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Ind. Eng. Chem. Res. 2009, 48, 10008–10014
SEPARATIONS Mass Transfer Enhancement in Liquid-Liquid Extraction with Very Dilute Aqueous Salt Solutions Javad Saien* and Fatemeh Ashrafi Department of Applied Chemistry, UniVersity of Bu-Ali Sina, Hamedan 65174, Iran
The effect of very low salt addition in the recommended liquid-liquid extraction chemical system of toluene-acetone-water was investigated with circulating single drops and in both of the mass transfer directions. By adding sodium chloride, potassium chloride, and potassium iodide, within the concentration range of 10-6-10-4 mol/L, into the continuous phase and under constant pH, the mass transfer rate finds enhancements to about 425%. A high trend of variation was observed for the salt concentrations until only about 10-5 mol/L. This phenomenon can be attributed to the hydration of ions which favors acetone to be transferred easier in the aqueous phase. Small drops are usually more benefited, and the rate of mass transfer is greater for the dispersed to continuous phase direction under similar conditions due to a higher dominant extraction fraction, whereas the drop size decreases in this direction. The effectiveness of salts in this matter appeared to be in the order of NaCl > KCl > KI. 1. Introduction Liquid-liquid extraction is a separation method that has found a vast application in various fields. The design of contactors for this process is often connected with expensive experimental investigations, and therefore, studies on the pilot plants and laboratory scale apparatuses are inevitable. To date, it is still difficult to describe the rate of mass transfer as a function of process and physical parameters. In a number of so far recommended chemical systems for liquid-liquid extraction investigations,1,2 water has been selected as the continuous phase. Also, in many industrial processes, at least one of the phases in the liquid-liquid extraction systems is aqueous. It is while the aqueous salt solutions are present in large amounts in an environment (e.g., seawater) that using them can greatly influence the mass transfer of organic solutes between phases and can change the equilibrium distribution of a mixture significantly. This phenomenon is often referred to as the saltingin or salting-out effect. During recent years, the salting-out effect has been investigated on liquid-liquid equilibrium systems in the separation of organic solutes, using the preferential organic solvents.3-7 Also, the influence of salts on the recovery of inorganic species and the equilibrium distribution coefficient of the extractable species has been intensively investigated, mainly for hydrometallurgy purposes.8 The ionic species in the aqueous phase are more easily hydrated than the metal complex being extracted with a solvating extractant. This reaction between extractant and solute is interfacial in nature. Thus, the presence of ionic species will compete with the solute for solvation water, the hydration degree of the solute will decrease, and its bond to the solvating extractant will be benefited. The extraction efficiency depends often on the ion strength of electrolytes, and empirically, it has been shown that, for good extraction, the aqueous phase should contain 2-3 mol/L of a nonextractable electrolyte salt.8 * To whom correspondence should be addressed. Tel. and Fax: +98811-8257407. E-mail:
[email protected].
The influence of some electrolytes and surface potential on ions and charged species has also been demonstrated in a number of publications9-11 and the strong influence of electrolytes on the partition behavior is reported. It has been shown that, for instance, the induced charge at the interface, which is caused by a hydrophilic-hydrophobic-oriented covering of surfactants at the interface has a strong influence on the extraction of ionic aqueous species, held with the interfacial reactions.9 Despite its high attraction, no investigation has been done in the case of the mass transfer rate of organic solutes in the presence of electrolyte salts for the recommended liquid-liquid extraction chemical systems. The disadvantage of adding extra agents to the continuous operating systems, with respect to the problems of contamination and the economic factors in separating the salts from the products, could be the reason for this case. In the present work, an attempt has been made to study the influence of adding very low amounts of different salts in the aqueous phase under constant pH, on the rate of mass transfer for single drops, moving in the extraction column. The aim is to find conditions for high efficiencies in extraction. 2. Experimental Section 2.1. Materials. The chemical system of toluene-acetonewater, a recommended system for liquid-liquid extraction studies,1 was chosen. The main specification of this system is its high interfacial tension. Toluene and acetone were Merck products with purities of more than 99% and 99.5%, respectively. Distilled water with a constant pH value of 6.1 was produced and used as the aqueous continuous phase. The usual salts, NaCl, KCl, and KI with common cation and anion and with minimum mass purities of 99.8%, 99.5%, and 99.5%, respectively, were used. These chemicals were the products of the Merck company. Experiments were carried out at the ambient temperature of 20 ( 2 °C. The physical properties of the used chemical system at the average temperature of 20 °C are given in Table 1. The
