Reactors sonochemical

Hydrodynamic cavitation reactors have been investigated for more than a decade now in the UDCT Department of Bombay University [63-66]. When applied to some industrially relevant reactions, the hydrodynamically created cavitation appeared to dehver on average an order of magnitude higher cavitation yields than the acoustic cavitation. In addition, the processing volumes could be up to about 100 times larger than in the conventional sonochemical reactors. So far, there is no information about the industrial applications of the hydrodynamic cavitation reactors, although some concepts have already been patented [67]. 

In chemical micro process technology there is a clear dominance of pressure-driven flows over alternative mechanisms for fluid transport However, any kind of supplementary mechanism allowing promotion of mixing is a useful addition to the toolbox of chemical engineering. Also in conventional process technology, actuation of the fluids by external sources has proven successful for process intensification. An example is mass transfer enhancement by ultrasonic fields which is utilized in sonochemical reactors [143], There exist a number of microfluidic principles to promote mixing which rely on input of various forms of energy into the fluid. 

Pandit and co-workers have shown that scale-up may be possible on a more rational basis if cavitation is employed, and some data have been reported by Pandit and Mohalkar (1996), Mohalkar et al. (1999), Senthil et al. (1999), and Cains et al. (1998). A variety of reactors can be used, viz. the liquid whistle reactor, the Branson sonochemical reactor, the Pote reactor, etc. The principal factors affecting the efficiency of a hydrodynamic cavitation reactor are irreversible loss in pressure head and turbulence and friction losses in the reaction rates. 

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

There are two types in sonochemical reactors. One is a bath-type reactor, and the other is an ultrasonic horn. 

Fig. 1.12 A typical experimental set-up for a bath-type sonochemical reactor...

In a bath-type sonochemical reactor, a damped standing wave is formed as shown in Fig. 1.13 [1]. Without absorption of ultrasound, a pure standing wave is formed because the intensity of the reflected wave from the liquid surface is equivalent to that of the incident wave at any distance from the transducer. Thus the minimum acoustic-pressure amplitude is completely zero at each pressure node where the incident and reflected waves are exactly cancelled each other. In actual experiments, however, there is absorption of ultrasound especially due to cavitation bubbles. As a result, there appears a traveling wave component because the intensity of the incident wave is higher than that of the reflected wave. Thus, the local minimum value of acoustic pressure amplitude is non-zero as seen in Fig. 1.13. It should be noted that the acoustic-pressure amplitude at the liquid surface (gas-liquid interface) is always zero. In Fig. 1.13, there is the liquid surface... 

Iida Y, Ashokkumar M, Tuziuti T, Kozuka T, Yasui K, Towata A, Lee J (2010) Bubble population phenomena in sonochemical reactor II Estimation of bubble size distribution and its number density by simple coalescence model calculation. Ultrason Sonochem 17 480-486... 

Yasui K, Kozuka T, Tuziuti T, Towata A, Iida Y, King J, Macey P (2007) FEM calculation of an acoustic field in a sonochemical reactor. Ultrason Sonochem 14 605-614... 

Dahnke S, Keil F (1998) Modeling of sound fields in liquids with a nonhomogeneous distribution of cavitation bubbles as a basis for the design of sonochemical reactors. Chem Eng Technol 21 873-877... 

Fig. 2.4 Steps for energy conversion in the case of sonochemical reactors...

The transducers operate at a fixed frequency of irradiation emitting radiations through a fixed area of irradiation. Thus, the type of transducer coupled with the total area of irradiation and the operating frequency are the key factors in the efficient design of the sonochemical reactors. The three main types of transducers are gas driven, liquid driven and electromechanical transducers out of which the electromechanical transducers are by far the most versatile and widely used. 

Characterization of the cavitational phenomena and its effects in sonochemical reactors are generally described through mapping. Mapping of sonochemical reactor is a stepwise procedure where cavitational activity can be quantified by means of primary effect (temperature or pressure measurement at the time of bubble collapse) and/or secondary effect (quantification of chemical or physical effects in terms of measurable quantities after the bubble collapse) to identify the active and passive zones. 

