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Energy & Fuels 2007, 21, 2226-2229
Photochemical Dissolution of Turkish Lignites in Tetralin at Different Irradiation Power and Reaction Times F. Karacan*,† and T. Togˇrul‡ Sarayko¨y Nuclear Research and Training Center, Turkish Atomic Energy Authority, Ankara, Turkey, and Department of Chemical Engineering, Engineering Faculty, Ankara UniVersity, 06100 Tandogˇ an, Ankara, Turkey ReceiVed NoVember 16, 2006. ReVised Manuscript ReceiVed April 3, 2007
The effect of the power of ultraviolet (UV) irradiation on the tetrahydrofuran (THF) solubles yield (the total soluble product) and the soluble product distribution of the dissolution of Turkish lignites (Beypazarı and Tunc¸ bilek lignite) in tetralin at ambient temperatures has been investigated. The lignite samples were exposed to UV irradiation for 1, 2, 3, 5, and 10 days in the power of irradiation ranging from 0 to 180 W at 60 W intervals. The yields of THF solubles and oils increased with increasing irradiation power and time. The optimum irradiation power depends on the irradiation time to obtain the highest degradation products. However, the yield of degradation products depends also on the lignite type. The largest fraction obtained from lignites by photochemical energy is oil. While the yields of THF solubles and oils sharply increased with irradiation power at longer reaction times, the yields of asphaltenes (AS) slightly decreased. Increasing oil yields is relatively larger when AS yields tend to decrease. These trends of AS and oil yields are ascribable to conversion of AS to oils at higher power. Small changes were observed in the PAS yields under all conditions.
1. Introduction The study of natural macromolecules which could substitute petroleum as energy and chemical feedstocks has been widely stimulated in the last decades, particularly those involving coal, shale oil, and biomass. Among them, coal has received special attention because of its large proven reserves as well as the similarity between the products obtained by its processing and crude oil.1,2 Coal-derived liquid can be produced by solvolysis of coals/lignites. The mechanism of coal solvolysis is described as the stabilization of thermally formed free radicals by hydrogen.3,4 The concentration of free radicals present in coal increases with increasing temperature and reaches a maximum between 400 and 500 °C.5 Therefore, the highest yields of soluble products were obtained from the dissolution reactions carried out at about 450 °C in the presence of a hydrogen donor solvent and/or under hydrogen atmosphere.6 Because thermally activated solubilization reactions of coals require high temperature and pressure, coal-derived liquid is expensive compared with petroleum. Many dissolution methods of coal, including oxidation, hydrogenation, alkaline hydrolysis, pyrolysis, and extraction with organic solvents, have not been economically established so far. Coals may undergo dissolution photochemically under ambient temperature and pressure, and thereby the production cost of the liquid reduces considerably. Although * Corresponding author. Tel.: +90-815 43 00/2122. Fax: +90 312 815 43 07. E-mail address:
[email protected]. † Turkish Atomic Energy Authority. ‡ Ankara University. (1) Lancas, F. M. J. Radioanal. Nucl. Chem. 1990, 142 (2), 425-431. (2) Lancas, F. M.; Carrilho, E.; Dibo, D. M. P. J. Radioanal. Nucl. Chem. 1992, 158 (2), 283-292. (3) Curran, G. P.; Struck, R. T.; Gorin, E. Prepr. - Am. Chem. Soc., DiV. Pet. Chem. 1966, 11 (2), C-130. (4) Curran, G. P.; Struck, R. T.; Gorin, E. Ind. Eng. Chem. Proc. Des. DeV. 1967, 6 (2), 166. (5) Petrakis, L.; Grandy, D. W. Anal. Chem. 1978, 50, 30. (6) Ceylan, K.; Olcay, A. Fuel 1992, 71, 539-544.
