Environ. Sci. Technol. 2002, 36, 4460-4466
Kinetics and Mechanisms of H2S Adsorption by Alkaline Activated Carbon RONG YAN,* DAVID TEE LIANG, LESLIE TSEN, AND JOO HWA TAY Institute of Environmental Science and Engineering, Nanyang Technological University, Innovation Centre, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723
Activated carbon adsorption is widely used to remove hydrogen sulfide (H2S), one of the major odorous compounds, from gas streams. In this study, the mechanisms of H2S adsorption by alkaline activated carbon were systematically studied. Two brands of commercial activated carbons were used as H2S adsorbents. A series of designed experiments were carried out to understand on a fundamental basis the differences in H2S removal capacity observed for the two types of carbons and samples for the same carbon obtained from different batches. The physicochemical and structural characteristics of the original and exhausted activated carbons were identified using several analytical approaches (i.e., XRF, SEM, XRD, and BET). The relationships between the adsorption performances of activated carbon for H2S and its physicochemical characteristics were discussed. The kinetics of the H2S adsorption was also studied using TGA/DSC system. Both physical adsorption and chemisorption played an important role in the H2S adsorption mechanisms with the studied carbons. Chemisorption was rapid and occurred mostly at the carbon surface whereas physical adsorption was relatively slow and mostly took place at the inner pores of carbon. Carbon II demonstrated the best performance of H2S removal due to its high capacity of both physical adsorption and chemisorption. Catalytic effects of transition metals might also contribute to enhancing the H2S oxidation.
Introduction Hydrogen sulfide (H2S) is one of the leading malodorants arising from sewage treatment facilities (1). Activated carbons (AC), modified using various physical and chemical methods, have been widely used for removal of H2S and other odorous compounds from gas streams. Carbons impregnated with alkaline materials such as NaOH and KOH are used for the removal of H2S based on an acid-base reaction occurring at the carbon surface (2-6). In the cases where an oxidationreduction reaction is predominant, the surface of activated carbons is impregnated with active species such as, for instance, potassium iodide, transition metals, or urea (7-9). The advantages of impregnated activated carbons in H2S removal, compared to virgin carbons, are their high efficiency and fast kinetic of reaction (6). The technique of impregnation is critical to ensure a homogeneous distribution of the chemicals on the surface of the activated carbon, thus maintaining maximum access to the pore structure to provide the target contaminants (H2S) with the maximum contact * Corresponding author phone: 65-67943244; fax: 65-67921291; e-mail:
[email protected]. 4460
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efficiency to the impregnant (5, 10). Although alkaliimpregnated carbons have been used so far as effective H2S adsorbents, the application of unimpregnated carbons is currently under consideration (11-14), driven by the following disadvantages of alkali-impregnated carbons: (i) low temperature of self-ignition, (ii) low capacity for physical adsorption due to the filling of the pore system with the impregnate, (iii) special precaution required to be used with alkalis, and (iv) difficulty for regeneration by washing with water. Many studies published so far have explored the effect of dynamic conditions such as concentration of H2S, content of air, and humidity, as well as carbon surface properties (i.e., pH and functional group) and carbon pore structure (15-23). However, the mechanisms of H2S removal by alkaline carbons are not yet fully understood. As proposed by Chiang et al. (5), dissociation of hydrogen sulfide occurred in the film of adsorbed water on the surface of alkaline activated carbon. The H2S was transported into and reacted with the water film by adsorption and acid-base reaction. On the other hand, Turk et al. (2) reported that the oxidation of hydrogen sulfide in the presence of alkalis results in the deposition of elemental sulfur in activated carbons. The carbon acts as an efficient adsorbent until the alkaline compound is exhausted. The adsorption isotherms for microporous adsorbents were studied by Aranovich et al. (24), and a multilayer theory was used for characterizing the adsorption behaviors of various adsorbates, including H2S, on activated carbons (25). This study is originally driven by the variable carbon performances resulted from H2S breakthrough capacity tests, which involved two types of commercial carbon and samples for the same carbon obtained from different batches. A fundamental study is thus carried out to understand the variations of the studied carbons in affinity to water sorption, porous structure, alkaline impregnation, and reaction kinetics, etc. On the basis of the results obtained, the relationships between the observed H2S breakthrough capacity and the major properties of carbons are analyzed. This may further indicate which features of activated carbons are important for their performance as H2S adsorbents, and consequently, help to better understand the physicochemical characteristics of the impregnated activated carbons and mechanism of the H2S adsorption process.
