Modelling liquid-solid catalytic reactions with gas production - decomposition of H2O2

In recent years, there has been a renewed interest in hydrogen peroxide propulsion systems. This technology was already in use in the 1960s but was later replaced by hydrazine propulsion systems, which were considered more efficient. However, due to increasing concern for environmental sustainability, hydrogen peroxide is being re-evaluated as a more environmentally friendly alternative. The engines' boost comes from the catalytic decomposition of hydrogen peroxide, producing oxygen and water. However, several technological challenges limit the large-scale use of these systems. One of the most significant complications is the presence of a gas phase generated by the liquid reagent's decomposition.The purpose of this work is to propose alternative set-ups that could improve the efficiency of the process by reducing the inhibition of the catalyst caused by the gaseous layer formed. Furthermore, the effectiveness and durability of the selected catalyst, namely silver, were assessed. The study also involved modelling the reactive system that was experimentally analysed to establish a correlation between the experimental and simulated outcomes. The laboratory experiments used three different set-ups with varying flow reactors. During the first experimental phase, a reactor with a corrugated wall covered by the catalyst was used. Tests were conducted to evaluate the impact of flow rate, concentration, and test reproducibility. The second experimental phase also conducted similar tests using a reactor of the same size but with a smooth inner wall covered by the catalyst. At the end of the tests, a comparison was made between the two setups to assess the efficiency of the corrugations on the decomposition process. The comparison showed that the corrugations did not affect the reaction in terms of increased conversion but allowed the catalyst to be more resistant along the reactor wall. This last aspect was of particular interest, since the type of catalyst used seems to be ideal for this reaction, achieving very high conversions, but its limited lifespan is a challenge to deal with, especially at high flow rates. For this reason, the last experimental campaign was dedicated to the evaluation of the durability of the catalyst distributed along the corrugated wall of the reactor at a sustained flow rate. The following modelling section, created using Comsol Multiphysics software, focused on developing two distinct models. The first model proposed is based on an experimental setup described in the literature, in which the H2O2 solution system is stagnant, and the decomposition reaction takes place on a tube completely immersed in the solution. The second model attempts to emulate the reactive system analyzed during this thesis work, where the H2O2 solution flows continuously through the reactive system. The aim of both models was to replicate the decomposition process as accurately as possible. However, this objective was only achieved qualitatively due to limitations in the physics used to describe the coexistence of the two phases.