Energy of Hydrogen

The flow of a reacting gas mixture in small channels is used in microchemical reactors to intensify the heat and mass transfer at chemical conversions. The advantages of small channels are the large specific surface of the reactor and small external diffusion resistance. This makes possible chemical conversions of gas mixtures at small residence times in the reaction zone and reactions under conditions of considerable thermodynamic nonequilibrium. Microchannel reactors are used, for instance, to convert hydrocarbons and alcohols into a synthesis gas and hydrogen and for a number of other energy-intense chemical  processes. The advantages of such reactors are most evident in the case of strongly exothermal or endothermal reactions; for instance, at partial oxidation of methane into a synthesis gas or steam reforming of methane. The considerable interest in these reactions is due to the fact that they are used in fuel processors to obtain a synthesis gas (hydrogen and carbon oxide) for fuel cells. In such processors, a catalyst is usually used to increase the reaction rate at low temperatures. The main reactions of methane steam reforming are the following:

CH4 + 2H2O = CO2+4H2     H298 K ° = 165 (kJ/mol);

CH4 + H2O = CO + 3H2        H298 K ° = 206 (kJ/mol).

These reactions are strongly endothermal, and an external heat supply is needed to sustain them. One thinks that this requirement prevents the development of methane vapor conversion reactors due to a decrease in their effectiveness. However, the use of the heat of the chemical reactions in the neighboring reactor channels proceeding in parallel can make steam reforming an attractive alternative of other methods to produce hydrogen.

The methane steam reforming is a complex multistage process including not only the exchange of reactants and reaction products between the gas phase and the reaction zone. It also consists of a series of reactions proceeding on the catalyst. Xu J. G. and Froment G. F. proposed a three-step mechanism of methane steam reforming, and above mentioned reactions are complemented by chemical conversions of their products:

CO + H2O = CO2 + H2      H298 K ° = –41.2 (kJ/mol).

A peculiarity of methane steam reforming is the need to supply heat to the reaction zone. At small heat flux density, the methane conversion degree is small and increases with increasing heat flux density. It has been found that the steam reforming reaction is more intensive at the reactor inlet and a heat supply is most effective at the reactor inlet. The heat supply at the reactor outlet mostly heats the gas mixture, thereby sustaining endothermal reactions. Thus, not only the external heat flux density but also the way it is supplied along the channel length affects the degree of methane conversion. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for producing a synthesis gas.

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