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HomeMy WebLinkAboutAppendix D- Cavitation A technology on the horizonRESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 35 *For correspondence. (e-mail: abp@udct.org) Cavitation: A technology on the horizon Parag R. Gogate1, Rajiv K. Tayal2 and Aniruddha B. Pandit1,* 1Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India 2Department of Science and Technology, Technology Bhavan, New Mehrauli Road, New Delhi 110 016, India An overview of the application of cavitation phenomenon for the intensification of chemical/physical processing applications has been presented here, discussing the causes for the observed enhancement and highlighting some of the typical examples. The important conside- rations required for efficient scale-up of the cavitatio- nal reactors and subsequent industrial applications have been depicted based on the work carried out as a result of sponsored projects at the Institute of Chemi- cal Technology, Mumbai. Overall, it appears that the combined efforts of physicists, chemists and chemical engineers are required to effectively use cavitational reactors for industrial applications. Some recommen- dations for further work to be carried out in this area have also been mentioned, which should allow the ex- ploitation of this technology on an industrial scale. Keywords: Acoustic cavitation, chemical processing, hydrodynamic cavitation, novel reactors, process intensi- fication. CAVITATION can be in general defined as the generation, subsequent growth and collapse of cavities resulting in very high energy densities of the order of 1 to 1018 kW/m3. Cavitation can occur at millions of locations in a reactor simultaneously and generate conditions of very high tem- peratures and pressures (few thousand atmospheres pres- sure and few thousand Kelvin temperature) locally, with the overall environment being that of ambient conditions. Thus, 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 vapours trapped in the cavitating bubbles, which results in either intensification of the chemical reactions or in the propagation of certain unexpected reactions. Cavitation also results in the gen- eration of local turbulence and liquid micro-circulation (acoustic streaming) in the reactor, enhancing the rates of transport processes. The four principle types of cavitation and their causes can be summarized as follows: Acoustic cavitation In this case, pressure variations in the liquid are effected using sound waves, usually ultrasound (16 kHz–100 MHz). The chemical changes associated with the cavitation indu- ced by the passage of sound waves are commonly termed as sonochemistry. Hydrodynamic cavitation Cavitation is produced by pressure variations, which is obtained using the geometry of the system creating velocity variation. For example, based on the geometry of the sys- tem, 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. Optic cavitation This is produced by photons of high intensity light (laser) rupturing the liquid continuum. Particle cavitation This is produced by a beam of elementary particles, e.g. a neutron beam rupturing a liquid, as in the case of a bubble chamber. Applications of cavitation phenomenon Among the various modes of generating cavitation given above, acoustic and hydrodynamic cavitations have been of academic and industrial interest due to the ease of operation and generation of the required intensities of cavitational conditions suitable for different physical and chemical transformations. It is worthwhile to overview different applications where cavitation can be used efficiently before discussing the design and scale-up aspects for these two types of cavitation in detail. Chemical synthesis In order to understand the way in which cavitational col- lapse can affect chemical transformations, one must con- sider 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 vapour of the liquid medium RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 36 or dissolved volatile reagents or gases. During the collapse, these vapours 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 their migration into the bulk liquid. Sec- ondly, the sudden collapse of the bubble also results in the rushing-in of the liquid to fill the void, producing shear forces in the surrounding bulk liquid capable of breaking the chemical bonds of many polymeric materials resulting into lower molecular weight polymeric forms, which are dissolved in the fluid or disturb the boundary layer facili- tating transport. The sonochemical activation in heterogeneous systems is mainly due to the mechanical effects of cavitation. In a heterogeneous solid/liquid system, the collapse of the cavitation bubble results in significant structural and mecha- nical defects. Collapse near the surface produces an asymmetrical rushing-in of the fluid to fill the void, form- ing a liquid jet targetted at the surface. This effect is equivalent to high-pressure/high-velocity liquid jets-based cutting and/or erosion and is the reason why ultrasound is used for cleaning solid surfaces. These jets activate the solid catalyst and increase mass transfer to the surface by disruption of the interfacial boundary layers as well as dislodging the material occupying the active sites. Col- lapse on the surface, particularly of powders, produces enough energy to cause fragmentation (even for finely divided metals). Thus, in this situation, ultrasound can in- crease the surface area for a reaction and provide addi- tional activation through efficient mixing and enhanced mass transport. 