A reactor cartridge includes a plurality of spaced-apart monoliths, formed along a tube or other mandrel. Each monolith is formed of a pair of flat and corrugated metal strips, spirally wound around the tube. These strips could be made of solid or screen material. The corrugations are skewed, such that the monolith imparts a swirl to gases flowing through it. The corrugations of the strips in adjacent monoliths are oriented differently, so that successive monoliths impart different swirls to the gases, so as to promote mixing of gases and better heat transfer from the exterior to the interior of the cartridge. An insertion and removal tool simplifies the procedure for stacking such cartridges in a long pipe, or for removing cartridges from the pipe. The all-metal construction facilitates heat transfer through the entire reactor, and avoids the problems associated with packed ceramic beds.