Numerical simulation of liquid velocity distribution in a sonochemical reactor

Ultrasonically induced flow is an important phenomenon observed in a sonochemical reactor. It controls the mass transport of sonochemical reaction and enhances the reaction performance. In the present paper, the liquid velocity distribution of ultrasonically induced flow in the sonochemical reactor with a transducer at frequency of 490 kHz has been numerically simulated. From the comparison of simulation results and experimental data, the ultrasonic absorption coefficient in the sonochemical reactor has been evaluated. To simulate the liquid velocity near the liquid surface above the transducer, which is the main sonochemical reaction area, it is necessary to include the acoustic fountain shape into the computational domain. The simulation results indicate that the liquid velocity increases with acoustic power. The variation of liquid height also influences the behavior of liquid velocity distribution and the mean velocity above the transducer centre becomes a maximum when the liquid height is 0.4 m. The liquid velocity decreases with increasing the transducer plate radius at the same ultrasonic power.     

A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 1. Overhead stirring

The effect of flow in an ultrasonic reactor is an important consideration for practical applications and for the scale-up of ultrasonic processing. Previous literature on the influence of flow on sonochemical activity has reported conflicting results. Therefore, this work examined the effect of overhead stirring at four different frequencies, 40, 376, 995 and 1179 kHz, in two different reactor configurations. Comparable power settings were utilised to elucidate the underlying mechanisms of interactions between the flow and sonochemical activity. The sonochemical activity was determined by the yield of hydrogen peroxide, measured by iodide dosimetry, and the active region was visualised with sonochemiluminescence imaging. The overhead stirring in the low frequency reactor altered the yield of hydrogen peroxide so it produced the maximum yield out of the four frequencies. The increase in hydrogen peroxide yield was attributed to a reduction in coalescence at 40 kHz. However at the higher frequencies, coalescence was not found to be the main reason behind the observed reductions in sonochemical yield. Rather the prevention of wave propagation and the reduction of the standing wave portion of the field were considered.     

The effects of liquid height/volume,initial concentration of reactant and acoustic power on sonochemical oxidation

Even though much knowledge on acoustic cavitation and its application has been accumulated over the past decades, further research is still required to develop industrial uses of acoustic cavitation. It is because the available information is mainly based on small-scale sonoreactors and the design and optimization of sonoreactors for large-scale applications have not been widely studied. In this study, the effects of liquid height/volume, initial concentration of the reactant and input acoustic power on sonochemical oxidation reactions including iodide ion oxidation, As(III) oxidation, and hydrogen peroxide generation were investigated using a 291 kHz sonoreactor with various liquid height/volumes (50, 100, 200, 300, 500, and 1000 mL) and input powers (23, 40, and 82 W). As the liquid height/volume and the input power changed, the power density varied from 23 to 1640 W/L and the maximum cavitation yields of triiodide ion for 23, 40, and 82 W were observed at 0.05, 0.1, and 0.2/0.3 L, respectively. It was found that low power was more effective for the small volume and the large volume required high power level and the moderate power density, approximately 400 W/L, was suggested for the sonochemical oxidation of iodide ion in the 291 kHz sonoreactor in this study. Similar results were observed in the generation of hydrogen peroxide and the sonochemical oxidation of As(III) to As(V). It was also revealed that KI dosimetry could be applicable for the estimation of the sonochemical reactions of non-volatile compounds such as As(III).     

Investigation of sonochemical activities at a frequency of 334 kHz: The effect of geometric parameters of sonoreactor

In this study, the effect of the dimensions of the bottom plate and liquid height was investigated for high-frequency sonoreactors under a vertically irradiated system. The dimensions of the bottom plate did not significantly influence sonochemical activity considering power density. However, as the bottom plate was increased in size, the hydroxyl radical generation rate decreased because of a decrease in power density. It is therefore recommended that sonoreactors with bottom-plate dimensions close to those of the ultrasonic transducer module be used. Liquid height had a significant effect on sonochemical activity, but the trend of the activity considering power density changed as the initial pollutant concentration changed. In the case of low initial concentration of As(III) (1 mg/L), the maximum cavitation yield for As(III) oxidation was observed at liquid heights of 150 mm.     

