Energy & Fuels 1996, 10, 941-947
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Coal Hydrocoprocessing with Tires and Tire Components Ana M. Mastral,* Ramo´n Murillo, Marı´a J. Perez-Surio, and Marisol Calle´n Instituto de Carboquı´mica, CSIC, Apdo 589, 50080-Zaragoza, Spain Received June 15, 1995X
This paper shows that the addition of rubber from old tires to dry coal hydrogenation always has a positive effect and promotes the formation of oils. This is the first time that components from tire rubber have been coprocessed with coal in order to gain knowledge of the specific role played by each in coal-tire hydrocoprocessing. A subbituminous coal, typical of the northeast of Spain, which shows good conversions in dry hydrogenation at nonsevere conditions because it reaches 75% at 400 °C and 10 MPa for 30 min without a catalyst, was hydrocoprocessed with rubber from old tires, and the results are reported in this paper. The rubber used came from a mixture of old tires, which had been ground and from which the steel thread and the textile netting had previously been removed. The coprocessing of both materials was batch run at 350, 375, and 400 °C, keeping constant the initial hydrogen pressure, 10 MPa, and the residence time, 30 min, but varying the ratio between both materials (1/0, 4/1, 2/1, 1/1, 1/2, 1/4, and 0/1) in the feed mixture. The influence of the feed composition was also studied for the first time. Despite the fact that total conversions and gas formation do not undergo significant variations in comparison with the ones when only coal is processed, oils conversion (43% at 350 °C and 45% at 400 °C with 20% coal-40% rubber feed) and oils selectivity (93% at 350 °C and 85% at 400 °C with 20% coal-40% rubber feed) show important increases, mostly when rubber-rich feeds are processed. Furthermore, asphalthenes formation decreases with increasing rubber percentages in the feed mixture. Results obtained show that the addition of rubber from old tires to coal hydrogenation always promotes the percentage of oils formation. To enter into the coal-tire interactions, three main components from the same tire rubber, carbon black (CB), polybutadiene (PB), and styrene-butadiene (SBR) were processed alone and with coal. In addition, polystyrene (PS) was coprocessed with coal. Conclusions show that CB catalyzes the process by promoting hydrocracking reactions, and secondary reactions, by breaking the hydrogenation products from direct hydrogenation, primary reactions, into smaller and lighter molecules leading to gas formation. PB addition has no affect at 350 °C, but at 400 °C it significantly improves total conversions (from 75% to 88%) and oils conversion (from 12% to 36%), giving intermediate conversions at 375 °C. With regard to SBR, significant improvements on results are already reached at 375 °C. Model compounds were not used in this work, but from the bulk of results obtained from the 29 different experiments carried throughout this research, it appears that from all the possible interactions between radicals involved in the coal-rubber hydrocoprocessing, those implying the alkylation of the aromatic radicals from rubber by all the radicals involved in the process are promoted.
Introduction In recent years, because of the growth of urban waste, there has been increasing attention paid to the coutilization of coal and waste materials.1-3 The intention of this is to reduce the high cost of the coal hydrogenation process 4 and at the same time to profitably employ some waste materials by taking advantage of their components,5 in addition to reducing environmental damage.6 This attention has mainly been focused on plastics 7,8 Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Symposium on Co-Utilization of Coal and Wastes, Proceedings of the ACS Division of Fuel Chemistry, Chicago; American Chemical Society: Washington, DC, 1993; Vol. 38 (3), and references therein. (2) Symposium on Co-Utilization of Coal and Wastes Materials, Proceedings of the ACS Division of Fuel Chemistry, Anaheim, CA; American Chemical Society: Washington, DC, 1995; Vol. 40 (1), and references therein. (3) International Congress on Solid Residues, Proceedings of the ANQUE, Tenerife; December 1994, and bibliograph there cited. (4) Williams, P. T.; Besler, S.; Taylor, D. T. Proc.sInst. Mech. Eng. 1993, 207, 55. (5) Farcasiu, M. CHEMTECH 1993, 22. X
S0887-0624(95)00113-7 CCC: $12.00
and rubber.9 Discarded automotive tires, with 60-70% of their composition originating from petroleum, have shown to be a very attractive material. In proportion to total waste material, discarded tires only amount to a small percentage, but this residue is not biodegradable, so it should not be disposed of as landfill or left in the open air. On the other hand, because of the vulcanization process, the recovery of its raw materials is impossible. The alternative, because of the high calorific power of tires, is incineration.10 However, apart from not being an environmentally acceptable solution, this is not an (6) Mastral, A. M.; Murillo, R.; Perez-Surio, M. J.; Callen, M.; Clemente, M. C. 3rd International Congress on Solids Residues, ANQUE, Tenerife, December 1994. (7) Xiao, X.; Zmierczak, W.; Shabtai, J. Proceedings of the ACS Division of Fuel Chemistry, Anaheim CA; American Chemical Society: Washington, DC, 1995; Vol. 40 (1), p 4. (8) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 12, 1474. (9) Taghiei, M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228. (10) Proceedings of the 8th Pittsburgh Coal Conference; 1991; p 859.