Experimental Section H2S Breakthrough Capacity Test. The dynamic test was carried out to evaluate the capacity of carbon for H2S removal. ASTM D28 (proposed in June 2000) was followed as the standard test method. The experimental setup is given in Figure 1. Activated carbon was packed in a glass tube (4.8 cm i.d.) at a fixed height of 22.9 cm. Glass beads were packed at the bottom and top of the carbon bed to facilitate the dispersal of gas flow and to ensure good mixing of adsorbates. The 1% (v/v) hydrogen sulfide together with moist air (RH 80%), prepared by mixing 5% (v/v) hydrogen sulfide at 1.05 L/min and moist air (RH 100%) at 4.15 L/min, was diverted into the bottom of the carbon bed. The linear flow rate and retention time of the gas passing through the carbon bed were kept at 4.77 cm/sec and 4.8 s, respectively. The experiments were carried out at room temperature. The H2S concentration was monitored at the outlet of the bed with a Draeger tube and PortaSens H2S monitor until a breakthrough at 50 ppm concentration was obtained. The time taken for breakthrough was recorded, and carbon capacity in terms of both weight (grams of H2S per gram of carbon) 10.1021/es0205840 CCC: $22.00
2002 American Chemical Society Published on Web 09/19/2002
FIGURE 1. Schematics of activated carbon breakthrough capacity test. and volume (grams of H2S per milliliter of carbon) were calculated from the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass/volume of carbon. The principal parameters influencing the test result could be the tube diameter, bed height, moist air, carbon prehumidification, and the total flow rate, etc. In most cases, the prehumidification was conducted before the capacity test: moist air (RH 100%) passed through the carbon bed for several hours at flow rate 1.5 L/min. The 5% (v/v) H2S gas used in the test was balanced with N2 in a cylinder (provided by MESSER Singapore), and the H2S concentration was guaranteed within 6 months after delivery. Carbon Characterization. Two brands of commercial activated carbons, referred to as Carbon I and II in the subsequent text, were studied. Six carbon samples (labeled Carbon I-1, 2, 3, 4, 5, and 6) of the same brand of activated carbon (Carbon I) but from different batches of production, and two samples from another brand (Carbon II) were tested. Both brands of carbons are coal-based and of alkali (KOH) impregnated type. The adsorption capacity by weight at breakthrough with pure H2S in a moist air stream is specified at minimum 20% for both brands of carbons. Significantly variable performances were observed from different carbon samples (Carbon I-1-I-6) in the H2S breakthrough test, probably related to differences in affinity to water sorption, porous structure, and reaction kinetics. To have a better understanding of these variations, the original and exhausted carbons were characterized using different analytical techniques. The surface areas and pore volumes of original carbons were measured by nitrogen adsorption using the Micromeritics BET analyzer model ASAP 2010. The chemical contents of carbons were analyzed by X-ray fluorescence (XRF) technique using the Bruker-axs SRS 3400. The distribution of sulfur (S) and potassium (K) in the surface and cross-section of carbons was measured by a scanning electronic microscope (SEM) together with energydispersive X-ray spectroscopy (EDS) by JEOL model JSM5310V (Japan). An X-ray diffraction (XRD) test using the Siemens D5005 X-ray diffractometer provided information about the crystalline and amorphous forms of sulfur in the carbons. The precision of XRF and SEM quantitative measurements was ensured by using standard samples and regular calibrations. Although errors might have been caused from the quantitative analysis of samples with different matrix, for the carbons used in this study, the results from XRF and SEM/EDS among the different samples should be comparable because they had a similar matrix. Variable dimensions of carbon pellets were also observed among the 6 samples of Carbon I. Average carbon size was measured based on the 10 representative pellets picked out from each sample. The average diameter of carbon pellets
