Generalized Kinetic Model for the Catalytic Wet Oxidation of Phenol

Apr 21, 2005 - Departamento de Ingenieria Quimica y Quimica Inorganica, ETSII y T, Universidad de Cantabria,. Santander, Spain. Catalytic wet oxidatio...
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Ind. Eng. Chem. Res. 2005, 44, 3869-3878

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Generalized Kinetic Model for the Catalytic Wet Oxidation of Phenol Using Activated Carbon as the Catalyst Aurora Santos,*,† Pedro Yustos,† Sara Gomis,† Gema Ruiz,‡ and Felix Garcia-Ochoa† Departamento Ingenierı´a Quı´mica, Facultad Quı´micas, Universidad Complutense, 28040 Madrid, Spain, and Departamento de Ingenieria Quimica y Quimica Inorganica, ETSII y T, Universidad de Cantabria, Santander, Spain

Catalytic wet oxidation of phenol using a commercial activated carbon (supplied by Chemviron Carbon) as the catalyst has been studied. The phenol oxidation has been carried out at middle temperatures (127-160 °C) and oxygen pressures (3.4-16 bar). An integral fixed-bed reactor with concurrent upflow of the gas and liquid phases that allows reaching of the steady state has been used as the experimental setup. The activated carbon employed is stable at the maximum temperature and pressure employed for periods higher than 200 h. It was found that the reaction takes place only at the catalyst surface, with the adsorption phenomena for both organic compounds and oxygen being significant. A generalized kinetic model is proposed for phenol oxidation and mineralization. The selected model, discriminated among several proposed, includes an adsorption term and fits the experimental data quite well. Moreover, small amounts of stable short-chain organic acids are produced in the phenol oxidation, and this fact is also taken into account in the kinetic model for the phenol mineralization rate description. Introduction Nowadays, huge volumes of industrial wastewaters containing organic compounds refractory to biological oxidation are produced. The petrochemical industry, coal plants, and production of many items (such as plastics, dyes, explosives, resins, disinfectants, biocides, etc.) cause these types of wastewaters,1-3 which cannot be treated by conventional biological processes. Among these nonbiodegradable organic compounds, phenol has often been employed as a model pollutant because it is widely used in industrialized countries.1 Alternative methods have been developed for the remediation of these effluents with nonbiodegradable organic compounds. Among these methods are adsorption on activated carbon (AC),4 advanced oxidation processes,5,6 and wet air oxidation (WAO).7-10 These techniques allow oxidation of refractory organic pollutants to CO2 and short-chain acids, which are more amenable to biodegradation.11 The use of heterogeneous catalysts in the process of wet oxidation is an attractive possibility, having been the object of numerous studies in the past decades,12 because it allows one to operate under smoother conditions of pressure and temperature. Moreover, a catalyst improves the oxidation of the most refractory compounds, reducing the number of intermediates, which makes the later application of conventional processing, for example, aerobic or anaerobic biological treatment, possible.13 Oxidation catalysts are the most commonly employed catalysts and can be classified into three groups: noble metals, metallic and metallic salts, and their complexes. Phenol oxidations in the aqueous phase has been performed by employing many different solid catalysts, such as oxides of Cu,14-24 Mn/Ce,25-29 V,25 and Zn,17,30 as active species, but noble metals (Ru,27,31-33 Pd, and * To whom correspondence should be addressed. E-mail: [email protected]. † Universidad Complutense. ‡ Universidad de Cantabria.

Pt27,34-37) have also been used. The main problem to be solved for the application of the catalytic wet oxidation (CWO) process at an industrial scale is the stability of the catalyst. Catalysts based on noble metals are expensive and suffer a quick deactivation by fouling because of the formation of a polymer at the catalyst surface.35,38 Metallic oxides (copper oxide shows the highest activity) are deactivated by leaching of the active phase due to the acidification of the media with the oxidation process.22,39,40 Moreover, this lixiviation introduces an additional toxicity in the media,41,42 and a further separation step to eliminate the metal ions in solution is required. If catalysts based on copper oxides are used, the pH of the media can be controlled by means of a buffer system,43 avoiding the leaching problem but making the process much slower than when carried out at acidic conditions.42 Because of the limitations quoted above, new catalysts are continuously prepared and tested. Thus, it would be very interesting to found a solid with significant activity in the CWO of organic compounds without the impregnation of any active phase. The AC is a candidate for this purpose because it not only is an excellent adsorbent but also has appropriate catalytic properties because of its different oxygen surface complexes.44,45 AC has a large number of applications as a catalyst support because it is relatively cheap and stable in both acidic and alkaline environments. AC has recently been used successfully as a catalyst for phenol oxidation with impregnation of different metals27,46 or even as a catalyst itself.47-50 AC without impregnation has also been successfully used as a catalyst for other oxidation reactions in the liquid phase, such as in the production of C4-C6 bicarboxylic acids from cyclohexanone.51 The main problem for the use of AC as a catalyst in the CWO is to prevent the burning off of the solid. Thus, the temperature and oxygen pressure must be fixed in an adequate operation range. The kinetics of the process under stable conditions is a helpful tool in order to understand and scale up the