10.1021/ie900542w CCC: $40.75 2009 American Chemical Society Published on Web 09/15/2009
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 1. Properties of Chemical System: Toluene-Acetone-Water at 20 °C property
dispersed phase
continuous phase
F [kg/m ] µ [kg/m · s] D [m2/s] γ [mN/m]
866.15 0.628 × 10-3 2.55 × 10-9
998.15 1.018 × 10-3 1.09 × 10-9
3
32.21
physical properties were measured using a self-adjustable temperature density meter (Anton Parr DMA 4500) for measuring density, an Ostwald viscometer for measuring viscosity, and employing the drop-weight method for measuring interfacial tension.12 Due to very low level of salt concentrations, used in this work, no significant change in the physical properties is assumed by adding the salts. The values of the equilibrium distribution of acetone between the phases at 20 °C, and under different salt concentrations, were examined with rigorous stirring of samples by a magnetic stirrer for 3 h and allowing to reach equilibrium for 24 h in closed cells. Samples were carefully taken and the acetone was analyzed with GC and correlated by C *d ) φC *c
(1)
where Cd*, Cc*, and φ are the equilibrium dispersed phase (organic) and continuous phase (aqueous) concentrations of acetone (g/L) and the distribution coefficient, respectively. The distribution coefficient differs slightly with adding salts in the continuous phase. The average φ values are 0.841, 0.866, 0.863, and 0.862 for a salt-free chemical system and with NaCl, KCl, and KI salts, respectively. Figure 1 shows the typically obtained equilibrium data in the presence of different NaCl concentrations. The distribution coefficients for salt-free equilibrium results are quite close to those reported by Brodkorb et al.13 2.2. Setup. The experimental setup is described in our previous work.14 A Pyrex glass column (11.4 cm diameter and 51 cm height) was used as the contactor. Drop forming was provided, using a variety of glass nozzles located at the bottom of this column. The toluene phase was held in a glass syringe, conducted by an adjustable syringe pump (Phoenix M-CP) and flowed through a rigid tube to the glass nozzle. A small inverted glass funnel attached to a pipet and vacuum bulb was used to catch a sample of about 1 mL of dispersed
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phase at the top of the column with 33 cm of distance from the initial point. The interfacial area in the funnel was minimized at the level of the pipet inlet by occasionally pulling toluene into the pipet. Three samples were prepared for each concentration and were kept in closed sample tubes in a cold medium until gas chromatograph (GC) analysis. In order to omit the influence of unsteady mass transfer during the drop formation and its transient velocity, the initial drop concentration was considered for a location of moving drops to be at 6.5 cm above the nozzle’s tip. Drop motion was observed to reach steady movement after about 40 mm of travel. To determine the initial concentrations at this location, an empty small column equipped with the same nozzle was used. Drops were collected at the same distance of 6.5 cm and under the same conditions and drop sizes of the main column. 2.3. Procedure and Analysis. The syringe and the connection tube to the nozzle tip were first filled with toluene or toluene + acetone (depending on the mass transfer direction) to produce drops. The solutions were prepared by adding accurate amounts of salt (with an uncertainty of (0.1 mg) to extra volumes of distilled water. The column was then filled with this aqueous phase and was agitated very often before introducing drops in each series of experiments. The salt concentration in the aqueous phase was adjusted at five values of 10-6, 5 × 10-6, 10-5, 5 × 10-5, and 10-4 mol/L, while the pH was constant at its natural value of 6.1. This constant pH value was checked with the pH meter (CorningM140 with an uncertainty of (0.01). As is mentioned in the literature,15 there is a range within which adding salts would not necessary give a change in the solution pH. Due to the unit ionic charges for the used salts, the concentration values and the ionic strength of solutions are the same. Meanwhile, in dilute aqueous solutions, the mean activity coefficient of a given strong electrolyte is the same in all solutions of the same ionic strength. The syringe pump was initially calibrated with respect to the specified volume scale on the calibrated syringe. Knowing the flow-rate and the number of generated drops, during a particular period, the drop size can easily be determined. Experiments were conducted in both mass transfer directions of from the dispersed to the continuous phase (d f c) and vice versa (c f d). The contact time of drops from the initial to the collection point was measured several times with a stopwatch, and its
Figure 1. Equilibrium distribution of acetone between phases with different concentrations of NaCl.
