Znd. Eng. Chem. Res. 1995,34, 3154-3158
3154
RESEARCH NOTES A Pulse-Echo Ultrasonic Probe for Local Volume Fraction Measurements in Liquid-Liquid Dispersions Costas TsourisJ Michael k Norato, and Lawrence L. Tavlarides” Department of Chemical Engineering and Materials Science, Syracuse Uniuersity, Syracuse, New York 13244-1190
A miniaturized ultrasonic probe has been constructed to measure the local volume fraction of immiscible phases in liquid-liquid contactors. The new probe has a 9 mm diameter and varying length and is based on sound pulse-echo measurements. It consists of a sound reflector and a n ultrasonic transducer which operates as a transmitter-receiver. The probe has been successfully used to measure the dispersed phase transient volume fraction (holdup) in stirred tanks. Measurements between 0.08 and 0.8 were obtained a t the top, bottom, and impeller discharge regions of the tanks. It has been found that local variations in the holdup exist, due to density differences of the two phases, which can be quantified by the new pulse-echo ultrasonic probe. Furthermore, measurements were obtained during phase inversion at which the continuous phase becomes dispersed and the dispersed phase continuous. Scattering and discontinuity is observed in the measurements in the proximity of phase inversion, which may be useful t o determine which phase is dispersed and which is continuous.
Introduction Dispersions of two immiscible or partially miscible liquids occur very often in chemical processes with the objective to increase the contact area between the two phases. The equipment design of such processes is usually based on the flow and residence time requirements and on mixing parameters such as mixing time and minimum agitation speed to achieve uniform liquidliquid dispersion. These parameters are measured in laboratory and pilot scale vessels and are correlated with the operating parameters and physical properties of the liquid system. Scale-up criteria are then used for the design of industrial-scale vessels. Mixing parameters in liquid-liquid dispersions are obtained from volume fraction measurements. For example,Armenante and Huang (1992)used a sampling technique t o measure locally the volume fraction of the dispersed phase. In the present work, a pulse-echo ultrasonic probe is developed to provide these measurements automatically with the possibility of real-time process monitoring and control. This probe is an extension of the ultrasonic technique developed by Bonnet and Tavlarides (1987)which originally consisted of a pulse generator, a transmitting ultrasonic transducer mounted on the outside wall of a cylindrical vessel, a receiver transducer mounted on the opposite wall of the vessel, and a digital oscilloscope for sound travel-time measurements. In subsequent developments (Tsouris et al., 19901, a data acquisition system was added for continuous monitoring of the volume fraction of the dispersed phase. Originally, the ultrasonic transducers were employed to measure the volume fraction through a cylindrical volume across the vessel diameter in between the transmitter and receiver
* To whom correspondence
should be addressed. Current address: Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 38731-6226. +
transducers. Modifications made for the pulse-echo ultrasonic mode include (i) a sound reflector which has replaced the receiver transducer to reflect the sound back to the transmitter transducer which works sequentially as a transmitter-receiver and (ii) conversion of the probe t o an immersible miniaturized instrument which provides measurements through a much smaller volume. The measurements are therefore considered “local”.
