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8 Scaling-Up Enabling the Full Potential of Industrial …
• Introduction of High Power into Sonoreactor
An efficient scaling operation requires introducing large irradiating powers in large
volumes and dissipating energy evenly over the reactor. Extensive literature shows
that the range of energy needed to achieve various transformations ranges from a few
hundred W/m
3 to several kW/m
3 , which can induce technological limitations. An
increase in the reactor volume leads to a simultaneous increase in the ultrasonic power
to maintain the same power density. Owing to the power limitation of each single
transducer, a large-scale reactor requires several ultrasonic transducers (Gonze et al.
1997) or as suggested by Gondrexon et al. (1999), a multistage reactor composed of
a series of several smaller ultrasonic units.
For effective processing scale-up using cavitation phenomenon, it is important
to maintain uniform cavitational activity distribution in the volume of processed
materials since a non-homogeneous distribution of cavitation can skew the scale-up
strategies. The sound wave streaming generates liquid circulation starting from the
transducers towards the medium largely perturbed by multiple reflections at reactor
walls and the surface of liquid or particles. The increase in the velocity of the liquid
can be obtained by stirring but it can cause negative effects if the mixing system is
placed near the transducers. Indeed, the bulk movement of the liquid due to stirring
may disturb the pressure fields and interfere in the propagation of sound waves due
to the scattering of ultrasound waves. Consequently, the diffusion of the acoustic
wave undergoes a loss of intensity of the ultrasonic waves. Convenient measurement
of the cavitation bubble fields is a significant stake to access a convenient mapping
of the sonochemical reactor.
• Transducers
Ultrasonic transducers are elements whose function is not only to generate an ultrasonic wave but also to receive it and convert it into an electrical signal. The optimization of this device is an important factor in increasing the mechanical and chemical
performance of the cavitational collapse. The positioning of the transducers must be
optimal to allow irradiation of the maximum volume of raw materials and also to
avoid disturbances related to the proximity of the ultrasonic capacitors.
Nowadays, for applications requiring high power, the most commonly used transducers are the prestressed sandwich piezoelectric ultrasonic transducers composed
of the piezoelectric ceramic active elements placed between the front metal cylinder
in contact with the medium and the back beam. The advantage of this technology is
to offer the possibility to tune the shape and structure of transducers to optimize the
electromechanical coupling coefficient. Nevertheless, these ultrasonic transducers
are restricted in power capacity and in their dimensioning and may be inappropriate
for large industrial applications. Indeed, the scaling of horn-type transducers is not
optimal for use on large volumes due to a small acoustic wave transmission surface,
erosion problems and a potential breakage of devices. Recent years have seen the
birth of a new family of power transducers with extensive radiators in order to furnish
high irradiative surface areas and good impedance. It consists of mounting a plate of
various forms and profiles on the vibrating surface of a horn sandwich transducer.
8 Scaling-Up Enabling the Full Potential of Industrial …
• Introduction of High Power into Sonoreactor
An efficient scaling operation requires introducing large irradiating powers in large
volumes and dissipating energy evenly over the reactor. Extensive literature shows
that the range of energy needed to achieve various transformations ranges from a few
hundred W/m
3 to several kW/m
3 , which can induce technological limitations. An
increase in the reactor volume leads to a simultaneous increase in the ultrasonic power
to maintain the same power density. Owing to the power limitation of each single
transducer, a large-scale reactor requires several ultrasonic transducers (Gonze et al.
1997) or as suggested by Gondrexon et al. (1999), a multistage reactor composed of
a series of several smaller ultrasonic units.
For effective processing scale-up using cavitation phenomenon, it is important
to maintain uniform cavitational activity distribution in the volume of processed
materials since a non-homogeneous distribution of cavitation can skew the scale-up
strategies. The sound wave streaming generates liquid circulation starting from the
transducers towards the medium largely perturbed by multiple reflections at reactor
walls and the surface of liquid or particles. The increase in the velocity of the liquid
can be obtained by stirring but it can cause negative effects if the mixing system is
placed near the transducers. Indeed, the bulk movement of the liquid due to stirring
may disturb the pressure fields and interfere in the propagation of sound waves due
to the scattering of ultrasound waves. Consequently, the diffusion of the acoustic
wave undergoes a loss of intensity of the ultrasonic waves. Convenient measurement
of the cavitation bubble fields is a significant stake to access a convenient mapping
of the sonochemical reactor.
• Transducers
Ultrasonic transducers are elements whose function is not only to generate an ultrasonic wave but also to receive it and convert it into an electrical signal. The optimization of this device is an important factor in increasing the mechanical and chemical
performance of the cavitational collapse. The positioning of the transducers must be
optimal to allow irradiation of the maximum volume of raw materials and also to
avoid disturbances related to the proximity of the ultrasonic capacitors.
Nowadays, for applications requiring high power, the most commonly used transducers are the prestressed sandwich piezoelectric ultrasonic transducers composed
of the piezoelectric ceramic active elements placed between the front metal cylinder
in contact with the medium and the back beam. The advantage of this technology is
to offer the possibility to tune the shape and structure of transducers to optimize the
electromechanical coupling coefficient. Nevertheless, these ultrasonic transducers
are restricted in power capacity and in their dimensioning and may be inappropriate
for large industrial applications. Indeed, the scaling of horn-type transducers is not
optimal for use on large volumes due to a small acoustic wave transmission surface,
erosion problems and a potential breakage of devices. Recent years have seen the
birth of a new family of power transducers with extensive radiators in order to furnish
high irradiative surface areas and good impedance. It consists of mounting a plate of
various forms and profiles on the vibrating surface of a horn sandwich transducer.