I n d . Eng. Chem. Res. 1995,34, 275-282
275
Supercritical Adsorption and Desorption Behavior of DDT on Activated Carbon Using Carbon Dioxide Stuart J. Macnaughton and Neil R. Foster' School of Chemical Engineering and Industrial C h e m i s t y , University of New South Wales, P.O. Box 1, Kensington, NSW, Australia
Supercritical adsorption isotherms were measured for the priority pollutant DDT on activated carbon at 313.1 and 318.1 K at a k e d carbon dioxide density of 0.658 g/cm3. Equilibrium loadings, measured at saturation DDT solubilities, ranged from 0.5 to 0.7 g/g of carbon, and the adsorption isotherms are well described by the Freundlich model. The desorption of DDT from activated carbon using supercritical carbon dioxide was examined to assess the influence of temperature, density, and flow rate. Concentration-time desorption profiles were measured using a supercritical extraction apparatus that incorporated a high-pressure W cell. The desorption efficiency for this system increases with both temperature and density, and the data indicate that the desorption process is limited by the adsorption equilibrium at low C02 flow rate. Overall the desorption of DDT from activated carbon using supercritical C02 is unfavorable with less than 60% removal and the bulk of the desorption occurring at very low relative DDT concentrations. A local equilibrium model combined with experimental adsorption data was able to predict the desorption profiles.
Introduction The extraction of solute(s) from solid matrices using supercritical fluids has many potential applications including analytical extractions, activated carbon regeneration, and soil remediation. Soil remediation and activated carbon regeneration are essentially desorption processes, and several recent studies have investigated the supercritical desorption characteristics of activated carbon systems (Tan and Liou, 1989a,c; Srinivasan et al., 1990; Madras et al., 1993) and also soil systems (Andrews et al., 1990; Erkey et al., 1993; Kothandaraman et al., 1992). The key to designing large-scale supercritical desorption processes is understanding how the desorption is influenced by process variables such as pressure, temperature, and flow rate. The majority of the published supercritical desorption studies have focused on relatively volatile compounds such as ethyl acetate (Tanand Liou, 1988),toluene and benzene (Tan and Liou, 1989a,c),and 2-chlorophenol (Tomasko et al., 1993). Recently several nonvolatile solids such as hexachlorobenzene and pentachlorophenol have been investigated (Madras et al., 1993). This investigation expands the range of solutes that have been studied to include DDT which is involatile and has a high molecular weight. DDT is a priority pollutant, and therefore there is considerable interest in extraction processes with the potential to remove DDT from solid substrates such as soil. The purpose of this study was to obtain a greater understanding of the dynamic extraction behavior of DDT from solid matrices with supercritical C02. Such knowledge is important for the fundamental understanding and design of environmental applications of SFE such as soil remediation and activated carbon regeneration. Activated carbon was selected as the solid matrix because it is well defined and has uniform properties. Properties such as organic content, particle size, and specific surface area differ significantly between soils and the varying influences of these properties could interfere with the interpretation of results. It would be very difficult to establish whether the observed behavior was due to the soil or due to the extraction conditions. The long-term objectives of the
research program are to carry out studies on actual soil samples after the desorption behavior of DDT is established on a well characterized solid. The specific objectives of this study are outlined below and have been formulated to complement the previous studies of supercritical desorption. The importance of the supercritical adsorption equilibrium has been clearly demonstrated and therefore the desorption measurements were accompanied by corresponding adsorption measurements. The influence of temperature, density, and flow rate on desorption behavior were investigated. The influence of temperature was examined using isochoric rather than isobaric measurements. Previous isobaric desorption studies were not suited to the separation of the combined effects of temperature and density.
Experimental Section Adsorption and desorption breakthrough curves were measured by monitoring the DDT concentration in a stream of supercritical C02 leaving a packed bed of activated carbon. The adsorption experiments involved passing a stream of C02 containing a known concentration of DDT through a bed of virgin carbon and monitoring the W absorbance of the column effluent until breakthrough occurred. The DDT-CO2 mixture was prepared by passing pure C02 through a saturator packed with DDT. The desorption experiments involved passing pure C02 through a bed of loaded carbon and monitoring the column effluent using the W cell. Monitoring the column effluent using a W detector provided continuous concentration measurements. The overall performance of the W detector was checked for consistency using gravimetric analysis of the DDT-CO2 stream.
