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Characterization of used tire vacuum pyrolysis oil: Nitrogenous compounds from the naphtha fraction
Journal of Analytical and Applied Pyrolysis, 22 (1992) 205-215 Elsevier Science Publishers B.V., Amsterdam
205
Characterization of used tire vacuum pyrolysis oil: nitrogenous compounds from the naphtha fraction S. Mirmiran, H. Pakdel and C. Roy * Unioersite’ Laual, Chemical Engineering Department, PaLSon Adrien-Pouliot, Ste. Foy, Qukbec GlK 7P4 (Canada) (Received
August
15, 1991; accepted
in final form October
3, 1991)
ABSTRACT Vacuum pyrolysis of used tires yielded 61% by weight hydrocarbon type oil from which 18% by weight naphtha fraction can be distilled off. The pyrolysis oil consisted of (i) the processing oil as part of the tire formulation, (ii) organic additives, and (iii) tire pyrolysis products. A procedure was developed for the separation and identification of nitrogenous compounds in used tire-derived oil, which were partially produced by pyrolysis of tire accelerants. The analytical separation technique involved liquid-solid chromatography on a dual packed silica gel and alumina column. n-Pentane and ethylacetate eluted the hydrocarbons. The nitrogenous compounds were eluted with methanol. Thirty-one nitrogenous compounds were identified by gas chromatography/mass spectrometry and gas chromatography/atomic emission detection. Atomic emission; gas chromatography; sis oil; rubber; tires; vacuum pyrolysis.
mass spectrometry;
nitrogenous
compounds;
pyroly-
INTRODUCTION
Used tire disposal is becoming a critical problem for the environment. In Canada for example, 24 million tires are discarded yearly. While some of these tires are recapped or ground up for special uses, most are simply dumped in rural farm land or in landfill sites. Used tires represent a source of energy and raw chemicals for the production of rubber parts. By thermal decomposition of rubber, useful products can be recovered [l-4]. Vacuum pyrolysis, a novel approach based on ,the decomposition reaction at low temperature and reduced pressure, is under development in our laboratory to transform used tires into liquid and solid fuels [4]. Materials used in tire compounding are the following with typical ranges in p.h.r. (parts per hundred of rubber): elastomers (100 p.h.r); vulcanizing agents (5-10 p.h.r); accelerators (l-3 p.h.r); activators and other additives (l-5 p.h.r); processing oil (S-30 p.h.r); carbon black (50-70 p.h.r). ’ 01652370/92/$05.00
0 1992 - Elsevier
Science
Publishers
B.V. AI1 rights reserved
Heating under vacuum of elastomers, vulcanizing agents, accelerators, additives and processing oil yields pyrolysis oils. The gaseous and vapour molecules formed under vacuum pyrolysis conditions are rapidly removed from the reaction chamber. This minimizes the extent of secondary decomposition reactions. The analysis of hydrocarbons in pyrolysis oils has been performed by several researchers [5-81. Limited work has been carried out, however, on the determination of sulfurous and nitrogenous compounds in rubber-derived oils. This study showed that naphtha extracted from used tire vacuum pyrolysis oil contains significant amounts of nitrogenous compounds which are derived from tire additives. N-compounds are undesirable components of oils because during the refining process they can poison the catalysts and promote gum formation [9-121. Several studies on the separation and characterization of nitrogenous compounds from petroleum have been reported earlier [13,14]. Jewel1 and Snyder [15] used complex formation on supported ferric chloride and ion-exchange resins to isolate the nitrogenous compounds from petroleum. Ford et al. [16] employed alumina adsorption chromatography to separate nitrogenous compounds. Ben’Kovski and Olzseva [17] removed nitrogen compounds from diesel fuel by complex formation with titanium tetrachloride. A preliminary study using brominated ilmenite as a sorbent demonstrated the possibility of using sorption for the large scale removal of nitrogenous compounds [18]. The objective of this work was to develop a procedure to concentrate the nitrogenous compounds in used tire vacuum pyrolysis oil naphtha fraction (b.p.: 30-204°C) by gradient elution chromatography on a dual packed silica gel and alumina column. A detailed chemical characterization was achieved by gas chromatography with mass spectrometry (GC/MS) and gas chromatography with atomic emission detectors (GC/AED).
