Catalytic wet air oxidation of phenol using active carbon: performance of discontinuous and continuous reactors

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1 Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 76:743± ) DOI: /jctb.441 Catalytic wet air oxidation of phenol using active carbon: performance of discontinuous and continuous reactors Frank Stüber, 1 * Isabelle Polaert, 2 Henri Delmas, 2 Josep Font, 1 Agustí Fortuny 1 and Azael Fabregat 1 1 Departament d Enginyeria Química, Escola Tècnica Superior d Enginyeria Química, Universitat Rovira i Virgili, Carretera de Salou, s/n, Tarragona, Catalunya, Spain 2 LGC-Laboratoire de Génie Chimique, UMR-CNRS 5503, 18 chemin de la loge, Toulouse Cedex, France Abstract: Catalytic wet air oxidation CWAO) of an aqueous phenol solution using active carbon AC) as catalytic material was compared for a slurry and trickle bed reactor.semi-batchwise experiments were carried out in a slurry reactor in the absence of external and internal mass transfer.trickle-bed runs were conducted under the same conditions of temperature and pressure.experimental results from the slurry reactor study showed that the phenol removal rate signi cantly increased with temperature and phenol concentration, whereas partial oxygen pressure had little effect.thus, at conditions of 160 C and 0.71 MPa of oxygen partial pressure, almost complete phenol elimination was achieved within 2h for an initial phenol concentration of 2.5g dm 3.Under the same conditions of temperature and pressure, the slurry reactor performed at much higher initial rates with respect to phenol removal than the trickle bed reactor, both for a fresh active carbon and an aged active carbon, previously used for 50h in the trickle bed reactor, but mineralisation was found to be much lower in the slurry reactor.mass transfer limitations, ineffective catalyst wetting or preferential ow in the trickle bed alone cannot explain the drastic difference in the phenol removal rate.it is likely that the slurry system also greatly favours the formation of condensation polymers followed by their irreversible adsorption onto the AC surface, thereby progressively preventing the phenol molecules to be oxidised. Thus, the application of this type of reactor in CWAO has to be seriously questioned when aiming at complete mineralisation of phenol.furthermore, any kinetic study of phenol oxidation conducted in a batch slurry reactor may not be useful for the design and scale-up of a continuous trickle bed reactor. # 2001 Society of Chemical Industry Keywords: catalytic wet air oxidation CWAO); active carbon; phenol; slurry reactor; trickle bed reactor NOTATION C 0 Initial phenol concentration g dm 3 ) D p Average particle diameter of active carbon mm) E A Activation energy of phenol destruction reaction kj mol 1 ) F liq Liquid feed ow rate of phenol solution g h 1 ) m AC Active carbon catalyst mass g) P O2 Oxygen partial pressure MPa) r 0 Speci c initial rate of phenol oxidation mg Phenol min 1 g AC ) r' 0 Absolute initial rate of phenol oxidation mg Phenol min 1 ) T Reaction temperature K or C) WHSV Weight Hourly Space Velocity m AC /F liq h 1 )) 1 INTRODUCTION Pollution control of aquatic environments is of primary concern especially in regions where water is not easily available. New legislation is progressively imposing more stringent environmental constraints for the disposal of industrial aqueous ef uents. Thus, waste water reduction and treatment is progressively gaining more attention. In particular, chemical, petrochemical and pharmaceutical industries generate large quantities of wastewater containing organic compounds, such as phenol and derivatives, which are poorly biodegradable or even toxic for the microorganisms. Phenol concentrations in the range of 50mg dm 3 cause a rapid decline of the active biomass due to the inhibition of reproduction of micro-organisms, and concentrations exceeding 1g dm 3 even lead to their total destruction. 1 Often, the economical * Correspondence to: Frank Stüber, Departament d Enginyeria Química, Escola Tècnica Superior d Enginyeria Química, Universitat Rovira i Virgili, Carretera de Salou, s/n, Tarragona, Catalunya, Spain fstuber@etseq.urv.es Contract/grant sponsor: University Rovira i Virgili (Received 13 November 2000; revised version received 21 February 2001; accepted 9 March 2001) # 2001 Society of Chemical Industry. J Chem Technol Biotechnol 0268±2575/2001/$