FIGURE 2. TGA/DSC curve. was found at 4.05-4.17 mm with standard deviation ranging from 0.15 to 0.41, and the average length of carbon pellets was at 6.83-7.25 mm, with standard deviation ranging from 1.77 to 2.43. Adsorption Kinetics of H2S on a Carbon Pellet. The adsorption kinetics of H2S on a carbon pellet was studied using a TGA/DSC system (NETZSCH STA409, Germany). First, experiments were conducted with absence of moisture, and four samples (Carbon I-1, 2, 4, and 5) were tested. A carbon pellet was put into the crucible that was located in the middle of the TGA/DSC furnace. Drying of the sample was carried out first; carbon was heated to 100 °C in ramp of 10 °C/min in the presence of nitrogen balance gas and isothermal for 1 h. After the sample was dried, the temperature was cooled at 10 °C/min to the desired reaction temperature (30 °C), and isothermal for 5 h. The 5% H2S gas was introduced into the TGA/DSC furnace once the constant reaction temperature and flat baseline of weight were achieved. When the H2S gas was passing through the carbon pellet, immediately both the weight increase and heat release were observed which resulted from the adsorption of H2S on the carbon pellet, respectively, through on-line monitoring of TGA and DSC curves (see Figure 2). To present the two curves at the same scale, the actual TGA data exported from the TGA/DSC system were transferred to (weight - 170) × 10 (mg), as shown in Figure 2. The rate of weight increase (DTGA curve) of a carbon pellet was used to describe the H2S adsorption rate, and kinetic parameters were compared among the studied carbons. Generally, the adsorptive saturation was achieved within 1 h. After each test, N2 was purged through for another 1 h to clean the reaction system to prevent the potential corrosion problem caused by H2S gas. VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Breakthrough Capacity of Activated Carbons in Adsorption of H2S Carbon I
initial weight (g) time to reach 50 ppm (min.) capacity in vol. (g/mL) capacity in weight (%) prehumidification time, at RH 100% (h) a
Carbon II
1
2
3
4
5a
6a
305 810 0.15 21.1 7
294 400 0.068 10.1 12
292 512 0.093 13.2 7
300 440 0.079 11.0 0
307 192 0.038 5.2 7
294 470 0.094 13.2 7
300 760 0.14 20.0 8
288 465 0.088 12.6 0
One test only, others are average of duplicates; all tests were conducted at room temperature and 80% RH.
To evaluate the moisture effect on the adsorption kinetics, a modified gas manifold was set up. Four carbon samples (Carbon I-1, 2, 4, and Carbon II) were tested in this case. Once again, after drying of sample, temperature was cooled to the desired reaction temperatures (30 and 60 °C). After the constant reaction temperature was achieved, the moist air (RH 100%) was purged into the reaction system for 20 min; some heat release was observed due to the adsorption of moisture but almost no change related to the weight curve was found. Then, 5% H2S gas was introduced into the system. The same as previously, significant weight increase and heat release were observed due to the adsorption of H2S on the carbon pellet, indicated by TGA and DSC curves, respectively. Within 1.5-2.5 h, the adsorptive saturation was achieved.
