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Ind. Eng. Chem. Res. 2003, 42, 5280-5286
PAH Mixture Removal from Hot Gas by Porous Carbons. From Model Compounds to Real Conditions A. M. Mastral,* T. Garcı´a, R. Murillo, M. S. Calle´ n, J. M. Lo´ pez, and M. V. Navarro Instituto de Carboquı´mica, CSIC, P. O. Box 589, M Luesma Casta´ n 4, 50015-Zaragoza, Spain
This paper reports an investigation of the abatement of polycyclic aromatic hydrocarbon (PAH) emissions using different carbonaceous materials as adsorbents. PAH model compounds (naphthalene and phenanthrene) were initially used to obtain a deeper understanding of the complex process of PAH adsorption in industrial hot gas cleaning systems. First, the adsorption of binary mixtures of these compounds was carried out at the laboratory scale in an experimental rig specially designed for this purpose, where gas-phase fluorescence is used for detection. Experimental conditions, mainly in terms of temperature and contaminant concentration, close to those observed in flue gases from energy generation systems were applied. The PAH adsorption process was analyzed in terms of experimentally obtained breakthrough curves. After the detection parameters had been optimized, the influence of the structural and chemical properties of the activated carbon was studied. It was found that the higher the adsorbent porosity, the higher the breakthrough time of both PAHs, with the microporosity and the micropore size distribution being the main factors controlling the adsorption process. Moreover, several naphthalene/phenanthrene binary mixtures were studied to assess the effect of the inlet concentration on PAH adsorption. It was observed that the presence of a second PAH in the inlet gas stream reduces the adsorbent efficiency and that, in general, the higher the naphthalene concentration, the lower the adsorbent efficiency. Finally, the conclusions obtained from model compounds were checked under real conditions. Introduction Activated carbons are extensively used to remove contaminants from exhaust gases.1-4 The gas is passed through a bed of activated carbon where the molecules of the contaminants are transferred to the solid phase (adsorption process). A significant amount of information is available from studies on the adsorption of pure organic vapors5-9 for environmental purposes. In these previous studies, it was demonstrated that the polycyclic aromatic hydrocarbon (PAH) adsorption capacity in different carbon materials depends on the textural characteristics of each material (with the micropore volume being the most important parameter for PAH removal). It was also shown that the adsorption of PAHs by sorbents from waste hot gas emissions is inversely proportional to their volatility. These studies also demonstrated the effects of moisture7 and CO28 on the phenanthrene (phe) adsorption capacity. First, the determined parameters showed that, for a carbonaceous material, the higher the steam percentage (in volume) in the gas stream, the lower its phe adsorption capacity. Second, a negative influence of CO2 on the adsorption capacity of phe was also observed. The latter effect is probably due to competition between CO2, H2O, and phenanthrene molecules for the adsorption sites within the carbonaceous material Because the inlet gas stream in industrial hot gas cleaning systems generally consists of a mixture of different contaminants, it would be interesting to obtain the necessary information to estimate the performance of activated carbons in the presence of vapor mixtures. * To whom correspondence should be addressed. Tel.: 34 976 733977. Fax: 34 976 733318. E-mail: amastral@ carbon.icb.csic.es.
Regarding organic emissions, the group of volatile organic compounds (VOCs)10 constitutes a general category with important subclasses, such as polycyclic aromatic compounds (PACs). More specifically, public concern about polycyclic aromatic hydrocarbons (PAHs) has increased during the past few years because of their carcinogenic character.11,12 This fact implies that the behavior of different adsorbents in the adsorption of PAH binary mixtures should be investigated to develop a deeper understanding of the complex processes that occur in industrial hot gas cleaning systems. Experimental data on the adsorption of vapor mixtures on solids are limited because it is difficult to obtain data for complete breakthrough curves for different concentrations of solutes in vapor mixture/adsorbent systems. Brosillon et al.13 have studied the coadsorption of acetone/heptane gaseous mixtures on zeolites. Their experiments showed that, for coadsorption of a mixture of polar and nonpolar components, selectivity depends on relative polarity (due to the presence of cation exchange in the framework of the zeolites), mixture ratio, and boiling point. Vahdat14 developed a theoretical study of the performance of activated carbon in the presence of binary vapor mixtures. Because of the difference in adsorption capacity and concentration of the two compounds, one of them is more strongly adsorbed by the carbon bed and has a longer breakthrough time. The component that has a higher ratio of adsorption capacity to concentration is the more strongly adsorbed component. After the first breakthrough point is reached, the weakly adsorbed component is replaced by the strongly adsorbed one until the latter reaches its breakthrough point. As for adsorbed components on activated carbon, the adsorption capacities in the breakthrough depend on several param-
10.1021/ie0302793 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5281
Figure 1. Experimental rig designed to study the adsorption behavior of binary PAH mixtures on carbonaceous materials.
