Diffusion of Solvents in Coals: 1. Measurement ofDiffusion Coefficients of Pyridine in Elbistan Lignite

Meryem Seferinoglu

Department of Chemistry, Hacettepe University, Beytepe, Ankara 06532, Turkey

Yuda Yurum*

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 about20-27 °C, the activation energies of diffusion, and to determine the type of transport mechanismof 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. Theraw and demineralized coal samples were extracted with pyridine. Pyridine uptake of the coalsamples was recorded at temperatures 20-27 °C in an adiabatic setup until a constant weightwas attained. The extent of swelling of original and treated coal samples in pyridine weremeasured. Pyridine extraction created a fraction of enlarged particle size due to irreversible solventswelling. 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 coalsamples. Extraction of the raw coal with pyridine extended the pyridine diffusion in the coalvery little. The formation of new carboxylic acid groups in acid-washed samples enhanced diffusionof pyridine. In all of the samples the diffusion constants increased linearly with an increase inthe temperature. The greater diffusion coefficients are encountered with the coal samples whichswelled more in pyridine. It seemed that the diffusion coefficients increased with swelling andacid washings as a result of structural variations decreasing the activation energy of diffusion ofthe solvent to the coal. The transport mechanism of pyridine in the macromolecular coal networkof Elbistan lignite was not relaxation controlled. The diffusion of pyridine in the low rank Elbistanlignite obeyed generally Fickian mechanism or an intermediate case of Fickian-anomalousmechanism.

Introduction

The dynamics of solvent swelling of macromolecularsystems provide significant knowledge about the struc-ture of the material and the interaction between thepenetrant and the macromolecular material. It mightbe possible to see the thermodynamic glassy and rub-bery states of the macromolecular system. If the systemis in the glassy state it is possible to differentiatewhether the diffusion of the solvent is due to Fickianand/or due to relaxation of the macromolecular system.Coals are glassy, strained, macromolecular solids.1 Theoptical anisotropy of Illinois No. 6 coal viewed in thinsection through a polarizing microscope was associatedwith the existence of strain in a glassy system.2 It isshown that this strain could be removed if the coal wasswollen with a good swelling solvent such as pyridine.Coals locked in this configuration by noncovalent in-teractions which serve as cross links. If these hydrogenbonds are removed either by the addition of a basicsolvent such as pyridine or by hydroxyl derivatization,

the coal becomes rubbery.2 Lucht et al.3 observed thatduring dynamic pyridine transport in coal samples, thevalue of Tg decreased as a function of solvent weightfraction. The softening temperature of Illinois No. 6 coaldecreased from 651 to 536 K after it was O-alkylated.4The data from these reports indicate that covalentcleavage is not necessary for glass-to-rubber transition.

Glassy coals and rubbery coals are different materi-als.5 In the glassy state, all large molecular motions arerestricted although segmental motion may still beexhibited. Diffusion rates are very low because diffusionthrough the macromolecular solid involves movingportions of the macromolecule around to allow thepassage of the diffusing molecule. As the temperatureis increased vibrational motions also increase, themacromolecular units move apart, and the density ofthe whole material decreases.6 In a rubbery solid,molecular motion is similar to that in a non-cross-linkedpolymer solution of the same composition. Bulk diffusion

* Author to whom correspondence should be addressed.(1) Larsen, J. W. In Clean Utilization of Coal; Yurum, Y., Ed.; NATO

ASI Series C, Vol. 370; Kluwer Academic Publishers: Dordrecht, 1992;pp 2.

(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.

135Energy & Fuels 2001, 15, 135-140

10.1021/ef000159x CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 12/02/2000

is much more rapid, often by as much as 103. Reactiveintermediates will have same opportunity to move aboutto seek the lowest energy reaction pathway. Sincediffusional limitations may be of significance in a largenumber of coal utilization processes,7 in order to removediffusional and steric constraints, it is best to runchemical reactions on rubbery coals rather than onglassy ones.8

