Environmental Engineering Reference
In-Depth Information
(a)
(b)
C
1
*
(1 -
α
)
α
CH
4
1
C20+
C
2
*
0.8
C10-C20
2
α
(1 -
α
)
α
C4
C
2
H
6
C5-C9
C3
C
3
*
C2
0.6
3
2
(1 -
α
)
C1
α
α
C
3
H
8
0.4
C
4
*
4
3
(1 -
α
)
α
α
0.2
C
4
H
10
C
5
*
0
5
4
(1 -
α
)
α
α
0
0.1
0.2 0.3 0.4 0.5
Chain growth probability
α
(-)
0.6
0.7
0.8
0.9
1
...
...
FIGURE 17.3
(a) Scheme of the chain reactions in FTS with the probability for each step,
according to theAnderson-Schulz-Florymechanism. (b) Plot of the resulting product distribution.
is also more expensive than iron and less resistant to poisons. Whereas iron-based cat-
alysts typically have Fe
3
O
4
as their main component, cobalt-based catalyst typically
consists of alumina support particles with cobalt deposited on it.
Both cobalt and iron are sensitive to high partial pressures of water. To prevent too
high partial pressures of water and to increase conversion, water can be removed, e.g.,
by interstage cooling and condensation or by membranes. In addition, a promoter (aid-
ing compound) can be added to iron (e.g., potassium), so that it catalyzes the water
-
gas shift reaction in addition to the FT reaction and converts water:
mol
−1
CO+H
2
O
CO
2
+H
2
Δ
r
H
=
−
41 kJ
ð
RX
:
17
:
2
Þ
⇄
An important design variable for FTS is the syngas ratio, the ratio of the hydrogen
concentration to the carbon monoxide concentration in the feed stream. The water
-
gas shift reaction (RX. 17.2) makes that an iron-based catalyst can work with a range
of syngas ratios. For the more widely used cobalt catalyst, H
2
and CO are only con-
sumed by FTS, in a ratio between 2 (for production of infinitely long hydrocarbon
chains) and 3 (for production of methane), depending on
. It can be shown mathe-
matically that the H
2
/CO consumption ratio exhibits the remarkably linear result (3
α
−
α
)
(Vervloet et al., 2012). Typically,
-values around 0.9 are desired, giving a syngas
ratio around 2.1 based on stoichiometric requirements. However, it may be advanta-
geous to choose a lower syngas ratio, as this will lead to a lower amount of water in the
reactor and a larger
α
α
-value (Vervloet et al., 2012).
17.2.3 Process Conditions and Reactor Types
FTS is typically carried out at pressures between 10 and 60 bar and temperatures in the
range of 480
−
620 K. In view of the required large scale of FTS reactors, the process is
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