Characterisation of mixing rate due to high power ultrasound

For meaningful assessment of results from laboratory and pilot plant trials, it is often necessary to know the mixing characteristics within the ultrasonic reactors. Previous workers have used conductivity measurements in an attempt to characterize the residence time distribution in ultrasonic reactors, but these results do not provide direct data on the mixing within the high power region adjacent to the ultrasonic probe tip. We overcome this difficulty through direct visualization of the mixing process within the high energy region close to the tip of the ultrasonic probe. Our analysis proceeds by determining an approximate turbulent diffusivity in a batch reactor arrangement for different values of ultrasonic energy input. For input electrical power levels between 70 and 120 W and a processing volume of 30 ml, the effective turbulent diffusivity varied from about 0.2 x 10(-3) to 0.7 x 10(-3)m(2)/s. We demonstrate that such results can be coupled to a suitable dispersion model to estimate the actual residence time distribution in flow-though arrangements when the through-put adds little additional mixing energy. Therefore, coupling the effective turbulent diffusivity identified in a batch reactor with a suitable dispersion model for the reactor offers an alternative approach to the deduction of RTD when determining the actual RTD in the high intensity zone of steady flow sonochemical reactors is problematic.     

Evaluation of different cavitational reactors for size reduction of DADPS

The present study deals with the size reduction based on the recrystallization (antisolvent approach using water) of 3,3′-Diamino Diphenyl Sulfone (DADPS) using different types of cavitational reactors as an alternative to the conventional process of mechanical size reduction, which is an energy intensive approach. Ultrasound was applied for fixed time specific to the reactors namely ultrasonic probes at different power dissipation levels and also ultrasonic bath. A High Speed Homogenizer was also used at varying speeds of rotation to establishing the efficacy for size reduction. The processed sample was analysed for particle size and morphology using particle size analyser and optical microscopy respectively. The final yield of recrystallization was also determined. The power density in W/L and power intensity in W/m² calculated for each equipment has been used to establish efficacy for size reduction since all devices had dissimilar configurations. Based on the studies of varying power intensity of the different US equipment, it was established that larger the power intensity and power density, smaller was the resultant final particle size after treatment for same time. Among the various ultrasonic devices used, Sonics VCX750 probe yielded the best size reduction of 85.47% when operated at 40% amplitude for 60 min for a volume of 200 ml. A High Speed Homogenizer used at 7000 rpm gave 92.35% of size reduction in 15 min operation and also demonstrated the best energy efficiency. The work has elucidated the comparison of different cavitational devices for size reduction for the first time and presented the best reactors and conditions for the desired size reduction.     

Qualitative characterization of ultrasound reactors for heterogeneous sonochemistry

Several measurement methods (thermocouple probe, perforation of aluminium foils by cavitation, chemical dosimeter) are used to characterize the sound fields of low and high intensity ultrasound reactors and to find optimal conditions for the ultrasonic irradiation of heterogeneous reaction mixtures. In this Paper it is shown that, due to the formation of standing waves, the local ultrasound intensity in a flask fixed in an ultrasonic cleaner is strongly susceptible to small changes in experimental conditions. It is also shown that high intensity ultrasound sources, such as an ultrasonic horn, tend to uncouple: that is, cavitation only occurs at the radiating surface and only marginal ultrasound intensity can be detected elsewhere in the surrounding liquid.     