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Table 2. Outside Tip Diameter of Nozzles (mm) and the Range of Drop Diameter (mm), Generated by Nozzles at Different Salt Concentrations nozzle no.
tip diameter
cfd
dfc
1 2 3 4 5
0.2 0.5 0.8 1.0 1.2
2.72-3.06 2.92-3.25 3.01-3.43 3.67-4.05 3.86-4.30
2.51-2.90 2.78-3.24 2.96-3.42 3.52-4.04 3.77-4.24
average was considered. The terminal velocity of drops was obtained with respect to the average times. Drops were spaced more than 60 mm apart typically with a range of flow rate between 21.37 and 116.84 mL/h. Acetone concentration in the collected samples was measured using a GC (Shimadzu, 14B) with a flame ionization detector (FID), calibrated with reference substances of toluene and acetone (Merck) for gas chromatography. The known concentration of acetone in the calibration samples was within the range of 1.63-44.14 g/L and the number of known samples, used in the calibration curve experiments, was 14. All the equipments and glass-wares were cleaned with Decon 90 solution, followed by several rinses with distilled water, prior to experiments. Experimental data were produced for either direction of mass transfer using five different nozzles, each of them with five concentrations of the three used salts. 3. Results and Discussion Toluene drop diameters, from 2.51 to 4.30 mm, were generated. The range of drop sizes by each nozzle is listed in Table 2. The outside tip diameter of the nozzles is also given. Figure 2 shows that in contrast to the presence of surfactants,16 the size of drops, formed at each nozzles’ tip, increases with the increase in salt concentration. Further investigation is required due to the presence of salts to justify the drop size variation. Moreover, larger drops were produced when the direction of mass transfer was c f d. In our previous work with different aqueous phase pH,14 similar behavior in generating larger drops was observed in this direction. The range of drops used in this work is within the conditions of circulating drops, since the values of dimensionless group H
defined in the model by Grace et al.17 are in the range of 5.1-53.8 (2 < H < 59.3), corresponding to circulating drops. The Weber number is also within 0.37 < We < 1.86 (less than 3.58), and the range of ratio Re/NPG0.15 (NPG is the inverse of the Morton number) is 4.71-13.72 (less than 20).18 Acetone was dissolved in the aqueous or the organic phase with initial concentrations of 43.3 (5%) and 29.9 g/L (3%), for the d f c and c f d directions, respectively. For each series of experimental data, initial and final concentrations, drop size, and contact time were obtained. The rate of transfer of acetone for a drop with size d can be written with respect to the overall mass transfer coefficient concept. For the mass transfer direction of c f d for instance: V
dCd ) KodS(C *d - Cd) dt
(2)
where V, S, Cd, t, and Kod are the drop volume, drop surface area, acetone concentration in drop, contact time, and the overall mass transfer coefficient, respectively. The ratio S/V is the surface area per volume of the spherical drop and equals to 6/d. With substitution for this ratio in eq 2 and integrating over a rise time t, appropriate to steady movement of the drop with initial and final concentrations of Cdi and Cdf, a time-averaged mass transfer coefficient can be obtained, Kod ) -
d ln(1 - E) 6t
(3)
where E is the extraction fraction, defined by E)
Cdf - Cdi C *d - Cdi
(4)
For d f c direction Cd* is zero; since the solute concentration in aqueous phase is zero for this case. With respect to the column capacity (5.2 L), the continuous phase is considered uniform in the bulk phase and thus having a unique bulk concentration. Introducing drops, even for a series of experiments with a nozzle does not cause a sensible change in the concentration of continuous phase.
Figure 2. Variation of generated drop size versus salt concentration for typical nozzle no. 5, in both mass transfer directions.
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Figure 3. Variation of contact time with the salt concentration, in both mass transfer directions for typical nozzle no. 3.
Figure 4. Variation of extraction fraction with the salt concentration, in both mass transfer directions for typical nozzle no. 3.
