Environmental Engineering Reference
In-Depth Information
Off gas
Fuel product
Water
Syngas
feed
(s)
Separator
r
,
S
i
r
⋅
S
i
(1-
r
⋅
S
i
)
Syngas recycle
(m)
Mixer
1
4
5
2
3
Species:
Ar
H
2
H
2
O
CO
CO
2
C
1
C
2
C
3
C
4+
Fischer-Tropsch conversion (c)
Reaction stoichiometry:
1 CO + 3 H
2
2 CO + 5 H
2
3 CO + 7 H
2
↔
1C
1
H
4
+ 1 H
2
O
→
1C
2
H
6
+ 2 H
2
O
→
1C
3
H
8
+ 3 H
2
O
C
4+
lump:
n
a
CO + (2
n
a
+ 1) H
2
→
1C
n
a
H
2
n
a
+2
+
n
a
H
2
O
WGS:
1 CO + 1 H
2
O
↔
1CO
2
+ 1 H
2
Extents of FT reactions:
e
k
=
α
e
k-1
for
k
= 2, 3, …,
n
a
Extent of WGS reaction:
e
WGS
=
β
∑
{
k e
k
}
FIGURE 7.12
FT conversion with syngas recovery and recycle; the process streams are
uniquely labeled 1-5.
The evaluation involves an assessment of the computed economic, ecological, and
technological performance indicators.
The FT conversion unit is related to subprocess 3 and covers also the recovery
and recycle of the unconverted syngas. The pertaining block diagram is shown in
Figure 7.12.
The streams and the function blocks in this diagram are all uniquely labeled. Also,
the FT reaction stoichiometry is shown as well as how the syngas recovery factors are
defined. The compressor for the recycle of the unconverted syngas is not shown as it
does not affect the mass balances, though it has definitely an energy cost. The follow-
ing nine species are accounted for in the analysis model: Ar, H
2
,H
2
O, CO, CO
2
,CH
4
,
C
2
H
6
,C
3
H
8
, and a
to collectively represent a range
of species needs some explanation. In reality, the hydrocarbon species range from C
1
to ~C
200
with a geometric distribution. The molar quantities of hydrocarbon species
(C
k
) with a chain length of
k
carbon atoms can be given as C
k
+1
=
“
lump
”
C
4+
. The use of a
“
lump
”
α
C
k
, with
k
> 0. The
chain growth probability,
0.95. The actual value of this factor can
be influenced by the design of the catalyst and operating conditions in the reactor. The
C
4+
lump can be obtained as the molar sum over all species ranging from C
4
up to C
∞
.
The lump is represented by a single
α
, is of the order 0.7
-
pseudocomponent: C
n
a
H
2
n
a
+2
. The
averaged carbon number
n
a
of this pseudocomponent is obtained by weighting of
the relative molar amounts of these species over the chain length distribution. It is just
“
averaged
”
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