A review and assessment of hydrodynamic cavitation as a technology for the future

In the present work, the current status of the hydrodynamic cavitation reactors has been reviewed discussing the bubble dynamics analysis, optimum design considerations, design correlations for cavitational intensity (in terms of collapse pressure)/cavitational yield and different successful chemical synthesis applications clearly illustrating the utility of these types of reactors. The theoretical discussion based on the modeling of the bubble dynamics equations aims at understanding the design information related to the dependency of the cavitational intensity on the operating parameters and recommendations have been made for the choice of the optimized conditions of operating parameters. The design information based on the theoretical analysis has also been supported with some experimental illustrations concentrating on the chemical synthesis applications. Assessment of the hydrodynamic cavitation reactors and comparison with the sonochemical reactors has been done by citing the different industrially important reactions (oxidation of toluene, o-xylene, m-xylene, p-xylene, mesitylene, o-nitrotoluene, p-nitrotoluene, m-nitrotoluene, o-chlorotoluene and p-chlorotoulene, and trans-esterification reaction i.e., synthesis of bio-diesel). Some recommendations have also been made for the future work to be carried out as well as the choice of the operating conditions for realizing the dream of industrial scale applications of the cavitational reactors.     

Investigation of induction of air due to ultrasound source in the sonochemical reactors

A detailed investigation into the phenomena of induction of air using a novel arrangement of the ultrasonic horn (tip is located just above the liquid surface) has been made with the quantification of the extent of induction in terms of the air entrainment rate and the gas–liquid mass transfer coefficient for the transfer of air into the system. The measurement of air entrainment rate was found to be quite difficult and hence focus was kept on the quantification in terms of the gas–liquid mass transfer coefficient. The effect of ultrasonic power dissipation and type of the liquid medium (water, sodium chloride and sodium laruyl sulphate [surfactant] solution) on the mass transfer coefficient has been studied and correlations have been developed for the prediction of the same. Comparison with the mechanically agitated surface aerators has enabled us to understand the controlling mechanism in the induction and subsequent distribution of the air i.e. turbulence or convective motion. The present work should open an entirely new field of research in the area of design of sonochemical gas–liquid reactors operating possibly as a combination of gas-inducing reactors and cavitational reactors.     

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.     

Mapping the efficacy of new designs for large scale sonochemical reactors

Sonochemical reactors have a great promise for many physical and chemical processing applications but its applicability at pilot or industrial scale levels is hindered by lack of novel designs which can reproduce the spectacular effects generated at the laboratory scale. The present work evaluates the efficacy of two new designs, operating at a liquid capacity of 7l. Mapping of the cavitational activity has been carried out using measurements of local pressure using hydrophone and cativational intensity using Cavitation Activity Indicator (Model IC-3, N. Deznukov, Belarus State University, Minsk, Belarus). Aim has been to identify the distribution of the cavitational activity in radial and axial directions and possibly characterizing the zones with very high and very low cavitational activity in these reactor configurations. It has been observed that the cavitational activity is substantially uniform in both the reactors unlike the conventional single transducer based reactors. The study clearly indicates the feasibility of these designs for future large scale applications.     

Mapping of an ultrasonic horn: link primary and secondary effects of ultrasound

The erratic behaviour of cavitational activity exhibited in a sonochemical reactor pose a serious problem in the efficient design and scale-up; thus it becomes important to identify the active and passive zones existing in the reactor so as to enable proper placement of the reaction mixtures for achieving maximum benefits. In the present work mapping of ultrasonic horn has been carried with the help of local pressure measurement using a hydrophone and estimation of amount of liberated iodine using the Weissler reaction and a quantitative relationship has been established. The measured local pressure pulses have been used in the theoretical simulations of the bubble dynamics equations to check the type of cavitation taking place locally and also estimate the possible collapse pressure pulse in terms of maximum bubble size reached during the cavitation phenomena. Relationship has been also established between the observed iodine liberation rates and the maximum bubble size reached. The engineers can easily use these unique relationships in efficient design, as the direct quantification of the secondary effect is possible.     

