## 2 Technologies for Purifying the Tail Gases from N2O and NOx
### Radial Flow Reactor
This amount depends on the mechanical properties of the catalyst and the number of plant shutdowns and start-ups;
* In a radial reactor, a sufficient space between the reactor and the basket side wall should be ensured to distribute the gas stream over the catalyst bed evenly;
* When two or more catalyst beds are used, the \(\mathrm{deNO_{x}}\) catalyst bed should be placed in the outer annular basket (inward flow of the gas stream). In a radial flow reactor with a co-axial arrangement of the annular catalytic baskets along the reactor diameter, it is not possible to introduce a reducing agent upstream of the second catalyst bed. Radial flow reactors can be used for high gas flow rates through the catalyst bed, where it is required to purify large amounts of gas per unit time while ensuring low pressure drops. This reactor provides lower gas flow resistances across the catalyst bed, compared with the axial ones, for the same volumetric gas flow rate. The efficiency of tail-gas purification from the nitrogen oxides in the radial reactor can be increased by increasing the reactor length. Despite the greater height of the bed, its lower thickness ensures a lower gas pressure drop. Such a reactor design enables the use of smaller sizes of the shaped catalyst bodies, which allow increasing the efficiency of the \(\mathrm{deNO_{x}}/\mathrm{deN_{2}O}\) technology without fear of excessive pressure drop. In a radial reactor, the catalyst bed settles during operation, and an empty space may form in the upper part of the bed, constituting a path of gas bypassing. It is, therefore, important to seal the catalyst bed in this area so that the gas does not reach the upper edge of the catalyst bed in the catalytic basket. The upper part of the catalyst bed is usually shielded by a gas-impermeable top cover plate. This part of the bed serves as a seal and is not used in gas purification. Using an excess catalyst as a sealing layer increases the cost of the apparatus and the \(\mathrm{deNO_{x}}/\mathrm{deN_{2}O}\) technology. Using a top cover plate of suitable weight and design also generates additional costs compared with the axial reactor. The radial reactor provides more efficient use of the available reaction zone than the axial one, while lower pressure drops translate into higher energy recovery in the expansion turbine. This design also has disadvantages. Catalyst loading and unloading in the radial reactor is more complicated and time-consuming. In addition, there is a need to install or remove the basket's top cover plate. Due to the low gas flow resistances, there can also be a problem with the uniform gas distribution over the catalyst bed, along the length of the reactor. Thus, the longer the reactor is, the poorer the gas distribution along the height of the basket. A way to slightly increase the flow resistance is to use a suitable design of basket walls in the upper and lower part of the catalyst bed. They should ensure a higher gas flow resistance in the upper part of the catalyst bed (near the top cover plate) and lower gas flow resistance in the lower part of the catalyst bed. This reduces gas recirculation in the annular space between the reactor and perforated side walls of the basket, as well as in the central part of reactor. The gas flow resistance through the perforated basket walls can be varied by, e.g., the use of different hole sizes and varying their location (the basket walls can be made of Vee Wire or Johnson screens). Radial reactors are often offered to nitric acid producers by BASF as part of the SCR-\(\mathrm{deNO_{x}}\) technology. Most often, catalysts based on vanadium oxide on TiO\({}_{2}\) support or iron-modified zeolites in the form of extrudates are installed in them [35; 61; 65; 80; 84; 86; 87; 88; 89]. Radial Flow Reactor with Triple-Bed Catalytic System  
The catalyst beds in the radial flow reactor can be installed in the three annular baskets, arranged coaxially along the reactor's diameter and/or axis. [PERSON] proposed such a solution.