The catalytic partial oxidation process could very suitably be used to provide the hydrogen feed for a fuel cell. In fuel cells, hydrogen and oxygen are passed over the fuel cell in order to produce electricity and water. Fuel cell technology is well known in the art.
One of the most challenging applications of fuel cells is in transportation. Transport means, such as automotive vehicles and crafts, powered by fuel cells are under development. The oxygen needed for the fuel cell may be obtained from the ambient air, the hydrogen feed could be obtained from a hydrogen fuel tank but is preferably produced on-board, for example by catalytic reforming of methanol. The on-board production of hydrogen by catalytic reforming of methanol has been proposed, for example by R. A. Lemons, Journal of Power Sources 29 (1990), p 251-264.
The on-board production of hydrogen by a catalytic partial oxidation process, for example as described in WO99/19249, has been proposed as an alternative for steam reforming of methanol. An important advantage of this catalytic partial oxidation process is its flexibility towards the choice of fuel.
In a catalytic partial oxidation process in a fixed catalyst bed, the temperature of the top layer, i.e. the layer at the upstream end of the catalyst bed, is typically higher than the temperature further downstream in the catalyst bed. This is due to the fact that the catalytic partial oxidation reaction is mass and heat transfer limited, i.e. full conversion is subject to mass and heat transfer limitations between the bulk of the gaseous feed mixture and the catalyst surface.
Typically, upon an increase in the average carbon number of the hydrocarbonaceous feedstock, the temperature of the top layer of the catalyst bed will increase. This is probably due to the fact that if the feedstock has a high carbon number, and thus a high molecular weight, the oxygen concentration at the upstream surface of the catalyst will be relatively high, i.e. higher than the average oxygen concentration in the feed mixture supplied, since the diffusion of the smaller oxygen molecules to the upstream surface will be faster than the diffusion of the larger hydrocarbon molecules. Thus, as the number of carbon atoms increases a larger part of the hydrocarbons will be completely oxidised at the upstream surface of the catalyst bed. Since the complete oxidation reaction is more exothermic than the partial oxidation reaction, more heat is produced, resulting in a very high temperature of the upstream part of the catalyst bed. Temperatures of the top layer of the catalyst bed above 1200° C. have been observed in the catalytic partial oxidation of a naphtha feedstock. It will be appreciated that the temperature of the top layer will not only depend on the feedstock, but also on the catalyst composition and structure, the composition of the feed mixture, the process conditions and the configuration of the reactor.
High temperatures in the top layer of the catalyst bed are unwanted, since the rate of catalyst deactivation increases with temperature. Therefore, there is a need in the art for a catalytic partial oxidation process wherein the temperature in the top layer of the catalyst bed can be reduced.