Materials The DDT used in this study was l,l-bis(4-chlorophenyl)-2,2,2-trichloroethane (Aldrich, 99+% purity) that had been purified using supercritical fluid extraction to remove the more volatile isomer OS'-DDT. A detailed description of the purification process is found elswhere
0888-5885/95/2634-0275$09.00/0 0 1995 American Chemical Society
276 Ind. Eng. Chem. Res., Vol. 34,No. 1,1995
Table 1. Properties of Filtrasorb 400 Activated Carbon and the Carbon Bed sa
total surface area (N2 BET method) (m2/g) bulk density (g/cm3) particle size (mm) bed length (mm) bed diameter (mm) bed voidage
900-1100 0.4320 0.55-0.75
8.9 8.6 0.48
(Macnaughton and Foster, 1994). Food-grade (minimum purity 99.8%) liquid carbon dioxide was used for all the experiments. The activated carbon used for the adsorption and desorption experiments was Calgon Filtrasorb 400 which is designed for use in watertreatment operations. The carbon was boiled in deionised and ultrafiltered water to remove fines and then dried to constant weight in a vacuum,oven at 60 "C. Fresh carbon samples were used for each adsorption experiment. Some relevant properties of the activated carbon and the carbon bed are listed in Table 1.
Apparatus The complete apparatus used during this study is shown schematically in Figure 1. The same apparatus was used for both adsorption and desorption measurements although it was configured differently for each case. The CO2 was supplied using an ISCO 260D highpressure syringe pump operating in constant volumetric flow mode. The barrel of the pump was jacketed with cold water (4.0 "C) so that the mass flow rate would not fluctuate with room temperature. The cold water was continuously recirculated through the coolingjackets of the ISCO pumps using two Iwaki magnetic pumps (Model MD-20R). The system pressure was monitored using Druck pressure transducers, and the temperature was monitored using type K thermocouples, coupled with Jenco temperature indicators. The temperature in the baths was controlled to within 0.1 "C using separate Thermoline heaterhtirrers. The volume of CO2 that was released during each experiment was passed through a 10 L pressure dampener and monitored using a mass flow meter (Brookes Model 5860). The signal from the mass flow meter was converted to flow rate and integrated using a signal processor (Red Lion Control Model IMDL). The first water bath contained a high-pressure sample bomb (volume 150 cm3) that was packed with purified DDT. The DDT was contained in the bomb with glass wool plugs and as a further check against entrainment a 0.5 pm filter unit was located at the exit of the bomb. The sample bomb could be completely isolated and bypassed as was necessary during the desorption runs. The second bath contained the carbon column. The column itself was made of a stainless steel tube (12.7 mm o.d., 8.6 mm i.d.) that was 83 mm in length with l/2 in. Swagelok fittings attached to each end. The carbon bed was only 8.9 mm in length and was positioned in the center of the column and held in place with porous Teflon frits. The dimensions of the carbon bed were chosen on the basis of a compromise between several competing factors. The ratio of bed diameter to particle diameter (approximately 13) exceeds the critical value of 10 which is the minimum value recommended by Tan and Wu (1988)to achieve an equilibrium flow distribution through a packed bed with a supercritical fluid.
COOLINWATER
DDT SATURATOR WATER BATH -1
l c A B W WATER BATH -2
I
I
INTECRATOR~
Figure 1. Experimental apparatus.
