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.