Carbon dioxide critical values

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,... 

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 is often used in SFC because its critical pressure and temperature are relatively easy to obtain. The critical point for this compound has the following values Tc = 31 C and Pc = 7 400 kPa (Fig. 6.1). Above these conditions, the supercritical domain is obtained. 

By using data from the small-scale extractions of dichlorophenol as an example, the maximum theoretical amount that can be collected at —76 °C can be calculated to be 77. Actual experimental values show recovery to be about 62 for the three small-scale supercritical fluid carbon dioxide extractions of dichlorophenol. These data support the suggestion that the vapor pressure of the compound being trapped is an extremely critical physical constant when large volumes of C02 relative to the aqueous sample volume are being used for the extraction process. 

The diffusion coefficient as defined by Fick s law, Eqn. (3.4-3), is a molecular parameter and is usually reported as an infinite-dilution, binary-diffusion coefficient. In mass-transfer work, it appears in the Schmidt- and in the Sherwood numbers. These two quantities, Sc and Sh, are strongly affected by pressure and whether the conditions are near the critical state of the solvent or not. As we saw before, the Schmidt and Prandtl numbers theoretically take large values as the critical point of the solvent is approached. Mass-transfer in high-pressure operations is done by extraction or leaching with a dense gas, neat or modified with an entrainer. In dense-gas extraction, the fluid of choice is carbon dioxide, hence many diffusional data relate to carbon dioxide at conditions above its critical point (73.8 bar, 31°C) In general, the order of magnitude of the diffusivity depends on the type of solvent in which diffusion occurs. Middleman [18] reports some of the following data for diffusion. 

We focus next on the thermal conductivity of gases at high pressure. At ambient pressure and temperature the thermal conductivity is in the range of 0.01 to 0.025 W/(K m), except for hydrogen and helium that can have higher values, around 0.18 W/(K m). The change of thermal conductivity around the critical state is seen on Fig 3.4-4 for carbon dioxide. Near the... 

The above equations were obtained from twenty non-polar gases including inert gases, hydrocarbons and carbon dioxide (but not hydrogen and helium). Hence, possible errors can be as large as 20%. The maximum pressure corresponds to a reduced density of 2.8. In the above equations, Zc represents the critical compressibility factor. The value of gamma is calculated using Eqn. (3.4-26). 

A substance with favorable critical parameter values and that best matches the other aforementioned criteria is carbon dioxide (C02). The critical temperature of C02 is +31.3°C, which is especially important for thermally unstable analytes, and its critical pressure of 72.9 bar (1 bar = 105 Pa) is easy to obtain under laboratory conditions. Moreover, C02 is nonflammable, nontoxic, does not pose any additional, serious threat to the environment, and is relatively inexpensive. For on-line solutions, it is important that C02 be compatible with most chromatographic detectors. 

The liquid-gas equilibrium line terminates at a point known as the critical point. The temperature and pressure that define the critical point are known as the critical temperature and the critical pressure. For example, nitrous oxide has a critical temperature of 36°C and a critical pressure of 72.45 bar (1051 psi). When the temperature and pressure exceed these critical values, the system becomes a supercritical fluid. Supercritical fluids have the flow properties of gases but densities similar to liquids, and supercritical fluids have no surface tension. Therefore, supercritical fluids are terrific solvents. For example, supercritical carbon dioxide is an excellent solvent for extracting caffeine from coffee without resorting to more toxic organic solvents like dichloromethane. 

Additional studies by Menon (32) have indicated the p can occur at lower pressures then those predicted by Equation 1 depending on the pore structure associated with the adsorbent. Empirically, adsorbents possessing microporosity exhibit a p that is 0.6-0.8 of the value predicted by Equation 1. This observation is attributed to the overlapping of potential fields in the adsorbent pores, thereby enhancing sorption of the gas at lower pressures. Experimental studies by Ozawa (33) have verified this trend as shown in Figure 4 for the C02/activated carbon system. Here the adsorption maxima for the gas occurs at a lower pressure than the critical pressure of carbon dioxide. It should also be noted that the amount of gas adsorbed is decreased at higher reduced temperatures and that additional compression is required to reach a defined adsorption maxima (i.e., at very high values of T it is sometimes difficult to discern a well-defined adsorption maximum). The above trend has also been found for other adsorbent/adsorbate systems, such as silica gel/C02. 

The carbon di oxi de/lemon oil P-x behavior shown in Figures 4, 5, and 6 is typical of binary carbon dioxide hydrocarbon systems, such as those containing heptane (Im and Kurata, VO, decane (Kulkarni et al., 1 2), or benzene (Gupta et al., 1 3). Our lemon oil samples contained in excess of 64 mole % limonene so we modeled our data as a reduced binary of limonene and carbon dioxide. The Peng-Robinson (6) equation was used, with critical temperatures, critical pressures, and acentric factors obtained from Daubert and Danner (J 4), and Reid et al. (J 5). For carbon dioxide, u> - 0.225 for limonene, u - 0.327, Tc = 656.4 K, Pc = 2.75 MPa. It was necessary to vary the interaction parameter with temperature in order to correlate the data satisfactorily. The values of d 1 2 are 0.1135 at 303 K, 0.1129 at 308 K, and 0.1013 at 313 K. Comparisons of calculated and experimental results are given in Figures 4, 5, and 6. 

The problem in choosing a polar SF for solubilizing polar solutes is that both the boiling point and the critical point are elevated by the polarity since supercritical operation requires T>TC, high temperatures are mandated in such cases. Thus for ammonia, the critical temperature, Tc = 132°C, must be exceeded for practical operation. A widely used compromise between polar substances with high values of Tc and nonpolar substances with low values of 5liq is carbon dioxide, for which Tc = 31°C and 5Uq = 8.9. Other factors involved in choosing an SF phase are elaborated by Schoen-makers et al. [191. 

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