Crystallization Case

Crystallization

Crystallization is the (natural or artificial) process for the formation of solid crystals from a uniform solution. A complete crystallization plant includes a crystallization unit, centrifuge, pump and condenser.

Crystallization types

1-Cooling crystallization

Whenever the conditions are favorable, crystal formation results from simply cooling the solution. An example of this crystallization process is the production of Glauber's salt, a crystalline form of sodium sulfate.

Cooling crystallizers

The simplest cooling crystallizers are tanks provided with a mixer for internal circulation, where temperature decrease is obtained by heat exchange with an intermediate fluid circulating in a jacket. These simple machines are used in batch processes, as in processing of pharmaceuticals and are prone to scaling. Batch processes normally provide a relatively variable quality of product along the batch.

A common practice is to cool the solutions by flash evaporation: when a liquid at a given T0 temperature is transferred in a chamber at a pressure P1 such that the liquid saturation temperature T1 at P1 is lower than T0, the liquid will release heat according to the temperature difference and a quantity of solvent, whose total latent heat of vaporization equals the difference in enthalpy. In simple words, the liquid is cooled by evaporating a part of it.

2-Evaporative crystallization

Another option is to obtain, at an approximately constant temperature, the precipitation of the crystals by increasing the solute concentration above the solubility threshold. To obtain this, the solute/solvent mass ratio is increased using the technique of evaporation. This process is of course insensitive to change in temperature.

Evaporative crystallizers

Most industrial crystallizers are of the evaporative type, such as the very large sodium chloride and sucrose units. The most common type is the forced circulation (FC) model. A pumping device keeps the crystal slurry in homogeneous suspension throughout the tank, including the exchange surfaces; by controlling pump flow, control of the contact time of the crystal mass with the supersaturated solution is achieved, together with reasonable velocities at the exchange surfaces.

3-Draft tube-baffle technology:

This type of crystallization is especially designed for low viscosity products when a narrow crystal size distribution is expected.

Super saturated product enters into a chamber containing (mixture of solids and solution). Instantaneously, the product crystallizes around the crystals that are already inside the vessel so that the solution reaches the saturation point. Inside the chamber, a stirrer creates a high circulation flow in order to give a good repartition of the crystals. On the side of the vessel, baffles create a quiet zone where an upward flow brings the smallest crystals out of the vessels. They are re-melted and put back in the vessel.

Process

The crystallization process consists of two major events, nucleation and crystal growth.

Nucleation is the step where the solute molecules dispersed in the solvent start to gather