t We herein report the effects of the following variables on demulsification using an alternating electric field: (1) type and volume fraction of an organic solvent, and (2) type and ionic strength of an electrolyte in water. Ethylbenzene, toluene, o-xylene, m-xylene, p-xylene, cyclohexane, and n-hexane were employed as organic solvents. The effect on demulsification of either KCl or CaCl2 addition to the aqueous phase was also examined. We observed that a square waveform alternating electric field was more effective than a sine waveform. In addition, the demulsification rate varied significantly upon altering the volume fraction of the organic solvent and the ionic strength of the electrolyte. Demulsification using an alternating electric field was effective for W/O type emulsions. Demulsification was promoted using an organic solvent with greater relative permittivity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the worldwide production of emulsions for use as industrial chemicals [1], cosmetic additives [2], food additives [3], and medicinal materials [4] has become widespread. However, emulsions are often generated as unwanted byproducts from the treatment of oil and aqueous solutions. Emulsions often form upon mixing of the organic and aqueous phases, thus rendering
∗ Corresponding author. E-mail address: [email protected] (A. Muto). http://dx.doi.org/10.1016/j.colsurfa.2016.06.037 0927-7757/© 2016 Elsevier B.V. All rights reserved.
separation of these phases difficult and time consuming, often requiring bulky equipment. Consequently, the development of a rapid, convenient, and continuous demulsification method would be of enormous benefit to many chemical processes. To date, several demulsification methods have been developed, such as heating [5,6], centrifugation [7,8], chemical treatment [9–11], and electric field treatment [12–20]. Electrical separation in particular is often favored for the following reasons: (i) No additional material is added, thus eliminating the requirement for further preor post-treatment; (ii) the demulsification process can be carried out at 20–25 ◦ C; and (iii) the demulsification device is structurally and operationally simple. However, this process also has several
A. Muto et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 228–233
disadvantages, such as the requirement for a high electric voltage and the possible occurrence of electrochemical reactions when the electrode is in direct contact with the emulsion. If these issues could be overcome, the electric field method could potentially yield good demulsification performances. We previously investigated the demulsification of a water-in-toluene emulsion by the application of an alternating current (AC) electric field [21]. A fluorocarbon tube of 1 mm diameter was used for the emulsion flow. The waterin-toluene emulsion was smoothly separated into clear water and toluene phases. Thus, we chose to investigate the effects of the following variables on demulsification using an alternating electric field: (1) Type of organic solvent in the emulsion; (2) volume fraction of the organic solvent; and (3) type and ionic strength of the electrolyte dissolved in water. Finally, a suitable emulsion structure for the demulsification using an alternating electric field is examined.
2. Materials and methods 2.1. Reagents All reagents were special grade and were purchased from Wako Pure Chemical Industries Ltd. A water/organic solvent emulsion was prepared in the presence of surfactant using a homogenizer (IKA, Germany, T18BS1). Ethylbenzene, toluene, o-xylene, m-xylene, p-xylene, cyclohexane, and n-hexane were employed as organic solvents. Sorbitan monooleate (Span® 80, reagent grade) was used as a surfactant, and was added to the organic solvent (4 vol%) prior to emulsion preparation. Potassium chloride and
229
calcium chloride were used as additives to the aqueous phase. Distilled and deionized water was employed in all experiments. 2.2. Experimental setup The experimental setup used in this study is outlined in Fig. 1. The apparatus comprises five devices, namely a plunger pump, a demulsification device, a measuring cylinder, a voltage supply, and a function generator. Two copper plates (100 mm × 100 mm × 5 mm) were used as electrodes and were connected to a voltage supply (HEOP-3B10, Japan, Matsusada Precision Inc.) and a wave generator (WF1974, Japan, NF Corporation) to control the waveform and the frequency of the alternating electric field. A fluorocarbon tube (Code No. 1024-01, Japan, ARAM Corporation; internal diameter 1 mm, external diameter 3 mm, length 760 mm), through which the emulsion flowed, was inserted between the two electrodes responsible for application of the electric field. The emulsion was fed into the fluorocarbon tube using a dual plunger pump (YMCK-11-13-P, Japan, YMC Co. Ltd.). The flow rate, and therefore the retention time of the device, was selected to correspond to the duration of the applied electric field. The output was collected in a measuring cylinder, thus allowing the volumes of the oil phase, emulsion phase, and aqueous phase to be measured. 2.3. Experimental procedure The surfactant was dissolved in the organic solvent, and calculated amounts of the organic solvent and water (total volume = 100 mL) were stirred at 24000 rpm for 1 min using a homogenizer to yield the stable emulsion. From preliminary
Fig. 1. Schematic representation of the demulsification device.
