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Energy & Fuels 2008, 22, 3420–3423
Pilot Preparation of Activated Carbon for Natural Gas Storage X. D. Dai, X. M. Liu, L. Qian, K. Qiao, and Z. F. Yan* State Key Laboratory of HeaVy Oil Processing, China National Petroleum Corporation (CNPC) Key Laboratory, China UniVersity of Petroleum, Dongying 257061, Shandong, China ReceiVed May 5, 2008. ReVised Manuscript ReceiVed July 2, 2008
Pilot preparation was carried out for activated carbon from petroleum coke/KOH chemical activation, to improve the methane storage capacity. The effects of activation temperature in the range from 973 to 1073 K and particle size from 0.10 to 0.45 mm were studied by nitrogen adsorption isotherms and scan electronic microscopy. It was found the activation temperature significantly influenced the porous structure of activated carbon, with the operation range from 1003 to 1043 K. Also the surface area and pore volume of activated carbon could be enhanced by the smaller particle size of petroleum coke, because of the relatively larger particle surface. Through the methane storage analysis, it was observed that higher surface area and pore volume were beneficial to higher adsorption capacity, while the lower methane delivery was attributed to more developed porosity, implying that desorption capacity was also closely related to the porous morphology.
1. Introduction Because of high surface area, excellent porous structures, and special surface properties, activated carbon is thought to be one of versatile adsorbents and widely used in purification, decolorization, deodorization, and other adsorption processes. With the increased research on activated carbon, its applications were extended to new fields, such as catalyst, catalyst support, and adsorbent for gas storage.1 The pores of activated carbon distribute from micro-, meso-, to macropores and play specific roles in adsorption. Although activated carbons are amorphous materials with a broad sense, they, in fact, are consist of graphite-like microcrystallines with disordered stacks.1,2 Hence, lots of edge-carbon atoms with unsaturated valences are presented and interact with heteroatoms, such as O, N, H, and others, to form different surface properties, which would also influence the adsorption.3 Generally, the properties of activated carbons depend upon the raw materials, the activation agents, and the activation process. Most of carbonaceous materials can be used as raw materials for activated carbons through two methods: physical and chemical activations. In the activation process, the disorganized carbons are also removed, exposing the crystallites to the activation agents and leading to the development of porous structures.4 Commonly, physical activation is the major technology for commercial production in China. However, activated carbons by the physical process are featured with low surface area, developed pore structures, which are not suitable for natural gas adsorption. The chemical process using KOH, NaOH, and ZnCl2 as activation agents could successfully synthesize acti* To whom correspondence should be addressed. Fax: 86-546-8391527. E-mail:
[email protected]. (1) Bansal, R. C.; Goyal, M. ActiVated Carbon Adsorption; CRC Press: New York, 2005; pp 243-469. (2) Shen, Z. M.; Zhang, W. H.; Zhang, X. J. Huoxingtan Cailiao de Zhibei yu Yingyong; Chemical Industry Press: Beijing, China, 2006; pp 23. (3) Zhao, Zh. G. Xifu Zuoyong Yingyong Yuanli; Chemical Industry Press: Beijing, China, 2005; pp 354-364. (4) Lozano-Castelloa, D.; Alcaniz-Mongea, J.; de la Casa-Lillob, M. A.; Cazorla-Amorosa, D.; Linares-Solanoa, A. Fuel 2001, 81 (14), 1777–1803.
