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Characterization of Sonochemical Reactor for Physicochemical Transformations Sravan Kumar S.,† A. Balasubrahmanyam,† Shyam Sundar P,‡ P. R. Gogate,† V. D. Deshpande,§ S. R. Shukla,‡ and A. B. Pandit*,† Department of Chemical Engineering, Department of Fibers and Textile Processing Technology, and Department of Physics, Institute of Chemical Technology (Deemed UniVersity), Matunga, Mumbai-400019, India
Complete knowledge of the pressure field distribution in a sonochemical reactor should be known for an efficient utilization of acoustic energy emitted by the transducers for targeted physicochemical transformations using cavitation phenomena. This work deals with the identification of the active and passive regions in the sonochemical reactor based on the measured pressure field intensities. Iodine liberation experiments based on the decomposition of aqueous potassium iodide (KI) as a representative of cavitationally induced chemical transformation were carried out at various locations in the bath type reactor and compared with the measured pressure intensities in the reactor. Similarly, extraction of natural dye from the bark of Pterocarpus marsupium tree has also been carried out as a representative of cavitationally induced physical transformation (extraction) and compared with the measured pressure intensities. Both the transformations indicate a similar trend of variation in the degree of transformation when correlated with the local pressure and indicated the existence of an optima, which is at a plane (≈3λ/2) away from the transducer surface. 1. Introduction Over the past few years, intense research has been going on for effective utilization of acoustic energy for cavitationally induced transformations. When a liquid is irradiated with a high intensity acoustic wave, small cavities will generate, expand, and collapse violently leading to high pressures (500-10 000 bar) and high temperatures (1000-10 000 K).1 Due to these severe conditions, free radicals are generated, which greatly enhance the rates of chemical reactions. Some of the chemical processing applications of acoustic cavitation include chemical synthesis,2-6 wastewater treatment7,8 etc. The physical effects of cavitation such as intensification of mass transfer and turbulence can be explained in terms of microjet formation due to asymmetric collapse of bubbles or shock wave formation due to symmetric collapse of oscillating cavitation bubbles causing the enhancement in the solid-liquid leaching or liquid-liquid extraction (physical processing applications).9-12 In spite of its potential applications, there is hardly any chemical processing carried out on an industrial scale using acoustic energy mostly due to lack of efficient scale-up strategies.1 The acoustic pressure distribution is not spatially uniform in the reactor resulting in active (high pressure intensities and high cavitational activity) and passive (low pressure intensities and low cavitational activity) zones. In the past,13-15 different techniques have been employed for the characterization of pressure distributions in a sonochemical reactor. These techniques are based on thermocouple probe, optical fiber tips and perforations (erosion) characterization of aluminum foils due to cavitation. For carrying out reactions in a sonochemical reactor, the active regions in the reactor should be known and should be maximized for an effective utilization of transmitted acoustic energy to carry out given physicochemical transformations through the bubble oscillation activity. Once this information is known, the reaction mixtures can be kept at that particular location or the transducer arrangement can be optimized to maximize the overall volumetric efficiency of energy utilization. * To whom correspondence should be addressed. Tel.: +91-22-24145616. Fax: +91-22-2414-5614. E-mail address:
[email protected]. † Department of Chemical Engineering. ‡ Department of Fibers and Textile Processing Technology. § Department of Physics.
In the present work, pressure distribution of the acoustic energy in a sonochemical reactor (cleaning bath type) has been measured with the help of hydrophone (pressure sensor) coupled to a high speed data acquisition system. Chemical and physical transformations of cavitation have been quantified with KI decomposition and extraction of natural dye from Pterocarpus marsupium, respectively. The variation of the physicochemical effects of cavitation at various locations of the sonochemical reactor have been explained with variation in measured pressures and interms of bubble activity and its size distribution. On the basis of the information of the measured pressures at various planes selected and scanned in the sonochemical reactor, the best possible plane has been suggested to carry out the physicochemical transformations. 2. Experimental Work and Techniques The sonochemical reactor used for the experiments is made by Trans-O-Sonic Ltd., Mumbai, and is shown in Figure 1. The reactor has a capacity of 5.6 L and operates at a driving acoustic frequency of 45 kHz, with maximum rated power of 230 W. The wavelength of the acoustic wave at this frequency measured in water medium is found to be 3.33 cm. The reactor was scaled in x-y-z direction, with one of its bottom corners chosen as the origin. The internal dimensions of the reactor were length ) 25 cm, breadth ) 15 cm, and height ) 15 cm. In our previous work,16 it has been established that maximum cavitational intensity is observed at antinodal points of the stationary wave formed due to the reflection of the wave from the air-water interface, and these planes are observed and measured to be multiples of wavelength of the acoustic wave passing through
Figure 1. Schematic view of the sonochemical reactor.