EXPERIMENTAL
Pyrolysis
A diagram of the Vacuum Pyrolysis ‘Process Development Unit (P.D.U.) used in this study is shown in Fig. 1. Run No. DO05 was performed in a multiple hearth furnace. The reactor was externally heated by electric elements surrounding the chamber. The maximum throughput of material across the reactor was 12 kg/h. The feedstock consisted of small, homogeneous, cross-ply (bias-ply) cylindric pieces of rubber cut from the wall section of used automobile tires. The elemental analysis of the feedstock rubber is given in Table 1. This material contained a relatively small amount of inorganic substances (3.6% ash), as indicated in Table 1. The
207
CO2 TRAP
SCRUBBER
CARBON SLACK SIN
SCRUBBER
1
GAS METER
FLARE
2
VACUUM PUMP
u-
FILTERS
ICE TRAP
t Fig. 1. Schematic
of the vacuum pyrolysis
process
development
unit.
heating plate temperatures increased from top to bottom of the reactor. A typical reactor temperature profile was 200°C to 500°C. The system was operated below 10.3 kPa absolute pressure. Air leakage through the system was negligible. Thermal destruction under vacuum yielded 61% pyrolysis oils, 26% pyrolytic carbon black (derived from the authentic carbon black plus solid residue from tire pyrolysis) and 13% gas on an as-received basis. The pyrolysis oil is a blend of (i) the processing oil used as part of the rubber formulation, (ii) organic additives in the rubber and/or their pyrolysis products and (iii) rubber pyrolysis products. The non-condensable
TABLE Ultimate
1 analysis of rubber
Tire Gas Oil Naphtha’ Carbon black
feedstock
and pyrolysis products
(% by wt., organic
C
H
N
0”
Sb
Ash (o/o by wt.)
88.20 85.76 85.94 87.25 94.78
8.53 14.24 10.62 11.20 1.11
1.07 trace 1.35 0.94 1.19
0.96 trace 1.20 0.22 0.56
1.24 trace 0.89 0.39 2.36
3.57 Nil Nil Nil 12.89
a By difference. b Total sulfur. weight of the pyrolysis oil.
’ Fraction
boiling
below
204°C and representing
basis)
18% by
1 2 3 4 5
CC peak No.
25 25 25 25 25
2.00 2.15 3.59 6.00 6.56 9.21 15.11 15.12 15.46 2.11 3.05 3.13 3.89 4.58
Pentanedinitrile 1H-Pyrrole 1-Hexene, 6-nitro3-(Methyhhio)propanenitrile Aniline 2-Methylbenzenamine N-Hexylacetamide N-Butylacetamide 2-Methylbenzothiazole
Pyridine y Picoline (YPicoline p Picohne 2,6-Lutidine
24 24 24 24 24 24 24 24 24
Retention time (min)
Compound Fraction
Nitrogenous compounds detected in used tire vacuum pyrolysis oil naphtha fraction
TABLE 2
3090 1000 5100 4980 710
2040 1560 3730 595 3175 170 1920 160 605
Concentration (ppm)
79 93 93 93 lti
*, 92,66
* ,78,66 * ,78,66
* ,52,39 * ) 66,40
66, 54, 41 67 * ,41,59 67,55,41 101*, 61,45 93 * ,66,65 107 *,@, 77 143 *, 73, 72, 30 115 *, 72,30 149 *, 108,63
Major fragment and molecular ion peaks, m/z a
a Underlined
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
m/z
is the major fragment;
4.98 5.90 6.64 7.98 10.37 11.08 11.34 13.42 13.42 13.77 14.00 14.11 14.11 16.02 16.50 19.44 20.55
* Molecular ion peak
2-Ethylpyridine 2,3-Lutidine Aniline 2-Ethyl, 6-methylpyridine 1H-Indole, 2,3-dihydroHexanedinitrile 2-(l-Methylethenyl)-aniline Thieno [2,3-c] pyridine Benzothiazole 1,2,3,4-Tetrahydro-quinoline Caprolactam 4-(Dimethylamino)-3-pentene 2-one 3,4,5-trimethyl-isothiazole 6-Methyl 91,29374-tetrahydroquinoline 4(1H)-Quinazolinone 4,8-Dimethylquinoline 2-Methylindole-3-carboxaldehyde 25 25 25 25 25 21-25 25 25 25 25 25 25 25 25 25 25 25
955 685 960 345 1440 520 355 820 630 1825 11810 2090 645 235 250 570 1420 107 *, 106,79 107 *, 92, 66,39 93 * ,66,65 1% *, _’ 120 93 119 *, 118,91 68,54Ti_ 133 *, 132,117 135 *, 108,91 135 *, 108,69 i% *, _’ 132 117 113 *, 84,55 127 *, 112,110 127 *, 1571 147 *, 146,132 146 *, 118,91 157 *, 142,128 159 *, _’ 158 130
210
gas was composed of H,, CO, CO,, CH, as well as other low molecular weight hydrocarbons and traces of sulfur-containing gases. Naphtha separation
The pyrolysis oil was distilled under atmospheric pressure and the naphtha fraction (30-204°C) was separated which yielded 18% by weight of the total water-free pyrolysis oil. The ultimate composition of the total oil and naphtha cut is shown in Table 1. Naphtha fractionation
A 0.8 g of the naphtha fraction was transferred to a dual column (1.2 x 90 cm) packed with 20 g of silica gel (BDH Chemicals, 60-120 mesh) and 25 g of aluminium oxide, activated, neutral, Brockmann I (Aldrich Chemical, ca. 150 mesh grade). The column was successively eluted with 200 ml n-pentane, 150 ml of 5% ethyl acetate in rt-pentane and 100 ml methanol by collecting c 17 ml fractions. GC /MS
analysis
A HP 5890 gas chromatograph with splitless injector and helium carrier gas with about 1 ml/min flow rate was used with HP fused silica column 12 m x 0.2 mm coated with 0.25 ,um film of crosslinked methyl silicone gum. The GC temperature was programmed from 35°C to 210°C at a rate of 3”C/min. The end of the column was directly introduced into the ion source of a HP 5970 series mass selective detector. Typical mass spectrometer operation conditions were as follows: transfer line 270°C ion source 250°C electron energy 70 eV. Data acquisition was done with HP-UX Chemstation software using a HP-UNIX computer and NBS library data base. The mass range m/z: 30-600 Dalton was scanned every 1.0 s. GC /AED
analysis
A HP 5221A multi-element 5890 gas chromatograph was compounds under similar gas analysis was kindly performed ssauga, Ontario.
