Article pubs.acs.org/IECR
Single-Pass Emulsification Processes in Two Different Inline High Shear Mixers Jintao Shi,† Shuangqing Xu,†,‡ Hongyun Qin,† Yulong Liu,† Wei Li,† and Jinli Zhang*,†,§ †
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China General Machinery Research Institute (GMRI), Hefei 230031, PR China § College of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, PR China ‡
ABSTRACT: Single-pass continuous emulsifications were studied in kerosene− and silicone oil−aqueous systems with two inline high shear mixers (HSMs) adopting two main commercial configurations, including the dual-row ultrafine-toothed and the single-row blade−screen units. The effects of the processing parameters on the measured drop sizes and power consumptions were investigated. Bimodal drop size distributions (DSDs) were observed within both inline HSMs due to the inhomogeneity of the turbulence and shear levels accompanied with the recirculation and re-entrainment flow patterns. The drop sizes increase with increases in the dispersed-phase volume fraction and the continuous-phase flow rate and decrease with increases in the rotor speed and the continuous-phase viscosity. Correlations for the Sauter mean diameters were also obtained. The results obtained here are fundamental for the assessment of emulsification capability as well as the design and selection of inline HSMs.
1. INTRODUCTION Emulsification of two immiscible liquids is an important unit operation in the chemical, petroleum, food, and pharmaceutical industries. High shear mixers (HSMs) have great potential applications to intensify this typical energy-intensive operation, with the characteristics of high rotor tip speeds, high shear rates, and highly localized energy dissipation rates near the mixing head.1−4 HSMs can be generally categorized into the batch and the inline operation modes. Within the batch mode, Calabrese and co-workers5−10 have intensively studied the emulsification process using different configurations of HSMs. Francis5 investigated low-viscosity dispersions in a bench-scale batch HSM with different stator geometries, suggesting that the stator geometry was more important than the shear gap width to adjust the drop size distribution (DSD), and no effect on the dispersed-phase volume fraction was observed. Phongikaroon7 studied the effects of the viscosities of the continuous and dispersed phases as well as the interfacial tension, indicating that the mean drop size decreased with increasing continuousphase viscosity while decreasing dispersed-phase viscosity and the interfacial tension. Padron10 investigated the effect of surfactants on the DSD and found that the surfactant not only lowered the interfacial tension and protected the newly formed droplets against coalescence but also modified the interfacial rheology of the system. These measured DSDs are close to the Kolmogoroff length scale ηK, indicating the existence of multiple breakage mechanisms.5,6,8 As for the inline HSMs, with the advantages of continuous operation, short residence time, and high throughput,3,11 the single-pass operation mode is often utilized for emulsifications; the dispersed phase can be introduced by the pipeline mixing tee,2,12 direct injection into the mixing head,13 or predispersion in a feed tank.14 The commercial inline HSMs are usually designed to be either rotor−stator toothed or a blade−screen mixer. For the single-pass emulsification in typical blade−screen inline HSMs, Thapar2 observed broad DSDs due to the © 2013 American Chemical Society
different turbulence and shear levels within the mixer, and found a critical rotational speed above which the drop size cannot be reduced further. Kevala13 found that the shape of the DSD varied with the drop viscosity, and the DSD shifted between monomodal and bimodal as the rotational speed and residence time increased. Gingras12 investigated bitumen emulsion using an inline pilot-scale blade−screen HSM, suggesting that the dispersed-phase viscosity, the rotor speed, and the dispersed-phase content have a significant effect on the drop size. Similar to the drop sizes generated by the batch HSMs,5,6,8 the inline HSMs can generate drop sizes lowered to the same order of ηK.14 However, note that the reports on emulsification using inline HSMs have so far been predominantly focused on the blade−screen configuration. Much less attention has been paid to emulsification accomplished by using inline toothed HSMs, although patented chemical productions by the toothed units have been issued increasingly in recent years.15−22 Emulsification process design and scale-up require accurate drop size correlations. Large amounts of theoretical and experimental work can be found in the literature concerning the prediction of the DSD in turbulent dispersions; most of that work refers to the classical mechanistic models of Kolmogoroff23 and Hinze,24 with the assumption of homogeneous isotropic turbulence. The surface area weighted mean drop diameter, d32, also known as the Sauter mean diameter, is a key parameter in describing the emulsification process due to its direct relation to the dispersed-phase content and the interfacial area per unit volume. Expressions for correlating d32 have been reported by many authors; these expressions depend on the relative magnitude between the drop size d and the Received: Revised: Accepted: Published: 14463
April 10, 2013 August 10, 2013 September 9, 2013 September 9, 2013 dx.doi.org/10.1021/ie401145j | Ind. Eng. Chem. Res. 2013, 52, 14463−14471
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Figure 1. Experimental geometry of the inline high shear mixer. (1) Process flow diagram of the continuous emulsification rig. (2) Geometric details of (a, c) the stator and (b, d) the rotor in (a, b) the toothed and (c, d) the blade−screen inline high shear mixers.
