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Effect of solvent basicity on the kinetics of solvent swelling of coal
Effect of solvent basicity solvent swelling of coal Francis
E. Ndaji and K. Mark
on the kinetics
of
Thomas
Northern Carbon Research Laboratories, Department of Chemistry, Newcastle upon Tyne, Newcastle upon Tyne NE7 7RU, UK (Received 79 June 7992; revised 79 February 7993)
University
of
This paper describes an investigation of the kinetics of the swelling of a coal of rank 502 in the British Coal classification scheme and the pyridine-extracted coal in solvents of the same steric properties but different basicity: pyridine, 2-chloropyridine and 2-fluoropyridine. The results show that the swelling of the coal and extracted coal in these solvents obeys an experimentally determined first-order rate law for typically all but the first 10% of the swelling. Rate constants and apparent activation energies of the swelling of the swelling of the coal increase with increasing solvent basicity. The swelling ratios and kinetic parameters for the pyridine-extracted coal do not vary markedly with the basicity of the pyridines. The results are discussed in terms of structural differences between the raw and pyridine-extracted coals. (Keywords coal solvent basicity; kinetics; swelling)
It has been established that coal has a cross-linked macromolecular structure’. The cross-links of the macromolecular network consist of two major types: the covalent cross-links and non-covalent interactions2. The covalent cross-links are mainly ethylenic, methylenic and ether linkages2, while hydrogen bonding, van der Waals forces and X-Z aromatic interactions constitute the non-covalent interactions3. Clearly there is a wide variety of interactions which occur to different extents. The density of cross-links and the relative abundance of each type of cross-link may influence the behaviour of the coal in conversion processes. For example, in coal liquefaction, the nature and density of cross-links influence the rate at which the solvent fraction diffuses into the coal matrix4’5. The rate of diffusion of the solvent no doubt influences the rate of the liquefaction process. The maximum fluidity observed in the Gieseler plastometer, an indication of the thermoplastic properties and suitability of coal for production of metallurgical coke, occurs in the region of coal rank where the solvent swelling is at a maximum and the cross-link density reaches a minimum6. Therefore a knowledge of the nature and the density of the cross-links, as well as the mechanism by which solvents diffuse through the bulk material, is important for understanding the behaviour of coal in various processes. Of the techniques usually used for probing the macromolecular structure of coal, solvent swelling appears to be the most favoured, especially in the study of the nature and density of cross-links in coal. The similarities in structure between coal and glassy polymers were noted by Sanada and Honda6. They obtained values of average molecular weight between cross-links, M,, of coal by applying the Flory-Rehner7 theory of cross-linked polymer networks to solvent swelling data for pyridineextracted coals. Larsen et al.* and Lucht and Peppas’ obtained M, by using non-Gaussian” and modified Gaussian models” respectively of cross-linked polymer
0016_2361/93/11,‘15314IS c~’ 1993 Butterworth-Heinemann
Ltd.
networks. Values of M, provide the basis for comparing the abundance of covalent cross-links in coals. It has been suggested that because of its character as a strong base, pyridine (pK, = 8.6) is capable of disrupting all the hydrogen bonds in coal12. A range of hydrogen bond strengths exists in coals*. Therefore the extent to which a coal will swell in a basic solvent will depend on the one hand on the relative abundance of covalent cross-links, hydrogen bonding etc. in the coal, and on the other hand on the ability of the solvent to disrupt the hydrogen bonds, which will in turn depend on the solvent basicity. Szeliga and Marzec13 reported an increase in the equilibrium swelling value of coals with increase in electron donor number (EDN) of the solvent. Hall et al. l4 found a poor co rrelation between solvent EDN and equilibrium swelling value of coals, but reported a trend of increasing equilibrium swelling value of coals with increasing basicity of solvent, as defined by pK, values, until a plateau was reached at pK, ~8-9. Further increases in basicity had little effect. It appears that in addition to the evaluation of the average molecular weight between cross-links, M,, previous work has concentrated mainly on the variation of equilibrium swelling values with coal rank, solvent EDN, basicity, solubility parameter and coal oxidation. Very little has been reported on the kinetics of solvent swelling of coal, and as a result there is a dearth of information on the effect of solvent properties on the rate of coal swelling, the apparent activation energy of the swelling process and the mechanism of solvent diffusion into the bulk material of coal. In this paper, the solvent swelling kinetics over the range 20-70°C for a coal of rank 502 (British Coal classification scheme) and its residue from pyridine extraction in pyridine, 2-chloropyridine and 2-fluoropyridine, which vary in basicity but have similar structural properties, are investigated, The data are analysed using the experimentally determined rate law
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Effect of solvent basicity on the kinetics of solvent swelling of coal: F. E. Ndaji and K. M. Thomas Table 1
Properties
of coal W
Proximate analysis (wt% db) Volatile matter Ash Ultimate analysis (wt% db) Carbon Hydrogen Chlorine Reflectance R, (max) (%) Maceral analysis (~01%) Vitrinite Exinite Inertinite C”,~,N!;; area (273 K) (m’ g-l) Extracted
36.5 3.1 81.3 5.0 0.42 0.85 74.0 12.0 14.0 113 78
coal
,.,1
tlme
(mlns
using the experimentally determined rate law15 for pyridine sorption by coals. The results are in agreement with the previous work’ 5. Plots of ln[(S, - S,)/S,], where S, is the equilibrium swelling and S, is the swelling at time t, against time (Figures 2-4) gave straight lines for the swelling of coal W in 2-chloropyridine and 2fluoropyridine as well as in pyridine. The swelling of coal W in the halopyridines obeys the experimentally determined first-order rate law despite the lower basicity of these solvents. The ranges of values of diffusional exponent n for the initial part of the swelling of the coal in the temperature range studied were 0.95-1.1, 0.864.93 and 0.63-0.76 for pyridine, 2-chloropyridine and 2-fluoropyridine respectively. The initial part of the swelling of the pyridineextracted coal was very rapid, preventing accurate determination of the corresponding values of n. However, plots of ln[(S, - SJS,] against time all gave straight lines, as shown in Figure 5. The swelling ratio, Q,, and rate constant, k, for the swelling process are shown in Table 2 for various temperatures. It is apparent that the rate constant for the coal is strongly influenced by basicity and temperature. In contrast, the rate of swelling of the pyridine-extracted coal shows only a small variation with
1
Figure 1 Variation of swelling ratio with time for coal W in basic solvents: 0, pyridine; +, 2-chloropyridine; n , 2-fluoropyridine
in order to determine the activation energy involved in the coal-solvent interaction for each of the solvents.
