Oslo Type Crystallizer

Oslo type crystallizer also called classified-suspension crystallizer is the oldest design developed for the production of large, coarse crystals.

The basic design criteria are twofold:

• desupersaturation of the mother liquor by contact with the largest crystals present in the crystallization chamber
• keeping most of the crystals in suspension without contact by a stirring device, thus enabling the production of large crystals of narrow size distribution

The equipment is schematically shown in Figure 1.

Figure 1 Oslo Type Crystallizer

The classifying crystallization chamber is the lower part of the unit. The upper part is the liquor-vapor separation area where supersaturation is developed by the removal of the solvent (water for most applications). The slightly supersaturated liquor flows down through a central pipe and the supersaturation is relieved by contact with the fluidized bed of crystals. The desupersaturation occurs progressively as the circulating mother liquor moves upwards through the classifying bed before being collected in the top part of the chamber. Then it leaves via the circulating pipe and after addition of the fresh feed, it passes through the heat exchanger where heat make-up is provided. It is then recycled to the upper part.

Additional devices, such as described for the forced circulation crystallizer, are of course available.

It is worth bearing in mind that the operating costs of the Oslo type crystallizer unit are much lower than with any other type when both large and coarse crystals are required. Since crystals are not in contact with any agitation device, the amount of fines to be destroyed is lower and so is the corresponding energy requirement.

This Oslo type crystallizer (classified - suspension crystallizer) allows long cycles of production between washing periods.

In addition to usual process operations, the Oslo type crystallizer has also found a number of interesting applications, e.g. for reaction-crystallization and for separation-crystallization when several chemical species are involved.

Most of the Oslo type crystallization units are of the "close type." However, the "open" type (refer to Figure 2) is worth to be considered when very large settling areas are required or when the vessel must be fabricated out of high cost alloys or metals.

Figure 2 Oslo Type Crystallizer Open Type

Typical products are:

• (NH₄)₂ SO₄
• Na₂SO₄
• AgNO₃
• hydrated mono sodium glutamate
• mono ammonium phosphate (MAP)

For more products see in section Product Experience.