Table 1. Physical Properties of All Fluids Investigated at 15 °C dispersed phase
continuous phase
μc (mPa s)
ρc (kg/m3)
μd (mPa s)
ρd (kg/m3)
σ (mN/m)
kerosene kerosene kerosene silicone oil
water gly #2 gly #3 water
1.2 2.7 13.0 1.2
999.5 1065.0 1143.0 999.5
1.5 1.5 1.5 585.0
785.0 785.0 785.0 970.0
25.3 23.1 20.5 22.3
Kolmogoroff length scale ηK. Padron10 summarized mechanistic models for correlating d32 in different subranges, namely, the inertial subrange (d ≫ ηK), the viscous subranges with the dominant drop breakup forces of the inertial stresses (d < ηK), and the viscous stresses (d ≪ ηK). The effect of the dispersedphase volume fraction Φ can be accounted for by the modification of mechanistic models with a function of Φ, when the dispersion is well stabilized against coalescence and the rheological behavior does not alter.25 d32 = C1(1 + C2 Φ)We−C3 D
rows of 52 teeth (axially straight, with outer diameters of the inner and outer rows of 47 and 59.5 mm, respectively) with 1 mm slots between teeth, while the stator has two rows of 30 teeth (15° backward inclined, with outer diameters of the inner and outer rows of 53.5 and 66 mm, respectively) with 2 mm slots between teeth. The shear gap width (i.e., the annular space between the assembled rotor and stator) is 0.5 mm, and the tipto-base clearance (i.e., the axial space from the rotor tip to the stator base, or that from the stator tip to the rotor base) is 1 mm. Geometric details of the stator and rotor teeth are shown in Figure 1(2). Alternatively, the mixer can be assembled into a blade− screen configuration in which the rotor has a single row of 6 blades (15° backward inclined, with the same outer diameter of 59.5 mm) while the single-row stator screen (with the same outer diameter of 66 mm) has two rows of 3 × 3 mm square holes with 30 holes in each row. The shear gap width and the tip-to-base clearance are kept the same as those of the toothed design, that is, 0.5 and 1 mm, respectively. In either case, the opening areas are calculated to be 23.6% of the outer circular face for the toothed and the blade−screen stators. In both inline HSMs, the heights of the rotors are 14 mm including the bases. Figure 1(2) shows the geometric details of the blades and screen holes. 2.2. Materials and Measurement Techniques. The Newtonian fluids of pure water and aqueous solutions of glycerin (Sinopharm Chemical Reagent Co., Ltd.) were used as the continuous phase. Two different oils were used as the dispersed phase: kerosene and silicone oil (Tianjin Guangfu Fine Chemical Research Institute). Tween 80 (Tianjin Guangfu Fine Chemical Research Institute, China) was added to the continuous phase as an emulsifier to avoid coalescence of the dispersed phase. The critical micelle concentration of Tween 80, reported to be about 0.0013 wt%,33 is less than the concentration of Tween 80 used in the present work (0.002 wt %). The densities of all fluids were measured by the
(1)
where Φ is the dispersed-phase volume fraction, the coefficient C1 depends on the geometry of the impeller, especially its power number,26 while C2 is a measure of the tendency of the drops to coalesce, with a wide range typically from 0.5 to 20 in the literature.27−32 In this study, the emulsification capabilities of the two main commercial inline HSMs that consist of the dual-row ultrafinetoothed and the single-row blade−screen units were evaluated under different operating conditions. The effects of the processing parameters on the measured drop sizes and power consumption were investigated, and correlations for the Sauter mean drop diameters were obtained in order to promote the understanding not only of the drop breakup mechanisms inside the inline HSMs but also of the emulsification process design and scale-up via inline HSMs.
