Desktop version

Home arrow Environment arrow Crystallization and Crystallizers

Source

Nucleation

Primary nucleation requires high supersaturation (o > 0.3) and is difficult to control. For this reason, industrial crystallization proceeds by seeding.

Secondary nucleation is a result of the friction of liquid on the crystals’ surface. This friction tears away clusters of molecules that subsequently act as seeds for secondary nucleation. Indeed, Brown et al. [BRO 92] demonstrated that the number of seeds created in this way is proportional to the crystalline surface present in the magma.

Liang et al. [LIA 87] showed that crystals smaller than 30 gm all grow at a constant rate, but that this rate is distributed according to a Gaussian law by which:

  • - the mean value is given by the BCF law;
  • - the standard deviation is given by:

These crystals (below 30 gm in size) are the result of crystals being affected by the agitator or impeller.

Note that secondary nucleation is not successful if the supersaturation is greater than 0.2. This is a necessary condition for the development of agglomerates which arise near crystal faces.

In practice, we try to avoid secondary nucleation, and often:

Sugar purity and weight ratio

Purity is defined by:

Impurity is defined as:

Of course:

In solution, the weight ratio of sugar is:

For purity equal to 1, the saturation rate driven by Grut (see [DE 83]) is defined according to temperature:

Where there are impurities present on saturation, we write:

According to Wagnerowski [WAG 62]:

with:

 
Source
Found a mistake? Please highlight the word and press Shift + Enter  
< Prev   CONTENTS   Next >

Related topics