Figure 2. Steam and carbon dioxide mole fraction contour plots in the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions.
The effect of catalyst layer thickness on the enthalpy of reaction in the oxidation and reforming processes is illustrated in Figure 3 in the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions. The steam reforming reaction is endothermic and is therefore typically carried out in an externally heated steam reforming reactor, usually a multi-tubular steam reformer comprising a plurality of parallel tubes placed in a furnace, each tube containing a fixed bed of steam reforming catalyst particles. The feedstock is typically first pre-heated, usually in heat exchange contact with flue gas from the burners of the furnace, before it is supplied to the catalyst-filled tubes. In catalytic steam reforming processes, fouling of the catalyst bed by coke formation is a major problem. Typically, at temperatures above 400 or 450 °C, carbon-containing deposits are formed on metal catalysts in the presence of hydrocarbons and carbon monoxide. Such carbon deposits result in for example pressure drop problems and reduced catalyst activity due to covering of active catalyst sites. When oxygenated hydro-carbonaceous feedstocks are used, the coke formation problem is more pronounced, since oxygenated hydro-carbonaceous feedstocks are more thermo-labile than hydrocarbons and therefore more prone to carbon formation. In steam reforming processes, the deactivated or spent catalyst is typically regenerated by burning off the carbon in a separate burner or by oxidizing the carbon by supplying steam to the reforming zone whilst stopping the supply of the feedstock. During a first period of time a feedstock comprising an oxygenated hydrocarbon and a hydrocarbon is converted into synthesis gas by contacting the feedstock and steam with a steam reforming catalyst. During the first period, oxygenated hydrocarbon, hydrocarbon and steam are supplied to the steam reforming catalyst under steam reforming conditions. As a result, synthesis gas is formed and the catalyst will gradually become deactivated due to deposition of carbon on the catalyst. Consequently, deactivated steam reforming catalyst is obtained during the first period of time. During a second period of time, consecutive to the first period of time, namely directly following the first period, the deactivated reforming catalyst is regenerated. The regeneration is carried out by stopping the supply of oxygenated hydrocarbon to the catalyst whilst the supply of hydrocarbon and steam is maintained. Also, the regeneration is carried out under steam reforming operating conditions. After the second period of time, namely the regeneration, the catalyst activity will be increased, typically to a level approaching the original catalyst activity, and the supply of oxygenated hydrocarbon is typically resumed. Another sequence of first period with supply of oxygenated hydrocarbon and second period wherein the supply of oxygenated hydrocarbon is stopped will then typically be carried out. The steam reforming process is preferably carried out in the absence of a molecular-oxygen containing gas both during the first and during the second period of time.