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Supercritical water oxidation of ion exchange resins: Degradation mechanisms
Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 213–222
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Supercritical water oxidation of ion exchange resins: Degradation mechanisms A. Leybros a , A. Roubaud a,∗ , P. Guichardon b , O. Boutin c a b c
CEA Marcoule, DEN/DTCD/SPDE/LFSM, BP17171, 30207 Bagnols/Ceze, France Ecole Centrale Marseille, Technopôle de Château-Gombert, 13451 Marseille Cedex 20, France Aix Marseille Universités, UMR-CNRS 6181, BP 80, Europôle de l’Arbois, 13545 Aix en Provence Cedex 4, France
a b s t r a c t Spent ion exchange resins are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation could offer a viable treatment alternative to destroy the organic structure of resins and contain radioactivity. IER degradation experiments were carried out in a continuous supercritical water reactor. Total organic carbon degradation rates in the range of 95–98% were obtained depending on operating conditions. GC–MS chromatography analyses were carried out to determine intermediate products formed during the reaction. Around 50 species were identified for cationic and anionic resins. Degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol and acetic acid are the main compounds. However, other products are detected in appreciable yields such as phenolic species or heterocycles, for anionic IERs degradation. Intermediates produced by intramolecular rearrangements are also obtained. A radical degradation mechanism is proposed for each resin. In this overall mechanism, several hypotheses are foreseen, according to HOO• radical attack sites. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Supercritical water oxidation; Ion exchange resins; Polymer; Aromatic compounds; Reaction mechanisms
1.
Introduction
Water treatment systems in nuclear power plants use organic ion exchange resins (IERs) in order to control system chemistry, to minimize corrosion and the degradation of system components as well as remove radioactive contaminants. Organic resins are used in chemical decontamination or cleaning processes for regeneration of process water by reagents and radionuclide removal. Large quantities of contaminated IERs are generated and often stored (the volume of spent IERs that arise from typical nuclear power plant is 5–7 m3 reactor unit−1 a−1 ). In the future, wastes must be treated to reduce volume and to improve stability. Unfortunately, spent resins are radioactive wastes for which there is no satisfactory industrial treatment (IAEA Report). Combustion has good performance according to treatment rate, waste volume reduction and decontamination factor. However, gas treatment system is needed to prevent emission of harmful substances such as sulphuric or nitric oxides and additionally radionuclides (137 Cs,
∗
etc.), which are volatile at temperature above 800 ◦ C. Moreover, resins tend to form large clusters by melting before burning, thus causing corrosion problems with the refractory. Embedded resins may also undergo radiolytic or chemical degradation, which can result in the emission of hazardous species in the environment. Then, according to (IAEA Report), chemical processes may be considered for the treatment of IERs, but additional studies are needed. In the case of IERs, supercritical water oxidation (SCWO) offers an alternative treatment for the destruction of organic structure of resins. At supercritical conditions (P > 22.1 MPa and T > 374 ◦ C), water acts as a non-polar medium and presents similar solvation properties as a low polarity organic solvent, completely miscible with liquid organics. Supercritical water also presents complete miscibility with oxygen, creating a homogeneous reaction medium, which makes supercritical water a suitable medium for the oxidation of liquid organics. If, however the organic material is a solid, this reaction is heterogeneous. Reaction time is about a few 106 Ru,
Corresponding author. Fax: +33 4 66 33 90 19. E-mail address: [email protected] (A. Roubaud). Received 19 June 2009; Accepted 2 November 2009 0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2009.11.001
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seconds and there is no formation of gaseous oxides (SOx and NOx), due to a relatively low temperature ranging from 450 to 500 ◦ C (compared to combustion, higher than 1000 ◦ C). Radionuclides will then remain in the cold liquid effluent and can be recovered by means of precipitation. The main problems of SCWO are corrosion and salt precipitation (leading to reactor plugging) (Bermejo and Cocero, 2006). To overcome these two problems, different kinds of reactor designs have been developed, such as tubular reactor (Cocero, 2001), cooled wall reactor (Cocero, 2001) or transpiring wall reactor (Bermejo et al., 2006), etc. CEA has developed and patented a double shell stirred reactor (Calzavara et al., 2004) as described later in this article. A few studies deal with the degradation of IERs in subcritical and supercritical conditions. IERs degradation rates between 80 and 96% were obtained in supercritical water (Dubois et al., 1996), depending on the nature of the resins and operating conditions. In order to improve degradation rates, greater than 99% TOC, different processes and ways were investigated: particle crushing (Faure et al., 1999), thermal liquefaction (Worl et al., 2000), the use of a metallic catalyst (Sugiyama et al., 2005), and a large oxidant stoichiometric excess (Akai et al., 2001, 2005). There is a lack of data regarding the reaction pathways for organic molecules and macromolecules. Polymers degradation mechanisms in supercritical water are scarcely investigated. Mechanisms of styrene monomer recovery from waste polystyrene by supercritical water partial oxidation have been studied (Douglas Lilac and Lee, 2001). Reaction mechanisms of petroleum derived hydrocarbons, e.g. biphenyl (Onwudili and Williams, 2007a), phenanthrene, naphthalene (Onwudili and Williams, 2007b) have been studied in subcritical and supercritical water conditions. Similar compounds are produced, such as phenol, acetophenone, benzaldehyde, or benzoic acid. The supercritical water oxidation of monosubstituted phenols (Martino and Savage, 1999) proceeds through multiple parallel reactions. One reaction involves ring opening and rapid formation of CO2 . Other reactions involve conversion or elimination of the substituent to form other phenolics. Chemistry reactions (oxidation, hydrolysis) in supercritical water for sulphur and nitrogen containing compounds, substituted phenols are reported by Savage (1999). In this study, a series of elementary organic chemical reactions have been identified under hydrothermal conditions. A detailed mechanism is given for methylamines (Li and Oshima, 2005; Li and Oshima, 2006) and explains formation of nitrous species such as NH4 + , NO3 − , N2 O, etc. Moreover, heterocyclic compounds such as quinoline, isoquinoline, pyridine supercritical water oxidation detailed mechanisms are also studied. Sulphur initially present could be completely converted into sulfates with a reactional medium temperature greater than 420 ◦ C (Wang and Zhu, 2003; Wang et al., 2003). In this case, reaction intermediates are S2 O3 2− and SO3 2− . This study deals with the oxidation of anionic and cationic IERs from nuclear industry in supercritical water. There is a need to evaluate the decomposition patterns of IERs intermediate compounds, in order to establish organic reaction models. It will help to throw more light on the elementary chemical reactions involved in the decomposition of organic molecules under hydrothermal conditions. The detailed mechanisms of the supercritical water oxidation of anionic and cationic IERs are presented to isolate the different stable intermediate products, which influence
the complete oxidation of the polymers. Although high total organic carbon reduction rates were observed, mechanisms are intricate and a brief description is attempted here. Hence, rate determining species are identified and it allows to create a simple mechanism that will be implemented in CFD software. The characterisation of specific reaction products is important for process optimisation and for developing thermodynamic (enthalpy of reaction, etc.) and thermophysical (viscosity, heat capacity, etc.) data.
