Energy & Fuels 2001, 15, 135-140
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Diffusion of Solvents in Coals: 1. Measurement of Diffusion Coefficients of Pyridine in Elbistan Lignite Meryem Seferinoglu Department of Chemistry, Hacettepe University, Beytepe, Ankara 06532, Turkey
Yuda Yu¨ru¨m* Faculty of Engineering and Natural Sciences, Sabanci University, Istanbul 81474, Turkey Received July 18, 2000. Revised Manuscript Received October 9, 2000
The aim of the present report is to measure the diffusion coefficients at temperatures about 20-27 °C, the activation energies of diffusion, and to determine the type of transport mechanism of diffusion of pyridine in macromolecular structure of Turkish Elbistan lignite (C: 53.0 wt %, dmmf). The raw coal sample was demineralized with HCl and HF by standard methods. The raw and demineralized coal samples were extracted with pyridine. Pyridine uptake of the coal samples was recorded at temperatures 20-27 °C in an adiabatic setup until a constant weight was attained. The extent of swelling of original and treated coal samples in pyridine were measured. Pyridine extraction created a fraction of enlarged particle size due to irreversible solvent swelling. The diffusion coefficients were measured from the slope of graphs of Mt/M∞ versus t1/2. The diffusion of pyridine in the raw coal seemed to be less, compared to those of the treated coal samples. Extraction of the raw coal with pyridine extended the pyridine diffusion in the coal very little. The formation of new carboxylic acid groups in acid-washed samples enhanced diffusion of pyridine. In all of the samples the diffusion constants increased linearly with an increase in the temperature. The greater diffusion coefficients are encountered with the coal samples which swelled more in pyridine. It seemed that the diffusion coefficients increased with swelling and acid washings as a result of structural variations decreasing the activation energy of diffusion of the solvent to the coal. The transport mechanism of pyridine in the macromolecular coal network of Elbistan lignite was not relaxation controlled. The diffusion of pyridine in the low rank Elbistan lignite obeyed generally Fickian mechanism or an intermediate case of Fickian-anomalous mechanism.
Introduction The dynamics of solvent swelling of macromolecular systems provide significant knowledge about the structure of the material and the interaction between the penetrant and the macromolecular material. It might be possible to see the thermodynamic glassy and rubbery states of the macromolecular system. If the system is in the glassy state it is possible to differentiate whether the diffusion of the solvent is due to Fickian and/or due to relaxation of the macromolecular system. Coals are glassy, strained, macromolecular solids.1 The optical anisotropy of Illinois No. 6 coal viewed in thin section through a polarizing microscope was associated with the existence of strain in a glassy system.2 It is shown that this strain could be removed if the coal was swollen with a good swelling solvent such as pyridine. Coals locked in this configuration by noncovalent interactions which serve as cross links. If these hydrogen bonds are removed either by the addition of a basic solvent such as pyridine or by hydroxyl derivatization, * Author to whom correspondence should be addressed. (1) Larsen, J. W. In Clean Utilization of Coal; Yu¨ru¨m, Y., Ed.; NATO ASI Series C, Vol. 370; Kluwer Academic Publishers: Dordrecht, 1992; pp 2.
the coal becomes rubbery.2 Lucht et al.3 observed that during dynamic pyridine transport in coal samples, the value of Tg decreased as a function of solvent weight fraction. The softening temperature of Illinois No. 6 coal decreased from 651 to 536 K after it was O-alkylated.4 The data from these reports indicate that covalent cleavage is not necessary for glass-to-rubber transition. Glassy coals and rubbery coals are different materials.5 In the glassy state, all large molecular motions are restricted although segmental motion may still be exhibited. Diffusion rates are very low because diffusion through the macromolecular solid involves moving portions of the macromolecule around to allow the passage of the diffusing molecule. As the temperature is increased vibrational motions also increase, the macromolecular units move apart, and the density of the whole material decreases.6 In a rubbery solid, molecular motion is similar to that in a non-cross-linked polymer solution of the same composition. Bulk diffusion (2) Brenner, D. Fuel 1985, 64, 167. (3) Lucht, L. M.; Larson, J. M.; Peppas, N. A. Energy Fuels 1987, 1, 56. (4) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277. (5) van Krevelen, D. W.; Hoffyzer, P. J. Properties of Polymers; Elsevier: New York, 1976. (6) Peppas, N. A.; Lucht, L. M. Chem. Eng. Commun. 1985, 37, 333.
