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Energy & Fuels 1998, 12, 476-478
Viscoelastic Behaviors of Loy Yang Coal-N-Methyl-2-pyrrolidinone Mixtures Toshimasa Takanohashi* and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-77, Japan
David E. Mainwaring Department of Applied Chemistry, Royal Melbourne Institute of Technology, Melbourne, Victoria 3001, Australia Received August 12, 1997. Revised Manuscript Received March 5, 1998
The mixture with 17 wt % Loy Yang (Australian) coal and 83 wt % N-methyl-2-pyrrolidinone (NMP) formed an elastic material at room temperature. Small-amplitude dynamic oscillatory measurements of the mixture by a rheometer showed that the mixture has a rubbery plateau, a characteristic behavior of gel. This viscoelasticity of the mixtures decreased with time (1-5 days), opposite to that for bituminous coal gels. The difference in viscoelastic behaviors between Loy Yang brown coal and Upper Freeport bituminous coal can be attributed to the diffusion rate of NMP into the both coals and the accompanied changes in the “solvated colloid” structure in the coals.
The mixtures of Upper Freeport coal extract fractions with N-methyl-2-pyrrolidinone (NMP) were reported1 to form a gelly material, indicating that the extract fractions form a physical network structure by coal particleNMP interactions. Kraemer and Williamson2 reported a model of “solvated colloids” which pictures the particles as loose porous frameworks of the dispersed material, swollen and permeated with solvent. The model contains two components, i.e., the solvent part, and the dispersed part distributed in the solvent as particles. Since coal extract fraction is an aggregates of small particles or molecules, coal extract permeated with solvent may be explained by the “solvated colloid” model by Kraemer and Williamson.2 The mixture of NMP with the extraction residue which is insoluble in NMP also formed a similar gel as those from the extract fractions,1 showing that a similar rubbery material formed from coal extraction residue-NMP mixture. The coal including a certain amount of solvent may show a transition from glass to rubber due to the strong interaction of the solvent with the coal.3-5 Brenner have found3 that Pittsburgh No. 8 coal swollen in pyridine showed rubbery behaviors. Cody et al.4 have reported a similar viscoelastic properties of pyridine-swollen pyridine-extracted coal (residue) and discussed the nature of the bonds which form cross-linked structure. Hall and Larsen5 have reported from measurements of DSC and TGA that Illinois No. 6 coal interacts with NMP strongly and they form a gel structure, when no (1) Takanohashi, T.; Kudo, T.; Iino, M. Energy Fuels 1998, 12, 470. (2) Kraemer, E. O.; Williamson, R. V. J. Rheology 1929, 1, 76. (3) Brenner, D. Fuel 1985, 64, 167. (4) Cody, G. D. J.; Davis, A.; Hatcher, P. G. Energy Fuels 1993, 7, 455. (5) Hall, P. J.; Larsen, J. W. Energy Fuels 1993, 7, 47.
free NMP exists in the mixture. Mainwaring et al. also reported that Loy Yang coal in alkaline solution showed elastic gel behaviors.6 Loy Yang coal gave high extraction yield (14.3% (daf)) and high swelling ratio (2.7) in NMP at room temperature.7 When NMP was gradually added to Loy Yang coal with mixing, NMP was instantly absorbed in the coal, and then in a composition of 1:5 of coal:NMP the mixture formed an elastic material. However, the time dependence of viscoelasticity for Loy Yang coal-NMP gels was opposite to that for Upper Freeport gels.1 In the present paper, the difference in viscoelastic behaviors between Loy Yang brown coal and Upper Freeport bituminous coal is discussed from the viewpoint of coal particle-solvent interaction and the structural changes in them. Australian Loy Yang brown coal (C 65.5%, H 4.8%, N 0.5%, S 0.3%, O (by difference) 28.9% (daf)) was used as a sample. Approximately 0.5 g of the coal (ground to G′′ at strains lower than about 3%, indicating that the mixture is an elastic solidlike material. At strains over 3%, G′ decreased and G′′ increased, and at 8% strain the crossover point (G′ ) G′′) was observed, indicating that the structure was broken at this point and became a fluidlike material. The “solvated colloid” structure of coal permeated with NMP would formed in the system, according to the physical model by Kramer and Williamson.2 Loy Yang coal contains a significant amount of NMPinsoluble parts, since the extraction yield of Loy Yang coal with NMP is 14.3% (daf),7 and the coal also contains 2% (db) ash. Loy Yang coal also gave a high swelling ratio, 2.7 in NMP,7 suggesting that a significant amount of NMP can permeate the inside of the coal. When the weight ratio of NMP added to coal was varied, the swelling ratio increased with increasing the solvent/coal ratio from 1 to 5, and became constant (around 2.6) above 5 of the solvent/coal ratio,7 suggesting that NMP more than this ratio does not seem to permeate the coal, i.e., no interaction with the coal. The composition of 17 wt % coal and 83 wt % NMP, at which the mixture forms a gel as shown in Figure 1, corresponds to the solvent/coal ratio of around 5. Figure 1 also shows the changes in the viscoelasticity when the mixture was allowed to stand for 1-5 days. G′ and G′′ decreased and both the strain range having
