INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 17 (END) docx

17 Catalytic Dehalogenation of Plastic- Derived Oil AZHAR UDDIN* and YUSAKU SAKATA Okayama University, Tsushima Naka, Japan I. PLASTIC WASTES AND ASSOCIATED TREATMENT The widespread use of plastics in modern industry has resulted in a huge volume of waste plastic requiring treatment. In the years between 1975 and 1998 in Japan, the amount of postconsumer plastic waste increased 3.8 times, plastic production 2.7 times, and consumption of plastics 3.2 times (Table 1) [1]. It can be antici- pated that this trend will continue unless more intensive recycling of plastic waste is undertaken. In the past, landfilling and incineration (without energy recovery) were widely practiced in many countries, but these options have been criticized in recent years for their adverse effects on the environment. Not only do the waste plastics make the environment unsightly, but also their energy and chemical content are lost. Presently, the alternative options for the treatment of plastic waste are: (1) mechanical recycling, (2) energy recycling, and (3) feedstock or chemical recycling. Mechanical recycling by melting and remolding of the used plastics is limited to particular types of polymers and applications of the recycled materials. If mechanical recycle is not feasible, energy recovery by incineration is another option, since the calorific value of plastics is very high. However, incineration may lead to the formation of pollutants, such as dioxins and other toxins, depending on the composition of the plastic waste and the nature of com- bustion. Feedstock or chemical recycling of plastic waste by pyrolysis or thermal degradation permits the recovery of valuable hydrocarbons, which can be used as feedstock materials or fuels. The plastic wastes are transformed into useful chemicals by thermal degradation in an inert atmosphere. Generally the gaseous and liquid products obtained are complex mixtures of hydrocarbons and other organic compounds, whose composition depends on the composition of the plas- * Current affiliation: The University of Newcastle, Callghan, New South Wales, Australia. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 330 Uddin and Sakata TABLE 1 Production, Consumption, and Disposal Rates of Polymer Resins in Japan 1975 1998 Increase Resin production 517 1391 2.7 times Domestic consumption of plastic goods 315 1020 3.2 times Disposal of plastic waste 261 984 3.8 times tic waste. The potentials and prospects of chemical recycling of plastic waste have been addressed in a recent review [2]. II. DEHALOGENATION ISSUES IN THE CHEMICAL RECYCLING OF PLASTIC WASTE Most of the practical research on plastic degradation to fuel oil is limited to PE, PP, and PS but excludes PVC, since it contains chlorine (56 wt%) and releases toxic and corrosive hydrogen chloride gas during the early stage of degradation at 280–320°C. Upon dehydrochlorination, PVC forms a polyene macromolecular structure and it decomposes at higher temperatures (380–600°C) to produce vola- tiles containing aliphatic, olefinic, and aromatic hydrocarbons and solid chars [3]. Dehydrochlorination of PVC has been studied extensively, in particular the kinetics and mechanism of PVC degradation [4–9]. Although more than 99% of the chlorine content of PVC is removed as HCl in the early stages, the remaining chlorine in the polyene macromolecular structure may lead to the formation of unwanted Cl-containing organics. Detailed studies on the identification of the products evolved from PVC degradation and their mechanisms of formation have also been reported [10–12]. We have reported that various types of organic chlo- rine compounds are produced when PVC mixed with PE, PP, and PS is thermally degraded at 430°C, and we have clarified the route of formation of these com- pounds [13–14]. Hydrogen chloride released from PVC degradation reacts with the hydrocarbons produced from the other polymers to form organic chlorine compounds. Generally, municipal plastic waves (MPW) contain all kinds of plas- tics, including PVC. In Japan, municipal plastic wastes contain about 5–10 wt% PVC [15]. Therefore, thermal degradation of MPW produces unwanted organic chlorine compounds in the oil. The waste