CFD Based Qualification of Mixing Efficiency of Stirred Vessels

Abstract

In this work, we focus on the most crucial units in a chemical technology, the chemical reactors. Using a commercially available CFD software package, COMSOL Multiphysics, 3D mathematical models of a batch reactor with different impeller geometries have been investigated. The reasonable agreement between the experimental and simulation results indicates the validity of the developed CFD model. The effect of the impeller design, e. g. number of blades on the mixing efficiency is evaluated based on the simulation studies.The proposed measure to determine the energy efficiency of mixing (i. e. mixing index) is based on the calculated velocity field and energy usage. The information about the homogeneity of the mixed phase in the system can be extracted from the developed velocity field. Hence, we proposed histograms of velocity fluctuations on a logarithmic scale as an efficient tool to measure the achieved homogeneity of the phase in case of different impellers and rotational speeds. IntroductionDuring the last few decades, computational fluid dynamics (CFD) method has become a very powerful tool in the process industry primarily to analyse the hydrodynamic behavior. In general, CFD methods rely on the numerical solution of the Navier-Stokes equations, thus flow velocity, velocity magnitudes, and pressure values can be calculated in the geometric space investigated. A particularly significant benefit of the CFD simulators is that they can include entire apparatuses in three dimensions, regarding either single phase or multiphase systems [1,2]. This property of the CFD simulators makes them useful in a large number of research areas, including the study of mixing phenomena. Fluid mixing is a fundamental operation in the chemical process industry and the stirred tank or autoclave is a commonly used reactor type in many kinds of chemical technologies; therefore these systems are well described. Regardless, when it comes to design work, simple experimental correlations are still commonly used, e.g. Reynolds number, Froude number, power number or the Euler number which is derived from the previous ones. This approach has its limitations when it comes to the design of stirred vessels with complex internal structure.

Multi‐impeller mixing performance prediction in stirred tanks using mean age theory approach

“…In transient simulations, firstly, a pseudo‐steady flow field 32 was obtained using the SLM model before the tracer injection started. Then, to mimic the experimental injection process, the passive scalar was introduced through the inlet for 0.5 s with m/s.…”

Mixing Efficiency of Viscous Fluid in a Multiple‐Impeller Agitated Tank

“…The results confirm that the multi-impellers systems are necessary to decrease the weakened zones. Till et al [22] used a commercially available computational fluid dynamics (CFD) software package, COMSOL Multiphysics, and investigated 3D mathematical models of a batch reactor with different impeller geometries. The proposed measure to determine the energy efficiency of mixing, i.e., mixing index, is based on the calculated velocity field and energy usage.…”

Numerical Study and Geometric Investigation of the Influence of Rectangular Baffles over the Mixture of Turbulent Flows into Stirred Tanks

“…Among the investigated shapes, a cylindrical shape is the most common [2]. Concerning the impellers, different configurations have also been explored, as well as their placement in the tank [3,4].…”

“…In general, an increase in the momentum of the flow leads to a decrease in dead spots in the volume of the tank, leading to a decrease in mixing time. Therefore, in order to reduce the dead zones and intensify the rate of mixing, baffles are distributed on the tank walls for directing the flow, breaking vortices and contributing to energy dissipation [3,6].…”

“…[16] performed a numerical study of the influence of an impeller with a regular blade and another with fractal blades on power consumption, and showed that fractal blades reduced torque compared to regular blades, since the drag coefficient of the fractal blade was distinctly lower than a regular blade. Till et al [3] studied the effect of the number of blades for the impeller and different rotation speeds on the mixing efficiency and power consumption in a stirred tank. They concluded that the electrical power requirement for an impeller depended largely on the shape and the size of the impeller and, of course, the rotational speed.…”