An ISCO V4variable wavelength absorbance detector containing a high-pressure cell (ISCO Model 68-0080073) was located just outside the water bath. The absorbance signal from the UV detector was monitored by an integratorhecorder (Shimadzu CR-6A Chromatopac). The cell was accessed via l/16 in. tubing through two small holes drilled in the wall of the perspex water bath. In this way it was possible to locate the cell only 5 mm from the water bath. The residence time of the fluid in the cell loop was calculated to be 3-6 s, which is small enough for temperature fluctuations to be insignificant. The pressure drop through the cell was less than 0.1 bar at flowrates of up to 3.0 standard liters per minute (SLPM), and above this the pressure drop was significant, The flow rates used during the adsorption and desorption experiments ranged from 0.16 to 0.48 SLPM, and thus pressure drops were not significant. The risk of DDT precipitating in the cell was minimized by operating below the DDT-CO2 crossover pressure for the bulk of the experimental program so that a slight drop in temperature would not result in the precipitation of DDT. In addition, the CO2 effluent leaving the column was normally below saturation during both the desorption and the adsorption experiments. The only time that saturation was encountered was after breakthrough during the adsorption runs. The response of the W cell as a function of DDT concentration was examined and found to obey Beer's law. Mer passing through the W cell, the column effluent was returned to the water bath and then flashed to atmospheric pressure through a metering valve. This resulted in the precipitation of any DDT in the C02 stream which was trapped either within the metering valve or in a 0.5 um filter located immediately downstream of the valve. Method Adsorption breakthrough curves were obtained by passing C02 through the DDT saturator and then through the carbon column that was packed with virgin carbon. Constant CO2 flow rate was achieved by operating the ISCO pump in constant flow mode and the pressure in the apparatus was maintained by adjusting the metering valve. The W absorbance was monitored until breakthrough was achieved and the absorbance signal matched that for CO2 saturated with DDT at the experimental conditions. As soon as the signal had stabilized at this value, the run was stopped.
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 277 Table 2. Equilibrium Adsorption of DDT onto Activated Carbon Using Supercritical Carbon Dioxide adsorption temp pressure (K) (bar) 313.1 104.4
318.1
119.8
323.1 333.1
135.4 167.2
DDT concn (moYcm3) 2.663-06 1.323-06 5.753-07 2.10E-07 3.293-06 1.953-06 7.633-07 4.193-06 7.433-06
saturator temp (K) 313.1 318.1 323.1 328.1 318.1 323.1 333.1 323.1 333.1
3
Oa60
carbon loading (g/g) 0.568 0.504 0.449 0.391 0.575 0.537 0.462 0.630 0.691
The column was then isolated and removed from the rig and weighed. Prior to weighing, it was necessary to dry the column under vacuum to remove any absorbed C02 or water clinging to the outside of the column. Trials determined that 15 min under full vacuum removed all traces of water or C02 but did not remove a measurable amount of DDT. The metering valve and filter were also removed and weighed. During the desorption experiments the first bath was bypassed, and pure C 0 2 was passed through the carbon column which was packed with a known mass of loaded carbon. The W absorbance was monitored and at the completion of the experiment the column was isolated, removed, dried, and weighed. The metering valve and filter were also weighed, and the total flow was recorded. There were several experimental features that were common to both the adsorption and the desorption experiments. The outlet flow rate displayed by the signal processor was maintained to within f2.5% of the desired value. The pressure was always maintained within f 0 . 5 bar. Most experimental runs required at least two full ISCO syringes (260mL capacity) of C02 and a second pump was switched on-line when required using a three-way valve which did not cause any disruptions to the overall flow. At the completion of each experiment the apparatus was flushed with COZ and the zero on the W detector was checked. This was necessary to check for drift caused by the long duration of the experiments. Normally the drift was less than 1% (0.01absorbance units). The adsorption isotherms require data that relates the equilibrium loading to the concentration of DDT in the fluid phase. To alter the DDT concentration, the temperature of the DDT saturator was changed. The first measurement was always taken at saturation, i.e., the temperature of the saturator and the carbon column were identical. Subsequent measurements were taken at lower concentrations that were achieved by raising the temperature of the saturator while the temperature of the carbon column remained constant. The equilibrium DDT solubilities had been determined previously (Macnaughton and Foster, 1994). This technique has its limitations as it will only work below the crossover pressure, and therefore a limited range of concentrations can be achieved. A superior technique would be to mix a stream of DDT saturated COZwith pure C 0 2 a t different ratios to achieve the desired concentration. The range of experiments and conditions investigated are listed in Table 2. Some of the desorption work was carried out using a bulk sample of activated carbon that had been loaded with DDT in a different apparatus. This apparatus was essentially a large scale version of the adsorption apparatus that did not include a W cell. A 10 g sample of carbon was loaded with DDT at 318.1K
Time (min) Figure 2. Reproducibility of the experimental desorption data (measurements a t 313.1 K and 104.4bar).
and 119.8 bar. Breakthrough of the equilibrium adsorption was detected by determining the DDT concentration in the column effluent using a gravimetric sampling technique described elsewhere (Macnaughton and Foster, 1994). Samples of this bulk loaded carbon were then used for desorption tests. The purpose of this was to measure desorption profiles at different conditions using identical samples of loaded carbon.
