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Energy & Fuels 2006, 20, 1150-1156
Diffusion of Solvents in Coals: 2. Measurement of Diffusion Coefficients of Pyridine in C¸ ayirhan Lignite Meryem Seferinogˇlu†,‡ and Yuda Yu¨ru¨m*,§ Department of Chemistry, Hacettepe UniVersity, 06532 Beytepe, Ankara, Turkey, and Faculty of Engineering and Natural Sciences, Sabanci UniVersity, 34956 Tuzla, Istanbul, Turkey ReceiVed December 1, 2005. ReVised Manuscript ReceiVed March 10, 2006
The aim of this study is to measure the diffusion coefficients of pyridine in Turkish C¸ ayirhan lignite (C: 57.1 wt %, dmmf) at temperatures about 20-27 °C and determine the type of transport mechanism of diffusion. The raw coal sample was demineralized with HCl and HF by standard methods, and the raw and demineralized coal samples were extracted with pyridine. To investigate the diffusion of pyridine vapor in coal samples, the mass of pyridine uptake per mass of coal sample (Mt/M∞) was calculated as a function of time. The diffusion coefficients were measured from the slope of graphs of Mt/M∞ versus t1/2. The diffusion coefficient of pyridine in the raw coal increased from 10.0 × 10-15 to 11.9 × 10-15 m2/s when the temperature was elevated from 21.1 to 26.9 °C, respectively. The diffusion coefficients of pyridine raw coal and of those treated with HCl and HF were 11.9 × 10-15, 4.3 × 10-15 , and 4.8 × 10-15 m2/s at 26.9 °C, respectively. The studies in the present work on pyridine vapor diffusion in raw coals have generally indicated that the diffusion obeyed the Fickian diffusion mechanism the temperatures 20.0-27.0 °C. Generally, the diffusion exponent values increased when the temperature elevated from 20.0 to 27.0 °C, but this raise placed the diffusion of pyridine between the Fickian diffusion and Case II diffusion mechanisms.
Introduction Diffusion of solvents through coal is the main limiting step of many coal processes; therefore, it is necessary to understand the mechanism and kinetics of solvent diffusion into the coal matrix.1 Coal has a cross-linked three-dimensional network gel structure in which different physical forces such as hydrogen bonds, London, and van der Waals forces exist. In addition to these π-π and charge-transfer interactions intermolecular forces take place between coal molecules. The solvent molecules must break these intermolecular forces and penetrate the solid coal.2 The dynamics of solvent swelling of the macromolecular system can provide significant knowledge about the structure of the material itself and the interaction between the penetrant and macromolecular material. It may be possible to identify the thermodynamic glassy and rubbery states of the macromolecular system. If the system is in the glassy state, it can be determined whether the diffusion of the solvent is controlled by Fickian diffusion and/or by relaxation of the macromolecular system. It is known that coals are glassy, strained, macromolecular solids.3 The optical anisotropy of Illinois No. 6 coal viewed in a thin section through a polarizing microscope was associated with the existence of strain in a glassy system.4 If the coal was * Corresponding author. Phone: 90 216 4839512. Fax: 90 216 4839550. E-mail:
[email protected]. † Hacettepe University. ‡ Present address. Directorate of Customs and Customs Enforcement, Ankara Center Laboratory, Behic¸ bey, Ankara, Turkey. § Sabanci University. (1) Yu¨ru¨m, Y. Clean Utilization of Coal, Coal Structure and ReactiVity and EnVironmental Aspects; NATO Advanced Study Institute Series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol. 370. (2) Sharma, D. K.; Giri. C. C. Fuel 2000, 79, 577. (3) Larsen, J. W. In Clean Utilization of Coal; Yu¨ru¨m, Y., Ed.; NATO Advanced Study Institute Series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol. 370, p 2.
swollen with a good solvent such as pyridine, noncovalent bonds such as hydrogen bonds between coal-coal molecules could be removed, and hence the coal could convert to rubbery state.4,5 Glassy coals and rubbery coals are different materials. 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 macromolecule allows 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. At some critical small range of temperature, there is not only sufficient space for rotations and inter- and intrachain movements, but also sufficient thermal energy.6 In a rubbery solid, molecular motion is similar to that in a non-cross-linked polymer solution of the same composition. Diffusional limitations are of concern in virtually all types of coal processing. It is generally necessary to diffuse reactants into coal, and/or products or moisture out of coal particles at some point in any process. The studies of solvents diffusion have generally indicated that diffusion in coals is similar in many respects to the diffusion of solvents through glassy polymers.7 The similarities in structure between coal and glassy polymers have led to the application of theories of solvent diffusion behavior of polymers to coals. In the modeling of solvent diffusion in coal, it is necessary to assume that the particles are spherical, isotropic, nonporous, and identical. These assumptions are major approximations. In particular, coals are heterogeneous with different physical and chemical properties. It possible to (4) Brenner, D. Fuel 1985, 64, 167. (5) Nomura, S.; Thomas, K. M. Fuel 1998, 77, 829. (6) Peppas, N. A.; Lucht, M. L. Chem. Eng. Commun. 1985, 37, 333. (7) Otake, Y.; Suuberg, F. M. Energy Fuels 1997, 11, 1155.