Intensification of delignification of Tectona grandis saw dust as sustainable biomass using acoustic cavitational devices

Delignification of sawdust was studied using ultrasound assisted alkali peroxide approach using longitudinal horn for the first time and the efficacy compared with more commonly used configurations of ultrasonic reactors. Comparison with the conventional approach based on stirring has also been presented to establish the process intensification benefits. Effect of different operating parameters such as sodium carbonate concentration (0.1, 0.15, 0.2, 0.25 M), hydrogen peroxide concentration (0.2, 0.4, 0.6, 0.8, 1 M) and biomass loading (2, 4, 6, 8, 10 wt%), on the efficacy of lignin extraction has been investigated for different ultrasonic reactors. The optimum conditions for probe type ultrasonic horn were established as 150 W, 50% duty cycle and 80% amplitude with optimum process conditions as Na₂CO₃ concentration as 0.2 M, H₂O₂ concentration as 1 M, biomass loading of 10 wt% and operating time of 70 min. Longitudinal horn resulted in best efficacy (both in terms of yield and energy requirements) followed by ultrasonic horn and ultrasonic bath whereas the conventional approach was least effective. The obtained lignin was also analyzed using different characterization techniques. The presence of peaks at wavelength range of 875–817, 1123–1110, and at 1599 cm⁻¹ for the extracted sample confirmed the presence of lignin. Increase in the crystallinity index of the processed sample (maximum for longitudinal horn) also confirmed the lignin removal as lignin is amorphous in nature. Overall it has been concluded that ultrasound can be effectively used for delignification with longitudinal horn as best configuration.     

不同超声作用方式对葛根有效部位提取率的影响

研究不同的超声作用方式对葛根总黄酮的提取率的影响.采用单频,双频（包括槽式双频以及槽式＋探头式双频）,及三频等不同的处理方法,对超声作用参数,如时间,声强及处理量的多少进行研究,采用两个超声特性参数即能量效率和空化产量来对比不同容积、频率的超声作用效果.目前的研究表明：多频超声耦合时,可以获得较高的能量效率和空化产量.     

Effects of ultrasonic intensity and reactor scale on kinetics of enzymatic saccharification of various waste papers in continuously irradiated stirred tanks

Based on the enzymatic saccharification of the various pulps in the previous 0.8 l ultrasonic stirred tank reactor, the ultrasound-enhanced saccharification of waste papers such as newspaper, carton paper, office paper etc. was carried out in the same reactor as well as larger scale stirred tank reactors of size 3.2 and 6.4 l. The saccharification of each waste paper was less enhanced in the larger reactor at a given ultrasonic intensity. This could be attributed to the decrease in the ultrasonic intensity per reaction volume, i.e., the specific ultrasonic intensity. Most waste papers were more efficiently hydrolyzed with increasing specific ultrasonic intensities, although newspaper was less efficiently done for a too high specific intensity. Such an adverse effect might be due to the fact that some impurities in the newspaper such as lignin were activated by an intensive ultrasonic irradiation to form a rigid and closed network, which inhibited the access and adsorption of cellulase on to the substrate surface. The previous kinetic model was found to be applicable to analyze and simulate the saccharification of each waste paper in the different ultrasonic reactors. The ultimate conversion of a substrate based on the total sugar concentration estimated for an infinite reaction time could be correlated as a function of the ratio of initial substrate to enzyme concentrations at a fixed specific ultrasonic intensity. Either the apparent rate constant or the ultimate conversion increased and tended to approach a constant with an increase in the specific ultrasonic intensity except for the case of newspaper, while neither the apparent Michaelis constant, product inhibition constant nor glucose formation equilibrium constant was influenced by the specific ultrasonic intensity.     

Sonochemical degradation of phenol in water: a comparison of classical equipment with a new cylindrical reactor

Cavitation due to ultrasonic waves produces highly reactive oxidising species in water. As a result, it can be used to oxidise organic pollutants such as aromatic compounds in dilute aqueous solutions. Recent studies have demonstrated that reactors operating in the high frequency range (e.g. 500 kHz) are more efficient than reactors working at lower frequency (20 kHz) for the destruction of these kinds of contaminants. Our study describes the degradation of phenol with the help of a cylindrical ultrasonic apparatus that operates at 35 kHz (Sonitube-SODEVA). To date, the use of this type of reactor has not been reported. The reaction rates thus obtained were compared to those obtained at the same ultrasonic power (50 W) with more classical devices operating at 20 and 500 kHz. The general result is that in aqueous solution, the rate of phenol destruction is higher at 500 kHz than at 35 or 20 kHz. Addition of hydrogen peroxide and copper sulphate to the medium provides a different oxidative system that proceeds more efficiently at 35 kHz; the time of destruction was about one-third of the time needed at 500 kHz. It was also observed that the intermediate organic compounds are eliminated much faster at 35 kHz in comparison with the two frequencies. The observation of such different behaviour is not necessarily a pure frequency effect, but can be due to a response to other parameters such as the acoustic field and intensity.     

