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Catalytic Wet-Air Oxidation of a Chemical Plant Wastewater Over Platinum-Based Catalysts

Posted on: Wednesday, 15 February 2006, 06:00 CST

By Cybulski, Andrzej; Trawczynski, Janusz

ABSTRACT:

Catalytic wet-air oxidation (CWAO) of wastewater (chemical oxygen demand [COD] = 1800 mg O2/dm^sup 3^) from a fine chemicals plant was investigated in a fixed-bed reactor at T = 393-473 K under total pressure of 5.0 or 8.0 MPa. Catalysts containing 0.3% wt. of platinum deposited on two supports, mixed silica-titania (SM1) and carbon black composites (CBC) were used. The CBC-supported catalyst appeared to be more active than the SM1-supported one. A slow decrease of activity of the platinum on SM1 (Pt-SM1) during the long- term operation is attributed to recrystallization of titania and leaching of a support component, while the Pt-CBC catalyst is deteriorated, owing to combustion of the support component. The power-law-kinetic equations were used to describe the rate of COD removal at CWAO over the catalysts. The kinetic parameters of COD reduction for the wastewater were determined and compared with the kinetic parameters describing phenol oxidation over the same catalysts. Rates of COD removal for the wastewater were found higher than those for phenol oxidation over the same catalysts and under identical operating conditions. Water Environ. Res., 78, 12 (2006).

KEYWORDS: wet air oxidation, wastewater, catalyst, support, lumped kinetics.

doi:10.1275/106143005X84468

Introduction

Processing of organic compounds generally generates various types of wastewater that contain toxic organic compounds. Disposal of such wastewaters into natural water reservoirs before wastewaters are treated is environmentally and legally unacceptable. The increasing demand for the reuse of water and stringent water-quality regulations have caused the need for treating all wastewater streams. Technologies used for the treatment of such streams should lead to the destruction of hazardous-waste components to give innocuous end products. Oxidation processes, which are used for treating wastewaters, can be classified as follows:

* Activated sludge treatment. This is the most widely used method because of its simplicity and relatively low cost. However, it is unsuitable for toxic waters or streams with high chemical oxygen demand (COD).

* Incineration. Effluents characterized by COD greater than approximately 100 g O2/dm^sup 3^ are appropriate for such treatment. However, this method is rather expensive (energy and equipment) process.

* Chemical oxidation. This method can be used only in specific situations and may be prohibitively expensive when used to achieve complete oxidation of all organics present in the wastestream.

* Oxidation in the liquid phase at high temperature (wet oxidation). This includes several possibilities, such as oxidation under supercritical conditions and wet-air oxidation. (Matatov- Meytal and Sheintuch, 1998; Mishra et al., 1995).

The catalytic oxidation of organic-containing wastewaters (CWAO) has gained much attention (Levee and Pintar, 1995; Luck, 1999). However, most of the investigations used model wastewaters containing a single organic compound. Reports on catalytic oxidation of real multicomponent wastewaters are scarce. Most of them deal with experiments that are performed by using autoclaves. Mixed manganese-cerium oxide (Mn-Ce oxide) and coppersupported mordenite [Cu(II)TNaY] catalysts were effective in total organic carbon (TOC) removal (up to 60%) from high-strength, alcohol-distillery waste liquors. However, the catalysts lost activity quickly because of deposition of carbonaceous materials (Belkacemi et al., 2000). Black liquor from a pulping mill was successfully treated by CWAO on platinum-palladium supported on aluminaceria (Pt-Pd/alumina-ceria) catalysts at 433 to 463 K and 1.5 to 2.2 MPa (Zhang and Chuang, 1998). Results of catalytic wet-air treatment of concentrated wastewaters from a petrochemical company (COD = 42.8 g O2/dm^sup 3^) showed that more than 50% of COD reduction could be achieved at T = 473 K and p = 3.0 MPa (Lin et al., 1996). An active carbon- supported, copper catalyst significantly improved treating of dyeing and printing wastewater by CWAO (Hu et al., 1999). Few reports have been published concerning CWAO of wastewaters in fixed-bed reactors. Pintar et al. (2001) reported that treating of Kraft bleach plant effluents by CWAO on a rubidium-titania catalyst makes it possible to remove nearly 90% of TOC, without leaching of the catalyst components.

