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2007, 111, 3081-3084 Published on Web 03/06/2007
Pressure-Induced Reduction of Shielding for Improving Sonochemical Activity Maikel M. van Iersel,* Jean-Paul A. J. van den Manacker, Nieck E. Benes,* and Jos T. F. Keurentjes Process DeVelopment Group, Department of Chemical Engineering and Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed: January 25, 2007; In Final Form: February 16, 2007
The effect of hydrostatic pressure on chemical reactions induced by 20 kHz ultrasound has been studied using three different methods: the oxidation of potassium iodide, bubble cloud visualization studies, and sound attenuation measurements. The latter two have demonstrated that shielding of the ultrasonic wave is less pronounced at elevated pressures. Accordingly, the yield of iodine liberation increases with increasing pressure. At high static pressures, however, the less efficient cavitation dynamics dominate and the chemical reactivity decreases rapidly.
1. Introduction The application of ultrasound in chemical processes has gained increasing interest over the past decades.1,2 Previous studies have demonstrated that ultrasound successfully initiates reactions, changes reaction pathways, and accelerates mixing. Often, acoustic cavitation is the basis of these physical and chemical effects. Acoustic cavitation refers to the sound-induced radial motion of small cavities in a liquid, resulting in an almost adiabatic compression of the cavity interior and a strong temperature rise. Even though acoustic cavitation provides unique high-energy conditions, its low energy efficiency and limited comprehension prevent an extensive utilization in large-scale industrial applications.2 To gain more understanding into the governing phenomena of acoustic cavitation, numerical modeling studies are employed in which the dynamics of a single cavity are studied.3 Additionally, the effect of various process conditions on ultrasound-induced reactivity can be predicted from these models to obtain a maximum energy efficiency.4,5 For example, singlecavity dynamics models predict a decrease in hot-spot temperature for higher bulk liquid temperatures. Accordingly, experimentally determined reaction rates are lower at elevated temperatures. However, some of the observed effects are difficult to explain using these numerical modeling studies. Findik et al. have demonstrated that the degradation rate of acetic acid displays a maximum as a function of the acoustic intensity for which the decrease in reactivity at high intensities is attributed to a less efficient sound field.6 A substantial amount of the acoustic energy is not converted to the formation of radicals.7 As the sound wave propagates through the liquid, bulk motion of the liquid in the direction of the wave arises, i.e., acoustic streaming.8 This sound-induced flow causes partial dissipation of the acoustic energy. Further* Corresponding authors. E-mail:
[email protected] (N.E.B.);
[email protected] (M.M.V.I.). Telephone: +31 40 247 5445 (N.E.B.). Fax: +31 40 244 6104 (N.E.B.).
10.1021/jp070635l CCC: $37.00
more, a large cloud of bubbles is formed close to the emitter for intense sound fields, which absorbs and scatters the sound wave. The volume of the bubble cloud will be reduced at elevated hydrostatic pressures, leading to a more intense sound field and a possible increase in sonochemical reactivity. On the contrary, single-cavity models predict that cavitation is hindered at elevated pressures. This inhibition is manifested by an increase in the acoustic threshold pressure at higher static pressures.9 Nevertheless, Henglein et al. observed a higher yield for the oxidation of potassium iodide for a small pressure increase using 1 MHz ultrasound.10 Because most of the research on sonochemistry is performed at ultrasonic frequencies in the range of 20-40 kHz, it seems desirable to study the effect of hydrostatic pressure at these sonication conditions. The aim of this study is to investigate the effect of hydrostatic pressure on sonochemical reactivity and on shielding of the sound field for intense sonication at 20 kHz. To study the sonochemical effect, the liberation of iodine from an aqueous potassium iodide solution has been chosen as the model reaction.11 In accordance with previous studies, microphone measurements have been performed to aid in the interpretation of these results.12,13 Additionally, optical methods have been employed to visualize the bubble cloud structures.14 2. Experimental Section In this study, ultrasound with a frequency of 20 kHz was produced using a Sonics and Materials VC750 ultrasonic generator. The piezoelectric transducer was coupled to the liquid with a full-wave titanium horn. The small tip diameter (13 mm) enables high acoustic intensities. The output of the ultrasonic generator was determined calorimetrically in a 1.8 L reaction calorimeter RC1e (Mettler-Toledo GmbH, HP60). The exact procedure has been described elsewhere.15 2.1. Bubble Cloud Visualization and Sound Attenuation Measurements. The visual observations and the sound attenuation measurements were performed in a thermostated 300 mL © 2007 American Chemical Society
3082 J. Phys. Chem. B, Vol. 111, No. 12, 2007 high-pressure view cell. The ultrasound horn was inserted at the top of the reactor and was fixed at its nodal point. To visualize the bubble cloud at the sonication tip, the reactor contents was illuminated from the bottom using a 200 mJ per pulse Nd:YAG laser system (Spectra-Physics Quanta Ray, GCR 150). The scattered light was recorded with a high-frame-rate CCD camera (Kodak, MEGAPLUS ES 1.0) and magnifying optics. The camera was positioned to capture the plane illuminated by the laser sheet, which was placed in line with the ultrasound horn. The images were acquired with Video Savant software, and the analysis was performed using mathematical software (Mathematica, Wolfram Research, Inc.). For the attenuation measurements, a microphone was used consisting of a lead zirconium titanate crystal embedded in polyurethane rubber to match it acoustically to water. The microphone was positioned 10 mm below the tip of the horn and at a radial distance of 10 mm from the symmetry axis of the horn. The signal was acquired with a 100 MHz, 8-bit analog-to-digital converter (National Instruments, NI PCI-5112). In LabVIEW, this digital signal was analyzed using fast Fourier transformation algorithms. The reactor was filled with 250 mL of Millipore filtered water and pressurized with argon (grade 5.0, Hoek Loos B.V.). The measurements were performed sequentially at 20 °C for an electrical input of approximately 100 W. To minimize temperature and pressure fluctuations, the measurements were done for short sonication times (
