## 2 Technologies for Purifying the Tail Gases from N2O and NOx
Above this temperature, a decrease in the NO\({}_{\rm x}\) reduction rate is observed, despite the increase in the NH\({}_{3}\)/NO\({}_{\rm x}\) molar ratio. This is due to the increased share of undesirable side reactions (e.g., ammonia oxidation to N\({}_{2}\)O). Therefore, for each catalyst, the temperature window for the deNO\({}_{\rm x}\) process is defined, i.e., the temperature range of its optimal operation [7; 7; 33; 57]. The deNO\({}_{\rm x}\) or deN\({}_{2}\)O catalysts can be formed in various shapes, i.e., extrudates, trillobes, spheres, stars, monoliths, foams or metal wire gauzes, which affect the design of the reactor interior. The shape of the catalyst determines the size of its geometric surface area available for the reactants, and the gas pressure drops across the catalyst bed. Higher gas flow resistances are caused by the catalyst beds in the form of grains, while the lowest are caused by the catalysts in the form of monoliths or gauzes. High-pressure drops and diffusion resistances characterize packed bed reactors, while these resistances are more significant the smaller the shaped catalyst body forming the bed. Monolithic catalysts usually have the form of honeycomb blocks. They are typically made of ceramic (cordierite) or metallic material (stainless steel, metal alloys), coated with the active phase of the catalyst. Bulk catalysts can also be shaped this way (without using support). Monolithic catalysts are characterized by a high specific surface area, low thermal expansion and high thermal shock resistance. Their manufacturing process is easy to scale up. Their disadvantagesare the lack of reactants radial mixing, low rates of interfacial mass and heat exchange. In addition to the solid catalyst blocks, monoliths can also have a foam structure, made of metal or ceramics. They are characterized by a high surface area-to-volume ratio. Their advantages are low pressure drop across the catalyst bed (however higher than in the case of standard monoliths), high mass and heat transfer coefficients, good radial mixing, higher gas flow disturbances in the material's tortuous channels, high specific surface area and porosity. Heat transfer occurs in three ways: via convection, radiation and conduction. Catalysts in the form of gauzes, made of metals or steel coated with an active phase (e.g., FeCeW catalysts supported on SSWM gauzes), are characterized by: high rates of mass and heat transfer and low pressure drops. In this case, the internal and external diffusion resistances and pressure drop are greatly influenced by the supports' structure. For the catalyst in the form of gauzes, the limitations related to the external diffusion resistances are eliminated (pore and axial diffusion), and a reactor with a dimension up to 10 times smaller than the reactor with a monolithic catalyst can be used. They can be used in nitric acid plants with high tail gases flow rates. The structure of such a catalyst is flexible, so its efficiency can be easily influenced by changing the size and shape of the gauzes, wire diameter or gauzes packing density. Fluidized bed reactors provide the best gas-solid body contact; therefore, their dimensions can be smaller than those of packed or monolythic bed reactors. However, this requires using catalysts with high mechanical strength, because during fluidization, abrasion of the catalyst bed may occur [37; 59; 60]. The pressure drop value is affected by the form and size of the shaped catalyst body and the linear gas flow velocity through the catalyst bed. The latter depends on the method of the catalyst bed installation inside the reactor. Figure 1 shows an exemplary dependence of the gas pressure drops on the linear gas flow velocity in the radial reactor, for the catalysts in the form of 3 mm diameter extrudates and 8 mm diameter tablets. It was determined based on deNOx tests in the SCR reactor in a pilot nitric acid plant.