TABLETING AND LAB CONSUMABLES
What is dry granulation? Granulation is a process in which powder particles are made to adhere to
each other, resulting in larger, multi-particle entities, so called granules. If such a process is performed without adding liquids, this is called dry granulation. In dry granulation, the powder blend is compacted by applying a force
onto the powder, which in general causes a considerable size enlargement. The compacts obtained are called ribbons, flakes or briquettes. In order to obtain the desired granules, the compaction process is followed by a milling step.
gâIncreasing the ap at the same
densification factor means the densification must start at larger nip angles
Figure 3
gap on ribbon or at-gap density can be found in literature, however, unfortunately, not always with the right explanation. Allesø [2] examined ribbons porosities for microcrystalline cellulose at two gaps and three different specific roll forces. His findings
and 4mm, towards the assumed at-gap solid fraction of 0.7. It gets obvious, that for each distance the solid fraction for the 4mm gap is larger than for the 2mm gap. Transferring this to the Thin Layer Model (Figure 2) means each horizontal layer must have a larger solid fraction for the 4mm gap than for the 2mm gap. But a larger solid fraction implicates a stronger densification and therefore a higher force which acts onto the rolls. The sum of all forces which act simultaneously onto the rolls must be higher. This is the reason changing the gap without adapting the specific roll force never results in the same granule properties. Unfortunately, this is one of
the most made mistakes in roller compaction especially in development and scale-up when increasing the throughput by increasing the gap. Several examples for the influence of
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(Figure 4) prove the above made considerations: at the same
Figure 4
specific roll force and a lager gap the resulting solid fraction of the ribbon is lower. The extend of this effect is caused by the densification properties of the material and substance specific.
LARGER GAPS REQUIRE LARGER NIP ANGLES Based on the Thin Layer Model and geometric calculations an explanation can be given why larger gaps require larger nip angles to achieve the same at-gap density. As consequence, changing the gap without adapting the specific roll force result in granules with different properties. This is an often made mistake in Scale- Up. To achieve the same granule properties upon increasing the gap an increase of specific roll force is mandatory but in its extent substance specific. n
References: [1] Barbara Fretter, Michael Schupp, Understanding the Nip Angle, Eurolab (Dec 2023),
https://content.yudu.com/web/15ex3/0A2nilh/EurolabDec2023/html/index. html?page=52&origin=reader
[2] Allesø,M., et al., Roller compaction scale-up using roll width as scale factor and laser- based determined ribbon porosity as critical material attribute, European Journal of Pharmaceutical Sciences (2015),
http://dx.doi.org/10.1016/j.ejps.2015.11.001
Contributors: - Barbara Fretter, Managing Partner, Solids Development Consult GmbH, Peter- Joseph-Lenné Str. 11, 51377 Leverkusen, Germany, +49 214 500 428 83,
www.solids-
development.com
- Michael Schupp, Head of Process Engineering, Gerteis Maschinen + Processengineering AG, Stampfstrasse 85, 8645 Jona, Switzerland, +49 55 222 55 22,
www.gerteis.com
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