Ultrasound enhancement factor

Figure 8.1.23 depicts the ultrasound enhancement factor Eus as a function of the surface activation degree with parameter f2. The circles represent measured values for Eus and . In the case of magnesium as the treated sohd with tetrahy-drofurane as solvent and different chlorobutanes as reactants a mechanical activation degree of approximately 10% has been observed. This means that 10% of the necessary activation energy has been delivered by the mechanical treatment by impinging liquid microjets. This is a typical value that can be found in other devices like reaction mills, too. This value of 4 to 6kJ/mol for mechanically treated... 

Figure 8.1.23 Ultrasound enhancement factor Bus as a function of the surface activation degree <3> with parameter describing the fraction of mechanical stored energy compared to the thermal activation energy.

Figure 8.1.24 Ultrasound enhancement factor activation are /to = 11 m/s for chlorobenzene, us as a function of temperature for three ko = 314 m/s for 2-chlorobutane, and ko = different reactions components. Lines 8800m/s for 2-chloro-2-methylpropane. The...

As in any solid-liquid reaction, when the solid is sparingly soluble, reaction occurs within the solid by diffusion of the liquid-phase reactant into it across the liquid film surrounding the solid. Thus two diffusion parameters are operative, the solid-liquid mass transfer coefficient sl and the effective diffusivity D. of the reactant in the solid. A reaction in the solid can occur by any of several mechanisms. The simpler and more common of these were briefly explained in Chapter 15. For reactions following the sharp interface model, ultrasound can enhance either or both these constants. Indeed, in a typical solid-liquid reaction such as the synthesis of dibenzyl sulfide from benzyl chloride and sodium sulfide ultrasound enhances SL by a factor of 2 and by a factor of 3.3 (Hagenson and Doraiswamy, 1998). Similar enhancement in was found for a Michael addition reaction (Ratoarinoro et al., 1995) and for another mass transfer-limited reaction (Worsley and Mills, 1996). 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

Since we still observed increased rates in the absence of inhibitor and in the presence of ultrasound (Fig. 5.35) we have explained the reduction or lack of an induction period in the presence of ultrasound in terms of two factors namely, a greater radical production both from enhanced initiator breakdown [74], and/or degradation of the polymer, and also from the creation of a far more stable emulsion. This latter point was confirmed visually. Depending upon the irradiation power and surfactant level employed, we were able to observe up to 40 % increase in initiator breakdown and a de-... 

These are fundamental considerations and are of interest not just to electrochemists and sonochemists, but care must be taken in correctly attributing an apparent shift in an experimentally observed potential under ultrasound. As already mentioned, system parameters and other factors may influence an observation beyond the effect under investigation. Thus there have been reports on the use of the titanium tip of the sonic horn itself, suitably electrically insulated, as the electrode material [50]. Dubbed the sonotrode , this is a clever idea to combine the two active components of a sonoelectrochemical system the authors noted the expected enhancements in limiting currents and an alteration in the morphology of copper electrodeposited from aqueous solution on to the titanium tip, which was the reaction under test. However, although titanium is widely used in sonochemistry because of its low-loss characteristics under vibration, it is not a common electrode material for electroanalysis because of its inferior electron transfer characteristics... 

Acoustic irradiation appears to be able not only to boost chemical reactions but also to intensify mass transfer processes in multiphase systems. A twofold increase of k a using ultrasound has been observed [150], but depends strongly on the reaction conditions. Other authors have reported instead higher intensification factors. The enhancement is probably related to a reduction of the boundary layer thickness due to the microscale turbulence and reduction of the viscosity in the boundary layer. 

Apart from the occurrence of cavitation inside the pores of the catalyst particles, ultrasound with kHz frequencies and a non-porous solid give an increase in the reaction rate with respect to silent conditions. In our opinion, this effect can be due to several factors, such as an enhancement of external transport phenomena and a local rise in temperature at the surface of the particle due to the cavitation of some bubbles next to the external surface. Such "heating" could propagate inside the catalyst, producing higher reaction rates. 

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