198
I&, Eng. Chem. Process Des. D ~ v 1983, . 22, 198-202
Nomenclature A = cross-sectional area of column, in.' D, = particle diameter, in. f = flow term coefficient F = total force on bed, lb-f FF = force on bed due to flow, lb-f F = force exerted on bed due to gravity, lb-f r;k, = force on bed resulting from walls of container, lb-f g = acceleration of gravity, ft/s2 g, = gravitational constant K = 150/@ K' = ratio of lateral to vertical pressure L = z, PF = solids pressure due to flow, psi P, = solids pressure due to Fg,psi Pa = total solids pressure exerted on bed, psi P, = solids pressure associated with F,, psi p* = dimensionless solids pressure AP = fluid pressure drop through bed, psi hp = dimensionless fluid pressure drop through bed R = radius of column, in. S = lateral surface area of column, in.2 uo = superficial linear velocity, in./s w = wall support coefficient, in:' Z = linear distance along axis of column, in. Z* = dimensionless axial reactor distance ZL= depth of bed Z*L= dimensionless depth of bed
a = compressibility of the enzyme bed, psi-' a* = dimensionless compressibility parameter
6 = exponential compressibility parameter psi-' e =
void fraction, dimensionless
eo = initial void fraction of unstressed I.I = viscosity of fluid phase, lb-s/in.2 pf = coefficient of wall friction
bed
density of liquid phase lbm/in.3 density of solid phase 1bm/i11.~ AP = (Pa - PL) g/g, p* = dimensionless density difference @ = sphericity, dimensionless Registry No. Glucose, 50-99-7;glucose isomerase, 9055-00-9; fructose, 57-48-7. pL = pa =
Literature Cited Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. "Transport Phenomena"; Wllay: New York, 1980; p 199. Buchdz, K.; (kdelman, B. Enzyme Eng. 1978, 4 , 89. Johanson, J. R. Chem. Eng. 1979, 86, 77. McCabe, W. L.; Smith, J. M. "Unit Operations of Chemical Englneerlng"; McQraw-HIU: New York, 1978; p 812. Norsker, 0.; (Wson, K.; Zittan, L. Stark8 1879, 37. 13. Tlller, F. M.; bynes, S.;Lu, W.M. A I C E J . 1979, 78, 13. Wakeman, R. J.; Trans, I. Chem. €q. 1978, 56, 258. Wlilis. M. S.; Shen, M.; @ay, K. R. Can J . Chem. Eng. 1974, 52, 331.
Received for review March 16, 1981 Revised manuscript received July 15, 1982 Accepted August 2, 1982
Sorptive Reclamation of Phenols from Coal Conversion Wastewater TaCHdng Cha and Larry A. blasgow' Department of Chemical Enginee&g, Kansas State Unlverslty, Mnhattan, Kansas 66506
The recovery of phenols from spent activated carbon after use In the sorpthre treatment of coal conversion wastewater is economicaWy and ecobgicelly desirable. Phendlc compounds, however, are desorbed and pyrolyzed at comparable rates; an experimental study of thermal desorption has shown that approximately 40% of the adsorbed phenol can be recovered in bench-scale apparatus at an optimum temperature of about 235 'C. The reclamatbn process has been successfu~modeled assuming firstorder pyrolytic decomposition and equilibrium desorption with a Langmuirian isotherm.
