Unconventional Strategies in Industrial Melt Crystallization

Unconventional strategies in industrial melt crystallization

GUILLOU L.

Fives Proabd, 20C rue de Chemnitz – F-68100 Mulhouse

The melt static crystallization (MSC) is usually described through an arrangement of steps consisting in:

-          Controlled cooling and crystallization

-          Draining of mother liquor containing impurities

-          Controlled partial melting of the crystals to remove remaining wetting impurities a.k.a. “sweating”

-          Melting of remaining purified crystals

This arrangement of steps defines one purification stage which can be multiplied in order to develop fractional crystallization processes that allow improvement of recovery yield or purity.

It is quite often considered that these features limit the scope of application of the MSC to very specific cases such as the treatment of concentrated feedstock and applicability to limited capacities only.

Through this contribution different cases of unconventional strategies for crystallization from the melt will be examined:

-          Supercharging and in situ recrystallization for lean concentrations

-          Substitution of the sweating step by a washing step

-          Extractive crystallization

Supercharging MSC	MSC with combined washing	Extractive MSC

We will show how supercharging is used to treat feedstock containing less than 20% of crystallizable compound and how it contributes to increase the recovery yield per pass vs. suspension crystallization.

We will present and discuss the advantages of washing vs. sweating in terms of improvement of impurity removal for very high capacity plants (beyond 100 ktpy).

Finally we will show how extractive crystallization can be advantageously used for separating a three crystallizable compounds blend and allow achieving better recovery than an alternate based distillation approach.

Through selected industrial examples, the declination will be introduced. It will be shown that they are not necessary meant to be considered as specific tools used for solving specific issues. They can in fact be considered as generic strategies For sure they are not as much widespread than the standard MSC step arrangement which remains the most common. But they are neither to be considered as being that rare: more than 20% of the industrial purification plants using a Proabd® MSC process nowadays involve one or more of such strategies.

Keywords: Crystallization, purification, energy optimization.