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is much more rapid, often by as much as 103. Reactive intermediates will have same opportunity to move about to seek the lowest energy reaction pathway. Since diffusional limitations may be of significance in a large number of coal utilization processes,7 in order to remove diffusional and steric constraints, it is best to run chemical reactions on rubbery coals rather than on glassy ones.8 If a glassy macromolecular system containing a solute originally dispersed in it is placed in contact with a solvent (penetrant), diffusion of the penetrant in the macromolecule may be observed. The diffusion of the solvent in the macromolecular system causes a dynamic swelling phenomenon which leads to a considerable volume expansion. Dissolution of the solute present in the macromolecular system is negligible at the start of the contact with the solvent, solute dissolution starts only as the swelling interface moves inward. It is possible to express the diffusional solvent penetration in terms of general equation
Mt ) ktn M∞
(1)
where, Mt is the amount of solvent diffused in the macromolecular structure at time t, M∞ is the amount of solvent diffused at steady state, k is a constant which depends on structural characteristics of the system, and n is an exponent characteristic of the mode of transport of the solvent in the macromolecular structure. When n ) 0.5, the diffusion is Fickian; when n ) 1.0, Case II transport occurs and values of n between 0.5 and 1.0 indicates anomalous transport.6 ln k is the intercept, and n is the slope of the graph of ln(Mt/M∞) versus ln t. Assuming the coal particles are of spherical shape, the solution of Fick’s second law of diffusion in spherical systems gives9
[ ]
Mt/M∞ ) 6
Dt πa2
1/2
-
3Dt a2
(2)
where Mt and M∞ represent the amount of solvent diffused entering the spheres with radius a, at times t, and steady state, respectively. D is the coefficient of diffusion of the solvent. Neglecting the contribution of the term 3Dt/a2, the value of D is found from the slope of a plot of Mt/M∞versus t1/2. Activation energies of diffusion are calculated using the equation below:10
D ) Doe-EA/RT
(3)
where, Do ) temperature-independent preexponential (m2/s), and EA ) the activation energy for diffusion. The activation energy was calculated from the slope of the straight line of the graph ln D versus 1/T. The aim of the present report is to measure the diffusion coefficients at temperatures about 20-27 °C, the activation energies, and to determine the type of (7) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155. (8) Larsen, J. W.; Green, T. K.; Choudry, P.; Kuemmerle, E. W. Adv. Chem. Ser. 1981, 192, 277. (9) Crank, J. Mathematics of Diffusion; Oxford University Press: London, 1970. (10) Callister, W. D., Jr. Material Science and Engineering, 2nd ed.; John Wiley: New York, 1991; p 104.
Table 1. Proximate and Ultimate Analyses of Elbistan Lignite Proximate Analysis moisture, % mineral matter, %, dry volatile matter, %, dmmf fixed carbon, %, dmmf Ultimate Analysis, %, dmmf carbon hydrogen nitrogen sulfur (total) oxygen (by difference)
2.7 33.6 68.5 31.5 53.0 5.8 1.8 3.6 35.8
transport mechanism of diffusion of pyridine in macromolecular structure of Turkish Elbistan lignite. Experimental Section Turkish Elbistan lignite with a carbon content of 53.0 wt % (dmmf) was used in the study. Analysis of the Elbistan is presented in Table 1. The coal sample was ground under a nitrogen atmosphere to -60 mesh ASTM and stored under nitrogen. Coal sample was Soxhlet extracted with tolueneethyl alcohol solvent couple (1:1) at its atmospheric boiling point to separate the resins of the coal and dried in a vacuum oven at 50 °C, for 24 h under a nitrogen atmosphere. This sample is called raw coal throughout the text of the present work. The raw coal sample was demineralized with HCl and HF by standard methods.11 A volume of 2 L of 6 N HCl was added to 200 g of coal. The slurry was stirred for 24 h under a nitrogen atmosphere, then it was filtered and washed with distilled water until the filtrate became neutral. Consecutively, 1.6 L of aqueous (40%) HF was added to HCl-washed coal and this mixture was stirred for ∼24 h under a nitrogen atmosphere. After filtering, the demineralized coal was washed with 1 L of distilled water and dried at 50 °C, for 24 h under vacuum. The raw and demineralized coal samples were extracted with pyridine in a Soxhlet extractor under a nitrogen atmosphere until the color of the solvent in the sidearm of the extractor became