Numerical investigations of flow and heat transfer of polymer melt in underwater extrusion pelletizers

Abstract

The thermoplastics compounding industry extensively utilizes underwater die-face pelletizers, where, the polymer melt is extruded through a capillary, then cut, stress-relaxed and cooled by a medium, which all have a significant impact on the final pellet quality. Some of the common practical problems in these applications are referred to as (1) “die-hole freeze-off”, due to the low temperatures and (2) “pellet marriages”, due to the polymer being too soft. In order to address some of these issues, three-dimensional (3D) computational fluid dynamics (CFD) calculations of non-Newtonian flow of a polymer in an extruder, along with the turbulent flow of heating oil and the conjugate heat transfer through the die are carried out using ANSYS Fluent. The computational model is validated by comparing predictions of a probed temperature in the die plate, and pressure drop within the polymer, to experimental measurements in the industrial-scale pelletizer, for five different operating conditions, with the maximum resulting error being <13%. In addition, this study investigates the effect of three control parameters: (Case Study I) thermal conductivity of the die plate surfacing, (Case Study II) inlet temperature of the polymer and (Case Study III) inlet mass flow rate of the polymer, on potential pellet quality, by analyzing the flow and thermal characteristics. Specifically, contours of temperature in the die plate and polymer, and also axial and radial profiles of pressure, viscosity, temperature, velocity magnitude, shear rate and shear stress, in the polymer, are analyzed in detail to assess the possibilities of freeze-off and higher yield loss. The temperature behavior showed that die plate surfacing with lower thermal conductivity would delay freeze-off. Shear-stress profiles helped identify the cases with higher probable freeze-off, which included those with polymer inlet temperatures >225∘C and mass flow rates <8 kg/hr and >12 kg/hr.