plastic–derived oil that contains organic chlorine compounds cannot be used safely as a fuel oil because there is a high possibility of producing toxic compounds, such as dioxins, dibenzofurans, and biphenyls, during combustion. It is necessary to remove the organic chlorine compounds from the oil before use. Halogens other than chlorine may also be present in the polymer as additives, for instance, brominated flame-retardant compounds, such as polybrominated TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Catalytic Dehalogenation of Plastic-Derived Oil 331 benzene compounds, are added to PS and ABS to inhibit or modify polymer combustion when heated in an oxidative atmosphere. Electrical and electronic home appliances are made of high-impact polystyrene (HIPS) containing bromi- nated flame retardants. Some of these brominated plastics end up in municipal plastic wastes [16]. Thus debromination of brominated hydrocarbons in the plas- tic-derived oil is another important issue. Dehalogenation of the plastic-derived oil is a key technology for the success of chemical recycling of mixed plastics containing chlorinated and brominated polymers into fuel oil or chemicals. Com- pared to PVC, very little work has been reported on the pyrolysis of brominated flame retardants containing plastic waste. The separation of halogenated flame retardants from polymer matrixes with extraction using supercritical fluids such as supercritical carbon dioxide (SC-CO 2 ) has been studied [17]. Thermal-behav- ior and degradation-mechanism studies on brominated polystyrenes have been studied mainly by thermogravimetry, thermal volatilization analysis (TVA), and Py-GC/MS technique and show that the degradation occurs mainly to the mono- mer via radical polymerization [18]. We have reported the formation of bromi- nated hydrocarbons when brominated-flame-retardant-containing HIPS and its mixture with other plastics are thermally degraded to obtain fuel oil [19]. III. OUR APPROACH TO THE DEHALOGENATION OF PLASTIC-DERIVED OIL Various methods are used to remove the chlorine by coprocessing catalysts or absorbent during the pyrolysis [20–22]. All of these efforts succeed to some extent in removing chlorine content in PVC-containing plastics before or during the pyrolysis. However, even the presence of a small amount of chlorine (Ͻ1%) leads to the formation of chloro-organic compounds. We observed that even when the PVC is dechlorinated to Ͼ98%, chloro-organic compounds are still produced [23]. There has been no detailed study of the removal of chloro-organic com- pounds from the plastic-derived oil thus far. Catalytic dechlorination is a promising method for the removal of organic chlorine compounds, compared to other methods, such as combustion [24]. There are many research papers on the catalytic dechlorination of various organic chlo- rine compounds [25–26], but most of them deal with the removal of chlorine by noble metal catalysts in the presence of hydrogen. We have been developing iron oxide–carbon composite catalysts for the dehydrochlorination and dehydrobrom- ination of plastic-derived oil. In our proposed catalytic process, waste plastic– derived oil containing chlorinated and brominated hydrocarbons is treated over solid catalysts in a fixed-bed flow-type reactor in a temperature range of 300– 400°C at atmospheric pressure in He or N 2 flow in order to decompose the haloge- nated hydrocarbons into hydrocarbons and hydrogen halides. We have demon- TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 332 Uddin and Sakata strated the effectiveness of our catalysts for the dehydrodechlorination of model compounds, such as chlorocyclohexane and 1-chloroheptane, in a fixed-bed reac- tor. These iron oxides are reported to be effective in the destruction or reduction of dioxins from municipal solid-waste incineration products [27]. In ABS degra- dation, iron oxides showed a catalytic effect in decomposition of N-containing heterocyclic compounds from the degradation oil [28–29]. This review summa- rizes our recent studies on (1) the catalytic dechlorination of PVC-containing mixed plastics–derived oil [30], (2) municipal waste plastic–derived oil [31], and (3) simultaneous dechlorination and debromination from the pyrolysis products