atomic emission detector coupled to a HP used to analyze the nitrogen containing chromatographic conditions as above. This by Hewlett Packard Canada Ltd., Missi-
Quantitative analysis
Benzothiazole, pyridine, caprolactame, aniline, 1H-pyrrole and quinoline were purchased and used as the external standards. Their concentra-
211
tion as well as those of similar chemical structures was measured in F24 and F25. A few other compounds were also quantified with estimated MS response factors.
RESULTS
AND DISCUSSION
During compounding and fabricating, tire ingredients are continuously subjected to heat. This is particular-y evident during the mixing process and it is the reason why accelerators are usually added toward the end of the mixing cycle, when the temperature of the mill or internal mixer is falling. Most accelerators used are nitrogen-based compounds. In case of internal mixing, sulfur is usually added on the sheeting mill for the same reason (vulcanization accelerator). During the development of the use of accelerators in tire compounding, it became evident that some were safer than others. For the same reason, accelerator mixtures were introduced in which the degree of cross-linking or the curing speed of the primary accelerator was boosted by the presence of small amounts of secondary accelerators without greatly affecting the scorch tendencies of the compound [19]. Organic vulcanization accelerators mainly comprise derivatives of benzothiazole. There is a large variety of accelerators, e.g. N,N-diisopropyl-2N, N ‘benzothiazole-sulfenamide, 2-(4-morpholinylthio)-benzothiazole, caprolactam disulphide, and 2-mercaptobenzothiazole. Due to its toxicity, aniline is no longer used but its derivatives such as diphenyl-p-phenyldiamine are used as principal accelerators [20]. The present method of separation enabled us to fractionate the pyrolysis oil naphtha fraction into three main chemical groups: hydrocarbons, mainly aromatics (fractions Fl to F20); oxygenated compounds (F21-F23); and nitrogenous compounds (F24 and F25). Sulfurous compounds were mainly eluted in F15 but a few of them were also present in F24 and F25. Aromatic hydrocarbons were distributed in a wide range of fractions, in order of increasing polarity and molecular dimension (e.g. mono to tetramethyl benzene and methyl to propyl benzene). Their detailed characterization will be published later. Alumina was found very effective in separating nitrogenous compounds from the oxygenated and aromatic hydrocarbons. All nitrogenous compounds found were eluted with about 30 ml of methanol without overlapping with the aromatic hydrocarbons. GC/MS analysis revealed various types of nitrogenous compounds including aliphatic amines; amides and nitriles; pyridine and alkyl derivatives; aniline and their alkyl derivatives; pyrroles; indoles; quinolines and benzothiazoles. From the total ion chromatograms of the F24 and F25, individual peaks were identified which are listed in Table 2. It was found that nearly all nitrogenous compounds were concentrated in fractions 24 and 25 (Figs.
212
4 oooooo-
10
Fig. 2. Gas chromatogram
of Fraction
30
20
24. (Key in Table 2). Abscissa:
Time (min).
2 and 3), except for hexanedinitrile and pentanedinitrile which were found in fraction 21, probably due to the presence of low polar linear hydrocarbon chains which decreased their retention in the column.
600000
10
Fig. 3. Gas chromatogram
of Fraction
20
25. (Key in Table 2). Abscissa:
30
Time (min).
213
~l~lll~lll~lll~lll~lll~lll~lll~l 4
6
8
10
12
Fig. 4. Nitrogen specific chromatogram oil. Abscissa: Time (min).