eters: the boiling temperature, the concentration ratio of the mixture, and the flow rate. Another approach to study the performance of adsorbent materials in the presence of binary mixtures is to develop mathematical models. However, these models15-17 use the data for pure-component adsorption equilibrium only to calculate the breakthrough curves of the two components in a binary mixture. In this paper, the adsorption process of binary PAH mixtures is investigated by obtaining the corresponding breakthrough curves. Experimental Section Adsorption Measurements. The adsorption experiments were performed in an experimental rig specially designed to assess the adsorption behavior of PAHs on carbonaceous materials.18 A schematic diagram of the apparatus is shown in Figure 1, where three different zones can be identified: (1) In the saturated gas stream generation zone, two small cylindrical reactors (6.8-mm i.d.) connected in parallel were used to generate the gas stream saturated with PAHs (see Figure 1). In these reactors, two different PAHs, naphthalene (np) and phe, are sublimated with a measured carrier gas stream. Two reactors were needed because the generation temperatures required to achieve the desired PAH concentrations (around 0.8 ppmv for each compound) were quite different. The flow through the saturation reactors was controlled using mass flow controllers. Helium was used as the carrier gas, and the total flow at the inlet of the adsorption reactor was 25 mL/min. The temperature of the saturators was regulated with two independent PID (proportional, integral, and derivative) controllers ((1 °C accuracy). These temperatures were allowed to increase slowly until the desired PAH concentrations were obtained and were then kept constant until the end of the experiment. (2) The adsorption zone contains an adsorbing bed composed of 25 mg of adsorbent (100-200-µm average particle size diameter). It was placed in a Teflon reactor (3.8-mm i.d.) and mixed with 1.0 g of sand as the inert material, with the same size to provide sufficient bed length (11 cm) to ensure uniform flow throughout and avoid axial dispersion. In this way, the gas linear and space velocity were 5.3 cm/s and 1736 h-1, respectively. Blank tests were carried out with sand to check its suitability as an inert material in adsorption experiments. The Teflon reactor was placed inside a gas chromatograph oven that allows for precise control of the adsorption temperature ((1 °C accuracy).
Figure 2. Excitation and emission gas-phase spectra of np and phe. Table 1. Response Factors Obtained for np and phe at the Wavelengths Selected to Perform the Resolution of Binary Mixtures naphthalene phenanthrene λ1 λ2
λ (nm) exc em
Lslit (nm) exc em
Kcalibration np phe
260 236
15 15
4.0 1.9
352 352
20 20
4.5 19
In the detection zone, the detection and resolution of the gas mixture was carried out using a gas-phase fluorescence system.19 To avoid PAH condensation, the gas stream leaving the Teflon reactor was driven through a heated transfer line to a gas cell specially designed for this purpose. The gas cell was placed in the spectrofluorimeter in such a way that the fluorescence signal for a stable PAH stream was maximum. Figure 2 shows the excitation and emission spectra obtained for np and phe. From these spectra, the optimum wavelength for resolution of binary mixtures of these PAHs were selected from the response factor of the pure compounds according to the method described in ref 19. These response factors and the optimum experimental conditions are compiled in Table 1. Adsorbents. Three carbon materials (CAs) of different origins were used to study their behavior in the adsorption of PAH vapor mixtures: CA-3 was a commercial coke from German Rhenish lignite supplied by RWE Rheinbraun, CA-8 was an activated carbon from apricot stones, and finally, CA-13, corresponded to an activated carbon from Spanish lignite. The activated carbons used in this work were characterized through their structural and chemical properties, and the data obtained are compiled in Table 2. Adsorbent Characterization. The three porous carbons were characterized by N2 adsorption at -196 °C using a Quantachrome Autosorb 1 instrument.
