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Textural changes during CO2 activation of chars: A fractal approach
Applied Surface Science 253 (2007) 6019–6031 www.elsevier.com/locate/apsusc
Textural changes during CO2 activation of chars: A fractal approach Manuel Marı´a Mahamud * University of Oviedo, Department of Chemical and Environmental Engineering, Faculty of Chemistry, Campus de El Cristo, 33071 Oviedo, Spain Received 10 July 2006; received in revised form 24 November 2006; accepted 28 December 2006 Available online 7 January 2007
Abstract In this paper two series of active carbons obtained at different flow rates of the activating agent, CO2, are characterized in order to establish the different mechanisms of pore development during the activation step. This study complements previous works on textural development during the different steps in the process of obtaining active carbons: coal oxidation, coal pyrolysis and char gasification. As the characteristics of the original and intermediate materials are of capital importance in the pore development of active carbons, the properties of the active carbons, precursor chars and coals were considered and analyzed together. Mercury porosimetry and helium picnometry were used to determine classical textural parameters as well as to perform a more detailed study of the pore volume generation during the different conditions of the activation step. Data obtained from the mercury porosimetry determinations was also employed for fractal determinations according to the methodologies proposed by Friesen and Mikula, Zhang and Li and the procedure of Neimark. Average surface fractal dimensions as well as fractal profiles and local surface fractal dimensions were calculated. The use of different flow rates during the activation step produces changes not only in the ordinary textural parameters but also in the fractal characteristics of the active carbons. Activation at higher flow rates leads to smoother fractal profiles and also to lower values of the average surface fractal dimensions of the active carbons. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05Uw; 91.60.Np Keywords: Mercury porosimetry; Fractal analysis; Active carbon texture; Carbon dioxide activation; Flow rate; Burn-off
1. Introduction To obtain active carbons from bituminous coals several steps should be followed in the manufacturing process. For the processes referred to as physical activation, the steps involved are described below. The first one is the oxidation of the coal, since this is necessary to eliminate the plastic properties of the coal if the aim is to obtain a good active carbon. When this oxidation is carried out with air, it also develops a primary pore network. The existence of this porosity in coals plays an important role in the oxidation process itself as well as in the processing operations ahead. The oxidized coals are carbonized in an inert atmosphere in order to obtain a char with a more developed pore network. Finally, the chars are gasified with an oxidant such as air, steam or carbon dioxide. During gasification, changes in the apparent and true densities lead
* Tel.: +34 985103668; fax: +34 985103434. E-mail address: [email protected]. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.109
to an increase in pore volume. Pore volume distribution is also modulated and the values of specific surface area increase compared with the values of the precursor char. Regarding the active carbons and the precursor materials used in the present and previous studies, it can be said that the carbonization step multiplies the total pore volume by an approximate factor of three. The activation leads to materials with total pore volumes that are roughly three times the total pore volume of the precursor char. The manufacture process of active carbons from bituminous coals has been explained in previous papers and studies [1–6]. Textural study of two series of active carbons has been carried out using classical methods of textural analysis such as helium picnometry, mercury porosimetry and physical adsorption of nitrogen and carbon dioxide [6]. The fractal theory is based on the concept of self-similarity. If an object is self-similar, its appearance does not depend on the degree of detail used for its observation. The original work of Mandelbrot [7] introducing fractal concepts was followed by other books focused on the applications of this new approach to technical and scientific fields [8,9]. The value of the fractal
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dimension that characterizes a surface can be related with the degree of complexity of this surface. In addition, the fractal dimension is an intrinsic property of a surface and, theoretically, does not depend on its size. This is an important fact that makes fractal dimension a useful parameter for characterizing the surface of porous solids, despite pore width or pore distribution. For actual objects the fractal character cannot be the same for the whole range of sizes. This fact opens up the possibility of obtaining fractal profiles of porous solids that constitute a very useful tool for characterizing porous materials [10–13]. Data from mercury porosimetry determinations are suitable for analysis from a fractal point of view. With this purpose, several methods have been proposed and applied [14–16]. Previous work by the author has shown how the application of these fractal analysis procedures are valuable tools not only for characterizing carbonaceous materials but for clarifying the phenomena underlying all the transformations of the materials. Fractal analysis is a complementary tool for analyzing textural characteristics and their evolution for series of oxidized coals [10,11], chars [12] and active carbons [13]. This analysis complements traditional approaches for clarifying the mechanisms of pore development during the steps of coal oxidation, coal carbonization and char activation. Fractal methodology is also applied to other techniques of textural characterization such as TEM [17], SEM [18,19] or optical microscopy image analysis [20]. Physical adsorption of gases [17,21,22,16,23–25,19], or SAXS [26,17,27–29] are also used for fractal determinations. All the mentioned methods have been extensively applied to carbonaceous materials. It should be said that fractal techniques are also employed to correlate properties such as the char yield in carbonization processes [30]. Maintaining the gasification temperature constant two other variables can be used to modulate the pore network of the final active carbon: the fraction of char mass eliminated during gasification (burn-off) and the flow rate of the activating agent. The influence of these variables has been extensively studied and reported for the active carbons whose textural data are used in this work [1–4,6]. The present article aims to complement the findings of previous studies with a more detailed analysis of pore volumes, their distribution and evolution, together with a fractal approach using data from the mercury porosimetry determinations. 2. Methods used for the determination of surface fractal dimension from mercury porosimetry analysis There exist several methods for the determination of surface fractal dimension from mercury porosimetry: the method of Friesen and Mikula [14], the analysis proposed by Zhang and Li [15] and the procedure of Neimark [16]. These methods have already been used to characterize series of materials constituted by oxidized coals [11], chars [12] and active carbons [13]. Recently, Zhang et al. [31] proposed an improvement of their procedure. The Laplace equation that represents an inverse relationship between the average radius of curvature of the meniscus (R) and
the intrusion pressure (P) is the basis for mercury porosimetry analysis. For very small pores, this technique may not be valid but mercury porosimetry permits the characterization of macropores – pores wider than 50 nm – and wide mesopores. In the present study the interval of mercury intrusion pressures ranges from 1 to 2000 bar. This leads to a pore width interval from 14.8 103 nm to 7.4 nm, that is the size of the smallest pore that can be penetrated under an intrusion pressure of 2000 bar. Mercury porosimetry data consist in pairs of values of cumulative mercury volume intruded V, up to a pressure P. These arrays of values are employed by the methods cited above in order to obtain the fractal dimension of pore surfaces, D. The method of Friesen and Mikula allows the obtaining of a value of the fractal dimension, DF, from the following expression: dV ln (I) / ðDF 4Þln P dP This expression corresponds to a point-to-point fit of the experimental data, there usually existing an important scatter of the individual points. As a consequence of this, the correlation coefficients for the Friesen’s fit are generally low [11–15]. According to Neimark, the fractal dimension, DN, is estimated as follows: RV d ln 0 P dV (II) DN ¼ 2 þ dðln PÞ The above expression was obtained from the equation of Rootare and Prenzlow [32] that considers thermodynamic principles relative to solid impregnation by a non-wetting fluid, the scaling law commonly employed for the calculation of fractal dimensions, and the Laplace equation [11,16]. From thermodynamic considerations and the scaling relationship of a fractal surface area and the circumscribed volume, Zhang and Li concluded that: Z V ln P dV ¼ constant þ lnðR2DZ V DZ =3 Þ (III) 0
that is usually written in an abbreviated form: lnðW V Þ ¼ constant þ lnðQV Þ
(IV)
The value, DZ that makes the slope of ln(WV) versus ln(QV) equal to unity – i.e. that satisfies Eqs. (III) and (IV) – is the fractal dimension. Zhang et al. [31], rearranging Eq. (III), found a simpler way to estimate DZ as follows: DZ ¼
dlnðW V R2 Þ dlnðV 1=3 R1 Þ
(V)
This procedure was applied by the author to several materials, giving essentially the same values for DZ as the figures obtained from Eqs. (III) or (IV). The coefficients of correlation obtained by fitting the data to Eq. (V) are much better than the corresponding to Eqs. (III) and (IV). There exists another known expression that was proposed to estimate the fractal dimension [14,21,33]. Its mathematical
M.M. Mahamud / Applied Surface Science 253 (2007) 6019–6031
expression is
dV / R2D dR
(VI)
In this case, it is necessary to know the pore volume distribution. 3. Experimental The samples characterized in this work are series of oxidized coal, series of chars obtained from the oxidized coals and series of active carbons obtained by means of char gasification with carbon dioxide. The raw material is a high-volatile A bituminous coal with a particle size in the interval 1–3 mm oxidized in air at a temperature of 543 K for periods of time of 1–4 days. The nonoxidized coal and the oxidized samples were carbonized in inert atmosphere at 1123 K, obtaining the corresponding series of chars. More detailed information on these materials regarding coal properties, coal oxidation and carbonization processes as well as coals and chars characterization has already been collected in previous papers [1,2,11,12,34] and studies [3]. The active carbons were obtained by gasification of chars with carbon dioxide at a temperature of 1123 K up to a fractional burn-off of about 0.52. The activation reactor consisted in a double-jacket quartz tube enclosing a fixed bed of about 4 g of char. The activation reaction took place at two different flow rates of CO2, 7.0 and 500 ml min1. Detailed information about the activation process has already been reported [3,6]. Table 1 shows the activation parameters as well as the pre-treatment conditions of active carbon precursors for each sample code. Table 2 compiles some textural parameters of the active carbons under study. Data analyzed in this study consist in results of helium picnometry and mainly in registers from the mercury porosimetry of the coals, chars and active carbons in the 1– 2000 bar pressure interval. Data from the mercury porosimetry constitute the starting point for the determination of the fractal dimensions by using the approaches suggested by Friesen and Mikula, Zhang and Li and the method proposed by Neimark. Despite the fact that data corresponding to coals and chars have been analyzed in depth in previous papers [10–12], textural data Table 1 Codes of active carbon samples indicating the activating conditions and the preoxidation conditions of the corresponding precursor coals Series
Active carbon samples
Oxidation conditions of precursor coal T (K)
Time (days)
AGL
A0GL A1GL A2GL A3GL A4GL
543 543 543 543 543
0 1 2 3 4
AGH
A0GH A1GH A2GH A3GH A4GH
543 543 543 543 543
0 1 2 3 4
CO2 flow rate (ml min1)
7.0 7.0 7.0 7.0 7.0 500 500 500 500 500
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Table 2 Codes of active carbon samples and their corresponding ash content and textural parameters Active carbon samples
Ash content (%)a
Mercury density (g cm3)
Helium density (g cm3)
Total pore volume (cm3 g1)
N2 BET surface area (m2 g1)
A0GL A1GL A2GL A3GL A4GL A0GH A1GH A2GH A3GH A4GH
14.1 13.5 15.2 16.1 20.8 14.1 13.8 13.7 16.5 20.8
1.229 0.994 0.820 0.866 0.801 1.215 0.991 0.889 0.868 0.818
1.894 2.010 2.100 2.104 2.053 1.836 1.984 2.040 2.055 2.007
0.285 0.509 0.744 0.679 0.761 0.278 0.505 0.635 0.666 0.724
120 445 605 666 704 74 331 489 504 548
Textural properties are expressed in dry ash-free basis. a Estimated from the ash content of the precursor char and the degree of burn-off.
