Znd. Eng. Chem. Res. 1992,31,333-339 current extraction-stripping system containing only one extractant. However, appropriate sequence of extractants and appropriate extraction conditions must be applied. Better results are obtained if the strong extractant is in the second loop and the weak one is in the first loop (Table V). The use of one extraction stage and one stripping stage in each loop is sufficient. In the case of more stages used two extraction stages should be in the loop containing the weak extractant or two stripping stages in the loop containing the strong extractant. The increase of sulfuric acid concentration used for stripping yields the increase of the total transfer of copper. The excessive sulfuric acid concentration can be compensated for by the use of appropriate excess of the organic phase in the stripping. Acknowledgment The work was supported by Grant DNS-T/09/144/90-2. Nomenclature F = flow rate of the aqueous phase (extraction) F’ = flow rate of the aqueous phase (stripping) k = total number of stripping stages 1 = number of extraction stages in the first processing loop m = number of stripping stag- in the second processing loop n = total number of extraction stages o = organic phase (subscript) S = flow rate of the organic phase (extraction) S’ = flow rate of the organic phase (stripping) x H , ~= feed sulfuric acid concentration x H , ~ +=~equilibrium sulfuric acid concetration x ~= feed , ~ copper concentration in the aqueous phase
333
x ~ , = ~ equilibrium + ~
copper concentration in the aqueous phase Registry No. I, 50849-47-3; 11, 37339-32-5; Cu, 7440-50-8.
Literature Cited Bogacki, M. B.; Szymanowski,J. Modeling of Extraction Equilibrium and Computer Simulation of Extraction Stripping Systems for Copper Extraction by 2-Hydroxy-5-nonylbenzaldehydeOxime. Znd. Eng. Chem. Res. 1990,29,601+06. Bogacki, M. B.; Szymanowski, J. Computer Simulation of Copper Multistage Extraction-Stripping System with 2-Hydroxy-5nonylbenzophenone Oxime. Inz. Chem. Proc. 1991, in press. Bogacki, M. B.; Alejski, K.; Szymanowski,J. The Fast Method of the Solution of a Reacting Distillation Problem. J. Comput. Chem. Eng. 1989,13, 1081-1085. Hughes, M. A.; Parker, N. A. Computer Study of Liquid-Liquid Stage-Wise Calculation in Typical and New Counter Current Contacting. J. Chem. Technol. Biotechnol. 1985, S a , 255-262. Hughes, M. A.; Parker, N. A Practical Proof of New Contacting Schemes for Copper Extraction. In Separation Processes in Hydrometallurgy; Davies, G. A., Ed.; Wiley: London, 1987; Part 2, Chapter 16. Piotrowicz, J.; Bogacki, M. B.; Wasylkiewicz, S.; Szymanowski, J. Chemical Model for Copper Extraction from Acidic Sulfate Solutions by Hydroxy Oximes. Ind. Eng. Chem. Res. 1989, 28, 284-288. Rod, V. Unconventional Extraction-stripping Flows Sheeta for the Separation of Metal by Liquid-Liquid-Extiaction. Chem. Eng. J. 1984,29. 77-83. Szymanowski, J.; Jeszka, P. Modeling of Simple Multistage and Counter Current Multistage Copper Extraction by Hydroxyoximes. Znd. Eng. Chem. Process Des. Dev. 1985,24, 244-250.
