Environmental Engineering Reference
In-Depth Information
10
8
Liquid
metal
H Metal
10
6
10
4
H
2
gas
10
2
Critical point
H
2
Liquid
10
0
H
2
solid
Triple point
H
gas
10
-2
10
0
10
1
10
2
10
3
10
4
10
5
10
6
Temperature [K]
FIGURE 5.6
A simple phase diagram of hydrogen.
Source
: Reproduced with permission from Leung
et al. [10].
The liquefaction process requires very clean hydrogen, several cycles of
compression, liquid nitrogen or helium cooling, and expansion taking the
advantage of the Joule-Thomson (JT) effect. When a real gas is allowed to
expand adiabatically through a porous plug or a fine hole into a region of
low pressure, it is accompanied by cooling (or heating). Cooling takes place
because some work is done to overcome the intermolecular forces of attrac-
tion. In JT effect, the enthalpy
H
of the system remains a constant, and the
JT coefficient is defined as
μ
=
∂
∂
T
P
H
.
(5.20)
For cooling
μ
> 0 and for heating
μ
< 0. A unique locus of P-T points
at
μ
= 0 is called JT inversion curve (see Fig. 5.7, the maximum points of
the
T
-
P
curve for a constant
H
). Thus, for a certain temperature, there exists
a pressure beyond which
μ
< 0 and isenthalpic expansion causes a tempera-
ture rise; while at lower pressures,
μ
> 0 and isenthalpic expansion induces
the cooling effect. Gases like H
2
, He, whose inversion temperature is low,
show heating effect at room temperature. However, if these gases are just
cooled below the inversion temperature and then subjected to JT effect, they
will also undergo cooling. For hydrogen, the maximum JT inversion tem-
perature is at 205 K (−68.15°C). Thus, hydrogen needs to be precooled to
below this temperature. This can be done using cold or liquid nitrogen. Then,
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