colorless. The extracted samples were washed with alcohol and dried in a vacuum oven at 60 °C for a day. These samples were referred as pyridine-extracted samples. Particle size distributions of the raw and pyridine-extracted coal samples were determined with a set of sieves of 800, 400, 200, 100, 63, and 40 µm mesh size, and the average radii of the samples were calculated. An adiabatic isothermal setup,12 designed and manufactured in our laboratories and made from Plexiglass, which contained a heater and a digital temperature control system, an electronic digital balance of 0.001 g accuracy, and a beaker filled with pyridine, was used in the diffusion experiments. At the start of experiment, about 0.25 g of coal sample was evenly distributed in a Petri dish and the initial weight of the coal was recorded. The temperature of the experiment was set and the system was closed, flushed with nitrogen, and the weight increase due to pyridine uptake was recorded until a constant weight was attained. The equilibrium time to reach a constant weight changed between about 3600 min and 2500 min depending on the temperature set at the start of the experiment which varied from 20.0 to 27.0 °C, respectively. The extent of swelling of original and treated lignite samples in pyridine were measured according to the method given by Liotta et al.13 and Larsen.1 Approximately 100 mg of a coal sample was placed in a 6 mm o.d. tube and centrifuged for 5 (11) Yu¨ru¨m, Y.; Kramer, R.; Levy, M. Thermochim. Acta 1985, 94, 285. (12) Seferinoglu, M. Ph.D. Thesis, Hacettepe University, Ankara, Turkey, 1999. (13) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277.
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Figure 1. Particle size distribution of the raw, HCl-, and HCl/ HF-washed Elbistan lignite samples. Figure 3. Mt/M∞ versus t1/2 graph for the pyridine diffusion in raw Elbistan lignite at 20.9 °C.
Figure 2. Particle size distribution of the raw/pyridineextracted, HCl-washed/pyridine-extracted, and HCl/HF-washed/ pyridine-extracted Elbistan lignite samples. min at 2500 rev/min. The height of the sample was measured as h1. Excess pyridine (∼1 mL) was added in the tube and the contents of the tube were mixed and the tube was centrifuged after 24 h and the height of the sample in the tube (h2) was measured. The volumetric swelling ratio was calculated as Qv ) h2/h1.
Results and Discussion Particle Size Distribution. Particle size distribution of the raw and acid-washed, and raw-pyridineextracted and acid-washed-pyridine extracted coal samples are presented in Figures 1 and 2, respectively. Particle size distribution of the acid-washed samples shifted toward a slightly bigger particle size from a maximum at 100 µm in the case of raw coal to maxima at 200 µm in the case of HCl- and HCl/HF-washed samples (Figure 1). It seemed that finer particles presumably with higher contents of mineral matter soluble in HCl solution were washed off during acid treatments. Pyridine extraction of the raw coal formed a sample which showed a maximum at 200 µm (Figure 2). In the case of acid-washed/pyridine-extracted samples the particle size distribution is bimodal (Figure 2). While the first maximum in the size distribution was observed at 200 µm for all of the pyridine-extracted samples a second maximum started to evolve at 800 µm. The situation is even more enhanced for the HCl/HF/ pyridine-extracted sample. It seemed that considerably greater fractions of bigger particles were produced when the acid-washed samples were extracted with pyridine. Pyridine extraction created a fraction of 800 µm in
particle size which constituted about 20% (by weight) of the whole sample. The reason for this phenomenon might be irreversible solvent swelling14 of some coal particles after pyridine extraction. Even when the pyridine was evaporated from these samples in a vacuum oven, these particles have retained their enlarged forms and thus some fraction of the coal might have become bigger in size. Enlargement of the size of particles might be due to aggregation of particles as a result of some physical and chemical attraction. In this way, very tiny coal particles containing some charges might join to form bigger particles. Diffusion Coefficients. In an experiment, the pyridine uptake of a coal sample was recorded until equilibrium. A graph of Mt/M∞ versus t1/2 for the pyridine diffusion in raw Elbistan lignite at 20.9 °C is presented in Figure 3. To determine the slope of the linear portion of a similar graph, a new graph which contained about 60% of the