Abstract: Plural vertical tube reactor provides downward flow only, in a serial manner, through a plurality of sets of tubes containing a particulate contact material such as a catalyst which are located in adjacent sectors or reigons of a shell and tube reactor. An empty tube(s) is provided to carry the reactant fluid upwardly from the bottom of one sector of tubes to the top of another. In a two sector configuration, the upper and lower end chambers are each partitioned in half, but the lower partition has a screened port to allow reacted fluid from the first sector to reach, and move upwardly through, an empty tube(s) in the second sector which is isolated from the other tubes in the second sector at its bottom end but not at its top end.