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

Available online at www.sciencedirect.com

Chemical Engineering and Processing 47 (2008) 515–527

Review

Cavitational reactors for process intensification of chemical processing applications: A critical review Parag R. Gogate ∗ Chemical Engineering Department, Institute of Chemical Technology, University of Mumbai (UICT), Matunga, Mumbai 400019, India Received 30 May 2007; received in revised form 7 August 2007; accepted 23 September 2007 Available online 29 September 2007

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. © 2007 Elsevier B.V. All rights reserved. Keywords: Cavitation phenomena; Process intensification; Optimization; Chemical processing; Bio-diesel synthesis

Contents 1. 2. 3.

4.

5. 6.

7.

∗

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of cavitation based process intensification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of applications of cavitation phenomena for process intensification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Chemical synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Water and effluent treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Biotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Miscellaneous applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactor designs for cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Sonochemical reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Hydrodynamic cavitation reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optimization of operating parameters in cavitational reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intensification of cavitational activity in the sonochemical reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Use of process intensifying parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Use of combination of cavitation and advanced oxidation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Combined use of microwave irradiation and sonochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specific case study for intensification of chemical synthesis using cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Reactor configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Important results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tel.: +91 22 2414 5616; fax: +91 22 24145614. E-mail address: [email protected].

0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.09.014

516 516 517 517 518 518 518 519 519 519 520 521 521 521 522 523 523 524

516

8. 9.

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

Efforts needed in the future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. 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 [4–6]. 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

525 525 526

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). 2. 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 to extreme conditions of high temperatures and pressures, causing molecules to fragment and generate highly reactive radical species. These radicals may then react either within the collapsing bubble or after migration into the bulk liquid. Secondly, the sudden collapse of the bubble also results in an inrush of the liquid to fill the void producing shear forces in the surrounding bulk liquid capable of breaking the chemical bonds of any materials, which are dissolved in the fluid or disturb the boundary layer facilitating the transport. When considering the reaction conditions for a cavitational process, the choice of the solvent and bulk operating temperature are significant factors and often interrelated. Any increase in the solvent vapor pressure decreases the maximum bubble collapse temperature and pressure. Thus, for a reaction where

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

517

Table 1 Process intensification effects of cavitation based on sound and flow energy Energy source

Form of application

Intensified element

Reported magnitude of possible improvement as compared to conventional technology (approx)

Acoustic field

Ultrasound irradiation

Reaction time Product yield

Flow

Gas liquid mass transfer

25 times In some cases 100% yield of the product which could not be synthesized at all via conventional method 5 times

Low frequency acoustics

Liquid solid mass transfer Gas solid mass transfer Gas liquid mass transfer

20 times 3 times 2 times

Hydrodynamic cavitation

Reaction time and product yield

Similar as with ultrasound, however 10 times higher cavitational yield for same energy input

Supersonic flow

Gas liquid mass transfer coefficient Fluid bed reactor capacity

10 times 2.25 times

cavitational collapse is the primary cause of the activation, a low operating temperature is recommended particularly if a low boiling solvent is used. Conversely, for a reaction requiring elevated temperatures, high boiling solvent is recommended. It is very important to decipher the controlling mechanism in the overall intensification of the chemical processing applications and then appropriate selection of the operating parameters needs to be done. Sometimes, when only mass transfer is the rate limiting step in deciding the overall rate of chemical reaction, use of cavitational reactors may not be needed and similar effects can be achieved using mechanical agitation at higher speeds of rotation or by using proper geometry/type of the stirring device. Cavitation phenomena need to be effectively used for chemical processing applications limited by intrinsic kinetics or where greener chemical synthesis routes need to be established. The cavitational activation in heterogeneous systems is mainly as a consequence of the mechanical effects of cavitation. In a heterogeneous solid/liquid system, the collapse of the cavitation bubble results in significant structural and mechanical defects. Collapse near the surface produces an asymmetrical inrush of the fluid to fill the void forming a liquid jet targeted at the surface. This effect is equivalent to high-pressure/highvelocity liquid jets and is the reason why ultrasound is used for cleaning solid surfaces. These jets activate the solid catalyst and increase the mass transfer to the surface by the disruption of the interfacial boundary layers as well as dislodging the material occupying the inactive sites. Collapse on the surface, particularly of powders, produces enough energy to cause fragmentation (even for finely divided metals). Thus, in this situation, ultrasound can increase the surface area for a reaction and provide additional activation through efficient mixing and enhanced mass transport. For heterogeneous reactions, a balance must be struck between ensuring enough cavitation to ensure reagent/catalyst activation without overly disturbing the thermodynamics of the reaction. It must also be remembered that in some cases where extended reaction time are required, the solvent itself may not be totally inert and discoloration and charring may occur. In this case, optimization needs to be done on the time of exposure to the cavitating conditions. However, in most synthetic reactions, solvent reaction can be ignored as the reaction times are substantially small.

In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface will cause disruption and mixing, resulting in the formation of very fine emulsions. When very fine emulsions are formed, the surface area available for the reaction between the two phases is significantly increased, thus increasing the rates of reaction. The emulsions formed using cavitation, are usually smaller in size and more stable, than those obtained using conventional techniques and often require little or no surfactant to maintain the stability. This is very beneficial particularly in the case of phase-transfer catalysed reactions or biphasic systems. 3. 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 variety of applications worldwide. Few of the important applications can be given as follows. 3.1. Chemical synthesis The different ways in which cavitation can be used in the chemical processing applications [4,7–12] are: a. Reaction time reduction. b. Increase in the reaction yield. c. Use of less forcing conditions (temperature and pressure) as compared to the conventional routes. d. Reduction in the induction period of the desired reaction. e. Possible switching of the reaction pathways resulting in increased selectivity. f. Increasing the effectiveness of the catalyst used in the reaction.

518

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

g. Initiation of the chemical reaction due to generation of highly reactive free radicals. It is important to remember following critical issues while selecting application of cavitating conditions for intensification of chemical processing applications: 1. In the case of homogeneous reactions, only those reactions which proceed via radical or radical-ion intermediates are usually sensitive to the cavitational effect. This means that cavitation can intensify reactions proceeding through radicals whereas ionic reactions are not likely to be modified. 2. In the case of heterogeneous reactions, reactions proceeding through ionic intermediates can also be stimulated by mechanical effects of cavitation. However, in this case appropriate balance needs to be maintained between the positive effects of cavitation and the associated costs of operation as sometimes same effects might be achieved by using improved agitation conditions. 3. In the case of heterogeneous reactions, where mixed mechanisms, i.e. radical and ionic exist, cavitation can improve both the mechanisms. In this case, if the two mechanisms lead to same product, an overall increase in the rate of reaction will be obtained but if the two mechanism lead to different product distributions, cavitational switching might occur due to enhancement in the radical pathway and the nature of reaction products is actually changed due to cavitational effects.

