Energy & Fuels 2001, 15, 1063-1068
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Articles Studies of Kinetics of Diffusion of N-Methyl-2-pyrrolidone (NMP), Ethylenediamine (EDA), and NMP+EDA (1:1, vol/vol) Mixed Solvent System in Chinakuri Coal by Solvent Swelling Techniques Shailaja Pande and D. K. Sharma* Centre for Energy Studies, Indian Institute of Technology, Delhi, New Delhi-110016, India Received November 15, 1999. Revised Manuscript Received May 30, 2001
Kinetics of swelling of Chinakuri coal in NMP, EDA, and NMP+EDA (1:1) (vol/vol) mixed solvent system were studied at temperatures ranging from 15 °C to 60 °C. The results obtained indicated that the diffusion of NMP, EDA, and NMP+EDA (1:1) in coal was described by Fickian diffusion. The values of diffusion exponent “n” were closer to Fickian diffusion in most cases though in some cases the value of “n” was even as low as 0.3. The activation energies for the combined diffusion and swelling process were in the range 20-30 kJ/mol suggesting that the activation energy barrier may be associated with the disruption of hydrogen-bonding interactions in coal. The present paper reports the details of the studies.
Introduction Coal is a cross-linked macromolecule.1-3 The crosslinks of the macromolecular network of coal consist of both covalent and noncovalent interactions.4,5 The covalent cross-links are mainly ethylenic, methylenic, and ether linkages,5,6 while the noncovalent coal-coal interactions include hydrogen bonds, van der Waals’ forces, and the π-π aromatic interactions.7,8 The swelling behavior of coal is considered to be a property of its cross-linked structure.4,9,10 Swelling of coal is the first step in any process (such as solvent extraction) where coal is brought in contact with an organic solvent for which it has an affinity.11 As the solvent penetrates the coal matrix, the coal-coal interactions are replaced by more favorable coal-solvent interactions.11 This causes the coal to swell to accommodate the solvent.12 Swelling is caused by the breakage of noncovalent interactions in coal such as hydrogen bonds in the case of polar solvents and dispersion forces in the case of nonpolar solvents.13 Painter et al.14 have applied theories of * Corresponding author. (1) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (2) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (3) Hall, P. J.; Thomas, K. M.; Marsh, H. Fuel 1992, 71, 1271. (4) Chen, C.; Gao, J.; Yan, Y. Energy Fuels 1998, 12, 1328. (5) Ndaji, F. E.; Thomas, K. M. Fuel 1993, 72, 1531. (6) Juntgen, H. Fuel 1984, 63, 731. (7) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (8) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107. (9) Schobert, H. H. The Chemistry of Hydrocarbon Fuels; Butterworths: London, 1990. (10) Takanohashi, T.; Iino, M. Energy Fuels 1995, 9, 788. (11) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 1538. (12) Larsen, J. W.; Lee, D. Fuel 1985, 64, 981. (13) Hall, P. J.; Marsh, H.; Thomas, K. M. Fuel 1988, 67, 863.
swelling to coal using pyridine as solvent and modified them to account for hydrogen bonding. In fact, Painter et al. have also proposed a model for coal swelling based on a process called disinterspersion.15 Swelling and extraction are specifically connected, extraction always being accompanied by coal swelling.16 Both extraction and swelling have common fundamental causes, i.e., coal-solvent interactions.17 During solvent extraction of coal, the solvent must diffuse inside the solid coal matrix, in the process swelling the matrix, and then diffuse out with soluble coal molecules.18 Diffusion is often the rate-limiting step in reactions of coal.3,19 Swelling of coal with organic solvents may help in understanding the diffusion of solvent inside the coal as well as help in understanding the macromolecular network properties of coal. 5,13,20 Dynamic swelling studies may help to determine the mechanism of solvent uptake that occurs in the network as well as to analyze the interaction between coal macromolecules and solvent.21 The solvent uptake can occur by Fickian diffusion, anomalous transport, Case(14) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 384. (15) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 393. (16) Szeliga, J.; Marzec, A. Fuel 1983, 62, 1229. (17) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (18) Giri, C. C. Ph.D. Thesis. Studies on development of a process for solvent deashing of coal to obtain environmentally clean fuels and characterization of products. I. I. T. Delhi, New Delhi, India, 1995. (19) Larsen, J. W.; Lee, D. Fuel 1983, 62, 1351. (20) Painter, P. C.; Park, Y.; Coleman, M. M. Energy Fuels 1988, 2, 693. (21) Barr-Howell, B. D.; Howell, J. M.; Peppas, N. A. Energy Fuels 1987, 1, 181.
