Znd. Eng. Chem. Res. 1995,34, 340-346
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Solubilities of Polychlorinated Biphenyls in Supercritical Carbon Dioxide Enping Yu,?Miroslav Richter,”Pairu Chen, Xiaorong Wang, Zhuohong Zhang, and Lawrence L. Tavlarides’ Department of Chemical Engineering & Materials Science, Syracuse University, Syracuse, New York 13244
The solubilities of three solid polychlorinated biphenyls, 4,4‘-dichlorobiphenyl, 2,3’,4’,5-tetrachlorobiphenyl, and 2,2’,3,3’,4,4’-hexachlorobiphenylin supercritical carbon dioxide, have been measured at temperatures from 308.1 to 323.1 K and pressures from 90 to 480 atm. The data were obtained in a static apparatus with a high pressure equilibrium view cell which was tested by measuring the solubilities of pyrene in compressed carbon dioxide. The experimental solubility data were correlated with a compressed gas model using the Peng-Robinson equation of state and a volume based linear model which has been recently developed. The unknown critical properties of the polychlorinated biphenyl congeners were estimated using group contribution methods, and the vapor pressure data were determined by the modified Watson method. The solubilities ranged from 1.6 x to 14 x molar fraction for 4,4’-dichlorobiphenyl and 2,3’,4‘,5-tetrachlorobiphenylover the conditions studied. The solubilities for 2,2’,3,3’,4,4‘hexachlorobiphenyl were less than 2.6 x molar fraction.
Introduction Supercritical fluid extraction (SCFE) has received much attention for the possible application to remove toxic organic mixtures from hazardous wastes and contaminated water. Supercritical fluids have the ability to solubilize heavy molecular weight organics. Attractive features of SCFE are that the low viscosity of the supercritical fluid combined with high solute diffusivities result in superior mass transfer characteristics. Furthermore, significant density gradients between the particle-fluid interface and the bulk SCF, which are caused by buoyant forces, result in enhanced mass transfer. Low surface tension of supercritical fluids enables facile penetration into microporous soil matrices. Owing to these properties the rate of removal of organics from soil is faster compared to conventional fluids and the entire extraction process is rapid. This enhanced mass transfer is in contrast to the traditional liquid solvent extraction, which generally takes several hours t o perform, requires relatively large amounts of solvent, and may result in incomplete recovery. SCFE is performed at relatively low temperatures and involves the use of fluids that can be easily separated from the end products by simple physical means. Various modes of contacting are possible. In a semicontinuous process, the dense gas could be recycled between the extractor, where it could flow through a fixed bed of soil material, and the separator. In this mode, the extracted waste material is separated from the supercritical fluid by expansion generally below critical conditions of the system. A solubility difference of several orders of magnitude can be achieved by an appropriate adjustment of pressure and temperature in the extractor and separator. Solids movement could be batch or semicontinuous, similar to extraction configurations employed to extract valuable substances from granular feedstocks (Wilhelm et al., 1988). To whom correspondence should be addressed. +Presentaddress: Dept. Chem. Eng., Beijing Institute of Chemical Technology, Beijing, China. Present address: Ferox-AirProducts,Ustecka 30, 40530 Dien, Czech Republic. 0888-588519512634-0340$09.00/0
