676
Energy & Fuels 1997, 11, 676-680
Assessment of the Tire Role in Coal-Tire Hydrocoprocessing Ana M. Mastral,* Ramo´n Murillo, Marisol Calle´n, M. Jesu´s Pe´rez-Surio, and M. Carmen Mayoral Instituto de Carboquı´mica, CSIC, Apdo. 589, 50080 Zaragoza, Spain Received June 10, 1996X
This paper analyzes the role performed by rubber from tire in coal-tire coprocessing when tire is added to coal hydrogenation. A low-rank coal (SAMCA, Spain) and discarded scrap tires (steel thread and the textile netting were previously removed) were batch processed in tube reactors at various hydrogenating atmospheres (10, 7.5, 5, and 1 MPa of hydrogen) and in inert atmosphere (1 MPa of nitrogen) for 30 min at 400 °C. Two feed mixtures, 80% coal/20% rubber (4/1 ratio), and 20% coal/80% rubber (1/4 ratio), and both raw materials were processed separately. While rubber does not show variations with the process atmosphere, coal significantly varies with the pressure and the hydrogenating inert atmosphere. The selected process conditions were critical in order to elucidate the role performed by rubber from tire in coal hydrocoprocessing. The percentages and characteristics of the conversion products help to clarify the performance of rubber as a hydrogen donor, as a hydrogen transport and as a solvent. It is concluded that the improvement in quantity and quality of conversion products caused by tire addition to coal hydroconversion does not seem to be due either to the as solvent role or to the hydrogen transport role of rubber. The results obtained point out that rubber behaves as a hydrogen donor. In addition, the synergism observed in coal-rubber hydrocoprocessing and the less aromatic nature of the radicals from tire pyrolysis involved in the process are very helpful in improving the nature of the obtained oils.
Introduction Today, the obtention of liquids from coal is not economically competitive compared to the cost of petroleum technology due to the cost of hydrogen. A way to try to reduce the expensive hydrogen addition is to make better use of the hydrogen supply through the utilization of different materials.1-3 The addition of some materials as an additional form of hydrogen supply is an attempt to obtain cheaper oils and at the same time to give environmentally friendly solutions by taking advantage of the high chemical potential of some4-6 nonvaluable materials. These objectives could be reached if the reused residue could perform one of these roles: (a) An inert material which could be added to reduce mass and heat transfer problems. If this material is liquid at reaction temperature, these problems would be minimized. Previous work has been reported in which an inert solvent has been used as the suspension medium in coal hydrogenation.7 Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Symposium on Co-Utilization of Coal and Wastes. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, (3) and references cited therein. (2) Symposium on Co-Utilization of Coal and Wastes Materials. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1) and references cited therein. (3) International Congress on Solid Residues, Proceedings of the ANQUE, Tenerife, December 1994 and references cited therein. (4) Taghiei, M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228. (5) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1994, 8, 607. (6) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1995, 9, 673. (7) Ratto , J. J.; Hededey, L. A.; Skowronsky, R. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1979, 24, 155. X
S0887-0624(96)00092-8 CCC: $14.00
(b) A hydrogen transport which takes hydrogen from the gas phase and carries it to the coal. A typical example of this, in classical coal hydrogenation, would be tetrahydronaphthalene (TTL), which in a first stage gives hydrogen atoms to coal and is converted into naphthalene and dihydronaphthalene. After that, it is hydrogenated to TTL, taking four hydrogen atoms from the gas phase, and again donates these hydrogen atoms to coal.8 In this way, tetralin is always taking hydrogen from the gas phase and transporting it to coal. (c) A material whose special characteristics could allow it to work as a donator of hydrogen.9 The hydrogen donation could be done in two different ways: (1) a internal reordering of hydrogen (in this case, by internal hydrogen redistribution, the part richer in hydrogen material could be incorporated to the coal matrix) and (b) by incorporating fragments to the coal matrix from a higher hydrogen content material. Small fragments with a high H/C ratio could be being incorporated, and in this way, the addition of hydrogen would be cheaper than working with higher hydrogen pressures. To improve coal hydrogenation, many catalysts have been tested and have shown activity in this process.10 The more used catalysts contain transition metals such as Fe, Mo, Ni, or Zn. The addition of these catalysts may be in three ways: supported over silica or alumina, mechanical mixture, and impregnation. Supported catalysts are not the best choice because it has been (8) Mastral, A. M.; Membrado, L.; Rubio, B. An. Quim. 1985, 80, 521. (9) Bengoa, C.; Font, J.; Moros, A.; Fabregat, A.; Giralt, F. Fuel 1995, 74, 1704. (10) Debyrshire, F.; Hager, T. Fuel 1994, 73, 7.