10.1021/ef9902395 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/28/2001
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II transport, or super Case-II transport.21 Several studies have been reported on the kinetics of diffusion of solvents in coal by solvent swelling techniques.3,5,22-27 These studies indicate that the nature of the swelling process varies from relaxation-controlled to Fickian diffusion-controlled. It has been found that the diffusion tends to follow the Case II rate law, i.e., a relaxationcontrolled mechanism at ambient temperatures and tends to become Fickian at higher temperatures. Otake and Suuberg24 have shown that diffusion mechanisms become increasingly Fickian with increasing temperature in the 298-320 K range. The low rank coals generally follow a relaxation-controlled mechanism.26 The kinetics of swelling are not dependent on particle size in the range from 150 to 600 µm, presumably because macropore transport allows for rapid transport of solvent into the particle interior,26 though it has been found that diffusion mechanisms become Fickian for small particle sizes.3,21 Barr-Howell et al. 21 investigated amine transport in coal particles. It was found that the amine transport was non-Fickian, approaching Case-II transport but tends to become pure Fickian diffusion for smaller particle sizes. Milligan et al.28 studied the kinetics of pyridine uptake of raw coals, maceral concentrates, the residual coals, and the corresponding maceral concentrates. It was found that swelling is anomalous or Fickian for raw coals and maceral concentrates and tends to become relaxation-controlled for residual coals and corresponding maceral concentrates. Gao et al.29 used a novel orthogonal microscope image analysis method for evaluating solvent-swelling behavior of single coal particles. They found that although Case-II and super Case-II are dominant, a few cases of Fickian and anomalous diffusion also exist for two kinds of coals. In addition, for some PDC-HV (bituminous coal, USA) coal samples the value of diffusion exponent, n, which indicates the type of solvent uptake, changes from 0.22 to 0.39, which does not match any classical diffusion mechanism, reflecting one diffusion mechanism of pyridine in coal. n is a number that indicates the nature of the swelling process.26 For nearly spherical particles and for mass uptakes up to about 60% of the equilibrium value, n ) 0.43 for Fickian diffusion, and n ) 0.85 for Case-II diffusion. A value of n above 0.85 corresponds to super Case-II diffusion. A value of n between that for Fickian and Case-II diffusion corresponds to anomalous transport. This will be further discussed in the present paper. Activation energies for the diffusion-relaxation processes have also been determined.5,22-26 These studies have shown that the activation energy barrier for swelling may be associated with the breakage of electron donor-acceptor interactions such as hydrogen-bonding interactions in coal.26 The activation energies for the combined diffusion and swelling process fall in the range of 20-60 kJ/mol, suggesting that the main activation barrier to swelling is dissociation of hydrogen bonds.23,25 (22) Ndaji, F. E.; Thomas, K. M. Fuel 1995, 74, 842. (23) Ndaji, F. E.; Thomas, K. M. Fuel 1993, 72, 1525. (24) Otake, Y.; Suuberg, E. M. Fuel 1989, 68, 1609. (25) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155. (26) Otake, Y.; Suuberg, E. M. Fuel 1998, 77, 901. (27) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379. (28) Milligan, J. B.; Thomas, K. M.; Crelling, J. C. Energy Fuels 1997, 11, 364. (29) Gao, H.; Artok, L.; Kidena, K.; Murata, S.; Miura, M.; Nomura, M. Energy Fuels 1998, 12, 881.