If a glassy macromolecular system containing a soluteoriginally dispersed in it is placed in contact with asolvent (penetrant), diffusion of the penetrant in themacromolecule may be observed. The diffusion of thesolvent in the macromolecular system causes a dynamicswelling phenomenon which leads to a considerablevolume expansion. Dissolution of the solute present inthe macromolecular system is negligible at the start ofthe contact with the solvent, solute dissolution startsonly as the swelling interface moves inward. It ispossible to express the diffusional solvent penetrationin terms of general equation

where, Mt is the amount of solvent diffused in themacromolecular structure at time t, M∞ is the amountof solvent diffused at steady state, k is a constant whichdepends on structural characteristics of the system, andn is an exponent characteristic of the mode of transportof the solvent in the macromolecular structure. Whenn ) 0.5, the diffusion is Fickian; when n ) 1.0, Case IItransport occurs and values of n between 0.5 and 1.0indicates 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 sphericalsystems gives9

where Mt and M∞ represent the amount of solventdiffused entering the spheres with radius a, at times t,and steady state, respectively. D is the coefficient ofdiffusion of the solvent. Neglecting the contribution ofthe term 3Dt/a2, the value of D is found from the slopeof a plot of Mt/M∞versus t1/2.

Activation energies of diffusion are calculated usingthe equation below:10

where, Do ) temperature-independent preexponential(m2/s), and EA ) the activation energy for diffusion.

The activation energy was calculated from the slopeof the straight line of the graph ln D versus 1/T.

The aim of the present report is to measure thediffusion coefficients at temperatures about 20-27 °C,the activation energies, and to determine the type of

transport mechanism of diffusion of pyridine in macro-molecular 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 ispresented in Table 1. The coal sample was ground under anitrogen atmosphere to -60 mesh ASTM and stored undernitrogen. Coal sample was Soxhlet extracted with toluene-ethyl alcohol solvent couple (1:1) at its atmospheric boilingpoint to separate the resins of the coal and dried in a vacuumoven at 50 °C, for 24 h under a nitrogen atmosphere. Thissample is called raw coal throughout the text of the presentwork. The raw coal sample was demineralized with HCl andHF by standard methods.11 A volume of 2 L of 6 N HCl wasadded to 200 g of coal. The slurry was stirred for 24 h undera nitrogen atmosphere, then it was filtered and washed withdistilled water until the filtrate became neutral. Consecutively,1.6 L of aqueous (40%) HF was added to HCl-washed coal andthis mixture was stirred for ∼24 h under a nitrogen atmo-sphere. After filtering, the demineralized coal was washed with1 L of distilled water and dried at 50 °C, for 24 h undervacuum.

The raw and demineralized coal samples were extractedwith pyridine in a Soxhlet extractor under a nitrogen atmo-sphere until the color of the solvent in the sidearm of theextractor became colorless. The extracted samples were washedwith 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-extractedcoal samples were determined with a set of sieves of 800, 400,200, 100, 63, and 40 µm mesh size, and the average radii ofthe samples were calculated.

An adiabatic isothermal setup,12 designed and manufacturedin our laboratories and made from Plexiglass, which containeda heater and a digital temperature control system, an elec-tronic digital balance of 0.001 g accuracy, and a beaker filledwith pyridine, was used in the diffusion experiments. At thestart of experiment, about 0.25 g of coal sample was evenlydistributed in a Petri dish and the initial weight of the coalwas recorded. The temperature of the experiment was set andthe system was closed, flushed with nitrogen, and the weightincrease due to pyridine uptake was recorded until a constantweight was attained. The equilibrium time to reach a constantweight changed between about 3600 min and 2500 mindepending on the temperature set at the start of the experi-ment which varied from 20.0 to 27.0 °C, respectively.

The extent of swelling of original and treated lignite samplesin pyridine were measured according to the method given byLiotta et al.13 and Larsen.1 Approximately 100 mg of a coalsample was placed in a 6 mm o.d. tube and centrifuged for 5(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.

(11) Yurum, 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.