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 such emulsions are formed, the surface area available for the reaction between the two phases is significantly increased, thus increasing the rates of reaction. This is beneficial, particularly in the case of phase transfer-cata- lysed reactions or biphasic systems. The different ways in which cavitation can be used bene- ficially in the chemical processing applications are1–10: (a) Reaction time reduction. (b) Increase in the reaction yield. (c) Use of less forcing conditions (temperature and pres- sure) compared to the conventional routes. (d) Reduction in the induction period of the desired re- action. (e) Possible switching of the reaction pathways resulting in increased selectivity. (f) Increasing the effectiveness of the catalyst used in the reaction. (g) Initiation of the chemical reaction due to generation of highly reactive free radicals. Water and effluent treatment Cavitation can be used effectively for the destruction of contaminants in water because of the localized high con- centration of oxidizing species such as hydroxyl radicals and hydrogen peroxide, higher magnitudes of localized temperatures and pressures and formation of transient su- percritical water. The type of pollutants in the effluent stream affects the rates of the degradation process. Hydro- phobic compounds react with OH. and H. at the hydro- phobic 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, energy intensity and nature and prop- erties 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 scales11,12 are p-nitrophenol, rhodamine B, 1,1,1 trichloroethane, parathion, pentachlo- rophenate, phenol, CFC 11 and CFC 113, o-dichloroben- zene and dichloromethane, potassium iodide, sodium cyanide and carbon tetrachloride among many others. Biotechnology Cell disruption is one of the important and vital unit op- erations in biotechnology for the recovery of intracellular proteins. This is an energy-intensive operation and hence there exists tremendous scope for the development of cheaper and energy-efficient methods. Cavitation can be used effectively for the rupture of cells with energy re- quirements as less as just 5 to 10% of the total energy consumed using conventional methods13,14. The intensity of the cavitation phenomenon can also be controlled so as to control the mechanism of rupture of cells, to selectively release the intracellular enzymes or enzymes present in the cell wall15. Lower intensity application of cavitation helps in retaining the activity of the leached-out enzymes and also reduces the cost of operation. Hydrodynamic cavitation has been found to be much more energy-effi- cient compared to acoustic cavitation and at the same time applicable at a larger scale of operation13,14. Sonocrystallization Sonocrystallization can be used to impart a variety of desir- able characteristics to high-value products. Dow Chemical, USA is already using sonocrystallization for adipic acid crystallization, but it is a closely guarded secret. Impuri- ties have been reduced from 800 to less than 50 ppm. Ul- trasound can be used beneficially in several key areas of crystallization such as: – Initiation of primary nucleation, narrowing the meta- stable zone width. RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 37 – Secondary nucleation. – Crystal habit and perfection. – Reduced agglomeration. – A non-invasive alternative to the addition of seed cry- stal (seeding) in sterile environment. – Manipulation of crystal distribution by controlled nu- cleation. The formation of primary nuclei is a function of ultrasonic parameters such as frequency of oscillations, intensity of irradiation and physical properties of the liquid such as degree of supersaturation and operating parameters such as temperature. Atomization Atomization is the process of formation of small droplets. Energy is imparted to the liquid to form a large surface area. The conventional way to atomize the liquid is to force it at high velocity through a small aperture. In the two-phase atomization system, the high-velocity air imparts its energy to the liquid sheet breaking it into droplets. When liquid in the form of thin film is allowed to flow at the tip of the vibrating surface (frequency >20 kHz), the film breaks up into fine droplets. This is called as ultra- sonic atomization. There are two hypotheses that explain the mechanism of liquid disintegration during ultrasonic atomization – capillary wave hypothesis and cavitation hypothesis. The means of controlling droplet size is de- sirable in many industrial applications, and this will only be achieved by a close study of the mechanism of break- up of liquid sheets/ligaments. The droplet size in case of ultrasonic atomization is a function of: – Physico-chemical properties of the liquid such as sur- face tension, density, viscosity, including non-Newtonian behaviour. – Ultrasonic parameters such as frequency of oscillation and intensity of irradiation. – Operating parameters such as liquid flow rate, liquid loading on the atomizing surface area and shape of the liquid film due to the geometry of the surface. These just cover the broad spectrum of applications of cavitation; there are many other specific applications of both acoustic and hydrodynamic cavitation. Acoustic cavitation has been found to be beneficial in polymer chemistry applications for initiation of polymerization re- actions or for destruction of complex polymers16–18, solid– liquid extraction19 as well as in the petroleum industry for refining fossil fuels, in the determination of composition of coal extracts, extraction of coal tars20,21, and in textile industry for enhancing the efficacy of dyeing techni- ques22,23. Some of the miscellaneous applications of hydro- dynamic cavitation can be given as floatation cells24,25, synthesis of nanocrystalline materials26, preparation of high quality quartz sand27, preparation of free disperse system using liquid hydrocarbons28,29 and dental water irrigator30. A careful analysis of the existing literature shows that though the application of cavitation to physical/chemical processing has been explored worldwide for a good 50 years, the first report from an Indian group regarding the use of hydrodynamic cavitation for chemical/physical processing applications dates back only to 1993 (study on hydrolysis of fatty oils by Pandit and Joshi31). We now discuss the evolution of technology with specific reference to work carried out in India (more than 95% of the work is by the group at the Institute of Chemical Technology, Mumbai), highlighting the current status of information re- lated to applications of cavitation. Overview of different reactor configurations In this section, we discuss the various equipment configu- rations and their scales commonly available for carrying out the cavitation phenomenon. For better understanding specific configurations as available with our research group have been discussed. It should be noted that different variants of these configurations are available with varying operating parameters like the maximum power supplied to the reactor, frequency of