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Microporosity of activated carbons produced from heat-treated and fractionated pitches
PERGAMON
Carbon 38 (2000) 59–64
Microporosity of activated carbons produced from heat-treated and fractionated pitches E. Daguerre, A. Guillot*, X. Py ´ ´ ´ ´ , 52 avenue de Villeneuve, 66860 Perpignan Cedex, France des Materiaux et Procedes C.N.R.S.-I.M.P., Institut de Science et Genie Received 7 May 1998; accepted 30 April 1999
Abstract Activated carbons with degrees of burn-off ranging from 12 wt.% to 60 wt.% have been prepared from toluene-insoluble (T.I.) fractions of a heat-treated A240 petroleum pitch. After toluene fractionation, the resulting pitches have been stabilized, carbonized and then activated using carbon dioxide at 1173 K. Characterization by high pressure carbon dioxide adsorption has been performed in order to determine the properties of activated carbons and to evaluate their performances for adsorption refrigerating machines. Microporous properties are influenced by pitch composition resulting from toluene fractionation and residual g-resins inhibit the microporosity development. Due to their high microporous volume, activated carbons prepared from extensively g-resins extracted pitches appear appropriate for cooling applications. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Petroleum pitch, Activated carbon; D. Adsorption refrigerating machines
1. Introduction Numerous industrial applications such as air purification [1], gas storage [2] and gas separation [3] take advantage of the adsorptive properties of activated carbons. Adsorption refrigerating machines form another application field of these materials which is currently undergoing rapid development [4,5], especially since the new international agreement on the use of CFC. Follin et al. [6] have shown, within the framework of the Dubinin analysis, that the performances of a basic adsorption refrigerating process (composed of an evaporator coupled with an adsorber) can be estimated from the characteristics of the activated carbon used as adsorbent. In this study, such characteristics were estimated using the Dubinin–Astakhov (DA) equation in the case of micropore volume Wo and mean pore size Lo , while the Dubinin– Stoeckli (DS) equation was used for pore size distribution (PSD) obtaining. As Jagiello and Schwarz [7] have shown that the DA exponent value n can be connected to the pore size dispersion s obtained by DS equation, Follin et al. used both parameters n and s to take into account the *Corresponding author. Tel.: 133-04-6866-2110; fax: 13304-6866-2141. E-mail address: [email protected] (A. Guillot)
microporous network homogeneity. This study has shown that, for a specific refrigerating system at a defined cold production temperature, the best performances of the machine are related to an optimal texture of the adsorbent. In the special case for which the refrigerant fluid (CO 2 ), after being loaded by the adsorbent, is released to the environment, Goetz et al. [8] have developed a new open sorption process. In such a process, the cooling energy is directly linked to the endothermic desorption of the adsorbate : in the case of carbon dioxide, the sorption process exhibits a high specific heat of desorption (500 kJ kg 21 ) compared to the enthalpy of the liquid–gas phase change (189 kJ kg 21 ). As the available energy is mainly correlated, in such experimental device, to the useful mass of CO 2 stored per kilogram of adsorbent, only high capacity adsorbents (MAXSORB [9] in that case) are efficient and the respective influences of micropore width and pore size dispersion are negligible. With such activated carbon, the authors have shown that a wearable air-conditioner can be developed [8]. In the case of cycling adsorption machines using ammonia as adsorbate, even if the increase of the micropore volume Wo always leads to an improvement of the performances, an optimal pore size exists, depending on the desired refrigeration temperature. On the other hand, even if Wo and Lo values are optimized, performances are
0008-6223 / 00 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00106-2
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60
Nomenclature Ws solvent weight fraction (g solvent / g pitch) Lo mean pore size (nm) Wo microporous volume (cm 3 g 21 ) s pore size dispersion (nm) n Dubinin–Astakov exponent (–) Eo characteristic energy (kJ mol 21 ) dp particle size (mm) COP coefficient of performance (cooling effect) (–) tg gasification time (h) T cold cold production temperature (K)
lowered when pore size dispersion increases. Due to those drastic conditions for obtaining the best performances and to the restrictive existing activated carbons available on the market, Follin et al. [6] concluded that new activated carbons with tailored microporous structure have to be developed for these special field of applications. Activated carbons made from conventional matters (wood, shells, anthracite...) present porous characteristics mainly prescribed by the initial frame of the precursor. Therefore, each tailored adsorbent needed for a specific application imposes its own elaboration process. This lack of flexibility is not compatible with the wide range of porous characteristics needed by the sole refrigeration process (depending on the temperature and power levels and required efficiency). As a matter of fact, pitches are known to develop, depending on the elaboration conditions, materials with various microtextures which can be used as activated carbons precursors. High surface area activated carbons can be produced by chemical activation with potassium hydroxide from petroleum coke [10] and from coal-tar mesocarbon microbeads [11]. Weishauptova´ et al. [12] studied the production of activated carbons from heat-treated coal-tar pitch in which quinoline-insoluble matter exhibits, after steam activation at 1173 K, porous characteristics. Alcaniz-Monge et al. [13] showed that CO 2 activation of petroleum pitch-based carbon fibers provides microporous activated carbons with high surface area (over 1700 m 2 g 21 ). The aim of the work described in this paper was to study the influence of pitch composition on the microporous network obtained after carbon dioxide activation and to determine if the use of pitches as activated carbons precursors allows the tailoring of microporous characteristics. The influence of fractionation process with toluene used for the control of pitch composition is described and the performances of the obtained activated carbons in the case of refrigeration applications are discussed.