, 20
0
40
EXPERIMENTAL
60
time
Samples
Figure 2
The properties of the coal used are shown in Table 1. The coal was stored in deoxygenated distilled water to prevent oxidation, and a test sample of 60&355pm particle size was obtained by crushing and sieving after drying at 20°C in vacuum. The solvents were 99% pure.
at 30°C
Variation
of ln[(S, -S,)/S.]
80
100
(mlns) with time for coal W in pyridine
Apparatus and procedure
The solvent swelling apparatus and the experimental procedure have been described previously4~‘4~‘5. RESULTS Figure 1 shows the swelling ratio versus time for coal W in pyridine, 2-chloropyridine and 2-fluoropyridine at 30°C. It is clear that the rate of swelling decreases with decrease in solvent basicity. Furthermore, the equilibrium swelling ratio also decreases with decreasing solvent basicity. The solvent swelling data obtained at various temperatures were analysed to obtain values of the diffusional exponent, n, using the empirical equation for solvent sorption into polymers’6. The data were also analysed
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20
40 time
Figure 3 Variation of ln[(S.-S,)/S,] chloropyridine at 70°C
6b
8b
1
(mlns)
with
time for coal
W in 2-
Effect of solvent basicity
on the kinetics
of solvent swelling Table 2
of coal: F. E. Ndaji and K. M. Thomas
Rate constants
and swelling
ratios of coal W Extracted
Raw coal
Temp. (“C)
Q”
Pyridine 20 30 40 50 60
3.57 5.15 8.84 1.34 2.25
X 1O-4 x lo-4 x lo-& x lo- 3 x 10-3
2-chloropyridine 20 _
-2.5
! 0
I 200 he
Figure 4 Variation of In[(S.-Q/S,] fluoropyridine at 40°C
0
I
1
400
600
30 40 50 60 70
I
(mlns)
with
time for coal
W in 2-
7.67 1.25 1.74 2.45 3.43
x x x x x
Q”
2.10 2.15 2.10 2.09 2.04
4.59x 1o-3 4.86 x IO-’ 5.5 X lo-3 6.12 x 1o-3 7.5 X 1o-3
_
3.26 3.56 4.01 5.09 5.23 _
IO-’ 1O-4 1o-i 10+ lo-&
1.64 1.62 1.61 1.69 1.70
2-fluoropyridine 20 _ 30 40 50 60 70
,
coal
_
2.59 x 3.27x 3.63 x 4.34 x 4.90 x _
1.27 1.34 1.41 1.41 1.38
3.36x IO-’ 5.16x 1O-5 6.05 x 1O-5 8.17 X 10-5 1.07 x 1o-4
x x x x x
I.92 1.89 I .90 1.88 1.85
10~m3 lo--’ lo- 3 1o--3 IO- 3
1.93
1.86 1.86 1.79 1.81 _
1O-3 1O-3 10-j 1o-3 1o-X
1.82 1.70 1.68 1.70 1.65 _
I
1.0 I
0.8 -
s
06-
&#
2
z
0.4 -
#
-411 0.0
10.0 time
Figure 5 Variation of In[(S.-Q/S.] 2-fluoropyridine at 3O’C
Crnlns)
for pyridine-extracted
coal in 10.0
0.0
20.0
36.0
t (mlns)
temperature. Figure 6 shows the observed and calculated values of M,/M, against time for the swelling of coal W in pyridine at 60°C. It is seen that there is good agreement between the observed and calculated values with the largest discrepancy at the very beginning of the swelling process. Figure 7 shows Arrhenius plots for the swelling of coal W in the three solvents. The apparent activation energies obtained are 37,3 1 and 24 kJ mol- ’ for pyridine, 2-chloropyridine and 2-fluoropyridine respectively. In contrast, the apparent activation energies for the extracted coal were similar (- 10 kJ mol- ‘) for all three pyridines. In a study of coal swelling in straight-chain amines with similar basicities, Green and WestI obtained values for the molar quantities of each solvent absorbed by each sample, using the equation mmol absorbed g dry coal
Figure 6 Variation of MJM, I?, observed; +, calculated
with time for coal W in pyridine
at 60°C:
= 1OOOQ-l PV
where V is the molar volume of the solvent, Q is the swelling ratio of coal in the solvent, and p is the density of the coal. Because coal W falls within the rank range studied by Green and West, it has been assigned a dry
0.0029
0.0030
0.0031
0.0032
0.0033
0.0034
o
l/T
Figure 7 Arrhenius plots for the swelling of coal W in basic solvents. Symbols as in Figure 1
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Effect of solvent basicity on the kinetics Table 3
Basicities and molar volumes pre-exponential factors for coal W
of so/vent swelling
of solvents,
and
specific
of coal: F. E. Ndaji and K. M. Thomas
quantities
of solvents
absorbed
at 3O”C, apparent
activation
energies
and
Solvent Raw coal
PK, Pyridine
Pyridine-extracted
Molar volume (cm3 mol- ‘)
(Mmmolg-‘)
$mol-L)
(Mmmolg-‘)
coal
$mol-‘)
8.6
80.9
10.93
31.4
8.46
9.7
2-Chloropyridine
13.5
94.6
5.20
31.1
7.00
10.5
1.04
2-Fluoropyridine
14.4
86.1
2.41
24.1
6.25
10.3
0.97
density of 1.3 g cme3. The same density value has also been assumed for the extracted coal. Table 3 shows the molar quantities of each solvent absorbed per gram of coal or extracted coal, the equilibrium swelling ratio of coal in each solvent, and the molar volume and pK, of each solvent. Since the molar volumes are similar, the amount absorbed is not unduly affected by the size of the substituted pyridine molecule. DISCUSSION Swelling of coal in pyridine and other amines involves the disruption of hydrogen bonds in the coal and the formation of bonds between the solvent and functionalities in the coal. It has been proposed that a solvent will disrupt only those coal-coal hydrogen bonds whose bond strengths are lower than those of the coal-solvent hydrogen bonds’. It has also been suggested that pyridine, because of its strong basicity, is capable of breaking nearly all hydrogen bonds in coali’. Studies14 of the solvent swelling of coals of different rank in a series of substituted pyridines showed that as the basicity increased, the swelling increased until a plateau was reached at pK,, -8, corresponding to the disruption of virtually