Page 211 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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108 Crystallization Processes

It is defined by: tion obtained from a continuous crystallizer. The effects
σ of operating variables on crystal size distributions will be
c.v. = (41) discussed in Section VI.
L D
The throughput per unit crystallizer volume is greater
and estimated from the moments of the distribution:
for a continuous system. Batch units have several operat-

1/2 ing steps in a cycle—charging, heating or cooling, crystal-
m 3 m 5
c.v. = 2 − 1 (42) lizing, discharging, and cleaning—and the unit production
m
4
rate is based on the total cycle time, even though the for-
This is especially useful for systems that cannot be de- mation of crystals may occur only during a small portion
scribed by an analytical distribution function. of the cycle.
It may be easier to operate a continuous system so that
it reproduces a particular crystal size distribution than it
V. CRYSTALLIZER CONFIGURATION is do reproduce crystal characteristics from a batch unit.
AND OPERATION Moreover, the coupling of several transient variables and
nucleation make it difficult to model and control the op-
Crystallization equipment can vary in sophistication from eration of a batch crystallizer.
a simple stirred tank to a complicated multiphase column,
and the protocol can range in complexity from simply al-
A. Relationship of Solubility
lowing a vat of liquor to cool to the careful manipulation
to Mode of Operation
required of batch cyclic operations. In principle, the objec-
tives of these systems are the same: to produce a product The driving force for crystal formation can be generated
meeting specifications on quality at an economical yield. through a variety of means, including cooling or heating
This section will examine some of the considerations that to reduce or increase the system temperature, evaporating
go into the selection of a crystallizer so as to meet these solvent, evaporative (flash) cooling, inducing a chemical
objectives. reaction (when the reaction product is sparingly soluble,
One of the first decisions that must be made is whether the process is called precipitation), adjusting pH, salting
the crystallizer operation is to be batch or continuous. In out through the addition of a nonsolvent, direct-contact
general, the advantages of each type of operation should cooling with a refrigerant, or some other means. All of
be weighed in choosing one over the other, but more often these modes of operation can be implemented in either a
the decision rests on whether the other parts of the process batch or a continuous process. In addition, two or more of
are batch or continuous. If they are batch, then it is likely the modes may be combined to enhance the product yield.
that the crystallizer also should be batch. As discussed in an earlier section, solubility is intrinsic
The equipment required for batch crystallization can be to the solute-solvent system, and the relationship of solu-
very simple. For example, some crystalline materials are bility to temperature often determines the mode by which a
produced by simply allowing a charge of hot liquor to cool. crystallizerisoperated.Recallforexamplethatthesolubil-
After the crystals have formed, the magma is discharged ity of NaCl (see Fig. 2) is essentially independent of tem-
through a filter or the liquor may be decanted and the set- perature, while Na 2 SO 4 · 10H 2 O has a solubility that ex-
tled slurry filtered. Very large crystals can be obtained by hibits a strong dependence on temperature. Consequently,
allowing encrustations formed on the walls of these crys- cooling a sodium chloride solution cannot generate sig-
tallizers to grow undisturbed; after the system has come nificant product yield; solvent evaporation is the primary
to equilibrium, the liquor is drained and the crystals are mode of NaCl production. On the other hand, reducing
removed by scraping them from the surface. the temperature of a saturated solution of sodium sulfate
Batch crystallizers can be used in a campaign to produce generates substantial product and may be used alone or in
a particular product and in a second campaign to produce combination with evaporation.
another product. Generally, it is not possible to operate Cooling crystallizers utilize a heat sink to remove both
continuous processes in this way. Batch crystallizers can the sensible heat from the feed stream and the heat of crys-
handle viscous or toxic systems more easily than can con- tallization released or, in some cases absorbed, as crystals
tinuous systems, and interruption of batch operations for are formed. The heat sink may be no more than the am-
periodic maintenence is less difficult than dealing with in- bient surroundings of a batch crystallizer, or (as is more
terruptions in continuous processes. The latter factor may likely) it may be cooling water or another process stream.
beespeciallyimportantinbiologicalprocessesthatrequire Evaporative crystallizers generate supersaturation by
frequentsterilizationofequipment.Batchcrystallizerscan removing solvent from the mixture, thereby increasing
produce a narrow crystal size distribution, whereas special the solute concentration. They may be operated under vac-
processing features are required to narrow the distribu- uum, and in those circumstances it is necessary to have a