irradiation, geometric arrange- ments of the transducers (in the case of sonochemical re- actors), geometric arrangement of the hole, capacity of the pump (in the case of hydrodynamic cavitation reactors). Sonochemical reactors Ultrasonic horn and bath A schematic diagram of these standard configurations has been depicted in Figure 1. The horn operates at a frequency of 22.7 kHz and a rated power dissipation of 240 W. Two different tips (irradiating surface area of 3.46 and 4.91 cm2) can be used so as to change the operating intensity of ir- radiation (defined as power dissipated per unit area of the horn). The capacity of the reactor generally ranges from 10 to 200 ml. Ultrasonic horn-type of configurations are generally suitable for laboratory-scale characterization/ feasibility experiments, as they provide intense but local cavitation. The ultrasonic bath has a fixed operating frequency of 22 kHz (one can get these at different operating frequen- cies) with a rated output power of 120 W (again variable power dissipation is possible). Three transducers are placed at the bottom of the reactor in a triangular pitch. The maximum operating capacity of the ultrasonic bath is about 3 l, but in general baths with higher capacity (up to 1000 l) can be used with some modification in terms of larger number of transducers in different configurations. RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 38 Again, ultrasonic bath-type of reactors are suitable for laboratory to pilot-scale operations, as there is a limitation on the number of transducers that can be incorporated in the system for a large-scale operation. Dual-frequency flow cell The flow cell (Figure 2) consists of a rectangular vessel with a diameter of 9.5 cm and height of 20 cm (1.5 l capa- city to hold the reacting mixture), with two sets of trans- ducers (three in each set) mounted on the two opposite faces. Transducers operating (independently or simulta- neously) at different frequencies, i.e. 25 and 40 kHz and having equal power rating of 120 W per set have been provided. The flow cell can be operated in a batch or con- tinuous mode. Reaction mixture Generator Ultrasonic horn Reaction mixture Ultrasonic bath Transducers Figure 1. Schematic representation of ultrasonic horn and ultrasonic bath. Thermocouple Cooling water-in Cooling water-out Cooling pipe 25 kHz 40 kHz Transducers Figure 2. Schematic representation of dual-frequency flow cell. Triple-frequency flow cell The hexagonal triple-frequency flow cell has a total capa- city of 7.5 l and can be operated in batch as well as con- tinuous mode. Transducers (three in each set per side) having equal power rating of 150 W per side have been mounted (thus the total power dissipation is 900 W when all the transducers with combination of 20 + 30 + 50 kHz frequencies are functional). The two opposite faces of the flow cell have a similar irradiating frequency. The operat- ing frequency of transducers is 20, 30 and 50 kHz and can be operated in different combinations (seven in total), ei- ther individually or in combined mode. Schematic repre- sentation of hexagonal flow cell is given in Figure 3. Ultrasonic bath with longitudinal vibrations The reactor is irradiated using a single longitudinally vibra- ting transducer (vibrations are away from the bottom of the reactor) kept at the bottom of the reactor. The advan- tage of such a configuration is that due to large area of the irradiating surface, the active cavitational volume in the reactor is higher resulting in better cavitational yields through a large number of cavities. The schematic repre- sentation of the reactor is depicted in Figure 4. The inner cross-section of the reactor has dimensions of 15 cm ´ 33 cm ´ 20 cm with a total holding capacity of 8 l (some part of the reactor is occupied by the transducer). There is provision for a drain as well as an outlet at the top, which facilitates continuous operation. An additional heater with a temperature controller has been provided so as to facili- tate high temperature reactions. The operating frequency of irradiation is 36 kHz and maximum power dissipation Ultrasound T1 T2 T3T5 T6 Quartz tube transducers Quartz tube Reactant T4 Hexagonal reactor with 10 cm sides. The central quartz tube can be used for simultaneous irradiation with UV light. Figure 3. Schematic representation of triple-frequency hexagonal flow cell. RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 39 into the system is 150 W (operation with varying power dissipation is also possible). Hydrodynamic cavitation reactors High-pressure homogenizer The high-pressure homogenizer (APV Gualin GmbH model) is basically a high-pressure positive displacement pump with a throttling device. This homogenizer operates according to the principle of high-pressure relief techni- que. The reactor configuration generally consists of a feed tank and multiple throttling valves designated as multiple stages of throttling. In the configuration available with our research group, the liquid from the feed tank (capacity of 1500 ml) is driven by a pump to the first-stage valve. Pressure up to 1000 psi (6.895 ´ 106 N/m2) can be attained by throttling this valve. Further increase in pres- sure is achieved using the second-stage valve. Upstream pressure up to 10000 psi (6.895 ´ 107 N/m2) can be ob- tained in the second stage. From the second stage valve, the liquid is recirculated back to the feed tank. The cavi- tating conditions are generated just after the second-stage throttling valve. When the liquid is suddenly released from the second-stage, evaporation takes place giving rise to cavities/bubbles. The cavitation intensity will be depen- dent on the magnitude of upstream pressure and also on the type of valve at the second stage. With an increase in the throttling pressure, there is a rise in the temperature of the liquid. To maintain the temperature at ambient conditions, a coil immersed in the feed tank can be used for circulation of the cooling water. High-pressure homo- genizers are especially suitable for emulsification processes in industries like food, chemical, pharmaceutical and bio- chemical and are available at industrial scales, though the configurations may not be the most ideal for these equipment to operate in optimized cavitating conditions. Ultrasonic generator Horn with longitudinal vibrations 24 