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

Abstract

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.