A catalytic reactor (16) is provided for purposes of effecting therewith the removal of nitrogen oxides from a process gas (F) that includes at least two catalyst bed segments (48, 50, 52), each of which is provided with a closing device (60, 62, 64). The catalytic reactor (16) is operative for causing said process gas (F) to flow through a first catalyst bed segment (48). Said process gas (F) is at a first temperature at which the sulphur trioxide that is entrained in said hot process gas is at least partially precipitated out on to the catalytic material that said first catalyst bed segment (48) embodies. Periodically said closing device (60) is operated in order to thereby isolate said first bed segment (48) from the flow therethrough of said hot process gas (F). A regeneration system (34, 36, 38) is also provided that is operative for purposes of causing a regenerating gas to flow through the first bed segment (48). In addition, a sulphur trioxide removal device (20) is provided, which is separate from said catalytic reactor (16), and which is operative for purposes of effecting therewith the removal of the sulphur trioxide from said regenerating gas.