Received for review March 8, 1991 Revised manuscript received September 3, 1991 Accepted September 19,1991
Supercritical Extraction of Hexachlorobenzene from Soil Aydin Akgerman,* Can Erkey, a n d Seyyed M.Ghoreishi Chemical Engineering Department, Texas A&M University, College Station, Texas 77843
Feasibility of supercritical extraction of hexachlorobenzene from soil by COz is investigated. A dynamic tracer response technique is employed to measure the adsorption equilibrium and rate at the temperature range 298-323 K and pressure range 1200-4000 psia at C02flow rates of 120-160 mL/ h. Thermodynamic consistency of the adsorption equilibrium constants is verified through isochoric temperature dependency and isothermal density dependency of the equilibrium constants. Overall mass transfer coefficients and axial dispersion coefficients are also determined at the experimental conditions. Introduction Supercritical extraction has been demonstrated in the literature at the bench scale for extraction of organic contaminants from a variety of solid matrices. Capriel et al. (1986) used supercritical methanol to extract bound pesticide residues from soil and plant residues. Hawthrone and Miller (1986) extracted polycyclic aromatic hydrocarbons (PAH) from diesel soot and Tenax packing for gas chromatographic columns by supercriticalcarbon dioxide. Schantz and Cheder (1986), similarly, used supercritical COP to extract polychlorinated biphenyls (PCB) from sediment and PAHs from urban particulate matter. Methanol/N20 mixtures are also used to extract PAHs from river sediments and urban particulate matter (Hawthrone and Miller, 1987). Supercritical COPis used to remove hexachlorocyclohexane, parathion, PCBs, and
* Author to whom correspondence should be addressed.
PAHs from Tenax packing (Raymer and Pellizari, 1987) and polyimide-based adsorbents (Raymer et al., 1987). SupercriticalCOPis also the solvent of choice for extraction of PAHs from various adsorbents selectively by varying the operating conditions (Wright et al., 1987a,b). The results of these studies have demonstrated at the bench scale that it is possible to extract compounds with molecular weight as high as 400 at mild conditions and selectively if desired. This conclusion constitutesthe basis of supercritical chromatography. Recently we have reported on the application of chromatography theory to supercritical extraction from solid matrices (Erkey and Akgerman, 1990). Concerning environmental applications, Kingsley (1985) applied subcritical and supercritical COPfor extraction of oil from metal fiies (mill scale) and bleaching clay in the pilot scale. The process operated on a semibatch mode, and the results indicated that the recovery of extractable material depended on the solvent flow rate. It was also
0888-5885/92/2631-0333$03.00/00 1992 American Chemical Society
334 Ind.
Eng.Chem. Res., Vol. 31, No. 1, 1992
20
1 '
15
VENT VENT
17
16
Figure 1. Experimental asasmbly.
observed that, for the bleaching clay, the recovery was improved by a static soaking period before extraction. Dooley et al. (1987) used a flow system to extract DDT from contaminated soil. The effect of entrainers on the solvent power was also determined. They demonstrated that 100% DDT extraction was possible when 5% by weight methanol was used as entrainer whereas when toluene was used as the entrainer (5%by weight) only 75% of the DDT was recovered. Other environmental applications of supercritical extraction are summarized by Akgerman et al. (1991). In this study we report on the supercritical extraction of hexachlorobenzene (HCB) from soil. A dynamic response technique is employed to measure the equilibrium adsorption coefficient and the mass-transfer coefficient. The axial dispersion coefficient for the experimental apparatus was determined separately using glass beads of the same size as the soil. Experimental Section The extraction parameters are determined by a dynamic response technique, Figure 1. The extractor (121, fitted with a porous metal fdter at the effluent, is ffled with soil. After the air in the system is vented by passing COz, the system is charged with GOz from the cylinder equipped with a dip tube with the piston in cylinder 4 fully retraded Valve 3 is shut off, and the system is pressurized to the set value employing cylinder 4, which is now pressurized by nitrcgen. This enables preasurizig of the whole system up to 2200 psia without using the syringe pump (5) and allows the use of the full volume of the syringe pump. The syringe pump (ISCO LC 2600) is then used to pressurize the system to the desired pressure and start the flow over the extractor. The effluent from the extractor passes through the cell of the UV detector (LDC, Critical Extraction Monitor) and the back-pressure regulators (18and 19) and is vented. A saturator (11)is used to dissolve some HCB in supercritical COP A pulse input of HCB/C02 is introduced through the sample loop of the switching valve (8) and the resulting peak is recorded on the computer (15). Figure 2 shows a typical peak from the detector. The system can be modeled using the standard chromatographic equations. The material balance in the mobile phase is given by
VUL"
t
-0.006
Time (dimensionless1
Figure 2. Typieal raaponse peak from the W detector.