When the US dose is increased, ultrasound cavitation can also destroy the cell walls and this effect is lethal to the microorganisms. Though the effects are spectacular, the technique is expensive as compared to conventional techniques of chemical disinfection. Some recent studies have reported the use of cavitation in combination with chemical disinfection techniques with overall reduced treatment times and much lower requirement of the chemical dosages [18,19]. 3.3. Biotechnology Cell disruption is one of the important and vital unit operations in biotechnology for the recovery of intracellular proteins. This is an energy intensive operation and hence, there exists a tremendous scope for the development of cheaper and energy efficient methods. Cavitation can be used effectively for the rupture of cells with energy requirements as less as just 5–10% of the total energy consumed by the conventional methods. The intensity of the cavitation phenomena can also be controlled so as to control the mechanism of the rupture of the cells to selectively release the intracellular enzymes or enzymes present in the cell wall. Lower intensity application of cavitation helps in retaining the activity of the leached out enzymes as well as it also reduces the cost of operation. Hydrodynamic cavitation has been found to be much more energy efficient as compared to acoustic cavitation and at the same time applicable at larger scale of operation [20,21].

3.2. Water and effluent treatment

3.4. Miscellaneous applications

Cavitation can be used effectively for the destruction of the contaminants in water because of the localized high concentrations of the oxidizing species such as hydroxyl radicals and hydrogen peroxide, higher magnitudes of localized temperatures and pressures and the formation of the transient supercritical water. The type of the pollutants in the effluent stream affects the rates of the degradation process. The hydrophobic compounds react with OH• and H• at the hydrophobic gas/liquid interface, while the hydrophilic species react to a greater extent with the OH• radicals in the bulk aqueous phase. Optimization of aqueous phase organic compound degradation rates can be achieved by adjusting the energy density, the energy intensity and the nature and properties of the saturating gas in solution. The variety of chemicals that have been degraded using acoustic cavitation though in different equipments and on a wide range of operating scales are p-nitrophenol, rhodamine B, 1,1,1 trichloroethane, parathion, pentachlorophenate, phenol, CFC 11 and CFC 113, o-dichlorobenzene and dichloromethane, potassium iodide, sodium cyanide, carbon tetrachloride among many others [13–16]. Apart from destruction of chemicals in the wastewater stream, cavitation can be effectively used for microbial disinfection and also for disintegration of the microbial sludge [17]. High power ultrasound, operated at low frequencies, is an effective means for disintegration of bacterial cells: first, at low ultrasound doses bacteria flocs can be deagglomerated by mechanical shear stress.

The applications discussed above just cover the broadspectrum applications of cavitation; there are many other specific applications of both acoustic and hydrodynamic cavitation [22,23]. Acoustic cavitation has been found to be beneficial in Crystallization operations (for controlling the crystal size distribution and also reduction in total operational time [24]), atomization (for controlling the droplet size distribution at minimum energy requirements [25]), Polymer chemistry applications for initiation of polymerization reactions or for destruction of complex polymers [26], solid–liquid extraction [27], intensification of leaching operation [28] as well as in petroleum industry for refining fossil fuels, determination of composition of coal extracts, extraction of coal tars, etc. and textile industry for enhancing the efficacy of dyeing of clothes and in wet processing [29]. Some of the miscellaneous application of hydrodynamic cavitation [23] can be given as floatation cells, synthesis of nanocrystalline materials, preparation of high quality quartz sand, preparation of free disperse system using liquid hydrocarbons and dental water irrigator. Though cavitation has been successful in intensification of majority of the physical and chemical processing applications, very few applications can be cited for industrial scale operation. Vinatoru [27] have reported the use of ultrasonic reactors operating at a working capacity of 750–800 l for solvent extraction of herbs. Horst et al. [30] have described a novel sonochemical reactor system which can be possibly used for synthesis of Grignard reagents at an annual capacity of 4 tonnes. Accentus,

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

UK and Prosonix, UK are some of the few companies that offer technologies based on the use of ultrasound for enhancing the efficacy of crystallization operations. 4. Reactor designs for cavitation 4.1. 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 work effectively if operated in geometry where most of the working liquid is constrained within the longitudinal highintensity region or where the liquid is stirred vigorously. Horst et al. [30] have reported a novel modification in terms of using high intensity ultrasound from a concentrator horn. It has been shown that the concept of a conical funnel fits the demands for nearly perfect radiation effectiveness and a good reaction management. The design used by Dahlem et al. [32,33] also needs a special mention here. Telsonic horn, which has radial vibrations as against conventional longitudinal vibrations for the immersion system gives dual advantages of higher irradiating surface (lower intensity of irradiation resulting in better yields) coupled with good distribution of the energy in the radial direction. Moreover even if the horn is radially vibrating, local measurements just below horn also give high cavitational activity, which will be again more beneficial in enhancing the global sonochemical yields. The scale up prospects of horn type systems is very poor as it cannot effectively transmit the acoustic energy into large process volume. Also, they suffer from erosion and particle shedding at the delivery tip surface, they may also be subject to cavitational blocking (acoustic decoupling), and the large transducer displacement increases stress on the material of construction, resulting in the possibility of failure. Thus, ultrasonic horn type systems are generally recommended for laboratory scale investigations. In another configuration commonly used, ultrasonic bath, the bottom of the reactor is irradiated with a single or multiple transducers. In this case, the active zone is restricted to a vertical plane just above the transducers with maximum intensity at the center of transducer. Thus, the area of irradiating surface should be increased (maximum possible) so as to get better distribution/dissipation of energy in the reactor. This has a twin advantage in terms of the decreased ultrasonic intensity (defined as power dissipation per unit area of the irradiating surface), which will increase the magnitude of the pressure pulse generated at the end of the cavitation events [34]. To increase the active zones existing in the reactor, one can easily modify the position of the transducers (if multiple transducers have been used which is likely to be the case at large scale operation due to the fact that it is quite difficult to successfully operate single transducer with very high power and