the use of radiation as a source of energy in coal reactions has rapidly increased in recent years, the application of photochemical energy is very rare. These applications are the oxidation of coals,7 acid-catalyzed depolymerization,8 and noncatalytic solvolysis of lignites.9 Microwave energy has been used to energize such reactions as the pyrolysis,10,11 desulfurization,12 dissolution of low rank coals in hydroiodic acid,13 and tetralin.14,15 The dissolution and desulfurization of coal and lignite with γ-irradiation have also been reported.16,17 Dissolution/degradation of Turkish lignites in tetralin at ambient temperature with UV irradiation have also been investigated.9,18 However, there is no study about the effect of irradiation power on the liquefaction of lignites by photochemical energy. Therefore, the objective of this work was to investigate the effect of UV irradiation power on the photochemical dissolution/degradation of lignites in tetralin at ambient temperature and pressure. In this study, the profile of the tetrahydrofuran (THF) solubles yield (the total soluble product) and (7) Hayatsu, R.; Winans, R. E.; Scott, R. G.; Moore, L. P. Fuel 1978, 37, 541. (8) Yu¨ru¨m, Y.; Yigˇinsu, I˙ Fuel 1982, 61, 1138-1140. (9) So¨gˇu¨t, F.; Olcay, A. Fuel Process. Technol. 1998, 55, 107. (10) Mirzai, P. M.; Ravidran, M.; McWhinnie, M. R.; Burchill, P. Fuel 1992, 71, 716-717. (11) Mirzai, P. M.; Ravindran, M.; McWhinnie, M. R.; Burchill, P. Fuel 1995, 74, 20-27. (12) Wang, J.; Jiekang, Y. Fuel 1994, 73, 155-159. (13) Andres, J. M.; Ferrando, A. C.; Ferrer, P. Energy Fuels 1998, 12, 563-569. (14) S¸ ims¸ ek, E. H.; Karaduman, A.; Togˇrul, T. Fuel Process. Technol. 2001, 73, 111-125. (15) S¸ ims¸ ek, E. H.; Karaduman, A.; Togˇrul, T. Energy Sources 2002, 24, 675-684. (16) Ram, L. C.; Tripathi, P. S. M.; Jha, S. K.; Sharma, K. P.; Singh, G.; Mishra, S. P. Fuel Process. Technol. 1997, 53, 1. (17) Tripathi, P. S. M.; Ram, L. C.; Jha, S. K.; Bandopadhyay, A. K.; Murty, G. S. Fuel 1991, 70 (1), 24. (18) Karacan, F.; S¸ ims¸ ek, E. H. Togˇrul, T. Energy Sources 2005, 27, 1523-1533.
10.1021/ef060576v CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007
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Energy & Fuels, Vol. 21, No. 4, 2007 2227
Table 1. Analysis of the Lignite Samples Beypazary´ moisture ash volatile matter fixed carbona C H N S Oa a
Tunc¸ bilek
Proximate Analysis (wt %) 13.00 25.55 29.19 32.26
2.88 49.69 24.85 22.58
Ultimate Analysis (wt % daf) 69.56 4.50 1.25 4.98 19.71
69.89 5.14 2.82 2.02 20.13
By difference.
Figure 2. Changes in THF solubles yield of Beypazarı lignite with dissolution conditions.
Figure 1. Schematics of the irradiation setup.
the soluble product composition with irradiation power have been reported in the various irradiation periods. 2. Experimental Section In the experiments, two lignites obtained from the Beypazarı and Tunc¸ bilek mining basins, which differ considerably in mineral matter, were used. The lignite samples were ground in a porcelain ball mill and sieved to -0.3 mm. The samples were analyzed for the proximate and ultimate analyses using the standard procedures. The results are given in Table 1. Photochemical dissolution of lignite samples was carried out in a 500 mL quartz flask equipped with a magnetic stirrer at UV cabin, which has six high-pressure 30 W mercury lamps (Philips UV-C). The self-designed batch irradiation setup is shown in Figure 1. All irradiated experiments were performed with constant inherent quantum yields of photochemical. The flask was first charged with a mixture of 75 g of tetralin as the solvent and 15 g of ground, air-dried lignite. Then the mixture of lignite-tetralin was exposed to UV irradiation in the desired irradiation power. The irradiation power was changed in the range of 0-180 W by means of regulating the number of lamps in the various periods of irradiation at ambient temperature. In this way, batch irradiation of lignite samples in tetralin was separately performed at varying power of UV radiation. During the reaction, the stirring rate was kept constant. It was expected that the UV light would sufficiently reach the lignite particles-tetralin mixture. Experiments, in the 0 W of irradiation power, were carried out in the dark under identical conditions. It was observed that the maximum temperature of 33 °C was reached and there was no gas formation during liquefaction. After irradiation the lignite samples were taken out of the UV cabin and filtered. The solid residue (char) was exhaustively extracted with THF in a Soxhlet apparatus. THF was then removed by rotary
evaporation from the Soxhlet extract, and the remaining product was combined with the tetralin solution. Tetralin was distilled off at 60-75 °C under vacuum at 100 mmHg. The soluble products were divided into three fractions: hexane solubles (oils), toluene solubles (asphaltenes, AS), and toluene insolubles (preasphaltenes, PAS). After adding 200 mL of hexane to soluble products, it was left overnight and the hexane solubles were separated. Hexane was removed from the oils by a rotary evaporator. Then 200 mL of toluene was added into hexane insoluble products and again left overnight, and toluene solubles were separated by filtration. Evaporation of toluene under vacuum from the fraction gave AS and the insolubles in toluene gave PAS. The percentage yield of the soluble products was calculated on a daf lignite basis and determined as a function of irradiation power. AS and PAS yields were calculated by weighing the dried products. Replicated runs at certain conditions were used to check the reliability of the data. Reproducibility of dissolution results are determined to within (4% (relative).