Evaluation of process parameters of ultrasonic treatment of bacterial suspensions in a pilot scale water disinfection system

In this study, several process parameters that may contribute to the efficiency of ultrasound disinfection are examined on a pilot scale water disinfection system that mimics realistic circumstances as encountered in an industrial environment. The main parameters of sonication are: (i) power; (ii) duration of treatment; (iii) volume of the treated sample. The specific energy (E_s) is an indicator of the intensity of the ultrasound treatment because it incorporates the transferred power, the duration of sonication and the treated volume. In this study, the importance of this parameter for the disinfection efficiency was assessed through changes in volume of treated water, water flow rate and electrical power of the ultrasonic reactor. In addition, the influences of the initial bacterial concentration on the disinfection efficiency were examined. The disinfection efficiency of the ultrasonic technique was scored on a homogenous and on a mixed bacterial culture suspended in water with two different types of ultrasonic reactors (Telsonic and Bandelin). This study demonstrates that specific energy, treatment time of water with ultrasound and number of passages through the ultrasonic reactor are crucial influential parameters of ultrasonic disinfection of contaminated water in a pilot scale water disinfection system. The promising results obtained in this study on a pilot scale water disinfection system indicate the possible application of ultrasound technology to reduce bacterial contamination in recirculating process water to an acceptable low level. However, the energy demand of the ultrasound equipment is rather high and therefore it may be advantageous to apply ultrasound in combination with another treatment.     

Application of ultrasound to biotechnology: an overview.

Application of ultrasound to biotechnology is relatively new, but several processes that take place in the presence of cells or enzymes are activated by ultrasonic waves. High intensity ultrasonic waves break the cells and denaturize the enzymes. Low intensity ultrasonic waves can modify cellular metabolism or improve the mass transfer of reagents and products through the boundary layer or through the cellular wall and membrane. In the case of enzymes, the increase in the mass transfer rate of the reagents to the active site seems to be the most important factor. Immobilized enzymes are more resistant to thermal deactivation produced by ultrasound than native enzymes. Reverse micelles can be used to carry out synthesis using enzymes. Several applications of ultrasound to the biotechnology are discussed.     

A computational modeling approach of the jet-like acoustic streaming and heat generation induced by low frequency high power ultrasonic horn reactors

High power ultrasound reactors have gained a lot of interest in the food industry given the effects that can arise from ultrasonic-induced cavitation in liquid foods. However, most of the new food processing developments have been based on empirical approaches. Thus, there is a need for mathematical models which help to understand, optimize, and scale up ultrasonic reactors. In this work, a computational fluid dynamics (CFD) model was developed to predict the acoustic streaming and induced heat generated by an ultrasonic horn reactor. In the model it is assumed that the horn tip is a fluid inlet, where a turbulent jet flow is injected into the vessel. The hydrodynamic momentum rate of the incoming jet is assumed to be equal to the total acoustic momentum rate emitted by the acoustic power source. CFD velocity predictions show excellent agreement with the experimental data for power densities higher than W(0)/V ≥ 25kWm(-3). This model successfully describes hydrodynamic fields (streaming) generated by low-frequency-high-power ultrasound.     

