Biomedical Engineering Reference
In-Depth Information
100
120
90
Dielectric constant
Specific heat
100
80
80
70
374 °C
60
60
50
40
40
30
20
20
0
10
0
-20
0
200
400
600
800
1000
1200
Temperature (°C)
FIGURE 9.2 Specific heat of water above its critical pressure shows a peak at it “pseudo-
critical” temperature. Dielectric constant at 22.1 MPa, also plotted on this graph, shows rapid
decline closer to the critical temperature.
although the viscosity starts rising with temperature above the critical value.
Above this critical point, water experiences a dramatic change in its solvent
nature primarily because of its loss of hydrogen bonding. The dielectric con-
stant of the water drops from a value of about 80 in the ambient condition to
about 10 at the critical point ( Figure 9.2 ). This changes the water from a
highly polar solvent at an ambient condition to a nonpolar solvent, like ben-
zene, in a supercritical condition.
The change in density in SCW across its pseudo-critical temperature is
much more modest. For example, at 25 MPa it can drop from about 1000 to
200 kg/m 3 while the water moves from a liquid-like to a vapor-like state
( Figure 9.3 ). At subcritical pressure, however, there is an order of magnitude
drop in density when the water goes past its saturation temperature. For example,
at 0.1 MPa or 1 atm of pressure, the density reduces from 1000 to 0.52 kg/m 3 as
the temperature increases from 25 Cto150 C (refer to Table 9.1 ).
The most important feature of SCW is that we can “manipulate” and con-
trol to a certain degree its properties around its critical point simply by
adjusting the temperature and pressure. SCW possesses a number of special
properties that distinguish it from ordinary or subcritical water. Some of
those properties relevant to gasification are as follows:
The solvent property of water can be changed much near or above its crit-
ical point as a function of temperature and pressure.
Subcritical water is polar, but SCW is nonpolar because of its low
dielectric constant. This makes it a good solvent for nonpolar organic
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