## 2 Technologies for Purifying the Tail Gases from N2O and NOx
For most catalyst beds, the gas flow is considered as turbulent, except for the flow inside the monolith chanels, where it is treated as laminar. The main chemical reactions rates, involving NO and NH\({}_{3}\) reactants, can be determined using the Chae kinetic model. To consider the reactor porosity, i.e., to obtain information on the permeability and pressure drop across the catalyst bed, the Ergun, Darcy-Forchheimer or Brinkman equations are also applied. The pressure equation can be obtained by combining continuity and momentum conservation equations. To solve the equations mentioned above, the appropriate solvers are used, depending on the nature of the fluid flow (laminar or turbulent, compressible or incompressible), the type and quantity of flowing media or the degree of gases mixing. A possible heat exchange should also be taken into account. The examples of solvers, that can be used, are: linear and non-linear solvers (pressure-based or density-based), fully coupled stationary solver with Direct module--UMFPACK, PCG--preconditioned (bi)-conjugate gradient solver for symmetric matrices, PbiCG--preconditioned stabilized (bi)-conjugate gradient solver for arbitrary matrices, smooth Solver--function smoothing, GAMG solver--using a generalized multi-mesh geometric-algebraic method, diagonal--for explicit methods, DIC with buffering--accelerated incomplete Choleski factorization, etc. [58; 63; 65; 69; 70; 71]. The deNO\({}_{\rm x}\)/deN\({}_{2}\)O reactor is a vertical apparatus installed on a steel supporting structure, inside which the catalyst bed is installed. Depending on the route of the tail gases pipeline and the available space in the technological line of nitric acid manufacturing, the arrangement of the reactor can be downflow or upflow. The gas inlet and outlet can be located in the upper and lower part of the reactor or on its side wall. Inside the reactor, the following zones can be distinguished:  
* Reducing agent injection (this zone can be located upstream of the reactor or inside it);
* Reactor inlet with possible internal elements improving gas flow distribution over the catalyst bed or tail gases mixing with reducing agent;
* Reaction zone with the catalyst bed;
* Reactor outlet. The efficiency of the SCR process is more remarkable the more homogeneous the gas flow field inside the reactor, and the more even the ammonia concentration distribution in the tail-gas stream. NH\({}_{3}\) concentration profile in the tail-gas stream should be similar to the NO\({}_{\rm x}\) concentration profile. To ensure a homogeneous gas flow field (conditioning of the gas flow) upstream of the catalyst bed, perforated plates, baffle plates, rectifying grids, guide vanes/plates or flow rectifiers are often installed inside the reactor. In the ammonia injection zone, static mixers can be installed. Flow rectifiers are used to ensure such a ratio value of the channel opening hydraulic diameter to the length of the gas flow path to create vertical flow lines upstream of the catalyst bed. Static mixers affect the flow structure by generating secondary cross-flow vortices and turbulent flow, increasing the mass and heat transfer in the cross-section of the pipeline/reactor while maintaining low-pressure drops. It can be a hollow pipe with openings or a channel with baffles mounted inside: straight, angled, corrugated or helical (Komax Custody Transfer, Kenics, Ross Engineering, Chemineer, Sulzer, JLS International). Static mixers can be of various types, e.g., made of metallic foam or suitably shaped sheets installed in the inlet section of the reactor (Iso Swirl, Shear, Swirl-Shear and Vortex mixers) [72; 73; 74; 75; 76]. Figure 2a,b show two simple static mixers: a pipe with holes and a helix-shaped sheet mounted inside the pipe. The red arrow marks the inlet of the reducing agent. \[4{\rm NO}+4{\rm NH}_{3}+{\rm O}_{2}\to 4{\rm N}_{2}+6{\rm H}_{2}{\rm O} \tag{8}\]2 Two types of static mixers: (a) a pipe with holes; (b) a helix-shaped sheet. The inlet of ammonia is marked with a red arrow.