If we require a crystallizer that is significantly greater in size than the pilot device, while keeping the same thermal program, we are obliged to introduce a heating coil into the vat.
Unfortunately, in most cases, a crystal deposit develops on the outside of the heating coil despite agitation. This is the phenomenon of crusting, which is detrimental to thermal transfer. The most common solution consists of recirculating the slurry through the external exchanger using a pump. The exchanger is used if the solubility varies significantly with temperature, and also acts as a means of heating the solution to evaporate the solvent should solubility vary little with temperature.
It could be asked why it is not more worthwhile to recirculate clear liquor. In fact, this can be done in case of evaporation, but for cooling, if we wish to avoid creating fines, supersaturation without crystals has to last for the time for the liquor to join the useful volume of the crystallizer. This time must not exceed the latency. Indeed, it increases with installation size. For these reasons, we simply circulate slurry in the external exchanger. In this way, the solid can be deposited immediately on the crystals rather than on the surfaces of thermal transfer, or rather than multiplying the seeds.
The exchanger is of the tubular type with one or two passes according to the flow. We must have easy access to clean the tubes in case of crusting.
With this type of exchanger, several hundred square meters can be reached. Blockages can be averted by selecting tubes of sufficient diameter (40 mm). On the other hand, the slurry velocity must be in the order of 1.5-2.2 m/s. The thermal transfer calculation for slurry is performed by adopting the physical properties of the liquid phase. Indeed, the presence of crystals scrapes and activates the boundary layer. In addition, the thermal conductivity of compact solids is clearly greater than that of liquids. Accordingly, this method favors reliability.
When the exchanger is used for heating, we must avoid boiling in the tubes. The temperature of the wall in contact with the slurry corresponds to a certain vapor pressure that must be lower than the hydrostatic pressure governing the upper part of the exchanger.
During crystallization by cooling, tube crusting can occur if the difference in temperature between the wall surface and the slurry exceeds a critical value ATcr according to:
- - the nature of the solvent-solute and the width of the metastable zone in particular;
- - slurry temperature;
- - velocity in the tubes (ATcr increases with this velocity);
- - the nature of the wall (roughness). A rough surface benefits crusting.
One solution consists of circulating the cooling fluid in an intermediate loop to ensure that the temperature difference between the slurry and the intermediate loop remains below 3 or 4°C.