It can be advantageous to use CWAO plants at an installation where wastewater is generated before it is mixed with other waters from the plant. Concentration of organics in streams leaving the installation is relatively high, and this makes it possible to operate CWAO plants autothermally or even with a positive balance of heat to be used within the plant.

Although the concept of CWAO is very interesting, successful application of this technology has not yet been fully demonstrated. Catalysts with appropriate activity and resistance are still needed. To our knowledge, now only Nippon-Shokubai Co., Ltd. (Japan) has reached success in commercial implementation of the CWAO technology applying heterogeneous catalysts. It is claimed that at least 10 industrial plants in Japan and worldwide (Interduct, 2003; Mitsui, 2003). As reported by Luck (1996), the Nippon-Shokubai process (NS- LC) involves platinum-palladium on titania-zirconia (Pt-PdTTiO^sub 2^-ZrO^sub 2^) honeycomb catalyst that prevents solids deposition on the catalytic surface.

Table 1-Characteristics of the catalysts.

The purpose of this work is to test platinum catalysts in a process of oxidation of a real waste water from Chemipan (Warsaw, Poland), a small chemical company producing organic fine chemicals. The wastewater contains mainly n-hexane, acetone, cyclohexanone, methanol, ethanol, polyethylene glycol, ethyl acetate, di- and trichloromethane, diglyme, other organic solvents, minor quantities of side products from organic syntheses, and inorganic salts such as sodium chloride, sodium sulfate, and potassium chloride. The initial COD value was 1800 mg O2/dm^sup 3^. This wastewater is treated by using a combination of physical-, chemical-, and biological- treatment units outside the company. Although this procedure is still satisfactory, there is a need to improve its efficiency and to reduce the load of the treatment system. Therefore, trials were made to use the CWAO process to deal with this wastewater. The Chemipan wastewater was oxidized in a high-pressure, fixed-bed reactor over Pt-cataly sts, which were also investigated in our work on oxidation of phenol (Cybulski and Trawczynski, 2004). Phenol has been widely used as the model compound in studies on CWAO, because of its relatively high refractoriness to oxidation. Only halogenated organics are commonly considered to be more resistant to oxidation. Having also in mind the availability of experimental data on CWAO of phenol over the catalysts investigated in the present work, we have decided to compare the performance of these catalysts in the oxidation of the real wastewater and of the phenol solutions, despite a lack of phenol in the wastewater investigated. A lumped kinetic model for CWAO of wastewaters was used to make comparisons possible.

Methodology

The catalysts were prepared by impregnation of supports with an aqueous solution of chloroplatinous acid. Impregnated precursors were dried for 24 h at room temperature, then for 24 h at 383 K, and reduced under hydrogen flow for 4 h at 723 K. Thereafter, they were cooled under hydrogen atmosphere and passivated (to avoid deep metal oxidation) by contacting with air through open ends of the reactor.

Two supports were used: carbon black composite (CBC) and mixed silica-titania (SMl). The CBC was prepared from a carbon black Carbex-330 (Carbochem, Gliwice, Poland) and a solution of partially polymerized polyfurfuryl alcohol (PFA) in acetone, which was prepared according to the Schmitt et al. (1975) procedure. The weight ratio of carbon black to PFA was 4:1. The mass was extruded, dried 24 h at room temperature and 24 h at 383 K, and then heated under an argon stream for 6 h at 973 K. This "raw" CBC was activated by boiling in concentrated nitric acid for 1 h. The SMl support was prepared from hydrated titania (from chemical plant Police in Poland) and hydrated silica gel (from chemical plant Inowroclaw in Poland). Both components were mixed, kneaded, passed through a screw extruder, dried, and calcined at 823 K for 2 h. Characteristics of the fresh and used catalysts are summarized in Table 1.