Results and Discussion Capacity of Activated Carbons in H2S Breakthrough Tests. In total, 7 activated carbons from 2 producers were measured in the H2S breakthrough capacity test and the results are collected in Table 1. Variable capacities (5.2∼21.1% in weight) of H2S adsorption were found from the 6 activated carbons (Carbon I-1, 2, 3, 4, 5, and 6) which are actually the same brand of carbon (Carbon I) but obtained from different batches of production. Carbon II was chosen as a reference that shows relatively constant performance in the test, where 4 runs with two batches of Carbon II samples were tested and around 20% in weight of carbon adsorptive capacities were obtained from all 4 tests. Carbon prehumidification enhances the capacity of activated carbons in H2S adsorption: with 8 h of prehumidification of Carbon II, the capacity increased significantly from 12.6 to 20.0% in weight (Table 1). This enhancement has been shown in other results (5, 12, 14, 26). The presence of water on activated carbons is supposed to contribute to the dissociation of hydrogen sulfide and facilitates its oxidation to sulfur and sulfur dioxide (27). Nevertheless, moisture seems less crucial for some samples of Carbon I: only 10.1% H2S adsorption in weight was achieved with Carbon I-2 although the carbon prehumidification at 12 h had been conducted. Several principle phenomena are crucial, and they jointly influence the overall performance of the studied carbons: i.e., physical adsorption, chemisorption, and H2S oxidation. The following equations indicate the mechanism involved (5, 27). Physical Adsorption. H2S adsorption on the carbon surface (eq 1), its dissolution in a water film (eq 2), and dissociation of H2S in an adsorbed state in the water film (eq 3):
H2S(g) f H2S(ads)
(1)
H2S(ads) f H2S(ads-liq)
(2)
H2S(ads-liq) f HS-(ads) + H+
(3)
where H2S(g), H2S(ads-liq), and H2S(ads) correspond to H2S in gas, liquid, and adsorbed phases. 4462
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Chemisorption. H2S reaction with KOH at carbon surface (eqs 4a and 4b):
H2S(g) + KOH(q)-C f KHS(q) + H2O
(4a)
H2S(g) + 2KOH(q)-C f K2S(q) + 2H2O
(4b)
where KOH(q)-C is the KOH impregnated on the activated carbon covered by the water film which is generated by the produced water from eqs 4a and 4b, or the adsorbed moisture in the case of carbon prehumidification. Moreover, other forms of H2S either as liquid or adsorbed phases might also participate in the reaction with KOH. Oxidation. Surface oxidation reaction with adsorbed oxygen (eq 5) or catalysts (i.e., transition metals Fe, Zn, and Cu in carbons) with formation of elemental sulfur or sulfur dioxide, and further oxidation of SO2 to H2SO4 in the presence of water:
HS-(ads) + O*(ads) f S(ads) + OH-
(5a)
HS-(ads) + 3O*(ads) f SO2(ads) + OH-
(5b)
SO2(ads) + O*(ads) + H2O(ads) f H2SO4(ads)
(6)
H+ + OH- f H2O
(7)
where O*(ads) is dissociatively adsorbed oxygen. It is believed that for the activated carbons, the significant decrease in the adsorption capacity corresponding to exhaustion is usually caused by the formation of sulfuric acid (11, 16, 27). Carbon Characterization. BET Test for Original Carbons. The pore distribution provides the key to understanding the performance of an activated carbon. The graphs of incremental pore area versus the average pore diameter within the range mesopores (1-2.5 nm) and macropores (>2.5 nm) for the five AC samples (Carbon I) are shown in Figure 1 of the Supporting Information (SI). The original adsorption/ desorption isotherms of two carbons (Carbon II and Carbon I-4, as representatives) are also provided in Figure 2 of the SI. A summary of the BET test results of Carbon I and Carbon II is given in Table 2. Six samples were tested; they were preheated at 130 °C until achieving the vacuum required by the instrument. The surface areas of the tested alkaline AC range from 619 to 777 m2/g which are much lower than that of virgin carbons as reported (>1000 m2/g) (5). Carbon II has a higher BET surface area than all 5 Carbon I samples (Table 2), which implies that it might have a better physical adsorption of H2S than the latter. In contrast, Carbon I-5 might have the worst performance due to the lowest level among the 6 carbons in terms of BET surface area, micropore volume, micropore area, and mesopore/macropore surface area. This might account for the high capacity of Carbon II (20.0%) and the lowest capacity of Carbon I-5 (5.2%) observed in the breakthrough tests. Nevertheless, the performance of alkaline activated carbons is dependent not only on the physical
