Barbara Fretter & Michael Schupp ask whether tamp auger is a misleading name
During the dry granulation process (Fig. 1), material is densified between two counter rotating press rolls – the core of a roller compactor. Material is conveyed by the dosing system to the rolls, but the dosing system is not responsible for the material movement during densification. The press rolls draw the material towards the gap (the smallest distance between the press rolls), and the draw-in is based on frictional forces between material and press rolls (Fig. 2).
Sometimes the influence of several feeding unit settings on ribbon or granulate properties is examined and additionally, it is stated that the feed auger speed is a main process variable. In principle, this is not correct. The feed auger is responsible for conveying the powder to the gliding area and the rolls. In general, the larger the feed auger speed, the more material will be transported. The applied specific roll force, the powder densification properties and the amount of powder conveyed to the rolls and by the rolls (= roll speed) are responsible for the resulting gap. Changing the powder flow results in a change of the gap. Usually, poorly flowable material is conveyed by the augers, which means that the mass flow towards the rolls might be unsteady. Even a constant auger speed will result in fluctuations of the gap. Therefore, not the auger speed but the resulting gap is the actual process variable, which is the second most important parameter in roller compaction, after the specific roll force. The control of the powder flow is the operating principle of the control loop with which variable gap roller compactors keep the gap constant: the speed of the feed auger is permanently adjusted to keep the gap at set value. This can be observed by analysing process diagrams carefully.
In the following two examples of those process diagrams are given for GranuLac 200 and Avicel PH 101 are shown, which were compacted at different specific roll forces (Fig. 3 and Fig. 4). For GranuLac 200 the gap fluctuates at the beginning of the process. The gap control loop tries to achieve the set value of 3mm by adjusting the tamp auger speed, which shows the same periodical fluctuations as the gap. By increasing the auger speed, more material is conveyed between the rolls and the gap starts to increase.
For both materials a larger specific roll force results in a larger degree of densification. To keep the gap at the set value, more material must be conveyed to the rolls correlating to a larger average auger speed (Table 1 and Table 2). This effect is more pronounced for the microcrystalline cellulose (Avicel PH 101), because its densification requires less force, which is a material property.
The diagrams also show that the tamp auger does not really tamp the material into the nip area. The material is not or at most only slightly densified by the tamp auger, which means no or only a small pressure is present in the slip region and at the beginning of the nip area. A large density in the nip area would require some force applied by the tamp auger which in return would result in a larger torque of the auger drive. This gets obvious by the tamp auger torque and its trend for both examples above. After an initial peak when starting up the process, the tamp auger torque for GranuLac 200 is small and almost constant (2.7 to 2.9 Nm, Table 1). Additionally, it is independent of the applied specific roll force and the amount of material conveyed to the rolls. For Avicel PH 101 an increase of the tamp auger torque can be realised for larger specific roll forces (1.4 Nm to 2.4 Nm, Table 2). This can be explained by some accumulation of powder in the gliding area and a small compression by the tamp auger. Therefore, a slightly higher resistance
of the rotational movement of the tamp auger can be detected.
Nevertheless, both examples show, that other than its name assumes, the tamp auger does not tamp the material into the nip area. The main function of the auger system is to convey the material to the gap. The draw-in itself is done by frictional forces between roll surface and powder particles.
Michael Schupp is with Gerteis Maschinen + Processengineering & Barbara Fretter is with Solids Development Consult.