Oxidation of the wastewater was studied by using a stainless- steel, fixed-bed, high-pressure flow reactor, which is described, in detail, elsewhere (Cybulski and Trawczynski, 2004). The reactor was run in the regime of a trickle-bed reactor. In a typical arrangement, the reactor was packed with 1OO cm^sup 3^ of glass balls (d = 1.5 mm), 50 cm^sup 3^ of catalyst particles (1 = 2 to 3 mm, d = 1.4 mm), and again 100 cm^sup 3^ of inert glass balls, each layer on top of another. The wastewater was preheated, mixed with air, and pumped into the reactor. The reaction products were quenched and separated into the liquid and gas by using high- and low-pressure separators. The conversions reported were obtained after a period of \1 to 8 h operation under fixed conditions. Tests were performed at 393 to 473 K, 5.0 or 8.0 MPa, and liquid hourly space velocity (LHSV) = 0.5 to 6 h^sup -1^.

The specific surface area and average pore radius of the investigated materials were determined by mercury porosimetry. Chemical oxygen demand was determined by the standard dichromate Cr^sub 2^O^sub 7^^sup 2-^/Cr^sup 3+^ method. The X-ray diffraction (XRD) patterns were taken with an X-ray diffractometer DRON-3 (Russia), by using a nickel-filtered copper target line K^sub α^ (CuK^sub α^). The pH of wastewater samples was measured by Inolab level l pH meter (Wissenschaftlich Technische Werkstatten, Weilheim, Germany). Mechanical resistance of the catalysts was determined as the strength necessary to crush the catalyst particle. The mean value of 20 measurements of strength for each sample was considered the characteristic value for the sample.

Results and Discussion

The dependence of COD removal on residence time of the liquid in the reactor is shown in Figures 1 and 2. The Pt-CBC catalyst is more active in COD removal than the Pt-SMl: the same degree of COD removal is obtained on the Pt-CBC at a temperature of approximately 30 K lower than for Pt-SMl. A common feature of these figures is that COD removal tends to level off as residence time becomes sufficiently long. Similar behavior was observed for CWAO of phenol solutions (Cybulski and Trawczynski, 2004). This originates from the formation of intermediates, which are difficult to oxidize, and these were identified as carboxylic acids (Mishra et al., 1995). The pH evolution, as the residence time prolonged, is shown in Figures 3 and 4. For both catalysts, the value of pH initially decreases fast (oxidation products are clearly more acidic than the raw wastewater) and then slowly increases up to 5 for the Pt-SM1 and up to 6.5 for the Pt-CBC when COD conversion approaches unity. These data confirm that acidic compounds are initially formed and then slowly oxidized. A higher end value of pH for the CBC-catalyst shows that intermediates are decomposed to a higher degree than for SM1- catalyst, under identical reaction conditions.

Figure 1-Effect of residence time on chemical oxygen demand removal on the Pt-SMI: [black circle] 423K, 8 MPa; [white circle] 453K, 8 MPa; [black square] 473K, 8 MPa; [white square] 473K, 5 MPa.

Figure 3-Effect of residence time on pH of products on the Pt- SMI: [black circle] 423K, 8 MPa; [white circle] 453K, 8 MPa; [black square] 473K, 8 MPa; [white square] 473K, 5 MPa.

