## 3 Results and Discussion
### Synthesis by Direct Ultrasonic Aerosolisation and Comparison with Non-Aerosolised Synthesis  
In this type of setup, the atomisation of water containing CaCl\({}_{2}\) and Na\({}_{2}\)CO\({}_{3}\), each contained in its own atomiser, was carried out under the operating conditions listed in Table 1. Samples were obtained using equimolar concentrations of both reagents in the atomisers, namely C\({}_{0}\) = 0.472 M and C\({}_{0}\)/4 in different experiments. It has been observed that the sequence of operations for starting the experimental setup must be carried out in a non-invertible order; otherwise, the system will not work. Specifically, the Venturi collector circuit must be started first, followed by switching the gas compressor upstream of the atomisers. The reason for this intriguing phenomenon could be explained by taking into account that the Venturi collector requires that there is no gaseous flow from the nebulisers at plant start-up. Hence, the commissioning of the apparatus described in Figure 1 must be carried out according to the following sequence of operations:  
1. Start the flow of liquid into the Venturi collector circuit, keeping the air compressor supplying the aerosolizers off. 2. Raise the water pressure at the Venturi nozzle inlet by increasing the supply voltage of the liquid pump, up to a value that allows the Venturi nozzle suction to be triggered. Generally, a liquid pressure of 1.5 bar is sufficient and higher values are useless, as they may overload the liquid pump without increasing the effectiveness of NP abatement. 3. Turn on the gas compressor and adjust its supply voltage so that the downstream compressor flow rate of gas does not exceed 6 L/min. Higher flow values may interfere with the operation of the Venturi nozzle. According to the previously described settings, the liquid contained in both aerosolsisers is atomised with a flow rate of approximately 9 \(\div\) 10 cm\({}^{3}\)/h. While the system is running, an intense formation of air bubbles is observed in the Venturi vessel. This fact can seriously hamper the operation of the liquid circulation pump, as a mixed gas-liquid phase regime in the pump may give rise to vibrations induced by cavitation phenomena. Consequently, it was necessary to prevent bubble trails from escaping from the Venturi vessel towards the liquid pump. To this end, a glass septum was placed inside it, so that the bubbles do not spread throughout the liquid holdup \begin{table}
\begin{tabular}{c c} \hline \hline
**Setup Component** & **Set Value** \\ \hline Venturi vessel total volume & 1000 [cm\({}^{3}\)] \\ Liquid holdup inside the vessel & 500 [cm\({}^{3}\)] \\ Total circulating liquid volume V\({}_{0}\) & 600 [cm\({}^{3}\)] \\ Maximum liquid holdup in each atomiser & 20 [cm\({}^{3}\)] \\ Liquid pressure at the Venturi reactor inlet & 1.5 [bar] \\ Volumetric gas flow rate at a single aerosoliser inlet & 50 [cm\({}^{3}\)/s] \\ Na\({}_{2}\)CO\({}_{3}\) and CaCl\({}_{2}\) concentration in aerosolisers & C\({}_{0}\); C\({}_{0}\)/4 [M] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Set of operating conditions adopted for the direct aerosol synthesis. C\({}_{0}\) = 0.472 [M]. within the vessel itself, but remain confined to a pre-determined volume, without leaving it. In addition, the vessel was equipped with special devices in the outlet gas piping, including two traps, in order to prevent the entrainment of liquid droplets towards the atomiser compressor. After an atomisation of 1.5 cm\({}^{3}\) of both reagents, the solution inside the Venturi vessel became cloudy, indicative of the formation of a dispersed solid phase of calcium carbonate. Once a volume of 5 cm\({}^{3}\) for each reagent has been atomised, the compressor of the aerosolsisers was turned off, and so was the circulating pump of the liquid supply to the Venturi nozzle. Finally, a liquid aliquot was extracted from the Venturi vessel in order to characterise its suspended solid phase.