14
16
of naphtha
18
fraction
from used tire vacuum pyrolysis
Figure 4 shows N-specific chromatogram of the total naphtha fraction produced by GC/AED. AED is a complementary GC detector which has been recently commercialized. It allows the selective screening of different elements to confirm MSD analysis and is used also to confirm LC fractionation and concentration methodology which was developed and reported in this paper. AED uses a microwave-induced plasma to fragment compounds into atoms and to excite the atoms so they emit light. The emitted light is then analyzed by a spectrophotometer. Thirty-one N-compounds were detected in F24 and F25 by GC/MS. Nearly the same number of compounds are shown in Fig. 4, which is a good indication that most of the N-compounds are eluted in F24 and F25. F15, for example, was analyzed by GC/AED for N-compounds but it exhibited no peak. The naphtha fraction has a relatively high nitrogen content, 0.94%. This figure compares quite well with the total nitrogen (0.80%) calculated from the data in Table 2. The presence of some nitrogenous compounds is explained by the thermal degradation of the accelerants incorporated in tires. It has been found that pyridine, a, p, y-picoline and the other isomers of pyridine could be obtained by the cleavage of the C-C bonds of certain accelerators such as 3,5-diethyl-1,2-dihydro-l-phenyl-2-propylpyridine [20]. Cleavage of the N-S and C-S bonds of N,N’-caprolactam and benzothiazolic additives produces caprolactam and benzothiazole in tire oils. Chao and Wharton [21] studied the preparation of benzothiazole by pyrolysis of a benzothiazole-containing nucleus compound such as mercapto-benzothiazole or 2,2’-dibenzothiazolyl disulfide or a zinc salt of mercaptobenzothiazole at 200 and 275°C. This is why mercaptobenzothiazole was not detected in the naphtha. Quinoline, diamine and aniline derivatives listed in Table 2, are likely the pyrolysis products of nitroge-
214
nous-based accelerators, such as 2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) and p-phenylendiamine (PPD) which are commonly used in tire compounding. Pyridine and their alkyl derivatives were identified as the most abundant compounds among all the nitrogenous compounds in F24 and F25. Cyclic nitrogenous compounds in general are identified by their most abundant molecular ion or CM+-- 1) peaks or both. The mass spectrum of pyridine itself reflects the great stability of the aromatic ring, the base peak corresponding to the molecular ion and the loss of hydrogenocyanide (M+- 27) furnishing the only abundant fragment ion. This situation is reminiscent of that found for indole and aniline. However, the decomposition modes of alkylpyridines are fundamentally different. The extent to which simple P-cleavage of an alkyl chain occurs is dependent on the position of the chain relative to the heteroatom. The molecular ions and m/z 51, 52, 66, 77, 78, 79 and 106 are characteristic peaks of alkyl pyridines.
CONCLUSION
The separation procedure described in this paper enables one to concentrate and characterize a large proportion of the nitrogen-based compounds in tire pyrolysis oils. Thirty-one individual compounds, mainly pyridine derivatives, were concentrated and separated by sequential elution solvent chromatography on a dual packed silica gel and alumina column. All the nitrogenous compounds were found in the methanol eluted fraction as confirmed by GC/AED analysis. Nitrogenous compounds identified in the oil distillate were mainly the pyrolysis products of tire additives.
ACKNOWLEDGMENTS
This work has been supported by NSERC (Natural Science and Engineering Research Council) and the Ministere de l’Energie, Ressources Quebec. The authors would also like to thank Dr. Alan C. Viau, Hewlett Packard Canada Ltd., for the GC/AED analysis.
REFERENCES 1 P.W. Dufton, The Value and Use of Scrap Tyres, Rapra Technology Ltd. England, 1987. 2 J. Doos, W.F. Domenico and D.R. Evans, November 1983. Scrap Tires: A Resource and Technology Evaluation of Tire Pyrolysis and Other Selected Alternate Technologies. N.T.I.S. Report # EGC-2241. Presented to the U.S. Dept. of Energy.
