Effect of particle addition on sonochemical reaction

Enhancement of chemical reaction with a photocatalyst of titanium dioxide (TiO(2)) by ultrasonic irradiation is studied through the absorbance measurements for liberation of iodine from a KI aqueous solution as an index of oxidation reaction. It is well known as a synergetic effect that the addition of TiO(2) fine particles under UV has an ability to enhance the yield in chemical reaction with OH-radical from hot spot at violent collapse of cavitation bubbles with intense ultrasound. In this study, the absorbance is measured after simultaneous irradiation of ultrasound and UV with the addition of TiO(2) much less than the usual concentration by two orders of content. It is shown that, even in case of quite a little TiO(2) addition where the photocatalytic effect is less, the yield is enhanced obviously in comparison with the summation in yield of independent procedure of ultrasound without TiO(2) and UV with TiO(2). The absorbance-peak deviation to the shorter wavelength implies the generation of titanium peroxide (TiO(3)). The effect of particle addition is due to the chemically activated particle surface on the TiO(2) and probably to the increase in the cavitation bubbles that results in promoting a transfer of OH-radical and other oxidants to bulk liquid region at the collapse.     

The dynamics of cavitation bubbles in a sealed vessel

A model of cavitation bubbles is derived in liquid confined in an elastic sealed vessel driven by ultrasound. In this model, an assumption that the pressure acting on the sealed vessel due to bubble pulsations is proportional to total volume change of bubbles is made. Numerical simulations are carried out for a single bubble and for bubbles. The results show that the pulsation of a single bubble can be suppressed to a large extent in sealed vessel, and that of two matched bubbles with same ambient radius can be further suppressed. However, when two mismatched bubbles have the same ambient radii, an interesting breathing phenomenon takes place, where one bubble pulsates inversely with the other one. Due to this breathing phenomenon the suppression effect becomes weak, so the maximum radii of two mismatched bubbles can be larger than that of a single bubble or that of two matched bubbles in sealed vessel. Besides that, for two mismatched bubbles with different ambient radii, the small one in sealed vessel under some certain parameters can pulsate as strong as or even stronger than that of a single bubble in an open vessel.     

Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics

This paper tries to discern the mechanistic features of sonochemical degradation of recalcitrant organic pollutants using five model compounds, viz. phenol (Ph), chlorobenzene (CB), nitrobenzene (NB), p-nitrophenol (PNP) and 2,4-dichlorophenol (2,4-DCP). The sonochemical degradation of the pollutant can occur in three distinct pathways: hydroxylation by OH radicals produced from cavitation bubbles (either in the bubble–bulk interfacial region or in the bulk liquid medium), thermal decomposition in cavitation bubble and thermal decomposition at the bubble–liquid interfacial region. With the methodology of coupling experiments under different conditions (which alter the nature of the cavitation phenomena in the bulk liquid medium) with the simulations of radial motion of cavitation bubbles, we have tried to discern the relative contribution of each of the above pathway to overall degradation of the pollutant. Moreover, we have also tried to correlate the predominant degradation mechanism to the physico-chemical properties of the pollutant. The contribution of secondary factors such as probability of radical–pollutant interaction and extent of radical scavenging (or conservation) in the medium has also been identified. Simultaneous analysis of the trends in degradation with different experimental techniques and simulation results reveals interesting mechanistic features of sonochemical degradation of the model pollutants. The physical properties that determine the predominant degradation pathway are vapor pressure, solubility and hydrophobicity. Degradation of Ph occurs mainly by hydroxylation in bulk medium; degradation of CB occurs via thermal decomposition inside the bubble, degradation of PNP occurs via pyrolytic decomposition at bubble interface, while hydroxylation at bubble interface contributes to degradation of NB and 2,4-DCP.     

Method for comparing the efficiency of ultrasound irradiation independent of the shape and the volume of the reaction vessel in sonochemical experiments

The method described herein this review compares the efficiency of ultrasound irradiation in sonochemical experiments in organic solvents. This method was shown to be independent of the shape and volume of the reaction vessel.

The principle of this method is based on the fact that the concentration of dissolved oxygen in the entire reaction volume during acoustic cavitation depends on the ultrasound power or intensity of ultrasound field respectively. The concentration of dissolved oxygen is determined by the measurement of the fluorescence intensity with fluorescence probes.     

Effect of nuclei concentration on cavitation cluster dynamics

Cavitation cluster dynamics after the passage of a single pressure wave is studied for different concentrations of artificial cavitation nuclei (30 to 3x10(5) nuclei/ml). With increasing concentration of cavitation nuclei the lifetime of the cavitation cluster is prolonged. Additionally, it is found that the spatial extent of the cluster decreases with higher nuclei concentration. The experimental data for concentrations less than 400 nuclei/ml are compared to simulations with a Rayleigh-Plesset-type equation, taking into account bubble-bubble interaction. For higher concentrations (more than 1000 nuclei/ml) the observed radial cluster dynamics is compared with calculations from an axisymmetric cavity-collapse model.     

