*3.4. Influence of Supersaturation*

The first three experiments (Exps. 1–3, Table 1) were utilized to identify a suitable operation window and, hence, four different supersaturation values were tested. A constant volumetric flowrate (10 L/h) and saturation temperature (35 ◦C) were ensured during all three experiments. The crystallization temperature was set to Tcrys = 27, 30, 31, and 32 ◦C, which corresponds to supersaturations of S = 1.40, 1.23, 1.18, and 1.13, respectively. Significant gain of the fluidized crystal bed height was not observed during 90 min operation time using the lowest driving force (1.13), and, thus, no product withdrawal could be realized. Previous studies [19] proved that crystal growth of Asn·H2O still takes place at this supersaturation. The absent gain of the crystal bed height could be explained by the reduced growth rate of the seed crystals, which are mainly discharged at the top of the crystallizer as a consequence. Nucleation within the tubular crystallizers and a significant decrease of the product purity were observed at the highest supersaturation (1.40). Thus, the operation window is limited by a maximum supersaturation, where nucleation of the counter-enantiomer prevents continuous racemate

resolution, as well as a minimal supersaturation, where seed crystals are mainly discharged due to insufficient growth.

The steady-state results of the applied supersaturations are presented in Figure 9 regarding the product crystal size distributions and their characteristics (Figure 9a), productivity (Figure 9b), and yield (Figure 9c). As depicted in Figure 9a, the influence of the supersaturation on the product crystal size is negligible, which again verifies the classifying effect of the fluidized bed. The standard deviation of the product crystal size distribution increases slightly at higher supersaturations. Figure 9b,c shows that the supersaturation clearly enhances both productivity and yield, which correlates with the growth kinetics (dotted lines) of the given substance system [19].

**Figure 9.** Mean steady-state crystal size distributions, q3, with their respective mean values and standard deviations (**a**), productivities, Pr (**b**), and yields, Y (**c**), for four different supersaturations, S = 1.40, 1.23, 1.18, and 1.13, and the same saturation temperature and volumetric flowrate. The respective crystallization temperatures, Tcrys = 27, 30, 31, and 32 ◦C, are indicated. The dotted lines (b and c) represent the change with respect to the operation point at Tcrys = 30 ◦C, assuming the withdrawn product mass is proportional to the growth rate [19]. Given results are from crystallizer C1 for Exps. 1–3.