Among the supercritical solvents, carbon dioxide is usually preferred, since it is nontoxic, nonflammable, environmentally acceptable, relatively inexpensive, and its critical point allows for extraction a t relatively low temperatures. Recent studies have demonstrated the use of supercritical fluids for extraction of organic contaminants from various solids such as sediments and fly ash. Brady et al. (1987),showed that when supercritical C02 was used to remove polychlorinated biphenyls (PCB’s) and other heavy molecular weight organics from contaminated topsoil and subsoil, over 90% of the PCBs were extracted in less than 1min, and 70% of organics were cleaned up from the topsoil in less than 10 min. Dooley et al. (1990,1987) demonstrated that cosolvents added to the supercritical C 0 2 enhanced the efficiency of extraction by 1 order of magnitude. Extraction and recovery of aromatic hydrocarbons from contaminated soils and waters by means of supercritical fluids have been investigated by Hawthorne and Miller (19871, Roop et al. (19891, and Akgerman et al. (1992). A major factor limiting the commercial success of the supercritical fluid extraction is the lack of reliable data for the design of the extraction unit. Solubility dependence of the extracted components on pressure and temperature forms the basis of design for the conditions in the extractor and separator. Brady et al. (1987) estimated the solubility of Aroclor 1245 (ClzHsCl) in supercritical carbon dioxide with the Peng-Robinson equation of state, but they had no experimental data available. Therefore, this paper presents carbon dioxide solubility data for three polychlorobiphenyl congeners. Since there are 209 PCB congeners (Erickson et al., 1990) and there is little knowledge of PCB physical properties, it is extremely difficult to study the phase equilibrium and t o set up a practical application model. In this respect, different components of PCBs were selected for this preliminary research, 4,4’-dichlorobiphenyl, 2,3’,4’,5-tetrachlorobiphenyl, and 2,2‘,3,3‘,4,4‘hexachlorobiphenyl,which had to meet the following requirements: (i) the solute should be in a solid state under experimental conditions; (ii) the congener should
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 1,1995 341
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VCR
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Fiber optics Camera
Monitor
Solvent Indicator
Water
Cold Trap
Figure 1. Schematic diagram of solubility apparatus.
be present in the Aroclor 1248 mixture; (iii) the cost should be relatively low. The solubility data were measured at temperatures from 308.1 K to 323.1 K and pressures from 90 atm to 480 atm and the experimental results were fitted to the Peng-Robinson equation of state with the quadratic mixing rules and the volume based linear model (Wang and Tavlarides, 1994).
Experimental Procedure Apparatus. A static apparatus equipped with a high pressure view cell was used to obtain equilibrium solubility isotherms of solid PCBs in supercritical CO2. A schematic diagram of the experimental static apparatus is presented in Figure 1. The major part is a high pressure equilibrium view cell (TEMCO). The internal volume of the cell can be adjusted from 3 to 15 cm3 with the piston movement, which is driven by a hydraulic pump (High Pressure Equipment, 62-6-10). The maximum working pressure allowed inside the cell is 680 atm. The maximum working temperature is 450 K. The cell is enclosed in a forced convection air bath. The temperature of the air bath is maintained constant within f O . l K by a temperature controller (Omega Engineering, CN9000A). The temperature in the equilibrium cell is measured by a calibrated K-type thermocouple with a digital temperature indicator (Omega, Model DP41-S2A) of which the resolution is f O . l K. The pressure in the cell is measured with a pressure sensor and registered by a Heise digital pressure indicator (Omega, 901A, 0-680 atm range). The maximum error of this gauge is f0.5 atm. The cell is illuminated by a fiber-light pipe (Volpi AG, Intralux 6000). The inside of the cell is observed through a visual system composed of a glass window, a camera, and a monitor. The binary mixture of PCB solute and CO2 inside the cell is mixed by a tiny magnetic stirring bar that is driven by a stirring engine located below the cell.
