Environmental Engineering Reference
In-Depth Information
in the 1930s when NG and other hydrocarbon feedstocks such as naphtha became
available on large scale. H
2
is currently produced from NG in large quantities in
mixtures with nitrogen or carbon oxides for manufacture of ammonia, alcohols
(mainly methanol) and for Gas to Liquid (GTL) processes [
6
]. In particular, SR
produces a mixture of H
2
and CO (synthesis gas or syngas) that could be used
directly for the synthesis of methanol or higher alcohols, and for Fischer-Tropsch
synthesis.
Natural Gas feedstock is mainly constituted by methane molecule (CH
4
), which
represents the hydrocarbon with the highest H/C ratio. The composition of the NG
could slightly change in dependence of the geographic region where it is extracted,
but generally the mixture contains mainly small amounts of light hydrocarbons
(C
2
-C
4
). The compound present in the highest concentration is the ethane (C
2
H
6
)
that can reach in some mixtures a volumetric concentration of 5%. Not negligible
traces of sulphur are often detectable in the hydrocarbon mixture.
A simplified scheme of methane SR is shown in Fig.
2.2
, which includes all
main process steps involved in hydrogen production plants based on the SR
reaction [
8
].
Two units remove the sulphur concentrations (ppm), added to natural gas as an
odorant for safety detection, or present in higher hydrocarbon feedstocks, to
protect downstream catalysts (sulphur is a poison for SR catalysts) and process
equipment. In particular, the organo-sulphur species are converted to H
2
Sat
pressures exceeding about 500 psig and temperatures higher than 350Cby
catalytic hydrodesulphurisation (HDS unit), and Co and Mo alumina-based par-
ticulates are used as catalysts. This step is not required for methanol but would be
necessary for any sulphur-containing petroleum-based fuels. A second unit permits
the H
2
S produced in the first step to be removed by a particulate bed of ZnO. When
necessary a further step for chloride removal should be included (not reported in
Fig.
2.2
).
The third step is the heart of the process (steam reformer). Ni-based (Ni-Al
2
O
3
)
catalysts, loaded in tubular reactors, favour the advancement of the following
reactions:
CH
4
þ
H
2
OCO
þ
3H
2
DH
¼þ
206 kJ/mol
ð
2
:
1
Þ
or for higher hydrocarbons:
C
n
H
m
þ
nH
2
OnCO
þð
m
þ
2n
Þ=
2
H
2
ð
2
:
2
Þ
Simultaneously in high- and low-temperature shift reactors, the so-called water
gas shift reaction produces further H
2
according to the exothermic equation:
CO
þ
H
2
OCO
2
þ
H
2
DH
¼
41 kJ/mol
ð
2
:
3
Þ
Thus SR process is highly energy intensive as the Eqs.
2.1
or
2.2
are highly
endothermic and requires high energy inputs, in dependence of the fuel. SR is
normally carried out at 800-900C and about 0.1-0.3 MPa. Expensive alloy
reaction tubes have to be used to withstand the severe operating conditions.
