Energy & Fuels 1996, 10, 709-717
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Coal Conversion with Selected Model Compounds under Noncatalytic, Low Solvent:Coal Ratio Conditions Jasna Tomic´† and Harold H. Schobert* Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received July 10, 1995. Revised Manuscript Received January 18, 1996X
The interaction between coal and model compounds at relatively mild conditions was investigated to explore effects of coal, solvent, and reaction conditions at low coal conversions. Five coals were reacted with five model compound solvents (eicosane, 1-phenyldodecane, 1,4diisopropylbenzene, 1,2,4,5-tetramethylbenzene, and pyrene) at 350, 400, and 450 °C. The coals were subbituminous and high-volatile bituminous rank and were selected to have very low concentrations of pyritic sulfur, to minimize catalytic effects of mineral matter. The effects of temperature, gas atmosphere (N2 or H2), and solvent on conversion to THF-solubles were investigated. Catalysts were not used. Conversions were influenced by temperature, which directly affected the coal and the solvent behavior. The optimum temperature for conversion appears to be 400 °C. Conversions were lowest at 350 °C, and, in some cases, were lower at 450 °C than at 400 °C. The thermal decomposition behavior of the coal is more important in determining total yields of THF-solubles than is the dissolving power of the solvent. Hydrogen improved conversion in the absence of a solvent, but the role of H2 was not so pronounced when a solvent was used. The most effective solvent for enhancing coal conversion was pyrene. Vapor/ liquid partitioning of the solvents was not a major factor in affecting conversions. The thermoplastic properties of the coals are related to the extent of coal conversion. Under our conditions, caking coals reacted in pyrene in H2 give the highest conversions. The maximum conversion achieved was 43 wt % which indicates certain limits in coal conversions under the given conditions.
Introduction Use of a liquid vehicle simplifies technological problems associated with moving and handling solid coal particles. When coals are processed in organic vehicles at elevated temperatures, and often in the presence of reactive atmospheres (as in direct liquefaction and coalpetroleum coprocessing), the vehicle often becomes another reactant in the complex, three-phase chemical system and may participate extensively in the chemistry as the coal is processed. Coal-petroleum coprocessing uses a petroleum residuum or other heavy petroleum product as solvent, in once-through operation. Direct liquefaction of coals would likely employ a process-derived liquid, recycled through the process, as the solvent. In either case, the solvent will contain dozens of individual components, some of which are of ill-defined structure. Use of welldefined solvents or pure compounds is valuable for acquiring more information on the relationship between solvent structure and coal conversion. Studies with model solvents lead to a better understanding of the reaction chemistry between the solvent, the coal particles, and the gas. The literature on coal-solvent interactions is large. Useful reviews of the field have been published by, among others, Dryden,1 Bockrath,2 Pullen,3 Berkowitz,4 † Current address: Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Dryden, I. G. C. In Chemistry of Coal Utilization. Supplementary Volume; Lowry, H. H., Ed.; Wiley: New York, 1963.
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and van Krevelen.5 A number of studies involving reactions of coal with model compound solvents have been conducted with the primary aim of investigating the influence of different types of solvents on coal conversion. Curran and co-workers6 postulated the transfer of H from a solvent to coal via a free-radical mechanism and indicated that the composition of the different solvents did not seem to affect the ultimate conversion.6 Petrakis and Grandy showed virtually no change in free-radical concentration at 400 °C, relative to that in the unreacted coal/tetralin slurry, but a dramatic increase in radical concentration was observed with further heating to 460 °C.7 In their work, the presence or absence of H2 seemed to have little effect on radical concentration. Neavel used H-donor and nondonor solvents for liquefaction of coals ranging in rank from lignite through low-volatile bituminous.8 At short residence times the conversions were similar regardless of the solvent type, but at longer residence times conversions with nondonor solvents decreased, (2) Bockrath, B. C. In Coal Science; Gorbaty,M. L.; Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1983; Vol. 2, pp 65124. (3) Pullen, J. R. In Coal Science Vol II; Gorbaty, M. L.; Larsen,J. W., Wender, I., Eds.; Academic Press: New York, 1983; Vol. 2, pp 174278. (4) Berkowitz, N. The Chemistry of Coal; Elsevier: Amsterdam, 1985. (5) van Krevelen, D. W. Coal: Typology-Physics-Chemistry-Constitution; Elsevier: Amsterdam, 1993. (6) Curran, G. P.; Struck, R. T.; Gori, E. Ind. Eng. Chem. Process Design Dev. 1967, 6, 166-173. (7) Petrakis, L.; Grandy, D. W. Free Radicals in Coals and Synthetic Fuels; Elsevier: Amsterdam, 1983. (8) Neavel, R. C. Fuel 1976, 55, 237-242.
