Development of the Pore-Size Distribution in Activated Carbon

Ind. Eng. Chem. Res. 2005, 44, 51-60

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Development of the Pore-Size Distribution in Activated Carbon Produced from Coconut Shell Char in a Fluidized-Bed Reactor P. M. Satya Sai Centralized Waste Management Facility, Bhabha Atomic Research Centre, Kalpakkam 603 102, India

K. Krishnaiah* Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

The adsorption capacity of an adsorbent depends on its surface area, pore size, pore-size distribution, and pore volume. The activated carbon produced in a fluidized-bed reactor using coconut shells1 was characterized in terms of the pore-size distribution, micropore volume, external surface area, and average pore size. The effects of various parameters, viz., reaction time, fluidizing velocity, particle size, and temperature of activation, using steam and CO2 as activating gases on the above-mentioned characteristics of activated carbon were evaluated. The reaction time, temperature, and fluidizing conditions showed significant effects on the poresize distribution, pore volume, and average pore diameter. Activating gases also had considerable effects on the characteristics of activated carbon produced in a fluidized-bed reactor. Introduction Adsorption takes place at the interphase boundary; therefore, the surface area of the adsorbent is an important factor in the adsorption process. Because the most important characteristic of a sorbent is its high porosity, physical characterization is more important than chemical characterization. Generally, the higher the surface area of the adsorbent, the higher is its adsorption capacity for all compounds. However, the surface area has to be available in a particular pore size within the adsorbent. At low partial pressure (concentration), the surface area in the smallest pores in which the adsorbate can enter is the most efficient. At higher pressures, the larger pores become more important, while at very high concentrations, capillary condensation takes place within the pores and the total macropore volume is the limiting factor. Thus, the most valuable information concerning the adsorption capacity of a certain adsorbent is its surface area and pore-volume distribution in different diameter pores. Pore-size distribution in activated carbon depends on various factors, which include the raw material, type of activation, chemical agents/activating gases employed, temperature and time of reaction, and other operating parameters. In addition, the type of reactor employed also influences the pore-size development.2 The type of application determines the desired pore-size distribution of the adsorbent. Commercial activated carbons are designated for either the gas or liquid phase, depending on its application.3 A majority of the pore volume is from pores near or larger than 30 Å in diameter for liquid-phase carbons, whereas the pores of gas-phase carbons are mostly in the range from 10 to 25 Å in diameter. The need for larger pores in liquidphase carbons is due to the large size of many dissolved adsorbates and the slower diffusion in the liquid than in gas for molecules of the same size. Thus, liquid-phase * To whom correspondence should be addressed. Tel.: 91044-2257 8211. Fax: 91-044-2257 0509. E-mail: krishnak@ iitm.ac.in.

carbons generally have about the same surface areas as gas-adsorbing carbons but have larger total pore volumes. A polymodal pore-size distribution is generally found in activated carbon. The pore structure may be pictured as having many small pores branching off from larger ones, which are open through the entire particle. The larger pores are called feeder or transport pores; the smaller ones, which may be dead-end pores, are called adsorption pores.3 Activation of carbonaceous materials apparently is only possible when an accessible initial porosity occurs. Pore-volume increase may proceed by either drilling or deepening of the existing pores. The extent of both mechanisms is determined by the type of feedstock and the process conditions.2 Not much published work is available on the effects of operating parameters on the development of the poresize distribution in activated carbon produced in a fluidized-bed reactor from coconut shell charcoal.1 Hashimoto et al.4 activated Miike coal (MC), Victoria coal (VC), and coconut shell (CS) char of 30-g batches in a fluidized bed of 36-mm inside diameter using steam. The particle size was 1.1 mm, the temperature was 850 °C, and the flow rates were such that aggregative fluidization was observed. For the above particle size and temperature and for all the three carbons, the total pore volume as a function of burnoff and pore-size distribution for burnoff of 0.476 was presented. Significant differences were found among the pore-size distribution of the products obtained from these three raw materials, establishing the importance of raw material in the production of activated carbon of a desired pore-size distribution. MC activated by steam showed a typical bimodal distribution in the micro- and macropore ranges. MC activated by CO2 had little micropore volume and a large amount of macropore volume, VC showed a gently sloped distribution except in the range of small pore radius, and CS had much micropore volume and little macropore volume. The micropore volumes passed through a maximum with an increase in burnoff. CS

