TABLETING AND LAB CONSUMABLES Dry
Understanding the nip angle by Barbara Fretter and Michael Schupp
GRANULATION T
he Thin Layer Model allows for understanding of some general aspects of powder densification
between the rolls in dry granulation and especially the nip angle. Part 1[1] describes the model and how the nip angle can be estimated easily. It also outlines that a stronger densification results inevitably in larger nip angles. Part 2 focuses on the interaction of nip angle and gap and the practical meaning in R&D and Scale-Up.
THE THIN LAYER MODEL Based on the geometric considerations of the Thin Layer Model it is possible to plot the nip angle versus the densification factor of the powder for different gaps (Figure 1). And an important relation can be derived: to achieve the same densification factor at a larger gap requires a larger nip angle - see Figure 1. For example, a densification factor of two requires a nip angle of 5o at a
Figure 1
gap of 1mm and a nip angle of 10o at a gap of 4mm. This has some practical consequences for R&D and Scale-Up. Let’s assume that in R&D you have found the right granule properties for your product at certain roller compactor settings. Often small gaps like 2mm or less are chosen in R&D because the available amount of Active Pharmaceutical Ingredient (API) is limited. Additionally, the specific roll force (or a range of it) is defined. Although force and gap are specified for this product, the actual parameter determining the granule properties is the ribbon or at-gap density and they coincide with a certain densification factor. For reproducing the granule properties the at-gap density must be reproduced. This is also valid
Figure 2
when the gap is increased. Because the powder density when being drawn-in is mainly independent of the gap, a general strategy for changing the gap is to keep the densification factor constant. And as shown in Figure 1, increasing the gap at the same densification factor means that the densification must start at larger nip angles. Otherwise, the granule properties will change.
MISTAKES MADE IN DRY GRANULATION Realising this correlation has some serious consequence for the specific roll force and ignoring it is one of the most made mistakes in Scale- Up. When increasing the gap at the same specific roll force, the resulting at-gap density cannot stay the same. Figures 2 and 3 give the explanation. By using the Thin Layer Model and simple geometric considerations (Figure 2) the solid fraction in each layer can be calculated for different gaps. Assuming that an at-gap solid fraction of 0.7 is the target value which should not be changed if the gap is increased. Figure 3 shows the progression of the solid fractions for two gaps, 2mm
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