2. Experimental
2.1. Heat-treatments and fractionation A 350 g amount of A240 isotropic petroleum pitch
(supplied by Ashland Co.) was heat-treated under nitrogen atmosphere in a standard stirred tank reactor at a heating rate of 2 K min 21 . Stirring was performed using a four bladed rushton turbine and was kept constant (300 rpm) throughout the heat-treatment. After a residence time of 5 h at 673 K, the residual pitch was cooled down to room temperature. The obtained pitch EP0 was ground and sieved to 100 mm. This powder was then submitted to various toluene fractionation in order to partially remove toluene-soluble (g-resins) content and to obtain an extracted pitch which composition is mainly toluene-insoluble but quinoline-soluble (b-resins) [14]. After filtration and drying (no warm drying was performed in order to avoid thermal effects [15]), residual solubilities in toluene and quinoline were determined for resins characterization.
2.2. Carbonization, stabilization and activation Extracted pitch stabilization was performed under a flowing air atmosphere (0.5 l min 21 ) in a conventional horizontal furnace. Extracted pitch powders were heated up to 553 K at the rate of 1 K min 21 , and kept at this stabilization temperature during 1 h. Those experimental conditions were chosen with respect to Drbohlav and Stevenson results [16]. As powders are expected to be oxidized more slowly than pitch fibers, Drbohlav and Stevenson stabilization conditions appear more appropriate to extracted pitch stabilization than numerous works concerning carbons fibers oxidation [17,18]. After stabilization, samples were carbonized at 1273 K under a flowing atmosphere of nitrogen (0.5 l min 21 ). The heating treatment procedure was managed as follows: 2 21 21 K min from 553 K up to 773 K; 4 K min up to 1273 21 K, 1 h residence time and finally 4 K min down to the activation temperature of 1173 K. A 1.6 g amount of the carbonized materials were then activated under a constant carbon dioxide flow of 0.5 l min 21 . Carbon dioxide was chosen as an activating agent in order to promote micropore formation [19,20] and to lower mesopore development. The activation process was performed for various residence times (0 to 17 h) and activated carbons were obtained in the burn-off range from 12 up to 60%. As stabilization, carbonization and activation conditions were the same for all samples, pitch composition was the sole parameter that differentiates the different obtained materials.
2.3. Activated carbons characterization The characterization of activated carbons by means of high pressure carbon dioxide adsorption and the determination of microporous characteristics using the same tools as Follin et al. [6] allow a direct estimation of the refrigerating machines performances. For this reason, isotherms for carbon dioxide adsorption were determined using a high pressure volumetric device [21] at pressures
E. Daguerre et al. / Carbon 38 (2000) 59 – 64
61
up to 2 MPa. Characterization was performed at three temperature levels (253, 273 and 298 K) which allow the analysis of the whole micropore size range and correspond to the different working conditions of adsorption machines [6,8]. About 0.6 g of samples were outgassed at 473 K for 24 h under a residual vacuum lower than 1310 24 Pa. The micropore volume Wo and the mean pore size Lo were determined using the Dubinin–Astakhov equation while the estimation of the PSD and the pore size dispersion s of the microporous network were estimated thanks to the Dubinin–Stoeckli equation.
3. Results Fig. 1. Pitch composition as a function of solvent weight fraction used for extraction. (h) g-resins, (D) b-resins, (o) a-resins.
3.1. Heat-treated pitches The raw A240 petroleum pitch used as starting material in this study is mainly composed of toluene soluble matter (96 wt.% g-resins) and appears completely isotropic under polarized optical microscopy observation. After heat-treatment (673 K, 5 h), the resulting pitch EP0 has developed a small quinoline-insoluble fraction (4 wt.% QI). This QI content was identified as Brooks and Taylor [22] anisotropic mesophase domains ranging from 10 to 15 mm, formed by the aggregation of five to ten individual spheres. The remainder of the heat-treated pitch still appears isotropic. Heat-treatment induces two main effects concerning g- and b-resins contents (Table 1, A240-EP0). g-resins content is lowered by the removal of the lightest fraction of the pitch which distils off the reactor through the nitrogen stream. Simultaneously, b-resins content increases by both effects of concentration (induced by lights removal) and thermal polymerization.
3.2. Extracted pitches The toluene extraction process applied to EP0 heattreated pitch is used to concentrate the higher molecular weight species by the specific removal of the lightest part (toluene soluble) as described in Riggs and Diefendorf patent [14]. The extraction process was quantified by the involved solvent weight fraction Ws which equals the mass
of toluene used per gram of pitch. As shown in Fig. 1, the use of an increasing amount of toluene up to a weight fraction of 85 leads to an extensive decrease of the g-resins content while the toluene-insoluble fraction (b- and aresins) is concentrated up to 80 wt.%. However, in the last stage of the extraction step (Ws .100), initial g-resins seem to be hardly solubilized and very large amounts of toluene are needed to reach very low resins content. Extraction seems to go through two different steps. First, pure isotropic g-resins (i.e. pitch which does not react during heat-soaking) are removed. Then, toluene solubilizes a more aromatic fraction, still toluene soluble, but certainly mixed with b-resins as already described by Lafdi et al. [15]. As the pitch softening point [23] is correlated with low molecular weight content, most of the partially extracted pitches soften before stabilization temperature is reached. Using the stabilization conditions previously described, only two pitches appeared infusible and were used directly as activated carbons precursors. Those two samples referenced EP1 and EP2 still contain 12 and 5 wt.% of unextracted g-resins, respectively (compositions of EP1 and EP2 extracted pitches are gathered in Table 1). As a consequence of toluene extraction, particle sizes are also modified and the EP2 powder (d p ,20 mm) exhibits a smaller mean diameter than EP1 (dp ¯40 mm).