Quantitative calculation of reaction performance in sonochemical reactor by bubble dynamics

In order to design a sonochemical reactor with high reaction efficiency, it is important to clarify the size and intensity of the sonochemical reaction field. In this study, the reaction field in a sonochemical reactor is estimated from the distribution of pressure above the threshold for cavitation. The quantitation of hydroxide radical in a sonochemical reactor is obtained from the calculation of bubble dynamics and reaction equations. The distribution of the reaction field of the numerical simulation is consistent with that of the sonochemical luminescence. The sound absorption coefficient of liquid in the sonochemical reactor is much larger than that attributed to classical contributions which are heat conduction and shear viscosity. Under the dual irradiation, the reaction field becomes extensive and intensive because the acoustic pressure amplitude is intensified by the interference of two ultrasonic waves.     

A standard method to calibrate sonochemical efficiency of an individual reaction system

Fricke reaction, KI oxidation and decomposition of porphyrin derivatives by use of seven types of sonochemical apparatus in four different laboratories were examined in the range of frequency of 19.5 kHz to 1.2 MHz. The ultrasonic energy dissipated into an apparatus was determined also by calorimetry. Sonochemical efficiency of Fricke reaction and KI oxidation was defined as the number of reacted molecule per unit ultrasonic energy. The sonochemical efficiency is independent of experimental conditions such as the shape of sample cell and irradiation instruments, but depends on the ultrasonic frequency. We propose the KI oxidation dosimetry using 0.1 moldm(-3) KI solution as a standard method to calibrate the sonochemical efficiency of an individual reaction system.     

Effects of ultrasonic frequency and liquid height on sonochemical efficiency of large-scale sonochemical reactors

The sonochemical efficiency of a cylindrical sonochemical reactor has been investigated as a function of frequency and liquid height. The irradiation frequencies were 45, 129, 231 and 490 kHz. The liquid height was varied from 10 to 700 mm. The sonochemical efficiency of the cylindrical reactor was evaluated by potassium iodide (KI) dosimetry and calorimetry. In our study, the sonochemical efficiency depended on the frequency and liquid height; further, the plots of sonochemical efficiency against liquid height exhibit one or two peaks for each frequency. The sonochemical efficiency up to the first peak increased monotonically with the logarithm of the frequency, and the liquid height for the first peak was inversely proportional to the frequency. From these results, the optimum frequency for a sonochemical reactor can be determined if the liquid height is specified for scale-up of the sonochemical reactor.     

Effect of supercritical water shell on cavitation bubble dynamics

Based on reported experimental data, a new model for single cavitation bubble dynamics is proposed considering a supercritical water(SCW) shell surrounding the bubble. Theoretical investigations show that the SCW shell apparently slows down the oscillation of the bubble and cools the gas temperature inside the collapsing bubble. Furthermore, the model is simplified to a Rayleigh–Plesset-like equation for a thin SCW shell. The dependence of the bubble dynamics on the thickness and density of the SCW shell is studied. The results show the bubble dynamics depends on the thickness but is insensitive to the density of the SCW shell. The thicker the SCW shell is, the smaller are the wall velocity and the gas temperature in the bubble. In the authors' opinion, the SCW shell works as a buffering agent. In collapsing, it is compressed to absorb a good deal of the work transformed into the bubble internal energy during bubble collapse so that it weakens the bubble oscillations.     

Sonoluminescence and sonochemical reactions in cavitation fields. A review

Contemporary ideas on the nature of cavitation are reviewed in this paper. The general theories of sonoluminescence and sonochemical reactions, the origin, stability and splitting of cavitation bubbles, the dynamics of cavitation field evolution, the peculiarities of cavitation effects at low intensity and low-frequency acoustic oscillations, the sonoluminescence quenching effect and some questions on the energetics of cavitation fields are discussed. The electrical theory of the splitting of cavitation bubbles may, as shown in the paper, become an alternative to the thermal theories of cavitation in the future.     