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suggesting that systems low in H donors will be deleterious to overall yields. Polycondensed aromatic compounds are considered to be nondonors, but conversion in their presence has been found to be relatively high. A comparison of the effect of pyrene on coal conversion with other solvents, such as tetralin and methylnaphthalene, showed higher coal conversion with pyrene.9 Liquefaction with mixtures of tetralin and pyrene was conducted under H2 and Ar.10 Conversion under H2 increased by 30%, indicating that interactions among pyrene, tetralin, and H2 induce pyrene hydrogenation and ultimately improve H transfer to coal. In a comparative study of H-donor model compounds and their aromatic analogs, Curtis et al. evaluated the ability of solvents to convert coal to THF-soluble products.11 Conversions varied even at the same level of donatable H, indicating that the H-donor content of the solvent is not the only parameter defining solvent effectiveness. The conversion in their work with aromatic compounds was lower (≈20%) than with the hydroaromatic analogs (40-70%). Snape and co-workers showed that good physical contact of the solvent with the coal is essential to minimize retrogressive reactions.12 This is especially important for low-rank coals. Snape and his colleagues also correlated overall or primary conversion with the nature of the solvents and their donor properties and showed that overall conversion is an indicative parameter when the effect of different solvents is examined.13 The reactions of five coals with five model compounds in the present study were conducted to investigate conversions in a variety of solvents and to investigate the role of reaction conditionssincluding solventssin terms of promoting or inhibiting coal conversion in noncatalytic reactions. The compounds were selected to represent molecular structural features that could be found in petroleum residua, but not necessarily be actual components of residua. This work was done primarily to obtain information on the effects of coal/ solvent/atmosphere combinations under relatively mild conditions. The coals were selected from the subbituminous and high-volatile bituminous ranks, since coals of these ranks are candidates for eventual commercial coprocessing or direct liquefaction. To minimize effects other than rank, we selected coals of low pyritic sulfur contents (90%. Because we had a general interest in investigating reactions under mild conditions, other than temperature,14,15 we conducted experiments without added catalyst and at low solvent: coal ratios, and with nondonor solvents. Many of the (9) Derbyshire, F. J.; Whitehurst, D. D. Fuel 1981, 60, 655-662. (10) Derbyshire, F. J.; Varghese, P.; Whitehurst, D. D. Fuel 1982, 61, 859-864. (11) Curtis, C. W.; Guin, J. A.; Kwon, K. C. Fuel 1984, 63, 14041409. (12) Snape, C. E.; Derbyshire, F. J.; Stephens, H. P.; Kottenstette, R. J.; Smith, N. W. In Coal Characterization of Conversion Processes; Prins, W., Nater, K. A., Chermin, H. A. G., Moulijn, J. A., Eds.; Elsevier: Amsterdam, 1990; pp 119-126. (13) Snape, C. E.; Derbyshire, F. J.; Stephens, H. P.; Kottenstett, R. J.; Smith, N. W. In Coal Science II; Schobert, H. H., Bartle, K. D., Lynch, L. J., Eds.; American Chemical Society: Washington, DC, 1991; Vol. 461, pp 182-192. (14) Tomic´, J.; Schobert, H. H. Fuel. Process. Technol. 1993, 34, 295312. (15) Tomic´, J. Ph.D. Dissertation Thesis, The Pennsylvania State University, 1993.
