Chemical Engineering May 2020 - 27

Facts At Your Fingertips
Nucleation Processes in Crystallizers
Department Editor: Scott Jenkins
ndustrial crystallization processes
are widely used in the chemical
process industries (CPI) for
their ability to separate and purify
products in a single step, often to
greater than 99.9% purity. Crystal
growth proceeds after nucleation,
a molecular aggregation process
whereby nuclei act as the centers
for crystallization. This one-page
reference provides information
about the chemistry and physics of
the nucleation step in crystallization
processes, and its ultimate impact
on solid crystal structure.
The period between the establishment
of supersaturation and the
formation of nuclei in the solution
plays a decisive role in determining
the properties of the resulting solid
products, including purity, crystal
structure, polymorphic form and
particle size. The type of nuclei that
are formed and their rate of formation
also influence the particle-size
distribution of the crystal population,
and other properties of the crystals,
so control of the nucleation process
is crucial in obtaining the required
product specifications [1].
I
Secondary
nucleation type
TABLE 1. SECONDARY NUCLEATION MECHANISMS (ADAPTED FROM REF. 1)
Characteristics
Initial breeding or
dust breeding
* Usually occurs if dry seed crystals are introduced into a solution
* Dry handling of the seed crystals causes small fragments by attrition that adhere
to the dry seeds. These fragments are liberated from the surface in the solution,
and become new nuclei
* It is recommended to suspend fresh seeds in an undersaturated solution to dissolve
the fragments before feeding the seeds as a slurry to the (generally batch)
crystallizer
Dendritic breeding
* Only happens at such high supersaturations that facet instabilities occur during
outgrowth of the crystals
* This implies that the corners and edges of the crystals experience a higher supersaturation
than the middle of the faces (facets) and hopper-like crystals or even
dendrites are formed
* These protruding crystal parts easily break off and become secondary nuclei
Contact nucleation
or attrition breeding
* The most important source of secondary nuclei in a crystallizer, and results from
collisions between crystals and impeller blades, between crystals and vessel walls
and from mutual collisions between the crystals
* The secondary nuclei are formed by attrition of the crystal corners, edges and
macro-steps on the surfaces
Fluid-shear breeding * Formation of secondary nuclei due to shear forces in turbulent fluid that are exerted
on a crystal surface
* Very high shear forces are, however, needed to exceed the yield stress of a parent
crystal leading to the formation of an attrition fragment. This breeding mechanism
is thus considered to be less important
Primary nucleation
Cluster formation
For nucleation to occur, solute molecules
that are dispersed within the
solvent must form clusters at nanometer
scales that persist (are stable)
under the given operating conditions.
Supersaturation refers to a state in
which a higher amount of solute is
dissolved in a volume of solvent than
would be predicted based on the
solubility characteristics of the substance
at a given temperature. Initiating
a phase change to begin crystallizing
the solute can be achieved by
further increasing the concentration
of solute, or decreasing the temperature.
Supersaturation is a prerequisite
condition in crystallization operations.
The spontaneous appearance
of a new phase can occur only when
a system is in a nonequilibrium condition.
Molecules dissolved in solution
begin to aggregate to relieve the
supersaturation and move the system
toward equilibrium [2].
In primary nucleation, crystals form
without the presence of other crystals.
This can occur homogeneously,
where nucleation is not influenced
by solids, including crystallizer vessel
walls or particles of foreign substances;
or heterogeneously, when
solid particles of foreign substances
cause an increase in the nucleation
rate.
In
industrial
practice, homogeneous
nucleation is rare because
it requires high energy to initiate
nucleation without a solid surface.
Most primary nucleation in industrial
crystallization processes is heterogeneous,
induced by foreign solid particles
present in working solutions.
Secondary nucleation
Secondary nucleation refers to the
formation of nuclei under the influence
of existing microscopic crystals,
where the birth of nuclei occurs
at the interface of parent crystals.
Secondary nucleation is the dominant
mechanism in industrial crystalizers,
and several types of secondary
nucleation are possible (Table 1).
Contact nucleation, the most important
source of secondary nuclei
in a crystallizer, results from collisions
between crystals and impeller blades,
between crystals and vessel walls
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2020
and from mutual collisions between
the crystals. The secondary nuclei
are formed by attrition of the crystal
corners, edges and macro-steps on
the surfaces. In this model, secondary
nucleation is determined by crystal
collisions. The number of nuclei produced
upon the impact of a crystal
is assumed to be proportional to the
impact energy [1]. The rate of nucleation
is then equal to the product of
the collision energy and the frequency
of collisions in a specific size range.
Attrition of the outgrown crystals
provides fragments that either dissolve
or grow into the crystal population.
In evaporative and cooling crystallization,
secondary nucleation is
thus the source of new crystals.
Secondary nucleation occurs at
lower degrees of supersaturation
than primary nucleation, which allows
control of crystal growth rate to
optimize product quality; as well as
lower energy, which avoids excessive
breakage of existing crystals. n
References
1. Lewis, A., Seckler, M., Kramer, H. and von Rasmalen, G. " Industrial
Crystallization: Fundamentals and Applications, " Cambridge
University Press, Cambridge, U.K., 2015.
2. Erdemir, D. and others. Crystal Nucleation, Chapter 3, in " Handbook
of Industrial Crystallization. " Cambridge University Press,
Cambridge, U.K., edited by Allan Myerson. 2019.
4. Ter Horst, J.H. and Meekes, H., Workshop at the 18th International
Symposium on Industrial Crystallization, Zurich, 2011.
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