Mt

M∞) ktn (1)

Mt/M∞ ) 6[ Dtπa2]1/2

- 3Dta2

(2)

D ) Doe-EA/RT (3)

Table 1. Proximate and Ultimate Analyses of ElbistanLignite

Proximate Analysismoisture, % 2.7mineral matter, %, dry 33.6volatile matter, %, dmmf 68.5fixed carbon, %, dmmf 31.5

Ultimate Analysis, %, dmmfcarbon 53.0hydrogen 5.8nitrogen 1.8sulfur (total) 3.6oxygen (by difference) 35.8

136 Energy & Fuels, Vol. 15, No. 1, 2001 Seferinoglu and Yurum

min at 2500 rev/min. The height of the sample was measuredas h1. Excess pyridine (∼1 mL) was added in the tube and thecontents of the tube were mixed and the tube was centrifugedafter 24 h and the height of the sample in the tube (h2) wasmeasured. The volumetric swelling ratio was calculated as Qv

) h2/h1.

Results and Discussion

Particle Size Distribution. Particle size distribu-tion of the raw and acid-washed, and raw-pyridine-extracted and acid-washed-pyridine extracted coalsamples are presented in Figures 1 and 2, respectively.Particle size distribution of the acid-washed samplesshifted toward a slightly bigger particle size from amaximum at 100 µm in the case of raw coal to maximaat 200 µm in the case of HCl- and HCl/HF-washedsamples (Figure 1). It seemed that finer particlespresumably with higher contents of mineral mattersoluble in HCl solution were washed off during acidtreatments. Pyridine extraction of the raw coal formeda sample which showed a maximum at 200 µm (Figure2). In the case of acid-washed/pyridine-extracted samplesthe particle size distribution is bimodal (Figure 2). Whilethe first maximum in the size distribution was observedat 200 µm for all of the pyridine-extracted samples asecond maximum started to evolve at 800 µm. Thesituation is even more enhanced for the HCl/HF/pyridine-extracted sample. It seemed that considerablygreater fractions of bigger particles were produced whenthe 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 phenomenonmight be irreversible solvent swelling14 of some coalparticles after pyridine extraction. Even when thepyridine was evaporated from these samples in avacuum oven, these particles have retained their en-larged forms and thus some fraction of the coal mighthave become bigger in size. Enlargement of the size ofparticles might be due to aggregation of particles as aresult of some physical and chemical attraction. In thisway, very tiny coal particles containing some chargesmight join to form bigger particles.

Diffusion Coefficients. In an experiment, the py-ridine uptake of a coal sample was recorded untilequilibrium. A graph of Mt/M∞ versus t1/2 for thepyridine diffusion in raw Elbistan lignite at 20.9 °C ispresented in Figure 3. To determine the slope of thelinear portion of a similar graph, a new graph whichcontained about 60% of the data from the start of theexperiment was reconstructed.15,16 The graph of thistype for the same experiment is presented in Figure 4.The diffusion coefficients were measured from the slopeof such graphs for all of the samples. Figure 5 gives thechange of the measured diffusion coefficients of pyridinein raw and treated Elbistan lignite with temperature.The diffusion of pyridine in the raw coal seemed to beless, compared to those of the treated coal samples. Thediffusion coefficient of pyridine in raw coal increasedslightly from 5.1 × 10-5 m2/s to 7.1 × 10-15 m2/s whenthe temperature was elevated from 20.9 to 26.3 °C,respectively. Extraction of the raw coal with pyridineextended the pyridine diffusion in the coal very little.The diffusion constants of pyridine in pyridine-extractedcoals increased from 6.4 × 10-15 m2/s to 7.5 × 10-15 m2/swhen the temperature was increased from 21.8 to 25.4°C. Although pyridine extraction might have increasedthe surface area, it seemed that the porosity created bypyridine extraction was not effective to enhance thepyridine diffusion. The reason for this result might bethe relaxation of the coal during its extraction withpyridine 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;

Yurum, Y., Ed.; NATO ASI Series C, Vol. 244; Kluwer AcademicPublishers: Dordrecht, 1988; p 85.

(16) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379.

Figure 1. Particle size distribution of the raw, HCl-, and HCl/HF-washed Elbistan lignite samples.

Figure 2. Particle size distribution of the raw/pyridine-extracted, HCl-washed/pyridine-extracted, and HCl/HF-washed/pyridine-extracted Elbistan lignite samples.

Figure 3. Mt/M∞ versus t1/2 graph for the pyridine diffusionin raw Elbistan lignite at 20.9 °C.

