Energy & Fuek 1991,5,463-468 hydrogens in functional groups such as -OH and -NH2. Although detailed analysis of such active hydrogens has not been done, the comparable analysis of bituminous coals has been reported?lS Bituminous coals which have a chemical composition of C 75-85% and H 5.0-5.4 w t 90 contain 6-12 atom % of phenolic OH hydrogen for total hydrogen. Yokoyama et al.29has reported that high-rank coals which have a chemical composition of C 75-8470 and H 5.8-6.4% (daf) contain 3-9 atom % of phenolic OH hydrogen and carboxylic acid COOH hydrogen for total hydrogen, while low-rank coals which have a chemical composition of C 61-70% and H 5.3-6.0% (daf) contain 12-14 atom 7% of those. Kotanigawa et al.% have reported that the exchange reaction between deuterium gas and aromatic hydrogen in phenol took place rapidly at 350 "C with ZnO-Fe20q catalyst and that no such exchange reaction occurred in the absence of catalyst. They did not refer to the exchange reaction between deuterium gas and hydrogen of hydroxyl group in phenol. We attempted the reaction of phenol with tritiated gaseous hydrogen at 340 "C for 2 h in the absence of catalyst; 8.8% of the hydrogen in phenol underwent tritium exchange. Since it can be assumed that only the hydrogen of the hydroxyl group in phenol would exchange, this indicated that 53 % of the hydrogen of the hydroxyl (21) Pestryakov, B. V. Solid Fuel Chem. 1986,20(6), 3. (22) Mrekawa, Y. J. Jpn. Pet. Inst. 1976,18,746. (23) Yokoyama, S.; Itoh, M.; Takeya, G.Kogyo Kagaku Za~Shi1967, 70, 133. (24) Kotanigawa, T.;Shimokawa, K.; Yoshida, T.; Tamamoto,M. J. Phye. Chem. 1979,83,3020.
463
group in phenol exchanged with gaseous hydrogen at 340 "C for 2 h. Further, the reaction of aniline with tritiated gaseous hydrogen was also performed at 300 "C for 2 h in the absence of catalyst; 13.7% of the hydrogen in aniline underwent tritium exchange. Since it can be assumed that only the hydrogen of the amino group in aniline would exchange, this indicated that 48% of the hydrogen of the amino group in aniline exchanged with gaseous hydrogen at 300 "C for 2 h. These results support the suggestion that, in the present reaction of coal with gaseous hydrogen, OH and NH2 hydrogens of polycondensed aromatic compounds would have exchanged at lower temperatures.
Conclusions The hydrogen-exchange reactions of Datong coal (bituminous coal), Wandoan coal (subbituminous coal), and Morwell brown coal with tritium-labeled hydrogen molecules were investigated. The hydrogen-exchangereaction of coal with gaseous hydrogen largely increased with a rise from 350 to 400 "C. The hydrogen-exchange reaction of tetralin with gaseous hydrogen was more difficult to proceed than that of coal. These results suggest that the radicals produced on coal would be strongly related to the hydrogen-exchangereaction with gaseous hydrogen. The hydrogen-exchange reactions between coals and gaseous hydrogen proceeded even at 300 "C and the hydrogenexchange ratio increased in the order of Datong N Wandoan < Morwell. It is suggested that hydrogens of functional groups such as -OH and -NH2 in coals would be exchangeable with gaseous hydrogen at 300 "C. Registry No. Tetralin, 119-64-2; tritium, 10028-17-8.
Effect of Nonuniform Surface Reactivity on the Evolution of Pore Structure and Surface Area during Carbon Gasification R. H. Hurt,* A. F.Sarofim, and J. P. Longwell Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusett,j 02139 Received August 30, 1990. Revised Manuscript Received February 22, 1991 There is much experimental evidence that the gasification of carbon surfaces is often nonhomogeneous on a fine scale. Gasification of impure carbons, for example, often occurs in the immediate vicinity of catalytically active particles of inorganic matter, proceeding by the formation either of irregular pita or of channels of definits size and orientation. In the present paper, a model is presented which predicts the effect of nonuniform gasification on the evolution of surface area and pore size distribution for low-temperature porous carbons. Nonuniformity of surface reactivity can dramatically reduce the extent of surface area development during gasification or activation. Resulta of the model are used to explain several seta of previously published measurements of surface area evolution as a function of carbon conversion.