Table 1 Pitches characterization by solvent fractionation
3.3. Activation process
Material
The weight-losses resulting of carbon dioxide activation at 1173 K of carbonized powders are plotted in Fig. 2. It is clearly shown that the gasification rate is constant for both precursors. However, the reactivity of the chars with respect to CO 2 is different and the gasification rate of EP1 (4% h 21 ) is lower than that of EP2 (6% h 21 ). This effect is certainly the consequence of the difference in particle size induced by toluene extraction: the simultaneous
A240 EP0 EP1 EP2
Solubilities (wt.%) TS (g-resins)
TI-QS (b-resins)
QI (a-resins)
96 76 12 5
4 20 69 74
0.1 4 19 21
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E. Daguerre et al. / Carbon 38 (2000) 59 – 64
However, the difference between both precursors can be easily observed since all the activated carbons from EP2 (even EP2-13%) adsorb more carbon dioxide than the most activated carbon from EP1 (EP1-60%). Simultaneously, the stronger microporous behavior of EP1 activated carbons is also visualized. Isotherms for EP1 activated carbons with burn-off lower than 40%, present a plateau at high relative pressure that accounts for the primary micropore filling. Moreover, all isotherms for EP2 (even at low burn-off) exhibit a more gradual approach to a ill-defined plateau located at higher relative pressure, as generally observed for heterogeneous microporous adsorbents.
Fig. 2. EP1 and EP2 gasification under carbon dioxide atmosphere at 1173 K (0.5 l min 21 ).
diffusion and reaction of CO 2 inside the particle is d p dependent and induce variations in effective gasification rates [24]. Moreover, the residual g-resins could induce differences in reactivity with respect to carbon dioxide.
3.4. Activated carbons characteristics 3.4.1. Isotherms The obtained isotherms for EP1 and EP2 activated carbons, determined at 253 K, are illustrated in Fig. 3. According to the IUAPC classification, they are of type I.
3.4.2. Micropore volume The micropore volumes Wo for EP1 and EP2 activated carbons were obtained from DA equation and are recorded in Table 2. It is seen that the burn-off increase induces a gradual development of the micropore volume Wo . However, Wo values are very different, depending on the precursor used for activated carbon production. Reducing the g-resins content from 12 wt.% down to 5 wt.% leads to a great improvement of adsorption properties. Whatever the burn-off level, micropore volume for EP2 is more than twice that for EP1. 3.4.3. Pore size distributions The pore size distributions (PSD) of each activated carbons, obtained through the DS equation, are plotted in Fig. 4 and the corresponding parameters Lo and s are recorded in Table 2. It is seen that all samples are microporous since the mean pore sizes Lo never exceed 1.4 nm. As usually observed during gasification, the increase of burn-off induces a gradual pore size widening expressed through an Lo increase from 0.7 nm up to, respectively, 1.26 and 1.4 nm for EP1 and EP2. However, PSD and the corresponding Lo values (Table 2) show that mean pore size, at the same burn-off value, is approximately equivalent for both series, except during the last stage of activation process (burn-off.60%) for which pores collapsing in EP2 (with respect to Wo and n values recorded in Table 2) certainly occurs. As already described [25,26], the activated carbons
Table 2 Microporous characteristics determined by carbon dioxide adsorption
Fig. 3. EP1 (s) and EP2 (h) carbon dioxide isotherms at 253 K for various burn-offs.
Sample burn-off
Wo (cm 3 g 21 )
Eo (kJ mol 21 )
n
Lo (nm)
s (nm)
EP1-12% EP2-13% EP1-37% EP2-37% EP1-61% EP2-60%
0.07 0.20 0.13 0.33 0.20 0.46
26.3 27.3 24.5 22.4 19.6 18.2
1.95 1.8 1.8 1.6 1.7 1.4
0.7 0.7 0.83 0.9 1.26 1.4
0.05 0.22 0.18 0.27 0.30 0.40
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4. Discussion
Fig. 4. Pore size distributions for EP1 ( ]]]) and EP2 (---) activated carbons.
homogeneity is lowered during the activation process, and the dispersion s calculated from the DS equation goes from 0.05 up to 0.4. However, the comparison of s values for EP1 and EP2 activated carbons (Table 2) demonstrates the strong influence of the g-resins removal on the microporous network. In the case of low burn-off (13%), carbon dioxide activation is known to develop homogeneous micropore network. Even if EP1-13% and EP2-13% activated carbons exhibit the same mean pore size value (Lo 50.7 nm), dispersion for EP2 appears to be four times higher than for EP1. The observation of low micropore size values with simultaneous high dispersion (s 50.22) which is generally observed in the case of high burn-off level, is certainly due to structural change in EP2 connected with g-resins extraction. As characterization by carbon dioxide adsorption does not explain such an effect, further studies based on immersion calorimetry [27] with molecular probes will be performed in order to quantify those possible structural changes.