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A. Muto et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 228–233
Organic solvent
Pure water
Addition of Span® 80, 4 vol%
Addition of Electrolyte; KCl or CaCl2
Homogenization (24,000 rpm, 1 min)
Emulsion Fig. 2. Outline of the emulsion preparation method.
experiments, we found that a reaction time of 185 s was sufficient to demulsify a test mixture. Consequently, the emulsion was fed into the demulsification device at constant flow rates of 20, 60, 100, 150, and 200 mL/h using a plunger pump. The preliminary experiment confirmed that the plunger pump did not affect demulsification. All emulsions demulsified to approximately ∼5 vol% after 24 h, with no significant changes being observed over 14 days. Demulsification experiments were carried out within 2 h of preparation to prevent spontaneous demulsification taking place. The volumes of the liquid output and the oil phase generated by demulsification were directly measured in the measuring cylinder. The demulsification experiment was conducted using a 1000 kV/m electric field oscillating as a square wave at a frequency of 100 Hz. All demulsification experiments were conducted under these conditions unless otherwise specified. The demulsification rate was defined according to Equation (1): =
Voil V0, oil + V0, water
(1)
where Voil is the volume of the separated organic phase, and V0,oil and V0,water are the original volumes of the organic solvent and water, respectively, prior to mixing. The water content of the oil phase was analyzed by Karl Fischer titrations (KF-21 moisture meter, Japan, Mitsubishi Chemical Analytec). 2.4. Determination of emulsion type The prepared W/O or O/W emulsion was then examined using the dilution method [22]. A drop of each emulsion was added either to water or to the organic solvent used for preparation of the emulsion. The emulsion drop was observed to determine whether it remained as a droplet or whether it was dispersed in either water or the organic phase (Fig. 2). 3. Results and discussion 3.1. Effect of alternating frequency on demulsification efficiency for the hexane-based emulsion An emulsion containing 50 vol% hexane was prepared according to the procedure outlined above. Hexane was selected as it is a typical aliphatic organic solvent used widely in the chemical industry. The emulsion was fed to the demulsification device at a constant flow rate and was applied to two types of alternating electric field, i.e., a square waveform or a sine waveform, for 62 s. The effect of the alternating electric field of the square and sine waveforms on the demulsification rate are shown in Fig. 3, where 0 Hz corresponds to a direct electric field. Upon increasing the frequency of the square waveform to 2 Hz, the demulsification rate
Fig. 3. Demulsification of a hexane/water emulsion (50 vol% hexane, electric field for 62 s).
increased from ∼5% to ∼50%, while the demulsification ratio of the sine waveform (at 2 Hz) increased to only ∼7%. However, for both waveforms, the demulsification rate increased with increasing frequency. At frequencies >8 Hz, the demulsification rate of the square waveform remained relatively constant at ∼80%. Indeed, the square waveform was more effective than the sine waveform, suggesting that a drastic change in the magnitude and direction of the electric field is important in the demulsification process. Upon reversing the electric field suddenly, vibration of the dispersed phase took place, resulting in collisions within the dispersed phase, promoting coalescence. 3.2. Effect of organic solvent volumetric fraction on demulsification The emulsion structure was examined to determine the reason for the dramatic variation in demulsification rate upon changing the volume fraction of the organic solvent. Fig. 4 shows the results of the demulsification experiments carried out using the p-xylene and toluene emulsions. Photographic representations of the dilution method are also shown. High demulsification rate emulsions represent the W/O type emulsion, while the O/W type emulsion exhibited no demulsification. Comparable results were obtained for other organic solvents, suggesting that electric field demulsification is effective for W/O type emulsions. The effect of the solvent volume fraction on demulsification was then examined. Fig. 4 shows the effect of the volume fraction when toluene or p-xylene was employed as the solvent. Demulsification did not occur at volume fractions <20 vol%. However, upon increasing the p-xylene volume fraction to 20 vol% or the toluene volume fraction to 30 vol%, the demulsification rate increased sharply to 80%. To clarify the reason for this remarkable increase in demulsification rate, the structure of the emulsion was examined using the diffusion method, where drops of each emulsion were added either to water or to the appropriate solvent (see corresponding photographic images in Fig. 4). The emulsions that were demulsified by the alternating field method dispersed well in water, but did not disperse in the organic solvent. This indicates that these emulsions are O/W (oil-in-water) type. In general, the opposite was observed for the emulsions that exhibited good demulsification. For example, Eow et al. [14] reported that “For drops with a permittivity greater than that of the suspension medium, as in the case of water drops in oil, they move toward the place of greatest field intensity. This does not require charged particles and it depends on the force felt by all polar materials when in a non-uniform electric field.” Our
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Fig. 4. Effect of emulsion structure on the relationship between demulsification and volume content of the organic solvent.