all the hydrogen bonds. Studies of the initial swelling mechanism for a coal of rank 802 in pyridine and substituted pyridines is showed that as the basicity increased, the mechanism changed from Fickian to case II. Therefore when coal containing an appreciable amount of hydrogen bonding is exposed to pyridine, it swells to a limit that is a function primarily of the covalent cross-link density, However, solvents having basicities lower than that of pyridine will disrupt only the weaker hydrogen bonds, leaving intact those of higher energy. In this case the number of hydrogen bonds disrupted is smaller, and the swelling of coal in such solvents will be lower. This suggestion is corroborated by Table 3, which shows that the equilibrium swelling ratios increase with increase in solvent basicity but do not vary markedly with temperature. The increase observed in the initial diffusional exponent n with increasing solvent basicity is in agreement with the change from Fickian to case II with increasing basicity observed previously”. It is apparent from Table 3 that, as expected from the earlier observations using straight-chain amines of the same basicity but different steric properties”, there is no relation between equilibrium swelling and the molar volume of the pyridines of similar molar volume used in this study. The coal absorbed different molar quantities of each solvent, the quantity absorbed increasing with solvent basicity (decreasing pK,). The increase with solvent basicity of molar quantity absorbed suggests that
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the number of hydrogen bonding sites reacting with solvent molecules increases with solvent basicity; this further explains the observed increase in equilibrium swelling with basicity of solvent. The swelling ratio varies slightly with temperature over the range 2CL7O”C. It has been demonstrated that the swelling of coals in pyridine involves their transformation from a glassy to a rubbery state”. Other workers” showed that transport of pyridine into coal sections is controlled by a case II or anomalous process. In a previous paper” it was shown that a graph of ln[(S, - Q/S,] versus time was a straight line, indicating a first-order rate law. This was valid no matter whether the initial diffusion mechanism was Fickian, anomalous or case II. It was proposed that the swelling mechanism changed as the coal swelled. Therefore the observed rate law typically accounts for -90% of the swelling, and it is possible to describe the swelling behaviour as involving a change from a first stage of anomalous behaviour to a stage involving first-order kinetics. It is clear that whereas the initial mechanism of diffusion of pyridine into coal W is case II, the initial mechanism is anomalous for the diffusion of 2-chloropyridine and 2-fluoropyridine. It is also apparent that the higher the pK, of a solvent, and hence the lower rate and extent of swelling, the closer the initial diffusion mechanism for this coal approaches Fickian. Table 2 also shows that the rate constant, k, of the swelling process changes with temperature and solvent basicity. It is apparent from Table 2 that both solvent basicity and temperature significantly affect the rate of the coal swelling process in these solvents. The values of the apparent activation energies already quoted represent the combined effects of solvent reaction, rearrangement, relaxation and swelling of the coal macromolecular network. The pre-exponential factors also show a similar trend of increase with solvent basicity, but the differences are much larger. The apparent activation energy also increases with increase in equilibrium swelling ratio, an indication that solvents of higher basicity react with hydrogen bonds of a wider range of bond strengths. A comparison of the solvent swelling of the coal with that of the extracted coal shows distinct differences. It is apparent that the extent of swelling is not markedly affected by either the temperature or the basicity of the solvent. The small dependence on temperature is similar to that observed for raw coal, whereas the independence of basicity contrasts with the marked dependence of the swelling of the coal. As already indicated, the activation energies and pre-exponential factors for the extracted coal are very similar for all three solvents. In particular, the
Effect of solvent basicity
on the kinetics
activation energies (m 10 kJ mol- ‘} are much lower than for the coal, suggesting a different type of interaction and mechanism. That the rates of swelling of the extracted coal are greater than those of the coal indicates structural differences. The pyridine extraction disrupts the hydrogen bonds in the coals and causes swelling. That the solvent swelling of the extracted coal does not vary greatly with the basicity of the substituted pyridine suggests that the formation of hydrogen bonds in the extracted coal after solvent removal is limited. This is supported by the faster rates of swelling and lower activation energies observed for the extracted coal, which are also not markedly dependent on the basicity of the substituted pyridines. Furthermore, the extent of swelling is not significantly dependent on solvent basicity. The swelling of the extracted coal in pyridine is markedly lower than for the coal, indicating some modification of the