519

frequency due to limitations over the material of construction for the transducers) so that the wave patterns generated by the individual transducers will overlap, also resulting into uniform and increased cavitational activity. More recent developments have employed direct bonding of the transducer to the surface of the vessel. Improvements in the bonding method, and a move to transducers with lower individual outputs, have enabled the move to systems with large numbers of transducers to give an acoustic pattern that is uniform and noncoherent above the cavitational threshold throughout the working volume. Arrangements such as triangular pitch in the case of ultrasonic bath [32,35], tubular reactors with two ends either irradiated with transducers or one end with transducer and other with a reflector [36], parallel plate reactors with each plate irradiated with either same or different frequencies [37,38] and transducers each on sides of hexagon [39,40] can be constructed. It is of utmost importance to have uniform distribution of the ultrasonic activity in order to get increased cavitational effects. The use of low-output transducers gives the additional advantage of avoiding the phenomenon of cavitational blocking (acoustic decoupling), which arises where power densities close to the delivery point are very high. In addition these multi-transducer units very effectively concentrate ultrasonic intensity towards the central axis of the cylinder and away from the vessel walls, thus reducing problems of erosion and particle shedding. Romdhane et al. [39] and Faid et al. [41] have described a sonitube reactor, which can be operated in continuous manner. Mapping of this reactor shows some cavitational activity even beyond the irradiation zone. Moreover as compared to various reactors studied in their work (ultrasonic horn, ultrasonic cleaner [bath], hexagonal reactor), sonitube gives a better homogeneity in terms of the cavitational activity; negligible variation in the radial direction has been reported. It is also important to predict the distribution of cavitational intensity in the reactor so that a proper design of the reactor can be achieved. Details of such analysis is beyond the scope of the present paper but work of Keil and coworkers [42–44] appears to be pioneering in terms of simulations of the pressure fields existing in the reactor and is recommended for further understanding. Such a detailed analysis can be used to identify the regions with maximum pressure fields in a large scale reactor and then may be small reactors or micro-reactors can be placed strategically at these locations in order to get maximum benefits and economic savings. It might happen that the threshold required for certain application is obtained at these locations but if considered globally these effects will be marginalized resulting into much lower yields from cavitational reactors. Thus, the location of the transducers on the irradiating surface and the location of micro-reactors will also depend on the type of application, which decides the required cavitational intensity for onset of the specific application under question. 4.2. Hydrodynamic cavitation reactors Hydrodynamic cavitation can simply be generated by the passage of the liquid through a constriction such as an orifice plate

520

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

Table 2 Optimum operating conditions for the sonochemical reactors Number

Property

Affects

Favorable conditions

1

Intensity of irradiation (range: 1–300 W/cm2 ) Frequency of irradiation (range: 20–200 kHz) Liquid Vapor pressure (range: 40–100 mm of Hg at 30 ◦ C) Viscosity (range: 1–6 cP) Surface tension (range: 0.03–0.072 N/m) Bulk liquid temperature (range: 30–70 ◦ C)

Number of cavities, collapse pressure of single cavity Collapse time of the cavity as well as final pressure/temperature pulse Cavitation threshold, intensity of cavitation, rate of chemical reaction. Transient threshold Size of the nuclei (cavitation threshold) Intensity of collapse, rate of the reaction, threshold/nucleation, almost all physical properties

Use power dissipation till an optimum value and over a wider area of irradiation Use enhanced frequencies till an optimum value Liquids with low vapor pressures

Gas content, nucleation, collapse phase Intensity of cavitation events

Low solubility Gases with higher polytropic constant and lower thermal conductivity (monoatomic gases)

Number of cavitational events and cavitational activity distribution

Higher number of transducers of optimum shape so as to achieve uniform cavitational activity in the reactor

2 3 4 5 6

7

8

Dissolved gas A. Solubility B. Polytropic constant and thermal conductivity Geometry of the reactor

or a valve [23]. When the liquid passes through the constriction, the kinetic energy associated with the liquid increases at the expense of the local pressure. If the throttling is sufficient, so that the local pressure falls below the threshold pressure for cavitation (usually vapor pressure of the medium at the operating temperature), cavities will be generated. Subsequently as the liquid jet expands, the pressure recovers and this results in the collapse of the cavities. During the passage of the liquid through the constriction, boundary layer separation occurs and substantial amount of energy is lost in the form of permanent pressure drop. Very high intensity turbulence is also present downstream of the constriction; its intensity depends on the magnitude of the pressure drop, which, in turn, depends on the geometry of the constriction and the flow conditions of the liquid, i.e. the scale of turbulence. A dimensionless number known as cavitation number (Cv ) has generally been used to relate the flow conditions with the cavitation intensity. Cavitation number can be mathematically represented as: Cv =

P2 − P v (1/2)ρl v2o

where P2 is the recovered pressure downstream of the constriction, Pv is the vapor pressure of liquid, vo is average velocity of liquid at the orifice and ρl is the density of liquid. The cavitation number at which the inception of cavitation occurs is known as cavitation inception number Cvi . Ideally, the cavitation inception occurs at Cvi = 1 and there are significant cavitational effects at Cv value of less than 1. Hydrodynamic cavitation can also be generated in the rotating machinery (e.g. high speed homogenizer) by adjusting the speed of rotation and the geometry of the system so that the local pressure just near the rotor falls below the vapor pressure. The flexibility of controlling the intensity of cavitation is however much less in these types of reactors as com-

Low viscosity Low surface tension Optimum value exits, generally lower temperatures are preferable

pared to the reactors based on the use of a constriction in the flow. 5. 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 the collapse pressure generated and the maximum size of the cavity during the cavitation phenomena have been studied using the bubble dynamics equation, which considers the compressibility of the medium and a single bubble in isolation [34]. In the present work only important considerations regarding the selection of operating parameters have been presented (Table 2). This study indicates the ways and means to manipulate cavitating conditions for maximum effect. In an earlier work [45], detailed theoretical analysis of the bubble dynamics in hydrodynamic cavitation type of reactors have also been investigated and only important results have been summarized here. The numerical simulations are based on bubble dynamics equations similar to acoustic cavitation; the only difference being the fact that surrounding fluctuating pressure field is driven by hydrodynamic conditions existing downstream of the constriction whereas in the case of acoustic cavitation, it is dependent on the time of irradiation (sinusoidal variation). The optimum set of operating parameters as obtained from these theoretical investigations has been given in Table 3.

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

521

Table 3 Optimum operating conditions for the hydrodynamic cavitation reactors Number

Property

Favorable conditions

1

Inlet pressure into the system/rotor speed depending on the type of equipment Physicochemical properties of the liquid and initial radius of the nuclei

Use increased pressures or rotor speed but avoid super-cavitation by operating below a certain optimum value The guidelines for selecting the physicochemical properties so as to achieve lower initial sizes of the nuclei are similar to those used for the sonochemical reactors Optimization needs to be carried out depending on the application. Higher diameters are recommended for applications which require intense cavitation whereas lower diameters with large number of holes should be selected for applications with reduced intensity Lower free areas must be used for producing high intensities of cavitation and hence the desired beneficial effects

2

3

Diameter of the constriction used for generation of cavities e.g. hole on the orifice plate

4

Percentage free area offered for the flow (ratio of the free area available for the flow, i.e. cross-sectional area of holes on the orifice plate to the total cross-sectional area of the pipe)

