Figure 7. Oxidation reaction rate profiles along the length of the
thermally coupled reactor for conducting simultaneous endothermic and
exothermic reactions.
The reforming reaction rate profiles are presented in Figure 8 along the
length of the thermally coupled reactor for conducting simultaneous
endothermic and exothermic reactions. This steam reforming reaction is
carried out using, as raw materials, methanol and steam fed into the
ceramic tubular reactors so as to yield a reformed gas containing
hydrogen and carbon dioxide. In such a steam reforming reaction,
methanol is reacted with steam while the reforming material containing
the methanol and steam is heated with part of combustion heat so as to
yield a reformed gas containing hydrogen and carbon dioxide. In a
low-temperature region where the reactivity is low, a steam reforming
reaction is carried out by the catalysis of a reforming catalyst. The
term catalyzed hardware is used for a catalyst system where a layer of
catalyst is fixed on a surface of another material, for example,
metallic surfaces. The other material serves as the supporting structure
giving strength to the system. This allows to design catalyst shapes
which would not have sufficient mechanical strength in itself. The steam
reforming technology makes use of reforming catalyst in the form of
pellets of various sizes and shapes. The catalyst pellets are placed in
fixed bed reactors. The reforming reaction is endothermic. In
conventional reformers, the necessary heat for the reaction is supplied
from the environment outside the tubes usually by a combination of
radiation and convection to the outer side of the reformer tube. The
heat is transferred to the inner side of the tube by heat conduction
through the tube wall and is transferred to the gas phase by convection.
Finally, the heat is transferred from the gas phase to the catalyst
pellet by convection. The catalyst temperature can be more than 80 °C
lower than the inner tube wall temperature at the same axial position of
the reformer tube. Heat transport is more efficient when catalyzed
hardware is used in the steam reforming process. The heat transport to
the catalyst occurs by conduction from the inner tube wall. This is a
much more efficient transport mechanism than the transport by convection
via the gas phase. The result is that the temperatures of the inner tube
wall and the catalyst are almost identical. Furthermore, the tube
thickness can be reduced, which makes the temperature difference between
the inner and outer side of the reformer tube smaller. It is hence
possible to have both a higher catalyst temperature and a lower tube
temperature, all other conditions being the same when replacing the
conventional reformer tubes with catalyzed hardware tubes. A low outer
tube wall temperature is desirable since it prolongs the lifetime of the
tube. A high catalyst temperature is advantageous since the reaction
rate increases with temperature and since the equilibrium of reaction is
shifted to the right-hand side resulting in a better utilization of the
feed. Finally, the catalyst amount is reduced when using catalyzed
hardware reformer tubes compared to the conventional reformer with a
fixed bed of reforming catalyst. The critical steam to carbon ratio
decreases when the operating pressure is increased. The operating
pressure in the thermally coupled reactor is the critical parameter for
suppressing soot formation. By increasing the operating pressure, it is
possible to operate advantageously at a lower steam to carbon ratio. The
actual critical pressure will depend on the burner design used in the
thermally coupled reactor.