# Experimental and Numerical Investigation of Spray Scrubber Dust Collection Efficiency
## 2 Materials and Methods
Before the experimental tests, the dust was examined under a microscope to determine the grain size. The results of microscopic examinations using different magnifications are presented in Figure 4a,b. Figure 2: Demister geometry used to perform 3D printing. Figure 4a,b show the characteristic dimensions of the various dust fractions found. The smallest pieces of dust have a diameter of about 0.5 \(\upmu\)m, the largest about 5 \(\upmu\)m, and there are also larger groups of combined smaller fractions. Dust of this size was suitable for the planned measurements. Magnesium oxide tends to coalesce and stick together into larger agglomerations, especially in the presence of water. In relation to the operation of the scrubber, this is a desirable phenomenon inside the cleaning chamber, which should have a positive effect on the final separation efficiency. The polluted gas is sprayed with droplets generated in the water nozzle, which causes the dust to combine not only with the drops but also with other particles of wet dust. However, a problem may occur during introducing dust into the system, where narrower pipes may become clogged in the presence of water or even humid air. For this reason, the dust introduced into the system must be appropriately crushed and distributed in the inlet pipeline. In order to enable uniform introduction of dust into the main air pipeline, the fluidizer design was proposed. Its design is shown in Figure 5a,b, where Figure 5a shows the fluidized with the external housing and Figure 5b shows the centrifugal inlet through which compressed air flows. The pipe inside the fluidizer has the internal diameter of 2 mm and the external housing has a volume of approximately 100 mL. Figure 4: The magnesium oxide used during the experiment, visible under the microscope. Full view (**a**) and zoom (**b**) with fine particles. Figure 3: Test stand for measuring the separation efficiency of the spray scrubber (1, 5—flowmeters, 2, 7—nanometers, 3—spray scrubber, 4—air filter, 6—water nozzle). The air flow in the fluidizer is generated by an additional compressor. A measured mass of dust was poured into the lower part of the fluidizer housing, then a centrifugal inlet located at the bottom, supplied with compressed air, generated a swirl flow inside the fluidizer. Fine, homogenously mixed dust flew out through the valve at the top of fluidizer into the main pipeline, where it mixed with the main air stream. Before the measurement, approximately 8 g of MgO was placed in the fluidizer. For the supplied pressure value on a level of 3 bar, around 6 g of dust was introduced into the main pipeline during a time period of 60 s.  
The particle size distribution was measured inside dry scrubber chamber, in the vicinity of the scrubber inlet. The measurement device was Topas LAP 322 Aerosol Spectrometer (Topas GmbH, Dresden, Germany), which allows to detect particle or aerosols of size from 0.2 m to 40 m. Obtained particle dimensions are presented in Figure 6. Three measurement series performed under the same conditions are presented in order to show the differences between the results caused by random circumstances. The characteristic peak value appears for the particle size of 6 m for all measurements. The authors also tried to measure the particle distribution on scrubber outlet; however, the quantity of the particle flowing through the device was significantly lower and the number of registered particle fractions was too small to perform a reliable measurement. Figure 5: Fluidizer—external view (a), fluidizer—internal view (b). A water nozzle was installed in the scrubber, which generated drops that caught contaminants. A full-cone axial water nozzle was selected. According to the author's estimations, a maximum vertical velocity of around 2 m/s should be expected in the scrubber chamber, depending on the gas flow rate at the inlet. This means that water drops with diameters smaller than the critical droplet diameter will be entrained by the air flow and may leave the system at the top and hit the air filter.