Numerical simulation of the 3D flow field in a ring die pellet mill considering viscous heating

Viscous heating has a substantial influence on the extrusion forming process and product quality of powder materials.This study selected the MUZL420 ring die pellet mill as the research object,from which a 3D flow physical model was established.The numerical simulation of 3D nonisothermal flow in the extrusion pelletizing process of granulated alfalfa was performed with POLYFLOW.The distribution laws of pressure,velocity,shear rate,viscosity,viscous heating and temperature in the flow field were revealed to thoroughly investigate the pelletizing process and provide a reference for structural optimization and process control.The results showed that two extrusion zones in the pelleting chamber were symmetrical with respect to the center,and the significant pressure gradient along the rotating direction of the ring die and the roller caused the material to flow back in the opposite direction.There were larger velocity gradients,shear rates and viscous heating levels in the deformation and compaction zone,the negative pressure zone behind the extrusion zone and the die holes.The distribution of viscosity was opposite to that of the shear rate.The temperature increase area caused by viscous heating gradually expanded from the material inlet to the bottom of the extrusion chamber along the Z-axis direction,and the temperature increased accordingly.The extrusion force and the forming temperature in the extrusion forming zone were captured in the numerical simulation.The extrusion forming density was calculated with the regression prediction model established through the simulation experiment of pelletizing with a ring die.Through a comparison with the results of mean alfalfa pellet density from the ring die pellet mill experiment,the relative error was less than 5%,which indicated that the numerical simulation method was reliable.

Development and experimental study on a pilot-scale feed pellet mill

Pilot-scale pellet mill has many advantages and great potential application prospect.In this work,a structure of pilot-scale pellet mill was designed and the effects of moisture content(12%-20%w.b.),temperature(55°C-95°C)and ring die speed(160-320 r/min)on the production and physical properties(pellet durability,bulk density and hardness)of the produced feed pellets were determined and analyzed.The results showed that pellet durability ranged from 90.47%to 96.92%,bulk density 537.4 kg/m3 to 62.2 kg/m3,hardness 5.26 kg to 9.65 kg and production 26.51 kg/h to 42.81 kg/h.Pellet durability was found to increase with the increase of temperature and moisture content,but to decrease with the increase of ring die speed.Bulk density showed a monotonic increase with temperature and ring die speed.Hardness increased with the increase of temperature,but exhibited a first ascending and then descending trends with the increase of moisture content.It also indicted that high ring die speed favored production.Due to its fast and cheap production in a suitable scale,the designed pilot scale pellet mill prototype and the comprehensive research would contribute to the production of feed pellets in various processing conditions with different feed formulas,like additive selection and so on,in order to meet diversified demands.