A structured packing for insertion in a reactor having an inlet, an outlet, a wall and an axis. The packing comprises a first part, i.e., a reactor core and a second part, i.e., a reactor casing. The second part is free to move relative to the first part. The first part and the second part are inserted in the reactor such that the first part is located proximate the axis and the second part is located between the first part and the reactor wall. In general, the second part will be in contact with the reactor wall.