Carbon dioxide, pressure-temperature phase

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

Carbon dioxide pressure-temperature phase diagram. 

It is impossible to have liquid carbon dioxide at temperatures above 31°C, no matter how much pressure is applied. Even at pressures as high as 1000 atm, carbon dioxide gas does not liquefy at 35 or 40°C. This behavior is typical of all substances. There is a temperature, called the critical temperature, above which the liquid phase of a pure substance cannot exist The pressure that must be applied to cause condensation at that temperature is called the critical pressure. Quite simply, the critical pressure is the vapor pressure of the liquid at the critical temperature. 

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. 

As the area is diminished below some thousands of sq. A., where the molecules cover only a small fraction of the surface, the surface pressure rapidly becomes much smaller than that of a perfect gas, and in the four acids with the longest chains becomes constant over a considerable region. The curves are indeed a very faithful reproduction of Andrews s curves for the relation between pressure and volume, for carbon dioxide, at temperatures near the critical. The horizontal regions in the curves correspond to the vapour pressure of liquids, and indicate the presence of an equilibrium between two surface phases, the vapour film, and islands of liquid, coherent film. 

Carbon dioxide pressures from 1 to 75 atmospheres and temperatures from 0° to 80°C were assessed. Since the liquid phase is in contact with the coal and is responsible for mineral matter dissolution, its composition would be expected to have a bearing on ash reduction in the coal. The solubility of CO2 in the liquid phase increases as the CO2 pressure increases and may be related to swelling of coal structure (although not linearly). Table I summarizes a set of experiments directed toward determining the effect of aqueous phase concentration of CO2 on the treated product ash content for Pittsburgh Seam coal. At low CO2 concentrations,... 

The mechanism of a sublimation process can be described with reference to the pressure-temperature phase diagram in Figure 8.28. The significance of the P-T diagram applied to one-component systems has already been discussed in section 4.2. The phase diagram is divided into three regions, solid, liquid and vapour, by the sublimation, vaporization and fusion curves. These three curves intersect at the triple point T. The position of the triple point in the diagram is of the utmost importance if it occurs at a pressure above atmospheric, the solid caimot melt under normal atmospheric conditions, and true sublimation, i.e. solid vapour, is easy to achieve. The triple point for carbon dioxide, for... 

In Fig. 1.5, two projections of the phase behavior of carbon dioxide are shown. In the pressure-temperature phase diagram (Fig. 1.5a), the boihng hne is observed, which separates the vapor and liquid regions and ends in the critical point At the critical point the densities of the equilibrium liquid phase and the saturat-... 

Phase Chemistry. The miscibility of supercritical carbon dioxide with solutions of polymer and organic solvent, in general, depends upon the carbon dioxide level, temperature, pressure, polymer level, and compatibility of the polymer with the solvent. As the carbon dioxide level in the ternary mixture is increased (for a fixed ratio of polymer to solvent), higher pressure is required to obtain complete miscibility, and it is found that there is a narrower temperature range for the region of miscibility (12,13), i.e., a single-phase liquid solution (L). This is illustrated schematically in Figure 4. The... 

Alvarez-Zepeda A (1991) Part I. A thermodynamic study of acetonitrile + water mixturesin reversed-phase liquid chromatography. Part n. A gas chromatographic study of the effect of temperature and column pressure on solute retention on a bonded phase using helium and carbon dioxide as mobile phases. Ph.D. thesis, Georgetown University, Washington DC... 

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. 

The cooled mixture is transferred to a 3-1. separatory funnel, and the ethylene dichloride layer is removed. The aqueous phase is extracted three times with a total of about 500 ml. of ether. The ether and ethylene chloride solutions are combined and washed with three 100-ml. portions of saturated aqueous sodium carbonate solution, which is added cautiously at first to avoid too rapid evolution of carbon dioxide. The non-aqueous solution is then dried over anhydrous sodium carbonate, the solvents are distilled, and the remaining liquid is transferred to a Claisen flask and distilled from an oil bath under reduced pressure (Note 5). The aldehyde boils at 78° at 2 mm. there is very little fore-run and very little residue. The yield of crude 2-pyrrolealdehyde is 85-90 g. (89-95%), as an almost water-white liquid which soon crystallizes. A sample dried on a clay plate melts at 35 0°. The crude product is purified by dissolving in boiling petroleum ether (b.p. 40-60°), in the ratio of 1 g. of crude 2-pyrrolealdehyde to 25 ml. of solvent, and cooling the solution slowly to room temperature, followed by refrigeration for a few hours. The pure aldehyde is obtained from the crude in approximately 85% recovery. The over-all yield from pyrrole is 78-79% of pure 2-pyrrolealdehyde, m.p. 44 5°. 

K-factors for vapor-liquid equilibrium ratios are usually associated with various hydrocarbons and some common impurities as nitrogen, carbon dioxide, and hydrogen sulfide [48]. The K-factor is the equilibrium ratio of the mole fraction of a component in the vapor phase divided by the mole fraction of the same component in the liquid phase. K is generally considered a function of the mixture composition in which a specific component occurs, plus the temperature and pressure of the system at equilibrium. 

Consider an experiment in which liquid carbon dioxide is introduced into an otherwise evacuated glass tube, which is then sealed (Figure 9.4, p. 232). At 0°C, the pressure above the liquid is 34 atm, the equilibrium vapor pressure of C02(Z) at that temperature. As the tube is heated, some of the liquid is converted to vapor, and the pressure rises, to 44 atm at 10°C and 56 atm at 20°C. Nothing spectacular happens (unless there happens to be a weak spot in the tube) until 31°C is reached, where the vapor pressure is 73 atm. Suddenly, as the temperature goes above 31°C, the meniscus between the liquid and vapor disappears The tube now contains only one phase. 

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