In Figure 5, COD conversions are presented as a function of time- on-stream (τ). The catalysts behaved in a different way in this stability test. The COD conversions for the SM!-supported catalyst decrease slowly in time, whereby, the higher the temperature, the faster is the deactivation. For the CBC-supported catalyst, COD conversions increase as the pressure and temperature are raised. However, contrary to the behavior of the Pt-SMl catalyst, COD conversions for the Pt-CBC catalyst increase initially and, thereafter, decrease in time. To transform conversions to catalyst activities, the following procedure was applied. Kinetic coefficients were calculated for all experimental data by using the second-order kinetic model. It turned out that the kinetic coefficients, for the first period of the test, were constant to the accuracy of the experimental procedure. These values were averaged, and the average was considered to be the initial value of the rate coefficient, k^sub 0^. Activities of the catalyst defined as the ratio k^sub τ^/k^sub 0^ were calculated for all other experimental points. Results of these calculations are shown in Figure 6. Lines on these plots represent spline functions, which were used to regress experimental data. Activity of the Pt-SMl catalyst remains unchanged for τ = 40 h. Thereafter, it starts to decrease and reaches the values of approximately 0.9 and 0.75 at τ = 100 h, at temperatures of 423 K and 473 K, respectively. Similarly, the activity of the Pt-SMl catalyst is practically constant for 35 to 40 h. Thereafter, its activity begins to rise and then decreases, whereby it reaches the value of more than 4 at the highest temperature and pressure.

Figure 2-Effect of residence time on chemical oxygen demand removal on the Pt-CBC: [black circle] 423K, 8 MPa; [white circle] 453K, 8 MPa; [black square] 393K, 8 MPa; [white square] 393K, 5 MPa.

Figure 4-Effect of residence time on pH of products on the Pt- CBC: [black circle] 423K, 8 Mpa; [white circle] 453K, 8 Mpa; [black square] 393K, 8 Mpa; [white square] 393K, 5 MPa.

X-ray diffraction patterns of the fresh and the used Pt-SMl (Figure 7) show that, under CWAO conditions, recrystallization of titania takes place. Anatase signals become more and more intensive when the reaction conditions become more severe. Increase in porosity (loss of surface area) and decrease of mechanical resistance is observed at the same time (Table 1). These data suggest that deactivation of the SM !-supported catalyst, in time, can be attributed to the progressive leaching of its component, presumably silica.

The CBC-supported catalyst exhibits an almost constant and moderate activity at 393 K and 5 MPa of total pressure. However, changes of its texture were observed at higher temperature: loss of mechanical resistance and development of porosity and surface-area changes so that no correlation with the reaction conditions can be found. In this case, the effects observed must be attributed to combustion of carbon material of CBC. Similar observations were reported by Fortuny et al. (1999), who used active carbon as a catalyst for CWAO of phenol. They also found that combustion of carbon material can be reduced by decreasing the oxygen partial pressure. Indeed, the decrease of total pressure from 8.0 to 5.0 MPa affects distinctly the rate of COD reduction and changes catalyst texture ( see data in Figure 7 and Table 1). Moreover, carbon black, which is the main component of CBC, is much more resistant to oxidation than active carbon. Hence, the catalyst binder (carbonized polyfurfuryl alcohol) is likely to be burnt preferentially during the reaction, with accompanying formation of fine carbon particles. This is probably the reason for the rapid loss of mechanical strength, hi separate experiments, with ceria supported on CBC, it was observed that the catalyst was destroyed to fine carbon-black particles under severe oxidizing conditions. Oxidation of the CBC surface, in the course of reaction, leads to an increase incatalytic activity of this material. Destruction of particles makes some metal crystallites in small pores more available to reactants. Moreover, particles of carbon black that are released from the CBC oxidized can participate in a "quasihomogeneous" reaction. Formation of more fine particles, which block some parts of the reaction zone by deposition in voids, explains also the drop of apparent catalyst activity after the first period of the activity rise. This would result in channeling and, accordingly, in the decrease of the catalytic surface area that participates in the catalytic reaction. Moreover, fines can be carried away from the reaction zone to the inert packing, and this might be also considered responsible for the drop of activity. More detailed explanation of the observed maximum in activity versus time-on-stream plot is not possible at this stage of research.

Figure 5-Chemical oxygen demand conversion as a function of time- on-stream. LHSV = 2 h^sup -1^, COD = 1800 g O2/m^sup 3^; [black circle] Pt/CBC, 393K, 5 MPa; [black square] PVCBC, 393K, 8 MPa; * Pt/ CBC, 423K, 8 MPa; [white circle] Pt/SM1, 423K, 8 MPa; [white square] Pt/SM1, 473K, 8 MPa.

