## 2 Technologies for Purifying the Tail Gases from N2O and NOx
The parameters of the catalyst bed were as follows: bed dimensions \(s\times h\) = 0.058 m\({}^{2}\times\) 0.39 m (\(s\)--cross-sectional area of a bed in an annular basket, \(h\)--height of the bed), porosity 50% (3 mm extrudates) and 60% (8 mm tablets). Operating parameters of the catalytic reactor: \(p\) = 450 kPa abs., T = 673 K.  
Figure 1: The dependence of the tail-gas pressure drop across the deNOx catalyst beds in the form of extrudates (\(\phi\) = 3 mm) and tablets (\(\phi\) = 8 mm) on the linear gas flow velocity (own research). The influence of the catalyst shaped body dimensions, bed porosity, shape factor and linear gas flow velocity on the pressure drop value can be determined based on the Ergun Equation (10):  
\[\frac{-\Delta\text{P}}{\text{H}}=\text{A}\frac{\mu\text{V}(1-\varepsilon)^{2}}{ \text{x}^{2}\varepsilon^{3}}+\text{B}\frac{\rho_{\text{f}}\text{V}^{2}(1- \varepsilon)}{\text{x}\varepsilon^{3}} \tag{15}\]  
H--height of the catalyst bed, m  
\(\Delta\text{P}\)--pressure drop of a gas flow through the catalyst bed, Pa  
V--linear gas flow velocity, m/s  
x--hydraulic diameter of the shaped catalyst body, m  
\(\varepsilon\)--bed porosity,  
\(\rho_{\text{f}}\)--density of the gas flowing through the catalyst bed, kg/m\({}^{3}\),  
\(\mu\)--gas viscosity, Pa's. For the shaped catalyst bodies, slightly differing from the shape of a sphere, the Blake-Kozeny-Carman and Plummers coefficients are equal to A = 150 and B = 1.75. The hydraulic diameter can be determined from Equation (16):  
\[4{\rm NO}+4{\rm NH}_{3}+{\rm O}_{2}\to 4{\rm N}_{2}+6{\rm H}_{2}{\rm O} \tag{8}\]0  
V\({}_{\text{i}}\)--volume of the shaped catalyst body, m\({}^{3}\)  
S\({}_{\text{i}}\)--surface of the shaped catalyst body, m\({}^{2}\). The average catalyst bed porosity can be determined based on the volume of the catalyst bed, mass and density of the shaped catalyst body, according to Equation (17):  
\[4{\rm NO}+4{\rm NH}_{3}+{\rm O}_{2}\to 4{\rm N}_{2}+6{\rm H}_{2}{\rm O} \tag{8}\]1  
The higher gas flow resistances promote better gas distribution over the catalyst bed. On the other hand, it is associated with less energy recovery in the expansion turbine, which translates into higher operating costs. The linear gas flow velocity affects the rate of catalytic reaction that runs in the diffusion regime, i.e., in the presence of the shaped catalyst. By lowering it, the efficiency of the deNO\({}_{\text{x}}\) and deN\({}_{\text{2}}\)O processes can be increased [61; 62; 63; 64; 65]. When developing the technology for the tail gases purifying from nitrogen oxides, the following aspects should be taken into account: the minimum N\({}_{\text{2}}\)O and NO\({}_{\text{x}}\) conversion expected by the industrial customer, or required to meet the applicable emission standards, the allowable pressure drop across the catalyst bed, and in case of using reducers, their permissible concentration in the purified gas stream (e. g. ammonia slip). On this basis, it is possible to select the appropriate type and/or shape of catalysts for N\({}_{\text{2}}\)O decomposition/reduction and NO\({}_{\text{x}}\) reduction, and determine the catalytic reactor design. The design of the reactor interior determines the value of the gas pressure drop across the catalyst bed and the linear gas flow velocity through the bed, which influences the efficiency of the catalyst operation. The assumed permissible pressure drop values across the catalyst bed are: \(\leq\)0.005 bars for low-pressure nitric acid plants (0-2 bars), \(\leq\)0.015 bar for the medium-pressure (3-6 bars), and \(\leq\)0.1 bar for the high-pressure ones (7-12 bars) [66].