5282 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 2. Carbon Material Structural and Chemical Properties DFT parameters
TPD parameters
Table 3. Column Characteristics from Breakthrough Curves of Binary Mixtures of np and phe in Terms of Adsorbenta adsorbent
S CO CO2 Vmicrop Vt Lmicrop sample (m2/g) (cm3/g) (cm3/g) (nm) Vmicrop/Vt (µmol/g) (µmol/g) CA-3 CA-8 CA-13
172 547 532
0.079 0.28 0.28
0.21 0.39 0.48
1.1 1.1 1.1
0.56 0.69 0.58
500 840 660
540 590 360
Before the experiment, the samples were heated at 200 °C and then outgassed overnight at this temperature under a vacuum of 10-5 Torr to constant pressure. The isotherms were used to calculate the specific surface areas (S), micropore volumes (Vmicrop), average micropore sizes (Lmic), and micropore size distributions, using density functional theory (DFT).20 The total pore volume (Vt) was obtained from the N2 volume adsorbed at a relative pressure of 0.95. The experimental error due to sample heterogeneity was around 3% depending on the sample. Characterization of the surface oxygen groups was carried out on a Pulse Chemisorb 2700 apparatus (Micromeritics) by temperature-programmed desorption (TPD) up to 1100 °C at a constant heating rate of 10 °C/min. The carrier flow rate was adjusted with an electronic mass flow controller. The gases evolved by thermal decomposition from the surface groups were measured by gas chromatography with Porapak N-120 and molecular sieve columns. The gas chromatograph temperature program was as follows: isothermal at 35 °C during 4 min, increasing temperature to 180 °C at a rate of 35 °C/min, and then isothermal at this temperature. The CO2 was released at low temperatures as a result of the decomposition of acid surface groups. The CO has its origin in weakly acidic, neutral, and basic groups, which are thermally more stable and therefore are released at higher temperatures.21 The measured experimental error was around 2%. Single-compound equilibrium adsorption capacities, q* (mg of PAH/mg of carbonaceous material), were calculated by integrating the corresponding breakthrough curves and applying eq 1
q* )
Q [C t w of
∫0t C(t) dt] f
(1)
where Q is the total inlet flow in mL/min; w is the weight of carbonaceous material introduced into the reactor in mg; Co is the inlet PAH concentration in mg/ mL; tf is the time, in minutes, at which the PAH outlet concentration coincides with Co; and C(t) is the outlet concentration in mg/mL continuously recorded as a function of time (breakthrough curve). For the binary mixture, the equilibrium adsorption capacity of each component was also calculated using eq 1 where tf corresponds to the equilibrium time. Fluidized Bed Combustion. The three porous materials studied were tested with the aim of studying their behavior on PAH retention under conditions similar to those existing in power stations. Experiments were performed in a fluidized bed combustion (AFBC) pilot plant at 850 °C and 2% oxygen excess with a lowrank coal (Samca coal) as the fuel.22 Under these experimental conditions, the PAH total mean concentration was between 100 and 150 ppbv.22,23 The sampling system comprised a Teflon filter (0.5-µm pore size), a fixed bed containing the activated carbon to be tested, and a second XAD-2 resin bed where, once the activated
CA-3
CA-8
CA-13
single-compound adsorption capacity np (mgnp/mgCA) 0.037 0.12 phe (mgphe/mgCA) 0.10 0.22
0.12 0.36
breakthrough point 1 (np breakthrough) MTZ height (cm) 1.5 1.2 column utilization degree (%) 62 74 total adsorption capacity (mgPAH/mgCA) 0.068 0.17
1.3 67 0.24
breakthrough point 2 (phe breakthrough) MTZ height (cm) 1.7 1.0 separation zone height (cm) 6.2 4.2
0.62 6.4
adsorption equilibrium np adsorption capacity (mgnp/mgCA) 0.017 phe adsorption capacity (mgphe/mgCA) 0.094 equilibrium ads. capacity (mgPAH/mgCA) 0.11 np substituted by phe (mgnp/mgCA) 0.018
0.023 0.34 0.36 0.077
a
0.042 0.18 0.22 0.036
L ) 10 cm, C0 ≈ 0.8 ppmv. Total gas flow ) 25 mL/min.
