Carbon density/pressure diagram

FIG. 2-8 Temperatnre-entropy diagram for carbon monoxide. Pressure P, in atmospheres density p, in grams per cubic enthalpy H, in joules per gram. (From Must and Stewart, NBS Tech. Note 202, 1963.)... 

Fig. 21.2. Projections of the phase diagram of carbon dioxide A) in the pressure-temperature plane and B) in the density-pressure plane [8, 9].

Figure 2 Density-pressure projection of the phase diagram for pure carbon dioxide (From Ref. 53).

Figure 6.4 On the left is a phase diagram for carbon dioxide. Broken lines indicate isotherm crossing at either constant pressure or density. On the right is illustrated the change in solubility of naphthalene as a function of temperature and pressure.

The melting point of carbon dioxide increases with increasing pressure, since the solid-liquid equilibrium line on its phase diagram slopes up and to the right. If the pressure on a sample of liquid carbon dioxide is increased at constant temperature, causing the molecules to get closer together, the liquid will solidify. This indicates that solid carbon dioxide has a higher density than the liquid phase. This is true for most substances. The notable exception is water. 

Figure 8, the carbon phase diagram, forms a basis for discussing the processes involved. Ideal graphite has a density of 2.2 and diamond, 3.52, so 1 ml of graphite becomes 0.63 ml of diamond, a relatively large change. Diamond is favored to form at pressures and temperatures where it is stable, but the carbon atoms must be in the proper environment, particularly at the milder conditions. 

Figure 11.2. Pressure-density diagram for carbon dioxide. Reprinted with permission from Chem. Eng. News 1982, 60(12), 46. Copyright 1982, American Chemical Society.

The phase diagram for carbon dioxide. The liquid state does not exist at a pressure of 1 atm. The solid/liquid line has a positive slope, since the density of solid carbon dioxide is greater than that of liquid carbon dioxide. 

Figure 5. Phase diagram of supercritical and near critical carbon dioxide, showing the density and pressure relationship. The shaded area represents the supercritical state.

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. 

Figure 7.2. Isotherm diagram for carbon dioxide. Broken lines indicate crossing of isotherms at either constant pressure or density.

The third instrumental approach is the use of supercritical fluid extraction (SEE). A supercritical fluid is a substance at a temperature and pressure above the critical point for the substance. (You may want to review phase diagrams and the critical point on the phase diagram in your general chemistry text.) Supercritical fluids are more dense and viscous than the gas phase of the substance but not as dense and viscous as the hquid phase. The relatively high density (compared with the gas phase) of a supercritical fluid allows these fluids to dissolve nonvolatile organic molecules. Carbon dioxide, CO2, has a critical temperature of 31.3°C and a critical pressure of 72.9 atm this temperature and pressure are readily attainable, making supercritical CO2 easy to form. Supercritical CO2 dissolves many organic compormds, so it can replace a variety of common solvents supercritical... 

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