A kinetic study of the dehydration of Northern Ireland lignite

A kinetic Northern

study of the dehydration Ireland lignite

Mark

Michael

E. Brady,

G. Burnett

The School of Chemistry, The Queen’s (Received 3 7 August 1993)

The kinetics

of dehydration

and Andrew

University

of Belfast,

of lignite have been characterized

of

K. Galwey Belfast

by isothermal

BT9 5AG,

measurements

UK

of the water

evolved under vacuum at 273-313 K. The rate of water release at low relative pressures is deceleratory and data are well expressed by the contracting volume rate equation. When the water vapour pressure, p, approaches the saturated value, pa, the rate becomes controlled by the relative vapour pressure, p=p/p,. These controls are combined in the kinetic analysis of the isothermal yield-time data. It is shown that the evaporation of water from lignite is satisfactorily expressed by the equation, dp/dt = k( 1 - cr)‘13(1 -/I), where c( is the fractional dehydration at time t. The activation energy was 35 + 5 kJ mol- ’ between 273 and 3 13 K. It was shown that the dehydration rates of different lignite samples were identical, and that they were the same as the rate of dehydration of a rehydrated reactant sample which had previously been dried. It is concluded that the low temperature drying of lignite occurs by the evaporation of liquid water previously immobilized within the continuous coherent carbonaceous matrix. The drying rate depends on the particle size, the rate of migration of the drying interface within the particles and on the ambient pressure of water vapour which tends to rehydrate the matrix. (Keywords

lignite; drying; dehydration kinetics)

The removal of water from solid materials is of considerable technological importance. Commercial product specifications often include an upper limit for water content and the calorific values of fuels, notably lignites, with which this article is concerned, can be significantly increased by drying. Potential industrial interest in lignite exploitation has resulted in research undertaken to characterize the constituent water in brown coals from various locations. The subject has been reviewed by Karr’ who included a consideration of the experimental difficulties. The rates of water removal from untreated and from rehydrated (previously dehydrated) lignites in nitrogen were shown2 to be dependent on sample size, history, coal rank and extent of water loss. The water release process depended on the degree of surface coverage, correlating with the functional groups present, and was regarded as analogous to an adsorption isotherm. Second dehydrations (of rehydrated material) showed no significant deviation from the water loss process of the original material. Other3 vacuum dehydration studies completed at higher temperatures, 293423 K, showed that radicals were formed at concentrations which increased with temperature, and that there was a concurrent evolution of other products including CO, CO, and volatile hydrocarbons. A recent article4 reported a detailed kinetic study of the dehydration of a ‘woody lignite’ from Northern Ireland, that was selected for investigation because the material was relatively hard and homogeneous. The results showed that reproducible kinetic measurements could be made of water loss under vacuum, and that at low temperatures between 273 and 293 K there was no

0016-2361/94/08/1343-05 0 1994 Butterworth-Heinemann

Ltd

accompanying chemical change. The constituent liquid water, 54.0% of the reactant mass, was removed from the interstices of the coherent and continuous carbonaceous matrix at rates satisfactorily expressed by the well known contracting cube equation’. This result implies that the rate determining step in water loss is through evaporation at the wetMry interface. Thus, an initially planar surface of the hydrated material generates a planar interface which moves at a constant speed into the interior of the particle. An initially cubic sample, therefore, dries by forming a cubic watercontaining residue of decreasing size as was confirmed by microscopic observations4. Partially reacted samples were sectioned and a square-shaped darker zone at the centre could readily be distinguished from the lighter coloured surrounding material from which the water had been lost. It was further shown that the rate of water loss was diminished by the reverse process of water adsorption if gaseous water vapour was present. Both geometric and water vapour adsorption controls were investigated individually. The effects were finally combined together in a single equation which fitted lignite dehydration rates across the wide range of conditions investigated. Dehydrations have been recognized in solid state science as particularly suitable as models for investigations of the fundamental chemical steps leading to the elimination of water of crystallization originally bound in various ways such as metal ion ligation6, hydrogen bonding or as hydroxyl groups. Studies of water removal from well-characterized hydrates have contributed notably to the theory of the steps involved in solid state reactions’. In particular, this work has led to the recognition that these reactions frequently take place by