© 1997 American Chemical Society
Coal-Tire Hydrocoprocessing
observed that they are deactivated by the deposition of carbonaceous materials10 which are the consequence of the internal hydrogen redistribution. These catalysts are more frequently used in the upgrading of coalderived liquids. Another option would be mechanical mixture, but a big amount of catalyst is needed and the homogeneity of the mixture is not as good as would be desired. High yields are obtained when active metals are deposited directly over the coal to be hydrogenated, but it is necessary that the corresponding salts used as precursors are soluble. High conversions have been reached dispersing Mo on coal,11,12 but that makes the process too expensive. Both, high hydrogen pressures and catalysts influence the final price of the oils. The alternative is to add very low cost materials which provide cheap hydrogen.13,14 Reusing of the rubber from old tires in its original form is not possible due to the vulcanization process which makes it impossible to recover its original components. On the other hand, tires are not biodegradable, so they should not be disposed of in landfills or left in the open air. Actually, based on the economy, this waste material is being burnt for its energetic value, but this should be the lowest priority in the reuse of old tire because it is not an environmentally acceptable solution. A fraction of the tire could be useful with important savings in natural resources.15 Hydrogenation of rubber from tire gives liquids which could be used to reduce mass transfer problems in different coprocessings.16 Besides, the rubber from old tires could be an additional hydrogen source in coaltire hydrocoprocessing, which could allow the hydrogen pressure to be reduced and therefore the cost of the process. In this paper, the performance of rubber from old tires is analyzed when added to coal hydrogenation from both points of view: as an additional hydrogen source and as a solvent (reaction medium) in dry coal hydrogenation. Experimental Section Discarded tires, supplied by AMSA (A. Mesalles, S.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. Low-rank SAMCA coal from Utrillas was used. The coal was ground and sifted to a particle size between 0.25 and 0.50 mm and stored in 250 g containers under nitrogen atmosphere. Coal and tire characteristics are shown in Table 1, where daf means dry and ash free, mf means moisture free, and af means ash free basis. The experimental installation used for hydrocoprocessing consists of a fluidized sand bath and a pneumatic agitation device, provided with a variable-speed electric motor and adjustable stroke. Minireactors, type tubing bomb of 60 cm3 capacity, are introduced into the fluidized and preheated sand bath hanging in a holder from the agitation device. The heat(11) Rivera-Utrilla, J.; Maldonado-Ho´dar, F.; Mastral, A. M.; Mayoral, M. C. Energy Fuels 1995, 9, 319. (12) Mastral, A. M.; Rubio, B.; Izquierdo, M. T.; Mayoral, M. C.; Pe´rez-Surio, M. J. Fuel 1994, 73, 897. (13) Farcasiu, M.; Smith, C. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 472. (14) Anderson, L.; Tuntawiroon, W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 816. (15) Williams, P. T.; Besler, S.; Taylor, D. T. Proc. Inst. Mech. Eng. 1993, 207, 55. (16) Mastral, A. M.; Murillo, R.; Pe´rez-Surio, M. J.; Calle´n, M. Energy Fuels 1996, 10, 941.