of PVC and brominated flame-retardant-containing high-impact polystyrene (HIPS) mixed plastics over iron-base catalysts. A. Catalytic Dechlorination of Chloro-Organic Compounds from PVC-Containing Mixed Plastic- Derived Oil over Iron Oxide Catalysts We have reported the catalytic activity of various iron oxide and iron oxide– carbon composite catalysts for the dechlorination of chloro-organic compounds formed during the thermal degradation of PVC-containing mixed plastics. PVC- containing mixed plastic-derived oil was prepared by thermal degradation in a separate facility, and the dechlorination of the derived oil was performed in a fixed-bed flow-type reactor. Emphasis was put on the stability of iron oxide– based catalysts in the presence of HCl gas produced during dechlorination of mixed plastic–derived oil. The PVC-containing waste mixed plastic (MX/PVC)– derived oil was prepared by degrading mixed plastics containing PE (33%), PP (33%), PS (33%), and PVC (1%) as a model sample at 410°C. Details of the experimental procedure for the preparation of mixed plastic (MX/PVC)–derived oil is given elsewhere [32]. Toda Kogyo Corporation, Japan, supplied the cata- lysts α-Fe 2 O 3 [PDC-03 (2)], γ-Fe 2 O 3 (TR99701), and iron oxide–carbon compos- ite (TR97305 and TR99300) catalysts used in this study. TR97305 and TR99300 were prepared from physical mixtures of iron oxide (Goethite: FeOOH) and phe- nol resins in a ratio of 9:1 by heat treatment. After the heat treatment at 500°C in N 2 flow, the product catalysts were identified as Fe 3 O 4 and carbon composites. The physical characteristics of the catalysts used in this study are presented in Table 2. TR97305 and TR99300 are two iron oxide–carbon composites with similar composition but prepared by different methods. These composite catalysts were prepared in order to increase the physical strength of the catalyst pellets. The dechlorination of mixed plastic–derived oil was carried out using a fixed- bed reactor at atmospheric pressure with a reaction temperature of 350°C. In a typical experiment, about 1 mL (0.1-mm average size) of the catalyst was loaded in between two quartz wool beds and treated in He atmosphere (60 cc/min) at reaction temperature for 1 h before feeding the mixed plastic–derived oil (10 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Catalytic Dehalogenation of Plastic-Derived Oil 333 TABLE 2 Physical Characteristics of Iron Oxide and Iron Oxide–Carbon Composite Catalysts Surface area Iron oxide Carbon Catalyst (m 2 -g Ϫ1 ) content (wt%) content (wt%) XRD γ-Fe 2 O 3 13 100 0 γ-Fe 2 O 3 α-Fe 2 O 3 4 100 0 α-Fe 2 O 3 TR97305 74 93.3 6.7 Fe 3 O 4 TR99300 60 93.2 6.8 Fe 3 O 4 mL/h) by a microfeeder. The product was collected in a trap downstream of the reactor. A separate coldwater condenser was provided to ensure the condensation of all volatile liquid products. The products were analyzed by gas chromatogra- phy equipped with a flame ionization detector (FID; YANACO G6800; column, 100% methyl silicone; 50 m ϫ 0.25 mm 0.25 µm; temperature program, 40°C (hold 15 min) → 280°C (rate 5°C/min; hold 37 min). The distribution of chlorine compounds and the quantity of chlorine content (organic) in the liquid products were analyzed by a gas chromatograph equipped with atomic emission detector (AED; HP G2350A; column, HP-1; cross-linked methyl siloxane; 25 m ϫ 0.32 mm ϫ 0.17 µm) using 1, 2, 4-trichlorobenzene as an internal standard. The conversion by the catalysts in the dechlorination of the mixed plastic– derived oil was calculated as follows: [Cl content in mixed plastic–derived oil–Cl content in product] [Cl content in mixed plastic–derived oil] (1) The physicochemical properties of the MX/PVC-derived oil estimated by stan- dard procedures and are tabulated in Table 3. The oil derived from mixed plastic- degradation oil contained 1894 ppm of organic chlorine compounds. The carbon number distribution of all compounds (C-NP gram) in MX/PVC derived oil and the carbon number distribution of chloro-organic compounds (Cl-NP gram) are shown in Figure 1a and 1b, respectively. The C-NP gram was obtained by plotting the weight percent of carbon-containing compounds in the MX/PVC oil against the carbon number of equivalent b.p. of normal paraffin. The Cl-NP gram was also obtained by plotting the content of Cl-containing compounds in the MX/ PVC oil against the carbon number of each normal paraffin (equivalent to boiling points) [33]. The chlorine compounds were distributed mainly in the b.p. range of nC 6 to nC 19 . The main organic chloro compounds are in the range of nC 6 – nC 11 . The major organic chlorine compounds were identified as 2-chloro, 2-methyl-propane, 2-chloro,2-methyl pentane, chloroethyl benzene, and 2-chloro, 2-phenyl propane [13]. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 334 Uddin and Sakata TABLE 3 Physicochemical Properties of MX/ PVC-Derived Oil Property Density (g/cm 3 ) 0.8520 Kinematic viscosity (cst @ 1500 50°C) Flash point (°C) 7 Pour point (°C) 10 Moisture content (wt%) 0.24 Conradson carbon residue (wt%) 0.11 Calorific value (cal/g) 10600 Ash content (wt%) Ͻ0.01 Chlorine content (wt%) 0.2 Nitrogen content (wt%) Ͻ0.03 Sulfur (mg/kg) Ͻ1 Carbonic acid (ppm) 42 Terephthalic acid (ppm) Ͻ5 Firstly, we carried out the dechlorination of MX/PVC-derived oil over TR97305 catalyst, which had shown high dehydrochlorination activity in our earlier studies on model chloro alkanes [34]. Here the experiment was carried out only with the plastic-derived oil feed without any carrier gas. The activity pattern against time is presented in Figure 2. The catalyst showed an initial high removal of chlorine content, but the dechlorination activity stabilized after initial deactivation. It is well known that in catalytic dechlorination the catalyst will be deactivated due to the adsorption of HCl produced during the reaction on the catalyst surface [35]. The used catalyst was characterized by XRD. The XRD patterns of the fresh and used TR97305 catalysts are presented in Figure 3. The Fe 2 O 3 phase is converted to FeCl 2 by interacting with the absorbed HCl. Blazso and Jakab, in their study of decomposition of PVC, reported that metal oxides with a large enough metal ion radius, such as iron oxide, are able to dehydrochlo- rinate the PVC by attracting chlorine and weakening the C–Cl bonds [36]. The conversion of iron oxide to chloride suggests that the oxide may act only as an absorbent. In order to elucidate this, a separate experiment was carried out by first treating the catalyst in HCl gas at 400°C for 3 h. During this treatment the iron oxide converted to iron chloride phase, and the dechlorination of MXPVC- derived oil was carried out using this catalyst. The dechlorination results on the HCl-treated catalysts are shown in Figure 4. A similar activity pattern to that of pure TR97305 catalyst is observed. This result suggests that the iron oxide ini- tially acts as catalyst and converts to iron chloride phase by reacting with the TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Catalytic Dehalogenation of Plastic-Derived Oil 335 FIG. 1 (a) Carbon number distribution of all compounds (C-NP gram) in MX/PVC derived oil and (b) carbon number distribution of chlorine organic compounds (Cl-NP gram). produced HCl, and that this iron chloride phase is also active for the dechlorina- tion of chloro-organic compounds. Based on the foregoing results, it is anticipated that the physically adsorbed HCl might be responsible for the initial decrease in activity. We carried out an experiment where the reaction was stopped after 10 h onstream; later, the catalyst was treated in He for 1 h at reaction temperature. These results are shown in Figure 5. After the first He treatment, the catalyst regained most of its activity. This behavior was found after this procedure was repeated twice. These results suggest that a continuous removal of reversible adsorbed HCl from the catalyst surface will be necessary to maintain a stable dechlorination activity of the cata- lyst. Further experiments were carried out using He as a carrier gas (5 mL/cm 3 ) over iron oxides α- and γ-Fe 2 O 3 and iron oxide–carbon composites TR97305 and TR97300 at a liquid hourly space velocity (LHSV) of 40 h 1 . Because the true TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 336 Uddin and Sakata FIG. 2 MX/PVC-derived oil dechlorination activity over TR97305 catalyst with time in the absence of carrier gas. value of gas hourly space velocity (GHSV) is difficult to determine due to the presence