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SEPARATIONS Vapor-Phase Adsorption of Hexane and Benzene on Activated Carbon Fabric Cloth: Equilibria and Rate Studies† Kunwar P. Singh,* Dinesh Mohan, G. S. Tandon, and G. S. D. Gupta Environmental Chemistry Division, Industrial Toxicology Research Centre, P.O. Box 80, Mahatma Gandhi Marg, Lucknow 226001, India
Volatile organic compounds (VOCs) comprise more than 60% of total hazardous air pollutants that are emitted by major industrial point sources. Adsorption by an activated carbon fiber has been recognized as one of the feasible regenerative control processes to separate and recover VOCs for reuse. The adsorption behavior of hexane and benzene in a single-component and in a mixture system onto activated carbon fabric cloth was studied. The adsorption capacities were evaluated using the Langmuir adsorption isotherm model. Depending upon the design of the experiment, we have concluded that internal diffusion controls the adsorption of benzene and n-hexane on activated carbon cloth. 1. Introduction Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere by industrial sources each year. Many of these VOCs are hazardous to human health and the environment. VOCs contribute to brown, hazy smog that surrounds many urban areas and are a leading cause of respiratory problems. VOCs, originating from anthropogenic sources, are the monocyclic aromatic hydrocarbons and the volatile chlorinated hydrocarbons. Both groups of compounds are considered priority pollutants in view of their high toxicity and volatility. Monocyclic aromatic hydrocarbons are mainly emitted by industrial processes and the combustion of fossil fuels, while chlorinated hydrocarbons are widely applied as solvents for cleaning, as degreasing agents in metal industries, and as fumigants.1 Adsorption has been recognized as one of the most practical regenerative methods for separating and recovering VOCs from the industrial flue gas streams. In recent years, new adsorbent materials2 and sorption methods have been actively studied for the efficient separation of VOCs from polluted air streams.3 Activated carbon is a logical choice for the removal and recovery of VOCs from air streams emanating from a range of industrial sources. The requirements of the carbon adsorbent are especially demanding in those applications with high flow rates and low VOC concentrations. In such situations, beds of granular activated carbon (GAC) must be relatively deep in order to provide sufficient contact time for the adequate removal of the adsorbates. An attendant disadvantage is the pressure drop over the bed. The rate of adsorption can be greatly increased by using narrow diameter activated carbons (micron vs millimeter dimensions) and can be achieved †
ITRC Publication No. 2189. * Corresponding author. Phone: +91-522-436144. Fax: +91522-228227. E-mail:
[email protected].
by using small particles or fibers. However, to circumvent problems of unacceptably high-pressure drop, handling, and containment, the particles or fibers must be incorporated into a more tractable flexible or rigid structure. Activated carbon fabric (ACF) cloth is a promising adsorbent that can be used for the efficient separation, purification, recovery, and storage of VOCs. A unique property of an ACF is the possibility to “reactivate” the fabric when it has become saturated and to reuse it. The activated carbon fabric is a flexible form of activated carbon. The activated carbon fabric is mechanically weak but highly porous in nature, and because of this fact, it possesses unique characteristics as compared to the conventional activated carbon, which is commonly used in granular, palletized, powdered, and moulded forms. Because of the thin fibrous shape in activated carbon fabric, a fast intraparticle adsorption kinetics takes place in gas- and liquid-phase adsorption. A number of adsorbents were used for the removal of VOCs from time to time. Chiang et al.4 studied the effects of surface characteristics of activated carbons on VOC adsorption. They reported that boiling point, critical temperature, cross-sectional area, and dipole moment of the VOCs are the most important properties governing activated carbon adsorption. Thermal waves resulting from the dynamic adsorption of organic vapor present in air on a GAC filter were studied.5 An experimental design was carried out to determine the influential factors among the relative humidity of air (0-60%), the initial water content of activated carbon (0-9.8%), and the VOC concentration range (0-50 g m-3; i.e., 0-20,700 ppm for acetone). They found that the moisture content of the air in the range of 0-95% is not found to be a prominent factor affecting both the adsorption capacity and warming of the GAC bed for the high VOC concentration tested. For multicomponent competitive adsorption process, a mathematical model was built6 which describes the mass transfer kinetics in a fixed-bed adsorber packed with activated carbon
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fibers. Lin and Lin7 investigated the gas-phase adsorption characteristics of 1,1,1,2-tetrafluoroethane (HFC134a) by activated carbon fiber, extruded activated carbon, GAC, activated alumina, and molecular sieve. The gas-phase adsorption equilibria for acetone, diethyl ether, and methanol at different temperatures on activated carbon fiber were also studied.8 Various workers have exploited activated carbon fibers for the removal of a variety of other solvents9-19 from time to time. The activated carbon cloth was also used to remove color19 using an ultrafiltration technique. Benzene is emitted into the workplace and the environment from industrial and other man-made sources, including gasoline from filling stations, smoking tobacco products, and auto exhaust. Benzene is carcinogenic in humans and animals by inhalation and in animals by the oral route of exposure.20 Occupational exposure to benzene increased incidences of acute myeloblastic or erythroblastic leukemias and chronic myeloid and lymphoid leukemias among workers. Hexane is used as solvent for glues, varnishes, cements, and inks. Acute inhalation exposure of humans to high levels of hexane causes mild central nervous