10.1021/ie050030g CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005

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CWO process. Important interactions (adsorption or chemisorption) between the AC surface and both the organic compounds and the dissolved oxygen are expected. Therefore, the experimental data to be used for the kinetic model determination must be obtained in a continuous setup. Only the continuous operation allows reaching of the steady state, with the phenol adsorption and oxidation on the carbon surface occurring at the same rates. There are some works in the bibliography modeling the mineralization of the organic pollutants in residual waters.24,25,52-58 Among these works, only some of them have employed fixed-bed reactors (FBRs)24,25,56 and no data are available if AC without impregnation is used as a catalyst. In this work, phenol, as a model pollutant, has been oxidized in the liquid phase using a commercial AC selected in a previous work50 that is mechanically and chemically stable and quite active in phenol oxidation and mineralization. The temperature, phenol concentration, and oxygen pressure have been changed in a wide experimental interval. Organic phenol oxidation intermediates obtained, before phenol mineralization, were both oxidizable compounds (hydroquinone, p-benzoquinone, and p-hydroxybenzoic acid) and refractory compounds (short-chain acids such as acetic and formic).49 The formation of intermediate compounds able to be oxidized and also those refractory compounds to oxidation are usual findings in the catalytic and noncatalytic WAO, independent of the initial composition of the toxic organic compounds present in the industrial wastewaters. Parallel simplified oxidation pathways54 or triangular schemes34,53,59 have been considered, assuming first-order kinetic equations with respect to the organic compound concentration (negligible adsorption term) and using potential kinetic equations to take into account the influence of the oxygen pressure. Experimental data obtained in the CWO of phenol by using AC as a catalyst will be analyzed by means of simplified reaction schemes involving the formation and destruction of intermediates. Thus, a generalized kinetic model for CWO of wastewaters containing refractory organic compounds using ACs as catalysts will be proposed; adsorption phenomena will be carefully considered because of the nature of the catalyst employed. Kinetic Modeling For engineering purposes, some simplified kinetic models are going to be proposed and then discriminated by the fitting of experimental results. For kinetic model formulation, two steps are accomplished: a simplified reaction scheme assumption and then kinetic equations for each one of the reactions in the scheme proposal. 1. Reaction Schemes Assumption. Scheme 1 (S-1). This scheme takes into account the phenol disappearance as a single reaction; all of the compounds obtained from phenol oxidation are lumped according to

Scheme 1 1

PhOH 98 P (-RPhOH) ) r1

(1)

Scheme 2 (S-2). This reaction scheme considers the mineralization of the total organic carbon (TOC) to carbon dioxide and water, as follows:

Scheme 2 2

TOC 98 CO2 (-RTOC) ) r2

(2)

To take into account the formation of refractory compounds that are not further oxidized to CO2, Scheme 3 is proposed, dividing the TOC in two components, A and B, which correspond to the oxidizable and refractory fractions, respectively, of the organic matter. This reaction scheme assumes that A produces B or CO2 by different reactions according to a parallel network as follows:

Scheme 3 (S-3) TOC ) A + B 3

A 98 CO2 4

A 98 B (-RA) ) r3 + r4 (-RTOC) ) r3

(3)

In the case that the same kinetic order for A can be assumed in r3 and r4 and if the activation energy of r3 coincides with that of r4, Scheme 3 can be simplified as follows:

Scheme 3 Simplified (S-3S) TOC ) A + B 5

A 98 xB + (1 - x)CO2 (-RA) ) r5 (-RTOC) ) (1 - x)r5

(4)

where x and 1 - x correspond to the fraction of the carbon content of A that reacts to B and CO2, respectively. The coefficient x can be obtained as the ratio r4/(r3 + r4). This ratio (x) will be constant with the temperature and A concentration under the hypothesis assumed above. 2. Kinetic Equation Proposal. Concentration Function. For each of the reaction schemes, different kinetic equations have been proposed considering or not an adsorption term for the organic compounds, thus yielding hyperbolic or potential models, respectively, for the concentration dependence with respect to phenol, TOC, and oxidizable fraction concentrations. Only the adsorption of A has been considered in Schemes 3 and 3S because the refractory compounds are short-chain acids that show weaker adsorption at the AC surface.60 On the other hand, the oxidizable fraction of the TOC (A) is composed of phenol, hydroquinone, p-benzoquinone, p-hydroxybenzoic acid, and traces of catechol; because most of the species A correspond to phenol, it is a reasonable approach to consider only the phenol adsorption in the adsorption term of Scheme 1 (model S2-E2 in Table 1). The different kinetic equations proposed are summarized in the first part of Table 1, being proposed for a constant oxygen pressure value.

