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.