Infrared imaging of temperature profiles in microreactors for fast and exothermic reactions

Though microreactors provide high heat transfer rates, temperature profiles are formed in case of fast and highly exothermic reactions. A method based on non-intrusive infrared thermometry has been developed to track these profiles quantitatively with a resolution of 250 data points/cm(2) and a precision of 1 degrees C. By placing the microreactor at a pressure of 10(-2) mbar, convective heat losses are efficiently suppressed which assures well defined boundaries and reproducible results. The temperature profiles of the fast and exothermic hydrolysis of tetraethoxysilane were measured inside a microcapillary at different flow rates via infrared transparent window. In the reactor provided with a cooling system, the heat transfer coefficient was determined by thermal imaging for different operating conditions. The data obtained are in good agreement with the values reported in literature. (C) 2012 Elsevier B.V. All rights reserved.

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

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.

2013

CFD modelling of liquid–liquid multiphase microstructured reactor: Slug flow generation

Madhvanand Namadeo Kashid, Lioubov Kiwi, Albert Renken

Microreactor technology, an important method of process intensification, offers numerous potential benefits for the process industries. Fluid–fluid reactions with mass transfer limitations have already been advantageously carried out in small-scale geometries. In liquid–liquid microstructured reactors (MSR), alternating uniform slugs of the twophase reaction mixture exhibit well-defined interfacial mass transfer areas and flow patterns. The improved control of highly exothermic and hazardous reactions is also of technical relevance for large-scale production reactors. Two basic mass transfer mechanisms arise: convection within the individual liquid slugs and diffusion between adjacent slugs. The slug size in liquid–liquid MSR defines the interfacial area available for mass transfer and thus the performance of the reactor. There are two possibilities in a slug flow MSR depending on the interaction of the liquids with the solid wall material: a dispersed phase flow in the form of an enclosed slug in the continuous phase (with film—complete wetting of the continuous phase) and an alternate flow of two liquids (without film—partial wetting of the continuous phase). In the present work, a computational fluid dynamics (CFD) methodology is developed to simulate the slug flow in the MSR for both types of flow systems. The results were validated with the experimental results of Tice et al. (J.D. Tice, A.D. Lyon and R.F. Ismagilov, Effects of viscosity on droplet formation and mixing in microfluidic channels, Analytica Chimica Acta 507 (1) (2004), pp. 73–77.).