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Feedstock Recycling from the Printed Circuit Boards of Used Computers C. Vasile,*,† M. A. Brebu,† M. Totolin,† J. Yanik,‡ T. Karayildirim,‡ and H. Darie† Romanian Academy, “Petru Poni” Institute of Macromolecular Chemistry, Department of Physical Chemistry of Polymers, 41A Grigore Ghica Voda Alley, 700487 Iasi, Romania, and Ege UniVersity, Faculty of Science, Department of Chemistry, 35100 Izmir, Turkey ReceiVed NoVember 5, 2007. ReVised Manuscript ReceiVed January 21, 2008
Here, we focused on the pyrolysis of printed circuit boards (PCBs) from used computers using various combined procedures of thermal and catalytic pyrolysis and dehalogenation (absorption) aiming to obtain pyrolysis oils with low amounts of heteroatoms (Br, Cl, N, and O) that might be suitable for use as fuel or feedstock. The composition of degradation oils was established by suitable methods, such as gas chromatography (GC-MSD and GC-AED), 1H nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR), and common methods for the analysis of petrochemical products. It has been found that catalytic cracking and dehalogenation procedures are simple and effective methods to convert PCBs into some useful products.
Introduction Plastic waste is one of the major problems facing the consumer societies in Europe, Japan, and North America. In recent years, there has been a growing interest in using pyrolysis to process scrap plastics from waste electrical and electronic equipment (WEEE). In the European Union, the disposal of WEEE is closely controlled by the WEEE directive of the European Commission, which requires that WEEE is collected separately and each type of material (e.g., plastics and metals) is separated and recycled.1 In Romania, disposal/recycling of such kind of waste is also starting to be controlled.2 However, the plastic fraction of WEEE is particularly problematic to recycle. This contains additives, such as heavy metals (Hg, Pb, Cd, and CrVI) and halogenated flame retardants, especially polybrominated ones (BFRs), such as polybrominated diphenyl ethers or tetrabromobisphenol A, which have a negative impact on the environment. WEEE consists mainly of (i) thermoplastics, such as acrylonitrile-butadiene-styrene (ABS), high-impact polystyrene (HIPS), and polycarbonate (PC), that are used for casings, and (ii) thermosets (e.g., epoxy resins) as major components of printed circuit boards (PCBs). Pyrolysis of epoxy resins in the presence of different additives showed that the interactions between the additives and the epoxy resin result in a change in the decomposition pathways and an increased char formation.3–6 BFRs are considered to be extremely toxic, and plastics that contain BFRs need careful processing to either * To whom correspondence should be addressed. E-mail: cvasile@ icmpp.ro. † Romanian Academy. ‡ Ege University. (1) Directive 2002/96/EC of the European Parliament and of the Council on Waste Electrical and Electronic Equipment. Official Journal of the European Commission, 2003, L37/24, Brussels, Belgium. (2) HG 448/2005 and HG 992/2005 Romanian Government decisions on WEEE and limitations of the use of certain hazardous substances in EEE. Romanian Ordin 901/2005 concerning specific measures for WEEE collection which present safety and health risks. (3) Chen, K. S.; Yeh, R. Z. J. Hazard. Mater. 1996, 49, 105–113. (4) Bychkov, S. G.; Desyatkov, A. V.; Biketov, A. A.; Ksandopulo, G. I. Combust., Explos. Shock WaVes 1986, 22, 340–342.