Energy & Fuels, Vol. 11, No. 3, 1997 677 Table 1. Tire and Coal Proximate and Ultimate Analyses tire
coal
C (daf) H (daf) N (daf) S (mf)
88.64 8.26 0.43 1.43
80.17 6.69 1.01 5.68
moisture (af) ash (mf) volatiles fixed carbon
0.94 3.83 67.30 31.14
22.05 26.93 48.62 28.45
calorific value (kcal/kg)
9.159
3.714
up time was lower than 2 min. After 30 min of reaction, the reactors are quenched in water. More details on the installation and the experimental procedure are described in ref 16. The repeatability working at the described conditions was (2%. Experiments were carried out by varying the feed mixture composition from 100% coal to 100% tire with the intermediate ratios of 80% coal/20% tire in daf weight (4/1 ratio) and 20% coal/80% tire in daf weight (1/4 ratio). Temperature (400 °C) and reaction time (30 min) were kept constant for all the runs. The starting pressures of the hydrogenating atmosphere were 10, 7.5, and 5 MPa of H2, and 1 MPa of N2 was the pressure of the inert atmosphere. The conversion products workup was performed according to the procedure described in ref 16, and the experimental and theoretical percentages were calculated according to eqs 1-4 and 5 and 6, respectively:
% conversion )
(mcoal + mtire)mf - THR insolubles (mcoal + mtire)daf
% oils )
moils (mcoal + mtire)daf
% asphaltanes )
masphaltenes (mcoal + mtire)daf
% gas ) % conversion - % oils - % asphaltenes m)
mcoal(daf) (mcoal + mtire)daf
Xt ) Xcoalm + Xtire(1 - m)
(1)
(2)
(3) (4) (5) (6)
where m is the daf coal/daf total mass ratio, mcoal is the mass of coal present in the feed mixture, mtire is the tire mass present in the feed mixture, Xt is the theoretical conversion, Xcoal is the experimental coal conversion, and Xtire is the experimental tire conversion. The gases were analyzed by GC with dual-column molecular sieve and Porapak N. C1-C4, H2S, and COx percentages in the gas mixture were calculated and are shown in Table 2. Oils were characterized by their elemental analysis and their hydrogen type by 1H-NMR in a Bruker CW-80 SY with DCCl3 in aliphatic (HR from 1.9 to 3.3 ppm, Hβ from 0.9 to 1.9, Hγ from 0.5 to 0.9 ppm, and Haromatic from 6 to 9 ppm). Their hydrocarbon type (saturated, aromatic, and polar) was analyzed by thin layer chromatography (TLC-FID) in a Iatroscan by elution of the chromrods with n-hexane, toluene, and dichloromethane/methanol.
Results and Discussion It is worth commenting that the rubber from the used tire has 1.55% more hydrogen and 4.25% less sulfur than the SAMCA coal. According to the two raw materials elemental analyses, there would not be rea-
678 Energy & Fuels, Vol. 11, No. 3, 1997
Mastral et al.
Table 2. Gas Composition by GC as a Function of the Coal-Tire Coprocessing Experimental Conditions (400 °C, 30 min) coal/tire ratio 1/0
4/1
1/4
0/1
pressure
C1-C4
COx
H2S
C1-C4
COx
H2S
C1-C4
COx
H2S
C1-C4
COx
H2S
10 MPa (H2) 7.5 MPa (H2) 5 MPa (H2) 1 MPa (H2) 1 MPa (N2)
21.3 17.0 17.2 13.0 11.2
55.9 60.3 60.7 70.0 70.7
22.8 22.9 22.1 16.9 18.2
14.1 21.2 20.6 17.9 18.3
66.4 52.3 54.2 64.7 67.8
19.6 26.5 25.4 17.5 11.8
35.2 35.5 37.7 39.1 51.3
46.9 48.3 50.0 49.4 41.6
17.9 16.3 12.4 11.5 6.3
60.3 70.6 70.1 63.5 62.1
35.8 25.3 28.5 30.9 35.4
4.0 4.1 1.4 5.7 2.1
Table 3. Total Conversion, Asphaltenes and Oils Percentages from Tire and Coal Processing as a Function of the Starting Pressure and of the Process Atmosphere tire
coal
initial pressure
conversion
asphaltenes
oils
conversion
asphaltenes
oils
10 MPa (H2) 7.5 MPa (H2) 5 MPa (H2) 1 MPa (H2) 1 MPa (N2)
71.4 70.7 70.0 70.3 67.8
0.5 0.8 0.8 0.4 0.3
48.2 41.9 40.4 43.8 44.4
94.5 88.4 63.9 36.7 18.5
55.7 49.0 31.5 4.4 1.4
18.9 15.1 14.4 8.2 4.4