Figure 6-Catalyst activity as a function of time-on-stream. LHSV = 2 h^sup -1^, COD = 1800 g O2/m^sup 3^; [black circle] Pt/CBC, 393K, 5 MPa; [black square] Pt/CBC, 393K, 8 MPa; * Pt/CBC, 423K, 8 MPa; [white circle] PI/SM1, 423K, 8 MPa; [white square] Pt/SM1, 473K, 8 MPa.

Figure 7-X-ray diffraction patterns of the Pt-SMI catalysts: A = fresh; B = after test at 423K and 8.0 MPa; C = 473K, 8.0 MPa; and D = 473K, 8.0 MPa, 104 h on stream.

Comparison between Catalytic Wet-Air Oxidation of Wastewater and Phenol Solutions: Kinetic Analysis of Experimental Data

Table 2-Kinetic parameters for Pt- SM1.

The mechanism of organic CWAO is very complex, and the exact reaction network has not been established, even for pure compounds. In the case of wastewater, which is a mixture of different components, each of its own specific reactivity, the oxidation network becomes still much more complicated. To model such a process, the rate equation, based on lumping of organics in a form of COD, was used. This is a very common way for kinetic modeling of mixtures consisting of many components, also in the case of modeling of CWAO processes.

Table 3-Kinetic parameters for Pt- CBC.

Rate coefficients for both CWAO processes cannot be compared directly in the case of the Pt-CBC catalyst, because of differences in reaction conditions for both processes. Therefore, the coefficients were regressed by using the Arrhenius equation for pressure of 8 MPa. The Arrhenius plots for oxidation of phenol and wastewater on the two investigated catalysts are shown in Figure 8, and parameters of the Arrhenius equation are given in Table 4. There are some minor biases on lines for phenol oxidation over the Pt-SMl and wastewater oxidation over the Pt-CBC, but regression can be considered satisfactory. The resulting coefficients of the pseudosecond-order-rate reaction (apart from CWAO of phenol on the Pt-CBC) are highly temperature-dependent, and this is characteristic for the kinetic regime.

The Arrhenius equations were used to generate rate coefficients at identical temperatures for both processes. These rate coefficients and ratios k'^sub wastewater^/k'^sub phenol^ calculated\, based on the coefficients for pressure of 8 MPa, are given in Table 5. Comparison of the second-order-rate constants for oxidation of phenol and the real wastewater at identical temperature and pressure and calculated ratios k'^sub wastewater^/ k'^sub phenol^ snow that the catalysts studied perform better in oxidation of the wastewater than in oxidation of the 0.5% phenol water solutions. The ratios k'^sub wastewater^/k'^sub phenol^ for the Pt- SM1 become smaller with the increase in temperature. Contrary to this, the ratios k'^sub wastewater^/k'^sub phenol^ for the Pt-CBC become higher as the temperature increased. These trends are difficult to explain, but it is clear that the rates of oxidation of the wastewater are, in general, higher than these for phenol. This is somewhat unexpected, in view of the literature data on CWAO of organics. Pintar and Levee (1992 and 1994) have found that phenol is oxidized faster than p-chlorophenol over the copper catalyst. Imamura et al. (1988) have stated that, in the presence of ruthenium supported on ceria (Ru/CeO^sub 2^), phenol is oxidized easier (conversion α = 94.8%) than n-propanol (α = 47.2%), polyethylene glycol of molecular weight 1000 (PEG 1000) (α = 54.3%), and n-butanol (α = 27.8%). The last mentioned compound was more difficult for oxidation than acetic acid (α = 44.5%), which is considered to be the one of the most refractory to oxidation in CWAO processes. Because organics in the wastewater used in this work contain over 50% mol. of alcohols and halogenated compounds, one could expect that this wastewater would be more refractory to CWAO than phenol. However, the wastewater also contains less refractory compounds, such as ketones. The measurements of rates for COD conversion (Qi et al., 1985) have set the following order for aliphatic compounds with the same number of carbon atoms in molecules: aldehydes > ketones > alcohols > carboxylic acids, whereby dibasic alcohols and acids appeared to be more reactive than their chemical mono equivalents. Imamura et al. (1988) reported that conversion of formaldehyde at high pH (α > 96%) was higher than phenol conversion under similar reaction conditions. Thus, it can be expected that aldehydes and ketones are easier oxidized than phenol. However, the wastewater used in this investigation contains less than 20 mol.% of ketones, so the effect of their presence on the overall rate of COD removal seems to be less important than the negative influence of alcohols and halogenated alkanes. It can be also supposed that the presence of inorganic salts in the feed solution exhibits no measurable effect on catalyst activity. So, the higher rates of oxidation of wastewater should be attributed to the specific activity of the catalysts studied, although the reasons of this observation remain still unknown. This finding requires further studies.