2.
Materials and methods
2.1.
Experimental set up
The double shell stirred reactor has been developed in order to overcome corrosion and salt plugging problems when oxidizing complex compounds which contain heteroatoms such as Cl, S and P or minerals (Calzavara et al., 2004). Experimental set-up is shown in Fig. 1. Degradation rates greater than 99% were obtained for simple compounds such as methanol, isopropanol, dodecane and also for nitrogenous compounds. No external vessel corrosion was observed for the treatment of 70% dodecane–30% tributylphosphate effluents and for chlorinated wastes (Calzavara et al., 2004). The external vessel made up of 316 stainless steel withstands high pressure. Along half of the vessel, four electric heaters (0.5 kW) are placed, following by a cooling shell. Within the autoclave, a titanium tube of 1 m length, 23.6 mm internal diameter and 0.9 mm thickness creates a double shell. The tube is placed so that incoming flows of water and oxidant are preheated before entering in the reactor, where the waste is fed. This tube is also used to prevent the autoclave from corrosion by confining the aggressive species. A titanium stirrer, constructed of a central axis and twenty blades, maintains a turbulent flow along the whole reactor. It enhances heat and mass transfer and prevents the precipitated salts from settling in the “supercritical zone” by bringing them into the “subcritical zone” of the reactor where they are dissolved again. Stirring rate generally ranges from 300 to 400 rpm. Pure water and oxidant (gaseous mixing of O2 /N2 at 20%/80% in volume) are fed in the annular zone at ambient temperature. Oxidant gas is fed into the reactor at ambient temperature and pressurized by a NOVA SWISS membrane compressor up to 30 MPa with a flow rate ranging from 1000 to 3000 NL h−1 . Demineralised water is pressurized by a LEWA membrane pump, named PM1. The suspension of resins in water is directly injected inside the inner zone. This suspension is pressurized by a membrane pump, named PM2, with flow rate ranges from 50 to 500 g h−1 . Particles diameter has to be lower than 200 m. For this reason, resins are first crushed and then sifted. For cationic resins (respectively anionic resins), the mean diameter of this particle size distribution is 128.6 m (respectively 68.6 m) and the standard deviation is 64.42 m (respectively 55.78 m). In addition, a magnetic stirrer is set inside the feed pot in order to prevent particles sedimentation and to obtain a homogeneous feed. Along the outer part of the autoclave, type K thermocouples are placed in order to get wall temperatures. Another type K thermocouple is placed directly inside the waste injector tube
Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 213–222
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Fig. 1 – Experimental set-up of the supercritical water oxidation process. where oxidation reaction occurs. The measured temperature will now be called injection temperature. The exit flow temperature is also controlled by a type K thermocouple placed after the reactor output. Experiments were carried out at a constant pressure of 30 MPa. Pressure was regulated by a backpressure regulator
placed downstream from the reactor. Just upstream, two filters, 60 and 15 m, respectively, were positioned to protect the backpressure regulator from solid particles. Upon passing through the backpressure regulator, effluent was flashed to atmospheric pressure and the two-phase effluent is separated using a gas–liquid separator.
Table 1 – Products of cationic resins degradation. Chemical species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Hexanone Hexandione Phenol (methylated) Phenol Benzoic acid (methylated) Phenylacetic acid (methylated) 2-Hydroxybenzoic acid (methylated) ␣-Methylphenylacetic acid (methylated) 4-Hydroxybenzaldehyde (methylated) Phenylpropanoic acid (methylated) 4-Ethylbenzoic acid (methylated) 4-Hydroxyacetophenone (methylated) 3-Hydroxybenzoic acid (methylated) 1(3H) isobenzofuranone Unknown species 4-Formylbenzoic acid (methylated) 2,4-Dihydroxybenzoic acid (methylated) Phthalic acid (methylated) Unknown species 1,4-Phenyldicarboxylic acid (methylated) 1,3-Phenyldicarboxylic acid (methyalated) 2-Methyl, 4H-1-benzopyran-4-one 2-Hydroxy, 5-méthylbenzaldehyde (methylated) 2-Hydroxy, 4-carboxyl benzenacetaldehyde Methylbiphenyl 4 carboxylic acid (methylated) [1,1 -Biphényl]-2,2 -diol 1-Hydroxy 4-phenoxybenzene Unknown species Unknown species 2-Hydroxybenzophenone (methylated)
Retention time (min) 4.96 6.95 8.91 14.54 23.28 30.12 31.58 33.58 40.06 41.79 43.37 44.38 46.27 46.99 47.58 47.98 48.95 50.95 51.78 52.47 52.89 54.20 54.93 55.56 58.36 60.42 61.09 64.91 67.88 75.78
Area (%) 1.624 0.154 4.199 11.571 54.991 1.034 3.206 0.422 1.060 0.211 0.623 0.421 0.475 1.201 0.790 0.486 3.864 0.839 1.400 0.960 0.992 0.509 0.326 0.734 0.952 0.100 1.084 0.657 0.624 0.554
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Fig. 2 – Global mechanisms of cationic resins degradation.
The elements in the output stream of the process are analyzed by a gas analyzer SICK MAIHAK. We can thus measure volumetric concentrations of CO2 , CO (by infrared spectroscopy) and O2 concentrations (by paramagnetic system). Liquid samples are collected at 10–15 min intervals.
2.2.
Reagents
Most of the IERs used in nuclear power plants are strong acid-cationic and base-anionic exchangers. The resins used in these experiments (Amberlite IRN77 and Amberlite IRN78) are based on a styrene–divinylbenzene copolymer and are representative of resins used in the nuclear industry. These resins are manufactured by Rohm & Haas. Both resins are supplied as beads with mesh size ranging from 0.3 to1.2 mm. Water used is deionised water (18 m) from a MilliQ system (manufactured by Millipore). Air (20 ± 2 wt% O2 and 80 wt% N2 ) was supplied by Air Liquide.
2.3.
Analysis
The compositional analysis of liquid effluents was carried out to determine the intermediate products formed during reactions. Samples obtained during experiments of {water + 20 wt% cationic IER} and of {water + 15 wt% anionic IER} SCWO were analyzed. Identification was aided by coupling gas chromatography/mass spectrometer (GC/MS). The system used was a HP 8590 Series gas chromatograph with a Varian CP-Sil 5CB Low Bleed 30 m long by 0,25 mm diameter column coupled to a Chrompak HP5972 mass spectrometer. The ion mass spectra derived was automatically compared to spectral librairies such as NIST. The oven temperature programme was held at 35 ◦ C for 2 min, heated to 60 ◦ C at 10 ◦ C min−1 , held at 60 ◦ C for 5 min followed by heating to 85 ◦ C at 1 ◦ C min−1 , held at 85 ◦ C for 5 min and heating to the final temperature of 250 ◦ C at 5 ◦ C min−1 .
