Carbon dioxide critical point

To prepare scanning electron microscopic specimens, the cells were fixed in 2.5% glu-taraldehyde for 30 min and in 1% osmium tetraoxide for 30 min. After dehydration with serial ethanol, the cells were treated with the carbon dioxide critical point drying technique followed by a gold-platinum coating in vacuum evaporation and examined by a scanning electron microscope (Japan Electric Co., Model TSM 840). 

Fig. 3.24 Test of the tensile strength hysteresis of hysteresis (Everett and Burgess ). TjT, is plotted against — Tq/Po where is the critical temperature and p.. the critical pressure, of the bulk adsorptive Tq is the tensile strength calculated from the lower closure point of the hysteresis loop. C), benzene O. xenon , 2-2 dimethyl benzene . nitrogen , 2,2,4-trimethylpentane , carbon dioxide 4 n-hexane. The lowest line was calculated from the van der Waals equation, the middle line from the van der Waals equation as modified by Guggenheim, and the upper line from the Berthelot equation. (Courtesy Everett.)...

Solvent Strength of Pure Fluids. The density of a pure fluid is extremely sensitive to pressure and temperature near the critical point, where the reduced pressure, P, equals the reduced temperature, =1. This is shown for pure carbon dioxide in Figure 2. Consider the simple case of the solubihty of a soHd in this fluid. At ambient conditions, the density of the fluid is 0.002 g/cm. Thus the solubiUty of a soHd in the gas is low and is given by the vapor pressure over the total pressure. The solubiUties of Hquids are similar. At the critical point, the density of CO2 is 0.47 g/cm. This value is nearly comparable to that of organic Hquids. The solubiHty of a soHd can be 3—10 orders of magnitude higher in this more Hquid-like CO2. 

Some values of physical properties of CO2 appear in Table 1. An excellent pressure—enthalpy diagram (a large Mohier diagram) over 260 to 773 K and 70—20,000 kPa (10—2,900 psi) is available (1). The thermodynamic properties of saturated carbon dioxide vapor and Hquid from 178 to the critical point,... 

Some volatile fluids are used once only, and then escape into the atmosphere. Two of these are in general use, carbon dioxide and nitrogen. Both are stored as liquids under a combination of pressure and low temperature and then released when the cooling effect is required. Carbon dioxide is below its critical point at atmospheric pressure and can only exist as snow or a gas. Since both gases come from the atmosphere, there is no pollution hazard. The temperature of carbon dioxide when released will be - 78.4°C. Nitrogen will be at - 198.8°C. Water ice can also be classihed as a total loss refrigerant. 

Fig. 10. The mole fraction of carbon dioxide in saturated solutions in air at — 110°C (above the lower critical end point). The full line is the experimental curve of Webster and the dashed curves are 1, an ideal gas mixture 2, an ideal gas mixture with Poynting s correction and 3, the solubility calculated from Eq. 8 and the principle of corresponding states.

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... 

Braune and Strassman8 measured the concentration of iodine in gaseous carbon dioxide at pressures up to 50 atm from 32° to 98°C. They passed the carbon dioxide over an excess of solid iodine and analyzed the effluent mixtures. Their pressures were too low to find the saturation vapor pressures or to show whether or not critical end points were formed. 

Raveau now calculated the values of p, v from van der Waals equation, plotted the logarithms, and compared the diagram with a similar one drawn from the experimental results. The results showed that the diagrams could not be made to fit in the ease of carbon-dioxide and acetylene, the divergencies being very marked near the critical point. 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

Liquid carbon dioxide is usually stored under 20 bar pressure at — 18°C. Compression and cooling of the gas between the temperature limits at the triple point and the critical point will cause it to liquefy. The triple point is the pressure temperature combination at which carbon dioxide can exist simultaneously as gas, liquid and solid. Above the critical temperature point of 31°C it is impossible to liquefy the gas by increasing the pressure above the critical pressure of 73 bar. Reduction in the temperature and pressure of liquid below the triple point causes the liquid to disappear, leaving only gas and solid. (Solid carbon dioxide is also available for cryogenic work and at —78°C the solid sublimes at atmospheric 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. 

Crystallization of CaCOj is highly dependent on nucleation condition. The precipitation of CaCOj in the absence or the presence of the G4.5 PAMAM dendrimer was carried out by a carbonate diffusion method similar to the method described by Addadi et al. [35]. A solution of the dendrimer with calcium chloride in 200 ml of distilled water was adjusted to pH 8.5 with aqueous NHj, and then placed in a closed desiccator containing crushed ammonium carbonate (Fig. 5). Carbon dioxide was introduced to the solution via vapor diffusion. The critical point of the appearance in the turbidity of the solution was observed at around 5 min. These solutions were kept at 30 °C under N2 for one day. The crys-... 

Liquid carbon dioxide is usually stored under 20 bar pressure at -18°C. Compression and cooling of the gas between the temperature limits at the triple point and the critic point will cause it... 

A supercritical fluid exhibits physical-chemical properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those in normal gaseous states. In the vicinity of the critical point the diffusion coefficient is more than 10 times that of a liquid. Carbon dioxide can be compressed readily to form a liquid. Under typical borehole conditions, carbon dioxide is a supercritical fluid. 

This sort of process works because the solubility of carbon dioxide in liquids increases with increasing pressure. This is true for all gases but carbon dioxide is much more soluble in liquids than the permanent gases because it is nearer its critical point. 

Transfer samples from 100% EtOH to the critical point dryer to complete dehydration with liquid carbon dioxide. 

Discs were critically point dried using liquid carbon dioxide. 

---