Sonochemical efficiency dependence on liquid height and frequency in an improved sonochemical reactor

Abstract

The aim of this study was to evaluate the effects of liquid height and frequency on sonochemical efficiency of a cup-horn sonoreactor. The selected frequencies (22, 371, 504 kHz) and liquid height (29–348 mm) were applied while measuring acoustic yield, ${\text{I}}_3^{-}$ formation rate and the resulting sonochemical efficiency. The frequency effect was shown to be coupled to liquid height effect. Indeed, acoustic zones (i.e. Fresnel and Fraunhöfer zones), which limits depends on both transducer diameter and frequency, significantly determine the production of radicalar species quantified by ${\text{I}}_3^{-}$ formation rate. An increase of ultrasonic frequency results in lower acoustic yield and higher sonochemical efficiency. Theoretical physical and chemical effects attributed to collapsing bubbles were considered. Sonochemical efficiencies obtained at 500 kHz were similar or higher than those at 371 kHz, depending on liquid height. The effect of reactor configuration was further investigated as an hypothesis to explain unsignificant effect of standing waves in our study. Therefore, the dependence of sonochemical efficiency with liquid height might be firstly attributed to reactor configuration prior to frequency effects.

Introduction

From the reverse piezoelectric effect, the conversion of electrical to mechanical energy is carried out by the ultrasonic transducer, enabling the formation, growth and collapse of transitory cavitation bubbles in the sonified liquid [1]. The dynamic and nonlinear nature of this phenomena called cavitation attributes the resulting acoustic field with characteristics that may be of complex approach. The dependence of the reflection of sound waves on liquid height [2] makes of this latter an interesting parameter to understand cavitational activity. Liquid height effect has been reported to result in contradictory consequences on sonochemical efficiency and cavitation dynamics [3]. Some studies have pointed out the dependence of acoustic field distribution with liquid height using chemiluminescence method [4] at frequencies of 45, 129, 231 [5] and 500 kHz [5], [6]. Nevertheless, the effect of liquid height either on acoustic yield or sonochemical efficiency was not systematically significant, even at similar frequencies, such as in the low range [5], [7] or high range [5], [6]. Therefore, in addition to frequency, failure to take into account other factors such as liquid physical properties as well as reactor and transducer configurations [5] might prevent from conclusively understanding the effect of liquid height or sonicated volume. Indeed, liquid height was suggested as limiting the sonochemical efficiency and cost-effectiveness provided by a 1.6-time decrease of emitting-to-sonified surface area ratio in our previous study [8]. Likewise, the decrease of sonochemical efficiency above an optimal reaction vessel diameter (90 mm in the range 20–120 mm), corresponding to an optimal sonicated volume, was attributed to decreasing number of active cavitation bubbles in the sonication zone, additionally to potential effect of transducer property, liquid height and frequency (200 kHz) [3]. Furthermore, coupled effect of frequency and acoustic power on acoustic field was investigated and modeled while taking into account fluid dynamics (e.g. density, viscosity) and properties (e.g. pressure, temperature, velocity) [9]. Therefore, these studies highlighted the dependence of both cavitational activity distribution and magnitude on several operating parameters. However, from our knowledge, some studies have taken into account and suggested the relation between both liquid height and frequency effects [2] or reactor configuration effect [3] on sonochemical efficiency [4] but no study simultaneously investigated the effect of these three factors.

The aim of this study was to elucidate the individual and coupled effects of liquid height and frequency on acoustic yield, ${\text{I}}_3^{-}$ formation rate and the resulting sonochemical efficiency in an improved sonochemical reactor configuration [8]. The effect of frequency and reactor configuration was firstly evaluated on sonochemical efficiency dependence with liquid height. Then, individual effect of frequency on sonochemical efficiency was assessed. Sonochemical effects, characterized by the production of radical species due to water sonolysis (Eq. (1)), are reported as being more effective in the frequency range of 300–500 kHz [5], [10], [11] rather than at lower frequencies (20 kHz) [12] or at higher frequencies like 900 kHz [13] or 1600 kHz [14]. The chosen frequencies extent ranged from the low (22 kHz) to high (371, 504 kHz) values with the view to evaluate both physical and chemical effects dependence with frequency.

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