macromolecular structure e.g. covalent cross-linking and 7c-7~interactions. It has been suggested3 that ~--7ccomplexes are formed in the extracted coal and that some of them are not disrupted in the solvent swelling experiments. However, it is possible that other structural modifications can occur during solvent extraction, for example some decomposition leading to modification of the cross-links. There are thus two possible explanations for the reduced swelling of the extracted coal in pyridine. First, the extraction process may cause decomposition, leading to the formation of cross-links. Second, the collapse of the structure due to the removal of soluble material leads to the formation of strong 7c--7~ interactions which act as effective non-covalent cross-links. It is difficult to distinguish unambiguously between the alternatives. Previous workers3 have strongly favoured z--71 bond formation during the rearrangement of the macromolecular structure due to removal of soluble material. CONCLUSIONS The extent of swelling and kinetics of solvent diffusion into coal are significantly affected by the basicity of the
of solvent swelling
of coal: F. E. Ndaji and K. M. Thomas
solvent, both the extent and rate constant decreasing with decreasing solvent basicity. The results also suggest that the number of hydrogen bonding sites in coal that are able to react with the solvent increases with solvent basicity, and that the rate of swelling of coal in a solvent also depends on the basicity of the solvent. The results support the proposal that swelling in pyridine and other bases involves the disruption of hydrogen bonding in the coal macromolecular structure. In contrast, the changes in solvent basicity do not have such a marked effect on the extent and kinetics of swelling of pyridine-extracted coal, but the rates are much faster than for the coal. The apparent activation energies for the swelling of the coal decrease with decreasing basicity of the pyridines used. The corresponding data for the activation energies for solvent swelling of pyridineextracted coal show no significant variation, and are much lower than for coal. REFERENCES 1 2 3 4
9 IO I1 12 13 14 IS 16 17 18 19 20
Van Krevelen, D. W. ‘Coal’, Elsevier, Amsterdam, 1981 Painter, P. C., Graf, J. and Coleman, M. M. Energy Fuels 1990, 4, 393 Larsen, J. W. and Mohammadi, M. Energy Fuels 1990,4, 107 Aida, T. and Squires, T. G. Am. Chem. Sot. Div. Fuel Chem. Preprints 1985, 30(l), 95 Neavel, R. C. Phil. Trans. R. Sot. Lord A 1981, A300, 141 Sanada, Y. and Honda, H. Fuel 1966.45,295 Flory, P. J. and Rehner, J., Jr. Chem. Phys. 1943, 11, 521 Larsen, J. W., Green, T. K. and Kovac, J. J. Org. Chem. 1985, 50,4129 Lucht, L. M. and Peppas, N. A. Fuel 1987, 66, 803 Kovac, J. Macromolecules 1978, 11, 362 Barr-Howell, B. D. and Peppas, N. A. Polym. Bull. 1985,13,91 Nishioka, M. and Larsen, J. W. Energy Fuels 1990, 4, 100 Szeliga, J. and Marzec, A. Fuel 1983, 63, 1229 Hall, P. J., Marsh, H. and Thomas, K. M. Fuel 1988, 67, 863 Ndaji, F. E. and Thomas, K. M. Fuel 1993,72, 1525 Ritger, P. L. and Peppas. N. A. Fuel 1987, 66, 1379 Green, T. K. and West. T. A. Fuel 1986, 65, 298 Hail, P. J., Thomas, K. M. and Marsh, H. Fuel 1992, 71, 1273 Brenner, D. Fuel 1985, 64, 167 Peppas, N. A. and Lucht, L. M. Chem. Eng. Commun. 1985,31, 333
Fuel 1993
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E. Ndaji and K. Mark
on the kinetics
of
Thomas
Northern Carbon Research Laboratories, Department of Chemistry, Newcastle upon Tyne, Newcastle upon Tyne NE7 7RU, UK (Received 79 June 7992; revised 79 February 7993)
University
of
This paper describes an investigation of the kinetics of the swelling of a coal of rank 502 in the British Coal classification scheme and the pyridine-extracted coal in solvents of the same steric properties but different basicity: pyridine, 2-chloropyridine and 2-fluoropyridine. The results show that the swelling of the coal and extracted coal in these solvents obeys an experimentally determined first-order rate law for typically all but the first 10% of the swelling. Rate constants and apparent activation energies of the swelling of the swelling of the coal increase with increasing solvent basicity. The swelling ratios and kinetic parameters for the pyridine-extracted coal do not vary markedly with the basicity of the pyridines. The results are discussed in terms of structural differences between the raw and pyridine-extracted coals. (Keywords coal solvent basicity; kinetics; swelling)
It has been established that coal has a cross-linked macromolecular structure’. The cross-links of the macromolecular network consist of two major types: the covalent cross-links and non-covalent interactions2. The covalent cross-links are mainly ethylenic, methylenic and ether linkages2, while hydrogen bonding, van der Waals forces and X-Z aromatic interactions constitute the non-covalent interactions3. Clearly there is a wide variety of interactions which occur to different extents. The density of cross-links and the relative abundance of each type of cross-link may influence the behaviour of the coal in conversion processes. For example, in coal liquefaction, the nature and density of cross-links influence the rate at which the solvent fraction diffuses into the coal matrix4’5. The rate of diffusion of the solvent no doubt influences the rate of the liquefaction