## 3 Development of Experimental Facility  
By means of some preliminary calculations run with CFD code, which will be detailed in following sections, a prototype of the bubble reactor has been designed and used to carry out the experimental validation of the CFD model used. The tests were carried out controlling the mass flow of air that passed through the water as well as the initial height of the water volume. The air mass flow tested was between 0.16 and 0.25 kg/s, and the water level was kept at the 0 cm mark in the measurement rule when at rest. Furthermore, the density of the air at the reactor outlet was kept at a similar value for every test, with an air pressure of 2.4 bar and a temperature of 50 \({}^{\circ}\)C. It should be noted that the range of mass flow selected, as well as the pressure and temperature conditions, were the available controlling range of the equipment (AS HORIBA MEDAS) in which the prototype designed was installed. The prototype built for the testing and validation is the one that appears in Figure 4, in which the external view of the bubbling reactor is depicted (Figure 4a). In this figure, the air inlet to the water can be seen (number 1 of Figure 4), as well as the air outlet (2). Furthermore, for the validation tests, the air circuit through the heat exchangers, present inside the reactor, has been closed, as shown in numbers 3 and 4 (inlet and outlet respectively). The reactor has been designed with a complex wall shape to help withstand the pressure difference between the high inside pressure and the ambient pressure, since it reduces the size of the flat areas. The bigger area inside the reactor has a footprint of 0.81 m\({}^{2}\), while the smaller one has an area of 0.64 m\({}^{2}\). In Figure 4b, a cut section of the reactor is shown, and the different internal components of the design are depicted. Firstly, a holed duct in the bottom (5), which is submerged in water and works as an air distributor. The holes have a diameter of 18 mm and follow a grid distribution in the first half (point 11 of Figure 4c) with a distance between holes of 25 mm in the axial direction and 34 mm in the tangential direction. In the second half of the duct (point 12 of Figure 4c) the distribution is scattered, which allows a higher hole  
Figure 4: Bubble reactor prototype description: (**a**) exterior; (**b**) cross section; (**c**) holed duct. (1) air inlet, (2) air outlet, (3) heat exchanger inlet, (4) heat exchanger outlet, (5) holed duct, (6) first heat exchanger, (7) reinforcing crosses, (8) visual access, (9) measurement rule, (10) second heat exchanger, (11) beginning of the holed duct and (12) end of the holed duct. density, as the distance in the tangential direction decreases to 17 mm. Next, a first heat exchanger (6) which also works as a flow homogenizer to avoid all the air following a single narrow path. Then, there is a pair of reinforcing crosses (7) to help withstand the pressure difference in the walls. Four visual access points are inside the reactor (8), and a measurement rule (9), with which the water level measurements during testing were carried out. A second heat exchanger (10) also serves as a droplet separator to avoid liquid water to flow out of the reactor. The heat exchangers are used to heat up the water using by-passed dry air at higher temperature than the water (see Figure 2). Finally, the testing procedure for the developed installation was as follows:  
* In the first place, before passing air through the water, the water level was adjusted to the 0 cm mark of the measurement rule
* Then, the installation was started and the mass flow, pressure, and the temperature of the air at the outlet of the bubble reactor was controlled
* Lastly, once the three controlled variables are stabilized, the water column height was measured through the sight glasses with the help of the measurement rule. Figure 5 shows the steady state results obtained from the model validation experiments carried out.