Effects of Fill Depth and Cake Morphology

The specific surface area of the plug is generally quite large, typically up to 3 g and will not only vary between different formulations but also 

Pikal et al studied a 5%-povidone (polyvinyl pyrrolidone, PVP) solution, dried from an 8-ml fill volume, of 2-cm depth, and from a 4-ml fill volume at a 1-cm fill depth. In this instance, the fill depth had little effect on the specific surface area of the dried product (2.5 m g compared to 2.3 m g )- It was found that the rates of secondary drying (as measured by the mean ratio of the (1 - F) values), the normalised water contents of the two products were essentially identical. A mean ratio of (1 - F) of 1.19 0.17 was found, indicating that cake thickness did not significantly affect secondary drying kinetics. It can thus be concluded that material at the top of the cake dries at a similar rate to material at the bottom, provided overheating is avoided. Similar experiments were carried out on an amorphous, formulated moxalactam product, as shown below. 

Three solutions were used in this study  

Although the sample with the lowest specific surface area has double the fill depth, the correlation between drying rate and specific surface area is good. It also demonstrates that cake depth does not significantly affect secondary drying kinetics. 

---