To explain clearly the variations of Kod values, investigations in the contact time (hydrodynamic term) and the extraction fraction (mass transfer term), included in eq 3, becomes useful. Figure 3 presents the variations of contact time with salt concentration for the typical nozzle no. 3. The decrease of contact time with salt concentration and the c f d mass transfer direction is expected due to the increase of terminal velocity which is influenced by the increase in drop size (Figure 2). Figure 4 shows the variation of extraction fraction, E, with salt concentration for the typical nozzle no. 3. As it is also obvious, the extraction fraction is increased with salt concentration and is higher in the d f c direction. The tending of acetone molecules to enter the aqueous phase and forming hydrogen bonds with the free water molecules can be assumed the reason for this case. This matter (higher mass transfer in d f c) is also observed for the salt-free system. Figures 5 and 6 show the variation of overall mass transfer coefficient versus salt concentration for three typical nozzles
in either of the mass transfer directions. The overall mass transfer coefficient finds a very remarkable enhancement, as high as 32-425% for the d f c direction and 21-245% for the c f d direction (for similar drop sizes and in comparison with the salt-free system), as the salt was added in the continuous phase. The reason can be attributed to the hydration of ions with water molecules and thus reducing the formation of acetone hydrogen bonds and facilitating its transfer in the aqueous phase. Correia and de Carvalho19 have investigated the salt effect on the equilibrium conditions of liquid-liquid extraction of phenol with Cyanex 923 extractant. In their work, adding salt (3.5 mol/L NaCl) was proposed to lead to the separation of an upper layer phenol concentrated phase from a low phenol content aqueous phase. For the later phase, an extraction process was applied. Meanwhile, the comparison between Figures 5 and 6 indicates that the enhancement in mass transfer rate is higher in the direction of d f c, under the same conditions. The range of
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Figure 5. Variation of overall mass transfer coefficient versus the salt concentration for three of the nozzles, d f c.
Figure 6. Variation of overall mass transfer coefficient versus the salt concentrations for three of the nozzles, c f d.
Kod values is within 9.6-192.8 µm/s for the d f c direction and within 6.1-100.1 µm/s for the c f d direction. As is obvious from Figure 4, the extraction fraction is significantly higher for the d f c direction, and this term as dominant parameter would provide higher overall mass transfer coefficient, despite the smaller drops corresponding to higher contact times (Figure 3), in this direction. Salt concentrations less than about 10-5 mol/L, exhibit a high trend of variation for extraction after which the variation becomes rather mild. This matter encourages adding salts as low as this concentration to enhance the mass transfer rate very significantly. The trend of variations in E and Kod are approximately similar and confirm this matter. It is noteworthy that a higher tendency in mass transfer coefficient is observed by adding extra salts in the aqueous phase; however, a high salty solution can be a limitation in practical points of view, since the subsequent removal of the salts is not economically viable. Moreover, adding extra amounts of salts can lead to an increase in the solution pH15 and therefore
providing its own influence in the mass transfer, as is reported in our recent work.14 An important observation, revealed from Figures 3 and 4, is that the effectiveness of salts in Kod enhancement is in the order of NaCl > KCl > KI. The ionic radius is an important factor in the free energy of solvation of ions. The cation of the salting agent does have a major effect on extractability as the cationic radius, in any one periodic group, decreases and as the charge on the cation increases.8,20 Since the Na+ has a smaller ionic radius than K+, it is easier hydrated with a more number of water molecules around itself and thus favoring the transfer of acetone molecules. The higher effect of KCl than that of KI can be attributed to the solvation of anion Cl- by water molecules, which is more effective compared to I- by water.21 It is notable that the difference of Kod values obtained with these salts is not so dramatic for small drops with higher contact times; however, the difference is significant for large size drops with short contact times.
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Figure 7. Variation of overall mass transfer coefficient versus drop size for both mass transfer directions and the maximum salt concentration of 10-4 mol/L.
As a criterion, to indicate the enhancement in the overall mass transfer coefficient Eh for a specified drop size is used here with the equation: Eh )
KodS - Kod0 Kod0
(5)
where KodS and Kod0 are the overall mass transfer coefficient in the presence of salt and without salt respectively. Figure 7 shows the variation of mass transfer coefficient with drop size for a typical salt concentration of 10-4 mol/L. Drops tend to higher internal circulating as the size increases, leading to enhancement in mass transfer. Meanwhile, large drops move faster with lower contact times. From Figure 7 it can also concluded that the mass transfer enhancement with the highest used concentration of 10-4 mol/L (for instance) of different salts is not the same for different drops. Small drops are more benefited based on the criterion of mass transfer enhancement. In the mass transfer direction of d f c and under the highest used salt concentration, for instance, drops generated from nozzle 1 find an average Eh value of 4.19 for different salts; while those from nozzle 5 find an average Eh value of 1.46. The higher contact time which in turn provides higher opportunity for the solute to be transferred with the influence of added salts, is the reason for this case. 4. Conclusions The results of this study show that adding very low amounts of electrolyte salts in aqueous phase (maximum of 10-4 mol/L) is very effective for extraction of nonelectrolyte solute acetone, in the recommended chemical system of toluene-acetone-water while the solute is transferring in both directions of continuous to dispersed and vice versa. Salt concentrations even less than 10-5 mol/L can enhance the rate of mass transfer drastically. The drop size increases with the salt concentration and for a specified drop size, the overall mass transfer coefficient increases with the salt concentration within the range of 10-6-10-4 mol/ L. The order of effectiveness of salts acting as the mass transfer promoter increases in the order of NaCl > KCl > KI, recommending the use of NaCl in favor with the economical points of view.