Procedure. The equilibrium cell was first flushed with low pressure carbon dioxide, and then the desired amount of solid solute (PCB), weighed to an accuracy of 1 x g, was charged into the cell. Low pressure C02 was carefully passed through the cell at a low flow rate again to remove the air trapped inside. Afterwards, liquid CO2 was pressurized by a compressor (Newport Scientific, 46-13421-2) or a hand pump (High Pressure Equipment, 62-6- 10) and then delivered into the cell. After reaching thermal equilibrium within the constant temperature air bath, the system pressure of the binary system PCB-CO2 was slowly increased by adding CO2 using the hand pump or by reducing of the cell volume by means of the movable piston driven by the hydraulic pump. The pressure was increased very slowly, on average 1a t d m i n , until all of the PCB particles were solubilized. in the supercritical CO2. When the last particle of PCB disappeared and only a single phase was present in the view cell, the equilibrium point was reached and the volume of the cell was recorded. This equilibrium solubilization process normally took 2-3 h. After the first solubilization point was obtained, the system was very slowly depressurized by increasing the cell volume with the moveable piston until PCB particles precipitated from the solution. The same solubilization and precipitation procedure were repeated at least twice or until reproducible data were obtained. The equilibrium pressure relative deviation was less than 5% for the majority of experimental runs. The amount of carbon dioxide inside the cell was determined from the known volume of cell and the density of carbon dioxide, which was calculated using the modified Bendict-Webb-Rubin equation of state (Ely, 1984) at the working conditions. The solubility of a PCB congener in carbon dioxide was calculated from the amounts of the solute and gas. Between experiments the cell was cleaned as follows. A syringe pump (ISCO, lOOD) in conjunction with a separator and cold trap was used to clean up the cell
342 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 6 .O
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Table 2. Experimental Solubilities in Supercritical Carbon Dioxide 4,4'-DCB T = 313.1 K T = 323.1 K Platm Y x 104 Platm Y 104 1.78 97.6 112.3 2.83 2.59 124.6 3.44 112.0 4.36 122.6 156.5 6.05 3.89 160.2 6.24 131.4 5.49 203.6 8.84 188.9 6.37 215.8 265.8 11.37 6.58 239.8 360.1 12.57 7.82 316.4 371.3 12.44 8.70 375.3
m
t P
l!
0
A
P AA
2,3',4',5-TCB
B
T = 308.1 K Platm 101.6 137.9 164.7 239.5 259.0 360.4
Yx
io4
1.69 2.84 3.00 4.00 4.52 4.95
T = 313.1 K Y x io4
Platm 93.1 105.9 115.9 133.7 155.4 160.8 215.2 248.3 273.9 318.2 384.7
T = 323.1 K Y
P/atm
1.83 2.86 3.39 4.43 5.05 5.48 6.09 6.64 6.70 7.48 8.60
118.4 125.0 154.9 187.1 228.5 297.6 364.9 448.2
x 104
2.13 1.99 4.85 5.72 8.18 9.89 11.20 13.60
I
0.0 100
200
xx) 400 500 otm Figure 2. Solubility of pyrene in carbon dioxide at 323.1 K this work 0, Johnston et al. (1982); A, Bartle et al. (1990). P,
m,
Table 1. Experimental Solubilities for Pyrene in Supercritical Carbon Dioxide at 323.1 K Platm 127.7 165.7 195.1 241.3
Y
104 1.14 1.88 2.68 3.39
Platm 295.9 321.5 433.3 476.2
Y x 104 4.16 4.23 5.46 5.82
and all piping with the aid of suitable liquid solvent (acetone, ethanol). To remove the remaining solvent from the system, the cell was heated and the solvent was vaporized. Finally, the high pressure carbon dioxide was used as a final rinsing of all piping and the equilibrium cell. Materials. 4,4'-Dichlorobiphenyl, 2,3',4',5-tetrachlorobiphenyl, 2,2',3,3',4,4'-hexachlorobiphenyl (AccuStandard) with certified purity of 99.9% and carbon dioxide (Matheson) of Coleman grade 99.99%were used without further purification. h d t 8 The reliabilities of the apparatus and the experimental method were tested by measuring the solubilities of pyrene in supercritical carbon dioxide at 323.1 K. These solubility data (Table 1)compared with those obtained by Johnston et al. (1982) and Bartle et al. (1990) are in good agreement (Figure 2). The largest relative deviations in the high pressure region are within 9%. It is noted that the solubility range of pyrene is similar to that of the polychorinated biphenyl congeners studied and provides a reliable check of the apparatus. The solubility isotherms for 4,4'-dichlorobiphenyl (4,4'-DCB) were obtained at 313.1 K and 323.1 K, for 2,3',4',5-tetrachlorobiphenyl(2,3',4',5-TCB) at 308.1 K, 313.1 K, and 323.1 K, and for 2,2',3,3',4,4'-hexachlorobiphenyl (2,2',3,3'4,4'-HCB) at 313.1K and 323.1 K. All experimental results are listed in Table 2 and plotted in Figures 3-7. The experimental data indicate that the solubilities of PCBs are much lower than those of aromatic hydrocarbon solids, such as naphthalene, phenanthrene, and