© 1996 American Chemical Society
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appropiate way to recover all their chemical and energy potential. A part of the tire could be used with important savings in natural resources. Specifically, composition depends on the brand of the tire and its specific use, but in general, about 60% of its components are synthetic elastomers and/or natural rubber, which could be reused to produce synthetic oils via pyrolysis,11,12 liquefaction,13 or a thermal process that turns them into useful products for the chemical industry, like fuels, road asphalt, or oils to produce, for instance, new tires. With all of these alternatives, the recovery of about 90% of old tires, based on the European Union production, could be achieved in the near future. Everyday it is becoming more and more difficult to burn some coals, especially those with a high sulfur content, because of environmental regulations. Pyrolyzing coal, instead of burning it, would contribute to C, N, and S oxides abatement, and copyrolyzing coal with other waste materials would mitigate solid residue disposal problems. Despite coal hydroconversion being an economically noncompetitive process, from an environmental point of view, some coals have shown themselves to be very reactive in hydrogenation.14 If the hydrogen supply could be cheapened, hydropyrolysis would be more attractive, since the emissions it produces are less polluting than those produced as a result of combustion. Although pyrolysis of organic material produces gas, liquids, and solids in a process involving radicals, if these radicals are generated under a hydrogen atmosphere and under controlled conditions, hydropyrolysis could lead to the selective production of liquids.15 Although their proximate analysis is quite different, coal and tires are materials with a quite similar ultimate analysis, showing a major organic structure, mainly aromatic, that can be coprocessed into products with commercial value. Up to now, coal rubber coprocessing has been undertaken from a practical point of view,16 but there is a lack of knowledge, from a mechanistic point of view, of the interaction between these two materials during their coprocessing and there is nothing about the specific behavior of each of the tire compounds on the positive effect of the coprocessing of these two materials. From a study that attempts to use carbon black (CB) from old tires as catalysts for methylene and ethylene bridges cleavage in coal liquefaction,17 through other papers showing attempts to convert plastic materials into valuable products, via liquefaction9 or via pyrolysis,18,19 (11) Hodek, W. ICCS Proceedings, Newcastle (UK); Elsevier: Amsterdam, 1991; p 782. (12) Serio, A.; Wojtowicz, H. A.; Tang, H.; Pines, D. S.; Solomon, P. R. Proceedings of the ACS Division of Fuel Chemistry, Chicago; American Chemical Society: Washington, DC, 1993; Vol. 38 (3), p 906. (13) Anderson, L.; Tuntawiroon, W. Proceedings of the ACS Division of Fuel Chemistry, Chicago; American Chemical Society: Washington, DC, 1993; Vol. 38 (3), p 816. (14) Mastral, A. M. Final Report CSIC to ECSC, Contract 7220/EC/ 755; Instituto de Carboquı´mica: Zaragoza, Spain, 1993. (15) Klavetter, E.; Mitchel, S. C.; R Garcı´a Snape, C. ICCS Proceedings, Banff, Canada, September 1993; Elsevier: Amsterdam, 1993. (16) Gra, D.; Tomlinson, G. Proceedings of the ACS Division of Fuel Chemistry, Anaheim, CA; American Chemical Society: Washington, DC, 1995; Vol. 40 (1), p 20 (17) Farcasiu, M.; Smith, C. Energy Fuels 1991, 5, 83. (18) Conesa, J. A.; Font, R.; Marcilla, A.; Garcı´a, A. N. Energy Fuels 1994, 8, 1238.
