Hydrodynamic CFD modeling of a pharmaceutical reactor vessel provided with a retreat-blade impeller under different baffling conditions

In the pharmaceutical industry, glass-lined reactors and vessels are often utilized to carry out a variety of different unit operations. Within these systems, both the vessel and impellers are typically glass-lined in order to provide superior corrosion resistance, prevent product contamination, and enhance cleanability. This approach, in turn, often requires the use of different, and sometimes sub-optimal, baffling conditions, which affect the hydrodynamics of the vessels and the reactor performance. Computational Fluid Dynamics (CFD) is a computational tool that employs numerical methods and algorithms to discretize and numerically solve partial differential equations (PDEs) representing mass, energy, and momentum conservation equations for the purpose of analyzing fluid flow problems. In recent years, CFD has been used successfully to model hydrodynamically complex systems such as stirred mixing systems. A variety of computational approaches and models are implemented in the CFD code to do so, including single reference frame (SRF), multiple reference frame (MRF), and sliding mesh (SM) models, also possibly combined with Volume of Fluid (VOF) models. In this study, a scaled-down version of a pharmaceutical glass-lined reactor vessel equipped with a retreat curve impeller (RCI) and a torispherical bottom is modeled using the CFD COMSOL software under a variety of setups, including variations in impeller speed, impeller clearance, and baffling conditions. Several modeling approaches are used. The CFD simulations result in the prediction of the power dissipated by the impeller and therefore the impeller Power Number. These predictions are then compared with the experimental results obtained in previous work by this group. In the fully baffled system, the values of the Power Numbers predicted by the simulations under turbulent conditions using MRF modeling are in close agreement with the experimental results across all tested impeller rotational speeds. In the partially baffled system, the results obtained with MRF modeling are very consistent with the experimental results. However, even better agreement is obtained when using the much more computationally expensive SM modeling technique. Finally, the simpler SRF approach proves to be very appropriate to model the unbaffled system, and good agreement between the simulation predictions and the experimental results is obtained, but only if the surface deformation of the liquid-air interface typically observed in unbaffled systems is small. It can be concluded that the computational method used to simulate the hydrodynamic behavior of a pharmaceutical reactor vessel generates predictions that are in close agreement with experimental results, thus validating the CFD approach used to model this system