Experimental Description The new probe shown in Figure 1 consists of a 2.25 MHz videoscan ultrasonic transducer (Panametrics)of 6 mm nominal element size mounted inside the end of a stainless-steel tube of 9 mm outer diameter. A brass reflector plate is mounted on the outside wall of the stainless-steel tube, at a distance of 20 mm from the surface of the transducer. The distance between the reflector and the surface of the transducer is comparable to path lengths used in commercial optical probes for concentration measurements. Hool(1992),for example, used a 20 mm total path-length probe (10 mm opening at the end of the probe) to measure mixing time in single phase mixers. Here, the total path length (round trip) of the sound is 40 mm. A smaller distance is avoided in order that acceptable accuracy is retained in the measurements. For liquids of much different sound velocity from the toluene-water system at room temperature, however, the distance between the reflector and the transducer can be smaller without loss of accuracy. The reflector is held by two legs of approximately 2 mm width, so that the constriction for flow through the gap between the transducer and the reflector is minimized. The equipment assembly is also shown in Figure 1. The central unit is a mixing vessel of approximately 13 L in volume with internal diameter and height each of 254 mm. Agitation is provided by a six-blade Rushton-
0888-588519512634-3154$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 9, 1995 3155 (2) phase volume fractions and tl, t 2 , and t* are the sound travel times in the pure phases 1 and 2 and in the dispersion, respectively. Equation 1 is applicable when the drops are big enough to be penetrated by sound waves. A more accurate estimation of the volume fraction of drops of one liquid phase dispersed into another can be obtained by an improved model discussed by Yi and Tavlarides (1990) and Tsouris and Tavlarides (1990):
Stirring Motor Motor Controller
Stirred Tank
w\
t* - t,
Dis ersion
f (BNCCable
t
zl
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42
= g 2 t 2 - g1t1
where gl and g2 are correction factors which account for the increase or decrease of the sound path length in each phase due to sound reflection and refraction at the drop interface. These factors are functions only of the sound velocities in the pure phases and are given in the above articles. For example, if U1 and U2 are the sound velocities in the liquid phases 1and 2, respectively, and phase 2 is the dispersed phase, then for y UdU1 I1:
g 1 = 1 + 31[ 1 - ( 1 - Y 2) 3121 Y
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-I
Reflector (top view)
Figure 1. Geometry of the pulse-echo ultrasonic probe and experimental assembly for volume fraction measurements.
type impeller of 127 mm diameter located at the center of the tank. The agitation speed is monitored continuously by an optical stroboscope and is controlled manually via a motor controller. The ultrasonic probe is mounted vertically in the tank at half the distance between the wall and the impeller blade tips and positioned at various levels from the top or bottom of the vessel. A pulse generator (Panametrics, Model 5052 PR) is used to excite the ultrasonic transducer. The pulse generator is also connected to a digitizing oscilloscope (Hewlett-Packard, Model HP54201A) which is controlled by an IBM PC XT digital computer through an HPIB interface board. Once a pulse is received by the ultrasonic transducer, a sound wave is generated which is transmitted through the dispersion, reflected by the sound reflector, and received back by the transducer (pulse-echomode). Then the sound wave received by the ultrasonic transducer causes an electric signal to be generated which is processed by the oscilloscope to determine the travel time of sound through the liquid dispersion. This sequence of events occurs in between two pulses sent by the pulse generator. A data acquisition frequency of 0.75 Hz is allowed by this equipment assembly. By comparison of travel-time measurements through the dispersion and through pure phases, calculation of the volume fraction of the two phases is possible. For example, the linear relation between sound velocities and volume fraction can be used which leads to
42 = 1- $1 where 41 and
42
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=2
t2 - tl
are the continuous (1)and dispersed
and (4) Similar functions have been derived for the case of Ud U ~ 1Z(Tsouris and Tavlarides, 1990). Equation 2 with eqs 3 and 4 have been used in this study for the analysis of the data. It should be pointed out that eq 2 is applicable only in the case of complete dispersion of one fluid into another in the form of drops. In the case of two liquid layers, eq 1is applicable. It is also apparent that in the limit of pure phase 2, eq 2 does not give the correct answer of 4 2 = 1in contrast to eq 1. The reason, again, is that eq 2 is applicable only for dispersions. It should also be mentioned here that eqs 1and 2 do not depend on the drop size as long as the drops are comparable or larger than the sound wavelength, i.e., d m i n > U1/2nv where Y is the ultrasonic frequency. A calibration of the in-situ probe with respect to toluene-water dispersions of known dispersed phase volume fraction was performed in a 0.8 L vessel with the probe positioned in the impeller discharge stream. The results of this calibration are shown in Figure 2. This figure shows the measurements of the probe to be rather accurate; especially when one considers that the probe actually measures local dispersed phase volume fraction (which may be different from the average dispersed phase volume fraction), and that the calibration was performed in a relatively small vessel where wall effects and disturbances to the flow caused by the instrument, itself, are more pronounced. The relative error in the dispersed phase volume fraction measurements is no greater than 11%at moderate values of dispersed phase fraction, 0.30 5 4 I0.40, but this error increases to as high as 26% at extreme values of phase fraction (0.05 I4 I0.10). The instrument output (oscilloscope traces) is shown in Figure 3 for the cases where the in-situ probe is immersed (a) in pure water, (b) in a 20% (vol) dispersion
3156 Ind. Eng. Chem. Res., Vol. 34,No. 9,1995 0.5
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0.1 0.2 0.3 0.4 0.5 Actual Volume Fraction Figure 2. Calibration of in-situ probe against toluene in water dispersions of known phase fraction.