vated carbons with high surface area and developed pore structures.5-7 It is a very efficient technique for natural gas storage, but there are fewer companies in the world that can provide activated carbons for natural gas adsorption. With the interest in alternative fuels in recently years, adsorption natural gas (ANG) presents a promising future for natural gas storage. In this application, natural gas is stored as an adsorbed phase in porous materials at relatively low pressure, with high energy density. Among kinds of porous materials, activated carbon is considered one of the best, with several advantages. In many of the literature,4,8-10 lots of efforts have being carried out on the optimization to surface area, porous structures, and surface chemistry of activated carbon. It also indicated that KOH chemical activation was a very effective method for activated carbon with controlled pore size distribution and developed pore structures.11-13 Until now, however, there are few reports on pilot-preparation of activated carbon with special focus on natural gas storage. Therefore, in the present study, continuous pilot-scale preparation is first carried out using a self-designed rotary oven with KOH/petroleum coke activation for natural gas adsorption. Investigations are focused on examining how activation conditions influence the structure of activated carbon with a pilot scale. Simultaneously, the (5) Sun, J.; Rood, M. J.; Rostam-Abadi, M.; Lizzio, A. A. Gas Sep. Purif. 1996, 10 (2), 91–96. (6) Lu, Ch. L.; Xu, Sh. P.; Gan, Y. X.; Liu, Sh. Q.; Liu, Ch. H. Carbon 2005, 43 (11), 2295–2301. (7) Marsh, H.; Yan, S. D.; O’Grady, T. M.; Wennerberg, A. Carbon 1984, 22 (6), 603–611. (8) Rodrı´guez-Reinoso, F.; Molina-Sabio, M. Carbon 1992, 30 (7), 1111–1118. (9) Menon, V. C.; Komarneni, S. J. Porous Mater. 1998, 5 (1), 43–58. (10) Biloe, S.; Goetz, V.; Mauran, S. Carbon 2001, 39 (11), 1653–1662. (11) Illan-Gomez, M. J.; Garcia-Garcia, A.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Energy Fuels 1996, 10 (5), 1108–1114. (12) Sun, J.; Brady, T. A.; Rood, M. J.; Lehmann, C. M. Energy Fuels 1997, 11 (2), 316–322. (13) Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A. Energy Fuels 2002, 16 (5), 1321–1328.
10.1021/ef800313f CCC: $40.75 2008 American Chemical Society Published on Web 08/06/2008
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Figure 1. Scheme of the gravimetric methane adsorption apparatus.
methane storage properties are carried out to investigate the relationship between adsorbate and adsorbent. 2. Experimental Section 2.1. Preparation. According the former studies,14,15 the petroleum coke (Shengli Refinery, China) with granularity of 0.10-0.45 mm, was used as raw material for the preparation. Potassium hydroxide power (Purity > 90%) without sieving was used as an activation agent (UNID Chemical Industries, Ltd., Korea). The whole activation process was carried out using a self-designed rotary reactor, with a yield of 10 kg/day. First, petroleum coke with different granularity and KOH power were well-mixed, with weight ratio of 3:1. The heating rate was set at 20 K/min. When the temperature reached the desired level, the mixture was feed in the reactor for 1.5 h activation under a N2 atmosphere. After activation, the activated carbon was washed thoroughly with water, filtered, and dried for 24 h. 2.2. Characterization. The nitrogen adsorption/desorption isotherms at 77 K of the samples were obtained using a gas sorption analyzer (Micrometrics 2010, USA). Samples were degassed for 4 h at 673 K prior to analysis. The total surface area was calculated by using the Brunauer-Emmet-Teller (BET) equation. The Dubinin-Radushkevich (DR) equation was used to calculate the micropore volume and distribution. The mesopore was determined by the Barret-Joyner-Halendar (BJH) equation. The morphology of the activated carbon surface was investigated with scan electronic microscopy (SEM) technology. After coated with a gold alloy layer for samples, SEM images were observed with a FEI Sirion 200 scanning electron microscope (FEI, USA) operated at 10 kV. 2.3. Methane Adsorption. The methane adsorption measurements were conducted on the self-design gravimetric difference apparatus, as shown in Figure 1. The apparatus was N2-leak-tested to 10 MPa and had a maximum operating pressure of 8 MPa. Dried samples were poured into a steel vessel with a volume of 81.7 cm3 and degassed at room temperature for 2 h without weight loss. The vessel was immersed into a thermostatic bath kept at 293 K in the entire adsorption procedure. With a constant methane (99.9%) flow, the system pressure grew gradually to 3.5 MPa and lasted for another 10 min, allowing systematic equilibrium. The weight of adsorbed methane was obtained by reducing the compressed one in vacancy. Also, the delivery was measured through weight loss, with delivery being the amount of methane released from the vessel until the pressure reduced to atmospheric pressure. Simultaneously, methane adsorption isotherms were measured from 0.5 to 6.0 MPa at the interval of 0.5 MPa with similar procedures.