Tomic´ and Schobert Table 1. Analyses of the Coals coal seam state ASTM rank moist,a wt % ash,b wt % % Cc %H %N %O % Stotal FSI max fluid T, °C a
PSOC 1488
PSOC 1498
PSOC 1501
PSOC 1504
PSOC 1448
Dietz
Wadge
Juanita C Upper York Sunnyside Canyon Montana Colorado Colorado Utah New Mexico subB hvCb hvBb hvAb hvAb
23.66
9.45
5.80
3.38
1.45
5.35
7.10
5.55
7.54
11.42
76.00 5.23 0.94 17.31 0.53 0.0 n/a
77.52 5.45 1.81 14.66 0.56 0.5 n/a
80.49 5.27 1.55 12.02 0.68 2.0 421
81.96 5.80 1.75 9.66 0.83 5.5 433
84.08 6.04 1.84 7.52 0.52 8.0 438
As received. b Dry basis. c Dry ash-free basis. Table 2. Model Compounds and Their Properties name
structure
eicosane 1-phenyldodecane
C20H42
H/C
fa
2.10 0.0 1.67 0.33 C12H25
1.50 0.50
1,4-diisopropylbenzene (CH3)2HC
1,2,4,5-tetramethylbenzene
CH(CH3)2
H3C
CH3
H3C
CH3
pyrene
1.40 0.60
0.63 1.00
experiments were conducted with a N2 atmosphere rather than H2. This work is an exploration of the lower end of the range of possible coal conversions. In subsequent papers, we will build on these “low-end” results to demonstrate effects of using process-derived solvents, catalysts, and optimized reaction conditions, as well as to further explore factors affecting retrogressive reactions. Experimental Section Selection of Materials. Coal samples used in this work were selected from the Penn State Coal Sample Bank and Data Base. They ranged in rank from subbituminous B to highvolatile A bituminous. Their origins and analyses are given in Table 1. The coals were ground to 150 µm (-100 mesh) and stored under Ar. Prior to reaction, they were dried under vacuum at 107 °C to 1% moisture. Five compounds were selected as representative of structural types that can be found in the petroleum residua. They range from a straight-chain alkane (eicosane), through alkylated aromatics (1-phenyldodecane, 1,4-diisopropylbenzene, 1,2,4,5-tetramethylbenzene), to fully aromatic (pyrene). They were chosen on the basis of aromaticities and H/C ratios to provide reasonably wide ranges of both properties. Table 2 gives the structures of the model compounds and their H/C ratios and aromaticities (fa). The compounds were obtained from Aldrich and used as-received. Reaction Procedure and Product Workup. The coals were reacted with the five model compounds under various conditions. Experiments were performed in vertical microautoclave reactors (tubing bombs) of nominal 22 mL capacity, constructed of type 316 3/4 in. stainless steel tubing. More complete details of these reactors are published elsewhere.15 Prior to reaction the microautoclaves were pressurized to 7
Coal Conversion with Selected Model Compounds MPa with N2 and tested for leaks. The reactors were heated in a fluidized sand bath with an oscillating mechanism for agitation. The sand bath temperature during the reaction was measured by a thermocouple in close proximity of the reactors. Each experiment was conducted with 2.5 g of coal and 5 g of model compound. The sealed reactor was purged with N2 three times to remove any captured air. Subsequently, the reactors were pressurized to 3.5 MPa (at room temperature) with H2 or N2 and were immersed in a preheated fluidized sand bath. The reaction temperatures used were 350, 400, and 450 °C. The reaction time was fixed at 30 min. At the end of the experiment, the microautoclave was rapidly quenched in a water bath. The microautoclave was disassembled and the contents were transferred to a preweighed ceramic thimble. The reactor was washed with THF, separating the products into THF-soluble and THF-insoluble fractions. The THF-insoluble matter in the thimble was further extracted using conventional Soxhlet apparatus with approximately 250 mL of THF under N2 until the solvent appeared colorless. The THF was removed from the soluble portion by rotary evaporation. Some THF remained in the solids even after vacuum drying. The solids were therefore rinsed with acetone and pentane successively to remove residual THF. Both soluble and insoluble portions were dried under vacuum for 12 h before being weighed. The final weight of the THFinsoluble matter was used to determine the overall coal conversion on a dry, ash-free (daf) basis. Several selected experiments were done in triplicates and the experimental error was determined to be (2.5 wt %. The conversion is defined as
conversion (wt %) ) [(wt coal (dry) wt THF-insoluble (dry))/wt coal (daf)] × 100% Baseline tests were conducted to determine whether insoluble matter was produced by the model compounds alone. In this case 5 g of model compound was reacted at the same temperatures as described above. In these experiments the amount of THF-insolubles from 5 g of model compound was e0.01 g; consequently, we neglected any contribution from the model compound to the total weight of insolubles. Product workup was identical for all cases. Similarly, the coals were reacted alone (i.e., in the absence of a solvent) under the same reaction conditions. Elemental analysis of selected THFinsolubles was performed using a Leco Model CHN-600 instrument.
Results and Discussion Effect of Temperature on Conversion. All of the 25 possible coal/solvent combinations were reacted at 350, 400, and 450 °C. To investigate the influence of temperature on conversion, the experiments were conducted under N2. Table 3 provides the results for conversion in N2 for the five coals without a solvent, and in combination with the solvents. The conversions vary depending on the coalsmodel compound combination and the temperature. The coal “blank” runs (no solvent present in the system) show the influence of temperature on conversion (Table 3). In general, conversions are all
