## 3 Results and Discussion
### The Effect of Volume Flow Rate and Temperature on Conversion
A flow in a domain with a sudden contraction, a so-called back-step geometry, can cause the formation of recirculation and stagnation areas and other instabilities [48]. The intensity of secondary flow raises at higher values and depends on the Reynolds number. Furthermore, the fluid elements which have become trapped in the stagnation zone can stay inside for a long time. This may cause a significant difference between the space time and the residence time in the reactor. The chemical reaction proceeds in stagnated fluid elements and the conversion of sodium hydroxide increases. This can contribute to measurement inaccuracies in conductivity measurements, especially when the stagnation and measurement areas are located in the same place as in the present study. The impact of this may result in higher reaction rate constants in the T-shaped reactor than those obtained in the kinetic model. Both of the effects described above persist in all of the experimental setups and are likely to belong to a systematic error present in all investigated experimental setups (Figures 8-10). Figure 8 shows the experimental data for a sodium hydroxide conversion over time in a microreactor system with an interdigital microreactor. Compared to the experimental data seen in the setup with a T-shaped reactor, there is no significant decrease in conversion values at low volume flow rates. The mixing process in the interdigital microreactor is based on the multilamination mixing principle and striation thickness reduction. The interdigital microreactor was developed especially for operation at low flow rates [16]. It gives a sufficient mixing  
Figure 10: Mixing in T-mixer at different volume flow rates: (a) 0.1 mL/min; (b) 5 mL/min; (c) 20 mL/min. rate across the entire range of the volume flow rates studied. The conversion values which have been achieved indicate good reactor performance. The measured conversion values versus space time in the experimental setup using a chicane microreactor are shown in Figure 9. The volume flow rate could be varied over a wider range, starting at 0.3, and reaching 20 mL/min. The experimental data indicates a good match of the experimentally achieved conversion with the results of the modeling. This type of microreactor has a chicane channel structure and belongs to a reactor type which is based on chaotic advection and is designed to be utilized at high flow rates. Nevertheless, even when flow rates are small, the chicane mixing channel provides good reactor performance due to a greater mixing rate, which is much faster than the reaction rate seen in a saponification reaction. Figure 11 shows the mixing process carried out at different volume flow rates in the initial region of the chicane microreactor, after passing through two rows, each with 33 cells. When there is a strongly stratified flow at a flow rate of 0.1 mL/min, the mixing quality is clearly low. The interface between the solutions is blurred due to the long diffusion time at low flow rate (Figure 11). With a higher flow rate of 5 mL/min, a significant change in hydrodynamics is observed, starting from the first cell (Figure 11). In this case, eddy structures of chaotic advection can be observed. The desired homogeneous mixing state is achieved after passing only a few mixing elements. The remaining elements are not necessary for mixing. In summary, the utilization of a complex microreactor system is not necessary to provide good mixing performance of slow homogeneous chemical reactions. For example, the reaction time of ethyl acetate saponification using sodium hydroxide (both solutions are 0.1 M) at a temperature of 17 \({}^{\circ}\)C is approximately 303 s. Even the investigated T-shaped reactor, coupled with a tube of 1.6 mm at high volume flow rates, provides a much faster mixing rate than reaction rate, resulting in good reactor performance but small conversions. In this case, the reaction is mainly limited by the reaction rate and a so-called chemical regime is provided [49].