Modeling of sonochemistry in water in the presence of dissolved carbon dioxide

CO₂ capture and utilization (CCU) is a process that captures CO₂ emissions from sources such as fossil fuel power plants and reuses them so that they will not enter the atmosphere. Among the various ways of recycling CO₂, reduction reactions are extensively studied at lab-scale. However, CO₂ reduction by standard methods is difficult. Sonochemistry may be used in CO₂ gas mixtures bubbled through water subjected to ultrasound waves. Indeed, the sonochemical reduction of CO₂ in water has been already investigated by some authors, showing that fuel species (CO and H₂) are obtained in the final products. The aim of this work is to model, for a single bubble, the close coupling of the mechanisms of bubble dynamics with the kinetics of gas phase reactions in the bubble that can lead to CO₂ reduction. An estimation of time-scales is used to define the controlling steps and consequently to solve a reduced model. The calculation of the concentration of free radicals and gases formed in the bubble is undertaken over many cycles to look at the effects of ultrasound frequency, pressure amplitude, initial bubble radius and bubble composition in CO₂. The strong effect of bubble composition on the CO₂ reduction rate is confirmed in accordance with experimental data from the literature. When the initial fraction of CO₂ in the bubble is low, bubble growth and collapse are slightly modified with respect to simulation without CO₂, and chemical reactions leading to CO₂ reduction are promoted. However, the peak collapse temperature depends on the thermal properties of the CO₂ and greatly decreases as the CO₂ increases in the bubble. The model shows that initial bubble radius, ultrasound frequency and pressure amplitude play a critical role in CO₂ reduction. Hence, in the case of a bubble with an initial radius of around 5 μm, CO₂ reduction appears to be more favorable at a frequency around 300 kHz than at a low frequency of around 20 kHz. Finally, the industrial application of ultrasound to CO₂ reduction in water would be largely dependent on sonochemical efficiency. Under the conditions tested, this process does not seem to be sufficiently efficient.