Separating the effect of die geometry and mash residence time in the die on biomass and livestock feed pellet manufacturing

Pellet manufacturing allows for the improvement of the handling properties of biomass streams. Increasing channel length of the ring-die holes generally increases physical pellet quality. This phenomenon is often attributed to an increase in pressure gradient over the die and greater densification of the mash during compaction. It remains unknown, however, to what extent this improvement in physical pellet quality is attributable to the concomitant increase in die volumetric content, and, therefore, longer mean residence time (MRT) of the mash in the die. Here, we aimed to separate the effect of die geometry and MRT on physical pellet quality, by combining a pilot scale pellet manufacturing experiment with capillary rheometric analysis, for mashes containing different fibre-rich coproducts from food production. Net energy costs of compaction (in kWh t⁻¹) were unaffected by production rate. Capillary rheometric analysis indicated that the work of friction between mash and the die hole wall did not increase at flow rates exceeding 1.0 mm³ s⁻¹, suggesting the occurrence of plug flow behaviour. Hence, the work of friction generated between mash and the die hole wall depends on the friction coefficient between mash and the die hole wall and the total surface area of the ring-die. Increasing production rate decreased physical pellet quality (−2.56 to −0.93 kPa (kg s⁻¹)⁻¹). Since the friction generated between mash and the die was unaffected by production rate, we conclude that this reduction in physical pellet quality is attributable to the concomitant reduction in MRT.