Fuel Processing Optimizing an Integrated Steam Reforming and Membrane Separation System 
 

Peter R. Bossard and Jacques Mettes

Power & Energy, Inc., 106 Railroad Drive, Ivyland, PA 18974 
 

Abstract The steam reformer bottleneck represented by the heat transport through the reactor wall is addressed in this work by a micro-channel wall-catalyzed design. The energy balance is demonstrated on a complete fuel processing system featuring the reformer reactor, an integrated burner, a palladium based hydrogen separator and multiple heat exchangers. Other issues discussed are size & capacity of the reformer, operating temperature range, coking, steam to carbon ratios, raffinate composition and the overall process energy efficiency. Compactness and fuel flexibility are some of the advantages of the wall-catalyzed approach and it offers a good outlook for an overall sulfur tolerant system when combined with a sulfur tolerant H₂ separation membrane.  
 

Introduction Larger differences in hydrogen partial pressure across a palladium membrane enable an efficient separation, making steam reforming the preferred method over auto-thermal or partial oxidation. In steam reforming, the heat for the endothermic step in the reforming process is entered externally avoiding the dilution of the generated hydrogen with combustion products. Heat transport through the wall into the catalyst bed, reference 1, becomes the limiting factor. Wall-catalyzed operation of a water gas shift, WGS, reactor was reported by reference 2 as well as by P&E, reference 3, while literature on steam reforming without catalyst is reported in reference 4 operating at extremely high temperatures. In this presentation these issues are addressed by an all Hastelloy micro-channel wall-catalyzed design, where the use of a distributed micro-channel reformer and burner reduces the temperature gradient required for the heat transfer.

Reference 5 mentions for methane steam reforming that the catalytic activity is rarely a limiting factor, which allows high space velocities in the reformer. However, utilization of the intrinsic catalytic activity is less than 10% because of transport restrictions. In this context, Rostrup-Nielsen in this reference mentions the effectiveness factor to become roughly proportional to the external surface area of the catalyst. This last observation provides an argument for current wall-catalyzed approach.

All group-8 metals are suited as catalyst for, e.g., methane steam reforming, but nickel is the most used one, as it is stable under steam reforming conditions and has the best cost/activity ratio, see reference 6. In this context Hastelloy is a good choice for the wall-catalyzed approach given its high nickel content.

No separate WGS reactor is present in the current setup and a surplus of steam in the reformer is used to shift the reaction equilibrium toward a high hydrogen product concentration. Also, raffinate burning will be applied in future applications requiring adequate combustible species in the raffinate stream. Given this surplus of steam, coking conditions can be avoided in the reformer and the separator. Further enhancement of mentioned partial pressure of hydrogen can be achieved condensing the water out of the reformate steam prior to the membrane separation which also provides an easy to burn dry raffinate. When writing this abstract measurements on the hereunder presented fuel processing system are still ongoing and resulting data will be shown at the time of the presentation. The energy balance associated with this process that makes the most cost effective use of the palladium membrane will be among the presented data. Hereunder presented is performance data of some of the key individual components such as the micro-channel wall-catalyzed reformer and the hydrogen/air burner while data regarding the membrane separator is described in reference 7.

Finally, at the presentation, data will be presented on the scalability and manufacturability of the involved components where, e.g., a large 2700 membrane separator was recently delivered to the Navy. Illustrative for the importance of the above outline process optimization is that this unit provides 50kW(e) in a non-optimal auto-thermal process while it is capable of 200kW(e) in the optimized SR scenario.

Some of the advantages of the current approach are the avoidance of traditional catalyst, fuel flexibility, sulfur tolerance and scalability. 
 

Experimental Setup

Figure 1 shows schematically the setup divided into three sections: Steam Reforming & Burner, Hydrogen Separation and Raffinate Analysis. A thermally insulated box contains the steam reformer with a built-in burner, the membrane separator and all the heat exchangers with the exception of the chiller coil. These components all have welded interconnects while thermocouple probes are placed in the various streams. Incoming to t