Fig. 3 – GC–MS chromatogram of cationic resins intermediate species.
Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 213–222
Diazoalkanes such as diazomethane are frequently employed for the methylation of carboxylic acids. The resulting methyl esters are ideal derivatives for the characterization of carboxylic acids. These acids are characterized by gas chromatography and readily identified by searching of their electron impact mass spectra. The details of methylation method can be found in literature. Diazomethane reacts rapidly with unesterified acids in the presence of methanol, which catalyses the reaction to form methylesters. The reagent is generally prepared by action of alkali on a nitrosamide, e.g. N-methyl-N-nitroso-ptoluenesulfonamide (or Diazald). Methylation takes place in a ventilated enclosure. 200 mg of N-methyl-N-nitroso-p-toluenesulfonamide and 1 mL of pure methanol are added in the reactional medium. Potassium hydroxide in methanol is added drop by drop. The gaseous emission obtained by this way is mixed with the sample of {water + IER} effluent, up to observe a yellow color that means a diazomethane excess. This excess
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is evacuated by a stream of nitrogen up to fade the solution.
3.
Results and discussion
3.1.
Decomposition of cationic resins
3.1.1.
Sulfur conversion for cationic resins
According to Wang and Zhu (2003), sulfur is present in the liquid effluents such as sulfides, sulfites or thiosulfates, and possibly in gaseous effluents such as sulfur dioxide. Analyses by ionic chromatography were made for samples resulting form IERs SCWO, with and without IPA. These analyses showed that an increase of reactional medium temperature promotes the formation of sulfates during the supercritical water oxidation of cationic IERs. These results confirm the sulfur recovery mechanism given in Wang et al. (2003): 2− S2− → S2 O3 2− → SO2− 3 → SO4
Table 2 – Products of anionic resins degradation. Chemical species 1 3 5 6 8 9 10 11 12 13 14 15 16 17 19 20 22 23 24 25 26 27 28 29 30 31 32 33 35 37 38 39 40 43 44 45 46 47 48 50 51 52 56 57 58
N,N-dimethyl-formamide Benzaldehyde Benzonitrile Phenol 4-Hydroxybenzaldehyde Benzyl alcohol Acetophenone 4-Methylphenol 2-Nitrophenol Isophtalaldehyde 4-Formylbenzonitrile 1,4-Benzenedicarboxaldehyde 3-Cyanobenzaldehyde 2,3-Dihydro-1H-Inden-1-one 5-Formylsalicylaldehyde Phenylpropanoic acid 4-Acetyl-benzonitrile 3-Hydroxybenzaldehyde Benzamide 4-Cyanobenzoic acid 2-Formylbenzoic acid 4-Formylbenzoic acid N,N-dimethyl-benzamide 5-Hydroxy-2-methylbenzaldehyde 4-Hydroxy-3-nitrobenzaldehyde 4-Hydroxybenzaldehyde 2-Methyl-1H-isoindole-1,3(2H)-dione 2-Hydroxy-5-nitro-benzaldehyde 4-Hydroxyacetophenone 1,1 -(1,4-Phenylene)bis-ethanone 2-Hydroxybenzonitrile 3-Hydroxybenzoic acid 2-Hydroxy-5-methylbenzaldehyde 3-Hydroxybenzoic acid 4-(Hydroxymethyl)benzoic acid 4-Nitrophenol 4,5-Dimethyl-3H-isobenzofuran-1-one 4-(2-Methylpropyl)acetophenone 3,4-Dimethylbenzamide 2-Formylbenzoic acid 4-Methyl-2(1H)-quinolinone 1,4-Benzenedicarboxaldehyde 3,7-Dimethoxy-2H-1-benzopyran-2-one 1,4-Benzenedicarboxylic acid Dibutylphthalate
Retention time (min) 3.99 8.17 8.97 10.51 12.41 12.48 13.99 16.48 18.81 26.52 27.08 27.1 28.2 32.37 37.44 39.06 39.47 41.11 41.42 41.77 42.1 42.68 43.27 43.71 44.06 45.37 45.64 46.12 46.49 47.14 47.43 47.95 48.05 49.21 49.59 51.72 52.16 52.31 53.5 54.59 54.97 55.77 60.23 60.59 73.81
Area (%) 0.6 0.9 0.1 15.7 0.1 0.8 1.9 0.2 0.1 13.8 2.7 0.7 0.1 0.1 0.2 0.2 0.6 0.3 0.5 0.3 5.9 0.5 0.7 1.4 0.6 2.4 2.8 0.7 0.4 0.7 0.4 1.2 0.2 4.9 3.2 4.8 0.3 0.6 0.2 1 0.2 0.2 0.5 1 1
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Fig. 4 – Attack sites on the polystyrenic backbone of cationic resins.
3.1.2.
Identified species
The identified reaction products of the oxidation of cationic IER are given in Table 1 and Fig. 2. Benzoic acid, phenol, methanol, acetic acid are the major degradation compounds. Acetophenone, hydroxybenzaldehydes are detected in significative yields. The multiring compounds (1(3H) isobenzofuranone, hydroxybenzophenones or [1,1 -biphenyl]2,2 -diol for example) or rearrangement products (such as phthalic acid or phthalic anhydrous) are present in very low concentrations. Such aromatic compounds are analogous to those arising from the SCWO of phenol (Thornton and Savage, 1990, 1992). These products are present, in addition to carbon dioxide and carbon monoxide, in the effluent gases and methanol, ethanol, formic acid and acetic acid in the liquid effluent. Other authors have reported similar intermediate compounds of IER oxidation (Dubois et al., 1996). Reaction pathways for SCWO of monosubstituted phenols have been investigated in Martino and Savage (1999). According to this study, phenol is directly formed as a primary product from
hydroxybenzaldehyde, hydroxyacetophenone or cresol. The appearance of phenol in the reactor network for most of the substituted phenols confirms that phenol can be considered as a good model pollutant for SCWO investigations.
3.1.3.
Cationic resins degradation mechanism
Fig. 3 proposes the reaction mechanism for the oxidation of cationic IER arising from this study. The first step is the Carbon-Sulphur bond cleavage reaction. Functional group is converted into sulphates as described earlier. The polystyrenic backbone oxidation process is probably initiated by the attack of OOH• radical at several positions on alpha or beta carbons of aliphatic chains near the aromatic ring (Fig. 4). This radical is formed by the reaction of O2 with the organic compound as described in Lyer et al. (1998). Then, the aromatic ring undergoes an addition of an OH• radical on the alpha carbon and hence was hydroxylated at these positions leading to the formation of phenolic species products: dicarboxylic acids (pathway 1), phenylacetic acid (pathway 2) and phenylpropanoic acid (pathway 3) depending on the attack sites.