carbon breakthrough had occurred, the PAH emissions were detected.24 The whole system was heated at 150 °C, with an aliquot of approximately 50% of the combustion gases flowing through the sampling system. Several sampling operations, every 500 L, were carried out, with an aliquot of combustion gases taken in each one and with the experiment stopped to replace the XAD-2 adsorbent until total activated carbon saturation is reached. Results and Discussion Single Compound Adsorption. To shed some light on the adsorption of binary PAH mixtures on the surface of activated carbons, it is interesting to take into account how the adsorption of a single PAH occurs. For np,6 it has been demonstrated that, in the studied concentration range, np is mainly adsorbed on the micropores, according to the results shown in Table 3. In this table, it can be observed that the np adsorption capacities of the three studied activated carbons exhibit the same relationship as their Vmicrop values, independently of the other adsorbent characteristics. On the other hand, it has been also demonstrated that not only does phe adsorption5 occur on the micropores, but also multilayer adsorption on the nonmicroporous surface makes a significant contribution. This is corroborated by the results shown in Table 3, where it is observed that the phe adsorption capacity of CA-13 is higher than that of CA-8, in accordance with the higher porosity of CA-13. With respect to the influence of surface chemistry on the adsorption process, it has been observed that the presence of strong acidic groups on the carbonaceous material surface (especially carboxylic acid sites) has a negative effect on the PAH adsorption process,24 according to the hydrophobic nature of these nonpolar compounds. Finally, it is worth commenting that, in the single adsorption of all studied compounds (molecules with two to four aromatic rings) on these three carbon materials, the obtained breakthrough curves showed that, independently of the studied PAH, the adsorption occurred very quickly, with minimum mass-transfer effects.25 Binary Mixture Adsorption. The breakthrough curves for the adsorption of binary mixtures of np and phe on the three different activated carbons (CA-3, CA-
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5283
Figure 3. Breakthrough curves for binary mixtures of np and phe in terms of the adsorbent. T ) 150 °C. Total gas flow ) 25 mL/min.
8, and CA-13) are presented in Figure 3. In addition, different column characteristics calculated from these measurements are summarized in Table 3. The adsorption temperature and gas linear velocity were 150 °C and 5.3 cm/s, respectively. These process variables were kept constant for the three experiments so that results obtained under the same conditions could be compared. The inlet np and phe concentrations were around 0.8 ppmv in all runs. First, it can be observed that these breakthrough curves are different from those obtained for the pure compounds because of the different kinetic behaviors of the two components that constitute the binary mixture. Therefore, during the binary adsorption process, once np breakthrough occurs (the weakly adsorbed component), the adsorbed np molecules are replaced by phe molecules (the strongly adsorbed component) until adsorption equilibrium is reached. In this figure, the np breakthrough determines the activated carbon efficiency (the adsorption capacity for the PAH binary mixture) in the hot gas cleaning process, and the phe breakthrough shows the adsorption equilibrium of the PAH binary mixtures. In addition, it can also be deduced from the slopes of the breakthrough curves that there is a minimum mass-transfer resistance, as was observed in the adsorption of single compounds. Influence of Surface Chemistry. To perform an integral study of the influence of adsorbent properties on PAH binary adsorption, both the chemical surface and the structural characteristics should be taken into account, as was previously described for single-compound adsorption. From this point, this study is focused on the structural characteristics, as it has already been demonstrated in previously published work6,9 that there is no direct relationship between the chemical surface properties [data from temperature-programmed desorption (TPD) experiments] and the adsorbent efficiency in single-compound adsorption for 16 carbonaceous materials. Working under the same conditions as the earlier paper6 and applying multivariate analysis, only a slight negative influence of surface CO2 groups was observed here, probably because these groups are hydrophilic groups and, therefore, are not appropriate for promoting the adsorption of hydrophobic molecules such as PAHs. On the other hand, data reported in the current paper show a positive relationship between the basicity, the CO/CO2 parameter, and the adsorbent efficiency (r2 ) 0.99) or the equilibrium adsorption capacity (r2 ) 0.98). Assuming, as will be shown later, that the binary mixture behaves similarly to the single adsorption, this direct relationship could be incorrect because it was also found that there is a direct relation-