Mastral et al.
to different studies in which the ground tire rubber is added to noncatalytic20 and catalytic21 coal liquefaction, increasing interest is being shown in the coprocessing of both materials.1-3,22 Althought the tire rubber used in this work did not undergo any selection process, the coal used was chosen from among 25 coals14,23 from different countries (U.S., UK, Germany, and Spain), which were previously hydrogenated under a wide range of conditions, because of its special characteristics: high reactivity in hydroconversion and high yields in conversion products. The chosen low-rank coal from NE Spain was hydrocoprocessed with old tire material from which the metallic component had been previously eliminated in order to achieve high conversions and to gain insight into coaltire interactions. The mechanistic aspects of the coalrubber interactions during their hydrocoprocessing are examined here and commented on through the results obtained from the processing of coal with tire, CB (carbon black), SBR (styrene-butadiene rubber), and PB (polybutadiene), the three main tire components. For comparison, coal, tire, and tire components, as well as polystyrene (PS), were processed separately under the same conditions. Experimental Section Discarded tires, supplied by AMSA (A. Mesalles, S. A., a rubber-recycling enterprise), ground, and sifted to a particle size of 0.9 mm, were used. The steel thread and the textile netting had previously been removed. The three main components in tire manufacturing, SBR, PB, and CB, were also obtained from the same supplier. Results reported on coal, tire, SBR, PB, and CB were obtained by working these materials separately and in coprocessing. Low-rank SAMCA coal from Utrillas was used. This coal corresponds to the average from this part of Spain, which in general, shows a high sulfur content, both organic and inorganic. In hydrogenation processes, pyritic sulfur, the main component of the coal mineral matter, is reduced to pyrrhotite, and in this conversion there is H2S release. On the other hand, part of the organic sulfur is converted into H2S during the process.24,25 Both, pyrrhotite and H2S, catalyze the hydrogenation process, giving high yields in conversion products. The coal was ground and sifted to a particle size between 0.25 and 0.50 mm and stored in 250 g containers under a nitrogen atmosphere. The coal and tire analyses are shown in Table 1. Data are given on daf basis (dry, ash-free), mf basis (moisture-free), and af basis (ash-free). The oxygen percentage is calculated by difference. The experimental installation used for hydrocoprocessing consists of a fluidized sand bath, provided with a temperature controller between 50 and 600 °C, and a pneumatic agitation device with a variable speed electric motor and adjustable stroke. The conditions used were 10 cm displacement and 120 rpm. At these conditions the mass and heat transfer problems are minimized for these materials.14 Minireactors are intro(19) Garcı´a, A. N.; Font, R.; Marcilla, A. Energy Fuels 1995, 9, 648. (20) Liu, Z.; Zondlo, J. W.; Dad burjor, D. B. Energy Fuels 1994, 8, 607. (21) Liu, Z.; Zondlo, J. W.; Dad burjor, D. B. Energy Fuels 1995, 9, 673. (22) Mastral, A. M.; Murillo, R.; Calle´n, M. S.; Pe´rez-Surio, M. J.; Clemente, M. C. Coal Science and Technology, ICCS Proceedings; Pajares, J., Tasco´n, J. M., Eds.; Elsevier: Amsterdam, 1995; pp 15351538. (23) Mastral, A. M.; Rubio, B.; Izquierdo, M. T.; Mayoral, M. C.; Pe´rez-Surio, M. J. Fuel 1994, 73, 897. (24) Mastral, A. M.; Mayoral, M. C.; Palacios, J. M. Energy Fuels 1994, 8, 94. (25) Rivera-Utrilla, J.; Maldonado-Ho´dar, F.; Mastral, A. M.; Mayoral, M. C. Energy Fuels 1995, 9, 319.