2,2',3,3',4,4'-HCB
T = 313.1 K Platm Y 104 239.1 242.7 307.1 378.1 484.1
1.10 0.90 0.85 1.34 1.74
T = 323.1 K Platm 151.5 210.3 291.1 342.1 426.5
Y x 104 0.87 0.89 1.52 2.18 2.59
anthracene, under the same conditions of temperature and pressure. The vapor pressures of PCB congeners are very low (below atm) and the molecular weights are relatively high. Generally, the lesser the vapor pressure and the higher the molecular weight, the less soluble is the solute in compressed gas. Comparison of the solubilities of all three measured PCB congeners showed that the solubilities of higher chlorinated congeners (such as 2,2',3,3',4,4'-HCB) are extremely low (Y .e 2.5 x mol fr); these solubility isotherms are flat and the extractibility from the contaminated soils would be less efficient than that of congeners with fewer chlorine substitutions. The solubility data for HCB are more scattered. It was very difficult to recrystallize the particles of HCB from the solution after the solubilization point was reached, especially a t the equilibrium pressure less than 300 atm. The amount of solid HCB charged into equilibrium cell for these experiments was smaller than 0.009 g. For these reasons, the solubility data for HCB have higher experimental error than more soluble PCB congeners. The shape of isotherms for 4,4'-DCB and 2,3',4',5-TCB are similar. The curves of the solubilities increase very steeply with pressure. The pressures above the region of steep increase are suitable for the supercritical extraction (especially from dry materials). As shown in Figures 3 and 4, there is a crossover point, which is the cross point of two or more isotherms. Beyond the crossover point, solubilities increase with increases of both the pressure and the temperature, while below the crossover point, solubilities increase with the increase of the pressure but decrease with the increase of the
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 343
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Table 3. Other Polychlorinated Biphenyl Congeners Considered melting point, reason data PCB congener K" not obtained 4-MCB 350.2 liquefied a t 313 K and 55 atm 2,2',4,4'-TCB 256, 314-315 liquefied a t 313 K and atmospheric pressure 2,2',5,5'-TCB 359-360 liquefied at 313 K and 53 atm 2,2',3,3',6,6'-HCB 387-388 did not recrystallize after dissolving
P
0
a
Hutzinger, 1983.
>.
5 .O
0.0 0
100
.,
300
xx) P, otm
400
Figure 3. Carbon dioxide solubility of 4,4'-DCB according to the Peng-Robinson EOS. Experimental: 323.1 K; 0, 313.1 K. Model: -.
In addition to the above mentioned PCB congeners, it was attempted to obtain solubility data from other polychlorinated biphenyls. Table 3 summarizes the PCB congeners which were considered with the explanation why solubility data were not obtained. The main problem was the formation of a liquid phase and liquefylng of solid crystals during the pressurization period of the experiment. Modeling of Solubilities. Two approaches were applied in the mathematical treatment of the experimental data. In the first one, the solubilities were correlated using the Peng-Robinson equation of state. In the second approach, a simple volume based linear model was used to correlate the solubility data, which was developed based on a dilute two-region solution theory (Wang and Tavlarides, 1994). Equation of State Modeling. The solubility of a heavy nonvolatile solid in supercritical fluid solvents can be derived from the condition of equal fugacities in both phases:
201)
158-
/
P
s 10.0
/ where p"2 is the vapor pressure of solid component, q5yt is the fugacity coefficient of the pure saturated vapor of the solute, us is the solid volume, P represents the system pressure and &cF is the fugacity coefficient of the solute in the supercritical fluid phase. The fugacity coefficient is derived from the PengRobinson equation of state (Peng and Robinson, 1976)
-
$iCF
5.0
-
p=--RT u -b
P, atm
.,
Figure 4. Carbon dioxide solubility of 2,3',4',5-TCB according to the Peng-Robinson EOS.Experimental: 323.1 K; 0,313.1 K, A, 308.1 K. Model: - fitted; - - - calculated a t 333.1 K.
