| dc.description.abstract | Olefins are important intermediates for many industrial processes and the demand for olefins is forecasted to grow for the next years. One of the promising alternative routes for the production of olefins is oxidative dehydrogenation of light alkanes.
Oxidative dehydrogenation of light alkanes was investigated at short contact time using different reactor configurations. VMgO and Pt monolithic catalysts were tested for oxidative dehydrogenation of propane. Oxidative dehydrogenation of ethane was studied over Pt and Pt-Sn monolithic catalysts.
Ethene is the main product formed during oxidative dehydrogenation of propane over different catalysts (Pt/10%Rh gauze, VMgO and Pt/monolith catalysts) at very short contact time with a maximum ethene yield of 50% at high temperatures. Propene gradually becomes a product of increasing importance at lower propane conversions. The maximum yields of propene were obtained at 800 °C; hence the highest yields of olefins (ethene and propene) were obtaine in the temperature range of 850 – 950 °C.
At high temperatures, other major products are methane and carbon monoxide. The production of methane is almost independent of the type of reactor-configuration used. The Pt monolithic catalysts promoted the formation of carbon oxides and CO2 was preferably produced at very low propane conversion levels. Important by-products formed during oxidative dehydrogenation of propane are ethyne, propyne and higher hydrocarbons favoured at high reactor temperatures. The formation of these by-products is enhanced by VMgO and Pt monolithic catalysts at low propane conversions.
Both the VMgO and the Pt monolithic catalysts clearly showed the ignition/extinction behaviour during oxidative dehydrogenation. The propane/oxygen mixture was ignited at lower temperatures than the ethane/oxygen mixture. Compared with the VMgO catalysts, the reaction mixture is ignited at much lower temperatures using the Pt catalyst under similar reaction conditions.
In mixtures of ethane/propane, the ethane conversion is almost not influenced by the addition of propane, while ethane affects the propane conversion. Ethane and propane do influence each other and they compete in the reactions with oxygen. This mutual effect was more remarkable in the case of pure gas-phase reactions than in the presence of the Pt/monolith.
Obviously, in mixtures of ethane, propane and oxygen, ethane and propane cannot be treated as if they were alone.
The comparative study of oxidative dehydrogenation of ethane over a Pt/Al2O3 catalyst,under conventional flow and at TAP reactor conditions, showed that ethene produced during oxidative dehydrogenation of ethane at low temperatures in the flow microreactor is formed in the gas-phase and not via a surface reaction mechanism. The main products formed via the surface reaction mechanism are methane, hydrogen and carbon oxides.
The TAP results also indicated that the trends in the deactivation behaviour with temperature and surface oxygen concentration are consistent with a balance between successive dehydrogenation steps leading to the possible formation of surface carbon and a substantial loss in activity. Periodic introduction of the reactants can minimize the deactivation processes.
Oxidative dehydrogenation of ethane studied over Pt and Pt-Sn monolithic catalysts revealed that the highest yield of ethene can be obtained using the Pt-Sn catalysts with addition of H2. Addition of H2 contributed also to the enhancement of the oxidation of H2 and suppression of the oxidation reactions of ethane. Ethane, which would normally be oxidized without addition of H2, was dehydrogenated and therefore a higher ethene production was achieved with H2 addition.
The oxidation of H2 provided sufficient heat for the dehydrogenation reactions of ethane. The activity of Pt catalyst was significantly improved by the addition of Sn. However, increasing the Sn loading did not show a significant influence on the catalytic performance. Furthermore, it was shown that a recycle of H2 would be possible to add to the process. | nb_NO |