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Table 1. Tire and Coal Proximate and Ultimate Analyses C (daf) H (daf) N (daf) S (mf) moisture (af) ash (mf) volatiles fixed carbon calorific value (kcal/kg)
tire
coal
88.64 8.26 0.43 1.43 0.94 3.83 67.30 31.14 9.159
80.17 6.69 1.01 5.68 22.05 26.93 48.62 28.45 3.714
duced into the preheated sand bath hanging in a holder from the oscillation device. Their capacity is 60 cm3, and they are built with Swagelok pieces and provided with a pressure manometer, which allows the pressure evolution to be followed through the process. The batch process carried out consisted of filling the reactor with the feed mixture (see Table 2), always maintaining the same dead volume. The original hydrogen pressure was constant and equal to 10 MPa. Once the absence of leaks was tested, the reactor was immersed in the preheated fluidized sand bath at the temperature at which the process was to be carried out. The residence time was always constant and equal to 30 min. At the end of the process, the reactor was quenched, and once room temperature was reached, the gases were vented into a gas-sampling bag and analyzed by GC (CO, CO2, H2S, and C1-C4) using two columns: molecular sieve and Porapak N. The reactor content was Soxhlet extracted with THF for 24 h. The THF insolubles were dried in a vacuum oven for 24 h and weighed. The THF solubles were fractionated with n-hexane into oils and asphaltenes, according to the standardized procedure, which has been described elsewhere.26 Oils and asphaltenes were dried in the vacuum oven and then weighed and analyzed by capillary GC, 1H NMR, and TLCFID. The total conversion1 and conversion product percentages2,3,5 were calculated according to the following equations:
% conversion )
(coal + tire)mf - THF-insolubles
% oils )
(coal + tire)daf oils (coal + tire)daf
% asphaltenes )
asphaltenes (coal + tire)daf
% oils selectivity )
oils THF-solubles
% gases ) % conversion - (% oils + % asphaltenes)
% residue )
THF-inslubles (coal + tire)mf
(1)
(2)
(3)
(4)
(5) (6)
The different experimental runs carried out depending on the process variables used are compiled in Table 2.
Results and Discussion The coal used in this work is a subbituminous coal from SAMCA, Utrillas mining area, which gives high conversions in hydrogenation process,14 showing itself to have special properties in conversion into liquids, probably because of its high sulfur content, both organic (5.7% daf) and pyritic (1.4% dry). During hydrogenation, the two new sulfur forms, the pyrrhotite formed (26) Mastral, A. M.; Rubio, B. Fuel 1984, 63, 355.
Table 2. Experimental Conditions Used in Coal Coprocessing with Tire and the Three Main Tire Components
in situ and the H2S involved in the process,24 seem to be involved in parallel catalytic mechanisms, heterogeneous and homogeneous, respectively,27 which would explain the high conversions reached with this coal, mainly when the process is catalyzed with iron precursors.25 Anyway, in this work it was not taken into account if the radicals came from thermal cleavages or from catalytic breaking (perhaps due to the mineral matter of coal or CB); it is only the basically aromatic or aliphatic chemical nature of the radicals that was considered. The hydrogen content of the rubber used in this work (see Table 1) is slightly lower than that which was previously reported.28 The availability of hydrogen during the process was ensured by using 10 MPa of H2 as the starting pressure. The reaction time in these kinds of processes has to be a compromise between the highest conversion and lowest retrogressive reactions. For this coal, 30 min hydrogenation time is optimal14 in order to reach maximum yields in conversion products, minimizing side reactions, repolymerization, and condensation. The organic tire material has as main components carbon black (CB) and a mixture of elastomers,29 styrene-butadiene (SBR) and polybutadiene (PB). These three components were the same as those used in the tire manufacture process and were worked separately in this research, and each one was coprocessed with coal, assuming that the CB percentage is about one-third (31% fixed carbon) and each of the two elastomers means another one-third (67% volatile matter). These materials were hydroprocessed, using the wide range of process variables shown in Table 2. Results (27) Mastral, A. M.; Mayoral, M. C.; Izquierdo, M. T.; Rubio, B. Energy Fuels, in press. (28) Farcasiu, M.; Smith, C. M. Proceedings of the ACS Division of Fuel Chemistry, San Francisco 1992; American Chemical Society: Washington, DC, 1992; Vol. 37, p 472. (29) Mastral, A. M.; Murillo, R. Caucho 1995, 436, 23.
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Figure 1. Yields percentages obtained as a function of the coal-tire feed composition at 35 °C.
