ARTICLE pubs.acs.org/IECR
Alternative Influence of Binary Surfactant Mixtures on the Rate of Mass Transfer in a LiquidLiquid Extraction Process Javad Saien* and Somayeh Rezabeigy Department of Applied Chemistry, BuAli Sina University, Hamedan, 65174, Iran ABSTRACT: The effect of binary mixtures of cationic and nonionic surfactants dodecyltrimethylammonium chloride (DTMAC) and Triton X100 on hydrodynamic and mass transfer of the liquidliquid extraction process was investigated. Mass transfer coefficients of acetone from toluene drops to the continuous aqueous phase and inverse direction were obtained. For the acetone analysis, gas chromatography was used. The rate of mass transfer closely relates to the adsorption behavior of surfactant mixtures at the interface, and the overall mass transfer coefficient, Kod, exhibits the similar variation of surfactants molecular interaction at the interface. The minimum mass transfer rate appears at Triton bulk mole fractions (surfactants based only) of about 0.01 and 0.2 and the maximum at about 0.6. The latter introduces the best composition to achieve the highest separation rate. Meanwhile, the mass transfer coefficient decreases rapidly with a tiny amount of total surfactants under a constant bulk mole fraction, and the solute transfer is always more for dispersed to continuous phase direction.
1. INTRODUCTION In a major series of liquidliquid extraction devices, the solute is transferred between drops of a dispersed phase and bulk of an immiscible continuous phase. Due to the presence of different contaminants such as surfactants and trace impurities in many industrial materials which are processed, the efficient design of contactors requires the study of solute mass transfer from/into the drops which their surface are imposed to contaminants.1 The reduction of mass transfer within dispersed drop systems, containing surfactants, has been attributed to the important changes in hydrodynamic characteristics of the moving drops and physicochemical properties of the phases.1,2 The contaminant species accumulate at the interface between phases, inhibit circulation within drops, cause adsorptive barriers to transfer across the interface, and change the pattern of drop behavior to some extent, depending on type and concentration of contaminants.3 A lot of studies have been carried out about this matter and attempted to identify the mechanisms of mass transfer in the presence of different contaminants.29 Moreover, the influence of individual surfactants has also been investigated for the liquid liquid extraction of the metals process, as well as reactive mass transfer at fluid interfaces.10,11 The widespread use of surfactant mixtures for industrial purposes has stimulated the interest of the researchers, and in recent years, many papers have been published on the solution properties of mixed surfactant systems. The reasons can be mainly due to (i) technical-grade surfactants are themselves mixtures, and the purification process may be difficult or excessively expensive and (ii) the surfactant mixtures, as a reality, exhibit different behaviors and may show synergism or antagonism effects with mole fraction; therefore, optimization of the surfactant mole fraction seems vital for reaching the best required physical properties such as interfacial tension and viscosity.1216 Many recent investigations are concerned about different structural aspects of solutions, composed from a two component surfactant mixture,17 and studies have been carried out to r 2011 American Chemical Society
concern the aggregation behavior of both ionic and nonionic surfactants in different solvent systems.1820 Moreover, the influences of mixed surfactants on the surface and interfacial tension and interaction between surfactants have been demonstrated in a number of publications.2123 Despite the abovementioned enormous works and while it has a high potential attraction, no investigation has been reported for the changes of a solute mass transfer rate when a mixture of surfactants is present in the liquidliquid extraction process. As a part of our continuous studies, the role of binary surfactant mixtures in the liquidliquid extraction process is studied in this work. The rate of mass transfer for single drops moving in the extraction column is determined, and both directions of mass transfer are considered. The findings from these studies provide a scientific basis to understand the role of different compositions of surfactants or to choose the materials and compositions to suit higher mass transfer requirements in extraction.