TABLE 2. BET Results of Original Carbons Carbon I
(cm3/g)
micropore volume average pore diameter (by BET) (nm) BET surface area (m2/g) micropore area (m2/g) mesopore/macropore surface area (m2/g)
1
2
3
4
5
Carbon II
0.16 1.95 639 ( 11 358 281
0.16 1.95 621( 9 356 265
0.19 1.96 649( 11 420 229
0.17 2.02 773 ( 11 371 402
0.13 2.01 619 ( 8 288 331
0.16 2.18 777 ( 10 350 427
TABLE 3. XRF Analysis for Exhausted Carbons, Wt. % part of bed
Carbon with High Capacity (Carbon I-1) 1 2 3 4 5 6
7
8
weight (g) 29.8 60.4 60.0 58.0 47.9 44.3 35.8 49.4 C 63.4 65.1 67.4 76.5 82.8 88.1 91.6 94.7 S 29.2 27.2 24.4 15.7 9.80 4.98 2.17 0.63 K 5.38 5.39 5.88 5.52 5.25 4.84 4.31 3.27
part of bed
Carbon with Low Capacity (Carbon I-4) 1 2 3 4 5 6
7
8
weight (g) 18.1 41.3 46.4 40.6 38.8 37.4 51.4 65.5 C 63.9 66.3 73.3 75.2 79.9 84.8 90.9 94.5 S 28.6 26.2 19.7 17.5 12.7 8.14 2.68 0.84 K 5.25 5.18 4.78 4.90 5.01 4.76 4.32 3.14
adsorption, but also on chemisorption (5). As seen in Table 2, Carbon I-4 has the largest BET surface area and mesopore/ macropore surface area among the 5 Carbon I samples, however, its H2S breakthrough capacity (11%) is worse than that of Carbon I-1 (21.1%) but better than that of Carbon I-5 (5.2%). Previously, impregnated AC were found to have a decreased surface area compared to that of virgin AC (5), therefore, a lower mesopore/macropore surface area in Carbon I-1 than in Carbon I-4 implies probably a better impregnation of alkali achieved in the former, which might account for its higher performance due to chemisorption. The incremental pore area/volume are both increased sharply when the pore diameters change to less than 3 nm (Figure 1, SI), implying the significant contribution of micropores and mesopores to the total pore volume/area of the studied carbons. Unfortunately, data are available only for those pores with diameter >2 nm, due to the limit of the current instrumental setup. Among the 5 samples of Carbon I, Carbons I-4 and I-5 both have shown the largest pore area and volume in the range of 2 to 3 nm, showing probably the relatively higher capacity in the physical adsorption. However, both Carbons I-4 and I-5 did not perform well in the H2S breakthrough test, again indicating that physical adsorption of carbon is not the only major contribution to the total carbon capacity, while chemisorption might also play an important role when using alkali-impregnated activated carbons (5). XRF/XRD Analysis for Exhausted Activated Carbons. Exhausted carbons (Carbon I) were collected after H2S breakthrough capacity tests, one from the high capacity carbons (Carbon I-1 with capacity 21% in weight) and another from low capacity carbons (Carbon I-4 with capacity 11% in weight). The exhausted carbon bed was divided into 8 parts, from bottom to top labeled 1, 2, 3, 4, 5, 6, 7, and 8. All 8 parts were analyzed by XRF and the results are present in Table 3. Sulfur content decreases significantly from bottom to top of the exhausted carbon bed for both Carbon I-1 and Carbon I-4. On the other hand, the content of potassium is relatively constant, although a tendency of slight decrease of K content is observed with no. 6, 7, and 8 samples in both cases. However, average K content in each part is not as high as
that specified by the supplier (∼10%). After H2S adsorption, the studied sample was transformed from mostly carbon basis (∼95 wt. %) to a combined basis of large carbon (∼63%) and part sulfur (∼30%). Besides C, S, and K that are present as the predominant elements, several elements are found in both exhausted carbons at relatively high concentrations: i.e., Fe (0.22-0.54%), Si (0.40-0.60%), Al (0.32-0.60%), Ca (0.22-0.43%), and Na (0.10-0.19%). Other elements (Cl, I, Mg, Mn, Ni, Ti, and V) are present at trace or impurity level below 0.1%. It is well-known that oxides of iron are used in industry as absorbents and/or catalysts of H2S removal from different gaseous media. Carbonates of calcium and magnesium are also supposed to have high affinity for H2S adsorption in wet conditions (27). The XRD spectrum shows a weak signal of crystalline species, implying most of the sulfur is present as amorphous forms in the tested sample (no. 1 of the exhausted Carbon I-1), although the sulfur content in the sample was quite high (29.2% in weight). Sulfur impregnated in activated carbons may be present as elemental sulfur in the form of ring (S8) or linear polymers (15-16). It was also previously demonstrated that, besides elemental sulfur as the predominant product during the oxidation of hydrogen sulfide on activated carbons, around 30% of the sulfur was in the form of sulfuric acid (16). However, further studies are needed in this case to understand the form of deposited sulfur in the exhausted carbons. Figure 3 shows more clearly the tendency of S content in the exhausted carbon bed. Higher efficiency of sulfur adsorption is found related to Carbon I-1. Nevertheless, the overall utilization of both carbon beds is estimated to be only around 50%, if supposing that all the carbons could be saturated by sulfur with its content close to 30% in weight thoroughly in the whole bed. Those carbons at the upper half of the bed are not effectively used and they could be re-utilized in other applications of H2S removal for purposes of carbon cost savings. Moreover, the main product of hydrogen sulfide oxidation is sulfur located in small pores in the case of alkaline carbons, which cannot be removed by washing with water. On the other hand, the deposited sulfur on porous carbons can be probably used to chemisorb the mercury vapor in flue gases (28); that is to say, the lower half of the exhausted carbon bed could possibly be used for flue gas mercury removal. SEM/EDS Analysis for Original/Exhausted Activated Carbons. SEM/EDS analysis provides information on the distribution of potassium (K) and sulfur in the original and exhausted carbons at both surface and cross section of a carbon pellet, which is necessary and helpful to understanding the effect of chemisorption occurring in the process of H2S adsorption. Two original activated carbons were analyzed using SEM/ EDS and the results of semiquantification elemental analysis are given in Table 4. In Carbon I-1, the distribution of potassium (K) at the surface of the carbon particle is not homogeneous, as a significant difference can be found between dark and bright areas. However, the potassium distribution is relatively more homogeneous at the crosssection than at the surface. Carbon content is much higher VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Sulfur content in the exhausted carbon bed with high/low capacity, at 50-ppm H2S breakthrough (Carbons I-1 and I-4).
TABLE 4. SEM/EDS Analysis for Original Carbons (Wt. %)
TABLE 5. SEM/EDS Analysis for Exhausted Carbon I-1 (Wt. %) no. 1 (bottom)
Carbon I-1 surface
Carbon I-2
element
bright area
dark area
cross section
surface
cross section
C O Al Si S K Fe Cu + Zn others Cl + Ca total
31.3 48.3 1.1 1.2 0.3 12.0 0.3 4.1 1.4 100
26.4 55.6 0.8 0.8 0.2 15.1 0.2 0 0.9 100
67.6 26.2 1.0 1.2 0.2 2.4 0.7 0 0.7 100
37.4 48.9 0.9 1.1 0.2 9.5 0.3 0 1.8 100
65.5 27.7 0.7 1.0 0.3 4.0 0.6 0 0.3 100
at cross-section than at the surface of the carbon pellet. A big difference of K concentration is found between surface (>12%) and cross-section (2.4%), showing impregnation of KOH takes effect mostly at the carbon surface. Consequently, the chemisorption based on the acid-base reaction between H2S and impregnated KOH should occur mostly at the carbon surface. In Carbon I-2, however, the distribution of potassium (K) at both surface and cross-section of a carbon pellet is quite homogeneous. Similarly, much more K is found at surface (9.5%) than at cross-section (4.0%). Furthermore, potassium content at the surface of Carbon I-2 (9.5%) is less than that of Carbon I-1 (>12%), implying the difference of KOH impregnation to the two carbons. Nevertheless, significantly more oxygen is detected at the surface than cross section for both carbons studied, which is consistent with other results (11) showing that all commercial carbons have some oxygen-containing groups on the surface. Those groups are known to significantly change the catalytic activity of carbons in oxidation reaction of H2S. Moreover, Cu, Zn, and Fe are well-known catalysts for H2S oxidation (27); Fe is found in all the studied samples, whereas Cu and Zn are detected only at bright areas of the original Carbon I-1 surface. Although carbon surface functional groups contribute significantly to the oxidation of H2S, impregnation of the carbon with transition metals was found to alter the catalytic activity of carbon toward sulfur and sulfuric acid (7). 4464