r1 )

r1 )

S1-E1-O2

S1-E2-O2

k1′PO2 CPhOH

1 + KOPO2 + K1′CPhOH

k1′PO2CPhOH

1 + KOPO2

k1′PO2CPhOH

1 + K1CPhOH

n1

S1-E2-O3

r1 )

Scheme 3

k4CA 1 + KCA

r2 )

r2 )

r2 )

1 + KOPO2 + K2′CTOC

k2′PO2CTOC

1 + KOPO2

k2′PO2CTOC

1 + K2CTOC

k2′PO2 CTOC n2

r2 )

(1 + KOPO2)(1 + K2CTOC) S3-E2-O3

k2′PO2CTOC

rj )

rj )

rj )

rj )

S3S-E2-O1

S3S-E1-O1

S3S-E2

S3S-E1

(1 + KOPO2)(1 + KCA)

kj′PO2CA

j ) 3 and 4

1 + KOPO2 + K′CA

k5CA 1 + K5CA

r5 )

r5 )

r5 )

1 + KOPO2 + K5′CA

k5′PO2CA

1 + KOPO2

k5′PO2CA

1 + K5CA

k5′PO2n5CA

r5 ) k′5 PO2n5CA

r5 )

r5 ) k5CA

Scheme 3S

S3S-E2-O3

r5 )

(1 + KOPO2)(1 + K5CA)

k5′PO2CA

k5′PO2CA S3S-E1-O3 ()S3S-E1-O2) r5 ) 1 + KOPO2

S3S-E2-O2

j ) 3 and 4 S3S-E1-O2

j ) 3 and 4

kj′PO2CA

1 + KOPO2

kj′PO2CA

1 + KCA

kj′PO2 CA

kj′PO2CA S3-E1-O3 ()S3-E1-O2) rj ) 1 + KOPO2

S3-E2-O2

S3-E1-O2

S3-E2-O1

nj

Dependence on the Oxygen Pressure r2 ) k2′PO2n2CTOC S3-E1-O1 rj ) kj′PO2njCA

r4 )

Dependence on the Organic Concentration (Constant PO2) r2 ) k2CTOC S3-E1 r3 ) k3CA r4 ) k4CA k2CTOC k3CA r2 ) r3 ) S3-E2 1 + K2CTOC 1 + KCA

Scheme 2

k2′PO2CTOC S2-E1-O3 ()S2-E1-O2) r2 ) 1 + KOPO2

S2-E2-O2

S2-E1-O2

S2-E2-O1

S2-E1-O1

S2-E2

S2-E1

(1 + KOPO2)(1 + K1CPhOH) S2-E2-O3

k1′PO2CPhOH

k1′PO2CPhOH S1-E1-O3 ()S1-E1-O2) r1 ) 1 + KOPO2

r1 )

r1 ) k1′PO2n1CPhOH

S1-E1-O1

S1-E2-O1

r1 )

k1CPhOH 1 + K1CPhOH

r1 ) k1CPhOH

S1-E2

S1-E1

Scheme 1

Table 1. Concentration Functions Proposed for the Kinetic Equations of Reaction Rates in Schemes 1-3 and 3S

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Table 2. Physical Properties of the AC Employed

carbon

Sg,BET (m2 g-1)

VP,meso, 2-50 nm (cm3 g-1)

IndReact FE01606A

745

0.120

VP,micro, dp < 2 nm (cm3 g-1) 0.322

With respect to the dependence of the oxygen pressure, the following possibilities have been checked depending on the type of oxygen adsorption considered: (i) No adsorption is considered for oxygen. In this case, it is assumed that the organic compounds (adsorbed or not) react with oxygen dissolved in the liquid phase. The kinetic equations proposed are referenced as SJ-EK-O1 (J ) 1, 2, 3, and 3S; K ) 1 and 2) in Table 1. (ii) It is supposed that the dissolved oxygen is adsorbed at the same active sites of the AC surface as the organic compounds. The kinetic equations proposed are referenced as SJ-EK-O2 (J ) 1, 2, 3, and 3S; K ) 1 and 2) in Table 1. (iii) It is supposed that the dissolved oxygen is adsorbed at active sites of the AC surface different from those of the organic compounds. The kinetic equations proposed are referenced as SJ-EK-O3 (J ) 1, 2, 3, and 3S; K ) 1 and 2) in Table 1. Obviously, the kinetic equations for SJ-E1-O2 and SJ-E1-O3 are the same because no adsorption is considered for the organic compounds. Temperature Function. Temperature influence has been taken into account in all of the kinetic equations in Table 1 according to an Arrhenius function for both kinetic and adsorption constants, as follows:

kj ) kj0 exp(-Eaj/RT)

tR )