Large amounts of toluene used during the extraction process allow the elaboration of pitches mainly composed of b-resins, which contain less than 12 wt.% of residual g-resins. Such extracted pitches lead, after carbonization and carbon dioxide activation, to activated carbons with well-developed microporosity. However, microporous characteristics of those activated carbons appear to be strongly influenced by the extraction process with toluene: the use of lower g-resins content precursors results in reduced required gasification time for activated carbons preparation. Simultaneously, greater microporous volumes are obtained without any significant change of mean pore size. According to Follin et al. [6], all those modifications of the porous characteristics will result in an improvement of the refrigerating machines performances. In fact, the authors have quantified the influence of all Wo , Lo and n variations on the refrigerating machines performances through the calculation of a coefficient of refrigerating performance (COP). The COP is defined as the ratio of the quantity of heat drawn from the environment (i.e. the cold production) to the quantity of heat that must be supplied to the system for its regeneration. As an increase of micropore volume always leads to an improvement of the COP because of the increase of the mass of refrigerant gas exchanged during cold production, performances of EP1 and EP2 activated carbons will be discussed in the case of 60% burn-off activation for which the Wo value is maximized. COP values in the case of refrigeration at 273 and 248 K are gathered in Table 3 for EP1-60%, EP2-60% and four commercial activated carbons. Those commercial adsorbents were chosen with respect to their micropore volume which is of the same order of magnitude as EP2-60% (Wo ¯0.4 cm 3 g 21 ). In both cases of cold production at 273 and at 248 K, COP for EP1-60% is much smaller than for EP2-60%. This difference is mainly due to their micropore volume values which are much higher when the extraction process is performed up to 5 wt.% of residual g-resins. Even if in the case of cold production at very low temperature (248 K) in which COP values are much more sensitive to pore size and dispersion values, the higher homogeneity (n5
Table 3 Characteristics of various activated carbons and corresponding performances estimated for an adsorption refrigerating cycle
EP1-60% EP2-60% AC35 [6] NORIT RB [6] BPL [6] TA60 [6]
Wo (cm 3 g 21 )
Lo (nm)
n
COP T cold 5273 K
COP T cold 5248 K
0.20 0.46 0.42 0.41 0.42 0.47
1.24 1.40 1.71 1.33 1.25 1.09
1.7 1.4 2.15 2.0 1.5 1.7
0.29 0.38 0.42 0.4 0.37 0.39
0.16 0.23 0.26 0.26 0.24 0.26
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E. Daguerre et al. / Carbon 38 (2000) 59 – 64
1.7, s 50.3 nm) for EP1-60% does not compensate the micropore volume difference between both samples. Therefore, the toluene extraction process investigated in this study must be extensively performed in order to decrease as much as possible the residual g-resins content of activated carbon precursors. This g-resins removal leads to an improvement of adsorption properties, mainly a Wo development, which is responsible for increasing COP values. Even if this Wo increase induces a loss of homogeneity in the microporous network, the performances of refrigerating machines using activated carbons from extensively extracted precursors are improved. As shown in Table 3, COP values for EP2-60% are, for both investigated cold temperatures, comparable to those calculated for several commercial activated carbons [6]. Despite extraction process improves the adsorbent performances for cold production applications, the COP would be greatly increased if a simultaneous control of micropore volume and pore size dispersion could be performed. Further studies will be focused on the control of the micropore development through activation process optimization. The study of the different steps of the elaboration process such as air stabilization or gasification using other activating agents than carbon dioxide could be helpful for the elaboration of carbon adsorbents appropriate for adsorption refrigerating machines.
5. Conclusion Toluene fractionation of heat-treated pitches appears to be a useful way for elaborating activated carbons with various microporous properties. As a matter of fact, the monitoring of pitch composition through extraction process leads to a control of adsorption properties after activation: an extensive g-resins removal induces increasing microporous volumes. Refrigerating machines performances estimated by Follin et al. [6], linked to the present results, show that such fractionation increases the COP for cycling adsorption machines and leads to performances similar to several commercial activated carbons. The numerous parameters (stabilization conditions; activation agent, activation temperature...) which can be optimized during the elaboration process presented in this study open large possibilities for a best control of microporous properties development.
