Effects of CO2 on the Phenanthrene Adsorption Capacity of

CO2 percentage, 4, 4 0%; 10%; 15% and O 30%. (T×bb ) 150. °C, 0.02 < C0 < 0.13 ppmv). Table 4. Parameters of Freundlich Isotherm for Phe. Adsorption...
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Energy & Fuels 2002, 16, 510-516

Effects of CO2 on the Phenanthrene Adsorption Capacity of Carbonaceous Materials A. M. Mastral,* T. Garcia, R. Murillo, M. S. Callen, J. M. Lopez, and M. V. Navarro Instituto de Carboquı´mica, CSIC, M Luesma Castan 4, 50015-Zaragoza, Spain Received October 16, 2001. Revised Manuscript Received December 19, 2001

Three-ring polycyclic aromatic hydrocarbons (PAHs), and phenanthrene (Phe) in particular, are the main pollutants found in the waste flue gases from organic material combustion in energy generation processes. Quantitatively, the most abundant gaseous contaminant emitted in these processes is CO2. Therefore, the aim of this paper has been to study the influence of different CO2 concentrations on the phenanthrene adsorption in a postcombustion cleanup process when carbonaceous materials are used to remove low concentrations of phenanthrene from a hot exhaust gas stream. Adsorption isotherms were obtained using CO2 and phenanthrene concentrations in the ranges of 0-30% and 0.025-3.2 ppmv, respectively. The process temperature was fixed to 150 °C in all runs. All the obtained isotherms were properly fitted to the Freundlich adsorption model and their parameters determined by regression analyses. A negative influence of CO2 in phenanthrene adsorption capacity was observed, probably due to a competitive effect between CO2 and phenanthrene for the adsorption sites within the carbonaceous material. However, it was found that high CO2 concentrations did not affect the phenanthrene adsorption more negatively than low CO2 concentrations. It was thus concluded that the most suitable carbonaceous materials for phenanthrene abatement in flue gases were those having a wide pore distribution with a high average micropore diameter. The development of mesoporosity in adsorbents promotes phenanthrene adsorption because it favors both the access of phenanthrene molecules into the micropores and the phenanthrene multilayer adsorption. Finally, the Yoon and Nelson mathematical model was applied to simulate the breakthrough curves. This model was able to predict the adsorption breakthrough in all the runs. Nevertheless, problems arose when trying to fit breakthrough curves at relative concentrations higher than 70% in those adsorbents with narrow micropore distributions.

Introduction Combustion processes of fossil fuels are currently the main source of energy generation. It is well-known that, in these processes, pollutants such as SO2, NOx, CO2 and CO are emitted to the atmosphere. Organic emissions, whose harmful nature has become the cause of growing concern, are another important source of pollution released in these processes. The inorganic emission abatement has been widely studied and as a result feasible solutions, such as fluidized bed combustion where lower temperatures reduce NOx formation and sorbents addition for SO2 abatement, have been developed. As for organic emissions, further investigations are still needed to provide similar solutions. Comparatively, organic emissions are released in much smaller amounts and show a wider variety of types than inorganic emissions. In particular, a special group of volatile organic compounds (VOC) constituted by the polycyclic aromatic hydrocarbons (PAHs) is remarkable because they are able to modify the normal metabolic functions of cells, originating mutagenic and carcinogenic effects.1-5 Nowadays, the study of PAH formation and abatement is receiving special attention.6 The influence of combus* Author to whom correspondence should be addressed. Phone: 34 976 733977. Fax: 34 976 733318. E-mail: [email protected]