2. EXPERIMENTAL SECTION 2.1. Materials. The chemical system of tolueneacetone water, a recommended high interfacial tension system for liquidliquid extraction studies,24 was employed. Toluene and acetone were Merck products with purities of more than 99.9%. Distilled water was produced from a new still and was used for the continuous phase. Mixtures of the following surfactants, as widely used simulating industrial contaminants, were used (i) the nonionic surfactant, octylphenoldecaethylen glycol ether (Triton X100) and (ii) the cationic surfactant dodecyltrimethylammonium chloride (DTMAC). Received: November 11, 2010 Accepted: April 11, 2011 Revised: January 31, 2011 Published: April 21, 2011 6925
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Table 1. Total Aqueous Phase Concentrations, Ct for Different Triton Bulk Mole Fractions and under the Used Interfacial Tensions and for the Constant Bulk Composition under Different Interfacial Tensions at 25 °C R
Figure 1. Chemical structure of Triton X100 (a) and DTMAC (b).
Triton X100 and DTMAC were Merck products with purities of more than 99.5% and 98%, respectively. Both the surfactants are watersoluble and were used without further purification. Figure 1 displays the chemical structure of the used surfactants. 2.2. Setup and Operating Procedure. The experimental setup has been described in our previous works.3,25 A Pyrex glass column (11.4 cm diameter and 51 cm height) was used as the contactor. Different drop size forming was provided using a variety of glass nozzles, located at the bottom of this column. The outside diameter of the nozzles was typically on the order of 0.3 mm. The toluene phase was held in a glass syringe, conducted by an adjustable syringe pump (JMS SP-500, Japan) and flowed through a rigid tube to the glass nozzle into the column, and the column was filled with aqueous phase. A small inverted glass funnel was used to catch samples of about 1 mL of dispersed phase at the top of the column with a 33 cm 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.25 Three samples were prepared for each concentration and immediately were analyzed with gas chromatography (GC; Shimadzu, 14B) with a flame ionization detector, calibrated with reference substances of toluene and acetone (Merck) for gas chromatography.3 For calibration, the normalization method was used and all calculations were based on response factor, based on reference solutions. In order to omit the influence of unsteady mass transfer during the drop formation and its transient rise velocity, the initial drop concentration was considered for a location of moving drops at 6.5 cm above the nozzles’ tip. Drop motion was observed to reach steady movement after about 40 mm of travel. To determine the initial concentrations, 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 as the main column. The influence of mixture of surfactants was investigated with both kinds of variations of (i) The use of different bulk mole fractions with their appropriate total concentration, under a specified constant interfacial tension. The appropriate results reveal the influence of composition and molecular interaction on the mass transfer. (ii) The use of different total bulk concentration (i.e., different interfacial tensions) for a specified bulk mole fraction. The results in this case reveal the influence of the amount of surfactant mixtures on the mass transfer. For the first above case, on the basis of data reported for interfacial tension variations with different bulk mole fractions of surfactants,21 two constant interfacial tensions of 26.0 and 28.0 mN 3 m1 were considered, and the appropriate aqueous phase concentrations were examined. The dropweight method has been used to determine the interfacial tension of this system.26,27 The total bulk concentration of surfactants, Ct, under different bulk mole fractions, R(= C1/Ct, C1, and Ct: Triton and total concentration of Triton þ DTMAC), are given in Table 1.