and fractal profiles corresponding to these materials are used together with those corresponding to the active carbons. The properties of coals, chars and active carbons, are helpful to understand the influence that the precursor materials have in the final properties of the active carbons obtained [12,13]. In this case, these properties where placed together with those corresponding to the series of active carbons obtained using the two different flow rates of activating agent. This was found useful in order to elucidate how the activating conditions determine the pore development of active carbons. 4. Results and discussion Findings from the fractal analysis of pore surfaces were used together with classical textural parameters in order to explain textural changes during gasification at different flow rates of the activating agent. The three previously mentioned procedures were employed for the fractal study of the pore surface of the materials. Results and their discussion are presented in the following sections. 4.1. Applicability of the fractal analysis methods of to the series of active carbons: evolution of the fractal dimensions For the series of active carbons obtained by char gasification at a flow rate of 7.0 ml min1, ‘‘AGL’’, the application of the method of Friesen was not found to be adequate for obtaining an average fractal dimension valid for the whole range of pore sizes reached by mercury porosimetry. This fact become evident observing the graphs – not shown here, but quite similar to others already published [13] – corresponding to the Friesen’s plots for the five materials in the series ‘‘AGL’’. The calculation of an average fractal dimension for the full range of pressures, DF, leads to a series of fractal dimensions that passes through a maximum as the oxidation time of the precursor coal increases. For the series of active carbons obtained by char gasification at a flow rate of CO2 of 500 ml min1, ‘‘AGH’’, the general trend of the values of the fractal dimension DF is essentially the same as that observed for the ‘‘AGL’’ series but
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some differences can be found. First, it is possible to obtain for the materials of the ‘‘AGH’’ series a value of the Friesen dimension more representative for the entire range of pore sizes. The coefficients of correlation of the Friesen fit for the ‘‘AGH’’ series always have values higher than those corresponding to the ‘‘AGL’’ series. Second, the values of the fractal dimensions for the series activated at the higher flow rate are always below the values for the corresponding materials of the series of active carbons obtained by activation at the lower flow rate. Third, the active carbons of the ‘‘AGH’’ series have fractal dimensions much closer to the fractal dimensions of the respective chars than the ‘‘AGL’’ series of active carbons, with the only exception of the activated coke (obtained from the non-oxidized coal). This last remark is related to the texture and the formation mechanism of chars and cokes together with the pore generation mechanism during the
activation step. When the non-oxidized coal is carbonized, it passes through a plastic phase leading to a coke that has closed pores with smooth surfaces. When this coke is activated, an important amount of these closed pores are opened leading to materials with very low fractal dimensions compared with the precursor coke. By applying the procedure proposed by Zhang and Li, the average fractal dimensions for the active carbons were calculated. By comparing the graphs of the Zhang and Li method for the ‘‘AGL’’ series of active carbons in Fig. 1, and ‘‘AGH’’ in Fig. 2, the pore surface – accessible to mercury – of the ‘‘AGH’’ active carbons series seems to be more homogeneous than the pore network of the ‘‘AGL’’ series. In the above-mentioned figures, the values of the average fractal dimensions DZ, are noted for every active carbon. The analysis of the evolution of the fractal dimension DZ is quite similar to
Fig. 1. Plots of ln WV vs. ln QV corresponding to samples of active carbons ‘‘AGL’’. The value of DZ represents the average fractal dimension. This satisfies Eq. (IV) for the full range of pressures, 1–2000 bar.
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Fig. 2. Plots of ln WV vs. ln QV corresponding to samples of active carbons ‘‘AGH’’. The value of DZ represents the average fractal dimension. This satisfies Eq. (IV) for the full range of pressures, 1–2000 bar.
that observed for the dimension DF. The Zhang and Li fractal dimension for the active carbon A0GH (obtained from the coke) has a lower and very different value to that corresponding to the precursor coke. The DZ figures for the rest of the active carbons obtained under conditions of the higher flow rate, almost coincide with the DZ values for the corresponding chars (differences are in the interval 0.02–0.07). By using the average fractal R V dimensions, DN, obtained by fitting the pairs of values ln 0 P dV versus ln P (method of Neimark) in the region of intermediate pressures as can be seen in Figs. 3 and 4—the considerations of the preceding paragraphs are applicable. In Fig. 5, the evolution of the fractal dimensions DF, DZ and DN of coals, chars and active carbons can be observed. This figure is in agreement and is helpful in summarizing what has been said in the above paragraphs. The activation
step tends to increase the fractal dimension of the materials, this increase being more important if the activation is carried out at the lower flow rate. If activation was carried out at the highest flow rate, changes in the fractal dimension during activation are less important and, very often, negligible. The exception is the activation of the coke (carbonized material obtained from the non-oxidized coal) that generates activated materials with a lower fractal dimension than the carbonized precursor. 4.2. Fractal profile analysis of active carbons and precursor chars and coals Previous work has shown how the Neimark’s graphs can be successfully used for performing a zonal fractal analysis of pore surfaces [11–13]. These graphs revealed the existence of
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Fig. 3. Plots of ln WV vs. ln P corresponding to samples of active carbons ‘‘AGL’’. Values of the fractal dimensions deduced from the straight lines are indicated on the graphs.
different fractal regions in the pore network of coals, oxidized coals, chars and active carbons. Apart from the fractal profiles themselves, interesting facts were found related to the ‘‘smoothing’’ of the transition between fractal zones as the oxidation of the precursor coal increases. Also noticeable was the phenomenon known as the ‘‘fingerprint’’ effect, consisting in the observation of similarities between related series of coals, chars and active carbons. It is clear that the fractal analysis of mercury porosimetry data reflects the influence of the porous texture of the precursor coal on the characteristics of the pore network of chars obtained from these coals. In a similar way, the textural properties of the char condition the properties of the active carbon obtained. More, as reported in a recent paper [13], the fractal profile of the active carbons even ‘‘discovers’’ features corresponding to the precursor coal that remain ‘‘hidden’’ in the fractal profile of the char.
The above-mentioned facts are a consequence of the importance of the textural properties of the precursor coals and chars on the textural development of the corresponding chars and active carbons. This evidence has already been pointed out in previous works where a ‘‘classical’’ textural analysis approach was applied [1–3,5,6]. The fractal profiles obtained by plotting the values ln(WV) ln P for the series of active carbons, ‘‘AGL’’ and ‘‘AGH’’, are plotted respectively in Figs. 3 and 4. In these figures the fitting lines corresponding to the linear regions of the graphs for the intermediate and high pressure intervals are also drawn. The corresponding values of the fractal dimensions DN are also noted. It is noteworthy that for the series of active carbons obtained at the lower flow rate of CO2 (‘‘AGL’’), the values of DN are always above the values of the corresponding active carbon of the ‘‘AGH’’ series. This behaviour of the
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Fig. 4. Plots of ln WV vs. ln P corresponding to samples of active carbons ‘‘AGH’’. Values of the fractal dimensions deduced from the straight lines are indicated on the graphs.
