Subcritical water phase diagram

This paper deals with the degradation of substances like PVC, Tetrabromobisphenol A, y-HCH and HCB in supercritical water. This process is called "Supercritical Water Oxidation", a process which gained a lot of interest in the past. The difference between subcritical and supercritical processes is easy to recognize in the phase diagram of water. The vapor pressure curve of water terminating at the critical point, i.e. at 374 °C and 221 bar. The relevant critical density is 0.32 g/cm3. This corresponds to approx. 1/3 of the density of normal liquid water. Above the critical point, a compression of water without condensation, i.e. without phase transition is possible. It is within this range that supercritical hydrolysis and oxidation are carried out. The vapor pressure curve is of special importance in subcritical hydrolysis as well as in wet oxidation. 

The propane system shown in Fig. 11.3 is clearly subcritical as the critical temperature of propane is about 96°C. An increase of the C02 fraction ((3) in the mixture of C02 and propane shifts the one-phase region (1), i.e. the bicontinuous microemulsion, to lower temperatures. For pure C02 the bicontinuous microemulsion (1) exists around 35°C, which is higher than the Tc = 31°C of C02. In other words, the C02 solubilised in the microemulsion is supercritical Knowing how to tune the phase behaviour of these systems, one can easily shift phase diagrams on the temperature scale by simply choosing an appropriate surfactant. Other tuning parameters are the oil-to-water fraction and the temperature which maybe adjusted such that, e.g. a C02-in-water droplet microemulsion forms. 

The large discrepancy for the run C at 0.6934 g/cm (see Table 5) could possibly be related to the fact that it was the only MD-simulation in this series actually performed at subcritical temperature, 630 K, and thus could correspond to a metastable thermodynamic state. An indirect indication of this might also be seen in the discrepancy of pressure calculated for the same point (Kalinichev and Heinzinger 1992 1995), which was also far beyond the qualitatively reasonable representation of the equation of state observable for all other simulation points. However, more extensive simulations for the BJH water over wider ranges of temperature and density need to be performed before the phase diagram for this particular water model could be better understood. 

Figure 17.1 The phase diagram of supercritical/subcritical water.

In the phase diagram of a liquid, as shown in Fig. 2.1, the region above the critical point is called the supercritical region. In the case of water, the critical point is at 374.2°C and 22.1 MPa. Above this temperature and pressure, the water is called supercritical water. Pressures below the critical point are referred to as subcritical pressures. Due to the relatively low viscosity of supercritical water with respect to its density and high specific heat enthalpy, it has a good ability as a coolant. 

It is observed experimentally that two miscible subcritical components such as water(l) and n-propanol(2) will change from one-phase liquid to two phases (LL or VL) and even to three phases (VLL) when ethylene(3) is added at pressures above 2 MPa and temperatures above the pure ethylene critical point Sketch a triangular diagram for P > 2 MPa and T > T. Include on your diagram the boundaries of the various regions and the expected orientations of the two-phase tie lines. 

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