3. Results and Discussion In the dissolution experiments, irradiation power was changed from 0 to 180 W and the irradiation period (reaction time) was changed from 1 to 10 days. The changes in the yields of THF solubles and in the distribution of soluble products with UV irradiation power in the various periods of irradiation are given in Figures 2 and 3 for Beypazarı lignite, respectively. It is clearly observed from the results that UV irradiation of lignite strongly affects its solubility at the all irradiation times and that the yields of THF solubles and oils is considerably higher as compared to the unirradiated case. As the irradiation power increased from 0 to 60 W, the yield of the THF solubles and oils increased significantly at all irradiation times. Relatively no changes in the formation of THF solubles and oils were observed for 1, 2, 3, and 5 days after 60 W of power, increasing continuously for 10 days. No changes in soluble product formation with power at shorter reaction times indicates that shorter reaction times are insufficient to stabilize or to form free radicals from coal and coal products. Increasing power promotes formation of free radicals from lignite and lignite products and the rate of hydrogen transfer to these radicals at longer reaction times (10 days). Therefore, the higher concentration of free radicals caused forming THF soluble products in high yields. Both longer irradiation times and higher irradiation power were required to obtain the highest yields of soluble products from the dissolution reaction. Although THF solubles and oil yields increased steadily with both irradiation time and power, the yields of AS and PAS first increased with power until 120 W, then after 120 W of power
2228 Energy & Fuels, Vol. 21, No. 4, 2007
Figure 3. Changes in soluble product yields of Beypazarı lignite with dissolution conditions.
Figure 4. Changes in THF solubles yield of Tunc¸ bilek lignite with dissolution conditions.
slightly decreased (Figure 3). Increase of THF solubles and oils and decerase of AS and PAS indicate that AS and PAS contribute to formation of oils at higher power and longer reaction times. The yield of THF solubles and the distribution of soluble products obtained at different reaction times are plotted against irradiation power in Figures 4 and 5 for Tunc¸ bilek lignite, respectively. In the case of irradiation, the yields of THF solubles increased dramatically in all times of irradiation. Further, the yields increased continuously with power for 2, 3, and 5 days while it increased slightly for 1 and 10 days. The solubility with Tunc¸ bilek lignite reached a maximum at the 180 W of irradaition power in the 3 and 5 days of time. However, the maximum solubility with Beypazarı lignite is observed at the 180 W of irradiation power in the 10 days of time (Figure 2). These indicate that the yields of THF solubles and their variations with irradiation power and time obtained from Tunc¸ bilek lignite were different from those of Beypazarı lignite. In addition, the yields of THF solubles obtained from Tunc¸ bilek lignite were higher than that of Beypazarı lignite as was the case in a previous study.9 But the yields of THF solubles obtained from Beypazarı
Karacan and Togˇ rul
Figure 5. Changes in soluble product yields of Tunc¸ bilek lignite with dissolution conditions.
