Energy & Fuels 1998, 12, 891-896
891
Sorption Behaviors of Various Organic Vapors to Argonne Premium Coal Samples Kazuhiko Shimizu, Toshimasa Takanohashi,* and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received December 17, 1997. Revised Manuscript Received May 27, 1998
Sorption of various organic vapors by Argonne Premium coals (APCS-1, 3, 5, and 8) was investigated to clarify the coal-organic interaction, sorption mechanism, and micropore and crosslinking structure of coals. Especially, the sorption at low vapor pressures could be measured under a temperature- and pressure-controlled gravimetric sorption system. Methanol was quickly sorbed by all the coals used. In contrast, pyridine was slowly sorbed by Beulah-Zap lignite coal, and a significant portion of pyridine was retained after desorption. The quantity of methanol sorption at saturation vapor pressure (P/P0) at 30 °C showed a good correlation with oxygen % (daf) in the raw coals, indicating that methanol is sorbed by the coals through interaction with oxygen functional sites. For high-rank Pocahontas No. 3 and Upper Freeport coals, the methanol isotherm could be explained by the both Langmuir-Henry and Dubinin-Radushkevitch-Henry dual-mode sorption equations, while for the other coals the Langmuir-Henry equation was better for the approximation. For Illinois No. 6 coal, at relative vapor pressures of 0.01-0.7, the sorption could be treated by the Langmuir-Henry equation regardless of the kinds of organic vapors (methanol, benzene, pyridine, and cyclohexane). For Beulah-Zap coal, the sorptions of benzene and cyclohexane were significantly small due to their small interactions with the coal surface, resulting in either dual-mode equation being inadequate. In contrast, for high-rank coals (Upper Freeport and Pocahontas No. 3) benzene or pyridine sorption isotherms could not be determined due to their slow sorption.
Introduction When a coal contacts an organic vapor, several phenomena occur, i.e., adsorption of the vapor on the coal surface, diffusion into the bulk of coal (cross-links), extraction, capillary condensation, swelling, and structural relaxation. The mechanism of solvent penetration into coal has not been well elucidated, though study of sorption of various organic substances into coals is useful for understanding coal structure, such as the porosity and cross-linking network, and for better utilization of coals. Franklin studied adsorption behaviors of helium, methanol, water, n-hexane, and benzene on coals of various ranks and reported that size of pores is the smallest for high-rank coals with C% of 89-93%.1 Hsieh and Duda have reported2 that organic vapor sorption can involve several complex phenomena, including the migration of molecules in the coal and solvent-induced changes in coal structure, and that the analyses that have been used to explain the sorption into synthetic macromolecules were not directly applicable to coals. Green et al.3 compared the rate of benzene sorption by O-alkylated Illinois No. 6 coal with * Corresponding author. Present address: Energy Resources Department, National Institute for Resources and Environment, Tsukuba 305-8569, Japan. (1) Franklin, R. E. Trans. Faraday Soc. 1949, 45, 274. (2) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170. (3) Green, T. K.; Ball, J. E.; Conkright, K. Energy Fuels 1991, 5, 610.
that for the original coal and found3 that O-methylation causes an increase in its microporosity, probably due to the disruption of the secondary structure of coal, i.e., hydrogen bonds. Otake and Suuberg4 showed the behaviors of solvent swelling of Argonne premium sample coals by diffusion of various organic solvents are quite different among the coals used, which did not correlate well with coal rank. Recently Green and Selby proposed5 that pyridine sorption isotherms for coals can be modeled by a dualmode sorption model which has been widely used to explain the sorption isotherms of glassy polymers.6 This model consists of physical adsorption on the coal surface described by a Langmuir isotherm and dissolution into coal bulk described by Henry’s law. A linear portion of the pyridine isotherms following initial adsorption represents dissolution of pyridine according to Henry’s law, i.e., sorption is linear with pressure. However, since chemical and physical (pore) structures of coal, and thus their interaction behaviors with solvents, are highly dependent on the type of coal, more detailed study on sorption is needed. In the present study, organic vapor sorption experiments were carried out by using Argonne premium coal samples (APCS-1, 3, 5, and 8), which are suitable for (4) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155. (5) Green, T. K.; Selby, T. D. Energy Fuels 1994, 8, 213. (6) Vieth, W. R. Diffusion in and Through Polymers; Hanser Publishers: Munich, 1991.
S0887-0624(97)00230-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/11/1998
892 Energy & Fuels, Vol. 12, No. 5, 1998
Shimizu et al.
Table 1. Ultimate and Proximate Analyses of Coals ultimate analysis wt % (daf)
proximate analysis, wt % (db)
coal
C
H
N
S
Oa
Pocahontas No. 3 Upper Freeport Illinois No. 6 Beulah-Zap
89.7 86.2 76.9 71.6
4.5 5.1. 5.5 4.8
1.1 1.9 1.9 1.0
0.7 2.2 5.6 0.9
4.0 4.6 10.1 21.7
a
VM
ash
FC
17.6 28.2 38.6 42.2
4.8 13.1 15.0 9.6
77.6 58.7 46.4 48.2
By difference.
sorption experiments since they have been stored under oxygen-free conditions. Sorption mechanisms are discussed from the viewpoints of coal porosity, cross-linking structure of the coals, and coal-solvent interactions. Experimental Section Sample Preparation. Four Argonne premium coal samples (APCS-1, 3, 5, and 8) were obtained in ampules of 5 g of