215 3 B. Labrecque, 1987. Etude du transfert de chaleur par radiation thermique dans un rtacteur de pyrolyse sous vide des vieux pneumatiques. M.Sc.A. Thesis. Universite de Sherbrooke, Sherbrooke, P. of Quebec, Canada. 4 C. Roy, B. Labrecque and B. de Caumia, Res. Conserv. Recy., 4 (19901203-213. 5 M. Galin-Vacherot, Eur. Polym. J., 7 (1971) 1455. 6 H. Sinn, W. Kaminsky and J. Janning, Angew. Chem. Int. Ed. Engl., 15 (11) (1976) 660. 7 G. Collin, Pyrolysis Oils from Wastes-New Chemical Raw Materials of the Future, Recycling Berlin ‘79, Vol. 1, Thome-Kozmiensky, Berlin, 1979, p. 700. 8 G.P. Bracker, Pyrolytic Resource Recovery, 4 (3)(19891 161. 9 A.A. Oswald and F.J. Noel, J. Chem. Eng. Data, 6 (1961) 294. 10 R.K. Artzmark and J.B. Gilbert, Hydrocarbon Process, 46 (1967) 143. 11 M.W. Schrepfer, R.J. Arnold and C.A. Stansky, Oil Gas J., 84 (2) (1984) 79. 12 J.W. Frakenfeld and W.F. Taylor, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) 608. 13 J.C. Escalier, M. Caude, C. Ballet and R. Rosset, Analusis, 5 (1977) 395. 14 1. Ignatiadis, J.M. Schmitter and P. Arpino, J. Chromatogr., 324 (1985) 87. 15 D.M. Jewel1 and R.E. Snyder, J. Chromatogr., 38 (19681 351. 16 C.D. Ford, S.A. Holmes, L.F. Thompson and D.R. Lathman, Anal. Chem., 53 (1981) 831. 17 V.G. Ben’Kovski and M.D. Olzseva, Chem. Technol. Fuels Oils, (1979) 474. 18 G. Jean, M. Poirier, and H. Sawatzky, Sep. Sci. Technol., 20 (1985) 541. 19 W. Hofman, Rubber Technology Handbook, Hanser Publishers, New York, 1988, pp. 233-264. 20 R.O. Babbit, The Vanderbilt Rubber Handbook, Vanderbilt Company, Norwalk, 1978, pp. 392-400. 21 T.H. Chao and E.M. Wharton, U.S. Patent 2,610,190, Sept. 9, 1952.
205
Characterization of used tire vacuum pyrolysis oil: nitrogenous compounds from the naphtha fraction S. Mirmiran, H. Pakdel and C. Roy * Unioersite’ Laual, Chemical Engineering Department, PaLSon Adrien-Pouliot, Ste. Foy, Qukbec GlK 7P4 (Canada) (Received
August
15, 1991; accepted
in final form October
3, 1991)
ABSTRACT Vacuum pyrolysis of used tires yielded 61% by weight hydrocarbon type oil from which 18% by weight naphtha fraction can be distilled off. The pyrolysis oil consisted of (i) the processing oil as part of the tire formulation, (ii) organic additives, and (iii) tire pyrolysis products. A procedure was developed for the separation and identification of nitrogenous compounds in used tire-derived oil, which were partially produced by pyrolysis of tire accelerants. The analytical separation technique involved liquid-solid chromatography on a dual packed silica gel and alumina column. n-Pentane and ethylacetate eluted the hydrocarbons. The nitrogenous compounds were eluted with methanol. Thirty-one nitrogenous compounds were identified by gas chromatography/mass spectrometry and gas chromatography/atomic emission detection. Atomic emission; gas chromatography; sis oil; rubber; tires; vacuum pyrolysis.
mass spectrometry;
nitrogenous
compounds;
pyroly-
INTRODUCTION
Used tire disposal is becoming a critical problem for the environment. In Canada for example, 24 million tires are discarded yearly. While some of these tires are recapped or ground up for special uses, most are simply dumped in rural farm land or in landfill sites. Used tires represent a source of energy and raw chemicals for the production of rubber parts. By thermal decomposition of rubber, useful products can be recovered [l-4]. Vacuum pyrolysis, a novel approach based on ,the decomposition reaction at low temperature and reduced pressure, is under development in our laboratory to transform used tires into liquid and solid fuels [4]. Materials used in tire compounding are the following with typical ranges in p.h.r. (parts per hundred of rubber): elastomers (100 p.h.r); vulcanizing agents (5-10 p.h.r); accelerators (l-3 p.h.r); activators and other additives (l-5 p.h.r); processing oil (S-30 p.h.r); carbon black (50-70 p.h.r). ’ 01652370/92/$05.00
0 1992 - Elsevier
Science
Publishers
B.V. AI1 rights reserved
Heating under vacuum of elastomers, vulcanizing agents, accelerators, additives and processing oil yields pyrolysis oils. The gaseous and vapour molecules formed under vacuum pyrolysis conditions are rapidly removed from the reaction chamber. This minimizes the extent of secondary decomposition reactions. The analysis of hydrocarbons in pyrolysis oils has been performed by several researchers [5-81. Limited work has been carried out, however, on the determination of sulfurous and nitrogenous compounds in rubber-derived oils. This study showed that naphtha extracted from used tire vacuum pyrolysis oil contains significant amounts of nitrogenous compounds which are derived from tire additives. N-compounds are undesirable components of oils because during the refining process they can poison the catalysts and promote gum formation [9-121. Several studies on the separation and characterization of nitrogenous compounds from petroleum have been reported earlier [13,14]. Jewel1 and Snyder [15] used complex formation on supported ferric chloride and ion-exchange resins to isolate the nitrogenous compounds from petroleum. Ford et al. [16] employed alumina adsorption chromatography to separate nitrogenous compounds. Ben’Kovski and Olzseva [17] removed nitrogen compounds from diesel fuel by complex formation with titanium tetrachloride. A preliminary study using brominated ilmenite as a sorbent demonstrated the possibility of using sorption for the large scale removal of nitrogenous compounds [18]. The objective of this work was to develop a procedure to concentrate the nitrogenous compounds in used tire vacuum pyrolysis oil naphtha fraction (b.p.: 30-204°C) by gradient elution chromatography on a dual packed silica gel and alumina column. A detailed chemical characterization was achieved by gas chromatography with mass spectrometry (GC/MS) and gas chromatography with atomic emission detectors (GC/AED).