10.1021/ie801467n CCC: $40.75 2009 American Chemical Society Published on Web 02/12/2009
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Figure 2. Experimental setup used for mapping of the Tran-O-Sonic reactor.
the water medium. On the basis of this information, measurement of pressure fields in the reactor was carried out at six different planes which are at a distance of 0.9 (≈λ/4), 1.7 (≈λ/ 2), 3.4 (≈λ), 5.1 (≈3λ/2), 6.8 (≈2λ), and 8.5 (≈5λ/2) cm planes along the “z” direction, and in each plane, readings were taken at different positions in a lengthwise (4, 8, 12, 16, and 20 cm) and breadthwise (2, 5, 7, 10, and 12 cm) manner, totalling 150 points in the reactor. Five equal powered transducers, cylindrical in shape and 3 cm in diameter, were located at the bottom of the reactor. A total power of 40.25 W was delivered (17.5% energy transfer efficiency, measured separately using calorimetric studies). The locations of these transducers were (4, 5), (12, 5), (20, 5), (8, 10), and (16, 10). The (x, y) coordinates refer to the x distance lengthwise and the y distance breadthwise. z is always zero as the transducers located at the base of the bath. The volume of the water taken in the reactor is 4200 mL, which corresponds to 11.2 cm of the liquid column height in the reactor. A thermocouple and a temperature indicator were used to measure the temperature inside the reactor. The temperature of water was maintained at 30 °C (ambient temperature). To avoid any significant rise in temperature, the bath was operated intermittently with a short duration of the irradiating periods. 2.1. Measurement of Local Pressure Field. To get the information about the pressure field distribution within the reactor acoustic emission spectra, measurements were carried out with a hydrophone (Bruel and Kjear Ltd., type 8103) connected to a charge amplifier (Bruel and Kjear Ltd., type 2635). The hydrophone used for the experimentation is cylindrical in shape with the following dimensions: length 50 mm and diameter 9.5 mm. It was placed in the reactor in such a way that its acoustic sensor is at the exact location (x-y-z coordinates) at which qualitative measurements were needed. The output of this amplifier was fed to a computer, equipped with data acquisition system (N. I. Ltd., PCI 6251), and the discrete data has been captured. Fast Fourier transform (FFT) power spectrum analysis has been carried out online using graphical interface programming done with LabVIEW software (N. I. Ltd., version 7.1).16 The discrete data of the samples were taken at a sampling frequency rate of 300 kHz, and each signal consisted of 300 000 digital points. The time scale of each signal is 1 s. The experimental setup used in the present investigation is shown in Figure 2. 2.2. KI Decomposition Studies. The cavitational yield (sonochemical yield) can be directly related to the collapse pressures and the number of cavity collapsing events.17 In the present study, the quantification of chemical effects has been made using a model reaction, decomposition/oxidation of aqueous KI solution, a widely studied reaction for the estimation of sonochemical activity. Collapse of cavities results in generation of high temperatures and pressures locally, which results
in the cleavage of water molecules (H2O) to yield OH• radicals. These OH• radicals are responsible for the oxidation of KI in aqueous solution form, resulting in the liberation of iodine.18 This liberated iodine was measured using the UV-spectrophotometer at a 354 nm wavelength. The concentration of KI solution used was 4% (w/v) after some initial trail experiments to get consistent detectable/measurable quantity. Five mL of this solution was taken in a test tube (made up of glass material with 1.7 cm internal diameter and depth of the liquid column in the test tube was 2.2 cm), and the same test tube has been used for all the experimental trials to avoid variation in the sound attenuation and cavitational activity due to different glass wall thickness in different samples.19 Every time this tube was irradiated with acoustic wave (45 kHz) for about 2 min at different locations and at different planes in the reactor (same as the location chosen for pressure measurements). At each location, experiments were repeated five times, and an average value of I3- concentration has been reported. The relationship between the absorbance and the I3- concentration has been established earlier, and that calibration chart was used to estimate the liberated iodine concentration in the irradiated solution. 