Review
Cavitational reactors for process intensification of chemical processing applications: A critical review

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Abstract

Cavitational reactors are a novel and promising form of multiphase reactors, based on the principle of release of large magnitude of energy due to the violent collapse of the cavities. An overview of this novel technology, in the specific area of process intensification of chemical processing applications, in terms of the basic mechanism and different areas of application has been presented initially. Recommendations for optimum operating parameters based on the theoretical analysis of cavitation phenomena as well as comparison with experimentally observed trends reported in the literature have been presented. A design of a pilot scale sonochemical reactor has been presented, which forms the basis for development of industrial scale reactors. Some experimental case studies using industrially important reactions have been presented, highlighting the degree of intensification achieved as compared to the conventional approaches. Guidelines for required further work for ensuring successful application of cavitational reactors at industrial scale operation have been presented. Overall it appears that considerable economic savings is possible by means of harnessing the spectacular effects of cavitation in chemical processing applications.

Introduction

Process intensification is becoming an immensely important area in scientific investigations in recent days and is essential for effective sustaining of the chemical process industry [1]. Process intensification can be achieved using multifunctional equipments [2], [3], increasing the rates of reactions by sophisticated equipments/operating reactor configurations or using completely newer energy sources. Among the available newer energy sources, use of sound energy or energy associated with the liquid flow, can be used to generate cavitation phenomena which can result in significant degree of process intensification. The different ways in which cavitation based on the use of sound and flow energy can be effectively used for process intensification have been summarized in Table 1.

Cavitation can be in general defined as the generation, subsequent growth and collapse of the cavities releasing large magnitudes of energy over a very small location resulting in very high energy densities [4], [5], [6]. Cavitation occurs at millions of locations in the reactor simultaneously and generates conditions of very high temperatures and pressures (few thousand atmospheres pressure and few thousands Kelvin temperature) locally with overall ambient conditions [6]. Thus, the chemical reactions requiring stringent conditions can be effectively carried out using cavitation at ambient conditions. Moreover free radicals are generated in the process due to the dissociation of vapors trapped in the cavitating bubbles, which results in either intensification of the chemical reactions or may even result in the propagation of certain reactions under ambient conditions [7], [8]. Cavitation also results in generation of local turbulence and liquid micro-circulation (acoustic streaming) in the reactor enhancing the rates of the transport processes [5], [7]. These mechanical effects of cavitation are mainly responsible for the intensification of physical processing applications and also chemical processing limited by mass transfer whereas the chemical effects such as generation of hot spots and reactive free radicals are responsible for intensification of chemical processing applications.

The method of energy efficiently producing cavities of a desired quality (type of the dynamic behavior) can be taken as the main criterion in distinguishing among different types of cavitation [5]. The four principle types of cavitation and their causes can be summarized as follows:

  • 1.

    Acoustic cavitation: In this case, the pressure variations in the liquid are effected using the sound waves, usually ultrasound (16 kHz–100 MHz). The chemical changes taking place due to the cavitation induced by the passage of sound waves are commonly known as sonochemistry.

  • 2.

    Hydrodynamic cavitation: Cavitation is produced by pressure variations, which is obtained using geometry of the system creating velocity variation. For example, based on the geometry of the system, the interchange of pressure and kinetic energy can be achieved resulting in the generation of cavities as in the case of flow through orifice, venturi, etc.

  • 3.

    Optic cavitation: It is produced by photons of high intensity light (laser) rupturing the liquid continuum.

  • 4.

    Particle cavitation: It is produced by the beam of the elementary particles, e.g. a neutron beam rupturing a liquid, as in the case of a bubble chamber.

Out of these four types of cavitation, only acoustic and hydrodynamic cavitation generates desired intensity suitable for chemical or physical processing. In the case of cavitation reactors, two aspects of cavity dynamics are of prime importance, the maximum size reached by the cavity before its violent collapse and the life of the cavity. The maximum size reached by the cavity determines the magnitude of the pressure pulse produced on the collapse and hence the cavitation intensity that can be obtained in the system. The life of the cavity determines the distance traveled by the cavity from the point where it is generated before the collapse and hence it is a measure of the active volume of the reactor in which the actual cavitational effects are observed. The aim of the equipment designer should be to maximize both these quantities by suitably adjusting the different parameters including the methodology used for the generation of cavities (type of the cavity generated is a crucial parameter in deciding the intensity of the cavitation phenomena).

Section snippets

Mechanism of cavitation based process intensification

In order to understand the way in which cavitational collapse can affect chemical transformations, one must consider the possible effects of this collapse in different systems. In the case of homogeneous liquid phase reactions, there are two major effects. First, the cavity that is formed is unlikely to enclose a vacuum (in the form of void)—it will almost certainly contain vapor from the liquid medium or dissolved volatile reagents or gases. During the collapse, these vapors will be subjected

Overview of applications of cavitation phenomena for process intensification

Among the various modes of generating cavitation, acoustic and hydrodynamic cavitation have been of academic and industrial interest due to the ease of operation and the generation of the required intensities of cavitational conditions suitable for different physical and chemical transformations. It is worthwhile to overview the different applications, where cavitation can be used efficiently. There are large illustrations where these spectacular effects have been successfully harnessed for a

Sonochemical reactors

Ultrasonic horns are the most commonly used reactor designs amongst the sonochemical reactors. These are typically immersion type of transducers and very high intensities (pressures of the order of few thousands atmosphere) are observed very near to the horn. The intensity decreases exponentially as one moves away from horn and vanishes at a distance of as low as 2–5 cm depending on the maximum power input to the equipment and also on the operating frequency [31]. Ultrasonic horn systems can

Optimization of operating parameters in cavitational reactors

The magnitudes of collapse pressures and temperatures as well as the number of free radicals generated at the end of cavitation events are strongly dependent on the operating parameters of the equipment namely, intensity and frequency of irradiation along with the geometrical arrangement of the transducers in the case of sonochemical reactors and the liquid phase physicochemical properties, which affect the initial size of the nuclei and the nucleation process. The effect of these parameters on

Intensification of cavitational activity in the sonochemical reactors

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 earlier. It is thus important to look into supplementary strategies with an aim of intensification of the cavitational intensity. Two types of operating strategies

Specific case study for intensification of chemical synthesis using cavitation

After looking into theoretical analysis of cavitational reactors and also general overview of the applications of cavitation phenomena, in this section we will discuss a specific industrially important application i.e. trans-esterification of vegetable oils using alcohol, based on the experimental investigations carried out at UICT with a view point of focusing the applications of cavitation phenomena.

Various products derived from vegetable oils have been proposed as an alternative fuel for

Efforts needed in the future

It should be noted that, in spite of extensive research on laboratory scale and immense potential applications as discussed above, there are only limited illustrations on an industrial scale. The possible major problems in the design and successful operation of cavitational reactors are:

  • 1.

    Exact quantification of the cavitation collapse intensity as a function of different operating and geometric properties is lacking.

  • 2.

    Lack of suitable design strategies linking the theoretically available

Conclusions

Cavitational reactors appears to be very effective for intensification of chemical processing operations and harnessing the spectacular effects of cavitation, chemical as well as mechanical, for physical and chemical processing applications would lead to considerable economic savings. The optimization strategies on the basis of theoretical analysis reported earlier should serve as useful guideline to the design engineers for selection of optimum set of operating parameters and design

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