EXPERIMENTAL
Pyrolysis
A diagram of the Vacuum Pyrolysis ‘Process Development Unit (P.D.U.) used in this study is shown in Fig. 1. Run No. DO05 was performed in a multiple hearth furnace. The reactor was externally heated by electric elements surrounding the chamber. The maximum throughput of material across the reactor was 12 kg/h. The feedstock consisted of small, homogeneous, cross-ply (bias-ply) cylindric pieces of rubber cut from the wall section of used automobile tires. The elemental analysis of the feedstock rubber is given in Table 1. This material contained a relatively small amount of inorganic substances (3.6% ash), as indicated in Table 1. The
207
CO2 TRAP
SCRUBBER
CARBON SLACK SIN
SCRUBBER
1
GAS METER
FLARE
2
VACUUM PUMP
u-
FILTERS
ICE TRAP
t Fig. 1. Schematic
of the vacuum pyrolysis
process
development
unit.
heating plate temperatures increased from top to bottom of the reactor. A typical reactor temperature profile was 200°C to 500°C. The system was operated below 10.3 kPa absolute pressure. Air leakage through the system was negligible. Thermal destruction under vacuum yielded 61% pyrolysis oils, 26% pyrolytic carbon black (derived from the authentic carbon black plus solid residue from tire pyrolysis) and 13% gas on an as-received basis. The pyrolysis oil is a blend of (i) the processing oil used as part of the rubber formulation, (ii) organic additives in the rubber and/or their pyrolysis products and (iii) rubber pyrolysis products. The non-condensable
TABLE Ultimate
1 analysis of rubber
Tire Gas Oil Naphtha’ Carbon black
feedstock
and pyrolysis products
(% by wt., organic
C
H
N
0”
Sb
Ash (o/o by wt.)
88.20 85.76 85.94 87.25 94.78
8.53 14.24 10.62 11.20 1.11
1.07 trace 1.35 0.94 1.19
0.96 trace 1.20 0.22 0.56
1.24 trace 0.89 0.39 2.36
3.57 Nil Nil Nil 12.89
a By difference. b Total sulfur. weight of the pyrolysis oil.
’ Fraction
boiling
below
204°C and representing
basis)
18% by
1 2 3 4 5
CC peak No.
25 25 25 25 25
2.00 2.15 3.59 6.00 6.56 9.21 15.11 15.12 15.46 2.11 3.05 3.13 3.89 4.58
Pentanedinitrile 1H-Pyrrole 1-Hexene, 6-nitro3-(Methyhhio)propanenitrile Aniline 2-Methylbenzenamine N-Hexylacetamide N-Butylacetamide 2-Methylbenzothiazole
Pyridine y Picoline (YPicoline p Picohne 2,6-Lutidine
24 24 24 24 24 24 24 24 24
Retention time (min)
Compound Fraction
Nitrogenous compounds detected in used tire vacuum pyrolysis oil naphtha fraction
TABLE 2
3090 1000 5100 4980 710
2040 1560 3730 595 3175 170 1920 160 605
Concentration (ppm)
79 93 93 93 lti
*, 92,66
* ,78,66 * ,78,66
* ,52,39 * ) 66,40
66, 54, 41 67 * ,41,59 67,55,41 101*, 61,45 93 * ,66,65 107 *,@, 77 143 *, 73, 72, 30 115 *, 72,30 149 *, 108,63
Major fragment and molecular ion peaks, m/z a
a Underlined
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
m/z
is the major fragment;
4.98 5.90 6.64 7.98 10.37 11.08 11.34 13.42 13.42 13.77 14.00 14.11 14.11 16.02 16.50 19.44 20.55
* Molecular ion peak
2-Ethylpyridine 2,3-Lutidine Aniline 2-Ethyl, 6-methylpyridine 1H-Indole, 2,3-dihydroHexanedinitrile 2-(l-Methylethenyl)-aniline Thieno [2,3-c] pyridine Benzothiazole 1,2,3,4-Tetrahydro-quinoline Caprolactam 4-(Dimethylamino)-3-pentene 2-one 3,4,5-trimethyl-isothiazole 6-Methyl 91,29374-tetrahydroquinoline 4(1H)-Quinazolinone 4,8-Dimethylquinoline 2-Methylindole-3-carboxaldehyde 25 25 25 25 25 21-25 25 25 25 25 25 25 25 25 25 25 25
955 685 960 345 1440 520 355 820 630 1825 11810 2090 645 235 250 570 1420 107 *, 106,79 107 *, 92, 66,39 93 * ,66,65 1% *, _’ 120 93 119 *, 118,91 68,54Ti_ 133 *, 132,117 135 *, 108,91 135 *, 108,69 i% *, _’ 132 117 113 *, 84,55 127 *, 112,110 127 *, 1571 147 *, 146,132 146 *, 118,91 157 *, 142,128 159 *, _’ 158 130
210
gas was composed of H,, CO, CO,, CH, as well as other low molecular weight hydrocarbons and traces of sulfur-containing gases. Naphtha separation
The pyrolysis oil was distilled under atmospheric pressure and the naphtha fraction (30-204°C) was separated which yielded 18% by weight of the total water-free pyrolysis oil. The ultimate composition of the total oil and naphtha cut is shown in Table 1. Naphtha fractionation