sons, according to what has been previously commented, for a positive effect in coal hydroconversion when rubber is added because the small difference in hydrogen content and because the lower sulfur content means a lower H2S partial pressure with the corresponding diminishing in the homogeneous catalysis that H2S has in coal hydrogenation.19,20 The coprocessing was carried out in hydrogenating and inert atmospheres. While for inert atmosphere only 1 MPa of nitrogen was used, the starting hydrogen pressures (10, 7.5, 5, and 1 MPa) ensure conditions from a large hydrogen excess to a lack of hydrogen through critical intermediate pressures when raw coal, raw old tires, and feed mixtures of the two products (80% coal/ 20% tire (4/1 ratio) and 20% coal/80% tire (1/4 ratio)) were worked. Results obtained in conversion products (Table 3) showed that each raw material behaves in a different way: While the distribution of conversion products from tire is practically constant, independently of whether nitrogen or hydrogen is used, with the starting pressure, coal shows important differences with this variable. As Table 3 shows, no noticeable differences with the initial pressure can be observed in the distribution of the conversion products from tire. However, the variations showed by total conversion, oils, and asphaltenes obtained when coal is hydrogenated at different initial hydrogen pressures are very significant. It is clear that the direct hydrogenation of coal into conversion products is a function of the hydrogen pressure. Although 1 MPa of hydrogen is a very low pressure, i.e., asphaltenes go from 55% at 10 MPa of hydrogen to 4% at 1 MPa of hydrogen, the hydrogen presence is really important if it is compared with the results obtained when 1 MPa of nitrogen was the process pressure. The gas composition shows (Table 2) higher C1-C4 formation at the higher pressures and with tire-rich feed mixtures. The COx (CO + CO2) formation is a function (17) Mastral, A. M.; Mayoral, M. C.; Izquierdo, M. T.; Rubio, B. Energy Fuels 1995, 9, 753. (18) Mastral, A. M. Final Report CSIC to ECSC, Contract 7220/EC/ 755, June 1993. (19) Mastral, A. M.; Murillo, R.; Calle´n, M. S.; Pe´rez-Surio, M. J.; Clemente, M. C. Coal Science and Technology. ICCS Proceedings, Volume II; Pajares, J., Tasco´n, J. M., Eds.; Elsevier: Amsterdam, 1995; pp 1535-1538. (20) Mastral, A. M.; Mayoral, M. C.; Palacios, J. M. Energy Fuels 1994, 8, 94.
Table 4. Hydrogen Nature by 1H NMR in the Oils from Coal-Tire Processing Depending on the Hydrocoprocessing Variables (400 °C, 30 min) feed ratio 4/1
1/4
pressure
1 MPa (H2)
1 MPa (N2)
1 MPa (H2)
1 MPa (N2)
aliphatic hydrogen (%) aromatic hydrogen (%)
84 13
82 16
88 11
85 12
of the coal ratio in the feed mixture, and it is quite bulky due to the high oxygen content of this low-rank coal. The partial H2S pressure in the gas is a direct function of the coal content of the feed mixture. The H2S origin is due20 to the organic sulfur components in coal and to the coal mineral matter pyrite transformation into pyrrhotite. The H2S percentage in the gas composition is quite bulky due to the high sulfur content of the coal, and its partial pressure decreases with decreasing starting pressures. The different behavior between tire and coal versus hydrogen pressure and process atmosphere will be very helpful to study the role played by rubber from old tires in its coprocessing with coal. In this process, rubber from tires could be performing the role simply as a solvent or working as a hydrogen donor and/or helping as a hydrogen transport: The coprocessing was carried out in the absence of any other material except coal and tire; no solvent was used to avoid any interference in the results of the study. The mass and heat transfer problems are accentuated in absence of a reaction medium. Coal hydrogenation in absence of solvent, dry hydrogenation, gives good yields when a catalyst has been previously dispersed on the coal.18 Although this is a noncatalyzed coprocessing, high conversion products yields were obtained so a positive effect of tire addition to coal hydrogenation can be deduced. The different possibilities of this positive effect are going to be analyzed. To the coal used, the suitable hydrogenation temperature has shown to be 350 °C.20 This temperature shows to be quite low for the rubber components depolymerization which has been reported22 to be totally (21) Mastral, A. M.; Mayoral, M. C.; Izquerdo, M. T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 121. (22) Williams, P. T.; Pesler, S. Fuel 1995, 74, 1277.