10.1021/ef050399i CCC: $33.50 © 2006 American Chemical Society Published on Web 04/06/2006
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Energy & Fuels, Vol. 20, No. 3, 2006 1151
express the diffusional solvent penetration in terms of the general equation:8
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 that depends on structural characteristic 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 and describes a system in which the process is controlled by the diffusion coefficient. When n ) 1.0, case II transport occurs and characterizes the moving-boundary phenomenon. In the case II mechanism, the velocity of a sharp advancing front between the inner glassy core and the outer swollen rubbery material controls the process. When the values of n are between 0.5 and 1.0, this indicates anomalous transport and is characterized by a situation where the diffusional and structural relaxation energy are comparable.9 The solution of Fick’s second law of diffusion in spherical systems gives10 the following equation:
Mt 3Dt ) 6[Dt/πa2]1/2 - 2 M∞ a
(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, since the absolute value of 3Dt/a2 is much smaller than 6[Dt/πa2], the value of D is found from the slope of a plot of Mt/M∞ versus t1/2. The activation energy of diffusion is calculated using the equation11
D ) D0 e-Ea/RT
(3)
where D0 is a temperature-independent pre-exponential (m2/s), Ea is the activation energy for diffusion, R is the gas constant, and T is temperature. 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 study is to measure the diffusion coefficients in a temperature range of 21.0-27.0 °C, to determine the effect of demineralization and extraction processes on the solvent diffusion into coal matrix, and to determine the type of transport mechanism of diffusion of pyridine in Turkish C¸ ayirhan lignite. Experimental Section Turkish C¸ ayirhan lignite with a carbon content of 51.7 wt % (dmmf) was used in the study. Analysis of the C¸ ayirhan lignite is presented in Table 1. The coal sample was ground under a nitrogen atmosphere to 60 mesh ASTM and stored under nitrogen. The coal sample was Soxhlet-extracted with toluene/ethyl alcohol solvent couple (1:1) at its atmospheric boiling point to separate the resins of coal and was 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 samples were deminer(8) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379. (9) Ndaji, F. E.; Thomas, K. M. Fuel 1993, 72, 1525. (10) Crank, J. Mathematics of Diffusion; Oxford University Press: London, 1970. (11) Callister, W. D., Jr. Material Science and Engineering, 2nd ed.; Wiley: New York, 1991.
Table 1. Proximate and Ultimate Analyses of C¸ ayirhan Lignite proximate analysis moisture, % mineral matter, %, dry volatile matter, %, dmmf fixed carbon, %, dmmf
1.8 22.0 30.9 45.3
ultimate analysis, %, dmmf carbon hydrogen nitrogen sulfur (total) oxygen (by difference) H/C O/C carbon aromaticity,a fa
57.1 4.5 1.8 5.7 30.9 0.96 6.5 0.6
a The carbon aromaticity (f ) for the C ¸ ayirhan coal sample was calculated a using an equation that was defined for low rank coal by Mazumdar. fa ) 12 1.20-0.617 H/C (Mazumdar, 1999).
