Energy & Fuels 2001, 15, 1409-1413
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Binary Solvent Extractions of Upper Freeport Coal† Gary R. Dyrkacz* and C. A. A. Bloomquist Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received February 13, 2001
Different organic bases, primarily with carbon disulfide as co-solvent, were used to probe the nature of the binary solvent extractions of Upper Freeport coal. The effect of tetracyanoethylene addition on extract yields was also examined. Amides and cyclic ureas where found to all be good extractants, but were not as efficient as N-methylpyrrolidone(I), with one exception. CS2/ N-methylpyridone was found to be a better solvent than CS2/(I). The results suggest that solvents which have similar basicities, but have a more flattened shape, such as aromatic rings compared to saturated rings, are better extraction agents.
Introduction There has been steady interest in the binary solvent extractions of coals. One of the combinations that has received recent attention is the use of carbon disulfide and N-methylpyrrolidone (CSNP). This solvent system was championed by Iino and co-workers.1 Subsequent work has shown this system to be a very effective solvent combination for extracting many coals.2-8 Further refinements to this media by the addition of a third component have led to even higher extraction yields in some cases. One of the most effective additives is tetracyanoethylene (TCNE).2,3,9 However, it now appears that TCNE is rapidly converted to the pentacyanopropenide anion in NMP and this is the species that enhances the extraction.9-11 Despite the amount of work done with both the binary and ternary systems, very little is known about why CSNP solvent is so effective for some coals. Iino and coworkers initially suggested that the role of the carbon disulfide was to reduce the viscosity of the N-methylpyrrolidone (NMP), permitting a higher diffusion rate.1 Painter et al. suggested that the role of the solvent * Corresponding author. † This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract number W-31-109-ENG-38. (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639-1647. (2) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579-580. (3) Liu, H.-T.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108-1111. (4) Takanohashi, T.; Iino, M. Energy Fuels 1991, 5, 708-711. (5) Takanohashi, T.; Iino, M. Energy Fuels 1996, 10, 1128-1132. (6) Takanohashi, T.; Ohkawa, T.; Yanagida, T.; Iino, M. Fuel 1993, 72, 51-55. (7) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588-1593. (8) Cody, G. D.; Obeng, M.; Thyagarajan, P. Energy Fuels 1997, 11, 495-501. (9) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, in press. (10) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 11801183. (11) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2000, 14, 513-514.
was as a diluent to lower the aggregation state of the NMP.12 This would provide a higher proportion of NMP monomers, which in turn would decrease the time for extraction. Aida et al. discussing swelling in binary solvents, suggested that a co-solvent “wedge effect” may explain some of the swelling behavior of coals in binary solvents. In this case, a small co-solvent molecule may penetrate more rapidly into the structure and overcome some coal-coal interactions, allowing the larger solvent partner to enter and break the stronger coal-coal interactions.13 Such a mechanism might also apply to extractions by CSNP. As a foundation for understanding the extraction mechanisms in multicomponent solvent systems, we examined the extraction of Upper Freeport coal (APCS1) from the Argonne Premium Coal Sample Program with a range of extractants and co-solvents. Iino and co-workers have already provided some information on different solvent combinations, but the data in some cases were sketchy. Furthermore, the large change in extract yields with a small addition of a third component is intriguing. We also wished to explore the relationship of TCNE addition to the solvent system. Our results show that carbon disulfide is indeed the solvent of choice for these binary solvent extractions, especially with a small amount of TCNE present. However, although CSNP is one of the best solvents, the related solvent, CS2/N-methylpyridone, is even better at extracting Upper Freeport coal. Moreover, the maximum extract yield with this solvent is equivalent to what can be extracted in ternary extractions. Tetracyanoethylene is ineffective at further increasing the extract yields with N-methylpryidone. Experimental Section Coal. Upper Freeport coal (APCS-1) from the Argonne Premium Coal Sample Program was ground under nitrogen (12) Painter, P. C.; Sobkowiak, M.; Gamble, V. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43 (4), (Aug 22-27), 913-916. (13) Aida, T.; Suzuki, E.; Yammanishi, I.; Sakai, M. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1999, 44 (3), (Aug 22-26), 623628.
10.1021/ef010036s CCC: $20.00 © 2001 American Chemical Society Published on Web 10/10/2001
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Energy & Fuels, Vol. 15, No. 6, 2001
using a fluid energy mill, and chemically demineralized using standard methods. The top size of the coal was 10 µm. The details of the pretreatment methods have been published.14 The elemental analysis (daf) of the coal after demineralization was: C, 87.41%; H, 5.23%; N, 0.82%; S + O, 6.54%; Ash 2.55%. Solvents. All solvents were reagent or spectrophotometric grade (Aldrich) and were used without further purification. Primary and secondary amines were not used in this study because they can react vigorously with CS2, resulting in very rapid heating. Extractions. A 200 mg amount of fluid energy mill ground, and chemically demineralized APCS 1 coal was placed into stainless steel centrifuge tubes. To each tube was added either 20 mL of the binary solvent, or17 mL of the binary solvent, and 3 mL of the binary solvent with approximately 9.4 mg of tetracyanoethylene dissolved in it. The TCNE had to be added without delay to the solutions.11 All the extractions were at room temperature, between 22 °C and 25 °C. Each tube was capped with a layer of plastic wrap and a large rubber septum to minimize moisture and oxygen access. The tubes were allowed to sit for at least 24 h with stirring every half hour for the first 6 h. Recovery of the solid residue consisted of the centrifuge tubes being centrifuged at RCFmax ) 72 000g for 1 h. The solvent was decanted, and the remaining CSNP/TCNE solvent removed by at least four cycles of 20 mL acetone addition, centrifugation, and decantation. Experiments were generally run in pairs, one tube with and one tube without TCNE.
Results Finely ground and demineralized Upper Freeport coal (APCS 1) from the Argonne Premium Coal Sample Program was used for all the extractions. The choice of this coal is based on the findings of Iino et al. that it is one of a small group of coals that are highly extractable in CSNP, but have a very low solubility in CS2.1 Thus, APCS 1 has a particularly broad extraction range, which translates to a high sensitivity to extraction conditions. In most of the binary solvent extractions, we used a single solvent composition ratio of 1:1 by volume. From the work of Iino et al., this is the solvent ratio where CSNP maximally extracts many coals.1,14 Our extraction procedures are similar to that reported by Takanohashi et al.4 However, because only 200 mg amounts of coal were used in each extraction, accurate quantitative results depended on minimizing material losses. Solvents such as carbon disulfide and especially NMP are quite aggressive at dissolving many polymers. Therefore, most filter membranes could not be used to recover the very fine (