data from the start of the experiment was reconstructed.15,16 The graph of this type for the same experiment is presented in Figure 4. The diffusion coefficients were measured from the slope of such graphs for all of the samples. Figure 5 gives the change of the measured diffusion coefficients of pyridine in raw and treated Elbistan lignite with temperature. The diffusion of pyridine in the raw coal seemed to be less, compared to those of the treated coal samples. The diffusion coefficient of pyridine in raw coal increased slightly from 5.1 × 10-5 m2/s to 7.1 × 10-15 m2/s when the temperature was elevated from 20.9 to 26.3 °C, respectively. Extraction of the raw coal with pyridine extended the pyridine diffusion in the coal very little. The diffusion constants of pyridine in pyridine-extracted coals increased from 6.4 × 10-15 m2/s to 7.5 × 10-15 m2/s when the temperature was increased from 21.8 to 25.4 °C. Although pyridine extraction might have increased the surface area, it seemed that the porosity created by pyridine extraction was not effective to enhance the pyridine diffusion. The reason for this result might be the relaxation of the coal during its extraction with pyridine which was irreversible and that some of the (14) Nishioka, M. Fuel 1993, 72, 997. (15) Quinga, E. M. Y.; Larsen, J. W. In New Trends in Coal Science; Yu¨ru¨m, Y., Ed.; NATO ASI Series C, Vol. 244; Kluwer Academic Publishers: Dordrecht, 1988; p 85. (16) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379.
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Seferinoglu and Yu¨ ru¨ m Table 2. Solvent Swelling Ratios and Activation Energies for Diffusion of Pyridine in Coal Samples
Figure 4. Mt/M∞ versus t1/2 graph for the pyridine diffusion in raw Elbistan lignite at 20.9 °C with the 60% of the data from the start of the experiment.
Figure 5. Change of the diffusion coefficients of pyridine in raw and treated Elbistan lignite with temperature.
increase in the diffusion coefficient in later experiments could be due to earlier structural relaxation. The situation in the case of acid-treated coal samples is particularly different. When the coal was treated first with HCl and then with HF the metal cations are washed away and they are replaced with hydrogen ions to form -COOH groups.17 When acid-treated coals will be in contact with pyridine, the pyridine molecules which are basic in chemical nature react with these carboxylic groups to form pyridine-coal hydrogen bonds. As the concentration of the carboxylic groups created with acid treatment were increased the number of bonded pyridine molecules would be increased, too. This of course would enhance the diffusion of pyridine in the coal. It seemed that an additional driving force for the pyridine diffusion in the acid-treated coals was the concentration of newly formed carboxylic groups. Removal of divalent cations which had contributed to the cross-link density might have also added to the formation of more relaxed structures which would allow higher diffusion rates of pyridine. Diffusion constants of pyridine in HCl-washed coals increased from 8.5 × 10-15 m2/s to 11.4 × 10-15 m2/s as the temperature was (17) Sugano, M.; Mashimo, K.; Wainai, T. Fuel 1999, 78, 945.
sample
Q
activation energy, kJ/mol
raw coal raw coal/pyridine-extracted HCl-washed/pyridine-extracted HCl/HF-washed/pyridine-extracted
1.3 1.2 1.6 1.5
52.1 47.9 40.7 29.5
raised from 21.7 to 24.8 °C. The case is more amplified in HCl/HF-washed coals in which presumably more new nascent carboxylic groups were produced. The diffusion constants increased from 16.9 × 10-15 m2/s to 24.0 × 10-15 m2/s when the temperature was increased from 22.0 to 25.5 °C, respectively. In all of the samples the diffusion constants increased linearly with an increase in the temperature. The slope of the linear relationship was similar in the case of raw and pyridine-extracted samples and it started to increase sharply for HCwashed coal and even more sharply for the HCl/HFwashed coal. The equilibrium swelling values of the coals in pyridine are parallel to the values of diffusion coefficients. Table 2 presents the equilibrium solvent-swelling values of the coal samples used in pyridine. The swelling ratios of the raw and pyridine-extracted coal samples were 1.3 and 1.2, respectively. The swelling ratios increased to 1.6 and 1.5 for HCl- and HCl/HF-washed coals, respectively. The removal of minerals from coal increased the surface area and the number of carboxylic groups in the structure of coal samples and the amount by which the coal samples