10.1021/ie0400090 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/02/2004

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showed the highest micropore volume among the three materials studied, which was obtained at relatively low burnoffs. Satya Sai et al.1 carried out experimental work for the production of activated carbon in a fluidized-bed reactor from coconut shell char using steam and CO2 as reacting fluidizing media. The effects of various parameters such as the reaction time, fluidizing velocity, particle size, static bed height (charge), temperature, and fluidizing gas on the development of the surface area were studied. In addition to the surface area, the pore-size distribution is the second most important parameter, which determines the choice of the adsorbent for any given application. The objective of the present paper is to study the effects of various operating parameters such as the reaction time, fluidizing velocity, particle size, temperature of activation, and activating gas on (i) pore-size distribution, (ii) micropore volume and external surface area, and (iii) average pore size of activated carbon produced in a fluidized-bed reactor. In this work, only micro- and mesopore volumes are determined. Experimental Determination of the Pore-Size Distribution The details of the experimental setup and experimental procedures are given elsewhere.1 The samples collected during the experiments for various process parameters and particle characteristics are analyzed to evaluate the pore-size distribution, micropore volume, and average pore size. Pores of widths/diameters below about 20 Å are classified as the micropores, above about 500 Å as the macropores, and between 20 and 500 Å as the mesopores.5 Micropores are mainly responsible for adsorption, with macro- and mesopores acting as channels and conduits for the passage of the adsorbate. Thus, an adsorbent should have a well-balanced proportion of all three types of pores, with the proportion being dependent on the desired application. The BrunauerEmmett-Teller surface area, which is essentially an estimate of the micropores, is determined by nitrogen adsorption. The pore-size distribution in the range of 20-150 Å (mesopores) is determined by nitrogen adsorption and by using the Kelvin equation.6 Micropore volume and external surface area including the surface area contributed by mesopores are determined from RS plots obtained from the pore-size distribution in the range of 20-150 Å.5 The total pore volume is usually determined by helium and mercury densities or displacements. Helium, because of its small atomic size and negligible adsorption, gives the total voids, whereas mercury does not penetrate into the pores at ambient pressure and gives interparticle voids. The total pore volume equals the difference between the two voids as determined above. The pore-size distribution is measured by mercury porosimetry for pores larger than 150 Å and by nitrogen adsorption (or desorption) for pores in the range of 10-250 Å3. In the present work, the pore-size distribution in the range of 20-150 Å is determined by nitrogen adsorption and desorption using a Quantasorb Jr. sorption system (Quantachrome Corp., Boynton Beach, FL). Results and Discussion The effect of various operating parameters on the pore-size distribution, micropore volume, and external surface area of the product is discussed below.

(1) Effect of the Operating Parameters on the Development of the Pore-Size Distribution. The pore-size distribution of selected samples is carried out to study the effects of the operating parameters on the development of pores. The pore-size distribution of the raw material is shown in Figure 1. It can be seen that pores are distributed around 32.66 and 86.52 Å, showing a bimodal distribution in this range, though the volumes associated are small. These pores are mostly responsible for the surface area (43 m2/g) of the raw material. Effect of the Reaction Time. Figures 2 and 3 show the effect of the reaction time on the pore-size distribution of the product for two different temperatures of 800 and 850 °C. The pore-size distribution at lower temperatures is not presented because the development of pores is not significant below 800 °C. It can be seen from Figure 2 that, at 800 °C, development of mesopores has taken place at pore sizes of around 32.66 and 86.52 Å for a reaction time of 0.5 h. At a reaction time of 1.0 h, the pores at 32.66 Å have grown considerably as compared to those at 0.5 h, while there is not much change in the pores at 86.52 Å. At 1.5 h, pore growth continues around 32.66 Å without much change at 86.52 Å. At 2.0 h, the pores of around 32.66 Å decrease while pores at 86.52 Å grow. With a further increase in the reaction time to 2.5 h, there is a further decrease in the pores at 32.66 Å and growth in the pores at 86.52 Å. At 850 °C, pore development is seen around 32.66 and 86.52 Å for a reaction time of 0.5 h (Figure 3). With an increase in the reaction time to 1.0 h, pores of around 32.66 and 86.52 Å grow. At 1.5 h, pores of around 32.66 Å slightly decrease, while pores at 86.52 Å increase to a large extent. For a reaction time of 2.0 h, pores of around 32.66 Å nearly disappear and pores of 86.52 Å completely disappear, while there is development of pores beyond 152.81 Å, as indicated by the sharp increase. After 1.5 h of activation, it can be seen that there is a trend in the formation of micropores (less than 20 Å) as indicated by the steep rise of the curve toward the micropore range (Figure 3). The highest iodine number (surface area), observed by Satya Sai et al.1 in their fluidized-bed experiments, was obtained for this sample. It can be seen from Figure 3 that this sample has the right combination of pores in the meso- and micropore range to give a high surface area. These pore-size distribution data, as explained above, establish that, with progress of the reaction time, the pores either grow or coalesce. Similar observations were reported by earlier studies.4 Comparing Figures 2 and 3, it can be clearly seen that there is a significant increase (more than double) in the pore volumes when the temperature is increased from 800 to 850 °C, indicating the pronounced effect of the temperature on the development of the pore-size distribution. The effect of the temperature is discussed in detail in a subsequent section. Effect of the Fluidizing Velocity. Figure 4 shows the effect of the fluidizing velocity on the pore-size distribution. At u/umf of 1, pore development is small at around 32.66 Å. When u/umf is increased to 2.5, it can be seen that there is no significant improvement in the pore development. For u/umf of 4, pores of around 32.66 and 86.52 Å are well developed and the micropores (below 20 Å) are also formed. This illustrates that slightly higher fluidizing velocities enhance pore devel-