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.     

A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 2. Fluid circulation at high frequencies

Sonochemical activity is dependent on flow patterns within the reactor and either no affect or a decrease in activity was observed at 376, 995, and 1179 kHz from overhead stirring. The interaction of fluid flow with ultrasound was further investigated in this study with circulatory flow. The effect of fluid circulation on radical production was investigated at two circulation speeds, with and without surface stabilisation. The sonochemical activity was determined by the yield of hydrogen peroxide, measured by iodide dosimetry. The sonochemically active region was pictured using sonochemiluminescence imaging and the flow fields were visualised with dyed flow videos. At 376 and 995 kHz, an increase in sonochemical activity was observed with the slower flow rate; however at 1179 kHz, the sonochemical activity was either not affected or decreased. The observed changes in sonochemical activity were attributed to an increase in asymmetry of the bubble collapse brought about by fluid motion.     

Comparison of characterization methods in high frequency sonochemical reactors of differing configurations

The aim of this study was to compare different characterization methods in order to evaluate the sonochemical efficiency of a cavitational reactor. The selected characterization methods were calorimetry and dosimetry based on potassium iodide oxidation or nitrite and nitrate ion formation. The effects of experimental parameters on physical and chemical effects of ultrasound were quantified with two transducers at a frequency of 366 kHz. The studied factors comprised temperature (16–28 °C), acoustic power (6–38 W), power density (4–61 W L^?1) and reactor configuration (D_{reactor 1} = 65 mm, D_{reactor 2} = 102 mm). Spectrophotometry was compared to ionic chromatography as a method to quantify nitrite and nitrate ions. Spectrometry was shown to be as representative as ionic chromatography. The reaction system based on the formation of both nitrite and nitrate ions was demonstrated to be as reliable as a potassium iodide dosimeter. The representativity of calorimetry was limited since part of acoustic energy was assumed to be used in the chemical reactions observed by dosimetry. Similar sonochemical efficiencies resulted from an increase of sonified surface (D_{reactor 1} = 65 mm vs. D_{reactor 2} = 102 mm) coupled to a 2-time decrease in power density at a constant emitting surface. The effect of emitting-to-sonified surface area ratio on the acoustic field was apparently limited by the height of the liquid.     

Estimation of absolute sound pressure in a small-sized sonochemical reactor

A small-sized sonochemical reactor in which the absolute value of the sound pressure amplitude can be estimated from the vibration velocity of the transducer was investigated. The sound pressure distribution in the reactor and the relationship between the vibration velocity and the sound pressure amplitude were derived through Helmholtz wave equation. The reactor consists of a bolt-clamped Langevin transducer and a rectangular cell with a tungsten reflector. A 3λ/4-standing-wave-field was generated in the reactor to simplify the sound pressure distribution. The sound pressure distribution was measured from the optical refractive index change of water using a laser interferometer. The experimental and theoretical results showed a good agreement in the absolute value of the sound pressure amplitude, and it was confirmed that the sound pressure in the sonochemical reactor can be estimated from the input current of the vibrator.     

Baeyer–Villiger oxidation of cyclohexanone to ε-caprolactone in airlift sonochemical reactor

Baeyer–Villiger oxidation of cyclohexanone to ε-caprolactone was studied in a new type reactor – the airlift loop sonochemical reactor. The reactor plays a synergistic effect of sonochemsity and higher oxygen transfer rate. The influences of ultrasound intensity, reaction temperature, the molar ratio of benzaldehyde to cyclohexanone and oxygen gas flow rate on the conversion and selectivity of cyclohexanone were investigated and discussed. Under ultrasound, the amount of benzaldehyde can be reduced from 75% to 67%. Ultrasound not only intensified the rates of reactions but also increased the yield of product. The optimized operation conditions are listed as follows: the reaction temperature is 30 °C, the molar ratio of cyclohexanone to benzaldehyde is 1:2, the oxygen gas flow rate is 1.15 cm s⁻¹, and ultrasonic irradiations 2 h at 40 kHz, 2.25 W cm⁻². Under the optimum operation conditions, the average molar yield of ε-caprolactone comes up to 87.7%.     