Ct (g 3 L1) at Ct (g 3 L1) Ct (g 3 L1) at 1 1 1 γ (mN 3 m ) at R = 0.2 γ = 26.0 mN 3 m γ = 28.0 mN 3 m
0.0
0.151
0.066
30.0
0.006
0.005
0.102
0.045
28.0
0.011
0.01
0.074
0.033
26.0
0.020
0.05
0.052
0.024
24.0
0.035
0.10
0.035
0.017
0.20 0.40
0.020 0.015
0.011 0.009
0.60
0.010
0.007
0.80
0.008
0.006
1.00
0.006
0.002
The used interfacial tensions include a range of total surfactant concentrations of 0.0060.151 g 3 L1 and 0.0020.066 g 3 L1 for constant interfacial tension values of 26.0 and 28.0 mN 3 m1, respectively; appropriate to the perfect surfactant mole fraction range of 01. It is notable that the interfacial tension of clean toluene and water is about 36.8 mN 3 m1 at 25.0 °C.21 Because of high activity of Triton, several low mole fractions of this surfactant (e0.1) were examined. It is worthy to note that the maximum used Triton and DTMAC concentrations were 0.006 and 0.151 g 3 L1, respectively, which are far from their critical micelle concentration (CMC) in aqueous solutions, reported as at least 0.138 g 3 L1 for Triton22,28 and 4.22 g 3 L1 for DTMAC.29 Experiments were also conducted with different total concentrations of surfactants while their composition was maintained constant. A typical Triton mole fraction of R = 0.2 was used for which the appropriate total surfactant concentrations and interfacial tensions are given in Table 1. Surely, higher interfacial tension values are corresponding to lower total surfactant concentrations under a specified surfactant composition. The aqueous solutions, with a specified surfactant mole fraction and total concentration, were prepared in a 6 L volumetric flask, and the appropriate weights of surfactants (Tables 1) were added. Then, the column was rinsed and filled with the solution as the continuous phase. Experiments were conducted in both directions of mass transfer, continuous to dispersed (cfd) and dispersed to continuous (dfc). The syringe and the connection tube to the nozzle tip were first filled with toluene or toluene þ acetone (depending on the mass transfer direction) to produce drops, and the column was then filled with distilled water containing the mixture of surfactants, as the continuous phase. Acetone was dissolved in the aqueous or organic phase with initial concentrations of 44.66 g 3 L1 (5.25%) and 28.2 g 3 L1 (2.85%), for dfc and cfd directions, respectively. For each series of experimental data, initial and final concentrations, drop size, and contact time were obtained. Drops were spaced more than 60 mm apart, typically employing a range of flow rate between 45.06 and 97.42 mL 3 h1. Skelland and Vasti30 have shown that interaction that drops is negligible for distances around this value or even less. The size of drops was determined by knowing the flow rate and the number of drops per a specified period; drop volume was 6926
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easily calculated. All the equipment and glassware were cleaned with Decon 90 solution, followed by several rinses with distilled water, prior to experiments. The contact time of drops (required for terminal velocity and mass transfer calculations) from the initial to the collection point was measured several times with a stopwatch. The terminal velocity (ut) was calculated using the time, t, that drops pass the travel distance, h, which was 33 ( 0.1 cm in the apparatus. 2.3. Viscosity Measurements. Regarding the variation of viscosity with the surfactant addition, this property was measured for the continuous phase, containing the appropriate surfactant concentrations (Ct) and composition (R). An Ubbleohde viscometer with an uncertainty of (2 103 mPa 3 s was used. The equation for viscosity, according to Poiseuille’s law, is ð1Þ
and aqueous phases were considered from those reported by Wegener et al. at 25 °C.33 2.4. Equilibrium Solute Distribution. Liquidliquid equilibrium data of acetone distribution between phases and in the presence of different concentrations and compositions of surfactants were determined. A weighed amount of aqueous solution, containing a known quantity of acetone (within 4.563 g 3 L1 in aqueous phase), was mixed with a known amount of solvent in a stopper funnel.34 Quantities of surfactants were added to make corresponding mixtures of different mole fractions. These mixtures were maintained at a constant temperature of 25 °C using a thermostat and stirring on a magnetic stirrer plate for 2 h, until acetone was completely distributed between two phases. The equilibrium was finally achieved by letting the mixture rest in closed containers for 12 h. After separation, concentration of acetone in the organic phase was measured
