A review on the importance of operating conditions and process parameters in sonic hydrogen production

This review article discusses the operating conditions and key parameters of the sonohydrogen process, including the acoustic pressure amplitude, ultrasonic frequency and ambient bubble radius, which help achieve better sonochemical activity and higher hydrogen production rate. It also clarifies some discrepancies in the values of various operating conditions and state properties assumed and/or utilized in the literature studies. It further reviews the history of experiments available in the literature on different techniques to measure the bubble radius-versus time curve and discusses the parameters that govern the operation of the sonoreactor and the sonohydrogen process. The boundary conditions of the sonohydrogen process for the expected sonochemical activity and the acoustic bubble pressure and temperatures are comprehensively illustrated. The classifications of the acoustic cavitation bubbles are presented, followed by a discussion of the Blake cavitation threshold and its application to single bubble simulation and full sonoreactor geometries. Furthermore, a review of the predicted hot-spot temperature at different boundary conditions is demonstrated, tailed with the dependence of the sonochemical activity on the medium, including dissolved gases specific heat ratio, thermal conductivity, and thermal diffusivity. Moreover, the findings of this review emphasize the differences in the maximum bubble temperature (hot spot's temperature) for different dissolved gases.