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Energy & Fuels 2004, 18, 1104-1107
Concerning the Nature of Coal Solutions and Suspensions Paul Painter,* Maria Sobkowiak, Jonathan Mathews, and Alan Scaroni The Energy Institute, Pennsylvania State University, University Park, Pennsylvania 16802 Received January 13, 2004. Revised Manuscript Received May 28, 2004
Coal extract and suspensions of coal particles are capable of forming viscoelastic gels. The nature of the connections or interactions in these systems is not clear. Simple experiments reported in this article indicate that NMP/CS2 extracts from Upper Freeport coal are predominantly solutions of molecular species that nevertheless form gels at certain concentrations. The physical gels formed in these mixtures must therefore involve some specific association that, in turn, can lead to aggregation, such as π-cation interactions, or a phase separation. This latter mechanism must involve bicontinuous domains where one of the phases is glassy, to provide the cohesive properties necessary for elastic properties. In contrast, gels formed from insoluble coal particles most probably associate through forces common to many colloidal suspensions. In such systems, it is not the local pairwise interactions between molecules alone that is important, but the sum of all interactions between the molecules in each particle. This results in a large interaction energy that is proportional to particle size and the association of these particles to form a space filling network and a gel with viscolelastic properties.
Coal suspensions and their extract solutions are complex systems, displaying a rich range of behavior that can vary from coal to coal and with the nature of the solvent. The results obtained by Iino and colleagues,1-15 using the mixed-solvent system N-methyl2-pyrrolidinone/carbon disulfide (NMP/CS2), are particularly intriguing. Certain coals, notably Upper Freeport coal from the Argonne Premium Coal Sample Program (APCS 1), have a significantly enhanced solubility in this mixed-solvent system, and suspensions of the extracted parent coal and certain solutions of the extracts form gels and display viscoelastic behavior.1-15 * Author to whom correspondence should be addressed. Telephone: 814-865-5767. Fax: 814-865-2917. E-mail address:
[email protected]. (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (2) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588-1593. (3) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452-455. (4) Fujiwara, M.; Ohsuga, H.; Takanohashi, T.; Iino, M. Energy Fuels 1992, 6, 859-862. (5) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579-580. (6) Liu, H. T.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108-1111. (7) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788-793. (8) Chen, C.; Iino, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 218, 19. (9) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 11801183. (10) Chen, C.; Iino, M. Energy Fuels 1999, 13, 1105-1106. (11) Chen, C.; Iino, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2000, 220, 89. (12) Norinaga, K.; Takahashi, K.; Iino, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2000, 219, 21. (13) Takanohashi, T.; Iino, M.; Mainwaring, D. Energy Fuels 1998, 12, 476-478. (14) Takanohashi, T.; Takeshi, K.; Iino, M. Energy Fuels 1998, 12, 470-475. (15) Norinaga, K.; Kuniya, M.; Iino, M. Energy Fuels 2002, 16, 6268.
The nature of the connections between particles or molecules in these systems is not fully understood, although Iino and co-workers have proposed that a physical network is formed by coal particle/NMP interactions in the framework of a solvated colloid model.13-15 A few years ago, we suggested that one possible basis for the enhanced solubility of certain coals in the NMP/ CS2 mixed-solvent system was a reduction in the degree of self-association of NMP through dilution, which may essentially tip the overall free-energy change in favor of mixing.16 We went on to argue that if this were so, then other NMP-based mixed solvents, particularly those where the second solvent is nonpolar, also should work. We presented results where NMP/toluene and NMP/cyclohexane mixtures gave apparently equivalent extraction yields to those obtained using NMP/CS2. Subsequently, other workers were not able to reproduce our results.17 The reason for this discrepancy is actually apparent in our paper. We reported that suspended particles could clearly be seen in an examination of the NMP/cyclohexane solutions, using an optical microscope. We came to the realization that the paper filters that we were using to separate the insoluble coal fraction from the extract solutions were sensitive to solvents or defective, allowing small coal particles through, thus leading to an erroneously high extraction yield for certain extracts. This leads us to the results we wish to report in this communication and the resulting comments we wish to make about the nature of association in coal gels. As we will discuss below, the nature of the interactions that occur in coal and coal extract gels is dependent on (16) Painter, P. C.; Sobkowiak, M.; Gamble, V. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43, 913. (17) Dyrkacz, G. R.; Bloomquist, A. A. Energy Fuels 2001, 15, 1409.
