Triple 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... 

Carbon dioxide plays a central role in the CNG process both as a pure component and in mixture with other compounds. The triple point of carbon dioxide is referred to frequently in the following discussion it is the unique temperature and pressure at which solid, liquid, and vapor phases of carbon dioxide can exist at equilibrium (-56.6°C, 5.1 atm). The carbon dioxide triple point is shown in Figure 2, a phase diagram for carbon dioxide. 

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 11.41) is generally similar to that of water, with one important exception—the 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 5.2 atm and -5TC. 

The triple point of carbon dioxide is at -57°C and 5.1 atm (Figure 11.12 top). Therefore, the solid sublimes if warmed at any pressure below 5.1 atm. This is why solid carbon dioxide sublimes at normal atmospheric pressure (1 atm). Above 5.1 atm, however, the solid melts if warmed. Sulfur has a more complicated phase diagram (Figure 11.12 bottom). It displays three triple points, one of them involving two different solid forms of sulfur (called rhombic sulfur and monoclinic sulfur), as well as the vapor. 

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 ... 

The dew point must be warmer than -56.6°C to permit use of liquid carbon dioxide absorbent because pure liquid carbon dioxide cannot exist below the triple point. The carbon dioxide partial pressure, i.e., gas phase CO2 mol fraction times total pressure, of synthesis gas mixtures with -56.6°C dew points is plotted versus synthesis gas pressure in Figure 4. Increasing the H2 CO ratio at fixed total pressure decreases the carbon dioxide partial pressure required for a -56.6°C dew point. Liquid carbon dioxide can be used to absorb sulfur molecules for any combination of gas pressure and carbon dioxide partial pressure which lies above the curves of Figure 4. 

Comparing the phase diagrams for water and carbon dioxide, we notice some interesting things. Even if it were not labeled, we could approximate the location of the 1 atm mark for either diagram. We know that at atmospheric pressure, water exists in all three phases at different temperatures. Thus, we know that the 1 atmosphere mark must be above the triple point. Since carbon dioxide (dry ice) sublimes (changes from solid to gas) at one atmosphere, we know that the triple point must be above the 1 atm mark. 

The phase diagram for carbon dioxide (Fig. 10.52) differs from that for water. The solid/ liquid line has a positive slope, since solid carbon dioxide is more dense than liquid carbon dioxide. The triple point for carbon dioxide occurs at 5.1 atm and -56.6X, and the critical point occurs at 72.8 atm and 31X. At a pressure of 1 atm, solid carbon dioxide... 

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... 

At a pressure below the triple point pressure, the solid can change directly to a gas (sublimation) and the gas can change directly to a solid, as in the formation of carbon dioxide snow from the released gas. 

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. 

When we look at the critical states and triple points of other gases, we find the situation shown in table 4.34. The liquid phase exists only when the pressure is between the critical and the triple-point pressures. If we cool down hydrogen, helium or water at room temperature and pressure, we will get liquids before we get solids. But if we cool down CO2 from room temperature and pressure, we get dry ice rather than liquid carbonic to obtain liquid carbon dioxide we have to raise the pressure to at least 5.1 atm to exceed the triple-point pressure. The melting point is not as sensitive to the pressure as the boiling point, which is stated usually for a room pressure of 1 atm, which prevails at sea level on Earth and not in Colorado or the Himalayas. 

The phase diagram in Figure 7.1 shows the effect of temperature and pressure on the state of carbon dioxide. At the triple point, carbon dioxide can exist in the three states as a solid, a liquid or a gas by just a small perturbation. All phases are in a state of equilibrium at the triple point, which is at 5.11 bar and 56.6°C. Above 31°C, it is impossible to liquefy the gas by increased pressure this is termed the critical point. At normal temperatures and pressures carbon dioxide is a colourless gas at high concentrations it has a slightly... 

The CNG process removes sulfurous compounds, trace contaminants, and carbon dioxide from medium to high pressure gas streams containing substantial amounts of carbon dioxide. Process features include 1) absorption of sulfurous compounds and trace contaminants with pure liquid carbon dioxide, 2) regeneration of pure carbon dioxide with simultaneous concentration of hydrogen sulfide and trace contaminants by triple-point crystallization, and 3) absorption of carbon dioxide with a slurry of organic liquid containing solid carbon dioxide. These process features utilize unique properties of carbon dioxide, and enable small driving forces for heat and mass transfer, small absorbent flows, and relatively small process equipment. 

The CNG acid gas removal process is distinguished from existing AGR processes by three features. The first feature is the use of pure liquid carbon dioxide as absorbent for sulfurous compounds the second feature is the use of triple-point crystallization to separate pure carbon dioxide from sulfurous compounds the third feature is the use of a liquid-solid slurry to absorb carbon dioxide below the triple point temperature of carbon dioxide. Pure liquid carbon dioxide is a uniquely effective absorbent for sulfurous compounds and trace contaminants triple-point crystallization economically produces pure carbon dioxide and concentrated hydrogen sulfide for bulk carbon dioxide absorption the slurry absorbent diminishes absorbent flow and limits the carbon dioxide absorber temperature rise to an acceptable low value. The sequence of gas treatment is shown in Figure 1, an overview of the CNG acid gas removal process. 

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. 

The process features of carbon dioxide triple-point crystallization and slurry absorption of carbon dioxide have been demonstrated with the first generation bench-scale apparatus. Current efforts are focused on the design and construction of an improved version of the carbon dioxide triple-point crystallizer in cooperation with the U S Department of Energy. Future efforts are planned to design and construct absorption units to study multi-stage slurry absorption of carbon dioxide, and the more conventional gas-liquid absorption of sulfuruous compounds with liquid carbon dioxide. 

The low-pressure phase diagram of carbon dioxide, shown in Fig. 10, is different from that of water in a number of respects. Carbon dioxide sublimates at l.Oatm, leading to solid C02 being called dry ice. The triple point is at 5.1 atm... 

Siiblinalion lemperalure. (At pressures belov the triple-point pressure of 518 kPa, carbon dioxide exists as s solid or gas. Also, tbe freezing-point temperature of carbon dioxide is the triple-point temperature of -56.5 C.)... 

The density data of carbon dioxide was obtained from the National Institute of Standards and Technology website for thermophysical properties (http //webbook.nist.gov/chemistry, July 2005). For CO2, this source employs Span and Wagner (1996) for the calculation of PVT properties. For the convenience of readers, pure CO2 density, covering both one phase and two phases, is provided in Appendix A (Table A. 1-Table A.3 and Figure A. 1) for temperatures in the range from the triple point 216.59 (the trible point of CO2) to 1073.15 K, and pressures in the range of 1 to 2000 bar. 

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