2.3. Natural Dye Extraction Studies. The physical effects of the cavitation, such as the shock waves and microjet formation, are enhancing the intraparticle convective diffusion and hence the internal (within the pore) mass transfer rate. Thus, the leaching solvent is able to penetrate into any solid matrix and undergo the reaction or dissolution of the solute.10 To quantify these physical effects of cavitation, extraction of natural dye from the bark of Pterocarpus marsupium tree was carried out in the same reactor. The bark of the Pterocarpus marsupium was first washed with water to remove the impurities sticking to it and then dried. The dried bark was then powdered to a small size (dp e 1 mm). One gram of this powder was taken in a test tube of 150 mL capacity, and 50 mL of selected solvent was added to it. The test tube was then placed at the predetermined locations (same as those used in KI decomposition studies) in the reactor and irradiated with acoustic wave for 1 h, each time 5 mL of extract was taken for the analysis after every 10 min interval. It is centrifuged in order to make it free from particles. The absorbance of the clear solution thus obtained was then measured using the UV-visible spectrophotometer at 230 nm with proper dilution. Ultrasonic extraction of Pterocarpus marsupium was tried out with five different solvents namely water, methanol, chloroform, ethyl acetate, and dimethyl formamide, each having different physicochemical properties shown in the Table 1, which is expected to affect the cavitational activity and hence the extent of extraction yield.
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Table 1. Physical Properties of the Various Solvents Used for the Experimentation ethyl water methanol chloroform acetate boiling point (°C) viscosity 103 (kg/(m s)) at 20 °C vapor pressure at 20 °C (mm Hg) surface tension (N/m)
DMF
100 0.89
64.7 0.0547
61.2 0.58
77.1 0.426
153 0.078
17.5
128
160
76
2.7
0.0271
0.02375 0.03676
0.0728 0.02255
Table 2. Average Overall Pressure Intensities (PI; atm) in Different Locations location
(4, 2) (4, 5) (4, 7) (4, 10) (4, 12) (8, 2) (8, 5) (8, 7) (8, 10) (8, 12) (12, 2) (12, 5) (12, 7) (12, 10) (12, 12) (16, 2) (16, 5) (16, 7) (16, 10) (16, 12) (20, 2) (20, 5) (20, 7) (20, 10) (20, 12)
PI (atm) PI (atm) PI (atm) PI (atm) PI (atm) PI (atm) 0.9 plane 1.7 plane 3.4 plane 5.1 plane 6.8 plane 8.5 plane
0.298 0.563 0.356 0.39 0.277 0.288 0.452 0.36 0.45 0.404 0.286 0.509 0.501 0.306 0.328 0.392 0.231 0.312 0.475 0.378 0.314 0.462 0.448 0.33 0.33
0.249 0.404 0.364 0.225 0.296 0.275 0.286 0.302 0.406 0.249 0.215 0.533 0.39 0.475 0.332 0.338 0.348 0.392 0.475 0.456 0.38 0.479 0.37 0.332 0.277
0.342 0.531 0.356 0.388 0.271 0.332 0.428 0.366 0.394 0.571 0.334 0.635 0.243 0.408 0.215 0.267 0.239 0.376 0.364 0.513 0.318 0.456 0.324 0.386 0.318
0.613 0.643 0.521 0.541 0.501 0.535 0.561 0.511 0.625 0.575 0.591 0.613 0.436 0.537 0.467 0.452 0.505 0.352 0.467 0.479 0.384 0.555 0.434 0.314 0.251
0.458 0.462 0.33 0.428 0.247 0.316 0.32 0.39 0.477 0.366 0.326 0.328 0.261 0.32 0.46 0.33 0.296 0.328 0.267 0.4 0.286 0.348 0.239 0.243 0.277
0.282 0.294 0.422 0.257 0.219 0.39 0.432 0.515 0.32 0.342 0.366 0.235 0.344 0.464 0.398 0.354 0.314 0.503 0.41 0.416 0.267 0.378 0.36 0.34 0.332
3. Results and Discussion 3.1. Mapping the Local Pressure Fields in the Reactor. Estimation of Overall Voltage Pressure Intensity. Let V1, V2, ..., Vn be the pressure pulse data (in terms of volts) measured by hydrophone (in this case number of samples in a single pressure pulse is (n) ) 300 000 at a sampling frequency rate of 300 000 samples/s). Average detected voltage is then given by the root-mean-square (rms) of the above data, and it is given as ν)
V12 + V22 + ... + Vn2 n
(1)
This average voltage is multiplied by the calibration factor of hydrophone (75 370 atm/V)16 to get average pressure intensity. As cavitation is a random phenomenon, two consecutive pressure pulse readings at the same location may yield completely different results in terms of the measured pressure intensities though the frequencies at which they were detected remained measurably constant and hence, it is important to collect large number of data sets at a particular location.12 Sixty pressure pulses were collected and averaged out at a particular location in the reactor to eliminate the stochastic error. The 60 readings ensured that a statistically significant number of samples were evaluated, which reduces the random variation in the measured pressure pulse due to the occasional and periodic cavity collapse. The average of overall intensities of 60 samples at different locations and in different planes are shown in the Table 2. The number in the first column e.g., (4, 2) indicates that x ) 4 and y ) 2 from the origin, and the subsequent columns indicate different planes. The data shown in Table 2