Introduction The evolution of pore structure and internal surface area during carbon gasification plays an important role in both the production of activated carbon and the kinetics of coal *Author to whom correspondenceshould be addressed. Current address: Combustion Research Facility, Sandia National Laboratories, Livermore, CA 945514989.
gasification. Several models treat the evolution of surface area and/or pore structure by considering the effect of widening of cylindrical pores due to uniform surface recession.'~ There is much experimental evidence, however, that the actual gasification process is more complex. The (1) Gavalae, G.R.AZChE J. 1980,36 (4), 577. (2) Simons, G. A. Combust. Sci. Tech. 1979, 19, 227.
0887-0624/91/2505-0463$02.50/0Q 1991 American Chemical Society
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464 Energy & Fuels, Vol. 5, No. 3, 1991
Combined pore production and widening al of pore
Figure 1. Various modes of gasification of amorphous carbons.
high surface area typical of many low-temperaturecarbons is located predominantly in pores with diameters < 50 A. At this length scale gasification often proceeds nonuniformly,= p r d i g , for example, by the formation of pits or channels. In addition, the gasification process induces atomic rearrangements which lead to densification and macroscopically observable particle shrinkage.' This densification/shrinkage phenomenon has been shown to dramatically influence surface area evolution? In contrast, it is not known to what extent nonuniform surface gasification affects pore structure and surface area evolution. Catalyzed gasification, in particular, often occurs in the immediate vicinity of particles of inorganic matter, proceeding by the formation of pits (of indefinite shape) or channels (of definite shape and perhaps orientation) instead of by uniform pore widening (see Figure 1). Bake$ has observed pitting and channeling during the gasification of graphite with a variety of metal catalysts. Pit or channel formation has been microscopically observed during the gasification of synthetic amorphous carbons both with impregnated metal catalyst3 and w i t h ~ u t .In ~ a study of numerous catalyzed systems it was reported that gasification rarely if ever proceeds uniformly over the entire ~ u r f a c e .(Cases ~ where augmentation of reactivity could occur uniformly over the surface are those in which the catalyst is mobile, e.g., potassium.) Several recent articles have dealt with the mechanism of particle channeling during the gasification of graphite~.~*l~ Sometimes, the formation of pits is observed in th8 absence of impregnated catalyst, possibly as a consequence of the inherent heterogeneity of the carbon substrate at the appropriate length scale. Lamond et al.," using the technique of electron microscopy with double replication, have observed the formation of 100-300 A diameter pits during the carbon dioxide gasification of polymer-derived carbons. Murrell et al.s have observed the appearance of a characteristic feature in the pore size distribution centered around 3.9 nm diameter during the gasification of a variety of chars and cokes. They postulate that these ~
~~
(3) Adair, R. R.; Boult, E. H.; F'reeman, E. M.; Jasienko, S.; Marsh, H. Carbon 1971,9,763. (4)Lamond, T.G.; Marsh, H. Carbon 1963,1,293. (5)Murrell, L.L.,Ratcliffe, C. T.,Pieters, W. J. M., Sherman, L. G. Dispenziere, N. C., Jr., Venero, A. F. Carbon 1988,26 (l),33. (6)Hurt, R.H.; Sarofim, A. F.; Longwell, J. P. The Role of Microporous Surface Area in the Gasification of a Sub-bituminous Coal Char. Fuel, in press. (7) Hurt, R. H.; Longwell, J. P.; Sarofim, A. F. Carbon 1988,26 (4), 433. (8)Baker, R. T.K.; Chludzineki, J. J., Jr. Carbon 1986,23 (6),635. (9)Tsamopouloa,J. A., Dandekar, H. W., Scholtz, J. H. J. Catal. 1989, 117,549. (10)Choi, A. S.,Devera, A. L., Hawlay, M. C. J. Catal. 1987,loS,313.