destroy or remove the flame-retardant additives.7 Therefore, the use of pyrolysis for processing WEEE plastics has been intensively investigated as a means of thermally degrading the toxic flame retardants. Various pyrolysis procedures8–10 have been applied, some of them using different catalysts, such as iron- and calcium-based catalysts11–13 or zeolites.14–18 We previously reported on the pyrolysis of various computer scraps and upgrading of pyrolysis oils by hydrogenation.19,20 Here, we focused on the degradation of waste PCBs using coupled thermal, catalytic, and dehalogenation (absorption) procedures, aiming to obtain pyrolysis products with low (5) Prabhu, T. N.; Hemalatha, Y. J.; Harish, V.; Prashantha, K.; Iyengar, P. J. Appl. Polym. Sci. 2007, 104, 500–503. (6) Schartel, B.; Balabanovich, A. I.; Braun, U.; Knoll, U.; Artner, J.; Ciesielski, M.; Döring, M.; Perez, R.; Sandler, J. K. W.; Alstädt, V.; Hoffmann, T.; Pospiech, D. J. Appl. Polym. Sci. 2007, 104, 2260–2269. (7) Tohka, A.; Zevenhoven, R. Brominated flame retardantssA nuisance in thermal waste processing? TMS Extraction and Processing Division Meeting on Recycling and Waste Treatment in Mineral and Metal Processing: Technical and Economic Aspects: Lulea, Sweden, June 16– 20, 2002. (8) Hall, W. J.; Williams, P. T. J. Anal. Appl. Pyrolysis 2006, 77, 75– 82. (9) Hall, W. J.; Williams, P. T. J. Anal. Appl. Pyrolysis 2007, 79, 375– 386. (10) Bhaskar, T.; Hall, W. J.; Mitan, N. M. M.; Muto, A.; Williams, P. T.; Sakata, Y. Polym. Degrad. Stab. 2007, 92, 211–221. (11) Mitan, N. M. M.; Brebu, M.; Bhaskar, T.; Muto, A.; Sakata, Y. J. Mater. Cycles Waste Manage. 2007, 9, 56–61. (12) Brebu, M.; Bhaskar, T.; Murai, K.; Muto, A.; Sakata, Y.; Uddin, M. D. A. Polym. Degrad. Stab. 2005, 87, 225–230. (13) Brebu, M.; Bhaskar, T.; Murai, K.; Muto, A.; Sakata, Y.; Uddin, M. D. A. Polym. Degrad. Stab. 2004, 84, 459–467. (14) Blazsó, M.; Czégény, Zs. J. Chromatogr., A 2006, 1130, 91–96. (15) Blazsó, M. J. Anal. Appl. Pyrolysis 2005, 74, 344–352. (16) Blazsó, M.; Czégény, Zs.; Csoma, Cs. J. Anal. Appl. Pyrolysis 2002, 64, 249–261. (17) Miskolczi, N.; Bartha, L.; Déak, G. Y. Polym. Degrad. Stab. 2006, 91, 517–526. (18) Bagri, R.; Williams, P. T. J. Inst. Energy 2002, 75, 117–123. (19) Vasile, C.; Brebu, M. A.; Karayildirim, T.; Yanik, J.; Darie, H. J. Mater. Cycles Waste Manage. 2006, 8, 99–108. (20) Vasile, C.; Brebu, M. A.; Karayildirim, T.; Yanik, J.; Darie, H. Fuel 2007, 86, 477–485.
10.1021/ef700659t CCC: $40.75 2008 American Chemical Society Published on Web 03/04/2008
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amounts of heteroatoms (Br, Cl, N, and O) that might be suitable to be used by the petrochemical industry. Materials and Methods Materials. The PCB sample was obtained from waste computer parts by a systematic disassembly followed by homogenization, size reduction, and sampling. PCBs and associated components represented 15 wt % of the initial WEEE, while thermoplastics, metal, and glass represented 30, 23, and 32 wt %, respectively. The average elemental composition (wt %) of the PCB sample was C, 24.69; H, 1.38; N, 0.85; Cl, 2.05; Br, 4.94; and S, 1.97. The ash content of PCBs (mainly metals and glass) was of 63.4 wt %, while the volatiles were of 36.6 wt %. The Fourier transform infrared spectroscopy (FTIR) spectrum of PCBs revealed signals in the range of 3300–3500 and 1600–1800 cm-1, indicating the presence of -OH and other oxygen-containing groups, while the presence of halogenated flame retardants was evidenced by bands at 1000–1200 and 600–750 cm-1. Some thermal properties of the organic fraction of PCBs have been compared to those of commercial epoxy resins (Ropoxid 501) obtained from bisphenol A hardened with diaminodiphenylmethane (ER/DDM) in a 9:2 ratio, with the results proving that the main component of PCBs is a flame-retarded epoxy resin. The amount of metals (in mg/g of sample) was determined by atomic absorption spectroscopy as following: Fe, 19.7; Ni, 1.7; Cr, 0.03; Mn, 1.4; Co, 0.01; Cu, 137.9; Zn, 4.84; Mg, 2.2; and Ca, 29.0. The cracking catalyst (Crack) used in the experiments was a silica–alumina amorphous one, with the following characteristics: SiO2, 86%; Al2O3, 12%; Fe2O3, 0.12%; and Na2O, 0.32%; density, 0.684 