where R is the rate of adsorption which is equal to the ma@ transfer to the surface
R = PB ap/at
= PBkf(CA- cd
and assuming linear adsorption aq/at = kf(CA - cs) sk.(Cs - I?/&)
(2) (3)
where 7 is similar to the effectiveness factor in heterogeneous reactions which accounts for the diffusional resistance into the pores. Since tracer response experiments involve very small concentrations, assumption of linear adsorption is normally justified. Eliminating CS a4/at = k,(CA
- q/KA)
(4)
where k. is an overall mass transfer coefficient given by
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 335 Normally adsorption rate constant k, is very high compared to the external mass transfer coefficient and if q is high enough k, = k f . The initial and boundary conditions are t = o C A = cs = =0 (6) z = 0: CA=O fortT (7) C A = CAo for O I t Ir acA/az = o
z = L:
(8)
These equations can be solved in Laplace's domain and have the solution C A = (CA"/s)[l - exp(-s~)]exp(az) (9) where
The first absolute moment (mean) and the second central moment (variance) are given by p1-
; [ 7 ] +[y] =
1
(12)
where T is the injection time defined by eq 7. The moments are calculated from the experimental data by finite summation. No matter what the diffusion resistances are, provided that the adsorption isotherm is linear, the first moment yields the adsorption equilibrium constant and the overall mass transfer coefficient, k,, can be obtained from the second moment if the axial dispersion coefficient is hown. In principle, it is possible to determine the axial dispersion coefficient by using a nonadsorbing tracer. However, in practice, it is very difficult to identify a nonadsorbing tracer that is also UV visible. An alternate technique is to use a similar packing with nonadsorbent particles of the same size. It may be argued that the particle size is not the only variable for axial dispersion and thus the technique of using nonadsorbent particles is subject to error. However, when the flow is close to plug flow, the approach can be justified. One pitfall in tracer response analysis concerns irreversible chemisorption of the adsorbate on the adsorbent. The zeroth moment establishes the material balance closure and hence can be used to determine that the full mass injected is recovered at the effluent. Thus a known amount is injected with the soil column removed from the system and mass injected is determined by the zeroth moment. The experiment is repeated with the soil column in place. This technique, however, is subject to error since the amount injected, at least in our equipment, varies because the saturator pressure is decreased by a small but a finite amount after each injection, which results in a decrease in the amount of solute injected due to change in solubility with pressure. An alternate approach is to load the soil with a known amount of HCB using 14C-labeled HCB and determine the amount removed from the column by passing supercritical C02over the soil column, i.e., by determining response to a step change.
Table I. Properties of Soil and the Packed Column bed length 20.32 cm bed diameter 1.75 cm 0.39 bed porosity particle diameter 0.1 cm (16-20 mesh) particle density 2.3 g/cm3 particle surface area 26.0 cm2/g soil decomposition 48.2% sand 15.2% silt 36.6% clay organic content of soil 1.4%
Results and Discussion The objective of our first experiments was to determine if any HCB remained on soil (chemisorbed)after the extraction is completed. Properties of the soil used and the packed bed parameters are given in Table I. In order to prepare HCB-loaded soil, sufficient amount of HCB containing a small amount of 14C-labeledHCB is dissolved in chloroform and the amount of HCB in the solution is determined exactly by liquid scintillation. About 5 mL of this solution is added to 50 g of soil to give loo0 ppm HCB loading on soil, and the soil is homogenized in a tumbler for 48 h followed by air drying to remove the chloroform. Three random samples of this soil were then mixed with scintillation cocktail, sonicated for 2 h, centrifuged for 1 h, and analyzed in a Beckman 3801 liquid scintillation counter. The results indicated that soil contamination was uniform with contamination level of loo0 f 12 ppm. This soil is extracted at 1700 psia and 25 "C by passing supercritical COOover it at a flow rate of 160 mL/h measured at the extraction conditions. The extractor effluent is expanded into a series of liquiscint traps (25-mL glass vials containing 15 mL of liquiscint). After the extraction is completed, (about 40 g of C02passed over soil) the system is depressurized and both the soil in the column and the liquiscint in the trap vials are analyzed. There was no detectable HCB remaining on soil, and all the HCB removed was trapped in the vials. The material balance closure was better than 97%. In supercritical extraction of a solid matrix, the three parameters of interest are the equilibrium partition coefficient that gives the extent of extraction, the masstransfer coefficient which determines the rate of extraction, and the axial dispersion coefficient which is a measure of the hydrodynamics of the system. The equilibrium partition coefficient KA has units cubic centimeters of supercritical fluid per gram of soil based on its definition by eqs 3 and 4. KA is the thermodynamic parameter of interest for design purposes, and it can be determined from the first moment of the response peak as explained above. The thermodynamic equilibrium constant, on the other hand, is defined in terms of the mole fraction ratio and is related to KA as