2 F StuÈber et al recovery of these contaminants is not feasible. In this case, speci cally designed chemical pre-treatment processes must be applied to reduce their impact on the classical activated sludge plants, otherwise biodegradation processes fail. Wet Air Oxidation WAO) is a well-established technique that permits the treatment of many refractory organic compounds giving a high detoxi cation of hazardous substances. 2,3 However, the complete mineralisation of the organic compounds into water and carbon dioxide is essentially accomplished at high pressure 20±200 bar) and temperature 150±325 C), which both negatively affect the economy of the process. 4 Catalytic Wet Air Oxidation CWAO) has permitted the operating conditions to be tempered by incorporating active oxidation catalysts, although the lack of both mechanical and chemical stability has prevented them from being more broadly implemented. 5±7 It is also well known that supported catalysts used in wet oxidation are severely deactivated not only by leaching of the active metals in the aggressive reaction environment, 8±12 but also by formation of carbonaceous deposits on the catalyst surface. 13 Recently, some newly developed catalysts have partially overcome the problems but the presence of noble metals or low surface supports make them too expensive or of low activity for them to be considered commercially. 14±17 On the other hand, activated carbon AC) has been applied for years as a non-selective adsorbent for the removal of organic compounds in dilute aqueous solutions. 18 Simultaneously, WAO has been extensively studied for the regeneration of AC spent with organic compounds after its use in adsorption processes. 19,20 Also, AC and other carbonaceous materials have been successfully tested as supports for catalysts, 21 even for CWAO of low molecular weight acids. 21±24 Recently, a study carried out in a trickle bed reactor has demonstrated that activated carbon AC) alone, without the help of any catalytic species, can effectively oxidise phenol solutions without the presence of any active metal. 25 It was found that the rate of phenol disappearance depends on the kind of AC used as some commercial AC yield only negligible destruction. 26,27 The role of the AC in WAO still remains uncertain, although its good performance could be related to its high adsorption capacity or its potential to generate oxygenated free radicals for initiating the oxidation reaction. The advantages of using AC are as follows. Firstly, mild pressure and temperature conditions can be simultaneously applied using a non-expensive material as oxidation promoter. Secondly, this is a single step process in contrast to other two stage schemes, 28 which use a preconcentration step by AC adsorption followed by the subsequent desorption and wet oxidation. On the other hand, the adequate choice of catalyst, the design and scale-up of process is an engineering feature of eminent importance. This requires reliable kinetic data, which are usually obtained from slurry batchwise experiments. 29 There is strong evidence that some reactions proceed and perform differently whether they are carried out in a slurry reactor or a trickle bed reactor. 30 Effects such as mass or heat transfer, as well as quite different liquid to solid ratios, could affect the macrokinetics observed or even modify the reaction mechanism. 31 For instance, formation of phenolic polymers was observed for CWAO of phenol using a copper oxide catalyst in slurry reactors 32±35 whereas no polymers were found when a trickle bed reactor was used. 8,36 As oxidative coupling of phenol has been also proved for phenol adsorption over AC in oxic conditions, 37 the different liquid to AC ratio presented by each reactor could enhance that path instead of phenol oxidation. The objective of this work was to perform the CWAO of phenol in a slurry reactor using active carbon as promoter, and to compare selected results with those coming from a continuous trickle bed system. Preliminary runs with varying stirring speed, AC catalyst mass and AC particle size were performed in a slurry reactor to determine operating conditions that eliminate both external and internal diffusion limitations. In these conditions, the in uence of oxygen pressure, temperature and initial concentration of phenol on phenol destruction rate were measured. Trickle bed data of phenol destruction were obtained at different liquid space time and otherwise constant operating conditions. To contrast these results, slurry experiments were carried out under the same conditions of temperature and pressure testing either samples of fresh AC or AC, previously used for 50h in the trickle bed reactor. The slurry run using the recycled AC was done to eliminate the fact that deactivation may take place in the trickle bed experiments as they last for several days compared with only 5±6h for batch tests. 