Figure 6. Steam mole fraction contour maps in 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 methanol conversion is
illustrated in Figure 7 in which combustion chambers are in direct
thermal contact to reaction chambers for an endothermic steam reforming
reaction. Within the reformer, the methanol and steam react
endothermically at high temperature to produce a gaseous product
consisting primarily of steam, hydrogen, and carbon dioxide. Methanol is
passed with steam over a catalyst at pressure typically ranging from
14.7 to 150 psia and temperatures in the range of about 373 K to about
548 K. Typical steam to methanol mole ratio are in the range of about
2.5:1 to about 4:1. The conversion of methanol may be affected in one
pass over the catalyst bed contained in the reformer. Conventional
methods for the production of hydrogen gas include steam reforming of
hydrocarbons [45, 46]. According to the usual method, carbon
monoxide and carbon dioxide are removed from a reformed gas containing
hydrogen gas, carbon monoxide and carbon dioxide obtained by the above
methods so as to produce hydrogen gas [47, 48]. The conventional
methods have several drawbacks in that the price of the raw material
hydrocarbons continues to rise, and the supply of the raw material
hydrocarbons is in unstable conditions, desulfurization of the raw
materials is required, and a high reaction temperature of 1100-1300 K is
required for the steam reforming process. Consequently, the conventional
methods are suitable for large scale hydrogen gas production, but they
are not adequate for middle to small scale hydrogen gas production. In
contrast, hydrogen gas production by the steam reforming of methanol has
various advantages in that the reaction temperature is relatively low,
separation of hydrogen gas from the reformed gas is easy, and no
desulfurization is required because methanol is the raw material. Also,
the method can easily cope with large to small scale plants, since it
uses inexpensive and easily transportable methanol as the raw material.
The steam reforming reaction of methanol yields a wet gas containing
condensable components such as methanol and water, and a reformed gas.
After the reaction, the reformed gas containing hydrogen and carbon
dioxide is taken out by cooling of the wet gas. Here, an industrial
problem is the treatment of the condensed liquid. Conventionally, the
condensed liquid is disposed without treatment or the condensed liquid
obtained by vapor-liquid separation treatment is recycled to the
reaction system so as to make reuse of the condensed liquid together
with the raw material methanol and water. However, the former has a big
problem from the view-point of pollution since the condensed liquid
contains a considerable number of organic components such as unreacted
methanol and high boiling point components. In the latter, a trace
amount of ethanol contained in the raw material methanol accumulates
because it is hardly converted by the usual reaction method. If the
treated condensed liquid is reused in the reforming reaction system, it
is necessary to adjust the amount of water so as to make the
methanol-water ratio at a predetermined value. Therefore, if steam
reforming of methanol is continuously carried out, a portion or the
whole of a predetermined amount of raw material water may be allotted
for water to be added to the condensed liquid. The steam reforming
reaction of methanol which discharges the condensed liquid or makes
reuse of the treated condensed liquid should not particularly be
limited, but the reaction is usually carried out as follows. Known
catalysts are used for the steam reforming reaction of methanol. For
example, they include copper catalysts such as those containing copper
oxides, chromium oxides and manganese oxides. The conditions of steam
reforming reaction of methanol vary with the catalysts used, and cannot
absolutely be specified. The reformed gas mainly contains hydrogen gas
and carbon dioxide gas. When carbon dioxide gas is removed from the
reformed gas by a conventional method such as absorptive removal with an
aqueous sodium carbonate solution, an aqueous potassium carbonate
solution or an aqueous monoethanolamide solution, a high purity of
hydrogen gas is obtained. Also, when the condensed liquid is reused
after treatment for the steam reforming reaction of methanol, it has no
bad effect due to ethanol and high boiling point components on the
reaction. The conversion of methanol is usually maintained high. The
durability of catalysts is increased, and the process causes no
pollution.