Optimisation et intensification du procédé de synthèse du dithionite de zinc

This thesis aims to analyse potential improvement pathways for the sodium dithionite production process, with a specific focus on the first step of the process: the reaction leading to the production of zinc dithionite. This work was carried out under the supervision of Professor Dominique Toye (University of Liège – Department of Chemical Engineering – PEPs), with the technical support of the company SILOX Belgium, located at the PRAYON s.a. industrial site in Engis, Belgium. The introductory chapter, Chapter 1, provides a detailed description of the batch process used at SILOX Belgium for zinc dithionite production and introduces mesofluidic reactors. These reactors offer several advantages compared to conventional batch systems, including enhanced heat transfer due to a high surface-to-volume ratio, more precise control of reaction conditions (temperature, concentration), and the possibility of continuous operation, which improves reproducibility. Industrial scale-up relies on the parallelisation of reactors in order to maintain a high surface-to-volume ratio. Nevertheless, several challenges remain, such as the handling of solids in flow (maintaining homogeneous suspensions and preventing fouling) as well as the implementation of reliable online monitoring. The first stage of this work focuses on developing a fundamental understanding of the reaction mechanisms involved. Chapter 2 is dedicated to this aspect. The quantification methods used are also presented, including iodometric titration, ATR-FTIR spectroscopy, and monitoring of hydrogen production in batch operation. This work also led to a scientific publication in Chemical Engineering Journal[1], in which some preliminary results were presented in dimensionless form for confidentiality reasons related to industrial data. Chapter 3 details the design and implementation of the continuous-flow experimental setup, including reactor geometry, heat exchange, and instrumentation used to monitor concentrations and temperatures under steady-state conditions. The experimental results obtained enabled the development and validation of a numerical reactor model. This model allowed the determination of reaction kinetics and was subsequently used in Chapter 4 to numerically optimise the reactor configuration and operating parameters with a view toward industrial application. Chapter 5 explores improvements in reaction control for the industrial batch reactor without major modifications to the existing installation. Based on the batch reactor modelling, the investigated strategies include increasing the initial feed rate to reduce reaction time, as well as adding acid during the reaction to improve selectivity and conversion. Finally, Chapter 6 presents the general conclusions and perspectives, including a synthesis of the experimental and modelling results, proposals for future improvements, and possible pathways toward industrial implementation.