Chemistry Reference
In-Depth Information
Figure 9.14.
Overview of spray drying equipment scales and expected particle property control
across scales. (PSD-4 picture courtesy of Hovione Inc.; other photos courtesy of Bend Research Inc.)
9.5.1 Scale-Up Considerations for Droplet Formation
As
M
soln
increases upon scale-up, the atomization process requires increasing energy
input to obtain the same droplet size and subsequent particle size of the SDD. For
atomization using a pressure-swirl nozzle, the increased energy is provided from the
hydraulic pressure used for atomization. As the nozzle diameter increases to allow a
higher
M
soln
, the applied pressure must increase to provide a similar droplet size.
Figure 9.15 shows an example correlation indicating the pressure required to
maintain droplet size across a wider range of
M
soln
values. It is important to keep this
in mind during early process development to ensure that reasonable particle size targets
are selected that can be achieved at commercial scales. This is particularly important for
SDDs that have particle size-dependent dissolution performance, which is common for
compounds with high lipophilicity.
9.5.2 Scale-Up Considerations for Droplet Drying Rate
There are two important considerations that impact the droplet drying rate upon scale-up:
(1) the introduction of solvent vapor in the drying gas and (2) increased droplet density or
flux in the spray plume.
To minimize the volume requirements for inert drying gas (most commonly,
nitrogen), it is typical to recycle the drying gas in large-scale spray dryers. As shown
in Figure 9.2, after passing through the cyclone and/or baghouse, the drying gas is
conveyed by fans through a condenser to remove the majority of the solvent vapor. The
