(146f) Modelling of a Filter Drying Process

Filter drying, also referred to as vacuum contact drying, is a process widely used in the pharmaceutical industry to
remove water or organic solvents. The advantage of this process over other
drying operations is that it can be used to dry solids that are sensitive to
oxygen and temperature.

The entire drying process is typically carried out in two
stages in a vessel. Typically a cylindrical vessel is used although conical
vessels are also encountered. In the first blowdown
stage, pressure in the form of nitrogen is applied to remove excess solvent by
convective drying. At the end of this stage, the wet filter cake typically has
around 45% solvent on a wet basis.

The second, longer, period involves heating the walls of the
filter cake while vacuum is applied to dry the cake to about 1% to 0.1%. During
this stage, heat conduction from the wall to the interior of the cake is often
the rate limiting step. While the process can be carried out without agitating
the bed, this can lead to excessively long drying times and heterogeneity of
the cake. On the other hand, continuous long agitation often results in
agglomeration and/or breakage of the solid. As a compromise, intermittent
periods of agitation are carried out to redistribute solvent and heat, resulting
in faster drying times but without significant agglomeration and/or breakage of
solid taking place.

A detailed model of the filter dryer unit operation has
previously been published in the literature (Murru et
al, 2011). The key features of the model are:

1.       The
second drying phase is modelled and drying is assumed to take place primarily
due to heat conduction from the equipment walls. The blowdown
phase where convective drying takes place is not accounted for.

2.       A
single solvent is considered. This allowed for process optimisation by allowing
different solvents to be examined for the effects on total drying times.

The model was successfully used to predict drying times for
various equipment sizes and solvents at both pilot plant and manufacturing
scales.

In this work, the model is further extended to consider the
following phemonena.

1.       Convective
drying is considered. This allows the modelling of the initial blowdown drying phase. The effects of the applied suction
during the second drying phase are also explicitly accounted for. The
application of  hot nitrogen flow over the bed
can significantly reduce the time of drying.

2.       The
model is extended to non-ideal mixtures of solvents. The presence of solvent
mixtures in the drying operation has a significant effect on the drying time
but most importantly lead to change of composition of the residual solvent
during drying. This can lead to stability issues (hydrates / solvates) or can
lead to particle bridging and agglomeration. Understanding the dynamics of the
solvent composition is crucial to avoid these situations. The ability to handle
multiple solvents simultaneously will allow for greater flexibility in
selecting solvents for process optimisation.

3.       Modelling
of the vapour processing step is undertaken. This allows for data from
downstream condensation and scrubbing unit operations to be used in model
validation.

References

M Murru, G Giorgio, S Montomoli, F Ricard, F Stepanek
(2011) Model-based scale-up of vacuum contact drying of pharmaceutical
compounds. Chemical Engineering Science 66, 5045-5054