process. The maximum fluidity observed in the Gieseler plastometer, an indication of the thermoplastic properties and suitability of coal for production of metallurgical coke, occurs in the region of coal rank where the solvent swelling is at a maximum and the cross-link density reaches a minimum6. Therefore a knowledge of the nature and the density of the cross-links, as well as the mechanism by which solvents diffuse through the bulk material, is important for understanding the behaviour of coal in various processes. Of the techniques usually used for probing the macromolecular structure of coal, solvent swelling appears to be the most favoured, especially in the study of the nature and density of cross-links in coal. The similarities in structure between coal and glassy polymers were noted by Sanada and Honda6. They obtained values of average molecular weight between cross-links, M,, of coal by applying the Flory-Rehner7 theory of cross-linked polymer networks to solvent swelling data for pyridineextracted coals. Larsen et al.* and Lucht and Peppas’ obtained M, by using non-Gaussian” and modified Gaussian models” respectively of cross-linked polymer
0016_2361/93/11,‘15314IS c~’ 1993 Butterworth-Heinemann
Ltd.
networks. Values of M, provide the basis for comparing the abundance of covalent cross-links in coals. It has been suggested that because of its character as a strong base, pyridine (pK, = 8.6) is capable of disrupting all the hydrogen bonds in coal12. A range of hydrogen bond strengths exists in coals*. Therefore the extent to which a coal will swell in a basic solvent will depend on the one hand on the relative abundance of covalent cross-links, hydrogen bonding etc. in the coal, and on the other hand on the ability of the solvent to disrupt the hydrogen bonds, which will in turn depend on the solvent basicity. Szeliga and Marzec13 reported an increase in the equilibrium swelling value of coals with increase in electron donor number (EDN) of the solvent. Hall et al. l4 found a poor co rrelation between solvent EDN and equilibrium swelling value of coals, but reported a trend of increasing equilibrium swelling value of coals with increasing basicity of solvent, as defined by pK, values, until a plateau was reached at pK, ~8-9. Further increases in basicity had little effect. It appears that in addition to the evaluation of the average molecular weight between cross-links, M,, previous work has concentrated mainly on the variation of equilibrium swelling values with coal rank, solvent EDN, basicity, solubility parameter and coal oxidation. Very little has been reported on the kinetics of solvent swelling of coal, and as a result there is a dearth of information on the effect of solvent properties on the rate of coal swelling, the apparent activation energy of the swelling process and the mechanism of solvent diffusion into the bulk material of coal. In this paper, the solvent swelling kinetics over the range 20-70°C for a coal of rank 502 (British Coal classification scheme) and its residue from pyridine extraction in pyridine, 2-chloropyridine and 2-fluoropyridine, which vary in basicity but have similar structural properties, are investigated, The data are analysed using the experimentally determined rate law
Fuel 1993
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72 Number
11
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Effect of solvent basicity on the kinetics of solvent swelling of coal: F. E. Ndaji and K. M. Thomas Table 1
Properties
of coal W
Proximate analysis (wt% db) Volatile matter Ash Ultimate analysis (wt% db) Carbon Hydrogen Chlorine Reflectance R, (max) (%) Maceral analysis (~01%) Vitrinite Exinite Inertinite C”,~,N!;; area (273 K) (m’ g-l) Extracted
36.5 3.1 81.3 5.0 0.42 0.85 74.0 12.0 14.0 113 78
coal
,.,1
tlme
(mlns
using the experimentally determined rate law15 for pyridine sorption by coals. The results are in agreement with the previous work’ 5. Plots of ln[(S, - S,)/S,], where S, is the equilibrium swelling and S, is the swelling at time t, against time (Figures 2-4) gave straight lines for the swelling of coal W in 2-chloropyridine and 2fluoropyridine as well as in pyridine. The swelling of coal W in the halopyridines obeys the experimentally determined first-order rate law despite the lower basicity of these solvents. The ranges of values of diffusional exponent n for the initial part of the swelling of the coal in the temperature range studied were 0.95-1.1, 0.864.93 and 0.63-0.76 for pyridine, 2-chloropyridine and 2-fluoropyridine respectively. The initial part of the swelling of the pyridineextracted coal was very rapid, preventing accurate determination of the corresponding values of n. However, plots of ln[(S, - SJS,] against time all gave straight lines, as shown in Figure 5. The swelling ratio, Q,, and rate constant, k, for the swelling process are shown in Table 2 for various temperatures. It is apparent that the rate constant for the coal is strongly influenced by basicity and temperature. In contrast, the rate of swelling of the pyridine-extracted coal shows only a small variation with
1
Figure 1 Variation of swelling ratio with time for coal W in basic solvents: 0, pyridine; +, 2-chloropyridine; n , 2-fluoropyridine
in order to determine the activation energy involved in the coal-solvent interaction for each of the solvents.
