Agriculture Reference
In-Depth Information
If the gas obeys Henry's law, then
C
L0
=
HC
G0
(
3
.
29
)
where
H
is the dimensionless Henry's law constant. Eliminating
C
G0
between
Equations (3.27) and (3.29) gives
1
k
G
(C
G
−
C
L0
/H )
=
1
k
L
(C
L0
−
C
L
)
F
=
(
3
.
30
)
where
k
G
(
=
D
G
/δz
G
)
and
k
L
(
=
D
L
/δz
L
)
are transfer coefficients for the gas and
liquid phases, respectively. Hence
C
L0
can be eliminated to obtain the following
equation for the flux through the water film
1
1
/k
L
+
H/k
G
(C
G
−
HC
L
)
F
=
(
3
.
31
)
In Equation (3.31), 1
/k
L
is the resistance to transfer through the liquid film and
H/k
G
is the resistance to transfer through the gas film. The relative importance
of these resistances is given by the ratio
resistance in liquid phase
=
H
k
L
resistance in gas phase
(
3
.
32
)
k
G
Table 3.8 gives values of this ratio for important gases in submerged soils. Most
of the gases are sparingly soluble, and the resistance in the liquid phase is much
greater than that in the gas. This is because diffusion coefficients in water are
two orders of magnitude smaller than those in air and because, for these gases,
H
is small. But for very soluble gases, such as NH
3
, resistance in the gas phase
may be limiting. Solubility varies much more between different gases than the
diffusion coefficients and is therefore the main determinant of whether gas or
liquid phase resistance is limiting. If
H
is less than about 5, transport in the
Table 3.8
Relative importance of resistances in air (
r
G
) and in water (
r
L
)to
gas transfer across an air-water interface at 25
◦
C and 1 atm (Equation 3.32)
D
G
(
dm
2
s
−
1
)
r
G
/r
L
a
D
L
(
dm
2
s
−
1
)
H
2
.
05
×
10
−
3
2
.
26
×
10
−
7
3
.
08
×
10
−
2
3
.
40
×
10
−
6
O
2
CO
2
1
.
55
×
10
−
3
1
.
93
×
10
−
7
8
.
29
×
10
−
1
1
.
03
×
10
−
4
2
.
20
×
10
−
3
1
.
73
×
10
−
7
3
.
16
×
10
−
2
2
.
48
×
10
−
6
CH
4
1
.
66
×
10
−
3
2
.
00
×
10
−
7
3
.
09
×
10
−
4
H
2
S
2.57
×
10
−
3
×
10
−
7
×
10
−
2
×
10
−
6
N
2
2
.
04
2
.
02
1
.
62
1
.
59
10
−
3
10
−
7
10
3
NH
3
2
.
19
×
2
.
49
×
1
.
39
×
0.159
N
2
O1
.
55
×
10
−
3
1
.
98
×
10
−
7
6
.
29
×
10
−
2
8
.
03
×
10
−
6
2
.
04
×
10
−
3
2
.
55
×
10
−
7
4
.
65
×
10
−
2
5
.
81
×
10
−
6
NO
a
For
δz
L
=
δz
G
and assuming gases do not react with water.
