Page 224 - Synthetic Fuels Handbook

210 CHAPTER SEVEN

Due to its endothermic character, reforming is favored by high temperature and, because
reforming is accompanied by a volume expansion, it is favored by low pressure. In contrast,
the exothermic shift reaction is favored by low temperature, while unaffected by changes
in pressure.
Increasing the amount of steam will enhance the methane conversion, but requires an
additional amount of energy to produce the steam. In practice, steam-to-carbon ratios [i.e.,
P(H O)/P(CH )] of approximately 3 are applied. This value for the steam-to-carbon ratio
4
2
will also suppress coke formation during the reaction (Rostrup-Nielsen, 1984; Rostrup-
Nielsen and Bak-Hansen, 1993; Rostrup-Nielsen et al., 2002).
Higher molecular weight feedstocks can also be reformed to hydrogen:
C H + 3H O → 3CO + 7H
3 8 2 2
That is,
C H + nH O → nCO + (0.5m + n)H 2 (7.2)
n
m
2
In the actual process, the feedstock is first desulfurized by passage through activated
carbon, which may be preceded by caustic and water washes. The desulfurized material is
then mixed with steam and passed over a nickel-based catalyst [730–845°C (1346–1553°F)
and 400 psi (2758 kPa)]. Effluent gases are cooled by the addition of steam or condensate to
about 370°C (698°F), at which point carbon monoxide reacts with steam in the presence of
iron oxide in a shift converter to produce carbon dioxide and hydrogen in which the carbon
monoxide is then shifted with steam to form additional hydrogen and carbon dioxide in an
exothermic (heat-releasing) reaction:
CO + H O = CO + H 2 ΔH 298 K =−41.16 kJ/mol (7.3)
2
2
The carbon dioxide (usually by amine washing), leaving hydrogen, is separated for its
commercial use; the hydrogen is usually a high-purity (>99 percent) material.
Since the presence of any carbon monoxide or carbon dioxide in the hydrogen stream
can interfere with the chemistry of the catalytic application, a third stage is used to convert
these gases to methane:
CO + 3H → CH + H O
2
2
4
CO + 4H → CH + 2H O
2
2
2
4
For many refiners, sulfur-free natural gas (CH ) is not always available to produce hydro-
4
gen by this process. In that case, higher-boiling hydrocarbons (such as propane, butane, or
naphtha) may be used as the feedstock to generate hydrogen (qv).
The net chemical process for steam-methane reforming is then given by:
CH + 2H O → CO + 4H ΔH =+165.2 kJ/mol (7.4)
4 2 2 2 298 K
Indirect heating provides the required overall endothermic heat of reaction for the
steam-methane reforming.
One way of overcoming the thermodynamic limitation of steam reforming is to remove
carbon dioxide as it is produced, hence shifting the thermodynamic equilibrium toward the
product side. The concept for sorption-enhanced methane-steam reforming is based on in
situ removal of carbon dioxide by a sorbent such as calcium oxide (CaO).
CaO + CO → CaCO 3
2
Sorption enhancement enables lower reaction temperatures, which may reduce catalyst
coking and sintering, while enabling use of less expensive reactor wall materials. In addition,