Fuel 1994

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Dehydration

kinetics

of Northern

Ireland

lignite:

M. E. Brady

means of an advancing reaction interface, and this concept is the basis of the nucleation7 and growth’ theory of solid state reaction. The dehydration of alums characteristically follows a sigmoid yield-time curve6, although that of Li2S0,.H,0 was predominantly deceleratory’. The dehydrations of solids have been reviewed5g”,” and the criteria have been discussed for assessing the agreement between the experimental rate data and a theoretical equation”. The value of complementing such kinetic interpretations by microscopic observations has been emphasized 13. This well established theory has not been extended to the dehydration of the less well defined materials that are of industrial importance. The present study continues and extends our previous4 investigations of lignite dehydration to a wider and less uniform set of samples, obtained from drilling cores, that are more representative of the extensive lignite deposits in Northern Ireland. The validity of our earlier conclusions are tested here with a more representative and non-uniform reactant material.

et al.

Reactants

The lignite used in this study was supplied by Antrim Coal Co.. Sample A is the ‘woody lignite’ which was the material used previously4. The present report is concerned with three further samples B, C and D all coming from the same bore-hole near to the source of sample A, close to Crumlin in County Antrim, Northern Ireland. After withdrawal, each core was wrapped and stored under air-tight conditions to minimize water loss, aerial oxidation and other changes which occur during weathering in the atmosphere. Sample details and composition data are shown in Table 1. Each lignite reactant cube was prepared as in the previous study4. It was cut from the specimen core using a sharp blade and rapidly trimmed into a cube of 5 mm edge. The specimen was transferred to the apparatus which was evacuated as quickly as possible to minimize water loss before the start of each experiment. Sample C, containing the greatest water content, (55.2 wt%), was the most extensively studied. Kinetic

EXPERIMENTAL Apparatus The apparatus was unchanged from that used earlier4x14 and it has been described previously in detail15. It is a conventional glass vacuum apparatus capable of attaining - 10m3 Pa. A Baratron MKS 222B absolute pressure diaphragm gauge is used to measure pressures from 0 to 1333 Pa with an accuracy of f 0.133 Pa. The gauge is interfaced with a computer programmed to store the pressure-time data, and which is also used to test14 the fit of these data to appropriate kinetic equations. Sample temperatures were maintained constant to within +l K. Water evaporation commenced immediately the sample was introduced into the apparatus. The evacuation period to remove air was reduced to a minimum, usually about 2min. The pressure was then only FZ30 Pa, and probably consisted almost exclusively of evaporated water. Amounts of water lost during initial evacuations were normally -5% of the total volatile product yield. The volume of the sample container, 0.811, was particularly convenient for determining the effect of water vapour on the drying rate, since the pressure of water increased rapidly in this relatively small volume. Two evacuated bulbs could be opened to the sample container to increase the total volume to either 6.15 or 11.57 1. These volumes provided conditions suitable for the study of dehydration in the presence of a low water vapour pressure.

Table 1

Sample

Depth

and compositions Location depth below ground level (m)

of lignite samples

equations

The rate of lignite dehydration is controlled by the competition between water evaporation and adsorption at the interface between the wet and dry material. Water loss occurs as the interface moves through the wet lignite at a constant rate, k,, whereas when water is adsorbed the interface moves back into the dry lignite at a rate proportional to the pressure of the water vapour, p. The drying process continues until either all the water has evaporated or the water vapour has reached its saturated pressure po. This model has previously4 been shown to yield the drying Equation (l), dp/dt = (6k,pJa,)(l-

c()“~(1 - /?)

