Beside the already mentioned criteria the design of the crystallization
processes is influenced by several additional factors. These are:
SURFACE-COOLING CRYSTALLIZATION
The surface-cooling produces
supersaturations directly on the heat exchanger surface. This supersaturation
is the highest in the entire crystallizer. Incrustations of the cooling surface
and a limitation of the plant operation are the normal consequences. That can
be accepted for discontinuous operation, because with the next batch all the
incrustations get dissolved again. For continuous processes the surface cooling
is only decided for, if too low operating pressures make vacuum cooling
crystallization uneconomically. One reason could be a high boiling point
elevation. In that cases especial large heat exchanger surface are chosen.
VACUUM-COOLING CRYSTALLIZATION
Vacuum-cooling crystallization is
the preferred cooling crystallization method under continuous operation.
Because cooling is generated by adiabatic expansion of the solvent no cooling
surfaces can be incrusted. Vacuum-cooling becomes uneconomical only if cooling
has to be effected at very low temperatures.
EVAPORATION CRYSTALLIZATION
The evaporation crystallization is a
vacuum process as well as the vacuum-cooling crystallization. Differing from
the vacuum-cooling this process method is independently from the concentration
and temperature of the feed solution. External heat can be added to the system
and the concentration of mother liquor can be adjusted independently. Like
vacuum-cooling crystallization there are no special incrustation problems in
evaporation crystallization if boiling on the heater surface is prevented. Some
difficulties may arise for inverse soluble substances, like the hardeners. In
those cases it has to be taken care as explained for the surface-cooling
crystallization. High velocities of the suspension flow and a high suspension
density can be helpful by erosion and faster desupersaturation behaviour. For a
better process economy the units are constructed as multiple-effect evaporation
plants.