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.