Energy Fuels 2010, 24, 6393–6399 Published on Web 11/18/2010
: DOI:10.1021/ef101055f
Simulation of Solvent Extraction of South African Vitrinite- and Inertinite-Rich Coals Daniel Van Niekerk* and Jonathan P. Mathews Energy and Mineral Engineering and the Earth and Mineral Sciences (EMS) Energy Institute, The Pennsylvania State University, 126 Hosler Building, University Park, Pennsylvania 16802, United States Received August 11, 2010. Revised Manuscript Received November 5, 2010
The application of molecular simulation and visualization approaches to solvent extraction was evaluated with large-scale molecular models (>14 000 atoms) of vitrinite- and inertinite-rich coals. A theoretical extraction yield was determined for the proposed molecular models using a molecule-specific solubility parameters approach. The theoretical extraction yields for these models agreed with the experimental trends. While this novel solubility estimation method did not predict the exact extraction yield of these models, there was agreement with extraction trends. Residue and extracted models were generated from the large-scale molecular coal models using solubility parameters and showed agreement with laser desorption-ionization time-of-flight mass spectrometry (LDTOF-MS) data. Thus, solvent extraction trends can be used to test the applicability of large-scale coal molecular models.
extraction yields.1 The extraction yield can also be influenced by additives (e.g., addition of amines),9 pretreatment (e.g., water treatment or pre-swelling),10,11 and the extraction method (flow reactor versus batch reactor).7,8 Various studies, taking into account the polymeric nature of coal, have investigated the use of Hildebrand’s solubility parameters (δ) for the solubilization of bituminous coal.12-14 For solubility, it is necessary that the difference between the solubility parameters of the coal and solvent is small (Δδ = δcoal - δsolvent).14,15 Two methods can be employed to calculate solubility parameters for coal: experimental swelling measurements or calculation of the group contributions (functional groups, e.g., -CH2-, -OH, -O-, etc.).16 The methods proposed by van Krevelen, Hoy, or Small can be used to determine the solubility parameters from group
Introduction Coal-to-liquids is a re-emerging technology that can provide alternatives to oil- and natural-gas-derived fuels and specialty chemicals. Solvent extraction at temperatures below the pyrolysis temperature (approximately 350 °C, depending upon the coal) and low pressures is one approach to obtain liquids directly from coal. The yield of solvent extraction is dependent upon various parameters: coal rank, maceral composition, solvent(s), and temperature. Iino et al. studied the extraction yields of various coals (from lignite to anthracite) using binary solvent carbon-disulfide/N-methylpyrrolidone (CS2/NMP, 1:1 by volume) at room temperature.1 Rank was shown to be important, with maximum extraction yields being obtained for bituminous coals [between 85 and 88% carbon, on a dry and ash-free basis (daf)]. Anthracite, subbituminous, and lignite coals did not generate high extraction yields.1 Maceral composition also has a significant impact on solvent extraction yields. Generally, there is a correlation between vitrinite content and extraction yields: the greater the vitrinite content, the higher the extraction yield.2-4 Various solvents (pyridine, CS2/NMP, dimethylnaphthalene, tetralin, etc.) have been evaluated for extraction of bituminous coals.1,5-8 In general, binary solvents, such as CS2/NMP, generate better
(6) Miura, K. Mild conversion of coal for producing valuable chemicals. Fuel Process. Technol. 2000, 62, 119–135. (7) Miura, K.; Nakagawa, H.; Ashida, R.; Ihara, T. Production of clean fuels by solvent skimming of coal at around 350 °C. Fuel 2004, 83, 733–738. (8) Miura, K.; Shimada, M.; Mae, K.; Sock, H. Y. Extraction of coal below 350 °C in flowing non-polar solvent. Fuel 2001, 80 (11), 1573– 1582. (9) Giray, E. S. V.; Chen, C.; Takanohashi, T.; Iino, M. Increase of the extraction yields of coals by the addition of aromatic amines. Fuel 2000, 79, 1533–1538. (10) Iino, M.; Takanohashi, T.; Li, C. Increase in extraction yields of coals by water treatment. Energy Fuels 2004, 18, 1414–1418. (11) Shui, H.; Wang, Z.; Cao, M. Effect of pre-swelling of coal on its solvent extraction and liquefaction properties. Fuel 2008, 87, 2908–2913. (12) Larsen, J. W.; Green, T. K.; Kovac, J. The nature of the macromolecular new structure of bituminous coals. J. Org. Chem. 1985, 50, 4729–4735. (13) Painter, C. P. Coal solubility and swelling. 2. Effect of hydrogen bonding on calculations of molecular weight from swelling measurements. Energy Fuels 1990, 4, 384–393. (14) Hombach, H.-P. General aspects of coal solubility. Fuel 1980, 7 (59), 465. (15) van Krevelen, D. W. Coal: Typology, Chemistry, Physics, Constitution, 3ed.; Elsevier Scientific Publishing Company: Amsterdam, The Netherlands, 1993. (16) Painter, C. P.; Graf, J.; Coleman, M. M. Coal solubility and swelling. 1. Solubility parameters for coal and the Flory χ parameter. Energy Fuels 1990, 4, 379–384.