Coal-Tire Hydrocoprocessing
Energy & Fuels, Vol. 11, No. 3, 1997 679
Figure 1. Asphaltene percentages reached as a function of the feed mixture composition and of the N2 or H2 pressure (1 MPa). Table 5. Experimental Percentages Obtained from Coal-Tire Hydrocoprocessing as a Function of the Starting Pressures and the Composition of the Mixture Feed 80% coal/20% tire
20% coal/80% tire
initial pressure
conversion
asphaltenes
oils
conversion
asphaltenes
oils
10 MPa (H2) 7.5 MPa (H2) 5 MPa (H2) 1 MPa (H2) 1 MPa (N2)
76.9 77.8 69.5 50.3 28.6
31.9 32.3 26.6 5.4 2.6
22.0 20.8 19.1 10.8 5.8
70.4 68.4 67.8 65.5 59.7
6.4 7.5 6.7 5.8 6.0
42.1 41.0 39.8 30.6 28.3
reached at much more higher temperatures (approximately 500 °C). These high temperatures have been avoided in this coprocessing because retrogressive reactions of radicals from coal are very significant.23 By hydrocoprocessing at 400 °C, even at the less favorable conditions, the hydrogenating effect, which is already observed, could be screening the role of rubber as a solvent. This fact does not allow the ability of rubber as a solvent to be known, but its lower thermal lability versus coal’s thermal lability (thermal cracking is already observed at 300 °C) suggests a low capacity for minimizing mass and heat transfer problems which, in the beginning, should be the role played by a good solvent. The critical conditions of the runs carried out with different feed mixtures were selected in order to allow the possible behavior of rubber as hydrogen donor and as hydrogen transport to be investigated. Tire as Hydrogen Donor. In this hydrogenation process, the higher the initial hydrogen pressure used, the higher the asphaltenes percentage from coal, and almost nothing happens regarding to rubber from tire (the higher percentage of asphaltenes is lower than 1%). Thus, the increase of rubber percentage in the feed mixture will theoretically decrease the asphaltenes percentages in the coal-rubber coprocessing. The data shown in Table 3 point out that the most relevant parameter for knowing the hydrogenating degree seems to be the asphaltenes percentage because, while these conversion products would come from the direct hydrogenation of the coal macromolecular component, oils and gas percentages could come from direct hydrogenation and from hydrocracking reactions as well.
The fact that the asphaltenes content increases when coal-rubber feed mixtures are coprocessed, even in nitrogen atmosphere (when the rubber contribution to asphaltenes formation is always lower than 1%), points out that the rubber is donating hydrogen to coal conversion in their coprocessing. This slight rubber hydrogenating effect (see Figure 1) is screened at high hydrogen pressures, equal or higher than 5 MPa, at which the hydrogenation by the hydrogen atmosphere is much more efficient than the rubber hydrogenation effect. Figure 1 shows the critical conditions at which there are no interferences due to a bulk hydrogen pressure. As Figure 1 shows, with the 4/1 (80% coal/20% tire) and the 1/4 (20% coal/80% tire) feed mixtures, the experimental yields are higher than the theoretically expected ones and those calculated with formulas 1 and 2; a higher tire proportion always means a higher hydrogenation mostly when the atmosphere is not hydrogenating. Despite the differences not being so great, all of these results demonstrate the hydrogenating role played by rubber in coal-tire hydrocoprocessing. This statement is corroborated by the nature of the corresponding oils according to the proton nuclear magnetic resonance analysis performed (see Table 4). The tire hydrogen donor effect is confirmed by the higher aliphatic hydrogen and the lower aromatic hydrogen shown by the corresponding oils obtained with higher tire proportion independent of whether the atmosphere is nitrogen or hydrogen. A good hydrogenation implies not only higher oils and asphaltenes percentages but also a more aliphatic structure because a good hydrogenation means the opening of aromatic rings with the corresponding in-
(23) Mastral, A. M.; Izquierdo, M. T.; Burchill, P.; McCaffrey, D. Fuel 1994, 73, 449.