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Time (s) Figure 4. Transient volume fraction measurements of the toluene (dispersed)-water (continuous)system: stirred tank volume 13 L; agitation speed 280 rpm.
the time of occurrence, t*, of the maximum in the received signal (marked by cursor X in the figure) relative to the times of occurrence, tl and t 2 , of the pure continuous and dispersed phases, respectively.
Results
Figure 3. Oscilloscope traces for the in-situ probe immersed in (a, top) pure water, (b, middle) a 20% (vol) dispersion of toluene in water, and (c, bottom) pure toluene.
of toluene in water, and ( c ) in pure toluene. The dispersed phase volume fraction calculation is based on
The chemical system used in the demonstration experiments of this study is distilled water (1)-toluene (2) which has a specific gravity difference of 0.14. The sound velocities at 20 "C are U l = 1484.7 m/s and U2 = 1360.6 m/s. Then, U2/U1 = 0.916 -= 1 andgl = 1.108, g2 = 1.114 (calculated from eqs 3 and 4). Measurements of the transient volume fraction of the dispersed phase are shown in Figures 4-9. The capability of the pulse-echo ultrasonic probe to monitor the evolution of the dispersed phase volume fraction over a wide range of values is shown in Figure 4. This experiment was conducted under continuousflow conditions where the input was pure toluene (dispersed phase), and the output was dispersion (continuous and dispersed phase). Initially, the volume fraction of toluene in water was 0.22. The probe was located at the middle of the tank, and the agitation speed was maintained at 280 rpm. After 2400 s, flows were terminated for batch operation, and steady state was obtained. If the continuous operation continued, then eventually phase inversion would occur. Measurements during phase inversion of the toluene-water system obtained from a smaller stirred tank of 0.8 L volume are shown in Figure 5. The geometry of the 0.8 L tank is similar to the geometry of the 13 L tank which has been used for all the other experiments reported here. The open squares in Figure 5 correspond to toluene dispersed in water before the inversion and water dispersed in toluene after the inversion. The data marked by filled triangles correspond to water dispersed in toluene before the inversion and toluene dispersed in water after the inversion. In the proximity of phase inversion some data scattering occurs, and at the inversion point a break in the volume fraction is observed. This break is predicted only by eq 2 and is discussed by Tsouris and Tavlarides (1990) who suggested that the travel times prior to (t',)and after (t',) phase inversion do not have to be equal. These travel
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 3157
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Figure 7. Steady-state volume fraction measurements of toluene in water a t the impeller region: stirred tank volume 13 L; parameter, volume fraction; agitation speed 250 rpm.
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times are given as
and W&d,Z t* = -
+
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L(l- &&c,2)
(6)
Ucont,a
where L is the distance between the transducers, u d i s , p = Ucont,a and Ucont,p = Udis,a, and 4 2 is the dispersed phase volume fraction (+zP = 1 - &). The subscripts 1 and 2 correspond to phases 1 and 2. Experimental results obtained from three batch experiments are shown in Figure 6. In the first experiment, the probe was located at the impeller region (middle);in the second experiment the probe was located a t the top of the tank immersed into toluene. The agitation was set to 250 rpm 100 s after the data acquisition was initiated. The volume fraction of the dispersed phase (toluene) in these experiments as determined by the added volume of the dispersed phase over the total volume of the vessel is 0.225. In the cases when the probe was initially immersed into water, the
volume fraction increased smoothly until steady state was reached. When the probe was initially immersed in toluene, however, large fluctuations around the steady-state value were observed right after agitation was started. By studying the steady-state values of these experiments from 200 to 400 s (Figure 7), one can observe local variations in the dispersed phase volume fraction. The average values of the volume fraction at the three locations are approximately 0.2114 at the bottom, 0.2199 at the impeller region (center), and 0.2389 at the top. There is a difference in volume fraction of 0.019 between the top and center locations. Similarly, there is a difference of 0.0085 between the center and bottom locations. These differences are mainly the result