Studies on Rheological Behavior and Structure Development of High‐Density Polyethylene in the Presence of Ultrasonic Oscillations During Extrusion

The effects of ultrasonic oscillations on properties and structure of extruded high‐density polyethylene (HDPE) were studied. The experimental results show that ultrasonic oscillations can improve the surface appearance of the HDPE extrudates; increase the productivity of the HDPE extrudates; and decrease the die pressure, melt viscosity, and flow activation energy of the HDPE. The processing properties of the HDPE improve greatly in the presence of ultrasonic oscillations. Linear viscoelastic properties tests show that dynamic shear viscosity and zero shear viscosity decrease in the presence of ultrasonic oscillations. Ultrasonic oscillations can improve crystal perfection and thermal stability of HDPE. At appropriate ultrasound intensity, ultrasonic oscillations could also increase the mechanical strength of extruded HDPE. The gel permeation chromatography (GPC) results show that at high ultrasound intensity and low rotation speed of extrusion, ultrasonic oscillations causes chain scission of HDPE, which result in a decrease of molecular weight and an increase of melt flow index.     

Respective contribution of cavitation and convective flow to local stirring in sonoreactors

The knowledge of respective parts of convection and cavitation to the stirring induced by ultrasound at one exact position into a sonoreactor is useful for all processes implementing surfaces exposed to sonication. PIV measurement allows real fluid motion determination, whereas the electrochemical technique gives an equivalent flow velocity considered as the sum of all stirring contributions to the electrode. Thus, by a simple subtraction between real fluid velocity and equivalent flow velocity, it is possible to identify the contribution of each phenomenon. Applied to low frequency reactors, it had been observed that cavitation is the preponderant phenomenon, with a contribution of stirring close to the electrode always more than 90%. High frequency reactors, frequently known to produce less cavitation, have shown that at the focal zone, if it concerns HIFU, cavitation becomes preponderant and reaches similar values to those close to the ultrasonic horn in low frequency sonoreactors.     

Application of low intensity ultrasonics to cheese manufacturing processes

Ultrasound has been used to non-destructively assess the quality of many foods such as meat, fish, vegetables and dairy products. This paper addresses the applications of low intensity ultrasonics in the cheese manufacturing processes and highlights the areas where ultrasonics could be successfully implemented in the future. The decrease of ultrasonic attenuation during the renneting process can be used to determine the optimum cut time for cheese making. The ultrasonic velocity increases during maturation for those types of cheese that become harder during this manufacturing stage, thus being an indicator of the maturity degree. Moreover, ultrasonic measurements could be linked to sensory parameters. From the ultrasonic velocity measurements at two different temperatures, it is possible to assess cheese composition, thus allowing an improvement in the quality and uniformity of cheese commercialization. In addition, in pulse-echo mode it is possible to detect cracked pieces due to abnormal fermentations and also to assess the distance of the crack from the surface.     

Comparison of two different ultrasound reactors for the treatment of cellulose fibers

The pulp and paper industry is in continuous need for energy-efficient production processes. In the refining process of mechanical pulp, fibrillation is one of the essential unit operations that count for up to 80% of the total energy use. This initial study explores the potential and development of new type of scalable ultrasound reactor for energy efficient mechanical pulping. The developed reactor is of continuous flow type and based on both hydrodynamic and acoustic cavitation in order to modify the mechanical properties of cellulose fibers. A comparison of the prototype tube reactor is made with a batch reactor type where the ultrasonic horn is inserted in the fluid. The pulp samples were sonicated by high-intensity ultrasound, using tuned sonotrodes enhancing the sound pressure and cavitation intensity by a controlled resonance in the contained fluid. The resonant frequency of the batch reactor is 20.8 kHz and for the tube reactor it is 22.8 kHz. The power conversion efficiency for the beaker setup is 25% and 36% in case of the tube reactor in stationary mode. The objective is to verify the benefit of resonance enhanced cavitation intensity when avoiding the effect of Bjerkenes forces. The setup used enables to keep the fibers in the pressure antinodes of the contained fluid. In case of the continuous flow reactor the effect of hydrodynamic cavitation is also induced. The intensity of the ultrasound in both reactors was found to be high enough to produce cavitation in the fluid suspension to enhance the fiber wall treatment. Results show that the mechanical properties of the fibers were changed by the sonification in all tests. The continuous flow type was approximately 50% more efficient than the beaker. The effect of keeping fibers in the antinode of the resonant mode shape of the irradiation frequency was also significant. The effect on fiber properties for the tested mass fraction was determined by a low-intensity ultrasound pulse-echo based measurement method, and by a standard pulp analyzer.     

