Reactor design covers the physical construction of the vessel in which the catalytic reaction takes place. It must ensure that the exhaust gas flows through the catalyst evenly, in order to avoid local overheating (hotspots) or “dead zones” without any cleaning effect. Important factors are flow distribution, thermal insulation to minimise heat losses, and easy access for maintenance work (e.g. drawer systems for the catalyst modules). A robust design made of high-quality stainless steel also prevents corrosion that could be caused by aggressive constituents in the exhaust gas. The engineering of the reactor is decisive for how compact the system can be and how flexibly it responds to fluctuations in exhaust gas volume.