2. EXPERIMENTAL METHODS 2.1. Experimental Apparatus. The experimental apparatus is shown in Figure 1, where the inline HSM is a custombuilt pilot-scale unit of FDX series provided by FLUKO. The rotor and stator of the mixer are designed to be interchangeable so that the two main commercial rotor−stator designs can be expediently tested in the same model. Originally, the mixer is a rotor−stator toothed design, where the rotor consists of two 14464
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Figure 2. Influence of the rotor speed on the volume drop size distributions for (a, b) kerosene and (c, d) silicone oil at the water flow rate of 0.2 m3/h and the dispersed-phase volume fraction of 0.01 in the (a, c) toothed and the (b, d) blade−screen inline high shear mixers.
influence on the measured bearing loss. The power consumption of the mixer is expressed as
pycnometer method. A viscometer (LVDV-II+Pro, Brookfield) was utilized for measuring the viscosities of the continuous phase and the dispersed phase. The interfacial tensions between the oils and the continuous phases were measured using an automatic surface tensiometer (JK99B, Powereach Co., Ltd.). The physical properties of the continuous and dispersed phases are presented in Table 1. The drop size distributions were measured by laser light diffraction using Mastersizer S (Malvern Instruments, Malvern, UK); the median volume diameters of the drops ranged from 0.02 to 900 μm ±1%. The relative refractive indices (RI) used were 1.33 for the continuous phase and 1.43 for the dispersed phase. The shaft torque and the rotor speed were measured on the drive shaft by using an AKC-215 transducer (China Academy of Aerospace Aerodynamics) and recorded by the data-logging system at a frequency of 1 Hz. The bearing losses were measured at zero flow rate by rotating the shaft at different rotor speeds with the rotors detached. The underlying assumption is that the power consumed by drag on the shaft is negligibly small when compared to the bearing loss. Therefore, the power measured with the rotors detached and with pure water as the working fluid accounts for the bearing losses in this paper. It was found that, when using low-viscosity water as the working fluid, the type of fitted stator (i.e., the toothed or the blade−screen configuration) shows no obvious
Pfluid = 2πNM − 2πNM n
(2)
where Pfluid is the power consumption (W), N is the rotor speed (s−1), M is the torque (N·m), and Mn is the torque (N·m) of the bearing loss correction, measured using water and no rotor attached. 2.3. Experimental Procedure. The continuous phase (with Tween 80) was fed by a centrifugal pump; therefore, the flow rate and the rotational speed of the inline HSMs were controlled separately. The flow rate Q was varied from 0.2 to 1.5 m3/h, and the rotor speed N was varied from 2000 to 3500 rpm. The oil phase was introduced to the aqueous line upstream of the HSM via a T-piece by a peristaltic pump. The oil-phase flow rate was controlled by the rotational speed of the peristaltic pump so that different dispersed-phase contents could be investigated. Emulsion samples were collected downstream from the HSM at the steady-state flow rate and rotor speed.