results therefore implied that these emulsions were W/O (waterin-oil) emulsions, indicating that demulsification by an electric field is optimal for W/O emulsions. Reversal of the dipole moment of the dispersed phase by an alternating current promotes coalescence, i.e., demulsification and fusion of the dispersed droplets. 3.3. Effect of organic solvent on demulsification efficiency The effect of the type of organic solvent on demulsification was then examined. Experiments were carried out using a range of aliphatic and aromatic organic solvents with varying relative permittivities, as shown in Table 1. The dispersion phase diameter of all emulsions was 5–20 m. Fig. 5 shows the effect of the oil phase volume fraction on demulsification using various solvents. In all organic solvents, the demulsification rate varied significantly with a change in the oil phase volume fraction, although the effect differed slightly between the different types of solvent. This suggests that the demulsification rate is affected by the structure and physical properties of the emulsion or the physical properties of the organic solvent. As shown in Fig. 5, the demulsification rate rose rapidly upon increasing the oil phase volume fraction of the emulsion. The initial and final demulsification rates (EL and EH ) shown in the figure represent the oil phase volume fractions VL and VH of the emulsion. In practice, EL is close to zero, and so the critical volume fraction (VC ) of the oil phase upon demulsification can be defined by Equation (2): VC = VL + (VH − VL )/2
The demulsification rate, E, corresponded to the VC expressed by the EC , which gave the relationship indicated in Fig. 6 when EL = 0. The value of the critical volume fraction VC therefore indicates the volume fractions where the emulsion type is W/O (water-in-oil) or O/W (oil-in-water). The relationship between the relative permittivity of the organic solvent used in the preparation of the emulsion and the critical volume fraction is shown in Fig. 7. When the relative permittivity of the organic solvent increased, the critical volume fraction also increased, indicating that the organic solvent was easily polarized with an increase in relative permittivity. These results suggest that a greater relative permittivity promotes demulsification using an alternating electric field. 3.4. Effect of aqueous phase ionic strength on demulsification Either potassium chloride or calcium chloride were dissolved in the aqueous phase to enhance the dielectric polarization of the dispersed phase, and the effect of the aqueous phase ionic strength on the demulsification is shown in Fig. 8. With an oil phase volume fraction of 16.7 vol%, demulsification was not observed for any of the organic solvents tested in the absence of electrolyte in the
(2)
Table 1 Physical properties of the organic solvents at 293 K [23]. Material
Relative permittivity
o-xylene ethyl benzene toluene m-xylene p-xylene cyclohexane n-hexane
2.562 2.446 2.379 2.359 2.274 2.024 1.887
Fig. 5. Effect of different types of organic solvent and volume contents on the demulsification rate (electric field = square waveform, 100 Hz, 62 s).
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100
(+
Demulsification η [%]
80
Linear approximation
60
Critical volume content of organic phase; Vc
(+
40
20
(/
0 0
20
40
60
Volume content of organic solvent [vol%] Fig. 6. Concept and evaluation of the critical volume content (VC ) of the organic phase.
Fig. 7. Effect of relative permittivity of the organic solvent on the critical volume fraction.