g/cm3; granulation, 3–4 mm; and activity index, 37. The red mud resulted as a byproduct of the electrochemical process of aluminum production was supplied by Seydis¸ehir Alumina Plant, Turkey. The components in Red Mud were detected by X-ray fluorescence spectrophotometry. The main analytical data (wt %) obtained by this method are Fe2O3, 37.72; Al2O3, 17.27; SiO2, 17.10; TiO2, 4.81; Na2O, 7.13; and CaO, 4.54. Silica and aluminum were detected in the AlSixOy form, and iron was found in the Fe2O3 form. CaCO3 (analytical grade, Merck) was used as received. We have used the system composed of a cracking catalyst relatively resistant to acidic medium in combination with two absorbers for halogenated compounds (CaCO3 and Red Mud), which demonstrated their efficacy in cracking21 and capture of some hazardous compounds,22 respectively. M-Ac catalyst was prepared by a wet impregnation method using metal salts. Activated carbon used as a catalyst support was obtained from pyrolytic carbon black from pyrolysis at 800 °C of used scrap truck tires. The demineralized pyrolytic carbon black was activated with carbon dioxide at a flow rate of 350 mL/min for 6 h at 900 °C and then loaded with metal by an impregnation method. M-Ac has the following characteristics: Mo, 2.89 wt %; Ni, 4.63 wt %; surface area, 215.13 m2/g. DHC-8 was a commercial catalyst commonly used for hydrocracking of vacuum gas oil in Izmir refinery, Turkey. DHC-8 is a bifunctional amorphous catalyst consisting of non-noble hydrogenation metals on a silica–alumina base, thus incorporating both hydrotreating and hydrocracking functions. It was used in the sulfide form of powder with a specific surface area of 102 m2/g. The selection of catalysts was performed on the basis of our previous experience because they are resistant to impurities, which was already tested in feedstock recycling of thermoplastics and waste plastics. (21) Moiceanu, E.; Ocneanu, I.; Vasile, C.; Sabliovschi, M.; Moroi, G.; Darie, H.; Zaharia, C. Romanian Patent 92783, 1987. (22) Karayildirim, T.; Yanik, J.; Yuksel, M.; Saglam, M.; Vasile, C.; Bockhorn, H. J. Anal. Appl. Pyrolysis 2006, 75, 112–119. (23) Vasile, C.; Pakdel, H.; Brebu, M.; Onu, P.; Darie, H.; Ciocalteu, S. J. Anal. Appl. Pyrolysis 2001, 57, 287–303.
Figure 1. Schematic experimental setup for thermal degradation and catalytic upgrading of PCBs.
Pyrolysis Procedure. Pyrolysis was performed under atmospheric pressure by semibatch operation (Figure 1)23,24 introducing 300 g of material in a metal reactor heated at 300–540 °C. For catalytic upgrading, the volatile pyrolysis products were directly passed to a secondary reactor that allows for a combination of various layers of catalysts. In our upgrading experiments, the catalytic reactor was filled with CaCO3/cracking catalyst or with CaCO3/cracking catalyst/Red Mud in 2.5:1 and 2.5:1:1 weight ratios, respectively, and it was heated to 470 °C. The residence time of the volatiles on the catalyst layer was of 1.0–2.5 s, varying with the evolution rate of volatile products from the pyrolysis reactor. The space velocity of volatiles in the catalyst bed was estimated to be of about 0.044 L/s. The volatile products of catalytic upgrading were passed through a cooling/collection system that separated liquids (pyrolysis oils and aqueous fraction) from gases. A gas meter measuring the evolution of gaseous products noncondensable at room temperature helped to estimate the end of pyrolysis experiments that occurred after about 120 min, when no gaseous product was observed. In a series of experiments, the PCB oils from thermal degradation were upgraded by thermal and catalytic hydrotreating in a shakingtype batch autoclave.25 Commercially available DHC-8 catalyst and a metal-loaded activated carbon (M-Ac) were used for catalytic hydrogenation. Reactions were carried out at 350 °C, 6.5 MPa initial hydrogen pressure, 15 g of oil feed, 3 g of catalyst, and reaction time of 120 min. Analysis and Characterization Methods. The distribution of bromine (Br), chlorine (Cl), nitrogen (N), and oxygen (O) containing organic compounds in pyrolysis oils was analyzed by a gas chromatograph equipped with an atomic emission detector (AED; HP G2350A; column, HP-1; cross-linked methyl siloxane; 25 m × 0.32 m × 0.17 µm). 