Sonoelectrochemical and sonochemical effects of cavitation: correlation with interfacial cavitation induced by 20 kHz ultrasound

Sonoelectrochemical measurements at macro-electrodes under extreme conditions with a very short distance between ultrasonic horn tip and electrode and different ultrasound intensity levels are shown to result in violent cavitation detected in form of current peaks superimposed on the average limiting current. Analysis of the current data obtained for the oxidation of ferrocene in dimethylformamide (0.1 M NBu4PF6) at a 4 mm diameter Pt disc electrode and for the reduction of Ru(NH3)6(3+) in aqueous 0.1 M KCl at a 6 mm diameter Pt disc electrode consistently indicate a change of the physicochemical nature of sonoelectrochemical processes under extreme conditions. The sonoelectrochemical measurement of the rate constant for the carbon bromide bond cleavage of a 3-bromobenzophenone radical anion electrogenerated at a glassy carbon electrode in dimethylformamide solution in the presence of power ultrasound is shown to yield evidence for a breakdown of the conventional mass transport model of a planar diffusion layer under extreme conditions. The change can be correlated to the number of current data points deviating more than 10% from the mean of the current due to violent cavitation processes superimposed onto the average limiting current. Further, a study of the sonochemical destruction of aqueous dilute cyanide solution (in 0.1 M NaOH) demonstrates a correlation between the electrochemically detected cavitation violence and the sonochemical activity. Factors that govern the violence of interfacial cavitation appear to be directly proportional to the factors that make cavitation in the bulk solution chemically efficient.     

Investigation of sonochemical treatment of heavy hydrocarbon by ultrasound-assisted cavitation

A highly viscous nature of heavy oil poses challenges to transportation leading to costly operation and difficult processing. Traditional methods of upgrading unconventional hydrocarbon sources involve catalytic and thermal upgrading and these methods require high temperature and pressure. In the present study, partial upgrading of heavy hydrocarbon is studied by using cavitation and the stimulator. Cavitation is a phenomenon comprising of formation, growth and collapse of bubbles in a liquid medium. The most well-known disruptive effect of cavitation occurs during the collapse phase of bubbles. Method of inducing cavitation involves transmitting 20 kHz of ultrasound through an ultrasonic horn. A model molecule used in this study is n-hexadecane (C16). The experiments were carried out at 230 °C, atmospheric pressure and 60 min time scale. The results indicated that the conversion of n-hexadecane into R1 fraction (<C16) and R2 fraction (>C16) was 4.46% for the cavitation-assisted cracking with the stimulator. The selectivity to R1 and R2 fractions were 71% and 29%, respectively. Adding 5 vol% decalin as hydrogen donor into the cracking process yielded 9.18% conversion of n-hexadecane into R1 and R2 fractions. In addition, the selectivity to R1 and R2 fractions were 87% and 13%. This study focuses on less energy intensive process for heavy hydrocarbon by utilizing cavitation and the stimulator and how ultrasound-assisted cracking with the stimulator could be a viable alternative to treat heavy hydrocarbon at the low temperature.     

Limitations of the Weissler reaction as a model reaction for measuring the efficiency of hydrodynamic cavitation

The Weissler reaction in which iodide is oxidised to a tri-iodide complex (I(3)(-)) has been widely used for measurement of the intensity of ultrasonic and hydrodynamic cavitation. It was used in this work to compare ultrasonic cavitation at 24kHz with hydrodynamic cavitation using two different devices, one a venturi and the other a sudden expansion, operated up to 8.7bar. Hydrodynamic cavitation had a maximum efficiency of about 5x10(-11) moles of I(3)(-) per joule of energy compared with the maximum of almost 8x10(-11)molJ(-1) for ultrasonic cavitation. Hydrodynamic cavitation was found to be most effective at 10 degrees C compared with 20 degrees C and 30 degrees C and at higher upstream pressures. However, it was found that in hydrodynamic conditions, even without cavitation, I(3)(-) was consumed at a rapid rate leading to an equilibrium concentration. It was concluded that the Weissler reaction was not a good model reaction for the assessment of the effectiveness of hydrodynamic cavitation.     

The effects of acoustic flow and mechanical flow on the sonochemical efficiency in a rectangular sonochemical reactor

Visualization of cavitation behavior in a rectangular sonochemical reactor at 490 kHz was carried out by a laser sheet technique and the distribution of liquid flow was measured by a laser Doppler velocimeter. The pattern of liquid flow and distribution of acoustic pressure of the rectangular sonochemical reactor were investigated as a function of the input power from 10 to 50 W. The liquid moved upward above the transducer at every power. As increasing the input power, the random flow out side the cylindrical part above the transducer changed into the convective one and the region of the visualized standing wave which was formed in the cylindrical part changed with the input power. The position showing the sonochemical luminescence exists inside or near the region where the standing wave was visualized. Introduction of a stirrer resulted in disturbance of liquid flow and expanded the position showing the sonochemical luminescence, but the luminescence intensity was weakened. The sonochemical efficiency was enhanced by about twice by introduction of the stirrer. From these results, we discussed the effects of liquid flow on sonochemical efficiency with and without a stirrer.     