6. 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 can be recommended depending on the type of applications: 1. Use of process intensifying parameters such as presence of dissolved gases and/or continuous sparging of gases such as air, ozone and argon (to a limited extent), presence of salts such as NaCl, NaNO2 and NaNO3 , presence of solid particles such as TiO2 , CuO and MnO2 which can also act as a catalyst in some cases. 2. Use of combination of cavitation with microwave irradiation or advanced oxidation processes such as ozonation, chemical oxidation using hydrogen peroxide and photocatalytic oxidation. 6.1. Use of process intensifying parameters The presence of gases and solid particles mainly provide additional nuclei for the cavitation phenomena and hence the number of cavitation events occurring in the reactor are enhanced resulting in a subsequent enhancement in the cavitational activity and hence the net chemical/physical effects. It must be noted that the presence of both these parameters also have a negative effect on the cavitational activity (aeration gives cushioning effect and incomplete collapse resulting in a decrease in the collapse pressure whereas solid particles result in scattering of the sound waves thereby decreasing the focused energy transferred into the system). The net effect of these two phenomena will be dependent on the system in question and hence optimization is a must before operating parameters are selected for actual operation. The earlier work of Gogate et al. [46] is recommended in this case to obtain an idea about the methodology to be used

for the optimization of operating parameters. The readers are also requested to refer to earlier literature illustrations [47–51] for a greater insight into the effect of presence of gases on the cavitational applications. Considering the specific application of chemical synthesis, the presence of solid catalyst (particles/salts in a typical concentration range of 1–10% by weight of the reactants; optimization is recommended in majority of the cases using laboratory scale studies) in the sonochemical reactors results in intensification due to the following mechanisms: 1. Formation of increased cavitation nuclei due to more number of discontinuities in liquid continuum as a result of the presence of particles to give larger number of collapse events resulting in increase in the number of free radicals. 2. In a biphasic solid–liquid medium irradiated by power ultrasound, major mechanical effects are the reduction of particle size leading to an increased surface area and the formation of liquid jets at solid surfaces by the asymmetrical inrush of the fluid into the collapsing voids. These liquid jets not only provide surface cleaning but also induce pitting and surface activation effects and increase the rate of phase mixing, mass transfer and catalyst activation. 3. Enhanced generation of free radicals due to some catalysts such as FeSO4 or elemental iron. 4. Better distribution of the organic pollutants increasing the concentration at reaction sites. 5. Alteration of physical properties (vapor pressure, surface tension) facilitating generation of cavities and also resulting in more violent collapse of the cavities. 6. Presence of salts might also result in preferential accumulation of the reactants at the site of cavity collapse thereby resulting in an intensification of the cavitational reactions [47]. 6.2. Use of combination of cavitation and advanced oxidation processes Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is

522

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters such as solid particles, sparging of gases etc., which results in overall increase in the cavitational intensity is recommended. Usually, the use of hydrogen peroxide in conjunction with ultrasound is beneficial only till an optimum loading [52–54]. The optimum value will be dependent on the nature of the chemical reactions and the operating conditions in terms of power density/operating frequency (these decide the rate of generation of the free radicals) and laboratory scale studies are essential to establish this optimum for the specific application in question. Literature reports may not necessarily give correct solutions (for optimum concentration) even if matching is done with respect to the primary components as it is the auxiliary components of the streams (most notably radical scavengers such as bicarbonate, carbonate ions, t-butanol, naturally occurring material, humic acids) that are crucial factors in deciding the effectiveness of the free radical attack. The synergistic effect of combining ozonation with ultrasonic irradiation is observed only when the free radical attack is the controlling mechanism and the rate of generation of free radicals due to ultrasonic action alone is somewhat lower (at lower frequencies of operation and power dissipation levels). At higher frequencies of operation, optimum for the ozone concentration exists and should be established by laboratory scale studies for the application under question. The effect of the presence of radical scavengers such as bicarbonate, chloride, sulfate ions, naturally occurring material will be again dependent on the specific reaction in question. For volatile materials such as chloroform, MTBE, the reactions takes place in the cavitating bubble and hence the effect will not be felt. But for majority of the other reactants, the reaction takes place usually in the liquid bulk or at gas–liquid interface and hence the decrease in the rates of synthesis processes will be significant. Acid pretreatment (only with a weak acid for the selective removal of ionic radical scavengers; also other type of treatment may be required if some of the non-ionic radical scavengers are present even under acidic conditions) along with sparging with neutral gas can be done for increasing the reactivity. It must also be noted that, large concentration of the weak acid will be required for achieving similar acidic conditions as compared to the stronger acid but if the selection of the weak acid is made in such a way that radical scavenging action of the dissociated acid is not there (strong acid counter-ions, i.e. Cl− , SO4 2− indeed have a scavenging action of the hydroxyl radicals), large number of free radicals will be present in the system available for the specific application. Thus, the use of proper weak acid is justified for the pretreatment. The extensive work done by the group of Hoffmann and coworkers [49,55–57], though in the area of wastewater treatment, is recommended for understanding more details about using combination of ozone and cavitation synergistically. The synergistic effects of combining photocatalytic oxidation with cavitation can be possibly attributed to: 1. Cavitational effects leading to an increase in the temperatures and pressure at the localized microvoid cavity implosion sites.

2. Cleaning and sweeping of the TiO2 surface due to acoustic microstreaming allows for an access to more active catalyst sites at any given time. 3. Mass transport of the reactants and products is increased at the catalyst surface and in the solution due to the facilitated transport as a result of shockwave propagation. 4. Surface area is increased by fragmentation or pitting of the catalyst. 5. Cavitation induced radical intermediates participate in the destruction of organic compounds. 6. The organic substrate reacts directly with the photogenerated surface holes and electrons. 7. Cavitation induced turbulence also enhances the rates of the desorption of intermediate products from the catalyst active sites and helps in continuous cleaning of the catalyst surface. Toma et al. [58] have given a brief overview of different studies pertaining to the effect of ultrasound on photochemical reactions concentrating on the chemistry aspects, i.e. mechanisms and pathways of different chemical reactions. It should be noted that in the situations where the adsorption of reactants at the specific sites is the rate controlling step, ultrasound will play a profound role due to substantial increase in the number of active sites and also due to the increased surface area available due to fragmentation of the catalyst agglomerates under the action of turbulence generated by acoustic streaming along with an increase in the diffusional rates of the reactants. It is of utmost importance to operate simultaneously rather than having sequential irradiation of ultrasound followed by photocatalytic oxidation. This is because the catalyst surface is kept clean continuously due to the cleaning action of ultrasound in the operation as a result of which maximum sites are available for the photocatalytic reaction. Also, the number of free radicals generated in simultaneous irradiation (more dissociation of water molecules due to more energy dissipation and also the presence of solid particles enhances the ultrasound effects due to surface cavitation and adsorption of reactants) will be the maximum as compared to the sequential operation. The details regarding the operating and design strategies for maximizing the synergistic effects have been described by Gogate and Pandit [59]. 6.3. Combined use of microwave irradiation and sonochemistry Combined use of microwave irradiation and sonochemistry is a recent advance that accelerates the chemical reactions and other processing applications significantly, especially in the case of heterogeneous systems. The dramatic acceleration effect may be attributed to a combination of enforced heat transfer due to microwave irradiation and intensive mass transfer at phase interfaces caused by ultrasonic irradiation. In many heterogeneous reactions, the rate-determining step is the mass transfer at the interface between two (or more) phases. With sonication, the liquid jet caused by cavitation propagates across the bubble towards the phase boundary at a velocity estimated at several hundreds of meter per second, and violently hits the surface. The intense agitate leads to the mutual injection of droplets of one liquid into