Neimark fractal dimensions is qualitatively similar to the trend observed for the Friesen and Zhang dimensions. It is also of note that in the case of the ‘‘AGH’’ series the plots are smoother and the transition between fractal regions is less marked. Nevertheless, the plots of Neimark and the graphs of Zhang and Li are still adequate for carrying out a zonal fractal analysis of the ‘‘AGH’’ series of active carbons. A method for comparing the fractal profiles of different materials is to place them together in the same graph. Fig. 6 simultaneously plots the Neimark graph for each active carbon pair of the series ‘‘AGL’’ and ‘‘AGH’’ as well as the plot of the corresponding precursor char and coal. In these figures the fractal dimensions calculated by the Neimark approach for the active carbons, chars and coals are also collected. It has previously been reported that the shape of the Neimark plots for
the active carbons of the ‘‘AGL’’ series are much more similar to the shape corresponding to the precursor coals than to the shape of the plots for chars [13]. For the series of active carbons ‘‘AGH’’ the same behaviour is noticeable. It also occurs that for the region of narrower pores, a zone of high values of the fractal dimensions -that can be appreciated for coals but is inexistent for chars-appears. Moreover, the following points must be considered. First, the shape of the Neimark plots of the ‘‘AGH’’ series resembles that corresponding to the plots of the precursor coals but this degree of mimicry is more evident for the ‘‘AGL’’ series. Second, the fractal profiles for the ‘‘AGH’’ series are much smoother than the profiles for the ‘‘AGL’’ series. Third, the active carbons of the ‘‘AGL’’ series from coals oxidized for 2 or more days have fractal dimensions – for the intermediate pressure range – lower than the dimensions of chars and almost
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Fourth, for the high-pressure region, the fractal dimensions of ‘‘AGH’’ active carbons tend to increase as in the case of the precursor coals. The values of this high-pressure range dimension for the ‘‘AGH’’ series are always below the values for the series of active carbons ‘‘AGL’’. The above fact may be explained taking into account the influence that the pore texture of the precursor coal has on the textural properties of the active carbon obtained from it [3,6,11–13,30]. The fractal dimensions over 3 detected for the active carbons ‘‘AGL’’ in the highpressure range has already been attributed to the filling with mercury of ink-bottle shaped pores generated by opening – actually, making accessible to mercury – of closed porosity during the activation process [13]. Classical textural determinations do not support the possibility of the existence of closed porosity that collapses under mercury pressure, leading to this increase of the fractal dimensions [3]. For the ‘‘AGH’’ series in the high-pressure range, high values of the fractal dimensions are also determined (see Fig. 4) but the values are clearly below the values of the fractal dimensions for the ‘‘AGH’’ series. This fact is understandable bearing in mind that when the activation is carried out at the lower flow rate, the reaction can take place in the smaller pores to a greater extent that if the CO2 flow rate is more elevated. As a consequence of this, the amount of small ink-bottle shaped pores that can be opened during the activation of the ‘‘AGL’’ series is more important than that corresponding to the ‘‘AGH’’ series. In addition, the generation of new porosity – that is usually associated with an increase in the fractal dimension [13] – in the region of the smallest pores reached by mercury, is more appreciable for the ‘‘AGL’’ series as will be indicated in Section 4.4. Let us now compare the fractal dimensions for chars and active carbons noted in Fig. 6, corresponding to the region of the intermediate pressures. Activation at the higher flow rates – ‘‘AGH’’ series – produce changes of the fractal dimensions – difference between the fractal dimension of the active carbon and the dimension of the precursor char – more important than that corresponding to the ‘‘AGL’’ series. This can be explained considering that for the ‘‘AGH’’ series, the generation of porosity in the interval of sizes detected by mercury porosimetry is much more important than for the ‘‘AGL’’ series. This leads to the series of active carbons obtained at the higher flow rate having more possibilities of modifying the surface corresponding to the pore sizes accessible to mercury.
Fig. 5. Evolution with time of oxidation of the fractal dimension for series of coals, chars, and the corresponding active carbons ‘‘AGL’’ (activated at a CO2 flow rate of 7.0 ml min1) and ‘‘AGH’’ (activated at a CO2 flow rate of 500 ml min1). The fractal dimensions are calculated by the methods of Friesen (DF), Neimark (DN) and Zhang (DZ).
coincident with the dimensions of the corresponding coals. In the case of the series of active carbons ‘‘AGH’’, the general trend is the same, but the differences between the fractal dimensions of chars and active carbons are more important. This leads to the fractal dimensions of active carbons reaching values lower than those corresponding to precursor coals.
4.3. Pore volume generation during activation and its distribution From classical data of textural analysis -apparent density, real density and pore volume intruded by mercury up to pressures of 2000 bar—an evaluation of the actual pore volume generation or destruction during the activation was carried out. The method is similar to that already used in previous studies [12,13]. The total pore volume – obtained from the real and apparent densities – that corresponds to pore sizes under 14.8 103 nm, was divided into two fractions: the pores reached by mercury intrusion between pressures 1 and 2000 bar (14.8 103 to 7.4 nm) and the pores smaller than 7.4 nm that are not accessible to mercury. For
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Fig. 6. Plots of ln WV vs. ln P corresponding to samples of coals, chars, and the corresponding active carbons ‘‘AGL’’ (activated at a CO2 flow rate of 7.0 ml min1) and ‘‘AGH’’ (activated at a CO2 flow rate of 500 ml min1). Values of the fractal dimensions deduced from the straight lines are indicated on the graphs.
a clear idea of the effective pore development during activation, the pore volume of active carbons are transformed to the corresponding equivalent volume of char (pore volume per unit of mass of precursor char). The pore volumes calculated are the following: V<7.4 char that represents the specific pore volume of chars with a size under 7.4 nm, non-accessible to mercury under a pressure of 2000 bar. V<7.4 act, i.e., the specific pore volume of active carbons with a size under 7.4 nm.
V<7.4 act trans, which corresponds to the pore volume of the active carbon under 7.4 nm per unit of mass of precursor char. This is calculated by taking into account the loss of mass during activation by multiplying the previous pore volume by the factor (1 X), being X the fractional burn-off achieved during activation. Dif V<7.4 corresponds to the difference V<7.4 act trans V<7.4 char, being the actual amount of pores generated during activation with a size below 7.4 nm. Dif V>7.4, is the actual pore volume generated with a size in the (7.4–14.8) 103 nm range.
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Dif Vtot, is the total volume effectively generated during the activation step and is calculated as the sum Dif V<7.4 + Dif V>7.4. All the pore volumes are expressed in a dry-ash-free basis (Vdaf). For the active carbons it is necessary to consider that the amount of ashes increases as the burn-off increases. The expression for transforming the pore volumes in dry basis (Vdb) into the volume in dry ash-free basis (Vdaf), is the following: ðV daf Þ ¼ ðV db Þ
ð1 XÞ þ ðAshÞX ð1 XÞ½1 ðAshÞ
(VII)
being X, as usual, the fractional burn-off and (Ash) the fraction of ash in the precursor char. From the graphs in Fig. 7, where some of the different pore volumes mentioned above for the ‘‘AGL’’ and ‘‘AGH’’ series are plotted versus the oxidation time of the precursor coal, the following facts can be observed. First: total pore volume generation is more important for the ‘‘AGL’’ series. This is understandable due to the fact that for the activation at the higher flow rate, the inhibition effect of the CO in the outermost sections of the particles may be negligible and the disappearance of material from the ‘‘external’’ region of the
particles is important. A different phenomenon occurs when activation is carried out at the lower flow rate [1,2,3,6], this inhibition affecting the whole particle volume, increasing in this way the fraction of the burn-off that takes place inside the porous network of the particles. It is also useful to consider that for the ‘‘AGL’’ series reaction times are about four to six times longer than the corresponding to homologous materials of the ‘‘AGH’’ series. Second: for the ‘‘AGH’’ series the generation of pore volume in the size range (7.4–14.8) 103 nm increases with the oxidation of the precursor coal. This generation represents up to 80% of the total pore volume generation. On the other hand, for the first four materials of the ‘‘AGL’’ series there is a slight loss of pore volume accessible to mercury. Only for the active carbon A4GL is there a net generation of pore volume for this size interval. For samples A0GL, A1GL, A2GL and A3GL the increase in porosity corresponds completely to pores under 7.4 nm and for the active carbon A4GL the generation of this porosity is about 3/4 of the total pore generation. 4.4. Pore volume evolution during activation and changes in fractal dimensions of the active carbons The analysis reported in the previous section only divided the total pore volume into two fractions, the pores nonaccessible to mercury and the pores accessible to mercury in the pressure interval 1–2000 bar. The use of the pore volume distribution obtained from mercury porosimetry was found useful to estimate the actual changes in pore volumes for the range of pore sizes detected by this technique [13]. For obtaining these distributions, the following variables were defined: Vchar, corresponds to the program of the char, i.e., the cumulative volume of mercury intruded into the char up to a pressure P. Vact trans, is the cumulative volume of mercury intruded into the active carbon up to a pressure P, based on mass of initial char. Dif V, is the cumulative volume actually generated during gasification and is calculated as the difference Vact trans Vchar.