is reported in the form of surface plots at different planes for qualitative understanding (representation) of the generated pressure field in the reactor (Figure 3). The locations (positions) of the five transducers arranged at the bottom are shown in Figure 3a. From this experimentally measured pressure field data, cavitationally active zones were identified in the reactor. On observing the data from Table 2 and Figure 3, it can be concluded that relatively high pressure intensities are observed at (4, 5), (8, 5), (8, 10), (12, 5), (12, 7), (16, 10), (20, 5), and (20, 7) locations in the 0.9 plane. Interestingly, these are the locations just above the transducers which are fitted to the base of the reactor. Similarly, relatively high intensities were observed at the same locations in remaining higher planes also. The highest intensities were observed in the 5.1 plane which corresponds to a (3λ/2) distance from the transducer surface. Gogate et al.1 also observed the high intensities in the (3λ/2) plane (acoustic frequency is 20 kHz). This can be attributed to constructive interference of all the sound waves coming from the different transducers and also due to the formation of standing wave patterns resulting from the reactor wall and by the reflection from the upper air-water interface. The reflection of the acoustic wave from the sides of the reactor walls also contribute to the higher intensity of the measured pressure signal to some extent. The material of construction of the reactor wall is stainless steel which has higher acoustic impedance (26 times) than the air-water interface. From this data, it can be concluded that the best place to keep the reaction mixture to get better physicochemical transformations is in the 5.1 plane. A typical fast Fourier transform of hydrophone signal (averaged over 60 readings) measured at the (8, 7) location in the 6.8 plane is shown in Figure 4 showing the peaks that were observed at driving frequency 45 (≈f) kHz as well as at other different frequencies such as, 22.5 (≈f /2), 67.5 (≈3f /2), 90 (≈2f), 113 (≈5f /2), and 135 (≈3f) kHz. These observed peaks are subharmonic (≈f/n), harmonics (nf), and ultraharmonics ((2n + 1)f/2) of the driving frequency (f). Some other interesting regular peaks at 27.5, 72.5, and 117.5 kHz have also been observed in all the experimentally measured FFT power spectra analysis. The magnitudes associated with these peaks are very small, and hence, we have not tried to explain any physical reason for the occurrence of these peaks. But, the observed harmonics can be explained in terms of forced nonlinear oscillations of the cavitational bubbles, and the noise background is explained in terms of the shock wave emitted by the collapsing bubbles.20 A subharmonic frequency peak (22.5 ()f/2) kHz) is also observed. Guth21 has demonstrated that a bubble with an equilibrium radius greater than the resonant size would pulsate in different modes of oscillation (other than zeroeth or volume oscillation mode) and as a consequence would radiate the subharmonic frequencies. Akulichev22 has explained it on the basis of the inertial forces (when the pressure amplitude of exciting field considerably exceeds the threshold) as the bubble continues to expand even during the compression half-period and passes the contraction phase. As a result, the time period of the pulsation of the bubble becomes a multiple of the period of the exciting field frequency (45 kHz in this case) and the subharmonic components appear in the noise spectrum of the bubble sound emission. In our recent study,16 it has been observed that the different oscillating components of this FFT spectra represents the oscillating bubbles of resonating size, and these resonating cavitational bubbles result into cavitational activity and the same has been observed in the present study. A typical cavity size distribution has been determined at the location (20, 12) of the 5.1 plane and is shown in Figure 5. It
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Figure 3. Surface plot of variation in overall pressure intensity (atm) in different planes.
