Environmental Engineering Reference
In-Depth Information
is equal to its calorific value
394 MJ/kmol, whereas the calorific value of CO
2
is zero
(as it is the reference substance for C).
The reference substances for the devaluation enthalpy and chemical exergy are
the same. Also, the reference temperature and pressure are the same. Thus, only the
devaluation enthalpy method, contrary to the formation enthalpy method, allows for
fair comparison of the values of chemical energy and chemical exergy. For example,
the chemical exergy of C is
∼
∼
413 MJ/kmol and devaluation enthalpy (calorific value)
of C is only
394 MJ/kmol. Therefore, only the devaluation enthalpy method should
be used in thermodynamic analysis, because then the comparative energy and exergy
analyses are simultaneously included.
More details on the devaluation method is discussed by Szargut et al. (1988).
Only the significance of the concept of devaluation reaction and the resulting con-
cept of devaluation enthalpy, used for calculation of chemical energy, is outlined here.
The method allows calculation of the following quantities: enthalpy devaluation of a
substance (appearing in the energy conservation equation), standard entropy of deval-
uation reaction (in the entropy considerations), and, consequently, chemical exergy of
substance (appearing in the exergy balance equation).
Devaluation enthalpy is determined based on the stoichiometric devaluation reac-
tion for a substance. The devaluation reaction has to be a combination only of the
considered substance and the various reference substances. A good example of a
devaluation reaction is reaction of photosynthesis:
∼
+
→
+
6O
2
(2.2.19)
in which, besides the considered substance of sugar (C
6
H
12
O
6
), only the reference
substances appear; CO
2
,H
2
O and O
2
. The devaluation enthalpy,
d
n
, is calculated
from the energy conservation equation for the chemical process in which substrates
are supplied, and products are extracted, all at standard temperature and pressure.
As shown, in comparison to the physical exergy
b
ph
of a substance, the calcu-
lation of the chemical exergy
b
ch
of a substance is more complex, depending on its
composition, and is based on the devaluation reaction. The calculation procedure is
discussed by Szargut et al. (1988), where the calculated standard values of the devalua-
tion enthalpy (
d
n
) and chemical exergy (
b
n
) are tabulated for the standard environment
temperature
T
0
6H
2
O
6CO
2
C
6
H
12
O
6
=
T
n
. If the environment temperature
T
0
differs from the standard envi-
ronment temperature
T
n
, then, when using the standard data on
d
n
and
b
n
, the formula
for the chemical exergy of condensed substances (solid or liquid), should be corrected
as shown, e.g., for the specific chemical exergy
b
chSU
of sugar:
T
n
−
T
0
b
ch
,
SU
=
b
nSU
+
(
d
nSU
−
b
nSU
)
(2.2.20)
T
n
where
b
n
,
SU
and
d
n
,
SU
are the standard tabulated values of the chemical exergy and
devaluation enthalpy of sugar, respectively. If a substance has a temperature different
from the surrounding environment, then also a physical component of energy or exergy
has to be included as shown, e.g., again for the physical exergy
b
ph
,
SU
of the sugar:
T
0
c
SU
ln
T
T
0
b
ph
,
SU
=
c
SU
(
T
−
T
0
)
−
(2.2.21)
where
c
SU
is the specific heat of sugar.
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