tor type7-8 and the process variablessfuel type,9-11 temperature,12,13 oxygen excess percentage,14,15 space velocity,16 and sorbent addition14,17shave been recently (1) Polynuclear Aromatic Compounds, Part 1. International Agency for Research on Cancer (IARC); Chem., Environm. and Experimental Data: Lyon, France 1983. (2) Lee, M. L.; Novotny, M.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press: New York, 1981. (3) Badger, G. M. Natl. Cancer Inst. Monogr. 1962, 9, 1. (4) Evaluation and estimation of potential carcinogenic risks of PAH. Carcinogen assessment group; Office of Health and Environmental Assessment; Office of Research and Development; US EPA: Washington, DC, 1985. (5) Scientific criteria document for multimedia standards development PAH, Part 1: Hazard identification and dose-response. Ministry of Environment and Energy: Ontario, 1997. (6) Mastral, A. M.; Calle´n, M. S. Environ. Sci. Technol. 2000, 34, 3051. (7) Davies, M.; Rantell, T. D.; Stokes, B. J.; Williamson, F. Characterization of trace hydrocarbon emissions from coal fired appliances; Final Report. ECSC project No. 7220/ED821; EUR-14866: Cheltenham, U.K., Coal Research Establishment, 1992; p 18. (8) Bayram, A. Ph.D. Generation of emission factors for PAH due to Turkish coals burned in different types of combustion units. Izmir, March 1995. (9) Fullana, A.; Font, R.; Conesa, J. A.; Blasco, P. Environ. Sci. Technol. 2000, 34, 2092. (10) Levendis, Y. A.; Atal, A.; Carlson, J. B. Combust. Sci. Techonol. 1998, 134, 407. (11) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Environ. Sci. Technol. 1999, 33, 4155. (12) Karunaratne, D. G. G. P.; Gulyurtlu, I.; Cabrita, I. accepted by the Int. J. Environ. Toxicol. Chem., accepted.

10.1021/ef010250g CCC: $22.00 © 2002 American Chemical Society Published on Web 01/24/2002

CO2 and Phenanthrene Adsorption of Carbonaceous Materials

reported, concluding that although process variables may be optimized to minimize the total PAH emissions it is not possible to achieve a total PAH abatement during the combustion process. Therefore, it is advisable to design new systems, outside the combustion process, to reduce the PAH emissions. The PAH emissions, due to their high volatility and reactivity,18 can be released and supported not only onto the particulate matter but also in the gas phase.19 While the more volatile compounds with 2 and 3 aromatic rings are mainly released in the gas phase, the heavier compounds, i.e., those containing 3 or more aromatic rings, can be emitted in association with the particulate matter.19 Some authors20,21 have studied the PAH gas/ solid partitioning, finding that it is associated with the liquid vapor pressure, the size and surface area of the suspended particulate matter, the particulate matter chemical composition, and the ambient temperature. These characteristics, together with the PAH volatile character, will determine their emission way being emitted during the combustion process. The main advantage of the PAHs supported onto the particulate matter is that they can be trapped using cyclones, electrostatic precipitators, scrubbers, etc. However, the more volatile compounds and those on the smallest particulate matter can be released to the atmosphere by chimney, and their control is more difficult. Two technologies are the most promising alternatives to reduce gaseous PAH emissions: catalytic PAH elimination22,23 and PAH adsorption on carbonaceous materials.24,25 Packed beds of carbonaceous materials have been used to remove organic emissions from air streams in a variety of applications and remain the most widely used method. They may be used in chimney technology to prevent environmental pollution or to provide clean air for personnel either on an individual basis, via a personal respirator, or collectively as in air conditioning.26 There are few papers about PAH retention onto carbonaceous materials in which a quantitative analysis of the adsorption capacity of the adsorbents has been carried out. In previous papers,25,27 the authors dem(13) Mastral, A. M.; Calle´n, M. S.; Murillo, R. Fuel 1996, 75 (13), 1533. (14) Liu, K. L.; Han, W. J.; Pan, W. P.; Riley, J. T. J. Hazard. Mater. 2001, 84 (2-3), 175. (15) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Fuel 1998, 77 (13), 1513-1516. (16) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Fuel 1999, 78, 1553-1557. (17) Mastral, A. M.; Garcı´a, T.; Calle´n, M. S.; Lopez, J. M.; Murillo, R.; Navarro, M. V. Energy Fuels 2001, 15, 1469. (18) Sloss, L. L.; Gardner, C. A. Sampling and analysis of trace emissions from coal-fired power stations; IEA Coal Research, IEACR/ 77: London, 1995; p 48. (19) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Environ. Sci. Technol. 1999, 33, 3177-3184. (20) Butler, J. D.; Crossley, P. Atmos. Environ. 1981, 15, 91-94. (21) Yamasaki, H.; Kuwate, K.; Miyamoto, H. Environ. Sci. Technol. 1982, 16, 189-194. (22) Liljelind, P.; Unsworth, J.; Maaskant, O.; Marklund, S. Chemosphere 2001, 42, 615. (23) Weber, R.; Plinnke, M.; Xu, Z.; Wilken, M. Appl. Catal. B: Environ. 2001, 31, 195. (24) Cudahy, J. J.; Helsel, R. W. Waste Manage. 2000, 20, 339. (25) Mastral, A. M.; Garcı´a, T.; Calle´n, M. S.; Navarro, M. V.; Galban, J. Energy Fuels 2001, 15, 1. (26) Karpowicz, F.; Hearn, J.; Wilkinson, M. C. Adsorption 2000, 6, 337. (27) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lopez, J. M.; Navarro, M. V. Energy Fuels, in press.