Diffusion of Solvents in Coals Energy & Fuels, Vol. 15, No. 1, 2001 137

increase in the diffusion coefficient in later experimentscould be due to earlier structural relaxation.

The situation in the case of acid-treated coal samplesis particularly different. When the coal was treated firstwith HCl and then with HF the metal cations arewashed away and they are replaced with hydrogen ionsto form -COOH groups.17 When acid-treated coals willbe in contact with pyridine, the pyridine moleculeswhich are basic in chemical nature react with thesecarboxylic groups to form pyridine-coal hydrogen bonds.As the concentration of the carboxylic groups createdwith acid treatment were increased the number ofbonded pyridine molecules would be increased, too. Thisof course would enhance the diffusion of pyridine in thecoal. It seemed that an additional driving force for thepyridine diffusion in the acid-treated coals was theconcentration of newly formed carboxylic groups. Re-moval of divalent cations which had contributed to thecross-link density might have also added to the forma-tion of more relaxed structures which would allowhigher diffusion rates of pyridine. Diffusion constantsof pyridine in HCl-washed coals increased from 8.5 ×10-15 m2/s to 11.4 × 10-15 m2/s as the temperature was

raised from 21.7 to 24.8 °C. The case is more amplifiedin HCl/HF-washed coals in which presumably more newnascent carboxylic groups were produced. The diffusionconstants increased from 16.9 × 10-15 m2/s to 24.0 ×10-15 m2/s when the temperature was increased from22.0 to 25.5 °C, respectively. In all of the samples thediffusion constants increased linearly with an increasein the temperature. The slope of the linear relationshipwas similar in the case of raw and pyridine-extractedsamples and it started to increase sharply for HC-washed coal and even more sharply for the HCl/HF-washed coal.

The equilibrium swelling values of the coals in pyri-dine are parallel to the values of diffusion coefficients.Table 2 presents the equilibrium solvent-swelling valuesof the coal samples used in pyridine. The swelling ratiosof the raw and pyridine-extracted coal samples were 1.3and 1.2, respectively. The swelling ratios increased to1.6 and 1.5 for HCl- and HCl/HF-washed coals, respec-tively. The removal of minerals from coal increased thesurface area and the number of carboxylic groups in thestructure of coal samples and the amount by which thecoal samples were swollen. These effects both increasedthe diffusion rate. The greater diffusion coefficients areencountered with the coal samples which swelled morein pyridine.

The activation energies of diffusion measured in thepresent study are given in Table 2. The activationenergies fall in the range from 29.5 to 52.1 kJ/mol. Thissuggested the activation barrier was associated with thebreakage of internal electron donor-acceptor (hydrogenbonding) interactions.7 Since it has been proposed thata solvent will disrupt only those coal-coal hydrogenbonds whose bond strengths are lower than those of thecoal-solvent hydrogen bonds.18 It has also been sug-gested that pyridine, because of its strong basicity, iscapable of breaking nearly all hydrogen bonds in coal.19

Larsen et al.20 observed that a selective associationbetween the hydrogen-bond acceptor (pyridine) andhydroxyl groups which were cross-links between mac-romolecular chains in the coal existed, the selectivityof the acceptor for cross-linking hydroxyls over otherhydroxyls was due to the much more favorable entropychange which occurred when one of these cross-linkswas disrupted by formation of a new hydrogen bond topyridine. The present results support other publishedresults which showed a decrease in activation energywith pyridine extraction.21 Activation energies mea-sured in the present work are also in accord with thevalues of diffusion coefficients of pyridine for the dif-ferent types of coal samples. The activation energies

(17) Sugano, M.; Mashimo, K.; Wainai, T. Fuel 1999, 78, 945.

(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.

Figure 4. Mt/M∞ versus t1/2 graph for the pyridine diffusionin raw Elbistan lignite at 20.9 °C with the 60% of the datafrom the start of the experiment.

Figure 5. Change of the diffusion coefficients of pyridine inraw and treated Elbistan lignite with temperature.