3. Results and Discussion 3.1. Effect of the Activation Temperature. In the KOH chemical activation process, the variables such as the temperature, the ratio of KOH/coke, and the granularity of raw (14) Xing, W.; Zhang, M. J.; Yan, Z. F. Acta Phys. Chim. Sin. 2002, 18, 340–345. (15) Chen, J. F; Li, X. C.; Li, Sh. Y. J. Fuel Chem. Technol. 2004, 32 (1), 57–61.
Figure 2. N2 isotherms of activated carbons at various activation temperatures.
materials have great influences on the porosity of activated carbon, especially for laboratory preparation.16-18 In the pilot study, the influence of the activation temperature was investigated first. Activated carbons were prepared with a KOH/petroleum coke ratio of 3, with an activation time of 1.5 h, in the temperature range of 973-1073 K. Those activated carbons were denoted as AC-973K, AC-1023K, AC-1043K, and AC-1073K, according their activation temperature. As shown in Figure 2, all activated carbons presented type-I adsorption isotherms, indicating microporous materials. The amount of nitrogen adsorbed presented a sharp increase at low relative pressure, attributing to micropore filling. Then, a plateau after relative pressure reaching about 0.2 could be ascribed to the adsorption of the larger pores. Because the higher adsorbed volume at low relative pressure meant that more micropores were presented, It can be concluded that sample AC-1003K possessed the most developed microporosity and the feasible activation temperature should range from 1003 to 1043 K in our pilot preparation. For a better understanding of the difference in porosity, Table 1 listed the pore structures at different activation temperatures. It was obvious that the activated carbons with a surface area more than 1200 m2/g had been successfully synthesized. The surface area and pore volume both increased rapidly with activation temperature until 1003 K and then decreased slowly up to 1073 K. At low temperature, it was a predominant reaction to form micorporosity by depth activation. With the increase of temperature, depth activation was reinforced for microporosity. Simultaneously, parts of micropores were enlarged or destroyed to the scale of mesopore by width activation.14,19 Until 1073 K, decreases in surface area and porosity were shown, attributing to overactivation. That meant that pre-existing microand mesopores were destroyed by activation. In particular, few differences between micropore and mesorpore volume were observed with activation from 1003 to 1043 K, because of the depth and width co-effects. This implied that activation in this range could be the operational temperature. As seen in SEM images (Figure 3), there were slice-layer structures and lack of pores on the surface with activation at (16) Arenas, E.; Chejne, F. Carbon 2004, 42 (12), 2451–2455. (17) Lee, S. H.; Choi, Ch. S. Fuel Process. Technol. 2000, 64 (2), 141– 153. (18) Daguerre, E.; Guillot, A.; Stoeckli, F. Carbon 2001, 39 (8), 1279– 1285. (19) Dai, X. D.; Liu, X. M.; Qian, L.; Yan, Z. F.; Zhang, J. J. Porous Mater. 2006, 13 (3), 399–405.
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Dai et al.