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onstrated that the Phe adsorption capacitysboth in dry and humid gas streamssof different microporous carbon materials depends on the material textural characteristics and shows a good correlation to the total micropore volume (VN2) from the N2 isotherm data calculated by the Dubinin-Radusckevich equation (DR). A competitive effect between Phe and water vapor molecules for adsorption sites was also reported.27 In this paper, the influence on Phe adsorption of another important component in the combustion flue gases, CO2, is studied within the concentration ranges normally emitted at energy generation. Experimental Section Sixteen carbon materials of different origins were used to study the moisture effect on their Phe adsorption capacity. CA-1, CA-2, and CA-4 were carbon blacks obtained from pyrolysis of different tire samples at 500 °C for 30 min in a swept fixed bed reactor with a continuous nitrogen flow of 2 L/min (STP conditions) and a heating rate of 25 °C/min. CA-1 and CA-2 were not subject to any activation process whereas CA-4 was activated to increase the micropore volume. The chars were activated at 850 °C in a nitrogen flow containing 10% steam for 8 h with a heating rate of 50 °C/min. CA-3 was a coke from German Rhenish lignite supplied by RWE Rheinbraun. This particular coke is well-known all over Europe for its interesting applications in the field of waste gas and wastewater cleaning. However, no information was given about the carbonization process. CA-5 and CA-6 were active carbons obtained from cherry stones and grape seeds, respectively. Both materials were activated with steam at 700 °C during 2 h and with a heating rate of 10 °C/min in a one-step method which combines the carbonization and activation process with a 10 g/min steam flow rate. CA-11 was a commercial activated carbon from coconut shells. CA-8, CA-14, and CA-16 were different activated carbons from apricot stones. CA-8 was obtained in a two-step process: Apricot stones were carbonized in nitrogen at 800 °C for 2 h, then the carbonization product was activated with steam at 800 °C for 2 h and a steam flow rate of 10 g/min. The heating rate was 20 °C/min. CA-14 and CA-16 were obtained in a one-step method which combines the process of carbonization at 800 °C and the subsequent activation with steam for 2 and 3 h, respectively, both with a heating rate of 20 °C/min and a steam flow rate of 10 g/min. Finally, CA -7, CA-9, CA-10, CA-12, CA-13, and CA-15 correspond to different activated carbons from Spanish lignites. Activated carbon preparation was carried out by carbonization of the original raw materials, followed by activation in both CO2 and steam. Carbonization and activation were carried out in a horizontal furnace. The carbonization was conducted in a N2 flow (80 mL/min) at 850 °C with a heating rate of 5 °C/min for 2 h. The activation process was carried out at the same temperature and heating rate. The flow gases used were CO2 (80 mL/min) for CA-9 and CA-15 adsorbents for 1 and 2 h, respectively, and steam/N2 mixture (0.2 g/min) for CA-7, CA-10, CA-12, and CA-13 for reaction times between 1 and 2 h. The textural parameters of these carbon materials are compiled in Table 1. The pore volumes and the micropore distribution parameters of the carbon samples were analyzed in an automatic volumetric sorption analyzer (model ASAP 2000, Micromeritics Instrument Co., Norcross, GA) using N2 and CO2 adsorption at 77 and 273 K, respectively. To determine the experimental error sample CA-3 analyses were reproduced 5 times. The RSD obtained was 3%. The surface oxygen groups characterization was carried out on a Pulse Chemisorb 2700 (Micromeritics Instrument Co., Norcross, GA) by temperature-programmed desorption (TPD). The carrier flow rate was adjusted with an electronic mass flow controller. The gases evolved by thermal decomposition