is observed that the mean cavity size distribution at this location for 45 kHz driving frequency of ultrasound determined to be 39 µm is lower than the average mean (60-80 µm) bubble size distribution for the 20 kHz driving frequency of ultrasound.16 It is generally observed that bubble size decreases as the driving frequency of the ultrasound increases. These cavitational bubbles shown in Figure 5 are possibly responsible for the chemical and physical effects of cavitation. 3.3. KI Decomposition. In order to correlate the measured pressure intensities, with the secondary effects (chemical reaction) such as aqueous KI decomposition, experiments were carried out at seven different locations in four different planes as a representative of cavitationally induced chemical transformation. Five repeated experiments were conducted at these locations, and the averages of five readings were then compared with measured pressure intensities as reported in Table 3. Figure
6 shows a graph of measured pressure intensities and the liberated iodine. The variation in the iodine liberation over these five repeated experiments is shown in the shaded area which is within a variation of 5%. From Figure 6, we can observe that initially the iodine liberation was found to increase with an increase in pressure intensity up to 0.39 atm (39526.6 N/m2) and then decreased with further increase in the measured pressure intensities. This may be due to the fact that at very high pressure intensities (measured by the hydrophone) at antinodal points of the stationary waves, due to the presence of primary Bjerknes forces, a large number of cavitational bubbles are likely to accumulate at these points16 which are causing the less violent collapse of the cavities and, hence, low cavitational effects are observed at these high intensity measured pressure points such as (12, 12) of plane 5.1 and (16, 7) of plane 8.5. This is clearly evident from Figure 7; the total number of
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Figure 4. FFT power spectrum of hydrophone signal measured at (20, 12) in plane 5.1.
Figure 6. Variation in iodine liberation with respect to pressure intensities. Figure 5. Bubble size distribution at location (8, 7) in plane 6.8. Table 3. Iodine Liberation and Methanol Extract Absorbance at Different Location (V) Measured Pressure Intensities
location
plane
pressure intensities (atm)
(20, 12) (12, 10) (20, 10) (16, 5) (8, 7) (12, 12) (16, 7)
5.1 6.8 1.7 1.7 6.8 5.1 8.5
0.251 0.32 0.332 0.348 0.39 0.467 0.503
average iodine (mg/L)
methanol extract absorbance
0.38 0.96 0.22 0.65 1.80 0.35 0.59
48.4 63.7 64.5 54.3 58.5
measured bubbles is quite high (≈429 120) at location (12, 12) of plane 5.1 compared to the total number of bubbles (≈189 956) at location (8, 7) of plane 6.8. Also, the number of cavitational collapses contributing to the aqueous KI decomposition may reduce (cavities inside the cavity clusters do not contribute significantly). This in turn leads to a decrease in the decomposition of KI beyond certain measured pressure intensity. Thus, the pressure intensities measured by hydrophone can be correlated with the chemical effects only partially (up to some critical pressure intensity), and this critical value may be different for different types of chemical and/or physical transformations as is shown later. 3.4. Extraction of Natural Dye from Pterocarpus marsupium. To check the correspondence between the measured pressure intensity and the physical effects of cavitation, i.e., extraction of natural dye from the bark of Pterocarpus marsupium tree, experiments were carried out with five different solvents at room temperature at location (12, 5) in 5.1 plane. Figure 8 shows the absorbance of the clear extracts with respect to time obtained with various solvents. It was found that the extract in methanol gives the maximum absorbance. This may be attributed to high vapor pressure, low viscosity, and low surface tension of methanol leading
Figure 7. Bubble size distribution at location (12, 12) in plane 5.1.
