Methane steam reforming improvements

De Falco M (2004) Pd-based membrane reactor a new technology for the improvement of methane steam reforming process. Thesis, University of Rome La Sapienza ... 

For a methane steam reforming fuel processor, more than 15% higher fuel processor efficiency was determined experimentally by Mathiak et al. [433] when utilising fuel cell anode off-gas compared with combustion of additional methane. Doss et al. analysed an autothermal gasoline fuel processor and found improved efficiency by utilisation of anode off-gas [434]. 

One particular application for which supported Au catalysts may find a niche market is in fuel cells [4, 50] and in particular in polymer electrolyte fuel cells (PEFC), which are used in residential electric power and electric vehicles and operate at about 353-473 K. Polymer electrolyte fuel cells are usually operated by hydrogen produced from methane or methanol by steam reforming followed by water-gas shift reaction. Residual CO (about 1 vol.%) in the reformer output after the shift reaction poisons the Pt anode at a relatively low PEFC operating temperature. To solve this problem, the anode of the fuel cell should be improved to become more CO tolerant (Pt-Ru alloying) and secondly catalytic systems should be developed that can remove even trace amounts of CO from H2 in the presence of excess C02 and water. 

If mixture 1 is now considered, it can be seen that it contains 19% methane but only approximately 3% water. The methane and water are expected to react at the fuel cell anodes via the steam reforming reaction. The large excess of methane will result in almost complete depletion of the water content of the fuel gas. It is this depletion of the water content and not the hydrogen concentration that leads to an increase in the Nemst voltage and hence to improved performance of the fuel cells. 

The UMR process can be improved by introducing calcium oxide, a carbon dioxide sorbent, into the packed bed. The potential advantages of using calcium oxide as a carbon dioxide sorbent have been previously recognized. The use of calcium oxide to enhance the steam reforming process has been patented by Gluud et al.( 1931). More recently Harrison and coworkers (Han and Harrison, 1994) have reported laboratory-scale data for the steam reforming of methane in the presence of calcium oxide. 

Without a doubt, a complete picture of the dynamics of dissociative chemisorption and the relevant parameters which govern these mechanisms would be incredibly useful in studying and improving industrially relevant catalysis and surface reaction processes. For example, the dissociation of methane on a supported metal catalyst surface is the rate limiting step in the steam reforming of natural gas, an initial step in the production of many different industrial chemicals [1]. Precursor-mediated dissociation has been shown to play a dominant role in epitaxial silicon growth from disilane, a process employed to produce transistors and various microelectronic devices [2]. An examination of the Boltzmann distribution of kinetic energies for a gas at typical industrial catalytic reactor conditions (T 1000 K)... 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

Poisoning of metal catalysts may provide a tool for improving selec> tivity by affecting the concentrations of ensembles required by different reaction paths. This is illustrated by steam reforming on sulfur passivated nickel catalysts and the results are compared with observations for sulfided platinum-rhenium catalysts for catalytic reforming and for a chlorine poisoned palladium catalyst for partial oxidation of methane. 

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