Results The experiments were designed to incorporate internal consistency checks. The purpose of these checks was effectively to measure the mass balance closure around the column and thereby detect if any solute was unaccounted for. During the adsorption experiments the mass balance around the column is given by
+
{mass entering column) = {mass adsorbed) {mass trapped in valve and filter} (1) The mass entering the column is determined from the DDT concentration and the total COZ volume. The average mass balance closure was 98% f 3.5%. The reproducibility of the final carbon loadings was examined by measuring five replicate data points at 313.1 K and 104.4 bar (DDT concentration 8.83 x mole fraction) producing a mean loading of 0.504g of DDT/g of carbon with a standard deviation of 1.5%. The range of loadings was 0.491-0.510g of DDT/gcarbon. The internal consistency of the desorption experiments was also examined by comparing the weight loss of the carbon with the mass trapped in the valve and filter. Theoretically these should be identical, and the average mass balance closure was 100.3% f 1.9%. The mass balance closure is better than that for the adsorption series because there are less potential sources of error associated with its calculation. The reproducibility of the desorption measurements can be seen in Figure 2, which illustrates the integrated desorption profiles for duplicate experiments a t 313.1K and 104.4 bar. Adsorption. The adsorption measurements were all performed at a C02 density of 0.658g/cm3. Experimental adsorption isotherms were measured at 313.1 and 318.1 K and additional equilibrium loadings were measured at the saturation DDT concentration at 323.1 and 333.1 K. The results obtained are listed in Table 2 and illustrated in Figure 3 as a function of concentration. The data exhibit features that are typical of previous studies (Tan and Liou, 1990;Madras et al.,
278 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
‘k
1.5E-3, h
=E“00
1.2E-3
9 0.6
Langmuir
Y
\
.-TI
1
Freundlich
(0
C
0 o.2
2
1 -
E
I
0.0 I
OE+O
I
6.OE-4
0
9 2E-6
4E-6
6E-6
’
O.OE+O O.OE+O
8E-6
DDT Concentration (mole/cc) Figure 3. Adsorption isotherms for DDT measured at a C02 density of 0.658 g/cm3.
1993). The adsorption isotherm is initially steep with loadings increasing very rapidly with concentration, and as the concentration approaches saturation, the isotherm flattens out with loading showing only a slight dependence on concentration. The temperature dependence of the adsorption isotherms is very weak (see Figure 3) although, as theoretical considerations indicate, an isochoric temperature increase would result in a decrease in carbon loading due to the increased affinity that the DDT would have with the supercritical phase. Previous studies have indicated that this is indeed the case (Tan and Liou, 1990; Andrews et al., 1990) and the two isotherms a t 313.1 and 318.1 K actually exhibit a slight decrease in loading as the temperature increases. The heat of adsorption can be calculated from the van’t Hoff equation (eq 2 ) which relates the equilibrium adsorption coefficient (K> to the temperature. The equi-
K = K,e-
MIRT
librium adsorption coefficient has been estimated from the equilibrium carbon loading at the saturation solubility of DDT at each temperature. It is calculated from the ratio of the concentration of DDT on the carbon to the concentration of DDT in the supercritical Con. A similar approximation has been used by Andrews et al. (1990). These adsorption equilibrium coefficients and also the calculated heat of adsorption apply only at saturation DDT concentrations. A logarithmic plot of the adsorption equilibrium coefficient as a function of the reciprocal of temperature is linear and the calculated heat of adsorption is 31.4 kJ/mol. This value compares favorably with previous estimates from supercritical adsorption of solid solutes-Madras et al. (1993) calculated heats of adsorption onto carbon ranging from 18.5to 31.8 kJ/mol, and Andrews et al. (1990) calculated the heat of adsorption of triphenylene onto soils a t 22.5 kJ/mol. The measured adsorption isotherms were correlated using both the Langmuir (see eq 3) and Freundlich isotherms (see eq 4). While both models described the q = QKc/(l
+ Kc)
Q = Kc’”
3.OE-4
(3)
(4)
general shape of the experimental data, the Freundlich isotherm provided a superior fit as can be seen in Figure 4 for the data at 313.1 K. The model parameters are listed in Table 3.