pores are produced as gasification preferentially attacks a primary structural unit having this characteristic size, which had formed during the pyrolysis process. A second type of uneven surface gasification occurs when reaction takes place primarily or exclusively on the surfaces of larger pores. This mode of gasification would occur if the micropores were inaccessible as a consequence of restricted diffusion limitations or in the presence of steric hindrance of the reaction itself in fine pores." Gasification would likewise occur predominately on large pore surfaces if the active sites were concentrated there, for example, in the form of catalyst particles that were too large to penetrate the microporous network.6 Ranish et al.12 have developed a model to predict the effect of pitting and channeling by inorganic impurities on the physical properties (surface area and porosity) of thin graphite flakes. The models consider pore production due to the channeling nature of catalytic gasification and subsequent channel widening by noncatalytic gasification on the newly created surface area. The intent of this paper is to present a model that predicts the effect of both types of nonuniform gasification (pore production and recession of large pore surfaces) on the surface area evolution for low-temperature porous carbons, and to use the results to explain previously published measurements of surface area evolution during carbon gasification.
Theory In this work we attempt to understand the effect of uneven surface gasification by considering two very different modes of nonuniform gasification: one in which the reaction takes place only on the surfaces of larger pores (having radii greater than r0),and one in which the reaction takes place exclusively by the formation of cylindrical channels of radius r,. In addition, the combined effect of channel formation and gasification-induced densification is modeled, in order to explain existing data on surface area evolution for Spherocarb carbon, which has previously been shown to exhibit pronounced densification and shrinkage.' In the next section we make use of several relations derived by Gavalasl for describing networks of randomly oriented cylindrical pores to develop numerical procedures to handle the above three cases. Gavalas' has treated the evolution of pore size distribution and total surface area during carbon gasification by uniform surface recession, for a model pore structure comprising randomly distributed, infmite, cylindrical pores (see Figure 1).The treatment is based upon a probability density function (r), defined such that (r) dr d S is the probability that the axis of a pore with a radius in [r, r + dr] intersects an arbitrary surface of differential area d S within the solid. The probability density function (PDF) X(r) contains all of the information about the pore structure and is related to, but differs from, measured pore volume distributions. The random distribution of pore axes in space results in pore overlap, in light of which the properties of the pore structure (such as surface area) cannot be obtained by simple summation or integration of the appropriately weighted function X(r). A statistical treatment of this overlap volume leads to an expression for the total porosity from the pore number density function X(r) (eq 1). Considering the effect of a differential amount of pore growth on the porosity, an accompanying (11) Hurt, R. H.; Sarofim, A. F.; Longwell, J. P. The Role of Microporous Surface Area in Noncatalyd Carbon Gasification. Energy ELels 1991,5,290. (12)Ranish, J. M.,Walker, P. L. Carbon 1990,B (6),887.
Evolution of Pore Structure during Carbon Gasification
expression for the total surface area has also been derived (eq 2).