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Table 1. Morphological and Chemical Surface Parameters DR equation VN2 VCO2 sample origin (cm3/g) (cm3/g) CA-1 CA-2 CA-3 CA-4 CA-5 CA-6 CA-7 CA-8 CA-9 CA-10 CA-11 CA-12 CA-13 CA-14 CA-15 CA-16

tire tire coal tire cherry grape coal apricot coal coal coconut coal coal apricot coal apricot

0.06 0.04 0.11 0.15 0.13 0.19 0.21 0.46 0.30 0.36 0.57 0.41 0.52 0.52 0.48 0.62

0.03 0.01 0.11 0.08 0.18 0.21 0.19 0.25 0.28 0.28 0.42 0.22 0.27 0.32 0.36 0.29

DA equationa n 2.4 2.2 2.0 1.6 2.4 2.3 2.0 1.9 2.0 1.9 1.4 1.5 1.6 1.6 1.7 1.5

TPDb

BJHc

L CO CO2 VBJH (nm) (µmol/g) (µmol/g) (cm3/g) 0.9 1.1 1.3 1.4 0.7 0.8 1.1 1.6 1.0 1.1 1.6 1.5 1.4 1.4 1.3 1.6

864 277 499 944 1860 1480 1400 839 669 678 681 1130 661 705 612 884

373 121 542 295 874 1010 578 592 353 392 465 344 357 161 321 482

0.13 0.39 0.11 0.39 0.01 0.07 0.06 0.22 0.11 0.26 0.10 0.25 0.43 0.57 0.14 0.51

a Dubinin-Astakhov equation. b Thermal Programmed Desorption. c Barnett, Joyner, Halenda method.

Figure 1. Experimental system used for Phe adsorption measurement. from the surface groups were measured by gas chromatography with Porapak N-120 and Molecular Sieve columns. The temperature program was as follows: 35 °C isothermal for 4 min from 35 to 180 °C at 35 °C/min. The experimental error of these calculated surface oxygenated groups was determined from 5 TPD analyses. The resulting percentage experimental standard deviations were calculated to be 2%. Experiments were carried out using the laboratory scale rig described in detail elsewhere26 where an additional mass flow controller was added in order to include CO2 in the gas stream (see Figure 1). The adsorption gas stream was passed through an adsorbent fixed bed reactor. The adsorbing bed, composed of 25 mg of adsorbent (a particulate size range between 100 and 200 µm) was mixed with 1.0 g of sand, inert material with the same particle size, to provide enough bed length (11 cm) ensuring a uniform flow throughout the reactor. Blank tests were carried out to check the inert material adsorption capacity. The temperature of the adsorption reactor was fixed at 150 °C ((1 °C accuracy) in the interior of a chromatographic furnace. Before proceeding with each experiment, it was necessary to reach a constant and known concentration of Phe and CO2 in the bed reactor inlet stream, by passing the outlet gas stream to the Flame Ionization Detector (FID) until these concentrations were obtained. Once reached, the gas stream was passed through the reactor starting the reaction time, which lasted until saturation was reached (C0 ) C).