3. RESULTS AND DISCUSSION 3.1. Effect of Rotor Speed. Emulsions were prepared using kerosene and silicone oil under different rotor speeds with a volume fraction of 0.01 and a water flow rate of 0.2 m3/h in the two inline HSMs. As shown in Figure 2, the single-pass 14465
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emulsification process produced bimodal drop size distributions (DSDs) for both oils in both HSMs. Increasing the rotor speed reduces the volume percentage of larger drops while increasing the volume percentage of smaller ones, especially for the case of silicone oil. For pure water as the continuous phase, the Kolmogoroff length scale ηK is calculated to be on the order of 9.9−13.9 μm, corresponding to the rotor speed of 2000−3500 rpm. Applying the Kolmogoroff theory to this data suggests that a turbulent inertial breakage mechanism is responsible for the kerosene-in-water emulsions since the majority of drop sizes are greater than the ηK.14 For the viscous silicone oil-inwater emulsions, the drop sizes are close to or less than the ηK, indicating the existence of multiple breakage mechanisms. For low-viscosity oil drops, simple shear flow is often considered to be the predominant breakage pattern, while the breakage of high-viscosity oil drops is more dependent on the extensional flow than it is on simple shear flow.10 In the deformation process, higher-viscosity drops will stretch more prior to breaking up, resulting in a larger population of smaller drops, increased bimodality, and reduced mean drop size (as shown Figure 2). In the continuous emulsification, most drops are discharged from the mixer after single-pass breakage with short residence time. After subjecting the emulsion to inhomogeneous turbulence and shear at different locations within the mixers, the bimodal daughter drop size distribution is generated. Because of the recirculation and re-entrainment flow pattern, some daughter drops go through multiple breakages and contribute to the smaller drop sizes in the DSD (no matter whether the large drops break up into smaller ones, or the relatively smaller ones break up into more even finer ones). Figure 3 presents the Sauter mean diameter d32 as a function of rotor speed for kerosene and silicone oil emulsions. The
Figure 4. Power consumption of the two inline HSMs as a function of the rotor speed for kerosene and silicone oil at the water flow rate of 0.2 m3/h and the dispersed-phase volume fraction of 0.01.
two HSMs presents similar variations with the rotor speed. In comparison, the silicone oil emulsions not only draw more power than the kerosene systems under all rotor speeds but also break the aforementioned drops into smaller sizes. 3.2. Effect of Dispersed-Phase Volume Fraction. Four oil volume fractions, namely, Φ = 0.01, 0.02, 0.05, and 0.1, were considered at a water flow rate of 0.2 m3/h, using the kerosene−water system in the toothed HSM. As shown in Figure 5, the measured DSDs are bimodal and shift slightly toward the larger drop size spectrum with increased oil volume fraction. An increase of the Sauter mean drop size with oil volume fraction Φ is observed accordingly (see Figure 6a), consistent with the results in the literature.2 For the kerosene− water system with the presence of surfactant to prevent coalescence, this increase of drop size with the oil volume fraction can probably be attributed to the turbulence dampening.28,34−36 The increased volume fraction of the oil phase results in a reduction of turbulence intensity, leaving the eddy lengths much larger than the drop diameters. As a result, the drops cannot be broken further. Figure 6b shows the Sauter mean diameter d32 as a function of the Weber number We at different dispersed-phase volume fractions. Since the interfacial tension is constant in the emulsion system, We is a measure of the inertial stresses caused by turbulent eddies. Therefore, the drop diameter decreases with increasing Weber number. The Sauter mean diameter d32 is correlated with the oil volume fraction Φ and the Weber number We by applying eq 1, with R2 = 0.980. d32 = 0.043(1 + 2.32Φ)We−0.53 D
Figure 3. Influence of the rotor speed on the drop size for kerosene and silicone oil at the water flow rate of 0.2 m3/h and the dispersedphase volume fraction of 0.01 in the two inline high shear mixers.
