Determining Local Mixing Time Distribution in Stirred Tank Reactors

There are many methods to measure mixing time and characterizing mixing processes in stirred tank reactors. In this paper, researchers at the Institute of Multiphase Flows and Boehringer Ingelheim collaborate to propose a novel image analysis method for the detailed characterization of mixing processes—and use M-Star CFD software to validate that new method.

Mixing plays a key role in chemical and bioprocess engineering because it controls the process performance in terms of solid suspension, homogenization, dispersion of phases, heat transfer, and chemical reactions. Rotating impellers, however, create turbulent flows, leading to a variety of different mixing structures. This variety requires engineers to subdivide the mixing into macro, meso, and micros scale contributions to better understand the conditions of the mixing.

Proper mixing performance—characterized by the global mixing time—is a critical requirement to ensure ideal transport processes. Historically, to determine mixing time researchers have recorded time until a tracer signal has reached a constant value after a peak has been introduced.

However, there are disadvantages to this method. For one, the position of tracer feeding and the probe significantly influence the detected mixing time. Further, the global mixing time does not provide any information about the spatial and temporal “history” of the mixing process to identify areas that are mixed poorly or areas that form stable compartments.

To overcome this disadvantage, researchers have presented a novel image analysis method for the detailed characterization of mixing processes. The method is based on the experimental determination of the local mixing time distribution by using a multi-color change caused by a pH-change in a bromothymol blue solution.

To demonstrate the suitability of the new characterization method, a Lattice Boltzmann–based calculation with M-Star CFD was employed. The exemplary application of image analysis to a numerical mixing time simulation shows good agreement with the corresponding experiment.

In the full paper, the researchers:

• explain the history of quantifying mixing efficiency and discuss past methods of measurement.
• show the disadvantages of the current measurement method.
• propose a new method to determine local mixing time, validated by M-Star CFD software.

Read the full paper to learn more about the determination of local mixing time distributions and see the full results.