lignite were comparable to those obtained from Tunc¸ blek lignite in the thermal degradation experiments.6 As shown in Table 1, although the carbon contents of the lignites are similar, their sulfur content and proximate analyses are distinctly different in some respects. The differences in the elemental composition and ash contents of the lignites are the main reasons affecting their dissolution behavior.6,19 Several previous studies have shown that mineral matter acts catalytically in thermal coal dissolution and hydrogen transfer reactions.20-22 In this study, because the mineral matter of Tunc¸ bilek lignite is higher than that of Beypazarı, the higher THF solubles yields from Tunc¸ bilek are obtained under identical reaction conditions. This can be ascribed to the fact that mineral matter inherent in lignites plays its role as a promoter for photochemical dissolution of lignite following the generation of free radicals in the coal structure and the stabilization of these free radicals by hydrogen. If the higher mineral matter exists in lignite it possibly causes the higher photochemically generated hydrogen atoms and free radicals by direct excitation of coal particles and attack of coal matrix by excitation of mineral matter, which presumably act as energy-transfer agents or as semiconductor agents. Sulfur compounds (especially pyrite) in coal also closely affect the dissolution yield and soluble product compositions.23,24 On the basis of the data from various coals, an approximately linear relation has been reported between the sulfur content of coals and their conversion in thermal liquefaction.25 But the higher conversion from Beypazarı lignite was not obtained although the sulfur content of Beypazarı lignite was higher than those of Tunc¸ bilek lignite in this study. Similar results have also been reported by So¨gˇu¨t and Olcay.9 They stated that this case might be explained by the different catalytic effects of (19) Gu¨ru¨z, G.; Olcay, A.; Yu¨ru¨m, Y.; Bac¸ , N.; Orbey, H.; Togˇrul, T.; S¸ enelt, A. Project no TBAG-575/B Tubitak, Ankara, 1987. (20) Mukherjee, D. K.; Mitra, J. R. Fuel 1984, 63, 722-723. (21) Guin, J. A.; Tarrer, A. R.; Prather, J. W.; Jhonson, D. R.; Lee, J. M. Ind. Eng. Chem. Proc. Des. DeV. 1978, 17 (2), 118. (22) Mukherjee, D. K.; Chowdhury, P. B. Fuel 1976, 55, 4-13. (23) Baldwin, R. M.; Vinciquerra, S. Fuel 1983, 62, 498. (24) Trewhella, M. J.; Guin, A. Fuel 1987, 66, 1315. (25) Abdel-Baset, M. B.; Yarzab, R. F.; Given, P. H. Fuel 1978, 57, 89.
Photochemical Dissolution of Turkish Lignites in Tetralin
sulfur compounds in UV irradiated and thermal dissolution processes. The data also obtained in this work support their suggestions. In addition, it must be emphasized here that the dissolution behavior of two lignites also depends on the intrinsic reactivities of lignites due to difference in the composition of organic matter. Tunc¸ bilek coal has more hydrogen than Beypazarı lignite (5.14% instead of 4.50%). This is a reason for higher intrinsic reactivity of organic matter of Tunc¸ bilek coal as compared to Beypazary´ coal. On the basis of these observations, it implies that some mineral compounds except for sulfur components and the intrinsic reactivity of organic matter of lignites play a promoter role in the photochemical dissolution reactions. Figure 5 indicates that the trends of oil, AS, and PAS yields from Tunc¸ bilek lignite are essentially similar to those of Beypazarı lignite. The highest increase was observed in oil yield. Increase in AS and PAS yield were relatively low. The yields of AS first increased with power until 120 W at longer reaction times and then decreased. Oil yields increase with power, and this increment is appreciably larger when the AS yields tend to decrease. These observations indicate that higher power promotes conversion of AS to oils at longer reaction times. S¸ ims¸ ek et al.15 reported similar findings in the liquefaction of the same lignites in tetralin with microwave radiaton energy at longer reaction times. The high-temperature dissolution of lignites in tetralin was different from their UV assisted dissolution reaction. While the largest fraction of thermal dissolution6,26 was the AS and the PAS, in UV assisted reactions8,9 the largest fraction was the oil fractions of the soluble products. Similar results were obtain in the microwave heated dissolution.14,15 It was concluded that radiation energy such as UV irradiation and microwave energy is more effective than thermal energy in the dissolution of lignites with respect to producing a lighter fraction. Photochemical dissolution of coal is still a new and rarely discussed problem. It is difficult to explain the mechanism of interaction of coal with UV radiation without some speculation. The photochemistry of molecules is the chemistry of excited states. A molecule can be promoted from the ground electronic state to an electronically excited state by the absorption of a quantum of light. By controlling the frequency of light used, it is possible to excite selectively the electrons associated with specific groups of atoms in a molecule without significantly altering the energy levels of other electrons. In the UV dissolution process, formation of coal free radicals is induced photochemically. These free radicals are stabilized by sufficient hydrogen donating ability forming an oil fraction of soluble products in high yields. When irradiation is applied to coal/ lignite the polymeric nature of coal, bridges have to be broken to obtain fragments of an appropriate size to be solubilized by organic solvents. Because of above reason, high-temperature dissolution and an assisted reaction process are different. The mechanism of reaction of high temperature and UV dissolution is very complex resulting in the formation of numerous compounds. Therefore, the approach in kinetics studies has been to attempt to separate these compounds into groups of similar character. Coal dissolution experiments conducted in an autoclave by thermal heating require relatively long periods of heating-up and cooling-down periods during which significant secondary reactions can occur. These factors might have effects on the order of the yields of oils, AS, and PAS during the dissolution process of lignite by thermal heating in tetralin. The general mechanism of the thermal dissolution (26) Karaca, H.; Ceylan, K.; Olcay, A. Fuel 2001, 80, 559.