References [1] Bailey A. In: Patrick JW, editor, Porosity in carbons, Great Britain: Edward Arnorld, 1995, p. 208. [2] Parkins ND, Quinn F. In: Patrick JW, editor, Porosity in carbons, Great Britain: Edward Arnorld, 1995, p. 291. [3] Yang YT. Gas separation by adsorption processes, New York: Butterworth, 1987. [4] Meunier F. Proceedings of Symposium on Solid Sorption Refrigeration, Paris, 1992:44. [5] Cacciola G, Restuccia G, Mercadante L. Carbon 1995;33:1205. [6] Follin S, Goetz V, Guillot A. Ind Eng Chem Res 1996;35:2632. [7] Jagiello J, Schwarz JA. J Colloid Interface Sci 1992;154:225. [8] Goetz V, Follin S. In: Extended abstracts, 23rd biennal conference on carbon, PennState: American Carbon Society, 1997, p. 156. [9] Otawa T, Tanibata R, Itoh M. Gas Separation & Purification 1993;7:241. [10] Marsh H, Crawford D, O’Grady TM, Wennerberg A. Carbon 1982;20:419. [11] Kasuh T, Scott DA, Mori M. Extended abstracts, Carbon ’88. Newcastle (UK), 1988:146. [12] Weishauptova Z, Medek J, Vaverkova Z. Carbon 1994;32:311. [13] Alcaniz-Monge J, Cazorla-Amoros D, Linares-Solano A, Yoshida S, Oya A. Carbon 1994;32:1277. [14] Diefendorf RJ, Riggs DN. US Patent 4208267, 1980. [15] Lafdi K, Oberlin A. Carbon 1994;32:61. [16] Drbohlav J, Stevenson WTK. Carbon 1995;33:693. [17] Lavin JG. Carbon 1992;30:351. [18] Matsumoto T, Mochida I. Carbon 1992;30:1041. [19] Molina-Sabio M, Gonzalez MT, Rodriguez-Reinoso F, Sepulveda-Escribano A. Carbon 1996;34:505. [20] Ehrburger P, Pusset N, Dziezinl P. Carbon 1992;30:1105. [21] Guillot A, Follin S, Poujardieu L. In: Mc Enaney B, Mays TJ, Rouquerol J, Rodriguez-Reinoso F, Sing KSW, Unger KK, editors. Characterization of porous solids IV, 1997:573. [22] Brooks JD, Taylor GH. In: Thrower PA, editor, Chemistry and physics of carbon, New York: Dekker, 1968, p. 243. [23] Barr JB, Lewis IC. Thermochim Acta 1982;52:297. ´ ´ [24] Villermaux J. Genie de la reaction chimique. TEC&DOCLavoisier. 1993. [25] Parra JB, De Sousa JC, Pis JJ, Pajares JA, Bansal R. Carbon 1995;33:801. [26] Verma SK, Walker PL. Carbon 1990;28:175. [27] Stoeckli HF. In: Patrick JW, editor, Porosity in carbons, Great Britain: Edward Arnorld, 1995, p. 67.
Carbon 38 (2000) 59–64
Microporosity of activated carbons produced from heat-treated and fractionated pitches E. Daguerre, A. Guillot*, X. Py ´ ´ ´ ´ , 52 avenue de Villeneuve, 66860 Perpignan Cedex, France des Materiaux et Procedes C.N.R.S.-I.M.P., Institut de Science et Genie Received 7 May 1998; accepted 30 April 1999
Abstract Activated carbons with degrees of burn-off ranging from 12 wt.% to 60 wt.% have been prepared from toluene-insoluble (T.I.) fractions of a heat-treated A240 petroleum pitch. After toluene fractionation, the resulting pitches have been stabilized, carbonized and then activated using carbon dioxide at 1173 K. Characterization by high pressure carbon dioxide adsorption has been performed in order to determine the properties of activated carbons and to evaluate their performances for adsorption refrigerating machines. Microporous properties are influenced by pitch composition resulting from toluene fractionation and residual g-resins inhibit the microporosity development. Due to their high microporous volume, activated carbons prepared from extensively g-resins extracted pitches appear appropriate for cooling applications. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Petroleum pitch, Activated carbon; D. Adsorption refrigerating machines
1. Introduction Numerous industrial applications such as air purification [1], gas storage [2] and gas separation [3] take advantage of the adsorptive properties of activated carbons. Adsorption refrigerating machines form another application field of these materials which is currently undergoing rapid development [4,5], especially since the new international agreement on the use of CFC. Follin et al. [6] have shown, within the framework of the Dubinin analysis, that the performances of a basic adsorption refrigerating process (composed of an evaporator coupled with an adsorber) can be estimated from the characteristics of the activated carbon used as adsorbent. In this study, such characteristics were estimated using the Dubinin–Astakhov (DA) equation in the case of micropore volume Wo and mean pore size Lo , while the Dubinin– Stoeckli (DS) equation was used for pore size distribution (PSD) obtaining. As Jagiello and Schwarz [7] have shown that the DA exponent value n can be connected to the pore size dispersion s obtained by DS equation, Follin et al. used both parameters n and s to take into account the *Corresponding author. Tel.: 133-04-6866-2110; fax: 13304-6866-2141. E-mail address: [email protected] (A. Guillot)
microporous network homogeneity. This study has shown that, for a specific refrigerating system at a defined cold production temperature, the best performances of the machine are related to an optimal texture of the adsorbent. In the special case for which the refrigerant fluid (CO 2 ), after being loaded by the adsorbent, is released to the environment, Goetz et al. [8] have developed a new open sorption process. In such a process, the cooling energy is directly linked to the endothermic desorption of the adsorbate : in the case of carbon dioxide, the sorption process exhibits a high specific heat of desorption (500 kJ kg 21 ) compared to the enthalpy of the liquid–gas phase change (189 kJ kg 21 ). As the available energy is mainly correlated, in such experimental device, to the useful mass of CO 2 stored per kilogram of adsorbent, only high capacity adsorbents (MAXSORB [9] in that case) are efficient and the respective influences of micropore width and pore size dispersion are negligible. With such activated carbon, the authors have shown that a wearable air-conditioner can be developed [8]. In the case of cycling adsorption machines using ammonia as adsorbate, even if the increase of the micropore volume Wo always leads to an improvement of the performances, an optimal pore size exists, depending on the desired refrigeration temperature. On the other hand, even if Wo and Lo values are optimized, performances are
0008-6223 / 00 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00106-2
E. Daguerre et al. / Carbon 38 (2000) 59 – 64
60
Nomenclature Ws solvent weight fraction (g solvent / g pitch) Lo mean pore size (nm) Wo microporous volume (cm 3 g 21 ) s pore size dispersion (nm) n Dubinin–Astakov exponent (–) Eo characteristic energy (kJ mol 21 ) dp particle size (mm) COP coefficient of performance (cooling effect) (–) tg gasification time (h) T cold cold production temperature (K)