Dependence of sonochemical parameters on the platinization of rutile titania – An observation of a pronounced increase in photocatalytic efficiencies

Using a standing wave sonochemical reactor (SWSR), the influences of parameters of ultrasonic power input, sonication time, sonication temperature and the amount of propanol (which generates the reducing radicals) were systemically investigated to ascertain and optimize the best conditions for the sonochemical reduction of Pt from its precursor hexachloroplatinic acid and then its deposition on rutile TiO₂ (platinization of rutile titania) catalysts. Catalytic activity of the prepared platinized catalysts was tested in the reaction of methyl orange degradation. The results of photocatalytic activity study in the degradation of methyl orange further demonstrate that sonochemically as-prepared Pt/TiO₂ catalysts show a pronounced increase (～2 times) in photodegradation, even with a deposition of small amounts of platinum (1.4 wt.%), as compared to the unsupported or naked rutile titania. Although there are various parameters that influence the sonochemical platinization of rutile titania, the present optimization results clearly indicate that the best photocatalytic degradation of methyl orange can be obtained when the experimental conditions of the preparation were with an input power of 50 W, an initial hexachloroplatinic acid volume of 70 ml (which results into 1.4 wt.% Pt on TiO₂), sonication time of 90 min, 0.18 g of propanol and a temperature of 10 °C were adopted. The method of ultrasound application to prepare metal supported semiconductors has many advantages such as convenience, safety and high efficiency. Furthermore, it is hopeful that this optimization study can also be extended to the generation of similar metal supported semiconductors.     

Synthesis of biodiesel from waste cooking oil using sonochemical reactors

Investigation into newer routes of biodiesel synthesis is a key research area especially due to the fluctuations in the conventional fuel prices and the environmental advantages of biodiesel. The present work illustrates the use of sonochemical reactors for the synthesis of biodiesel from waste cooking oil. Transesterification of used frying oil with methanol, in the presence of potassium hydroxide as a catalyst has been investigated using low frequency ultrasonic reactor (20 kHz). Effect of different operating parameters such as alcohol–oil molar ratio, catalyst concentration, temperature, power, pulse and horn position on the extent of conversion of oil have been investigated. The optimum conditions for the transesterification process have been obtained as molar ratio of alcohol to oil as 6:1, catalyst concentration of 1 wt.%, temperature as 45 °C and ultrasound power as 200 W with an irradiation time of 40 min. The efficacy of using ultrasound has been compared with the conventional stirring approach based on the use of a six blade turbine with diameter of 1.5 cm operating at 1000 rpm. Also the purification aspects of the final product have been investigated.     

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.     

Intensification of sonochemical decolorization of anthraquinonic dye Acid Blue 25 using carbon tetrachloride

In this work, the influence of CCl₄ on the sonochemical decolorization of anthraquinonic dye Acid Blue 25 (AB25) in aqueous medium was investigated using high frequency ultrasound (1700 kHz). This frequency, reputed ineffective, was tested in order to introduce the ultrasound waves with high frequency in the field of degradation or removal of dyes from wastewater, due to its limited use in this field, and to increase the application of high frequency ultrasound wave in the field of environmental protection. The effects of various parameters such as the concentration of CCl₄, frequency (22.5 and 1700 kHz), solution pH, temperature and tert-butyl alcohol adding on the decolorization rate of AB25 was studied. The obtained results clearly demonstrated the significant intensification of AB25 decolorization in the presence of CCl₄. The enhancement effect of CCl₄ increased by decreasing temperature and by increasing the CCl₄ concentration. The pH has a significant influence on the bleaching of dye both in the absence and presence of CCl₄. The three investigated dosimeter methods (KI oxidation, Fricke reaction and H₂O₂ production) well corroborate the improvement of the sonochemical effects in the presence of CCl₄. The best sonochemical decolorization rate of AB25 in aqueous solution both in the absence and presence of CCl₄ is observed to occur at 1700 kHz compared to 22.5 kHz. The sonochemical oxidation of CCl₄ generates oxidizing species in the liquid phase that are highly beneficial for oxidation of hydrophilic and non-volatile pollutant, such as dyes, because they are less susceptible to free radical attack due to lower stability of the generated free radicals.     

