1740: Reactive Bubbly Flows

Coordinator: Prof. Dr.-Ing. Michael Schlüter

The conversion of chemical substances with high selectivity and yield is one of the major tasks in Chemical Process Engineering. For the production processes of many bulk chemicals it is necessary to bring gaseous substances in contact with a continuous liquid phase (e.g. oxidation, hydration and chlorination). In the 1960s Bubble Column Reactors have been developed for this purpose, which allowed an intensive mixing for large reaction volumes and long residence times. In the 1980s an increased amount of Bubble Columns with defined mixing zones (e.g. loop reactors) showed up the potential of specific flow configuration. In the late 1980s an increased detailed consideration of bubbly flows began and started to boom in the 1990s. Once it was shown for single-phase flows that chemical reactions are influenced by the mixing on different time and length scales, large optimization potential was identified for reactive bubbly flows due to the additional transport resistances through phase boundaries and boundary layers. However, multi-scale transport processes with a coupled reaction could not be adequately described so far. With the result, that reaction rates were often assumed and therefore include an unknown part of mass transfer in addition to the intrinsic kinetics. Thus, the transferability of models and the predictive forecast of yield and selectivity are very limited. Up to now, there were neither experimental nor numerical tools with sufficiently high spatial and temporal resolution available to determine the uninfluenced reaction rates.

Due to the rapid development of microelectronics, LCD, OLED and image sensor technology this situation has improved significantly. The experimental determination of intrinsic kinetics and the elucidation of single reactions steps as well as local mass transfer processes are today available through new miniaturized reactors, research methods and measurement techniques, which allow resolving the processes on the smallest time and length scales. In addition tremendous progress was made in the field of numerical simulation through new methods (e.g. consideration of concentration jumps at phase boundaries and interfacial contamination) and the new high-performance computers. To handle the challenges of a predictive process control, these experimental and numerical progresses have to be harnessed for the process technology and mass transfer and reaction steps locally coupled.

The new possibilities for the elucidation of reaction networks and local transport processes as well as for the numerical simulation of gas-liquid interfaces shall be used in the priority program specifically on the systematic analysis of complex technical processes. Therefore it is necessary to characterize the reaction systems as accurate as it is required for the detailed registration of the interaction of gas-liquid mass transfer and reaction. New methods for setting defined mixing ratios (micro reactors, turbulence generators) shall here also be used, as new measurement equipment (e.g., Rotary Chamber, Taylor-Flow Capillaries) and new analytical methods (e.g., Resonance Raman, Coherent Anti-Stokes Raman and Two-Photon Spectroscopy). In the numerical simulation, new methods for the calculation of mass transfer and the implementation of reactions are necessary (e.g. dynamically adapted meshes in combination with parallel computing techniques). For the research program experimental and numerical methods should therefore be equally developed and applied to the analysis and calculation of reactive bubbly flows to allow a predictive forecast of yield and selectivity for chemical reactions overlaid with mass transfer limitations.

Explicitly excluded are basic investigations exclusively on single bubbles, on the phase transition (condensation/evaporation/boiling), on three-and multiphase flows, on particulate flows, on two-phase flows in porous media and the exclusive new development of measurement techniques and numerical methods.