Parametric study of the partial oxidation of propane over nickel and platinum based catalysts.

摘要

Hydrogen production though the partial oxidation of propane over 1%Pt/CeO 2 and 1%Ni/CeO2 catalysts was studied in a fixed-bed reactor. The purpose of the experiments was to study the pathways, priority and the sequence of reactions which occur over each catalyst system. A temperature of 600°C and O2/C3H8 ratio of 1.78 was used for all the runs. The space velocity was varied by varying the flowrates (100, 200, 300, 400 sccm), and also the catalyst loadings. Seven species were found at the outlet of the reactor (C3H8, O2, H2, CO, CO2, H2O and C3H 6). All the species except H2O were analyzed quantitatively by the gas chromatograph. The following reactions can occur during the process at 600°C: total oxidation, partial oxidation, steam reforming, dry reforming, water gas shift, dehydrogenation and water formation. Of these, a Gaussian elimination process yields four independent reactions. This results in ten sets of possible independent reactions. For each set, a material balance on the six outlet compositions allows the calculation of rates of each of the four reactions in the set. Sets containing negative rates for irreversible reactions are discarded. To confirm the validity of sets containing dry reforming, steam reforming and water gas shift, these reactions were carried out over the catalysts at the experimentally determined outlet conditions for the propane partial oxidation process. For the 1%Ni/CeO2 catalyst, both dry and steam reforming reactions were favorable, but the water gas shift reaction was not favorable. The activities of the 1%Pt/CeO2 catalyst for dry reforming and steam reforming were insignificant. The water gas shift reaction was not conducted over the 1%Pt/CeO2 catalyst as no feasible set contained this reaction. These results, coupled with the effect of weight hourly space velocity, allows us to evaluate the relative importance of each reaction in each allowable set as a function of contact time.;The results indicate that the pathways supported by both the catalysts are completely different. For the Pt-based catalysts, hydrogen is formed directly by the exothermic partial-oxidation route at lower contact times. This is consistent with earlier work in our laboratory. On the other hand, for Ni-based catalysts, hydrogen is formed by the endothermic steam-reforming reaction which occurs after total oxidation. The 1%Pt/CeO2 catalyst favors higher hydrogen production at lower contact times, whereas the 1%Ni/CeO 2 catalyst favors higher hydrogen production at higher contact times. These results suggest that the 1%Pt/CeO2 catalysts could be used in micro-reactors, at low contact times. On the other hand, since the 1%Ni/CeO 2 catalyst favors the indirect partial oxidation pathway through the endothermic steam reforming process (which is in-turn driven by the heat and products from the total oxidation reaction) and supports higher hydrogen production at higher contact times, large scale reactors or industrial reactors providing larger contact times would be more effective for the 1%Ni/CeO2 catalyst.