ship between the phe adsorption capacity as a single compound (r2 ) 0.99) and the basicity, which was not found in the above-mentioned deeper analysis.6,9 Influence of Morphological Properties. With respect to the influence of the structural characteristics of the activated carbons on the adsorption behavior of PAH binary mixtures, Table 3 shows that the height of the mass-transfer zone (HMTZ)25 for np is almost independent of the adsorbent properties. However, other column characteristics such as the degree of column utilization and the sorbent efficiency are a function of these textural parameters. The degree of column utilization was calculated as the ratio of the total adsorption capacity in the first breakthrough (adsorbent efficiency) to the total adsorption capacity at equilibrium. It was observed that this parameter is a function mainly of the adsorbent characteristics. Table 3 shows that the degree of column utilization for CA-8 is higher than those observed for CA-3 and CA-13. By comparing these results with those compiled in Table 2, where the structural and chemical properties of the activated carbons are shown, it can be deduced that the degree of microporosity (Vmicrop/Vt) is the most important variable. It was observed that the higher the Vmicrop/Vt value, the higher the degree of column utilization and, therefore, pores close to the adsorbate molecular size will improve the degree of column utilization in hot gas cleaning. More details about the differences in pore sizes can be obtained from an analysis of the pore size distributions (PSDs) presented in Figure 4. This figure shows that CA-8 has the narrowest micropore size distribution, with the majority of its pores being smaller than 1.2 nm (ca twice the adsorbate size), which is the appropriate size to retain both molecules according to the single-compound adsorption mechanism. However, CA-3 and CA-13 have wider unimodal distributions with significant volumes in pores between 1.2 and 2 nm. In these pores, according to the previously published work,6 phe adsorption takes place through cooperative effects, whereas np cannot be adsorbed because it needs pores with a higher adsorption potential. In this way, the presence of adsorbate-size pores will enhance the degree of column utilization and, therefore, the performance of adsorbents in the adsorption of PAH binary mixtures. Adsorption Mechanism. The relationship between adsorbent efficiency and adsorbent characteristics for the binary mixture is similar to that observed for phe as a pure compound, with a high correlation between the porosity, Vt, and the adsorbent efficiency (r2 ) 0.99). However, it can be also observed that adsorbent efficiency value is quite a bit lower than the phe adsorption capacity. These facts clearly show again that, up to the first breakthrough point, np and phe molecules are competing for molecular-size pores (micropores) and, in addition, phe molecules are also retained on the nonmicroporous surface. Therefore, porosity development and pore size distribution are the main factors controlling adsorbent efficiency. After np breakthrough begins an important zone of breakthrough curves, which lasts until phe breakthrough, where the separation of components is achieved. It can be observed in Table 3 that the height of this zone depends on the adsorbents characteristics. These data show that CA-3 has the highest separation zone and CA-8 the lowest, which is opposite to the trend found in the degree of microporosity. In this way, the presence of molecular-size pores seems not to favor np and phe
5284 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 4. Pore Volumes Filled (cm3/g) by np and phe as Single Compounds and in the Binary Mixture single adsorption
Figure 4. Micropore size distributions: (A) CA-3, (B) CA-8, and (C) CA-13.
separation from hot gas. This conclusion is supported by an analysis of the PSDs presented in Figure 4. They clearly show that the micropore size distribution is the narrowest for CA-8 and the widest for CA-13. Therefore, it can be concluded that, although these type of distributions decrease the adsorbent efficiency, as mentioned above, wide micropore distributions are suitable for the separation of np/phe mixtures. Regarding the amount of np substituted by phe, it has been found that, in addition to the increase expected with the porosity accessible for phe in the activated carbon, there is also a relationship with the pore size distribution. This fact is demonstrated by calculating the percentages of np substituted for phe from the np breakthrough until the equilibrium is achieved: 52% for CA-3, 46% for CA-8, and 77% for CA-13. Therefore, the wider the micropore size distribution, the higher the percentage of np substituted. The same trend was found between the separation zone length and the pore size distribution of the activated carbons. The single and total PAH equilibrium adsorption capacities in the binary mixtures are reported in Table 3. First, as could be expected, the same trend is found for the total adsorption capacity and the adsorbent
binary adsorption
adsorbent
np
phe
np + phe
CA-3 CA-8 CA-13
0.043 0.14 0.14
0.11 0.25 0.40
0.13 0.25 0.41
