An investigation of ultrasonic based hydrogen production

Abstract

A comprehensive numerical study is performed to establish a link between the primary effect of the acoustic cavitation bubble activity and the consequent effect of the chemical kinetics mechanism associated with the sonochemical process. In this work, we studied a possible reaction kinetics mechanism for the sonochemical hydrogen production, which we called the sonohydrogen process. The reaction kinetics mechanism consists of 19 reversible reactions taking place inside the acoustic cavitation micro-bubble at different conditions. The simulation of the reaction kinetics is validated and utilized to quantify the amount of hydrogen produced by a single bubble that is initially saturated with water vapor/oxygen. The results from the bubble dynamics model and the chemical kinetics model are compared with two different experiments available in the literature. Furthermore, the work evaluated the energy efficiency of this technology to produce 1 μmol of hydrogen in kWh. The present results revealed that the bubble temperature governs the chemical reaction mechanism of the water vapor dissociation. The minimum bubble temperature required for a slight production of hydrogen is 3000 K, the hydrogen is in the range of 5.46E-06 – 8.59E-06 μmol/h. At the following conditions of an ultrasonic frequency of 20 kHz, an acoustic power of 30 W, and a bubble temperature of 6000 K, the amount of hydrogen produced is 1.05E-03 μmol/h. The higher the bubble temperature, the faster the chemical reaction rate, which will lead to a higher hydrogen production rate. In terms of performance, the sonohydrogen process produces 2.22E-02 μmol/kWh.