Figure 3. Effect of wall thermal conductivity on the average Nusselt number in the exothermic process of the combined parallel plate heat exchanger-reactor.
The effect of wall thermal conductivity on the average Nusselt number are illustrated in Figure 4 in the endothermic process of the combined parallel plate heat exchanger-reactor. The process may be carried out in a single autothermal reactor without the need to provide multiple sequential catalyst zones in the reactor. The catalytic partial oxidation reaction is exothermic in nature and the heat generated thereby is used to carry out the steam reforming reaction which is endothermic in nature. By having the catalytic partial oxidation layers in intimate contact with the steam reforming catalyst layers, the process heat can be more effectively managed in an adiabatic reactor, namely an autothermal reactor. By having the two catalyst layers in contact with one another, heat loss which otherwise occurs from the use of multiple autothermal reactors or an autothermal reactor containing multiple catalyst zones is significantly minimized. The process results in savings in reactor volume and monolith substrate costs as well as less pressure drop throughout the catalytic partial oxidation and steam reforming reactions. The process thereby provides more efficient utilization and uniform usage of the heat generated by the exothermic catalytic partial oxidation reaction, thus allowing the endothermic steam reforming reaction to be carried out at a somewhat higher temperature due to lower heat loss and concomitant higher reaction rate and under adiabatic conditions. The result is that the catalytic partial oxidation reaction temperature is somewhat lowered, estimated to be by about 60 degrees and concomitantly, the steam reforming reaction temperature is estimated to be raised by about 60 degrees, thereby improving catalyst life and resulting in higher steam reforming reaction rates. Moreover, by utilizing the catalytic partial oxidation and steam reforming catalysts as layers in contact with one another, adverse reactions such as the reaction of oxygen with rhodium and the reaction of oxygen with platinum, may be avoided. The overall process consists of first preheating the reactants to the required temperature. It ensures good thermal management for the products leaving the reactor to be used to preheat the incoming reactants to a temperature close to the reaction temperature. The methanol, oxygen, and associated nitrogen flow through the inlets of the inlet manifold and into the reaction channels of the flow path. Heterogeneous oxidation occurs in the catalyst attached to the wall. As the stream flows down through the flow path, the conversion increases until the stream passes through the outlets. In the adjacent flow paths, preheated methanol and steam enter the second discrete set of channels through inlets, contact the catalyst coated onto the wall, and reaction occurs. The heat for the reaction is supplied directly through the wall from the oxidation channels occurring on the opposing side of the dividing wall. As the heat transfer characteristics are highly independent of the bulk reactants velocity, a velocity can be chosen to ensure that the reactants exiting the reactor has attained the desired level of conversion or indeed reached any equilibrium. It is interesting to note that in such an arrangement it is desirable to operate the reactants in a co-current flow arrangement. This ensures that the area with the greatest heat generation is adjacent to the area with the greatest heat requirement. However, cases may exist where a countercurrent flow arrangement is desirable.