Micro-batch reactor for catching intermediates and monitoring kinetics of rapid and exothermic homogeneous reactions

The mechanism and kinetics of rapid and exothermic reactions is difficult to study using conventional offline techniques. This paper deals with experimental investigations of highly exothermic and fast model reaction (cyclisation of pseudoionone) using slug flow (micro-batches) microstructured reactor (MSR). The slug flow was obtained using a cross-junction and identifying suitable inert carrier fluid (perfluorohexane) immiscible with the species participating in the reaction. MSR provided short reaction time in a continuous slug-flow that allowed catching intermediates and monitoring the kinetics. The small reaction volumes suppress the temperature rise by removing the reaction heat. The kinetics has been studied under isothermal conditions and more reliable reaction network has been proposed. Finally, the mixing within the slugs and its influence on the obtained results was accessed. This methodology has a generic nature and can be applied for a wide variety of fast exothermic homogeneous reactions. (C) 2012 Elsevier B.V. All rights reserved.

Heat Management in Single and Multi-injection Microstructured Reactors: Scaling Effects, Stability Analysis, and Role of Mixing

Julien Haber, Madhvanand Namadeo Kashid, Lioubov Kiwi, Albert Renken

The exothermic reactions with fast intrinsic kinetics when carried out in conventional reactors are often limited by heat transfer. This results in the formation of hot spots, which affects product distribution of complex reactions and may lead to reactor runaway. Microstructured reactors (MSR) have been successfully used for this type of reaction, gaining increased temperature control and enhanced safety. Although different MSR concepts have been applied to various processes, the general design criteria are still poorly discussed in the literature. In this review, temperature management in conventional single injection MSR is described. The important issues, such as reactor safety and stability, along with the role of mixing, are discussed in detail. Subsequently, the multi-injection reactor concept is introduced and a simplified model developed to investigate the temperature profile is presented. Finally, the benefits and critical issues in the design of multi-injection MSR are highlighted.

2012

Heat Management for Process Intensification of Fast Exothermic Reactions in Microstructured Reactors

Julien Haber

Nowadays, the production in fine chemical and pharmaceutical industry is mostly carried out in large scale batch reactors having typically dimensions of a few meters to satisfy the demand of the market. Even though this technology has been widely used and developed for centuries, it is by far not optimal for every type of reaction. For example, when working with exothermic reactions, the produced heat can’t be always fully evacuated. To avoid run-aways, high amounts of solvents are used to increase the heat capacity of the mixture, or semi-batch mode with a slow addition of one of the reactants. In both cases, the space-time yield, i.e. mass of product produced per unit of time and per unit of volume, drastically diminishes. One of the main enabling technologies allowing process intensification are the microstructured devices, characterized by high heat and mass transport rates due to the small characteristic dimensions (< 1mm). Using this type of equipment, almost isothermal conditions can be achieved while carrying out fast exothermic reactions (with characteristic reaction times down to tr ≈ 10 s). Thereby, the target throughput is reached by numbering-up, i.e. parallel connection of several identical microreactors. For very fast exothermic reactions, especially for quasi-instantaneous reactions, dimensions smaller than 100 μm are needed to prevent the formation of unwanted hot spots. As such small dimensions are not suitable for industrial scale due to possible clogging and high pressure drops, other solutions are warranted. The aim of this thesis is to develop alternative microstructured reactors enabling quasi-instantaneous reactions to be carried out under intensified conditions while suppressing the large hot spots. The work is divided into two main parts: determination of suitable strategies for the microstructured reactor design via numerical simulations (Chapter 3) and the experimental validation of the best microstructured reactor concept (Chapter 4-6). Three strategies for enhanced temperature control within microstructured reactors for quasi-instantaneous reactions are taken for analysis using numerical simulation: 1) reduction of hot spot temperature by increased axial heat transfer in the reactor wall, 2) by injection of one reactant in multiple points along the reactor length and 3) by continuous injection of one reactant through a porous wall in a concentric reactor geometry. The multi-injection reactor (option 2) is the most effective design since with an optimized dosing with only 4 injection points the temperature rise is 5-fold smaller as compared to the adiabatic temperature rise. Furthermore, the key design requirements for an efficient multi-injection reactor are identified: 1) complete mixing after each injection and 2) evacuation of the produced heat before reaching the next injection point. To experimentally validate the simulation results, in the subsequent chapter, an experimental method to monitor temperature in microstructured reactors is developed (Chapter 4). To track axial temperature profiles quantitatively, a method based on non-intrusive infrared thermography is developed yielding a resolution of 100 points/mm2 and a precision of 1 °C. In the first validation experiments, the heat transfer coefficient determined in a micro heat exchanger (574 W/m2K) is in good agreement with prior estimations. While carrying out the hydrolysis of tetraethoxysilane as a fast model reaction, incomplete mixing of the reactants is detected via the temperature profile, and is ascribed to the high difference in density of the inlet flows. Applying the method of quantitative IR-thermography to a T-micromixer with circular cross section gives insight into the mixing phenomenon (Chapter 5). The latter is studied via the temperature profile of the reactions strongly controlled by mixing, i.e. dilution of sulfuric acid with water and cyclization of pseudoionone. The mass transfer coefficients determined are in the order of 0.1-9 1/s. It is shown that at high Fourier numbers Fo = tdiff/τ (mixing by shearing), the Damköhler number DaI= τ/tmix remains constant with respect to flow rate at the reactor outlet, as both, mixing time and residence time decrease proportionally with the latter parameter. To enhance the mixing performance, two approaches are applied: 1) the introduction of a carrier phase leading to travelling micro-batches with up to 4-fold faster mixing and 2) the structuring of the channel walls leading to the formation of vortices, and thus, to a substantially improved mixing efficiency. For efficient mixing in the multi-injection reactor (Chapter 6), two types of mixing structures, i.e. the tangential mixer and the herringbone mixer, are developed using low temperature co-fired ceramics, and compared using quantitative infrared thermography. The best mixing performance is obtained by the herringbone structure, providing efficient mixing in a large range of flow rates corresponding to Reynolds numbers Re = 20-130. Finally, a multi-injection reactor comprising three injection points and the herringbone microstructure is developed. Using the quasi-instantaneous and exothermic cyclisation of pseudoionone to α-ionone and β-ionone as model reaction, it is demonstrated that the temperature rise can be reduced 8-fold compared to the adiabatic temperature rise due to 1) the high volumetric heat transfer coefficient in the order of 4·106 W/(m3K), 2) the reduced overall transformation rate due to gradual mixing within the herringbone structure and 3) the injection of pseudoionone at three injection points. Yields of α-ionone and β-ionone above 98 % are achieved at a residence time of 3.7 s while efficiently avoiding the unwanted consecutive polymerization in a temperature range of 30-60 °C. Compared to the conventional semi-batch process, where such high yields can only be attained at temperatures below 10 °C, a 500-fold increased space-time- yield is achieved. In addition to the intensification of the process, the required mass of solvent is halved while maintaining good temperature control, rendering the overall process safe.