8 = 1 - exp[-2rSrmA(r) dr] rldn
In eqs 1 and 2 , 8 is porosity (pore volume/particle volume), A is specific surface area (m2/m3),r is pore radius, and X(r) is the pore PDF defined earlier. The development leading to eq 1 and 2 is presented elsewhere.' Equations 1 and 2 provide a framework for modeling of pore structure evolution in which the distribution X(r) can be altered by reaction (e.g., to represent pore widening) and the new porosity and surface area calculated at any instant from the new distribution X(r). The PDF, X(r),for the initial unreacted carbon can be obtained from measured pore volume distributions by additional equations.'J The model has, in the past, been related to measured rate/conversion curves, assuming that the concentration of active sites is constant throughout conversion and using the zeroth and first moments of the initial pore PDF as adjustable parameters.' This treatment has also been used in connection with measured initial pore size distributions to generate numerical predictions of surface area evolution with no adjustable parameter^.^ For the numerical treatments the pore size distribution is divided into discrete narrow classes, of width log (ri/ri+J = 0.1, and converted into the PDF form. Reaction is then accomplished by directly altering the pore PDF to produce an incremental amount of carbon conversion, after which relations 1 and 2 are invoked to compute the porosity and total surface area from the new PDF. The entire pore structure evolution is obtained by stepping through carbon conversion in this way. This approach gives the modeler the flexibility to change the pore structure in any conceivable way (i.e., to investigate various modes of gasification), while rigorously calculating the effect of the changes on pore structure and surface area. The production of channels of uniform diameter was effected numerically by increasing the value of X(r) (creating new pores) in the interval [ri, ri+'] of the discrete PDF, as shown in eq 3. for ri < r < ri+' X(r),+& = X(r), + 6, (3) X(r)x+b = X(r),
for r
> ri+l or r < ri
for r 1 r, for r < ro
In some simulations, the PDF was additionally altered after each time step to simulate the phenomenon of gasification-inducedcarbon densification. This was necegsary in order to make a comparison with experimental results reported for gasification of Spherocarb carbon, which has been observed to undergo densification and particle shrinkage during reaction in zone I.' Densification was effected by reducing the diameters of large pores, after each time step, by an amount that is proportional to the extent of particle shrinkage observed by microscopy. These pores act as holes in a homogeneously shrinking matrix in geometric analogy to the shrinkage of voids in a free body undergoing thermal contraction. The solid-state rearrangements which are ultimately responsible for densification/shrinkage are simulated by eliminating fine pores until the volume balance within the particle is satisfied. The incorporation of the densification/shrinkage step is described in detail elsewhere.' In both cases one has, after each complete gasification step, a new pore axis distribution, from which any property of the pore structure (such as surface area) can be calculated. The version of the model without adjustable parameters requires as input the complete pore size distribution of the unreacted carbon. These data were available to us for Spherocarb carbon in the form of mercury porosimetry measurements courtesy of Niksa13 and carbon dioxide surface areas from Dudek.14 For most other chars this complete characterization is not available. It is possible, however, to estimate the initial distribution using measured values of total surface area and pore volume (or porosity), if the shape of the pore number density distribution is known. Simons2has shown that many measured pore size distributions can be adequately represented by a pore number density proportional to l / d 3 . The d-ll3 scaling law, in combination with measured surface area, porosity, and maximum pore size, can be used to completely define the pore PDF as follows. X(r) = c / ?
(4)
= Mr), where S1, b2 are constants chosen to produce a small AX at each step. The following procedure was used to simulate the combined effects of pore widening (responsible for a fraction P of the carbon consumption) and channel production (responsiblefor a fraction 1 - P). All pores were uniformly widened by increasing their radii by 62 to incrementally increase carbon conversion. Channel production was then simulated by increasing the probability density in an interval [ri, ri+J of the discrete PDF containing r,, iteratively, in small steps, until the ratio of the incremental conversions due to the two mechanisms in this time step was P/(1- P).
(5)
where r is the pore radius, X(r) is the pore PDF, and c is a constant (a property of the pore structure). The surface area and porosity are given by 0 = 1 - exp[-2*Jrmc/r
dr] = 1 - [rmax/rmh]-2*c
rldn
A = 4 m ( l - 8)[l/rmi,, - l/r-]
Pore widening was accomplished by increasing the value of r at constant number density in each of the size intervals. For reaction in pores of radius greater than r,, eq 4 represents the difference expression used. h(r+62),+b = X(r),
Energy & Fuels, Vol. 5, No. 3, 1991 465
(7)
After r- is chosen (here taken to be one-tenth of the particle radius), the above two algebraic equations can be solved simultaneously, for known porosity and surface area, to yield c and rmin.Several calculations in this study were performed for a carbon with a diameter of 200 pm, a surface area of 450 m2/g, and a porosity of 40%. This yields a maximum pore radius of 10 pm, a c value of 0.007 16, and a minimum pore radius of about 1 A.16
Results The surface area evolution calculated by the model for the case of gasification by pore production appears in (13) N i b , S. Privata communication. (14) Dudek, D. Ph.D. Thesis, Department of Chemical Engineering, Masaachusetta Institute of Technolopy, Cambridge, MA, 1988. (15) The appropriate value for the true minimum pore radius for carbon gasification ia thought to be 1 . 6 2 A, somewhat larger than 1 A. The actual distribution scales with an exponent slightly higher than 3 and would therefore have, over the anme range of radii, more volume in fine pores. For the 1/P distribution to reproduce the high measured surfacea areas, ita range of radii must simply be extended to 1 A.