alized with HCl and HF by standard methods.13 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, and then it was filtered and washed with distilled water until the filtrate became neutral. Consecutively, 1.6 L of distilled aqueous (40%) HF was added to HCl-washed coal, and the mixture was stirred for about 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 exhaustively in a Soxhlet extractor under a nitrogen atmosphere until the color of solvent in the sidearm of the extractor became colorless. After extracting the coal exhaustively with the carbon disulfide (CS2)/N-methyl-2-pyrrolidinone (NMP) mixed solvent, Takahashi et al.14 washed the residue with acetone to remove CS2 and retain NMP. A similar technique was employed in the present study; the extracted samples were stirred with 500 mL of ethanol (98%, by volume) in a flask for 24 h and dried in a vacuum oven at 60 °C to remove pyridine retained in the pores of the coal. Particle size distributions of raw and pyridine-extracted coal samples were determined with a set of sieves of 800, 400, 200, 63, and 40 µm mesh size, and the average radii of the samples were calculated. An adiabatic isothermal setup,15 designed and manufactured in our laboratories and made from Plexiglas, which contained a heater and 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 the 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 up, the system was closed and 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 and 2500 min depending on the temperature set at the start of the experiment, which varied from 21.0 to 27.0 °C, respectively. The extent of swelling of original and treated lignite samples in pyridine was measured according to the methods given by Larsen3 and Liotta et al.16 Approximately 100 mg of a coal sample was placed in a 6-mm o.d. tube and centrifuged for 5 min at 3500 rev/ min. The height of the sample was measured as h1. Excess pyridine (∼1 mL) was added to the tube, the contents of the tube were mixed, the tube was centrifuged after 24 h, and the height of the sample (12) Mazumdar, B. K. Fuel 1999, 78, 1097. (13) Yu¨ru¨m, Y.; Kramer, R.; Levy, M. Thermochim. Acta 1985, 94, 285. (14) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, 141. (15) Seferinogˇlu, M. Ph.D. Thesis. Hacettepe University, Ankara, Turkey, 1999. (16) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277.
1152 Energy & Fuels, Vol. 20, No. 3, 2006
Seferinogˇ lu and Yu¨ru¨m
Figure 1. Particle size distribution of raw, HCl-, and HCl/HF-washed C¸ ayirhan lignite samples. Figure 3. Mt/M∞ versus t1/2 for the pyridine diffusion in raw C¸ ayirhan lignite at 21.1 °C.
Figure 2. Particle size distribution of the raw/pyridine-extracted, HClwashed/pyridine-extracted, and HCl/HF-washed/pyridine-extracted C¸ ayirhan lignite.
Figure 4. Mt/M∞ versus t1/2 for the pyridine diffusion in raw C¸ ayirhan lignite at 21.1 °C with 60% of the data from the start of the experiment.
in the tube (h2) was measured. The volumetric swelling ratio was calculated as Qv ) h2/h1.
Results and Discussion Particle Size Distribution. Particle size distributions of the raw, acid-washed, raw-pyridine-extracted and acid-washed/ pyridine-extracted coal samples are presented in Figures 1 and 2, respectively. Particle size of the raw coal showed a maximum at 200 µm. This fraction constituted about 70% (by weight) of the whole sample. In the case of HCl- and HCl/HF-washed samples, particle size shifted toward a slightly smaller particle size to maximal at 100 µm (Figure 1). Some bigger particles of coal presumably contained higher contents of mineral matter which was soluble in HCl and in HF solutions and those minerals were washed off during acid treatment and therefore a size reduction was observed. Krzesin´ska17 claimed that HCl acid removes oxide, carbonates (such as calcite), exchangeable cations, and monosulfides from coal structure and affects the physical structure of coal. Similarly, HF acid removes silicate minerals.17,18 Particle size distribution of pyridine-extracted raw and acidwashed coal samples showed a similar trend (Figure 2). Pyridine-extracted raw and acid-washed coal formed a sample that was composed of approximately 30% (by weight) of the whole sample and showed a maximum particle size at 200 µm. This indicated that pyridine extraction was not effective to change the particle size of C¸ ayirhan lignite. Diffusion Coefficients. A C¸ ayirhan lignite sample with an average particle radius that changed in the range of 5.9 × 10-5 to 10.9 × 10-5 m was used. The coal particles were assumed to be of spherical shape which had completely similar features. (17) Krzesin´ska, M. Energy Fuels 1997, 11, 686. (18) Davidson, R. Models of Occurrence of Trace Elements in Coal; IEA Coal Research CCC/36; The Clean Coal Centre: London, June 2000.
Figure 5. Change of diffusion coefficients of pyridine in raw and acidwashed C¸ ayirhan lignite with temperature. ], Raw coal. 0, HCl-washed coal. 4, HCl/HF-washed coal.
In all experiments, the pyridine uptake of C¸ ayirhan lignite was recorded until equilibrium was reached. A graph of Mt/M∞ versus t1/2 for pyridine diffusion in raw C¸ ayirhan lignite at 21.0 °C is presented in Figure 3. To determine the slope of the linear portion of a similar graph, a new graph that contained approximately 60% of the data from the start of the experiment was reconstructed.6,9 This type of graph for the same experiment is given in Figure 4. The diffusion coefficients were measured from the slope of such graphs for all the samples. Figure 5 gives the change of the measured diffusion coefficients of pyridine in raw and acidtreated C¸ ayirhan lignite with temperature. Increasing temperature led to an increase in diffusion coefficients. This situation might be attributed to an increase of the concentration of pyridine vapor when temperature was increased. Therefore, the pyridine molecules diffused into the coal matrix at a faster rate. In previous reports,19,20 it was observed also that there was a (19) Wargadalam, V. J.; Norinaga, K.; Lino, M. Fuel 2002, 81, 1403.
