Ultrasound stick

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]. 

Figure 2. Experimental apparatus for the investigations of acoustic cavitation in a liquid metal. (1) Signal-Generator (2) Amplifier (3) Ultrasonic Generator (4) Transducer (5) Frequency meter (6) Valve voltmeter (7) Sensor of a waveguide stick (8) Recorder (9) Cavitometer (10) Potentiometer (11) Probe (12) Crucible with a melt (13) Source of ultrasound (14) Receiving stick (15) Electric furnace (16) Detector of the first bubble.

One of the important application area of ultrasound (US) is catalytic reactions with the participation of low molecular mass compounds and heterogeneous catalysts. The effect of ultrasound on catalytic reactions in the presence of platinum and rhodium catalysts of various dispersities was investigated in Ref [1]. It was demonstrated that ultrasound can provide for the occmrence of chemical processes that cannot be performed even in the presence of catalysts. It is assumed that the main mechanism of its action on catalytic processes consists in the dispersion of catalyst particles however, as was shown in Ref [1], the adhesion of particles can occur during the action of the so-called Bjerknes forces, that is, forces that promote the attraction of particles (primarily small particles) to a deformed bubble followed by their sticking together. As a consequence, the diffusion of reagents to the surface of a particle becomes more pronoimced and the rate of the process inereases. 

A high frequency generator triggers a piezoceramic sonic modifier. The sonic modifier oscillates with the frequency of stimulation (normally around 20 kHz) at a low amplitude of around 5 pm. The amplitude of the ultrasound can be amplified by coupling the sonic modifier with a transformer and sonotrode. These elements are tuned to the frequency and the amplitude intensified to 20-40 pm. The sonotrode also acts as locator for the tool [281,282]. The tool is joined with the sonotrode by brazing, sticking or press-bonding. 

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