Kj ) Kj0 exp(-∆Hj/RT) Kj′ ) Kj0′ exp(-∆Hj′/RT) (5)

Experimental Section A commercial AC from Chemviron Carbon (Industrial React FE01606A) has been used, and its physical properties are summarized in Table 2. Experimental runs have been carried out in a FBR with concurrent upflow of gas and liquid phases. The FBR reactor is made of a stainless steal tube of 0.75 cm internal diameter and 25 cm length. The experimental setup is schematically shown in Figure 1. A bed of catalyst pellets crushed to 0.65 mm diameter was placed, employing catalyst weights between 1 and 5 g. Nonporous glass spheres of 1 mm diameter were used at the reactor entrance if necessary. The reactor was placed in an oven with a proportional-integral-derivative temperature controller ((1 °C). Two different preheaters were used for the gas and liquid pipes to reach the fixed values as operational conditions. A solution of 1000 or 400 ppm of phenol at a pH0 of 3.5 was fed to the reactor at different liquid flow rates. Mass flow controllers were used for both gas and liquid phases. The reactor was kept at constant pressure and temperature, with the temperature interval studied being from 127 to 160 °C. The oxygen pressure in the reactor was fixed in the interval of 3.4-16 bar, and the total pressure was set to 16 bar with a backpressure

VL hLVR ) QL QL

(6)

The ratio between the volume of the liquid phase (VL) and the reactor volume (VR) is the liquid holdup (hL). This parameter has been experimentally determined for the particle diameter and liquid and gas flow rates employed here; an almost constant value of the liquid holdup of about 0.28 was found. The space time and the liquid residence time can be related by the following equation:

τ)

kj′ ) kj0′ exp(-Eaj′/RT)

KO ) KO0′ exp(-∆HO/RT)

valve. The equilibrium for dissolved oxygen was reached at the reactor pressure and temperature conditions. First, some runs changing the particle diameter (in the range of 0.1-2 mm) and the liquid and gas velocities (in the ranges of 0.01-0.15 and 1-8 cm s-1 STP, respectively) were carried out under standard conditions (P ) 16 bar and T ) 160 °C) to check the absence of intra- and interphase mass-transfer resistances. Then, several runs under kinetic control at different liquid flow rate values (12-200 mL h-1) were carried out in order to obtain data of phenol and TOC conversion at different space time (τ ) W/FA0) values at constant temperature. A blank run was also carried out by filling the reactor with nonporous glass of 1 mm diameter (T ) 160 °C, PO2 ) 16 bar, at the minimum liquid flow rate value used in the kinetic runs, QL ) 12 mL h-1). The residence time of the liquid phase in the FBR has been calculated as

trCCAT W W ) ) FA0 QLCA0 CA0

(7)

with CCAT being the weight of the catalyst in the reactor volume occupied by the liquid phase. In all of the runs, for all of the operational conditions employed, the steady state for the outlet composition of the reactor was achieved in the first 20 h of operation. Liquid samples were periodically drawn and analyzed. Phenol and organic compounds were identified and quantified by a high-performance liquid chromatograph (Hewlett-Packard, model 1100) using a diode array detector (HP G1315A); a Chromolith Performance column (monolithic silica in rod form, RP-18e 100-4.6 mm) was used as the stationary phase; a mixture of acetonitrile, water, and a solution of 3.6 mM H2SO4 in the ratio 5/90/5 (v/v/v) was used as the mobile phase. The flow rate of the mobile phase was 1 mL min-1, and the UV detector was used at wavelengths of 192, 210, and 244 nm. Organic acids were analyzed by ionic chromatography (Metrohm, model 761 Compact IC) using a conductivity detector; a column of anion suppression Metrosep ASUPP5 (25 cm length and 4 cm diameter) was used as the stationary phase and an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as the mobile phase, at a constant flow rate of 0.7 mL min-1. TOC values in the liquid phase were determined with a Shimadzu TOC-V CSH analyzer by oxidative combustion at 680 °C, using an infrared detector. Results and Discussion Preliminary Experiments. Some preliminary runs were carried out to determine the stability of the catalyst and the significance of the homogeneous reaction in the liquid phase.

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Figure 1. Experimental FBR setup.