, 20
0
40
EXPERIMENTAL
60
time
Samples
Figure 2
The properties of the coal used are shown in Table 1. The coal was stored in deoxygenated distilled water to prevent oxidation, and a test sample of 60&355pm particle size was obtained by crushing and sieving after drying at 20°C in vacuum. The solvents were 99% pure.
at 30°C
Variation
of ln[(S, -S,)/S.]
80
100
(mlns) with time for coal W in pyridine
Apparatus and procedure
The solvent swelling apparatus and the experimental procedure have been described previously4~‘4~‘5. RESULTS Figure 1 shows the swelling ratio versus time for coal W in pyridine, 2-chloropyridine and 2-fluoropyridine at 30°C. It is clear that the rate of swelling decreases with decrease in solvent basicity. Furthermore, the equilibrium swelling ratio also decreases with decreasing solvent basicity. The solvent swelling data obtained at various temperatures were analysed to obtain values of the diffusional exponent, n, using the empirical equation for solvent sorption into polymers’6. The data were also analysed
1532
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0
20
40 time
Figure 3 Variation of ln[(S.-S,)/S,] chloropyridine at 70°C
6b
8b
1
(mlns)
with
time for coal
W in 2-
Effect of solvent basicity
on the kinetics
of solvent swelling Table 2
of coal: F. E. Ndaji and K. M. Thomas
Rate constants
and swelling
ratios of coal W Extracted
Raw coal
Temp. (“C)
Q”
Pyridine 20 30 40 50 60
3.57 5.15 8.84 1.34 2.25
X 1O-4 x lo-4 x lo-& x lo- 3 x 10-3
2-chloropyridine 20 _
-2.5
! 0
I 200 he
Figure 4 Variation of In[(S.-Q/S,] fluoropyridine at 40°C
0
I
1
400
600
30 40 50 60 70
I
(mlns)
with
time for coal
W in 2-
7.67 1.25 1.74 2.45 3.43
x x x x x
Q”
2.10 2.15 2.10 2.09 2.04
4.59x 1o-3 4.86 x IO-’ 5.5 X lo-3 6.12 x 1o-3 7.5 X 1o-3
_
3.26 3.56 4.01 5.09 5.23 _
IO-’ 1O-4 1o-i 10+ lo-&
1.64 1.62 1.61 1.69 1.70
2-fluoropyridine 20 _ 30 40 50 60 70
,
coal
_
2.59 x 3.27x 3.63 x 4.34 x 4.90 x _
1.27 1.34 1.41 1.41 1.38
3.36x IO-’ 5.16x 1O-5 6.05 x 1O-5 8.17 X 10-5 1.07 x 1o-4
x x x x x
I.92 1.89 I .90 1.88 1.85
10~m3 lo--’ lo- 3 1o--3 IO- 3
1.93
1.86 1.86 1.79 1.81 _
1O-3 1O-3 10-j 1o-3 1o-X
1.82 1.70 1.68 1.70 1.65 _
I
1.0 I
0.8 -
s
06-
&#
2
z
0.4 -
#
-411 0.0
10.0 time
Figure 5 Variation of In[(S.-Q/S.] 2-fluoropyridine at 3O’C
Crnlns)
for pyridine-extracted
coal in 10.0
0.0
20.0
36.0
t (mlns)
temperature. Figure 6 shows the observed and calculated values of M,/M, against time for the swelling of coal W in pyridine at 60°C. It is seen that there is good agreement between the observed and calculated values with the largest discrepancy at the very beginning of the swelling process. Figure 7 shows Arrhenius plots for the swelling of coal W in the three solvents. The apparent activation energies obtained are 37,3 1 and 24 kJ mol- ’ for pyridine, 2-chloropyridine and 2-fluoropyridine respectively. In contrast, the apparent activation energies for the extracted coal were similar (- 10 kJ mol- ‘) for all three pyridines. In a study of coal swelling in straight-chain amines with similar basicities, Green and WestI obtained values for the molar quantities of each solvent absorbed by each sample, using the equation mmol absorbed g dry coal
Figure 6 Variation of MJM, I?, observed; +, calculated
with time for coal W in pyridine
at 60°C:
= 1OOOQ-l PV
where V is the molar volume of the solvent, Q is the swelling ratio of coal in the solvent, and p is the density of the coal. Because coal W falls within the rank range studied by Green and West, it has been assigned a dry
0.0029
0.0030
0.0031
0.0032
0.0033
0.0034
o
l/T
Figure 7 Arrhenius plots for the swelling of coal W in basic solvents. Symbols as in Figure 1
Fuel 1993 Vofume 72 Number ‘I 7
‘1533
Effect of solvent basicity on the kinetics Table 3
Basicities and molar volumes pre-exponential factors for coal W
of so/vent swelling
of solvents,
and
specific
of coal: F. E. Ndaji and K. M. Thomas
quantities
of solvents
absorbed
at 3O”C, apparent
activation
energies
and
Solvent Raw coal
PK, Pyridine
Pyridine-extracted
Molar volume (cm3 mol- ‘)
(Mmmolg-‘)
$mol-L)
(Mmmolg-‘)
coal
$mol-‘)
8.6
80.9
10.93
31.4
8.46
9.7
2-Chloropyridine
13.5
94.6
5.20
31.1
7.00
10.5
1.04
2-Fluoropyridine
14.4
86.1
2.41
24.1
6.25
10.3
0.97