where KIA has units gram of soil per gram of COz. Since the total mass of HCB is very small compared to soil and/or the supercriticalphase, ita amount in these p h are neglected in calculating the mole fractions. Table I1 summarizes the values of KIA obtained at different experimental conditions. As expected, the partition coefficients were independent of the carbon dioxide flow rate. Through the van't Hoff equation
336 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Table 11. Absorption Conatants (WA)for HCB Distributed between Soil and Supercritical Carbon Dioxide KL,g of soil/g of COP coz flow press., psia rate, mL/h 298 K 308 K 323 K 1200 120 2.70 140 2.70 160 2.70 1700 160 2.78 2500 160 2.89 2.76 2.62 3200 120 3.12 2.71 140 3.12 2.72 160 3.13 2.87 2.71 4000 120 3.32 2.84 140 3.32 2.84 160 3.31 3.08 2.85 1.20
-
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.
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0
.
0.86
/
i
/
I
I
-0.3
-0.1
-0.2
I"
0.0
P
Figure 4. Isothermal density dependency of the partition coefficients.
and Akgerman, 1990). It is interesting to note that data at different temperatures all fall on the same curve. The temperature dependency of ln K b, at constant density is given by the expression
J 0.86
'
3.0
I
"
"
3.1
"
'
'
3.2
'
'
"
3.3
'
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1/T x 1Oa(1/K)
Figure 3. Isobaric temperature dependency of the partition coefficients.
where AHi" is the partial molar enthalpy of adsorption at infinite dilution and is similar to the heat of adsorption. The variation of Kb, with 1/T at constant pressures of 2500,3200,and 4OOO psia is given in Figure 3. The plots are consistent with thermodynamic analysis, and the slope gives an enthalpy of adsorption of 750, 1080,and 1150 cal/mol at pressures of 2500, 3200, and 4000 psia, respectively. The slight increase in AHi" is indicative of stronger adsorption of HCB on soil with increasing pressure as expected. The isothermal density dependence of the distribution coefficient is given by (Erkey and Akgerman, 1990)
where Vp and Vp" are the partial molar volumes of the solute at infinite dilution in the supercritical fluid and nolid phases, mpectively. Figure 4 shows the isothermal density dependence of the partition coefficient. The poeitive slope of the curve at all points indicates that the partial molar volume of HCB in the fluid phase approaches large negative values in the vicinity of the critical point due to the magnitude of the term R,TKT, consistent with the observations of other investigators (Eckert et al., 1986;Erkey
Both of the terms on the right-hand side of eq 18 are negative. Therefore, due to the minus sign in front of the second term, the direction of temperature dependency at constant density will depend on the magnitude of these terms. However, the magnitude of both terms is so small that the temperature dependency at constant density is very small. Hence it is possible for data at different temperatures to fall on the same curve. Entrainers, cosolvents added in small amounts, enhance the solvent power of supercritical fluids. Methanol is frequently employed as an entrainer with C02,and it is shown that the solubility of various organics in C02 is increased significantly when methanol is used. We performed a set of experiments at 298 K and three different presaures using 5% by weight methanol as entrainer. The results shown in Figure 5 indicate that methanol when used as an entrainer does not offer any advantages over pure COP In fact, the partition coefficients decrease slightly, indicating a negative effect when it is interpreted as the amount of C02required for equilibration at a given condition. However, the values are within the experimental accuracy of the measurements. This observation confirms our earlier findings concerning the entrainer effect ( h o p and Akgerman, 1989). The results indicate that although an entrainer may result in significant increases of solubilities of solutee in supercritical fluids,this does not necessarily translate into more advantageous adsorption equilibrium coefficients, and that interactions between the entrainer and the medium from which ex-
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 337 0
---
carbon dloxldo
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Figure 7. Isothermal density dependency of the Peclet numbera for the bed at subcritical and supercritical conditions. 8.0
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1.0
2.0
3.0
4.0
6.0
6.0
7.0
Time (dimensionless) Figure 6. Difference between the response peaks from the UV detector when the bed is packed with same size glass beads or soil.
