Biology Reference
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the solution, which is thereby cooled. Direct contact cooling has the
advantage of avoiding heat exchanger surfaces, where incrustation can
occur. The technique is not often used, however (Bohlin, Rasmuson,
1986). If the temperature dependence of the solubility is weak, cooling is
an inefficient way of generating supersaturation, and the yield will be low.
In that case, one usually turns to evaporation crystallization (Nyvlt,
1982). The process is similar to a normal evaporation process, but the pur-
pose is to obtain the substance in the form of crystals. Evaporation crys-
tallizations are normally performed under vacuum.
In another type of crystallization, a second solvent or substance is
added, which causes the substance to be supersaturated. “Salting-out” is
the term for when, for example, potassium chloride is made to crystallize
by adding sodium chloride. The chloride concentration is increased until
the solubility product is exceeded. “Drowning-out” denotes a process
where a solvent is added, such as water to an alcohol, so that the solubil-
ity is lowered. In a reaction crystallization , the substance to be crystal-
lized is generated in situ in concentrations exceeding the solubility. An
example is an organic substance precipitated through the addition of
hydrochloric acid. In such processes, the local supersaturation at the feed
point is often very high, which makes mixing conditions particularly
important.
A crystallization process can be continuous, semi-continuous or
batch-wise operated (Nyvlt, 1982). Often, a batch process is simpler, eas-
ier to scale up, and less demanding in terms of maintenance and training
of staff. Any remaining incrustations vanish when a new batch is started,
and relatively big crystals can be obtained. When particularly narrow size
distributions are desired, batch processes have the greatest potential
(Larson, 1978). A batch crystallization is more flexible than a continuous
one, but also more sensitive to changes in the process. Even quite small
changes in solubility, supersaturation, and rate of cooling and evaporation,
etc., can cause large changes in the product size. This can lead to sig-
nificant variations from one batch to the next. Furthermore, batch
processes are more labor intensive. In batch processes, the equipment can
be thoroughly cleaned between batches, thereby avoiding contamination or
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