(1)

where pf is the final pressure of water vapour corresponding to the complete dehydration of a cubic specimen of side a,, CIis the fractional dehydration at any time t, a =p/pf, and j? is the relative humidity, b =p/p,. The value of pf may either be measured directly, in the large volume apparatus, or obtained as the sum of the contributions where several consecutive rate measurements are made for the same sample. The constants in any experiment are controlled by the water content and weight of the lignite specimen, its temperature and the volume of the apparatus. The pressure p,, is determined by the reactant temperature. The final pressure, pr, depends on the total water content of the reactant specimen used. In practice the specimens were either: (i) completely dehydrated in a large volume apparatus where the change

studied

&%)

&%)

Ash (wt%)

Wet sample moisture (wt%)

63.4

5.2

0.0

0.3

1.5

54.0

B

17.0

56.4

3.8

0.8

0.1

4.4

46.4

C

30.0

68.7

8.4

0.4

0.2

2.0

55.2

5.5

0.6

0.2

4.0

51.9

A

D

2.0”

61.4

30.8

a Ref. 4

1344

Fuel 1994

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Dehydration 0.8

of Northern

Ireland

lignite:

M. E. Brady

et al.

reliably because of the sensitivity’ of the fit to the value of pf. The dehydration in these experiments is expected to overlap with the onset of the release of water more strongly bonded to the carbonaceous residue16. It is important to emphasize that pr values should be determined directly and not measured by standard moisture determinations, e.g. at 378 K, because this may result in a significant release of bonded water16.

r

10

0

20 Time

30

40

(min)

Figure 1 Representative linear plots of [1 -(l -a)“‘] versus time for the isothermal dehydration of sample C shaped as a cube of side 5 mm. The contracting cube Equation (2) is obeyed to at least c(= 0.88 for low humidity where BcO.2. Lines (i) and (ii) illustrate maximum variations ofdehydration rate at 308 K for two different samples of the reactant

in /? was relatively small, /? ~0.25, or (ii) were partially dried in a small volume apparatus where the change in a was still relatively small, c(< 0.3, when the water vapour became saturated. 1 If fi is approximately zero then Equation (1) can be reduced to the conventional contracting cube expression5 (2), 1 -(l -t~)l’~ =(2k,/a,)t

(2)

since c1=

kinetics

[a,3-(a, - 2W31 a,”

2. If I_Y is approximately zero then Equation (1) becomes a simple ‘first order’ rate law for the approach of the gaseous water pressure, p, to the saturated value p,,, dpldt = k(p, -P)

(3)

where k = (6k,p,/a,p,) The limiting rate laws (2) and (3) were tested directly4,14 in cases where the conditions were suitable. Otherwise the full rate Equation (1) was used to compute the theoretical reaction profiles for comparison with experiment so that the fit could be assessed and the best value of the rate function, k,, could be estimated. RESULTS

AND DISCUSSION

I/arying humidity. Kinetic measurements were carried out under conditions when it was impossible to neglect either the varying relative humidity, 8, or the fractional dehydration, CI.These data satisfactorily fit Equation (1) and are poorly represented by either Equations (2) or (3). Six identical experiments were performed at 292 k 1 K with specimens from sample C, in which the relative humidity, p, varies from 0 to 0.7, while the fractional dehydration, ~1,varies from 0 to 0.8. The observations could be analysed using Equation (2) for a limited range of a values, 0 < c(~0.6, but the rate constants k, varied between experiments and covered a range from 1.13 to 3.15 x 10e2 mm min- I. Equation (3) provided an even less satisfactory fit, and the measured rate constants varied from 1.16 to 7.10 x 10m2mm min-‘. Equation (l), on the other hand, could be used to model the reaction profile successfully over the observed range as illustrated in Figure 2, and the measured rate constants varied much less, from 2.49 to 3.67 x 10m2 mm min-‘, giving a mean value of (3.09 f 0.38) x 10e2 mm min- ‘. The validity of the equation is further confirmed by restarting experiments after partial dehydration. The effect of water vapour is shown in Figure 3 where it was pumped away after 34% dehydration. The rate of dehydration then increased sharply, although the results could still be analysed using Equation (1) and the calculated value of k, remained unaltered, (k, = 3.3 x 10 -2 mm min- ‘). Dehydration at high humidity. When dehydrations were performed using a relatively large 7 mm cube of lignite in a small volume apparatus, 0.811, at a low temperature, 273 K, the dehydration became strongly deceleratory since the water vapour pressure quickly approached saturation. The change in CI in these experiments was negligible since the percentage dehydration was so small and the results, therefore, fitted Equation (3). The results of two runs, shown in Figure 4, agree with the integrated form of Equation (3), ln(1 -B) versus time, for more than three half-lives. In