A 0.8 g of the naphtha fraction was transferred to a dual column (1.2 x 90 cm) packed with 20 g of silica gel (BDH Chemicals, 60-120 mesh) and 25 g of aluminium oxide, activated, neutral, Brockmann I (Aldrich Chemical, ca. 150 mesh grade). The column was successively eluted with 200 ml n-pentane, 150 ml of 5% ethyl acetate in rt-pentane and 100 ml methanol by collecting c 17 ml fractions. GC /MS
analysis
A HP 5890 gas chromatograph with splitless injector and helium carrier gas with about 1 ml/min flow rate was used with HP fused silica column 12 m x 0.2 mm coated with 0.25 ,um film of crosslinked methyl silicone gum. The GC temperature was programmed from 35°C to 210°C at a rate of 3”C/min. The end of the column was directly introduced into the ion source of a HP 5970 series mass selective detector. Typical mass spectrometer operation conditions were as follows: transfer line 270°C ion source 250°C electron energy 70 eV. Data acquisition was done with HP-UX Chemstation software using a HP-UNIX computer and NBS library data base. The mass range m/z: 30-600 Dalton was scanned every 1.0 s. GC /AED
analysis
A HP 5221A multi-element 5890 gas chromatograph was compounds under similar gas analysis was kindly performed ssauga, Ontario.
atomic emission detector coupled to a HP used to analyze the nitrogen containing chromatographic conditions as above. This by Hewlett Packard Canada Ltd., Missi-
Quantitative analysis
Benzothiazole, pyridine, caprolactame, aniline, 1H-pyrrole and quinoline were purchased and used as the external standards. Their concentra-
211
tion as well as those of similar chemical structures was measured in F24 and F25. A few other compounds were also quantified with estimated MS response factors.
RESULTS
AND DISCUSSION
During compounding and fabricating, tire ingredients are continuously subjected to heat. This is particular-y evident during the mixing process and it is the reason why accelerators are usually added toward the end of the mixing cycle, when the temperature of the mill or internal mixer is falling. Most accelerators used are nitrogen-based compounds. In case of internal mixing, sulfur is usually added on the sheeting mill for the same reason (vulcanization accelerator). During the development of the use of accelerators in tire compounding, it became evident that some were safer than others. For the same reason, accelerator mixtures were introduced in which the degree of cross-linking or the curing speed of the primary accelerator was boosted by the presence of small amounts of secondary accelerators without greatly affecting the scorch tendencies of the compound [19]. Organic vulcanization accelerators mainly comprise derivatives of benzothiazole. There is a large variety of accelerators, e.g. N,N-diisopropyl-2N, N ‘benzothiazole-sulfenamide, 2-(4-morpholinylthio)-benzothiazole, caprolactam disulphide, and 2-mercaptobenzothiazole. Due to its toxicity, aniline is no longer used but its derivatives such as diphenyl-p-phenyldiamine are used as principal accelerators [20]. The present method of separation enabled us to fractionate the pyrolysis oil naphtha fraction into three main chemical groups: hydrocarbons, mainly aromatics (fractions Fl to F20); oxygenated compounds (F21-F23); and nitrogenous compounds (F24 and F25). Sulfurous compounds were mainly eluted in F15 but a few of them were also present in F24 and F25. Aromatic hydrocarbons were distributed in a wide range of fractions, in order of increasing polarity and molecular dimension (e.g. mono to tetramethyl benzene and methyl to propyl benzene). Their detailed characterization will be published later. Alumina was found very effective in separating nitrogenous compounds from the oxygenated and aromatic hydrocarbons. All nitrogenous compounds found were eluted with about 30 ml of methanol without overlapping with the aromatic hydrocarbons. GC/MS analysis revealed various types of nitrogenous compounds including aliphatic amines; amides and nitriles; pyridine and alkyl derivatives; aniline and their alkyl derivatives; pyrroles; indoles; quinolines and benzothiazoles. From the total ion chromatograms of the F24 and F25, individual peaks were identified which are listed in Table 2. It was found that nearly all nitrogenous compounds were concentrated in fractions 24 and 25 (Figs.
