# CFD Simulation of Solid Suspension for a Liquid-Solid Industrial Stirred Reactor
## 3 Results and Discussion  
For chemical multiphase reactions, the hydrodynamic field plays a major role in the mixing of the two phases and in the mass transfer. The impact of the hydrodynamic field upon the performances of an industrial stirred reactor is determined by the specific geometry of the reactor. For reactors operating with a two-phase flow, solid and liquid, it is recommended to monitor the solid particle distribution to check if the distribution is in line with the prescribed condition of the chemical process. For certain chemical reactions, it is mandatory that the solid particles float inside the liquid phase close to the lower part of the reactor. For other situations, such as in the case we analyzed, it is required that the solid particles are distributed uniformly throughout the entire liquid-phase volume. The experimental methods of investigations based on PIV or LDV are recommended to analyze the hydrodynamic field from inside the reactor; however, because of technical limitations, in our case, the reactor was made out of steel, and only CFD could be employed  
Figure 2: Computational domain: (**a**) components of the reactor; (**b**) mesh of the computational domain. to determine the flow structure. The performance of the industrial reactor requires the solid particles to remain in the suspension in the entire volume of the liquid phase for a long enough period of time so that the chemical reactions between the two phases take place. If the solid phase is not homogenously distributed in the liquid phase and it settles on the bottom of the reactor, it results in the waste of the chemical ingredients and the failure to obtain the desired final chemical product with the prescribed characteristics. There are two cases studied in this paper and the results analyzed: the first case considers that the solid particles are already settled at the bottom of the reactor before the impeller starts to rotate (Figure 3a), and for the second case, it was considered that the solid particles are all in the suspension when the impeller starts to rotate (Figure 3b). Figure 3a,b represents the distribution of the volume fraction of the solid phase in the entire 3D reactor and on a vertical plane. The most relevant quantitative representation for the analysis of the performance of the stirring mechanism is represented by the histogram of the distribution of the volume fraction of the solid phase. Similar representations of the numerical results were given by [PERSON] et al. [3], [PERSON] et al. [7] and [PERSON] [21]. Thereby, the initial state with the homogenous distribution of the solid phase in the entire volume of the liquid phase corresponds to an optimum solid volume concentration of 18.2% (Figure 3b). For the first case (Figure 3a), where the solid phase is completely sedimented in the lower part of the reactor, the solid volume concentration has a value of 63%, and the remaining 42% of the entire volume of liquid contains no solid phase. The analysis of the volume fraction of the solid phase distribution presented in Figure 4 underlines the fact that the rotation of the impeller did not avoid the sedimentation of the solid phase on the bottom of the reactor for both analyzed situations. It results that the first situation analyzed (Figure 4a) is the most unfavorable situation from the point of view of solid-phase sedimentation. The histograms of the distribution of the volume fraction of the solid phase presented in Figure 4a, b confirm that this impeller did not perform properly. In the first case (Figure 4a), almost 40% of the volume of the liquid phase does not contain solid particles, and for the second case (Figure 4b), almost 23% of the volume of the liquid phase does not contain solid particles. By analyzing Figure 4a,b, it results that it is the best option to start the operation of the stirring mechanism when the solid phase is not completely sedimented on the lower part of the reactor.