Carbon dioxide critical behavior

Catchpole-Kinp examined binaiy diffusion data of near-critical fluids in the reduced density range of 1 to 2.5 and found that their data correlated with average deviations of 10 percent and a maximum deviation of 60 percent. They observed two classes of behavior. For the first, no correction fac tor was required R = 1). That class was comprised of alcohols as solvents with aromatic or ahphatic solutes, or carbon dioxide as a solvent with ahphatics except ketones as solutes, or... 

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. 

Kamath, K. and Salazar. M., The role of the critical temperature of carbon dioxide on the behavior of wells injecting hydrochloric acid into carbonate formations, in Proc. Int. Symp. Subsurface injection of Liquid Wastes, New Orleans, National Water Well Association, Dublin, OH, 1986, pp. 638-655. 

Mixtures of C02 and methanol were selected for the initial investigation of the solvatochromic behavior in supercritical fluid systems. This combination is of interest as it combines the low critical temperature and pressure of carbon dioxide with a polar, less volatile modifier. This system exhibits relatively simple Type I phase behavior and several groups have published measurements of mixture critical points (19-21). At intermediate compositions the critical pressure for this fluid is much higher than that of either pure C02 or pure methanol, reaching a maximum of approximately 2400 psi (20). 

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

Ternary Systems. As one of a series of model systems, we studied the carbon dioxide - acetone - water ternary system at 313 and 333 K. The most interesting feature of the system behavior is an extensive three-phase region at both temperatures. The three-phase region is first observed at a pressure of less than 30 bar at 313 K and approximately 35 bar at 333 K, extending up to approximately the critical pressure of the binary carbon dioxide - acetone system. Table I summarizes our experimental results for the composition of the three phases at equilibrium as a function of pressure and temperature. 

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. 

Very few experiments have been performed on vibrational dynamics in supercritical fluids (47). A few spectral line experiments, both Raman and infrared, have been conducted (48-58). While some studies show nothing unique occurring near the critical point (48,51,53), other work finds anomalous behavior, such as significant line broadening in the vicinity of the critical point (52,54-60). Troe and coworkers examined the excited electronic state vibrational relaxation of azulene in supercritical ethane and propane (61-64). Relaxation rates of azulene in propane along a near-critical isotherm show the three-region dependence on density, as does the shift in the electronic absorption frequency. Their relaxation experiments in supercritical carbon dioxide, xenon, and ethane were done farther from the critical point, and the three-region behavior was not observed. The measured density dependence of vibrational relaxation in these fluids was... 

In this chapter, we describe the density- and temperature-dependent behavior of the vibrational lifetime (TO of the asymmetric CO stretching mode of W(CO)6( 2000 cm-1) in supercritical ethane, fluoroform, and carbon dioxide (C02). The studies are performed from low density (well below the critical density) to high density (well above the critical density) at two temperatures one close to the critical temperature and one significantly above the critical temperature (68-70). In addition, experimental results on the temperature dependence of Ti at fixed density are presented. Ti is measured using infrared (IR) pump-probe experiments. The vibrational absorption line positions as a function of density are also reported in the three solvents (68,70) at the two temperatures. 

Figure 15 shows the lifetime as a function of temperature at the critical density of carbon dioxide. With CO2 as the solvent there is no inverted region in which the lifetime becomes longer as the temperature is increased. Instead, the lifetime decreases approximately linearly. Thus the inverted behavior is not universal but is specific to the properties of the particular solvent. The fact that the nature of the temperature dependence changes fundamentally when the solvent is changed from ethane to C02 demonstrates the sensitivity of the vibrational relaxation to the details of the solvent properties. The solid line is the theoretically calculated curve. The calculation of the temperature dependence is done with no adjustable... 

Figure 15 Tj (p, T) vs. temperature for the solvent carbon dioxide at the critical density and the theoretically calculated curve. The frequency u> and the hard sphere diameters are the same as those used in the fit of the 33°C data. The theory is scaled to match the data at 33°C and the critical density, 10.6 mol/L. Unlike ethane at the critical density, there is no inverted region, and the vibrational lifetime decreases nearly linearly with temperature. The theory does not quantitatively fit the data, but it does show the correct general behavior. Most importantly, the hydrodynamic/thermodynamic theory shows the existence of the inverted region in ethane and the lack of one in carbon dioxide.

In order to obtain a commercial loading of the near-critical extractant, the extraction is sometimes carried out at enhanced pressures in the droplet regime. In such cases the liquid phase does not flow downwards as a film adhering to the packings of a column as is usually assumed, rather it falls down as a swarm of droplets. On the basis of the separation of a mixture of partial glycerides the behavior of packed columns in the droplet regime (instable flowing films) the efficiency of different column installations are compared. A mixture of 55 wt.% propane and 45 wt.% carbon dioxide is used as an extractant. 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

We propose the study of Lennard-Jones (LJ) mixtures that simulate the carbon dioxide-naphthalene system. The LJ fluid is used only as a model, as real CO2 and CioHg are far from LJ particles. The rationale is that supercritical solubility enhancement is common to all fluids exhibiting critical behavior, irrespective of their specific intermolecular forces. Study of simpler models will bring out the salient features without the complications of details. The accurate HMSA integral equation (Ifl) is employed to calculate the pair correlation functions at various conditions characteristic of supercritical solutions. In closely related work reported elsewhere (Pfund, D. M. Lee, L. L. Cochran, H. D. Int. J. Thermophvs. in press and Fluid Phase Equilib. in preparation) we have explored methods of determining chemical potentials in solutions from molecular distribution functions. 

The equilibrium solubility of Lovastatin in carbon dioxide at 55 C and 75 C and for pressures up to 400 bar was obtained using the HPLC apparatus and reported in Table II and Figure 9. The data exhibit both the abrupt change in solubility above the solvent s critical point, as well as the retrograde behavior, both of which characterize supercritical extraction processes. The solubilities were reproducible to within 5%. 

Enick, R.M. Holder, G.I. Morsi, B.I. Critical and 3 phase-behavior in the carbon-dioxide tridecane system. Fluid Phase Eq. 1985, 22, 209-224. 

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