Pande and Sharma Table 1. Proximate and Ultimate Analyses of Chinakuri Coal air-dried basis moisture (%)
ash (%)
volatile matter (%)
fixed carbon (%)
C (%)
dmmf basis H N S (%) (%) (%)
O (%)
5.1
13.6
32.9
48.4
78.5
5.5
13.1
2.4
0.5
Studies carried out in the authors’ laboratory have shown that at room temperature an enhanced amount of coal is rendered extractable by using NMP+EDA (1: 1; vol/vol) mixed solvent system.30 The studies have also shown that addition of a small amount of EDA to NMP can increase extraction yield of coals in NMP under reflux conditions at atmospheric pressure.31 For example Chinakuri coal, the coal of study in the present paper, gave an extraction yield (on dmmf basis) of only 1% in NMP, 15% in EDA, and 28% in NMP+EDA (1:1) mixed solvent system at room temperature in an extraction time of 4 h. The extraction yield of Chinakuri coal under reflux conditions was 10% in NMP, 21% in EDA, and 34% in NMP containing a small amount of EDA in an extraction time of 2 h. The increased solvent power of the mixed solvent systems may be due to the synergistic effects. Swelling studies may help in understanding the mechanism of solvent extraction of coal. The present paper reports the kinetics of solvent diffusion by swelling of coal in NMP, EDA, and NMP+EDA (1:1) and thereby attempts to understand the synergistic effects of the mixed solvent systems in obtaining higher extraction yields. Experimental Section The kinetics of solvent diffusion in coal was determined by a solvent swelling method.24 Chinakuri coal, -60 to +120 BSS (120 to 250 µm), a bituminous, noncoking coal, was used for the studies. The coal was dried in an oven at 105 °C for 24 h and stored in a desiccator. Commercially available solvents (E Merck (India) Limited), N-methyl-2-pyrrolidone (NMP), and ethylenediamine (EDA) were used for studies. For the swelling experiments, the coal sample (0.5 g) was placed in a graduated tube and centrifuged for 5 min at 2000 rpm. The height h1 of the coal column was measured. Solvent (NMP, EDA, NMP+EDA (1:1)) prewarmed or precooled to the required temperature was added and the tube was shaken properly. The tube was then placed in a thermostat at the required temperature for a desired time. After the required time, the samples were removed from the thermostat and kept in an ice-bath to slow the swelling to a negligible rate and then again centrifuged for 5 min and the height of the coal column was measured. The error in temperature measurement was 2%. Swelling ratio at a particular temperature and time was calculated as follows:
Q ) height of coal column at time “t” ÷ height of coal column at zero time All experiments were performed in duplicate to ensure reproducibility. The reproducibility of the swelling experiments was (2% for NMP and (5% for EDA and NMP+EDA (1:1). (30) Pande, S.; Sharma, D. K. Organo-dissolution of coals in N-methyl-2-pyrrolidone + ethylenediamine (1:1) mixed solvent system - Studies of the synergistic effect of the mixed solvent system on the extraction of coals. Energy Fuels, submitted. (31) Pande S.; Sharma, D. K. Ethylenediamine assisted solvent extraction of coal in N-methyl-2-pyrrolidone. Synergistic effect of ethylenediamine on extraction of coal in N-methyl-2-pyrrolidone. Energy Fuels, submitted.)
Diffusion of Solvents in Chinakuri Coal
Energy & Fuels, Vol. 15, No. 5, 2001 1065
Figure 2. Analysis of the swelling data in NMP+EDA at 15 °C using eq 2.
Figure 1. (a) Kinetic data of swelling of Chinakuri coal in NMP+EDA (1:1) at 15, 25, 35, and 45 °C. (b) Kinetic data of swelling of Chinakuri coal in NMP+EDA (1:1) at 60 °C.
