Figure 4. Effect of catalyst layer thickness on the methanol conversion and hydrogen yield in the oxidation and reforming processes of the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions.
The methanol and oxygen mole fraction contour plots are illustrated in Figure 5 in the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions. In a steam reforming process, a mixture of a hydrocarbon feedstock and steam, and in some cases also carbon dioxide or other components, is passed at an elevated pressure through particulate catalyst-filled heat exchange tubes, which are externally heated by means of a suitable heating medium, generally a hot gas mixture [51, 52]. The catalyst is normally in the form of shaped units, for example, cylinders having a plurality of through holes, and is typically formed from a refractory support material, for example, alumina, impregnated with a suitable catalytically active metal such as nickel [53, 54]. Structured steam reforming catalysts offer higher heat transfer, higher activity and lower pressure drop than particulate steam reforming catalysts. Therefore, there have been proposals to use them throughout the entire depth of the tube to maximize the performance of the steam reformer in terms of obtaining the lowest tube wall temperature, the lowest pressure drop and the maximum hydrocarbon conversion [55, 56]. Structured steam reforming catalysts however are typically manufactured from metals, typically high temperature alloys [57, 58]. The strength of these materials reduces substantially at the temperatures encountered at the outlet of steam reformers. Therefore, as the structured catalyst is often formed from leaves of very thin material with little weight bearing capability, it is often mounted on a central core structure that supports all of the weight of the catalyst along with the imposed pressure drop load. Structured catalysts offer heat transfer benefits and extra activity, which is more effective in the inlet zone of the steam reformer. However, in the outlet zone of the steam reformers where the duty is lower, the structured catalyst may be replaced with a conventional particulate catalyst to provide the desired conversion at an overall lower cost than if structured catalysts were used along the entire length of the tubes. Another key benefit of the present arrangement is that it overcomes the need for extensive support structures often required for structured catalysts, in particular at the at the bottom of the tubes due to high temperature, total weight and pressure drop. Furthermore, loading and unloading of the structured catalyst may be shortened and the flexibility to provide tailored reforming solutions improved. Therefore, using structured catalyst at the inlet of the tubes and particulate catalyst at the outlet of the tubes offers a more cost effective and more robust catalyst arrangement than particulate catalyst alone, structured catalyst alone or alternative arrangements of particulate catalyst and structured catalyst. The steam reformer contains a plurality of vertical tubes through which the gas mixture may be passed, and to which heat is transferred by means of a hot gas flowing around the tubes. The tube inlets are typically at the top end such that the feed gas mixture is typically fed to the top of the steam reformer and flows downward through the tubes. The steam reforming reactions are endothermic and heat is transferred to the tubes by means of a hot gas flowing around the exterior surfaces of the tubes. Various steam reformer arrangements may be used. Consequently, the steam reformer may be a conventional top-fired steam reformer or a side-fired steam reformer. In such reformers the hot gas is provided by combusting a fuel gas using a plurality of burners disposed either at the top end or along the length of the tubes. Alternatively, the steam reformer may be a gas-heated reformer in which the hot gas may be provided by a flue-gas from a combustion process, or may be a gas generated by catalytic or non-catalytic partial oxidation of a hydrocarbon, or by autothermal reforming of a hydrocarbon and the reformed gas mixture. Furthermore, the hot gas may be mixed with the reformed gas that has passed through the plurality of tubes.