(578f) Steady State CFD for Determining Just Suspended Speed

A new method for determining the just suspended speed (Njss) in stirred tank reactors was developed using steady state CFD simulations. This method can be used for determining Njss for those systems in which it would be difficult with traditional experimental techniques. It also removes subjectivity associated with traditional experimental visualization techniques. Additionally, since the technique employs steady state simulations, it is advantageous over time-dependent simulations, which require much longer computational times due to the multiple phases and transient nature. Using an application of mean age theory, the method solves scalar transport equations within the settling zone along the tank bottom. Three-dimensional simulations have been performed for a range of solids concentrations and various tank geometries. Computational results compare very favorably with published experimental data. Plotting average age for the solid phase in this region versus impeller speed reveals three distinct regions, with the narrow central region corresponding to Njss. The three regions are believed to be caused by the interaction between solid fraction in the region and z-direction velocity of the solid phase: above Njss, the average age is low, due to large z-velocities; below Njss, the average age is also low, due to the higher concentration of the solid phase allowing for greater scalar transport; at Njss, the mean age spikes sharply due to the lack of either of these two factors.