Very thick slurry
When the system is such that the crystal volume (including the interstitial voids) exceeds 30% of the slurry volume, the slurry flows poorly. These are typically crystals in plates or fibers, whose porosity at rest exceeds 80%. Not much liquid remains free to ensure the flow of slurry, leading to considerable attrition.
In such cases, we use a continuous cooled tube crystallizer fitted with internal helical scrapers and a double envelope in which the cooling liquid circulates.
In terms of crystallization, the process is comparable to that of a ‘discontinuous’ batch, and whereas the residence time of the vat is in the order of several hours (8-24 h for sugar), it is only in the order of a dozen minutes for the cooled tube. The crystals cannot grow much.
In theory, it is batch crystallization that allows for the greatest control, particularly if prior seeding has been performed. The crystals are also all the same age and, hopefully, also the same size, and the variation coefficient will be in the order of only 10%.
The variation coefficient of a continuous homogenous crystallizer without attrition is of 50%, but can be reduced to 30% or even 20% if the device is fitted with fines destruction associated with product classification.
The V.C. of a fluidized bed equipped with fines destruction is below 20%.