100
Demulsification
[%]
80
m-xylene p-xylene cyclohexane
n-hexane
60
n-hexane cyclohexane
40
m-xylene p-xylene
0.00
0.05
0.10
0.15
aqueous phase. The black legends show the results obtained upon dissolving KCl in the aqueous phase, while the white legends show the results obtained using CaCl2 . In both cases, the demulsification rates varied dramatically with the ionic strength of the aqueous phase. This suggests that dissolution of the electrolyte promotes demulsification due to enhancement of the dielectric polarization of the aqueous phase in the emulsion, and also due to the difference in ionic behavior between CaCl2 and KCl. For example, Ca2+ affects the oil-water interface in the interface structure due to ionic interactions. The critical ionic strength, IC , was then determined for KCl and CaCl2 , and the relationship between the relative dielectric constant of the organic solvent and the IC is shown in Fig. 9. When KCl was dissolved in the aqueous phase, the critical ionic strength increased with an increase in the relative permittivity of the organic solvent. Upon application of an electric field to the emulsion, a localized charge was generated in the dispersed phase. If the ionic strength of the aqueous phase and the relative permittivity of the organic solvent are high, this effect is expected to be more pronounced. However, upon increasing the relative permittivity and the ionic strength, the positive and negative charges at the oilwater interface cancel each other out, and thus demulsification is not promoted. Therefore, as a high organic solvent relative permittivity is required for demulsification, the aqueous phase must have a high ionic strength. When CaCl2 was dissolved in the aqueous phase, the critical ionic strength remained constant at ∼0.04 mol/L for all organic solvents. Indeed, calcium-containing emulsions, with adsorbed surfactants at the oil-water interface have been reported to be stable [24]. However, the addition of calcium chloride is beneficial for all organic solvents in the demulsification using an electric field. Further studies into this phenomenon are therefore required. 4. Conclusions
20 0
Fig. 9. Effect of relative permittivity of the organic solvent on critical ionic strength. ,䊉 = m-xylene, , = p-xylene,䊐,䊏 = n-hexane, ♦, = cyclohexane.
0.20
Ionic strength [mol/L] Fig. 8. Effect of aqueous phase ionic strength on demulsification (16.7 vol% organic phase, solid line = NaCl, dotted line = CaCl2 ).
We herein discussed the results of our studies into the effects of the following factors on demulsification using an alternating electric field: (1) Type of organic solvent present in the emulsion; (2) volume fraction of the organic solvent; and (3) type and ionic strength of the electrolyte dissolved in water. The organic solvents employed were ethylbenzene, toluene, o-xylene, m-xylene, p-xylene, cyclohexane, and n-hexane. In addition, sorbitan monooleate (Span® 80) was employed as a surfactant, and the effects of KCl or CaCl2 addition on the demulsification were examined. The main conclusions that could be drawn from the
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study were as follows: (1) A square waveform alternating electric field was more effective than a sine waveform electric field; (2) the demulsification rate varied significantly with changes in volume fraction of the organic solvent and ionic strength; (3) demulsification using an alternating electric field was effective for W/O type emulsions; and (4) demulsification was promoted when the organic solvent exhibited a greater relative permittivity. We therefore expect that our system will be useful for application in the treatment of emulsions in the chemical industry. Studies into demulsification using other types of surfactants and clarification of the mechanism of surfactant demulsification will be carried out in the near future. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 24560943. We would like to thank Editage (www.editage.jp) for their help with English language editing. References [1] H.M. Shewan, J.R. Stokes, Review of techniques to manufacture micro-hydrogel particles for the food industry and their applications, J. Food Eng. 119 (2013) 781–792. [2] K.R. Agnieszka, J. Teofil, Zinc oxide—from synthesis to application: a review, Materials 7 (2014) 2833–2881. [3] B.C. Claire, K. Schroën, Pickering emulsions for food applications: background, trends, and challenges, Annu. Rev. Food Sci. Technol. 6 (2015) 263–297. [4] L.J. Wang, X. Zhang, T. Wang, C. Huang, Z.L. Xiang, A review on phospholipids and their main applications in drug delivery systems, Asian J. Pharm. Sci. 10 (2015) 81–98. [5] C.C. Chan, Y.C. Chen, Demulsification of w/o emulsions by microwave radiation, Sep. Sci. Technol. 37 (2002) 3407–3420. [6] R. Martínez-Palou, R. Cerón-Camacho, B. Chávez, A.A. Vallejo, D. Villanueva-Negrete, J. Castellanos, J. Karamath, J. Reyes, J. Aburto, Demulsification of heavy crude oil-in-water emulsions: a comparative study between microwave and thermal heating, Fuel 113 (2013) 407–414. [7] A. Cambiella, J.M. Benito, C. Pazos, J. Coca, Centrifugal separation efficiency in the treatment of waste emulsified oils, Trans IChemE Part A, Chem. Eng. Res. Des. 84 (2006) 69–76. [8] T. Krebs, C.G.P.H. Schroen, R.M. Boom, Separation kinetics of an oil-in-water emulsion under enhanced gravity, Chem. Eng. Sci. 71 (2012) 118–125.
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