## 2 Technologies for Purifying the Tail Gases from N2O and NOx
### Lateral Flow Reactor  
In the lateral flow reactor (LFR), as in the radial one, the gas flow direction through the catalyst bed is perpendicular to the reactor axis. Therefore, it is characterized by a lower linear gas flow velocity through the catalyst bed than the axial flow reactor. The catalytic basket of the LFR reactor consists of a series of cassettes of small width, the side walls of which are perforated, while the top and bottom parts are gas-impermeable (Figure 10). This design allows only for installing one type of catalyst. Therefore, it is impossible to combine the two catalytic processes, deNO\({}_{x}\) and deN\({}_{2}\)O.  
Figure 10: (**a**,**b**) Lateral flow reactor (LFR): 1—solid bottom plate of the basket, 2—solid upper part of the basket, 3—perforated side walls of the cassette, A—cassette with a catalyst bed. The tail-gas stream can be introduced in the top or bottom part of the reactor. It initially flows in the axial direction into the space between the two catalyst beds. Then, it hits the solid plate installed between the two cassettes in their upper part and flows through their perforated side walls. After passing through the catalyst bed, the gas stream exits the cassette through the second perforated wall and joins with the gas stream from the adjacent cassette. The gas streams from the spaces between the cassettes merge and leave the reactor through the outlet. The gas stream inside the cassette flows quite slowly, which to some extent, limits the mass and heat exchange between the gas phase and the catalyst surface. Therefore, LFR reactors are used for low-rate reactions, especially those running mainly in the kinetic regime. CFD calculations of the LFR reactors are performed to assess the reactor's geometrical parameters and the operating conditions impact on the hydrodynamic properties, i.e., gas pressure drops across the catalyst bed or flow velocity distribution (transverse/lateral) along the catalyst bed height. They are also used to estimate the friction factor against the walls of the channels between the cassettes. In LFR reactors, the pressure drop does not exceed 2 kPa. Typically, it is in the range of 0.1-2.0 kPa at a gas flow rate of 2000-50,000 \(\mathrm{Nm^{3}/(m^{3}\;h)}\), and cassette dimensions of height \(\times\) depth \(\times\) width = (0.5 \(\div\) 2.0) \(\times\) (0.5 \(\div\) 2.0) \(\times\) (0.02 \(\div\) 0.1) m. A low pressure drop (especially at a level of several hundred Pa) translates into a poorer gas distribution over the catalyst bed, along the height of the cassette (uneven distribution of transverse/lateral velocity). The uniformity of the gas flow through the LFR reactor can be improved by using very small catalyst bodies or by increasing the width of the cassettes. However, this translates into investment costs increase. The total pressure drop value is also affected by the friction factor, which depends on the roughness of the channel walls [94; 95; 96; 97]. Shell company offers the LFR reactors as a Shell DeNO\({}_{\mathrm{x}}\) system technology. Titania-supported vanadium oxide catalysts in the form of trilobes, with dimensions 0.6-5.0 mm (S-096), are installed in them. The design of the LFR reactor ensures very low gas flow resistances across the catalyst bed (lower than in the axial or radial flow reactor). Therefore, installing very small catalyst bodies without the risk of excessive pressure drop value is possible. In this type of reactor, high efficiency of the gas purification from nitrogen oxides is achieved, exceeding 95% (Shell deNO\({}_{\mathrm{x}}\) System, titania/vanadia catalyst). A particular problem is the uneven gas distribution along the length of the cassettes. The construction of the catalytic basket is quite complicated, as it consists of a series of perforated and solid plates, directing the flow of the gas stream. Unloading and loading the catalyst into the cassette is also problematic due to their large depth and small width.