efficiency (high correlation to Vt). In addition, the total equilibrium adsorption capacity is always similar to the phe single adsorption capacity. This means that the same pores are taken into account during both adsorption processes, single and binary. Regarding the pores filled by each compound in the adsorption of the binary mixtures, it is also observed that, whereas the phe adsorption capacity is correlated with Vt, the np adsorption capacity depends on the degree of microporosity. These facts suggest that, during the adsorption process, the mechanism followed by the np/phe binary mixture is a physical adsorption that depends on the porosity and pore size distribution of the activated carbon. Therefore, until first breakthrough, both molecules are retained: np in the molecular-size pores and phe in the total porosity. The first breakthrough occurs when all of the pores that the np molecule (weakly adsorbed component) can occupy are filled by the binary mixture. After np breakthrough, separation of the binary mixture can be achieved until phe breakthrough. In addition, phe molecules are retained in the nonmicroporous surface by multilayer interactions. In this binary mixture, np does not seem to affect the multilayer adsorption of phe. Finally, once the accessible pore volume for this concentration range is filled, phe breakthrough occurs throughout the activated carbon bed. Because of the different molecular sizes of np and phe, a possible molecular sieve effect could also be observed, because the small size of the np molecules allows them to enter pores into which other larger PAH molecules cannot penetrate. Therefore, the pore volume filled by the mixture could become higher than that filled by the strongly adsorbed component as a pure compound. This behavior is demonstrated by the data in Table 4, where the pore volumes filled by the single compounds and the binary mixture are compiled. These values were obtained by assuming that the adsorbed molecules were in a liquid phase with np and phe molar volumes of 148 and 199 cm3/mol, respectively.26 Figure 5 shows the amount of binary mixture adsorbed on CA-3 in terms of phe inlet concentration for different np concentrations in the first breakthrough. In fact, this parameter shows the efficiency of this carbonaceous material in PAH adsorption processes because, under real conditions, to prevent undesired contaminant emissions, it should be necessary to use a new adsorptive bed and probably regenerate or dispose of the old one. In Figure 5, it can be observed that the presence of np molecules in the inlet gas stream clearly reduces the sorbent efficiency for PAH removal, compared to that for phe as a pure compound, because the sorbent breakthrough occurs when the np molecularsize pores have been filled by the binary mixture, as was deduced above. In the same way, the sorbent efficiency of the binary mixture will be higher than that of np as a pure compound. As shown in Figure 5A, it was also observed that the higher the np concentration in the binary mixture, the lower the adsorbent efficiency, except at very low phe
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5285
Figure 6. Fluorescence synchronic spectra with ∆λ ) 5 nm. Blank and XAD-2 resin for different volumes taken when the adsorption bed is composed of CA-13 adsorbent. Table 5. Normalized Efficiency Comparison of Adsorbents between Real Conditions (AFBC) and Model Compound Adsorption Using CA-3 Adsorption Capacity as the Reference Value single adsorption binary adsorption
Figure 5. Efficiency of CA-3 activated carbon in terms of phe inlet concentration for different np concentrations. T ) 150 °C. Total gas flow ) 25 mL/min. (A) Whole concentration range and (B) low concentration range.
concentrations, where the opposite trend was found (Figure 5B). On one hand, in the range of low concentrations, both components are adsorbed only in the molecular-size pores, competing for the same range of micropores,27 and therefore, the binary mixture behaves as in the adsorption of a single compound. On the other hand, it is also observed that, at phe concentrations higher than 0.8 ppm (Figure 5A), the behavior of the binary mixture is opposite. As mentioned above, phe adsorption in this concentration range is occurring not only in the molecular-size pores, but also as a result of multilayer interactions. Thus, in addition to a competitive effect between np and phe molecules for the molecular-size pores, phe molecules can also adsorb on the nonmicroporous surface. Therefore, an increase in the np inlet concentration would cause a decrease in the time needed for the adsorbent breakthrough and, in this way, a decrease in the pores filled by multilayer interactions. PAH Adsorption under Real Conditions. The breakthrough volume of each material was determined in a noncontinuous method. In this method, the fluorescence spectra of XAD-2 blanks were compared with those obtained with the test adsorbent in each stage into which the sampling process was divided. The qualitative analysis of PAHs emitted during the process was performed by comparison with the model compound spectra.12,28 Figure 6 shows the fluorescence synchronic spectra of blank and XAD-2 resin obtained for different volumes of combustion gases that have been passed through the CA-13 activated carbon bed. First, it was observed that, for this activated carbon, the PAH emissions were under the detection limit, 10 ng/m3 for the most sensitive compound under these experimental conditions,29 until 2500-3000 L of combustion gases had passed through the bed, because no change was recorded