EPFL2013

Développement de réacteurs microstructurés pour la conversion du propane en produits valorisés

Olivier Wolfrath

Industry requires continuously new chemical processes to convert raw materials to valuable products limiting the formation of by-products. Since a few years, an increasing interest has been given to the intensification of chemical processes. This represents a new approach in chemical reaction engineering and calls for a new type of chemical reactors: the microstructured reactors. They are characterized by miniature structures whose components have a sub-millimeter size. The flow channels, for example, have a diameter from ten to several hundred microns. The distinctive feature of these structures is a very large surface/volume ratio, namely between 10'000 and 50'000 m2/m3. In these conditions, heat transfer coefficient is very high, up to 25 kW · m-2 · K-1. It is accepted that the residence time distribution in a reactor strongly influences the product selectivity and yield. In these thin channels, the RTD is particularly narrow due to a short radial diffusion time (ms), allowing a precise control of the residence time. Another advantage of these microstructures is an increase of the inherent safety due to a small amount of the compounds used. However, fabrication of microreactors, especially the introduction of active catalyst in microchannels remains a difficult operation. And integration of microsystems in conventional chemical processes is still a challenge. The aim of this work is first to develop a novel membrane reactor based on a new microstructured catalytic packing in the filamentous form and to apply it to the nonoxydative dehydrogenation of propane. A second objective is to develop a reactor with micro heat exchangers allowing an optimized control of homogeneous reactions. This reactor was tested with the partial oxidation of propene to propylene oxide. The microstructured catalyst above is made of thin and long catalytic filaments placed in parallel with a tubular reactor. This new kind of catalyst is based on silica fibres covered by a γ-alumina layer. The position in parallel of these filaments of 7 μm diameter forces the gases to flow in the fibres direction between the filaments. The hydraulic diameter is about 70 μm. Due to this laminar and regular flow, the pressure drop in the bed was divided by 5 compared to a conventional bed (100-160 μm spheres) and the residence time distribution was particularly narrower. Such novel packing brings new opportunities to the heterogeneous catalysis domain because it can be used for many other reactions improving highly the hydrodynamics and keeping an excellent contact with the reactants. The microstructured catalyst was installed in two concentric zones of a tubular reactor separated by a palladium-silver membrane permeable to hydrogen. This product was removed from the gas phase to shift the thermodynamic equilibrium, enhancing the propane conversion from 22% up to 30%. Another significant advantage in this reactor is the increase of propene selectivity up to 97% in comparison to conventional industrial processes reaching 80-90%. This difference is due to the diminution of hydrogenolysis and hydroisomerization reactions. The dehydrogenation is known to be endothermic. Oxidation of the hydrogen passing through the membrane produced heat for this reaction along the catalytic zone. Moreover, a periodic regeneration by air allowed recovering the activity of the catalyst deactivated by coke formed during dehydrogenation. Periodic reaction operations allowed producing continuously propene with a high selectivity, and regenerating simultaneously the catalyst. The second microstructured reactor was developed integrating two micro heat exchangers at the inlet and outlet of a macroscopic tube. The aim is to heat up very fast the gas and to cool it down as well. Without these exchangers, heating and cooling duration corresponded each to about 20% of the total residence time. The use of the microstructures allowed diminishing significantly these durations (4-5% each). Applied to partial oxidation of propene, this reactor reached a more precise control of reaction temperature and duration. The propylene oxide (PO) selectivity was not increased with this system but, due to the microstructures, it was possible to work at higher temperatures where the reaction becomes very fast, without reaching an oxygen conversion of 100%. Indeed, it is essential to avoid this situation where oligomers and coke form dividing by 2 the PO selectivity. So another new kind of reactor was developed suitable for any fast exothermic reaction whose progression has to be limited because of potential consecutive reactions reducing, for example, the selectivity of an intermediate desired compound.