1-Bromohexane and 1,2,4-trichlorobenzene were used as internal standards for the quantitative determination of Br and Cl, and nitrobenzene was used as the internal standard for N and O. This allows for quantitative determination of Br, Cl, N, and O in pyrolysis oil; however, because of the lack of available methods for quantitative analysis of compounds in solid residue, water phase, and gaseous products, we were unable to calculate the balance of heteroelements in pyrolysis products. The qualitative composition of liquid products was analyzed by a gas chromatograph using a mass-selective detector (MSD; HP 5973; column, HP-1; cross-linked methyl siloxane; 25 m × 0.32 mm × 0.17 µm; temperature program, 40 °C (hold for 10 min) and 300 °C (rate of 5 °C/min, hold for 5 min). The composition of the liquid products was characterized using the C-NP gram method (C stands for carbon, and NP stands for normal paraffin) based on chromatographic results.26,27 In a similar way, the organic Br-, Cl-, N-, and O-containing compounds were characterized using Br-, Cl-, N-, and O-NP grams. High-performance liquid chromatography (HPLC) has been applied for the analysis of aqueous fraction by means of a Spectroflow 783 instrument from ABI Analytical Kratos Division (24) Vasile, C.; Calugaru, C. M.; Sabliovschi, M.; Cas¸caval, C. N. Romanian Patents 74577, 1980, and 78462, 1981. (25) Karagoz, S.; Yanik, J.; Ucar, S.; Song, C. Energy Fuels 2002, 16, 1301–1308. (26) Murata, K.; Hirano, Y.; Sakata, Y.; Uddin, M. A. J. Anal. Appl. Pyrolysis 2002, 65, 71–90. (27) Shiraga, Y.; Uddin, M. A.; Muto, A.; Narazaki, M.; Sakata, Y.; Murata, K. Energy Fuels 1999, 13, 428–432.
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Figure 2. TG/DTG curves of PCBs comparatively with ER (a) un-cross-linked and (b) cross-linked with DDM. Table 1. Thermogravimetric Data for Studied Samplesa sample PCB PCB plus ER/ DDM (1 + 1) ER ER/DDM (9:2)
Ti (°C) Tm (°C)
Tf (°C)
182 180
268 258
298, 348 312, 376
107 172
302 325
390 448
°C
W700 pyrolyzing (wt %) mass (wt %) 64.8 89.6
35.2 10.4
0.0 6.6
100.0 93.6
a T , onset temperature; T , temperature corresponding to the i m maximum rate of mass loss; Tf, final temperature of the degradation step; W700 °C, residual mass at 700 °C.
with a Spectroflow 491 Injector at a wavelength of 254 nm and a LKB Bromma 2150 HPLC Pump. The column with a length of 250 mm and inner diameter of 4 mm was packed with Nucleosil 5C18 from Macherey and Nagel. The elution was made with a 60: 40 (v/v) methanol/water mixture, in isocratic regime, with a flow rate of 1 or 2 mL/min. Sample volumes of 200 µL were injected. 1H nuclear magnetic resonance (NMR) spectra of liquid products were recorded with a Bruker GMBH DPX-400 (Germany) using CDCl3 as the solvent, and they were used to determine the hydrocarbon types and research octane number (RON) of the oils.28 FTIR spectra were recorded on a FTIR Bomem MB-104 spectrometer (Canada) with a resolution of 4 cm-1, with a very thin layer of pyrolysis oil being deposited on KBr tablets. Common analysis methods for petrochemical products [paraffins, isoparaffins, olefins, naphthenes, and aromatics (PIONA)] were also considered for the analysis of pyrolysis liquid products. The thermogravimetric/derivative thermogravimetric (TG/DTG) curves were recorded on a Paulik-Paulik-Erdey-type derivatograph (MOM, Budapest, Hungary) under the following operational conditions: heating rate, 12 °C/min; temperature range, 25–600 °C; and sample mass, 50 mg, using platinum crucibles and a selfgenerated atmosphere. Two curves were recorded for each sample.
Results and Discussion The decomposition pathway was envisaged from the shape of TG/DTG curves and from the characteristic temperatures of degradation (Figure 2 and Table 1). A comparison of the thermal behavior of PCBs with that of an epoxy resin (ER) cross-linked (hardened) with diamonodiphenylmethane (DDM) showed similar TG/DTG curves; however, the one of PCBs is shifted to lower temperatures. The results confirmed that an epoxy resin is the main polymeric component in the composition of the PCBs. (28) Myers, M. E.; Stollsteimeir, J. R.; Wins, A. M. Anal. Chem. 1975, 47, 2010–2015.
Figure 3. Material balance for PCB pyrolysis at various temperatures.