Figure 2. Yields percentages obtained as a function of the coal-tire feed composition at 400 °C. Table 3. Composition of Gases Obtained in the Coprocesing of Coal with Tire and Tire Components run number
COx
H2S
C1-C4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
56 36 17 0 0 54 61 53 55 46 54 55 56 61 55 54 57 65 8 70 68 66 59 66 66 65 66
23 4 3 0 0 32 26 23 17 15 26 25 28 22 33 33 34 27 9 15 20 19 21 7 24 24 26
21 60 80 100 0 14 14 24 28 38 20 20 16 17 12 13 9 8 82 14 11 15 20 27 10 11 8
obtained when different feeds of coal and rubber were coprocessed at 350 and 400 °C are compiled in Figures 1 and 2, respectively, and when each raw material was hydrogenated, in Table 4. From the results obtained, it can be observed that at 350 °C the tire rubber already gives higher conversion products than coal when they are separately hydrogenated although 350 °C is not the ideal temperature for rubber hydrogenation because at this temperature its depolymerization is not complete
as can be noted from its behavior during the extraction of the reactor content. A temperature of 400 °C is seen to be proper for rubber processing because the maximum conversion with the minimum secondary reactions takes place at this temperature. Thus, as the conversion increases to 71%, the asphaltene percentages decrease from 1.7 to 0.5 and the oils selectivity reaches 97%. These two latter facts show that 400 °C is a good rubber hydrogenation temperature because hydrocracking reactions leading to condensation products are minimal. The yields obtained in this research are a little higher in comparison to other reported yields when half-half coal-rubber feeds were coprocessed,20,28 which could be a result of both the experimental installation optimization and the subbituminous coal studied. The ease with which rubber is converted into liquids during the hydroprocessing is helpful in reducing problems because of mass and heat transfers in dry coal hydrogenation, mostly in dry coal catalytic hydrogenation where the solid-liquid contact must be optimized. On the other hand, rubber is an additional source of hydrogen supply30 to atmospheric hydrogen. Feed mixture composition is very important in the distribution of conversion products, and this distribution can be controlled depending on the aim of the coprocessing. If the goal is to eliminate residues, with rubberrich feeds, the nonconverted product is 49% at 350 °C and 38% at 400 °C when 20% coal-80% tire is coprocessed. These percentages are generally lower than those obtained in the pyrolysis of urban waste materials, which means about the 50% of the starting material. The residues from coal-rubber hydrocoprocessing, with an average caloric power of around 5000 kcal/kg, are mostly composed of CB, about one-third of the original tire, and the coal mineral matter. Although this last causes most problems in its reutilization, the CB could be recovered and reused. From a gas formation point of view, the percentages range from 6% to 35% at 350 °C and from 17% to 27% at 400 °C, depending on the feed composition. These gases, with an average caloric power of around 8000 kcal/Nm3, are composed of CO, CO2, H2S, and C1-C4 hydrocarbons (see Table 3). As expected, the H2S percentage gradually increases with increasing coal ratios in the feed mixture, the same as with the COx (CO + CO2) formation. H2S and COx percentages in the gas formation are high, since what is being processed is a low-rank coal and therefore with high heteroatom content (see Table 1). The formation of each of the C1C4 hydrocarbons increases with increasing rubber ratios in the feed mixture, and it is much higher at 400 °C than at 350 °C. For instance, with a 80% coal-20% tire feed, the total C1-C4 formation is 14% at both temperatures, while with a 20% coal-80% tire feed the total C1-C4 formation increases in the gas mixture from 27.3% to 38.4% (see Table 3). A synergism in total conversion seems to occur when the feed is made up of 80% coal-20% tire. By comparison of the experimental conversion with the one that theoretically would correspond to both starting materials worked alone, the former conversion is three points higher than the latter at 350 °C and four points higher at 400 °C. Since the experimental error is (2%, despite (30) Mastral, A. M.; Murillo, R. Manuscript in preparation.
