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Energy & Fuels 1998, 12, 798-800
Cooperative Effects in Solvent Swelling of a Bituminous Coal Yongseung Yun† and Eric M. Suuberg* Division of Engineering, Brown University, Providence, Rhode Island 02912 Received January 23, 1998. Revised Manuscript Received March 25, 1998
The relative abilities of mixtures of nonspecifically and specifically interacting solvents to swell a Utah Blind Canyon bituminous coal have been explored. The results are consistent with published reports of the efficacy of such mixed solvent systems but show sensitivity to the choice of specifically interacting solvent. The dynamics of swelling in such systems also serve to confirm the general nature of the process.
Larsen et al.1 recently reported on a method for counting hydrogen bond cross-links in coals, involving examination of the swelling behavior of coals in nonhydrogen-bonding solvents (chlorobenzene and toluene) to which were added small amounts of hydrogenbonding solvents (pyridine and tetrahydrofuran). The hypothesis was advanced that the latter bases first titrate the internally hydrogen-bonded functional groups of the coal (mainly hydroxyls), rendering the coals more swellable in non-hydrogen-bonding solvents. A related system has been the subject of study in this laboratory. Experimental Section The solvent swelling experiments were performed on a sample of Utah Blind Canyon (UBC) bituminous coal obtained from the Argonne National LaboratorysPremium Coal Sample Program. Detailed petrographic, chemical, and physical analysis data on this coal can be found elsewhere.2 The coal was always dried at 100 °C in a vacuum, prior to beginning the swelling tests. Coal samples were placed in constant diameter glass tubes (3 mm i.d., ca. 5 cm high) and centrifuged at 7500 rpm, after which the initial height of the sample was measured. Solvent was then added and the system was vigorously mixed. At the desired measurement times, the sample tubes were centrifuged again (7500 rpm for 3 min) and the swollen coal height measured. The volumetric swelling ratio, Q, is equal to the ratio of the final to initial height of the coal. The solvent was frequently replaced with the clean solvent to ensure that the concentration of extractables was not so high as to interfere with swelling. Further details can be found elsewhere.3 In a number of cases, samples of the UBC coal were subjected to solvent swelling, following heating to a particular temperature. Heating was performed in inert gas at a rate of 8 °C/min, after which the samples were immediately quenched to room temperature. This method has been used to study the occurrence of irreversible relaxation events.3 † Present address: Institute for Advanced Engineering, Seoul, South Korea. (1) Larsen, J. W.; Gurevich, I.; Glass, A.; Stevenson, D. S. Energy Fuels 1996, 10, 1269. (2) Vorres, K. S. Energy Fuels 1990, 4, 420. (3) Yun, Y.; Suuberg, E. M. Fuel 1993, 72, 1245.
Results and Discussion Pyridine and tetrahydrofuran (THF) are commonly quite good swelling solvents for bituminous coals. These are specifically interacting solvents, meaning that they are able to dissociate specific interactions (particularly hydrogen-bonding interactions) within the coal. The room temperature swelling ratio Q of fully dried UBC coal in pyridine was observed to be about 2.5, and in THF the value was about 2.0. It is generally accepted that the organic portion of coals have a structure in which most elemental carbon is contained in aromatic ring structures and that these aromatic structures have various alkyl substituents. Thus it would be expected that a solvent with this character should, except for the role of heteroatoms, have a good solubility parameter match with the coal. Alkyl naphthalenes have such a structure. We tested the swelling of UBC in 1-ethylnaphthalene, 2-ethylnaphthalene, and 1-methylnaphthalene (MN) at room temperature, and obtained Q values of 1.08, 1.12, and 1.10, respectively. In the pure state, these are not effective swelling agents, as is apparent in comparison with the values obtained in pyridine and THF. These alkyl naphthalenes are unable to dissociate the very strong hydrogen-bonding interactions which exist in the dried, raw coal. It was suspected that the addition of a cosolvent which is able to dissociate the hydrogenbonding interactions should render the mixture with alkyl naphthalenes effective for swelling. Figure 1 shows the equilibrium swelling behavior (measured to equilibrium over several days) of the same coal in 1:1 v/v mixtures of MN with various linear alcohols, ranging from methanol to hexanol. The effect of the added hydrogen-bonding solvent (the alcohol) is immediately apparent. The swelling behavior in pure alcohols is also shown in Figure 1 for reference, and it is seen that the mixed solvent is far superior. The much greater efficacy of pure THF and pyridine, in comparison with the pure alkyl alcohols, may be partly due to the greater electron donor strengths of THF and pyridine (especially the latter), but may also be due to the greater affinity of these two solvents for the coal.
