(501f) Hydrogen Production Via Steam Reforming in a Reverse-Flow Reactor

As one begins to contemplate a broader use of hydrogen in our energy infrastructure, it is instructive to consider that our most economical route to hydrogen, Steam-Methane Reforming (SMR) is only 70-75% efficient to hydrogen. SMR inefficiency is rooted in a high-temperature endothermic nature of the reaction. Too much heat is lost in heating streams to reaction temperature and ineffectively using the sensible heat in product streams as they are returned to ambient conditions. Heat exchange can be added to improve efficiency, but the corrosive nature of the syngas makes this heat exchange costly and problematic.

Our approach to raising the efficiency of steam reforming is to perform this heat exchange within the catalyst bed itself. We operate the steam reforming bed as a reverse flow reactor, in which heat addition (by combustion) alternates with heat consumption (by reforming), with the flow being reversed between these steps. By properly designing the bed and other reactor components, the entire bed becomes a high-efficiency heat exchanger. With appropriate design, SMR efficiency can be boosted into the 85-90% range, within a few percent of the theoretical maximum.

We call this approach "Pressure Swing Reforming" (PSR) because we swing the pressure of the reactor such that low pressure air can be used during combustion, while high pressure syngas is produced as reform-product. A consequence of using the bed as heat reservoir is that space velocity becomes inversely proportional to cycle time for a fixed amount of heat storage. In addition, the catalyst system must be concerned with more than just kinetics and mass transfer. In this new reverse-flow reactor, providing sufficient heat transfer is the key to achieving the efficient heat exchange that is needed from the catalyst bed.

We have found that the heat transfer rate must increase in proportion to the desired space velocity of the system. The primary way to increase heat transfer rate is to increase heat transfer surface area - in essence to make the catalyst particles smaller. Thus, while conventional 0.5 to 1.0-inch reforming catalysts may be adequate at GHSV of 100, new bed systems are applied to achieve high space velocity. Achieving high GHSV has been critical to attaining energy-efficient and economical reactor designs.

This presentation will provide a brief introduction to PSR and discuss the key reactor features that are needed to provide a commercially practical reverse-flow reactor. We will discuss some structure-property relationships that are key to the selection of materials for application in PSR, and present illustrative examples.