Review
A review of applications of cavitation in biochemical engineering/biotechnology

https://doi.org/10.1016/j.bej.2008.10.006Get rights and content

Abstract

Cavitation results in the generation of hot spots, highly reactive free radicals, and turbulence associated with liquid circulation currents, which can result in the intensification of various physical/chemical operations. The present work provides an overview of the applications of the cavitation phenomenon in the specific area of biochemical engineering/biotechnology, discussing the areas of application, the role of cavitation, the observed enhancement and its causes by highlighting some typical examples. The different methods of inducing cavitation and the dominance of one over the other, mostly with respect to energy requirements, in different areas of biotechnological application are discussed. The major applications discussed in the work include microbial cell disruption for the release or extraction of enzymes, microbial disinfection, wastewater treatment, crystallization, synthesis of biodiesel, emulsification, extraction of bio-components, freezing and gene transfer into cells or tissues. Some recommendations for optimal operating/geometric parameters have also been made. Overall, it appears that the combined efforts of physicists, chemists, biologists and chemical engineers are required to effectively use cavitational reactors for industrial applications.

Introduction

The process industry demands that operations be performed in the most efficient way with respect to either product quality, energy or time, or in terms of economics. Alternative novel technologies are constantly being sought to reduce the total processing cost while maintaining or enhancing product quality in an environmentally benign manner. Cavitation offers immense potential for intensification of physical or chemical processing in an energy-efficient manner. Cavitation is generally defined as the generation, subsequent growth and collapse of cavities, resulting in very high local energy densities [1]. Cavitation, when it occurs in a reactor, generates conditions of very high temperatures and pressures (100–5000 atmospheres of pressure and 500–15000 K of temperature) locally, but with the overall environment remaining equivalent to ambient atmospheric conditions [1]. This enables the effective execution under ambient conditions of the various physical processes or chemical reactions that require stringent conditions [2], [3]. 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 in alteration of reaction mechanism. Cavitation also results in the generation of local turbulence and liquid micro-circulation (acoustic streaming) in the reactor, enhancing the rates of transport processes; in addition, they also eliminate mass transfer resistances in heterogeneous systems [2]. Based on the degree of intensity, which may be described in terms of the magnitude of pressure or temperature, cavitation can also be classified as either transient or stable. The energy requirements for the generation of these two types are significantly different, and hence proper care must be taken when selecting the operating parameters for the specific type of application [4]. Transient cavitation is a process where the generated bubble/cavity will eventually collapse to a minute fraction of its original size, at which point the gas present within the bubble dissipates into the surrounding liquid via a rather violent mechanism, releasing a significant amount of energy in the form of an acoustic shock-wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand Kelvin, and the pressure may be several hundred atmospheres. In the case of stable or non-inertial cavitation, small bubbles in a liquid are forced to oscillate in size or shape due to some form of energy input, such as an acoustic field, when the intensity of the energy input is insufficient to cause total bubble collapse. This form of cavitation causes significantly milder cavitational effects than the transient cavitation.

Cavitation is also classified into four types based on the mode of generation: acoustic, hydrodynamic, optic and particle. Only acoustic and hydrodynamic cavitation have been found to be efficient in producing the desired chemical/physical changes in processing applications [2], [5], whereas optic and particle cavitation are typically used for single bubble cavitation, which fails to induce any physical or chemical change in the bulk solution. The spectacular effects of cavitation phenomena generated using ultrasound (acoustic cavitation) have been more commonly harnessed in food and bioprocessing industries [6]. Similar cavitation phenomena can also be generated relatively easily in hydraulic systems. Engineers have generally been cautious regarding cavitation in hydraulic devices due to the problems of mechanical erosion, and thus all initial efforts to understand it were mainly with the objective of suppressing it in order to avoid the erosion of exposed surfaces. However, a careful design of the system allows for generation of cavity collapse conditions similar to acoustic cavitation. This enables different applications requiring varying cavitational intensities that have been successfully carried out using acoustic cavitation phenomena but with much lower energy input as compared to sonochemical reactors. In the last decade, concentrated efforts were made by few researchers around the world to harness the spectacular effects of hydrodynamic cavitation for chemical/physical transformation [7]. The present work provides an overview of different applications of cavitational reactors with an emphasis on different operations in biochemical engineering/biotechnology.

Section snippets

Reactor designs

Reactors in which cavitation is generated by ultrasound are usually described as sonochemical reactors, whereas reactors in which cavities are generated by virtue of fluid energy are described as hydrodynamic cavitation reactors.

Microbial cell disruption

A key factor in the economical production of industrially important microbial components is an efficient large-scale cell disruption process [16], [17]. The need for an efficient microbial cell disruption operation has always hindered the large-scale production of commercial biotechnological products of intracellular derivation [16]. For the large-scale disruption of micro-organisms, mechanical disintegrators such as high-speed agitator bead mills and high-pressure homogenizers are commonly

Concluding remarks

The present work has enabled us to clearly exemplify the importance of cavitation phenomena generated by using both ultrasound and hydrodynamic means in the general area of biotechnology/biochemical engineering. Generally it has been observed that the mechanical effects are more dominant in these applications as compared to the chemical effects of cavitation phenomena. The efficacy of hydrodynamic cavitation is well established as compared to ultrasound generated cavitation, especially in

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