Environmental Engineering Reference
In-Depth Information
potential problems. In particular, the thermal stability of catalysts needs to be
carefully verified [
9
], because steam tends to favour catalyst and support sintering
[
10
]. However, the major problem lies in the formation of coke, according to the
following thermodynamically possible reactions:
CH
4
C
þ
2H
2
methane decomposition
ð
Þ
DH
¼þ
74
:
7 kJ/mol
ð
2
:
5
Þ
or for higher hydrocarbons:
C
n
H
2n
þ
2
nC
þð
n
þ
1
Þ
H
2
ð
2
:
6
Þ
2COC
þ
CO
2
Boudouard equilibrium
ð
Þ
DH
¼
173 kJ/mol
ð
2
:
7
Þ
CO
þ
H
2
C
þ
H
2
O
DH
¼
131 kJ/mol
ð
2
:
8
Þ
The coke can affect the performance of active sites of SR catalysts [
11
,
12
],
determining their partial deactivation, with progressive loss of selectivity towards
synthesis gas production, blockage of reformer tubes and increasing pressure drop.
The above reactions are in equilibrium and the formation of coke via reactions
(
2.7
) and (
2.8
) becomes less favoured as the temperature increases. However, coke
formation via reactions (
2.5
)or(
2.6
) becomes increasingly important at higher
temperatures and, depending on the nature of the feed, can rapidly deactivate the
SR catalyst and block the reactor [
12
].
Therefore, the minimization of coking is one of the major factors controlling
the industrial application of SR. The thermodynamic of the process dictates
reaction conditions that favour coke formation cannot be avoided, but operating
conditions can be chosen to minimize coke. Temperature, pressure and feed
composition must be carefully controlled to avoid catalysts deactivation due to
coking. Perhaps, the most obvious way is to increase the steam to hydrocarbon
ratio to favour the reverse of reaction (
2.8
). Rostrup-Nielsen et al. [
13
] have
presented carbon limit diagrams which relate the propensity of the catalyst to coke
formation as function of to the H/C and O/C ratios in the gas phase.
The outlet from the secondary reformer contains about 10-14% CO (dry gas)
which is fed to a high-temperature water gas shift (WGS) reactor (Fig.
2.2
),
typically loaded with Fe or Cr particulate catalyst at about 350C. This further
increase the H
2
content lowering CO content to about 2% as governed by the
thermodynamic and kinetics of the Eq.
2.3
, that is an exothermic reaction. Water
gas shift reaction equilibrium is sensitive to temperature with the tendency to shift
towards products when temperature decreases.
Then the product gas is fed to a low-temperature reactor where a Cu/Zn-Al
2
O
3
particulate WGS catalyst works at about 200C. Outlet CO concentration is
decreased to \0.5%, while the remaining CO, which can poison downstream
ammonia or methanol synthesis catalysts, is removed by pressure swing adsorption
(PSA) unit. This method exploits the adsorption capacity of different molecular
sieves or active carbon, which selectively permit the crossover of hydrogen but not
of the other compounds present in the effluents. This technology has been
