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y = dimensionless amount adsorbed (= q/qo),dimensionless
dimensionless axial position in the column (= kpG/u), dimensionless z = axial position in the column, m or cm ZE = column length, m or cm Greek Letters t = void fraction of the bed, dimensionless { = dimensionless parameter (= kfR/5De),dimensionless 0 = dimensionless time (= kp,(t - ('/u)z)/pb(qo/co)), dimensionless I.L = viscosity of the gas, g/cm s [ = dimensionless radial position (= r / R ) ,dimensionless pb = bulk density of the bed, g/cm3 pf = density of the gas, g/cm3 ps = particle density, g/cm3 r = dimensionless parameter (= kp,(u/ V)/Pb(qo/Co)),dimension1ess
2
Subscripts in = value at the column inlet out = value at the column outlet = value at equilibrium Registry No. CH,Br, 74-83-9; C, 7440-44-0.
Literature Cited Carberry, J. J. AIChE J . 1960, 6 , 460. Chang, F. H. I.; Tan, K. S.; Spinner, I. H. AZChE J . 1973, 79, 188, Chu, J. C.; Kaiii, J.; Wetteroth, W. A. Chem. Eng. Rog. 1953, 49, 141. Cooney, D. 0.; Shieh, D. AIChE J. 1972, 78, 245. Hashimoto, K.; Miwa, K.; Negate, S. J. W m . Eng. Jpn. 1975, 8 , 367. Mor, L.; Mor, L. A.; Sldeman, S.; Brandes, J. Chem. Eng. Sci. 1980, 35, 725.
Received for review April 28, 1982 Accepted October 27, 1982
Aerobic Coupling of Aqueous Phenol Catalyzed by Cuprous Chloride: Basis of a Novel Dephenolization Scheme for Phenolic Wastewaters Phool K. Llm,' John A. Cha, and Champaklal P. Palel Depadmnt of Chemical Engineering, North Carolina Stste University, Raleigh, North Carolina 27650-5035
The discovery that cuprous chloride can effect specifically the aerobic coupling of aqueous phenol under mild reaction conditions ( T < 60 O C , Po?< 1 atm) has led us to propose a novel dephenoiization scheme for high strength phenolic wastewaters. As envisioned In the proposed scheme, aqueous phenols will be converted aerobically and catalytically into insoluble coupling products which may then be easily separated from the wastewaters. The recovered s o l i organics may be disposed of as a fuel. The low costs of cuprous chloride and oxygen, the simplicity of the scheme, and the fuel value of the recovered organics make the scheme highly promising. Kinetic studies with ammoniacal phenol solutions indicate that the coupling reaction is first order with respect to cuprous chloride catalyst, half order with respect to oxygen, approximately 1.7th order with respect to phenol, and second order with respect to base. At 26 O C and pH 9.7, the rate constant is such that the half-life of a 0.05 M aqueous phenol solution in the presence of 1.58 g/L of capper catalyst and 1 atm oxygen pressure is about 2 h. The reaction activation energy is 12.5 kcal/g-mol. Humic acids are believed to the the major components of the coupling products. A preliminary study with actual coal gasification wastewater confirms the promising nature of the proposed coupling-dephenolization scheme.
