Figure 3. Schematic illustration of the heat integrated reactor with
multiple parallel channels for thermochemically producing hydrogen from
methanol by steam reforming.
The computational domain of the heat integrated reactor is illustrated
schematically in Figure 4 for thermochemically producing hydrogen from
methanol by steam reforming. A methanol-steam mixture is supplied to the
reforming channels to be reformed, and a methanol-air mixture is
supplied to the combustion channels to be combusted. The temperatures
and pressures of the two streams entering the combustion channels and
the reforming channels, respectively, are the same. The temperature of
the two streams is 373 K at the flow inlets. The system operates at a
pressure of up to 1.5 MPa. Typically, high pressure combustion is widely
practiced, although combustion can take place at low or near-atmospheric
pressure. Steam reforming can take place at pressures somewhat above
atmospheric to moderately high, up to 5.0 MPa. The channel walls should
be of sufficient strength to allow for the pressure differential between
the two streams entering the reactor. The composition of the mixture
entering the combustion channels should be such as to ensure complete
combustion of the fuel. Although a stoichiometric ratio of fuel to air
is sufficient, an equivalence ratio of 0.8 is employed with the present
study. The composition of the mixture entering the reforming channels is
determined by the stoichiometries of the reforming reaction for the
given fuel. It is typical practice to provide a higher than
stoichiometric steam-to-fuel ratio to minimize possible side reactions
that can cause carbon formation to the detriment of the catalyst and the
reactor. A steam-to-carbon molar ratio of 1.4 is employed with the
present study. The fluids flow essentially parallel to the axes of the
channels. The velocity of the fluid flowing into the reforming channels
is 2.0 meters per second at the flow inlets. In contrast, the velocity
of the fluid flowing into the combustion channels varies depending on
the desired design requirements. Theoretically, the washcoats can be of
any shape. The washcoats can be shaped into any of various
configurations, but they must be designed to increase the area available
for heat exchange, thereby minimizing the length of the channels and the
associated pressure drop. The mass transfer coefficient can be estimated
from the relation between the pore Reynolds and Schmidt numbers and the
asymptotic Sherwood number. The local mass transport coefficient is
dependent upon the local velocity of the fluid, reaction rate, and local
pore structure. The term bulk flow region refers to open areas or open
channels within the reaction chamber. A contiguous bulk flow region
allows rapid gas flow through the reaction chamber without large
pressure drops. Equilibrium conversion is defined in the classical
manner, where the maximum attainable conversion is a function of the
reactor temperature, pressure, and feed composition. For the case of
hydrocarbon steam reforming reactions, the equilibrium conversion
increases with increasing temperature and decreases with increasing
pressure.