*2.3. Experimental Procedure*

Table 2 depicts the conditions of the executed experiments, one with thiamine hydrochloride (Exp. 5) and four with KH2PO4 (Exp. 1–4). For Exp. 1–3 the saturation temperature was 35 ◦C and the cooling rate, the final temperature, and the mass of the seed loading was varied for each experiment to evaluate the limits, with respect to the suspension density and crystal size. The seed fraction was sieved and had a normal distributed initial size of 212–300 μm for all experiments, except for Exp. 3. Smaller seeds with a range of 150–212 μm were used in this case to alter the initial suspension and optical density, to evaluate the impact of these parameters on the crystal size measurements. The fourth experiment was carried out with a larger initial concentration (according to a saturation temperature of 56.5 ◦C), since it is well known that the bypass of online microscopes often tends to block under these conditions. In addition, an anti-solvent crystallization, Exp. 5, of thiamine hydrochloride was performed to evaluate the applicability of the shadowgraphic probe to fragile crystal systems.

**Table 2.** Process conditions of the performed experiments with KH2PO4 and thiamine hydrochloride.


KH2PO4 was added to the reactor according to Equation (1) (see Table 2), the impeller speed was set to 250 rpm and the crystallizer was heated a priori for 0.5 h to 5–10 K above the respective saturation temperature to ensure complete dissolution and equal starting conditions for all experiments. Afterwards, the clear solutions were slightly subcooled (0.1–1 K) and the seeds were added at temperature Tseed (see Table 2) and time t = 0 h. Subsequently, cooling was executed as a simple linear cooling ramp after the seed's addition, with a certain slope between −7.5 ◦C/h and −12 ◦C/h (see Figure 4a for an exemplary temperature curve of Exp. 3—KH2PO4). Immediately after seeding (t = 0 h), the particle size was measured simultaneously every 5 min by the online microscope and the shadowgraphic probe. The experiments ended at the final temperature, Tend, if either the suspension density was too high, resulting in too much overlapping of the single crystals, or excessive nucleation was observed. During the last measurement of the crystal size by the optical methods an unclassified suspension sample was taken from the bottom valve of the reactor, and was immediately filtered using a strainer and a filter paper. Then, the filter cake was washed with an adjusted ethanol/water-mixture to prevent nucleation or dissolution of the crystals through the residual

mother liquor. Afterwards, the crystals were dried and sieved to determine the mass-based size distribution. During the experiments with KH2PO4, the state of the liquid phase was monitored by a calibrated ATR-FTIR and by solid-free liquid samples, which were taken every 5 min, simultaneous to the particle size measurements (see Figure 4b).

**Figure 4.** (**a**) Temperature profile of Exp. 3—KH2PO4: saturation, seeding, linear cooling ramp, and final temperature. (**b**) Concentration profile of Exp. 3—KH2PO4: FTIR, offline samples, and saturation curve.

From ATR-FTIR spectra the mass fraction (*mKDP*/*msolution*) was calculated by an existing calibration, successfully applied in the past [68] to evaluate the suspension density, where *cFTIR* is the concentration of the FTIR and *c*<sup>0</sup> is the initial concentration:

$$
\rho\_{\text{Susp}}(t) = (c\_0 - \varepsilon\_{FTIR}(t)) + \frac{m\_{\text{seed}}}{m\_{\text{solution}}} \tag{2}
$$

$$S(t) = \frac{c\_{FTIR}(t)}{c\_{\text{sat}}(T(t))}\tag{3}$$

The supersaturation, *S*, is the driving force in crystallization, and was calculated according to Equation (3), with the concentration at saturation, *csat*(*T*(*t*)).

The experiments started at small supersaturations, which gradually increased due to cooling. After about 0.6 h sufficient solid surface was present in the crystallizer to counterbalance the supersaturation generation and the driving force started to decrease, exemplarily shown in Figure 5a for Exp. 3—KH2PO4.

The concentration of the liquid phase versus temperature in the binary phase diagram for all KH2PO4 experiments is given in Figure 5b. Exp.1–3 have almost identical conditions with a saturation temperature of 35 ◦C and only Exp. 4 was saturated at an elevated temperature. A significant influence of the seed load or the cooling ramp on the concentration profile is not clearly visible due to the fast crystallization kinetics of KH2PO4.

In addition, a fifth experiment with thiamin hydrochloride was carried out as an anti-solvent crystallization via primary nucleation. The thiamine hydrochloride was dissolved in water and added to the reactor. At tstart = 0 h the ethanol was added as the antisolvent. The initial masses were calculated based on literature data [67]. The experiment was carried out at a constant temperature of 25 ◦C, and the impeller speed was set to 250 rpm, similar to the experiments with KH2PO4.

The shadowgraphic probe took pictures of the suspension every minute from the beginning, and after a significant number of crystals were visible (t = 1 h), the online microscope in the bypass was put into operation.

**Figure 5.** (**a**) Supersaturation profile calculated for Exp. 3—KH2PO4 with the FTIR data, (**b**) Liquid phase concentration for Exp. 1–4 KH2PO4, measured with the FTIR depicted in a part of the binary phase diagram of KH2PO4/H2O. The dashed line is the saturation curve according to Equation (1).

The experiment was carried out until the concentration was too high to identify single crystals. Taking a suspension sample similar to the experiments with KH2PO4, was not possible since the thin, needle-like crystals of the thiamine hydrochloride monohydrate broke during filtration and further handling. A representative sieving analysis was, therefore, not possible and the crystal length and width were only determined via image processing.