cm 6 cm 5 cm3 cm Drain Outlet 15 cm 33 cm Drain is at 8 cm height from the bottom of reactor whereas the outlet is at a height of 32 cm from the bottom so as to facilitate continuous operation Figure 4. Schematic representation of ultrasonic bath equipped with longitudinally vibrating horn. High-speed homogenizer The high-speed homogenizer consists of an impeller (ro- tor) and a stator, which are made up of stainless steel. The impeller is driven by a variable voltage motor (the limit permitted for the homogenizer is 30 V or 3.5 A, re- sulting into a maximum rotational speed of 16000 rpm). The distance between the impeller and stator can be varied using different geometric configurations of the impeller and stator, and this distance along with the operating para- meters decides the cavitational zone. Various combinations are available with our group, e.g. a combination having an impeller with nine blades, while the stator has 13 blades. The impeller blades are 6 mm apart whereas the distance between two stator blades is 6 mm. The distance between the OD of the impeller blade and the ID of the sta- tor blade can be varied over a range of 0.5 to 2 mm. A plate with holes attached to the stator top has been pro- vided, which can be used for inserting baffles so as to avoid vortex formation and surface aeration that de- creases the intensity of cavitation. The cavitating condi- tions are generated after the liquid passes through the stator–rotor assembly, according to principle similar to the orifice plates set-up, which is described later. As one increases the rotor speed, the liquid velocities generated increase and beyond a certain speed defined as the critical inception speed for the cavitation, cavities are formed due to the fact that local pressure falls below the vapour pres- sure of the medium. A re-circulating loop similar to the one depicted for the case of high-pressure homogenizer can be used for operation with high-speed homogenizers. Orifice plates set-up The set-up consists of a closed loop fluid circuit comprising a holding tank, a centrifugal pump, control valve and flanges to accommodate the orifice plates, as shown in Figure 5. The suction side of the pump is connected to the bottom of the tank. Discharge from the pump branches P1 P2 BYE PASS LINE TANK P1, P2 – Pressure gauges V1 V2 V3 V1, V2, V3 – Control valves Centrifugal pump CW out CW in Figure 5. Schematic representation of orifice plate set-up. RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 40 into two lines, which help in the control of inlet pressure and inlet flow rate into the main line housing the orifice with the help of valves V2 and V3. The main line consists of a flange to accommodate the orifice plates (single or multiple holes); different configurations of the orifice plates have been shown in Figure 6, along with a hard glass tube next to these plates to make visual observation. The cavitating conditions are generated just after the ori- fice plates in the main line and hence the intensity of the cavitating conditions strongly depends on the geometry of the orifice plate. When the liquid passes through the ori- fice plates, the velocities at the orifice increase due to the sudden reduction in the area offered for the flow, result- ing in a decrease in the pressure. If the velocities are such that their increase is sufficient to allow the local pressure to go below the medium vapour pressure under operating conditions, cavities are formed. Such cavities are formed at a number of locations in the reactor, which also depends strongly on the number of holes in the orifice plates. At the downstream of the orifice, however, due to an increase in the area of cross-section, the velocities decrease giving rise to increasing pressures and pressure fluctuations, which control the different stages of cavitation, namely formation, growth and collapse. The holding tank is pro- vided with a cooling jacket to control the temperature of the circulating liquid. The inlet pressure and the fully re- covered downstream pressure can be measured with the pressure gauges P1 and P2 respectively. Comparison of cavitational reactors After giving an insight into the different aspects of the reactor configurations, we now discuss the comparison of Plate 1 Plate2 Plate 6 Plate 4 Plate 3 Plate 5 Figure 6. Multiple-hole orifice plates having different combinations of number and diameter of holes. different cavitational reactors to provide recommendations for selection of reactor type to carry out specific cavita- tional transformation. Energy efficiency calculations Calorimetric method is generally used to determine the energy efficiency of the equipment. In this method, the rise in temperature of a fixed quantity of water in an insu- lated container over a given time is measured. Using this information, the actual energy (power) dissipated into a liquid can be calculated from the following equation: Power (watt) = mCp (dT/dt), (1) where Cp is the heat capacity of the solvent (J kg–1 K–1), m the mass of solvent (kg), dT the difference between the initial and final temperature after a specific reaction time (K), and dt is time (s). Energy efficiency can be then cal- culated as follows: Energy efficiency = Power dissipated in the liquid/ electric power supplied to the system. Energy efficiency gives an indication of the quantity of energy effectively dissipated in the system, a fraction of which is actually utilized for the generation of cavities. Thus both should be as high as possible for the particular equipment. Cavitational yield calculations The cavitational yield of an acoustic equipment indicates the ability of the equipment to produce the desired cavitatio- nal change based on the total electric energy supplied to the equipment. Details about the estimation of cavitational yield have been discussed in detail in the earlier sections. Important results In all cavitational equipments, the primary form of energy supplied as input is electrical energy. This energy goes through various changed forms, i.e. pressure, velocity, vibrations, etc. before it is used to generate cavitation, re- sulting in the desired chemical change. In most cases, the electrical energy is converted to mechanical energy (vi- bratory motion of the transducer in acoustic equipments and liquid flow with pressure in hydrodynamic cavitation