Fig. 5 – GC–MS chromatogram of anionic resins intermediate species.
Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 213–222
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Losses of CO from dicarboxylic acids lead to benzoic acid and then, benzaldehyde and phenol. The formation of dicarboxylic acids and subsequent intramolecular rearrangement produced phthalic anhydride or 1(3H)-isobenzofuranone. The recombination of bicyclic compounds, such as phenoxyphenol or hydroxybenzophenone was also possible by phenoxy radicals addition. The mechanism of multiring compounds (phenoxyphenol, etc.) is described in Savage (1999). In an oxidizing environment, phenol is converted to hydroquinone and then to benzoquinone (Onwudili and Williams, 2007b). Oxidation of benzoquinone lead to ring opening products to form the dialdehyde, which was further, oxidized to low molecular weight acids such as acetic acid. This last one is commonly known to be refractive to the oxidation reaction.
Fig. 6 – Attack sites on the functional group of anionic resins.
3.2.
Decomposition of anionic resins
3.2.1.
Nitrogen conversion for anionic resins
Compared with the carbon chemistry, the study of nitrogenous compounds is scarce. During the SCWO of the resin, a fraction of trimethylamine evolves and forms, for example, ammonium, nitrite and nitrate ions (Dubois et al., 1996). Some studies deal with methylamines behaviour during supercrit-
Fig. 7 – Mechanism of anionic resins degradation (pathway B).
Fig. 8 – Mechanism of anionic resins degradation (pathway C1 ): formation of benzamide.
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Fig. 9 – Mechanism of anionic resins degradation (pathway C2 ): formation of benzonitriles. ical water oxidation. Methylamines appear as intermediate in the degradation of nitrogen containing compounds in Li and Oshima (2005, 2006). It is also possible to find nitrogen or nitrous oxide in the gas phase.
3.2.2.
Identified species
The identified reaction products of the oxidation of anionic IER resins are summarized in Table 2 and Fig. 5. Similarly to cationic IERs, major degradation compounds are phenolic species such as phenol, isophthalaldehyde, acetophenone, benzoic acid, but benzamide, nitrophenol, benzonitrile or derived species from all these products are also detected in appreciable yields. Multi-ring compounds (such as dimethyl1(3H)-isobenzofuranone, (1H)-isoindole-1,3(2H)-dione, intermediates derived from benzopyran-2-one or from quinolinone are also present in very low concentration. In addition, CO2
and CO are present in the gaseous effluent and methanol, ethanol, formic acid and acetic acid in the liquid effluent. Species such as phenolic and methylphenolic compounds have been reported for anionic IER oxidation in Dubois et al. (1996). The great number of methylated species could be explained by the presence of CH3 • radicals resulting from the functional group of the IERs that swap with H• radicals. Multiring compounds are analogous to those arising from the SCWO of nitrogen containing species such as quinoline, isoquinoline (Katritzky et al., 1994) or pyridine (Crain et al., 1993).
3.2.3.
Anionic resins degradation mechanism
Concerning anionic IER degradation mechanism, the same pathway than for cationic IER degradation for polystyrenic backbone is considered. Nevertheless, the attack on the func-
Fig. 10 – Rearrangements and formation of bicyclic compounds.
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tional groups –CH2 –N(CH3 )3 must be considered too. These attack sites (A–C) are investigated and explained in Fig. 6. According to pathway A, the mechanism after the carbon–carbon bond cleavage reaction leads to the formation, one hand of the polystyrenic backbone and on the other hand of CH2 –N(CH3 )3 radical functional group. This group can be converted into methanol, CO, CO2 , NO, N2 , N2 O according to Li and Oshima (2006). Theses species can interact with other radicals. Indeed, nitrophenol or hydroxynitrobenzaldehyde result from NO2 addition on phenolic radicals. Concerning polystyrenic structure, the degradation mechanism is identical to cationic resins one. According to pathway B (Fig. 7), trimethylamine and a «methylated polystyrenic backbone» are obtained. By the same way than cationic resins mechanism, methylacetophenone is formed after attack of a HOO• radical. Methylbenzaldehyde and methylphenol formations are explained according to the same way than Martino and Savage (1999) mechanism. H• –OH• or H• –CH3 • radical exchanges are probable, owing to their presence in the reactional medium. Due to H• –CH3 • radical exchanges, two hypothesis for HOO• attack sites remain (pathway C). In case C1 (Fig. 8), HOO• attack is followed by H2 O elimination that leads to N, N-dimethylbenzamide. This reaction intermediate can be separated to phenol and dimethylformamide. Considering H• –CH3 • exchange and by an identical pathway, formation of N-methylbenzamide (respectively benzamide) and then of N-methylacetamide (respectively formamide) is explained. In case C2 (Fig. 9), HOO• attack on polystyrenic chain leads to the formation of acetylbenzylamine. Two successive H2 eliminations leads to the formation of triple bond CN. As in Martino and Savage (1999), hydroxybenzonitrile and benzonitrile appears and are hydrolyzed to form benzaldehyde. Rearrangements and cyclisations explain the formation of bicyclic compounds, owing to the presence of radicals such as COOH• , CH2 OH• or C3 H7 O• . Some examples are shown in Fig. 10.
4.
Conclusion
The oxidation of IERs has been studied in our continuous supercritical water reactor. A lot of intermediate species has been identified for both types of resins. The degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol, acetic acid are the preponderant compounds. However, other products are detected in appreciable yields. A degradation mechanism was proposed both for cationic and anionic resins. The degradation pathways are numerous and lead to the formation of different kind of species. Investigation of the reaction mechanism suggests that HOO• radical is a participant in the polymer decomposition process to produce aromatic acids and alcohols that will be converted into acetic acid, formic acid, methanol, ethanol and then carbon dioxide. Minor recombination reactions produce bicyclic compounds. Considering that he overall decomposition reaction depends on the reactivity of stable intermediates, it is necessary to determine rate-determining species. The ratedetermining steps were identified and it allows us to create a simple mechanism that will be implemented in a CFD software Fluent© . Indeed, CFD modelisation of IER suspensions behaviour in POSCEA double shell stirred supercritical water reactor, ever previously studied in Moussiere et al. (2007) for
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the oxidation of liquid wastes, can be undertaken thanks to these experimental results.
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Savage, P., 1999, Organic chemical reactions in supercritical water. Chem. Rev., 99: 603–622. Sugiyama, W., Park, K., Yamamura, T., Okada, H., Sugita, Y. and Tomiyasu, H., 2005, Decomposition of radioactive organic wastes with supercritical water medium containing RuO2 . J. Nucl. Sci. Technol., 42: 256–258. Thornton, T. and Savage, P., 1990, Phenol oxidation in supercritical water. J. Supercrit. Fluids, 3: 240–248. Thornton, T. and Savage, P., 1992, Phenol oxidation pathways in supercritical water. Ind. Eng. Chem. Res., 31: 2451–2456.
