*2.4. Suspension Freeze Crystallization*

The experimental setup of suspension freeze crystallization is shown in Figure 2.

**Figure 2.** Experimental setup of suspension freeze crystallization: (1) crystallizer, (2) scraper, (3) mixer, (4) thermostat, (5) thermocouple, (6) Pt 100 thermosensor, (7) coolant streams circulating through jacketed vessel, (8) data processing device.

The crystallizer setup consisted of a 250 mL jacketed glass vessel (1) with a rotating scraper (2) that prevents encrustation of ice crystals at the inner surface of the crystallizer. A rotation speed of 18 rpm was set for the scraper (3). The coolant (approx. 50 wt.% aqueous ethylene glycol solution) was circulating at a flow rate of 1.64 L/min through the jacketed vessel connected to a Lauda ECO RE

1050 thermostat (3), and the flow rate of the coolant was measured by a Kytola EH-5SA rotameter. The thermocouple (5) and the Lauda PT 100 thermosensor (6) were used to determine solution temperature. LabVIEW and WinTherm software were used to monitor and store measured temperature data (8).

Suspension crystallization experiments with 3 and 6 wt.% [DBNH][OAc] aqueous solutions were conducted at five different temperatures of sub-cooled solutions for freezing times of 40 and 60 min. In the case of suspension freeze crystallization, the degree of sub-cooling presents the difference between the temperature of the sub-cooled solution and its freezing point. Sub-cooling of the solution depended on the temperature of coolant circulated by the thermostat pump through the jacket of the crystallizer. When the sub-cooling temperature of solution reached the desired value and became stable, ice crystal seeds were placed inside the crystallizer to induce the freeze crystallization. For suspension FC experiments, the cooling area is the vessel inner wall surface area. The ice seeds were produced by a Scotsman AF 103 Ice Flaker and transported inside an insulated container.

As the main challenge was to properly separate ice crystals from the mother liquor, a gravity-based filtration procedure with a perforated plate setup was used (Figure 3), where partial melting of washed ice samples could take place.

**Figure 3.** Gravity-based filtration and partial melting of ice crystals.

Approximately 15 g of surface floating ice crystals and mother liquor were taken directly from the reactor with a spoon and pressed on to the perforated plate in order to squeeze the mother liquor from the sample. Additionally, the sample was washed with 7 mL of de-ionized water at 0 ◦C to remove the remaining mother liquor. Washed samples were then left to partially melt and, with each step, the ice became more purified. The last fraction of ice crystals, with a mass of approx. 5 g, were considered to be pure crystals that had properly separated from the mother liquor, and its melt was used for the ice crystal purity analysis.
