Page 206 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-ID159 June 8, 2001 15:47

Crystallization Processes 103

Occlusions find their way intothecrystal structure when The characteristic macroscopic shape of a crystal re-
thesupersaturationincloseproximitytothecrystalsurface sults, in large measure, from the internal lattice structure;
is high enough to lead to an unstable surface. Such insta- surfaces are parallel to planes formed by the constituent
bility leads to the creation of dendrites, which then join units of the crystal. Moreover, although the Law of Con-
to trap mother liquor in pools of liquid within the crystal. stant Interfacial Angles is a recognition that angles be-
Occlusions are often visible and can be avoided through tween corresponding faces of all crystals of a given sub-
careful control of the supersaturation in the crystallizer. stance are constant, the faces of individual crystals of that
Mother liquor can be flushed from a cake of crystals substance may exhibit varying degrees of development.
on a filter or centrifuge by washing with a liquid that also As a result, the general shape or habit of a crystal may
may dissolve a small portion of the cake mass. To be ef- vary considerably.
fective, the wash liquid must be spread uniformly over Crystal morphology (i.e., both form and shape) affects
the cake and flow through the porous material without crystal appearance; solid–liquid separations such as filtra-
significant channeling. Such washing is hindered when tion and centrifugation; product-handling characteristics
the crystals themselves have significant cracks, crevices, such as dust formation, agglomeration, breakage, and
or other manifestations of breakage or the mother liquor washing; and product properties such as bulk density,
has a viscosity that is significantly greater than the wash dissolution kinetics, catalytic activity, dispersability, and
liquid. In the latter event, significant channeling (also caking.
called fingering) may reduce the effectiveness of the wash The shape of a crystal can vary because the relative rates
process. of growth of crystal faces can change with system con-
Lattice substitution requires that the incorporated im- ditions; faster growing faces become smaller than faces
purity be of similar size and function to the primary crys- that grow more slowly and in the extreme may disappear
tallizing species. In other words, the impurity must fit from the crystal altogether. For illustration, consider the
into the lattice without causing significant dislocations. two-dimensional crystal shown in Fig. 7a and the process
An example of such a system is found in the crystalliza- variables that would cause the habit to be modified to the
tion of L-isoleucine in the presence of trace quantities of forms shown in Figs. 7b and c. The shape of the crystal
L-leucine. The two species have similar molecular struc- depends on the ratio of the growth rate of the horizontal
tures, differing only by one carbon atom in the position faces, G h , to the growth rate of the vertical faces, G v .For
of a methyl side group. In this system, the incorporation the shapes shown in Fig. 7,
of L-leucine in L-isoleucine crystals is proportional to the
G h G h G h
concentration of L-leucine in the mother liquor. Moreover, < < (27)
the shape of the recovered crystals changes as the content G v b G v a G v c
of L-leucine in recovered crystal increases. Growth rates depend on the presence of impurities,
system temperature, solvent, mixing, and supersaturation,
and the importance of each may vary from one crystal face
B. Crystal Morphology
toanother.Consequently,analterationinanyorallofthese
Both molecular and macroscopic concepts are important variables can result in a change of the crystal shape.
in crystal morphology. Molecular structures (i.e., the ar- Modeling intermolecular and intramolecular inter-
rangements of molecules in specific lattices) can greatly actions through molecular mechanics calculations has
influence the properties of a crystalline species and varia- advanced significantly in the past decade, and it has pro-
tions from a single structure lead to the prospect of poly- vided the basis for prediction of the equilibrium shape of
morphic systems. In such systems, the molecular species
of the crystal can occupy different locations depending
on the conditions at which the crystal is formed, and both
microscopic and macroscopic properties of the crystal can
varydependingonthepolymorphformed.Thereis,ingen-
eral, a single stable polymorph for prevailing conditions,
but that polymorph may not have been formed during the
crystallization process. In such cases, system thermody-
namics will tend to force transformation from the unstable
polymorph to the stable one at rates that may vary from
being nearly instantaneous to infinitely slow. Additional
discussion of the molecular structures of crystalline ma-
terials has been provided elsewhere. FIGURE 7 Effect of facial growth rates on crystal shape.