Phenol distribution coefficient pressure

For determination of phenol distribution coefficients the extraction proceeded for 15 minutes in order to reach equilibrium. The time required to reach equilibrium was determined by making five replicate injections of the headspace onto the SFC system. The first injection was after the extraction had proceeded for 15 minutes at 50°C and 100 atm. Following the equilibration time, four further injections at ten minute intervals were made, after which the pressure inside the extraction apparatus was increased and the system was again allowed to equilibrate (i.e. 15 minutes). The five replicate injection process was then repeated. The amount of phenol in each injection was then noted by referring to an external phenol standard calibration curve. As the total volume of the system was known, the amount of phenol in the SF could be calculated. The amount of phenol in the aqueous phase could then be calculated by mass balance. 

Figure 7. Phenol distribution coefficient as a function of pressure at four temperatures.

A detailed description of the experimental apparatus and procedure used for the aqueous study are given elsewhere (Roop and Akgerman, Ind. Eng. Chem. R., in review) Static equilibrium extractions were carried out in a high pressure equilibrium cell (300 mL Autoclave). After the vessel is initially charged with 150 mL of water containing 6.8 wt.% phenol and supercritical carbon dioxide (and a small amount of entrainer, if desired), the contents were mixed for one hour followed by a two hour period for phase separation. Samples from both the aqueous phase and the supercritical phase were taken for analysis and the distribution coefficient for phenol calculated. 

The distribution coefficients of phenol obtained for the aqueous system as a function of pressure and temperature using pure supercritical carbon dioxide are shown in Figure 2. The values increase proportionately with pressure for each isotherm, but decrease overall at higher temperatures. At 298 K, reproduction of the distribution coefficients yielded an average standard deviation of 1.5 %. At 323 K the data axe somewhat more scattered due to fluctuations in the temperature caused by control. Figure 3 shows the effect of various concentrations of benzene and methanol used as entrainers in supercritical carbon dioxide at 17.3 and 27.6 MPa at 298 K. Methanol, a commonly used entrainer in studies concerning solid organics, was found to have little effect on the distribution of phenol in the aqueous system. The presence of a small amount of benzene, however, did increase the distribution coefficient up to 50 % over those obtained with pure carbon dioxide. 

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). 

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