Infoscience

Thesis

Scale-down approach: chemical process optimisation using reaction calorimetry for the experimental simulation of industrial reactors dynamics

This thesis deals with the combined utilisation of a reaction calorimeter, the RC1® commercialised by Mettler Toledo and equipped with a 2 L glass reactor, and the heat transfer dynamics modelling of industrial reactors. By doing so, the temperature evolution of the reaction medium of full scale equipment during a chemical process can be forecast already at laboratory scale. Thus, the selectivity, quality and safety issues arising during the transfer of a new process, respectively the optimisation of an existing one, from the laboratory to the production scale are earlier detected and more correctly apprehended. It follows that the proposed methodology is a process development tool aiming to accelerate the rate at which innovative processes can be introduced into the market, and for which the global safety can be guaranteed. Chapter 3 of the thesis devotes to the heat transfer dynamics modelling of industrial reactors. To this intention, heating/cooling experiments have been performed at plant scale. First, it consisted in filling up the industrial reactor with a measured quantity of a solvent with known physical and chemical properties (typically water or toluene). Second, after a stabilisation phase at low temperature, the setpoint of the liquid was modified to a temperature about 20 °C below its boiling point, followed by a stabilisation phase at high temperature. Then, the setpoint was changed to a value about 20 °C higher than the fusion point, again followed by a stabilisation phase at low temperature. During the experiment, the solvent and jacket temperatures are measured and registered. The stirrer revolution speed or the liquid amount are changed, and the whole measurement cycle repeated. Not only the heat transfer between the utility fluid and the reaction medium was modelled, but also the thermal dynamics of the jacket itself. Nine industrial reactors have been characterised, their sizes ranging from 40 L to 25 m3. Chapter 4 presents the developed methodology allowing to predict the thermal behaviour of full scale equipment during a chemical process. It is based on two on-line heat balances, namely one over the reaction calorimeter to determine the instantaneous heat release rate and the other over the industrial reactor dynamics to compute its hypothetical thermal evolution. The dynamic model of the industrial reactor is introduced in an Excel sheet. A Visual Basic window allows to establish the connection between the reaction calorimeter and the Excel sheet, meaning that the data from the various sensors of the RC1® can be sent at regular intervals of 10 s to the Excel sheet. By controlling its jacket temperature, the calorimeter is then forced to track the predicted temperature of the industrial reactor. The advantage of the proposed methodology is that the kinetics modelling of the reaction, often a time-consuming and expensive step, is here not mandatory. In chapter 5, the precision of the on-line heat balance over the RC1® was tested and validated with the help of an external voltage source controlling the power delivered by the calibration probe. In this way, the heat provided to the reaction medium was known with great accuracy. The error of the on-line heat balance on the heat release rate, qrx, lies in the generally acceptable 5 % range for bench scale calorimeters. Afterwards, the chosen test reaction, the hydrolysis of acetic anhydride, has permitted, at laboratory scale using the RC1®, to highlight that the thermal dynamics of industrial reactors has a great influence on the temperature evolution of the reaction medium and, hence, on process safety. Finally, the simulation of a polymerisation reaction with the help of a thickener permits to conclude that the "scale-down" methodology and the on-line heat balance over the reaction calorimeter are also applicable to reactions accompanied with large variations of the reaction medium viscosity. Chapter 6 compares the temperature evolution of the reaction medium predicted in the calorimeter with that actually recorded at plant scale. Three reactions are presented: a neutralisation, a three steps reaction and an alkene oxidation by a peroxycarboxylic acid. For the neutralisation, the results precisely tallied with a mean temperature difference lesser than 0.5 °C. Due to technical difficulties, the results of the three steps reaction slightly differ. For the oxidation reaction, the temperature predicted in the reaction calorimeter corresponds to that of full scale equipment to the nearest 0.5 °C. Moreover, the final compositions of the reaction medium are from the gas chromatography analyses also comparable. Moreover, this reaction being thermosensitive, a final selectivity decrease of 13 % is obtained at laboratory scale if this reaction took place in the 25 m3 reactor. This effect is due to its slower dynamics, smaller cooling capacity and more unfavourable heat transfer area to volume ratio compared with smaller reactors. The effect being highlighted already at laboratory scale, the elaborated tool results in a shorter process development time, a safer process and, hence, a shorter time-to-market. Finally, chapter 7 concludes with some outlooks concerning the continuation of the project. As this thesis did not deal with mixing issues, its logical continuation would be the scale-down of mixing effects. A few general guidelines are given for this field.

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