Chemistry Reference
In-Depth Information
Figure 9.17.
Use of scale factor to describe effect of higher droplet in the spray plume for larger
scale spray dryers, resulting in amultiscale correlation. (a) Correlation of bulk density versus RS
out
for three spray dryer scales and (b) multiscale correlation of bulk density versus RS
out
using scale
factor.
limitations dictated by plant utilities (e.g., maximum achievable
T
in
or minimum
achievable
T
cond
). These constraints can be depicted as planes in the three-dimensional
plots similar to the constraints shown as lines in Figure 9.16 to guide the identi
cation of
target spray drying conditions and to illustrate the operating space.
As described previously, correlations with
T
out
,RS
out
, or other combinations of key
process parameters can be used to predict the important SDD attributes. An example
correlation between RS
out
and SDD bulk density is shown in Figure 9.17a for three spray
dryer scales. All of these spray dryers were operated in closed-cycle mode and the target
median droplet size was the same for all of the conditions shown. The three different
spray dryer scales have separate correlations with SDD density. SDD manufactured with
the same RS
out
results in denser SDD on the larger PSD-5 spray dryer than on the smaller
PSD-1 spray dryer. This illustrates the scale effect of the denser droplet plume and thus
slower droplet drying rate resulting in denser SDD described above. The droplet-to-gas
mixing ef
ciency can be modeled by CFD or studied with carefully designed experi-
ments across spray dryer scales. Scale correlations can be developed and used to predict
the impact of the slower droplet drying due to denser spray plumes. These correlations
can be widely applicable to SDDs with similar formulations (e.g., cellulose-based
polymer with drug loadings less than 50%). The use of an example scale correlation
is shown in Figure 9.17b. A dimensionless Damköhler number, which is the ratio of
reaction rate to convective mixing rate, can be adopted [17]. In this case, the Damköhler
number is de
ned as the ratio of evaporation rate to convective droplet and gas mixing
and a simpli
ed equation is shown below.
L
diffusion
=
D
L
mixing
=
u
∝
L
mixing
u
Da
:
