Chemical Engineering and Processing: Process Intensification
ReviewCavitational reactors for process intensification of chemical processing applications: A critical review
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
References (72)
- et al.
Process intensification using multifunctional reactors
Chem. Eng. Sci.
(2001) Reactive separations for process intensification: an industrial perspective
Chem. Eng. Process.
(2003)- et al.
Sonochemistry in China between 1997 and 2002
Ultrason. Sonochem.
(2005) - et al.
Ultrasonic disintegration of biosolids for improved biodegradation
Ultrason. Sonochem.
(2007) - et al.
The effect of sonication on microbial disinfection using hypochlorite
Ultrason. Sonochem.
(2004) - et al.
Use of hydrodynamic cavitation for large scale microbial cell disruption
Chem. Eng. Res. Des.
(1997) - et al.
Microbial cell disruption: role of cavitation
Chem. Eng. J.
(1994) - et al.
Ultrasonic atomization: effect of liquid phase properties
Ultrasonics
(2006) An overview of the ultrasonically assisted extraction of bioactive principles from herbs
Ultrason. Sonochem.
(2001)- et al.
Ultrasound: a powerful tool for leaching
TrAC Trends Anal. Chem.
(2003)
Design, modeling and performance of a novel sonochemical reactor for heterogeneous reactions
Chem. Eng. Sci.
Quantification of cavitation intensity in fluid bulk
Ultrason. Sonochem.
The radially vibrating horn: a scaling up possibility for sonochemical reactions
Chem. Eng. Sci.
Semiquantitative characterization of ultrasonic cleaner using a novel piezoelectric pressure intensity measurement probe
Ultrason. Sonochem.
Standing waves in a high frequency sonoreactor: visualisation and effects
Chem. Eng. Sci.
Local investigation of some ultrasonic devices by means of a thermal sensor
Ultrasonics
A comparative study of local sensors of power ultrasound effects: electrochemical, thermoelectrical and chemical probes
Ultrason. Sonochem.
Modelling of linear pressure fields in sonochemical reactors considering an inhomogeneous density distribution of cavitation bubbles
Chem. Eng. Sci.
Modelling of three dimensional pressure fields in sonochemical reactors with an inhomogeneous density distribution of cavitation bubbles. Comparison of theoretical and experimental results
Ultrason. Sonochem.
Destruction of formic acid using high frequency cup horn reactor
Water Res.
Effect of ionic strength on the acoustic generation of nitrite, nitrate and hydrogen peroxide
Ultrason. Sonochem.
Sonochemical degradation of chlorophenols in water
Ultrason. Sonochem.
Sono-oxidation treatment of humic substances in drinking water
Ultrason. Sonochem.
Sonochemical degradation of toxic halogenated organic compounds
Ultrason. Sonochem.
The effect of ultrasound on photochemical reactions
Ultrason. Sonochem.
Utilization of electromagnetic and acoustic irradiation in enhancing heterogeneous catalytic reactions
Appl. Catal. A Gen.
Energy matters: alternative sources and forms of energy for intensification of chemical and biochemical processes
Chem. Eng. Res. Des.
High-intensity ultrasound and microwave, alone or combined, promote Pd/C-catalyzed aryl–aryl couplings
Tetrahedron Lett.
Biodiesel production from waste cooking oil. 1. Process design and technological assessment
Bioresour. Technol.
Ultrasonic versus silent methylation of vegetable oils
Ultrason. Sonochem.
Fatty acids methyl esters from vegetable oil by means of ultrasonic energy
Ultrason. Sonochem.
Process intensification: transforming chemical engineering
Chem. Eng. Prog.
Practical Sonochemistry: Users Guide in Chemistry and Chemical Engineering
Cavitation
The Acoustic Bubble
Synthetic Organic Chemistry
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