of a large number of hydrocarbons, only the LHSV, without considering He carrier flow rate, was used; the results are presented in Figure 6. The iron oxides and iron oxide–carbon composite showed a high activity in dechlorination. These catalysts showed similar high activity in the dechlorination of model chloroalkane, like chlorocyclohexane and 1-chloroheptane [37]. The effect of LHSV over γ-Fe 2 O 3 catalyst was studied; the results are presented in Figure 7. The removal of chlorine compounds at lower space felocity was very high. None of the catalysts studied in this reaction greatly affected the carbon number distri- bution (C-NP gram) during the dechlorination. It is reasonable to think that the removal of chlorine may result in a change in the carbon number (equivalent to the b.p. of normal paraffin) distribution. However, the change in carbon number distribution was negligible, since the amount of chlorine-containing compounds in the original mixed plastics–derived oil was not high enough to produce any significant change in the C-NP gram. Figure 8 shows the time-on-stream analysis over TR97305 and TR99300 cata- lysts. It is worth mentioning that these catalysts are very active and that no appre- ciable deactivation was observed in 24 h of time on stream. It has also reported on a Ni/SiO 2 catalyst system that a rapid deactivation (60%) occurred within 4 h during the dechlorination of chloro alkanes in the presence of He carrier gas, but the catalyst deactivation was suppressed by hydrogen [38]. It is well known that the suppression of catalyst deactivation in the presence of H 2 is due to a displacement of the hydrogen halide by dissociated hydrogen, which acts to clean the surface of the metal catalyst [40]. Surprisingly, in the present iron oxide catalyst system, in the presence of He carrier, high dechlorination activity with higher stability was achieved without using any hydrogen atmosphere. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. Catalytic Dehalogenation of Plastic-Derived Oil 337 FIG. 3 X-ray diffraction patterns of the (a) fresh and (b) used TR97305 catalyst. (᭹) Fe 3 O 4 ;(᭡) FeCl 2 ⋅ 4H 2 O. B. Removal of Organic Chlorine Compounds from Municipal Waste Plastic–Derived Oil by Catalytic Dehydrochlorination over Iron Oxides (Fe 3 O 4 - Carbon), Zinc Oxide, Magnesium Oxide, and Red Mud In this study, the dehydrochlorination of chloro-organic compounds from the mu- nicipal waste plastic–derived oil was carried out using various metal oxides, such as iron oxides, MgO, ZnO, and Red mud. The dehydrochlorination of model chloro alkanes, like chlorocyclohexane and 1-chloroheptane, was also carried over these catalysts as a test reaction. The municipal waste plastics (MWP) col- TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 338 Uddin and Sakata FIG. 4 Activity patterns of TR97305 catalysts before and after HCl treatment in the dechlorination of MX/PVC-derived oil. (᭺) Fresh catalyst; (■) HCl-treated catalyst. lected from Kamagaya City of Japan were thermally graded to obtain MWP- derived oil. The composition of the municipal waste plastics from Kamagaya City is shown in Table 4. The thermal degradation was carried out at 410°Cin a continuous-flow stirred-tank reactor. The oil obtained from municipal waste plastics contained about 600 ppm organic chlorine compounds. The physico- chemical characteristics of the derived oil were estimated using available standard methods; the results are summarized in Table 5. The catalysts iron oxide (TR99701) and iron oxide–carbon composite (TR99300) were obtained from Toda Kogyo Corporation, Japan. The Red mud, FIG. 5 Activity patterns during the sequential regeneration of the TR97305 catalyst dur- ing the dechlorination of MX/PVC-derived oil in the absence of carrier gas. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... brominated flameretardant-containing high-impact polystyrene (HIPS-Br) mixed with polyvinyl chloride (PVC) into halogen-free hydrocarbons The effect of catalyst weight on the complete removal of halogen content during the degradation and the distribution of hydrocarbons in the waste plastic–derived fuel oil were discussed The HIPS-Br [brominated high-impact polystyrene (Br-HIPS) contained decabromodiphenylethane... plastics containing polybrominated diphenyl ether as a flame retardant has been reported [41] In the present study we have not found any such compounds in the degradation products IV CONCLUDING REMARKS When PVC-containing (model) mixed plastics and municipal plastic wastes are thermally degraded to obtain fuel oil, various chloro-organic compounds, such as 2-chloro 2-methyl propane, 2-chloro 2-methyl pentane,... catalyst (TR-00301) was observed, and one of the main compounds was identified as 1-( bromoethyl-4-methyl benzene The main chlorinated compound in the liquid products of thermal degradation and FIG 17 Cl-NP gram of liquid product from thermal and catalytic degradation of HIPSBr/PVC (4/1) at 430°C TM Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved 350 Uddin and Sakata FIG 18 Br-NP gram of... pretreated in He flow (60 mL/min) at 300°C for 1 h The reactant was fed into the reactor at a flow rate of 1.2 mL/min along with He carrier gas (30 mL/min) FIG 7 Effect of space velocity during the dechlorination of MX/PVC-derived oil over TR97305 and TR99701 catalysts (■) γ-Fe2 O 3 ; (᭹) TR97305 TM Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved 340 Uddin and Sakata FIG 8 Time-on-stream analysis:... Vehlow Municipal Solid Waste Incinerator Residues Elsevier, Amsterdam, 1997 17 T Gamse, F Steinkellner, R Marr, P Alessi, and I Kikic Industrial and Engineering Chemistry Research 39:4888–4890, 2000 18 F Bertini, G Audisio, and K Kiji Journal of Analytic and Applied Pyrolosis 33: 213–230, 1995 19 MA Uddin, K Ikeuchi, N Lingaiah, H Tanikawa, A Muto, and Y Sakata Proceeding of 2nd International Symposium... Time-on-stream analysis: Dechlorination of MX/PVC-derived oil over different iron oxide–carbon composite catalysts The distribution of all the carbon compounds (C-NP gram) in MWP oil and the carbon number distribution of chloro-organic compounds (Cl-NP gram) are presented in Figures 9a and 9b, respectively The C-NP gram shows that the hydrocarbons in the MWP-derived oil were distributed mainly in the b.p range of... polybrominated flame retardant degradation Richard et al reported that high-temperature degradation of polybrominated flame-retardant materials produced Br-benzenes, Br-phenols, polybrominated dibenzodioxins (PBDD), and polybrominated dibenzofurans (PBDFs) [40] However, they were subsequently destroyed at high temperature (800°C) Dioxin (PHDD) and dibenzofuran formation (PHDF) during the thermal treatment in. .. cracking of higher-molecular-weight compounds As a result, the density of the fuel oil also decreased, due to the catalytic degradation When the catalyst amount was increased from 1 g to 2 g, the bromine was completely removed, but 840 ppm of chlorine was still present in the oil Further increase in the amount of catalyst (4 g and 8 g) resulted in the complete removal of both chlorinated and brominated... of the recycling movement REFERENCES 1 Monthly report of the Plastic Waste Management Institute, Japan, June 1998 2 J Aguado and D Serrano Feedstock Recycling of Plastic Waste Series editor: James H Clark, RSC Clean Technology Monographs: The Royal Society of Chemistry, UK, 1999, p 1–29 3 C-H Wu, C-Y Chang, J-L Hor, S-H Shih, L-W Chen, and F-W Chang Canadian Journal of Chemical Engineering 72:644–650,... presented as a ClNP gram and brominated compounds as a Br-NP gram in Figures 17 and 18, respectively The hydrocarbons containing chlorine atoms are distributed in the range of C9 –C14 , with a peak at C11 , and brominated compounds were observed at C11 only The liquid products were analyzed by GC-MS to identify the compounds The presence of brominated compounds during the thermal degradation and catalytic . and brominated flame-retardant-containing high-impact polystyrene (HIPS) mixed plastics over iron-base catalysts. A. Catalytic Dechlorination of Chloro-Organic Compounds from PVC-Containing Mixed. chloro-organic compounds (Cl-NP gram) are shown in Figure 1a and 1b, respectively. The C-NP gram was obtained by plotting the weight percent of carbon-containing compounds in the MX/PVC oil against the. since the amount of chlorine-containing compounds in the original mixed plastics–derived oil was not high enough to produce any significant change in the C-NP gram. Figure 8 shows the time-on-stream