PCB decomposes at 180–348 °C, leaving a high quantity of residue because of the presence of metals. The particular behavior of PCBs during decomposition can be explained by the radicals easily formed from flame retardants, which are presented in greater amounts in PCB scraps. It is known that most flame retardants decompose around 300 °C, mainly by the removal of hydrogen bromide. In contrast, the decomposition of epoxy resin is finished at 450 °C. Luda and others29 observed a decrease in the decomposition temperature of epoxy resin in the presence of flame retardants, which implies that the radicals derived from the flame retardants induced decomposition of epoxy resin.30 The residue amount from PCBs is almost equal with the ash content. The amount of volatiles below 700 °C is of about 35 wt %. Effect of the Temperature on Thermal Degradation of PCBs. PCB waste, because of its high metal content, leaves after pyrolysis at 350–540 °C a high amount of 70–80 wt % residue (Figure 3) decreasing with an increased pyrolysis temperature as expected. Considering the ∼63% ash content of PCBs, it means that more than 80% of organic fraction from PCBs could be converted at 540 °C into recoverable products as oils, aqueous fraction, and gases, from which we focused in (29) Luda, M. P.; Balabanovich, A. I.; Hornung, A.; Camino, G. Proceedings of ISFR’2002, The 2nd International Symposium on Feedstock Recycling of Plastics and Other Innovative Recycling Techniques: Ostend, Belgium, September 8–11, 2002; CD-A66. (30) Kamo, T.; Kondo, Y.; Kodera, Y.; Kushiyama, S. Proceedings of ISFR’2002, The 2nd International Symposium on Feedstock Recycling of Plastics and Other Innovative Recycling Techniques: Ostend, Belgium, September 8–11, 2002; CD-A21.
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Figure 4. Gas volume versus time for PCB decomposition at different temperatures.
Figure 5. C-NP grams of oil products resulting from PCB pyrolysis at different temperatures.
our study on pyrolysis oils. Increasing the pyrolysis temperature from 350 to 540 °C enhances the thermal degradation, with the oil yield increasing more that 2 times, while the amount of gaseous products increased from 5 to about 8 wt %. The amount of water fraction resulted from pyrolysis was not affected by the temperature of the process. PCB that consists mainly of epoxy resin decomposes with a higher rate than other components of computer scraps (such as cases or keyboards that are made by ABS or HIPS),15 giving a considerable volume of gas, even when the pyrolysis temperature was decreased below 350 °C. The gas volume and the rate of evolution increase with an increasing temperature (Figure 4). PCB pyrolysis oils have a high quantity of light fraction, about 70–80 wt % of the compounds in oils belonging to the n-C10-nC14 range in C-NP gram (Figure 5), with two peaks at n-C10 and n-C13. Another small peak is present at n-C17. The distillation curves are similar at all temperatures, with more than 85 wt % of oils being volatile below 250 °C. The refractive indices of oils have values between 1.4513 and 1.5325; therefore, the main components are aromatics. This was expected considering the structure of epoxy resins, the main component of the pyrolyzing fraction in PCBs. Oxygen is the heteroatom with the highest concentration in PCB oils. O-Containing compounds are represented mainly by phenol and its alkyl derivatives, with the most important compounds being phenol at n-C10, isopropylphenol at n-C13, and p-hydroxybiphenyl at n-C16 (Figure 6a). The main peaks in O-NP gram correspond with the main ones in C-NP gram, proving that O-containing compounds are the main ones in the pyrolysis oil of PCBs. Benzofurane, methylbenzofurane, and bisisopropylphenol were also identified by GC-MSD. Nitrogen
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is represented by light aliphatic nitriles at n-C5, n-C6, methylbenzonitrile and benzeneacetonitrile at n-C10, benzenebutanenitrile and methylquinoline at n-C13 and octadecanenitrile at n-C18 (Figure 6b). Halogens are represented mainly by chlorophenol and bromophenol (parts c and d of Figure 6). The amounts of O, Cl, and Br compounds had maximum values at the pyrolysis temperature of 400 °C and minimum values at 300 °C. In contrast, the amount of N compounds is continuously increasing with the pyrolysis temperature. No sulfur-containing compound was identified in PCB pyrolysis oils. PCB gives considerable higher amounts of halogens compared to other components of computer scraps (e.g., casings and keyboards).15 Therefore the elimination of the hazardous toxic compounds (mainly those containing bromine) is necessary before safely using the pyrolysis oils as fuels or in refinery or petrochemical industry flows. Catalytic Decomposition/Absorption Procedures Applied in PCB