were swollen. These effects both increased the diffusion rate. The greater diffusion coefficients are encountered with the coal samples which swelled more in pyridine. The activation energies of diffusion measured in the present study are given in Table 2. The activation energies fall in the range from 29.5 to 52.1 kJ/mol. This suggested the activation barrier was associated with the breakage of internal electron donor-acceptor (hydrogen bonding) interactions.7 Since it has been proposed that a solvent will disrupt only those coal-coal hydrogen bonds whose bond strengths are lower than those of the coal-solvent hydrogen bonds.18 It has also been suggested that pyridine, because of its strong basicity, is capable of breaking nearly all hydrogen bonds in coal.19 Larsen et al.20 observed that a selective association between the hydrogen-bond acceptor (pyridine) and hydroxyl groups which were cross-links between macromolecular chains in the coal existed, the selectivity of the acceptor for cross-linking hydroxyls over other hydroxyls was due to the much more favorable entropy change which occurred when one of these cross-links was disrupted by formation of a new hydrogen bond to pyridine. The present results support other published results which showed a decrease in activation energy with pyridine extraction.21 Activation energies measured in the present work are also in accord with the values of diffusion coefficients of pyridine for the different types of coal samples. The activation energies (18) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (19) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (20) Larsen, J. W.; Gurevich, I.; Glass, A. S.; Stevenson, D. S. Energy Fuels 1996, 10, 1269. (21) Ndaji, F. E.; Thomas, K. M. Fuel 1993, 72, 1525.
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Table 3. Diffusion Rate Constants, Diffusion Exponents, and Transport Mechanisms of Pyridine in Coal Samples sample raw Elbistan lignite
raw Elbistan lignite/pyridine-extracted
HCl-washed Elbistan lignite/pyridine-extracted
HCl/HF-washed Elbistan lignite/pyridine-extracted
T, °C 20.9 22.9 23.3 24.3 26.3 21.8 22.4 23.8 24.3 25.4 21.7 22.5 23.5 24.3 24.8 22.0 23.3 23.9 24.4 25.5
might be thought of as the energy required to produce the diffusive motion of one mole of penetrant molecules. A large activation energy results in a relatively small diffusion coefficient. The diffusion coefficient of pyridine in the raw coal was measured to be the smallest among those of other coal samples and the activation energy of pyridine diffusion in the raw coal was the greatest as 52.1 kJ/mol (Table 2). Activation energy results measured by Otake and Suuberg22 for lignites, (51-59 kJ/mol) are in accord with the activation energy of diffusion measured for the raw Elbistan lignite. Activation energy decreased to 47.9, 40.7, and 29.5 kJ/mol for the raw/pyridine-extracted, HCl/pyridine-extracted, and HCl/HF/pyridine-extracted samples, respectively, (Table 2). The greatest diffusion coefficients were measured for HCl/HF/pyridine-extracted coal sample and this was in parallel with the lowest activation energy of diffusion of this sample (29.5 kJ/mol) which indicated the moderate diffusion of pyridine in these samples. Activation energies measured in the present work for the raw Elbistan lignite, 53.0% C, was relatively greater than those of measured (31.8-44.6 kJ/mol), for higher rank coals with carbon contents changing from 76.3 to 85.3%. The effect of coal rank is found to be effective in the diffusion of solvents in coals; activation energy of diffusion of pyridine in low rank coals is greater than those of in high rank coals.23 It seemed that the diffusion coefficients increased with swelling and acid washings as a result of structural variations decreasing the activation energy of diffusion of the solvent to the coal. Therefore, it can be concluded that the diffusion process in the coal-pyridine system is associated with swelling and chemical reactions occurring during diffusion of the solvent. Type of Transport of Pyridine in Coal Structure. Table 3 presents the diffusion rate constants, diffusion exponents, and transport mechanisms of pyridine in coal samples. R2 values in all of the experiments were equal to or greater than 0.99, indicating a linear relationship between ln(Mt/M∞) and ln t. Using this fact it seemed that diffusion of pyridine in coal samples can be approximated with a first-order rate law for all of the coals (22) Otake, Y.; Suuberg, E. M. Fuel 1998, 77, 901. (23) Otake, Y.; Suuberg, E. M. Fuel 1989, 68, 1609.