1.OE-6
2.OE-6
3.OE-6
DDT Concentration (molelcc) Figure 4. Comparison between the Freundlich and Langmuir isotherms (measurements at 313.1 K and 104.4 bar). Table 3. Freundlich and Langmuir Parameters for DDT Adsorption from Supercritical Carbon Dioxide (COz Density 0.668 dcm5) temp (K) 313.1 318.1
Freundlich model K (mol/cm3) n 0.00876 0.146 0.0109 0.163
Langmuir model K (cm3/mol) 0.00143 5.95 x lo6 0.00149 3.61 x lo6
Q (mol/cm3)
Previous supercritical adsorption studies have found that both the Langmuir (Tan and Liou, 1990) and Freundlich isotherm models (Madras et al., 1993) were able to represent the experimental data. While the experimental adsorption data at high relative concentrations can be adequately correlated with existing models, caution must be exercised when extrapolating with these models down to low concentrations. It is clear from the data that at low concentrations the isotherms are very steep and there are no data to verify the shape of the isotherm in this region. The most important information that can be derived from the measured adsorption data concerns the general shape of the isotherms. The isotherm shape is generally favorable for adsorption and consequently unfavourable for desorption. The consequences of this for supercritical desorption are that assuming equilibrium considerations, the bulk of the desorption will occur with DDT concentrations well below 10% of saturation. This conclusion is assuming the best case scenario in which all mass-transfer resistances are negligible and complete local equilibrium is achieved. At 313.1 K for example, the equilibrium indicates that the DDT concentration will fall to below 8%of saturation after only 30% of the DDT has been removed. This means that for most of the duration of the desorption, the column effluent will contain very low DDT concentrations which is an unavoidable but inefficient use of the supercritical solvent. In addition the separation of the DDT from the COz prior to recycling will be more difficult at these very low concentrations. Desorption. The experimental desorption profiles were integrated to convert the absorbance signal to concentration and also to generate curves of fraction desorbed as a function of time. The total area under the desorption trace was then equated with the total mass desorbed from the carbon and in this way the concentration profile was calculated. This technique, also employed by Tomasko et al. (19931, has the advantage that the W signal does not have to be calibrated against DDT concentration at each new temperature, pressure, and flow rate. The technique
0.70 I
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o *
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Moles of Carbon Dioxide Figure 5. Isochoric desorption profiles at a COz density of 0.658 g/cm3.
Figure 7. Influence of flow rate on desorption at 313.1 K and 134.3 bar as a function of desorption time.
1
0.50
x
.*
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e0
.**
*
**
8 0.30
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,.**
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0.145 0.241 0.338 0.434 SLPM SLPM SLPM SLPM
0.10
2
4
6
8
10
Moles of Carbon Dioxide Figure 6. Isothermal desorption profiles at 313.1 K.
0.00
0
2
4
6
8
1
0
12
Moles of Carbon Dioxide Figure 8. Influence of flow rate on desorption at 313.1 K and 134.3 bar as a function of the total moles of COz.
assumes a linear relationship between absorbance and DDT concentration which was verified as described earlier. The experiments performed at a carbon density of 0.658 g/cm were all carried out on carbon for which the equilibrium adsorption measurements had been taken. The remainder of the experiments were performed using carbon samples taken from a bulk supercritical adsorption experiment which achieved a carbon loading of 0.551 g of DDT/g of carbon. The overall desorption efficiency is poor for this system. The highest fraction desorbed was only 60% which would be unsatisfactory for both soil remediation and activated carbon regeneration applications. The fraction of DDT desorbed is comparable to that achieved by Brady et al. (1987) with DDT-contaminated soils using pure COz and also the initial results observed by de Filippi et al. (1980) for the pesticides Carbaryl and Diazinon. DDT clearly has a strong aflinity for activated carbon. Desorption increases with isochoric increases in temperature as shown in Figure 5 . The reason for the increase is due to the increased affinity that the DDT has for the supercritical phase as the temperature increases isochorically. This trend correlates with the behavior of the solubility of pure DDT in COz with isochoric temperature increases. A similar dependency exists between desorption and COz density (see Figure 6). The increase in desorption with isothermal increases in COz density are due to the greater solvating power of the COz which results in more rapid desorption. Most of the desorption experiments were conducted at a constant molar flow rate of COz of 0.338 SLPM.