Fig. 7. Pore volumes of chars and the corresponding transformed volumes for active carbons ‘‘AGL’’ and ‘‘AGH’’. Generation of pore volume of different range of sizes during the activation step.
All volumes are expressed in daf basis. Derivative, corresponds to the derivative of the curve Dif V ln P. This indicates for a specific pore size, if there is a net generation (positive values) or destruction (negative values) of pore volume. The pore development for the active carbons of the ‘‘AGL’’ series has already been reported [13]. Here, Fig. 8 shows cumulative pore volumes of chars and active carbons, cumulative pore volume generation and the derivative of the cumulative pore volume for the series of active carbons ‘‘AGH’’. The value of Dif V for the maximum intrusion pressure is, obviously, that calculated as Dif V>7.4 in the previous section. The graph of the cumulative pore volume generation for the active carbon from coke A1GH is quite similar to that of the
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Fig. 8. Cumulative pore volumes of chars, of active carbons (‘‘AGH’’) and cumulative pore volume generation during the activation step. The values corresponding to the cumulative pore volume generation and its derivative are shown on the right ordinate axis.
homologous active carbon obtained by activation at higher flow rate, A1GL. Evolution for the two materials corresponds to an important pore generation for the large pore sizes (pores wider than 2.0 103 nm) due to the increase in size of smaller pores and to the opening of wide closed pores of the cokes. For the rest of the sizes accessible to mercury up to 2000 bar (pore sizes between 7.4 and 2.0 103 nm) there is a net pore volume destruction due to a widening of the pore network. For the active carbons of the ‘‘AGH’’ series coming from oxidized coals the general pattern of pore generation is as follows: an important pore generation for the widest pore sizes and very few changes in pore volume for pores narrower than 2.0 103 nm. This fact is evidenced by the analysis of the curves of cumulative volume generation in Fig. 8. These conclusions are consistent with those obtained from the analysis of the information in Fig. 7 as seen in the previous section. Pore generation in the size ranges not accessible to mercury – pores
under 7.4 nm width – for the ‘‘AGH’’ series is much less important (maximum values in the interval 30–50 mm3 g1) than for the ‘‘AGL’’ series, with pore generations of about 70– 80 mm3 g1. For the small pores, the reaction rates are reduced, due to diffusion and to the inhibitory effect of the reaction product, CO. In the case of activation at the lower CO2 flow rate, reaction times are between 4 and 6 times longer, allowing a more important pore development in the internal zones of the particles. Let us now discuss the relationship between changes in fractal dimension and the generation of porosity during the activation step. By comparing the fractal profiles for chars and active carbons in Fig. 6, and the pore generation curves in Fig. 8, it can be seen that in the regions where a generation of pores takes place during activation there is an increase in the fractal dimension of the active carbon compared with the fractal dimension of the precursor char. This fact is more evident in the high-pressure range and in the low-pressure interval, where
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there is a net pore generation. This is understandable considering that the generation of new porosity accessible to mercury – by means of the opening of the initially closed porosity – reveals pore surfaces with fractal characteristics closer to those of the parent coals, with higher fractal dimensions in the high and low pressure regions. The increase of fractal dimensions in the high-pressure range is more important for the ‘‘AGL’’ series than for the ‘‘AGH’’ series. This is not surprising as the pore generation in this zone is more important for the active carbons of the ‘‘AGL’’ series. For the mid-pressure regions there is, generally, a net pore destruction (widening of pores), leading to fractal dimensions of the active carbons of both series under the values of the fractal dimension of the corresponding chars. For the active carbons A3GH and A4GH there is no net destruction of pores in the mid-pressure region but this decrease in the fractal dimension during activation is also observed. Even if there is no decrease in the pore volume, what does happen in fact, is a pore widening process which is the same that produces the destruction of the widest pores. For the low-pressure region the behaviour is essentially the same: where there exists a pore generation (all samples of the ‘‘AGH’’ series and the samples A1GL and A4GL) there is an increase in the fractal dimension of the active carbon compared to the fractal dimension of the char. For the active carbons A1GL, A2GL and A3GL, where there is a net pore destruction in this size interval, the fractal dimensions of these materials are below the fractal dimensions of the corresponding active carbons of the ‘‘AGH’’ series. As happened for the series of active carbons AGL, the most important changes in the fractal dimension of the series of activated carbons AGH correspond to the first materials of the series, i.e., active carbons obtained from coals in the first stages of oxidation.
3.
4.
5.
6.
7.
8.
9. 5. Conclusions Characterization of the active carbons was carried out by means of classical textural techniques and a detailed study of the evolution of pore volumes during the activation step. Fractal analysis techniques were applied to the data from mercury porosimetry analysis. The results obtained from the conventional techniques, from the fractal analysis and from the comparison of both techniques allow us to formulate the following conclusions: 1. The active carbons produced by gasification at the higher flow rate can be characterized by using the method of Friesen and Mikula and that proposed by Zhang and Li, obtaining a fairly valid average fractal dimension from the whole pore surface accessible to mercury. For the active carbons obtained by activation at a flow rate of 7 ml min1 the average fractal dimensions are valid for following the evolution of the series despite the fact they are not representative for all the pore sizes. 2. The activation of cokes leads to materials with a fractal dimension below the values of the corresponding nonactivated coke. This decrease is more noticeable for the
10.
11.
12.
13.
active cokes obtained by gasification at the higher flow rate of the activating agent. For the rest of the activated carbons, as the coal oxidation increases, the fractal dimensions of the active carbons tend to be closer to the fractal dimensions of the corresponding chars. This fact is more obvious for the series of active carbons ‘‘AGH’’ that have very close fractal dimensions – sometimes coincident – to those of the corresponding precursor chars. The use of the Neimark’s plots allows us to see that during the activation process there is a destruction of the fractal homogenization achieved during the carbonization step. This destruction is more evident when the activation is carried out at the lower flow rate of CO2. The effect that the textural properties of the precursor coals have on the characteristics of the corresponding active carbons is more evident for the active carbons activated at the lower flow rate. This phenomenon is more obvious in the high-pressure region. The above-mentioned facts can be seen by comparing together the Neimark plots for the coals and to the corresponding active carbons. During activation, for the two flow rates employed, there is a net generation of total pore volume. This generation is more important when the activation was carried out with the lower flow rate. This pore volume generation corresponds almost completely to pore volume under 7.4 nm for the activation at the lower flow rate. If the activation was carried out at the higher flow rate, the generation of pore volume with size interval (7.4– 14.8) 103 nm becomes noticeable. For the active carbons from coals oxidized for 3 and 4 days the generation of this kind of porosity is prevalent. Considering the pore sizes accessible to mercury in our determinations ((7.4–14.8) 103 nm), the activation at the lower flow rate gives a net pore destruction, with the only exception of the active carbon coming from the coal oxidized for 4 days. The activation with the higher flow rate leads to a net generation of pore volume accessible to mercury that increases almost linearly with the oxidation time of the precursor coal. The only exception is the activated coke. For the two flow rates of CO2 employed, the effect of pore generation on the evolution of the fractal dimension of the high-pressure zone was the same: the value of the fractal dimension rises as the amount of pore generation increases. For the intermediate pressure range, where a net pore volume destruction takes place during activation, the fractal dimensions of the active carbons of both series are under the values corresponding to the precursor chars. In the low-pressure region, the values of the fractal dimensions of active carbons with generation of porosity are above the values for the homologous materials with destruction of porosity. Fractal analysis furnishes us with useful tools that are able to detect changes in active carbons due to different activation conditions.