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

the other, resulting in the formation of fine emulsions. These ultrasonically produced emulsions are smaller in size and more stable than those obtained conventionally and require little or no surfactant to maintain stability. Microwave irradiations compliment these effects by way of enhanced heating as well as by way absorption by the molecules resulting in significantly enhanced rates of reactions. The main advantage of microwave heating is the instantaneous heating of solids and liquids, which is not the case of conventional heating. The details about the use of microwave irradiations in intensifying the chemical reactions can be referred in the earlier published work of Toukoniitty et al. [60], Stankiewicz [61] and Strauss and Varma [62]. Peng and Song [63] reported the use of combination technique for hydrazinolysis of methyl salicylate and compared the yields of reaction under different operating combinations comprising of conventional reflux, sonication alone, microwave irradiation alone and combination technique. The yield of reaction was reported to be 73% in 9 h of reaction time under conventional reflux conditions whereas use of sonication (50 W power dissipation) resulted in a reaction yield of 79% in only 1.5 h. Microwave irradiation at 200 W power dissipation further intensified the process resulting in a yield of 80% in only 18 min of reaction time. Combination technique was the fastest of all investigated processes giving about 84% yield in only 40 s. The combination technique was also applied to different methyl esters and yields reported were around 80–85% in less than a minute depending on the type of the ester investigated. Cravotto et al. [64] also reported that the combined use of microwave irradiation and ultrasound resulted in intensification of the Suzuki reactions. The system for combined effect consisted of an air-cooled probe equipped with a titanium horn, working at about 20.5 kHz and an US-reaction vessel (made of titanium) in which the horn is inserted. The reaction vessel is cooled by a flow of either tap-water or refrigerated oil. The reaction mixture was pumped using two lengths of thermostatted coaxial tubing and a peristaltic pump for circulation between the US-cell and another vessel (the MW-cell, made of Teflon) placed inside a MW oven. The coupling reaction between 3bromoanisole and phenylboronic acid resulted in a reaction yield of 54% using ultrasonic irradiation and 64% using microwave irradiation when operated individually. The combined operation resulted in a yield of 88% in similar reaction periods. Trotta et al. [65] and Cravotto et al. [66] have also reported the superiority of using a combination of ultrasound and microwave irradiations.

diesel” is the term applied to the esters of simple alkyl fatty acids used as an alternative to petroleum based diesel fuels. Importance of bio-diesel in the recent context increases due to increasing petroleum prices; limited fossil fuel reserves and environmental benefits of bio-diesel (decrease in acid rain and emission of CO2 , SOx and unburnt hydrocarbons during the combustion process). Due to these factors, and due to its easy biodegradability, production of bio-diesel is considered as an advantage over that of fossil fuels. The conventional techniques for the synthesis of bio-diesel [67–69] refer to a catalysed chemical reaction involving vegetable oil and an alcohol to yield acid alkali esters and glycerol. The conventional techniques typically utilize temperatures in the range of 70–200 ◦ C, pressures in the range of 6–10 atm and reaction times of up to 70 h for achieving conversions in the range of 90–95% based on the type of raw material used (usually mixtures of fatty acids obtained as waste). The present work aims at using cavitation as an alternative technique for the synthesis of bio-diesel. The main objectives of this study were, comparison of hydrodynamic cavitation and acoustic cavitation in terms of their energy utilization, the study of this reaction at pilot plant scale of operation and preparation of alkyl esters of fatty acids which is used as bio-diesel fuel. 7.1. Reactor configurations The hydrodynamic cavitation reactor consists of a reservoir or a collecting tank with (10 l) capacity that is connected to the multistage centrifugal pump with power rating of 1.5 kW. A schematic representation of the setup has been shown in Fig. 1. The pipe connected to the discharge side of the pump branches into main and bypass lines. The main line has the facility to incorporate different orifice plate to generate cavitation of different intensities and characteristics. The main line and bypass line having the throttling valves and pressure gauges for adjusting the pressure upstream of the cavitating orifice. The operating temperature of the reactor was maintained to near ambient conditions by circulating water within the jacket surrounding the tank as the energy dissipated by the pump can increase the temperature of the mixture. The sonochemical reactor used in the present work is a conventional cleaning tank type reactor (ultrasonic bath) equipped with three transducers at the bottom of the tank arranged in

7. 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 diesel engines. Today “bio-

523

Fig. 1. Schematic representation of orifice plate setup.

524

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527 Table 5 Comparison of energy efficiency for different techniques No

Technique

Time (min)

Yield (%)

Yield (Kg/KJ)

1 2

Acoustica Hydrodynamicb

10 15

99 98

8.6 × 10−5 3.37 × 10−3

3

Conventionalc With stirring Under reflux

180 15

98 98

2.27 × 10−5 7.69 × 10−6

a

Fig. 2. Schematic representation of sonochemical reactor generating acoustic cavitation.

a triangular pitch and operates at an irradiating frequency of 20 kHz and power dissipation of 120 W (calorimetrically measured exact power dissipation was 45 W indicating about 37.5% energy conversion and transfer efficiency). The bath had dimensions of 15 cm × 15 cm × 15 cm. The schematic representation of the ultrasonic bath and the reactor assembly is shown in Fig. 2. The reactor was immersed at location of maximum cavitational intensity obtained using mapping studies [70]. 7.2. Important results The results obtained for variety of oils as a starting material has been shown in Table 4. It can be seen from the table that cavitation can be very successfully applied to transesterification reactions with more than 90% yield of the product as per stoichiometry in as low as 15 min of the reaction time. The technique hence appears to be very effective as compared to the conventional approach which is also evident from the comparison of different techniques based on quantitative criteria of energy efficiency as shown in Table 5. It can be seen from this table, that hydrodynamic cavitation is about 40 times more efficient as compared to acoustic cavitation and 160–400 times more efficient as compared to the conventional agitation/heating/refluxing method. Stavarache et al. [71,72] have also shown a significant process intensification using acoustic cavitation for these trans-esterification reactions using pure vegetable oil.