Four experiments were conducted to examine the influence of C02 flow rate on the desorption behavior. These experiments were conducted at 313.1 K with a COz density of 0.753 g/cm3 using a sample of the bulk adsorption carbon, and the results are illustrated in Figure 7. The data in Figure 7 are presented as a function of desorption time, and as the flow rate falls the desorption becomes slower but not with respect t o dimensionless time. Similar results have been reported previously (Srinivasan et al., 1990; Tan and Liou, 1988, 1989a), and these have been attributed to an external film resistance that is significant at low flow rates and effectively slows down the rate at which the DDT can enter the bulk flowing fluid and be carried out of the bed. When the flow rate data are plotted as a function of the total number of moles of C02 passed through the bed, the curves for the three lowest flow rates are coincident (see Figure 8). This result is not fully predicted by the explanation of an external film resistance, and therefore another mechanism may also be influencing the desorption. The flow rate data are also presented in Figure 9 which illustrates the average carbon loading as a function of the eMuent concentration. This plot is an approximation (made valid by the short bed length) of the desorption path that is followed during the desorption. This plot is directional and the course of the desorption with time is indicated by the arrow on Figure 9. At the three lowest flow rates the curves are essentially coincident which is typical of equilibrium-limiteddesorption. At a particular loading the concentration of the fluid in equilibrium with the solid cannot exceed the concentration predicted by the
280 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 I
1
0.6 1
0.6 1
/.
U 0.5
/'
0.4
C
0 0.2
s
0 0.1
SLPM SLPM SLPM SLPM
I %.\E+O
3.OE-5
6.OE-5
9.OE-5
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'
0.1 2.OE-5
5.OE-5 I.OE-4 2.OE-4
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DDT Solubility (mole fraction)
DDT Concentration (mole fraction) Figure 9. Influence of flow rate on the calculated desorption path
J
Figure 10. Relationship between desorption and equilibrium
at 313.1 K and 134.3 bar.
solubility.
adsorption isotherm. In summary, at the three lowest flow rates desorption is limited by equilibrium considerations while other mass-transfer mechanisms (e.g. external film) are more significant at the higher flow rates. This mechanism explains why local equilibrium theory (see Modeling) is capable of predicting the desorption data accurately. At the highest flow rate, equilibrium is not achieved, Le., the effluent concentrations at each average loading are below the equilibrium concentration. This behavior indicates the presence of a mass-transfer resistance which is possibly intraparticle diffusion or an external film resistance. The concept of equilibrium-limited desorption is compatible with the data illustrated in Figure 7. Instead of a mass-transfer film resistance at lower flow rates, the desorption at lower flow rates is constrained by the equilibrium, and therefore the effluent concentration is effectively fured. The desorption occurring at the lower flow rates is primarily influenced by adsorption equilibrium constraints rather than masstransfer limitations. Although the data of Srinivasan et al. (1990) exhibit trends similar to those seen in Figures 7 and 8, the average desorption isotherms cannot be calculated, so the trends in Figure 9 cannot be compared. Most previous workers have concluded that solubility limitations do not influence supercritical desorption and that the shape of the adsorption isotherm has the dominant influence on the desorption process (Kander and Paulaitis, 1983; McHugh and Krukonis, 1986; Madras et al., 1993). The relationship between desorption and equilibrium solubilities was investigated. The desorption results are difficult to quantify into a single number or measurement; however, for the purposes of comparison, the fraction desorbed after 6 mol of COZ had passed through the bed was selected as a potential measure of desorption behavior. These data, calculated only from the desorption profiles measured at constant molar flow rate (0.338 SLPM) using the bulk adsorption carbon (initial loading 0.551 g of DDTIg of carbon) were then plotted as a function of the DDT solubility (Macnaughton and Foster, 1994) corresponding to the desorption temperature and pressure (see Figure 10). A relationship does exist between these data and the equilibrium DDT solubility at the desorption conditions. Although this relationship can be explained in terms of changes in C 0 2 density and temperature, as described earlier, it is nevertheless of some interest because it provides an insight into how the desorption efficiency can be increased.