Table 1. Porous Structures of Activated Carbons at Various Activation Temperatures activation temperature (K)
surface area (m2/g)
micropore volume (cm3/g)
micropore diameter (nm)
mesopore area (m2/g)
mesopore volume (cm3/g)
micro/meso ratio
973 1003 1023 1043 1073
1205 2127 1915 2000 1706
0.39 0.63 0.57 0.55 0.50
1.59 1.58 1.51 1.53 1.62
328 655 681 933 756
0.22 0.41 0.44 0.60 0.47
64:36 61:39 56:44 43:57 52:48
973 K (Figure 3a). During the activation process, noncarbon elements and disorganized carbons on the surface were first eliminated and resulted in coarse surface and then depth and width activations could give rise to porous structures. At 973 K, attributing to less reactivity and fluidity of KOH, K2O, and K2CO3,20 activation was partially limited to the surface with smaller micropores and coarse surface. As shown at 1003 K (Figure 3b), through liquid-phased potassium with better reactivity, the pore structures were further developed and enhanced.11,14,15 More and larger porosity on the surface were obvious in images. While at 1073 K (Figure 3c), it can be observed that parts of formed pores on the surface were destroyed because of excessive width activation. At that time, the activation process mostly occurred in the interior of carbons and changed the inner pores greatly. The micropore size distribution of the activated carbons can be illustrated by the DR equation derived from the adsorption isotherms (Figure 4). It can be seen that each activated carbon had a similar distribution curve, with one peak around 1.55 nm. The higher peak reflected the micropore volume enhancement.
However, with the temperature up to 1023 K, activated carbons presented a slight decrease in micropore volume. The main effect of temperature on microporosity is attributed to the depth and width activations. On one hand, the smaller micropore size for activated carbon prepared at 973 K can be attributed to an insufficient activation. On the other hand, the higher temperature resulted in the less micropore volume by width activation.15 In general, these results supported the data given in Table 1, and it can be concluded that the activated carbons obtained at 1003 K or slightly higher temperatures were characterized by welldeveloped microporosity. 3.2. Effect of the Granularity of Petroleum Coke. Granular petroleum coke with a particle size of 0.10-0.45 mm was used as raw material, with a KOH/petroleum coke ratio of 3, an activation time of 1.5 h, and an activation temperature of 1003 K. These activated carbons were denoted as AC-S (0.10-0.15 mm), AC-M (0.15-0.28 mm), and AC-L (0.28-0.45 mm), according to petroleum coke particle size. Figure 5 showed the nitrogen adsorption isotherms on activated carbon from different raw material granularity. It can be seen in Figure 5 that, the smaller granularity of petroleum coke, the higher adsorption volumes at the same relative pressure were observed, indicating that the development of pores was promoted with a smaller particle size. This result was further confirmed by the porous structures data in Table 2. In Table 2, it can be observed that surface area and pore volume both increase with the decrease of the particle size, indicating that the development of pores was promoted with smaller particle size. In the activation process, the smaller particle size meant a larger active surface and easy activation to the chemical agent, and then more developed porosity was formed. However, few differences of porous structure were demonstrated for a particle size between 0.28-0.45 and 0.15-0.28 mm. In our experiments, a rotary furnace was designed to a continuous operation. The premixed KOH and petroleum coke were added to the furnace with a setting temperature and speed, and then the rotary furnace will promote
Figure 3. SEM images of activated carbons at various activation temperatures.
Figure 4. Micropore size distributions of activated carbons at various activation temperatures.
Figure 5. N2 isotherms of activated carbons with different granularity.
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Energy & Fuels, Vol. 22, No. 5, 2008 3423
Table 2. Porous Structures of Activated Carbons with Different Granularity granularity (mm)
surface area (m2/g)
micropore volume (cm3/g)
micropore diameter (nm)
mesopore area (m2/g)
mesopore volume (cm3/g)
0.10-0.15 0.15-0.28 0.28-0.45
2319 1915 1970
0.65 0.57 0.58
1.62 1.58 1.60
985 681 757
0.60 0.40 0.49