Hydrodynamic cavitation as a novel approach for delignification of wheat straw for paper manufacturing

The present work deals with application of hydrodynamic cavitation for intensification of delignification of wheat straw as an essential step in the paper manufacturing process. Wheat straw was first treated with potassium hydroxide (KOH) for 48 h and subsequently alkali treated wheat straw was subjected to hydrodynamic cavitation. Hydrodynamic cavitation reactor used in the work is basically a stator and rotor assembly, where the rotor is provided with indentations and cavitational events are expected to occur on the surface of rotor as well as within the indentations. It has been observed that treatment of alkali treated wheat straw in hydrodynamic cavitation reactor for 10–15 min increases the tensile index of the synthesized paper sheets to about 50–55%, which is sufficient for paper board manufacture. The final mechanical properties of the paper can be effectively managed by controlling the processing parameters as well as the cavitational parameters. It has also been established that hydrodynamic cavitation proves to be an effective method over other standard digestion techniques of delignification in terms of electrical energy requirements as well as the required time for processing. Overall, the work is first of its kind application of hydrodynamic cavitation for enhancing the effectiveness of delignification and presents novel results of significant interest to the paper and pulp industry opening an entirely new area of application of cavitational reactors.     

Experimental study of the hydrodynamic behaviour of a high frequency ultrasonic reactor

In relation to design and modeling of sonochemical reactors, the hydrodynamic behaviour of a high-frequency ultrasonic reactor has been investigated. Residence time distribution (RTD) measurements have been performed by means of a tracer method. The influence of ultrasound on the response to an inlet pulse was evidenced. It was shown that the reactor behaves like a completely stirred tank reactor (CSTR) as soon as ultrasonic irradiation operates. Preliminary observations on acoustic streaming occurring within the reactor will also be presented.     

Intensification of hydroxyl radical production in sonochemical reactors

The efficacy of sonochemical reactors in chemical processing applications has been well established in the laboratory scale of operation though at a given set of operating parameters and no efforts have been directed in terms of maximizing the free radical production. In the present work, the effect of different operating parameters viz. pH, power dissipation into the system, effect of additives such as air, haloalkanes, titanium dioxide, iron and oxygen on the extent of hydroxyl radical formation in a sonochemical reactor have been investigated using salicylic acid dosimetry. Possible mechanisms for oxidation of salicylic acid in the presence of different additives have also been established. It has been observed that acidic conditions under optimized power dissipation in the presence of iron powder and oxygen result in maximum liberation of hydroxyl radicals as quantified by the kinetic rate constant for production of 2,5- and 2,3-dihydroxybenzoic acid. The study has enabled the optimization of the conditions for maximum efficacy of sonochemical reactors where free radical attack is the controlling mechanism for the chemical processing applications.     

Investigation of design parameters in ultrasound reactors with confined channels

This paper presents a three-dimensional numercial simulation of sonochemical degradation upon cavitational activity. The model relates the simulation of the acoustic pressure distribution to the sonochemical reaction rate. As a case study, the thermal degradation of carbon tetrachloride during sonication is studied in a tubular milliscale reactor. The model is used to optimize the reactor diameter, ultrasound frequency and power dissipated to the ultrasound transducers. The results indicate that multiple transducers at a moderate power level are more efficient than one transducer with high power level. Furthermore, the average cavity volume fraction is proposed as a reaction independent parameter to estimate the optimal reactor design. Within the results obtained in this paper, it appears possible to optimise reactor design based on this parameter.     

Recent trends in the applications of sonochemical reactors as an advanced oxidation process for the remediation of microbial hazards associated with water and wastewater: A critical review

Water is one of the major sources that spread human diseases through contamination with bacteria and other pathogenic microorganisms. This review focuses on microbial hazards as they are often present in water and wastewater and cause various human diseases. Among the currently used disinfection methods, sonochemical reactors (SCRs) that produce free radicals combined with advanced oxidation processes (AOPs) have received significant attention from the scientific community. Also, this review discussed various types of cavitation reactors, such as acoustic cavitation reactors (ACRs) utilizing ultrasonic energy (UE), which had been widely employed, involving AOPs for treating contaminated waters. Besides ACRs, hydrodynamic cavitation reactors (HCRs) also effectively destroy and deactivate microorganisms to varying degrees. Cavitation is the fundamental phenomenon responsible for initiating many sonochemical reactions in liquids. Bacterial degradation occurs mainly due to the thinning of microbial membranes, local warming, and the generation of free radicals due to cavitation. Over the years, although extensive investigations have focused on the antimicrobial effects of UE (ultrasonic energy), the primary mechanism underlying the cavitation effects in the disinfection process, inactivation of microbes, and chemical reactions involved are still poorly understood. Therefore, studies under different conditions often lead to inconsistent results. This review investigates and compares other mechanisms and performances from greener and environmentally friendly sonochemical techniques to the remediation of microbial hazards associated with water and wastewater. Finally, the energy aspects, challenges, and recommendations for future perspectives have been provided.     