Results and Discussion Table 1 shows the proximate and ultimate analyses of Chinakuri coal. The equilibrium swelling ratios, Q∞, of Chinakuri coal in NMP, EDA, and NMP+EDA (1:1) obtained after 24 h were 2.0, 2.85, and 3.75, respectively. It was found that Q∞NMP+EDA(1:1) > Q∞EDA > Q∞NMP showing the greater tendency of the mixed solvent to swell the coal. Figure 1a,b shows the kinetic data of the swelling of Chinakuri coal in NMP+EDA (1:1) at different temperatures. The following rate law, as suggested by Ritger and Peppas,32 was used to determine the nature of diffusion of solvents into coal:
Q-1 M ) ) ktn M ∞ Q∞ - 1
(1)
where M refers to mass uptake of solvent by the coal and the subscript ∞ refers to final equilibrium values. k is a constant (whose units depend on the value of n) related to the rate of swelling, and n is a number that indicates the nature of the swelling process. For nearly spherical particles and for mass uptakes up to about 60% of the equilibrium value, n ) 0.43 for Fickian diffusion, and n ) 0.85 for Case-II diffusion.25 A value of n above 0.85 corresponds to super Case-II diffusion. A value of n between that for Fickian and Case-II diffusion corresponds to anomalous transport. (32) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 815.
Figure 3. Analysis of the swelling data in NMP+EDA at 25 °C using eq 2.
The value of n was determined by taking the natural log of both sides of eq 1.25 The resulting equation was
ln
Q-1 ) ln k + n ln t Q∞ - 1
(2)
A plot of ln (Q - 1/Q∞ - 1) vs ln t gave straight line fits with n being determined from the slope of the line. Figures 2-6 show the plots for NMP+EDA (1:1) at different temperatures. Table 2 shows the values of n and k obtained in case of swelling of coal in NMP, EDA, and NMP+EDA (1:1) at different temperatures. Error in estimation of the value of k was about 2% for NMP and about 5% for NMP and NMP+EDA (1:1). The values of n are all low and closer to that for Fickian diffusion in most cases though in some cases the value of n is even as low as 0.3. Studies carried out on diffusion of pyridine in coal have shown that although Case-II and super Case-II have been found to be dominant for diffusion of pyridine in coal,26,27 a few cases of Fickian and anomalous diffusion have been observed.25 A noted exception was Illinois No. 6 coal in which case the diffusion of pyridine into coal was unusually fast and the value of n for diffusion of pyridine corresponded to a pure Fickian
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Pande and Sharma Table 2. Summary of Swelling Results of Chinakuri Coal in NMP, EDA, and NMP+EDA (1:1) solvent system
Q∞
NMP
2.0
EDA
2.85
N+E (1:1)
3.75
T (°C)
t50% (min)
N
K × 102
25 35 45 60 20 35 45 15 25 35 45 60
240 120 50 30 9 5 1.7 150 60 12 5 3
0.42 0.40 0.40 0.35 0.45 0.30 0.37 0.38 0.42 0.31 0.44 0.30
5.0 7.36 10.4 15.0 20.0 33.3 44.4 7.6 9.12 23.5 25.0 36.6
Ea (kJ/mol) 19.9 (25.9)
18.9 (22.9) 26.9 (29.7)
a Figure in bracket is the value of activation energy determined using eq 4
Figure 4. Analysis of the swelling data in NMP+EDA at 35 °C using eq 2.
than relaxation phenomenon. A Fickian diffusion mechanism is controlled by a concentration gradient that is set up between the center of the particle and its surface.3,22 A relaxation phenomenon is characterized by a sharp front separating the swollen and the unswollen regions of the coal, i.e., a solvent uptake process whose rate is controlled by slow relaxation of the macromolecular network structure of coal. The low values of n obtained in case of NMP, EDA, and NMP+EDA (1:1) in Chinakuri coal suggest a very fast transformation of the coal from glassy to rubbery state, thus setting up a concentration gradient inside the coal particle. The relaxation of the macromolecular structure is very fast as solvent penetrates the coal. It has been found that diffusion mechanisms become Fickian for small particle sizes.3,21 In the present studies, the particle size used was relatively small (120-250 µm). The evaluation of activation energies for the swelling process was done using the method described by Otake and Suuberg.25 For any particular extent of swelling Qr:
Figure 5. Analysis of the swelling data in NMP+EDA at 45 °C using eq 2.