equipments). The mechanical energy is used to generate cavitation and finally the violent collapse of generated cavities dissipates this diffused form of energy by con- centrating it through a number of cavitational events and induces chemical reactions. In each case, there is a conti- nuous loss of energy during its transformation from one RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 41 form to another and hence the comparison of the energy efficiency can aid in understanding the productive energy utilization of various equipments for the desired cavita- tional activity. The results of the study for energy efficiency as obtained in various cavitational equipment described earlier are given in Figure 7 a. It can be observed that amongst sono- chemical reactors, the triple-frequency flow cell is the most energy efficient (energy efficiency of about 75%) due to uniform energy dissipation over a wider area and through multiple transducers (three each on six sides of the hexagonal cross-section), rather than concentrated en- ergy dissipation in the horn (energy efficiency <10%). Efficiency of the dual-frequency flow cell (56%) was also found to be equally good, due to the fact that the energy is dissipated over a wider area and through multiple transducers. The ultrasonic bath which has three trans- ducers, attached at the bottom of the reactor gives an energy efficiency of about 20%, twice compared to ultrasonic horn. Thus multiple transducer irradiation is the key for Figure 7. Comparison of different cavitational equipments in terms of energy efficiency (a) and cavitational yield (b). Numbers at the top of the columns represent the exact values obtained for the particular dataset. maximizing the transfer of energy into the system for sonochemical reactors. Amongst the hydrodynamic cavi- tation equipments, high speed and high pressure homogeniz- ers (typically laboratory-scale equipment with capacity of 1.5 and 2 l respectively) have energy efficiency of 43 and 54% respectively. The orifice type of hydrodynamic cavi- tation reactor having a capacity of 50 l (typically a pilot- plant scale) has an observed energy efficiency of 60%. Conventionally speaking, hydrodynamic cavitation equip- ment are more energy-efficient compared to the acoustic counterparts (except for the multiple-frequency flow cells), though the exact cavitational effects may or may not fol- low similar trends, as the fraction of this energy utilized for the cavitational activity is different. Figure 7 b also gives the values of cavitational yields obtained for different equipment for the Weissler reaction. It can be seen from Figure 7 b that amongst sonochemical reactors, triple-frequency flow cell gives an order of magnitude higher cavitational yield as compared to all the other equipment. Dual-frequency flow cell also gives 20% higher yield compared to ultrasonic bath, whereas the cavitational yield obtained in the case of single trans- ducer-based ultrasonics is the lowest (two orders of magni- tude lower). Similar results were also obtained for another reaction (degradation of formic acid), which requires sig- nificantly higher cavitational activity compared to the Weissler reaction, indicating that the intensity requirements for a particular reaction do not necessarily affect the trends. Thus multiple transducer irradiation also results in an enhancement in the cavitational yield and these types of reactors should play a key role in the design of indus- trial-scale reactors. Another feature of the flow cells used in the present work is the possibility of continuous opera- tion, which is a key requirement for the industrial-scale operation. It should also be noted at this stage that the tri- ple-frequency flow cell as well as the dual-frequency flow cell have a flexibility of multiple frequency operation (for comparison purposes in the present work, only single frequency irradiation has been considered). At this stage, it is not possible to make any recommendation about the applicability of multiple frequency irradiations for process in- tensification and synergistic effects, but the results are found to be dependent on the type of reaction32. It can be also seen from Figure 7 b that the desired chemical change is an order of magnitude higher for a given amount of electrical energy supplied to the system for the hydrodynamic cavitation reactors compared to the sono- chemical reactors. However, it should be noted that the comparison made here is valid only for a model reaction (decomposition of potassium iodide) and the efficiencies of the various equipment may or may not be the same for a variety of cavitational transformations and also other applications. To get more insight into the relative efficacies of the hydrodynamic and acoustic cavitation reactors, we now discuss some chemical synthesis applications of cavitational reactors. RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 42 Table 1. Comparative results for different industrially important reactions in hydrodynamica and acousticb cavitation reactors Cavitational yield in hydrodynamic Cavitational yield in acoustic Reactant Productc cavitation reactor (g/J) cavitation reactor (g/J) Toluene Benzoic acid 3.3 ´ 10–6 5.6 ´ 10–7 p-Xylene Terephthalic acid 2.1 ´ 10–6 3 ´ 10–7 o-Xylene Phthalic acid 1.9 ´ 10–6 3 ´ 10–7 m-Xylened Isophthalic acid 1.9 ´ 10–6 – Mesitylene Trimesic acid 7 ´ 10–6 1 ´ 10–7 o-Nitrotoluene o-Nitrobenzoic acid 1.9 ´ 10–6 1 ´ 10–7 m-Nitrotoluene m-Nitrobenzoic acid 1.3 ´ 10–6 1 ´ 10–7 p-Nitrotoluenee p-Nitrobenzoic acid – 3 ´ 10–7 o-Chlorotoluene o-Chlorobenzoic acid 1.1 ´ 10–6 1 ´ 10–7 p-Chlorotoluenef p-Chlorobenzoic acid 2 ´ 10–6 – Sunflower oil Bio-diesel (methyl ester of sunflower oil) 2.1 ´ 10–6* 5.1 ´ 10–7* aToluene (1 mol), (o-/p-/m)-xylene (0.5 mol), mesitylene (0.4 mol), (o-/m)-nitrotoluene (1 mol) and (o-/p)-chlorotoluene (1 mol), sunflower oil (1 mol) with excess of methanol, KMnO4 for all reactions except for sunflower oil (2 mol), and for all above reactions, water (5 l), pressure 3 kg/cm2, orifice plate no. 1. Time = 5 h, except for oxidation of toluene, where it is 3 h and trans-esterification, where it is 30 min. bToluene (10 mmol), (o-/p)-xylene (5 mmol), mesitylene (4 mmol), (o-/m-/p)-nitrotoluene (10 m mol) and o-chlorotoluene (10 mmol), sunflower oil (1 mol) with excess of methanol, KMnO4 for all