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.     

Ultrasonic bath with longitudinal vibrations: a novel configuration for efficient wastewater treatment

Efficacy of a novel configuration for large-scale wastewater treatment applications has been investigated using formic acid degradation as a model reaction. The reactor is first characterized using energy efficiency measurements and the optimum operating volume for maximum transfer of supplied energy and hence maximum cavitational effects has been established. Effect of initial concentration of the pollutant on the rates of degradation has been investigated. Comparison has been also made with the conventional ultrasonic horn in terms of energy efficiency and cavitational yield for the model reaction. With an aim of possible reduction in the total treatment time, some intensification studies have been undertaken considering hydrogen peroxide as an additional source of free radicals.     

Dynamics of cavitational bubbles and design of a hydrodynamic cavitational reactor: cluster approach

In the present work, a cavity cluster of predetermined size has been considered to study the bubble dynamics in the hydrodynamic cavitation reactor. The effect of different operating and system parameters on the cavitational intensity has been numerically investigated. The yield of any cavitationally induced physical/chemical transformations depends not only on the collapse pressure of the cavities but also on the active volume of cavitation within the reactor. Empirical correlations have been developed to predict the collapse pressure and the active volume of cavitation as a function of different operating parameters based on the bubble dynamics studies. Recommendations are made for designing a cavitational reactor on the basis of the proposed empirical correlations. This work is a first step towards the designing and optimization of hydrodynamic cavitational reactor with cluster approach.     

Velocity study in an ultrasonic reactor

In order to determine the parameters required to describe and to optimize sonochemical reactors, we have investigated the water flow inside such a reactor. With this aim, the experimental velocity field has been measured by tomography laser. The influence of certain parameters such as the electric power, the water height and the fluid viscosity has been evaluated. At the same time, the water movement has been studied theoretically using Nyborg's model. We have tried to improve this model by considering a three-dimensional velocity.     

Intensification of esterification of acids for synthesis of biodiesel using acoustic and hydrodynamic cavitation

Cavitation results in conditions of turbulence and liquid circulation in the reactor which can aid in eliminating mass transfer resistances. The present work illustrates the use of cavitation for intensification of biodiesel synthesis (esterification) reaction, which is mass transfer limited reaction considering the immiscible nature of the reactants, i.e., fatty acids and alcohol. Esterification of fatty acid (FA) odour cut (C(8)-C(10)) with methanol in the presence of concentrated H(2)SO(4) as a catalyst has been studied in hydrodynamic cavitation reactor as well as in the sonochemical reactor. The different reaction operating parameters such as molar ratio of acid to alcohol, catalyst quantity have been optimized under acoustic as well as hydrodynamic cavitating conditions in addition to the optimization of the geometry of the orifice plate in the case of hydrodynamic cavitation reactors. Few experiments have also been carried out with other acid (lower and higher)/methanol combination viz. caprylic acid and capric acids with methanol with an aim of investigating the efficacy of cavitation for giving the desired yields and also to quantify the degree of process intensification that can be achieved using the same. It has been observed that ambient operating conditions of temperature and pressure and reaction times of <3h, for all the different combinations of acid (lower and higher)/methanol studied in the present work, was sufficient for giving >90% conversion (mol%). This clearly establishes the efficacy of cavitation as an excellent way to achieve process intensification of the biodiesel synthesis process.     

