Critical point of carbon dioxide

Donnelly and Katz19 have made a very thorough study of this system from —79° to 31°C, the critical point of carbon dioxide. The triple point of carbon dioxide is at — 56.6°C. They found that... 

Look at the phase diagram for carbon dioxide (CO2) in Figure 10-lb. If you put carbon dioxide under a pressure of 4.5 atm at a temperature of 23°C, in what phase of matter would the carbon dioxide be What are the triple point and the critical point of carbon dioxide according to the phase diagram ... 

In addition, the polymerization itself can be performed in subcrit-ical carbon dioxide as solvent. Not that the critical point of carbon dioxide is at 37°C and 73 bar. 

In general, volatile components can be extracted from the dried raw materials under conditions close to the critical point of carbon dioxide. Temperatures should be within the range of 32 to 60°C. However, some heat-sensitive components may decompose, even below this range. Extraction pressures should be between 74 and 120 bar, since at higher pressures the increased solvent power of CO2 also increases the solubility of unwanted components. The yields obtained by SFE are very similar to those found by steam distillation. However, even under mild extraction conditions, some small amount of cuticular waxes is co-extracted with the volatiles. The major constituents of the waxes are -paraffins, ranging from C25 to... 

Figure 5 shows a typical solubility vs. pressure curve for a nonvolatile solute such as naphthalene in a solvent such as carbon dioxide. As the pressure increases from a low value, the solubility first decreases and then dramatically increases before it attains an approximately constant value. The dramatic increase in solubility occurs near the critical point of carbon dioxide, so that a small change in pressure near the critical point can lead to large changes in solubility. Large supersaturations can therefore be generated by small changes in pressure near the critical point of the solvent. 

The values that Dr Andrews reported for the critical point of carbon dioxide, 30.92°C and 74.0 bar, are in close agreement to presently accepted values, 31.1°C and 73.8 bar. 

The critical point of carbon dioxide is near ambient conditions 7 = 304.2 V, pc = 72.9 atm.The SCCO2 is the most commonly used supercritical fluid, because it is easier to use as compared to other substances it is not corrosive as SCNH3, not flammable (the SCN2O has better solvent properties as SCCO2, but it is explosive), does not damage the... 

Critical point of carbon dioxide 73.8 bar Critical point of sulfur dioxide 78.6 bar X Molar fraction of carbon dioxide in the liquid phase y Molar fraction of carbon dioxide in the vapor phase tf Temperature... 

The structure of biomaterials deforms when dried. This is due to the surface tension of water during drying. In order to avoid this phenomenon, evaporation can be done at a critical point where there is no surface tension observed. Unfortunately, it is not practical because the critical point of water is 374°C at 22.06 MPa. In contrast, the critical point of carbon dioxide is 31.4°C at 7.375 MPa. 

Y Garrabos, R Tufeu, B Le Neindre, G Zalczer, D Beysens. Rayleigh and Raman scattering near the critical point of carbon dioxide. J Chem Phys 72 4637, 1980. 

P Etesse, AM Ward, WV House, R Kobayashi. Spin-lattice relaxation and selfdiffusion near the critical point of carbon dioxide. Physica B 183 45, 1993. 

By definition, an SCF is a gas compressed to a pressure greater than its critical pressure (Pc) and heated to a temperature higher than its critical temperature (Tc). For example, the critical point for carbon dioxide occurs at a pressure of 73.8 bar and a temperature of 31.1 °C, as depicted in Figure 3.7. In this phase, regardless of the pressure applied, the fluid will not transcend to the liquid phase. 

Carbon dioxide has a vapour pressure of about 50 bar at room temperature. It is therefore more difficult to handle, and it can only be pumped as a liquid when it is cooled down to sub-ambient temperatures. However, carbon dioxide is non-flammable and non-toxic, which makes it very attractive from a practical point of view. Also, the critical properties of carbon dioxide are very mild. 

The isotherm EFGH at 21.5 C shows a similar behaviour except that now the liquefication commences at a higher pressure and the horizontal portion FG, representing decrease in volume, becomes smaller. At still higher temperatures, the horizontal portion of the curve becomes shorter and shorter until at 31.1 C it reduces just to a point (represented by X). At this temperature, therefore, the gas passes into liquid state imperceptibly. Above 31.1 C, the isotherm is continuous. There is no evidence of liquefaction at all. Andrews concluded that if the temperature of carbon dioxide is above 31.1 C, it cannot be- liquefied, no matter how high the pressure may be. He called 31.1 C as the critical temperature of carbon dioxide. Since then, other gases have been... 

Recently, systems suppliers have developed subcritical cleaning systems (i.e., systems operating below the critical point for carbon dioxide) that are fabricated using low cost alloy steel coated on the inside with an appropriate plastic to protect it from attack. This approach promises to significantly reduce system costs and enhance the competitive position of carbon dioxide cleaning in less demanding applications. 

The triple point of carbon dioxide is at -56.7°C and 518 kPa. The critical point is at 31.1°C and 7.38 x 10 kPa. Vapor pressure above solid carbon dioxide is 101.3 kPa at -78.5°C. Solid carbon dioxide is denser than liquid carbon dioxide. Sketch the phase diagram. 

The densities as well as critical properties and the triple point of carbon dioxide were obtained from Chemistry Webbook, NIST (http //webbook.nist.gov/chemistry/fluid/, July 2005). 

The phase diagram of carbon dioxide (Figure 5.8) is generally similar to that of water, with one important exception— Ihe slope of the curve between solid and liquid is positive. In fact, this holds true for almost all other substances. Water behaves differently because ice is less dense than liquid water. The triple point of carbon dioxide is at 5.16 bar and 56.6°C. The aitical pressure of carbon dioxide is 73.6 bar and the critical temperature is 31.0°C—just above standard room temperature. 

Figure 1.3 Phase behaviour of carbon dioxide/water system at temperatures between the critical hydrate temperature and the upper critical solution temperature, (a) Typical pressure/composi-tion diagram for carbon dioxide/water (a Class B2 system) at temperatures below the critical temperature of carbon dioxide but above the critical hydrate formation temperature. Data for arms B and C are shown in (b) and (c) respectively, (b) Solubility of liquid CO2 in water as a function of temperature and pressure (arm C in (a)), (c) Solubility of water in liquid CO2 as a function of temperature and pressure (arm B in (a)), (d) The three phase pressure curve compared with the vapour pressure curve of carbon dioxide showing the critical locus CsU (i.e. locus of points such as C on (e) where vapour properties merge with those of solvent-rich liquid). (Data reference [75].) (e) Detail of the isothermal pressure/composition diagram at 25°C (on left) and at temperature between Tc and Tu (on right). Subscripts 1 and 2 denote water-rich and C02-rich phase. Critical point C is shown as blocked-in circle. (Data reference for (b) and (c) is [81].)...

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