lowered when pore size dispersion increases. Due to those drastic conditions for obtaining the best performances and to the restrictive existing activated carbons available on the market, Follin et al. [6] concluded that new activated carbons with tailored microporous structure have to be developed for these special field of applications. Activated carbons made from conventional matters (wood, shells, anthracite...) present porous characteristics mainly prescribed by the initial frame of the precursor. Therefore, each tailored adsorbent needed for a specific application imposes its own elaboration process. This lack of flexibility is not compatible with the wide range of porous characteristics needed by the sole refrigeration process (depending on the temperature and power levels and required efficiency). As a matter of fact, pitches are known to develop, depending on the elaboration conditions, materials with various microtextures which can be used as activated carbons precursors. High surface area activated carbons can be produced by chemical activation with potassium hydroxide from petroleum coke [10] and from coal-tar mesocarbon microbeads [11]. Weishauptova´ et al. [12] studied the production of activated carbons from heat-treated coal-tar pitch in which quinoline-insoluble matter exhibits, after steam activation at 1173 K, porous characteristics. Alcaniz-Monge et al. [13] showed that CO 2 activation of petroleum pitch-based carbon fibers provides microporous activated carbons with high surface area (over 1700 m 2 g 21 ). The aim of the work described in this paper was to study the influence of pitch composition on the microporous network obtained after carbon dioxide activation and to determine if the use of pitches as activated carbons precursors allows the tailoring of microporous characteristics. The influence of fractionation process with toluene used for the control of pitch composition is described and the performances of the obtained activated carbons in the case of refrigeration applications are discussed.
2. Experimental
2.1. Heat-treatments and fractionation A 350 g amount of A240 isotropic petroleum pitch
(supplied by Ashland Co.) was heat-treated under nitrogen atmosphere in a standard stirred tank reactor at a heating rate of 2 K min 21 . Stirring was performed using a four bladed rushton turbine and was kept constant (300 rpm) throughout the heat-treatment. After a residence time of 5 h at 673 K, the residual pitch was cooled down to room temperature. The obtained pitch EP0 was ground and sieved to 100 mm. This powder was then submitted to various toluene fractionation in order to partially remove toluene-soluble (g-resins) content and to obtain an extracted pitch which composition is mainly toluene-insoluble but quinoline-soluble (b-resins) [14]. After filtration and drying (no warm drying was performed in order to avoid thermal effects [15]), residual solubilities in toluene and quinoline were determined for resins characterization.
2.2. Carbonization, stabilization and activation Extracted pitch stabilization was performed under a flowing air atmosphere (0.5 l min 21 ) in a conventional horizontal furnace. Extracted pitch powders were heated up to 553 K at the rate of 1 K min 21 , and kept at this stabilization temperature during 1 h. Those experimental conditions were chosen with respect to Drbohlav and Stevenson results [16]. As powders are expected to be oxidized more slowly than pitch fibers, Drbohlav and Stevenson stabilization conditions appear more appropriate to extracted pitch stabilization than numerous works concerning carbons fibers oxidation [17,18]. After stabilization, samples were carbonized at 1273 K under a flowing atmosphere of nitrogen (0.5 l min 21 ). The heating treatment procedure was managed as follows: 2 21 21 K min from 553 K up to 773 K; 4 K min up to 1273 21 K, 1 h residence time and finally 4 K min down to the activation temperature of 1173 K. A 1.6 g amount of the carbonized materials were then activated under a constant carbon dioxide flow of 0.5 l min 21 . Carbon dioxide was chosen as an activating agent in order to promote micropore formation [19,20] and to lower mesopore development. The activation process was performed for various residence times (0 to 17 h) and activated carbons were obtained in the burn-off range from 12 up to 60%. As stabilization, carbonization and activation conditions were the same for all samples, pitch composition was the sole parameter that differentiates the different obtained materials.
2.3. Activated carbons characterization The characterization of activated carbons by means of high pressure carbon dioxide adsorption and the determination of microporous characteristics using the same tools as Follin et al. [6] allow a direct estimation of the refrigerating machines performances. For this reason, isotherms for carbon dioxide adsorption were determined using a high pressure volumetric device [21] at pressures
E. Daguerre et al. / Carbon 38 (2000) 59 – 64
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up to 2 MPa. Characterization was performed at three temperature levels (253, 273 and 298 K) which allow the analysis of the whole micropore size range and correspond to the different working conditions of adsorption machines [6,8]. About 0.6 g of samples were outgassed at 473 K for 24 h under a residual vacuum lower than 1310 24 Pa. The micropore volume Wo and the mean pore size Lo were determined using the Dubinin–Astakhov equation while the estimation of the PSD and the pore size dispersion s of the microporous network were estimated thanks to the Dubinin–Stoeckli equation.
3. Results Fig. 1. Pitch composition as a function of solvent weight fraction used for extraction. (h) g-resins, (D) b-resins, (o) a-resins.
3.1. Heat-treated pitches The raw A240 petroleum pitch used as starting material in this study is mainly composed of toluene soluble matter (96 wt.% g-resins) and appears completely isotropic under polarized optical microscopy observation. After heat-treatment (673 K, 5 h), the resulting pitch EP0 has developed a small quinoline-insoluble fraction (4 wt.% QI). This QI content was identified as Brooks and Taylor [22] anisotropic mesophase domains ranging from 10 to 15 mm, formed by the aggregation of five to ten individual spheres. The remainder of the heat-treated pitch still appears isotropic. Heat-treatment induces two main effects concerning g- and b-resins contents (Table 1, A240-EP0). g-resins content is lowered by the removal of the lightest fraction of the pitch which distils off the reactor through the nitrogen stream. Simultaneously, b-resins content increases by both effects of concentration (induced by lights removal) and thermal polymerization.
