| Popis: |
The most significant cost associated with partial oxidation of methane to syngas is that of the oxygen plant. In this paper, the authors offer a technology that is based on dense ceramic membranes and that uses air as the oxidant for methane-conversion reactions, thus eliminating the need for the oxygen plant. Certain ceramic materials exhibit both electronic and ionic conductivities (of particular interest is oxygen-ion conductivity). These materials transport not only oxygen ions (functioning as selective oxygen separators) but also electrons back from the reactor side to the oxygen/reduction interface. No external electrodes are required and if the driving potential of transport is sufficient, the partial oxidation reactions should be spontaneous. Such a system will operate without an externally applied potential. Oxygen is transported across the ceramic material in the form of oxygen anions, not oxygen molecules. In principle, the dense ceramic materials can be shaped into a hollow-tube reactor, with air passed over the outside of the membrane and methane through the inside. The membrane is permeable to oxygen at high temperatures, but not to nitrogen or any other gas. Long tubes of La-Sr-Fe-Co-O (SFC) membrane were fabricated by plastic extrusion, and thermal stability of the tubes was studied more » as a function of oxygen partial pressure by high-temperature XRD. Mechanical properties were measured and found to be acceptable for a reactor material. Fracture of certain SFC tubes was the consequence of an oxygen gradient that introduced a volumetric lattice difference between the inner and outer walls. However, tubes made with a particular stoichiometry (SFC-2) provided methane conversion efficiencies of >99% in a reactor. Some of the reactor tubes have operated for up to {approx} 1,000 h. « less |
