Comparison of conventional and membrane reactor fuel processors for hydrocarbon-based PEM fuel cell systems

Several reactor types for the autothermal reforming (ATR) of hydrocarbon fuels are evaluated for the production of hydrogen in PEM fuel cell systems. Each ATR reactor is integrated into an overall process model including the fuel cell, heat integration exchangers, and water recycle. Hydrogen permselective membrane reactors (Pd-based and proton conducting oxide) are compared to the three-step reactor system consisting of an adiabatic ATR reactor, cooled water gas shift (WGS) reactor, and a preferential oxidation (PrOx) reactor. We have attempted to optimize the flowsheet and process variables in the context of an integrated equilibrium model of the fuel processor and fuel cell systems. These sensitivity analyses define ranges of the ratios of water and oxygen to the hydrocarbon feed to the ATR reactor for maximum system efficiency. The study utilizes kinetic rate expressions modified using literature data on the autothermal reforming of n-tetradecane. We show that the Pd membrane reactor eliminates the need for a separate WGS reactor, as the selective removal of hydrogen enables sufficient hydrogen production and recovery. Hydrogen recoveries of up to 97% are achieved in the membrane reactor at moderate reforming pressures by incorporating a countercurrent steam sweep on the permeate side. The overall efficiency of the Pd membrane reactor system is comparable to the conventional system; a small 1% efficiency gain is attributed to elimination of the hydrogen losses associated with the PrOx reactor. We show that the volume of the Pd membrane reactor for a 50 kW fuel cell system is less than 20% of the conventional ATR, WGS, and PrOx reactors. A drawback of the Pd-based membrane is the uncertain durability of the Pd at the high operating temperature of the C14 reforming chemistry (900°C), the cost of the requisite quantity of Pd (100 g), and coking brought about by hydrogen removal by the membrane. Some of these issues may be overcome with the use of the oxide-based membrane, which we show has reasonable performance in terms of productivity.