Figure 6. Chemical enthalpy and sensible enthalpy contour plots in the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions.
The oxidation reaction rate profiles are presented in Figure 7 along the length of the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions. In top-fired or side-fired reformers, the burners are typically fed with a fuel gas mixture comprising a hydrocarbon, such as methane, and which may further comprise hydrogen or other suitable fuel gases. Combustion is performed using an oxidant such as air, which is also fed to the one or more burners to form the hot combustion gas. In the case of a top-fired reformer the inlets for the feed gas mixture are typically located at the top end of the reformer and the outlets for the reformed gas mixture at the bottom end [59, 60]. The burners are located at the top end and the combusted gas outlet is typically located at the bottom end [61, 62]. In the case of a side-fired reformer the inlets for the feed gas mixture are typically located at the top end of the reformer and the outlets for the reformed gas mixture at the bottom end [63, 64]. The burners in this case are located at multiple levels between the top end and the bottom end and the combusted gas outlet is typically located at the top end [65, 66]. The feed gas mixture may be passed to distribution means, such as header pipes which distribute the feed gas mixture to the heat exchange tubes. The tubes pass through a heat exchange zone in which heat is transferred to the reactants passing through the tubes. Collector pipes may be connected to the bottom of the tubes, which provide channels for collection of the reformed gas. Such distribution and collection means define an inlet zone and an outlet zone above and below the heat exchange zone. They may be termed boundary means as they define boundaries between the heat exchange zone and the inlet and outlet zones. In gas-heated reformers, the inlet for the feed gas mixture is typically located at the top end of the reformer. The feed gas mixture may be passed to distribution means, such as header pipes which distribute the feed gas mixture to the heat exchange tubes. The tubes pass through a heat exchange zone in which heat is transferred to the reactants passing through the tubes. Collector pipes may be connected to the bottom of the tubes and the reformed gas outlet which may be at the bottom end of the steam reformer. Alternatively, tube-sheets may be provided to separate the inlet and outlet zones from the heat exchange zone. Consequently, a tube-sheet may separate the heat exchange zone through which the hot gas passes from a zone, such as a plenum chamber, communicating with the interior of the heat exchange tubes to permit feed of feed gas mixture to the tubes or off-take of reformed gas from the tubes. Alternatively, there may be a combination of tube-sheets and header pipes. Alternatively, the heat exchange tubes may discharge the reformed gas into the heat exchange zone containing the hot gas to form a reformed gas mixture which is recovered from the reformed gas outlet. The reformed gas may be recovered from the top end or bottom end of the steam reformer. Again, the tube-sheets or header or collector may be termed boundary means as they define boundaries between the heat exchange zone and the inlet and outlet zones. Preferably hot gas distribution means, such as baffles, are provided within the reformer that causes the hot gas to flow evenly through the reformer. Desirably all of the tubes contain the same proportions of structured catalyst and particulate catalyst, although this is not essential. This provides the benefits of the higher activity, higher heat transfer, and low pressure drop of the structured catalyst at the inlet end and the benefit of the cheaper and stronger particulate catalyst at the outlet end. Each tube has an inlet for the feed gas mixture, an outlet for the reformed gas mixture, and the tubes contain a particulate steam reforming catalyst adjacent the outlet and a structured steam reforming catalyst adjacent the inlet, so that the feed gas mixture contacts the structured steam reforming catalyst and then the particulate steam reforming catalyst. In the case of a methanol steam reformer, where feed stream contains water and methanol, these components may be mixed together and delivered as a single stream. Alternatively, these components may be separately delivered to the reforming region. Although the reformate stream contains a substantial amount of hydrogen gas, the stream may also be referred to as a mixed gas stream because it also contains gases other than hydrogen gas. Examples of these gases include carbon dioxide, carbon monoxide, water, methane and unreacted methanol or other carbon-containing feedstocks. A feed stream may be delivered to the steam reformer at an elevated temperature, and accordingly may provide at least a portion of the required heat. When a burner or other combustion chamber is used, a fuel stream is consumed and a heated exhaust stream is produced. The feed stream is vaporized prior to undergoing the reforming reaction, and the heating assembly may be adapted to heat and vaporize any liquid components of the feed stream.