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 53

Figure 1. Pore-size distribution of raw material.

Figure 2. Effect of time on the pore-size distribution.

opment because of better gas-solid contact and effective transport of heat and mass in and around the particle. Further, at these velocities, the rate of gas-solid reaction is enhanced because of better heat- and masstransfer processes, resulting in the formation of large quantities of product gases, which surround the particle.

These gases are effectively scavenged at these fluidization velocities, exposing a fresh particle surface for further reaction and thus leading to better pore development, as explained by Satya Sai et al.1 Effect of the Particle Size. The effect of the particle size on pore development is shown in Figure 5. For a

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Figure 3. Effect of time on the pore-size distribution.

Figure 4. Effect of the fluidizing velocity on the pore-size distribution.

particle size of 0.55 mm, pores are developed around 21.82 and 86.52 Å. For a particle size of 1.1 mm, enhanced pore development is observed around 32.66 and 86.52 Å. For a particle size of 1.55 mm, pores of around 32.66, 86.52, and also less than 20 Å are well developed. The pore-size distribution for the other particle sizes, viz., 0.55 and 1.1 mm, also shows the presence of micropores as indicated by the left-hand side

of the curves. The reasons for better pore development in the case of bigger particles are explained by Satya Sai et al.,1 indicating that the small particles burn out easily, without providing a pore structure. Also, large particles (>5 mm) do not develop pore structure because of the slow rate of reaction inside the particle. Particles in the intermediate size range provide a good matrix for a well-developed pore structure.


Figure 5. Effect of the particle size on the pore-size distribution.

Figure 6. Effect of the temperature on the pore-size distribution.

Effect of the Temperature. Figure 6 shows the effect of an increase in the reaction temperature from 550 to 850 °C on pore development for a reaction time of 1.5 h. At 550 °C, there is formation of pores at 19.51 and 32.66 Å, indicating very low activation. At 700 °C, pore development is at 21.82, 32.66, and 86.52 Å but only to a small extent. At 800 °C, pore development is quite prominent compared to 700 °C. At 850 °C, pores are further developed with prominent micropore formation, as seen by the left part of the curve. This shows

that pore formation and growth increase with the temperature of activation, resulting in a product of higher surface area and well-developed pore structure. Under the present experimental conditions with steam activation, it is observed that significant pore formation cannot be obtained below 800 °C. Pore drilling, which leads to a steady increase in the pore diameter, is the predominant mechanism at lower temperatures, whereas pore deepening, which hardly effects the pore diameter, is favored at high temperatures.2

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Figure 7. Effect of the activating gas on the pore-size distribution.

Figure 8. Effect of time on the micropore volume and external surface area.

Effect of the Activating Gas. Figure 7 shows the variation in the pore-size distribution when two different activating gases, viz., steam and CO2, are used for activation. When steam is used as an activating gas, pores are developed below 20 Å, around 32.66 Å to a large extent, and around 61.21 Å to a small extent. With CO2 as an activating gas, pores are developed around 38.77 and 86.52 Å to a lesser extent, while there is no significant development in micropores. Activation with CO2 promotes surface oxidation and hence the development of larger pores compared to activation with steam.7 This can be construed as due to the phenomenon of

molecular drilling. Steam, a lighter molecule compared to CO2, helps in the development of smaller pores. (2) Effect of the Operating Parameters on the Development of the Micropore Volume and External Surface Area. The surface of any piece of porous or nonporous solid contains cracks and fissures. Some of these penetrate very deeply, and these contribute to the internal surface. The superficial cracks and indentations on the outside surface contribute to the external surface. The external surface is taken to include all of the prominences and all of the cracks that are wider than they are deep. The internal surface


Figure 9. Effect of time on the micropore volume and external surface area.