of the density difference between the two liquid phases. Local variations in the drop size may also exist with the larger drops accumulated at the top. Furthermore, by studying Figure 7, one can obtain an estimation of the fluctuations observed in the holdup measurements which are due t o local variations in the volume fraction. The larger fluctuations are of the order of k0.015 unit which corresponds to &7% of the overall volume fraction. Measurements from three batch experiments with the probe located at the impeller region and the volume fraction varying from 0.08 to 0.16 to 0.24 are shown in Figure 8. The system is at rest between 0 and 100 s, and the agitation speed is set to 250 rpm a t 100 s. Data shown in Figure 8 reveal that the process transient time is approximately the same in all cases. Also, from Figures 6 and 8, it is clear that the pulse-echo ultrasonic probe provides measurements very close t o the correct steady-state values which are known a priori and which provides a good test for the measurements of the new probe. Experimental data obtained with different agitation speeds and with the probe located in the impeller region are shown in Figure 9. In all three experiments, the two phases are a t rest between 0 and 100 s when the agitation is initiated. In the first experiment (squares),the volume fraction is 0.12 and the agitation speed is 175 rpm, while in the second and third experiments, the volume fraction is 0.22. In the second experiment (crosses), the agitation speed is set to 200 rpm a t 100 s, while in the third experiment (triangles), the agitation speed is set t o 250 rpm. It is clear from
3168 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
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Figure 8. Transient volume fraction measurements of toluene in water in the impeller region: stirred tank volume 13 L; parameter, volume fraction; agitation speed 250 rpm.
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Acknowledgment Financial support of this work provided by the Dow Chemical Company, Cooperative Research Grant, is gratefully acknowledged. The authors are also thankful to Mr. Dick Chave for his creative machining work which minimized the volume of the probe. Also, the authors would like to thank Ms. Nicole Jones for preparing the manuscript. Literature Cited
l
‘+
the pulse-echo ultrasonic probe described in this paper can monitor a wide range of volume fractions in liquidliquid dispersions under various mixing conditions. Local variations of the volume fraction in stirred tanks are for the first time observable by the ultrasonic probe. Furthermore, the transient time to reach steady state in terms of the volume fraction is readily measurable even in the case of low viscosity systems where the response time is very short. No comparable measurements are provided by other techniques currently used for such measurements. A good test is provided by the average steady-state value of the volume fraction which is known a priori in batch systems. It is the authors’ opinion that the presented probe is very useful in dispersion studies, process monitoring, and process control.
400
Armenante, P. M.; Huang, Y. Experimental determination of the minimum agitation speed for complete liquid-liquid dispersion in mechanically agitated vessels. Ind. Eng. Chem. Res. 1992, 31, 1398-1406. Bonnet, J. C.; Tavlarides, L. L. Ultrasonic technique for dispersedphase holdup measurements. Ind. Eng. Chem. Res. 1987, 26, 811-815. Hool, K. Mixing time measured using a recyclable electrochemically generated chromophore. MChE J. 1992,38, 473-476. Tsouris, C.; Tavlarides, L. L. Comments on model for hold-up measurements in liquid dispersions using an ultrasonic technique. Ind. Eng. Chem. Res. 1990,29, 2170-2172. Tsouris, C.; Tavlarides, L. L.; Bonnet, J. C. Application of the ultrasonic technique for real-time holdup monitoring for the control of extraction columns. Chem. Eng. Sci. 1990,45,30553062. Yi, J.; Tavlarides, L. L. Model for hold-up measurements in liquid dispersions using an ultrasonic technique. Ind. Eng. Chem. Res. 1990,29, 475-482.
Figure 9. Transient volume fraction measurements of toluene in water in the impeller region: stirred tank volume 13 L; (0)175 rpm, 0.12 volume fraction; (+) 200 rpm, 0.22 volume fraction; (A) 250 rpm, 0.22 volume fraction.
Figure 9 that the transient time is decreasing as the agitation speed increases. From the results of the demonstration experiments shown in Figures 4 through 9, it can be concluded that
Received for review July 19, 1994 Revised manuscript received May 30, 1995 Accepted J u n e 20, 1995@ IE940449Z Abstract published in Advance ACS Abstracts, August 1, 1995. @