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Solvent Swelling of Bituminous Coal
Figure 1. Swelling of Utah Blind Canyon bituminous coal in pure linear alkyl alcohols (open points) and in 1:1 v/v mixtures of 1-methylnaphthalene and linear alkyl alcohols (closed points). Also shown as the solid line with no points is the swelling ratio in pure 1-methylnaphthalene.
The trend observed in experiments involving swelling in different alcohol mixtures appear to suggest that steric factors, associated with the alcohol, are playing a role in determining the ability of the alcohols to relax the structure. All of the alcohols are comparable in hydrogen-bonding strength, and differ only in alkyl chain length. This alkyl “tail” needs to be accommodated within the coal structure, in order for the polar head to be able to interact with a hydrogen-bonding site. With alcohols of increasingly large carbon number, a progressively larger molecular entity must move through the coal network to effect relaxation. Recently, results have been presented showing how dramatically the size of an alkyl tail affects both the relaxation rate and the activation energy barrier for relaxation in linear alkyl amines of a bituminous coal.4,5 The activation energy barrier has been suggested to reflect the dissociation of hydrogen-bonding interactions in the coal. It should also be noted that there is a decrease in molar uptake of alcohol, with increase in chain length, evident from the results for pure alcohol; if Q is roughly constant for increasing size of alcohol, then the number of moles taken up must be decreasing, with increasing size. In addition to the diffusional aspects, there is also a mismatch in the solubility parameter between the alkyl chain and the (aromatic) structure of the coal. This is what we believe controls the final equilibrium swelling ratio. Coals do not swell well in alkanes, regardless of what is done with the hydrogen-bonding interactions.6 Thus, the unfavorable interaction of the alkyl chain with the coal is apparently not balanced by the favorable enthalpy of mixing of the polar end of the alcohol with structures in the coal, as it is in the case of alkyl amines.7 In that case, the swelling ratio increases with chain length for the pure alkyl amine. This is clearly not seen in the case of alcohols. Mixtures of methanol with the ethylnaphthalenes behaved comparably to methanol/MN mixtures, but were a bit less effective than those with MN. It is also worth noting that in a few cases, mixtures with methanol showed formation of bubbles and a third liquid phase, prior to decanting the extract-rich phase. This (4) Ndaji, F. E.; Thomas, K. M. Fuel 1995, 74, 842. (5) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155. (6) Larsen. J. W.; Green, T. K.; Kovac, J. W. J. Org. Chem. 1985, 50, 4729. (7) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247.
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Figure 2. Swelling of Utah Blind Canyon bituminous coal in various solvent mixtures: mixture of 1-methylnaphthalene and methanol (triangles), toluene and methanol (solid circles), 1-methylnaphthalene and phenol (open circles), pure toluene (solid squares), pure 1-methylnaphthalene (open squares). All mixtures were 1:1 v/v.