Figure 10. Effect of the fluidizing velocity on the micropore volume and external surface area.

comprises the walls of all cracks, pores, and cavities that are deeper than they are wide.5 A wide range of porous solids have an internal surface greater by several orders of magnitude than the external surface, with the total surface of the solid thus being predominantly internal. Further, the term internal surface is usually restricted to those cavities that have an opening to the exterior of the grains; i.e., it does not include the walls of the sealed pores. The micropore volume and external surface area along with the mesopore area can be determined from the

pore-size distribution using the RS method.5 RS is the ratio of the volume of gas adsorbed at any relative pressure to the volume of gas adsorbed at a relative pressure of 0.4. RS obtained from the isotherm of a reference sample is used to construct RS plots, which are plots of RS against the volume of gas adsorbed. From the intercept and slope of the linear branch of the RS plots, the micropore volume and external surface area along with the mesopore area can be estimated. Mesoporosity, in which capillary condensation (hysteresis) occurs, shows an upward deviation in the RS plot after

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Figure 11. Effect of the particle size on the micropore volume and external surface area.

Figure 12. Effect of the temperature on the micropore volume and external surface area.

completion of micropore filling. Micropores cause enhanced adsorption in the low relative pressure region, and this is reflected as a downward distortion.8 The reference standard chosen to determine the RS values in this work is nonporous graphite and is chemically similar to activated carbon.5 Using the above procedure, the RS plots are constructed for all samples, and from the slope and intercept of the straight-line portion, the external surface area (including the meso-

pore area) and micropore volume are calculated. For example, the micropore volume and external surface area of the raw material are 0.001 895 cm3/g and 9.35 m2/g, respectively. Effect of the Reaction Time. The micropore volume and external surface area for varying reaction times at a temperature of 800 °C are presented in Figure 8. The micropore volume increases up to a reaction time of 1.5 h and decreases for higher reaction times. The decrease


Figure 13. (a) Effect of the reaction time on the average pore radius. (b) Effect of the temperature on the average pore radius.

in the micropore volume at higher reaction times is due to pore coalescence. The external surface area (including the mesopore area) increases with the reaction time, confirming pore coalescence, which results in the formation of more mesopores. It can be seen from Figure 8 that the external surface area increases steeply after 1.5 h, indicating the growth in mesopores at the cost of micropores. Figure 9 shows the effect of time at 850 °C. In contrast to 800 °C, at this temperature both the micropore volume and external surface area increase up to a time of 1.5 h and decrease further. This shows that the pore coalescence is faster at this temperature, with pores growing beyond 150 Å, as shown in Figure 3. As was already mentioned, Hashimoto et al.4 also observed that the micropore volume passed through a maximum with burnoff in their experiments. Because burnoff is equivalent to the reaction time, the observation made in the present work is similar to that of

Hashimoto et al.4 The pore volume created during activation invariably increases with increasing burnoff.2 However, an optimum in the surface area or micropore volume is observed. This means that pore enlargement, and therefore a shift from micro- to mesoporosity or even macroporosity, occurs. Wigmans2 observed that at all temperatures a maximum was observed in the micropore volume and this maximum was higher for higher temperatures. The mesopore development was observed to exhibit a steady rise that is more pronounced at lower activation temperatures. Effect of the Fluidizing Velocity. The micropore volume and external surface area for different fluidizing velocities are shown in Figure 10, which shows that the micropore volume and external surface area increase with u/umf. This shows the significant effect of fluidization velocities on the pore development in micro- and mesopore ranges, the effect of which was reported by Satya Sai et al.1 in terms of iodine numbers.