The mass transfer direction itself influences the rate of mass transfer, no matter the variation and level of salt concentration; however, the trend of variation of the overall mass transfer coefficient is similar. The mass transfer from dispersed to continuous phase is more influenced and the percentage of enhancement is higher for small drops particularly under higher salt concentrations. Acknowledgment We express thanks the university authorities for providing the financial support to carry out this work. Nomenclature C ) solute concentration (g/L) d ) drop diameter (mm) D ) diffusivity (m2/s) E ) extraction fraction defined by eq 4 Eh ) overall mass transfer coefficient enhancement H ) dimensionless group defined in the Grace et al.15 model K ) overall mass transfer coefficient (µm/s) NPG ) inverse of the Morton number (Fc2γ3/gµc4∆F) Re ) drop Reynolds number (FcVtd/µc) S ) drop surface area (m2) t ) contact time (s) V ) drop volume (m3) Vt ) terminal velocity of drops (m/s) We ) drop Weber number (FcVt2d/γ) Greek Letters γ ) interfacial tension (mN/m) µ ) viscosity(kg/m · s) F ) density (kg/m3) φ ) solute distribution coefficient Subscripts c ) continuous phase d ) dispersed phase i ) initial value f ) final value h ) enhancement o ) overall value s ) in the presence of salt
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Superscript * ) equilibrium
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(11) Pfennig, A.; Schwerin, A.; Gaube, J. Consistent View of Electrolytes in Aqueous Two-Phase Systems. J. Chromatogr. B 1998, 711, 45–52. (12) Saien, J.; Akbari, S. Interfacial Tension of Toluene + Water + Sodium Dodecyl Sulfate from (20 to 50) °C and pH between 4 and 9. J. Chem. Eng. Data 2006, 51, 1832–1835. (13) Brodkorb, M. J.; Bosse, D.; von Reden, C.; Gorak, A.; Slater, M. J. Single Drop Mass Transfer in Ternary and Quaternary Liquid-Liquid Extraction Systems. Chem. Eng. Proc. 2003, 42, 825–840. (14) Saien, J.; Daliri, S. Mass Transfer Coefficient in Liquid-Liquid Extraction and the Influence of Aqueous Phase pH. Ind. Eng. Chem. Res. 2008, 47, 171–175. (15) Skoog, D. A.; West, D. M.; Holler, F. J. Analytical Chemistry: An Introduction, Fifth ed.; Saunders College Pub.: USA, 1990. (16) Saien, J.; Riazikhah, M.; Ashrafizadeh, S. N. Comparative Investigation on the Effect of Contamination and Mass Transfer Direction in Liquid-Liquid Extraction. Ind. Eng. Chem. Res. 2006, 45, 1434–1440. (17) Grace, J. R.; Wairegi, T.; Nguyen, T. H. Shapes and Velocities of Single Drops and Bubbles Moving Freely Through Immiscible Liquids. Trans. Inst. Chem. Eng. 1976, 54, 167–174. (18) Skelland, A. H. P. Interphase Mass Transfer. In Science and Practice of Liquid-Liquid Extraction; Thornton, J. D., Ed.; Oxford Science Pub.: New York, 1992; Vol 1. (19) Correia, F. M. M. P.; de Carvalho, M. R. Salt Effects on the Recovery of Phenol by Liquid-Liquid Extraction with Cyanex 923. Sep. Sci. Technol. 2005, 40, 3365–3380. (20) Bostro¨m, M.; Ninham, B. W. Contributions from Dispersion and Born Self-free Energies to the Solvation Energies of Salt Solutions. J. Phys. Chem. 2004, 108, 12593–12595. (21) Izutsu, K. Electrochemistry in Nonaqueous Solutions; Wiley-VCH Pub: Verlag GmbH, Weinheim, 2002.
ReceiVed for reView April 3, 2009 ReVised manuscript receiVed August 2, 2009 Accepted August 28, 2009 IE900542W