Geometric and operational optimization of 20-kHz probe-type sonoreactor for enhancing sonochemical activity

The use of a 20-kHz probe-type sonicator irradiating downward in a 500 mL vessel was optimized for the enhancement of the sonochemical activity in terms of the geometric and operational factors. These factors included the probe immersion depth (the vertical position of the probe), input power, height of the liquid from the bottom, horizontal position of the probe, and thickness of bottom plate The sonochemical oxidation reactions were investigated both quantitatively and qualitatively using calorimetry, KI dosimetry, and luminol (Sonochemiluminescence, SCL) techniques. The sonochemical activity was very positively affected by the vertical boundaries. The highest sonochemical activity was obtained when the probe was placed close to the bottom of the vessel (immersion depth of 60 mm), with a high input power (input power of 75%), and optimal liquid height condition (liquid height of 70 mm). The SCL image analysis showed that the cavitational activity zone gradually expanded around the probe body and changed into a circular shape as the experimental conditions were optimized, and consequently the sonochemical activity increased. The formation of a large bright circular-shaped activity zone could be attributed to the strong reflections of the ultrasound firstly, at the vessel bottom and secondly, at the liquid surface. On the other hand, the cavitational activity zone and the sonochemical activity were negatively affected by the horizontal boundaries when the probe was placed close to the side wall of the vessel. In addition, it was found that the sonochemical activity was also significantly affected by the thickness of the support plate owing to the reflection and transmission of the ultrasound at the boundary between the liquid and the solid media.     

Sonochemistry in non-aqueous liquids

The chemical effects of high-intensity ultrasound on organic liquids are reported. In order to probe the factors which affect sonochemistry in non-aqueous solvents, two very different chemical dosimeters have been used: radical trapping by diphenylpicrylhydrazyl and decomposition of Fe(CO)₅. In both cases, good correlation is found between the logarithm of the sonochemical rate and the solvent vapour pressure. This result is justifiable in terms of the cavitation ‘hot-spot’ mechanism of sonochemistry. Thus, decreasing solvent vapour pressure increases the intensity of cavitational collapse, the peak temperature reached during such collapse, and, consequently, the rates of sonochemical reactions.     

Experimental study of the ultrasound attenuation in chemical reactors

Ultrasound is used in different domains, and in sonochemistry particularly, for different purposes and in various flow configurations: monophasic, two-phase and polyphasic reactors. In order to optimize and to design sonochemical reactors, it is important to describe the ultrasonic intensity space and time distribution. In addition, it is important to study the different parameters influencing the intensity profiles of the ultrasonic wave. In this work, a thermoelectric probe has been used to measure the ultrasonic intensity. This procedure has shown that the ultrasound propagation is influenced by the presence of cavitation bubbles, the flow regime and the presence of solid particles.     

Comparison of sonochemiluminescence images using image analysis techniques and identification of acoustic pressure fields via simulation

One common method to determine the existence of cavitational activity in power ultrasonics systems is by capturing images of sonoluminescence (SL) or sonochemiluminescence (SCL) in a dark environment. Conventionally, the light emitted from SL or SCL was detected based on the number of photons. Though this method is effective, it could not identify the sonochemical zones of an ultrasonic systems. SL/SCL images, on the other hand, enable identification of ‘active’ sonochemical zones. However, these images often provide just qualitative data as the harvesting of light intensity data from the images is tedious and require high resolution images. In this work, we propose a new image analysis technique using pseudo-colouring images to quantify the SCL zones based on the intensities of the SCL images and followed by comparison of the active SCL zones with COMSOL simulated acoustic pressure zones.     

Comparative study of sonochemical reactors with different geometry using thermal and chemical probes

Laboratory scale 20 kHz sonochemical reactors with different geometries have been tested using thermal probes, the kinetics of H(2)O(2) formation, and the kinetics of diphenylmethane (DPhM) sonochemical darkening. Results revealed that the overall sonochemical reaction rates in H(2)O and DPhM are driven by the total absorbed acoustic energy and roughly independent the geometry of the studied reactors. However, the sonochemical efficiency, defined as eta=VG/S, where G is a sonochemical yield of H(2)O(2), V is a volume of sonicated liquid, and S is a surface of the sonotrode, was proved to increase with the decrease of S. This phenomenon was explained by growing of the maximum cavitating bubble size with ultrasonic intensity and its independence towards the specific absorbed acoustic power. For the cleaning bath reactor the kinetics of the sonochemical reactions in H(2)O and DPhM depends strongly on the reaction vessel materials: the reaction rates decreased with the increase of the materials elasticity. Kinetic study of H(2)SO(4) sonolysis using a sonoreactor without direct contact with titanium sonotrode showed that sulphate anion is an effective scavenger of OH() radicals formed during water sonolysis.