adsorbent
np
phe
np + phe
real conditions
CA-3 CA-8 CA-13
1.0 3.2 3.2
1.0 2.2 3.0
1.0 2.5 3.3
1.0 2.0-2.5 3.0-3.5
in the baseline of the fluorescence spectra. Second, comparing these spectra with those obtained from the model compounds, it was observed that, as happened when the binary adsorption of model compounds was studied, the compound breakthrough occurred according to decreasing volatility in actual combustion processes. In this way, once the breakthrough had taken place, in the next two samples, corresponding to total volumes of 3000 and 3500 L, respectively, two peak zones were detected (300-320 and 320-350 nm) in the fluorescence spectra. These peaks were mainly attributed to compounds with one, two, or three conjugated aromatic rings. In the 3500 L sample, the higher fluorescence intensity showed that the activated carbon followed saturation. Finally, in the 4000 L sample, a new zone in the fluorescence spectrum was detected. Compounds with four or more aromatic rings exhibit fluorescence in this zone. The adsorbent saturation was checked by taking samples at 4500 and 5000 L and obtaining results similar to those obtained before. The CA-3 and CA-8 activated carbons showed the same behavior as described for CA-13 but with different breakthrough volumes. The breakthrough volumes found for CA-3, CA-8, and CA-13 were 1000-1500, 2000-2500, and 3000-3500 L, respectively. Therefore, the efficiency followed the order CA-3 < CA-8 < CA-13, as was observed when phe and np/phe binary mixtures were studied. These data are shown in Table 5, where a comparison of the normalized efficiency of adsorbents between real conditions (AFBC) and model compound adsorption is listed. The normalization was performed taking the CA-3 adsorption capacity as the reference value. In this way, it can be deduced that the total porosity and wider pore size distribution are the most important textural characteristics of the activated carbons controlling the adsorption process. Although hot gas cleaning using activated carbons seems to be an appropriate process for PAH abatement, the final process optimization should take into account
5286 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003
regeneration of the adsorbents to improve the process economy. Other possibilities could be studied such as the disposal of the saturated activated carbon in landfills or their reuse in the process as fuels. However, the final decision should be made after an economic balance in the global process is performed, taking into account the costs of the different options such as raw material price, regeneration cost, environmental cost, etc. Acknowledgment The authors thank ECSC for partial financial support (Project 7220/EC/089); the Autonomic Government of Arago´n, DGA, Spain (fellowships for T.G. and J.M.L.); and the Spanish Ministry of Science and Technology, “Ramo´n y Cajal” Program (contracts for M.S.C. and R.M.). Literature Cited (1) Cousins, I. T.; Beck, A. J.; Jones, K. C. Review of the processes involved in the exchange of semi-volatile organic compounds (SVOC) across the air-soil interface. Sci. Total Environ. 1999, 228, 5. (2) Rubio, B.; Izquierdo, M. T.; Mastral, A. M. Influence of low rank coal char properties on their SO2 removal capacity from flue gases. 2. Activated chars. Carbon 1998, 36, 263. (3) Cudahy, J. J.; Helsel, R. W. Removal of products of incomplete combustion with carbon. Waste Manage. 2000, 28 (56), 339. (4) Adib, F.; Bagreev, A.; Bandosz, T. J. Analysis of the relationship between H2S removal capacity and surface properties of unimpregnated activated carbons. Environ. Sci. Technol. 2000, 34, 686. (5) Mastral, A. M.; Garcı´a, T.; Calle´n, M. S.; Navarro, M. V.; Galba´n, J. Assessment of phenanthrene removal from hot gas by porous carbons. Energy Fuels 2001, 15 (1), 1. (6) Mastral, A. M.; Garcı´a, T.; Calle´n, M. S.; Navarro, M. V.; Galba´n, J.; Removal of naphthalene, phenanthrene, and pyrene by sorbents from hot gas. Environ. Sci. Technol. 2001, 35, 2395. (7) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lo´pez, J. M.; Navarro, M. V. Moisture effects on the phenanthrene adsorption capacity by carbonaceous materials. Energy Fuels 2002, 16 (1), 205. (8) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lo´pez, J. M.; Navarro, M. V. Effects of CO2 on the phenanthrene adsorption capacity of carbonaceous materials. Energy Fuels 2002, 16 (2), 510. (9) Mastral, A. M.; Garcı´a, T.; Calle´n, M. S.; Lo´pez, J. M.; Navarro, M. V.; Galba´n, J. Three-Ring PAH Removal from Waste Hot Gas by Sorbents: Influence of the Sorbent Characteristics. Environ. Sci. Technol. 2002, 36 (8), 1821. (10) Foley, P.; Gonzalez-Flesca, N.; Zdanevitch, I.; Corish, J. An investigation of the adsorption of C-5-C-12 hydrocarbons in the ppmv and ppbv ranges on Carbotrap B. Environ. Sci. Technol. 2001, 35 (8), 1671. (11) Lee, M. L.; Novotny, M.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press: New York, 1981.