Figure 2. Phenol and TOC conversion as a function of the tos. T ) 160 °C, W/QL ) 2.3 gcat min-1 mL-1, PO2 ) 16 bar, QG ) 90 mL min-1, C0 ) 1000 mg L-1, and pH0 ) 3.5.

For the study of the catalyst stability, the change of the phenol and TOC conversions with the time on stream (tos) under standard operational conditions was determined; the results are given in Figure 2. As can be seen in Figure 2, the AC is stable at least during 200 h at 160 °C and 16 bar of oxygen pressure (this time interval was used in the rest of the experimental study). The initial loss of activity of the AC observed in Figure 2 with the tos corresponds to the time required to achieve the same rates for the pollutant adsorption and oxidation on the carbon surface. From this point, the steady-state operation is reached. During this time of stabilization, the surface and pore-size distribution of the AC change and so does the organic adsorption equilibrium. After approximately 20 h of operation, almost constant surface area (about 1/4 of the original one) and pore-size distribution are obtained with the tos increase. After 200 h of operation, the AC recovered from the reactor had negligible weight changes (lower than 3%). The homogeneous reaction contribution was analyzed by comparison of the phenol and TOC conversions obtained at the same value of the space time (W/FA0) but changing the catalyst weight in the reactor, thus modifying the residence time of the liquid phase, as shown in Figure 3; as can be seen, no significant changes were obtained. Therefore, as can also be deduced from Figure 3, the reaction in the liquid phase (phenol oxidation and mineralization) can be neglected, at least under the operational conditions studied. As expected, if heterogeneous catalysis is taking place and no reaction in the liquid phase is considered, the phenol

Figure 3. Influence of the homogeneous contribution on the phenol and TOC conversion. C0 ) 1000 mg (L of phenol)-1. Table 3. Experimental Runs Carried Out under a Kinetic Control Regime runs

T C0,PhOH P O2 (bar) (°C) (mg L-1)

1-6 16 7-11 12-16 17-22 23-27 8 28-32 33-38 39-44 3.4

160 140 127 160 140 127 140

1000 400 1000 1000 1000 1000 1000 1000

W/QL (g min-1 mL-1) 0.50, 1.00, 2.33, 3.23, 4.67, 10.50 0.30, 0.60, 1.20, 2.40, 4.80 1.05, 2.44, 4.20, 8.40, 17.00 0.30, 2.40, 4.83, 8.40, 10.50, 17.00 1.05, 2.33, 4.70, 8.40, 17.50 1.05, 2.33, 4.70, 8.40, 17.50 1.05, 2.33, 4.68, 8.40, 10.50, 17.50 1.05, 2.33, 4.68, 8.40, 10.50, 17.50

and TOC conversions depend on the W/QL values and are not influenced by the liquid residence time (in the range studied). Moreover, the conversion obtained at the blank run carried out was lower than 5%. On the other hand, previous to the kinetic experiments, the operational interval for the gas and liquid velocities and the particle catalyst diameter were also determined in order to avoid the external and internal transport limitations. It was found that gas velocities between 3 and 6 cm s-1 (STP conditions), liquid velocities in the range of 0.01-0.12 cm s-1, and particle diameters lower than 0.8 mm ensured that the oxidation and mineralization rates were obtained under chemical kinetic control. Kinetic Experiments. The kinetic models proposed have been discriminated according to the experimental data obtained in the absence of transport limitations; these runs are summarized in Table 3. The experimen-

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Figure 4. Influence of the inital phenol concentration in the liquid feed on the (a) phenol conversion and (b) TOC conversion vs W/QL. T ) 160 °C, and PO2 ) 16 bar. Experimental results are given as symbols. Predicted values are given as lines.

Figure 6. Results obtained for (a) phenol conversion and (b) TOC conversion. PO2 ) 8 bar. Experimental results are given as symbols. Predicted values are given as lines.

Figure 7. Results obtained for phenol conversion and TOC conversion. PO2 ) 3.4 bar. Experimental results are given as symbols. Predicted values are given as lines.

mass balance of phenol in Scheme 1 and the mass balances of TOC and A in Schemes 1, 3, and 3S can be written as follows:

S-1: CPhOH ) CPhOH0 S-3: CA ) CTOC0 -

∫0W/Q r1 d(W/QL) L

∫0W/Q (r3 + r4) d(W/QL) L

CTOC ) CTOC0 Figure 5. Results obtained for (a) phenol conversion and (b) TOC conversion. PO2 ) 16 bar. Experimental results are given as symbols. Predicted values are given as lines.

