Intensification of highly exothermic fast reaction by multi-injection microstructured reactor

Navid Borhani, Julien Haber, Bo Jiang, Lioubov Kiwi, Thomas Maeder, Albert Renken, John Richard Thome
Elsevier, 2014
Article

Résumé

Microstructured reactors (MSR) with characteristic dimensions below 100 μm are warranted to maintain close to isothermal conditions when carrying out quasi-instantaneous highly exothermic reactions. Unfortunately, such small dimensions increase the risk of clogging, create high pressure drop and are costly to number-up. The multi-injection (MI) MSR, where one of the reactants is added stepwise along the reactor length, allows working with larger dimensions (diameter >500 μm) while maintaining good temperature control. Herein presented MI-MSR is made of low temperature co-fired ceramics (LTCC) with herringbone mixing structure inside the reactor channels and is shown to mix efficiently in a large range of Reynolds numbers Re = 20–130. The cyclization of pseudoionone is studied as a model of a highly exothermic fast reaction. The temperature profiles are characterized by a quantitative infrared thermography. The developed LTCC MI-MSR allows ∼ 8-fold reduction of hot spot temperature as compared to the adiabatic temperature rise. Moreover, ∼500-fold intensification is achieved as compared to the conventional semi-batch process with reduced solvent mass by a factor of 2 while attaining a yield of target product above 98%.

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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.

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Chargement

Effect of hot spots in microstructured reactors on product distribution during quasi-instantaneous exothermic reactions

Navid Borhani, Julien Haber, Bo Jiang, Lioubov Kiwi, Thomas Maeder, Albert Renken, John Richard Thome

Background. The small characteristic sizes (in the range of 102 - 103 μm) of Microstructured reactors (MSR) lead to very high specific interfacial area which, in turn, increases heat- and mass transfer efficiency. However, for quasi instantaneous exothermic reactions, the overall reaction kinetics in MSR is strongly influenced by mixing. The characteristic time of the latter phenomenon varies from millisecond to seconds depending on the operating conditions used. If mixing is the rate determining step for heat release, fast homogenization of reactants may lead to an unwanted hot spot that even in MSR cannot be avoided completely (1). Aims. To study the product distribution of a model reaction carried out in a cooled MSR as a function of mixing quality and temperature profile for the optimization of performance towards the target product. Methods. As model reaction, the quasi instantaneous cyclisation of Pseudoionone to beta-Ionone was chosen. This reaction, which is commercially used in the synthesis of vitamin A and in perfumery, exhibits a reaction enthalpy of -120 kJ/mol. The product distribution can be monitored via gas chromatography and displays sensitivity towards several parameters, including the quality of mixing and the reaction temperature. The MSR used to carry out the reaction was fabricated by Low Temperature Co-Fired Ceramic Technology, which allows the integration of a cooling unit. A newly developed method based on infrared thermometry (2) was applied to investigate the temperature profile inside MSR. It allows quantitative measurements of the axial temperature profile with a precision of 1°C. The adiabatic boundary conditions required for better reproducibility and precision is created by placing the reactor inside a vacuum chamber. Results. By monitoring the axial temperature profile inside the uncooled reactor (quasi adiabatic case), the mixing profile could be deduced. It was demonstrated that the onset of mixing in this MSR lies in the range of Re = 60 leading to a temperature rise close to adiabatic temperature. Subsequently, the coolant temperature was varied between 0 - 40 °C for Reynolds numbers between 30 – 120 and samples were taken at the outlet. As expected, small conversions were observed for the lower range of Re. Best performance was found around Re = 70. A further increase of the latter lead to loss of selectivity due to higher hot spot temperatures. Summary/conclusions. For the first time, axial temperature profiles of a quasi-instantaneous and highly exothermic reaction were quantitatively measured in a cooled MSR. Using this non-intrusive technique, the influence of several parameters, such as the flow rate and cooling temperature, on mixing quality and hot spot temperature was monitored. The low performance at small Re could be attributed to slow mixing, whereas working at too high throughput lead to a loss of selectivity ascribed to limited temperature control. (1) Haber, J.; Kashid, M. N.; Renken, A.; Kiwi-Minsker, L. Industrial and Engineering Chemistry Research 2012, 51, 1474-1489 (2) Haber, J.; Kashid, M. N.; Borhani, N.; Thome, J.; Krtschil, U.; Renken, A.; Kiwi-Minsker, L. Chemical Engineering Journal 2013, 214, 97-105.

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Chargement

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

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Effect of hot spots in microstructured reactors on product distribution during quasi-instantaneous exothermic reactions

Navid Borhani, Julien Haber, Bo Jiang, Lioubov Kiwi, Thomas Maeder, Albert Renken, John Richard Thome

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