(12) Vo-Dinh, T.; Chemical Analysis of Polycyclic Aromatic Compounds; John Wiley & Sons: New York, 1989. (13) Brosillon, S.; Manero, M. H.; Foussard, J. N. Adsorption of acetone/heptane gaseous mixtures on zeolite coadsorption equilibria and selectivities. Environ. Technol. 2000, 21, 457. (14) Vahdat, N.; Theoretical study of the performance of activated carbon in the presence of binary vapor mixtures. Carbon 1997, 35 (10/11), 1545. (15) Moon, H.; Lee, W. K. A lumped model for multicomponent adsorptions in fixed beds. Chem. Eng. Sci. 1986, 41 (8), 1995. (16) Stoeckli, F.; Wintgens, D.; Lavanchy, A.; Sto¨ckli, M. Binary adsorption of vapours in activated carbons described by the combined theories of Myers-Prausnitz and Dubinin (II). Adsorpt. Sci. Technol. 1997, 15 (9), 677. (17) Lavanchy, A.; Stoeckli, F. Dynamic adsorption of vapour mixtures in activated carbon beds described by the MyersPrausnitz and Dubinin theories. Carbon 1997, 35 (10-11), 1573. (18) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lopez, J. M.; Navarro, M. V.; Galba´n, J. Study of the adsorption of polyaromatic hydrocarbon binary mixtures on carbon materials by gas-phase fluorescence detection. Energy Fuels, in press. (19) Patent, in progress. (20) Lastoskie, C. M.; Gubbins, K. E.; Quirke, N.; Pore Size Distribution Analysis of Microporous Carbons: A Density Functional Theory Model. J. Phys. Chem. 1993, 97, 4786. (21) Raymundo-Pin˜ero, E.; Cazorla-Amoro´s, D.; Salinas-Martı´nes de Lecea, C.; Linares-Solano, A. Factors controlling the SO2 removal by porous carbons: Relevance of the SO2 oxidation step. Carbon 2000, 38, 335. (22) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Assessment of PAH emissions as a function of coal combustion variables in fluidised bed. 2. Air excess percentage. Fuel 1998, 77 (13), 1513. (23) Calvo-Revuelta, C.; de la Fuente-Santiago, E.; Rodrı´guezVazquez, J A. Characterization of Polycyclic Aromatic Hydrocarbons in Emissions from Coal-Fired Power Plants: The Influence of Operation Parameters. Environ. Technol. 1999, 20, 61. (24) Garcia, T. Control de emisiones de HAP en procesos de generacio´n de energı´a. Doctoral Thesis, Instituto de Carboquı´mica, Zaragoza, Spain, 2002. (25) Suzuki, M. Adsorption Engineering; Kodansha/Elsevier: Amsterdam, 1990. (26) Patrick, J. W. Porosity in Carbons: Characterization and Applications; Edward Arnold: London, 1995. (27) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lopez, J. M.; Navarro, M. V.; Measurements of PAH adsorption on activated carbons at very low concentrations. Ind. Eng. Chem. Res. 2003, 42, 155. (28) Katoh, T.; Yokoyama, S.; Sanada, Y. Analysis of a coalderived liquid using high-pressure liquid chromatography and synchronous fluorescence spectrometry. Fuel 1980, 59, 845. (29) Mastral, A. M.; Pardos, C.; Rubio, B.; Galban, J. Analytical determination of polycyclic aromatic hydrocarbons in gases from coal conversion by synchronous fluorescence spectrometry. Anal. Lett. 1995, 28 (10), 1883.
Received for review April 2, 2003 Revised manuscript received July 9, 2003 Accepted July 27, 2003 IE0302793