Table 2. Solvent Swelling Ratios and ActivationEnergies for Diffusion of Pyridine in Coal Samples

sample Qactivation energy,

kJ/mol

raw coal 1.3 52.1raw coal/pyridine-extracted 1.2 47.9HCl-washed/pyridine-extracted 1.6 40.7HCl/HF-washed/pyridine-extracted 1.5 29.5

138 Energy & Fuels, Vol. 15, No. 1, 2001 Seferinoglu and Yurum

might be thought of as the energy required to producethe diffusive motion of one mole of penetrant molecules.A large activation energy results in a relatively smalldiffusion coefficient. The diffusion coefficient of pyridinein the raw coal was measured to be the smallest amongthose of other coal samples and the activation energyof pyridine diffusion in the raw coal was the greatestas 52.1 kJ/mol (Table 2). Activation energy resultsmeasured by Otake and Suuberg22 for lignites, (51-59kJ/mol) are in accord with the activation energy ofdiffusion measured for the raw Elbistan lignite. Activa-tion energy decreased to 47.9, 40.7, and 29.5 kJ/mol forthe raw/pyridine-extracted, HCl/pyridine-extracted, andHCl/HF/pyridine-extracted samples, respectively, (Table2). The greatest diffusion coefficients were measured forHCl/HF/pyridine-extracted coal sample and this was inparallel with the lowest activation energy of diffusionof this sample (29.5 kJ/mol) which indicated the moder-ate diffusion of pyridine in these samples. Activationenergies measured in the present work for the rawElbistan lignite, 53.0% C, was relatively greater thanthose of measured (31.8-44.6 kJ/mol), for higher rankcoals with carbon contents changing from 76.3 to 85.3%.The effect of coal rank is found to be effective in thediffusion of solvents in coals; activation energy ofdiffusion of pyridine in low rank coals is greater thanthose of in high rank coals.23

It seemed that the diffusion coefficients increasedwith swelling and acid washings as a result of structuralvariations decreasing the activation energy of diffusionof the solvent to the coal. Therefore, it can be concludedthat the diffusion process in the coal-pyridine systemis associated with swelling and chemical reactionsoccurring during diffusion of the solvent.

Type of Transport of Pyridine in Coal Structure.Table 3 presents the diffusion rate constants, diffusionexponents, and transport mechanisms of pyridine in coalsamples. R2 values in all of the experiments were equalto or greater than 0.99, indicating a linear relationshipbetween ln(Mt/M∞) and ln t. Using this fact it seemedthat diffusion of pyridine in coal samples can be ap-proximated with a first-order rate law for all of the coals

studied. Diffusion rate constants remained generallyunchanged as the temperature of diffusion was in-creased for all of the samples in the range of 20-26 °C.Ndaji and Thomas21 observed an increasing trend of 1order of magnitude in the diffusion rates of pyridine incoals of 76.3-85.3% C content when the temperaturewas raised from 20 to 60 °C. Diffusion rates measuredin the present work for the Elbistan lignite, 53.0% C,are smaller than those measured for higher rank ofcoals.21 The effect of transport rates has also beenstudied by Peppas and Lucht6 and Ritger and Peppas16

and a comparable conclusion of higher rates in higherrank of coals was reported.

The chain relaxation time is the reciprocal of thediffusion rate constant k obtained from analysis usingeq 1. For the coal samples studied in the present work,the relaxation time is of the order of 1800-6000 s. Itmust be stated that these values are much lower thanthose measured for coals with carbon contents in therange of 70.0-94.0%, 33 000 to 200 000 s, respectively.16

Thus, the transport mechanism of pyridine in themacromolecular coal network of Elbistan lignite can beconsidered as nonrelaxation controlled. It seemed thatpyridine diffusion in a young lignite like Elbistan lignite(Pleistocene-Pliocene, 2-5 million years B. P.)24 whichshould contain smaller macromolecules compared tocoals of higher ranks obeyed a mechanism close toFickian. The discussion below on the diffusion exponentssupport this claim.