Wang, T. and Zhu, W., 2003, Sulfur transformations during supercritical water oxidation of a Chinese coal. Fuel, 82: 2267–2272. Wang, T., Xiang, B., Liu, J. and Shen, X., 2003, Supercritical water oxidation of sulfide. Environ. Sci. Technol., 37: 1955– 1961. Worl, L., Buelow, S., Harradine, D., Lanning, R., Padilla, D., Roberts, J. and Shao, X., 2000, Hydrothermal oxidation of radioactive combustible waste. Waste Manage., 20: 417–423.
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Supercritical water oxidation of ion exchange resins: Degradation mechanisms A. Leybros a , A. Roubaud a,∗ , P. Guichardon b , O. Boutin c a b c
CEA Marcoule, DEN/DTCD/SPDE/LFSM, BP17171, 30207 Bagnols/Ceze, France Ecole Centrale Marseille, Technopôle de Château-Gombert, 13451 Marseille Cedex 20, France Aix Marseille Universités, UMR-CNRS 6181, BP 80, Europôle de l’Arbois, 13545 Aix en Provence Cedex 4, France
a b s t r a c t Spent ion exchange resins are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation could offer a viable treatment alternative to destroy the organic structure of resins and contain radioactivity. IER degradation experiments were carried out in a continuous supercritical water reactor. Total organic carbon degradation rates in the range of 95–98% were obtained depending on operating conditions. GC–MS chromatography analyses were carried out to determine intermediate products formed during the reaction. Around 50 species were identified for cationic and anionic resins. Degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol and acetic acid are the main compounds. However, other products are detected in appreciable yields such as phenolic species or heterocycles, for anionic IERs degradation. Intermediates produced by intramolecular rearrangements are also obtained. A radical degradation mechanism is proposed for each resin. In this overall mechanism, several hypotheses are foreseen, according to HOO• radical attack sites. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Supercritical water oxidation; Ion exchange resins; Polymer; Aromatic compounds; Reaction mechanisms
1.
Introduction
Water treatment systems in nuclear power plants use organic ion exchange resins (IERs) in order to control system chemistry, to minimize corrosion and the degradation of system components as well as remove radioactive contaminants. Organic resins are used in chemical decontamination or cleaning processes for regeneration of process water by reagents and radionuclide removal. Large quantities of contaminated IERs are generated and often stored (the volume of spent IERs that arise from typical nuclear power plant is 5–7 m3 reactor unit−1 a−1 ). In the future, wastes must be treated to reduce volume and to improve stability. Unfortunately, spent resins are radioactive wastes for which there is no satisfactory industrial treatment (IAEA Report). Combustion has good performance according to treatment rate, waste volume reduction and decontamination factor. However, gas treatment system is needed to prevent emission of harmful substances such as sulphuric or nitric oxides and additionally radionuclides (137 Cs,
∗
etc.), which are volatile at temperature above 800 ◦ C. Moreover, resins tend to form large clusters by melting before burning, thus causing corrosion problems with the refractory. Embedded resins may also undergo radiolytic or chemical degradation, which can result in the emission of hazardous species in the environment. Then, according to (IAEA Report), chemical processes may be considered for the treatment of IERs, but additional studies are needed. In the case of IERs, supercritical water oxidation (SCWO) offers an alternative treatment for the destruction of organic structure of resins. At supercritical conditions (P > 22.1 MPa and T > 374 ◦ C), water acts as a non-polar medium and presents similar solvation properties as a low polarity organic solvent, completely miscible with liquid organics. Supercritical water also presents complete miscibility with oxygen, creating a homogeneous reaction medium, which makes supercritical water a suitable medium for the oxidation of liquid organics. If, however the organic material is a solid, this reaction is heterogeneous. Reaction time is about a few 106 Ru,
Corresponding author. Fax: +33 4 66 33 90 19. E-mail address: [email protected] (A. Roubaud). Received 19 June 2009; Accepted 2 November 2009 0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2009.11.001
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seconds and there is no formation of gaseous oxides (SOx and NOx), due to a relatively low temperature ranging from 450 to 500 ◦ C (compared to combustion, higher than 1000 ◦ C). Radionuclides will then remain in the cold liquid effluent and can be recovered by means of precipitation. The main problems of SCWO are corrosion and salt precipitation (leading to reactor plugging) (Bermejo and Cocero, 2006). To overcome these two problems, different kinds of reactor designs have been developed, such as tubular reactor (Cocero, 2001), cooled wall reactor (Cocero, 2001) or transpiring wall reactor (Bermejo et al., 2006), etc. CEA has developed and patented a double shell stirred reactor (Calzavara et al., 2004) as described later in this article. A few studies deal with the degradation of IERs in subcritical and supercritical conditions. IERs degradation rates between 80 and 96% were obtained in supercritical water (Dubois et al., 1996), depending on the nature of the resins and operating conditions. In order to improve degradation rates, greater than 99% TOC, different processes and ways were investigated: particle crushing (Faure et al., 1999), thermal liquefaction (Worl et al., 2000), the use of a metallic catalyst (Sugiyama et al., 2005), and a large oxidant stoichiometric excess (Akai et al., 2001, 2005). There is a lack of data regarding the reaction pathways for organic molecules and macromolecules. Polymers degradation mechanisms in supercritical water are scarcely investigated. Mechanisms of styrene monomer recovery from waste polystyrene by supercritical water partial oxidation have been studied (Douglas Lilac and Lee, 2001). Reaction mechanisms of petroleum derived hydrocarbons, e.g. biphenyl (Onwudili and Williams, 2007a), phenanthrene, naphthalene (Onwudili and Williams, 2007b) have been studied in subcritical and supercritical water conditions. Similar compounds are produced, such as phenol, acetophenone, benzaldehyde, or benzoic acid. The supercritical water oxidation of monosubstituted phenols (Martino and Savage, 1999) proceeds through multiple parallel reactions. One reaction involves ring opening and rapid formation of CO2 . Other reactions involve conversion or elimination of the substituent to form other phenolics. Chemistry reactions (oxidation, hydrolysis) in supercritical water for sulphur and nitrogen containing compounds, substituted phenols are reported by Savage (1999). In this study, a series of elementary organic chemical reactions have been identified under hydrothermal conditions. A detailed mechanism is given for methylamines (Li and Oshima, 2005; Li and Oshima, 2006) and explains formation of nitrous species such as NH4 + , NO3 − , N2 O, etc. Moreover, heterocyclic compounds such as quinoline, isoquinoline, pyridine supercritical water oxidation detailed mechanisms are also studied. Sulphur initially present could be completely converted into sulfates with a reactional medium temperature greater than 420 ◦ C (Wang and Zhu, 2003; Wang et al., 2003). In this case, reaction intermediates are S2 O3 2− and SO3 2− . This study deals with the oxidation of anionic and cationic IERs from nuclear industry in supercritical water. There is a need to evaluate the decomposition patterns of IERs intermediate compounds, in order to establish organic reaction models. It will help to throw more light on the elementary chemical reactions involved in the decomposition of organic molecules under hydrothermal conditions. The detailed mechanisms of the supercritical water oxidation of anionic and cationic IERs are presented to isolate the different stable intermediate products, which influence
the complete oxidation of the polymers. Although high total organic carbon reduction rates were observed, mechanisms are intricate and a brief description is attempted here. Hence, rate determining species are identified and it allows to create a simple mechanism that will be implemented in CFD software. The characterisation of specific reaction products is important for process optimisation and for developing thermodynamic (enthalpy of reaction, etc.) and thermophysical (viscosity, heat capacity, etc.) data.
