Chemical Engineering March 2023 - 21

Facts At Your Fingertips
Fractional Solvent-Free Melt Crystallization
Department Editor: Scott Jenkins
ractional crystallization is a
stage-wise separation technique
that relies upon the liquid-solid
phase change and enables the purification
of multi-component mixtures,
as long as none of the constituents
can act as a solvent to the others.
Thanks to the level of selectivity that
can be achieved in solid-liquid equilibrium,
it is ultimately possible to reach
high purities for the selected components.
This one-page reference provides
basic information on fractional
solvent-free melt crystallization.
F
Principle of separation
The crystallization process relies on
the partial freezing of the initial liquid
mixture by gradually decreasing its
temperature. The frozen solid phase
exhibits a different chemical composition
to the remaining residual liquid.
This phenomenon is the basic physical
principle behind the fractional melt
crystallization process.
Crystals are allowed to grow on a
cooling surface, which separates the
feedstock from the coolant medium,
as polycrystalline layers. The heat released
by the solidification process is
withdrawn through the crystalline layer
or from the bulk. The driving force
results from the net effect of temperature
and concentration gradients
across both solid and liquid phases.
In a hypothetical case where crystallization
continued indefinitely, purified
products could theoretically
be recovered in 100% pure forms.
In practice, trade-offs required at
industrial scale make it impossible
to operate under these ideal conditions.
Nevertheless, a number of
strategies, such as partial melting of
the solid fraction (sweating), address
these challenges and help reach extremely
high purity levels.
Process steps
In a fractional crystallization process
(Figure 1), the main steps are crystallization,
draining, sweating and total
melting. During the crystallization
step, high-purity crystals are formed
on the cooling surface, while the impurities
are mostly concentrated in the
remaining liquid
(draining).
The sweating
phase then
enhances the
purification
process and
end-prod -
uct quality
through partial
melting of
the impurities
entrapped
within or between
crystals.
Finally,
the remainFIGURE
1. Fractional crystallization can be used to purify multi-component mixtures
and can achieve high purities
ing crystallized material (final purified
product), can be melted to remove it
from the column and continue with
downstream activities.
Equipment
Three different technologies and their
respective equipment can be used for
fractional melt crystallization:
Falling-film crystallizer. In falling-film
crystallization, the crystals grow from
an agitated melt (falling-film product)
inside tubes that are cooled by a cocurrent
cooling medium flow, which
runs on the other side of the tubes. In
this case, crystals grow on the inside
of the tube from the falling film of melt.
Reproducible (and high) transfer rates
are achieved on both side of the tube.
The resulting shear at the crystalliquid
interface transports impurities
rapidly into the bulk of the melt.
This technology is robust and easy
to operate. In addition, it can handle
high throughputs while achieving extremely
high purities. More precisely,
a typical feed has concentrations between
90 and 99%, and end products
can reach 99.99% purity or greater.
Glacial acrylic acid, optical-grade bisphenol
A and battery-grade ethylene
carbonate are purified to their highest
grade using this technology.
Static crystallizer. In static crystallization,
crystals are grown from a stagnant
melt. Versatility and robustness
are intrinsic to this technology, which
can purify the most challenging products,
including those characterized by
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high viscosities and high or low melting
points. Isopulegol, phosphoric
acid, wax and paraffins, anthracene/
carbazole and satellite-grade hydrazine
are chemicals that benefit from
static crystallization processes.
Suspension crystallizer. In suspension
crystallization, crystals are generated
on a cooling surface wall and
then scraped off, so they continue to
grow in size within a stirred vessel, in
suspension (slurry). The solid-liquid
separation of the slurry can be performed
either by using a wash column
or a centrifuge. More complex to
operate than the technologies mentioned
previously, suspension crystallization
has a key advantage of high
separation efficiency. In effect, products
with purities of 99.5% and above
can be achieved in a single crystallization
stage, from feedstocks with concentrations
of 75-90%. This can drive
considerable energy savings. Paraxylene
and halogenated aromatics
are purified using this technology,
which can also concentrate aqueous
feedstocks, such as food and drink
concentrates, or wastewater.
Combining these technologies can
enhance their respective features.
For example, combining falling-film
with suspension and static crystallization
is common.
n
Editor's note: The content for this column was authored by Sulzer
Chemtech (www.sulzer.com).
Sponsored by
21
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