(Φ ≤ 0.1, N ≤ 3500 rpm)
(3)
The C3 of 0.53 is close to the theoretical value of 0.6 from the mechanistic model.25 The C2 of 2.32 is within typical values from 0.5 to 20 in the literature and indicates a relatively low coalescence tendency. For a double-row blade−screen Silverson inline HSM, the values of C1, C2, and C3 are reported to be 0.250, 0.459, and 0.58, respectively.14 That is to say, when processing emulsions at the same mixer scale and We, the
plots clearly show a stepped drop in d32 with increasing rotor speed in the HSMs due to the increased energy dissipation rate and shear rate. There were minor differences between the d32 at different rotor speeds for kerosene-in-water emulsions in the two mixers. For silicone oil emulsions, similar trends were observed and finer drops were produced than in kerosene emulsions. Shown in Figure 4, the power consumption of the 14466
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Figure 5. Influence of the dispersed-phase volume fraction on the volume drop size distributions for kerosene−water systems at the water flow rate of 0.2 m3/h in the toothed inline high shear mixer with rotor speed (a) 2000 rpm and (b) 3500 rpm.
Figure 6. (a) Influence of the dispersed-phase volume fraction on the drop size for kerosene−water systems at the water flow rate of 0.2 m3/h in the toothed inline high shear mixer. (b) The drop size as a function of Weber number We at different dispersed-phase volume fractions.
slight increase of the Sauter mean diameter with the flow rate as shown in Figure 9. For the single-pass inline HSMs, an increase in flow rate results in the reduction of residence time, which means that the fluids have not been broken down to a critical drop size before discharge. On the other hand, an increase in flow rate increases the velocity of the jets emanating from the holes in the stator,2 which is beneficial to the drop breakage. Hence, the drop size shows a small dependence on the continuous-phase flow rate combining the two effects. However, note that the rotor tip speeds at 2000−3500 rpm were 6.23−10.90 m/s (calculated on the basis of the outer swept rotor diameter), while the jet velocity through the slots at 0.2−1.5 m3/h was 0.10−0.77 m/s (calculated through the open area of the outer stator). Since the jet velocity is relatively milder than the rotation, the effect from the increased jet turbulence is not so pronounced. As presented in Figure 9, the Sauter mean diameters have minor differences for the two HSMs at Q = 0.2 m3/h, but these differences step up with increasing continuous-phase flow rate. The drop sizes are relatively larger and more dependent on the continuous-phase flow rate in the blade−screen HSM as
ultrafine-toothed inline HSM can produce smaller drop sizes than the Silverson unit. As shown in Figure 7, the power draw remains almost flat across the range of the dispersed-phase volume fraction at lower rotor speeds but slightly increases at higher rotor speeds. This means the power delivered by the rotor to the fluid is not increased with the dispersed-phase volume fraction. This may help to explain some of the dependency of drop size on the dispersed-phase volume fraction. 3.3. Effect of Continuous-Phase Flow Rate. For singlepass continuous emulsification, the continuous-phase flow rate is a main operational variable to adjust processing time. Limited studies were performed to investigate the relationship between flow rate and drop size in the emulsification using inline HSMs. In this section, the effect of continuous-phase flow rate on drop size was investigated with the kerosene−water system in the two inline HSMs. Figure 8 shows the measured DSDs at different continuousphase flow rates with the oil volume fraction of 0.01 and a rotor speed of 3500 rpm. The DSD shifts toward the larger size range with increasing continuous-phase flow rate. This leads to a 14467
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Figure 9. Influence of the continuous-phase flow rate on the drop size for kerosene−water systems at the dispersed-phase volume fraction of 0.01 in the two inline high shear mixers.