reactions except for sunflower oil (20 mmol), and for all above reactions, water (50 ml). Time = 5 h except for oxidation of toluene where it is 3 h and trans-esterification, where it is 15 min. cIdentification of compounds was done by TLC and melting point. d,fNot used in acoustic cavitation. eThis compound is not used in hydrodynamic cavitation. *In mol/J. Different chemical reactions (oxidation of toluene, (o-/p- /m)-xylenes, mesitylene, (o-/m)-nitrotoluenes and (o-/p)- chlorotoluenes and trans-esterification reaction) have been carried out in 10-l capacity orifice plate hydrodyna- mic cavitation reactor (under optimized conditions, viz. inlet pressure 3 kg/cm2, 0.4 mol/l of the oxidant – beyond these values, the increase in the cavitational yield is only marginal – and orifice plate with more number of holes) and also in conventional sonochemical reactor (ultrasonic bath, reaction volume 55 ml, power dissipation 120 W and operating frequency 20 kHz). For comparison purpose, cavitational yield has been used, which is defined as the quantity of product formed per unit of supplied energy. Table 1 shows the values of cavitational yields obtained for all the reactions in hydrodynamic and acoustic cavitation reactors (the specific operating conditions are mentioned as footnote for Table 1). It can be seen from Table 1 that the cavitational yield values in the hydrodynamic cavitation reactors are an order of magnitude higher for all the reac- tions considered in the work. Also the processing volume is about 100 times more compared to the conventional sonochemical reactor. The results have conclusively proved the better efficacy of the hydrodynamic cavitation reactors compared to the ultrasonic bath reactor consid- ered in the work. Similar results have been also obtained earlier (degradation of potassium iodide33, destruction of p-nitrophenol34 and microbial cell disruption13,14). What can cavitation achieve? After looking into the general applications of cavitation, in this section we will discuss specific industrially impor- tant applications. Transesterification of vegetable oils using alcohol Various products derived from vegetable oils have been proposed as an alternative fuel for diesel engines; crude vegetable oil, a mixture of vegetable oil with petroleum diesel fuel and alcohol esters of vegetable oil appear to be the most promising alternative. Today ‘bio-diesel’ is the term applied to esters of simple alkyl fatty acids used as an alternative to petroleum-based diesel fuels. Vegetable oil could be used as bio-diesel, but its proper- ties (high viscosity leads to poor atomization of the fuel, incomplete combustion and fuel injector fouling) are not perfectly suitable for diesel engines. Esters of fatty acids with alcohols have been prepared by the transesterfication of triglycerides in the presence of a catalyst to produce al- cohol esters of fatty acids (bio-diesel fuel) and glycerine as a by-product. Bio-diesel fuels have advantages over petroleum diesel fuel as they produce less carbon monoxide, smoke and particles, have higher cetane number, are bio- degradable, non-toxic and have lower viscosity than vegetable oils. The transesterfication reaction of vegetable oils using base catalyst and short-chain alcohols was studied in the presence of hydrodynamic cavitation and compared with the results of the reaction under acoustic cavitation in terms of their energy utilization. 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 used as bio- diesel fuel. This is a rapid technique for preparing alkyl esters from triglycerides at pilot plant-scale of operation. Various parameters, such as ratio of vegetable oil to alco- RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 43 Table 2. Transesterfication of different vegetable oils Time (in min) Yield (%) Vegetable oil Product Acoustic Hydrodynamic Acoustic Hydrodynamic Soyabean oil Soyabean oil ester 15 15 97 98 Castor oil Castor oil ester 10 10 99 99 Peanut oil Peanut oil ester 10 10 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. Table 3. Comparison of energy efficiency for different techniques Technique Time (min) Yield (%) Yield/kJ of energy Acoustic+ 10 99 8.6 ´ 10–5 Hydrodynamic* 15 98 3.37 ´ 10–3 Conventional** with stirring 180 98 2.27 ´ 10–5 under reflux 15 98 7.69 ´ 10–6 +In acoustic cavitation, 4 ml of methanol is mixed with 4 g of vegetable oil and catalyst concentration (NaOH) used is 0.5% of oil. Ultrasonic bath is the sonochemical reactor with 20 kHz frequency and 85 W as power dissipation. *Operation with hydrodynamic cavitation is under optimized condi- tions: 4 : 4 ratio (w/v) of oil to alcohol, catalyst concentration (NaOH) is 1% of oil. Orifice plate 1 has 16 holes with 2 mm diameter. Volume of methanol is 4000 ml, with 4000 g of oil. **For conventional approach, 4 ml of methanol is mixed with 4 g of vegetable oil and catalyst concentration (NaOH) used is 0.5% of oil (Case I: a stirrer is used for uniform mixing which consumes energy. Case II: a heater is used for maintaining reflux conditions). hol, type of catalyst, ratio of catalyst to oil and cavitation parameters such as geometry of orifice plate were optimi- zed. The results obtained for a variety of oils as starting material are shown in Table 2. It can be seen from Table 2 that cavitation can be successfully applied to transesterifi- cation reactions with more than 90% yield of the product according to stoichiometry in as low as 15 min of the re- action time. The technique hence appears to be effective compared to the conventional approach, which is also evi- dent from the comparison of different techniques based on quantitative criteria of energy efficiency, as shown in Table 3. It can be seen from Table 3 that hydrodynamic cavitation is about 40 times more efficient compared to acoustic cavitation and 160 to 400 times more efficient com- pared to the conventional agitation/heating/refluxing method. Oxidation of sulphide to sulphone with 30% H2O2 Oxidation of sulphides is the most straightforward method for synthesis of sulphones, which are important as speci- ality chemicals and in some cases, as agrochemicals, pharmaceuticals, lubricants, etc. Hydrogen peroxide is used as an oxidant mainly because of its effective oxygen content, produces only water by side reaction, safety in storage and transportation. In this case, the effect of