2 EXPERIMENTAL 2.1 Materials Analytical grade phenol Ph) was purchased from Aldrich and used without further puri cation. Phenol solutions were prepared using deionised water to serve as feed solution. The gaseous oxidant was high purity synthetic air. Activated Carbon AC) was supplied by Merck Ref 2514) in the form of 2.5mm pellets. This AC is obtained from wood and has a low ash content 3.75%). The nitrogen BET method Micromeritics ASAP 2000) gives a speci c surface area of 990m 2 g 1, a pore volume of 0.55cm 3 g 1 and an average pore diameter of 1.4 nm. Phenol adsorption tests performed elsewhere 25,38 showed a maximum capacity of 370mg Phg 1 AC, at 20 C in oxic conditions. Prior to use, the AC was crushed and sieved. Table 1 lists the different particle size fractions collected and separated. Every fraction was washed to remove all nes, then dried at 110 C overnight, allowed to cool and stored under an inert atmosphere. 744 J Chem Technol Biotechnol 76:743± )

3 Catalytic wet air oxidation of phenol using active carbon Table 1. Particle size fractions of activated carbon Fraction Range mesh) Range mm) D p mm) a F1 8± ± F2 16± ± F3 25± ± F4 50± ± F5 100± ± F6 >200 < a D p : Mean average particle diameter expected. 2.2 Slurry reactor experiments Phenol oxidation was conducted in a slurry system at constant oxygen partial pressure and temperature using a magnetically stirred high-pressure reactor with a nominal capacity of 300cm 3. All experiments were carried out batchwise for the liquid and continuous for the gas with a ow rate of 2dm 3 min 1 STP) to avoid eventual accumulation of CO 2 in the course of reaction. Typically, the reactor was loaded with 175 ± 200cm 3 of feed solution and a given mass of AC. For comparison with the trickle bed reactor, the AC samples a fresh one and an aged one) for the slurry runs were previously equilibrated overnight with a 5gdm 3 phenol solution to start the experiment in the same conditions of presaturation as in the trickle bed system. The slurry run using the aged AC, previously used for 50h in the trickle bed reactor, was undertaken to consider an eventual deactivation that may have occurred in the trickle bed experiments as they last for several days compared with only 5±6h for batch tests. Once loaded, a leak test was performed by pressurising the reactor with nitrogen. Then, the reactor was jacketed with a preheated oven to provide a rapid heating. When the desired temperature of reaction was reached, generally in 10 min, the reactor was pressurised with synthetic air and a rst liquid sample was taken. Only then, the stirring was turned on to prevent oxidation reactions during the heating period. The reaction was allowed to run either for half an hour in the preliminary runs or for 5 ± 6h in the long runs, and was then stopped by rapid cooling. The oven temperature was automatically controlled but, for proper reaction temperature control, cold water was circulated through a coil placed inside the reactor. Throughout the reaction, the pressure was maintained constant by continuously feeding fresh air. Periodically, liquid samples of 10 3 dm 3 were withdrawn, quickly cooled, ltered and analysed. In order to select operating conditions in which kinetic control was assured, the extent of the external mass transfer was determined in preliminary runs by varying the AC mass from 1 to 4g, as well as varying the stirring speed o) up to 2000rpm. Likewise, different average particle diameters D p ) between 0.1 and 2mm were tested in order to highlight the effect of the internal mass transfer. The in uence of temperature, oxygen partial pressure and initial phenol concentration after saturation of AC) on phenol destruction were measured respectively in the ranges of 100 to 160 C, 0.35 to 0.95 MPa and 200 to 7000 ppm. 2.3 Trickle bed reactor experiments Continuous oxidation of phenol was conducted in a packed bed reactor performing in trickle ow conditions. The equipment consists of an SS-316 tubular reactor, 20cm long and 1.1cm id, lled with usually 7.7g of AC D p =0.5mm) and placed in a temperature controlled oven 1 C). Independent inlet systems for gas and liquid feed permit the experiments to be performed at variable liquid to gas ow rate ratios. The liquid feed 5g phenoldm 3 ) is stored in a 5dm 3 stirred glass tank, which is connected to a highpressure metering pump that can dispense ow rates between 0.01 and 0.3dm 3 h 1. The air used as oxidant comes from a cylinder equipped with a pressure controller that allows the operating pressure to be constant. A owmeter coupled with a high precision valve is used to measure and control the gas ow rate. The liquid and gas streams are mixed and then passed through a heating coil placed in the oven in order to reach the reaction temperature. The mixture enters the reactor and down ows over the AC bed, which is placed between two sintered metal discs. The exited solution goes to a liquid±gas separation and sampling system. Regularly, liquid samples were taken out for analysis. A detailed scheme of the experimental set-up is available in the literature. 