To achieve higher desorption efficiencies, the DDT solubility at the extraction conditions needs to be increased. Increasing the temperature andlor pressure is unlikely to be sufficient because above 200 bar and 333.1 K the solubility increases are relatively small. An alternative method of raising the solubility would be through the addition of cosolvents to the supercritical C02. Cosolvent addition could raise the solubility of DDT significantly, the extrapolation of the relationship in Figure 10 indicates higher desorption efficiencies would result. This conclusion is somewhat speculative; however, it is consistent with the findings of Dooley et al. (19871, who found that a methanol cosolvent significantly enhanced the extraction of DDT from soils. To date there is only one report of cosolvent use during desorption from activated carbon. Tomasko et al. (1993) found that the weight of activated carbon actually increased when methanol was used as a cosolvent in C02 during the desorption of 2-chlorophenol. The conclusion drawn was that some of the cosolvent was adsorbed onto the carbon; however, no information is given as to the chlorophenol desorption behavior. It is possible that despite the adsorption of some of the methanol onto the carbon the desorption of the primary adsorbate was still enhanced. It would be relatively simple to remove any adsorbed cosolvent after desorption of the primary adsorbate by using pure COZas the solvent. Previous studies have found (Groves et al., 1985)that the desorption of volatile organic compounds such as methanol is efficient using supercritical COZ. Modeling. Equilibrium theory, which assumes that mass-transfer resistances are negligible, is the simplest method of describing the measured desorption data and has been used previously to describe supercritical desorption processes (de Filippi et al., 1980; Tan and Liou, 1989b; Madras et al., 1993). When equilibrium theory is applied to isothermal, fixed-bed, plug-flow desorption, the desorption profile will be a function of the adsorption equilibrium relationship. The behavior of an adsorber can be described by eq 5 (Ruthven, 1984) when equilibrium is assumed. On account of the assumption of equilibrium, eq 5 can be used to develop an expression for the breakthrough curve for the desorption of a uniformly saturated bed with a pure carrier. The derivative term dqldc in eq 5 is obtained from the relevant adsorption isotherm expression, and after integration an equation describing the desorption profile can be obtained:
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 281
,
Freundlich
~
In this study the adsorption data were described most accurately using the Freundlich isotherm (eq 41, and combining eq 4 and 5 and integrating yields an expression (eq 6) that relates the effluent concentration c to the desorption time t. The desorption profile at 313.1
~
~
'
Langmuir
1 1
Tan and Liou ...
1
1
I
(6) "."L.
K and 104.4 bar, predicted using eq 6 and the Freundlich parameters listed in Table 3, is illustrated in Figure 11, and there is good agreement between the experimental and predicted data. A similar modeling approach was adopted by Madras et al. (1993) with satisfactory results. Equation 6 is constrained by the concentration inequality co 2 c > 0 which can be converted to a time domain and means the expression is inapplicable at times below 18.7 min. The desorption profile predicted by eq 6 was integrated to yield the fraction desorbed as a function of time. The predicted effluent concentrations were truncated at low times to the maximum experimental concentration prior to integration. The results of this work are illustrated in Figure 12, and again the prediction using the Freundlich isotherm is satisfactory. The equilibrium model when coupled with an experimentally determined adsorption isotherm provides a good representation of the desorption data without any reliance on fitting parameters. The equilibrium model was also used to predict the desorption profiles at 318.1 K with a C02 density of 0.658 g/cm3 using the experimental adsorption data. The predicted desorption profile at 318.1 K exhibits good agreement with the data. The Langmuir isotherm (see eq 2) was also incorporated into the local equilibrium model producing eq 7. The desorption profile that was predicted using eq 7is
(7) illustrated in Figure 11. This modeling approach was also used by de Filippi et al. (1980); however, the Langmuir adsorption constants were fitted to the experimental desorption data. (Note: eq 7 is equivalent to an expression derived by Ruthven (1984)). The comparison in Figure 11 clearly demonstrates the importance of an accurate description of the adsorption isotherm when using the equilibrium model. The predicted desorption concentrations are much higher than the experimental data. This inaccuracy is compounded when the predicted desorption profile is integrated (see Figure 12). Tan and Liou (1988,1989a-c) utilized an equilibrium model that incorporated a linear adsorption equilibrium. Mass-transfer resistances and adsorption effects were lumped together in the assumption of linear desorption kinetics described by eq 8. dqldt = -kq
C,= 71 - qE
(8)
y)) - exp(-kt)}
O exp(k(t [ - EL
The solution to the model is given by eq 9 with the desorption rate constant It being an adjustable param-
"
0
200
400
600
1000
800
Time (min) Figure 11. Predicted desorption profiles at 313.1 K and 104.4 bar (flow rate 0.338 SLPM). Freundlich Langmuir Tan and Liou
.._
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n
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0
.