Introduction A principal environmental problem associated with coal gasification is the generation of wastewater containing significant quantities of phenols. Schmidt et al. (1974) have reported a mass spectrometric analysis of gasifier condensate from the Synthane process; the contaminants were found to include phenol, cresols, dihydric phenols, naphthols, pyridines, and indenols at total concentrations as high as 7000 to loo00 ppm by weight. Existing methods for the treatment of such wastewaters include biological oxidation, destructive chemical oxidation, incineration, solvent extraction, and adsorption. Only solvent extraction and adsorption offer the possibility of phenol reclamation, and as Boyer et al. (1980) have noted, the raffinate from solvent extraction may require polishing by adsorption to meet effluent requirements. Greminger et al. (1982) have evaluated methyl isobutyl ketone and &isopropyl ether as candidate aolventa for the extraction of phenol from water; the principal problems include solvent recovery from the product water (they suggest vacuum steam stripping) and recovery of
phenol from the extract (by distillation). Energy costs and solvent losses are major factors in the application of extractive recovery processes. The purpose of the present investigation was to examine adsorption alone as a means of phenol reclamation from coal conversion wastewaters, and to lay the groundwork for an economic comparison with extractive processes. The economic feasibility of the use of activated carbon in water treatment processes is highly dependent upon the regeneration costa of the spent carbon. The most common industrial process for regeneration of exhausted carbon, according to Remirez (19771, involves treatment in multiple hearth furnaces, in which the sorbate molecules are volatilized and destroyed. The disadvantages of this approach include the large energy requirement, adsorbent attrition, furnace corrosion, and perhaps most importantly, the loss of the sorbate species. Commercial thermal regeneration normally consists of drying at about 370 K, thermal desorption and pyrolysis at about 1075 K, and a final high-temperature heat treatment in the presence of limited quantities of oxidizing
0196-4305/83/1122-0l98$01.S0/00 1983 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983
gases such as water vapor, flue gas, and oxygen in which the pyrolytic residue is gasified. Chihara et al. (1981) have suggested that the intermediate phase of regeneration is a complicated process involving decomposition of the original absorbates and desorption of low-molecular weight pyrolysis products, leaving an adsorbed carbon residue. Himmelstein et al. (1973) have made an economic comparison of alternate regeneration schemes including thermal regeneration, solvent regeneration, and reactive regeneration in which adsorbed phenols were removed by caustic soda. They found that as the contaminant concentration in the wastewater increases, solvent and reactive regeneration systems become increasingly attractive visa-vis thermal regeneration, especially when valued materials are recovered. Picht et d. (1981) have presented data obtained from treatment of phenol-loaded sorbents with a supercritical fluid (Cod; their results indicate that 50% recovery is possible with activated carbon, and nearly 100% recovery can be obtained with certain styrene-divinylbenzene polymeric adsorbents. It is important to note that reclamation of phenols prior to carbon regeneration may generate a significant economic credit for the overall process. Furthermore, the recovered compounds could relieve pressure exerted by the increasing demand for petroleum used in the synthesis of phenol and its derivatives; Haberstroh and Collins (1974) report that in 1970, 210 million gallons of benzene-obtained from petroleum-were used in the manufacture of phenol. Suzuki et al. (1978) have utilized the technique of thermogravimetric analysis (TGA) to measure the weight change when spent, granular carbon samples were heated in an inert atmosphere. The base carbon was loaded with different, single organic adsorbates, and the resultant weight change vs. temperature curves were divided into three groups according to the ease of decomposition. Single desorption and cracking steps were then used to represent the numerous and complex processes that occurred. Phenol, P-naphthol, lignin, humic acid and other phenolic organica were categorized as group 111compounds; these exhibited a gradual change of weight with increasing temperature and high residues on activated carbons after the temperature attained 800 "C. This suggests that adsorbed species of group I11 are not easily recovered by thermal methods alone. However, the fraction of phenol decomposed and the fraction of phenol which was desorbed while retaining the phenol structure were not determined. For the purpose of reclamation, it is important to know how much of the phenol adsorbed on activated carbon can be thermally desorbed while retaining the chemical structure, C6H,0H. In this investigation, a reclamation process was studied to determine the extent of thermal desorption.
Experimental Procedure Phenol was used as the sorbate species throughout this phase of the investigation. Desorption-recovery experiments were initiated by placing granular activated carbon samples loaded with phenol in a stainless steel heating chamber (16 cm long and 1.6 cm i.d.) upon a mesh support. Dissipative heat was supplied by resistance tapes and a Variac was used to adjust the power input and thereby regulate gas temperature. Nitrogen was selected as the inert carrier gas; it was preheated and passed through the supported granular carbon bed. An iron-constantan thermocouple was used to measure the outlet gas temperature. Because the gas flow rate was very small (about 2.2 L/min STP),the gas temperature was assumed to be representative of the average temperature inside the chamber.
