# CFD Simulation of Solid Suspension for a Liquid-Solid Industrial Stirred Reactor
## 1 Introduction  
The main idea behind this paper, which is the first in a series of three papers, is to develop a tractable and robust numerical approach for the design and assessment of mixing impellers, beyond the rather classical approach of selecting on-the-shelf impellers based on empirical correlations. In the chemical industry, the employment of stirred reactors containing liquid and solid phases to obtain chemical products from the reaction between the solid-phase component and the liquid-phase component is very common. The quality of the finite chemical product is determined by the fact that the solid particles must remain in the suspension inside the liquid phase for as long as possible for the chemical reaction to take place. The sedimentation of the solid particles must be prevented by the stirring mechanism and must ensure a homogenous distribution of the solid particles in the entire volume of liquid. Another important aspect for obtaining a quality product from the chemical reaction inside the reactor is to prolong the contact time between the solid particles and the liquid phase, and the impeller plays a major role in this matter. The quality of the suspension of the solid particles generated by the impeller can be characterized by several parameters such as solid particle distribution, cloud height and suspension velocity [1; 2; 3]. The hydrodynamics of the flow process inside the stirred tanks are complex, three-dimensional and turbulent. The efficiency of the mixing process that takes place inside the reactor depends on the interaction between the solid particles and the liquid, the geometry of the reactor and the design and position of the impeller. In the past three decades, computational fluid dynamics (CFDs) has proved to be a useful and substantial instrument in the chemical industry to learn about and enhance the complex phenomena of multiphase flow. A great number of studies from all over the world have performed numericalanalyses of the turbulent multiphase flow specific to stirred reactors with different types of configurations employing different professional CFD codes (ANSYS Fluent, ANSYS CFX) [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. [PERSON] et al. [5] presented the numerical results of investigations carried out in a mixing tank. The aim was to identify the type of approach, URANS equations or the large eddy simulation (LES) with the Smagorinski subgrid-scale model, best suited for modeling the complex multiphase turbulent flow. The results obtained with the software Fluent, velocity distribution, were compared with PIV experimental results. The conclusion is that the URANS approach provides accurate enough results, but for greater accuracy, the LES approach is recommended. [PERSON] and [PERSON] [6] analyzed the performances of various types of impellers and tried to assess three types of turbulence models, standard \(k\)-\(\varepsilon\), the Reynolds stress model (RSM) and LES, by using the professional software Fluent. Validation of the numerical results was performed by comparison to the LDA measurement. The conclusion of the research is that the LES model provides the most accurate results but needs high computation power and a long period of time. For certain types of impellers, the standard \(k\)-\(\varepsilon\) and RSM models generate satisfactory results. [PERSON] and [PERSON] [8] conducted an extensive review of the CFD results obtained by numerous studies in order to establish the most suitable settings for the numerical simulation of the complex flow from stirred reactors. Experimental results were also reviewed. For grid generation, it is recommended that finer grids are generated in regions of high turbulence and near the walls. From their study results, the employment of the turbulence model RNG \(k\)-\(\varepsilon\) generates accurate results without a large computational power and in a decent period of time.