Figure 7. Power consumption of the toothed HSM as a function of the dispersed-phase volume fraction for kerosene−water systems at the water flow rate of 0.2 m3/h.
use the estimated power draw based on Kowalski’s model38 with the experimentally determined model constants.
compared to the toothed unit. A previous study suggests that the blade−screen HSM has a better pump capacity than the toothed one due to the configurational difference.37 For identical rotor speeds the power consumed in the dispersion within the blade−screen HSM is lower than that in the toothed unit (see Figure 10). Figure 10 also shows that the dependence of energy dissipation rate on the continuous-phase flow rate is close for the two HSMs at 2000 rpm, and the discrepancy becomes more obvious at 3500 rpm. The relatively smaller drop sizes in the toothed HSM are also attributed to the dual rows and ultrafine-toothed design of the mixer, which can avoid the channeling and/or short-circuiting defects that may occur in the single-row coarse-toothed units.1,4,8 The drop size from the toothed HSM is correlated using eq 4 with R2 = 0.90, which is analogous to eq 1 with a substitution of energy density for We. The measured power draw is used in the data fitting here. However, if the equation is applied to the mixer design where the actual power input is unknown, one can
⎛ Pfluid ⎞−0.23 d32 = 0.029(1 + 3.46Φ)⎜ ⎟ D ⎝ Q ⎠ (Φ ≤ 0.1, N ≤ 3500 rpm, 0.2 ≤ Q ≤ 1.5 m 3/h)
(4)
3.4. Effect of Continuous-Phase Viscosity. The continuous-phase viscosity μc was changed by adding varying amounts of glycerin to water in the emulsion preparation. As shown in Table 1, the glycerin addition also produced a slight decrease in the interfacial tension, which is beneficial for breaking drops into smaller drops. The effect of the continuousphase viscosity on the drop size was studied at the kerosene volume fraction Φ = 0.01 and the continuous-phase flow rate Q = 0.2 m3/h. Figure 11 presents the influence of the continuousphase viscosity on the DSD at 3500 rpm for the two inline HSMs. The volume percentage of the small drop size increases with the continuous-phase viscosity in two HSMs due to the
Figure 8. Influence of the continuous-phase flow rate on volume drop size distributions for kerosene−water systems at the dispersed-phase volume fraction of 0.01 in the (a) toothed and the (b) blade−screen inline high shear mixers. 14468
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Figure 10. Energy dissipation rates of the two inline HSMs as a function of the continuous-phase flow rate for kerosene−water systems at the dispersed-phase volume fraction of 0.01.
Figure 12. Influence of the viscosity ratio on the drop size for kerosene−water systems at the dispersed-phase volume fraction of 0.01 and the water flow rate of 0.2 m3/h in the two inline high shear mixers.
⎛ μ ⎞−0.021 d32 = 0.11⎜⎜ d ⎟⎟ We−0.63 μ D ⎝ c⎠
more effective transmission of shear stress to the droplet interface.14 As shown in Figure 12 the drop size decreases slightly with decreasing viscosity ratio. According to the Kolmogoroff theory, the increase of the continuous-phase viscosity μc leads to an increase in the size of the smallest turbulent eddy ηK. As a result, some drop sizes are smaller than ηK, and the viscous stresses become an important role in the drop breakage.14 The Sauter mean diameter d32 is correlated with the following equations. For the toothed inline HSM ⎛ μ ⎞0.0074 d32 = 0.048⎜⎜ d ⎟⎟ We ‐0.55 D ⎝ μc ⎠
⎛μ ⎞ (0.1 ≤ ⎜⎜ d ⎟⎟ ≤ 1, N ≤ 3500 rpm) ⎝ μc ⎠
(6)
These correlations indicate that the effect of the viscosity ratio on drop size is relatively weak. The discrepancy in the dispersion capability of the two HSMs may be considerable when processing high-viscosity materials or operating at high rotor speed. On the other hand, despite the approximate d32, the DSDs from the toothed HSM are in general slightly narrower than those from the blade−screen unit, indicating superior dispersion performance from the dual-row ultrafinetoothed design. For example, the span of DSDs, which is a measurement of the width of the distribution (calculated by eq
R2 = 0.990
⎛μ ⎞ (0.1 ≤ ⎜⎜ d ⎟⎟ ≤ 1, N ≤ 3500 rpm) ⎝ μc ⎠
R2 = 0.996
(5)
For the blade−screen inline HSM
Figure 11. Influence of the continuous-phase viscosity on volume drop size distributions for kerosene−water systems at the dispersed-phase volume fraction of 0.01 and the water flow rate of 0.2 m3/h in the (a) toothed and the (b) blade−screen inline high shear mixers. 14469
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mixers, while the discrepancy in these diameters becomes more obvious at higher flow rates. Generally speaking, the drop size distributions from the toothed HSM are narrower than those from the blade−screen unit, suggesting that inline HSMs with multiple rows of rotor−stator effectively overcome the defect of fluid bypass that is present in the single-row devices.