IS9
Desorbed phenol vapor and the volatile products of pyrolysis were transported by the N2 carrier gas and introduced into the bottom of a gas absorption column (1.5 m high, 5 cm 0.d.) packed with 1.6 cm 0.d. plastic Pall rings and operated countercurrently. An alkaline (NaOH) absorbent solution was recycled by a centrifugal pump. A bypass loop around the pump was used to regulate the liquid flow rate and a l-L hold-up bottle was installed upstream of the pump to avoid the problem of cavitation. During the experiment, the hold-up bottle was placed in an ice bath to remove the heat produced by viscous dissipation. The activated carbon used throughout this investigation was obtained from Fisher Scientific Co. (6 to 14 mesh, No. 5-685B) and was manufactured from coconut charcoal. A size-distribution analysis was performed on a representative sample of the carbon with a Bausch & Lomb Omnicon Image Analysis System. The data were well described by the log-normal density function, as shown in Figure 1. The geometric mean and standard deviation were 0.261 cm and 0.008 cm, respectively. Specific surface area determinations were performed using a Perkin-Elmer Shell Model 212D Sorptometer; this method produced an average N2 BET area of 1004 m2/g. The carbon samples were loaded with phenol by immersing a known amount of carbon into a phenol solution which was agitated by a magnetic stirrer for 4 h. After adsorption, the granular carbon was decanted and placed in a desiccator for one week at room temperature. The phenol solutions were prepared by dissolving accurately weighed phenol crystals in distilled water; the change in concentration of the aqueous solution was used to calculate the phenol loading. The final concentration of the phenol solutions was roughly 600 mg/L after adsorption. The amount of phenol adsorbed on the carbon samples (16 g) was about 2.5 g, providing typical loadings of from 0.15 to 0.18 g/g. The phenol desorbed from the carbon sample was captured and accumulated in the caustic absorbent. In each experiment, 2.3 L of 0.2 N sodium hydroxide solution was used as the absorbent which was recycled at a flow rate of about 3 L/min. The hold-up liquid level of absorbent in the gas absorber column was about 20 cm height during operation. The phenol vapor carried by nitrogen was assumed to be completely captured by the adsorbent because phenol is easily converted to the phenoxide ion in alkaline solutions and good contact between the liquid and gas phases was provided. A preliminary absorption experiment indicated that the capture efficiency would exceed 99%. The rate of phenol recovery was obtained by monitoring the phenol concentration of the absorbent at selected time intervals. Absorbent samples (5 mL) were collected with a syringe from the outlet of the hold-up bottle. Phenol content was determined spectrophotometrically by the 4-aminoantipyrine method (ASTM D 1783-70) in which the characteristic antipyrine color is formed in the presence of potassium ferricyanide at a pH of 10. Absorbance was measured against blank solutions at a wavelength of 510 nm; good agreement with Beer's law was obtained for aliquot concentrations as high as 60 mg/L. Samples of higher concentration required dilution. Sample collection for analysis resulted in a small decrease in the total absorbent available. The total amount of sample removed was less than 50 mL; the maximum error due to the change in volume was less than 2% and was considered to be insignificant. Since the total space time for the liquid in the system was very short (about 45 s), the solution inside the hold-up bottle was assumed to
200
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Figure 1. Activated carbon particle size distribution.
be perfectly mixed and the phenol concentration of the absorbent sample was assumed to represent the average concentration throughout the absorbent phase. Experimental Results Four preliminary experiments'were conducted by elevating the gas temperature to desired levels in about 30 min, with sufficient heating for maintenance thereafter. The fraction of phenol recovered vs. time is presented in Figure 2. A striking feature of these results is that the fraction of phenol recovered is very low at low temperature but increases rapidly for moderately higher temperatures. The maximum recovery of phenol achieved was about 45% by weight when the gas temperature was elevated to around 230 "C. However, the fractional recovery is reduced dramatically when the nitrogen temperature is further elevated. The desorption-recovery occurred mainly in the first hour of each experiment and was almost insignificant after the second hour. Another four experiments were performed under conditions where temperatures were carefully controlled, and attention was focused on the first 2 h of operation. The first three experiments were carried out by elevating the gas temperature linearly with the same rate (6.8 OC/min) and maintaining the gas temperature after 172,243,and 278 "C were reached, respectively. The fourth experiment was conducted by elevating the gas temperature with a higher rate (11.1"C/min) until 280 O C was reached. These experimental results are presented in Figure 3, which provides a clearer picture of how the desorptionrecovery process is influenced by elevated gas temperature. It is evident that high heating rates result in reduced recovery because pyrolytic temperatures are attained before the desorption process can reach completion. The observed behavior in the desorption-recovery process can be attributed to the result of the competition between thermal desorption and pyrolytic decomposition. At low temper-
I-
Phenol Recovery at Various Temperatures
T.234-C
1'
4
/
0
2
3
Time,hr.