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no. 4 (middle)
cross cross element surface section surface section C O Al Si S K Fe others
278.0 30.1 1.3 1.4 32.7 6.1 0.4 0
total
100
53.9 26.1 0.5 0.5 14.9 3.8 0.4 0 100
39.0 29.8 1.3 1.4 23.4 4.5 0.5 0 100
59.4 29.8 0.5 0.6 5.5 4.1 0.2 0 100
no. 8 (top) surface
cross section
48.4 36.2 0.9 1.2 1.8 9.5 0.6 1.4 (Na + Cl + Mg) 100
65.1 24.7 0.8 0.9 1.4 6.2 0.7 0.3 (Na) 100
Three parts coming from an exhausted carbon bed (Carbon I-1), labeled as 1, 4, and 8 from bottom to top of the bed, were identified using SEM/EDS (Table 5). In the top (no. 8) sample, sulfur is just slightly higher(1.4-1.8%) than that in original carbons (0.2-0.3%). However, a lot of sulfur (maximum 32.7%) was detected in the bottom and middle parts. The decreasing tendency of S content from bottom to top is quite obvious in both surface and cross section. The ratio of S contents at surface and cross section from bottom to middle and top is 32.7/14.9, 23.4/5.5, and 1.8/1.4, respectively. The increase of sulfur content due to adsorption of H2S is much more significant at the carbon surface than cross-section. It is suggested that the adsorption of sulfur at the surface of Carbon I-1 is faster than that at cross-section, probably because of the limitation of gas diffusion into the carbon pellet and also because a much larger quantity of potassium was impregnated at the carbon surface. Furthermore, there is still some sulfur (14.9%) detected at the cross section of the bottom part of exhausted Carbon I-1, indicating that physical adsorption still plays an important role in H2S adsorption in the case of alkaline activated carbons. However, the rate of physical adsorption is relatively slow compared to that of chemisorption that accounts for the significant difference of S contents detected between surface and cross-section (ratio 23.4/5.5) in the middle sample. Kinetics of H2S Adsorption on a Carbon Pellet. The results of kinetic study using TGA/DSC, at temperature 30 °C and with absence of moisture in the reaction system, are
highlighted in Table 1 of the SI. Moisture was absolutely excluded under the designed experimental conditions. Therefore, chemisorption might be restrained at a certain level considering moisture was suggested to enhance the chemical reaction between H2S and the impregnated KOH (5, 12, 14, 26). Among the 4 tested carbons (Carbons I-1, 2, 4, and 5), Carbon I-4 was found to have the best adsorptive capacity with the highest total weight increase (3.6%) in dry basis. It also has the fastest adsorption rate (0.34 mg/min) and the largest heat release (39.7 µVs/mg), as highlighted in bold. However, the overall adsorption capacity is low compared to that observed in the H2S breakthrough test (Table 1). It is supposed that carbon physical adsorption controls this process. As seen previously, Carbon I-4 demonstrated good physical properties (Table 2) which accounts for its better adsorptive capacity observed in this case. Apart from the moisture, temperature may also be a principle parameter. Higher temperature will probably enhance chemisorption but definitively restrain physical adsorption. The results of kinetic study at the experimental condition with presence of moisture are summarized in Table 2 of the SI. Here Carbon II was considered as a reference for comparison. Two reaction temperatures (30 and 60 °C) are tested for both Carbon I-1 and Carbon I-4 samples, whereas only one temperature (30 °C) is considered for Carbon I-2 and Carbon II. The total weight increase of Carbon I-1, 2, and 4 samples in the presence of moisture, are significantly augmented compared to the case with absence of moisture (Table 1 of SI). The maximum weight increase due to adsorption of H2S is found at 40.3% with Carbon II, which is significantly increased compared to the result (20.0%) from the carbon breakthrough test (Table 1). This is probably due to the saturation of H2S on the carbon pellet in the case of the kinetic study, whereas during the breakthrough test only less than half of the carbon bed is fully utilized. High adsorptive capacities are also observed for Carbons I-4 and I-1 at 28.7% and 19.7%, respectively, but only at 30 °C. When a higher reaction temperature of 60 °C is used, lower capacities are observed with both Carbons I-4 and I-1 at 12.6% and 10.1%, respectively. With increased temperature, the physical adsorption capacity of carbon