Hurt et al.
466 Energy & Fuels, Vol. 5, No. 3, 1991
$
4/
all pores = 1
37
/A/AO
subbit r h o r . meosured oreos
A
/
d = 2 0 h
/
/
0
e
,,
:
I
B
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/
/
widening uniform
,
d = 5 h
d = 5 0 h
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d=100A
d = 2 0 h A/Ao = 0.1 A
$1
d = 500 A
u
A/Ao
v1
v
= 0.04
$ 1
Q
0
10
30
20
40
50
60
70
80
90
OTf
100
Figure 2. Surface area evolution during gasification by pore
production.
h
E
95%
0.7-
v
.-732
0.6-
0.5Y
01
3
2
0.4-
0.3-
01
3
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v
l-
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-10
-9
-a
210
30
40
50
$0
$0
do
90
160
% Conversion
% Conversion
- 0.81
1'0
-7
-6
-5
Log[pore r a d i u s ( m ) ]
Figure 3. Surface area evolution during gasification by uniform recession of the surfaces of larger pores.
Figure 2 as a function of the diameter of the created channels. The solution of the original Gavalas model considering uniform pore widening is also plotted for comparison. From the figure it is evident that the evolution of surface area during pore production is a strong function of the diameter of the created channels, and, depending on this diameter, can deviate widely from the values obtained for pore widening. As channel size increases, the amount of area developed during the gasification process decreases. For the production of very large channels, the carbon structure and its specific surface area (area per gram of remaining material) remains largely unchanged by the gasification process. It is interesting that the production of very fine pores results in surface area values which exceed those achieved by pore widening, as seen in Figure 2. Under most conditions, however, pores of larger size are produced, resulting in a suppression of the area development. The production of channels with diameters even as small as 100 A results in a surface area development that is much smaller than that resulting from pore widening. The production of large channels can be expected to have, therefore, a negative impact on gasification rates and on the absorptive properties of activated carbon. The activation process would be completely ineffective if gasification took place exclusively by production of large channels, consuming carbon without developing the microporous structure whatsoever. The second type of uneven surface gasification occurs when reaction takes place primarily or exclusively on the surfaces of larger pores. Model predictions of the surface area evolution with conversion are seen in Figure 3 for the
Figure 4. Evolution of pore size distribution during gasification by uniform pore widening.
cases in which gasification is assumed to be confined to pores larger than 5, 20, and 50 A, as well as for the case of gasification in all pores (dashed line). The calculations were made for a char with an initial surface area of 450 m2/g, porosity of 40%, and an assumed l/? distribution. It is clear from the figure that surface area development is greatly suppressed when fine pores do not participate in the reaction. Even when the minimum accessible pore diameter is as small as 20 A, the surface area development is much less pronounced than in the case of uniform pore widening,"j It is interesting to note from Figure 3 that the extent of surface area development roughly scales with the fraction of the total area on which the reaction takes place ( A J A J . As with the production of large channels, reaction on large pore surfaces does not affect the carbon fine structure where most of the surface area lies and thus does not result in high surface area development. In the limit, where reaction takes place only on the external particle surface, the carbon structure in the remaining unreacted core remains completely unchanged by gasification, as does its surface area, being itself a property of the carbon structure. A mode of reaction in which only the large pore surfaces participate is therefore undesirable from the standpoint of commercial gasification or activation, as was gasification by large channel production. We consider next the evolution of pore size distribution occurring during the various modes of gasification. The numerical solutions were carried out for the case of a nonshrinking carbon having a l/? pore PDF and 40% initial porosity, and the results are presented in Figures 4-6 in the form of distributions of pore volume per initial gram of carbon. Gasification by uniform pore widening produces volume mainly in fine pores, having little effect on the largest pore classes, as seen in Figure 4. The limited reaction in larger pores does, however, cause some engulfment of smaller pores, which leads to some loss of fine pore volume and eventually to a maximum in the pore size distribution a t high conversion. For a high surface area carbon, displacement of the pore walls uniformly by approximately 20 A (for an initial area of 500 m2/g) will result in a complete consumption of carbon, consistent with the observed (16) When considering absolute values of pore diameter in this analysis, it is important to remember that ap lication of the 1/P dbtribution required a minimum pore d i e t e r of 2 to reproduce measured surface areas. The smallest pores which contribute to the measured surface area in real carbons have somewhat larger diameters (3-4 A) and thua the absolute values of the minimum pore sizes required to attain the effects seen in Figure 3 would be slightly higher.
