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

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.