3.2. Extracted pitches The toluene extraction process applied to EP0 heattreated pitch is used to concentrate the higher molecular weight species by the specific removal of the lightest part (toluene soluble) as described in Riggs and Diefendorf patent [14]. The extraction process was quantified by the involved solvent weight fraction Ws which equals the mass
of toluene used per gram of pitch. As shown in Fig. 1, the use of an increasing amount of toluene up to a weight fraction of 85 leads to an extensive decrease of the g-resins content while the toluene-insoluble fraction (b- and aresins) is concentrated up to 80 wt.%. However, in the last stage of the extraction step (Ws .100), initial g-resins seem to be hardly solubilized and very large amounts of toluene are needed to reach very low resins content. Extraction seems to go through two different steps. First, pure isotropic g-resins (i.e. pitch which does not react during heat-soaking) are removed. Then, toluene solubilizes a more aromatic fraction, still toluene soluble, but certainly mixed with b-resins as already described by Lafdi et al. [15]. As the pitch softening point [23] is correlated with low molecular weight content, most of the partially extracted pitches soften before stabilization temperature is reached. Using the stabilization conditions previously described, only two pitches appeared infusible and were used directly as activated carbons precursors. Those two samples referenced EP1 and EP2 still contain 12 and 5 wt.% of unextracted g-resins, respectively (compositions of EP1 and EP2 extracted pitches are gathered in Table 1). As a consequence of toluene extraction, particle sizes are also modified and the EP2 powder (d p ,20 mm) exhibits a smaller mean diameter than EP1 (dp ¯40 mm).
Table 1 Pitches characterization by solvent fractionation
3.3. Activation process
Material
The weight-losses resulting of carbon dioxide activation at 1173 K of carbonized powders are plotted in Fig. 2. It is clearly shown that the gasification rate is constant for both precursors. However, the reactivity of the chars with respect to CO 2 is different and the gasification rate of EP1 (4% h 21 ) is lower than that of EP2 (6% h 21 ). This effect is certainly the consequence of the difference in particle size induced by toluene extraction: the simultaneous
A240 EP0 EP1 EP2
Solubilities (wt.%) TS (g-resins)
TI-QS (b-resins)
QI (a-resins)
96 76 12 5
4 20 69 74
0.1 4 19 21
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However, the difference between both precursors can be easily observed since all the activated carbons from EP2 (even EP2-13%) adsorb more carbon dioxide than the most activated carbon from EP1 (EP1-60%). Simultaneously, the stronger microporous behavior of EP1 activated carbons is also visualized. Isotherms for EP1 activated carbons with burn-off lower than 40%, present a plateau at high relative pressure that accounts for the primary micropore filling. Moreover, all isotherms for EP2 (even at low burn-off) exhibit a more gradual approach to a ill-defined plateau located at higher relative pressure, as generally observed for heterogeneous microporous adsorbents.
Fig. 2. EP1 and EP2 gasification under carbon dioxide atmosphere at 1173 K (0.5 l min 21 ).
diffusion and reaction of CO 2 inside the particle is d p dependent and induce variations in effective gasification rates [24]. Moreover, the residual g-resins could induce differences in reactivity with respect to carbon dioxide.
3.4. Activated carbons characteristics 3.4.1. Isotherms The obtained isotherms for EP1 and EP2 activated carbons, determined at 253 K, are illustrated in Fig. 3. According to the IUAPC classification, they are of type I.
3.4.2. Micropore volume The micropore volumes Wo for EP1 and EP2 activated carbons were obtained from DA equation and are recorded in Table 2. It is seen that the burn-off increase induces a gradual development of the micropore volume Wo . However, Wo values are very different, depending on the precursor used for activated carbon production. Reducing the g-resins content from 12 wt.% down to 5 wt.% leads to a great improvement of adsorption properties. Whatever the burn-off level, micropore volume for EP2 is more than twice that for EP1. 3.4.3. Pore size distributions The pore size distributions (PSD) of each activated carbons, obtained through the DS equation, are plotted in Fig. 4 and the corresponding parameters Lo and s are recorded in Table 2. It is seen that all samples are microporous since the mean pore sizes Lo never exceed 1.4 nm. As usually observed during gasification, the increase of burn-off induces a gradual pore size widening expressed through an Lo increase from 0.7 nm up to, respectively, 1.26 and 1.4 nm for EP1 and EP2. However, PSD and the corresponding Lo values (Table 2) show that mean pore size, at the same burn-off value, is approximately equivalent for both series, except during the last stage of activation process (burn-off.60%) for which pores collapsing in EP2 (with respect to Wo and n values recorded in Table 2) certainly occurs. As already described [25,26], the activated carbons
Table 2 Microporous characteristics determined by carbon dioxide adsorption
Fig. 3. EP1 (s) and EP2 (h) carbon dioxide isotherms at 253 K for various burn-offs.