constant G′ and the difference between G′ and G′′ reduced with increasing time, showing that the mixture gradually became more fragile to deformation (strain) and lower viscoelasticity with time, while the crossover point moved to higher strain with increasing time, showing that the mixture also became a more entangled structure. These changes with time have not been observed for the usual synthetic polymer systems. The frequency dependences of G′ and G′′ at 3% strain for the sample are shown in Figure 2. G′ is almost independent of frequency over the 3 orders of magnitude examined and G′ > G′′. Clark and Ross-Murphy have shown8 that a criterion for the gel state is the frequency independence of G′. Thus, the result shows that the mixture is a gel and has a network structure. When the frequency sweep for the mixture with 14% coal content was measured, G′ and G′′ were slightly increasing with increasing frequency. Figure 2 also shows that G′′ is increasing with frequency and the difference between G′ and G′′ is decreasing on the fifth day, indicating that the lowering of viscosity and elasticity occurs by long standing. When a solvent contacts with a coal, relaxation and swelling of coal structure and extraction of solubles in the coal occur. Generally, their changes will enhance the viscosity of the mixture. It is expected that a viscosity increase gradually occurs, since the solvent penetration into the coal bulk is usually slow. However, in this coal-solvent mixture, the viscosity reduced with time, as shown in Figure 2. Coal has a strong tendency to form associates in solvents.9-11 The solubility of coal decreased due to the formation of associates among the coal molecules.9,10 The swelling measurement of Loy Yang coal in NMP showed that it took at least 2 days for the swelling to be reached to equilibrium,7 i.e., the coal first swelled in NMP, giving 2.9 swelling ratio, and then the swelling ratio was gradually decreased to reach an equilibrium, 2.7 swelling ratio after 2 days. These results can be explained as follows. Since low-rank coals have more polar functional groups, the coal-NMP interaction is relatively favorable and incorporation of (8) Clark, A. H.; Ross-Murphy, S. B. Adv. Polym. Sci. 1987, 83, 57. (9) Takanohashi, T.; Iino, M. Energy Fuels 1991, 5, 708. (10) Liu, H.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108. (11) Hayasaka, K.; Takanohashi, T.; Iino, M. Energy Fuels 1996, 10, 262.
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the solvent occurs relatively fast compared to relaxation and rearrangement of coal segments. Thus, much coalNMP interaction is formed, resulting in formation of a gelly material, i.e., no free NMP in the system, although its state is nonequilibrium state. The formation of gel increases the mobility of the coal, leading to the replacement of coal-NMP interaction by coal-coal interaction, while NMP is expelled from the gel and the solvent state will appear, where the coal has been reported to shrink.7 In this state, more dispersed parts of coal exist in more free NMP than those in gel state, resulting in a decrease in the viscosity of the system. On the other hand, in the case of gels of extract fractions from Upper Freeport coal which is a medium-volatile bituminous coal, the viscoelasticity increased with time,1 opposite to that for Loy Yang coal in this study. We have reported that permeation of NMP into bituminous coals is slow and the solvent extraction with NMP alone is incomplete at
Takanohashi et al.
room temperature.12 It was also reported that Upper Freeport coal consists of aggregate structure through mainly aromatic-aromatic interactions.13 This aggregate structure may make the permeation of solvent slow. Thus, the difference in viscoelastic behaviors between Loy Yang coal gel and Upper Freeport gel can be attributed to the diffusion rate of NMP into the both coals and the accompanied changes in the “solvated colloid” structure in the coals. Acknowledgment. This work was supported by the Ministry of Education, Science and Culture, Japan (Japan-Australia Joint Research Program). EF9701403 (12) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (13) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395.