Pyrolysis. Catalytic degradation of PCBs at 540 °C over CaCO3/crack and over crack/CaCO3/Red Mud systems decreases the oil yield from 18 to 12 and 7.7 wt % with respect to the thermal procedure and increases the aqueous fraction from 4.3 to 5.3 and 7.3 wt % (Figure 7). The yield of gaseous product remains almost constant (7.7-8.1 wt %), with its evolution being only insignificantly changed using the second step of catalytic cracking/dehalogenation (absorption). About 2.3-3.3 wt % of coke is deposited on the cracking catalyst. The amount deposited on the adsorbing layers of CaCO3 and Red Mud is very low (0.3–0.7 wt %). The main part of the initial material (70–74 wt %) remains as the decomposition residue from the first step of thermal decomposition, as it was expected from TG analysis discussed above. There are no important variations in the gas evolution during pyrolysis (Figure 8) and in distillation curves of oils (Figure 9a) and the refractive indices of the cuts (Figure 9b). The most important effect of the catalytic/absorbing systems appears on the composition of the oily pyrolysis products as found by chromatographic analysis. All GC-AED chromatograms are simpler with fewer peaks in the catalytic procedure, showing the cracking effect of the catalysts simultaneous with the partial removal of heteroelements from PCB pyrolysis oils (Figure 10). The identification of heteroatom-containing compounds, most of them in small amounts in pyrolysis oils and thus difficult to see in Figure 10, was obtained by GC-MSD. These are given in Figure 11, while the peak area of the main identified compounds is given in Table 2. From the global distribution of the compounds in pyrolysis oil (Figure 11a), it can be remarked that some peaks corresponding to the oil obtained from catalytic/dehalogenation (absorption) processes are shifted to a lower carbon number and are larger. The main components in pyrolysis oil are phenols, while several aromatic hydrocarbons have been also identified. They are shown in parts b and c of Figure 11 at the corresponding carbon number. Some of the peaks (e.g., at n-C14 in Figure 11b and n-C15, n-C17, and n-C19 in Figure 11c) correspond to many compounds with a smaller peak area whose clear identification of the structure was difficult to obtain. The GC-MSD data in Table 2 shows that the CaCO3/crack system compared to the thermal process leads to a slight increase of toluene at n-C8 and dimethylbenzene at n-C9, a significant increase of phenol derivatives (2- and 4-methylphenol at n-C11, ethylphenols at n-C12, and dimethylphenols and propylphenol at n-C13), and a strong increase of 2-methylbenzofurane at n-C12. The crack/Red Mud/CaCO3 system compared to the CaCO3/
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Figure 6. (a) O-, (b) N-, (c) Cl-, and (d) Br-NP grams of oil products from the pyrolysis of PCBs at various temperatures.
Figure 7. Product yield of PCB decomposition by thermal and catalytic/ adsorbing procedures.
Figure 8. Evolution of gaseous products from the decomposition of PCBs by various procedures.
crack system and thermal process leads to a slight increase of 2-butanone at n-C7, a significant increase of acetone at n-C6, a strong increase of aromatics (benzene at n-C7, toluene, and ethylbenzene at n-C9), and an increase of phenol at n-C10, and in comparison to the CaCO3/crack system, it showed a decrease of phenol derivatives (methylphenols, ethylphenols, dimeth-
Figure 9. Distillation curves of PCB pyrolysis oils and refractive indices of the cuts.
ylphenols, and propylphenols) and an increase of methylbenzofuran at n-C12; n-buthylether at n-C9 was not found in other samples. The changes in the pyrolysis oil composition after applying various cracking/absorbing procedures is also evident from FTIR spectra (Figure 12). It can be easily remarked the disappearance of the bands at 551, 831, 974, 1381 1514, and 1654 cm-1 and significantly diminishing of the 1705 cm-1 band from the spectrum of the pyrolysis oil obtained by uncatalytic procedures when catalytic/absorbing procedures are applied. Most of the bands correspond to halogen-C bonds; therefore, the quantities of halogen containing compounds are reduced by cracking/ absorbing treatment of the thermal degradation products. No significant difference was found in 1H NMR spectra of the oily products resulting from studied procedures (Figure 13), but signals of different protons, mainly the saturated ones, become much simpler, which is in good concordance with GC-MS data. From all data, it can be remarked that, after applying catalytic pyrolysis coupled with different procedures of adsorption/ dehalogenation, lighter compounds are formed, nitrile com-
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Figure 10. C, Cl, Br, N, and O chromatograms (GC-AED) of PCB pyrolysis oils.