25 The tests in trickle regime were performed at 140 C and 0.55 MPa of oxygen partial pressure and a liquid feed concentration of phenol of 5g dm 3. The air ow rate was set at STPdm 3 s 1, which assures the oxygen in the gas phase to be in excess with respect to the stoichiometric demand for complete oxidation of phenol that enters the reactor with the liquid feed stream. Several liquid WHSV Weight Hourly Space's Velocity) values were evaluated by feeding proper liquid ow rates. In all cases, the trickle regime was maintained Analytical method Phenol concentration was determined by HPLC using a C 18 reverse phase column Spherisob ODS-2). Separation of phenol from the partial oxidation products was accomplished using an eluent with 35% v/v) of methanol and the balanced distilled water at a ow rate of 10 1 dm 3 min 1. UV absorbency at 254 nm permitted the phenol to be detected. Periodically, calibration curves were made for phenol concentration and used to correct deviations in retention time because of variations in ow rates. 3 RESULTS AND DISCUSSION 3.1 Kinetic regime in slurry reactor In heterogeneous catalysis, the conversion of one reactant is very often affected by mass transfer limitations, so apparent kinetics are actually governed by J Chem Technol Biotechnol 76:743± ) 745

4 F StuÈber et al Figure 1. Influence of the stirring speed on the initial rate of phenol removal: C Ph,0 =5gdm 3, m AC =2g, T =140 C, P O2 =0.55MPa. external or internal mass transfer resistances. Intrinsic kinetics can only be evaluated if the above effects are minimised. Thus, a rst set of runs was carried out in the slurry reactor to select suitable conditions that eliminate any mass transfer resistances. External mass transfer effects were evaluated at 140 C, MPa of oxygen partial pressure and AC particle size of 0.1mm by varying the stirring speed and the AC weight in the range of 0 to 2000 rpm and 1 to 4 g, respectively. The initial speci c rate of phenol disappearance, r 0, was found to be almost independent of stirrer speed o). As shown in Fig 1, increments of o beyond 400±500 rpm did not improve the initial reaction rate, which suggests the absence of external mass transfer limitations on phenol destruction. This was con rmed by using different AC loading. Figure 2 shows the effect of the AC mass on the absolute initial rate of phenol oxidation, r' 0, which is equal to the speci c initial rate of phenol removal, r 0, times the AC mass, m AC. Proportionality between r' 0 and m AC is evident, which also demonstrates that the gas±liquid mass transfer is not a limiting step. Figure 2. Influence of the active carbon mass on the apparent initial rate of phenol removal: C Ph,0 =5gdm 3, T =140 C, P O2 =0.55MPa, D p =0.1mm,! =1500rpm. Figure 3. Influence of the AC particle size on the initial rate of phenol removal: C Ph,0 =5gdm 3, m AC =2g, T =140 C, P O2 =0.55MPa,! =1500rpm. Experiments with AC having different particle sizes highlighted the internal mass transfer limitations. These tests were conducted at 140 C, MPa of oxygen partial pressure and 1500 rpm using a loading of 2g of AC. Figure 3illustrates the in uence of the average particle diameter on r' 0. As can be seen, the effect of the internal mass transfer is negligible for D p below 0.3±0.4 mm. For further experiments in the slurry reactor, AC size fractions F5 and F and mm) were selected since the lower the particle size the easier it is to make a suspension and the more dif cult it is to break the particles. In turn, the F3 0.5mm) size fraction was chosen for experiments in the trickle bed in order to minimise both pressure drop and internal mass transfer limitations, the latter should not exceed 10% for the AC size fraction used. 3.2 Phenol oxidation in slurry reactor Liquid sample analyses indicate progressive destruction of phenol as well as, to only a small extent, formation of intermediate reaction products in the course of reaction. Summed concentrations of aromatic compounds hydroquinone, catecol and p-benzoquinone) reach only about 1% of the initial phenol concentration while the carboxylic acids