Characterization of transient cavitation activity during sonochemical modification of magnesium particles

Investigation of the cavitation activity during ultrasonic treatment of magnesium particles during nanostructuring has been performed. Cavitation activity is recorded in the continuous mode after switching the ultrasound on with the use of ICA-5DM cavitometer. It has been demonstrated that this characteristic of the cavitation zone may be varied in a wide range of constant output parameters of the generator. The speed and nature of the cavitation activity alteration depended on the concentration of Mg particles in the suspension and the properties of the medium in which the sonochemical treatment has been performed. Three stages of the cavitation area evolution can be distinguished: 1 – the initial increase in cavitation activity, 2 – reaching a maximum with a subsequent decrease, and 3 – reaching the plateau (or the repeated cycles with feedback loops of enlargement/reduction of the cavitation activity).The ultrasonically treated magnesium particles have been characterized by scanning electron microscopy, X-ray diffraction analysis and thermal analysis. Depending on the nature of the dispersed medium the particles can be characterized by the presence of magnesium hydroxide (brucite) and magnesium hydride. It is possible to reach the incorporation of magnesium hydride in the magnesium hydroxide/magnesium matrix by varying the conditions of ultrasonic treatment (duration of treatment, amplitude, dispersed medium etc.). The influence of the magnesium reactivity is also confirmed by the measurements of cavitation activity in organic dispersed media (ethanol, ethylene glycol) and their aqueous mixtures.     

Energy balance of high-energy stable acoustic cavitation within dual-frequency sonochemical reactor

The acoustic cavitation bubble as an open energetic system is the seat of conversion of various forms of energy accompanying the bubble oscillation. The energy conversion would explain specific dynamical, thermal and kinetical behaviors. In the present paper, the energy balance related to a stable bubble irradiated by dual-frequency field is simulated numerically and interpreted in accordance with the phenomena occurring inside it. The study particularly focuses on the comparison of the energetic behavior of high-energy stable cavitation with bubbles that are non-active in sonochemistry, submitted to couples of 35, 140, 300 and 515 kHz. The simulation results revealed that pressure forces work is the major energetic input during the bubble oscillation lifetime, while the main energetic loss comes from heat transfer by diffusion and enthalpy loss accompanying water condensation. Besides, high rates of condensation of water molecules and low amounts of accumulated energy inside the bubble volume were identified as the key factors preventing the achievement of the sonochemical activity threshold.     

Effect of ultrasonic cavitation on dynamics and characteristics of electric discharge in liquid

Characteristics of electric discharges in liquid media under intense ultrasonic vibrations are studied. The difference in current dynamics and discharge voltages in the presence and in the absence of cavitation is shown.     

Ultrasonic cavitation applied to the treatment of bisphenol A. Effect of sonochemical parameters and analysis of BPA by-products

Bisphenol A (BPA), a chemical compound largely used in the plastics industry, can end up in aquatic systems, which it disturbs by its endocrine disrupting effect (EDE). This study investigated the BPA degradation upon ultrasonic action under different experimental conditions. The effect of saturating gas (oxygen, argon and air), BPA concentration (0.15-460 micromol L(-1)), ultrasonic frequency (300-800 kHz) and power (20-80 W) were evaluated. For a 118 micromol L(-1)-BPA solution, with the best performance obtained at 300 kHz, 80 W, with oxygen as saturating gas. In these conditions, BPA can be readily eliminated by the ultrasound process (approximately 90 min). However, even after long ultrasound irradiation times (9 h), more than 50% of chemical oxygen demand (COD) and 80% of total organic carbon (TOC) remained in the solution. Analyses of intermediates using HPLC-MS investigation identified several products: monohydroxylated bisphenol A, 4-isopropenylphenol, quinone of monohydroxylated bisphenol A, dihydroxylated bisphenol A, quinone of dihydroxylated bisphenol A, monohydroxylated-4-isopropenylphenol and 4-hydroxyacetophenone. The presence of these hydroxylated aromatic structures showed that the main ultrasonic BPA degradation pathway is related to the reaction of BPA with the *OH radical. After 2h, these early products were converted into biodegradable aliphatic acids.     

Realization of cavitation fields based on the acoustic resonance modes in an immersion-type sonochemical reactor

Different modes of cavitation zones in an immersion-type sonochemical reactor have been realized based on the concept of acoustic resonance fields. The reactor contains three main components, namely a Langevin-type piezoelectric transducer (20 kHz), a metal horn, and a circular cylindrical sonicated cell filled with tap water. In order to diminish the generation of cavitation bubbles near the horn-tip, an enlarged cone-shaped horn is designed to reduce the ultrasonic intensity at the irradiating surface and to get better distribution of energy in the sonicated cell. It is demonstrated both numerically and experimentally that the cell geometry and the horn position have prominent effects on the pressure distribution of the ultrasound in the cell. With appropriate choices of these parameters, the whole reactor works at a resonant state. Several acoustic resonance modes observed in the simulation are realized experimentally to generate a large volume of cavitation zones using a very low ultrasonic power.