tal results of phenol and TOC conversions are shown as symbols in Figures 4-7. According to those results given in Figures 4, 5b, 6b, and 7, an asymptotic value can be observed for the TOC conversion with W/QL, this value being around 0.75 throughout the temperature and oxygen pressure ranges studied. Scheme 2 is not able to explain this asymptotic behavior. Therefore, this scheme and all of the kinetic equations derived from it in Table 1 (S2-EK-OL; K ) 1 and 2; L ) 1-3) will not be able to describe correctly the TOC evolution with W/QL and can be neglected for further analysis. Because the experimental runs have been carried out in a FBR at constant pressure and temperature, the

(8)

∫0W/Q r4 d(W/QL) L

(9)

S-3S: CTOC ) CTOC0 - (1 - x)(CTOC0 - CA) CA ) CTOC0 -

∫0W/Q r5 d(W/QL) L

(10)

To discriminate between first-order or hyperbolic kinetic models for the function depending on the organic concentration (E1 and E2 in Table 1), the following qualitative and quantitative analysis has been accomplished. First, the conversion values of phenol and TOC, obtained at a temperature of 160 °C and 16 bar of oxygen pressure by feeding 1000 or 400 mg L-1 of a phenol solution to the FBR, have been plotted vs the ratio W/QL, as shown in Figure 4. As can be seen, higher conversions are obtained when the initial concentration of the pollutant fed to the FBR decreases. This fact cannot be explained by a first-order kinetic equation (E1), and hyperbolic models (E2) are needed.

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3875 Table 4. Kinetic Parameters Obtained at Constant Temperature by Using Kinetic Models SJ-EK (J ) 1, 3, and 3S; K ) 1 and 2) in Table 1 (PO2 ) 16 bar) model

parameter

k1 (L gAC-1 min-1) ΣR2 [(mg of C L-1)2] S1-E2 k1(L gAC-1 min-1) K1 [L (mg of C)-1] ΣR2 [(mg of C L-1)2] S3-E1 k3 (L gAC-1 min-1) k4 (L gAC-1 min-1) ΣR2 [(mg of C L-1)2] S3-E2 k3 (L gAC-1 min-1) k4 (L gAC-1 min-1) K [L (mg of C)-1] ΣR2 [(mg of C L-1)2] S3S-E1 k5 (L gAC-1 min-1) x ΣR2 [(mg of C L-1)2] S3S-E2 k5 (L gAC-1 min-1) K5 [L (mg of C)-1] x ΣR2 [(mg of C L-1)2]

S1-E1

T )160 °C

T ) 140 °C

T ) 127 °C

0.570 27750 2.76 9.29 × 10-3 2930 0.36 9.59 × 10-2 36850 0.947 0.295 3.81 × 10-3 14820 4.56 × 10-2 0.237 36880 1.24 3.81 × 10-3 0.237 14830

0.294 1813 0.411 8.21 × 10-3 845 0.16 5.76 × 10-2 6439 0.252 9.72 × 10-2 1.18 × 10-3 1517 2.17 × 10-2 0.265 6439 0.349 1.18 × 10-3 0.278 4214

0.164 4142 0.176 1.51 × 10-4 4106 9. 54 × 10-2 3.10 × 10-2 4733 0.122 4.78 × 10-2 6.85 × 10-4 4080 1.26 × 10-2 0.245 4734 0.22 6.81 × 10-4 0.245 4103

Table 5. Discrimination between Schemes 3 and 3S by Fitting the Data Obtained at PO2 ) 16 bar, with Temperature as a Variable Scheme 3 Kinetic Equation S3-E2 k3,0 (L gAC-1 min-1) 1.05 × 104 Ea3/R (K) 10072 k4,0 (L gAC-1 min-1) 2.54 × 102 Ea4/R (K) 8959. K0 [L (mg of C)-1] 3.02 × 105 ∆H/R (K) 7962 ΣR2 [(mg of C L-1)2] 26450 Scheme 3S Kinetic Equation S3S-E2 k5,0 (L gAC-1 min-1) 1.24 × 104 Ea5/R (K) 10017 K5,0 [L (mg of C)-1] 4.65 × 105 ∆H5/R (K) 8137 x 0.25 ΣR2 [(mg of C L-1)2] 27060