oxalic, formic, acetic, malonic, etc.) never exceed 10%. Also, the end product carbon dioxide was detected in the outlet gas stream. Little effect of oxygen partial pressure order close to zero) on phenol removal was observed in the range of 0.35 to 0.95 MPa. It is known from the literature 32,33,36,40±42 that for even lower oxygen partial pressures the reaction order with respect to oxygen in aqueous phenol oxidation over metal supported catalyst is mostly equal to or less than 0.5. In fact, the small order in oxygen found indicates that oxygen transfer to the catalytic sites is not a limiting step within the concentration range of phenol studied. This result is rather interesting in the sense that it should be possible to operate the slurry reactor at relatively low oxygen partial pressures. 746 J Chem Technol Biotechnol 76:743± )

5 Catalytic wet air oxidation of phenol using active carbon Figure 4. Phenol concentration time profiles for different initial concentrations of phenol: m AC =2g, T =140 C, P O2 =0.765MPa,! =1500rpm, F air =2dm 3 min 1 (STP). Figure 6. Time liquid concentration profiles at different reaction temperatures: C Ph,0 =2.5g dm 3, m AC =2g, P O2 =0.71 and 0.82MPa,! =1500rpm, F air =2dm 3 min 1 (STP). Figure 4 shows the time±phenol concentration pro les for various initially equilibrated phenol concentrations C 0 ). The degradation pro les in Fig 4 reveal high initial phenol destruction and no induction period. CWAO at 140 C and MPa of oxygen partial pressure using 2 g of AC almost completely destroys phenol solutions up to 1g dm 3 within 1h and solutions up to 2g dm 3 within 3h. For more concentrated solutions 3±7g dm 3 ), the pro les show a tendency to level off. The conversion of phenol signi cantly slowed down despite the availability of liquid reactants, oxygen and fresh AC catalyst. Very similar behaviour has been observed in the case of CWAO of phenol over a mixed oxide catalyst of MnO 2 /CeO The authors proved that the activity loss observed is due to the deposition of foulant deposits carbonaceous polymers) which irreversibly adsorb on the active sites until their total blockage. For each initial phenol concentration C 0 ), the initial rate of phenol disappearance was calculated from experimental data and plotted in Fig 5. As shown in this gure, r 0 increases faster at low phenol concentration <0.5g dm 3 ) than at high concentration. The sensitivity of r 0 to the phenol concentration may also be related to the laydown type of AC deactivation, as this effect should become more important at increasing concentrations. The effect of temperature on phenol conversion in the range of 100 to 160 C, for oxygen partial pressure between 0.71 and 0.82 MPa, loaded phenol concentration of 2.5g dm 3 and 2g of AC is depicted in Fig 6. Phenol removal of 99% was achieved at 160 C within 2h, while at 100 C more than 7h were required to convert 99% of initial phenol. In order to determine the activation energy E A ) of the phenol disappearance reaction, an Arrhenius plot Fig 7) was constructed using experimental initial reaction rate data. From the slope of the plot, E A was evaluated to be 37kJ mol 1 which seems to be relatively small compared with those frequently reported in the literature for the oxidation of phenol over metal supported catalysts 85kJ mol 1 for CuO/Al 2 O 3 and CuO.ZnO/ Al 2 O 3 33,34,36,43 and 65kJ mol 1 for MnO 2 /CeO 2 13 ). This value may be taken as an apparent one that accounts not only for the oxidation of phenol to Figure 5. Initial rate of phenol disappearance as a function of initial concentrations of phenol: C Ph,0, m AC =2g, T =140 C, P O2 =0.765MPa,! =1500rpm, F air =2dm 3 min 1 (STP). Figure 7. Arrhenius plot for the initial phenol disappearance rate, ln r 0 versus 1/T: C Ph,0 =2.5g dm 3, m AC =2g, P O2 =0.71 and 0.82MPa,! =1500rpm, F air =2dm 3 min 1 (STP). J Chem Technol Biotechnol 76:743± ) 747

6 F StuÈber et al seen in Fig 8, abcisses gives a normalised time coordinate, t, that permits both group of results to be represented in the same graph. Assuming that the reaction order with respect to phenol concentration is one 25 and that the trickle bed reactor follows an ideal plug ow model, 27 the evolution of phenol conversion, X Ph, versus the liquid ow-rate as well as the kinetic constant can be expressed as: X Ph ˆ 1 exp k 1 m AC Q ; t 1 ˆ mac Q 1 k 1 ˆ 1 t 1 ln 1 X Ph 2 Figure 8. Comparison between the performance of slurry and trickle systems for CWAO: T =140 C, P O2 =0.55MPa, m cat =4g (slurry) and 7.7g (TBR), D p =0.5mm, F air =2.4dm 3 min 1 (STP), F liquid = dm 3 h 1 (STP). intermediate products, but also for the supposed fast formation of polymers by oxidative coupling due to its free-radical nature. Results on homogeneous aerobic coupling of aqueous phenol catalysed by cuprous chloride at temperatures and partial pressures of oxygen below 60 C and 1 bar, respectively, reported a comparable activation energy of about 50kJ mol According to the authors the relatively low value was consistent with the free-radical nature of the reaction. 