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92 Crystallization Processes

in recent years to improve the understanding of crystal- product that meets purity specifications. For example, in
lization, and a large portion of the research on the topic the manufacture of the amino acid L-isoleucine, the prod-
has dealt with mechanisms of nucleation and growth. Es- uct is first recovered in acid form, redissolved, neutralized,
pecially important has been elucidation of the effects of and then recrystallized in order to exclude the impurity
process variables on the rates at which these phenomena L-leucine and other amino acids from the product.
occur. Additionally, extensive progress has been achieved A simple change in physical properties also can be
in modeling both steady-state and dynamic behavior of achieved by crystallization. In the process of making soda
crystallization systems of industrial importance. The pri- ash, referred to earlier, the sodium bicarbonate crystals are
mary elements of the discussion that follows are the princi- subjected to heat that causes the release of carbon dioxide
ples that influence yield, morphology, and size distribution and produces low-density sodium carbonate crystals. The
of crystalline products. density of these crystals is incompatible with their use in
glass manufacture, but a more acceptable crystal can be
obtained by contacting the sodium carbonate crystals with
I. OBJECTIVES OF CRYSTALLIZATION water to form crystalline sodium carbonate monohydrate.
PROCESSES Drying the resulting crystals removes the water of hydra-
tion and produces a dense product that is acceptable for
Several examples of objectives that may be satisfied in glass manufacture.
crystallization processes are given in the following dis- Separation of a chemical species from a mixture of sim-
cussion. Soda ash (sodium carbonate) is recovered from ilar compounds can be achieved by crystallization. The
brine by contacting the brine with carbon dioxide that re- mode of crystallization may fall in the realm of what
acts with sodium carbonate to form sodium bicarbonate. is known as melt crystallization. In such processes, the
Sodium bicarbonate, which has a lower solubility than mother liquor largely is comprised of the melt of the crys-
sodium carbonate, crystallizes as it is formed. The pri- tallizing species, and, subsequent to its crystallization,
mary objective of the crystallizers used in this process crystals formed from the mother liquor are remelted to
is separation of a high percentage of sodium bicarbonate produce the product from the crystallizer. Para(p)-xylene
from the brine in a form that facilitates segregation of the can be crystallized from a mixture that includes ortho
crystals from the mother liquor. The economics of crystal and meta isomers in a vertical column that causes crys-
separation from the mother liquor are affected primarily tals and mother liquor to move countercurrently. Heat is
by the variables that control the flow of liquid through the added at the bottom of the column to melt the p-xylene
cake of crystals formed on a filter or in a centrifuge. For crystals; a portion of the melt is removed from the crys-
example, the flow rate of liquid through a filter cake de- tallizer as product and the remainder flows up the col-
pends on the available pressure drop across a filter, liquid umn to contact the downward-flowing crystals. Effluent
viscosity, and the size distribution of crystals collected on mother liquor, consisting almost entirely of the ortho and
the filter. With a fixed available pressure drop and defined meta isomers of xylene, is removed from the top of the
liquid properties, the crystal size distribution controls filter column.
throughput and, concomitantly, the production rate from Production of a consumer product in a form suitable
the process. for use and acceptable to the consumer also may be an ob-
Crystallization can be used to remove solvent from a jective of a crystallization process. For example, sucrose
liquid solution. For example, concentration of fruit juice (sugar) can be crystallized in various forms. However, dif-
requires the separation of solvent (water) from the natural ferent cultures are accustomed to using sugar that has a
juice. The common procedure is evaporation, but the de- particular appearance and, unless the commodity has that
rived juices may lose flavor components or undergo ther- appearance, the consumer may consider the sugar to be
mal degradation during the evaporative process. In freeze unacceptable.
concentration, the solvent is crystallized (frozen) in rela- In all these processes, there are commmon needs: to
tively pure form to leave behind a solution with a higher form crystals, to cause them to grow, and to separate the
solute concentration than the original mixture. Significant crystals from the residual liquid. While conceptually sim-
advantages in product taste have been observed in the ap- ple, the operation of a process that utilizes crystallization
plication of this process to concentrations of various types can be very complex. The reasons for such complexity in-
of fruit juice. volve the interaction of the common needs and process re-
The elimination of small amounts of an impurity from a quirements on product yield, purity, and, uniquely, crystal
product species may be an objective of crystallization. In morphology and size distribution. In the following discus-
such instances, a multistep crystallization–redissolution– sion, the interactions will be discussed and general princi-
recrystallization process may be required to produce a ples affecting crystallizer operation will be outlined. More