traction is performed (in this case methanol/soil interactions) must be considered in addition to entrainer/solute and entrainer/supercritical fluid interactions. We have also used ethylene as the supercritical fluid at a single experimental condition (298K, 2500 psia, supercritical fluid flow rate of 160 mL/h) and the single data point is shown in Figure 5 as well. Ethylene seems to be a better solvent for HCB extraction from soil yielding a partition coefficient of 3.77 g of soil/g of COz compared to 2.89 for COZ, about 30% improvement.
In order to determine the axial dispersion coefficient, the bed was packed with 0.1-cm-diameter glass beads (same size as the average size of soil particles). Figure 6 shows the difference in the response between the glass beads and soil column. The response peak from glass beads is very sharp and has no tail, indicating no or very weak adsorption. The dimensionless Peclet number (ud,/D,) exhibita different behavior at subcritical and supercritical conditions. As shown in Figure 7, the Peclet number is strongly density dependent at 298 K (subcritical liquid COJ, but is almost density independent at supercritical conditions. All the data are taken at the same flow rate of 160 mL of COz/h at the experimental conditions. The Peclet number for a packed bed is affected by molecular diffusion and convective eddies and is usually ex-
338 Ind. Eng. Chem. Res., Vol. 31, No. 1,1992 6.3
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4.8
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-
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needed to justify the hypothesis. Employing the axial dispersion coefficients obtained from the experiments with the glass beads, we extracted the overall mass transfer coefficients from the second moment of the response curve for the soil packing. The mass-transfer coefficient was independent of the density at a constant flow rate as shown in Figure 8 and increased slightly with the flow rate of the supercritical fluid, Figure 9. We have compared the real-time response curve to the simulated solution using the parameters obtained from the method of moments. Figure 10 indicates the agreement between the calculated and measured response peaks for an experimental condition. Good agreement was obtained for all experimental conditions. The numerical solution was obtained by using the method of line algorithm (Ghoreishi, 1990). Our results indicate that the dynamic response technique is fast and accurate for determination of equilibrium partition coefficients. The first moment of the response c w e is model independent and will yield accurate values for the partition coefficient. The second moment, however, is model dependent, and the axial dispersion coefficients obtained and the overall mass transfer coefficients must be regarded with caution.
4-
0.006
u u
m I 0.000
t -0.006
Time (dimensionless) Figure 10. Comparison of experimental and predicted response in the real-time domain.