cavi- tation on the reaction has been studied. Various parame- ters affected by cavitation have been considered in this study. All the reactions have been carried out under acoustic cavitating conditions. Methyl phenyl sulphide (thioanisole) was used as a model substrate. Different types of catalysts have been used, such as sodium tungstate and ammonium moly- bdate. This reaction was carried out at room temperature (28°C). The yield of sulphone was 5–6 times higher un- der ultrasonic irradiation compared to just agitation. With sodium tungstate as catalyst, different parameters such as molar ratio of MPS : H2O2, catalyst loading, solvent and reactant concentration have been studied. The yield of sulphone increased with increase in molar ratio, i.e. higher H2O2 concentration. The yield of sulphone also in- creased with catalyst loading, but there was no formation of sulphone as well as sulphoxide in the absence of catalyst for 3 h. With the polar solvents, i.e. methanol and etha- nol, the rate of sulphide conversion is higher and as the vapour pressure of the solvent increases, the yield also increases. This may be due to more number of cavitating species and higher exposure of reactants to cavitating conditions. In the case of non-polar solvents, the yield is higher in acetonitrile. This may be due to its miscibility with water, which forms a homogeneous phase. It is obser- ved that the cavitating conditions are severe in water. With an increase in sulphide concentration, the yield also increases. Esterification of fatty acids Methyl esters have applications in making fatty alcohols, which can be used in making plasticizers, lubricants, ag- ricultural chemicals, detergents, emulsifiers, antioxidants, cosmetics and pharmaceutical products. Other intermedi- ates made from methyl esters are used to make soaps, shampoos, germicides and antifoaming agents. They are also used in aluminum rolling and as a synthetic flavouring agent in foods. The esterification of C8–C10 fatty acid (FA) cut with methanol using concentrated H2SO4 as catalyst has been carried out with different molar ratios of FA to methanol under hydrodynamic cavitation. The kinetic parameters of this reaction have been estimated. It is observed that the RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 44 order of the reaction is zero with respect to FA concentration. To achieve nearly complete conversion to methyl esters, the different parameters such as catalyst concentration, and molar ratio of FA to methanol have been optimized. These reactions have been carried out under ultrasonic cavitation. The kinetic parameters and energy efficiencies of these runs have also been estimated. The reaction be- tween FA and methanol has also been carried out with superacid clay (solid, heterogeneous catalyst) as a catalyst under ultrasonic cavitation. With 2 wt% catalyst, almost complete conversion was obtained. The kinetic parame- ters of these reactions have been estimated. The wax esters have potential applications as premium lubricants, parting agents and antifoaming agents as well as in cosmetics, pharmaceuticals and food additives. The reaction between FA cut (C8–C10 fatty acid) and fatty alcohol (C12–C14) with conc. H2SO4 as a catalyst, has been carried out using hydrodynamic cavitation reactor. The objective of this reaction was to produce wax esters. A comparison between hydrodynamic cavitation and ultrasonic cavitation has been studied for this reaction on the basis of cavita- tional yield. The kinetic parameters and energy efficien- cies have also been estimated. Ultrasonic synthesis of benzaldehyde from benzyl alcohol using H2O2 An industrially important organic reaction, namely synthesis of benzaldehyde from benzyl alcohol using ultrasound has been selected for investigation as our next system. Hydrogen peroxide, being an environmentally clean oxi- dant, is selected as an oxidizing agent. The oxidation reaction has been carried out in the presence of a phase transfer catalyst (PTC) and a co-catalyst to form the active peroxo complex. PTC is used for facilitating the transfer of active peroxo complexes, which are present in the aqueous phase to the organic phase. The use of ultrasound is expected to en- hance the rate of reaction by various mechanisms: 1. It can result into emulsions of very small sizes increasing the surface area available for the reaction. 2. Cavitation results into a faster decomposition of hy- drogen peroxide into active oxygen, thereby increas- ing the rate of formation of active peroxo complexes. The effect of various parameters like amount of co-cata- lyst, amount of catalyst, temperature, presence and absence of agitation, speed of agitation, concentration of substrate, concentration of oxidizing agent and the effect of pH were studied. The analysis was done using gas chromatog- raphy. From this detailed study of ultrasonic oxidation of benzyl alcohol with hydrogen peroxide in the presence of commercially available homogeneous catalyst and phase transfer catalyst, the following conclusions can be arrived at: 1. It is possible to oxidize benzyl alcohol to benzaldehyde at room temperature in the presence of ultrasound. 2. The rate and selectivity of benzaldehyde formation is more in the presence of ultrasound than in the pres- ence of mechanical agitation alone. 3. The presence of PTC and catalyst is absolutely neces- sary. Thus ultrasound cannot replace PTC and cata- lyst, but it increases the effectiveness of the catalyst. 4. The physical effect of ultrasound, viz. microagitation caused due to intense cavitational collapse, is the con- trolling mechanism for the observed intensification of the oxidation process. 