## 3 Results  
The geometry of the air reactor considered in this simulation is taken to be similar to that in the experimental work of [PERSON] et al. [22]. The height of the riser is 7.3 m, with an interior diameter of 0.1 m, while the diameter of the inlet and outlet pipes is 0.04 m. Figure 3a,b show the mesh generated on the upper and lower parts of the computational domain. Figure 3c shows the top view of the mesh. The grid size with the mesh count of 516,332 elements is found to be optimum. At the inlet and outlet of the particles, the meshing is coarse because of the maximum number of interactions between the particles. In the current simulation, hematite is chosen as the OC with a particle diameter of 60 \(\mathrm{\SIUnitSymbolMicro m}\) and a density of 5150 kg/m\({}^{3}\). Simulation is done to understand the distribution of the pressure and volume fraction of particles at different bed heights. During simulation, the solid mass flow circulation is kept constant, considering the experimental conditions. An accurate and stable solution is achieved using the second-order upwind numerical scheme. For injection purposes, the standard parcel release method is used [30; 31]. Figure 3: Mesh in the (**a**) upper part and (**b**) lower part; (**C**) top view of the air. Initially, the inlet velocity of the air from the bottom and the velocity of the injection of the OC is taken as zero. The injection of OC occurs at the particle inlet for 1 s only and almost 198,565 particles are released. The velocity of the air is maintained at zero for the next second so that the particle that falls due to gravitation settles down at the bottom, and its kinetic energy becomes negligible. This is the initial boundary condition of the simulation. After the bed of OC is formed, the inlet velocity of the air is increased gradually up to 2.50 m/s, which is slightly greater than the minimum fluidization velocity for the particle [22]. This velocity is supposed to lift the particle outside the reactor towards the mixture outlet. When the air velocity is increased, the particles in the settled bed experience sudden lift force, and they begin to move upward. Particles that are at the top of the bed reach the outlet of the reactor at around 15 s. The whole simulation runs for almost 45 days due to the large number of particles. The results of the simulation are monitored at different heights, i.e., 0 m (air inlet), 0.5 m, 0.75 m, 6 m, 7 m, and mixture outlet. The mass flow rate of the particle is set as 0.04 kg/s, similar to the solid circulation rate of [PERSON] et al. [22]. Various other key modeling parameters required to run the simulation are given in Table 1.  
Figure 4 shows the variation in the mass flow rate with respect to time at the outlet of the reactor. It can be concluded that at the start of the simulation, particles escape the reactor at a discontinuous rate. However, after 28 s, the particles start to escape almost at a continuous rate of 0.0007 kg/s. This is of the same order as was found in the experiment performed by [PERSON] et al. [22]. The stable circulation of hematite is achieved, which shows it can be used as an OC. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Value** \\ \hline Particle diameter & 60 \(\upmu\)m \\ \hline Density of particles & 5150 kg/m\({}^{3}\) \\ \hline Material in primary phase & Air \\ \hline Material in discreet phase & Hematite \\ \hline Discretization scheme & Second-order Upwind \\ \hline Particle inlet boundary condition & Wall, 0.04 kg/s \\ \hline Gas inlet boundary condition & Velocity inlet at 2.50 m/s \\ \hline Outlet boundary condition & Pressure Outlet \\ \hline Numerical scheme & Phase Coupled Simple \\ \hline Drag model & Syamlal-O’Brien \\ \hline Time step & Fluid: 10\({}^{-3}\) s, Particle: 10\({}^{-4}\) s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Modeling parameters considered for simulation. Figure 5 shows the variation in static pressure (gauge) with time at different heights in the reactor.