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References [1] J.A. Pajares, J.J. Pis, A.B. Fuertes, A.J. Pe´rez, M. Mahamud, J.B. Parra, in: F. Rodrı´guez-Reinoso, J. Rouquerol, K.S.W. Sing, K.K. Unger (Eds.), Characterization of Porous Solids II, Elsevier, Amsterdam, 1991, p. 347. [2] J.A. Pajares, J.J. Pis, A.B. Fuertes, J.B. Parra, M. Mahamud, A.J. Pe´rez, in: G. Poncelet, P.A. Jacobs, P. Grange, B. Delmon (Eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, p. 439. [3] M. Mahamud, Doctoral Thesis, University of Oviedo, Oviedo, Spain, 1992. [4] J.C. Sousa, M. Mahamud, J.B. Parra, J.J. Pis, J.A. Pajares, Fuel Process. Technol. 36 (1993) 333. [5] J.J. Pis, M. Mahamud, J.B. Parra, J.A. Pajares, R.C. Bansal, Fuel Process. Technol. 50 (1997) 249. [6] J.J. Pis, M. Mahamud, J.A. Pajares, J.B. Parra, R.C. Bansal, Fuel Process. Technol. 57 (1998) 149. [7] B.B. Mandelbrot, Les Objects Fractals: Forme, Hasard et Dimension, Flammarion, Paris, 1975. [8] J. Feder, Fractals, Plenum Press, New York, 1988. [9] D. Avnir (Ed.), The Fractal Approach to the Heterogeneous Chemistry, Wiley, Chichester, 1989. [10] M. Mahamud, O. Lo´pez, in: Abstracts of the XI International Congress on Industry, Mining & Metallurgy, Zaragoza, Spain, June 2002, p. 295 (Full paper edited on CD-ROM). [11] M. Mahamud, O. Lo´pez, J.J. Pis, J.A. Pajares, Fuel Process. Technol. 81 (2003) 127. [12] M. Mahamud, O. Lo´pez, J.J. Pis, J.A. Pajares, Fuel Process. Technol. 86 (2004) 135. [13] M.M. Mahamud, Fuel Process. Technol. 87 (2006) 907. [14] W.I. Friesen, R.J. Mikula, J. Colloid Interface Sci. 120 (1) (1987) 263.
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Textural changes during CO2 activation of chars: A fractal approach Manuel Marı´a Mahamud * University of Oviedo, Department of Chemical and Environmental Engineering, Faculty of Chemistry, Campus de El Cristo, 33071 Oviedo, Spain Received 10 July 2006; received in revised form 24 November 2006; accepted 28 December 2006 Available online 7 January 2007
Abstract In this paper two series of active carbons obtained at different flow rates of the activating agent, CO2, are characterized in order to establish the different mechanisms of pore development during the activation step. This study complements previous works on textural development during the different steps in the process of obtaining active carbons: coal oxidation, coal pyrolysis and char gasification. As the characteristics of the original and intermediate materials are of capital importance in the pore development of active carbons, the properties of the active carbons, precursor chars and coals were considered and analyzed together. Mercury porosimetry and helium picnometry were used to determine classical textural parameters as well as to perform a more detailed study of the pore volume generation during the different conditions of the activation step. Data obtained from the mercury porosimetry determinations was also employed for fractal determinations according to the methodologies proposed by Friesen and Mikula, Zhang and Li and the procedure of Neimark. Average surface fractal dimensions as well as fractal profiles and local surface fractal dimensions were calculated. The use of different flow rates during the activation step produces changes not only in the ordinary textural parameters but also in the fractal characteristics of the active carbons. Activation at higher flow rates leads to smoother fractal profiles and also to lower values of the average surface fractal dimensions of the active carbons. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05Uw; 91.60.Np Keywords: Mercury porosimetry; Fractal analysis; Active carbon texture; Carbon dioxide activation; Flow rate; Burn-off
1. Introduction To obtain active carbons from bituminous coals several steps should be followed in the manufacturing process. For the processes referred to as physical activation, the steps involved are described below. The first one is the oxidation of the coal, since this is necessary to eliminate the plastic properties of the coal if the aim is to obtain a good active carbon. When this oxidation is carried out with air, it also develops a primary pore network. The existence of this porosity in coals plays an important role in the oxidation process itself as well as in the processing operations ahead. The oxidized coals are carbonized in an inert atmosphere in order to obtain a char with a more developed pore network. Finally, the chars are gasified with an oxidant such as air, steam or carbon dioxide. During gasification, changes in the apparent and true densities lead
* Tel.: +34 985103668; fax: +34 985103434. E-mail address: [email protected]. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.109
to an increase in pore volume. Pore volume distribution is also modulated and the values of specific surface area increase compared with the values of the precursor char. Regarding the active carbons and the precursor materials used in the present and previous studies, it can be said that the carbonization step multiplies the total pore volume by an approximate factor of three. The activation leads to materials with total pore volumes that are roughly three times the total pore volume of the precursor char. The manufacture process of active carbons from bituminous coals has been explained in previous papers and studies [1–6]. Textural study of two series of active carbons has been carried out using classical methods of textural analysis such as helium picnometry, mercury porosimetry and physical adsorption of nitrogen and carbon dioxide [6]. The fractal theory is based on the concept of self-similarity. If an object is self-similar, its appearance does not depend on the degree of detail used for its observation. The original work of Mandelbrot [7] introducing fractal concepts was followed by other books focused on the applications of this new approach to technical and scientific fields [8,9]. The value of the fractal
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dimension that characterizes a surface can be related with the degree of complexity of this surface. In addition, the fractal dimension is an intrinsic property of a surface and, theoretically, does not depend on its size. This is an important fact that makes fractal dimension a useful parameter for characterizing the surface of porous solids, despite pore width or pore distribution. For actual objects the fractal character cannot be the same for the whole range of sizes. This fact opens up the possibility of obtaining fractal profiles of porous solids that constitute a very useful tool for characterizing porous materials [10–13]. Data from mercury porosimetry determinations are suitable for analysis from a fractal point of view. With this purpose, several methods have been proposed and applied [14–16]. Previous work by the author has shown how the application of these fractal analysis procedures are valuable tools not only for characterizing carbonaceous materials but for clarifying the phenomena underlying all the transformations of the materials. Fractal analysis is a complementary tool for analyzing textural characteristics and their evolution for series of oxidized coals [10,11], chars [12] and active carbons [13]. This analysis complements traditional approaches for clarifying the mechanisms of pore development during the steps of coal oxidation, coal carbonization and char activation. Fractal methodology is also applied to other techniques of textural characterization such as TEM [17], SEM [18,19] or optical microscopy image analysis [20]. Physical adsorption of gases [17,21,22,16,23–25,19], or SAXS [26,17,27–29] are also used for fractal determinations. All the mentioned methods have been extensively applied to carbonaceous materials. It should be said that fractal techniques are also employed to correlate properties such as the char yield in carbonization processes [30]. Maintaining the gasification temperature constant two other variables can be used to modulate the pore network of the final active carbon: the fraction of char mass eliminated during gasification (burn-off) and the flow rate of the activating agent. The influence of these variables has been extensively studied and reported for the active carbons whose textural data are used in this work [1–4,6]. The present article aims to complement the findings of previous studies with a more detailed analysis of pore volumes, their distribution and evolution, together with a fractal approach using data from the mercury porosimetry determinations. 2. Methods used for the determination of surface fractal dimension from mercury porosimetry analysis There exist several methods for the determination of surface fractal dimension from mercury porosimetry: the method of Friesen and Mikula [14], the analysis proposed by Zhang and Li [15] and the procedure of Neimark [16]. These methods have already been used to characterize series of materials constituted by oxidized coals [11], chars [12] and active carbons [13]. Recently, Zhang et al. [31] proposed an improvement of their procedure. The Laplace equation that represents an inverse relationship between the average radius of curvature of the meniscus (R) and
the intrusion pressure (P) is the basis for mercury porosimetry analysis. For very small pores, this technique may not be valid but mercury porosimetry permits the characterization of macropores – pores wider than 50 nm – and wide mesopores. In the present study the interval of mercury intrusion pressures ranges from 1 to 2000 bar. This leads to a pore width interval from 14.8 103 nm to 7.4 nm, that is the size of the smallest pore that can be penetrated under an intrusion pressure of 2000 bar. Mercury porosimetry data consist in pairs of values of cumulative mercury volume intruded V, up to a pressure P. These arrays of values are employed by the methods cited above in order to obtain the fractal dimension of pore surfaces, D. The method of Friesen and Mikula allows the obtaining of a value of the fractal dimension, DF, from the following expression: dV ln (I) / ðDF 4Þln P dP This expression corresponds to a point-to-point fit of the experimental data, there usually existing an important scatter of the individual points. As a consequence of this, the correlation coefficients for the Friesen’s fit are generally low [11–15]. According to Neimark, the fractal dimension, DN, is estimated as follows: RV d ln 0 P dV (II) DN ¼ 2 þ dðln PÞ The above expression was obtained from the equation of Rootare and Prenzlow [32] that considers thermodynamic principles relative to solid impregnation by a non-wetting fluid, the scaling law commonly employed for the calculation of fractal dimensions, and the Laplace equation [11,16]. From thermodynamic considerations and the scaling relationship of a fractal surface area and the circumscribed volume, Zhang and Li concluded that: Z V ln P dV ¼ constant þ lnðR2DZ V DZ =3 Þ (III) 0
that is usually written in an abbreviated form: lnðW V Þ ¼ constant þ lnðQV Þ
(IV)
The value, DZ that makes the slope of ln(WV) versus ln(QV) equal to unity – i.e. that satisfies Eqs. (III) and (IV) – is the fractal dimension. Zhang et al. [31], rearranging Eq. (III), found a simpler way to estimate DZ as follows: DZ ¼