Enhancement of sonochemical reaction of terephthalate ion by superposition of ultrasonic fields of various frequencies

The ultrasonic reactor with dual frequency was used and the effect of frequency on the fluorescence intensity of terephthalate ion was experimentally investigated in the frequency range from 176 to 635 kHz. The sonochemical reaction fields were visualized by using sonochemical luminescence of luminol solution. Compared with the fluorescence intensity of terephthalate ion for single frequency, the fluorescence intensity for dual frequency increased. The fluorescence intensity ratio of dual frequency to single frequency had maximum value when the frequency of transducer attached at the bottom wall was comparable in magnitude to that at the side wall. In the case of dual frequency, the sonochemical reaction fields became more extensive in the reactor and more intensive around the center of the reactor.     

Generation of microbubbles from hollow cylindrical ultrasonic horn

In this study, we found that microbubbles with diameters of less than 100 μm can be easily generated by using a hollow cylindrical ultrasonic horn. Consecutive images of bubbles obtained by using high-speed and high-resolution cameras reveal that a capillary wave is formed on the gas–liquid interface under weak ultrasonic irradiation and that the wave head is detached in the form of bubbles by the fragmentation of the interface as the power of ultrasonic irradiation increases. Moreover, consecutive images of the bubble interface obtained by an ultra-high-speed camera indicate that the breakup of bubbles oscillating harmonically with the ultrasonic irradiation generates many microbubbles that are less than 100 μm in diameter. With regard to the orifice diameter of the horn end, we found that its optimum value varies with the ultrasonic power input. When the orifice diameter is small, the capillary wave generated from the horn end easily propagates all over the gas–liquid interface, thereby starting the generation of microbubbles at a lower ultrasonic power input. When the orifice diameter is large, the capillary wave is attenuated because of viscosity and surface tension. Hence, in this case, microbubble generation from the horn requires a higher ultrasonic power input. Furthermore, the maximum yield of microbubbles via primary and secondary bubble generation can be increased by increasing the gas flow rate.     

A study of the influence of the input electrical power on the spectral width of the 510.6 nm line of an atomic copper vapor laser

The effect of the input electrical power on the spectral width of the 510.6 nm line of an atomic copper vapor laser (CVL) is investigated. An analysis of the gas temperature inside the discharge tube and the line broadening mechanism of the CVL is reported. The input electrical power was varied from 2.0 to 4.2 kW in a cylindrical discharge tube of inner radius 2.35 cm and length 150.0 cm. A Fabry–Perot etalon and imaging camera-based setup interfaced with personal computer was used to measure the spectral width of the 510.6 nm (green) laser line. The Doppler broadened spectral profile of the laser emission varies with input electrical power and an additional broadening of almost 1 GHz at the highest operating input power was observed.     

Template-free sonochemical synthesis of hierarchically porous NiO microsphere

A novel template-free sonochemical synthesis technique was used to prepare NiO microspheres combined with calcination of NiO_2.45C_0.74N_0.25H_2.90 precursor at 500 °C. The NiO microspheres samples were systematically investigated by the thermograviometric/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), fourier-transformed infrared spectroscopy (FT-IR), Brunnauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms, laser particle size analyzer, and ultraviolet–visible spectroscopy (UV–Vis). The morphology of the precursor was retained even after the calcination process, and exhibited hierarchically porous sphericity. The morphology changed over the ultrasonic radiation time, and the shortest reaction time was 70 min, which was much less than 4 h for the mechanical stirring process. The mechanical stirring was difficult to form the complete hierarchically porous microsphere structure. The BET specific surface area and the median diameter of the hierarchically porous NiO microspheres were 103.20 m²/g and 3.436 μm, respectively. The synthesized NiO microspheres were mesoporous materials with a high fraction of macropores. The pores were resulted from the intergranular accumulation. The ultraviolet absorption spectrum showed a broad emission at the center of 475 nm, and the band gap energy was estimated to be 3.63 eV.     

