Left: Figure 1. 0.6m² Filter Dryer part
of scaled-up pilot plant API facilities.
Right: Figure 2. Direct base heating with minimum gap design between the filter media
and the heated base.
Pharmaceuticals and fine chemicals are often devel- oped in the laboratories and pilot plants before they are transferred or scaled-up to production. Most equipment, even from the same manufacturer, have
differences in heat transfer area, utilities, and mechanical attrition, just to name a few. First to understand these differences,
the basic functions of the filter dryer should be discussed.
BASIC FILTER DRYER CHARACTERISTICS
The most important part of filtration is actually the forma-
tion of the product crystals in the crystallizer. Many products
are developed with a temperature-cooling curve, or through
precipitation, but they are rarely repeatable. In fact many com-
panies have a crystallization “guru”, who tries to develop
repeatable crystallization techniques for each product.
Once the product has formed the ap-
propriate crystal size and achieved
the correct particle distribution,
the slurry needs to be filtered.
Filtration - Filtering in a pressure
nutsche filter dryer is becoming the
most common type of equipment for
Active Pharmaceutical Ingredient
(API) manufacturing as it combines
filtering and drying with excellent
containment methods [see Figure
1]. At the beginning of
the filtration, the slurry
is brought into the
filter, either all at once,
in steps, or is filtered
as it is fed to the filter
dryer. In most
cases the agita-
tor is not running,
unless the product is
difficult to filter. Then
the agitator is run
close to the filter media in order to limit the cake height from
hindering the filtration rate. Other times, a cloudy filtrate may
be recycled back to the filter until the product cake is thick
enough to act as a finer filter media, blocking all of the un-dis-
solved product from going through the regular filter media.
This is continued until the filtrate clears.
Once the solid and liquid are separated by applying gas pressure (usually nitrogen), it is important to remove the residual
filtrate and wash out any impurities. In order to improve the
efficiency of the filtration and washing, the agitator is used
to smooth out any cracks or preferential channels that have
formed in the product cake. There are basically two types of
agitators: one with teeth and one without. Both types, however,
have a sloped surface to smooth the surface of the product
cake. The one with teeth has alternating teeth on each arm,
which forces the product back and forth across the surface.
The one without teeth works by compressing the product cake
and pushing the product into the cracks and crevices.
Washing - Another advantage of the agitated nutsche filter
dryer (ANFD) is its capability to perform displacement or
reslurry washes. In the displacement wash, a solvent is loaded
into the filter on top of the product cake, and then is pressed
out. This is used to displace the residual filtrate from between
the product crystals. These are good product washes, but
usually need to be performed multiple times to eliminate many
impurities. The reslurry wash takes advantage of the filter
dryer’s agitator. A solvent wash is, again, loaded into the filter
and the agitator stirs down into the cake, before the wash is
pressed out after the agitator is removed from the cake. The
reslurry wash provides a better wash with less solvent, especially when the product needs to be washed extensively.
Drying - Once the product has been filtered and washed, the
product is ready to be dried, or the product can be discharged
■ By Michelle Frisch, MBA, Sr. Manager, Global Technical Systems, Powder Systems Limited (PSL)
Understanding filter dryer operation can assist with tech transfer
Filter Dryer Scale-Up
PHARMACEUTICAL PROCESSING | JULY/AUGUST 2014 15 ■