dlnðW V R2 Þ dlnðV 1=3 R1 Þ
(V)
This procedure was applied by the author to several materials, giving essentially the same values for DZ as the figures obtained from Eqs. (III) or (IV). The coefficients of correlation obtained by fitting the data to Eq. (V) are much better than the corresponding to Eqs. (III) and (IV). There exists another known expression that was proposed to estimate the fractal dimension [14,21,33]. Its mathematical
M.M. Mahamud / Applied Surface Science 253 (2007) 6019–6031
expression is
dV / R2D dR
(VI)
In this case, it is necessary to know the pore volume distribution. 3. Experimental The samples characterized in this work are series of oxidized coal, series of chars obtained from the oxidized coals and series of active carbons obtained by means of char gasification with carbon dioxide. The raw material is a high-volatile A bituminous coal with a particle size in the interval 1–3 mm oxidized in air at a temperature of 543 K for periods of time of 1–4 days. The nonoxidized coal and the oxidized samples were carbonized in inert atmosphere at 1123 K, obtaining the corresponding series of chars. More detailed information on these materials regarding coal properties, coal oxidation and carbonization processes as well as coals and chars characterization has already been collected in previous papers [1,2,11,12,34] and studies [3]. The active carbons were obtained by gasification of chars with carbon dioxide at a temperature of 1123 K up to a fractional burn-off of about 0.52. The activation reactor consisted in a double-jacket quartz tube enclosing a fixed bed of about 4 g of char. The activation reaction took place at two different flow rates of CO2, 7.0 and 500 ml min1. Detailed information about the activation process has already been reported [3,6]. Table 1 shows the activation parameters as well as the pre-treatment conditions of active carbon precursors for each sample code. Table 2 compiles some textural parameters of the active carbons under study. Data analyzed in this study consist in results of helium picnometry and mainly in registers from the mercury porosimetry of the coals, chars and active carbons in the 1– 2000 bar pressure interval. Data from the mercury porosimetry constitute the starting point for the determination of the fractal dimensions by using the approaches suggested by Friesen and Mikula, Zhang and Li and the method proposed by Neimark. Despite the fact that data corresponding to coals and chars have been analyzed in depth in previous papers [10–12], textural data Table 1 Codes of active carbon samples indicating the activating conditions and the preoxidation conditions of the corresponding precursor coals Series
Active carbon samples
Oxidation conditions of precursor coal T (K)
Time (days)
AGL
A0GL A1GL A2GL A3GL A4GL
543 543 543 543 543
0 1 2 3 4
AGH
A0GH A1GH A2GH A3GH A4GH
543 543 543 543 543
0 1 2 3 4
CO2 flow rate (ml min1)
7.0 7.0 7.0 7.0 7.0 500 500 500 500 500
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Table 2 Codes of active carbon samples and their corresponding ash content and textural parameters Active carbon samples
Ash content (%)a
Mercury density (g cm3)
Helium density (g cm3)
Total pore volume (cm3 g1)
N2 BET surface area (m2 g1)
A0GL A1GL A2GL A3GL A4GL A0GH A1GH A2GH A3GH A4GH
14.1 13.5 15.2 16.1 20.8 14.1 13.8 13.7 16.5 20.8
1.229 0.994 0.820 0.866 0.801 1.215 0.991 0.889 0.868 0.818
1.894 2.010 2.100 2.104 2.053 1.836 1.984 2.040 2.055 2.007
0.285 0.509 0.744 0.679 0.761 0.278 0.505 0.635 0.666 0.724
120 445 605 666 704 74 331 489 504 548
Textural properties are expressed in dry ash-free basis. a Estimated from the ash content of the precursor char and the degree of burn-off.
and fractal profiles corresponding to these materials are used together with those corresponding to the active carbons. The properties of coals, chars and active carbons, are helpful to understand the influence that the precursor materials have in the final properties of the active carbons obtained [12,13]. In this case, these properties where placed together with those corresponding to the series of active carbons obtained using the two different flow rates of activating agent. This was found useful in order to elucidate how the activating conditions determine the pore development of active carbons. 4. Results and discussion Findings from the fractal analysis of pore surfaces were used together with classical textural parameters in order to explain textural changes during gasification at different flow rates of the activating agent. The three previously mentioned procedures were employed for the fractal study of the pore surface of the materials. Results and their discussion are presented in the following sections. 4.1. Applicability of the fractal analysis methods of to the series of active carbons: evolution of the fractal dimensions For the series of active carbons obtained by char gasification at a flow rate of 7.0 ml min1, ‘‘AGL’’, the application of the method of Friesen was not found to be adequate for obtaining an average fractal dimension valid for the whole range of pore sizes reached by mercury porosimetry. This fact become evident observing the graphs – not shown here, but quite similar to others already published [13] – corresponding to the Friesen’s plots for the five materials in the series ‘‘AGL’’. The calculation of an average fractal dimension for the full range of pressures, DF, leads to a series of fractal dimensions that passes through a maximum as the oxidation time of the precursor coal increases. For the series of active carbons obtained by char gasification at a flow rate of CO2 of 500 ml min1, ‘‘AGH’’, the general trend of the values of the fractal dimension DF is essentially the same as that observed for the ‘‘AGL’’ series but
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some differences can be found. First, it is possible to obtain for the materials of the ‘‘AGH’’ series a value of the Friesen dimension more representative for the entire range of pore sizes. The coefficients of correlation of the Friesen fit for the ‘‘AGH’’ series always have values higher than those corresponding to the ‘‘AGL’’ series. Second, the values of the fractal dimensions for the series activated at the higher flow rate are always below the values for the corresponding materials of the series of active carbons obtained by activation at the lower flow rate. Third, the active carbons of the ‘‘AGH’’ series have fractal dimensions much closer to the fractal dimensions of the respective chars than the ‘‘AGL’’ series of active carbons, with the only exception of the activated coke (obtained from the non-oxidized coal). This last remark is related to the texture and the formation mechanism of chars and cokes together with the pore generation mechanism during the
activation step. When the non-oxidized coal is carbonized, it passes through a plastic phase leading to a coke that has closed pores with smooth surfaces. When this coke is activated, an important amount of these closed pores are opened leading to materials with very low fractal dimensions compared with the precursor coke. By applying the procedure proposed by Zhang and Li, the average fractal dimensions for the active carbons were calculated. By comparing the graphs of the Zhang and Li method for the ‘‘AGL’’ series of active carbons in Fig. 1, and ‘‘AGH’’ in Fig. 2, the pore surface – accessible to mercury – of the ‘‘AGH’’ active carbons series seems to be more homogeneous than the pore network of the ‘‘AGL’’ series. In the above-mentioned figures, the values of the average fractal dimensions DZ, are noted for every active carbon. The analysis of the evolution of the fractal dimension DZ is quite similar to
Fig. 1. Plots of ln WV vs. ln QV corresponding to samples of active carbons ‘‘AGL’’. The value of DZ represents the average fractal dimension. This satisfies Eq. (IV) for the full range of pressures, 1–2000 bar.
M.M. Mahamud / Applied Surface Science 253 (2007) 6019–6031
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Fig. 2. Plots of ln WV vs. ln QV corresponding to samples of active carbons ‘‘AGH’’. The value of DZ represents the average fractal dimension. This satisfies Eq. (IV) for the full range of pressures, 1–2000 bar.
that observed for the dimension DF. The Zhang and Li fractal dimension for the active carbon A0GH (obtained from the coke) has a lower and very different value to that corresponding to the precursor coke. The DZ figures for the rest of the active carbons obtained under conditions of the higher flow rate, almost coincide with the DZ values for the corresponding chars (differences are in the interval 0.02–0.07). By using the average fractal R V dimensions, DN, obtained by fitting the pairs of values ln 0 P dV versus ln P (method of Neimark) in the region of intermediate pressures as can be seen in Figs. 3 and 4—the considerations of the preceding paragraphs are applicable. In Fig. 5, the evolution of the fractal dimensions DF, DZ and DN of coals, chars and active carbons can be observed. This figure is in agreement and is helpful in summarizing what has been said in the above paragraphs. The activation
step tends to increase the fractal dimension of the materials, this increase being more important if the activation is carried out at the lower flow rate. If activation was carried out at the highest flow rate, changes in the fractal dimension during activation are less important and, very often, negligible. The exception is the activation of the coke (carbonized material obtained from the non-oxidized coal) that generates activated materials with a lower fractal dimension than the carbonized precursor. 4.2. Fractal profile analysis of active carbons and precursor chars and coals Previous work has shown how the Neimark’s graphs can be successfully used for performing a zonal fractal analysis of pore surfaces [11–13]. These graphs revealed the existence of
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Fig. 3. Plots of ln WV vs. ln P corresponding to samples of active carbons ‘‘AGL’’. Values of the fractal dimensions deduced from the straight lines are indicated on the graphs.
different fractal regions in the pore network of coals, oxidized coals, chars and active carbons. Apart from the fractal profiles themselves, interesting facts were found related to the ‘‘smoothing’’ of the transition between fractal zones as the oxidation of the precursor coal increases. Also noticeable was the phenomenon known as the ‘‘fingerprint’’ effect, consisting in the observation of similarities between related series of coals, chars and active carbons. It is clear that the fractal analysis of mercury porosimetry data reflects the influence of the porous texture of the precursor coal on the characteristics of the pore network of chars obtained from these coals. In a similar way, the textural properties of the char condition the properties of the active carbon obtained. More, as reported in a recent paper [13], the fractal profile of the active carbons even ‘‘discovers’’ features corresponding to the precursor coal that remain ‘‘hidden’’ in the fractal profile of the char.