Figure 11. Distribution of various compounds in the pyrolysis oil obtained by different procedures versus the carbon number: (a) global, (b) oxygen-containing compounds, and (c) hydrocarbons.
pounds disappear, but some amines are formed. After catalytic cracking, light compounds, such as bromomethane, bromobezene, and halogeno-phenols, vanish but some amines and quinolines are new compounds formed by these procedures. No nitriles are found after using the crack/Red Mud/CaCO3 system. However, further dehalogenation treatment is needed before using the pyrolysis oils as fuel or feedstock. The organic compounds from the aqueous fraction have been analyzed by GC-MS and HPLC after extraction with n-butyl ether. The main compounds found in this fraction were benzene, xylene, phenol, and their derivatives. The proportion of the phenols is very high (>80%). The identified compounds in the aqueous fraction of catalytic pyrolysis of PCBs are given in Table 3. There are some compounds characteristic for each procedure, such as butanol, formic acid, benzofuran, and
naphthalene, in oil resulting from CaCO3/crack and benzene, 2,5-dihydrofuran, butanoic acid, and isopropylphenol in oil resulting from crack/Red Mud/CaCO3. The residue from thermal degradation of PCBs still contains some undecomposed organic material because three steps of mass loss were found in the TG/DTG curves recorded up to 800 °C (Figure 14). The thermogravimetric characteristics are given in Table 4. The presence of organic material is also proven by the high carbon content (∼61 wt %) of the residue, as found by elemental analysis. The concentration (wt %) of other elements in the residue was as following: H, 2.0; N, 0.6; S, 0.29; Br, 8.24; and Cl, 3.64. This shows a concentration of Br and Cl in the residue. Organic compounds volatile up to 800 °C count for 29.4 wt %, while the residue is of 70.6 wt %, having the composition
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Table 2. GC-MSD % Area of Main Identified Compounds in Pyrolysis Oils from PCBs compound
RT (min)
acetone propanenitrile 2-butanone benzene toluene octane ethylbenzene dimethylbenzene n-buthylether styrene nonane 1-methylethylbenzene propylbenzene ethylmethylbenzene phenol 2-methylphenol 4-methylphenol 2-methylbenzofuran 2,4-dimethylphenol 2-ethylphenol naphthalene 4,7-dimethylbenzofuran 2-isopropylphenol 2,4,6-timethylphenol 2-propylphenol
1.44 1.74 1.83 2.32 3.7 4.23 5.38 5.55 5.81 5.94 6.1 6.59 7.14 7.29 7.98 9.21 9.73 10.05 10.77 11.09 11.37 11.65 12.19 12.24 12.47
Table 3. Identified Compounds in Aqueous Pyrolysis Fractions from PCBs
thermal
CaCO3/ crack
crack/Red Mud/CaCO3
RT (min)
0.36 0.07 0.1 0.09 0.28 0.01 0.76 0.27
0.38 0.01 0.03 0.06 1.01 0.01 0.45 1.31
1.17
1.44 1.81 2.31 2.43 2.56 6.75
0.75
0.01 0.03 0.11 0.15 0.52 34.42 9.12 10.62 1.73 2.76 5.72 2.53 0.26 9.46 0.24 2.52
0.08 0.05 0.09 32.02 0.36 2.58 0.05 1.89 2.06 9.91 1.04
0.11 2.08 2.5 0.11 1.04 1.33 0.64 0.08 0.28 0.14 0.46 46.8 6.63 7.7 2.42 1.65 4.73 1.52 0.17 8.07 0.11 1.57
similar with that of PCB ash. The residue composition is approximately identical to that of PCB ash (see above).