Figure 8-Arrhenius plot for the rate coefficients determined by pseudo-second-order-kinetic model (eq 1): [black circle] Pt/SM1 = phenol oxidation; [white circle] PI/SM1 = wastewater oxidation; [black square] Pt/CBC = phenol oxidation; and [white circle] Pt/ CBC, wastewater oxidation.

Table 4-Estimates of the parameters in the Arrhenius equation.

Table 5-Ratio of phenol and wastewater-oxidation-rate coefficients.

Conclusions

Catalytic wet-air oxidation, of real wastewater from the fine chemical plant on platinum catalysts supported on silica-titania and CBC, was investigated. The CBC-supported catalyst appeared to be more active than the silica-titania supported one: maximum COD conversions are 77.2 and 94.2%, respectively, under identical operating conditions. Both catalysts change their physical characteristics and chemical activity during long-term operation. Slow decrease in the activity of the Pt-SMl can be attributed to recrystallization of titania and leaching of a support component in the course of prolonged oxidation, and this results also in the increase of its porosity and loss of the mechanical strength. The activity of the Pt-CBC catalyst increases initially, passes through the maximum, and drops down in time. Its porosity develops, while mechanical strength decreases during operation, presumably owing to burning carbon component(s) of the support. This can lead to disintegration of the catalyst. Oxidation of the catalyst surface and formation of fine catalyst particles can be considered responsible for the temporary rise of the catalyst activity. On the other hand, entrapping of fine catalyst particles from the reaction zone can be considered the reason of further decrease of the CBC- catalyst activity.

Gross kinetic analysis of CWAO process was carried out, based on the power-law-kinetic model of second order, with respect to COD. Pseudo-second-order-rate coefficients were used for comparison of the catalyst performance in oxidation of the real wastewater and the model compound (i.e., phenol). Surprisingly, rates of COD removal for the wastewater turned out to be higher than those for phenol oxidation over the same catalysts, although the reverse trend could be expected based on a high proportion (approximately 50 mol%) of alcohols and halogenated hydrocarbons, which are more refractory to oxidation than phenol.

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Acknowledgments

Credits. This work was supported by the Polish State Committee for Scientific Research (Warsaw, Poland) under research project No. 3 T09B 054 18. The authors are grateful to the reviewers for their comments and suggestions made after reading the manuscript.

Authors. Andrzej Cybulski is Ph.D. graduate from the Institute of Industrial Chemistry, Warsaw, Poland. Janusz Trawczynski is Ph.D. graduate from the Institute of Chemistry and Technology of Petroleum and Coal at Wrocl[bar]aw University of Technology, Wrocl[bar]aw, Poland. Correspondence should be addressed to Janusz Trawczyrlski, Institute of Chemistry and Technology of Petroleum and Coal, Wrocl[bar]aw University of Technology, Wrocl[bar]aw, Poland. 50-344 Wrocl[bar]aw, ul. Gdanska 7/9, Poland; e-mail: janusz.trawczynski@ pwr.wroc.pl.

Submitted for publication February 3, 2003; revised manuscript submitted January 29, 2004; accepted for publication September 1, 2004.

The deadline to submit Discussions of this paper is April 15, 2006.

Copyright Water Environment Federation Jan 2006

Source: Water Environment Research

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