where μ, F, and t are dynamic viscosity, density, and efflux time and k and c are the viscometer constants, respectively. Density of the pure compounds and mixtures was measured by an Anton Parr DMA4500, provided with automatic viscosity correction. The uncertainty for density measurements was (0.01 kg 3 m3. The apparatus was calibrated with dry air and bidistillated water. The temperature in the cell was regulated to (0.01 °C with a solid state thermostat. The k and c parameters were obtained by measurements on double distilled water and benzene. The variation of solution viscosity with total surfactant concentration (under constant typical Triton mole fraction of 0.2) and with Triton mole fraction (under constant interfacial tensions of 26 and 28 mN 3 m1) is presented in Figure 2. It is clear that higher dosages of surfactants (corresponding to the lower interfacial tensions) give rise to the variation in viscosity which agrees with the previous reports.3,31 An alternative variation is corresponding to the viscosity with composition of surfactants under constant interfacial tensions. The ascending and descending viscosity variation with composition of ionic and nonionc surfactants has been reported.32 This sort of variation can be due to the interactions between molecules of surfactants and between them and surrounding water molecules. Here, the viscosity of the aqueous continuous phase, containing surfactants, is mainly influenced by two parameters: (i) hydration of the surfactants’ molecules, which is more appropriate for the Triton molecules due to its long polyoxyethylene group in its structure (Figure 1), and (ii) an electroviscous effect for DTMAC molecules which provides iondipole interaction with water molecules. Moreover, the concentration of surfactants decreases with Triton bulk mole fraction to have a constant interfacial tension (Table 1). It has to be noted that a maximum tolerance of about (7 103 mPa 3 s, around average viscosity, is observed while the bulk mole fraction varies. The range of physical properties of the chemical system at 25 °C are given in Table 2. The molecular diffusivities in organic
Figure 2. Viscosity of continuous phase versus total surfactant concentration, under constant Triton bulk mole fraction of 0.2 (a) and versus mole fraction of Triton under the used constant interfacial tensions (b).
μ ¼ Fðkt ðc=tÞÞ
Table 2. Physical Properties of the Chemical System at 25 °C phase dispersed continuous
a
F (kg 3 m3) 870.11
μ (mPa 3 s1) 0.619
(996.91997.09)a
(0.9350.948)a
(997.05997.10)b
(0.9640.975)b (0.9320.984)c
1010 D (m2 3 s1)
γ (mN 3 m1)
29
(26 or 28)
12.5
(24.030.0)c
Constant interfacial tension of 28 mN 3 m1. b Constant interfacial tension of 26 mN 3 m1. c Constant Triton bulk mole fraction of 0.2. 6927
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Figure 4. Comparison between experimental terminal velocity with the Grace correlation for γ = 28 mN 3 m1 and both mass transfer directions.
where H is defined as: H ¼ ð4=3ÞE€oM 0:149 ðμc =μw Þ0:14
Figure 3. Variation of drop size with bulk mole fraction of Triton for dfc (a) and cfd (b) mass transfer directions, γ = 28.0 mN 3 m1.
using the GC calibrated with reference substances of toluene and acetone (Merck).3 The remaining amount of acetone in the aqueous phase was then calculated from simple material balance.
3. RESULTS AND DISCUSSION 3.1. Hydrodynamic Investigations. The size of drops does not vary much, while using different surfactant compositions under constant interfacial tension and the changes in density and viscosity of phases (Table 2) do not alter in this regard. As results show (Figure 3), a maximum deviation of about 12% is appropriate for drop size with R for either of the used nozzles. Drops of larger sizes were obtained when the mass transfer direction was cfd, as previously reported too.3,25 The correlation of Grace et al.,35 which has been recommended for terminal velocity of systems with some contamination, can be used for the comparison with experimental results:
ut ¼ ðμc =dFc Þ½ðJ 0:857Þ=M 0:149
ð2Þ
where d, μc, and Fc stand for the drop size, viscosity, and density of continuous phase, respectively. The parameter J is defined as J ¼ 0:94H 0:757 ;
ð2 < H e 59:3Þ
ð3Þ
ð4Þ
where μw is the viscosity of water at 4 °C. Morton number (M) and E€otv€os number (E€o) are two dimensionless numbers that are defined as M = gμc4ΔF/Fc2γ3 and E€o = gΔFd2/γ. The value of 59.3 for H corresponds, approximately, to the transition between circulating and oscillating drops. Figure 4 shows that there is relatively a good agreement between experimental data and the Grace correlation. The effect of surfactants on the hydrodynamic characteristics of the moving drops includes the variation in the internal circulation rate and hindering the interfacial movements due to the gradients of the interface accumulation. Drops in this work lie within 5.07 < H