Measuring Diffusion Coefficients in C¸ ayirhan Lignite
tendency toward an increase in diffusion with increasing temperature. It is suggested that the final extent of swelling was not sensitive to temperature, but the rate of swelling increased significantly with increasing temperature.20 Sharma and Giri2 studied the kinetics of diffusion of N-methyl-2-pyrolidone (NMP) swelling of coal. It was also found that the rate of swelling of coal in NMP was faster at higher temperature. It is proposed that the probable reason for this phenomenon at higher temperatures is the increase of kinetic energy of the solvent molecules, and hence, the molecules diffuse into the coal matrix at a faster rate. It was observed in the present work that the diffusion coefficient of pyridine in the raw coal sample slightly increased from 1.0 × 10-14 to 1.2 × 10-14 m2/s when the temperature was elevated from 21.1 to 26.9 °C (Figure 5). All of the acidtreated coal samples also showed a similar trend. In the case of the HCl-washed coal samples, the diffusion coefficients ranged between 3.8 × 10-15 and 4.3 × 10-15 m2/s with an increase in temperature. With regard to HCl/HF-treated coal samples, the diffusion coefficients changed from 3.0 × 10-15 to 4.8 × 10-15 m2/s when temperature was elevated from 22.6 to 25.6 °C. The diffusion of pyridine in the raw coal was rather high compared to those of acid-treated coal samples. With respect to this result, demineralization of the raw coal might be responsible for a reduction in the pyridine uptake of the demineralized coal sample. It seemed that the minerals in coal enhanced the pyridine diffusion. It is possible that pyridine interacted preferentially and strongly with the minerals in coal. Pyridine is known to be an organic Lewis base and hydrogen bond acceptor stronger than any other nitrogen-containing compound. It is also well-known that lignite and lower rank coals are highly oxygenated.21 Hence, it is thought that the noncovalent interaction such as hydrogen bonds would be between pyridine molecules and mineral components in coal. As the concentration of minerals that are capable of forming hydrogen bonds increases, the number of bonded pyridine molecules can be increased. Therefore, diffusion of pyridine in raw coal must have increased. When the coal was treated with HCl and HF acids, HCl removed carbonate and ion-exchangeable minerals, while HF removed silicate and clay minerals from the coal. It was estimated that pyridine molecules could interact with carbonate and/or ion-exchangeable mineral matter and with silicate and/or clay minerals on the original coal surface. When these minerals were removed by acid treatment, the interaction between pyridine molecules and minerals might be reduced. That might be the possible reason for the lower values of the diffusion coefficients of pyridine in acid-treated coal samples compared to those in raw coal. Another reason for the reduction of the diffusion of pyridine in acid-treated coal samples might be the collapse of the physical structure; the total porosity of subbituminous coals is about 25-30%.22 Using the dependence of coal porosity on coal rank, one can deduce that the lignite used in the present work was also a highly porous material. Porosity as well as defects (e.g., cracks) associated with minerals can affect measurements of diffusion parameters. The removal of minerals from coal may cause the collapse of the structure and decrease of number of cracks resulting in decrease of observed diffusion. Some organic bases such as alcohols and amines have both an unshared electron pair and a hydrogen atom to donate in the (20) Otake, Y.; Suuberg, E. M. Fuel 1989, 68, 1609.(21) Sugano, M.; Mashimo, K.; Wainai, T. Fuel 1999, 78, 945.(22) van Krevelen, D. W. Coal: Typology, Physics, Chemistry, Constitution, 3rd ed.; Elsevier: Amsterdam, 1993; p 197.
