Chemical Engineering March 2023 - 17

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EKATO Crystallizers for batch and
continuous processes
C
rystallization as a basic process engineering operation
is of paramount importance in the process industry. It
is used for the production and purification of solids in
numerous chemical, pharmaceutical and food industry
processes. The use of mixing technology adapted to the process is
the key to achieving the desired product properties.
Crystallization of a solution can be initiated by changing
temperature or solute concentration. This can generally be
triggered by cooling or evaporating the solvent, but also by adding
an antisolvent or adding a new component as in precipitation or
reaction crystallization. The process
variants as well as the apparatusbased
process solutions are strongly
dependent on the properties
of the material system and are
correspondingly diverse.
For successful crystallization, the
most suitable process and thus the
method for exceeding the solubility,
i.e. generating a supersaturation
must be determined. A helpful
tool for process selection is the
solubility curve, which shows the
concentration of the dissolved solid
over the temperature. Depending
on the influence of temperature on
the solute concentration, cooling or
evaporation is chosen for crystallization, or a combination of both
by applying a vacuum.
Crystallization can be initiated by shear, by foreign substances
to smaller set-ups, this equipment allows not only variation of
pressure and temperature and their rates, but also variation of
container geometry, stirrer geometry and speed and thus an
extended investigation of the target variables product purity, yield
or crystal size distribution.
In addition, already on this small scale various inline probes
like FBRM or Infrared Spectrometry can be used as powerful tools
to clarify the significant process parameters which in later project
phases will become decisive for a successful scale-up of the process
solution.
Mixing requirements
Solids are formed during
3-liter-scale test stand in R&D Center
crystallization, which must be
considered when designing
agitators. The basic mixing task
of suspending is therefore of great
importance as a critical success
factor for every crystallization
process. Furthermore, a good
homogeneity in the entire filling
volume is required to ensure, for
example, uniform growth conditions
for the crystals in the entire liquid
volume. Depending on the type of
crystallization mechanism,
heat
transfer and renewal of the liquid
surface are also relevant.
In addition to these tasks, which are easy to describe in terms of
such as impurities, but also by the vessel wall or internals, or by the
introduction of seed crystals. If the degree of supersaturation is too
high, nucleation clearly dominates crystal growth, and the process
becomes uncontrollable. This effect can lead to a broad particle
size distribution with an excessive fines content at the same time.
Depending on the material system, further significant
such as nucleation and crystal growth kinetics,
the option of seeding the
maximum local energy input or
crystallization need to be elucidated, which are usually determined
by experiments on a geometrically similar test scale.
Thus, an important aspect of process control is supersaturation
control. By setting the operating point in the metastable region
close to the solubility curve, crystal growth is promoted over
primary nucleation.
The determination of solubility and metastable range is
an elementary investigation in the process development of a
crystallization process. These values can be determined in the
laboratory by repeated heating and cooling at different substance
concentrations.
The picture shows a test stand on a 3-liter scale. Compared
mixing technology, there are other effects such as crust formation
or deposits. These can usually be influenced by the agitator design
in combination with the mode of crystallization as well.
EKATO solutions for crystallization processes
EKATO can support customers already during the development
parameters
phase. This starts with basic laboratory investigations on basic
parameters, selecting the mode of crystallization and finding the
right strategy for a robust process. Taking this as a basis, EKATO is
in the position to deliver a complete solution starting from a first
estimation until detail engineering, manufacturing and operation.
EKATO has excellent experience in designing not only the agitators,
but also can assist with expertise in vessel design and process
know-how. Furthermore, we can support with CFD simulations
and trials in our laboratory or on site.
For more insights in EKATO R&D capabilities:
https://youtu.be/p1ZCk9UyGO8
More Information under:
https://www.ekato.com/solutions/processes/crystallization/
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MARCH 2023
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Chemical Engineering March 2023

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