212
4 oooooo-
10
Fig. 2. Gas chromatogram
of Fraction
30
20
24. (Key in Table 2). Abscissa:
Time (min).
2 and 3), except for hexanedinitrile and pentanedinitrile which were found in fraction 21, probably due to the presence of low polar linear hydrocarbon chains which decreased their retention in the column.
600000
10
Fig. 3. Gas chromatogram
of Fraction
20
25. (Key in Table 2). Abscissa:
30
Time (min).
213
~l~lll~lll~lll~lll~lll~lll~lll~l 4
6
8
10
12
Fig. 4. Nitrogen specific chromatogram oil. Abscissa: Time (min).
14
16
of naphtha
18
fraction
from used tire vacuum pyrolysis
Figure 4 shows N-specific chromatogram of the total naphtha fraction produced by GC/AED. AED is a complementary GC detector which has been recently commercialized. It allows the selective screening of different elements to confirm MSD analysis and is used also to confirm LC fractionation and concentration methodology which was developed and reported in this paper. AED uses a microwave-induced plasma to fragment compounds into atoms and to excite the atoms so they emit light. The emitted light is then analyzed by a spectrophotometer. Thirty-one N-compounds were detected in F24 and F25 by GC/MS. Nearly the same number of compounds are shown in Fig. 4, which is a good indication that most of the N-compounds are eluted in F24 and F25. F15, for example, was analyzed by GC/AED for N-compounds but it exhibited no peak. The naphtha fraction has a relatively high nitrogen content, 0.94%. This figure compares quite well with the total nitrogen (0.80%) calculated from the data in Table 2. The presence of some nitrogenous compounds is explained by the thermal degradation of the accelerants incorporated in tires. It has been found that pyridine, a, p, y-picoline and the other isomers of pyridine could be obtained by the cleavage of the C-C bonds of certain accelerators such as 3,5-diethyl-1,2-dihydro-l-phenyl-2-propylpyridine [20]. Cleavage of the N-S and C-S bonds of N,N’-caprolactam and benzothiazolic additives produces caprolactam and benzothiazole in tire oils. Chao and Wharton [21] studied the preparation of benzothiazole by pyrolysis of a benzothiazole-containing nucleus compound such as mercapto-benzothiazole or 2,2’-dibenzothiazolyl disulfide or a zinc salt of mercaptobenzothiazole at 200 and 275°C. This is why mercaptobenzothiazole was not detected in the naphtha. Quinoline, diamine and aniline derivatives listed in Table 2, are likely the pyrolysis products of nitroge-
214
nous-based accelerators, such as 2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) and p-phenylendiamine (PPD) which are commonly used in tire compounding. Pyridine and their alkyl derivatives were identified as the most abundant compounds among all the nitrogenous compounds in F24 and F25. Cyclic nitrogenous compounds in general are identified by their most abundant molecular ion or CM+-- 1) peaks or both. The mass spectrum of pyridine itself reflects the great stability of the aromatic ring, the base peak corresponding to the molecular ion and the loss of hydrogenocyanide (M+- 27) furnishing the only abundant fragment ion. This situation is reminiscent of that found for indole and aniline. However, the decomposition modes of alkylpyridines are fundamentally different. The extent to which simple P-cleavage of an alkyl chain occurs is dependent on the position of the chain relative to the heteroatom. The molecular ions and m/z 51, 52, 66, 77, 78, 79 and 106 are characteristic peaks of alkyl pyridines.
CONCLUSION
The separation procedure described in this paper enables one to concentrate and characterize a large proportion of the nitrogen-based compounds in tire pyrolysis oils. Thirty-one individual compounds, mainly pyridine derivatives, were concentrated and separated by sequential elution solvent chromatography on a dual packed silica gel and alumina column. All the nitrogenous compounds were found in the methanol eluted fraction as confirmed by GC/AED analysis. Nitrogenous compounds identified in the oil distillate were mainly the pyrolysis products of tire additives.
ACKNOWLEDGMENTS
This work has been supported by NSERC (Natural Science and Engineering Research Council) and the Ministere de l’Energie, Ressources Quebec. The authors would also like to thank Dr. Alan C. Viau, Hewlett Packard Canada Ltd., for the GC/AED analysis.
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