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Crystallization Processes 109

vacuum pump or ejector as a part of the unit. If the boiling
point elevation—the increase in boiling temperature due
to the presence of the solute—is low, mechanical recom-
pression of the vapor obtained from solvent evaporation
may be used in some cases to produce a heat source to
drive the operation.
Evaporative-cooling crystallizers are fed with a liquor
whose temperature is such that solvent flashes upon feed
entry to the crystallizer. They typically are operated under
vacuum, and flashing of solvent increases the solute con-
centration in the remaining liquor while simultaneously
reducing the temperature of the magma. The mode of this FIGURE 12 Schematic diagram of a forced-circulation evapora-
operation can be reduced to that of a simple cooling crys- tive crystallizer.
tallizer by returning condensed solvent to the crystallizer
body.
schematic diagram of an evaporative forced-circulation
Salting-outcrystallizationoperatesthroughtheaddition
crystallizer that withdraws a slurry from the crystallizer
of a nonsolvent to the magma in a crystallizer. The selec-
body and pumps it through a heat exchanger. Heat trans-
tion of the nonsolvent is based on the effect of the solvent
ferred to the circulating magma causes evaporation of sol-
on solubility, cost, properties that affect handling, inter-
vent as the magma is returned to the crystallizer. This
action with product requirements, and ease of recovery.
type of unit is used to control circulation rates and veloc-
Adding a nonsolvent to the system increases the complex-
ities past the heat transfer surfaces, and the configuration
ity of the process; it increases the volume required for a
shown is especially useful in applications requiring high
given residence time and produces a highly nonideal mix-
rates of evaporation. A calandria that provides heat trans-
ture of solvent, nonsolvent, and solute.
fer through natural convection is an alternative to forced-
Melt crystallization operates with heat as a separating
circulation systems.
agent, but a crystalline product is not generated in the
Scale formation on the heat exchanger surfaces or at the
process. Instead, crystals formed during the operation are
vapor–liquid surface in the crystallizer can cause opera-
remelted and the melt is removed as the product. Such op-
tional problems with evaporative crystallizers. These can
erations are often used to perform the final purification of
be overcome by avoiding vaporization or excessive tem-
products after prior separation units; for example, the pu-
peratures within the heat exchanger and by properly in-
rity of an acrylic acid feed may be increased from 99.5 to
troducing the circulating magma into the crystallizer. For
99.9%. Melt crystallizers do not require solids handling
example, introducing the circulating magma a sufficient
units nor do they utilize solid–liquid separation equip-
distance below the surface of the magma in the crystallizer
ment. Finally, in some instances the use of melt crystal-
prevents vaporization upon re-entry and forces it to occur
lization can eliminate the use of solvents, thereby reducing
at a well-mixed zone above the point of re-entry. Alter-
the environmental impact of the process.
natively, the magma may be introduced so as to induce a
swirling motion that dislodges encrustations from the wall
B. Crystallizers of the crystallizer at the vapor-liquid interface.
Figure 13 shows a schematic diagram illustrating the
The basic requirements of a crystallization system are (1) a
configuration of a surface cooling (indirect heat transfer)
vessel to provide sufficient residence time for crystals to
crystallizer. Heat can be transferred to a coolant in an
grow to a desired size, (2) mixing to provide a uniform
external heat exchanger, as shown, or in coils or a jacket
environment for crystal growth, and (3) a means of gener-
ating supersaturation. Crystallization equipment is manu-
factured and sold by several vendors, but some chemical
companies design their own crystallizers based on exper-
tise developed within their organizations. Rather than at-
tempt to describe the variety of special crystallizers that
can be found in the marketplace, this section will provide
a brief general survey of types of crystallizers that utilize
the modes outlined above.
The forced-circulation crystallizer is a simple unit de-
signed to provide high heat-transfer coefficients in either FIGURE 13 Schematic diagram of a forced-circulation, surface-
an evaporative or a cooling mode. Figure 12 shows a cooling crystallizer.