2.
Materials and methods
2.1.
Experimental set up
The double shell stirred reactor has been developed in order to overcome corrosion and salt plugging problems when oxidizing complex compounds which contain heteroatoms such as Cl, S and P or minerals (Calzavara et al., 2004). Experimental set-up is shown in Fig. 1. Degradation rates greater than 99% were obtained for simple compounds such as methanol, isopropanol, dodecane and also for nitrogenous compounds. No external vessel corrosion was observed for the treatment of 70% dodecane–30% tributylphosphate effluents and for chlorinated wastes (Calzavara et al., 2004). The external vessel made up of 316 stainless steel withstands high pressure. Along half of the vessel, four electric heaters (0.5 kW) are placed, following by a cooling shell. Within the autoclave, a titanium tube of 1 m length, 23.6 mm internal diameter and 0.9 mm thickness creates a double shell. The tube is placed so that incoming flows of water and oxidant are preheated before entering in the reactor, where the waste is fed. This tube is also used to prevent the autoclave from corrosion by confining the aggressive species. A titanium stirrer, constructed of a central axis and twenty blades, maintains a turbulent flow along the whole reactor. It enhances heat and mass transfer and prevents the precipitated salts from settling in the “supercritical zone” by bringing them into the “subcritical zone” of the reactor where they are dissolved again. Stirring rate generally ranges from 300 to 400 rpm. Pure water and oxidant (gaseous mixing of O2 /N2 at 20%/80% in volume) are fed in the annular zone at ambient temperature. Oxidant gas is fed into the reactor at ambient temperature and pressurized by a NOVA SWISS membrane compressor up to 30 MPa with a flow rate ranging from 1000 to 3000 NL h−1 . Demineralised water is pressurized by a LEWA membrane pump, named PM1. The suspension of resins in water is directly injected inside the inner zone. This suspension is pressurized by a membrane pump, named PM2, with flow rate ranges from 50 to 500 g h−1 . Particles diameter has to be lower than 200 m. For this reason, resins are first crushed and then sifted. For cationic resins (respectively anionic resins), the mean diameter of this particle size distribution is 128.6 m (respectively 68.6 m) and the standard deviation is 64.42 m (respectively 55.78 m). In addition, a magnetic stirrer is set inside the feed pot in order to prevent particles sedimentation and to obtain a homogeneous feed. Along the outer part of the autoclave, type K thermocouples are placed in order to get wall temperatures. Another type K thermocouple is placed directly inside the waste injector tube
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Fig. 1 – Experimental set-up of the supercritical water oxidation process. where oxidation reaction occurs. The measured temperature will now be called injection temperature. The exit flow temperature is also controlled by a type K thermocouple placed after the reactor output. Experiments were carried out at a constant pressure of 30 MPa. Pressure was regulated by a backpressure regulator
placed downstream from the reactor. Just upstream, two filters, 60 and 15 m, respectively, were positioned to protect the backpressure regulator from solid particles. Upon passing through the backpressure regulator, effluent was flashed to atmospheric pressure and the two-phase effluent is separated using a gas–liquid separator.
Table 1 – Products of cationic resins degradation. Chemical species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Hexanone Hexandione Phenol (methylated) Phenol Benzoic acid (methylated) Phenylacetic acid (methylated) 2-Hydroxybenzoic acid (methylated) ␣-Methylphenylacetic acid (methylated) 4-Hydroxybenzaldehyde (methylated) Phenylpropanoic acid (methylated) 4-Ethylbenzoic acid (methylated) 4-Hydroxyacetophenone (methylated) 3-Hydroxybenzoic acid (methylated) 1(3H) isobenzofuranone Unknown species 4-Formylbenzoic acid (methylated) 2,4-Dihydroxybenzoic acid (methylated) Phthalic acid (methylated) Unknown species 1,4-Phenyldicarboxylic acid (methylated) 1,3-Phenyldicarboxylic acid (methyalated) 2-Methyl, 4H-1-benzopyran-4-one 2-Hydroxy, 5-méthylbenzaldehyde (methylated) 2-Hydroxy, 4-carboxyl benzenacetaldehyde Methylbiphenyl 4 carboxylic acid (methylated) [1,1 -Biphényl]-2,2 -diol 1-Hydroxy 4-phenoxybenzene Unknown species Unknown species 2-Hydroxybenzophenone (methylated)
Retention time (min) 4.96 6.95 8.91 14.54 23.28 30.12 31.58 33.58 40.06 41.79 43.37 44.38 46.27 46.99 47.58 47.98 48.95 50.95 51.78 52.47 52.89 54.20 54.93 55.56 58.36 60.42 61.09 64.91 67.88 75.78
Area (%) 1.624 0.154 4.199 11.571 54.991 1.034 3.206 0.422 1.060 0.211 0.623 0.421 0.475 1.201 0.790 0.486 3.864 0.839 1.400 0.960 0.992 0.509 0.326 0.734 0.952 0.100 1.084 0.657 0.624 0.554
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Fig. 2 – Global mechanisms of cationic resins degradation.
The elements in the output stream of the process are analyzed by a gas analyzer SICK MAIHAK. We can thus measure volumetric concentrations of CO2 , CO (by infrared spectroscopy) and O2 concentrations (by paramagnetic system). Liquid samples are collected at 10–15 min intervals.
2.2.
Reagents
Most of the IERs used in nuclear power plants are strong acid-cationic and base-anionic exchangers. The resins used in these experiments (Amberlite IRN77 and Amberlite IRN78) are based on a styrene–divinylbenzene copolymer and are representative of resins used in the nuclear industry. These resins are manufactured by Rohm & Haas. Both resins are supplied as beads with mesh size ranging from 0.3 to1.2 mm. Water used is deionised water (18 m) from a MilliQ system (manufactured by Millipore). Air (20 ± 2 wt% O2 and 80 wt% N2 ) was supplied by Air Liquide.
2.3.