The changes of diffusion exponents are presented inTable 3. The diffusion exponent, n, was calculated tobe equal or less than 0.5 in all experiments done attemperatures equal to or less than 22 °C for all of thesamples, indicating Fickian diffusion mechanism. As thetemperature was raised to higher values than 22 °C thediffusion coefficients increased to values less than 0.60except in the case of pyridine diffusion at 26.3 °C in rawElbistan lignite that is n ) 0.65. In all of the cases thetransport mechanism of pyridine might be consideredas anomalous (non-Fickian) transport. The n valuesmeasured in the present work are very close to Fickiantransport mechanism boundaries since the non-Fickiandiffusion coefficients in the literature usually are in the

(22) Otake, Y.; Suuberg, E. M. Fuel 1998, 77, 901.(23) Otake, Y.; Suuberg, E. M. Fuel 1989, 68, 1609.

Table 3. Diffusion Rate Constants, Diffusion Exponents, and Transport Mechanisms of Pyridine in Coal Samples

sample T, °C k, s-1 n R2 transport mechanism

raw Elbistan lignite 20.9 4.18 × 10-4 0.49 0.9986 Fickian22.9 2.77 × 10-4 0.56 0.9969 Fickian-anomalous23.3 2.55 × 10-4 0.56 0.9937 Fickian-anomalous24.3 3.57 × 10-4 0.53 0.9979 Fickian-anomalous26.3 1.66 × 10-4 0.64 0.9876 Fickian-anomalous

raw Elbistan lignite/pyridine-extracted 21.8 3.72 × 10-4 0.52 0.9929 Fickian-anomalous22.4 2.57 × 10-4 0.57 0.9956 Fickian-anomalous23.8 3.76 × 10-4 0.52 0.9940 Fickian-anomalous24.3 2.70 × 10-4 0.57 0.9944 Fickian-anomalous25.4 3.67 × 10-4 0.53 0.985 Fickian-anomalous

HCl-washed Elbistan lignite/pyridine-extracted 21.7 4.14 × 10-4 0.49 0.9866 Fickian22.5 4.00 × 10-4 0.52 0.9970 Fickian-anomalous23.5 4.03 × 10-4 0.52 0.9932 Fickian-anomalous24.3 2.84 × 10-4 0.56 0.9970 Fickian-anomalous24.8 3.04 × 10-4 0.56 0.9970 Fickian-anomalous

HCl/HF-washed Elbistan lignite/pyridine-extracted 22.0 5.35 × 10-4 0.47 0.9910 Fickian23.3 3.08 × 10-4 0.55 0.9969 Fickian-anomalous23.9 4.00 × 10-4 0.52 0.9940 Fickian-anomalous24.4 3.83 × 10-4 0.53 0.9931 Fickian-anomalous25.5 2.92 × 10-4 0.58 0.9939 Fickian-anomalous

Diffusion of Solvents in Coals Energy & Fuels, Vol. 15, No. 1, 2001 139

range of 0.6-1.0,6,21,23,25 therefore it will not be errone-ous to claim that the diffusion of pyridine in a low rankcoal like Elbistan lignite (53.0% C) was with a processsomewhere between Fickian and relaxation controlledas discussed by Ritger and Peppas.16

Conclusions

Pyridine extraction created a fraction of enlargedparticle size due to irreversible solvent swelling. Thediffusion of pyridine in the raw coal seemed to be less,compared to those of the treated coal samples. Extrac-tion of the raw coal with pyridine, extended the pyridinediffusion in the coal very little. The formation of new

carboxylic acid groups in acid-washed samples enhanceddiffusion of pyridine. In all of the samples the diffusionconstants increased linearly with an increase in thetemperature. The greater diffusion coefficients areencountered with the coal samples which swelled morein pyridine. It seemed that the diffusion coefficientsincreased with swelling and acid washings as a resultof structural variations decreasing the activation energyof diffusion of the solvent to the coal. The transportmechanism of pyridine in the macromolecular coalnetwork of Elbistan lignite was not relaxation con-trolled. The diffusion of pyridine in the low rankElbistan lignite obeyed generally Fickian mechanism oran intermediate case of Fickian-anomalous mechanism.

EF000159X(24) Karayigit, A. I.; Akdag, Y. Turk. J. Earth Sci. 1996, 7, 1.(25) Peppas, N. A. Polymer 1997, 38, 3425.

140 Energy & Fuels, Vol. 15, No. 1, 2001 Seferinoglu and Yurum