temperature. The different effects of temperature on the solubilities are due to influences of temperature on vapor pressure, density, and molecular interaction of the supercritical fluid phase. A comparison of solubilities of 4,4'-DCB, 2,3',4',5-TCB, and 2,2',3,3',4,4'-HCB at 313.1 K and 323.1 K indicates that the solubilities of 4,4'-DCB and 2,3',4',5-TCB are almost the same at 313.1 K. At higher temperature, there is a difference in solubilities and the curves are more separated. The solubilities of PCB congeners decrease in the following succession: 4,4'-DCB =. 2,3',4',5-TCB =- 2,2',3,3',4,4'HCB.
a(T)
U(U
+ b ) + b ( -~ b )
(2)
The values of parameters aii and bii of the PengRobinson equation of state for pure components are calculated by using the critical properties P,, T,,and the acentric factor o. For binary mixtures, appropriate mixing rules and corresponding combination rules are chosen to evaluate the a and b parameters (McHugh and Krukonis, 1986). The adjustable size parameter was assumed to be zero, whereas, the interaction parameter k l z was determined by a regression method minimizing the objective function
when applying the model to fit the experimental data. The absolute average relative deviation (AARD) for y2 is given by
344 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
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AARD (%) = N
N
lYFP - Y 3
x 100
(4)
yexp 2
Physical Properties. Since the critical properties of the majority of PCB congeners are not known, they were estimated using Joback contribution method (Reid et al., 1987). The boiling point for 4,4’-DCB was found in the literature (Ericksson, 1990; Hutzinger et al. 1983),whereas, for 2,3’,4’,5-TCB and 2,2’,3,3’,4,4‘-HCB, they were calculated by means of the Meissner and McGowan method (Lyman et al., 1982). The vapor pressure of 4,4‘-DCB was interpolated from the published data (Smith et al., 1964; Stephenson et al., 1987). The vapor pressures of 2,3’,4‘,5-TCB and 2,2‘,3,3‘,4,4‘HCB were estimated by the modified Watson method (Lyman et al., 1982) whose parameters were fitted to the experimental values found in the literature (Erickson, 1990; Stephenson et al., 1987; Burkhard et al., 1985). The acentric factor was then calculated from the defining equation
where p“, is the reduced vapor pressure at reduced temperature T, = 0.7. The solid densities of PCB congeners were determined by the additivity method of Immirzi and Perini (1977). Table 4 lists the estimated physical properties of PCBs. Using these physical property values and the above model, the results of data analyses are illustrated in Figures 3 and 4. For comparison the calculated solubility curve at 333.1 K is shown for the 2,3’,4’,5-TCB-C02 system. It is seen that at 200 atm the solubility increase above 323.1 K is only 25%, which suggests much higher pressure would be necessary to increase the extraction potential of PCBs from soil matrices. The binary interaction parameters k12 and the absolute average relative deviations are listed in Table 5 . The phase equilibria of 4,4’-DCB-C02 and 2,3’,4‘,5-TCB-C02 systems were modeled with sufficient accuracy with only one interaction parameter k12 (Figures 3 and 4). For 2,2‘,3,3’,4,4‘-HCB,a sufficient number of reliable results were not obtained because the solubilities below 1.5 x molar fraction have a higher experimental error, and the agreement between experimental data and calculated values was poor. This disagreement is a result of experimental difficulties mentioned above and the inaccuracies in the estimation of the physical properties, especially vapor pressure.