(24) Mastral, A. M.; Rubio, B. Final Report CSIC to EU, Joule Programme, Contract EN-3V-0044 (E) A, March 1991.
680 Energy & Fuels, Vol. 11, No. 3, 1997
Mastral et al.
Table 6. Theoretical Percentages from Coal-Tire Hydrocoprocessing as a Function of the Starting Pressures and the Composition of the Mixture Feed 80% coal/20% tire
20% coal/80% tire
initial pressure
conversion
asphaltenes
oils
conversion
asphaltenes
oils
10 MPa (H2) 7.5 MPa (H2) 5 MPa (H2) 1 MPa (H2) 1 MPa (N2)
89.9 84.9 65.1 43.4 29.0
44.7 39.4 25.4 3.6 1.2
24.8 20.5 19.6 15.3 12.9
76.0 74.2 68.8 63.6 57.8
11.5 10.4 6.9 1.2 0.5
42.3 36.5 35.2 36.7 36.4
Table 7. Oils Hydrogen Nature from Coal-Tire Processing Depending on the Hydrocoprocessing Variables (400 °C, 30 min, hydrogen atmosphere) by 1H NMR feed ratio 4/1
1/4
pressure
5 MPa
10 MPa
5 MPa
10 MPa
aliphatic hydrogen (%) aromatic hydrogen (%)
86 12
90 9
90 9
91 8
crease the aliphatic/aromatic ratio due to the total or the partial transformation of aromatic compounds into hydroaromatic and/or aliphatic compounds.24 Thus the higher aliphatic hydrogen in oils is an indicative parameter of the hydrogenating role of rubber. Tires in Hydrogen Transport. The critical conditions to study the role of tires in hydrogen transport will be noticed when the hydrogen pressure is not so high as to screen the rubber hydrogen transport effect and not so low that the hydrogen is so scarce that is made difficult its transport. Besides, only a small amount of tire would be necessary to notice its hydrogen transport effect. According to the above comments, 7.5 and 5 MPa of hydrogen pressure and 4/1 feed mixture have been selected to analyze the role of the rubber from tire as hydrogen transport in coal-tire hydrocoprocessing. Comparing the experimental16 data obtained with the theoretically calculated ones (see Table 5 and Table 6, respectively, it can be seen that both are so similar, even the asphaltenes percentages in some cases are lower than could be expected, that the hydrogen transport effect in this process can be ruled out. On the other hand, the nature of the oils (see Table 7) shows very small differences with hydrogen pressure
and feed composition. The aliphatic and aromatic hydrogens do not show appreciable variation as a function of these two variables at the highest hydrogen pressure, and the slight differences at 5 MPa with feed composition, slightly higher aliphatic hydrogen, and slightly lower aromatic hydrogen at the lower tire ratio could be due more to the conversion products incorporation from tire than to the possible hydrogen transport effect of the tire. Conclusions It could be summarized that the positive effect of rubber from tire addition to coal hydroconversion does not seem to be due to the solvent role of rubber or hydrogen transport role during coal-rubber hydrocoprocessing. The data shown in this work point out that rubber performs a role as a slight hydrogen donor which partially justifies the higher oil and asphaltene formations and the more aliphatic character of the oils reached in coal-tire hydrocoprocessing. Complementary to this positive effect seems to be the synergism observed16 in the coal-rubber hydrocoprocessing and the more accentuated, as the mass spectroscopy shows, saturated character of the radicals from tire pyrolysis involved in hydrocoprocessing of these two materials. Acknowledgment. The authors thank the Spanish CICYT (Project Reference AMB95-0059) for the financial support and the Autonomic DGA a fellowship (R.M.). EF9600927