3.3 Comparison of slurry reactor and trickle bed reactor Figure 8 compares the results obtained at same conditions of temperature and pressure in trickle bed reactor and slurry reactor. For the runs in the slurry reactor both a fresh and spent AC sample D p = 0.5mm) were tested. Figure 9 shows the evolution of the kinetic constants as calculated from eqns 2) trickle bed) and 4) slurry reactor), respectively. As Figure 9. Evolution of kinetic constants in slurry and trickle bed reactor as a function of normalised time co-ordinate t (for operating conditions, refer to Fig 8). where k 1 g liq g 1 cat h 1 ) is the apparent kinetic rate constant of the trickle bed reactor, m AC g cat ) is the weight of catalyst, Q dm 3 h 1 ) is the volumetric ow rate, r g dm 3 ) is the liquid density, and t 1 h) is the inverse of the WHSV h 1 ). Similarly, the evolution of the conversion against the reaction time and the kinetic constant for the batch slurry reactor can be written as: X Ph ˆ 1 exp k 2 m AC V t k 2 ˆ 1 t 2 ln 1 X Ph ; t 2 ˆ mac V t 3 4 where k 2 g liq g cat 1 h 1 ) is the true kinetic rate constant, V dm 3 ) the liquid volume, r g dm 3 ) the liquid density and t h) the reaction time. Figure 8 shows that, even in case of the aged AC, the phenol removal of the slurry system is signi cantly better with respect to the trickle bed reactor. Values of the initial rate constants at very small conversion) calculated using eqns 2) and 4) even indicate that the one in the slurry reactor is about 50 times higher see also Fig 9). This is surprising since trickle bed systems were originally designed to improve the performance of continuous three-phase systems, whereas in our case the experimental results apparently showed an opposite effect. Mass transfer limitation, certainly present to some extent, in the trickle bed reactor cannot, alone, satisfactorily explain these ndings. First, the study on AC particle size see Fig 4) shows that diffusion within the pores is not a limiting step for the used AC particle size of 0.5mm. Also, external mass transfer limitations may not explain the drastic difference in the phenol destruction rates as the operating conditions of the trickle bed reactor are set to provide a trickle bed ow regime. 39 Comparing the ratio of the measured apparent reaction rate and the respective rate of external mass transfer allows the in uence of gas±liquid and liquid±solid mass transfer to be checked. If the ratio is less than 10%, it is generally accepted that mass transfer is not a limiting step. The measured apparent initial reaction rate is 0.038mol/ m 3 s 1 ) resulting in a k l a and k s a s value of 0.065s 1 in the absence of mass transfer limitation. Values of k l a and k s a s have not been experimentally 748 J Chem Technol Biotechnol 76:743± )

7 Catalytic wet air oxidation of phenol using active carbon measured but can be estimated with the help of the experimental values reported in the review of Gianetto and Specchia 39 for the low interaction regime trickle bed ow). In our experiments, the power dissipated in the liquid DP/DZ) LG v L was varied from 20 to 100W m 3. According to their graph, the corresponding experimental values of k l a in the low interaction regime typically vary from 0.02s 1 to 0.06s 1, and from 0.01s 1 to 0.03s 1 for k s a s. These values are comparable to the required one based on the apparent initial reaction rate), hence external mass transfer can not explain alone the differences observed in the slurry and trickle-bed reactors. Also, the effect of AC deactivation in the trickle bed reactor should not play an important role. Phenol oxidation is known to occur through a complex reaction pathway 45 that could include in the rst steps the formation of polymers of high molecular weight 28,46 that could be partially adsorbed on the active carbon surface. Due to the longer duration of the trickle bed tests, the polymers if formed) could accumulate on the surface, thus reducing the effective surface area. This type of deactivation process certainly occurred in the trickle bed, 25 but as seen in Fig 8, phenol removal in the slurry reactor both for a fresh and a used AC as close at short reaction times, although the differences increased up to 50% in the course of reaction. The phenol removal in the slurry system was still much faster than in the trickle bed reactor see Figs 8 and 9). It is known also that incomplete and/or ineffective catalyst wetting as well as preferential ow can occur in trickle bed reactors at low liquid ow rates and inadequate reactor diameter/particle size ratio. 