ln
Qr - 1 ) ln k + n ln t Q∞ - 1
(3)
Activation energy may be defined as
d ln k -E ) R d(1/T)
(4)
E ) -nR [d(ln 1/tr)/d(1/T)]
(5)
from which
Figure 6. Analysis of the swelling data in NMP+EDA at 60 °C using eq 2.
diffusion control.25 Earlier studies carried out in the authors’ laboratory have shown that diffusion of NMP in coal is anomalous and tends to become Fickian at higher temperatures.18,33 In the present case, the swelling of coal in NMP, EDA, and NMP+EDA (1:1) was governed by diffusion rather (33) Giri, C. C.; Sharma, D. K. Fuel 2000, 79, 577.
The time scales for swelling are characterized by the times to achieve 50% of the maximum swelling t50%.26 Figure 7 shows the plot of ln 1/t50% vs 1/T for NMP+EDA(1:1). Table 2 also gives the values of activation energies obtained using eq 5 for NMP, EDA, and NMP+EDA (1: 1) diffusion into coal. Since the values of n did not vary much with temperature, the activation energies were also estimated from the values of k using eq 4. These values are given in parentheses in Table 2. The two estimates of activation energy are in fair agreement. Error in estimation of activation energies obtained using eq 4 was carried out using the following equation:
dk -1 dE - E dt ) k RT T
[
]
(6)
Diffusion of Solvents in Chinakuri Coal
Energy & Fuels, Vol. 15, No. 5, 2001 1067 Table 3. Molecular Size of EDA and NMP
Figure 7. Arrhenius plot for determining activation energy Eact for NMP+EDA (1:1) diffusion using eq 5.
Figure 8. Arrhenius plot for determining activation energy Eact for NMP+EDA (1:1) diffusion using eq 4.
Error in activation energies obtained for NMP, EDA, and NMP+EDA (1:1) diffusion into coal were in the range 1.5-2%. Figure 8 shows the Arrhenius plot of ln k vs 1/T for NMP+EDA (1:1). The values of activation energies obtained suggest that the swelling of Chinakuri coal in NMP, EDA, and NMP+EDA (1:1) was limited by ordinary diffusion. Hydrogen-bond strengths in coal vary from 20 to 30 kJ/mol and may reach even 40-70 kJ/mol.2,25 The values of activation energies obtained by the two methods fall in the range 20-30 kJ/mol which suggests that the main activation energy barrier to swelling is dissociation of hydrogen bonds. Both NMP and EDA are hydrogen-bonding accepting solvents.34,35 EDA is also a hydrogen-bond donor solvent. The values of activation energies obtained are in agreement with the fact that swelling by specific solvents involves disruptions of specific interactions such as hydrogenbonding interactions in coals.25 (34) Larsen, J. W.; Shawver, S. Energy Fuels 1990, 4, 74. (35) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247.
