Comparison between a twin-screw extruder and a rotary ring die press. Part II: influence of process variables.

The influence of different processing steps on pellet quality was investigated: granulation/extrusion and spheronization. Pellets were produced at two levels of water content on two different types of extruders: a twin screw extruder and a rotary ring die press. In order to control the spheronization process each extrudate was rounded in two spheronizers using two radial velocities, respectively. Pellet shape and size were selected to describe the pellet quality. Under constant spheronization conditions the extrudates behaved dissimilar on the two spheronizers. This could be attributed to the geometry of the friction plates. The spheronizer with the rougher surface applied more mechanical energy to the extrudate and wet pellets which reduced the water content necessary for the formation of good pellets. Eliminating the influence of the spheronization process, high differences were observed in the quality of the extrudates produced by the two extruders. This confirmed the results from the first part of this study. Due to the crystallite-gel-model the different extruder types apply different mechanical stress on the extrudate which affect the network structure of the microcrystalline cellulose gel. The twin-screw extruder produced a more delicate network with a lower water movement. This led to a shift of the optimal moisture content towards higher values. Compared with spheronization, process changes in the granulation/extrusion process were more critical.