Abstract: In an apparatus and method for improved operation of a catalytic reactor, a preferred reactor comprises a vessel having concentric first and second annular catalyst beds. Heat transfer fluid flows through expandable heat transfer U-tubes passing through the catalyst beds, either concurrently or countercurrently with respect to the flow of process gas through the catalyst beds. This arrangement delivers improved heat transfer between the process and heat transfer fluids and allows the heat transfer U-tubes to expand longitudinally to withstand the pressure differential between the process gas and heat transfer fluids.