system (CNS) depression and irritation of the mucous membranes. CNS effects include dizziness, giddiness, slight nausea, and headache in humans.21-23 Acute exposure to hexane vapors may cause dermatitis and irritation of the eyes and throat in humans.21,23 Efficient utilization and design of an ACF-VOC sorption system requires optimization of various adsorption parameters. In this paper, we have studied the removal efficiency for benzene and hexane in single- and multicomponent systems on activated carbon fabric cloth to determine the optimum set of parameters necessary to design the pilot plant on a large scale. Background. Activated carbon fiber adsorbents are synthetic materials manufactured from an array of uniform polymeric substrates, including polyacrylonitrile, pitch-based polymers, and phenolic-based resins.24 These materials can be used to produce ACFs with uniform and continuous pore structures; in addition, they contain very low amounts of inorganic impurities as compared with the conventional activated carbon feedstocks. ACF uses chiefly cellulose or acrylic precursors. The production of ACF involves stabilizing the acrylic precursor in the same way as by the structural carbon fibers, in air up to 300 °C. The resulting oxidized fiber can be directly activated or, more usually, made into a fabric through conventional textile means (felting, spinning and weaving, or knitting). The oxidized acrylic cloth is then activated through heating the fabric at a temperature up to 1300 °C, not in an inert atmosphere like in structural carbon fiber but in an oxidizing atmosphere such as CO2 or H2O (steam). The action of the oxidizing agent induces a huge porous surface area. The size and configuration of the pore greatly influences the power of the carbon atoms on the surface to act as chemical “hooks” to attach to many chemical substances which may pass through the ACF filter. Basically, ACF carbon is a form of graphite, free of nitrogen, hydrogen, halogens, sulfur, or oxygen. Most of ACFs have a fiber diameter of 7-15 µm, which is even smaller than the powdered activated carbon. The activated carbon employed for purification purpose usually contains macro(10-30%), meso-(20-40%), and micropores (40-50%) (radii >50, 50-2, and internal transport; (case II) external transport < internal transport; and (case III) external transport ≈ internal transport. In cases I and II, the rate is governed by film and particle diffusion, respectively. In case III, the transport of ions to the boundary may not be possible at a significant rate, thereby leading to the formation of a film with a concentration gradient surrounding the sorbent particles. Depending upon the design of the experiment, it may be concluded that internal diffusion controls the adsorption of benzene and n-hexane on activated carbon cloth. It is interesting to note that the adsorption capacities of ACF for hexane and benzene obtained from equilibrium studies are found to be more or less same obtained from the kinetic studies at the point where the adsorption saturation is nearly reached. 4. Conclusions
Figure 7. Logarithmic rate of solvent uptake. Table 5. Constants m and Kt for the Sorption of Benzene and n-Hexane on Activated Carbon Cloth Fabric benzene concentration (mg/L)
n-hexane concentration (mg/L)
constants
10
100
200
50
250
450
slope m intercept Kt
0.321 1.611
0.148 1.418
0.016 0.688
0.590 1.098
0.1314 0.7665
1.020 0.358
can give the idea about the rate of adsorption can be written as33
∆S ) Kt tm
(6)
where ∆S is the percent solvent removal, t is the contact time in minutes, and m and Kt are constants. Slope m depicts the adsorption constant, and Kt represents the rate factor. Fairly linear plots between the log of time versus the log of percentage solvent removal are obtained, as shown in Figure 7. The values of m and Kt are given in Table 5. The lower values of m for hexane indicate a better adsorption or stronger bonds between hexane and ACF cloth. This justifies subsequent removal of hexane by diffusion through carbon fabric cloth, when concentration gradient is large at solidsolvent interface. Higher values indicate an increase in solvent removal rate. For proper interpretation of the experimental data, it is necessary to determine which of the steps in the adsorption process governs the overall removal rate under the specified experimental conditions. The three consecutive steps in the adsorption of an organic/inorganic species by a porous adsorbent are (i)
The ACF cloth was used as an adsorbent for the adsorption of benzene and n-hexane in the singlecomponent and in mixture systems. The data were correlated with Langmuir and Freundlich adsorption isotherms. The adsorption of n-hexane was found to be more in comparison to benzene. The activated carbon cloth also worked well for the mixture of benzene and hexane. Kinetics studies were undertaken to determine various rate constants. Depending upon the design of the experiment, it may be concluded that internal diffusion control the adsorption of benzene and n-hexane on activated carbon cloth. The detailed studies on organic desorption and chemical regeneration of the ACF cloth in fixed-bed columns without dismantling the same are in progress. Acknowledgment The authors are thankful to the Director, Industrial Toxicology Research Centre, Lucknow, for providing all the necessary facilities for this work and consistent encouragement and guidance throughout the studies. Literature Cited (1) Keymeulen, R.; Van Langenhove, H. In Application of gas chromatography/mass spectrometry for determination of volatile organic pollutants in plants. II; Buszewski, B., Ed.; Oı`olnopolskie Sympozium Chromatograficzve: Torun´, 1995. (2) Lo, I. M.-C.; Cheng-Hao, L.; Liljestrand, H. M. Tricaprylmethylammonium bentonite complexes as adsorbents for benzene, toluene, ethylbenzene and xylene. Water Sci. Technol. 1996, 34 (7/8), 319. (3) Lordgooei, M.; Rood, M. J.; Rostam-Abadi, M. Modeling Effective Diffusivity of Volatile Organic Compounds in Activated Carbon Fiber. Environ. Sci. Technol. 2001, 35 (3). (4) Chiang, Y.-C.; Chiang, P.-C.; Chang E.-E. Effects of Surface Characteristics of Activated Carbons on VOC Adsorption. J. Environ. Eng. (ASCE) 2001, 127 (1), 54.
