Carbon dioxide density

Figure 4. Carbon dioxide density depending on pressure and temperature...

The relative effects of supercitical carbon dioxide density, temperature, extraction cell dimensions (I.D. Length), and cell dead volume on the supercritical fluid extraction (SFE) recoveries of polycyclic aromatic hydrocarbons and methoxychlor from octadecyl sorbents are quantitatively compared. Recoveries correlate directly with the fluid density at constant temperature whereas, the logarithms of the recoveries correlate with the inverse of the extraction temperature at constant density. Decreasing the extraction vessels internal diameter to length ratio and the incorporation of dead volume in the extraction vessel also resulted in increases in SFE recoveries for the system studied. Gas and supercritical fluid chromatographic data proved to be useful predictors of achievable SFE recoveries, but correlations are dependent on SFE experimental variables, including the cell dimensions and dead volume. 

The SFC separation of selected estrogens (estrone, equilin, a-estradiol, /3-estradiol, and d-equilenin) was achieved on a SB-cyanopropyl-50 capillary column using a carbon dioxide density gradient at an oven temperature of 73°C [5]. A typical analysis time on the 7-m column was 21... 

In the manufacture of steel, pure oxygen is blown through molten iron to remove some of the carbon impurity. If the combustion of carbon is efficient, carbon dioxide (density = 1.80 g/L) is produced. Incomplete combustion produces the poisonous gas carbon monoxide (density = 1.15 g/L) and should be avoided. If you measure a gas density of 1.77 g/L, what do you conclude ... 

Phenol was successfully extracted from water using pure supercritical carbon dioxide at pressures up to 31 MPa for two isotherms 298 and 323 K. The distribution coefficient increased with increasing pressure, but decreased with increasing temperature. This is expected since increasing the temperature severely drops the carbon dioxide density and hence the solubility of the phenol in it. Increased volatility at the higher temperature is not sufficient to off-set the density effect, since phenol has a low vapor pressure. Benzene was foimd to be a suitable entrainer since its solubility in water is very small and it enhances the distribution of phenol into the supercritical phase. The presence of methanol was found to have no effect. Since methanol is polar and completely soluble in water, it favors the aqueous phase and therefore does not change the characteristics of the supercritical phase. Others have found that the distribution of short chain alcohols between water and supercritical carbon dioxide highly favors the aqueous phase (ifl). 

In SFC either capillary or packed columns may be used. Consider first a comparison between packed column LC and SFC systems. Experimentally obtained (5) plots of the HETP as a function of mobile phase linear velocity for a packed column are shown In Figure 1a. In the LC operating mode a mixture of acetonitrile and water Is the mobile phase and In the SFC mode carbon dioxide Is the mobile phase. The solvent powers were adjusted In each operating mode so that the capacity factor of pyrene was about the same. In one case the adjustment Involved the acetonitrile/ water ratio and In the other, the carbon dioxide density. 

A limited number of experiments were conducted to evaluate the last two possibilities. First, the take-up solvent for two test solutes, anthracene and naphthalene, was cyclopentane (with solution concentrations still about 1 mg/mL and injection aliquots still about 0.3 - 0.5 pL into a 20vL loop) instead of 2-methoxy-ethanol. The distortion persisted even when a gentle stream of air was used after aliquot deposition in the sampling loop to blow off the take-up solvent. Furthermore, the onset of the distortion for naphthalene occurred at a lower carbon dioxide density than for anthracene. When the column was changed to a partially chemically deactivated column, Hypersll SAS, where short alkyl chains are bonded to active sites, onsets of peak distortion for anthracene and for naphthalene were shifted to lower carbon dioxide densities (and still different from each other) than for the fully active Hypersll SIL adsorption column. Only slight peak distortion was observed for a Hypersll ODS column at even lower column midpoint densities (0.33 g/mL with the back pressure at 1170 psig, the FLOW setting at 1.90 mL/min, and 40 C distortion was observed for only anthracene and not naphthalene). 

The chemical description of this interaction is still to be determined. It appears that there exists some threshold solvent power (defined either by the pure carbon dioxide density or the modifier identity and concentration in a modifier/carbon dioxide mixture) at which the solvent can begin to compete successfully with a particular stationary phase for a particular solute. Whether this involves a deactivation of active sites amenable to specific solute adsorption on the silica surface or a secondary solvent effect (43) where the mobile phase interacts with the solute as well as with the adsorption surface is unknown. 

With different gases, under the same conditions, any difference in the value of vnJvQ must be due to the different densities of the gases. The mean of a series of experiments with chlorine (density, 35 5), carbon dioxide (density, 22), and ozone (density, ), gave the following for the value of this ratio — C02, 0 29 Ozone, 0 271 Cl2, 0 227. 

Figure 24. (b) log k of several ethyleneglycol oligomers as a function of carbon dioxide density at 395 K. 

It uses a double-acting piston pumping system, providing precise control of the liquid carbon dioxide. The system allows re-circulation at pressures of up to 3000 psig, and the control system provides for very tight control of temperature of the re-circulating liquid, with built-in capacity to account for changes in carbon dioxide density with respect to temperature. 

Figure 5 1 Zosel s first process for the decaffeination of green coffee beans with supercritical CO2. (Operating conditions are 90°C, 160-200 atm, carbon dioxide density in range 0.4-0.65 g cm ). In this process the caffeine is removed from the CO2 by washing with water. Reproduced with permission from K. Zosel (see caption to Figure 5.3).

Natural gas contains 85 mol% methane and 15 mol% carbon dioxide (density, p = 2.879 kg/m and the viscosity, p = 1.2 x 10" kg/m s) is pumped through a horizontal Schedule 40, 6-in.-diameter cast-iron pipe at a mass flow rate of 363 kg/h. If the pressure at the pipe inlet is 3.45 bars and 25°C, the pipe length is 20 km downstream, assume incompressible flow. Calculate the pressure drop across the pipe. Is the assumption of incompressible flow reasonable ... 

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