the moving of raw materials in the column furnace. Therefore, it is hard to know the mixed feature in the furnace, and it is possible for insufficient thermal effect and activation, which could be the reason. Micropore size distributions of the activated carbons synthesized at different granularity were shown in Figure 6. It can be seen that each activated carbon had a general similar distribution curve, with one peak centered at 1.60 nm. However, little decrease of the micropore size was found for larger particle sizes. Thus, the pore size distributions shown in the figure support the results given in Table 2, and it can be concluded that the activated carbons obtained with a smaller particle size were characterized by well-developed microporosity. Because of a larger active surface, there were more reaction sites on the surface, giving rise to sufficient activation to developed porosity. While the activation degree was dominated by the activation temperature through the depth and width process, the smaller particle size at the same temperature changed little on the pore size distribution. 3.3. Application to Methane Storage. Because of these unsaturated and unbalanced forces, there was a file of force in pores for adsorption. Commonly, for activated carbon-gas adsorption, physical adsorption bound to the Van der Waals forces was the main reason for gas condensation in pores. It was thought that the porous structure and pore morphology of the adsorbent were the main parameters in gas adsorption. As a good adsorbent, activated carbon should not only possess higher methane uptake but also delivery capacities, namely, the methane storage capacity. Table 3 showed the methane adsorption/desorption capacities of activated carbons at 3.5 MPa and 298 K. The carbon with the largest surface area (AC-S) showed the highest methane uptake 23.86%, while AC-973K had the lowest uptake of 13.19%. It was demonstrated that activated carbon with a high methane adsorption capacity could be produced in our research. Correlating the adsorption capacity with porous structures, it was indicated that the methane adsorption capacity of activated carbons were significantly influenced by surface area and pore volume. It was known that
Table 3. Methane Storage Capacities of Activated Carbons sample
methane uptake (mass %)
methane delivery (mass %)
AC-973K AC-1003K AC-1023K AC-1043K AC-1073K AC-S AC-M AC-L
13.39 20.99 19.63 19.51 18.26 23.86 19.63 20.52
10.68 17.18 17.17 16.17 16.35 19.92 17.17 17.92
methane adsorption on activated carbon was a spontaneous and exothermic process.21,22 For the adsorption process, a larger internal surface area and pore volume could uptake more methane molecules. On the contrary, it is observed that a much higher surface area was not favorable for obtaining a higher delivery for desorption. This result could be attributed to the smaller micropores and the complexity of pore walls.23,24 Methane desorption from porous adsorbents depended upon not only the surface area and pore volume but also characteristics of porous morphology. Activated carbons had a disorder microcrystallite layer arrangement and lots of structural defects, such as vacant lattice sites on pore walls, which were easy for methane adsorption but hard to be released. Once methane was attracted and retained in smaller micropores or on inner pore walls, it was hard and a long way for methane to become released. Therefore, a higher developed porosity would result in the decrease on methane desorption. Accordingly, further improvement in methane storage should be focused on the optimization of porosity and morphology. 4. Conclusions To realize the improvement of adsorption for activated carbon as an adsorbent for methane storage, preparation of activated carbons by KOH chemical activation was designed with pilot scales. The porous structure and methane storage performance at 3.5 MPa and 298 K were investigated. It was found that all activated carbons prepared in our pilot studies had welldeveloped porosity and a favorable micropore size for methane storage. The porosity of activated carbons was significantly influenced by activation temperature, with the favorable range of 1003-1043 K. The surface area and pore volume were affected by the depth and width activations, which generated pores with different sizes. Because of the more active surface for a smaller particle, the decrease in petroleum coke granularity could also produce an increase in surface area and pore volume. The pore size of products was largely dependent upon temperature and was not greatly affected by granularity. The methane adsorption capacity of activated carbon was significantly decided by surface area and pore volume, while desorption capacity was not only decided by porous structure but also influenced by porous morphology. The low methane delivery was closely related to the complexity of porous morphology. EF800313F
Figure 6. Pore size distributions of activated carbons with different granularity.
(20) Lillo-Rodenas, M. A.; Juan-Juan, J.; Cazorla-Amoros, D.; LinaresSolano, A. Carbon 2004, 42 (7), 1371–1375. (21) Barobosa, Mota, J. P.; Rodrigues, A. E.; Saatdjian, E.; Tondeur, D. Carbon 1997, 35, 1259–1270. (22) Yang, X. D.; Zheng, Q. R.; Gu, A. Z.; Lu, X. S. Appl. Therm. Eng. 2004, 25, 591–601. (23) Biloe, S.; Goetz, V.; Guillot, A. Carbon 2002, 40, 1295–1308. (24) Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A.; Quinn, D. F. Carbon 2002, 40, 989–1002.