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(5) Delage, F.; Pre´, P.; Le Cloirec, P. Effects of Moisture on Warming of Activated Carbon Bed during VOC Adsorption. J. Environ. Eng. (ASCE) 1999, 125 (12), 1160. (6) Li, P.; Xiu, G.; Jiang, L. Competitive Adsorption of Phenolic Compounds onto Activated Carbon Fibers in Fixed Bed. J. Environ. Eng. (ASCE) 2001, 127 (8), 730. (7) Lin, S. H.; Lin, R. C. Adsorption of 1,1,1,2-Tetrafluoroethane by Various Adsorbents. J. Environ. Eng. (ASCE) 1999, 125 (9), 802. (8) Tomoshige, N.; Suzuki, T.; Katayama, T. Gas-phase adsorption equilibria for acetone, diethyl ether, methanol, and water on activated carbon fiber. J. Chem. Eng. Jpn. 1991, 24 (2) 160. (9) Foster, K. L.; Fuerman, R. G.; Economy, J.; Larson, S. M.; Rood, M. J. Adsorption characteristics of trace volatile organic compounds in gas streams onto activated carbon fibers. Chem. Mater. 1992, 4, 1068. (10) Cal, M. P.; Larson, S. M.; Rood, M. J. Experimental and modeled results describing the adsorption of acetone and benzene onto activated carbon fibers: Comparison of experimental and modeled isotherms. Environ. Prog. 1994, 13 (1), 26. (11) Cal, M. P.; Rood, M. J.; Larson, S. M. Removal of VOCs from humidified gas streams using activated carbon cloth. J. Gas Sep. Purif. 1995, 10 (2), 117. (12) Cal, M. P.; Rood, M. J.; Larson, S. M. Gas-phase adsorption of VOCs and water vapor on activated carbon cloth. Energy Fuels 1998, 11, 3311. (13) Lordgooei, M.; Carmichael, K. R.; Kelly, T. W.; Rood. M. J.; Larson, S. M.; Activated carbon cloth adsorption cryogenic system to recover toxic volatile organic compounds. J. Gas Sep. Purif. 1996, 10 (2), 123. (14) Lordgooei, M.; Sagen, J.; Rood, M. J.; Rostam-Abadi, M. Sorption and mass transfer of toxic chemical vapours in activated carbon fiber cloth fixed bed adsorbers. Energy Fuels 1998, 12 (6), 1079. (15) Lordgooei, M.; Carmichael, K. R.; Rood, M. J.; Larson, S. M. Development of an Activated Carbon Fiber Cloth Adsorption/ Regeneration System to Recover and Reuse Toxic Organic Compounds. Final Report, ENR HWR-94115; Waste Management Research Center: Champaign, IL, 1998. (16) Lordgooei, M. Adsorption Thermodynamics and Mass Transfer of Toxic Volatile Organic Compounds in Activated-Carbon Fiber-Cloth for Air Pollution Control. Ph.D. Dissertation, University of Illinois, Urbana-Champaign, IL, 1999. (17) Lordgooei, M.; Rood M. J.; Rostam-Abadi, M. Prediction of Adsorption Characteristics of Volatile Organic Compounds. Activated Carbon Fiber. Recent Developments in Air Pollution Control; AIChE Publications: New York, 2000; pp 207-212. (18) Dimotakis, E.; Cal, M. P.; Economy, J.; Rood, M. J.; Larson, S. Chemically treated activated carbon cloths (ACCs) for removal of volatile organic carbons from gas streams: Evidence for Enhance physical adsorption. Environ. Sci. Technol. 1995, 29, 1876. (19) Pignon, H.; Brasquet, C.; Le Cloirec, P. Coupling ultrafiltration and adsorption onto activated carbon cloth: application to the treatment of highly coloured wastewaters. Water Sci.
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Resubmitted for review January 29, 2002 Revised manuscript received January 29, 2002 Accepted February 4, 2002 IE0105674