Degradation of dichlorvos containing wastewaters using sonochemical reactors

The present work deals with application of sonochemical reactors for the degradation of dichlorvos containing wastewaters. The sonochemical reactor used in the work is a simple ultrasonic horn type operating at 20 kHz with a power rating of 270 W. The effect of different operating parameters such as operating pH, temperature and power density on the extent of degradation has been investigated initially followed by intensification studies using additives such as hydrogen peroxide, Fenton's reagent and CCl(4). It has been observed that low frequency sonochemical reactors can be effectively used for treatment of pesticide wastewaters and acidic conditions and optimum values of temperature and power dissipation favors the degradation of dichlorvos. The efficacy of sonochemical reactors can be further enhanced by using different additives at optimized loadings. Complete removal of the pesticide at the given loading has been obtained using an optimized combination of ultrasound and Fenton's chemistry. The controlling mechanism for the sonochemical degradation has been confirmed to be the free radical attack based on the studies involving radical scavengers. The novelty of the present work is clearly established as there have been no earlier studies dealing with degradation of dichlorvos pesticide using sonochemical reactors operating at low frequency which offers distinct advantage in terms of cost and the stability of the reactor.     

Inverse effects of the gas feed positioning on sonochemistry and sonoluminescence

Purging of solutions to enhance sonochemical reactions is a common practice. A fundamental study combining sonoluminescence spectroscopy and sonochemical activity is adopted to study the effects of continuous Ar gas flow in the solution and of the position of the gas inlet tube on high-frequency sonolysis of aqueous solutions. It has been observed that neither sonochemical activity nor sonoluminescence intensity is controlled by the gas solubility only. Besides, the change in position of the gas inlet tube leads to opposite effects in sonoluminescence intensity and sonochemical activity: while the former increases, the latter decreases. Such an observation has never been reported despite sonochemical reactions have been carried out under different gas environments. Sonoluminescence spectroscopy indicates that more extreme conditions are reached at collapse with the gas inlet on the side, which could be explained by a more symmetrical collapse. Finally, it is shown in certain conditions that it is possible to favor the formation of some sonochemical products simply by positioning the gas inlet at different positions, which has practical significance in designing large scale sonochemical reactors for industrial applications.     

Study on degradation of dimethoate solution in ultrasonic airlift loop reactor

In this work, the degradation of dimethoate solution in ultrasonic airlift loop reactor (UALR) assisted with advanced oxidation processes was studied. The effects of O₃ flow rate, ultrasonic intensity, pH value and reaction temperature on the degradation rate were investigated. UALR imposed a synergistic effect combining sonochemical merit with high O₃ transfer rate. Under the optimal operation conditions: ultrasonic irradiation time was 4 h, O₃ flow rate was 0.41 m³ h⁻¹, ultrasonic intensity was 4.64 W cm⁻², pH value was 10.0, reaction temperature was 25 °C, and initial concentration of dimethoate was 20 mg L⁻¹, degradation rate of dimethoate increased to 90.8%. The experimental results indicated that the method of UALR degradation of organic pollutants in the presence of gas could reduce reaction time and improve degradation rate. UALR was an advisable choice for treating organic waste waters and this device could be easily scale up. Thus this process has wide application prospect in industry.     

