(394c) Residence Time Distribution Studies of a Hydrodynamic Cavitation Reactor

Process intensification aims to design more efficient and hence more sustainable processes by developing “next-generation” technologies for the chemical manufacturing industry. Cavitation reactors, a reactor concept with a long history but very limited application and few systematic studies to-date, constitute one such technology. In these reactors, cavitation, i.e. the local boiling of a liquid reactant or solvent, is induced either via acoustic waves (sonication) or local acceleration of the flow (hydrodynamic cavitation). The resulting formation and subsequent rapid collapse of vapor bubbles results in strong localized heating and the creation of pressure waves and microjets that induce intense local mixing.

In the present work, we focus on the evaluation of a Hydrodynamic Cavitation Reactor (HCR). In our system, hydrodynamic cavitation is induced via a ”dimpled” rotor inside a cylindrical reactor housing. The deep indentations in the rotor capture fluid and accelerate it to a velocity at which cavitation occurs. The cavitation itself furthermore assures efficient exchange of fluid between the indentations and the bulk flow. However, the complex flow pattern in this reactor renders description of the (macroscopic) flow and mixing pattern challenging. This not only makes fundamental understanding of the interplay between reaction and cavitational heating and mixing difficult, but also constitutes a hurdle for quantitative design of these units for reaction purposes.

To address these issues, we conducted detailed Residence Time Distribution (RTD) studies of the HCR. Mineral oil was used as main working fluid since it is widely used as a diluent in the production of specialty chemicals, the intended application for the present reactor. In addition to measurement of the response to a step-change in inlet concentrations, a transparent reactor housing is used for visualization of the flow pattern using a dye. We find that a relatively simple 2-CSTR-in-series model with dead volume captures the measured RTD behavior with good accuracy. Interestingly, we furthermore find a dependence of the dead volume on the fluid flow rate which closely matches actual physical volume elements in the reactor. A key conclusion from these observations is that the deep indentations in the rotor do not contribute to the effective reactor volume, i.e. that these volumes apparently serve as “cavitational mixers” only. We are currently combining the derived reactor model with detailed reaction kinetics for the production of a class of dispersants which were determined in a parallel task in order to verify the HCR model via comparison to experimental measurements under reaction conditions.