39 Both effects will reduce the reactor performance, and thus may explain smaller overall reaction rates as observed in our study. However, the drastic differences in initial reaction rates suggest an additional explanation that could be based on the simultaneous presence of two different reactions for phenol removal: the classical oxidation reaction and the condensation reaction that form polymers. According to this assumption and considering our results, the very high liquid to solid ratio in the slurry reactor would greatly enhance the formation of heavy polymers through oxidative coupling. These polymers could irreversibly adsorb on the AC surface and progressively block the active carbon sites, thereby lowering the rate or rate constant) of phenol removal. As a matter of fact, Fig 9 indicates such behaviour. The rate constant k 2 in the slurry system, calculated assuming a rst order reaction eqns 3) and 4)), decreases drastically in the course of reaction, whereas k 1 eqn 4)) was found to be constant in the trickle bed reactor. Liquid sample analysis and total organic carbon balance provides further evidence. In the slurry reactor, only a certain quantity of the destroyed phenol was found as intermediate products such as aromatic compounds and carboxylic acids, or the end product carbon dioxide. In addition, less intermediate products appeared during reaction in the slurry reactor at a given phenol conversion. In the case of the fresh AC sample, for instance, only four intermediate peaks were detected by HPLC analysis compared with 14 peaks in the trickle bed reactor. Since carbon balance is not accomplished in the slurry system, the missing carbon should be accumulated as phenolic polymers adsorbed on the AC surface. 4 CONCLUSIONS CWAO of aqueous phenolic solutions using active carbon as a catalyst was performed in slurry and trickle bed reactors. At relatively mild operating conditions of 160 C and 0.71 MPa of oxygen partial pressure, almost complete phenol removal was achieved in the slurry reactor for an initial phenol concentration of 2.5g dm 3 in 2h. When compared with the trickle bed reactor, the slurry system gave extremely faster initial rates of phenol destruction, even in case of an aged AC sample previously used for 50 h in the trickle bed reactor. On the other hand, the oxidation of phenol towards intermediates aromatic compounds, carboxylic acids) and end product carbon dioxide) was found to be strongly enhanced in the trickle bed system. Mass transfer limitations, ineffective catalyst wetting as well as preferential ow, certainly existing to some extent in the trickle bed reactor, can only partially explain the drastic differences in phenol removal rate observed. From a chemical point of view, these contradictory results may also be due to the preferential formation in the slurry reactor) of heavy polymers which irreversibly adsorb on the active carbon, thereby progressively preventing further oxidation of phenol molecules. On the other hand, the higher catalyst to liquid ratio given in the trickle bed reactor should favour the heterogeneous oxidation pathways rather than the phenol condensation reactions. Ongoing research in the trickle bed aims to determine the activation energy of phenol removal, test catalyst dilution and gas±liquid up ow operation in order to clarify the in uence of mass transfer and hydrodynamic effects in the trickle bed reactor. In addition, the assumed laying down of polymers on the AC surface will be quantitatively analysed to show up possible differences in reaction mechanisms. The results obtained in this work are important in the context of modelling and scaling of catalytic reactors that are used for CWAO treatment of industrial wastewater. Design procedure needs reliable kinetic data that, in the case of catalytic phenol oxidation, should also account for possible catalyst deactivation via polymer formation and laydown on the active catalyst site. As pointed out in this work, it is not appropriate to apply directly experimental kinetic information obtained in the slurry system to meet the design of more complex continuous three phase reactors, since it undoubtedly could lead to wrong designs. J Chem Technol Biotechnol 76:743± ) 749

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