(548c) Modeling the Transient Response of Heat Exchange Reformers

One method of producing hydrogen for fuels cells is the steam reforming of conventional hydrocarbons, an established refinery process. It has been proposed to carry out this process in a heat exchange reformer – a reformer that has a flow configuration analogous to a heat exchanger. Usually a catalyst, in the form of a conventional pellet, is placed in the tubes of the reformer and a hot gas (i.e. from a combustor) or reformer effluent is circulated in the shell side. Key design objectives of the Heat Exchange Reformer are a high, steady state, thermal efficiency and a compact unit both in volume and mass.

It is well known that many applications of fuel cells require transient operation. The production of hydrogen also needs to vary otherwise excess hydrogen is generated and not consumed, leading to a significant loss in overall energy efficiency. The implication is that the transient response of the heat exchange reformer also needs to be considered in its design

In this work, the transient response of a simplified heat exchange reformer is modeled. The geometry of the reformer is two concentric cylinders, filled with conventional pellet catalyst, a common design element in many heat exchange reformers. The model can be characterized as one-dimensional, heterogeneous packed bed reactor with thermal coupling between different parts of the reactor. The model incorporates the kinetics of methanol steam reforming.

Typical transients considered are step changes (both positive and negative) in the feed flowrate or the composition of the feed (i.e. steam to carbon ratio). Other investigations are changes in heat transfer coefficients and feed composition.