Coal Hydrocoprocessing
Energy & Fuels, Vol. 10, No. 4, 1996 945
Figure 3. Possible interactions between radicals grouped according to their chemical nature and involved in coal-tire hydrocoprocessing
the low increase, the fact that it happens at both temperatures corroborates the potenciation between the two materials at the mentioned ratio. At these conditions, an increase in oils formation (18.8% theoretical to 19.8% real) and in asphaltenes formation (29.2% theoretical to 35.9% real) together with an important reduction in gas formation (26.1% theoretical to 21.5% real) can be observed. From these results it could be deduced that at these conditions while direct hydrogenation reactions with oils and asphalthenes formation are promoted, those reactions implying product insolubilization and gas formation are being inhibited. Despite the huge complexity of a high-pressure and temperature hydrogenation process carried out with organic material, mostly when this material is as complex as coal and, in addition, another material, rubber, is involved, an attempt has been made to deduce some mechanistic aspect by grouping the different radicals involved in the hydropyrolysis into five main groups: hydrogen radicals, aromatic radicals from coal, aliphatic radicals from coal, aromatic radicals from rubber, and aliphatic radicals from rubber. The possible interactions among them are shown in Figure 3. When heated, a great generation of radicals is produced from the coal organic matrix, which will then be reduced to (coal-aromatic)• and (coal-aliphatic)•, and from rubber, (rubber-aromatic)• and (rubber-aliphatic)•. If the pyrolysis is carried out in a hydrogenating atmosphere, there are hydrogen radicals as well and the five radicals involved in the hydropyrolysis could be stabilized (1) by capturing hydrogen from the bulk phase, producing lighter products and (2) by reacting with each other, giving a wide range of possible molecules that can range from light ones, a consequence of hydrogenation and hydrocracking reactions, to heavy products from side reactions like condensation and repolymerization. The control of variables of the pyrolysis process would help to minimize the side reactions, but when the pyrolyzed organic material is as complex as are the coal-rubber mixtures, all these reactions would happen simultaneously and some retrogressive reactions would take place. In general, the hydroprocessing is performed seeking high conversions to liquids and, at the temperatures usually used, the bond-breaking selectivity is quite low. In this work, the selected temperatures were 350, 375, and 400 °C. At these temperatures and 10 MPa (H2, cold), the bond cleavage from both materials processed is significant, but as the radicals are released in a rich hydrogen atmosphere, the incidence of retrogressive reactions is not. The gradual decrease in THF-insolubles and in asphaltenes percentages with feeds rich in rubber corroborates this fact. However,
Figure 4. Yields obtained when a rich coal feed (coal/tire ) 4/1) is coprocessed for 30 min, 10 MPa (H2 cold) at 350 and 400 °C.
with a coal/rubber ratio feed equal to 4, from 350 to 400 °C, (350 °C for coal and 400 °C for tire are the compromising temperatures at which the conversion reactions vs undesired reactions, retrogressive reactions, are more provable), the relevance of the hydrocracking reactions is noticeable because while the oils recovery increases from 15% to 17% (see Figure 4), the gas formation and the asphaltenes percentages are practically duplicated. These results point out that the conversion products from coal hydrogenation are more labile to the temperature than the rubber ones and undergo disproportionation reactions that convert them into lighter (gas) and heavier (asphaltenes) products. To elucidate the role played in the coprocessing by each of the more abundant tire components, CB, SBR, and PB were processed alone (see Table 4) and with coal (see Figures 5-8) in the same proportion as they are when the coal-tire ratio feed is 1/1, assuming that the tire is composed of one-third carbon black, one-third styrene-butadiene and one-third polybutadiene. These three components have different natures: while PB shows an aliphatic nature, one-third of SBR is aromatic. In addition, poly(vinylbenzene) (PS), the aromatic component of SBR, was coprocessed with coal. Results obtained with each of these components are commented below. Carbon Black. According to previous publications,20,28 there is no agreement on the role played by carbon black during coal-tire coprocessing. First of all, it has to be taken into account that CB has a high surface area and it will at least help to improve the contact between reagents and so diminish mass transfer problems, mostly when the process is carried out in the absence of solvent. In addition, CB was treated by itself at 400 °C and under the same conditions it was coprocessed with coal. In this way it would be certain that nothing happened to it by hydrogenation. As it was, there was neither gas formation nor THF-solubles. For Illinois No. 6, although Farcasiu et al.17,28 report that CB catalyzes this coprocessing, by performing the same experiments in the presence of rubber-treatment and with rubber not containing carbon black, with conversions falling from 71% to 60%, respectively, Liu et al.20 do not find differences either in conversions or oils + gas yields. Despite the fact that it could be due to the use of different carbon blacks, as Liu points out, and the attempt to make light of these controversial results, this coal was processed in the presence and absence of the same carbon black used in the tire manufacture, avoiding in this way other tire compo-
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Table 4. Results Obtained in the Raw Materials Processing at 400 °C, 30 min, and 10 MPa raw material coal tire SBR PB CB
conversion (%)
THF-insol (%)
asphaltenes (%)
oils (%)
oils selectivity
75 72 100 100 0
51 32 0 0 100
37 1 0.3 0.1 0
12 48 80 78 0
20 97 99 95 0
Figure 5. Yields obtained in CB-coal coprocessing at different temperatures (30 min, 10 MPa H2).