Adsorption capacities, w* (mg Phe/mg carbonaceous material), were calculated as tb × C0 × Q/W, where tb was the breakthrough time for a 2% of penetration time (min), C0 was the Phe inlet stream concentration (mg/mL), Q was the flow (mL/min), and W was the adsorbent weight (mg). The PAH concentration (C) in the outlet gas stream was directly measured by a previously calibrated FID. The reproducibility analyses of Phe adsorption capacity were carried out at an influent concentration of 2.0 ( 0.1 ppmv (mean ( standard deviation). A value of 0.11 ( 0.01 mg Phe/mg CA-3 was obtained for five experimental runs.

Results and Discussion Sample Characterization. The characteristics of the sixteen carbon materials are shown in Table 1. The parameter VN2 included in Table 1 is the total micropore volume (pore size < 2 nm), calculated from the Dubinin-Radushkevich (DR) equation for the N2 isotherm data. VCO2 (pore size < 0.7 nm) is the narrow micropore volume, calculated from the DR equation for the CO2 isotherm data. The mesopore volume, pore size 2-50 nm, was calculated using the BJH method for the N2 isotherm data (VBJH). The n exponent was obtained from applying the Dubinin-Astakhov equation to the CO2 isotherm data. For most of the carbon adsorbents, n ranged from 1 to 4. An n value higher than 2 indicates a highly homogeneous, small micropore carbon adsorbent, while an n value lower than 2 means a strongly activated and heterogeneous carbon. The L value was obtained at the maximum value of the distribution curve using the Stoeckli equation when the characteristic adsorption energy, from the DA equation for the CO2 isotherm data, ranges from 20 to 42 kJ/mol and using the Dubinin equation when the E0 values were lower. There are great differences among the adsorbent morphological and chemical surface parameters compiled in Table 1 where a considerable variety in microporosity development and distribution can be observed. Although samples CA-3, CA-7, and CA-9 have different porosities, the value of parameter n in the DA equation (close to 2) and the similar values of VN2 and VCO2 indicate that they all have a homogeneous micropore distributions. In the case of CA-5 and CA-6 adsorbents, where n > 2 and VN2 higher than VCO2, the micropore distribution was very homogeneous and narrow. Finally, the remaining adsorbents, where n < 2 and VCO2 < VN2, will have a wide micropore distribution with an average micropore diameter with different values depending on their precursor nature and the experimental conditions used during activation (Table 1). Regarding the mesoporosity of adsorbents, the different values obtained will clarify their role in the adsorption process. Table 1 also shows the decomposition products from the surface oxygen-containing groups (CO and CO2) which were determined by TPD. The CO2 was released at low temperatures, as a result of the decomposition of acid surface groups. The CO groups have their origin from weakly acidic, neutral, and basic groups, which are thermally more stable and are, therefore, released at higher temperatures. In Table 1, we can see the great differences between the amount of CO and CO2 desorbed for the various samples during the TPD experiments. Table 1 shows that the oxygen content of the samples depends on the precursor nature and the experimental conditions used during activation. There-

CO2 and Phenanthrene Adsorption of Carbonaceous Materials

Energy & Fuels, Vol. 16, No. 2, 2002 513 Table 3. Correlation Coefficients, r, and Statistical Significance Levels, p, of the Relationships between Textural Parameters and Adsorption Capacities VN2 0% CO2 15% CO2 % diff a

Figure 2. Breakthrough curves for Phe adsorption on the CA-3 sample at different CO2 percentages: 4 0%; 10%; 15%, and O 30%. (T×bb ) 150 °C, C0 ≈ 1 ppmv). Table 2. Phe Adsorption Capacities (w*/C0 [mL/mg]) for the Studied Carbonaceous Materials at 0% and 15% CO2 Gas Stream adsorbent