is believed to generally decrease, but the effect on chemisorption and catalytic oxidation is not clear in this case. Further studies are needed to understand the different influences of temperature on physical adsorption, chemisorption, and catalytic oxidation. In terms of reaction kinetics, Carbons I-1 and II have both demonstrated a high initial adsorption rate around 0.330.34%/min (Table 2 SI). Carbon I-1 has the highest adsorption rate observed at 0.69%/min. The initial adsorption rate in this case may represent the rate of chemisorption, as most of the potassium found by SEM/EDS is associated with the carbon surface. The lower overall capacity of Carbon I-1 (19.7%) compared to that of Carbon II (40.3%) is most possibly due to the lower physical adsorption of the former as seen previously by BET (Table 2). Carbon II demonstrates both a high physical adsorption ability and initial adsorption rate, which account for the highest capacity in adsorption of H2S gas. Furthermore, although Carbon I-1 has also a higher initial adsorption rate than Carbon I-4, the latter has a better adsorption capacity (28.7%) than the former (19.7%) probably because Carbon I-4 has the best physical adsorption ability among the samples of Carbon I. Contrary to its low carbon capacity found in the breakthrough test where no prehumidification was conducted (Table 1), Carbon I-4 has demonstrated a much better performance in the kinetic study due to a sufficient exposure to moist air in the latter case. Interpretation of Variable Carbon Performances. The variable carbon capacities of H2S removal related to Carbon I (1, 2, 3, 4, 5 and 6) as observed in the breakthrough capacity
tests (Table 1), can possibly be interpreted on the basis of the following observations. Physical Adsorption. The five samples of Carbon I (1, 2, 3, 4, and 5) present quite similar porosity properties (Table 2) because they are all actually from the same coal basis, except that Carbon I-4 has a much larger surface area closer to that found for carbon II. It is believed that Carbon I-4 might have a relatively better physical adsorptive capacity than the other Carbon I samples. Chemical Adsorption. Chemisorption is found to play an important role in H2S removal with alkaline activated carbons. Impregnated KOH is concentrated at the surface of a carbon pellet as shown by SEM/EDS analysis (Table 4). Differences regarding KOH impregnation are found related to Carbon I-1 and carbon I-2. The rapid chemisorption of H2S on alkaline Carbon I-1 takes place mostly at the carbon surface (see Table 5), whereas physical adsorption that is slower and mostly takes place at the inner pores of carbon still contributes a lot to the overall H2S adsorption. Catalytic Oxidation of H2S. Besides physical and chemical adsorption, catalytic oxidation of H2S also has a great influence on the carbon performances. Iron (Fe) was found in all the studied carbon samples, which may significantly increase the capacity of the studied activated carbons (27). Some Cu and Zn were also detected at the surface of original Carbon I-1 (Table 4), which probably accounts for the highest adsorption rate (0.69%/min) of Carbon I-1 observed in kinetic studies (Table 2 in SI). Kinetics of H2S Adsorption. The good performance of Carbon I-1 and Carbon II in H2S breakthrough tests is consistent with their high initial adsorption rates as seen in the kinetic study, indicating that chemisorption contributes a lot to the capacity of these two carbons in H2S removal. Those carbons with both high capacity of physical adsorption (evaluated by BET results) and chemisorption (evaluated by initial adsorption rate in kinetic study) are supposed to be able to achieve the best performance such as Carbon II. Intermediate carbon performances are found related to Carbon I-4 and Carbon I-1 because the former has better physical properties while the latter has higher initial adsorption rate, compared to other samples of Carbon I such as, for instance Carbon I-2, 3, 5, and 6, which have demonstrated the worst performances.
Supporting Information Available Tables reporting kinetic studies of H2S adsorption in carbon pellet, without and with moist air, BJH adsorption pore distribution, and isotherm plots of carbon II and carbon I-4. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review February 7, 2002. Revised manuscript received June 24, 2002. Accepted August 19, 2002. ES0205840