CaCO3/crack acetone butanal butanol 2,2,4-trimethyl-pentane
6.78 6.91 6.97 7.96 7.99 8.12 9.03
crack/Red Mud/CaCO3 acetone butanal benzene 2,2,4-trimethyl-pentane pentane, 2-butoxy (C4H9-O-CHCH3C3H7) butanoic acid, 3,3-dimethyl (CH3)3CCH2COOH)
butane, 1-butoxy-2-methyl 2,5-dihydro furan phenol
phenol benzofuran 4-ethyl pyridine plus 2,4-dimethyl-pyridine 3,5-dimethyl-pyridine 2-methyl-benzofuran
9.42 9.95 9.97 10.5 10.62 11.03 11.24 11.94 12.99
2-ethyl phenol 2,4-dimethyl-phenol 4-ethyl phenol naphthalene
butanoic acid 4-ethyl pyridine plus 2,4-dimethyl-pyridine 3,5-dimethyl-pyridine 2-propenal, 3-phenyl (C6H5-CHdCH-CHdO) 2-ethyl phenol 4-ethyl phenol 3-(1-methylethyl)-phenol
1-methyl-naphthalene
Effect of Hydrogenation on the Upgrading of PCB Oil. Hydrogenation converted 37–60 wt % of PCB oils into gases, while 6–11 wt % of the initial load remained as a solid residue (Table 5). Hydrogenation over metal-loaded activated carbon catalyst (M-Ac) gave the highest gas yield and the lowest amount of liquid products, showing an excellent cracking activity combined with hydrogenation activity. Catalytic hydrogenation decreased the amount of paraffins and the H/C ratio while increasing the amount of aromatics
Figure 12. FTIR spectra of the pyrolysis oil resulted from various procedures applied to PCBs.
Figure 14. TG/DTG curves of the residue from thermal degradation of PCBs. Table 4. Characteristic Parameters from TG Analysis of the Thermal Degradation Residue of PCBsa peak 1
peak 2
peak 3
total
Ti Tm Tf Wf Ti Tm Tf Wf Ti Tm Tf Wf Wt (°C) (°C) (°C) (%) (°C) (°C) (°C) (%) (°C) (°C) (°C) (%) (%) 53 a
Figure 13. 1H NMR spectra of the pyrolysis oil of PCBs.
90
262 2.3 263 416 530 11.7 530 660 770 15.4 29.4
Ti, onset temperature; Tm, temperature corresponding to the maximum rate of mass loss; Tf, final temperature; Wf, mass loss at the end of the degradation step; Wt, total mass loss at the end of decomposition (800 °C); this is equivalent to the total mass of still decomposable material in the PCB residue.
Feedstock Recycling from the PCBs
Energy & Fuels, Vol. 22, No. 3, 2008 1665
Table 5. Product Yield and Hydrocarbon Content (from 1H NMR Data) of Liquid Products from the Hydrogenation of PCB Oils product yield of hydrogenation (wt %)
hydrocarbon content of the liquid products (vol %)
hydrogenation method
liquid
solid
gasa
aromatics
paraffins
olefins
H/C
isoparaffin index
RONb
no treatment thermal M-Ac DHC-8
56.5 33.2 50.9
5.9 7.7 11.0
37.5 59.1 38.1
80.40 84.56 89.33 88.05
15.95 15.44 10.67 11.95
3.65 0.00 0.00 0.00
1.02 1.04 0.92 0.93
0.03 0.17 0.10 0.14
89.05 90.75 90.61 90.89
a
Evaluated by difference. b RON ) research octane number.
Figure 15. Proportion of the (a) main aromatics and (b) phenol and main phenol derivatives found in pyrolysis oil from PCBs before and after hydrotreating.
(mainly phenol derivatives). Olefin compounds were totally removed. This is a proof that DHC-8 and M-AC have both hydrocracking and hydrotreating effects. Hydrogenation increased the amount of benzene derivatives from about 6 to 14–17 wt % (Figure 15a). Styrene was removed during upgrading. In the case of O-containing compounds, isopropylphenol was decreased by hydrotreating, whereas the other ones were slightly increased (Figure 15b). All aliphatic and aromatic nitriles were removed from oil by thermal and catalytic hydrotreating. However, new nitrogen-containing heterocyclic compounds (e.g., methylpyridine at n-C11) were formed after hydrogenation, while they were not identified in untreated oils. Chlorine-containing compounds in PCBs are represented by chloromethane, chlorocyclopentane, 2-chlorophenol, and 2-chloro-6-methylphenol; they were found in traces after DHC-8 treatment and totally removed by the other two
hydrogenation procedures. Bromine-containing compounds were not found in upgraded (hydrogenated) pyrolysis oils. Conclusions Pyrolysis of PCBs from waste computer scraps gave less than 20 wt % oils and pyrolytic water that are rich in phenol and phenol derivatives but also contain organic Cl, Br, N, and other O compounds. Catalytic pyrolysis decreased the amount of all heteroatoms in PCB oils. The hydrogenation over commercial DHC-8 catalyst and over metal-activated carbon upgraded the PCB oils, removing almost all of the hazardous toxic compounds. Acknowledgment. The authors acknowledge the financial support from ANCS, Romania, by COMBERG 761/2006 research project. EF700659T