density of 1.3 g cme3. The same density value has also been assumed for the extracted coal. Table 3 shows the molar quantities of each solvent absorbed per gram of coal or extracted coal, the equilibrium swelling ratio of coal in each solvent, and the molar volume and pK, of each solvent. Since the molar volumes are similar, the amount absorbed is not unduly affected by the size of the substituted pyridine molecule. DISCUSSION Swelling of coal in pyridine and other amines involves the disruption of hydrogen bonds in the coal and the formation of bonds between the solvent and functionalities in the coal. It has been proposed that a solvent will disrupt only those coal-coal hydrogen bonds whose bond strengths are lower than those of the coal-solvent hydrogen bonds’. It has also been suggested that pyridine, because of its strong basicity, is capable of breaking nearly all hydrogen bonds in coali’. Studies14 of the solvent swelling of coals of different rank in a series of substituted pyridines showed that as the basicity increased, the swelling increased until a plateau was reached at pK,, -8, corresponding to the disruption of virtually all the hydrogen bonds. Studies of the initial swelling mechanism for a coal of rank 802 in pyridine and substituted pyridines is showed that as the basicity increased, the mechanism changed from Fickian to case II. Therefore when coal containing an appreciable amount of hydrogen bonding is exposed to pyridine, it swells to a limit that is a function primarily of the covalent cross-link density, However, solvents having basicities lower than that of pyridine will disrupt only the weaker hydrogen bonds, leaving intact those of higher energy. In this case the number of hydrogen bonds disrupted is smaller, and the swelling of coal in such solvents will be lower. This suggestion is corroborated by Table 3, which shows that the equilibrium swelling ratios increase with increase in solvent basicity but do not vary markedly with temperature. The increase observed in the initial diffusional exponent n with increasing solvent basicity is in agreement with the change from Fickian to case II with increasing basicity observed previously”. It is apparent from Table 3 that, as expected from the earlier observations using straight-chain amines of the same basicity but different steric properties”, there is no relation between equilibrium swelling and the molar volume of the pyridines of similar molar volume used in this study. The coal absorbed different molar quantities of each solvent, the quantity absorbed increasing with solvent basicity (decreasing pK,). The increase with solvent basicity of molar quantity absorbed suggests that
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Volume
72 Number
11
0.97
the number of hydrogen bonding sites reacting with solvent molecules increases with solvent basicity; this further explains the observed increase in equilibrium swelling with basicity of solvent. The swelling ratio varies slightly with temperature over the range 2CL7O”C. It has been demonstrated that the swelling of coals in pyridine involves their transformation from a glassy to a rubbery state”. Other workers” showed that transport of pyridine into coal sections is controlled by a case II or anomalous process. In a previous paper” it was shown that a graph of ln[(S, - Q/S,] versus time was a straight line, indicating a first-order rate law. This was valid no matter whether the initial diffusion mechanism was Fickian, anomalous or case II. It was proposed that the swelling mechanism changed as the coal swelled. Therefore the observed rate law typically accounts for -90% of the swelling, and it is possible to describe the swelling behaviour as involving a change from a first stage of anomalous behaviour to a stage involving first-order kinetics. It is clear that whereas the initial mechanism of diffusion of pyridine into coal W is case II, the initial mechanism is anomalous for the diffusion of 2-chloropyridine and 2-fluoropyridine. It is also apparent that the higher the pK, of a solvent, and hence the lower rate and extent of swelling, the closer the initial diffusion mechanism for this coal approaches Fickian. Table 2 also shows that the rate constant, k, of the swelling process changes with temperature and solvent basicity. It is apparent from Table 2 that both solvent basicity and temperature significantly affect the rate of the coal swelling process in these solvents. The values of the apparent activation energies already quoted represent the combined effects of solvent reaction, rearrangement, relaxation and swelling of the coal macromolecular network. The pre-exponential factors also show a similar trend of increase with solvent basicity, but the differences are much larger. The apparent activation energy also increases with increase in equilibrium swelling ratio, an indication that solvents of higher basicity react with hydrogen bonds of a wider range of bond strengths. A comparison of the solvent swelling of the coal with that of the extracted coal shows distinct differences. It is apparent that the extent of swelling is not markedly affected by either the temperature or the basicity of the solvent. The small dependence on temperature is similar to that observed for raw coal, whereas the independence of basicity contrasts with the marked dependence of the swelling of the coal. As already indicated, the activation energies and pre-exponential factors for the extracted coal are very similar for all three solvents. In particular, the
