Abstract
In sonochemistry, acoustic bubbles are a population of microreactors where hydrogen and oxidants are produced. Optimizing the effectiveness of sonochemical processes and, as a result, designing ultrasonic reactors for diverse uses, including hydrogen generation, requires determining the number density of acoustic microreactors.
The number density of micro-bubbles during water sonolysis was determined in this study using a novel semi-empirical method developed (for the first time) using hydrogen sono-production. The technique is based on relying on the overall molar production rate of hydrogen (i.e. resulted from the sonicated solution) to the amount of hydrogen produced per a single collapsing bubble, either from its internal gas phase reaction (pyrolysis) or from both the bubble inside and its liquid shell (via H•+H•→H2). The retrieved number density of bubbles varied between ∼108 to ∼1013 L−1 s−1 (depending on empirical conditions), showing an excellent order with that reported in the literature. As the frequency increased, the number of active bubbles increased, regardless of whether the number density is calculated through the amount of hydrogen formed inside the bubble or the total single-bubble yield (gas phase + liquid shell). However, a reduced number density was obtained as it was calculated via the total single-bubble yield, where this decrease goes up with the rise of ultrasound frequency (from 210 to 724 kHz) and the decrease of the liquid temperature. It has been deduced that hydrogen is mainly formed at the bubble's liquid shell (via H•+H•→H2), particularly at higher frequency and cold liquid.
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•Hydrogen sonoproduction was quantified and used for determining the bubbles number.•A full cavitation-based model was applied for predicting the H2's single bubble yield.•The H2-based technique relies the overall and the single bubble yield during sonication.•The bubbles' number density was ∼108 to ∼1013 L−1 s−1, depending on circumstances.•The number density increased with frequency increase and liquid temperature decrease.