Figure 2. Total phenol recovery at various ultimate temperatures.
atures, decomposition is not significant and higher energy input leads to higher desorption rates. Conversely, high temperatures enhance pyrolytic decomposition; if the high-temperature region is reached too early, phenol on the carbon surface decomposes rapidly with little desorption. Kinetic Interpretation of the Recovery Process Suzuki et al. (1978)have set up two kinetic models to explain the TGA curves obtained in their experiments, i.e.,
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983 201
particles that can be desorbed. K'is a constant which is proportional to mass transfer coefficient, and the entire term of K'(q - qmbP/(l+bP) represents the rate at which phenol is desorbed, carried away, and captured by the gas absorbent. K', bo, and qm are assumed to be temperature-independent. A further assumption necessary for the solution of eq 1 is that the amount of phenol initially adsorbed, qo, is equal to qmfor the desiccated carbon samples; this simplification was also employed by Suzuki et al. (1978). After rearrangement and introduction of eq 2 and 3, eq 1 may be written
'"t
I
/
ii/
The fractional amount of phenol recovered by thermal desorption as a function of time (4) may be obtained by integration r
Tm,nnn
Figure 3. Temperature-recovery history for linear heating.
a thermal desorption model and a thermal cracking model. The models are based on two simple physicochemical characteristics of adsorbed organics during the temperature rise period: (a) thermal desorption of volatile organics initially adsorbed on the activated carbon, and (b) decomposition of organics on the adsorbent surface where some of the volatile fragmenta produced are vaporized. Suzuki et al. further suggest that the behavior of adsorbed organic compounds during heating can be classified on the basis of boiling point and aromatic carbon content. Since phenol boils at 182 "C and has a fractional aromatic content of 1, it falls into a category identified by Suzuki et al. as group 111-compounds not easily desorbed by thermal means. Indeed, the kinetic behavior of adsorbed phenol during heating cannot be represented by either a desorption or pyrolysis model exclusively because the processes occur at comparable rates. In this investigation, a model for the heat treatment of phenol-bearing carbon was formulated that mcludes both desorption and cracking terms. As an initial approximation, equilibrium desorption with a Langmuirian isotherm was assumed and the pyrolytic decomposition of phenol is represented by a first-order kinetic expression
-dq-- -kq dt
-K(q
-
gp)
where q is the weight of adsorbed material retaining the structure of phenol, q- is the saturation adsorption capacity, b is the Langmuirian energy constant, and P is the equilibrium pressure, which is treated in this model as a constant. The temperature dependence of eq 1 is due to the Arrhenius form of the velocity constant, k
k = ko exp(-E/RT)
(2)
and the dependence of b upon the heat of adsorption, Q b = bo exp(Q/RT)
(3)
+
' I
1
The difference between q and q,bP/ (1 bP) represents the amount of "free" phenol species inside the carbon
dt (5) 1 1 + - exp(-Q/RT) bop There are five unknown parameters in eq 4; the criterion function for parameter evaluation was based upon the sum of the squares of the differences between 32 pairs of calculated and experimental 4 values. Some thought was given to the application of an initial guess generation method, e.g., Glowinski and Stocki (1981). However, application is complicated by the fact that q ( t ) must be integrated in eq 5 since only @ is available as a function of t. Fortunately, some experimental evidence is at handactivation energies for the cracking of benzophenone, vanilin, and polyethylene glycol on activated carbon ranged from 41 to 109 kJ/mol according to Suzuki et al. (1978). These relatively low values may be due to the catalytic activity of the carbon surface. Typical values for the heat of adsorption are on the order of 20 to 41 kJ/mol, e.g., Herbes (1977). Further support for Q s of this magnitude is offered by Mattaon et al. (1969), who suggest that weak chemical interactions account for the adsorption of phenolic compounds upon activated carbon. A Hooke and Jeeves pattern search was used to improve parameter values; eq 4 was solved with a fourth-order Runge-Kutta scheme and @ values were determined by integration of eq 5 with Simpson's rule. A reasonably good fit was obtained with E = 141.5 kJ f mol, Q = 45.6 kJ f mol, ko = 1.72 X 10l2min-', and K' = 0.037 min-'; this is illustrated in Figure 4. Although most pyrolysis reactions follow first-order kinetics as Stiegel et al. (1981) have noted, free-radical recombination is a distinct possibility. Furthermore, the assumptions of equilibrium desorption and a single sorptive energy for the association between phenol and the carbon surface are not realistic. Despite these obvious problems, the proposed model offers a means by which thermal recovery strategies can be compared without resort to laborious experimentation. Model Application A n illustration of the comparison of thermal desorption modes is provided here, using an exponential temperature increase instead of the linear function used experimentally. An unsteady-state energy balance employing a lumped-
202
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983 0
0
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,
,
,
,
,
,
,
30
40
50
60
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90
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100
, 110
120
TIME (MINUTES)
Figure 4. Comparison of model and experimental reaulta after parameter optimization.