Analysis
The compositional analysis of liquid effluents was carried out to determine the intermediate products formed during reactions. Samples obtained during experiments of {water + 20 wt% cationic IER} and of {water + 15 wt% anionic IER} SCWO were analyzed. Identification was aided by coupling gas chromatography/mass spectrometer (GC/MS). The system used was a HP 8590 Series gas chromatograph with a Varian CP-Sil 5CB Low Bleed 30 m long by 0,25 mm diameter column coupled to a Chrompak HP5972 mass spectrometer. The ion mass spectra derived was automatically compared to spectral librairies such as NIST. The oven temperature programme was held at 35 ◦ C for 2 min, heated to 60 ◦ C at 10 ◦ C min−1 , held at 60 ◦ C for 5 min followed by heating to 85 ◦ C at 1 ◦ C min−1 , held at 85 ◦ C for 5 min and heating to the final temperature of 250 ◦ C at 5 ◦ C min−1 .
Fig. 3 – GC–MS chromatogram of cationic resins intermediate species.
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Diazoalkanes such as diazomethane are frequently employed for the methylation of carboxylic acids. The resulting methyl esters are ideal derivatives for the characterization of carboxylic acids. These acids are characterized by gas chromatography and readily identified by searching of their electron impact mass spectra. The details of methylation method can be found in literature. Diazomethane reacts rapidly with unesterified acids in the presence of methanol, which catalyses the reaction to form methylesters. The reagent is generally prepared by action of alkali on a nitrosamide, e.g. N-methyl-N-nitroso-ptoluenesulfonamide (or Diazald). Methylation takes place in a ventilated enclosure. 200 mg of N-methyl-N-nitroso-p-toluenesulfonamide and 1 mL of pure methanol are added in the reactional medium. Potassium hydroxide in methanol is added drop by drop. The gaseous emission obtained by this way is mixed with the sample of {water + IER} effluent, up to observe a yellow color that means a diazomethane excess. This excess
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is evacuated by a stream of nitrogen up to fade the solution.
3.
Results and discussion
3.1.
Decomposition of cationic resins
3.1.1.
Sulfur conversion for cationic resins
According to Wang and Zhu (2003), sulfur is present in the liquid effluents such as sulfides, sulfites or thiosulfates, and possibly in gaseous effluents such as sulfur dioxide. Analyses by ionic chromatography were made for samples resulting form IERs SCWO, with and without IPA. These analyses showed that an increase of reactional medium temperature promotes the formation of sulfates during the supercritical water oxidation of cationic IERs. These results confirm the sulfur recovery mechanism given in Wang et al. (2003): 2− S2− → S2 O3 2− → SO2− 3 → SO4
Table 2 – Products of anionic resins degradation. Chemical species 1 3 5 6 8 9 10 11 12 13 14 15 16 17 19 20 22 23 24 25 26 27 28 29 30 31 32 33 35 37 38 39 40 43 44 45 46 47 48 50 51 52 56 57 58
N,N-dimethyl-formamide Benzaldehyde Benzonitrile Phenol 4-Hydroxybenzaldehyde Benzyl alcohol Acetophenone 4-Methylphenol 2-Nitrophenol Isophtalaldehyde 4-Formylbenzonitrile 1,4-Benzenedicarboxaldehyde 3-Cyanobenzaldehyde 2,3-Dihydro-1H-Inden-1-one 5-Formylsalicylaldehyde Phenylpropanoic acid 4-Acetyl-benzonitrile 3-Hydroxybenzaldehyde Benzamide 4-Cyanobenzoic acid 2-Formylbenzoic acid 4-Formylbenzoic acid N,N-dimethyl-benzamide 5-Hydroxy-2-methylbenzaldehyde 4-Hydroxy-3-nitrobenzaldehyde 4-Hydroxybenzaldehyde 2-Methyl-1H-isoindole-1,3(2H)-dione 2-Hydroxy-5-nitro-benzaldehyde 4-Hydroxyacetophenone 1,1 -(1,4-Phenylene)bis-ethanone 2-Hydroxybenzonitrile 3-Hydroxybenzoic acid 2-Hydroxy-5-methylbenzaldehyde 3-Hydroxybenzoic acid 4-(Hydroxymethyl)benzoic acid 4-Nitrophenol 4,5-Dimethyl-3H-isobenzofuran-1-one 4-(2-Methylpropyl)acetophenone 3,4-Dimethylbenzamide 2-Formylbenzoic acid 4-Methyl-2(1H)-quinolinone 1,4-Benzenedicarboxaldehyde 3,7-Dimethoxy-2H-1-benzopyran-2-one 1,4-Benzenedicarboxylic acid Dibutylphthalate
Retention time (min) 3.99 8.17 8.97 10.51 12.41 12.48 13.99 16.48 18.81 26.52 27.08 27.1 28.2 32.37 37.44 39.06 39.47 41.11 41.42 41.77 42.1 42.68 43.27 43.71 44.06 45.37 45.64 46.12 46.49 47.14 47.43 47.95 48.05 49.21 49.59 51.72 52.16 52.31 53.5 54.59 54.97 55.77 60.23 60.59 73.81
Area (%) 0.6 0.9 0.1 15.7 0.1 0.8 1.9 0.2 0.1 13.8 2.7 0.7 0.1 0.1 0.2 0.2 0.6 0.3 0.5 0.3 5.9 0.5 0.7 1.4 0.6 2.4 2.8 0.7 0.4 0.7 0.4 1.2 0.2 4.9 3.2 4.8 0.3 0.6 0.2 1 0.2 0.2 0.5 1 1
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Fig. 4 – Attack sites on the polystyrenic backbone of cationic resins.
3.1.2.
Identified species
The identified reaction products of the oxidation of cationic IER are given in Table 1 and Fig. 2. Benzoic acid, phenol, methanol, acetic acid are the major degradation compounds. Acetophenone, hydroxybenzaldehydes are detected in significative yields. The multiring compounds (1(3H) isobenzofuranone, hydroxybenzophenones or [1,1 -biphenyl]2,2 -diol for example) or rearrangement products (such as phthalic acid or phthalic anhydrous) are present in very low concentrations. Such aromatic compounds are analogous to those arising from the SCWO of phenol (Thornton and Savage, 1990, 1992). These products are present, in addition to carbon dioxide and carbon monoxide, in the effluent gases and methanol, ethanol, formic acid and acetic acid in the liquid effluent. Other authors have reported similar intermediate compounds of IER oxidation (Dubois et al., 1996). Reaction pathways for SCWO of monosubstituted phenols have been investigated in Martino and Savage (1999). According to this study, phenol is directly formed as a primary product from
hydroxybenzaldehyde, hydroxyacetophenone or cresol. The appearance of phenol in the reactor network for most of the substituted phenols confirms that phenol can be considered as a good model pollutant for SCWO investigations.
3.1.3.