5. Less concentrated reagents are required when reac- tions are carried out in the presence of ultrasound than in the presence of mechanical agitation. Synthesis of benzonitrile Enhancement of reaction rate by combining the beneficial effects of PTC and ultrasound has been explored by con- sidering the transformation of benzamide by dehydration to give benzonitrile as a model system. It was found that there is a substantial reduction in the reaction time when ultrasound was coupled with PTC. The important results obtained in the study are shown in Table 4. The observed increase in the rates of reaction can be attributed to the fact that ultrasound plays a dual role in creating higher interfacial area as well as facilitating the process of inter- facial transport. Physical processes Crystallization: Ultrasonic irradiation (Dakshin horn operating at 22 kHz and rated power of 240 W) was em- ployed during the partial crystallization of diphenyl oxide, dimethyl phenyl carbinol from its respective crude melts. The important results obtained in the study are given in Table 5. It can be seen from Table 5 that the crystals obtained during both stages of sonication (nucleation and growth) Table 4. Typical results obtained for synthesis of benzonitrile Condition Time (min) Yield (%) With PTC Only stirring 120 84.76 Stirring + ultrasound 5 29.43 Stirring + ultrasound 10 45.90 Stirring + ultrasound 15 52.85 Stirring + ultrasound 20 80.12 Only ultrasound 10 45.22 Only ultrasound 20 84.61 Without PTC Stirring + ultrasound 60 0 RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 45 Table 5. Results for application of cavitation to crystallization operation Observed melting Crystal sample point (°C) Comments Typical results for diphenyl oxide: Observed melting point = 26.8–28°C Without sonication 24–28 About 55% crystals melt before 27°C. Balance 45% melts between 27 and 28°C. Sonication during nucleation stage 25–28 About 35% crystals melt between 25 and 27°C. Balance 65% melts between 27 and 28°C. Sonication during nucleation and crystal growth 25–28 About 15% crystals melt between 25 and 27°C. About 85% crystals melts between 27 and 28°C. Typical results for dimethyl phenyl carbinol: Observed melting point = 35–37°C Without sonication 21–26 50% crystals melt between 21 and 24°C. Balance melts between 24 and 26°C. 15 s sonication 23–32 50% crystals melt between 23 and 25°C. Balance melts between 24 and 26°C. 2 min sonication 24–33 50% crystals melt between 24 and 26.5°C. Balance melts between 26.5 and 33°C. 5 min sonication 29–34.5 50% crystals melt between 29 and 31°C. Balance melts between 31and 34.5°C. showed considerable improvement in purity (as assessed by increase in melting point). Microscopic observation indicated that the crystals obtained with the use of ultra- sound were thin, long, needle-shaped and of much smaller size (40 to 60% smaller) compared to those obtained in the absence of ultrasound. Thus ultrasound during cry- stallization is beneficial for improving the quality and pu- rity of the compounds. Scope for future work Combination of hydrodynamic cavitation reactors and sono- chemical reactors, where the cavity is generated using hydrodynamic means and, the collapse of the cavities is achieved in the sonochemical reactor. The distance be- tween the two events (generation and collapse) will be a crucial aspect in the expected synergism and should be established with theoretical simulations and/or experi- mental validation for a particular application. The devel- oped reactor should be operated in a continuous mode and needs to be tested for different reactions. The effect of process intensifying parameters such as the presence of solid particles and bubbles should be studied in detail for small-scale reactors such as the ultra- sonic horn. Its efficacy cannot be underestimated as in- tense cavitation is produced and it may be useful in the chemical synthesis of novel materials and also for laboratory- scale characterization of acoustic cavitation phenomenon. Concluding remarks Acoustic cavitation reactors generate much more intense cavitation and it appears at a first glance that one should go in for sonochemical reactors. However, it should be noted that these reactors are associated with scale-up problems and information in a variety of fields is required for an efficient design of large-scale equipment, which is not readily available. Moreover, hydrodynamic cavitation re- actors offer versatility in terms of operation and condi- tions similar to acoustic cavitation can be generated more efficiently. The scale-up of hydrodynamic cavitation units is much easier as knowledge regarding the hydrodynamic conditions existing downstream of the orifice is easily available in the literature or can be obtained with the help of modern CFD codes. Centrifugal pumps offer higher operating energy efficiency at larger scales of operation. Overall, cavitation can be effectively applied for a variety of physical/chemical transformations, including chemical synthesis, biotechnology, environmental engineering, polymer engineering, etc. Also, the rates of transforma- tion are at times, order of magnitude higher compared to the conventional approach, and energy consumption is relatively less. At this stage of development of sono- chemistry/cavitation, it seems that there are some techni- cal, economical limitations and practically no processing on an industrial scale is being carried out, though some efforts have been made with success in pilot-scale appli- cation of cavitational reactors by few research groups, in- cluding one at the Institute of Chemical Technology. More insight into intensification studies using process in- tensifying parameters and/or a combination of different reactor configurations/processes based on the guidelines established in the present report should help in achieving the goal of industrial-scale application. Undoubtedly, com- bined efforts of chemists, physicists, chemical engineers and equipment manufacturers, according to the guidelines provided here, will be required for the Chemical Process Industry to harness cavitation as a viable option for process intensification. RESEARCH ACCOUNT CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006 46 1. Ando, T., Sumi, S., Kawate, T., Ichihara, J. and Terukiyp, H., Sonochemical switching of reaction pathways in solid–liquid two phase reactions. J. Chem. Soc. Chem., Commun., 1984, 45, 439– 440. 2. Javed, T., Mason, T. J., Phull, S. S., Baker, N. 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Fluids Eng., 2000, 122, 465–470. ACKNOWLEDGEMENTS. We thank the Department of Science and Technology, New Delhi for sponsoring four research projects in the In- stitute of Chemical Technology, Mumbai since 1992 till date. The work presented here has been mainly due to continuing support from DST and has helped in developing an understanding of the new technology and its industrial exploitation seems to be a reality in the near future. Received 29 November 2005; revised accepted 19 April 2006