Abstract: An apparatus for contacting reactants with a particulate catalyst while indirectly contacting the reactants with a heat exchange medium amid simultaneous exchange of catalyst particles by an operation that sequentially restricts reactant flow while moving catalyst through reaction stacks in which the reactant flow has been restricted. The apparatus permits a change out of catalyst in a channel type reactor arrangement that would normally restrict catalyst flow during operation. Moving catalyst through a heat exchange type reactor having reactant and heat exchange channels permits control of catalyst activity as well as temperatures.