Chemical Engineering Journal 155 (2009) 26–36 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Review Design aspects of sonochemical reactors: Techniques for understanding cavitational activity distribution and effect of operating parameters Vinayak S. Sutkar, Parag R. Gogate ∗ Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 400019, India article info Article history: Received 22 April 2009 Received in revised form 25 June 2009 Accepted 4 July 2009 Keywords: Cavitation Sonochemical reactors Cavitational activity distribution Mapping Design aspects abstract Cavitation is a phenomenon having enormous potential for intensification of physical and chemical pro- cessing applications such as chemical synthesis, industrial wastewater treatment, cell disruption for release of intracellular enzymes, crystallization, extraction and leaching. However, the dynamic behavior of cavitational activity, especially in sonochemical reactors based on the use of ultrasonic irradiations, creates problems in proposing reliable design and operating strategies. The present work presents an overview of different techniques to understand the cavitational activity distribution in the reactor, high- lighting the basic aspects, its applicability and relative merits/demerits. A detailed analysis of the literature has also been made with an aim of explaining the dependency of the cavitational activity on the design of sonochemical reactors and also the operating parameters. Recommendations for optimum operating parameters and design of reactor based on the experimental as well as theoretical analysis have been reported. Some trends in the future reactor designs useful in large scale applications have also been discussed. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Cavitation is a phenomenon of nucleation, growth and sub- sequent collapse (quasi-adiabatic) of micro bubbles in a liquid medium. Cavitation results in generation of high temperature (in the range 1000–15,000 K) and pressure (in the range 500–5000 bar) locally but at millions of locations in the reactor [1]. In addi- tion to generation of hotspots, cavitation also results in strong acoustic streaming (liquid circulation), high shear stress near the bubble wall, formation of micro-jets near the solid surface (due to asymmetric collapse of bubbles), generation of highly reac- tive free radicals and turbulence resulting in enhanced transport properties of the species [2–4]. These effects can be effectively used for the intensification of physical and chemical processing applications such as chemical synthesis (in homogenous and het- erogeneous reaction systems in terms of acceleration of the rate of reaction, increase in reaction yield, use of less forcing conditions, reduction in induction time, switching of reaction pathway to have better selectivity), wastewater treatment (degradation of biore- fractory or complex chemicals such as p-nitrophenol, rhodamine B, phenol, dichloromethane, etc.), textile processing (enhancing the efficacy of dyeing technique), biotechnology (homogenization, water disinfection and cell disruption for release of intracellular ∗ Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614. E-mail address: parag@udct.org (P.R. Gogate). enzymes and foam control in bioreactors), crystallization, polymer chemistry (degradation of polymers and initiation of reactions), extraction, emulsification and petrochemical industries (determi- nation of composition of coal extracts), etc.[5–8]. However, it should be noted that in spite of extensive research and vital potential appli- cations proven on laboratory scale, there are limited number of chemical processing applications being carried out on an indus- trial scale owing to the lack of expertise required in diverse fields such as material science, acoustics, chemical engineering etc. for scaling up successful laboratory scale processes and also due to the lack of suitable reactor design and scale-up strategies. Rate of sonochemical reactions is not only influenced by frequency and intensity of ultrasonic irradiations but also by the shape of reactor, operating power density, fraction of dissolved gases, physicochem- ical properties of liquid medium, surrounding pressure field in the sonochemical reactor and operating temperature [8,9]. The major problems in efficient design and operation of sonochemical reactor are: 1. Cavitation is a dynamic phenomenon and its effects strongly depend on the operating parameters and geometry of the reac- tor system. The reaction mechanism and the overall yields of sonochemical reactions are influenced by the bulk temperature, the acoustic intensity or the static pressure in the fluid. A small change in the temperature or gas content in the liquid medium may alter the conditions dramatically leading to a completely different cavitational effect and hence the yield and selectivity 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.07.021