w*/C0 0% CO2

w*/C0 15% CO2

% difference

CA-1 CA-2 CA-3 CA-4 CA-5 CA-6 CA-7 CA-8 CA-9 CA-10 CA-11 CA-12 CA-13 CA-14 CA-15 CA-16

13 8 62 64 12 74 102 199 161 189 272 234 260 285 274 332

13 13 47 61 9 46 70 102 97 101 175 161 212 172 166 235

0 -63 24 5 25 38 31 49 40 47 36 31 18 40 39 29

fore, a set of samples have been selected, with important differences regarding both porous texture and surface chemistry. Phe Adsorption. The Phe adsorption capacity was obtained from the breakthrough curves. Figure 2 shows the CA-3 sample breakthrough curves with the same Phe concentration (0.5 ppmv) and different CO2 percentages (0%, 10%, 15%, and 30%). The high slope indicates that the Phe adsorption was very quick, with minimum mass transfer effect, and very fast kinetics.28 The Phe adsorption capacities of the sixteen carbonaceous materials are reported in Table 2. This Table shows w*/C0 values in 0% and 15% CO2 gas streams, as well as the difference in percentage between them. The ratio w*/C0 is given because the experimental design did not permit the same Phe inlet concentration constant in all runs, and so, w* could not be compared. The inlet average concentration was 2.0 ( 0.1 ppmv. The influence of adsorbent characteristics on Phe adsorption has been reported in previous works.25 The Phe adsorption process occurs preferentially in those pores with molecular size diameter (ca 0.7 nm) where the adsorption potential is maximum.25 Therefore, adsorbents with a high microporosity developed around an average pore diameter higher than 0.7 nm favor the Phe retention. It can also be determined that the mesopo(28) Vermeulen, T.; LeVan, M. D.; Hiester, N. K.; Klein, G. Perry’s Chemical Engineers Handbook; Perry, R. H., Green, D. W., Maloney, J. A., Eds.; McGraw-Hill: New York, 1984.

r p r p r p

0.98 0.00b 0.95 0.00b 0.54 0.03a

VCO2

n

0.86 -0.83 0.00b 0.00b 0.77 -0.83 0.00b 0.00b 0.76 -0.37 0.00b 0.16

L

VBJH

CO

CO2

Oa

0.74 0.45 -0.28 -0.28 -0.30 0.00b 0.05a 0.30 0.29 0.26 0.76 0.52 -0.27 -0.30 -0.30 0.00b 0.04a 0.31 0.26 0.25 0.19 -0.23 0.30 0.39 0.37 0.47 0.39 0.25 0.14 0.16

95% significance. b 99% significance.

rosity presence is suitable for Phe adsorption, because not only are the Phe molecules allowed to enter the micropores but also the retention increase through multilayer adsorption is favored. The presence of CO2 in the gas stream with a concentration similar to the one existing in the new systems for energy generation decreases the total amount of Phe adsorbed. This is probably due to a competitive effect between both molecules to access the adsorption sites (see Table 2). In Table 3, the Pearson correlation coefficients and the significance levels between the Phe adsorption capacities and the adsorbent textural parameters are reported. It is observed that the incorporation of CO2 to the gas stream does not change the main textural parameters that control the adsorption process. The total micropore volume, VN2, which is the main property in the Phe adsorption process, is positively correlated to Phe adsorption capacity (see Table 3). The narrow micropore volume, VCO2, is also positively correlated to Phe adsorption capacity but with a correlation coefficient lower than VN2. This is probably due to diffusional and steric problems of Phe molecules to get into the narrow micropores. According to the correlation coefficients obtained, the more suitable micropore distribution for Phe retention would be wide micropore distributions (n value lower than 2) with a high micropore average size to avoid diffusional problems. Regarding mesoporosity, the Phe adsorption is positively correlated to the VBJH with a 95% significance level. Mesopores not only favor the Phe diffusion into the micropores but also increase the Phe retention by multilayer adsorption because of adsorbate-adsorbate interactions. The influence of adsorbent surface chemistry was determined from the correlation coefficients between the Phe adsorption capacities and the oxygen surface groups desorbed in TPD. Statistical significance between Phe adsorption capacity and oxygen surface groups was not found probably due to the physical character of the Phe adsorption process at the operating conditions. In Table 3, the influence of the textural parameters in the change of the Phe adsorption capacity when CO2 (15% CO2) is added to the gas stream is also shown. It is remarkable that there is a high statistical significance with VCO2. This is probably because the CO2 molecules are retained mainly in the molecular size micropores (ca. 0.3 nm)sand therefore, the higher the narrow microporosity, the higher the CO2 adsorptionscompeting for the Phe adsorption into the micropores. Adsorption Isotherm Modeling. Adsorption isotherms of Phe vapors on CA-3 sample were obtained at different CO2 percentages, 0%, 10%, 15%, and 30% at 150 °C (see Figures 3 and 4). The adsorption isotherms are close to type I of the BDDT (Brunauer, Deming,