The kinetics of dehydration Low humidity. The dehydration

of cubes of sample C at 308 K and at low humidity throughout, pcO.2 using 11.56 1volume, gave deceleratory a-time plots. These data were well represented by the contracting cube, Equation (2), up to at least cr=O.88, Figure 1. This contrasts with the difficulty of designing an experiment in which control by humidity, Equation (3), applies throughout. Some variation was observed between dehydration rates of successive apparently identical lignite specimens, but this is not unexpected due to the difficulty of cutting precisely identical cubes and to variations in the amounts of water lost at the start of experiments. The two lines shown in Figure 1 represent the maximum variation observed for the [l -(l - a)‘j3] versus time plots for these conditions and have slopes of 1.2 and 1.7 x 10m2 min-‘. The rate equation applicable at high c1values cannot be established

3 4

0.63

-

0.56

-

0.49

-

0.43

-

0.36

-

i

0.29

-

3

0.24

-

a

0.16

-

0.09

7:

0.03 0

,__ ., ,_ ___. ..-.. .,-” _.

Calculated data +.

‘.

i

,, : ,I‘

'bbserved data

,,’ i'

L

I

I

I

I

10

20

30

40

50 Time

I, 60

I

70

80

I 90

I,,

100

110

120

(min)

Figure 2 Observed pressure-time values for the dehydration of a 5 mm edge cube of sample C at 273 K compared with the prediction of Equation (1) for 0.02
Fuel 1994 Volume 73 Number 8

1345

Dehydration kinetics of Northern Ireland lignite: M. E. Brady et al 1.10

-

1.04

-

0.97

-

2 r

0.90 0.84

-

z

0.77

-

$ F a

0.70 0.64

-

0.57

-

0.50

-

0.45

-

0.37 24

dehydrations were also reported by Vorres and Kolman’. Although the reversibility of dehydration under mild conditions was confirmed, the influence of water vapour on dehydration rates was not investigated. The observation that an appreciable amount of water is adsorbed from the vapour by the dried lignite shows that the hydrophilic character is not lost during low temperature drying. This contrasts with the high temperature treatment’ 7 which leads, perhaps via the elimination of surface -OH and -COOH groups, to the surface becoming hydrophobic, and thus ensures that the water-free material can be stored as a dry fuel.

Observed data + 4 Calculated data

34 I

44 I

54 1

64 I

74 I

84 I

94 1 -;fifi1147;4

Time (min)

Figure 3 Observed pressure-time plots for lignite dehydration at 273K in 0.81 1 apparatus following water removal after 34% dehydration compared with pressures calculated using Equation (1)

A I/

4

3 [

_:-

_:-

m 2

5 c -i-

J..

_./

,-.

A-::-

1

;_ I

0

I

I

I

I

I

1

5

10

15

20

25

30

Time (min)

Figure 4 Typical ln(l-j?) versus time plots for lignite under high humidity at 273 K in 0.811 apparatus

dehydration

the first run, A, the value of c(increased from zero to 0.15 while in the second, B, the water vapour was pumped away and the sample was again allowed to dehydrate so that CIchanged from 0.16 to 0.47. Dehydration

of rehydrated

Fuel 1994

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Lignite dehydration in low pressures of air. Comparative experiments showed that the presence of small pressures of air did not change dehydration rates. The value of k, in -200Paofairat293K,(2.0+0.3)x10-2mmmin-’, does not significantly differ from experiments in the absence of added air, (2.lkO.3) x 10m2mm min- ‘. This showed that the necessarily short evacuation at the start of all experiments did not affect the rate constants. However, further studies would be required to determine the effects of higher pressures of inert gases, particularly to extend the results to the dehydration of lignite in the atmosphere.