Energy & Fuels, Vol. 20, No. 3, 2006 1153
Figure 6. Change of the diffusion coefficients of pyridine in raw and pyridine-extracted C¸ ayirhan lignite with temperature. ], Raw coal. 0, Raw coal/pyridine extracted. 4, HCl-washed coal/pyridine extracted. O, HCl/HF-washed coal/pyridine extracted.
interactions with a surface. Those adsorbates may form hydrogen bonds to the surface by accepting or donating a hydrogen atom. Some adsorbates interact preferentially and strongly with coal mineral matter.23 Glass and Larsen23 reported that alcohol, amines, and pyridine molecules had more exothermic specific adsorption heats on the original coal (17% ash) compared to that of demineralized (2% ash) coal. Pyridine, alcohols, and amines were found to interact more strongly with carbonate and/ or ion-exchangeable minerals and with silicate or clay minerals on the original coal surface from the IGC data. It was proposed that pyridine, alcohol, and amines have additional interactions with the mineral matter-containing coals.23 Figure 6 shows the change of the diffusion coefficients of pyridine in raw and pyridine-extracted C¸ ayirhan lignite with temperature. The diffusion coefficients of pyridine in all of the pyridine-extracted coal samples seemed to be less, compared to that of the raw coals. Moreover, extraction of raw coal with pyridine decreased the pyridine diffusion in the coal very much. Similar results were also obtained by Seferinogˇlu and Yu¨ru¨m24 for Elbistan lignite. The diffusion coefficients of pyridine in pyridine-extracted raw coal increased from 2.9 × 10-15 to 4.1 × 10-15 m2/s when temperature was elevated from 22.7 to 26.7 °C, respectively. Although pyridine extraction might have increased the surface area, it appears 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 created irreversible solvent swelling of coal. It is well-known that removing pyridine from coal after extraction is very difficult, and some pyridine can remain in the coal structure even after drying under vacuum at their boiling points.7,25 Therefore, one possible reason for the reduced diffusion of pyridine into coal might be the presence of some amounts of pyridine that remained after pyridine extraction of the coal that prevented the diffusion of extra amounts of pyridine into the coal matrix. Yu¨ru¨m et al.26 reported that pre-extraction of Zonguldak bituminous coal with pyridine caused a sharp decrease in pyrolysis yield and the initial glass transition temperature increased compared with that of untreated coal. The reason for this result was that small amounts of pyridine remained sorbed in the dried coal after pyridine extraction. In the pyridine (23) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 629. (24) Seferinogˇlu, M.; Yu¨ru¨m, Y. Energy Fuel 2001, 15, 135. (25) Nishioka, M. Fuel 1993, 72, 997. (26) Yu¨ru¨m, Y.; Karabakan, A. K.; Altuntas¸ , N. Energy Fuels 1991, 5, 701.
1154 Energy & Fuels, Vol. 20, No. 3, 2006
extraction process, while pyridine swells the solid coal matrix it also diffuses into the micropores of the solid coal. Pyridine breaks the intermolecular forces between coal molecules such as hydrogen-bonding, van der Waals and London forces, and π-π and charge-transfer interactions and then diffuses out with coal molecules soluble in pyridine during extraction. It was estimated that the noncovalent association such as hydrogen bonds could occur between pyridine and coal molecules during pyridine extraction,27 and these proportionally could reduce the diffusion coefficient of pyridine in pyridineextracted raw coal. In the event of acid-treated/pyridine-extracted coal samples, diffusion coefficients were found to be higher than those of pyridine-extracted raw coal. Diffusion coefficients of pyridine in HCl-washed/pyridine-extracted coals increased from 4.1 × 10-15 to 5.3 × 10-15 m2/s as the temperature was raised from 22.3 to 25.8 °C. In the case of HCl/HF-washed/pyridineextracted coal samples, diffusion coefficients have also been observed to increase from 5.3 × 10-15 to 6.4 × 10-15 m2/s when the temperature was increased from 22.0 to 26.6 °C, respectively. The situation in the event of acid-treated coal samples is far different than that for pyridine-extracted coal samples. In the first step, coal was treated with HCl and then with HF acids. In the second step, the acid-treated coal sample was extracted with pyridine. When the coal was treated with HCl and HF acids, the metal cations (such as K+, Na+, Mg2+, Ca2+, Fe3+, and Al3+) which were known to be the major elements in coal mineral structure were removed from coal, and they were replaced with hydrogen ions to form -COOH groups.21 When acid-treated coal was in contact with pyridine, the pyridine molecules which are basic in chemical nature reacted with these carboxylic groups to form pyridine-coal hydrogen bonds. The number of bonded pyridine molecules would be increased while the concentration of carboxylic groups created by acid treatment was increased. This may enhance the diffusion of pyridine in the coal. It was estimated that the concentration of newly formed carboxylic groups was probably the driving force for the pyridine diffusion in the acid-treated coals. Further, removal of divalent cations which had contributed to cross-link density of coal could have also added to formation of a more relaxed structure which would allow higher diffusion rates of pyridine. When the diffusion coefficient of pyridine in HCl-washed coal was compared with that of HCl/HF-washed/pyridineextracted coal, it was found that pyridine diffusion in HCl/HFwashed coal was slightly higher than that of only HCl-washed coal. This result might be also concerned with producing the newly nascent carboxylic groups during HCl/HF washing of coal. Consequently, in all of the samples, the diffusion coefficients of pyridine increased linearly with an increase in the temperature. It has also been found that the mineral matter that existed in raw coal was effective to increase the rates of diffusion of pyridine in coals. When the coal was treated with HCl/HF acids, new carboxylic groups probably formed after pyridine-extracted coal samples. As the concentration of the carboxylic groups increased, the diffusion coefficients of pyridine in pyridineextracted coal probably increased. The equilibrium swelling values of coal samples in pyridine were determined for raw, acid-treated, and pyridine-extracted coals in the present study and are presented in Table 2. The swelling ratios slightly decreased from 1.6 in the case of the (27) Larsen, J. W.; Flower, R. A.; Hall, P. J. Energy Fuels 1997, 11, 998.