In acoustic cavitation, 4 ml of methanol is mixed with 4 g of vegetable oil and catalyst concentration (NaOH) used in 0.5% of oil. Ultrasonic bath is the sonochemical reactor with 20 kHz frequency and 85 W as power dissipation. b Operation with hydrodynamic cavitation is under optimized conditions: 4:4 ratio (w/v) of oil to alcohol, catalyst concentration (NaOH) is 1% of oil, orifice plate 1 having 16 holes with 2 mm diameter), volume of methanol is 4000 ml with 4000 g of oil. c For conventional approach, 4 ml of methanol is mixed with 4 g of vegetable oil and catalyst concentration (NaOH) used in 0.5% of oil (Case I: a stirrer is used for uniform mixing which consumes energy; Case II: heater is used for maintaining reflux conditions).

Usually waste vegetable oils as against virgin (pure) vegetable oil have been used for the synthesis with an aim to reduce the cost of production. To check feasibility of applying cavitation for synthesis based on waste vegetable oils, esterification of fatty acid (FA) odour cut (C8 –C10 ) with methanol in the presence of concentrated H2 SO4 as a catalyst has been studied in hydrodynamic cavitation reactor as well as in the sonochemical reactor. Few experiments have also been carried out with other acid/alcohol combination, viz. coconut fatty acids with methanol and ethanol and FA odour cut with fatty alcohols with an aim of investigating the efficacy of cavitation for giving the desired yields and also to quantify the degree of process intensification that can be achieved using the same. The different reaction operating parameters such as molar ratio of acid to alcohol, catalyst quantity have been optimized under acoustic as well as hydrodynamic cavitating conditions in addition to the optimization of the geometry of the orifice plate in the case of hydrodynamic cavitation reactors. It has been observed that ambient operating conditions of temperature and pressure and reaction times of less than 3 h, for all the different combinations of acid/alcohol studied in the present work, was sufficient for giving more than 90% conversion. This clearly establishes the efficacy of cavitation as an excellent way to achieve process intensification of the bio-diesel synthesis process. To cite a specific illustration as regards to the degree of process intensification achieved in the present work, with an operating ratio of FA cut (waste fatty acids) to methanol as 1:10,

Table 4 Trans-esterfication of different vegetable oils Number

1 2 3

Vegetable oil

Soyabean oil Castor oil Peanut oil

Product

Soyabean oil ester Castor oil ester Peanut oil ester

Time (min)

Yield (%)

Acoustic

Hydro

Acoustic

Hydro

15 10 10

15 10 10

97 99 99

98 99 90

Acoustic cavitation: vegetable oil (4 g), methanol (4 ml), NaOH (0.5%). Hydrodynamic cavitation: vegetable oil (4000 g), methanol (4000 ml), NaOH (1%), P = 1 kg/cm2.

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

0.1% by weight loading of the catalyst and at operating temperature of 30 ◦ C, 92% conversion was achieved using hydrodynamic cavitation in only 90 min of reaction time whereas conventional method for the esterification of waste cooking oil using methanol required about 69 h to obtain more than 90% oil conversion to methyl esters at 65 ◦ C operating temperature and a molar ratio of methanol to oil as 30:1. Comparison of energy efficiencies of hydrodynamic cavitation reactors with sonochemical reactors indicated that hydrodynamic cavitation is more energy efficient as compared to acoustic cavitation. Depending on the type of acid/alcohol combination used in the present work the energy efficiency for hydrodynamic cavitation varied in the range of 1 × 10−4 to 2 × 10−4 g/J whereas for acoustic cavitation it was order of magnitude lower, i.e. in the range of 5 × 10−6 to 2 × 10−5 g/J. The present work has clearly illustrated the efficacy of cavitation as a novel tool for process intensification of synthesis of bio-diesel which has been looked at an affordable alternative to conventional petroleum based fuels.

525

geometry to study the effect of these on cavity/bubble/cluster dynamics and hence on the cavitational activity. 3. In the case of hydrodynamic cavitation reactors, realistic modelling of the turbulence phenomena which then can be used to model the cavity/bubble dynamics either in isolation or in the form of cavity clusters in high-velocity flow. The modern sophisticated CFD codes can be employed to get the flow field information, i.e. mean and fluctuating velocity components, Reynolds stresses, turbulent pressure fluctuations, which can be then used to understand the role of these flow field parameters in altering cavity dynamics. 4. Combination of the hydrodynamic cavitation reactors and sonochemical reactors where the cavity is generated using the hydrodynamic means and the collapse of the cavities is taking place in the sonochemical reactor, needs to be tested for different cavitational transformations. 5. The effect of process intensifying parameters such as the presence of solid particles, dissolved salts and gases should be studied in details at different operating scales. 9. Conclusions

8. 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 information with the experimental results. 3. The existing information is available mainly on laboratory scale, which would give very large scale up ratios and hence very high degree of uncertainty. 4. Local existence of the cavitation phenomena just near the irradiating surface in the case of ultrasonic transducers and wide distribution of the energy dissipation patterns in the reactor. 5. Design information is required from diverse fields such as chemical engineering (gas–liquid hydrodynamics and other reactor operations), material science (for construction of transducers efficiently operating at conditions of high frequency and high power dissipation) and acoustics (for better understanding of the sound field existing in the reactor). Efforts are required in the following directions so as to realize the dream of application of cavitational reactors in actual practice: 1. Detailed theoretical analysis of the bubble dynamics phenomena under different operating conditions is essential to predict the cavitational intensity and its effect on the rates of physical or chemical processing applications. 2. Design and fabrication of different types of cavitational reactors differing in flow field, turbulence characteristics and

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 configuration to achieve maximum benefits. The future of sonochemical reactors lies in the design of multiple frequency multiple transducer based reactors whereas for hydrodynamic cavitation reactors, orifice plate type configuration appears to be most suitable. It can be said that the hydrodynamic cavitation reactors offer immediate and realistic potential for industrial scale applications as compared to the sonochemical reactors and the scale up of these reactors is comparatively easier as vast amount of information about the fluid dynamics downstream of the constriction is readily available and the operating efficiency of the circulating pumps which is the only energy dissipating device in the system is always higher at large scales of operation. The experimental illustrations depicted in the work have clearly illustrated the efficacy of this novel technology. At this stage, it should be noted that the operating costs of cavitational reactors especially the sonochemical reactors are on a higher side as compared to the conventional reactors but the advantages obtained for a specific application (e.g. better product distribution ratios or global ambient operating conditions, etc.) should be weighed against the higher processing costs. Optimum selection of operating parameters with the use of process intensifying parameters/techniques as discussed earlier is a must for obtaining overall benefits. Overall, it can be said that, cavitation is a well established technology at laboratory/pilot scale and combined efforts of chemists, chemical engineers and physicists are required to effectively harness this technology on an industrial scale of operation.