Effect of solvent basicity
on the kinetics
activation energies (m 10 kJ mol- ‘} are much lower than for the coal, suggesting a different type of interaction and mechanism. That the rates of swelling of the extracted coal are greater than those of the coal indicates structural differences. The pyridine extraction disrupts the hydrogen bonds in the coals and causes swelling. That the solvent swelling of the extracted coal does not vary greatly with the basicity of the substituted pyridine suggests that the formation of hydrogen bonds in the extracted coal after solvent removal is limited. This is supported by the faster rates of swelling and lower activation energies observed for the extracted coal, which are also not markedly dependent on the basicity of the substituted pyridines. Furthermore, the extent of swelling is not significantly dependent on solvent basicity. The swelling of the extracted coal in pyridine is markedly lower than for the coal, indicating some modification of the macromolecular structure e.g. covalent cross-linking and 7c-7~interactions. It has been suggested3 that ~--7ccomplexes are formed in the extracted coal and that some of them are not disrupted in the solvent swelling experiments. However, it is possible that other structural modifications can occur during solvent extraction, for example some decomposition leading to modification of the cross-links. There are thus two possible explanations for the reduced swelling of the extracted coal in pyridine. First, the extraction process may cause decomposition, leading to the formation of cross-links. Second, the collapse of the structure due to the removal of soluble material leads to the formation of strong 7c--7~ interactions which act as effective non-covalent cross-links. It is difficult to distinguish unambiguously between the alternatives. Previous workers3 have strongly favoured z--71 bond formation during the rearrangement of the macromolecular structure due to removal of soluble material. CONCLUSIONS The extent of swelling and kinetics of solvent diffusion into coal are significantly affected by the basicity of the
of solvent swelling
of coal: F. E. Ndaji and K. M. Thomas
solvent, both the extent and rate constant decreasing with decreasing solvent basicity. The results also suggest that the number of hydrogen bonding sites in coal that are able to react with the solvent increases with solvent basicity, and that the rate of swelling of coal in a solvent also depends on the basicity of the solvent. The results support the proposal that swelling in pyridine and other bases involves the disruption of hydrogen bonding in the coal macromolecular structure. In contrast, the changes in solvent basicity do not have such a marked effect on the extent and kinetics of swelling of pyridine-extracted coal, but the rates are much faster than for the coal. The apparent activation energies for the swelling of the coal decrease with decreasing basicity of the pyridines used. The corresponding data for the activation energies for solvent swelling of pyridineextracted coal show no significant variation, and are much lower than for coal. REFERENCES 1 2 3 4
9 IO I1 12 13 14 IS 16 17 18 19 20
Van Krevelen, D. W. ‘Coal’, Elsevier, Amsterdam, 1981 Painter, P. C., Graf, J. and Coleman, M. M. Energy Fuels 1990, 4, 393 Larsen, J. W. and Mohammadi, M. Energy Fuels 1990,4, 107 Aida, T. and Squires, T. G. Am. Chem. Sot. Div. Fuel Chem. Preprints 1985, 30(l), 95 Neavel, R. C. Phil. Trans. R. Sot. Lord A 1981, A300, 141 Sanada, Y. and Honda, H. Fuel 1966.45,295 Flory, P. J. and Rehner, J., Jr. Chem. Phys. 1943, 11, 521 Larsen, J. W., Green, T. K. and Kovac, J. J. Org. Chem. 1985, 50,4129 Lucht, L. M. and Peppas, N. A. Fuel 1987, 66, 803 Kovac, J. Macromolecules 1978, 11, 362 Barr-Howell, B. D. and Peppas, N. A. Polym. Bull. 1985,13,91 Nishioka, M. and Larsen, J. W. Energy Fuels 1990, 4, 100 Szeliga, J. and Marzec, A. Fuel 1983, 63, 1229 Hall, P. J., Marsh, H. and Thomas, K. M. Fuel 1988, 67, 863 Ndaji, F. E. and Thomas, K. M. Fuel 1993,72, 1525 Ritger, P. L. and Peppas. N. A. Fuel 1987, 66, 1379 Green, T. K. and West. T. A. Fuel 1986, 65, 298 Hail, P. J., Thomas, K. M. and Marsh, H. Fuel 1992, 71, 1273 Brenner, D. Fuel 1985, 64, 167 Peppas, N. A. and Lucht, L. M. Chem. Eng. Commun. 1985,31, 333
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72 Number
11
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