Experimental data, phenol or TOC, obtained at 16 bar of oxygen pressure at the three temperatures used (127, 140, and 160 °C), have been fitted to equations SJ-EK (J ) 1, 3, and 3S; K ) 1 and 2) in Table 1 by using eqs 8-10 for Schemes 1, 3, and 3S, respectively, and corresponding kinetic parameters have been determined. A Runge-Kutta method (for differential equation integration), combined with a nonlinear regression (Mardquart algorithm), has been used to calculate the kinetic parameter values. The results of these calculations are given in Table 4. As can be seen, the residual sums of squares (ΣR2) for Schemes 1, 3, and 3S are significantly lower if kinetic model E2 is used. This fact confirms that hyperbolic models fit the experimental data better, indicating that there is a significant adsorption of the reactant at the catalyst surface, as is expected when AC is used as the catalyst. Residuals obtained by considering Schemes 3 or 3S are quite close, as shown in Table 4. Moreover, the x value is almost constant with temperature. This indicates that the activation energy of reaction (3) coincides with that of reaction (4), validating Scheme 3S. A final discrimination between Schemes 3 and 3S has been accomplished by fitting the TOC data obtained at 16 bar of oxygen pressure by introducing the temperature as a variable, according to eq 5. Results are given in Table 5; as can be seen, the simplified Scheme 3 (3S) yields again almost the same residual that as those obtained with the nonsimplified Scheme 3. Therefore, Scheme 3 and all of the kinetic equations derived from

it in Table 1 (S3-EK-OL; K ) 1 and 2; L ) 1-3) will not be considered for further analysis. The kinetic constant values summarized in Tables 4 and 5 have been obtained at a constant value of the oxygen pressure (16 bar). To quantify the influence of the oxygen pressure on the kinetic equations remaining (S1-E2 for Scheme 1 and S3S-E2 for Scheme 3S), data obtained in all runs in Table 3 have been fitted to kinetic equations SJ-E2-OL (J ) 1 and 3S; L ) 1-3) by using eqs 8 and 10 for Schemes 1 and 3S, respectively, and corresponding kinetic parameters have been determined, with the values obtained being summarized in Table 6. Models considering a potential dependence for the oxygen pressure yield a reaction order value for the oxygen pressure of about 0.5 for both phenol and A oxidation rates. Other kinetic equations, considering oxygen adsorption, yield lower residuals than those using potential laws. Besides, lower residuals are obtained with the kinetic equations considering that oxygen adsorption takes place at different sites from those used for the organic compound adsorption. Moreover, kinetic equation SK-E2-O3 (K ) 1 and 3S) gives almost the same values of the adsorption oxygen constant in Schemes 1 and 3S. Thus, the generalized models discriminated for the phenol oxidation and mineralization by using AC as the catalyst are as follows:

S1-E2-O3: r1 ) 0.059e-5605/TPO2CPhOH/[(1 + 7.30 × 10-7e5022/TPO2)(1 + 3.01 × 106e-9215/TCPhOH)] (11) S3S-E2-O3: r5 ) 0.032e-5528/TPO2CA/[(1 + 4.70 × 10-7e5096/TPO2)(1 + 4.98 × 105e-8542/TCA)]

x ) 0.246 (12)

Validation of these models has been accomplished by simulation of the phenol and TOC values at the experimental conditions in Table 3. This simulation has been made by employing eqs 8 and 10, where the rate expressions are those given by eqs 11 and 12. The simulated values are plotted as lines in Figures 4-7, while points are the experimental values; as can be seen, the predicted and experimental values are very close. Parity plots of phenol and TOC concentrations are shown in parts a and b of Figure 8, with the calculated values (y axis) being those obtained by using eqs 8 and 10, where the rate expressions are those given by eqs 11 and 12, respectively. Conclusions The experimental results of the CWO of phenol by using a commercial AC as the catalyst indicate that this solid presents good chemical and mechanical stability when used in a continuous process, at least under the operational conditions studied in this work. The nonimpregnated AC tested is more active in both oxidation and mineralization of phenol than the catalysts based on copper oxides as the active phase, avoiding the leaching of the metallic ion, Cu2+, which is a problem associated with the use of these latter catalysts.42 The activity of the AC can be related to the interaction between the catalyst surface and both the organic

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Table 6. Kinetic Parameters Obtained in Schemes 1 and 3S, with the Oxygen Pressure and Temperature as Variables Scheme 1 S1-E2-O1

S1-E2-O2

S1-E2-O3

Scheme 3S

k1,0′ (L gAC-1 min-1) bar-n1 Ea1′/R (K) K1,0 [L (mg of C)-1] ∆H1/R (K) n1

42.52 8258 2.66 × 108 11166 0.52

ΣR2 [(mg of C/L-1)2] k1,0′ (L gAC-1 min-1) bar-1 Ea1′/R (K) K1,0′ [L (mg of C)-1] ∆H1′/R (K) KOo (bar-1) ∆H0/R (K) ΣR2 [(mg of C/L-1)2]

38719 0.79 6601 8.36 × 106 9369 1.19 × 10-3 -2159.73 38209

k1,0′ (L gAC-1 min-1) bar-1 Ea1′/R (K) K1,0 [L (mg of C)-1] ∆H1/R (K) KO0 (bar-1) ∆H0/R (K)

