Carbon dioxide critical region

Figure 2.8 Isotherms of carbon dioxide near the critical point of 31.013 °C. The shaded parabolic region indicates those pressures and volumes at which it is possible to condense carbon dioxide...

Promotion of Lipase-Catalyzed Esterification of N-Valeric Acid and Citro-neUol in Supercritical Carbon Dioxide in the Near-Critical Region (Ikushima et al 1996). 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

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. 

As the area is diminished below some thousands of sq. A., where the molecules cover only a small fraction of the surface, the surface pressure rapidly becomes much smaller than that of a perfect gas, and in the four acids with the longest chains becomes constant over a considerable region. The curves are indeed a very faithful reproduction of Andrews s curves for the relation between pressure and volume, for carbon dioxide, at temperatures near the critical. The horizontal regions in the curves correspond to the vapour pressure of liquids, and indicate the presence of an equilibrium between two surface phases, the vapour film, and islands of liquid, coherent film. 

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... 

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.

The present paper gives an overview of results on high-pressure phase equilibria in the ternary system carbon dioxide-water-1-propanol, which has been investigated at temperatures between 288 and 333 K and pressures up to 16 MPa. Furthermore, pressure-temperature data on critical lines, which bound the region where multiphase equilibria are oberserved were taken. This study continues the series of previous investigations on ternary systems with the polar solvents acetone [2], isopropanol [3] and propionic add [4], A classification of the different types of phase behaviour and thermodynamic methods to model the complex phase behaviour with cubic equations of state are discussed. 

The phase behaviour of systems with low molecular alcohols methanol and ethanol as well as of systems with acetone and propionic acid is relatively simple (pattern I). At lower pressures the single three-phase region is bound by a critical line (L3=L2)Vy at higher pressure the three-phase region is limited by either an upper critical line Lj(L2=V) or the binary three-phase line of the system carbon dioxide-water depending on temperature. 

There is a relatively large database of experimental data for the solubility of carbon dioxide in water, much of which is at low pressure. Clever (1996) critically reviewed the data and generated salting-out coefficients. He separated the data into low pressure and high pressure regions. 

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