*2.5. Reactor Setup*

Two Plane flange vessels with a tempering jacket and a bottom drain valve of 250 mL (Pfaudler GmbH, Germany) were used as crystallizer and saturator. The membrane reactor consists of two Teflon chambers separated by a polyvinylidene fluoride transfer membrane (PVDF) with a cut-off of 0.2 μm (Bio-Rad Laboratories GmbH, Germany) (Figure 1). The vessels were connected with PharMed - BPT and Tygon LMT - 55 tubes (Saint - Gobain, France), tempered with the cooling bath thermostat Huber CC-K6 attached to a Pilot ONE control panel (both by Peter Huber Kältemaschinenbau AG, Germany) and stirred with overhead stirrers MICROSTAR 7.5 control. The membrane reactor was stirred with magnetic stirrers (IKA Werke, Germany). Two peristaltic pumps—Hei-FLOW Precision 01 (Heidolph Instruments, Germany)—were used to transfer the mother liquors from the saturator to the membrane reactor and from the crystallizer to the saturator. To ensure a constant filling level in the membrane reactor, an overflow was integrated from which the reaction solution can flow into the lower positioned crystallizer. In order to keep the solid salts in the saturator and the crystallizer, 11 μm filters have been placed at the inlet of the tubes.

**Figure 1.** Design of the membrane reactor with polyvinylidene fluoride membrane (PVDF).
