Residence Time Distribution of Variable Viscosity Fluids in the Stirred Tank

Stirred tanks are widely used in polymerization processes, where the residence time distribution (RTD) significantly affects monomer conversion and polymer quality. In this study, the RTD in the stirred tank with both constant and variable viscosity fluids was investigated numerically. To account for the viscosity evolution during polymerization, a model relating fluid viscosity to the mean age of the fluid was developed. After verifying mesh and time step independence, the effects of impeller speed, fluid space time, and viscosity varying on RTD were examined in both single-tank and two-tank configurations. Compared to the constant-viscosity fluids, the variable-viscosity fluid shows different flow behaviors such as dead zones and short-circuiting. Analysis based on the number of tanks in series showed that increasing impeller speed and extending space time can enhance mixing efficiency, where the improved mixing in the second stage of the two-tank configuration eliminated the concentration fluctuations caused by recirculating flow in the first tank, which may result in a more uniform RTD curves.