Yasui et al. [29] have used solution of wave equation based on finite element method for characterization of the acoustic field distribution. A unique feature of the work is that it also considers contribution of the vibrations occurring due to the reactor wall and have evaluated the effect of different types of the reactor walls or in other words the effect of material of construction of the sonochemical reactor. The work has also contributed to the understanding of the dependence of the attenuation coefficient due to the liquid medium on the contribution of the vibrations from the wall. It has been shown that as the attenuation coefficient increases, the influence of the acoustic emission from the vibrating wall becomes smaller and for very low values of the attenuation coefficient, the acoustic field in the reactor is very complex due to the strong acoustic emission from the wall. 

The reactor design in terms of ratio of the diameter of the immersion transducer to reactor diameter, liquid height, position of the transducers and characteristics of the cell plays a important role in deciding the cavitational activity distribution and hence the efficacy of sonochemical reactors for the specific application. Based on a critical analysis of the existing literature, following important design related information can be recommended ... 

At times the net rates of chemical/physical processing achieved using ultrasonic irradiations are not sufficient so as to prompt towards industrial scale operation of sonochemical reactors. This is even more important due to the possibility of uneven distribution of the cavitational activity in the large scale reactors as discussed... 

In the sonochemical reactors, selection of suitable operating parameters such as the intensity and the frequency of ultrasound and the vapor pressure of the cavitating media is an essential factor as the bubble behavior and hence the yields of sonochemical transformation are significantly altered due to these parameters. It is necessary that both the frequency and intensity of irradiation should not be increased beyond an optimum value, which is also a function of the type of the application and the equipment under consideration. The liquid phase physicochemical properties should be adjusted in such a way that generation of cavitation events is eased and also large number of smaller size cavities are formed in the system. 

Design of sonochemical reactors is a very important parameter in deciding the net cavitational effects. Use of multiple transducers and multiple frequencies with possibility of variable power dissipation is recommended. Theoretical analysis for predicting the cavitational activity distribution is recommended for optimization of the geometry of the reactor including the transducer locations in the case of multiple transducer reactors. Use of process intensifying parameters at zones with minimum cavitational intensity should help in enhancing the net cavitational effects. 

Horst C, Chen Y-S, Kunz U, Hoffmann U (1996) Design, modeling and performance of a novel sonochemical reactor for heterogeneous reactions. Chem Eng Sci 51 1837-1846... 

Dahlem O, Demaiffe V, Halloin V, Reisse J (1998) Direct sonication system suitable for medium scale sonochemical reactors. AIChE J 44 2724-2730... 

Gogate PR, Tatake PA, Kanthale PM, Pandit AB (2002) Mapping of sonochemical reactors Review, analysis and experimental verification. AIChE J 48 1542-1560... 

Gogate PR, Pandit AB (2000) Engineering design methods for cavitation reactors I Sonochemical reactors. AIChE J 46 372-379... 

Gogate PR, Mujumdar S, Pandit AB (2003) Large scale sonochemical reactors for process intensification Design and experimental validation. J Chem Tech Biotech 78 685-693... 

Keil F, Dahnke S (1996) Numerical calculation of pressure field in sonochemical reactor. Chem Ing Tech 68 419 -22... 

Dahnke S, Keil F (1998) Modeling of three dimension linear pressure field in sonochemical reactores with homogenous and inhomogenous density distribution of cavity bubbles. Ind Eng Chem Res 37 848-864... 

Dahnke S, Keil F (1999) Modeling of linear pressure fields in sonochemical reactor considering an inhomogeneous density distribution of cavitation bubble. Chem Eng Sci 54 2865-2872... 

Kumar A, Gogate PR, Pandit AB (2007) Mapping of acoustic streaming in sonochemical reactors. Ind Eng Chem Res 46 4368 1373... 

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