A:
Evolution of Pore Structure during Carbon Gasification
=: E
0.71
v
cn ,
-io
-9
-7
-8
-6
-5
Log[pore radius m]
Figure 5. Evolution of pore size distribution during gasification
only in pores larger than 50 A.
60%
2
IiI *
J
0.84
30% Conversion
Log[pore rodius (m)]
Figure 6. Evolution of pore size distribution dur' gasification by production of pores in the size range 1W3M?.
pore enlargement (displacementalong the X axis in Figure 4) a t 99.8% conversion. A similar process occurs when gasification is restricted to pores with a certain minimum diameter, as seen in Figure 5 for a minimum diameter of 50 A. In this w e the gasification produces volume primarily in the smallest pores in which gasification takes place, the large pores remaining, again, largely unaffected. The pores with diameters smaller than the minimum diameter, in contrast, are eliminated over the course of conversion by the process of engulfment by larger pores. Gasification by the production of 100-300 A diameter pores results in the evolution of the pore size distribution seen in Figure 6. Within the range 100-300 A, pores were created by increasing the value of the pore PDF by a constant value. A characteristic pore size quickly becomes apparent upon gasification. Over the course of conversion, the larger pores are completely unaffected, the small pores are lost to engulfment, and the spike in the distribution between 100 and 300 A becomes more pronounced, eventually comprising most of the existing pore volume. Discussion The three modes of gasification considered (uniform pore widening, large pore widening, and channel production) produce carbons with distinctly different pore structures. The model results show that nonuniform surface reactivity in the form of both channel production and the restriction of reaction to larger pores usually reduces the amount of surface area development signifi-
Energy & Fuels, Vol. 5, No. 3, 1991 467
cantly. In the limit of very large channel production or confinement of reaction to very large pore surfaces, the process of activation is completely ineffective, consuming carbon without developing the microporous structure whatsoever. It is interesting that these results differ from those predicted by the models of Ranish et al.12 for the gasification of graphite flakes. This fact can be readily explained on the basis of differences in pore structure between low-temperature and graphitic carbons as follows. A reacting catalyst particle increases the sample surface area by contributing the inner surface of its channel, while simultaneously destroying the surface area that had previously existed in the channel volume. Depending on the channel diameter and the amount of existing specific surface, the net effect can be either an increase or a decrease in total area. As the area contributed by the channel scales with D,,the channel diameter, while the loss of existing area scales with D,2 multiplied by the sample specific surface area (in m2/m3),any combination of low initial surface area or small channel diameter will tend to promote area development. In general it can be said that the area development of materials with initially low surface area (e.g., graphite) will be enhanced by channeling, while the further area development of microporous materials will be impeded by channeling. The model results will prove to be useful in the interpretation of several previously published sets of measurements of surface area evolution during the gasification of microporous carbons. The surface area evolution of a subbituminous coal char was measured during gasification in 1atm of COz in a thermal gravimetric analyzer at 860 OC, yielding reaction rates of O.OO2-0.006 g/(min g)! Rates under these conditions were shown to be independent of sample size, indicating freedom from the mass- and heat-transfer effects. Carbon dioxide adsorption was used to measure the surface areas of the partially gasified subbituminous coal cham6 Gasification reactivities and nitrogen surface areas were also measured in that study as a function of conversion and heat treatment conditions. The gasification behavior of this char was explained by postulating that the reaction took place outside of the microporous network, only on the surfaces of large pores where catalytic impurities had collected. (During heat treatment, catalyst particles have been observed to undergo random motion on carbon surfaces and to coalesce to form particles that are too large to reenter