Ultrasonic degradation of acetaminophen in water: Effect of sonochemical parameters and water matrix

This paper deals about the sonochemical water treatment of acetaminophen (ACP, N-acetyl-p-aminophenol or paracetamol), one of the most popular pharmaceutical compounds found in natural and drinking waters. Effect of ultrasonic power (20–60 W), initial ACP concentration (33–1323 μmol L⁻¹) and pH (3–12) were evaluated. High ultrasonic powers and, low and natural acidic pH values favored the efficiency of the treatment. Effect of initial substrate concentration showed that the Langmuir-type kinetic model fit well the ACP sonochemical degradation. The influence of organic compounds in the water matrix, at concentrations 10-fold higher than ACP, was also evaluated. The results indicated that only organic compounds having a higher value of the Henry’s law constant than the substrate decrease the efficiency of the treatment. On the other hand, ACP degradation in mineral natural water showed to be strongly dependent of the initial substrate concentration. A positive matrix effect was observed at low ACP concentrations (1.65 μmol L⁻¹), which was attributed to the presence of bicarbonate ion in solution. However, at relative high ACP concentrations a detrimental effect of matrix components was noticed. Finally, the results indicated that ultrasonic action is able to transform ACP in aliphatic organic compounds that could be subsequently eliminated in a biological system.     

Investigation of flow features and acoustic radiation of a round cavity

In this work, the interaction between a boundary layer and a circular cylindrical cavity is studied. Experimental pressure and velocity results for a cavity of diameter 10 cm and depth ranging from 10 to 15 cm are described, for flow velocities between 50 and 110 m s^?1. This flow configuration is found to generate intense discrete depth- and flow-dependent tones, resulting in modes similar in appearance to Rossiter modes found in shallow rectangular cavities. Differences between the cylindrical cavity's mean flow and that of a similarly sized rectangular cavity are highlighted. The development of the shear layer is quantified, in terms of thickening and of velocity statistics profiles. Radial and azimuthal acoustic modes are observed in the acoustic field inside the cavity. A feedback model based on the coupled behaviour of the fundamental acoustic depth mode of the cavity and the large scale dynamics of the shear layer is constructed, and its response is compared to experimental data. A good qualitative agreement between available data and modeled behaviour is observed, allowing the two acoustic modes found in this work to be attributed to the interaction of the shear layer with the cavity's fundamental depth mode.     

Creaming enhancement in a liter scale ultrasonic reactor at selected transducer configurations and frequencies

Recent research has shown that high frequency ultrasound (0.4–3 MHz), can enhance milkfat separation in small scale systems able to treat only a few milliliters of sample. In this work, the effect of ultrasonic standing waves on milkfat creaming was studied in a 6 L reactor and the influence of different frequencies and transducer configurations in direct contact with the fluid was investigated. A recombined coarse milk emulsion with fat globules stained with oil-red-O dye was selected for the separation trials. Runs were performed with one or two transducers placed in vertical (parallel or perpendicular) and horizontal positions (at the reactor base) at 0.4, 1 and/or 2 MHz (specific energy 8.5 ± 0.6 kJ/kg per transducer). Creaming behavior was assessed by measuring the thickness of the separated cream layer. Other methods supporting this assessment included the measurement of fat content, backscattering, particle size distribution, and microscopy of samples taken at the bottom and top of the reactor. Most efficient creaming was found after treatment at 0.4 MHz in single and double vertical transducer configurations. Among these configurations, a higher separation rate was obtained when sonicating at 0.4 MHz in a vertical perpendicular double transducer setup. The horizontal transducer configuration promoted creaming at 2 MHz only. Fat globule size increase was observed when creaming occurred. This research highlights the potential for enhanced separation of milkfat in larger scale systems from selected transducer configurations in contact with a dairy emulsion, or emulsion splitting in general.