## 1 Introduction
As for the Eulerian-Eulerian model, it treats both phases as continuous flow. As the upward gas flow and downward liquid flow have a significant slip velocity, two sets of Navier-Stokes equations are required [5; 10]. The two-phase mass and force interaction is modelled similar to that in the Eulerian-Lagrangian model, except all the source terms activate in the continuous phase without summation from the dispersed particles. [PERSON] et al. [5] used an Eulerian-Eulerian model to simulate the desulfurization process by limestone and oxidation for the production of gypsum in the bottom tank. There are six additional species transport equations to tackle the complex chemical reactions. Depending on the complexity of the geometry of the FGD tower, it can be impractical to solve all the chemical species, which could cause convergence issues and increase computational cost. As a result, instead of solving every transport equation, a prepared chemical database that couples with the flow solver [22] was developed in our previous study [10], which significantly reduces the required number of transport equations. The implementation of a sieve tray can enhance the uniformity of the flow, which is usually favorable to the operation conditions [5; 12; 23]. The tray-equipped FGD tower is found to increase the SO\({}_{2}\) removal efficiency by 7% compared to the empty tower [24]. The detailed two-phase mixing of a small-scale FGD tower implemented by sieve tray was already simulated in our previous study [5]. The number of the perforated holes at each tray is 300 for this small-scale FGD tower. However, this number exceeds 200,000 for the large-scale FGD tower [4]. Considering the extremely large ratio of hole at tray to height and width of the tower, the computation cost can be very large for simulation of the full-scale tower. As a result, the porous media model [5; 25] that replaces the perforated details at the sieve tray is used in this study. The porous media model regards the tray region as a flow domain and compensates for the pressure drop by a momentum sink term. The porous media model has already been found to be able to replace the detailed structures for flow past packed beds, filter papers, and perforated plates [24; 25; 26; 27]. The empirical coefficients of the porous media model can be tabulated from Idelchik [28], and the velocity and pressure for a gas flow past tray tower by using porous media model are very consistent with that with a real perforated structure [4], saving significant computational time from the grid resolution near the trays. In this study, the two-phase chemical flow model is used together with the porous media model, which further validates the capability of the porous media model to replace the complex structures of the FGD tower. It avoids having a tremendous number of grid points near the regions of sieve trays. The required computational time speeds up by 13 times, and hence, it becomes easier to conduct different designs for the small-scale tower, such as the influences of sieve tray and inlet gas flow rate on the two-phase mixing and the ensuing desulfurization efficiency within the FGD tower. The model replacement can also be extended to the full-scale tower simulations of more practicality. This study provides a reliable and fast computational framework and insightful information for the design guidelines for the FGD tower in the relevant industries.