Figure 8. Extraction of Pterocarpus marsupium in different solvents.
to significantly higher number of cavitational events and their intensity of collapse. However, observed effects are not only due to the high vapor pressure, as chloroform having even higher vapor pressure does not show a similar effect. The solubility of the target dye in the chosen solvent (methanol in this case) could also have contributed to this effect. The trend of gradual (even though only marginal) increase in the absorbance seems to be common for all the solvents.
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near the wall. The variation in iodine liberation and extraction of Pterocarpus marsupium with respect to measured pressure intensities goes through a maximum. This indicates that physicochemical effects are somewhat similarly dependent on the cavity dynamics but the extent of variation is different as different forces (cavity dynamics) are responsible for the different types of transformations originating from the chemical and physical effect of cavitational events. Literature Cited
Figure 9. Variation of absorbance of methanol extract with respect to measured pressure intensities.
On the basis of these studies, methanol was selected for further study. The extraction of Pterocarpus marsupium was carried out at five different locations of the reactor (the same locations chosen for KI decomposition studies), in order to have a comparison between the measured pressure fields and their physical effects i.e., extraction. The absorbance value of the methanol extract with respect to the pressure intensities (measured in water) is shown in Table 3 and Figure 9. In Figure 9, the variation of the absorbance of the solute in the methanol (solvent) with measured pressure intensities is shown in the shaded area, which is found to be again within the variation of 5%. The absorbance of methanol (solvent) also reaches a maxima (optima) and then marginally reduces with an increase in the measured pressure intensities (indicating lower extraction of the solute in the specified time period). Even though the actual pressure intensities in methanol may be different due to the difference in the cavitational events and their intensity in methanol as compared to the water due to the difference in their physical properties, a relative comparison can always be made as the pressure intensities are geometrically decided (transducer location and arrangement) and may differ for different solvents23 but are likely to show a similar spatial variation. From Table 3 and Figure 9, it is evident that, the nature of physical effects (natural dye extraction) and the chemical effects (KI decomposition) can be correlated on the basis of the measured acoustic pressure with the hydrophone (initial increase with an increasing pressure, attainment of maxima, and then decrease with further increase in pressure). However, the rates of magnitudes are different, as different forces are responsible for these two types of effects. The KI decomposition is expected to be governed by OH- radical generation as a result of sonolytic dissociation of water (sonochemical transformation) where as the dye extraction is likely to be governed by the propagation of the shock wave due to the cavity collapse. Even though the effects are different, they are ultimately decided by the dynamics of the cavity and hence are expected to show similar general trend. Thus, the maxima observed for the chemical transformation with respect to the measured pressure intensity between 0.36-0.4 atm (i.e., 36 674.2-407 549 N/m2), also seems to coincide well (same range of pressure intensities for maxima) with the physical process, indicating a similar type of correspondence. 4. Conclusions In this work, mapping of the sonochemical reactor has been carried out. On the basis of the local pressure measurements, the 5.1 plane (≈3λ/2) is concluded to be the best location (active region) in the reactor under study for carrying out cavitational transformations. Low intensities (passive region) are observed
