Reactor hydrodynamic cavitation

Purposeful hydrodynamic cavitation in a chemical reactor can be created in two ways. One alternative is to let the hquid pass through a throttling valve, orifice 

Hydrodynamic cavitation reactors have been investigated for more than a decade now in the UDCT Department of Bombay University [63-66]. When applied to some industrially relevant reactions, the hydrodynamically created cavitation appeared to dehver on average an order of magnitude higher cavitation yields than the acoustic cavitation. In addition, the processing volumes could be up to about 100 times larger than in the conventional sonochemical reactors. So far, there is no information about the industrial applications of the hydrodynamic cavitation reactors, although some concepts have already been patented [67]. 

Hydrodynamic cavitation can also be generated in 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 controUing the intensity of cavitation is, however, much less in these types of reactors as compared to the reactors based on the use of a constriction in the flow. 

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Pandit and co-workers have shown that scale-up may be possible on a more rational basis if cavitation is employed, and some data have been reported by Pandit and Mohalkar (1996), Mohalkar et al. (1999), Senthil et al. (1999), and Cains et al. (1998). A variety of reactors can be used, viz. the liquid whistle reactor, the Branson sonochemical reactor, the Pote reactor, etc. The principal factors affecting the efficiency of a hydrodynamic cavitation reactor are irreversible loss in pressure head and turbulence and friction losses in the reaction rates. 

Moholkar et al. [11] studied the effect of operating parameters, viz. recovery pressure and time of recovery in the case of hydrodynamic cavitation reactors and the frequency and intensity of irradiation in the case of acoustic cavitation reactors, on the cavity behavior. From their study, it can be seen that the increase in the frequency of irradiation and reduction in the time of the pressure recovery result in an increment in the lifetime of the cavity, whereas amplitude of cavity oscillations increases with an increase in the intensity of ultrasonic irradiation and the recovery pressure and the rate of pressure recovery. Thus, it can be said that the intensity of ultrasound in the case of acoustic cavitation and the recovery pressure in the case of hydrodynamic cavitation are analogous to each other. Similarly, the frequency of the ultrasound and the time or rate of pressure recovery, are analogous to each other. Thus, it is clear that hydrodynamic cavitation can also be used for carrying out so called sonochemical transformations and the desired/sufficient cavitation intensities can be obtained using proper geometric and operating conditions. 

It is always important to choose an optimum design configuration of the hydrodynamic cavitation reactor so as to maximize the cavitational effects and achieve cost effective operation. In this section, we will discuss available reactor configurations and give some guidelines, based on theoretical analysis coupled with experimental results, for selection of optimum design and operating parameters for hydrodynamic cavitation reactors. 

From the above discussion about various hydrodynamic cavitation reactors, it can be easily concluded that the orifice plate set-up offers maximum flexibility and can also be operated at relatively larger scales of operation. It should be also noted that the scale-up of such reactors is relatively easier as the efficiency of the pump increases with an increase in size (flow rate and discharge pressure) which will necessarily result into higher energy efficiencies. 

Guidelines for Selection of Hydrodynamic Cavitation Reactor Configurations... 

Table 3.1 Optimum operating conditions for the hydrodynamic cavitation reactors No. Property Favorable conditions...

Ambulgekar et al. [30] investigated the oxidation of toluene using aqueous KMn04 as an oxidizing agent in the hydrodynamic cavitation reactor with an objective of optimization of the operating parameters. The reaction scheme can be depicted as follows ... 

Fig. 3.4 Schematic representation for experimental setup for the orifice plate hydrodynamic cavitation reactor...

No Reactants Product0 Cavitational yield in hydrodynamic cavitation reactor (g/J) Cavitational yield in acoustic cavitation reactor (g/J)... 

Sivakumar and Pandit [13] reported the use of hydrodynamic cavitation for decolorization of dye effluent stream. It has been observed that for same flow area, plate with larger number of small diameter holes gives higher extent of color removal as compared to the plate with smaller number of larger diameter holes. The observed results have been explained on the basis of frequency of turbulence for the two cases. The hydrodynamic cavitation reactor using multiple hole orifice plates has been found to give cavitational yields, which are two times higher than the best acoustic cavitation device (Dual frequency flow cell with capacity of 1.5 L). 

This fact is also confirmed by the discussion made earlier regarding the effect of various design parameters on the performance of hydrodynamic cavitation reactors. 

Gogate PR, Pandit AB (2001) Hydrodynamic cavitation reactors A state of the art review. Rev Chem Eng 17 1-85... 

Moholkar VS, Pandit AB (2001) Modeling of hydrodynamic cavitation reactors a unified approach. Chem Eng Sci 56 6295-6302... 

Kanthale PM, Gogate PR, Wilhelm AM, Pandit AB (2005) Dynamics of cavitational bubbles and design of a hydrodynamic cavitational reactor cluster approach. Ultrason Sonochem 12 441 -52... 

Sharma A, Gogate PR, Mahulkar A, Pandit AB (2008) Modeling of hydrodynamic cavitation reactors using orifice plates considering hydrodynamics and chemical reactions occurring in bubble. Chem Eng J 143 201-209... 

Sampathkumar K, Moholkar VS (2007) Conceptual design of a novel hydrodynamic cavitation reactor. Chem Eng Sci 62 2698-2711... 

Nano Calcium Carbonate Crystallization using New Hydrodynamic Cavitation Reactor Sonawane S, Mahajan C, Gumfekar S, Kate K, Meshram S, Kunte K, Ramajee L, Ashokkumar M (2010) Int J Chem Eng. Article ID 242963,1-8 doi 10.1155/2010/242963... 

Cavitation is not exclusive of ultrasound. Thus, hydrodynamic cavitation can simply arise from passage of the liquid through a constrictor such as a throttling valve, orifice plate, Venturi tube, etc. On passage through the constrictor, the kinetic energy — velocity — of the liquid increases at the expense of the pressure. Various types of hydrodynamic cavitation reactors have been reported and their most salient features reviewed [17]. Also, the effects of ultrasound and hydrodynamic cavitation on oxidation processes have recently been compared [18]. 

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