Energy & Fuels, Vol. 21, No. 4, 2007 2229
process was different UV irradiated dissolution. However, as stated by earlier researchers6,14,27-30 about not only thermal dissolution but also dissolution with microwave heating, it is not possible to give a general mechanism about the formation of the soluble coal products during the UV-irradiated dissolution process. The effect of irradiation power on the dissolution behavior differed from one lignite to another. Increasing power strongly affects the yields of THF solubles and oils with Beypazarı lignite at longer reaction times. But the effect of increasing power on the yields of THF solubles and oils obtained from Tunc¸ bilek lignite is appreciable at relatively shorter reaction times (Figures 4 and 5). The reaction period to reach the maximum solubilization depends on the lignite type. These results are in accordance with the earlier reports,9,14 stating that the rate of radical formation was different for different coals in the process of liquefaction with radiation energy. In this study, the variations in the product yield obtained from the two Turkish lignite support this idea. For Tunc¸ bilek lignite, the remaining constants of the solubization with power at shorter reaction times (1 days) and longer reaction times (10 days) indicate that recombination of the radicals of lignite and lignite products, which are formed by the effect of UV light to insolubles products, occurred because of an insufficient amount of H-donor solvent at 10 days and 1 day is an insufficient reaction time to stabilize the radicals. The yields of THF solubles obtained from photochemical dissolution are comparable to those obtained from thermal dissolution experiments carried out at 350 °C in tetralin at the solvent/coal ratio of 5:1 under hydrogen or nitrogen atmospheres for 30 min6 and at 375 °C in tetralin under nitrogen atmospheres for 60 min.26 This conclusion supports an earlier suggestion9 that considerable amounts of coal radicals were formed by UV irradiation under ambient conditions to generate soluble products. 4. Conclusions The data obtained in this study indicate that, for the two lignites used, their dissolution generally increased with power and time of the irradiation. However, the yields of soluble products and their variations with irradiation power obtained at different times for the Beypazarı lignite were different from those of Tunc¸ bilek lignite. While the highest yields of THF solubles were obtained at 10 days of irradiation time for Beypazarı lignite, they were obtained at 3 days of irradiation time for Tunc¸ bilek lignite. The yields of AS and PAS slightly changed with power under all conditions while the yields of THF solubles and oils significantly increased with power and time of the irradiation. It has been determined that 30% of Beypazarı lignite and 50% of Tunc¸ bilek lignites can be dissolved at ambient temperature with the aid of UV light. The photochemical dissolution offers an alternative method for milder degradation of coals, from which direct or indirect information about the structure of coal can be deduced and is also helpful in providing a guideline for selection of coal for solubilization processes. Acknowledgment. The authors would like to thank the Scientific Research Projects of Ankara University for financial support under Project No. 2002.07.45.005. EF060576V (27) S¸ ims¸ ek, E. H.; Karaduman, A.; Togˇrul, T. Fuel 2001, 80, 21812188. (28) Youthceff, J. S.; Given, P. H. Fuel 1982, 61, 980. (29) Philip, C. V.; Anthony, R. G. Fuel 1982, 61, 351. (30) Berkowitz, N.; Calderon, J.; Liron, A. Fuel 1988, 67, 626.