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Figure 3. Phe adsorption isotherms on the CA-3 sample at different CO2 percentages, 4 0%; 10%; 15% and O 30%. (T×bb ) 150 °C, 0.02 < C0 VCO2, n < 2 and high VBJH, the Phe molecules did not have mass transfer problems to get into the micropores, independently of the percentage of surface coverage. In Figure 6, the breakthrough curves for an adsorbent with a narrow and homogeneous micropore distribution, CA-6, are shown. It is observed that the model did not fit the breakthrough experimental data obtained for Phe relative concentrations higher than 70%. This fact is probably due to the mass transfer and diffusional problems of Phe molecules when the percentage of surface coverage caused by a micropore entrance blocking increases. The Yoon and Nelson model does not seem to be an appropriate tool to predict the behavior of an active carbon at high surface coverage, probably because it does not take into account changes in the Phe diffusion coefficient. Other mathematical models37,38 consider the change of this parameter at high bed conversions and probably will be able to simulate the breakthrough curve in the whole range of surface coverage. In Table 5, τ and k, two parameters of the Yoon and Nelson model for Phe adsorption on the studied adsorbents are reported. The τ parameter is positively correlated to the Phe adsorption capacity, r2 ) 0.98, so that τ values depend on the microporosity, the micropore distribution, and the mesoporosity, as reported for the Phe adsorption capacity. According to Yoon and Nelson’s postulates,39 the k parameter should be a constant for (37) Suzuki, M. Adsorption Engineering; Kodansha, Ed.; Elsevier: Amsterdam, 1990. (38) Ruthven, D. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984.

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Table 6. Values of Simulation Parameters for Phe onto CA-3 Adsorbent % CO2

a

kw

0% 10% 15% 30%

1.9 1.9 1.9 1.9

2.4 1.4 1.8 1.0

a given adsorbate and a specified type of adsorbent, and also independent of the adsorbate concentration and the flow rate. The k values reported in Table 5 are dependent on the adsorbent nature, but no relationship has been found between them and the physical and chemical adsorbent properties with a significance level higher than 95%. According to previous results, comparing τ and k parameters obtained from 0% and 15% CO2 gas streams, it could be observed that the τ values were always lower working with CO2, due to the competitive effect between CO2 and Phe for the adsorption sites. Yoon and Nelson, on the basis of chemical kinetic principles, postulated that the adsorptive reaction is an nth-order reaction, and formulated the following relationship between τ and the reactant concentration: (39) Tsai, W. T.; Chang C. Y.; Ho, C. Y.; Chen, L. Y. Sep. Sci Technol. 2000, 35 (10), 1635.

t)

kw

(3)

Cia-1F

where kw is a proportionality constant, Ci is the inlet effluent concentration, a is the reaction order, and F is the flow rate of the gas stream. This equation can be transformed into eq 4 to determine the reaction order, and the proportionality constant kw:

ln

()

( )

1 F ) (a - 1) ln(Ci) + ln τ kw

(4)

Plotting ln(1/τ) versus ln(Ci), the reaction order can be calculated from the slope and the proportionality constant from the intercept. In Table 6, the reaction order, a, and the proportionally constant, kw, are shown. It is observed that the reaction order is not a function of either CO2 presence or the CO2 percentage. This conclusion seems to support the hypothesis above whereby the presence of CO2 does not affect the Phe adsorption mechanism. Acknowledgment. The authors would like to thank the European Community for its partial support and to the DGA (Aragon, Spain) for its fellowships (T. G. and J. M. Lopez). EF010250G