526

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527

References [1] A. Stankiewicz, J.A. Moulijn, Process intensification: transforming chemical engineering, Chem. Eng. Prog. 96 (2000) 22. [2] F.M. Dautzenberg, M. Mukherjee, Process intensification using multifunctional reactors, Chem. Eng. Sci. 56 (2001) 251. [3] A. Stankiewicz, Reactive separations for process intensification: an industrial perspective, Chem. Eng. Process. 42 (2003) 137. [4] T.J. Mason, Practical Sonochemistry: Users Guide in Chemistry and Chemical Engineering, Ellis Horwood Series in Organic Chemistry, Chichester, UK, 1992. [5] F.R. Young, Cavitation, McGraw Hill, London, UK, 1989. [6] T.G. Leighton, The Acoustic Bubble, Academic Press, London, UK, 1994. [7] J.L. Luche, Synthetic Organic Chemistry, Plenum Press, New York, USA, 1999. [8] J. Lindley, T.J. Mason, Use of ultrasound in chemical synthesis, Chem. Soc. Rev. 16 (1987) 275. [9] T.J. Mason, Ultrasound in synthetic organic chemistry, Chem. Soc. Rev. 26 (1997) 443. [10] L.H. Thompson, L.K. Doraiswamy, Sonochemistry: science and engineering, Ind. Eng. Chem. Res. 38 (1999) 1215. [11] G. Cravatto, P. Cintas, Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications, Chem. Soc. Rev. 35 (2006) 180. [12] Y. Zhao, R. Feng, Y. Shi, M. Hi, Sonochemistry in China between 1997 and 2002, Ultrason. Sonochem. 12 (2005) 173. [13] Y.G. Adewuyi, Sonochemistry: environmental science and engineering applications, Ind. Eng. Chem. Res. 40 (2001) 4681. [14] P.R. Gogate, Cavitation: an auxiliary technique in wastewater treatment schemes, Adv. Environ. Res. 6 (2002) 329. [15] Y.G. Adewuyi, Sonochemistry in environmental remediation. 1. Combinative and hybrid sonophotochemical oxidation processes for the treatment of pollutants in water, Environ. Sci. Technol. 39 (2005) 3409. [16] Y.G. Adewuyi, Sonochemistry in environmental remediation. 2. Heterogeneous sonophotocatalytic oxidation processes for the treatment of pollutants in water, Environ. Sci. Technol. 39 (2005) 8557. [17] K. Nickel, U. Neis, Ultrasonic disintegration of biosolids for improved biodegradation, Ultrason. Sonochem. 14 (2007) 450. [18] H. Duckhouse, T.J. Mason, S.S. Phull, J.P. Lorimer, The effect of sonication on microbial disinfection using hypochlorite, Ultrason. Sonochem. 11 (2004) 173. [19] T. Blume, U. Neis, Improving chlorine disinfection of wastewater by ultrasound application, Water Sci. Technol. 52 (2005) 139. [20] S.S. Save, A.B. Pandit, J.B. Joshi, Use of hydrodynamic cavitation for large scale microbial cell disruption, Chem. Eng. Res. Des. 75 (1997) 41. [21] S.S. Save, A.B. Pandit, J.B. Joshi, Microbial cell disruption: role of cavitation, Chem. Eng. J. 55 (1994) B67. [22] Y.T. Shah, A.B. Pandit, V.S. Moholkar, Cavitation Reaction Engineering, Plenum Publishers, NY USA, 1999. [23] P.R. Gogate, A.B. Pandit, Hydrodynamic cavitation: a state of the art review, Rev. Chem. Eng. 17 (2001) 1. [24] L.J. McCausland, P.W. Cains, Power ultrasound—a means to promote and control crystallization in biotechnology, Biotechnol. Genet. Eng. Rev. 21 (2004) 3. [25] B. avvaru, M.N. Patil, P.R. Gogate, A.B. Pandit, Ultrasonic atomization: effect of liquid phase properties, Ultrasonics 44 (2006) 146. [26] J.M.J. Paulusse, R.P. Sijbesma, Ultrasound in polymer chemistry: revival of an established technique, J. Poly. Sci. A Poly. Chem. 44 (2006) 5445. [27] M. Vinatoru, An overview of the ultrasonically assisted extraction of bioactive principles from herbs, Ultrason. Sonochem. 8 (2001) 303. [28] J.L. Luque-Garciˇıa, M.D. Luque De Castro, Ultrasound: a powerful tool for leaching, TrAC Trends Anal. Chem. 22 (2003) 41. [29] A.K. Patra, M. Das, Ultrasound and its utility in wet processing, Melliand Int. 12 (2006) 60. [30] C. Horst, Y.-S. Chen, U. Kunz, U. Hoffmann, Design, modeling and performance of a novel sonochemical reactor for heterogeneous reactions, Chem. Eng. Sci. 51 (1996) 1837.