5 illustrates the predicted phenol recovery achieved during the first 75 min of thermal desorption using the experimentally determined parameters and 7 = 20 min. The effect of the exponential temperature increase upon total phenol recovery is slight; however, maximum recovery occurs at slightly higher ultimate temperature (-260 "C) than in the case of linear heating (-235 "C). Conclusions The motivation for phenol recovery is predicated upon the need for coal conversion, the necessity of petroleum conservation, and the increasing demand for phenols by the chemical industry. This experimental study has shown that fixed-bed thermal desorption can lead to the reclamation of more than 40% of phenol adsorbed on activated carbon by employing operating temperatures between 230 and 260 "C and a pressure of 1 X lo5 Pa. It should be possible, therefore, to recover approximately 1000 to 5000 kg of phenol per day from a large-scale (4 X lo6 L/day wastewater) coal gasification plant resulting in a sizable economic credit for the operation. Furthermore, the recovery process will not impose a significant energy burden upon spent sorbent treatment since thermal regeneration requires thorough drying anyway. Registry No. Carbon, 7440-44-0; phenol, 108-95-2.
Literature Cited Boyer, 0.
T.; DeGeorge, C. W.; Wasserstrom, D. H. "Coal Technology", V d . 6; AIChE-CEP New York, 1960 p 1.
Process
CMhara, K.; Smlth, J. M.; Suzukl, M. A I M J . 1981, 27, 213. Glowhskl, J.; Stockl, J. A I M J . 1981, 27, 1041. Qemlnger, D. C.; Bums, (3. P.; Lynn, S.;Hanson, D. N.; King, C. J. Ind. Eng. Chem. Process Des. Dev. 1082, 21, 51.
Habemtroh, W. H.; cdllns,D. E. "Rbgd's Handbook of Indwtrkrl Chemlstty"; Van Nostrand-Relnhdd: New York, 1974; p 813. H e h , S.E. Wet. Res. 1977, 1 7 , 493. HlmmeI8teln. K. J.; Fox, R. D.; Wkrtsr, T. H. aSem.€4.Rug. 1973, 69. 65. Mattilon, J. B.; Mark, H. B.; Mslbln, M. D.; Weber, W. J.; Crittenden. J. C. J . COMokl Intertam SGI. 1060, 31, 116. Plcht, R. D.; Dllman, T. R.; Burke, D. J.; deFIUpp1, R. P. Paper presented at Annual Meetlng of AIChE, New Orleans, Nov 1981.
Ko=1,72xIO mi"-'
E =33.8 kcol/gmole b,P: l . i l x I 0 ~ ' P = 10.9 kcol/gmole T - 2 0 min
200
250 Ult#mOle Temperature,
300
'C
Figure 5. Predicted phenol recovery with exponential heating.
parameter assumption produces the following relation for spent-carbon bed temperature T - Ti= AT(1 (6) where Ti is the initial carbon temperature and AT is the ultimate change. The time constant, 7 , can be estimated from the thermal resistance and capacitance of the bed; a typical value might be of the order of 20 min. Figure
Remkez, R. Chem. €ng. 1977, 84, 95. Schmldt, C. E.; Sharkey, A. G.; Frledel, R.
A. US. Bureau of Mines TPR 86, 1974. Stlegel, 0. J.; Shah, Y. T.; Krlsnamuthy, S.: Panvelker, S.V. "Reaction Englneerlng In Dlrect Coal Liquefactlon"; Addlson-Wesley: Readlng. MA, 1981; p 297. Suzukl, M.; Mlslc, D. M.; Koyema, 0.; Kawazoe, K. Chem. Eng. Sci. 1078, 33, 271.
Received for review January 1, 1982 Accepted August 27, 1982 One of the authors (T. H. Cha) received partial support from Kansas Energy Study funds administered by the Engineering Experiment Station a t Kansas State University.