Volume Based Linear Correlation Modeling. Recently, a linear relationship has been developed (Wang and Tavlarides, 1994) based on a dilute tworegion solution theory. This relationship has the capability to quantitatively describe the solubility behavior of a heavy solute in a compressed gaseous solution and is of the form: (6) where A and B are constants. Here E = y2p/P2 represents the solubility enhancement, Z = PvIRT is the compressibility factor, u is the molar volume of solvent. This linear relation requires no use of an empirical equation of state and critical parameters, which may not be available. It also enables other thermodynamic properties to be predicted, such as the partial volume
Table 4. Physical Properties of PCBs and COz properties 2 C1-DCB 4 Cl-TCB 6 C1-HCB 360.9 292.0 Mw [gh”l 223.1 660 Tb tK1 590 630 889 Tc [Kl 847.5 878 24.2 26.8 P, [atml 30.1 204 Vs[cdmoll 155 180 0.562 0.716 w 0.453 pd x lo7 [atml 308.1 K 0.233 0.449 0.281 313.1 K 1.922 1.569 0.998 323.1 K 6.594
COz 44.01 194.7 304.2 72.8 0.239 -
-
Table 5. Modeling of Solid Gas-PhaseEquilibria by Peng-Robinson Equation of State 308.1 K 313.1 K 323.1 K 4,4‘-DCB kiz 0.0605 0.0582 AARD (%) 17.6 24.5 2,3’,4‘,5-TCB kiz 0.0881 0.0772 0.0760 AARD (%) 8.9 25.3 15.3 2,2’,3,3’,4,4‘-HCB 0.1606 kiz 26.3 AARD (9%)
and the partial molar heat capacity of a solute. Further, it has been shown that the linear relation can be used over a wide range from the ideal gas state to highly nonideal states (e.g., supercritical state). Application of this model on the solubility data of PCB congeners shows the expected linear plots. For each congener, the experimental solubilities collapse to a single linear line when they are plotted as l/(T[ln E - In ZI} versus u (Figures 5-71, Exception is only for 2,3’,4‘,5-TCB at 313.1 K, which deviates from those at 308.1 K and 323.1 K. Table 6 summarizes the results calculated by using the linear equation.
Conclusions The static high-pressure view cell can be used to measure the solubility of solids in supercritical fluids. The advantages of this experimental equipment are that only a minimum amount of solute is required for an experiment and no analytical procedure is necessary to determine the composition of the fluid phase. Also, it is possible to observe the solute to see if melting occurs. Solubility data for 4,4‘-dichlorobiphenyl, 2,3’,4‘,5tetrachlorobiphenyl, and 2,2’,3,3’,4,4‘-hexachlorobiphenyl were measured at pressure from 90 t o 480 atm and at temperatures from 308.1 to 323.1 K. The solubilities of the PCB congeners mentioned above are in the range from 8 x to 1.5 x molar fraction. The solubilities of PCB’s congeners studied decrease with the number of chlorines on the benzene rings. The experimental solubility data for 4,4’-DCB and 2,3’,4’,5-TCB were correlated by the Peng-Robinson equation of state with only one interaction parameter k12. The low reliability and limited data for 2,2’,3,3‘,4,4’hexachlorobipheyl precludes any reliable correlation with this approach. The absolute average relative deviations (AARD’s)between experimental and calculated data range from 9 to 27% for 4,4‘-DCB and 2,3’,4’,5-TCB. Further, a simple volume based linear model was also used to correlate the solid solubility data. The experimental solubility data at different temperatures collapse together in a linear line. Despite the very low solubilities, the correlation of the linear equation t o the experimental data of PCBs was quite good and the average deviations range from 11to 19%.
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