[31] M.M. Chivate, A.B. Pandit, Quantification of cavitation intensity in fluid bulk, Ultrason. Sonochem. 2 (1995) S19. [32] O. Dahlem, V. Demaiffe, V. Halloin, J. Reisse, Direct sonication system suitable for medium scale sonochemical reactors, AIChE J. 44 (1998) 2724. [33] O. Dahlem, J. Reisse, V. Halloin, The radially vibrating horn: a scaling up possibility for sonochemical reactions, Chem. Eng. Sci. 54 (1999) 2829. [34] P.R. Gogate, A.B. Pandit, Engineering design methods for cavitation reactors. I. Sonochemical reactors, AIChE J. 46 (2000) 372. [35] S.R. Soudagar, S.D. Samant, Semiquantitative characterization of ultrasonic cleaner using a novel piezoelectric pressure intensity measurement probe, Ultrason. Sonochem. 2 (1995) S49. [36] E. Gonze, Y. Gonthier, P. Boldo, A. Bernis, Standing waves in a high frequency sonoreactor: visualisation and effects, Chem. Eng. Sci. 53 (1998) 523. [37] G. Thoma, J. Swofford, V. Popov, M. Som, Sonochemical destruction of dichloromethane and o-dichlorobenzene in aqueous solution using a nearfield acoustic processor, Adv. Environ. Res. 1 (1997) 178. [38] P.R. Gogate, I.Z. Shirgaonkar, M. Sivakumar, P. Senthilkumar, N.P. Vichare, A.B. Pandit, Cavitation reactors: efficiency analysis using a model reaction, AIChE J. 47 (2001) 2326. [39] M. Romdhane, C. Gourdon, G. Casamatta, Local investigation of some ultrasonic devices by means of a thermal sensor, Ultrasonics 33 (1995) 221. [40] P.R. Gogate, S. Mujumdar, A.B. Pandit, Large scale sonochemical reactors for process intensification: design and experimental validation, J. Chem. Technol. Biotechnol. 78 (2003) 685. [41] F. Faid, M. Romdhane, C. Gourdon, A.M. Wilhelm, H. Delmas, A comparative study of local sensors of power ultrasound effects: electrochemical, thermoelectrical and chemical probes, Ultrason. Sonochem. 5 (1998) 63. [42] S. Dahnke, F.J. Keil, Modelling of linear pressure fields in sonochemical reactors considering an inhomogeneous density distribution of cavitation bubbles, Chem. Eng. Sci. 54 (1999) 2865. [43] S. Dahnke, K.M. Swamy, F.J. Keil, 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. 6 (1999) 31. [44] S. Dahnke, F.J. Keil, Modelling of three-dimensional linear pressure fields in sonochemical reactors with homogenous and inhomogeneous density distribution of cavitation bubbles, Ind. Eng. Chem. Res. 37 (1998) 848. [45] P.R. Gogate, A.B. Pandit, Engineering design methods for cavitation reactors. II. Hydrodynamic cavitation, AIChE J. 46 (2000) 1641. [46] P.R. Gogate, A.M. Wilhelm, B. Ratsimba, H. Delmas, A.B. Pandit, Destruction of formic acid using high frequency cup horn reactor, Water Res. 40 (2006) 1697. [47] J. Seymore, R.B. Gupta, Oxidation of aqueous pollutants using ultrasound: salt induced enhancement, Ind. Eng. Chem. Res. 36 (1997) 3453. [48] C.A. Wakeford, R. Blackburn, P.D. Lickiss, Effect of ionic strength on the acoustic generation of nitrite, nitrate and hydrogen peroxide, Ultrason. Sonochem. 6 (1999) 141. [49] J.-W. Kang, H.-M. Hung, A. Lin, M.R. Hoffmann, Sonolytic destruction of methyl tert butyl ether by ultrasonic irradiation: role of O3 , H2 O2 , frequency and power density, Environ. Sci. Technol. 33 (1999) 3199. [50] O. Krugar, Th-L. Schulze, D. Peters, Sonochemical treatment of natural ground water at different high frequencies: preliminary results, Ultrason. Sonochem. 6 (1999) 123. [51] Y. Nagata, M. Nagakawa, H. Okuno, Y. Mizukoshi, B. Yim, Y. Maeda, Sonochemical degradation of chlorophenols in water, Ultrason. Sonochem. 7 (2000) 115. [52] J.R. Chen, X.-W. Xu, A.S. Lee, T.F. Yen, A feasibility study of dechlorination of chloroform in water by ultrasound in the presence of hydrogen peroxide, Environ. Technol. 11 (1990) 829. [53] F. Chemat, P.G.M. Teunissen, S. Chemat, P.V. Bartels, Sono-oxidation treatment of humic substances in drinking water, Ultrason. Sonochem. 8 (2001) 247. [54] K.C. Teo, Y. Xu, C. Yang, Sonochemical degradation of toxic halogenated organic compounds, Ultrason. Sonochem. 8 (2001) 241.

P.R. Gogate / Chemical Engineering and Processing 47 (2008) 515–527 [55] J.-W. Kang, M.R. Hoffmann, Kinetics and mechanism of the sonolytic destruction of methyl tert butyl ether by ultrasonic irradiation in the presence of ozone, Environ. Sci. Technol. 32 (1998) 3194. [56] L.K. Weavers, F.H. Ling, M.R. Hoffmann, Aromatic compound degradation in water using a combination of sonolysis and ozonolysis, Environ. Sci. Technol. 32 (1998) 2727. [57] L.K. Weavers, N. Malmstadt, M.R. Hoffmann, Kinetics and mechanism of pentachlorophenol degradation by sonication, ozonation and sonolytic ozonation, Environ. Sci. Technol. 34 (2000) 1280. [58] S. Toma, A. Gaplovsky, J.-L. Luche, The effect of ultrasound on photochemical reactions, Ultrason. Sonochem. 8 (2001) 201. [59] P.R. Gogate, A.B. Pandit, Sonophotocatalytic oxidation based reactors for wastewater treatment: a critical review, AIChE J. 50 (2004) 1051. [60] B. Toukoniitty, J.-P. Mikkola, D.Yu. Murzin, T. Salmi, Utilization of electromagnetic and acoustic irradiation in enhancing heterogeneous catalytic reactions, Appl. Catal. A Gen. 279 (2005) 1. [61] A. Stankiewicz, Energy matters: alternative sources and forms of energy for intensification of chemical and biochemical processes, Chem. Eng. Res. Des. 84 (2006) 511. [62] C.R. Strauss, R.S. Varma, Microwaves in green and sustainable chemistry, Top. Curr. Chem. 266 (2006) 199. [63] Y. Peng, G. Song, Simultaneous microwave and ultrasound irradiation: a rapid synthesis of hydrazides, Green Chem. 3 (2001) 302. [64] G. Cravotto, M. Beggiato, A. Penoni, G. Palmisano, S. Tollari, J.-M. L´evˆeque, W. Bonrath, High-intensity ultrasound and microwave, alone or

[65]

[66]

[67]

[68] [69]

[70]

[71] [72]

527

combined, promote Pd/C-catalyzed aryl–aryl couplings, Tetrahedron Lett. 46 (2005) 2267. F. Trotta, K. Martina, B. Robaldo, A. Barge, G. Cravotto, Recent advances in the synthesis of cyclodextrin derivatives under microwaves and power ultrasound, J. Inclusion Phenom. Macrocyclic Chem. 57 (2007) 3. G. Cravotto, S. Di Carlo, M. Curini, V. Tumiatti, C. Rogerro, A new flow reactor for the treatment of polluted water with microwave and ultrasound, J. Chem. Technol. Biotechnol. 82 (2007) 205. Y. Zhang, M.A. Dube, D.D. Mclean, M. Kates, Biodiesel production from waste cooking oil. 1. Process design and technological assessment, Bioresour. Technol. 89 (2003) 1. B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soyabean oil, J. Am. Oil. Chem. Soc. 63 (1986) 1375. B. Freedman, E.H. Pryde, T.L. Mounts, Variables affecting the yields of fatty esters from transesterified vegetable oils, J. Am. Oil Soc. Chem. 61 (1984) 1638. P.R. Gogate, P.A. Tatake, P.M. Kanthale, A.B. Pandit, Mapping of sonochemical reactors: review, analysis and experimental verification, AIChE J. 48 (2002) 1542. C. Stavarache, M. Vinatoru, Y. Maeda, Ultrasonic versus silent methylation of vegetable oils, Ultrason. Sonochem. 13 (2006) 401. C. Stavarache, M. Vinatoru, Y. Maeda, Fatty acids methyl esters from vegetable oil by means of ultrasonic energy, Ultrason. Sonochem. 12 (2005) 367.