A steam reforming furnace has a plurality of substantially vertical reformer tubes. Each reformer tube has a feed inlet at its lower end, an outlet at its upper end, and a catalyst containment zone disposed intermediate its upper and lower ends and contains a charge of a particulate steam reforming catalyst which is insufficient to fill completely the catalyst containment zone. An upper retainer means is mounted at the upper end of the catalyst containment zone and is permeable to gas or vapor but retains particles of the catalyst in the catalyst containment zone. A follower means is movably mounted in the catalyst containment zone beneath the charge catalyst for movement upwardly from a lower end of the containment zone upon upward flow of gas through the catalyst containment zone at a rate beyond a threshold rate.