The above-mentioned facts are a consequence of the importance of the textural properties of the precursor coals and chars on the textural development of the corresponding chars and active carbons. This evidence has already been pointed out in previous works where a ‘‘classical’’ textural analysis approach was applied [1–3,5,6]. The fractal profiles obtained by plotting the values ln(WV) ln P for the series of active carbons, ‘‘AGL’’ and ‘‘AGH’’, are plotted respectively in Figs. 3 and 4. In these figures the fitting lines corresponding to the linear regions of the graphs for the intermediate and high pressure intervals are also drawn. The corresponding values of the fractal dimensions DN are also noted. It is noteworthy that for the series of active carbons obtained at the lower flow rate of CO2 (‘‘AGL’’), the values of DN are always above the values of the corresponding active carbon of the ‘‘AGH’’ series. This behaviour of the
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Fig. 4. Plots of ln WV vs. ln P corresponding to samples of active carbons ‘‘AGH’’. Values of the fractal dimensions deduced from the straight lines are indicated on the graphs.
Neimark fractal dimensions is qualitatively similar to the trend observed for the Friesen and Zhang dimensions. It is also of note that in the case of the ‘‘AGH’’ series the plots are smoother and the transition between fractal regions is less marked. Nevertheless, the plots of Neimark and the graphs of Zhang and Li are still adequate for carrying out a zonal fractal analysis of the ‘‘AGH’’ series of active carbons. A method for comparing the fractal profiles of different materials is to place them together in the same graph. Fig. 6 simultaneously plots the Neimark graph for each active carbon pair of the series ‘‘AGL’’ and ‘‘AGH’’ as well as the plot of the corresponding precursor char and coal. In these figures the fractal dimensions calculated by the Neimark approach for the active carbons, chars and coals are also collected. It has previously been reported that the shape of the Neimark plots for
the active carbons of the ‘‘AGL’’ series are much more similar to the shape corresponding to the precursor coals than to the shape of the plots for chars [13]. For the series of active carbons ‘‘AGH’’ the same behaviour is noticeable. It also occurs that for the region of narrower pores, a zone of high values of the fractal dimensions -that can be appreciated for coals but is inexistent for chars-appears. Moreover, the following points must be considered. First, the shape of the Neimark plots of the ‘‘AGH’’ series resembles that corresponding to the plots of the precursor coals but this degree of mimicry is more evident for the ‘‘AGL’’ series. Second, the fractal profiles for the ‘‘AGH’’ series are much smoother than the profiles for the ‘‘AGL’’ series. Third, the active carbons of the ‘‘AGL’’ series from coals oxidized for 2 or more days have fractal dimensions – for the intermediate pressure range – lower than the dimensions of chars and almost
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Fourth, for the high-pressure region, the fractal dimensions of ‘‘AGH’’ active carbons tend to increase as in the case of the precursor coals. The values of this high-pressure range dimension for the ‘‘AGH’’ series are always below the values for the series of active carbons ‘‘AGL’’. The above fact may be explained taking into account the influence that the pore texture of the precursor coal has on the textural properties of the active carbon obtained from it [3,6,11–13,30]. The fractal dimensions over 3 detected for the active carbons ‘‘AGL’’ in the highpressure range has already been attributed to the filling with mercury of ink-bottle shaped pores generated by opening – actually, making accessible to mercury – of closed porosity during the activation process [13]. Classical textural determinations do not support the possibility of the existence of closed porosity that collapses under mercury pressure, leading to this increase of the fractal dimensions [3]. For the ‘‘AGH’’ series in the high-pressure range, high values of the fractal dimensions are also determined (see Fig. 4) but the values are clearly below the values of the fractal dimensions for the ‘‘AGH’’ series. This fact is understandable bearing in mind that when the activation is carried out at the lower flow rate, the reaction can take place in the smaller pores to a greater extent that if the CO2 flow rate is more elevated. As a consequence of this, the amount of small ink-bottle shaped pores that can be opened during the activation of the ‘‘AGL’’ series is more important than that corresponding to the ‘‘AGH’’ series. In addition, the generation of new porosity – that is usually associated with an increase in the fractal dimension [13] – in the region of the smallest pores reached by mercury, is more appreciable for the ‘‘AGL’’ series as will be indicated in Section 4.4. Let us now compare the fractal dimensions for chars and active carbons noted in Fig. 6, corresponding to the region of the intermediate pressures. Activation at the higher flow rates – ‘‘AGH’’ series – produce changes of the fractal dimensions – difference between the fractal dimension of the active carbon and the dimension of the precursor char – more important than that corresponding to the ‘‘AGL’’ series. This can be explained considering that for the ‘‘AGH’’ series, the generation of porosity in the interval of sizes detected by mercury porosimetry is much more important than for the ‘‘AGL’’ series. This leads to the series of active carbons obtained at the higher flow rate having more possibilities of modifying the surface corresponding to the pore sizes accessible to mercury.
Fig. 5. Evolution with time of oxidation of the fractal dimension for series of coals, chars, and the corresponding active carbons ‘‘AGL’’ (activated at a CO2 flow rate of 7.0 ml min1) and ‘‘AGH’’ (activated at a CO2 flow rate of 500 ml min1). The fractal dimensions are calculated by the methods of Friesen (DF), Neimark (DN) and Zhang (DZ).
coincident with the dimensions of the corresponding coals. In the case of the series of active carbons ‘‘AGH’’, the general trend is the same, but the differences between the fractal dimensions of chars and active carbons are more important. This leads to the fractal dimensions of active carbons reaching values lower than those corresponding to precursor coals.
4.3. Pore volume generation during activation and its distribution From classical data of textural analysis -apparent density, real density and pore volume intruded by mercury up to pressures of 2000 bar—an evaluation of the actual pore volume generation or destruction during the activation was carried out. The method is similar to that already used in previous studies [12,13]. The total pore volume – obtained from the real and apparent densities – that corresponds to pore sizes under 14.8 103 nm, was divided into two fractions: the pores reached by mercury intrusion between pressures 1 and 2000 bar (14.8 103 to 7.4 nm) and the pores smaller than 7.4 nm that are not accessible to mercury. For
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Fig. 6. Plots of ln WV vs. ln P corresponding to samples of coals, chars, and the corresponding active carbons ‘‘AGL’’ (activated at a CO2 flow rate of 7.0 ml min1) and ‘‘AGH’’ (activated at a CO2 flow rate of 500 ml min1). Values of the fractal dimensions deduced from the straight lines are indicated on the graphs.
a clear idea of the effective pore development during activation, the pore volume of active carbons are transformed to the corresponding equivalent volume of char (pore volume per unit of mass of precursor char). The pore volumes calculated are the following: V<7.4 char that represents the specific pore volume of chars with a size under 7.4 nm, non-accessible to mercury under a pressure of 2000 bar. V<7.4 act, i.e., the specific pore volume of active carbons with a size under 7.4 nm.
V<7.4 act trans, which corresponds to the pore volume of the active carbon under 7.4 nm per unit of mass of precursor char. This is calculated by taking into account the loss of mass during activation by multiplying the previous pore volume by the factor (1 X), being X the fractional burn-off achieved during activation. Dif V<7.4 corresponds to the difference V<7.4 act trans V<7.4 char, being the actual amount of pores generated during activation with a size below 7.4 nm. Dif V>7.4, is the actual pore volume generated with a size in the (7.4–14.8) 103 nm range.
