Cavitation ultrasonic baths

We have observed a dependence of the yield, polymerization degree, and polydispersity of polysilanes on temperature and also on the power of ultrasonication. In the ultrasonication bath the simplest test of the efficiency of cavitation is the stability of the formed dispersion. It must be remembered that the ultrasonic energy received in the reaction flask placed in the bath depends on the position of the flask in the bath (it is not the same in each bath), on the level of liquid in the bath, on temperature, on the amount of solvent, etc. When an immersion probe is used the cavitation depends on the level of the meniscus in the flask as well. The power is usually adjusted close to 50% of the output level but it varies with the reaction volume, flask shape, and other rection conditions. The immersion-type probe is especially convenient at lower temperatures. 

Moholkar VS, Sable SP, Pandit AB (2000) Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission. AIChE J 46 684—694... 

Frequency and Intensity. Most ultrasonic baths operate in the 30 -80 kHz range. Frequency is rarely an important factor, provided the frequency is low enough to permit cavitation. The cell disruptors normally adapted for sonochemical uses operate at 20 kHz. The intensity must be enough to produce cavitation. Beyond that, optimum intensities for heterogeneous reactions have not been determined. 

In 1983 Suslick reported the effects of high intensity (ca. 100 W cm, 20 kHz) irradiation of alkanes at 25 °C under argon [47]. These conditions are of course, well beyond those which would be produced in a reaction vessel immersed in an ultrasonic bath and indeed those normally used for sonochemistry with a probe. Under these extreme conditions the primary products were H2, CH4, C2H2 and shorter chain alk-l-enes. These results are not dissimilar from those produced by high temperature (> 1200 °C) alkane pyrolyses. The principal degradation process under ultrasonic irradiation was considered to be C-C bond fission with the production of radicals. By monitoring the decomposition of Fe(CO)5 in different alkanes it was possible to demonstrate the inverse relationship between sonochemical effect (i. e. the energy of cavitational collapse) and solvent vapour pressure [48],... 

Two different types of ultrasonic devices are used in laboratories ultrasonic bath and ultrasonic probe. However, as a result of inhomogeneity of ultrasonic energy distribution in the whole solution and a decrease in power with time, the repeatability and reproducibility of experimental conditions for ultrasonic baths is often unsatisfactory. With ultrasonic probes the energy is focused on a small sample area, which significantly improves cavitation efficiency and, thereby, extraction effectiveness [56]. 

Many of the disadvantages in using a simple cleaning bath in sonochemistry can be avoided by using an ultrasonic probe (also called a sonotrode ) instead. A sonotrode delivers its energy on a specific zone, cavitation in which is thus dramatically boosted. Also, probes are not subjected to any exhaustion restrictions, so they are much more suitable for use in analytical chemistry than are ultrasonic baths. 

Position of the sampie vessei. Although the position of the sample container with respect to the transducer is not a characteristic variable of US application, it determines the amount of energy that is received by the sample. This variable, exclusive of ultrasonic baths, should be optimized in both DUSAL and CUSAL methods. When only one sample is leached, the precision is not affected provided the sample vessel is always in the same position — cavitational effects can be maximal or not in this situation, however. If several samples are simultaneously treated, then the precision is probably affected because the irradiation profile is not uniform throughout the bath. One example is the DUSAL of cadmium and lead from foods, where the iodine method was used to locate the best position for cavitational effects [5]. This requires the use of mapping techniques. 

Ultrasounds can be applied to chemical systems by using ultrasonic baths or probes. Although baths are more widely used, probes are more efficient as a result of (a) the lack of uniformity in the transmission of ultrasounds (in baths, only a small fraction of the total liquid volume in the immediate vicinity of the ultrasound source experiences the effects of cavitation) and (b) the decline in power with time, which leads to exhaustion of the energy applied to baths. Both phenomena result in substantially decreased experimental repeatability and reproducibility. For this reason, the use of baths should be restricted to cleaning operations and removal of dissolved gases, their intended applications. A wide variety of commercially available ultrasonic baths exists ranging from laboratory to industrial-scale models. 

As noted earlier, most applications of ultrasound-assisted leaching involve discrete systems using a bath or an ultrasonic probe. Although ultrasonic baths are more common, ultrasonic probes have the advantage that they focus their energy on a localized sample zone, thereby providing more efficient cavitation in the liquid. 

The major operating problem of piston pumps is dissolved air and the formation of bubbles in the eluent. Bubbles in the pump heads cause pulsation of volume flow and pressure pulsation. Bubble formation and cavitation problems are promoted at the inlet check valve because the minimum pressure in the system is reached here. For this reason the eluent must be suitably degassed. This can be done online by a membrane degasser, by pearling helium offline through the eluent or by the use of an ultrasonic bath. 

Figure 37-30, p. 443, shows an ultrasonic bath sieve cleaner. When ultrasonic energy waves are transmitted to a liquid, a pattern of microscopic bubbles forms and collapses immediately after generation. This rapid cavitation keeps particles in constant motion and frees lodged particles. It is most effective for fine mesh sizes up to about No. 30. Cleaning time is t ically 2 to 5 minutes. 

It is worthwhile at this stage to compare the predictions of the cavitational yield from the obtained relationship with some of the earlier studies on decomposition of potassium iodide. Naidu et al. (1994) have studied the decomposition of potassium iodide in an ultrasonic bath having a cross-sectional area of 0.0404m and a height of 15 cm. The operating parameters used in the experimentation were 25 kHz driving frequency and 0.6188W/cm intensity. The results obtained indicate a linear variation in the iodine liberation with the initial concentration of potassium iodide and time of the reaction, which confirms the consideration of inclusion of the concentration of the reactant (i.e. KI in this case) and the reaction time in the proportionality constant. 

The data set obtained with the local measurements of cavitational yield for the Weissler reaction (Gogate et al., 2002) in the case of an ultrasonic bath can be effectively used for the development of the design equation for cavitational yield in terms of the maximum size of the cavity (recommended when free-radical attack is the controlling mechanism). The mathematical equation relating the two can be given as follows ... 

Cavitational Yield Results Table 8.2.4 also gives the values of cavitational yields obtained for different equipments for the Weissler reaction, ft can be seen from the table that a triple-frequency flow cell gives 2 to 3 orders of magnitude higher cavitational yield as compared to all the other equipments. A dual-frequency flow cell also gives 20% higher yield as compared to an ultrasonic bath, whereas the... 

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