the microporous network. This phenomenon has been shown to produce a net migration of material out of micropores and on to the surfaces of larger pores.17J8) The measured carbon dioxide surface areas are plotted in Figure 3 for comparison with the model predictions. The measured surface area evolution is clearly not in agreement with that expected for gasification by the uniform widening of all pores. The observed behavior is described well by the numerical solution based on the occurrence of reaction only on the surfaces of pores having diameters greater than 20 A, in which only 10% of the total surface area is located. The area evolution provides support for the previous interpretation6 that gasification of this carbon did not occur fully within the carbon micropores. An adequate description of the measured surface area evolution for this char could only be achieved by (17) Wigmans, T.; Auwerda, K.; Gem, J. W.; Moulijn, J. A. 15th Biennial Conf. Carbon, Extended Abstr. 1981,144. (18) Radovic, L. R. Ph.D. Thesis, Department of Materiale Science and Engineering, The Pennsylvania State University, 1982.
Hurt et al.
468 Energy & Fuels, Vol. 5, No. 3, 1991
2 \
'1
A
gasification by pore widening and simultaneous production of 200 A diameter pits. Densification was assumed to occur by pore elimination in pores with diameters smaller than
Measured areas, Spherocarb
'
50% widening, 50% production.
------&
Q o ! 0
I
I
I
I
,
10
20
30
40
50
,
60
20 A.
pore widenina
,
70
and densification I
I
I
80
90
100
% Conversion
Figure 7. Surface area evolution during the oxidation of spherocarb carbon.
accounting for nonuniform surface reactivity. The evolution of surface area during the air oxidation of Spherocarb carbon in the temperature range 500-600 O C has previously been shown to deviate sharply from predictions of classical models considering uniform pore widening. Spherocarb carbon also undergoes a pronounced densification/shrinkage which affects area development. A previous model which incorporated the effect of densification was able to explain much, but not all, of the A potential source of the remaining disdi~crepancy.~ crepancy is the presence of nonuniform surface gasification. Lamond's results, referred to in the Introduction, suggest that gasification of numerous carbons, even those containing no doped catalyst, may occur in part formation of 100-300 A diameter pits. In the present study we investigate the possibility that nonhomogeneous surface gasification in the form of pit formation is responsible for the remaining discrepancy between the model predictions and measured surface areas for Spherocarb carbon. The measured surface areas are compared to predictions of a model accounting for carbon densification/shrinkage and
Surface areas of Spherocarb samples partially oxidized in air in the temperature range 500-600 "C are plotted in Figure 7, together with the model predictions based on the measured pore volume distribution. Under these conditions, reaction rates were 0.005-0.1 g/ (min g) and the reaction has been shown to take place in zone I. Both densification/shrinkage and gasification by pore production have an important effect on surface area evolution. A quite satisfactory description of Spherocarb surface area development is obtained when 50% of the carbon gasification is accomplished by pore production, the remaining 50%, then, by uniform pore widening.
Conclusions Previously existing pore structure models are too simple to describe or predict the surface area conversion relationship in carbon gasification. In the present work, experimental and theoretical evidence has been presented that the nonunifor? gasification of carbon surfaces plays a important role in the determination of surface area and pore structure development. Nonuniformity of surface reactivity can dramatically reduce surface area development for microporous materials. The opposite trend is observed for materials of initially low surface area such as graphite. Modifications of pore evolution models, which account for nonuniform reaction and the phenomenon of densification/shrinkage,are shown to provide an adequate description of carbon gasification and contribute to our fundamental understanding of this complex process. Acknowledgment. Support of this research by Exxon Research and Engineering Co. is gratefully acknowledged. This article is based on work supported also under a National Science Foundation Graduate Fellowship. Registry No. C,7440-44-0.
