3.1.1. Effect of Concentration Factor α on Solubility of Product Substance

With increasing concentration factors, the concentration of impurities or side compounds in the mother liquor increase accordingly (if not co-crystallizing or volatile). This normally has an impact on the solubility of the product. Figure 5 shows the observed depressive effect on product solubility of an increasing concentration factor.

**Figure 5.** Solubility of the product as function of the concentration factor of soluble impurities/ byproducts during multi-stage crystallization at 60 ◦C in aqueous solution (blue) and the solubility of the product in pure water at 60 ◦C (orange).

The solubility of the product is decreasing linearly in this matrix, with an increasing concentration factor corresponding to the concentration of impurities/byproducts. This information is crucial to close the overall heat and mass balance, as it affects the potential process yield. The extrapolated curve is not going to affect the solubility of the pure product substance in water, as the feed solution used for the tests contained impurities (feed solution is determined by α = 1). No data between 1 < α < 2 are available, as the feed solution was undersaturated and needed to be pre-concentrated before any crystallization take place.
