A new process for the production of hydrogen with a low carbon monoxide content
04-Sep-2003
fuel cells are on their way. But their use for apllications such as in vehicles
will only become widespread when the expensive process of "refueling" with
hydrogen can be made unnecessary. Transportable hydrogen sources are thus under
investigation. The problem with this is that hydrogen produced by the usual
processes contains rather large amounts of carbon monoxide (CO), which has a
negative effect on the efficiency of fuel cells and can only be removed through
multiple expensive production steps. James A. Dumesic and Rupali R. Davda at the
University of Wisconsin have now developed a process that allows for the
production of hydrogen with a low CO content.
The researchers are not using steam reforming of petroleum products, but rather
of oxygen-containing compounds that can be derived from biomass, such as
carbohydrates. Ethylene glycol is a simple molecule in this class of reactants
and acts as the starting material. In a catalytic reforming process under
pressure and at about 225 °C, these molecules in liquid water are split into
hydrogen and carbon monoxide. In a second reaction, called the water-gas shift,
the CO and water are then converted to CO2 and more hydrogen. Because both
reactions occur in the same, relatively low, temperature range, they can be
carried out together in the same reactor, which is especially advantageous for
transportable hydrogen sources.
In the reforming process, CO and hydrogen are produced as gases, which form
bubbles within the liquid phase. The water-gas shift occurs within these
bubbles. It is an equilibrium reaction, which means that the reactants are not
completely converted to products; instead, there is always a certain ratio of
one to the other. In order to minimize the amount of CO, the conditions within
the bubble have to be set so as to push the equilibrium as far as possible
toward the product side. This can be achieved by maximizing the amount of water
vapor in the bubbles. However, under these conditions, the starting materials
for the reforming process can decompose. Davda and Dumesic found a trick to get
around this problem: they divided the reactor into two areas. The reforming
process takes place in the lower area, resulting in gas bubbles with relatively
little water vapor. These then rise into the upper area, where the temperature
is raised by about 10 °C. This causes a large amount of water to vaporize, so
that the water-gas shift occurs under optimal conditions, bringing the CO
content down to a level tolerable in fuel cells.
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