A method for predicting the number of active bubbles in sonochemical reactors

Abstract

Knowledge of the number of active bubbles in acoustic cavitation field is very important for the prediction of the performance of ultrasonic reactors toward most chemical processes induced by ultrasound. The literature in this field is scarce, probably due to the complicated nature of the phenomena. We introduce here a relatively simple semi-empirical method for predicting the number of active bubbles in an acoustic cavitation field. By coupling the bubble dynamics in an acoustical field with chemical kinetics occurring in the bubble during oscillation, the amount of the radical species OH and HO2 and molecular H2O2 released by a single bubble was estimated. Knowing that the H2O2 measured experimentally during sonication of water comes from the recombination of hydroxyl (OH) and perhydroxyl (HO2) radicals in the liquid phase and assuming that in sonochemistry applications, the cavitation is transient and the bubble fragments at the first collapse, the number of bubbles formed per unit time per unit volume is then easily determined using material balances for H2O2, OH and HO2 in the liquid phase. The effect of ultrasonic frequency on the number of active bubbles was examined. It was shown that increasing ultrasonic frequency leads to a substantial increase in the number of bubbles formed in the reactor.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…An increase in the number of active bubbles and reactive species with an increase in frequency has been widely observed [42,106,121,164]. At higher frequencies, collapse and fragmentation occurs more quickly due to a decrease in the oscillation period, thus more fragmentation bubbles are produced over a shorter space of time that can then grow to become active [106].…”

“…Mason suggested a fourth site originating from a two site model that considers a liquid phase surrounding the hot bubble, that can either be a heated shell of liquid around the collapsing bubble or liquid droplets injected into the bubble by instabilities at the surface [100]. Of these sites the interfacial region is the greatest area of •OH population and recombination [20,30,[105][106][107][108][109][110]. Merouani et al through numerical simulation determined that of the numerous products, H 2 occurred most abundantly and more readily in the gas phase via reaction (8) [111].…”

“…All these things are dependent on the reactor geometry, ambient pressure, and temperature, viscosity and gas composition of the liquid [42]. In general, higher frequencies will result in increased nucleation and production of bubbles that are comparatively small in size [106,[151][152][153]. This is because the resonance size of a bubble is inversely related to the applied frequency [25].…”

“…At higher frequencies, collapse and fragmentation occurs more quickly due to a decrease in the oscillation period, thus more fragmentation bubbles are produced over a shorter space of time that can then grow to become active [106]. This could be a result of bubble surface instabilities which have been calculated to occur more readily in the presence of travelling waves that are inherent at high frequencies [165].…”

Interplay Between Mechanochemistry and Sonochemistry

“…Once again, such effects depend on the characteristics of the ultrasonic field. Numerical methods and simulations have been introduced to predict the active zones while bearing in mind that, by definition, a cavitating field is a heterogeneous system (liquid and bubbles) [32][33][34]. In general, reproducible results are attained as long as external factors and reactor shape are optimized, although some practitioners often ignore the critical issue of power control.…”