Influence of the dimensions of spheroniser plate protuberances on the production of pellets by extrusion-spheronisation

Abstract

The influence of the dimensions of square-patterned pyramidal protuberances on a spheronisation plate on pellet yield, size and shape distributions, surface tensile strength and surface morphology was investigated using a 45 wt% microcrystalline cellulose/water paste.Tests were conducted using four extrudate diameters (1.0, 1.5, 2.0 and 2.5 mm) generated by screen extrusion and seven plate geometries, including a flat plate as a control. Geometrical analyses of the protuberance shapes provide some insight into the observed differences.Sharper protuberances reduced yield (promoting breakage and attrition) but tended to give narrower size distributions and less even pellet surfaces. The pellet yield for 1 mm extrudates was also subject to losses caused by fragments passing through the 1.0 mm gap between the plate and the spheroniser wall. Pellet tensile strengths were noticeably greater for 1.0 mm diameter extrudates, which is attributed to the greater extensional strain imparted on the paste during the extrusion step. For some geometries there is an optimal ratio of extrudate to protuberance dimensions.

Modeling the breakage stage in spheronization of cylindrical paste extrudates

“…Paste with 47 wt% MCC was prepared in batches of 250 g. A planetary mixer (Kenwood Chef KM200, Kenwood Ltd) was used to mix the MCC powder and deionized water based on the procedure in Reference 11. Briefly, the mixer was run at speed settings “min,” 1, 2, 3, and 4 for 1, 2, 3, 3, and 2 min, respectively.…”

“…The breakage stage is nevertheless important as it determines the number of pellets and thus their average size, as well as the formation of fines, which may either be retained or lost from the process. 11 Fines play an important role in rounding as they re-attach to the pellets. 12,13 The tendency to generate fines is governed by the energy involved in collisions (and thus operating conditions), the rheology of the material (and thus formulation), and drying (changing the binder content and hence affecting the rheology, driven by operating conditions such as humidity, temperature, and rotational speed).…”

“…The breakage stage is nevertheless important as it determines the number of pellets and thus their average size, as well as the formation of fines, which may either be retained or lost from the process 11 . Fines play an important role in rounding as they re‐attach to the pellets 12,13 .…”

“…A second characteristic length scale in spheronization is that associated with the features on the friction plate. Zhang and co‐workers 11,15 investigated the influence of protuberance geometry on spheronization behavior. For the pyramidal cross‐hatched patterns employed in this study (see Figure 1B) they identified a critical extrudate diameter, D c , below which cylindrical fragments are expected to sit in the cavities between the protuberances: …”

“…The influence of geometry was also investigated by varying the extrudate diameter from 2 to 3.5 mm with friction plate S4, giving 1.2 ≤ w* ≤ 2.1 and 0.7 ≤ D* ≤ 1.2. Smaller extrudate diameters suchas those investigated by Zhang et al11 were not investigated as they tended to dry out quickly and were therefore not suitable for interrupted experiments due to the time required to prepare extrudates. They also tended to fall through or be caught in the gap between the friction plate and the spheronizer wall.Figure 6shows similar values of β (given the limited number of tests conducted).…”