Figure 7. Effect of catalyst layer thickness on the methanol conversion
of the integrated combustion-reforming reactor in which combustion
chambers are in direct thermal contact to reaction chambers for an
endothermic steam-methanol reforming reaction.
The effect of catalyst layer thickness on the hydrogen yield is
illustrated in Figure 8 in which combustion chambers are in direct
thermal contact to reaction chambers for an endothermic steam reforming
reaction. The process comprises contacting a gas phase comprising
methanol and steam, with a solid catalyst. The gas phase may or may not
comprise other gases, in addition to the methanol and steam. For
instance, the gas phase may comprise an inert gas, for example, nitrogen
or argon, which could for example be present as a carrier gas. The inert
gas, when present, is typically nitrogen. Additionally, or
alternatively, the gas phase may further comprise oxygen. Blending
oxygen or air into the gas phase may encourage combustion and may also
balance the total thermodynamic requirements of the non-syngas direct
steam reforming system. Thus, the gas phase may further comprise oxygen
or air. The gases in the gas phase may be pre-mixed, namely mixed
together before the mixture is brought into contact with the catalyst.
Alternatively, the gases can be fed into a reactor separately, so that
the reactant gases are mixed together in the presence of the solid
catalyst. The exothermic and endothermic reaction channel flows are in
the same direction, although in this design, the reformer flow enters
and leaves in a direction perpendicular to the direction of flow during
reaction to accommodate manifolding connections on a different face of
the device than the combustion flow manifolding. At a given reactor
length or height it is not difficult to obtain nearly uniform ratios of
catalyst area to heat transfer area and thereby uniform temperature
distribution in the interior of the catalyst bed cross section, namely
toward the center of the reactor. This ratio can be kept constant if,
for instance, the tube pitch is kept constant for the same heat transfer
tube diameter. By tube pitch is meant the center-to-center distance of
neighboring tubes. Even a change from for instance triangular pitch in
the center of the bed to a rectangular pitch near the periphery of the
bed can be obtained without experiencing too large variations in the
ratio of catalyst area to heat transfer area. However, at the periphery
of the heated or cooled catalyst bed, the surrounding external reactor
wall defining the periphery of the reactor does not heat nor cool the
catalyst bed. The peripheral heat transfer tubes are preferably
positioned in direct contact with the external reactor wall. The
peripheral heat transfer tubes may be simple tubes having a single wall
and may be shaped so that they have a substantially semi-circular or
triangular cross-section. Other types of tube can be envisaged, for
example double-tubes. The temperature toward the external reactor wall
can be kept at about the same level as in the center of the reactor.
Note that the catalytic combustion is most likely accompanied by some
homogeneous combustion in the flow-by gap. The outermost steam reforming
channels are sized to have half the flow of the inner channels since the
outer channels received only half of the heat. Each outermost channel
has catalyst only against the innermost wall and is designed to admit
roughly half the flow going through a full channel for a given pressure
drop. Some process characteristics of some preferred processes include
the following factors: Operate safely at a fuel: oxygen ratio near
stoichiometric for the use of combustion as the exothermic reaction.
This reduces the required air which improves the overall system thermal
efficiency and reduces the required duty for the external air blower or
compressor. Operate steam reforming at short contact times or conversely
at high gas hourly space velocities. This is required to create a
compact device. Operate with a high heat flux. This is required to
operate at short contact times. Operate with a low pressure drop per
unit length of reactor. This enables a higher productivity per unit
volume. Optional: quench-inhibit gas phase reactions. As the channel
dimension nears the quench diameter or drops below, the contribution of
the unwanted gas phase homogeneous combustion reaction is reduced. The
devices may be made of materials such as plastic, metal, ceramic and
composites, depending on the desired characteristics. Walls separating
the device from the environment may be thermally insulating; however,
the walls separating adjacent exothermic and endothermic reaction
chambers should be thermally conductive. The devices can be made by
forming chambers within a single block of material, by joining multiple
components, and by stacking and bonding shims. A preferred integrated
reactor body can be made from a single block of metal. Its channels
could be created with a wire electric discharge machining in the main
body, and the headers and footers could be made separately and welded
on, adding to the flexibility of the design. Wire electric discharge
machining is used to create slots or holes in a block of metal that are
the microchannels through which flow passes and a unit operation occurs.
Sinker electric discharge machining, laser machining, and in some larger
channels conventional milling can also be used to make channels from a
single block of metal. The aperture-containing shims can be formed by
processes including: conventional machining, wire electric discharge
machining, laser cutting, photochemical machining, electrochemical
machining, molding, water jet, stamping, etching and combinations
thereof. For low cost, stamping is especially desirable. Alternately,
laser welding shims could join the devices or sheets to form seals
between flow paths. Devices could alternatively be joined by the use of
adhesives. The combustion chamber and reforming chamber are oriented so
that heat is transferred from the combustion chamber into the reforming
chamber. Both the combustor and reformer should have a separate
preheater or a preheat zone integrated within the device in which
reactants are preheated prior to contacting a catalyst.