Seferinogˇ lu and Yu¨ru¨m Table 2. Solvent-Swelling Ratios and Activation Energy of Diffusion of Pyridine in Coal Samples
sample
Q
activation energy of diffusion, EA, kJ/mol
raw C¸ ayirhan lignite HCl-washed C¸ ayirhan lignite HCl/HF-washed C¸ ayirhan lignite raw C¸ ayirhan lignite/pyridine-extracted HCl-washed C¸ ayirhan lignite/pyridine-extracted HCl/HF-washed C¸ ayirhan lignite/pyridine-extracted
1.6 1.8 1.8 1.3 1.6
22.5 33.1 75.1 65.4 56.0
1.9
28.6
raw coal to 1.3 in the case of pyridine-extracted coal. This result was confirmed with diffusion coefficient values obtained for raw and extracted coal samples. The swelling values provided evidence that it formed a newly nascent noncovalent interaction such as a hydrogen bond between pyridine and coal during pyridine extraction. It would appear that this effect was responsible for reducing the swelling value and diffusion coefficient of pyridine in coal. The swelling ratios increased to 1.8 for only demineralized coals and to 1.6 and 1.9 for HCl- and HCl/HF-washed/pyridineextracted coals, respectively. The removal of minerals from coal almost certainly increased the surface area and the number of carboxylic groups in structure of coal samples and the amount by which coal samples were swollen. It seemed that this effect increased the diffusion rates for acid-washed/pyridine-extracted coal samples and pyridine-extracted raw coal. The removal of minerals from coals caused increases in the swelling values but conversely decreased the diffusion constants of pyridine in coal. The reason for this phenomenon might be enhanced absorption of pyridine in minerals in raw coal. Activation Energy of Diffusion of Pyridine in Coal. In the present study, the activation energy of diffusion for all coal samples was calculated by the slope, -EA/R, of the straight line of the plot of ln D versus 1/T. The calculated activation energy values for raw and treated coals are given in Table 2. It was observed that the values of activation energy for diffusion process all fall in the range from 22.5 to 75.1 kJ/mol. These results are supported by a previous observation28 that the activation barrier is associated with the breakage of an internal electron donor-acceptor (such as hydrogen bonding) interaction. It has been suggested that a solvent will disrupt only those coalcoal hydrogen bonds whose bond strengths were lower than those of coal-solvent hydrogen bonds. The activation energies measured are in accord with the values of diffusion coefficients of pyridine for raw and treated C¸ ayirhan lignite. The activation energy might be thought of as the energy required to initiate the diffusive motion of 1 mol of the penetrant molecules. A large activation energy results in a relatively small diffusion coefficient. It was observed in the present work that the diffusion coefficient of pyridine in raw coal was the biggest compared to the others. The activation energy of pyridine diffusion in the raw coal was the smallest as 22.5 kJ/mol (Table 2). In the case of acid-treated coals, activation energy increased from 22.5 to 33.1 kJ/mol for HCl-washed and to 75.1 kJ/mol for HCl/HF-washed coal, in conformity with decreased diffusion coefficients of pyridine in these coal samples. These results provided evidence that pyridine molecules interact preferentially and more strongly with the inorganic component than with the organic component of the coal surface. (28) Otake, Y.; Suuberg, F. M. Fuel 1998, 77, 901.
