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.