The main aim of mixing in most industrial processes, especially those in agitated tanks, is to control the dispersion state by homogenizing the material concentration and fluid temperature. Most agitated tank reactors have simple geometrical configurations comprising a tank and a mechanical impeller. Nevertheless, the design of these tanks and determination of favorable operating conditions is relatively complicated, considering the differences in the properties of fluids and complex transport phenomena. Therefore, a thorough understanding of the transport phenomena in agitated vessels, such as heat, momentum and mass transfer, is imperative. This is important in designing appropriate processes using tank reactors and estimating other critical parameters like heat transfer coefficient, mixing time and power consumption.
Most chemical reactions require high controllability of thermal properties to improve the rate of reaction to efficiently and safely remove heat from processes. Helical coils are commonly used to control thermal heat transfer in agitated tanks. In addition, heat transfer at the tank wall has also drawn significant research attention owing to its benefits in simplifying the tank geometry for efficient operation and cleaning. To this end, several researchers have studied the heat transfer coefficients of the tank’s sidewall in different flow regimes. Although mass transfer coefficients at the sidewall of the agitating tank are predominantly dependent on the impeller configurations, recent studies show that it is also influenced by the friction factor. Unfortunately, the friction factor distribution in agitated vessels with large impellers is not fully explored. This is of great significance due to the benefits of large impellers, including superior performance in different processes.
To fill this research gap, Dr. Yusuke Ochi, Professor Yoshiyuki Komoda and led by Professor Naoto Ohmura from Kobe University in collaboration with Dr. Katsuhide Takenaka from Sumitomo Heavy Industries Process Equipment Company, studied the local distribution of the friction factor on the sidewall in a turbulent agitated vessel equipped with MAXBLEND impeller using computational fluid dynamics simulation. The numerical procedure was validated by calculating the fluid flow in the vessel and the resulting velocity near the wall. The work is published in the journal, Industrial and Engineering Chemistry Research.
The research team reported a larger friction factor value at the sidewall in a range characterized by strong discharge flow and a smaller friction factor value in the upper part of tank wall. A drastic increase in the friction factor, especially near the baffles, was observed when the baffle clearance was added. Consequently, the friction factor in the baffled vessel exhibited large-scale fluctuations when the baffle cleared was installed. These results suggested the possible improvement of heat/mass transfer as the sidewall of the turbulent agitated vessel. In addition, the impeller induced a circulation flow that was, however, deformed upon adding the baffle clearance, resulting in a more complex flow pattern.
In a nutshell, a numerical investigation of the effects of the clearance between the sidewall and baffles on the friction factor of a turbulent agitated vessel with a large impeller was reported. The velocity distribution of the paddle impeller obtained numerically agreed well with the theoretical and experimental values, indicating the reliability and accuracy of the numerical procedure. In a statement to Advances in Engineering, Professor Naoto Ohmura explained that the study provides valuable insights that would enhance heat and mass transfer and the overall performance of agitated vessels with larger impellers used in various industrial processes.
Ochi, Y., Takenaka, K., Komoda, Y., & Ohmura, N. (2022). Friction Factor Distribution at the Side Wall of a Turbulent Agitated Vessel with Baffles Using a MAXBLEND Impeller. Industrial & Engineering Chemistry Research, 61(3), 1514-1522.