Figure 6. Yields obtained in SBF-coal coprocessing at different temperatures (30 min, 10 MPa H2).
Figure 7. Yields obtained in PS-coal coprocessing at different temperatures (30 min, 10 MPa H2).
nents that could be interfering and screening the obtained results. Temperatures used were 350 and 400 °C. At both temperatures (see Figure 5), in presence of CB there is an increase in coal conversion but it does not mean an increase in either oils or asphaltenes formation. On the contrary, oils and asphaltenes percentages decrease. The conversion increase is due to a higher gas formation, which triples at 350 °C and doubles at 400 °C, when the same process is performed in presence of CB. This increase in gas formation is mainly due to C4-C6 hydrocarbons formation, according to the performed GC analyses.
Figure 8. Yields obtained in PB-coal coprocessing at different temperatures (30 min, 10 MPa H2).
Hence, it seems that indeed CB is catalyzing the hydrocoprocessing, according to Farcasiu, but not in the desired direction, and the gas + oils formation might remain practically constant, as Liu observed. CB seems to be catalyzing the breaking of molecules from oils and asphaltenes produced in the coal hydrogenation and to be converting them into smaller molecules responsible for the gas formation. On the other hand, the oils selectivity increase at both temperatures in the presence of CB corroborates the catalytic breaking into lighter molecules. SBR Elastomer. The tire SBR elastomer is composed of 1,4-butadiene and vinylbenzene in a ratio of 3/1. Their copolymerization needs an excess of the diolefin, which undergoes 1,4 diaddition, with remaining double bonds with a trans configuration. The most likely structural unit will be formed by the addition of three molecules of butadiene and one of vinylbenzene according to
The chains composed of the repetition of this unit will be cross-linked among themselves by the double bonds and will be connected by bridges through the para position of the aromatic rings. The rolling up of these chains is what provides elastic properties to rubber. During hydropyrolysis, under the working conditions, at temperatures equal to or higher than 350 °C and at 30 min residence time, these chains will break preferentially at the bonds to give the more stable radicals. All breakings that give rise to benzylic, allylic, or arylic radicals will be favored, while the double bonds and phenyl carbon bonds will remain. The tertiary aliphatic radicals will be released preferentially to secondary ones and these preferentially to the primary aliphatic radicals because the required energy is lower. In this way the most branched radicals will be preferentially produced. To know how SBR behaves, it was coprocessed with coal at 350, 375, and 400 °C. Its addition to coal
Coal Hydrocoprocessing
hydrogenation always had a positive effect. The yields at 375 °C are much higher than those obtained at 350 °C (see Figure 6), showing its thermal lability at this intermediate working temperature. According to the above comments, in the SBR with coal coprocessing the radicals involved in the process will be (SBR-aliphatic)• and (SBR-aromatic)• from SBR and (coal-aliphatic)• and (coal-aromatic)• from coal. The aliphatic radicals could be stabilized (a) by hydrogen capture, giving preferential C1-C4 hydrocarbons, (b) by dimerization reactions among themselves, giving C2-C8, and (c)by interaction of the aromatic radicals, both from the elastomer and from coal. In this way, gas and light oils will be preferentially produced. With regard to the aromatic radicals, (SBR-aromatic)•, and basically aromatic units from coal, (coal-aromatic)•, could be stabilized (a) by hydrogen capture, (b) by the aromatic radical interactions with the aliphatic radicals, and (c) by aromatic-aromatic interactions. With these last three possibilities, aromatic compounds will always be obtained, but while the (a) and (b) possibilities will preferentially lead to oils, with the (c) possibility asphaltenes formation or even the THF-insolubles percentage will increase. To analyze the incidence of the aromatic-aromatic interactions, a run at 400 °C was performed with PS and coal, with a PS amount equivalent to that which exists when coal and SBR are worked in a 1/1 ratio, avoiding in this way the presence of aliphatic radicals from tire. The asphaltenes conversion and gas formation percentages were practically equal. Only the oils and total conversions were different (see Figure 7 where all the coal-PS ratios are shown). Since oils formation take place when only aromatic radicals are involved, in the case of PS-coal coprocessing, the aromaticaromatic interactions seem to occur with less incidence