7), is 1.76 for the toothed HSM while it is 2.02 for the blade− screen unit at N = 3500 rpm and μd/μc = 0.1. span =
d0.9 − d0.1 d0.5
(7)
■
As presented in Figure 13, the power input for emulsification increases with increases in the continuous-phase viscosity in
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: 86-22-27890643. Fax: 8622-27890643. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by NSFC (21076144), the Special Funds for Major State Basic Research Program of China (2012CB720300), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1161). The authors gratefully acknowledge FLUKO Equipment Shanghai Co., Ltd. for providing the custom-built inline high shear mixer of FDX series and offering technical support. The authors also wish to thank colleagues from Chemical Engineering Experimental Center (CEEC) of Tianjin University for the debugging of the data-logging system.
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Figure 13. Power consumption of the two inline HSMs as a function of the continuous-phase viscosity at the dispersed-phase volume fraction of 0.01 and the water flow rate of 0.2 m3/h.
both mixers. And the toothed HSM draws higher power than the blade−screen unit under all continuous-phase viscosities. This helps to explain the effect of the continuous-phase viscosity and the configuration of the HSM on drop sizes.
4. CONCLUSIONS Drop size distributions of kerosene- and silicone oil-in-water emulsions in the presence of Tween 80 are presented in the single-pass continuous emulsification process using inline HSMs, for various oil-phase volume fractions, rotor speeds, continuous flow rates, and viscosities. The results indicate that the bimodal distribution of oil-drop size is obvious for all the cases in the single-pass emulsifications with inline HSMs. This type of distribution is caused by the different drop-breakup mechanisms, inhomogeneous turbulence, and shear at different locations within the mixer, as well as the fluid recirculation and re-entrainment. The volume percentage of small drops increases and the Sauter mean diameter d32 decreases with rotor speed due to the increased energy dissipation rate and shear rate. Drop sizes for silicone oil-in-water emulsions close to or below the Kolmogoroff length scale ηK suggest the existence of multiple breakage mechanisms. Both the oil-phase volume fraction Φ and the continuous-phase flow rate Q have a weak effect on the mean drop size. Also, the drop size decreases with increases in the continuous-phase viscosity due to the more effective transmission of shear stress to the drop interface. Correlations for the Sauter mean diameters have been developed for the emulsification process design. Within the confines of the emulsion systems studied here (i.e., noncoalescing, low to moderate oil viscosity, and volume fraction), the resulting Sauter mean diameters d32 are approximately equal at lower flow rates for the two types of
NOMENCLATURE C1, C2, C3 = constants, − D = outer rotor diameter, m d = drop size, m d32 = Sauter mean drop diameter, m d0.1, d0.5, d0.9 = drop diameters defined by cumulative volume frequencies, m FV = volume frequency, % M = torque, N·m Mn = torque measured without the rotor, N·m N = rotational speed, s−1 Pfluid = net power delivered to fluid, W Q = volumetric flow rate, (m3/h)
Greek symbols
ηK = Kolmogoroff length scale, m μc = continuous-phase viscosity, Pa·s μd = dispersed-phase viscosity, Pa·s ρc = continuous-phase density, kg/m3 ρd = dispersed-phase density, kg/m3 σ = interfacial tension, N/m Φ = dispersed-phase volume fraction, − Dimensional groups
■
We = Weber number, (ρcN2D3)/(σ)
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dx.doi.org/10.1021/ie401145j | Ind. Eng. Chem. Res. 2013, 52, 14463−14471