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Dif Vtot, is the total volume effectively generated during the activation step and is calculated as the sum Dif V<7.4 + Dif V>7.4. All the pore volumes are expressed in a dry-ash-free basis (Vdaf). For the active carbons it is necessary to consider that the amount of ashes increases as the burn-off increases. The expression for transforming the pore volumes in dry basis (Vdb) into the volume in dry ash-free basis (Vdaf), is the following: ðV daf Þ ¼ ðV db Þ
ð1 XÞ þ ðAshÞX ð1 XÞ½1 ðAshÞ
(VII)
being X, as usual, the fractional burn-off and (Ash) the fraction of ash in the precursor char. From the graphs in Fig. 7, where some of the different pore volumes mentioned above for the ‘‘AGL’’ and ‘‘AGH’’ series are plotted versus the oxidation time of the precursor coal, the following facts can be observed. First: total pore volume generation is more important for the ‘‘AGL’’ series. This is understandable due to the fact that for the activation at the higher flow rate, the inhibition effect of the CO in the outermost sections of the particles may be negligible and the disappearance of material from the ‘‘external’’ region of the
particles is important. A different phenomenon occurs when activation is carried out at the lower flow rate [1,2,3,6], this inhibition affecting the whole particle volume, increasing in this way the fraction of the burn-off that takes place inside the porous network of the particles. It is also useful to consider that for the ‘‘AGL’’ series reaction times are about four to six times longer than the corresponding to homologous materials of the ‘‘AGH’’ series. Second: for the ‘‘AGH’’ series the generation of pore volume in the size range (7.4–14.8) 103 nm increases with the oxidation of the precursor coal. This generation represents up to 80% of the total pore volume generation. On the other hand, for the first four materials of the ‘‘AGL’’ series there is a slight loss of pore volume accessible to mercury. Only for the active carbon A4GL is there a net generation of pore volume for this size interval. For samples A0GL, A1GL, A2GL and A3GL the increase in porosity corresponds completely to pores under 7.4 nm and for the active carbon A4GL the generation of this porosity is about 3/4 of the total pore generation. 4.4. Pore volume evolution during activation and changes in fractal dimensions of the active carbons The analysis reported in the previous section only divided the total pore volume into two fractions, the pores nonaccessible to mercury and the pores accessible to mercury in the pressure interval 1–2000 bar. The use of the pore volume distribution obtained from mercury porosimetry was found useful to estimate the actual changes in pore volumes for the range of pore sizes detected by this technique [13]. For obtaining these distributions, the following variables were defined: Vchar, corresponds to the program of the char, i.e., the cumulative volume of mercury intruded into the char up to a pressure P. Vact trans, is the cumulative volume of mercury intruded into the active carbon up to a pressure P, based on mass of initial char. Dif V, is the cumulative volume actually generated during gasification and is calculated as the difference Vact trans Vchar.
Fig. 7. Pore volumes of chars and the corresponding transformed volumes for active carbons ‘‘AGL’’ and ‘‘AGH’’. Generation of pore volume of different range of sizes during the activation step.
All volumes are expressed in daf basis. Derivative, corresponds to the derivative of the curve Dif V ln P. This indicates for a specific pore size, if there is a net generation (positive values) or destruction (negative values) of pore volume. The pore development for the active carbons of the ‘‘AGL’’ series has already been reported [13]. Here, Fig. 8 shows cumulative pore volumes of chars and active carbons, cumulative pore volume generation and the derivative of the cumulative pore volume for the series of active carbons ‘‘AGH’’. The value of Dif V for the maximum intrusion pressure is, obviously, that calculated as Dif V>7.4 in the previous section. The graph of the cumulative pore volume generation for the active carbon from coke A1GH is quite similar to that of the
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Fig. 8. Cumulative pore volumes of chars, of active carbons (‘‘AGH’’) and cumulative pore volume generation during the activation step. The values corresponding to the cumulative pore volume generation and its derivative are shown on the right ordinate axis.
homologous active carbon obtained by activation at higher flow rate, A1GL. Evolution for the two materials corresponds to an important pore generation for the large pore sizes (pores wider than 2.0 103 nm) due to the increase in size of smaller pores and to the opening of wide closed pores of the cokes. For the rest of the sizes accessible to mercury up to 2000 bar (pore sizes between 7.4 and 2.0 103 nm) there is a net pore volume destruction due to a widening of the pore network. For the active carbons of the ‘‘AGH’’ series coming from oxidized coals the general pattern of pore generation is as follows: an important pore generation for the widest pore sizes and very few changes in pore volume for pores narrower than 2.0 103 nm. This fact is evidenced by the analysis of the curves of cumulative volume generation in Fig. 8. These conclusions are consistent with those obtained from the analysis of the information in Fig. 7 as seen in the previous section. Pore generation in the size ranges not accessible to mercury – pores
under 7.4 nm width – for the ‘‘AGH’’ series is much less important (maximum values in the interval 30–50 mm3 g1) than for the ‘‘AGL’’ series, with pore generations of about 70– 80 mm3 g1. For the small pores, the reaction rates are reduced, due to diffusion and to the inhibitory effect of the reaction product, CO. In the case of activation at the lower CO2 flow rate, reaction times are between 4 and 6 times longer, allowing a more important pore development in the internal zones of the particles. Let us now discuss the relationship between changes in fractal dimension and the generation of porosity during the activation step. By comparing the fractal profiles for chars and active carbons in Fig. 6, and the pore generation curves in Fig. 8, it can be seen that in the regions where a generation of pores takes place during activation there is an increase in the fractal dimension of the active carbon compared with the fractal dimension of the precursor char. This fact is more evident in the high-pressure range and in the low-pressure interval, where
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there is a net pore generation. This is understandable considering that the generation of new porosity accessible to mercury – by means of the opening of the initially closed porosity – reveals pore surfaces with fractal characteristics closer to those of the parent coals, with higher fractal dimensions in the high and low pressure regions. The increase of fractal dimensions in the high-pressure range is more important for the ‘‘AGL’’ series than for the ‘‘AGH’’ series. This is not surprising as the pore generation in this zone is more important for the active carbons of the ‘‘AGL’’ series. For the mid-pressure regions there is, generally, a net pore destruction (widening of pores), leading to fractal dimensions of the active carbons of both series under the values of the fractal dimension of the corresponding chars. For the active carbons A3GH and A4GH there is no net destruction of pores in the mid-pressure region but this decrease in the fractal dimension during activation is also observed. Even if there is no decrease in the pore volume, what does happen in fact, is a pore widening process which is the same that produces the destruction of the widest pores. For the low-pressure region the behaviour is essentially the same: where there exists a pore generation (all samples of the ‘‘AGH’’ series and the samples A1GL and A4GL) there is an increase in the fractal dimension of the active carbon compared to the fractal dimension of the char. For the active carbons A1GL, A2GL and A3GL, where there is a net pore destruction in this size interval, the fractal dimensions of these materials are below the fractal dimensions of the corresponding active carbons of the ‘‘AGH’’ series. As happened for the series of active carbons AGL, the most important changes in the fractal dimension of the series of activated carbons AGH correspond to the first materials of the series, i.e., active carbons obtained from coals in the first stages of oxidation.
3.
4.
5.
6.
7.
8.
9. 5. Conclusions Characterization of the active carbons was carried out by means of classical textural techniques and a detailed study of the evolution of pore volumes during the activation step. Fractal analysis techniques were applied to the data from mercury porosimetry analysis. The results obtained from the conventional techniques, from the fractal analysis and from the comparison of both techniques allow us to formulate the following conclusions: 1. The active carbons produced by gasification at the higher flow rate can be characterized by using the method of Friesen and Mikula and that proposed by Zhang and Li, obtaining a fairly valid average fractal dimension from the whole pore surface accessible to mercury. For the active carbons obtained by activation at a flow rate of 7 ml min1 the average fractal dimensions are valid for following the evolution of the series despite the fact they are not representative for all the pore sizes. 2. The activation of cokes leads to materials with a fractal dimension below the values of the corresponding nonactivated coke. This decrease is more noticeable for the
10.
11.
12.
13.
active cokes obtained by gasification at the higher flow rate of the activating agent. For the rest of the activated carbons, as the coal oxidation increases, the fractal dimensions of the active carbons tend to be closer to the fractal dimensions of the corresponding chars. This fact is more obvious for the series of active carbons ‘‘AGH’’ that have very close fractal dimensions – sometimes coincident – to those of the corresponding precursor chars. The use of the Neimark’s plots allows us to see that during the activation process there is a destruction of the fractal homogenization achieved during the carbonization step. This destruction is more evident when the activation is carried out at the lower flow rate of CO2. The effect that the textural properties of the precursor coals have on the characteristics of the corresponding active carbons is more evident for the active carbons activated at the lower flow rate. This phenomenon is more obvious in the high-pressure region. The above-mentioned facts can be seen by comparing together the Neimark plots for the coals and to the corresponding active carbons. During activation, for the two flow rates employed, there is a net generation of total pore volume. This generation is more important when the activation was carried out with the lower flow rate. This pore volume generation corresponds almost completely to pore volume under 7.4 nm for the activation at the lower flow rate. If the activation was carried out at the higher flow rate, the generation of pore volume with size interval (7.4– 14.8) 103 nm becomes noticeable. For the active carbons from coals oxidized for 3 and 4 days the generation of this kind of porosity is prevalent. Considering the pore sizes accessible to mercury in our determinations ((7.4–14.8) 103 nm), the activation at the lower flow rate gives a net pore destruction, with the only exception of the active carbon coming from the coal oxidized for 4 days. The activation with the higher flow rate leads to a net generation of pore volume accessible to mercury that increases almost linearly with the oxidation time of the precursor coal. The only exception is the activated coke. For the two flow rates of CO2 employed, the effect of pore generation on the evolution of the fractal dimension of the high-pressure zone was the same: the value of the fractal dimension rises as the amount of pore generation increases. For the intermediate pressure range, where a net pore volume destruction takes place during activation, the fractal dimensions of the active carbons of both series are under the values corresponding to the precursor chars. In the low-pressure region, the values of the fractal dimensions of active carbons with generation of porosity are above the values for the homologous materials with destruction of porosity. Fractal analysis furnishes us with useful tools that are able to detect changes in active carbons due to different activation conditions.
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