Page 144 - Chemical process engineering design and economics

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to supply some of the enthalpy of reaction for the endothermic reforming reaction.
If it is economical, the hydrogen in the purge stream could also be recovered.
Thermodynamically, the reforming reaction, Equation 3.5.1, shows that the
reformer should be operated at the lowest pressure and highest temperature possi-
ble. The reforming reaction occurs on a nickel-oxide catalyst at 880 °C (1620 °F)
and 20 bar, which results in a 25 °C approach to the equilibrium temperature
[25,29]. Methane conversion increases by reducing the pressure, but natural gas is
available at a high pressure. It would be costly to reduce the reformer pressure
and then recompress the synthesis gas later to 100 bar (98.7 arm) for the converter.
The steam to carbon monoxide ratio is normally in the range of 2.5 to 3.0 [30].
The ratio favors both the conversion of methane to carbon monoxide and the car-
bon monoxide to carbon dioxide as indicated by Equations 3.5.1 and 3.5.3. If the
ratio is decreased, the methane concentration increases in the reformed gas, but if
the ratio is set at three, the unreacted methane is small. The methane is a diluent in
the synthesis reaction given by Equation 3.5.2.

Process Description

The process generates three hot gas streams: flue gas, reformer gas, and converter
gas. We must recover the enthalpy of these streams to have an economically viable
process. Thus, methanol synthesis plants are designed to generate 70% of their
energy requirements internally [30]. The excess enthalpy generates high-pressure
steam for steam-turbine drivers needed to compress the synthesis gas and the con-
verter recycle gas. This is an example of a process where the process engineer
must integrate several energy-transfer steps with reaction and separation steps for
an energy-efficient process.
Figure 3.5.1 is the flow diagram for the Imperial Chemical Industries (ICI)
process. The solid lines in the diagram are for the process streams, and dashed
lines are for the steam system, which is really a subprocess of the main process -
just as the cooling-water supply system is also a subprocess. Sulfur-containing
compounds present in most natural gas streams will poison the reforming and syn-
thesis catalysts. A hydrodesulphurization reaction removes these compounds by a
using a catalyst in a packed bed. If there is no hydrogen present in the natural gas,
purge gas from the synthesis loop, which is hydrogen rich, can be mixed with the
natural-gas feed stream. Hydo-desulpurization forms hydrogen sulfide, which
then reacts with zinc oxide in a packed bed to form zinc sulfide. Both the hydro-
genation-catalyst and the zinc-oxide beds may be contained in the same vessel.
After removing hydrogen sulfide and mixing the stream with steam, the
mixture flows to the reformer. Combustion gas heats the reformer to supply the
enthalpy of reaction. To cool the hot reformed gas, steam is generated first and
then vapor in the reboilers of the methanol-recovery section of the process. Cool-
ing the reformed gas reduces the temperature and therefore the gas volume, which
reduces the energy of compression. During cooling, water condenses and is re-

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