## 2 Technologies for Purifying the Tail Gases from N2O and NOx
Various Aspects of Designing Reactors for Purifying the Tail Gases from N\({}_{\text{2}}\)O and NO\({}_{\text{x}}\)  
Several aspects should be considered when designing a reactor for the nitrogen oxides removal from the tail gases emitted from nitric acid plants:  
* The required reactor size (dependent on a gas flow rate and catalyst activity);
* The gas temperature at the reactor inlet, required for the effective catalyst operation (eventually, selection of a heat exchanger for the tail gases preheating before they enter the catalytic reactor);
* The allowable gas flow resistances across the catalyst bed;* Permissible content of nitrogen oxides downstream of the \(\mathrm{deNO_{x}/deN_{2}O}\) reactor;
* Method of the tail gases purifying from the nitrogen oxides (selective or non-selective catalytic reduction of N\({}_{2}\)O, direct catalytic N\({}_{2}\)O decomposition);
* The method of mixing the tail gases with a reducing agent (selection of the static mixer) and distributing of this mixture inside the reactor [67; 68];
* The arrangement (sequence) of the catalyst beds in reactors for simultaneous purifying of the tail gases from N\({}_{2}\)O and NO\({}_{x}\) and a method of reducing agent introduction upstream of the second catalyst bed;
* The available free space in the technological line of HNO\({}_{3}\) plant, in which the reactor could be installed (downstream of the absorption column). The process of reactors designing is often supported by numerical calculations (CFD--Computational Fluid Dynamics). They allow for optimising the proposed solution or determining the distribution of gas velocity, pressure and temperature or the degree of reactants conversion within the reactor volume for different reactor design solutions. The CFD method enables to simulate the influence of key process parameters on the efficiency of the NOx reduction process, such as temperature, NH\({}_{3}\)/NO\({}_{x}\) molar ratio and GHSV. Ansys Fluent, OpenFOAM, or Comsol Multiphysics software can be used to simulate the gas flow throughout the reactor. When building a reactor model that would best reflect the actual process conditions, the following factors should be taken into account: reactor design, the process of ammonia with the tail gases mixing, mass and heat transfer processes in a multi-component system, as well as the main chemical reactions. Due to the slight temperature difference between the outlet and inlet of the reactor, the modelled system can be treated as an adiabatic. The computational domain, reflecting the interior of the modelled reactor, is built based on the spatial dimensions of the existing or designed reactor (2D axisymmetric modelling or 3D modelling in the COMSOL program). It is divided into many subdomains, within which the CFD equations are discretized (e.g., by GAMBIT software) and calculated. It is crucial to select the correct geometry of the mesh elements (e.g., cuboid, tetrahedron, prism, pyramid), the number of subdomains and their sizes, which affect the accuracy of the calculations. The computational mesh is densified in the area of higher turbulence. Then, boundary, initial and operating conditions (based on industrial practice) are determined. The model parameters can be determined from the experimental results, using an optimization procedure based on a Taguchi method (used to design the experiment), a real-coded genetic algorithm (to search for optimal solutions) and a shape-tunable neural network (to bind the operational data together). The gas flow through the catalyst bed can be modeled as a compressible and viscous fluid flow through a porous medium. A \(k\)-\(\epsilon\) turbulent flow model, e.g., of the RANS type (Reynolds-Averaged Navier-Stokes), as well as the equations of continuity and conservation of mass, momentum and energy can be used. The fixed bed reactors are modelled as a plug flow or axially dispersed plug flow. The gas density, dynamic viscosity, temperature, pressure and composition are determined based on the characteristics of the real gas.