Measurement of hydroxyl radical production in ultrasonic aqueous solutions by a novel chemiluminescence method

Measurement methods for ultrasonic fields are important for reasons of safety. The investigation of an ultrasonic field can be performed by detecting the yield of hydroxyl radicals resulting from ultrasonic cavitations. In this paper, a novel method is introduced for detecting hydroxyl radicals by a chemiluminescence (CL) reaction of luminol-hydrogen peroxide (H2O2)-K5[Cu(HIO6)2](DPC). The yield of hydroxyl radicals is calculated directly by the relative CL intensity according to the corresponding concentration of H2O2. This proposed CL method makes it possible to perform an in-line and real-time assay of hydroxyl radicals in an ultrasonic aqueous solution. With flow injection (FI) technology, this novel CL reaction is sensitive enough to detect ultra trace amounts of H2O2 with a limit of detection (3sigma) of 4.1 x 10(-11) mol L(-1). The influences of ultrasonic output power and ultrasonic treatment time on the yield of hydroxyl radicals by an ultrasound generator were also studied. The results indicate that the amount of hydroxyl radicals increases with the increase of ultrasonic output power (< or = 15 W mL(-1)). There is a linear relationship between the time of ultrasonic treatment and the yield of H2O2. The ultrasonic field of an ultrasonic cleaning baths has been measured by calculating the yield of hydroxyl radicals.     

Enhancement of the knowledge on the ultrasonic reactor behaviour by an interdisciplinary approach

Our work is a step to a better understanding of high frequency ultrasonic reactors behaviour. Using finite elements calculations, it was demonstrated that localization of chemical and physical effects can be well correlated with mechanical behaviour of ultrasound emitter. This complementary approach enables us to propose a full interpretation of the sonochemical reactor behaviour. A major reason of scientific interest on ultrasound is the well-known enhancement of chemical or physical phenomena. This is so important that "Enhancement" is probably the most used word in the title of related publications. To fully understand experimental results, present work demonstrates that ultrasound needs also to significantly enhance a very difficult knowledge transfer operation that might be named interdisciplinary co-working. Hence, ultrasound is now used and studied in many different fields of science such as acoustic, chemistry, medical imaging, disease treatment (lithotripsy), non-destructive testing. Each one has his own vocabulary, approach, and method to describe the phenomenon. In this work four different methodologies were involved to study of the same effect but using a chemical, chemical engineering, physical and mechanical approach respectively. All these viewpoints were then brought together in order to explain new original results.     

Understanding the ultrasound field of high viscosity mixtures: Experimental and numerical investigation of a lab scale batch reactor

In this work, mixtures of increasing viscosity (from 0.9 to ≈720 mPas) are sonicated directly using an ultrasonic horn at 30 kHz to investigate the effect of viscosity on the ultrasound field both from an experimental and numerical point of view. The viscosity of the mixtures is modified by preparing water-polyethylene glycol solutions. The impact of the higher viscosity on the acoustic pressure distribution is studied qualitatively and semi-quantitatively using sonochemiluminescence. The velocity of light scattering particles added in the mixtures is also explored to quantify acoustic streaming effects using Particle Image Velocimetry (PIV). A numerical model is developed that is able to predict cavitationally active zones accounting for both thermoviscous and cavitation based attenuation. The results show that two cavitation zones exist: one directly under the horn tip and one around the part of the horn body that is immersed in the liquid. The erosion patterns on aluminum foil confirm the existence of both zones. The intensity of the cavitationally active zones decreases considerably with increasing viscosity of the solutions. A similar reduction trend is observed for the velocity of the particles contained in the jet directly under the tip of the horn. Less erratic flow patterns relate to the high viscosity mixtures tested. Finally, two numerical models were made combining different boundary conditions related to the ultrasonic horn. Only the model that includes the radial horn movements is able to qualitatively predict well the location of the cavitation zones and the decrease of the zones intensity, for the highest viscosities studied. The current findings should be taken into consideration in the design and modelling phase of horn based sonochemical reactors.