(1) Gogate, P. R.; Tatake, P. A.; Kanthale, P. M.; Pandit, A. B. Mapping of sonochemical reactors: Review, analysis, and experimental verification. AIChE J. 2002, 48, 1542. (2) Pandit, A. B.; Joshi, J. B. Microbial cell disruption in hydrodynamic cavitation. Chem. Eng. Sci. 1993, 48, 3440. (3) Lindley, J.; Mason, T. J. Sonochemistry. Part 2- Synthetic applications. Chem. Soc. ReV. 1987, 16, 275. (4) Naresh, N. M.; Gogate, P. R.; Pandit, A. B. Ultrasound-Accelerated Green and Selective Oxidation of Sulfides to Sulfoxides. Ind. Eng. Chem. Res. 2006, 45 (26), 8829. (5) Mason, T. J. Sonochemistry: Current Uses and Future Prospects in the Chemical and Processing industries. Philos. Trans. R. Soc. London 1999, 355. (6) Ondruschka, B. J. L.; Hofmann, J. Aquasonolysis of Ether - Effect of Frequency and Acoustic Power of Ultrasound - Communication. Chem. Eng. Technol. 2000, 23, 588. (7) Gogate, P. R. Cavitation: An auxiliary technique in waste water treatment schemes. AdV. EnViron. Res. 2002, 6 (3), 329. (8) Pandit, A. B.; Gogate, P. R.; Mujumdar, S. Ultrasonic degradation of 2:4:6 trichlorophenol in presence of TiO2 catalyst. Ultrason. Sonochem. 2001, 8, 3361. (9) Thakore, K. A. Physico-Chemical Study on Applying Ultrasonics in. Z. Textile Dyeing. Amer. Dyestuff Rep. 1990, 79, 45. (10) Avvaru, B.; Roy, S. B.; Chowdhury, S.; Hareendran, K. N.; Pandit, A. B. Enhancement of leaching rate of uranium in the presence of ultrasound. Ind. Eng. Chem. Res., 2006, 45 (22), 7639. (11) Yoshiyuki, A.; Masahiro, M.; Tatsuro, M.; Koda, S. Electro. Commun. Jpn. 2007, 90 (3), 716. (12) Moholkar, V. S.; Sable, S. P.; Pandit, A. B. Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission. AIChE J. 2000, 46 (4), 684. (13) Pugin, B. Qualitative characterization of ultrasound reactors for heterogeneous sonochemistry. Ultrasonics 1987, 25, 49. (14) Mason, T. J.; Berlan, J. Dosimetry in Sonochemistry, 4th ed.; Advances in Sonochemistry; JAI Press, 1996. (15) Jenderka, K.-V.; Koch, C. Investigation of spatial distribution of sound field parameters in ultrasound cleaning baths under the influence of cavitation. Ultrasonics 2006, 44, e401-e406. (16) Avvaru, B.; Pandit, A. B. Oscillaitng bubble concentration and its size distribution using acoustic emission spectra. Ultrason. Sonochem. 2009, 16 (1), 105. (17) Gogate, P. R.; Pandit, A. B. Engineering design method for cavitational reactors: I. Sonochemical reactors. AIChE J. 2000, 46 (2), 372. (18) Prasad Naidu, D. V.; Rajan, R.; Gandhi, K. S.; Arakeri, V. H.; Chandrasekaran, S. Modelling of a batch sonochemical reactor. Chem. Eng. Sci. 1994, 49 (6), 877. (19) Tatake, P. A.; Pandit, A. B. Modelling and experimental investigation into cavity dynamics and cavitational yield: influence of dual frequency ultrasound sources. Chem. Eng. Sci. 2002, 57, 4987. (20) Frohly, F.; Labouret, S.; Bruneel, C.; Looten-Baquet, I.; Torguet, R. Ultrasonic cavitation monitoring by acoustic noise power measurement. J. Acoust. Soc. Am. 2000, 108 (5), 2012. (21) Guth, W. Acustica, Nichtlineare schwingungen von luftblassen in wasser in german. Acustica 1956, 6, 532. (22) Akulichev, V. A. The Structure of Solutions of Equations Describing Pulsations of Cavitation Bubbles, in Russian. Akusticheskii J. 1967, 13, 533. (23) Soudagar, S. R.; Samant, S. D. Investigation of ultrasound catalyzed oxidation of arylalkanes using aqueous potassium permanganate. Ultrason. Sonochem. 1995, 2 (1), S15.
ReceiVed for reView September 30, 2008 ReVised manuscript receiVed November 25, 2008 Accepted January 7, 2009 IE801467N