10.1021/ef049976d CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004
Nature of Coal Solutions and Suspensions
whether the system consists of suspended particles or is a true molecular solution. Swelling stresses in glassy polymers exposed to solvents invariably lead to some cracking and fracture and segments or particles can, in some cases, break off. In this regard, more than twenty years ago, Hombach18 observed that solutions of many coal extracts actually consist of two componentssa “true” solution of molecules with a range of molecular weights, together with some suspended particles that have dimensions characteristic of many colloidal systemsssuggesting that they are broken-off parts of the insoluble network. He also noted that the cutoff between a particle and a very-high-molecular-weight molecule can only be considered arbitrary; however, he found a gap in the population density of pyridine-soluble material such that it seems reasonable to assume that material with a size greater than ∼0.7 µm consists of suspended particles, whereas material that has a particle size that is much less than this value should be considered molecular in nature. It is important to note that, in some (but not all) of the studies reported in the literature, NMP/CS2 solutions are centrifuged and filtered through membrane papers with a pore size of 0.8 µm, and, in terms of the definition previously given, are probably true solutions. However, much is dependent on the nature of these membranes. As noted previously, we experienced problems with some materials. Accordingly, we performed two sets of experiments to determine the effect that NMP/CS2 extracts have on a suspended particle component. Both involved the soluble material obtained by extracting APCS 1 coal with NMP and the 50/50 (by weight) mixed solvents NMP/CS2, NMP/cyclohexane, and NMP/toluene. The extractions were exhaustive, with 2.5 g of coal being mechanically stirred in 400 mL of solvent for 7 days. The suspensions were then allowed to settle overnight and the supernatant was carefully decanted and replaced with fresh solvent. Our goal was to continue the extractions until the solvent or solvent mixture in contact with the coal became clear. This occurred after one or two changes of solvent with the NMP/cyclohexane and NMP/toluene mixed solvents. Pure NMP usually required at least one more change of solvent. The NMP/CS2 mixed solvent in contact with the coal did not become optically clear after several exchanges, and, for the purposes of this study after more than one month, we stopped the extraction. (We are presently trying to determine how far this process can go.) Although the extracts obtained in many studies are centrifuged prior to filtration, we did not include this step, because we were seeking information on the size of particles suspended in the solvent, as we will discuss below. In the first set of experiments, the extracts were separated once more from their residues, using paper filters. The averages of the extraction yields from three separate extractions are shown in Table 1. It can be seen that the extract yields obtained using the NMP/cyclohexane and NMP/toluene binary solvents are now much less than the yield obtained using NMP/CS2, indicating that the filters maintained their integrity to a much better extent than those used in our previous study.16 NMP gave an extract yield of ∼32%, which is signifi(18) Hombach, P.-H. Fuel 1982, 61, 215-220.
Energy & Fuels, Vol. 18, No. 4, 2004 1105 Table 1. Extract Yields Obtained from Upper Freeport Coal solvents
extraction residue (daf)
extraction yield (daf) (by difference)
NMP/CS2 NMP NMP/cyclohexane NMP/toluene
51% 68% 79% 82%
49% 32% 21% 18%
cantly larger than the 18% reported in some studies. However, these extractions are often conducted for much shorter periods of time and involve ultrasonic irradiation. Takanohashi et al.19 found that soaking APCS 1 coal in NMP for 2 weeks increased the extraction yield in NMP to ∼28%, which is a result that is comparable to ours. To determine if the extract solutions contained some large particles, we attempted to perform a particle size analysis using a laser-diffraction particle size measuring system (Coulter, model LS 230). This instrument uses a He-Ne laser to detect large particles (>5 µm) and a PIDS lamp and detector array to detect small (3 µm (wt %)
fraction >1 µm (wt %)
fraction
