Biomedical Engineering Reference
In-Depth Information
The selectivity of the products of FTS reactions depends on several fac-
tors including:
Process parameters (temperature, pressure, residence time).
Catalyst (type, support).
Choice of reactor type.
Composition of the syngas.
Higher temperature, for example, results in faster reaction, but it leads to
higher methane formation, faster carbon deposition on catalysts, and reduc-
tion in average chain length (Steynberg, 2004).
11.4.2.3 Reactions
The FTS reaction, which is typically carried out in the range 200
350
C and
20
300 atm (Reed, 2002, p. II-238), may be written in a generic form as:
Catalyst
2nH
2
!
ð
CH
2
Þ
n
1
Heat (11.7)
where the hydrocarbon product is represented by the generic formula (CH
2
)n.
The main reaction is (Dayton et al., 2011):
nCO
1
nH
2
O
1
227
C
CO
2H
2
52
CH
2
21
H
2
O
165 kJ
=
mol
ð
Þ
(11.8)
1
2
For cobalt-based catalyst reaction, the required H
2
/CO molar ratio is
2.15. So, the reaction may be written as (Dry, 2002, p. 228):
Cobalt
CO
2
:
15H
2
!
hydrocarbon
H
2
O
(11.9)
1
1
When Fe catalyst is used, the water gas shift (WGS) reaction occurs
simultaneously:
Fe
CO
H
2
O
-
H
2
1
CO
2
41 kJ
=
mol
(11.10)
1
The water produced in main FTS reaction
(11.8)
is converted into H
2
in
WGS reaction (
Eq. (11.10)
). As this hydrogen could be utilized in the main
reaction, the net hydrogen requirement of the FTS process is less when Fe
catalyst is used. For low-temperature synthesis reaction, the overall H
2
/CO
ratio is typically 1.7 (Dry, 2002, p. 229).
FTS produces a number of undesired (aldehyde, carbon etc.) and desired
(paraffins, alcohol) products. For both desired and undesired products the
reactions may be written as (Dalai and Davis, 2008) follows:
Desired Products:
Paraffins: nCO
1
ð
2n
1
Þ
H
2
-
C
n
H
2n
1
2
1
nH
2
O
(11.11)
1
Olefins: nCO
1
2nH
2
-
C
n
H
2n
1
nH
2
O
(11.12)
Alcohol: nCO
1
2nH
2
-
C
n
H
2n
1
1
OH
1
ð
n
2
1
Þ
H
2
O
(11.13)
Search WWH ::
Custom Search