solvent
molar volume (Å3)
length (Å)
breadth (Å)
height (Å)
EDA NMP
53.2 76.6
5.324 5.500
2.35 4.10
1.63 1.96
EDA is a good swelling solvent for coal.36 Much of the early work on EDA has been reviewed.1 The equilibrium swelling ratio of Chinakuri coal in EDA was found to be higher than that in NMP (Table 2). Activation energy (determined using eq 4) for EDA diffusion into coal was found to be lower than that for NMP diffusion into coal (Table 2). The lower value of activation energy obtained for EDA compared to that for NMP diffusion was investigated. Otake and Suuberg26 have reported that the activation energy for swelling depends on steric factors in addition to base strength. These steric factors depend on both the size and shape of the diffusing molecule. Attempts were made to determine the molecular shape and size of EDA and NMP using web lab viewer software program. Molecular size of EDA was found to be smaller than that of NMP (Table 3). Also, the shape of EDA appeared to be cylindrical compared to that of NMP, which was diffused and appeared triangular. A cylindrical shape will have comparatively a better penetrating power. Hence the activation energy barrier for EDA is comparatively reduced. EDA is easily able to penetrate the coal structure, sterically unhindered. NMP, owing to its larger size, i.e., the pyrrolidone ring, cannot penetrate the coal structure before breaking a coal-coal hydrogen bond. This increases the activation energy barrier for NMP to that required for breaking a hydrogen bond in coal. The swelling of coal in NMP was less than that in EDA, which shows that NMP has lower ability than EDA in disrupting hydrogen-bonding interactions present in coal. This may be because EDA is also a hydrogen-bond donor solvent. The activation energy for swelling of coal in NMP+ EDA (1:1) mixed solvent system was found to be more than that in either NMP or EDA. The mixed solvent system has greater ability to disrupt more number of stronger noncovalent interactions as is also reflected in the highest equilibrium swelling ratio (3.75) of coal in NMP+EDA (1:1). The greater ability of a solvent to break strong, noncovalent interactions in coal may result in higher activation energies.24 Cooperative effects of NMP and EDA in swelling the coal become important. Yun and Suuberg37 have shown that cooperative effects between a specifically and a nonspecifically interacting solvent play an important role in determining the course and extent of swelling of coals. Present studies show that cooperative effects exist between specifically interacting solvents as well. The reduced activation energy barrier for EDA improves the penetration power of NMP into coal as the coal structure loosens up by EDA penetration as EDA swells the coal. Also, the rate of swelling at any given temperature is fastest in EDA (Table 2) among the solvents studied which shows its greater ability to relax the network structure of coal thereby permitting the solvent swelling to occur more readily. The relatively open structure of (36) Dryden, I. G. C. Nature 1949, 163, 141. (37) Yun, Y.; Suuberg, E. M. Energy Fuels 1998, 12, 798.
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coal is easily accessible by NMP. This results in greater coal-solvent interactions i.e., coal-EDA and coal-NMP interactions. Both EDA and NMP act on coal by disrupting more number of different noncovalent interactions present in coal thereby resulting in greater swelling of coal in the mixed solvent system. Higher activation energy for NMP+EDA (1:1) shows greater ability of the mixed solvent to break strong, noncovalent interactions in coal.24 Solvent swelling studies may help in understanding the mechanism of solvent extraction of coal. A comparison of the extraction yields of Chinakuri coal in NMP, EDA, and NMP+EDA (1:1) at room temperature shows that the combination of EDA and NMP not only has a greater swelling power but also has additional ability to break other types of coal-coal interactions such as stronger hydrogen bonds and π-π aromatic interactions. Rate and extent of swelling in EDA is faster than that in NMP. When NMP+EDA (1:1) mixture is used, the faster coal swelling solvent EDA opens the coal structure for NMP penetration. Since a larger number of extractable sites are rendered accessible to both EDA and NMP the extraction yield is also more in the mixed solvent system. The enhanced extraction yield of Chinakuri coal in NMP containing a small amount of EDA under reflux conditions is due to the improved penetra-
Pande and Sharma
tion power of NMP into the EDA pre-swollen coal network, which renders more extractable sites accessible to NMP thereby resulting in greater solubilization. Conclusions 1. The swelling kinetics of Chinakuri coal in NMP, EDA, and NMP+EDA (1:1) was characterized by a Fickian diffusion process. 2. The values of activation energies for the combined diffusion and swelling process all fall in the range 2030 kJ/mol. They are a measure of not only the ability of solvent to penetrate the macromolecular structure of coal (i.e., a measure of the steric effect of the penetrant species) but also the ability of solvent to cleave coalcoal hydrogen bonds as determined by the equilibrium extent of swelling of coal in that solvent system. Acknowledgment. The authors thank the Department of Science and Technology, Ministry of Science and Technology, Govt. of India, for the financial assistance to carry out the research work. The authors also thank Prof. T. R. Rao of Chemical Engineering Department, I. I. T. Delhi for some useful discussions. EF9902395