Cationic resins degradation mechanism
Fig. 3 proposes the reaction mechanism for the oxidation of cationic IER arising from this study. The first step is the Carbon-Sulphur bond cleavage reaction. Functional group is converted into sulphates as described earlier. The polystyrenic backbone oxidation process is probably initiated by the attack of OOH• radical at several positions on alpha or beta carbons of aliphatic chains near the aromatic ring (Fig. 4). This radical is formed by the reaction of O2 with the organic compound as described in Lyer et al. (1998). Then, the aromatic ring undergoes an addition of an OH• radical on the alpha carbon and hence was hydroxylated at these positions leading to the formation of phenolic species products: dicarboxylic acids (pathway 1), phenylacetic acid (pathway 2) and phenylpropanoic acid (pathway 3) depending on the attack sites.
Fig. 5 – GC–MS chromatogram of anionic resins intermediate species.
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Losses of CO from dicarboxylic acids lead to benzoic acid and then, benzaldehyde and phenol. The formation of dicarboxylic acids and subsequent intramolecular rearrangement produced phthalic anhydride or 1(3H)-isobenzofuranone. The recombination of bicyclic compounds, such as phenoxyphenol or hydroxybenzophenone was also possible by phenoxy radicals addition. The mechanism of multiring compounds (phenoxyphenol, etc.) is described in Savage (1999). In an oxidizing environment, phenol is converted to hydroquinone and then to benzoquinone (Onwudili and Williams, 2007b). Oxidation of benzoquinone lead to ring opening products to form the dialdehyde, which was further, oxidized to low molecular weight acids such as acetic acid. This last one is commonly known to be refractive to the oxidation reaction.
Fig. 6 – Attack sites on the functional group of anionic resins.
3.2.
Decomposition of anionic resins
3.2.1.
Nitrogen conversion for anionic resins
Compared with the carbon chemistry, the study of nitrogenous compounds is scarce. During the SCWO of the resin, a fraction of trimethylamine evolves and forms, for example, ammonium, nitrite and nitrate ions (Dubois et al., 1996). Some studies deal with methylamines behaviour during supercrit-
Fig. 7 – Mechanism of anionic resins degradation (pathway B).
Fig. 8 – Mechanism of anionic resins degradation (pathway C1 ): formation of benzamide.
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Fig. 9 – Mechanism of anionic resins degradation (pathway C2 ): formation of benzonitriles. ical water oxidation. Methylamines appear as intermediate in the degradation of nitrogen containing compounds in Li and Oshima (2005, 2006). It is also possible to find nitrogen or nitrous oxide in the gas phase.
3.2.2.
Identified species
The identified reaction products of the oxidation of anionic IER resins are summarized in Table 2 and Fig. 5. Similarly to cationic IERs, major degradation compounds are phenolic species such as phenol, isophthalaldehyde, acetophenone, benzoic acid, but benzamide, nitrophenol, benzonitrile or derived species from all these products are also detected in appreciable yields. Multi-ring compounds (such as dimethyl1(3H)-isobenzofuranone, (1H)-isoindole-1,3(2H)-dione, intermediates derived from benzopyran-2-one or from quinolinone are also present in very low concentration. In addition, CO2
and CO are present in the gaseous effluent and methanol, ethanol, formic acid and acetic acid in the liquid effluent. Species such as phenolic and methylphenolic compounds have been reported for anionic IER oxidation in Dubois et al. (1996). The great number of methylated species could be explained by the presence of CH3 • radicals resulting from the functional group of the IERs that swap with H• radicals. Multiring compounds are analogous to those arising from the SCWO of nitrogen containing species such as quinoline, isoquinoline (Katritzky et al., 1994) or pyridine (Crain et al., 1993).
3.2.3.
Anionic resins degradation mechanism
Concerning anionic IER degradation mechanism, the same pathway than for cationic IER degradation for polystyrenic backbone is considered. Nevertheless, the attack on the func-
Fig. 10 – Rearrangements and formation of bicyclic compounds.
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tional groups –CH2 –N(CH3 )3 must be considered too. These attack sites (A–C) are investigated and explained in Fig. 6. According to pathway A, the mechanism after the carbon–carbon bond cleavage reaction leads to the formation, one hand of the polystyrenic backbone and on the other hand of CH2 –N(CH3 )3 radical functional group. This group can be converted into methanol, CO, CO2 , NO, N2 , N2 O according to Li and Oshima (2006). Theses species can interact with other radicals. Indeed, nitrophenol or hydroxynitrobenzaldehyde result from NO2 addition on phenolic radicals. Concerning polystyrenic structure, the degradation mechanism is identical to cationic resins one. According to pathway B (Fig. 7), trimethylamine and a «methylated polystyrenic backbone» are obtained. By the same way than cationic resins mechanism, methylacetophenone is formed after attack of a HOO• radical. Methylbenzaldehyde and methylphenol formations are explained according to the same way than Martino and Savage (1999) mechanism. H• –OH• or H• –CH3 • radical exchanges are probable, owing to their presence in the reactional medium. Due to H• –CH3 • radical exchanges, two hypothesis for HOO• attack sites remain (pathway C). In case C1 (Fig. 8), HOO• attack is followed by H2 O elimination that leads to N, N-dimethylbenzamide. This reaction intermediate can be separated to phenol and dimethylformamide. Considering H• –CH3 • exchange and by an identical pathway, formation of N-methylbenzamide (respectively benzamide) and then of N-methylacetamide (respectively formamide) is explained. In case C2 (Fig. 9), HOO• attack on polystyrenic chain leads to the formation of acetylbenzylamine. Two successive H2 eliminations leads to the formation of triple bond CN. As in Martino and Savage (1999), hydroxybenzonitrile and benzonitrile appears and are hydrolyzed to form benzaldehyde. Rearrangements and cyclisations explain the formation of bicyclic compounds, owing to the presence of radicals such as COOH• , CH2 OH• or C3 H7 O• . Some examples are shown in Fig. 10.
4.
Conclusion
The oxidation of IERs has been studied in our continuous supercritical water reactor. A lot of intermediate species has been identified for both types of resins. The degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol, acetic acid are the preponderant compounds. However, other products are detected in appreciable yields. A degradation mechanism was proposed both for cationic and anionic resins. The degradation pathways are numerous and lead to the formation of different kind of species. Investigation of the reaction mechanism suggests that HOO• radical is a participant in the polymer decomposition process to produce aromatic acids and alcohols that will be converted into acetic acid, formic acid, methanol, ethanol and then carbon dioxide. Minor recombination reactions produce bicyclic compounds. Considering that he overall decomposition reaction depends on the reactivity of stable intermediates, it is necessary to determine rate-determining species. The ratedetermining steps were identified and it allows us to create a simple mechanism that will be implemented in a CFD software Fluent© . Indeed, CFD modelisation of IER suspensions behaviour in POSCEA double shell stirred supercritical water reactor, ever previously studied in Moussiere et al. (2007) for
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the oxidation of liquid wastes, can be undertaken thanks to these experimental results.
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