Environmental Engineering Reference
In-Depth Information
recycled; the remainder is purged. Some separation parameters are scalable
by a separation technology factor
λ
(see Equation (7.9)). This factor is a design
variable
—
0.9 <
λ
< 1.1:
S
ðÞ
Ar
=
S
ðÞ
H
2
=
S
ðÞ
CO
=
S
ðÞ
CO
2
=
S
ðÞ
Gas
CH
4
=1
:
0
:
S
ðÞ
;
S
ðÞ
“
Light
”
condensables
C
2
H
6
=0
:
90
λ
C
3
H
8
=0
:
80
λ
:
S
ðÞ
;
S
ðÞ
C
4+
=0
:
10
=λ
H
2
O
=0
:
25
=λ
:
:
“
Heavy
”
condensables
:
ð
Eq
7
9
Þ
There are 4 × 9 + 2 = 38 variables and 27 equations in these sets (Equations (7.8)
and (7.9)). The inlet flows for nine species are determined by the supply from an
upstream unit. Hence, there are two design degrees of freedom (
r
,
λ
) in the
model of the separator unit for syngas recovery.
3.
Molar Balances for the Mixer
The molar balances for the mixer are
φ
1
,
i
+
φ
4
,
i
=
φ
2
,
i
ð
i
=1,2,
…
,9
Þ
ð
Eq
:
7
:
10
Þ
and no design targets are needed; the flows entering the mixer are determined by
their respective upstream units. Hence, the number of flow variables and equa-
tions match, leaving no design degree of freedom.
4.
Feed Composition Specification
The molar fractions of feed stream (1) are given by
x
1
,
i
φ
1
,
total
−
φ
1
,
i
=0
ð
i
=1,
…
,9
Þ
X
9
i
=1
φ
1
,
i
−
φ
1
,
total
=0
:
:
ð
Eq
7
11
Þ
The feed composition is given by
x
1
,
Ar
=0
:
02;
x
1
,
H
2
=0
:
62;
x
1
,
H
2
O
=0
:
02;
x
1
,
CO
=0
:
31;
x
1
,
CO
2
=0
:
01;
x
1
,
CH
4
=0
:
02;
x
1
,
C
2
H
6
=0
:
00;
x
1
,
C
3
H
8
=0
:
00;
x
1
,
C
4+
=0
:
00
ð
Eq
:
7
:
12
Þ
There are only eight independent values because of normalization to unity.
There are 19 variables and 18 equations in the sets of equations (7.11 and
7.12), leaving 1 degree of freedom. One may wonder if the total feed flow rate
should not be specified. In process design, there are two options to specify proc-
ess capacity: either the (nominal) intake rate of the main feed or the (nominal)
production rate of the main product. The latter option is chosen here.
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