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Energy & Fuels, Vol. 20, No. 3, 2006 1155
Table 3. Diffusion Rate Constants, Diffusion Exponent, and Transport Mechanisms of Pyridine in Raw and Acid-Washed Coal Samples T, °C
sample raw C¸ ayirhan lignite
HCl-washed C¸ ayirhan lignite
HCl/HF-washed C¸ ayirhan lignite
21.1 22.3 23.5 25.3 26.9 22.6 23.5 24.1 24.7 25.6 21.6 22.6 24.7 25.1 26.5
k, s-1 10-4
3.62 × 3.51 × 10-4 3.15 × 10-4 2.11 × 10-4 4.22 × 10-4 4.36 × 10-4 2.67 × 10-4 4.52 × 10-4 4.21 × 10-4 4.96 × 10-4 3.02 × 10-4 8.51 × 10-4 3.35 × 10-4 3.04 × 10-4 3.48 × 10-4
n
R2
transport mechanism
0.52 0.53 0.54 0.60 0.50 0.51 0.58 0.51 0.52 0.50 0.52 0.39 0.54 0.55 0.54
0.9991 0.9965 0.9985 0.9937 0.9961 0.9991 0.9950 0.9985 0.9992 0.9990 0.9958 0.9873 0.9959 0.9967 0.9975
Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian Fickian-anomalous Fickian Fickian-anomalous Fickian-anomalous Fickian-anomalous
Table 4. Diffusion Rate Constants, Diffusion Exponent, and Transport Mechanisms of Pyridine in Raw/Pyridine-Extracted and Acid-Washed/ Pyridine-Extracted Coal Samples sample raw C¸ ayirhan lignite/pyridine-extracted
HCl-washed C¸ ayirhan lignite/pyridine-extracted
HCl/HF-washed C¸ ayirhan lignite/pyridine-extracted
T, °C 22.7 23.5 23.9 24.4 26.7 22.3 22.6 24.4 25.8 22.0 22.7 24.0 24.8 26.6
It was observed that activation energy increased to 65.4 kJ/ mol for pyridine-extracted raw coal contrary to its raw coal. This behavior was thought to be caused by the removal of pyridine from the swollen coal by the refluxing treatment, with consequential rearrangement, reassociation, and shrinking of the coal structure to a conformation of lower free energy containing more noncovalent interaction after pyridine extraction.9 Pyridine, because of its strong basicity, is capable of breaking nearly all hydrogen bonds in coal. Therefore, pyridine extraction disrupts the hydrogen bonds in the coals and causes swelling. Ndaji and Thomas9 reported that the swelling of the extracted coal in pyridine is markedly lower than that for the coal, indicating some modification of the macromolecular structure (e.g., covalent cross linking and π-π interactions). They suggested that the extraction process may cause decomposition, leading to the formation of cross-links or the collapse of the structure because the removal of soluble material leads to the formation of strong π-π interactions which act as effective noncovalent cross links. When the activation energy of pyridine diffusion in pyridineextracted coals is compared with each other, the value of activation energy decreased to 65.4, 56.0, and 28.6 kJ/mol for the raw/pyridine-extracted, HCl/pyridine-extracted, and HCl/ HF/pyridine-extracted samples, respectively (Table 2). These results are in agreement with increasing diffusion coefficients of pyridine in these coals. The biggest diffusion coefficient was measured for HCl/HF-washed/pyridine-extracted coal samples, and this was parallel with the lowest activation energy of diffusion of this sample (22.6 kJ/mol). These results might be associated with the formation of carboxylic groups during acid treatment of coal. It can be declared that the value of activation
k, s-1 10-4
2.86 × 2.63 × 10-4 4.44 × 10-4 3.12 × 10-4 3.80 × 10-4 6.22 × 10-4 2.67 × 10-4 2.07 × 10-4 2.24 × 10-4 3.11 × 10-4 4.19 × 10-4 4.22 × 10-4 3.00 × 10-4 3.91 × 10-4
n
R2
transport mechanism
0.55 0.56 0.49 0.56 0.53 0.46 0.60 0.62 0.60 0.54 0.50 0.50 0.55 0.52
0.9982 0.9943 0.9971 0.9956 0.9978 0.9880 0.9942 0.9956 0.9968 0.9911 0.9969 0.9981 0.9964 0.9957
Fickian-anomalous Fickian-anomalous Fickian Fickian-anomalous Fickian-anomalous Fickian Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian-anomalous Fickian Fickian Fickian-anomalous Fickian-anomalous
energy of diffusion of pyridine in acid-treated coal decreases as the concentration of carboxylic groups was increased. Type of Transport of Pyridine in Coal Structure. Tables 3 and 4 present 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 studied. The values of k and n are constants for particular diffusion systems. The diffusion rate constant may only be viewed as a proportionality constant in the empirical rate law and does not clearly represent either the diffusion coefficient or the relaxation constant for the coal. The unit of k depends on the value of n, but the values are consistent with t in minutes. Therefore, values of k obtained at two different temperatures cannot be directly compared unless the values of n are the same at the two temperatures.7,28 It might be better to compare the chain relaxation times. The chain relaxation time is the reciprocal of diffusion rate constant k obtained from analysis using eq 1 at constant n value (n ) 1). For the coal samples in the present work, the relaxation time is of the order of 1100-4900 s. The relaxation time for Elbistan lignite with 53.0% C content obtained in our previous study was of the order of 1800-6000 s.22 It must be stated that these values obtained for C¸ ayirhan and Elbistan lignite are much lower than those measured for coals with C contents in the range from 70.0 to 94.0%, 33 000 to 200 000 s, respectively.8 The effect of transport rates has also been studied by Peppas and Lucht.6 It was observed that the diffusion rate of pyridine in coals
