Dedicated to Professor Rüdiger Lange on the occasion of his 65th birthdayA flexible reactor and measurement setup to safely obtain thermokinetic data for exothermic chemical reactions in plate-type microreactors is presented. Precise heat flux measurement is realized by means of Seebeck elements and allows for direct as well as space-and time-resolved heat flux determination across the reactor. The microreactor used in this work is manufactured from poly(vinylidene fluoride) foils and consists of 11 SZ-shaped mixing channel elements. The Seebeck elements are calibrated by the Joule effect, while its performance is demonstrated in heat transfer and neutralization reaction experiments. Furthermore, the local resolution enables an estimation of mixing time scale for rapid reactions. A comparison of obtained results indicates good accordance with literature data and is the base of further investigations using the calorimeter.

Reaction calorimetry is one of the most important steps in designing chemical reactors. This contribution describes a continuously operated micro calorimeter using Seebeck elements for microreactors made of PVDF-foils. Seebeck elements allow for local and temporal resolution of heat flux profiles. Various calibration methods for the Seebeck effect based heat flux sensors are presented. Here, the direct correlation between measured thermoelectric voltage and heat flux is found to be the most promising one. Commissioning of the calorimeter and validation of its performance are done by means of heat transfer measurement of warm water and an acid base reaction. Obtained reaction enthalpy values of the neutralization reaction of acetic acid and sodium hydroxide agree very well with literature data. The progression of the reaction can be followed optically using phenolphthalein as color indicator and can be compared to measured data. Heat profiles over the course of the microreactor were shown and checked for consistency. Consequently, this approach helps to characterize reactors and aids reactor development.

Transmission infrared in situ measurement technique was used to assess its feasibility for real-time process analysis. In particular, the advanced chemometric analysis by band-target entropy minimization (BTEM) was implemented to obtain pure component spectral estimates from the multicomponent reaction system. In this case study, the rhodium-catalyzed reductive amination of 1-decanal in thermomorphic multiphase systems was investigated. This complex reaction/separation approach consisted of more than 10 IR-active components so that the herein-implemented BTEM technique, built for improved data handling, was highly challenged. Seven parameters with significant impact on the BTEM algorithm were identified and investigated regarding the general resolution quality of each spectral estimate. Herein, the BTEM algorithm was extended with an automatic band-target selection and parameter limits regarding the Savitzki–Golay filter for tuning raw data. Finally, the source code of the algorithm for MATLAB is published to find use in extended spectroscopic applications.

Continuous reaction calorimetry in microreactors is a powerful technology for the investigation of fast and exothermic reactions regarding thermokinetic data. A Seebeck element based reaction calorimeter has been designed, manufactured, and its performance has been shown in previous works using neutralization reaction in a microreactor made from PVDF-foils [1]. The Seebeck elements allow for spatial and temporal resolution of heat flux profiles across the reactor. Therefore, hot spots and regions of main reaction progress are detected. Finally, heat of reaction has been determined in good agreement with literature data [1]. However, more information can be retrieved related to chemical transformations using the continuously operated reaction calorimeter. In this work, mixing time scale is determined for instantaneous and exothermic reactions. Volumetric flow rate is varied and the region of main reaction progress is shifted within the microreactor. The reaction occurs near the reactor outlet for low flow rates. Here, mixing is dominated by diffusion. However, the reaction and hot spot are shifted towards the reactor inlet for high flow rates as convective mixing regime is reached and secondary flow profile with Dean vortices develop due to curvature of the reaction channel. Finally, mixing time scales can be derived from the location of heat flux peaks. Results display a decrease in mixing time at increased flow rates. Additionally, passive micromixers can be evaluated regarding their efficiency and comparison can be drawn. Moreover, pumps can be characterized and evaluated regarding low-pulsation dosing using the Seebeck element based reaction calorimeter.