0.059 5605 3.01 × 106 9215 7.30 × 10-7 -5022

ΣR2 [(mg of C/L-1)2]

35759

compounds and dissolved oxygen, with the role of the functional oxygenated groups being a question to clarify in further analysis. These interactions could explain the positive value of the enthalpy obtained for the adsorption constant of the organic matter. The oxidation reactions take place almost quantitatively at the catalyst surface, and the reaction in the liquid phase has been proven to be negligible under the reaction conditions hereby used. Because of this fact, the adsorption must be taken into account for oxidation and mineralization rates and the first-order kinetic models usually employed in the literature are not able to describe correctly the results obtained here. On the contrary, data are obtained by using AC as the catalyst in the CWO process, required of hyperbolic functions to take into account the dependence of the reaction rates on the organic compounds and the oxygen pressure. Although phenol has been used as a model pollutant, the generalized kinetic model discriminated could be applied to other wastewater with refractory organic compounds by changing the kinetic parameters. In

S3S-E2-O1

S3S-E2-O2

S3S-E2-O3

k5,0′ (L gAC-1 min-1) bar-n3 Ea1′/R (K) K5,0 [L (mg of C)-1] ∆H5/R (K) n5 x ΣR2 [(mg of C/L-1)2] k5,0′ (L gAC-1 min-1) Ea5′/R (K) K5,0′ [L (mg of C)-1] ∆H5′/R (K) KO0 (bar-1) ∆H0/R (K) ΣR2 [(mg of C/L-1)2] x k5,0′ (L gAC-1 min-1) Ea5′/R (K) K5,0 [L (mg of C)-1] ∆H5/R (K) KO0 (bar-1) ∆H0/R (K) x ΣR2 [(mg of C/L-1)2]

31.18 8257 1.39 × 106 9002 0.531 0.247 78700 1.95 6611 2.59 × 107 9930 2.10 × 10-4 -2740 76650 0.244 0.032 5528 4.98 × 105 8542 4.70 × 10-7 -5096 0.246 74630

general, it can be considered that the original organic pollutants are oxidized, yielding both readily oxidizable organic intermediates and final intermediates quite refractory to the chemical oxidation. On the other hand, the latter are usually short-chain carboxylic acids (maleic, oxalic, acetic, and formic), which are compounds that are easily biodegradable. The generalized kinetic model discriminated for the mineralization of the organic pollutant is able to describe the formation of these final-end organic compounds. A constant value of the overall fractional yield is obtained for the oxidation of phenol, as a target pollutant, to CO2. It was found that a mineralization of about 75% of the initial organic content is achieved for the whole interval of temperature and oxygen pressure used. Acknowledgment The authors acknowledge financial support for this research from the Spanish MCYT (Project PPQ200301452 and the Ramon-Cajal Program). The authors also thank Chemviron Carbon for kindly supplying the commercial catalyst used in this work. Nomenclature

Figure 8. Parity plot for (a) phenol conversion and (b) TOC conversion.

A ) oxidizable fraction of the organic compounds in the liquid phase AC ) activated carbon B ) nonoxidizable fraction of the organic compounds in the liquid phase CA ) concentration of A expressed as the carbon content (mg of C L-1) CCAT ) catalyst concentration [g (L of liquid phase)-1] C0 ) initial concentration of the target pollutant (mg of compound L-1) CPhOH ) phenol concentration expressed as the carbon content (mg of C L-1) FA0 ) organic carbon flow rate fed to the reactor (g of C min-1) hL ) liquid holdup K ) adsorption constant [L (mg of C)-1] k ) kinetic constant for the phenol oxidation or mineralization n ) potential order for the oxygen pressure influence

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3877 PO2 ) oxygen pressure (bar) QL ) liquid flow rate (mL min-1) r ) reaction rate (mg of C gAC-1 min-1) Rj ) production rate of the j compound (mg of C gAC-1 min-1) T ) temperature (°C or K) TOC ) organic carbon concentration (mg of C L-1) TOC0 ) initial organic carbon concentration (mg of C L-1) tos ) time on stream (h) tR ) residence time of the liquid in the FBR (min) VL ) reactor volume occupied by the liquid phase (mL) VR ) reactor volume (mL) W ) catalyst weight in the reactor (g) X ) conversion x ) overall fractional yield of the organic carbon in the liquid phase to B ΣR2 ) residual sum of squares, Σ(Cexpt - Ccalc)2 [(mg of C L-1)2]

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Received for review January 10, 2005 Revised manuscript received March 10, 2005 Accepted March 30, 2005 IE050030G