Supercritical fluid separations density-pressure diagram

In the subsurface the density of a gas increases with depth, despite increasing temperature, because of the pressure-induced compression. When a fluid s critical temperature (Tc) and pressure (pc) are exceeded there are no longer separate gas and liquid phases only a single supercritical fluid can exist. For methane Tc = -82.6 °C and pc = 4.6 MPa, whereas for carbon dioxide the corresponding values are -31.0°C and 7.4MPa (a typical phase diagram is shown in Fig. 4.28). A supercritical fluid has a much higher density than a gas and many of its properties are intermediate between those of a gas and a liquid. Consequently, supercritical methane and carbon dioxide are potentially excellent solvents for oil. 

When a fluid substance such as carbon dioxide or water is heated under pressure, the density of the liquid phase will decrease as the temperature is increased, whereas the density of the gas phase will increase with increased pressure. If a temperature-pressure phase diagram is constructed, and if we move upward along the curve separating the liquid phase from the gas phase, a temperature and pressure will be reached where the densities of the liquid and gas phases become identical and there will be no distinction between the gas and liquid phases. This point on this phase diagram is known as the critical point, and a supercritical fluid is any fluid that is at a temperature and pressure greater than those at the critical point. The critical temperature and pressure will vary according to the chemical structure of the fluid. For example, carbon dioxide (CO ) has a critical temperature of 304 K and a critical pressure of 74 bar, whereas the critical temperature and pressure of water are 647 K and 221 bar, respectively (Williams et al, 2002). Above the critical temperature, a gas cannot be liquefied by pressure. 

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