k, s-1 10-4
4.18 × 2.77 × 10-4 2.55 × 10-4 3.57 × 10-4 1.66 × 10-4 3.72 × 10-4 2.57 × 10-4 3.76 × 10-4 2.70 × 10-4 3.67 × 10-4 4.14 × 10-4 4.00 × 10-4 4.03 × 10-4 2.84 × 10-4 3.04 × 10-4 5.35 × 10-4 3.08 × 10-4 4.00 × 10-4 3.83 × 10-4 2.92 × 10-4
n
R2
0.49 0.56 0.56 0.53 0.64 0.52 0.57 0.52 0.57 0.53 0.49 0.52 0.52 0.56 0.56 0.47 0.55 0.52 0.53 0.58
0.9986 0.9969 0.9937 0.9979 0.9876 0.9929 0.9956 0.9940 0.9944 0.985 0.9866 0.9970 0.9932 0.9970 0.9970 0.9910 0.9969 0.9940 0.9931 0.9939
transport mechanism Fickian Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous
studied. Diffusion rate constants remained generally unchanged as the temperature of diffusion was increased for all of the samples in the range of 20-26 °C. Ndaji and Thomas21 observed an increasing trend of 1 order of magnitude in the diffusion rates of pyridine in coals of 76.3-85.3% C content when the temperature was raised from 20 to 60 °C. Diffusion rates measured in the present work for the Elbistan lignite, 53.0% C, are smaller than those measured for higher rank of coals.21 The effect of transport rates has also been studied by Peppas and Lucht6 and Ritger and Peppas16 and a comparable conclusion of higher rates in higher rank of coals was reported. The chain relaxation time is the reciprocal of the diffusion rate constant k obtained from analysis using eq 1. For the coal samples studied in the present work, the relaxation time is of the order of 1800-6000 s. It must be stated that these values are much lower than those measured for coals with carbon contents in the range of 70.0-94.0%, 33 000 to 200 000 s, respectively.16 Thus, the transport mechanism of pyridine in the macromolecular coal network of Elbistan lignite can be considered as nonrelaxation controlled. It seemed that pyridine diffusion in a young lignite like Elbistan lignite (Pleistocene-Pliocene, 2-5 million years B. P.)24 which should contain smaller macromolecules compared to coals of higher ranks obeyed a mechanism close to Fickian. The discussion below on the diffusion exponents support this claim. The changes of diffusion exponents are presented in Table 3. The diffusion exponent, n, was calculated to be equal or less than 0.5 in all experiments done at temperatures equal to or less than 22 °C for all of the samples, indicating Fickian diffusion mechanism. As the temperature was raised to higher values than 22 °C the diffusion coefficients increased to values less than 0.60 except in the case of pyridine diffusion at 26.3 °C in raw Elbistan lignite that is n ) 0.65. In all of the cases the transport mechanism of pyridine might be considered as anomalous (non-Fickian) transport. The n values measured in the present work are very close to Fickian transport mechanism boundaries since the non-Fickian diffusion coefficients in the literature usually are in the
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range of 0.6-1.0,6,21,23,25 therefore it will not be erroneous to claim that the diffusion of pyridine in a low rank coal like Elbistan lignite (53.0% C) was with a process somewhere between Fickian and relaxation controlled as discussed by Ritger and Peppas.16 Conclusions Pyridine extraction created a fraction of enlarged particle size due to irreversible solvent swelling. The diffusion of pyridine in the raw coal seemed to be less, compared to those of the treated coal samples. Extraction of the raw coal with pyridine, extended the pyridine diffusion in the coal very little. The formation of new (24) Karayigit, A. I.; Akdag, Y. Turk. J. Earth Sci. 1996, 7, 1. (25) Peppas, N. A. Polymer 1997, 38, 3425.
Seferinoglu and Yu¨ ru¨ m
carboxylic acid groups in acid-washed samples enhanced diffusion of pyridine. In all of the samples the diffusion constants increased linearly with an increase in the temperature. The greater diffusion coefficients are encountered with the coal samples which swelled more in pyridine. It seemed that the diffusion coefficients increased with swelling and acid washings as a result of structural variations decreasing the activation energy of diffusion of the solvent to the coal. The transport mechanism of pyridine in the macromolecular coal network of Elbistan lignite was not relaxation controlled. The diffusion of pyridine in the low rank Elbistan lignite obeyed generally Fickian mechanism or an intermediate case of Fickian-anomalous mechanism. EF000159X