pressed as an empirical function of the Reynolds and Schmidt numbers. At subcritical conditions (liquids) the infinite dilution molecular diffusion coefficient is a very weak function of density at constant temperature (Matthews and Akgerman, 1987), yet the convective eddies are dependent on density. Thus axial dispersion may be affected by the convective term only at these conditions and the Peclet number may be a strong function of density. At supercritical conditions, however, the diffusion coefficient is a very strong function of pressure at constant temperature and its contribution may result in a density-independent Peclet number, although more data are
Literature Cited Akgerman, A.; Roop, R. K.; Hess, R. K.; Yeo, S. D. Environmental Application of Supercritical Extraction. In Supercritical Fluid Technology-Reviews on Modern Theory and Applications; Bruno, T. J., Ely, J. F., Eds.; CRC Press: Boca Raton, FL, 1991; Chapter 14, pp 479-509. Capriel, P.; Haisch, A.; Khan, S. U. Supercritical Methanol: An Efficacious Technique for Extraction of Bound Pesticide Reaiduea from Soil and Plant Samples. J. Agric. Food Chem. 1986, 34, 7e73. Dooley, K. M.; Kao, C. P.; Gambrell, R. P.; Knopf, F. C. The Use of Entrainers in the Supercritical Extraction of Soils Contaminated with Hazardous Organics. Znd. Eng. Chem. Res. 1987, 26, 2058-2062. Eckert, C. A.; Ziger, D. H.; Johnston, K. P.; Kim, S. Solute Partial Molal Volumes in Supercritical Fluids. J. Phys. Chem. 1986,90, 2738-2746. Erkey, C.; Akgerman, A. Chromatography Theory: Application to Supercritical Extraction. AIChE J. 1990, 36,1715-1721. Ghoreishi, S. M. Supercritical Extraction of Polychlorinated Biphenyls from a Solid Matrix. Ph.D. Dissertation, Texas A&M University, 1990. Hawthrone, S. B.; Miller, D. J. Extraction and Recovery of Organic Pollutants from Environmental Solids and Tenax-GC Using Supercritical Carbon Dioxide. J. Chromatogr. Sci. 1986,24,258-264. Hawthrone, S. B.; Miller, D. J. Extraction and Recovery of Polycyclic Aromatic Hydrocarbons from Environmental Solids Using Supercritical Fluids. Anal. Chem. 1987,59, 1705-1709. Kingsley, G. S. 'Pilot Plant Evaluation of Critical Fluid Extractions for Environmental Applications"; EPA/600/2-85/081, 1985. Matthews, M. A.; Akgerman, A. Diffusion Coefficients for Binary Alkane Mixtures at Temperatures to 573 K and Pressures to 3.5 MPa. MChE J. 1987,33,881-885. Raymer, J. H.; Pellizzari, E. D.Toxic Organic Compounds Recoveries from 2,6-Diphenyl-pPhenylene Oxide Porous Polymer Using Supercritical Carbon Dioxide and Thermal Desorption Methods. Anal. Chem. 1987,59,1043-1048. Raymer, J. H.; Pellizzari, E. D.; Cooper, S. D. Desorption Characteristice of Four Polyimide Sorbent Materials Using Supercritical Carbon Dioxide and Thermal Desorption Methods. Anal. Chem. 1987,59, 2069-2073. Roop, R. K.; Akgerman, A. Entrainer Effect for Supercritical Extraction from Aqueous Systems. Znd. Eng. Chem. Res. 1989,28, 1542-1546.
Ind. Eng. Chem. Res. 1992,31,339-346 Schantz, M. M.; Cheder, S. N. SupercriticalFluid Extraction Procedure for the Removal of Trace Organics from Soil Samples. J. Chromutogr. 1986,363, 397-401. Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R. D. Analytical Supercritical Fluid Extraction of Adsorbent Materials. Anal. Chem. 1987a, 59, 38-44.
339
Wright, B. W.; Frye, S. R.; McMinn, D. G.; Smith, R. D. On-line Supercritical Fluid Extraction-Capillary Gas Chromatography. Anal. Chem. 1987b, 59,640-644.
Received for review February 22, 1991 Accepted September 6,1991
Assessment of Activated Carbon Stability toward Adsorbed Organics Edmond C. Akubuiro* and Norman J. Wagner Calgon Carbon Corporation, P.O. Box 717, Pittsburgh, Penmylvania 15230-0717
Activated carbon adsorption technology is an important industrial process used in solvent recovery and air pollution abatement systems. The safe use of activated carbon requires an understanding of the conditions that might promote carbon bed exotherms. A test method has been developed which evaluates the relative oxidative activity characteristics of carbons containing organic molecules and their relative potentials for resulting in carbon bed exotherms. Results indicated that the degree of oxidizability of adsorbed organic molecule plays an important role. However, various carbons exhibited different levels of oxidative behavior toward adsorbed oxidizable organic solvents. The solvent reaction initiation temperature for methyl ethyl ketone oxidation on the carbons investigated ranged from 329 to 383 K. Observed reaction enthalpies indicated a difference of more than a fador of 5 between the least reactive and most reactive carbons. This test method predids trends in carbon and solvent reactivities similar to those determined from column studies reported in the literature.
