Cavitation intensification

An interesting way to retard catalyst deactivation is to expose the reaction mixture to ultrasound. Ultrasound treatment of the mixture creates local hot spots, which lead to the formation of cavitation bubbles. These cavitation bubbles bombard the solid, dirty surface leading to the removal of carbonaceous deposits [38]. The ultrasound source can be inside the reactor vessel (ultrasound stick) or ultrasound generators can be placed in contact with the wall of the reactor. Both designs work in practice, and the catalyst lifetime can be essentially prolonged, leading to process intensification. The effects of ultrasound are discussed in detail in a review article [39]. 

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 cavita-tional reactions [63]. However it should be noted that an optimum salt concentration can exist beyond which the cavitational activity decreases. Wall et al. [64] have reported that the optimum concentration is usually in the range 1-2 M and above this optimum, the sonoluminescence intensity was observed to reduce drastically. 

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 the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

Kelkar MA, Gogate PR, Pandit AB (2008) Intensification of esterification of acids for synthesis of biodiesel using acoustic and hydrodynamic cavitation. Ultrason Sonochem 15 188-194... 

Gogate PR (2008) Cavitational reactors for process Intensification of chemical processing applications A critical review. Chem Eng Proc 47 515-527... 

Intensification of different processes through a special organization of interaction of different oscillation or wave processes such as cavitational destruction, cleaning, emulgation of non-mixing liquids and of substances in the liquid phase, the development of different wave technologies. 

The paper gives an overview of effects occurring or acoustic treatment of dissolved and molten polymers. Emphasis is made on acoustic cavitation discovered recently not only in low-viscous fluids but also in molten polymers. Major guidelines have been specified for practical utilization of acoustic treatment of flowable polymers in molding intensification of extrusion processes, reduction in thickness of produced films, directed mechanical destruction, chemical activation of melts, etc. Efficiency of overlapping high-frequency vibrations in molding of molten thermoplastics is discussed in terms of power consumption. 

Moser, W. R., Giang, T., Nguyen, S., and Kozyuk, O. V., A new route to cavitational chemistry and chemical processing by controlled flow cavitation, in Process Intensification for the Chemical Industry, 3rd International Conference (A. Green, Ed.), pp. 38, 173, BHR Group, London (1999). 

In Great Britain, Crowford [2], Notlingk and Neppiras [24], Chalmers [25], and a number of other scientists have devoted their work to the ultrasound effect on metallurgical processes. Recently the investigations on intensification of various chemical and metallurgical processes under acoustic cavitation field are centered at Coventry University (Mason [26]). 

Table 3.8 Process intensification effects of cavitation based on sound and flow energy. Source adapted from Cogate [151].

It is important to cite the following critical issues while selecting the application of cavitating conditions for intensification of chemical processing applications [151] ... 

The 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 (Seymore and Gupta, 1997). 

The effect of process-intensifying parameters such as the presence of solid particles, dissolved salts and gases should be studied in details. The effect of scale of operation and different designs of the cavitational reactors on the expected intensification also needs to be evaluated. 

Cavitation generates conditions of high temperature and pressure along with the release of active radicals, which results in intensification of many of the physical and chemical transformations. The magnitudes of pressure and temperature and number of free radicals can be easily manipulated by adjusting the operating and... 

There are two possible further intensification of the cavitation activity ... 

Gogate, P Cavitational reactors for process intensification of chemical processing appUca-tions a critical review. Chem. Eng. Proc. 47, 515-527 (2008)... 

Gogate, P.R. (2007). Cavitational reactors for process intensification of chemical processing applications a critical review. Chemical Engineering Processing, doi 10.1016/j.cep. 2007.09.014.. 

---