0.7 [

0.61

lignite.

The results of this study clearly indicate that low temperature drying has little effect on the adsorptive and desorptive properties of lignite. Lignite, after dehydration at 293 K, readsorbed only - 10 wt% water after exposure for 3 days to the saturated vapour. However, dehydrated reactant cubes immersed in liquid water readsorbed -40 wt% water so that they then contained 70-80 wt% of that originally present, as shown in Table 1. The rates of the second dehydration were closely comparable with those of the first dehydration of the same specimen as illustrated in Figure 5. This experiment was performed at low humidity and analysed by Equation (2). The second dehydration is slightly more rapid than the first, perhaps because of minor textural modifications during the initial dehydration. The restricted uptake of water from the vapour suggests that an open pore structure is not filled by condensation, but that it can admit liquid by capillary action. This, and the similarity of the first and second dehydration rates, implies that either the retexturing during water loss is small or that the continuous phase exerts little influence on the water evaporation. Similar rates of first and second

1346

Reproducibility of dehydration results. All four samples of lignite were dehydrated in identical experiments at 293 K using the usual procedure. The experiments were at low humidity and were analysed using Equation (2) for CI varying from zero to 0.5. The rate constants summarized in Table 2 are identical within the limits of experimental error. The results are consistent with the conclusion that the controlling process is the evaporation of liquid water from the interstices of the carbonaceous matrix. The matrix structure is either unimportant in controlling water release, or it is sufficiently similar within the different materials to give identical rate constants.

73 Number

8

(b)

0.5 I-

I

n

‘=

2

0.4

2

0.3

.:’

‘;.

.’

.’

(a)

L. 0.2 F 0.1

:

t

)’

0

,

,

1

1

I

c

5

10

15

20

25

30

Time (min)

Figure 5 The dehydration of a 5 mm edge cube of sample C lignite (a) as initially prepared, and (b) after rehydration using liquid water; both experiments under low humidity analysed by the contracting cube Equation (2)

Table 2 Comparison of the dehydration lignite samples at 293 K

rate constants

for different

Sample

A

B

C

D

IO* k, (mmmin-‘)

1.86kO.15

2.5* 1.0

2.1 kO.7

2.0*0.3

Dehydration 313 K 0.4

308 K

0.1

-

273 K ,

0

I 5

1 10 Time

1 15

(min)

Figure 6 Comparable experiments using 5 mm cube specimens of sample C at temperatures from 273 to 313 K at low humidity and analysed by the contracting cube Equation (2)

Activation energy. The vapour pressure, pO, varies with temperature so that in order to find the activation energy of the desorption process, the rates were measured at low humidity where its value has no effect on the rate (Equation (2)). Representative results are shown in Figure 6. Three or more rate constants were measured at each temperature, and the mean activation energy was found to be 35 f 5 kJ mol-’ by the method of least squares fit. The more restricted set4 of data at 273 and 293 K using sample A suggested a value of 24 + 18 kJ mol- ‘. The energy barrier to the evaporation of water from lignite is, therefore, either equal to or less than the molar enthalpy of evaporation of water, 44 kJ mol- ‘, thereby confirming our earlier conclusion4 that it is either low or negligible.

CONCLUSIONS The low temperature dehydration of Northern Ireland (Crumlin) lignite involves the evaporation of liquid water retained within, but not chemically bonded to, a hydrophilic carbonaceous and coherent matrix. The rate of evaporation is determined by the size of the lignite particle, and may be shown both kinetically and microscopically to obey a contracting cube equation provided the gaseous pressure of water vapour is low enough to prevent water readsorption. Dehydration is occurring predominantly4 at the wet-dry interface, tnoving at a constant rate from the surface into the interior of the lignite particle. This is an interesting instance of a reaction ‘interface’5*“*‘3 appearing during a purely physical transformation in the absence of any detectable ‘reactant to product’ chemical process. The reversal of the dehydration process by an increase in relative humidity is similar to the case of alum’*. These two rate controlling parameters combined in the rate Equation (1) satisfactorily describe the dehydration kinetics of different lignite samples from the extensive Antrim deposit. The observations in Table 2 confirm that the rates of interface advance are the same in all four samples and provide evidence that the detailed structure of the carbonaceous residue exerts little influence on the rate of dehydration. The mean rate of water

kinetics

of Northern

Ireland

lignite:

M. E. Brady

et al.

evaporation from these lignites is estimated to be -8x10-7molmin-‘mm-2 at 293K. This value is greater than that calculated4 to occur at the same temperature during the growth of nuclei in the course of the dehydration of chrome alum and aluminium alum6, -0.6 and 0.3 x lo-’ mol min-’ mme2, respectively. The similarities are interesting in providing support for the proposed mechanism of alum dehydration, which is based on the temporary retention of water within growth nuclei6. Product escape may influence the rate of dehydration in both lignite and alum, but only in the alum dehydration will the water vapour pressure within the nuclei be reduced by bonding to the cations present. This may explain the reduced rate of water loss in alum but the broad similarity of the rates is consistent with the similarity of the mechanism. Dehydration kinetic characteristics reported here are different to the behaviour described by McKay et al.” using material from the same deposit but employing higher temperatures, 313-343 K in air. These workers identified diffusion as rate determining during the relatively rapid evolution of steam. Their activation energy, 17.0 + 0.3 kJ mol- 1 was lower than our estimates, and again lower than the molar heat of vaporization of water.

ACKNOWLEDGEMENTS We thank Mr David Watt and other members of staff of the Antrim Coal Co. for supplying the coal samples. We also thank Mr A. E. Griffith, Director of the Geological Survey for Northern Ireland, for helpful discussions and for advice in the selection of material for study.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Karr, Jr, C. (Ed.) ‘Analytical Methods for Coal and Coal Products’, Vol. 2, Academic Press, New York, 1978, p. 294 Vorres, K. S. and Kolman, R. Prepr. Pap. Am. Chem. Sot., Div. Fuel Chem. 1988,33, 7 Goldberg, I., McKinney, T. M., Chung, K. E. and Galli, R. Fuel 1986 65, 241 Brady, M. E., Burnett, M. G. and Galwey, A. K. J. Chem. Sot., Farahay Trans. I 1990, 86, 1573 _ Brown. M. E.. Dollimore. D. and Galwev. A. K. ‘Comnrehensive Chemical Kinetics’, Vol.‘22, Elsevier, A-Asterdam, 1680 Galwey, A. K., Spinicci, R. and Guarini, G. G. T. Proc. R. Sot. Lond. 1981, A378,477 Galwey, A. K. and Laverty, G. M. Solid State Ionics 1990,38,155 Galwey, A. K. React. Solids 1990, 8, 211 Brown, M. E., Galwey, A. K. and Li Wan PO, A. Thermochim. Acta 1992, 203, 221; 1993, 220, 131 Galwey, A. K. and Laverty,G. M. J. Chim. Phys. 1990,87,1207 Galwey, A. K. J. Therm. Anal. 1992, 38, 99 Brown, M. E. and Galwey, A. K. Thermochim. Acta 1979,29,129 Galwey, A. K. Thermochim. Acta 1985,96, 259 Brady, M. E. PhD Thesis, The Queen’s University of Belfast, 1991 Carr,N. J.andGalwey, A. K. Proc. R.Soc. Lond. 1986, A404.101 Allardice, D. J. and Evans, D. G. Fuel 1971 50, 201 and 236 KGuDelman. E. US Patent No. 4.052.168. 1977 Gaiwey, A. K. and Guarini, G. 6. T.’ Prk. R. Sot. Lond. 1993, A441, 313 McKay, G., Magee, T. R. A. and Diamond, N. C. Fuel 1990, 69. 189

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