Sample burn-off
Wo (cm 3 g 21 )
Eo (kJ mol 21 )
n
Lo (nm)
s (nm)
EP1-12% EP2-13% EP1-37% EP2-37% EP1-61% EP2-60%
0.07 0.20 0.13 0.33 0.20 0.46
26.3 27.3 24.5 22.4 19.6 18.2
1.95 1.8 1.8 1.6 1.7 1.4
0.7 0.7 0.83 0.9 1.26 1.4
0.05 0.22 0.18 0.27 0.30 0.40
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4. Discussion
Fig. 4. Pore size distributions for EP1 ( ]]]) and EP2 (---) activated carbons.
homogeneity is lowered during the activation process, and the dispersion s calculated from the DS equation goes from 0.05 up to 0.4. However, the comparison of s values for EP1 and EP2 activated carbons (Table 2) demonstrates the strong influence of the g-resins removal on the microporous network. In the case of low burn-off (13%), carbon dioxide activation is known to develop homogeneous micropore network. Even if EP1-13% and EP2-13% activated carbons exhibit the same mean pore size value (Lo 50.7 nm), dispersion for EP2 appears to be four times higher than for EP1. The observation of low micropore size values with simultaneous high dispersion (s 50.22) which is generally observed in the case of high burn-off level, is certainly due to structural change in EP2 connected with g-resins extraction. As characterization by carbon dioxide adsorption does not explain such an effect, further studies based on immersion calorimetry [27] with molecular probes will be performed in order to quantify those possible structural changes.
Large amounts of toluene used during the extraction process allow the elaboration of pitches mainly composed of b-resins, which contain less than 12 wt.% of residual g-resins. Such extracted pitches lead, after carbonization and carbon dioxide activation, to activated carbons with well-developed microporosity. However, microporous characteristics of those activated carbons appear to be strongly influenced by the extraction process with toluene: the use of lower g-resins content precursors results in reduced required gasification time for activated carbons preparation. Simultaneously, greater microporous volumes are obtained without any significant change of mean pore size. According to Follin et al. [6], all those modifications of the porous characteristics will result in an improvement of the refrigerating machines performances. In fact, the authors have quantified the influence of all Wo , Lo and n variations on the refrigerating machines performances through the calculation of a coefficient of refrigerating performance (COP). The COP is defined as the ratio of the quantity of heat drawn from the environment (i.e. the cold production) to the quantity of heat that must be supplied to the system for its regeneration. As an increase of micropore volume always leads to an improvement of the COP because of the increase of the mass of refrigerant gas exchanged during cold production, performances of EP1 and EP2 activated carbons will be discussed in the case of 60% burn-off activation for which the Wo value is maximized. COP values in the case of refrigeration at 273 and 248 K are gathered in Table 3 for EP1-60%, EP2-60% and four commercial activated carbons. Those commercial adsorbents were chosen with respect to their micropore volume which is of the same order of magnitude as EP2-60% (Wo ¯0.4 cm 3 g 21 ). In both cases of cold production at 273 and at 248 K, COP for EP1-60% is much smaller than for EP2-60%. This difference is mainly due to their micropore volume values which are much higher when the extraction process is performed up to 5 wt.% of residual g-resins. Even if in the case of cold production at very low temperature (248 K) in which COP values are much more sensitive to pore size and dispersion values, the higher homogeneity (n5
Table 3 Characteristics of various activated carbons and corresponding performances estimated for an adsorption refrigerating cycle
EP1-60% EP2-60% AC35 [6] NORIT RB [6] BPL [6] TA60 [6]
Wo (cm 3 g 21 )
Lo (nm)
n
COP T cold 5273 K
COP T cold 5248 K
0.20 0.46 0.42 0.41 0.42 0.47
1.24 1.40 1.71 1.33 1.25 1.09
1.7 1.4 2.15 2.0 1.5 1.7
0.29 0.38 0.42 0.4 0.37 0.39
0.16 0.23 0.26 0.26 0.24 0.26
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1.7, s 50.3 nm) for EP1-60% does not compensate the micropore volume difference between both samples. Therefore, the toluene extraction process investigated in this study must be extensively performed in order to decrease as much as possible the residual g-resins content of activated carbon precursors. This g-resins removal leads to an improvement of adsorption properties, mainly a Wo development, which is responsible for increasing COP values. Even if this Wo increase induces a loss of homogeneity in the microporous network, the performances of refrigerating machines using activated carbons from extensively extracted precursors are improved. As shown in Table 3, COP values for EP2-60% are, for both investigated cold temperatures, comparable to those calculated for several commercial activated carbons [6]. Despite extraction process improves the adsorbent performances for cold production applications, the COP would be greatly increased if a simultaneous control of micropore volume and pore size dispersion could be performed. Further studies will be focused on the control of the micropore development through activation process optimization. The study of the different steps of the elaboration process such as air stabilization or gasification using other activating agents than carbon dioxide could be helpful for the elaboration of carbon adsorbents appropriate for adsorption refrigerating machines.
5. Conclusion Toluene fractionation of heat-treated pitches appears to be a useful way for elaborating activated carbons with various microporous properties. As a matter of fact, the monitoring of pitch composition through extraction process leads to a control of adsorption properties after activation: an extensive g-resins removal induces increasing microporous volumes. Refrigerating machines performances estimated by Follin et al. [6], linked to the present results, show that such fractionation increases the COP for cycling adsorption machines and leads to performances similar to several commercial activated carbons. The numerous parameters (stabilization conditions; activation agent, activation temperature...) which can be optimized during the elaboration process presented in this study open large possibilities for a best control of microporous properties development.
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