1156 Energy & Fuels, Vol. 20, No. 3, 2006
increased while the C content of coals was increasing. It was found that the value of the relaxation constant depended on the C content of coal and was on the order of 10-4.6 Thus, the transport mechanism of pyridine in the macromolecular coal network of C¸ ayirhan lignite may be considered as nonrelaxation controlled. It was concluded that pyridine diffusion in C¸ ayirhan lignite that contained a macromolecule smaller than that compared to coals of higher ranks obeyed a mechanism close to Fickian. Tables 3 and 4 show the variation of diffusion exponents to determine the mechanism of pyridine diffusion in the coals. In general, the values of n changed when the temperature was increased, but the diffusion mechanism was not changed. Otake and Suuberg7 reported that the values of n are reasonably constant over the temperature range of interest (10-60 °C). The diffusion exponent, n, was calculated to fall in the range from 0.39 to 0.60 for raw and acid-treated coal samples. In case of pyridine-extracted raw and acid-treated coal samples, the diffusion exponent values were observed to change in the range of 0.46-0.60. These values remained generally between 0.5 for Fickian diffusion and 1.0 for relaxation (case II) controlled diffusion. These results indicated that diffusion of pyridine through C¸ ayirhan lignite was controlled by an anomalous (nonFickian) diffusion mechanism. In general, diffusion mechanisms for solvent trough coals have been shown to usually vary between the extremes that are Fickian and relaxation controlled diffusion.7,29 The n values measured in the present work were nearer to the values for Fickian diffusion control than the values for relaxation control. The anomalous diffusion coefficients in the literature usually are in the range of 0.5-1.0.2,9 Therefore, it will not be erroneous to claim that the diffusion mechanism of pyridine in a low rank coal was with a process somewhere between Fickian and relaxation controlled. (29) Hall, P. J.; Thomas, M. K.; Marsh, H. Fuel 1992, 71, 1271.
Seferinogˇ lu and Yu¨ru¨m
Conclusions The acid treatments of coal created a fraction of reduced particle size due to removal of mineral contents from coal. In all of the samples, the diffusion coefficients of pyridine increased linearly with an increase in the temperature. The diffusion of raw coal was observed to be higher, compared to those of the treated coal samples. It seemed that the mineral contents of coal were much more effective to increase the rates of diffusion of pyridine in coals. Extraction of raw coal with pyridine decreased the pyridine diffusion into coal matrix. But in the case of acidwashed/pyridine-extracted coal samples, the diffusion coefficients of pyridine were observed to increase. The formation of new carboxylic acid groups in the acid-washed sample enhanced diffusion of pyridine. It was observed that, while the diffusion of pyridine in coal increased, the activation energy gradually decreased. It might be noted that the coals with higher rates of pyridine diffusion generally exhibited lower activation energy for the diffusion process. The smaller diffusion coefficients were encountered with the coal samples which were extracted with pyridine. It seemed that the diffusion coefficient decreased with irreversible pyridine swelling of coal, as a result of structural variations increasing the activation energy of diffusion of pyridine to the coal. The values of diffusion exponents were found to vary in the range of 0.39-0.62 for raw, acid-treated, and pyridine-extracted coal samples. It was also observed that the n values were closer to Fickian transport mechanism boundaries than that of case II transport mechanism. Therefore, it was assumed that the diffusion of pyridine in the low rank C¸ ayirhan lignite was governed generally by Fickian mechanism or an intermediate case of Fickian anomalous mechanism. Furthermore, the temperature did not changed diffusion mechanism at the range studied in the present work. EF050399I
