Carbon dioxide phase diagrams

The triple-point crystallization of carbon dioxide is illustrated in Figure 7, which shows a schematic carbon dioxide phase diagram expanded about the triple-point and a closed-cycle triple-point crystallizer operating with pure carbon dioxide. The operation of this closed-cycle unit is identical to that of a unit in the stripping section of a continous crystallizer cascade, except that in the cascade vapor would pass to the unit above, and liquid would pass to the unit below. 

Extractions or extractive distillations with supercritical solvent need to be performed at as high as possible a solubility of oil in the extract or vapor phase in order to reduce the solvent or carrier gas requirement. From our lemon oil-carbon dioxide phase diagrams, it appears that the highest practical solubility level is 0.9 mole % (2.8 wt ) essential oil. This is obtainable at 313 K. At lower temperature, sensitivity of solubility to pressure requires that solubility be lower (e.g., 0.3 mole % at 308 K). 

Capacity factor, 11 Capillary columns, 110, 118-121 Carbon dioxide phase diagram, 65, 251, 252... 

The carbon dioxide phase diagram (see Figure 7.8) has three phases gaseous, liquid, and solid. The triple point (pressure 5.1 atm, temperature -56.7 °C) is defined as the temperature and pressure where three phases (gas, liquid, and solid) can exist simultaneously in thermodynamic equilibrium. 

Solubility diagrams were prepared for the phases that separated in the lemon oil extractions performed in this study with carbon dioxide. Such diagrams can serve only as-guides, since solubility is composition-dependent and is a function of the amount extracted. The data are shown in Figures 4, 5, and 6 at 303, 308, and 313 K, respect vely. 

The locations of the tietriangle and biaodal curves ia the phase diagram depead oa the molecular stmctures of the amphiphile and oil, on the concentration of cosurfactant and/or electrolyte if either of these components is added, and on the temperature (and, especially for compressible oils such as propane or carbon dioxide, on the pressure (29,30)). Unfortunately for the laboratory worker, only by measuriag (or correcdy estimatiag) the compositions of T, Af, and B can one be certain whether a certain pair of Hquid layers are a microemulsion and conjugate aqueous phase, a microemulsion and oleic phase, or simply a pair of aqueous and oleic phases. 

Phase Behavior. One of the pioneering works detailing the phase behavior of ternary systems of carbon dioxide was presented ia the early 1950s (12) and consists of a compendium of the solubiHties of over 260 compounds ia Hquid (21—26°C) carbon dioxide. This work contains 268 phase diagrams for ternary systems. Although the data reported are for Hquid CO2 at its vapor pressure, they yield a first approximation to solubiHties that may be encountered ia the supercritical region. Various additional sources of data are also available (1,4,7,13). 

FIGURE 8-7 The phase diagram for carbon dioxide (not to scale). The liquid can exist only at pressures above 5.1 atm. Note the slope of the boundary between the solid and liquid phases it shows that the freezing point rises as pressure is applied. 

Self-Test 8.4A From the phase diagram for carbon dioxide (Fig. 8.7), predict which is more dense, the solid or the liquid phase. Explain your conclusion. 

Self-Test 8.5A The phase diagram for carbon dioxide is shown in Fig. 8.7. Describe the physical states and phase changes of carbon dioxide as it is heated at 2 atm from — 155°C to 25°C. 

All phase diagrams share the ten common features listed above. However, the detailed appearance of a phase diagram is different for each substance, as determined by the strength of the intermolecular interactions for that substance. Figure 11-40 shows two examples, the phase diagrams for molecular nitrogen and for carbon dioxide. Both these substances are gases under normal conditions. Unlike H2 O, whose triple point lies close to 298 K,... 

N2 and CO2 have triple points that are well below room temperature. Although both are gases at room temperature and pressure, they behave differently when cooled at P = 1 atm. Molecular nitrogen liquefies at 77.4 K and then solidifies at 63.3 K, whereas carbon dioxide condenses directly to the solid phase at 195 K. This difference in behavior arises because the triple point of CO2, unlike the triple points of H2 O and N2, occurs at a pressure greater than one atmosphere. The phase diagram of CO2 shows that at a pressure of one atmosphere, there is no temperature at which the liquid phase is stable. 

Phase diagrams for molecular nitrogen and carbon dioxide, two substances that are gases at room temperature and pressure. 

The phase diagrams in Figures 11-39 and 11-40 do not show critical points, because the critical points of water, carbon dioxide and nitrogen occur at higher pressures than those shown on these diagrams. The critical point of water is P = 218 atm, T = 647 K that of CO2 is P = 72.9, T — 304 K and that of N2 is P = 33.5 atm, P = 126 K. 

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.

Figure 9.1. Carbon dioxide pressure-temperature phase diagram adapted from McHugh and Krukonis (1994).

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 5.5 Phase diagram of a system that sublimes at room temperature phase diagram of carbon dioxide. (Note that the y-axis here is logarithmic)...

What does the phase diagram above show to be the normal boiling point of carbon dioxide ... 

E) Normal means 1 atm (760 mm Hg) pressure. Boiling occurs at a temperature at which the substance s vapor pressure becomes equal to the pressure above its surface. On this phase diagram, at 1 atm pressure, there is no intercept on a line separating the liquid phase from the gas phase. In other words, carbon dioxide cannot be liquefied at 1 atm pressure. It is in the liquid form only under very high pressures. At 1.0 atm pressure, solid C02 will sublime — that is, go directly to the gas phase. 

The discovery of supercritical fluids occurred in 1879, when Thomas Andrews actually described the supercritical state and used the term critical point. A supercritical fluid is a material above its critical point. It is not a gas, or a liquid, although it is sometimes referred to as a dense gas. It is a separate state of matter defined as all matter by both its temperature and pressure. Designation of common states in liquids, solids and gases, assume standard pressure and temperature conditions, or STP, which is atmospheric pressure and 0°C. Supercritical fluids generally exist at conditions above atmospheric pressure and at an elevated temperature. Figure 16.1 shows the typical phase diagram for carbon dioxide, the most commonly used supercritical fluid [1]. 

Fig. 16.1. Phase diagram for carbon dioxide critical temperature 31.3°C critical pressure 72.9 atm.

Brady et al. [52] have discussed pressure-temperature phase diagrams for carbon dioxide polychlorobiphenyls and examined the rate process of desorption from soils. Supercritical carbon dioxide was used to extract polychlorobiphenyls and DDT and Toxaphene from contaminated soils. 

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