Introduction A major problem associated with a large-scale coal gasification project is the generation of a large quantity of high strength phenolic wastewater which has to be treated. It has been estimated (Beychok, 1974) that a commercial-scale coal gasification plant, equivalent to a 250 X lo6 ft3/day synthetic natural gas plant, will generate about 100 tons/day of phenols in 4 X lOe gal of wastewater. In view of the toxicity (Baker et al., 1978; Throop, 1977) and offensive odor of phenols, the coal-conversion wastewater must be rigorously dephenolized prior to its discharge into the environment. Among the dephenolization schemes which have been proposed-solvent extraction (Beychok, 1974; Cavanaugh et al., 19771, biological oxidation (Benefield et al., 1977; Cavanaugh et al., 1977; Holladay et al., 1978; Luthy and Tallon, 1980),physical adsorption (Cavanaugh et al., 1977), and anaerobic oxidation (Eisenhauser, 1968; Ficek and Boll 0196-4305/83/1122-0477801.50/0
, 1980; Keating et al., 1978)-solvent extraction and biooxidation schemes have received the most attention, although both have their drawbacks. Phenol extractants are inherently polar in nature; they are sufficiently soluble in water to warrant an expensive solvent recovery step in an extraction scheme. Biooxidation, on the other hand, is limited to dilute phenolic wastewaters (typically less than 1000 mg of phenols/L); it is slow and highly sensitive to fluctuations in temperature, solution pH, nutrients, and loadings (Holladay, et al., 1978; Luthy and Tallon, 1980; Vela and Ralston, 1978). The pronounced difference (>3 or 4 decades on a weight basis) between the aqueous solubilities of phenols and their coupling products has prompted us to consider the possibility of using an oxidative coupling reaction as a means to dephenolize high strength phenolic wastewaters. We have investigated the possibility of effecting the oxidative coupling of aqueous phenol aerobically and catalytically, 0 1983 American Chemical Society
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and we wish to report our discovery that cuprous chloride can bring about the desired coupling reaction under mild reaction conditions. In this paper, we present the kinetic results of the catalyzed coupling reaction and consider the possible application of the coupling reaction as a means to dephenolize coal-conversion wastewaters. Experimental Section The catalyzed aerobic coupling of aqueous phenol was studied in a glass Morton reactor. Simulated and actual coal gasification wastewaters were used in the study. Simulated wastewaters were made up of reagent-grade phenol, ammonium hydroxide, ammonium chloride, ammonium acetate, and sodium chloride; they were used to study the kinetics of the coupling reaction. Actual coal gasification wastewaters were generated by a pilot-plant scale, fluidized-bed coal gasifier located within the Department of Chemical Engineering at North Carolina State University. The actual wastewaters were used to confirm the promising nature of the coupling-dephenolization scheme. Cuprous chloride was used as supplied, i.e., in the powder form. Vigorous mixing of the reacting solution was provided by a mechanical stirrer designed for a closed system (available from Fisher Scientific). Temperature control was achieved in a water bath. A constant oxygen pressure was maintained throughout each run. The catalyzed aerobic coupling reaction was followed by monitoring the phenol concentration of the reaction mixture by use of high-performance liquid chromatography for the analyses. Solution samples were withdrawn from the reactor at timed intervals, quenched in dilute acetic acid solutions to arrest the reaction, and then analyzed for phenol. A Waters Associates Model 273 HPLC System equipped with a p-Bondapak C18 cartridge column was used for the phenol analyses and product fractionation. The eluent for the phenol analyses was a mixture of 40% methanol, 59% water, and 1% acetic acid, and ita flow rate was 4.0 mL/min. Results and Discussion A. Catalyst Screening. Several transition metal compounds and complexes, which are known to catalyze various aerobic oxidation reactions, were evaluated for their ability to effect the aerobic coupling of aqueous phenol. They included copper pyridine complex, a known aerobic coupling catalyst in organic solvents (Hay et al., 1959; Hay, 1962), and horseradish peroxidase enzyme, a known peroxide coupling catalyst in aqueous solutions (Brignac and Yu, 1975; Danner et al., 1973; Westerfeld and Lowe, 1942). None was found to be effective until cuprous chloride was tested. The peroxidase enzyme was found to be easily deactivated by the coupling products, which apparently blocked the substrate accessibility to the active sites of the enzyme. Cuprous chloride was chosen for the screening study for several reasons. It is known to catalyze various reactions, including oxidation, condensation, and polymerization reactions (Berkman et al., 1940; Chaltykyan, 1966), and it undergoes autoxidation reaction readily (Cotton and Wilkinson, 1972) CU'Cl+ 0 2 + H2O Cu"Cl+ + HOy + OH-
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It has a zinc blende structure (in which the cuprous and chloride ions are each tetrahedrally coordinated) and its predominantly covalent character makes it nondissociable and sparingly soluble in aqueous solutions (Wells, 1962; Remy, 1956). In analogy with the peroxidase enzyme which has adjacent active metal sites, we speculated that cuprous chloride might mimic the enzyme activity by
providing neighboring copper sites for the formation and the subsequent coupling of phenoxy, aryloxy, and other free radicals. Our kinetic study confirmed the suspected catalytic property of cuprous chloride; it showed that cuprous chloride readily catalyzed the aerobic coupling of alkaline phenol under very mild reaction conditions (T< 60 OC and PO, < 1atm). Other insoluble cuprous compounds, such as cuprous bromide and cuprous sulfide, also exhibited catalytic activities, but they were less potent than cuprous chloride . B. Characteristics of the Phenolic Coupling Products. A mixture of gray, dark brown, and black precipitates was formed in the aerobic coupling reaction. Soluble products, such as hydroquinone, catechol, p quinone, and o-quinone, were also formed but only in a relatively small amount (