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Effect of the Particle Size. Figure 11 shows the micropore volume and external surface area for a change in the particle size. It can be seen that both the micropore volume and external surface area increase with the particle size. As observed by Satya Sai et al.,1 in the case of iodine numbers, bigger particles provide a better matrix for pore development. Effect of the Temperature. The change in the micropore volume and external surface area with a change in the temperature of the reaction is shown in Figure 12. It can be seen that both the micropore volume and external surface area increase steadily from 550 to 800 °C and steeply from 800 to 850 °C. Wigmans2 also observed that high temperature enhanced the microporosity for steam activation of peat in the range of 860-1040 °C. Effect of the Activating Gas. The micropore volume and external surface area for two reacting gases, steam and CO2, are estimated using RS plots for t ) 1.5 h, u/umf ) 2.5, dp ) 1.55 mm, H0/D ) 1, and T ) 850 °C. Both the micropore volume and external surface area are higher for steam compared to CO2. The micropore volumes are 0.366 and 0.106 cm3/g and the external surface areas are 159.035 and 48.147 m2/g for steam and CO2, respectively. (3) Effect of the Operating Parameters on the Average Pore Radius. The average pore radius in the range of 15.806-152.81 Å is calculated from the poresize distribution data. The effect of various operating parameters on the average pore radius is discussed below. Effect of the Reaction Time. The effect of the reaction time on the average pore radius is shown in Figure 13a. The average pore radius decreases with reaction time up to 1.0 h and increases subsequently. This may be due to the formation of new pores in the range of 20-40 Å up to a reaction time of 1.0 h, as seen in Figures 2 and 3. For higher reaction times, pores in the range of 20-40 Å coalesce, resulting in an increase in the average pore size. Effect of the Temperature. The effect of the temperature on the average pore radius is shown in Figure 13b. The average pore radius increases from 550 to 700 °C. This is due to the pore development in the range of 60-150 Å. For 800 °C, pore development is more pronounced in the range of 30-50 Å, as a result of the formation of new small pores, and hence the average pore radius decreases. At 850 °C, the pore size increases because of coalescence of smaller pores. This phenomenon can be observed in Figure 6. It is observed that the average pore radius increased with an increase in the fluidization velocity and also with an increase in the particle size for the raw material used. This is in conformity with the observations made with respect to the effect of these parameters on pore development. Conclusions The operating parameters of a fluidized-bed reactor have a considerable effect on the pore size and pore-

size distribution of the product. The most significant effect is that of reaction time and temperature. Formation of micropores is more at 1.5 h of reaction time and large pores beyond 150 Å developed for a reaction time of 2 h. A pronounced effect of temperature is noticed between 800 and 850 °C, in which the pore volume more than doubled with the corresponding pore-size distribution. High fluidizing velocities (u/umf ) 4) and large particles are required for the formation of meso- and micropores. The activating gas employed also exhibits a pronounced effect on the pore-size distribution. Steam promotes smaller pore size compared to CO2. This is an important observation because tailor-made carbons can be produced by properly choosing the activating gases. The above conclusions based on the experimental observations confirm the significant effect of the operating parameters on the development of the surface area presented by Satya Sai et al.1 Nomenclature D ) diameter of the bed, m dp ) average particle size, mm H0 ) static bed height, m P ) partial pressure of the adsorbate, mmHg P0 ) saturated vapor pressure of the adsorbate, mmHg rp ) pore radius, Å t ) reaction time, h T ) temperature of the bed, °C u ) fluidizing velocity, m/s umf ) minimum fluidization velocity, m/s Vgas ) volume of gas adsorbed (STP) at a given relative pressure, cm3/g Vp ) pore volume in the given pore range, cm3/g RS ) ratio of the volume of gas adsorbed at a given relative pressure to the volume of gas adsorbed at a relative pressure of 0.4

Literature Cited (1) Satya Sai, P. M.; Ahmed, J.; Krishnaiah, K. Production of Activated Carbon from Coconut Shell Char in a Fluidized Bed Reactor. Ind. Eng. Chem. Res. 1997, 36, 3625. (2) Wigmans, T. Industrial Aspects of Production and Use of Activated Charcoal. Carbon 27, 13, 1989. (3) Yang, R. T. Gas Separation by Adsorption Processes; Series in chemical engineering; Butterworths: Sydney, Australia, 1987. (4) Hashimoto, K.; Miura, K.; Yoshikawa, F.; Imai, I. Change in Pore Structure of Carbonaceous Materials during Activation and Adsorption Performance of Activated Carbon. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 72. (5) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (6) Defay, R.; Prigoine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Longmans: London, 1966. (7) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (8) Byrne, J. F.; Marsh, H. Introductory Overview. In Porosity in Carbons; Patrick, J. W., Ed.; Edward Arnold: London, 1995.

Received for review January 2, 2004 Revised manuscript received July 20, 2004 Accepted September 2, 2004 IE0400090

Development of the Pore-Size Distribution in Activated Carbon

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