Introduction Activated carbon technology is widely employed in solvent recovery and air pollution abatement systems. Activated carbon beds containing organic molecules sometimes come into contact with air or oxygen, in some cases, as a consequence of their normal operations. A number of investigators (Miller et al., 1987;Chapman and Field, 1979) have reported exothermic runaway reactions under these conditions. In almost all cases, oxidizable organic solvents such as ketones, aldehydes, and the like were present, to some extent, during the thermal runaway processes (Naujokas, 1979,1985). Concern for carbon bed combustion has led to numerous studies reported in the literature. These investigators (Miller et al., 1987;Chapman and Field, 1979;Naujokas, 1979,1985;Takeuchi et al., 1990;Mathewes, 1986;Wildman, 1988;Cameron and MacDowall, 1972;Bowes and Cameron, 1971;Johnson and Woods, 1971;Boiston, 1968;Hardman and Street, 1980; Hardman et al., 1980)have studied the problems associated with carbon bed exotherm, employing various conditions conducive to simulation of this process. However, the test procedures employed in most of the cited studies generally require elaborate equipment and/or long durations. This study reports the development of a comparatively simple and rapid test procedure for assessment of the relative oxidative activity characteristics of activated carbons containing organic solvents. It also compares observed results to those of literature reports on column studies (Naujokas, 1979,1985). Experimental Section System Description and Calibration. All experiments were performed on a Du Pont Model 1090 thermal analysis system fitted with Model 910DSC and Model 951 TGA accessory modules. The DSC (differential scanning calorimeter) measures heat flow into or out of a sample, while the TGA (thermogravimetric analyzer) measures sample weight changes. The DSC cell was calibrated using an indium standard. Indium samples of 15-18 mg were
Table I. DSC and TGA Experimental Parameters N2,02,or air gas type 100 mL/min gas flow rate initial temperature 293 K 17 K/min heating rate final temperature 723 K none hold temperature 7-15 mg sample size" 50 X 200 mesh (0.297 X 0.074 mm) sample particle size sample organic loading 30 w t % (mass ratio) "7 mg for DSC (based on solvent-free carbon); 10-15 mg for TGA.
typically used. The fusion endotherm was recorded usually after one initial 'conditioning" heat-cooled cycle through the transition (up to 533 K) which allowed for good thermal contact between the pan and the indium sample. Adipic acid was also employed as a DSC calibration standard. Fusion temperatures (Tf) and heat of fusion (AHf)were calculated using a Du Pont DSC interactive analysis program. The DSC system amplifier and recorder were wired 80 that exothermic peaks are displayed upward and endothermic peaks displayed downward. The TGA system was calibrated using high-purity tin, zinc, or silver wires (depending on temperature range of interest) and calcium oxalate. All calibration experiments were conducted in a nitrogen atmosphere. Nitrogen flow rates and heating rates for calibration experiments were the same as listed in Table I. Materials. The characteristics of activated carbons investigated are listed in Table 11. The solvents (>99% purity) investigated were employed with no further pretreatment. Also, oxygen (99.6%),nitrogen (99.99%), and air (zero grade) were used as received from the supplier with no further treatment. Sample Preparation and Test Procedure. All activated carbons were crushed, sized to 50 X 200 mesh, and dried overnight (approximately 18 h) at 378 K. Enough of the dried carbon (usually 1 g) was loaded with 30 wt ?% (mass ratio) of organic solvent of interest, by thoroughly mixing the carbon and the solvent in a tightly capped vial
0SSS-5SS5/92/263~-0339$03.O0/00 1992 American Chemical Society