suggests that the thermodynamics of the system are by no means simple. It is therefore difficult to unequivocally state that the differences observed using different alcohols are of purely steric origin, as the thermodynamic properties of the solutions will also be influenced by the alcohol’s chain length. In any case, the present results suggest that care must be exercised in concluding that any hydrogenbonding solvent can be equally effective at loosening the structure for a nonspecifically interacting solvent. In the work by Larsen et al.,1 THF and pyridine were equally effective. Those experiments were, however, performed differently, inasmuch as just enough hydrogenbonding solvent was added so as to give maximum swelling. Here, the mixtures were kept at a constant concentration. The results of swelling tests performed in other mixtures are shown in Figure 2. The swelling of the UBC coal in a mixture of methanol and toluene is observed to follow a course which is generally similar to that in methanol and MN, but leads to a lower value of total swelling. This is in spite of the fact that in a pure state toluene is a better swelling solvent than is MN. The results for a mixture of MN with phenol are also shown in Figure 2. The room temperature solid phenol is not an obvious choice as a “swelling solvent”, and yet, it shows itself to be quite effective in liquid mixture with MN. The swelling process in the presence of phenol is quite slow. It is probable that its rate is limited by the larger size of phenol, compared with alkyl alcohols, and therefore its slower rate of diffusion through the coal. This reinforces the view that the diffusion of the hydrogen-bonding solvent is key to determining the rate of swelling. Again, the hydrogen-bonding solvent is the agent that relaxes the structure, allowing the solvent with the better solubility parameter match to swell the coal.1 Figure 3 shows the results of an experiment in which the coal is exposed, in alternating fashion, to methanol and MN. No attempt was made to “wash” the structure of the coal of the other solvent prior to immersion in the new solvent. Clearly the MN does less to open the structure to swelling by methanol than the methanol does to open the structure to MN. This may be concluded by comparing the slopes of the curves after the changes in solvent.
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Figure 3. Swelling of Utah Blind Canyon bituminous coal upon exposure to the indicated pure solvents, in alternating fashion. At the times indicated by the thin lines, the first solvent was decanted and replaced by the second solvent. No attempt was made to wash the first solvent out of the coal at the time of solvent switchover.
Yun and Suuberg
Unfortunately, the experiments at 200 °C proved difficult, again due to the evolution of considerable quantities of gas, which generally caused the contents of the swelling tube to overflow after the first half-hour. Thus the point indicated at 200 °C is a minimum value. Since pure MN does not boil until well above 200 °C, the origin of the gas is not clear, but complicated phase behavior is certainly possible: as the first extracts issue from the coal, the composition of the solvent phase is quickly changing. Solvent swelling of UBC coal was also examined in pyridine and THF, following heating to 200 °C. These experiments were not conducted as were the experiments with MN. Instead, the coal was heated to 200 °C and then cooled prior to the solvent swelling experiment. There was no major change in swelling behavior as a result of the preheating. The swellability of the coal in pyridine only appeared to decrease slightly following heating because the extractability of the coal increased as a result of heating. Extraction of the coal during a swelling experiment results in a decrease of Q with time as a result of mass loss. The increased extractability is generally confirmed visually. This behavior has been noted before with other samples and is associated with an irreversible structural relaxation upon heating.3 Thus there is what may be interpreted as a small degree of such irreversible structural relaxation near 200 °C, consistent with an increase in swelling in MN at that temperature. Conclusions
Figure 4. Swelling of Utah Blind Canyon bituminous coal in 1-methylnaphthalene at various temperatures.
An experiment was performed in which the swelling in pure MN was performed at increased temperatures. The preliminary hypothesis was that dissociation of coal-coal hydrogen-bonding interactions could also be promoted by increasing temperature. The results are seen in Figure 4, which show that indeed temperature is somewhat effective at increasing swelling in MN.
This work supports the concept that cooperative effects between solvents can play an important role in determining the course and extent of swelling of coals. It is again seen that the hydrogen-bonding solvent serves to relax the structure, allowing a nonspecifically interacting solvent to interact with what is effectively a less-cross-linked structure. Acknowledgment. The authors gratefully acknowledge the support of this work by the U.S. Department of Energy under Grant DE-AC22-PC91027. EF9800157
