that provides the best overall consistent heat distribution. To avoid
product balling up, it is important to insert the agitator into the wet
cake slowly, inch-by-inch, because, as stated above, the agitator is
the biggest active heated surface in the filter dryer.
Drying can also be performed in two ways, either with heated, pressurized gas being blown through the product cake, or with vacuum
assist. Of these two types, vacuum drying is the best method for most
heat sensitive products, as it reduces the production times and out of
specification product due to over heating. The pressurized gas also
has the disadvantage of recycling and recompression of the gas after
the condensation of the evaporated solvent, but has advantages as a
closed loop system and adds another heat transfer driver in the gas.
TRANSFERRING FILTER DRYER TECHNOLOGY AND
How does the above filter dryer processing of crystallized product
slurry behave during a technology transfer and scaling-up processes?
What are the parameters that must be taken into consideration for a
positive technological transfer and scale-up process on a filter dryer?
To answer these questions, we have to again look at the heat transfer area, the agitator type and the product cake depth.
The heat transfer area efficiency must be determined for both agitated nutsche filter dryers. This can be performed through tests of
boiling off a known quantity of water or solvent at the maximum flow
of the heat transfer medium. Then, the time to boil off this amount
can be compared to show the efficiency of each filter dryer. Hopefully,
the development equipment is more efficient than the production
unit, because one can always cut back on the heat transfer rate, but it
cannot be increased. So, once the heat transfer potential of each piece
of equipment is determined, the developed equipment can be scaled
back for test batches to be comparable to the production equipment.
When scaling up, it is important to also determine the heat transfer
area to the maximum cake volume for each piece of equipment.
The agitator type is the next important factor [see Figure 3]. The
amount of heated surface on the agitator is important. Most machines have two arms, but some have three or even four arms to
increase the amount of heat transfer area. Because these agitators
are cantilevered and are subjected to high torque of the solids, there
maximum speeds are very low, usually in the range of 5 to 25 rpm.
When scaling up, it is important to match the tip speed of the agitator to simulate the same agitator to product contact times. Agitators
in filter dryers have been thought to cause attrition on the product
crystals. Of course this is very dependent on the product, and crystal type, but the potential of change in the size of the crystals are
in general rarely greater than 1%, especially considering the force
that the agitator can inflict on the crystals at a low rpm is minimal.
Obviously, more blades may cause more attrition, but it would still
be minimal. It is important to point out again, that crystal formation
during the crystallization step is much more crucial than any attrition that the filter dryer agitator could cause.
When simulating a filtration scale-up the main factor is to maintain
the same cake depth. As the product builds up on the filter media,
the product cake starts to act as the filter media. So, as long as the
SMALL SCALE SIMULATION AND FEASIBILITY STUDY
cracks and preferential channeling are eliminated with a spatula or
agitator, the filtration time should be comparable with the same pa-
rameters of cake depth and applied pressure. This has been proven
to work even from lab scale to large-scale production.
Lab scale filter dryer technology has not always been available for
accurate scaling up study. During laboratories studies and R&D product development, chemists need to work on similar technology to
evaluate and determine the parameters discussed above. Traditional
filtration and drying methods usually found in laboratories are Buchner
filtration, centrifuge and oven drier. These technologies cannot be
transferred successfully onto commercial production scale. Therefore,
smaller agitated nutsche filter dryer have been developed to provide
same technology for less than 200 grams batch of final product. They
can be found in glass for complete process visibility or stainless steel
for higher pressure requirement. [see Figure 4] These lab filter dryers
perform the same filtration under vacuum and pressure, washing and
re-slurry capabilities and thermal drying through a heating jacket.
In summary, most equipment, even from the same manufacturer,
will have issues and problems when scaling-up or even transferring
the production to another machine. Consistent and reliable operation of the filter dryer will only lead to quicker development times
and a better understanding of the process. This in turn will lead to
easier transfer and scale-up of processes. It is important to do the
“homework” of evaluating each piece of equipment before attempting any technology transfer or scale-up [see Figure 5]. ■