*To whom correspondence should be addressed. E-mail: dxv910@ gmail.com. (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Extraction of coals with CS2-N-methyl-2-pyrrolidinone mixed solvent at room temperature: Effect of coal rank and synergism of the mixed solvent. Fuel 1988, 67, 1639–1647. (2) Takanohashi, T.; Ohkawa, T.; Yanagida, T.; Iino, M. Effect of maceral composition on the extraction of bituminous with carbon disulphide-N-methyl-2-pyrrolidinone mixed solvent at room temperature. Fuel 1993, 72, 51–55. (3) Dyrkacz, G. R.; Bloomquist, C. A. A. Solvent extraction of separated macerals in carbon disulfide/N-methylpyrrolidone. Energy Fuels 2001, 15, 1403–1408. (4) Keogh, R. A.; Taulbee, D.; Hower, J. C.; Chawla, B.; Davis, B. H. Liquefaction characteristics of the three major maceral groups separated from a single coal. Energy Fuels 1992, 6, 614–618. (5) Marzec, A. Towards an understanding of the coal structure: A review. Fuel Process. Technol. 2002, 77-78, 25–32. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 6393–6399
: DOI:10.1021/ef101055f
Van Niekerk and Mathews
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contributions. However, functional group distributions of coal are not always known, and therefore, the group contribution method is not always applicable. Painter et al. developed a method to determine the solubility parameters using aromaticity and elemental composition data.16 This method was developed from a data set of 255 non-hydrogen bonding organic liquids using a matrix approach. Solubility parameters calculated for coals using this method were in the same range as predicted by van Krevelen’s plot of δ versus the carbon content of coal.16 Even though group contribution methods are flawed when applied to coal, they at least appear to give values in the right range and, thus, are useful in providing an initial estimate for solubility parameters.16 Molecular modeling is a tool that can be used to investigate and probe the structure and behavior of coal.19 Molecular modeling has been used to study the solvent-coal interaction in coals, in particular solvent swelling.20-23 The interactions between the coal structure and water have also been modeled by various groups.24,25 To date, no information is available regarding the prediction of the solvent extraction of coal using molecular modeling. The goal of this study was to determine if previous constructed large-scale vitrinite- and inertinite-rich coal models26 can be used to predict the solvent extraction yield for two solvents (pyridine and NMP). These vitriniteand inertinite-rich coal models were previously constructed using nuclear magnetic resonance (NMR) spectroscopy, highresolution transmission electron microscopy (HRTEM), and laser desorption-ionization time-of-flight mass spectrometry (LDTOF-MS).26,27
(ASTM D 2798-06) and microscopic determination of the maceral composition (ASTM D 2799-05a)29 were conducted. Demineralization was performed on -200 mesh (U.S. standard sieve) coal; the proximate ash percentage was reduced from 28 to 2% in Highveld coal and from 8 to
