Modern Engineering Thermodynamics

72 CHAPTER 3: Thermodynamic Properties
CASE STUDY: A NEW SUPERCRITICAL WATER TREATMENT
A major engineering challenge today is the development of an
effective waste disposal or destruction technique that produces no
toxic waste or emission itself. Hazardous wastes have historically
been discarded in ocean dumping, landfills, incineration, or longterm
storage. However, ocean dumping is now illegal and landfill
sites are becoming increasing difficult to manage due to concerns
about groundwater contamination.
A new technology has emerged as an effective way to eliminate thousands
of tons of organic wastes that are the by-products of modern
society. Called supercritical water oxidation, the technique exploits the
fact that, while most organic wastes are not soluble in water at normal
temperatures, they are dissolved at high pressure and temperature
(Figure 3.14). Once the organic wastes are dissolved, oxygen is
added and an oxidation-reduction reaction occurs, much like a
controlled combustion process. Operating at supercritical conditions
results in a single-phase homogenous reaction environment that
causes rapid oxidation of the organics, producing carbon dioxide,
water, nitrogen, and small amounts of other compounds, such as
ammonia and acids. Since the entire process is in a closed system,
no harmful products are released into the environment.
This technique has also been found to be an effective disposal
method for surplus military chemical wastes, such as nerve agents,
mustard gas, rocket fuels, TNT, and other explosives. More than
99.9% of the explosives are destroyed in less than 30 s at 600.°C.
Even radioactive wastes can be concentrated and stabilized by eliminating
their organic components. The resulting radioactive components
can then be encased in molten glass and stored deep
underground.
218 atm
Supercritical Water Oxidation (SCWO)
Supercritical liquid water
Supercritical
water vapor
Critical point
Pressure
Solid water (ice)
Normal
liquid water
Water vapor
0.006 atm
Triple point
0.01°C
Temperature
374.14°C
FIGURE 3.14
Supercritical water oxidation.
3.8 QUALITY
As mentioned earlier, in an equilibrium two-phase mixture, temperature and pressure cannot be varied independently;
therefore, either one or the other can be taken as an independent thermodynamic property, but not
both. Figure 3.15a shows the actual p-v diagram for water on log-log coordinates. Notice that, in the two-phase
regions (liquid plus vapor and solid plus vapor), the isotherms (lines of constant temperature) are parallel to
the isobars (lines of constant pressure), showing that pressure and temperature are not independent in this
region. To determine other thermodynamic properties of a mixture of phases, we need to know the amount of
eachphasepresent.Wedothiswithalever rule applied to one of the phase diagram coordinates. Consider the
simplified liquid-vapor p-v diagram shown in Figure 3.15b.
Substances whose states lie on the saturation curve are called saturated. Substances whose states lie under the
saturation curve are called wet. Substances whose states are on the saturation curve but to the left of the critical
state are called saturated liquids, and those on the saturation curve to the right of the critical state are called saturated
vapors. Substances whose states are to the left ofthesaturationcurvearecalledcompressed or subcooled
liquids, and those to the right are called superheated vapors.
To help identify the properties of a system, we adopt the convention of using an f subscript on the symbols of
all thermodynamic properties of saturated liquids and a g subscript on the symbols of all thermodynamic properties
of saturated vapors. Thermodynamic properties in the compressed (or subcooled) liquid region, the wet
(or mixture) region, and the superheated vapor (or gas) region carry no subscripts. Consequently, the specific