Chemistry Reference
In-Depth Information
Figure 9.15.
Example correlation for predicting the required nozzle pressure to maintain
target droplet size for a high-viscosity spray solution (12wt% polymer in acetone).
drying gas is then passed back through the inlet gas heater and into the drying chamber
through the gas disperser. Recycling the drying gas is referred to as
“
closed cycle
”
operation, whereas smaller scale spray dryers without recycle operate in
”
operation (i.e., without the condenser loop). Although the purpose of the condenser is to
remove solvent from the exhaust vapor in closed-cycle operation, the recycled drying gas
will contain signi
“
single-pass
cant solvent vapor. The amount will be the concentration in
equilibrium at
T
cond
. This recycled solvent vapor can result in slower droplet drying
rates and higher residual solvent contents relative to smaller open-cycle spray dryers.
Slower drying rates typically result in denser SDD particles with raisin-like morphology.
If the formulation is sensitive to drying rate and solvent content, this could also impact
the physical state of the SDD since the API will have higher mobility in the SDD and thus
increase the potential for amorphous phase separation or inhomogeneity.
The effect of this solvent vapor can easily be mitigated by adjusting drying
parameters (e.g.,
T
in
,
M
gas
, and
M
soln
) and
T
cond
, which controls the quantity of recycled
solvent vapor in the recycled gas stream. The recycled solvent vapor can be accounted for
in heat and mass balance equations, and thermodynamic models can be developed to
predict target drying parameters at larger scales.
A second scale-up consideration that impacts droplet drying rate is the droplet and
drying gas mixing ef
ciency. Since typical nozzles have spray angles of less than 90
and
°
are essentially a
flux of droplets in the key
control volume (near the exit of the atomizer) increases proportionally to the spray rate.
Higher
M
soln
in similarly sized control volumes leads to denser spray plumes upon scale-
up. The denser plumes are an added resistance to mass and energy transfer and slow
down droplet drying. The effects of the denser plume can also be easily mitigated by
adjusting drying parameters (e.g.,
T
in
,
M
gas
,
M
soln
, and
T
cond
). The effect of denser droplet
plumes can be modeled with CFD, and scale correlations can be developed to predict
target drying parameters at larger scales. It is important to consider the recycled solvent
vapor and denser droplet plumes early in development to evaluate the impact of slower
“
point source
”
of droplet formation, the
