Controlling Law of Flow and Mixing Behavior in an Autoclave for the Production of Low-Density Polyethylene

Abstract

This work investigates how typical impellers control flow and mixing behaviors within low-density polyethylene (LDPE) autoclaves via well-verified CFD simulations. Results show that turbine impellers mainly control axial circulation flow, and the input power per unit volume (p) and blade-to-wall distance (D – d)/2 severely affect its action strength. Defining the circulation flow ratio (r c) to quantify circulation flow strength, increasing p or decreasing (D – d)/2 will enlarge the effective zone of the turbine impeller and average circulation flow ratio (r a) in it. Furthermore, a correlation of r a is established as r a = true( D − d 2 true) 2 · false( a · log ( p ) + b false) + c , and r a must be larger than 0.45 to maintain stable operation. For pitched-blade impellers, it can enhance local micromixing, and the pitched blade number and tilt angle of each blade control the enhancement effects. Adjusting the pitched blade number but keeping the tilt angle below 10° is the most recommended to enhance micromixing. The above results provide significant guidance for the design of LDPE autoclaves.