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0
4
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0
400
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600
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1 1000
Time (min) Figure 12. Predicted fraction desorbed at 313.1 K and 104.4 bar (flow rate 0.338 SLPM).
eter that was fitted to the experimental data at 313.1 K and 104.4 bar. The model correlation at 313.1 K and 104.4 bar is illustrated in Figure 11. The description of the data is poor, and the general shape of the predicted desorption profile is not accurate. As a consequence of this, the estimated fraction desorbed as a function of time (see Figure 12) is grossly inaccurate. A similar model has also been used by Kothandaraman et al. (1992) to correlate desorption data for anthracene and phenanthrene from soils. Their findings are similar to those of this study with a generally poor representation of the shape of the desorption profile. The available adsorption data for PAHs onto soils indicate that Freundlich rather than linear isotherms are applicable (Andrews et al., 19901, and thus the poor modeling result is not unexpected. The poor correlation that was achieved using the model of Tan and Liou (19881, despite the use of a fitting parameter, can be directly attributed to the incorrect description of the adsorption equilibrium. The local equilibrium model yields a good description of the desorption behavior and if the appropriate adsorption data are available, then no fitting parameters are needed. The desorption data measured at different flow rates indicate that mass-transfer resistances are significant, and therefore an improved model should account for this. A comprehensive desorption model that uses a nonlinear adsorption equilibrium (i.e., Freundlich) and accounts for mass-transfer resistances is described by a set of differential equations that has a nonlinear solution (Garg and Ruthven, 1973). Such a model is currently being developed.
282 Ind. Eng. Chem. Res., Vol. 34, No. 1,1995
The findings of this study coupled with previous investigations allows some general comments t o be made on supercritical sorption phenomena. The desorption of DDT from activated carbon was not completely successful because the maximum desorption achieved was only 60%. This compares with the study of Madras et al. (1993) in which approximately 75% desorption was achieved for several solid solutes and the earlier studies of Tan and Liou (1988, 1989a,c) and Srinivasan et al. (1990) in which complete (i.e., 100%) desorption was achieved for ethyl acetate, benzene, and toluene. The difference in behavior can be explained in terms of the shape of the adsorption isotherm and the relative solubilities of the solutes. A review of existing supercritical adsorption data reveals that despite the wide range of adsorbates and fluid-phase concentrations, the carbon loadings all range from 0.1 t o 0.7 g/g of carbon and the shape of the isotherms is similar when expressed in terms of relative solubility. The DDT data measured in this study confirm this trend. In effect the supercritical adsorption isotherms for these solutes are not influenced at all by the equilibrium solubility of the solutes in supercritical CO2. The importance of solute solubility lies in the fact that it will determine the CO:!requirement for desorption. In general, a solute with low equilibrium solubilities such as DDT will require much larger quantities of C02 t o achieve a desired desorption efficiency when compared t o a more volatile compound like ethyl acetate. Desorption efficiency can be improved if the equilibrium solubility of the solute can be enhanced.
Conclusions The adsorption of DDT onto activated carbon from supercritical COz was characterized successfully at 313.1 and 318.1 K a t a C02 density of 0.658 g/cm3. The adsorption isotherms are well described using a Freundlich isotherm. The shape of the adsorption isotherms and the absolute carbon loadings (0.5-0.7 g/g of carbon) were consistent with previous supercritical adsorption studies despite the low solubility of DDT with respect to most previously investigated solutes. The desorption of DDT from activated carbon using supercritical CO2 was enhanced by isochoric temperature increases and isothermal density increases. This behavior is very similar to the behavior of the DDT solubility as a function of these two variables. The desorption of identical carbon samples a t four different flow rates indicates that desorption is limited by the adsorption equilibrium except at the highest flow rate. At the highest flow rate equilibrium desorption is not achieved due, most likely, to intraparticle mass-transfer resistances. The desorption data were successfully modeled using local equilibrium theory and the experimental adsorption data. Acknowledgment The authors wish t o acknowledge an ARC Grant Physical, Thermal and Transport Properties of Supercritical Fluids, No. A89131800.
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Received for review February 23, 1994 Accepted August 25, 1994@ Abstract published in Advance ACS Abstracts, October 15, 1994. @