than the aliphatic-aromatic ones, which is quite logical because of the sterical impediments, mostly due to the substituents on the aromatic units from coal and to the branching of the aliphatic radicals. Polybutadiene. The role played by PB is less important than that played by SBR in coal coprocessing, the process temperature being a determinant variable. The lower lability to thermal cleavage of PB vs SBR is demostrated at 350 °C by the lower total conversion and oils percentages reached, which vary from 24% to 35% for total conversions and are duplicated in the case of oils formation. According to the results obtained in this work and shown in Figure 8, at 350 and 375 °C the bond breaking in the PB macromolecule is not as great as with SBR, as the lower conversions show. On the other hand, the nature of the released radicals from PB depolymerization7 will be mostly aliphatic, alkanes and cycloalkanes, mostly at these lower temperatures at which the cyclation reactions are not as significant as at 400 °C. The results obtained when only PB was hydropyrolyzed at 400 °C show that total conversion, asphaltenes, oils, and gas formation are quite similar to those obtained with SBR (see Table 4), probably because of the fact that at this high temperature the previously commented aromatization reactions are significant. When PB is coprocessed with coal, results are slightly lower at this temperature, while at 350 and 375 °C they are noticeably lower, as Figure 8 shows.
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It is not easy to explain the differences between these two synthetic elastomers, but a possible interpretation could be that the basically aromatic radicals from the organic coal matrix are more difficult to alkylate by the aliphatic radicals released by PB,31 because of steric impediments, than the aromatic radicals from rubber (SBR and PS) and from aromatization reactions. Assuming this fact, the conversion products would come from hydrogen capture because the alkylation reactions will be inhibited by the absence of aromatic nuclei from the PB at low temperatures. In addition, the radical mechanism could be reinforced by an ionic mechanism due to the acid environment in which the process takes place32 because of the H2S emitted as consequence of the reduction of pyrite from the coal mineral matter to pyrrhotite and from partial coal organic sulfur release. The complexity of the starting materials and the complexity of the hydrocoprocessing make it very difficult to know about the incidence of each of these possibilities. Conclusions According to the results shown in this work, it can be concluded that tire rubber has a positive effect as an additive for coal hydropyrolysis. Its effect is more relevant when rich-in-tire feeds are coprocessed, showing a synergism in total conversions when 80% coal20% tire feed is hydrocoprocessed. The role played by each of the three major tire components has been studied and analyzed. This work demostrates that CB catalyzes the process by promoting hydrocracking reactions with the formation of lighter and smaller molecules, principally leading to gas formation. The SBR facilitates the radicals stabilization involved in the process, probably through alkylation reactions due to its aromatic component. Even 350 and 375 °C an improvement in gas, oil, and asphaltenes formation is observed. It seems to be that reactions implying the alkylation of the aromatic ring from this elastomer by the radicals involved in the process are promoted. The role played by PB is not as noticeable at 350 and 375 °C as the one played by SBR, maybe because of the absence of aromatic rings from the PB elastomer. However, at 400 °C, when aromatization reactions of the aliphatic radicals have taken place, the difference between the two elastomers is minimum. These results seem to point out that the aromatic fragments from the coal organic matrix are not being alkylated with the same facility as the aromatic radicals from tire. Among all the possible equilibriums of the radicals involved in coal-tire hydrocoprocessing, those that imply the alkylation of the aromatic from tire are favored. The absence of aromatic rings in the PB macromolecule and its lower positive effect corroborate this fact. Acknowledgment. The authors thank the Spanish Interministerial Commission for Science and Technology, CICYT, for financial support of this work (No. AMB-94 0933) and AMSA for the tire rubber supply. EF950113H (31) Mastral, A. M.; Cebolla, V.; Gavila´n, J. M. Fuel 1984, 63, 1422. (32) Mastral, A. M.; Ma oral, M. C.; Palacios, J. M. Proceedings of the ACS Division of Fuel Chemistry; American Chemical Society: Washington, DC, 1993; Vol. 38 (1), p 12.
