Multi-stage reactors are routinely used in chemical plants. Traditionally, the design and scale-up of these reactors rely heavily on empirical correlations or expensive experiments. Computational Fluid Dynamics (CFD) has been widely accepted as an effective replacement or complementary tool to the conventional methods in reactors' design. Although CFD has become a routine tool in the chemical industry, when complicated physics are coupled, numeircal modeling is still very challenging. In this study, a numerical simulation of a two-stage acid reactor is presentated. Liquid acid spray and droplets' evaporation, complex chemical reactions among evaporated species and interactions between chemical reactions and turbulence are all carefully considered by using some advanced numerical submodels. The predicted flow field and product conversion are compared with the experimental data.